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+The Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Disease in Plants
+
+Author: H. Marshall Ward
+
+Release Date: February 29, 2012 [EBook #39011]
+
+Language: English
+
+Character set encoding: ISO-8859-1
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DISEASE IN PLANTS ***
+
+
+
+
+Produced by Chris Curnow, Lisa Reigel, and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+
+Transcriber's Notes: Variations in spelling and hyphenation have been
+left as in the original. Words in italics in the original are surrounded
+by _underscores_. Words in bold in the original are surrounded by =equal
+signs=. Ellipses match the original.
+
+A few typographical errors have been corrected. A complete list as well
+as other notes follows the text.
+
+
+
+
+                           DISEASE IN PLANTS
+
+
+
+
+                           DISEASE IN PLANTS
+
+
+                                   BY
+
+                    H. MARSHALL WARD, Sc.D., F.R.S.
+
+  FELLOW OF SIDNEY SUSSEX COLLEGE, HONORARY FELLOW OF CHRIST'S COLLEGE
+         AND PROFESSOR OF BOTANY IN THE UNIVERSITY OF CAMBRIDGE
+
+    PRESIDENT OF THE BRITISH MYCOLOGICAL SOCIETY, AND FELLOW OF THE
+       LINNEAN AND ROYAL HORTICULTURAL SOCIETIES; HONORARY FELLOW
+          OF THE MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY
+               AND OF THE BOTANICAL SOCIETY OF EDINBURGH
+
+
+                                 London
+
+                       MACMILLAN AND CO., LIMITED
+
+                    NEW YORK: THE MACMILLAN COMPANY
+
+                                  1901
+
+                         _All rights Reserved_
+
+
+
+
+               GLASGOW: PRINTED AT THE UNIVERSITY PRESS
+
+                     BY ROBERT MACLEHOSE AND CO.
+
+
+
+
+PREFACE.
+
+
+It has often been represented to me that the cultivators of plants,
+among whom are to be included planters and foresters, as well as
+agriculturists and gardeners of every kind, are more particularly
+concerned with, and interested in, the maladies themselves of the plants
+they grow, than in the life-history of the fungi, insects or other
+organisms to which they are due, or in the physiological processes which
+are involved; and although it is impossible to really understand any
+disease unless we also understand the processes by which it is brought
+about, there is room for sympathy with the point of view of the
+cultivator. He says, in effect, "I do not want to know all about the
+biology of the fungus of wheat-rust, or of the _phylloxera_, nor do I
+want to learn what experts can tell me about the action of bacteria in
+soil, or the process of starch-formation in the leaves: I have neither
+the time nor the means to master these details. What I want is guidance
+as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir
+trees, turnips, etc., and what I am to do to set things right." Just
+so. With the latter part of this cry one must sympathize, much as a
+doctor does with the wail of the parent who calls him in to cure his
+sick child--we need not stop to classify or compare the motives of the
+parent and the cultivator, and perhaps I had done better to select a
+breeder of sheep with his flock and a veterinary doctor in the
+illustration, but we will let it pass; and as regards the former part of
+the cry, I do not know that the plant-doctor can expect the cultivator
+to be initiated in the aetiology of the disease any more than the
+physician expects the parent to understand the biology of the typhoid
+bacillus. That both the cultivator and the parent would be the better
+for a real knowledge of the disease in either case must be admitted--nay
+insisted on, provided the knowledge _is_ real--but we have to deal with
+facts, and it is a fact that the clients of both doctors are impatient
+of the details of the case.
+
+Now, of course, I am aware that no short cut or "royal road" to science
+exists, and if a man is going to train up trees or other plants, he
+ought to know all about them in health and in sickness, in youth and in
+old age, and he ought to learn everything about the soil they grow in,
+the air that surrounds them, the enemies that beset them, and all the
+multifarious relations of these one to another; but when I look at my
+boy and reflect how much his nurse, his schoolmaster, his tutor, his
+doctor, and his parents _ought_ to know successively and simultaneously
+about him in sickness and in health, and about his surroundings, etc., I
+begin to wonder whether there is not after all something to be said for
+the cultivator's point of view.
+
+Moreover, the cultivator knows a good deal about his plants which I do
+not know, and although I should much like to know it, his plea of want
+of time rings in my ears and the conviction strikes home that one ought
+to try and meet his views, and tell him something about disease as
+manifested in plants without insisting on his becoming a professional
+mycologist, entomologist, agricultural chemist, and philosopher.
+
+Of course, beyond a certain point, it is his lookout how much the
+information is worth, and its educational value--a very different
+matter--is sure to suffer from any restrictions imposed on the treatment
+of the subject; but if the theme of disease in plants, treated from a
+general point of view--I was about to write "treated in a popular
+manner," but that is impossible until physiology and mycology are more
+widely taught--enables him to understand better the questions he puts to
+himself, and, still more, if it stimulates him to enquire further into
+the inexhaustible field of science glimpsed at, something may come of
+it.
+
+The purpose of these essays is to treat the subject of disease in plants
+with special reference to the patient itself, and to describe the
+symptoms it exhibits and the course of the malady, with only such
+references to the agents which induce or cause disease as are necessary
+to an intelligent understanding of the subject, and of the kind of
+treatment called for. Consequently I have avoided any unnecessary
+classification or elaborate descriptions of parasitic fungi or insects,
+histological details of the tissues of plants, chemical and physical
+details regarding the soil, and even matters purely physiological as far
+as possible. Several admirable works on these subjects are already
+available, and must be referred to for further details.
+
+It is, however, quite out of the question to avoid technicalities,
+though I have chosen the simpler course wherever it was found feasible,
+and have tried to so employ the examples selected that the student who
+wishes to go further into the matters dealt with may turn to special
+treatises for further information. For one eminently technical section I
+ought perhaps to apologise, but the temptation to try and set forth, in
+concrete form and suitable for the purposes of this book, some account
+of what is known of the most essential and profound factors concerned in
+the difficult question of the nature of life and death, health and
+disease, was great. Probably my apology should go further, and apply to
+what after all must be failure to explore this mystery to the bottom: my
+only excuse must be that it may stimulate others to go further.
+
+It was an afterthought to add, in Part I., the considerations on the
+factors which influence the plant regarded as a living machine, so to
+speak, in order that the student may the better apprehend the point of
+view taken of the bearings of the matters discussed in Part II.
+
+With regard to references, it seemed a better plan to give, in the form
+of notes after each chapter, the titles of the principal books and
+papers on which a student may base a further course of reading, than to
+overweight the pages of what is, after all, merely an introductory
+sketch to a huge subject, with detailed quotations from the numerous
+sources of information made use of. I have freely expressed my own
+opinions, but the sources for others are, I hope, as freely given. It
+will, however, be understood that I have not aimed at a complete
+bibliography, and, particularly, I have only given foreign references
+where it seemed that adequate treatment of the subject could not be
+found in English.
+
+My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly
+looked through many of the proofs, and given me the benefit of several
+suggestions: and to my wife for the very material aid she has afforded
+me in the preparation of the index.
+
+                                                H. MARSHALL WARD.
+
+     CAMBRIDGE,
+       _November, 1900_.
+
+
+
+
+CONTENTS.
+
+
+  _PART I.--SOME FACTORS._
+
+
+  CHAPTER I.
+                                                    PAGE
+  THE PLANT AND ITS SURROUNDINGS,                      1
+
+
+  CHAPTER II.
+
+  THE PLANT AND ITS FOOD,                              7
+
+
+  CHAPTER III.
+
+  THE PLANT A LIVING MACHINE,                         15
+
+
+  CHAPTER IV.
+
+  METABOLISM,                                         23
+
+
+  CHAPTER V.
+
+  ROOTS AND ROOT-HAIRS,                               35
+
+
+  CHAPTER VI.
+
+  THE FUNCTIONS OF ROOT-HAIRS,                        45
+
+
+  CHAPTER VII.
+
+  THE BIOLOGY OF SOIL,                                56
+
+
+  CHAPTER VIII.
+
+  HYBRIDISATION AND SELECTION,                        69
+
+
+  _PART II.--DISEASE IN PLANTS._
+
+
+  CHAPTER IX.
+
+  PHYTOPATHOLOGY. DERIVATION AND MEANING,             85
+
+
+  CHAPTER X.
+
+  HEALTH AND DISEASE,                                 91
+
+
+  CHAPTER XI.
+
+  CAUSES OF DISEASE,                                  99
+
+
+  CHAPTER XII.
+
+  CAUSES OF DISEASE. THE LIVING ENVIRONMENT,         108
+
+
+  CHAPTER XIII.
+
+  NATURE OF DISEASE,                                 119
+
+
+  CHAPTER XIV.
+
+  NATURE OF DISEASE (_Continued_),                   130
+
+
+  CHAPTER XV.
+
+  SPREADING OF DISEASE AND EPIDEMICS,                142
+
+
+  CHAPTER XVI.
+
+  THE FACTORS OF AN EPIDEMIC,                        149
+
+
+  CHAPTER XVII.
+
+  REMEDIAL MEASURES,                                 159
+
+
+  CHAPTER XVIII.
+
+  VARIATION AND DISEASE,                             168
+
+
+  CHAPTER XIX.
+
+  SYMPTOMS OF DISEASE,                               179
+
+
+  CHAPTER XX.
+
+  SYMPTOMS OF DISEASE (_Continued_),                 186
+
+
+  CHAPTER XXI.
+
+  ARTIFICIAL WOUNDS,                                 194
+
+
+  CHAPTER XXII.
+
+  NATURAL WOUNDS,                                    204
+
+
+  CHAPTER XXIII.
+
+  EXCRESCENCES,                                      212
+
+
+  CHAPTER XXIV.
+
+  EXCRESCENCES (_Continued_),                        222
+
+
+  CHAPTER XXV.
+
+  EXUDATIONS AND ROTTING,                            227
+
+
+  CHAPTER XXVI.
+
+  NECROTIC DISEASES,                                 240
+
+
+  CHAPTER XXVII.
+
+  MONSTROSITIES AND MALFORMATIONS,                   246
+
+
+  CHAPTER XXVIII.
+
+  PROLIFERATIONS,                                    257
+
+
+  CHAPTER XXIX.
+
+  GRAFTS,                                            262
+
+
+  CHAPTER XXX.
+
+  LIFE AND DEATH,                                    271
+
+
+  INDEX,                                             293
+
+
+
+
+_PART I._
+
+SOME FACTORS.
+
+
+
+
+CHAPTER I.
+
+THE PLANT AND ITS SURROUNDINGS.
+
+     _The plant the central object of study--soil, climate,
+     atmosphere, etc., are factors of its environment. Agricultural
+     chemistry. The plant a machine. Physiology._
+
+
+If I were asked to sum up the most important result of the numerous
+advances made during the past decade in agriculture and forestry, I
+should reply--the clearer and wider recognition of the fact that the
+plant itself is the centre of the subject, and not the soil, climate,
+season, or other factors of its environment. Until comparatively recent
+times it was the habit of farmers, foresters, planters, and gardeners,
+all the world over, to look upon the plant as a mere item or as a
+mysterious if important one in their calculations, and to regard the
+soil as the chief factor in their studies.
+
+Now all is changing, and the world is gradually awakening more and more
+to the recognition of the truth that the soil and the clouds and the
+atmosphere are merely reservoirs of more or less inert materials, from
+which the living plant draws its supplies, and works them up, by means
+of energy focussed from the sun, into new plant substance.
+
+In other words, the more far-seeing pioneers of scientific agriculture
+and forestry, etc., are recognising that agricultural chemistry is not
+the be-all and end-all of agricultural science; but that, in place of
+the study of the chemical analyses of dead soil, water, air, and
+plant-remains, which has so long held sway, largely owing, I think, to
+the influence of Liebig, the student should have his attention more
+concentrated on the living plant itself and on the physiological actions
+which make up its life. He must regard the living plant as a sort of
+working machine--infinitely more complex than any machine made by man,
+but a machine nevertheless--the purpose of which is to store up energy
+from the sun, and so to add to our wealth on this planet, at the expense
+of the extra-terrestrial universe.
+
+It is not, be it noted, that the new study proposes to ignore or abandon
+the old studies: modern physiology owes too much to the physics and
+chemistry on which it is partly based, and to the labours of De
+Saussure, Ingenhousz, Priestley, and others, for that. But it is that
+the new study recognises that the central point, to which all views must
+be focussed, is not the one that it was formerly supposed to be. The
+student is still taught that the chemistry of soils yields valuable
+information, and that lessons of importance are derived from comparisons
+of the analyses of the ashes, etc., of plants; but he is no longer able
+to cherish the hope, however remotely, that such studies solve his most
+important problems.
+
+The scene--or rather the point to which attention is now directed--is
+the living, working, energy-accumulating plant itself, and not the dead
+store of materials in the soil. The reason for the change is not far to
+seek: it is due to the enormous strides made in the study of the
+physiology of plants during the last quarter of a century, and the
+subject abounds in examples illustrating the marvellous advances that
+have been made, and at the same time showing how, in the progress of
+researches, made for their own sake--_i.e._ in pursuit of satisfaction
+for the intense curiosity of the scientific man--all kinds of side
+issues turn up which prove to be of value in practice, and suggestive of
+further thinking.
+
+At the beginning of the nineteenth century--_i.e._ about 1820--the best
+thinkers were giving up the old ideas that the environment supplied
+food, as such, to plants, and had recognised that the plant takes up
+substances from without and rearranges these in its own body.
+
+The next twenty years or so form a very dark interval in plant
+physiology, chiefly owing to the influence of the assumption of a
+special "vital force," an assumption which was not allowed merely to
+serve as a hypothesis put forward to stimulate research and suggest
+better ideas, but which gained a hold over men's powers of reasoning to
+an extent which now appears monstrous and phenomenal.
+
+Many errors crept in during this reign of terror, one of the most fatal
+of which was De Candolle's revival of the idea of "spongioles"; and
+another, equally disastrous in many of its effects, was the conception
+of a sort of vegetable food-extract, humus, existing in the soil in a
+form peculiarly suitable for direct use by plants. It was during this
+period that the confusion between the processes of respiration and
+carbon-dioxide assimilation arose, and exerted its effects for evil into
+our own day.
+
+The now astounding statement that oxygen-respiration in plants did not
+occur, laid the foundation of many subsequent difficulties, and so did
+the positive and authoritative views on the uses of minerals to the
+plant. Liebig, in fact, stood in the invidious position of being a high
+authority on purely chemical questions, who was impelled to give
+opinions on matters which can only be solved by physiological
+experiments: his great service was to clear up mistakes as regards the
+chemistry of soils and of plants--his great mistakes were due to his
+pronouncing on physiological matters; and it may be doubted whether his
+great services to the purely chemical side of subjects connected with
+agricultural matters are the more to be admired, or the disastrous
+influence of his statements on subjects which do not belong to the
+domain of chemistry should be the more deplored. Be that as it may, he
+handed on to succeeding generations some weighty errors as regards
+plant-life, and taught the agriculturist to regard chemical analyses of
+soils and plant ashes with a reverence which obstructed progress for
+some time. As a set-off to this we must place his contributions to the
+destruction of the bugbear vitalism, which was simply preventing
+enquiry, and his services in bringing together and sifting with power
+and originality all that had been then acquired as regards the chemistry
+of the plant, the soil, and the atmosphere.
+
+That Liebig was indispensable in 1840-1850 is one thing; but that his
+influence should extend to the present day is quite another, and his
+inevitable mistakes were almost as powerful for future evil, as his
+clear exposition of the chemistry of his day was productive of immediate
+good.
+
+Boussingault, working at the same time, 1837-1855, but experimentally
+with the living plant, taught us more about these matters than
+any investigator of the time, though it is very probable that the
+stimulus of Liebig's speculations, good and bad, had its effect in
+impelling Boussingault to devote his splendid methods to problems of
+plant-nutrition. Boussingault's contributions to our knowledge of the
+composition of the dead plant cannot be over-estimated; but he did more
+than this, because he so clearly apprehended the necessity for asking
+his questions directly of the living plant, instead of deducing from
+chemical principles what might be supposed to occur in it; and although
+future researches showed that even so careful an investigator solved a
+problem of first importance--viz. the question of the fixation of free
+nitrogen--the wrong way, it will be found that so far as he did go his
+conclusions were sound, and well calculated to inspire the confidence
+with which the world received them. As we are here concerned more
+especially with the botany of agriculture, however, it is unnecessary to
+dwell longer on these matters, or on the similar and even more extensive
+experiments, of world-wide reputation, carried on for so many years, and
+still being carried on under the liberal auspices of Sir John Lawes, at
+Rothamsted. Moreover it may be necessary to return to some of these
+points later on.
+
+
+NOTES TO CHAPTER I.
+
+     The reader will find a further general account of these
+     matters in Sachs' _Lectures on the Physiology of Plants_,
+     especially Lectures I. and XII., Engl. ed., Oxford, 1887. He
+     may then proceed to Pfeffer's _Physiology of Plants_, Engl.
+     ed., 1899, chapter I., and to the account of the history of
+     the subject in Sachs' _History of Botany_, Oxford, 1890,
+     especially pp. 359-375 and 445-524. References to more special
+     literature will be found in Pfeffer.
+
+
+
+
+CHAPTER II.
+
+THE PLANT AND ITS FOOD.
+
+     _The food of plants--"Vital force"--Other errors--Liebig and
+     Boussingault--The botany of agriculture. The synthesis of
+     carbohydrates--The physiology of plant-nutrition. The
+     persistence of misconceptions._
+
+
+The year 1860 may be regarded as a landmark of importance in the history
+of plant physiology, for it was in that year that Sachs discovered that
+the bringing together of water and carbon-dioxide, in the green
+chlorophyll-corpuscles of the plant exposed to sunlight, results in the
+formation of the grains of starch found in these corpuscles.
+
+Previous to this date Dutrochet (1826-37) had introduced the then crude
+idea of osmosis into physiology; vegetable anatomy had improved, and the
+modern conceptions of the living cell, protoplasm, nucleus, etc., were
+slowly looming; sieve-tubes had been discovered, and the proteids and
+starch in various parts of the plant examined; and the suggestion was
+abroad, replacing Liebig's idea that plant acids were the first
+products of carbon-assimilation, that some substance, of a slimy nature,
+was manufactured in the cells of the leaves and thence distributed as
+the formative material from which the plant constructed its parts. Davy
+and Boussingault had even surmised that a carbohydrate might be the
+first-formed product in assimilation.
+
+There can be little doubt that Sachs' classical proof, by direct
+physiological observation and experiment, first brought forward the
+truth of organic synthesis in the plant in a concrete and convincing
+form.
+
+But it did more than that. It laid the foundation of the modern
+physiology of plant-nutrition on ground already prepared by De Saussure
+and the earlier workers; for, in addition to emphasising the truth of
+organic synthesis--a truth which had been gradually impressing itself on
+the world for some years--Sachs' discovery showed clearly the real
+meaning of carbon-assimilation as a process for obtaining combustible
+food, which the plant then proceeds to make use of.
+
+Many points were rapidly cleared up at once, or if not explained were at
+least put into a strong light for further enquiry, and plant-nutrition
+soon ceased to be the mysterious subject for all kinds of wild
+conjectures that it had hitherto been.
+
+The meaning of thin leaves, with numerous stomata and finely ramified or
+divided vascular bundles, became more apparent, as also did the
+significance of the ascending transpiration current; the storage of
+starch-grains in tubers, medullary rays, roots, seeds, etc., obtained
+meanings not understood before; the spread of roots in the soil, and the
+gradually discovered properties of the finer rootlets and of the
+root-hairs, fitted naturally into their places; and, in short, a
+thousand facts, otherwise isolated, became collated into an intelligible
+system, full of suggestions for new work, such as has since gone on and
+is now being pursued with an activity and success never before realised
+in the history of science.
+
+As time went on, while the general truth of Sachs' views was confirmed,
+a number of detailed discoveries were made which seemed to contradict
+them in certain points. It was found that not all leaves form starch,
+for some contain sugar or oil; but Holle and Godlewski proved
+experimentally that this oil may be replaced by starch if the conditions
+of assimilation are slightly modified. More recently Hébert discovered
+that the stalks and leaves of grasses contain a peculiar form of gum,
+which was formerly confounded with starch, a substance not abundant in
+them. Then came Schimper's discovery of starch-forming corpuscles,
+which, if supplied with sugar, are able to form starch-grains in the
+dark, as in tubers, etc., underground; and as subsequent researches have
+proved that the chlorophyll-corpuscles--which are morphologically the
+same as the starch-forming corpuscles and can be replaced by them--are
+also able to form starch-grains from sugar, as proved by the experiments
+of Boehm, Acton, Meyer, Laurent, Bokorny, Saposchnikoff, and others, it
+soon became evident that nothing essential needed altering in Sachs'
+view that starch is the first visible product of carbon-dioxide
+assimilation, only it became clearer that the starch-grains are built up
+by the protoplasm from glucose or some similar body, and represent so
+many packets of reserve materials put by for the present because not
+required for the immediate needs of the cell.
+
+Boussingault showed, about thirty years ago, that assimilation soon
+stops in green leaves if cut off from the plant, not because the leaves
+die, but owing to some "maximum capacity" being attained. Sachs had
+shown that the starch passes down to other parts of the plant in
+solution as glucose.
+
+Neither time nor space will permit me to go into the enormous field of
+research and results opened up by these and similar observations made
+between 1860-70. It must suffice to say that they led to the discovery
+and study of the diastatic and other enzymes in the leaves and other
+green parts of plants, and to a clearer understanding of what was
+already known of them in seeds, and this knowledge reacted at once on
+our insight into the processes of transport of reserve materials and
+constructive materials from one part of the plant to another, matters
+which will be referred to later on.
+
+It remains to explain Boussingault's difficulty as regards the cessation
+of assimilation. Recent researches confirm the view that at least three
+causes are at work to bring about the inhibition of the
+carbon-assimilation: first, the chlorophyll-corpuscles become filled to
+excess with starch, which cannot get away because all the passages are
+full and the products are inhibiting the further action of the enzymes
+which should dissolve the solid granules; secondly, the leaf being
+detached from the plant explains why the soluble products cannot get
+away, for this makes a great difference in the rate of exhaustion of the
+leaf; and, thirdly, the same fact involves that the leaf can obtain no
+further supply of salts of potassium, etc., without which elements the
+processes in question cannot go on.
+
+These and numerous other deeper insights into the process of
+assimilation, obviously strengthen the force of Sachs' discovery; though
+it by no means necessarily follows that starch-grains are always the
+resting form of the products of assimilation, and we now know that such
+is often not the case: we now have much deeper glimpses into the initial
+products of carbon-assimilation than Sachs had in 1860, but this
+enhances rather than detracts from the importance of his splendidly
+worked-out discovery. Put more generally, we may now say that the
+process of carbon-dioxide assimilation in green leaves under the
+influence of light is a process of synthesis--photo-synthesis--resulting
+in the building up of a carbohydrate such as sugar, inulin or starch
+from the elements carbon, hydrogen and oxygen.
+
+But it must not be supposed that the importance of Sachs' discovery, and
+the rapid consequent extensions of our knowledge, did their work
+forthwith in disabusing men's minds of old and erroneous notions. To say
+nothing of numerous smaller misconceptions which still held their ground
+owing to the stupendous ignorance of plant-physiology which prevailed,
+we find incompetent teachers and text-books were still propagating ideas
+worthy of ancient times. The confusion between oxygen-respiration and
+the gas interchanges in carbon-assimilation was by no means eliminated
+even recently, though it can no longer withstand the deliberate
+onslaughts now made on it. That the roots take up food as such from the
+soil, and that that food is directly employed by the plant for its
+nutrition is even yet implied in daily conversation around us; and
+although matters have advanced so far that everyone now knows that the
+substances at the roots must be in solution, ere they can be received
+into the plant, it sometimes leads to astonishing replies, if we press
+the question very far as to how the absorption takes place, in an
+elementary examination of agricultural students. That manures are foods
+to the plant, that sap circulates, that transpiration is of use to keep
+the plant cool, and wood is a "porous body," etc., are only a few of the
+misconceptions still current, in a decade that has found publishers for
+a work advocating that roots are congealed sap, and that the leaves of
+plants absorb the moisture and dust of the air, and so provide the plant
+with food, and for a paper explaining the action of root-hairs as tubes
+with open pores at their tips. But the gravest misapprehensions current
+among us are due to the crude ideas as to what a plant really is: this,
+I take it, is owing to the difficulty of grasping what physiologists
+mean by organised structure, and leads to regarding the living being
+either as a mere aggregation of chemical compounds, built up by the
+ordinary play of chemical forces, as we know them, acting on dead
+matter, or, as in the days before organic chemistry, as a mysterious
+entity endowed with "vital force," and with properties not amenable to
+scientific investigation. The mistaken notions as to the powers of roots
+to "select" those substances which the plant requires, and to reject
+useless ones was merely an expression of this belief.
+
+The rock on which all are liable to come to grief--the chemist or
+physicist who requires all his facts in terms of analyses and
+proportions by weight, and therefore takes too mechanical a view of the
+subject, or the man who is not scientifically trained at all, and
+therefore is more liable to go to the other extreme and regard the plant
+as a mysterious something which grows and has poetical associations and
+traditions--is the great fact of organised structure, and it is the
+recognition of this fact and some of its consequences which has altered
+the whole position of the subject, and brought the study of the plant
+into the domain of physiology. The living plant, its structure and
+organisation, the functions of its mechanism, and its relations to the
+environment, thus forms a subject apart from that which concerns the
+chemical composition of the plant and its environment, and this
+distinction designates, in a word, as it were, the change which has been
+brought about by modern biology.
+
+A point to be emphasised to the utmost where agricultural students are
+concerned is that the essential process of feeding is the same in a
+green plant, a fungus, and an animal; the greatest confusion still
+exists with regard to this matter, owing to misconceptions as to the
+real meaning of the functions of the chlorophyll-corpuscles when
+supplied with carbon-dioxide and water and the energy of the sun's rays.
+The plant does not feed on carbon-dioxide, any more than it feeds on
+oxygen--it feeds on the organic material after it has been constructed,
+and the chlorophyll-function is merely one mode of obtaining supplies of
+such organic substance.
+
+
+NOTES TO CHAPTER II.
+
+     In addition to the references in the last chapter, the student
+     should consult Sachs' _Lectures_, XVII.-XIX., and Pfeffer's
+     _Physiology_, pp. 287-329, for the further development of this
+     subject. An excellent résumé, with new facts and points of
+     view, will be found in Dr. Horace Brown's "Address to the
+     Chemical Section," _British Association Reports_, Dover, 1899;
+     and "Chemistry and Physiology of Foliage Leaves" in _Trans.
+     Chem. Soc._, 1893, p. 604. See also Blackman, "Experimental
+     Researches on Vegetable Assimilation and Respiration," _Phil.
+     Trans._, 1895; and Parkin, "Formation, etc., of Carbohydrates
+     in Monocotyledons," _Phil. Trans._, 1899.
+
+
+
+
+CHAPTER III.
+
+THE PLANT A LIVING MACHINE.
+
+     _The plant a machine into which energy and material are
+     taken--Carbon assimilation--Feeding--Accumulation and
+     transformations in the plant. The action of light--The
+     chlorophyll-function._
+
+
+The relations of the plant to the environment can only be understood by
+taking into account the results of modern physiological discoveries.
+These teach us that the living plant is a highly complex machine, the
+details of its organisation and structure being much more numerous and
+much more closely correlated at numerous points, than the parts of any
+other machine known to us.
+
+They also teach us that it is supplied with energy from without, as any
+other machine; and that when so supplied, and properly working, the
+living structure or machinery does work, also as other machines. But
+modern physiology goes further, in that it renders some account of the
+ways by which the external energy is taken into the plant, and there
+applied to do work, or stored up for a time in order that it may be used
+to do work at some future time.
+
+The accumulation of energy thus ensured is associated with corresponding
+changes of material substance, and the principal means for bringing this
+about is recognised in the assimilation of carbon-dioxide--photo-synthesis.
+
+In this process energy enters the chlorophyll-corpuscle in the form of
+the radiant energy of the sun, it is there directed in the mechanism of
+the protoplasm, so as to do work on the molecules of water and
+carbon-dioxide which have also been brought into the machinery; this it
+does, breaking asunder their stable structure into unstable bodies,
+which then re-combine in different ways to form a carbohydrate, such as
+starch, and this starch is temporarily stored as grains, while oxygen
+escapes.
+
+Each starch-grain, therefore, is to be regarded as a packet of matter
+and of potential energy, as it were, capable of yielding up the latter
+at any future time, when put under such circumstances that it must do
+so. Such stores of energy-yielding substance, if I may use the
+much-abused phrase, form the principal food of the plant--or of an
+animal, if it steps in and takes them--and we now see that the process
+of carbon-dioxide assimilation, as it has perhaps unfortunately been
+called, is not the same thing as the process of feeding, for the
+_feeding_--_i.e._ the nutrition proper--of the plant does not begin
+until the _food_ has been thus obtained.
+
+We now see what the real position of the plant is, to its environment,
+whether the latter be living or dead. From our point of view, the plant
+serves as a centre for bringing together the substances obtainable from
+the soil, and those derived from the atmosphere, and so focussing and
+directing the radiant energy of the sun upon these substances, that they
+are broken up, and some of their constituents synthesised, with
+absorption of energy, into a body, such as starch, containing more
+energy than did the original substances taken together or separate. It
+matters little whether the actual carbohydrate thus synthesised is
+starch, or sugar or inulin: the point is that energy has been gained
+from outside and bound up with the acquired material for further use.
+But modern physiology has carried matters much further than this, and
+especially in the three following directions.
+
+In the first place, it has shown that much of the energy thus stored
+from without in the plant is again liberated in the process of oxygen
+respiration, and expended partly as appreciable heat and partly as
+driving force for stimulating the machinery of the living plant to
+further activities.
+
+In the second place, part of it is rearranged with the rearrangement of
+the molecules with which the energy is bound up, as it were, so that
+work of various kinds is done _in_ the machinery of the plant: I refer
+to various metabolic and surface-actions resulting from the peculiar
+mode of presentment of the resulting substances, for instance the
+production of osmotic pressures in the cell.
+
+And, thirdly, part of the synthesised substance is worked up into higher
+bodies, by processes which obviously entail the further doing of work on
+the constituents.
+
+The further pursuit of this theme would evidently carry us beyond the
+more immediate subject of this book; but I want to make clear that
+recent researches render it more and more certain that the living plant
+is a complex piece of co-ordinated machinery which brings together
+matter and energy from the external universe, and then gets work out of
+these.
+
+This proposition is the more important because the whole question of the
+enrichment of our planet with new food, new building materials, and new
+fuel, to compensate the daily losses, depends on it, and is of course to
+be referred fundamentally to the acquirement of new supplies of energy
+from the sun. Enormous activity has been displayed by physiologists,
+since 1860, in attempting to solve the question, which of the many
+different rays known to proceed from the sun are absorbed by the
+chlorophyll-corpuscle, and directed to the performance of the work above
+referred to.
+
+The names of Draper, Sachs and Pfeffer stand forth prominently as
+pioneers in this; while those of Lommel, Engelmann, Timiriazeff and
+Langley have been among the most active in making important
+contributions to the subject, and in attempting to answer the further
+questions connected with the mode in which the chlorophyll is concerned
+in utilising the energy of the solar radiations. The point is one of
+supreme importance, because it goes on all fours with modern questions
+as to the rays of light absorbed or dispersed in our atmosphere at
+different seasons of the year, or in special climatic conditions, to say
+nothing of its other scientific aspects. Unfortunately, however, we have
+no satisfactory explanation of the actual rôle played by the chlorophyll
+substance itself, in spite of much industrious work which has been done
+in the subject in this country and elsewhere. As regards the rays
+employed, it was first proved that the most effective belong to the red
+end of the visible spectrum, and that the effect as measured by the
+amounts of oxygen given off, and of starch formed in given periods of
+time, is more or less proportionable to the intensity of the solar
+light. Then it was established that no monochromatic light is so
+powerful as the white light from which it was obtained, though the
+relative numbers expressing the activity in the red and yellow regions
+may stand to those in the blue as something like 12:1. The latest
+results place the maximum assimilation in the red-orange, and this
+coincides with the maximum absorption in the chlorophyll. If we may
+accept the current views as to the distribution of energy in the
+spectrum of solar light, which depends on the complete absorption of all
+the rays by a black body, where they are estimated as heat, we have the
+interesting result that the agricultural or forest plant is adapted to
+catch and retain, broadly speaking, just those particular rays which
+possess most energy.
+
+The probability is increasing that the protoplasmic machinery is the
+really effective mechanism in the process, and we may figure this
+machinery as so holding or presenting the molecules of carbon-dioxide
+and water to the impact of the light-vibrations, that the latter are
+enabled to undo the molecular structure; the atomic combinations thereby
+liberated may then be supposed to form a body like formic-aldehyde,
+which by polymerisation becomes a carbohydrate of the nature of a sugar
+such as glucose, which the protoplasm then builds up into its substance
+and subsequently deposits as starch, and stores temporarily in the form
+of grains or as amorphous material.
+
+This is partly hypothetical, and is largely due to the careful
+deductions of the chemists, but there are very many facts now to hand
+which bear out its probability, especially the recent advances in our
+knowledge of the sugars, and the experimental feeding of leaves and
+plants deprived of starch with such substances as dextrose, levulose,
+maltose, and other sugars, as well as glycerine and other bodies which
+should be convertible into, or yield them, if the theory is true. In
+this last connection, the careful and extensive experiments of Acton, A.
+Meyer, Boehm, and Laurent should be mentioned. It would be interesting
+to enlarge upon Engelmann's beautiful physiological experiments in
+connection with this subject of absorption of solar energy, where the
+maximum accumulation of oxygen-loving bacteria at those parts of a green
+alga which lie in the red-orange of the spectrum, are used as indicators
+of the maximum oxygen evolution (and therefore of the maximum
+carbon-dioxide assimilation), but space will not admit of this. For a
+similar reason I must also pass over the same observer's experiments
+with plants which assimilate in protoplasm behind a red instead of a
+green substance, and which absorb chiefly other rays between the yellow
+and blue, with the remark that they also seem to imply that it is the
+protoplasmic machinery which turns the energy on to the carbon-dioxide
+molecule, the coloured screen being secondary in the matter. Recent
+experiments which show that green plants will not assimilate
+carbon-dioxide in a light which has passed through a solution of
+chlorophyll--and therefore left its red rays behind; nor behind a screen
+of iodine dissolved in carbon-dioxide--which lets no visible rays
+between the red and blue pass--should be noticed, as showing the
+importance of the chlorophyll and the special rays referred to, however;
+and I ought at least to mention Timiriazeff's beautiful proof, published
+in 1890, that if, on the leaf of a plant left in the dark long enough to
+render it free of starch, a bright solar spectrum is steadily projected
+for 3-6 hours, the chlorophyll then removed by alcohol and the
+decolorised leaf placed in iodine, the image of the spectrum is
+reproduced by the different intensities of the starch bands, blue with
+iodine, in the different parts. Here, again, the maximum coloration
+coincides with the maximum absorption in and near the red.
+
+Microscopic observations and photo-chemical experiments alike convince
+us that the chlorophyll-corpuscle is itself a complex piece of
+protoplasmic machinery, working for and with the rest of the plant, and
+there can be little question as to the greater accuracy of our reasoning
+on the whole question I am discussing, since Meyer, Schimper,
+Pringsheim, and others have established the importance of its structural
+peculiarities.
+
+I must now pass on to consider another aspect of the question of
+carbon-assimilation.
+
+
+NOTES TO CHAPTER III.
+
+     In addition to the references in the last chapter, the reader
+     may be referred to Sachs' _Lectures_, XXV., and Pfeffer's
+     _Physiology_, pp. 329-356, where the voluminous literature is
+     given.
+
+
+
+
+CHAPTER IV.
+
+METABOLISM.
+
+     _Quantities of starch formed, and their significance for the
+     plant. The absorption of energy--the conversion of energy in
+     the plant. The plant is a complex machine for concentrating
+     and storing energy and material from without._
+
+
+Sachs measured the increase in dry weight (due to the carbohydrates
+formed in the chlorophyll-corpuscles) per square meter of leaf-surface,
+exposed for a definite period, by drying rapidly at 100° C. equal areas
+of the leaves concerned, and comparing the weights.
+
+Of course the results are not to be pushed too far, in view of the fact
+that some of the starch is continually passing away to be utilised, and
+of the difficulties of comparing the weather, the intensity of light,
+currents of air, hygroscopic conditions of atmosphere, and other
+variable factors which influence the matter. For instance, the stomata
+open and close to different extents according to the conditions of
+light and moisture, and this affects the whole mechanism of
+transpiration especially, and therefore the supplies of water and
+mineral salts. Nevertheless, some interesting and valuable results have
+been obtained in connection with this important subject.
+
+It was found, for instance, that the foliage of a sun-flower or of a
+vegetable-marrow may be forming starch at a rate of considerably over a
+gram per hour in every square meter of leaf-surface exposed on a fine
+day; while in particularly clear and warm sunny weather Sachs obtained
+as much as 24 to 25 grams per square meter per diem.
+
+When one reflects that 200 square meters is not an extravagant estimate
+for the area of leaf-surface exposed on a tree, for a period which even
+in our latitudes may be considerably over 100 days of, say, ten hours'
+light, we need no longer wonder at the rapidity with which wood is
+produced in the stems, and similar estimates (which I have purposely
+kept lower than the estimates for continental and tropical climates) may
+suffice to show how quickly potatoes or the ears of corn, etc., may fill
+up with the starch or other carbohydrates which render them valuable as
+crops. We want more measurements in these connections, moreover, for
+there are several ways in which they are of scientific value and
+practical importance.
+
+It is evident from what has been said that every grain of starch formed
+represents so much energy, packed away for the moment in the
+storehouses of the plant; and we know that--quite apart, however, from
+intermediate transformations of the energy thus stored--this energy
+reappears in the kinetic state eventually, when the starch is burned
+off, in presence of oxygen, and transformed into carbon-dioxide and
+water. It matters not how quickly or how gradually this combustion
+occurs, or whether it is accomplished by burning in a fire, or by slow
+and complex stages in respiration or metabolism: the point is that the
+unit of weight of starch yields so many units of heat when its structure
+tumbles down to the original components, carbon-dioxide and water.
+
+Clearly, if we know how many units of heat are yielded by the combustion
+of one gram of starch, we can obtain an estimate of the amount of
+energy, measured in terms of heat, which the foliage gains and stores
+up--an estimate which will approach the truth in proportion as our
+estimate of the total assimilative activity is correct.
+
+A word of warning is necessary here, however, for those best acquainted
+with physiology recognise that however useful such calculations as the
+above may be, and undoubtedly are, to give a general idea of the fact
+that the energy represented is large, it would be a mistake to suppose
+that such estimates give even an approximate measure of the energy of
+potential which may be got from the carbohydrate, and still less of the
+amount of work that may be got from its employment, according to the way
+it is employed or presented in the plant. To take a single instance
+only. If the carbohydrate is rapidly burned off to carbon-dioxide and
+water, very little is got out of it in the way of work--most, if not
+all, of the energy set free escapes as heat: whereas if the carbohydrate
+is slowly and gradually oxydised, passing through various stages and
+giving rise to powerfully osmotic bodies in the process, or if it is
+built up into protoplasm, or into the structure of a cell-wall,
+relatively enormous quantities of work may be got out of its
+surface-energy, and heat may be absorbed. Whence it follows that we
+cannot measure the power for physiological work of a body by merely
+obtaining its heat of combustion, any more than we can infer its
+significance in metabolism from its chemical properties.
+
+The general conclusion that the plant stores large quantities of energy
+may of course be arrived at by simply estimating the enormous quantities
+of food-material which we obtain annually from agricultural plants.
+
+Modern physiologists have attempted to proceed further than this,
+however, in their essays to form an estimate of the relations between
+the available energy in the solar rays and that used and stored in the
+plant.
+
+If we reflect on such phenomena as the cool shade of a tree, and the
+deep gloom of a forest, and on experiments which show that an ordinary
+leaf certainly lets very little of the radiant energy of the spectrum
+pass through it, it becomes evident that many of the rays which fall on
+the leaf are absorbed in some form, and it becomes very probable that
+much of the solar energy, other than that we term light, is retained in
+the leaf for other purposes than assimilation--or, at least, no other
+conclusion seems possible in view of all the facts. Engelmann's
+researches with purple bacteria are almost conclusive on this point, and
+we may regard it as extremely probable that the plant makes other uses
+of rays, perceived by us as heat-rays, as sources of energy. Researches
+on the influences of temperature on assimilation and other functions
+point to the same conclusion; and Pfeffer and Rodemann definitely state
+that heat is converted into work in the osmotic cells. And the study of
+the absorption bands in the spectrum of the living leaf becomes more
+intelligible in the light of these conclusions. Moreover, the fact that
+a plant still carries on processes of metabolism when active
+transpiration has lowered its temperature below that of the surrounding
+air--and the plant therefore receives heat from the environment--points
+to similar conclusions.
+
+The importance of the conclusion is immense, for even if the plant had
+no other sources of energy than the darker heat rays of the solar
+spectrum, it is clear that it ought to be able to do work.
+
+The above may suffice for the general establishment of the conclusion
+that the plant absorbs more radiant energy than it employs solely for
+assimilation, and emphasises our deduction that it is a machine for
+storing energy.
+
+The question now arises, how is this relatively enormous gain in energy
+employed by the plant? Our answer to the question is not complete, but
+modern discoveries in various directions have supplied clues here and
+there which enable us to sketch in some degree the kinds of changes that
+must go on.
+
+Not the least startling result is that, important as carbon-assimilation
+is as the chief mode of supplying energy, it is not the only means that
+the plant has of obtaining such from the environment, and it is even
+possible--not to say probable--that energy from the external universe
+may be conveyed into the body of the plant in forms quite different from
+those perceptible to our eyes as light.
+
+In the most recent survey of this domain, it is pointed out that we may
+distinguish between radiant energy, as not necessarily or obviously
+connected with ponderable matter, and mechanical energy, which is always
+connected in some way with material substance. All mechanical
+performances in the plants depend on transformation of some form of
+these, evident either as actual energy doing mechanical work, or as
+energy of potential ready to do work.
+
+In so far as molecular movements are concerned, we have the special form
+of chemical energy. The evolution of heat, light and electricity by
+plants are instances of radiant energy, and so on.
+
+Many transformations of energy in the plants are due to non-vital
+processes--_e.g._ transpiration, warping actions, etc., but we cannot
+always draw sharp lines between the various cases. Nor can we directly
+measure the work done in the living machinery; but from the effects of
+pressures and strains, the lifting of heavy weights, driving of
+root-tips into soil, osmotic phenomena, etc., it is certain that the
+values may be very high.
+
+The following classes of processes in living protoplasm and cells may be
+taken as indicators. First we have transformation of chemical energy,
+without which continued life is impossible: in many cases--_e.g._ the
+processes connected with oxygen respiration--these result in the
+development of heat. Secondly, we have those remarkable manifestations
+of energy known as osmotic processes, which depend on surface actions,
+and with which may be associated other surface effects, such as
+imbibition, secretion, etc., and in connection with which heat may be
+evolved or absorbed. It is true the substances which exhibit the
+properties here referred to may be produced, or placed in position, by
+chemical energy, or they may be absorbed by roots, etc.; but the
+proximate energy exhibited by them is not derived from chemical energy,
+and may be out of all proportion to the chemical energy of the substance
+or substances concerned. Moreover it is significant to note that a
+highly oxydised body may develop much osmotic energy, as well as a
+highly combustible one.
+
+It is of the greatest importance to realise the truth that much work can
+be, and is done in the living plant, by conversions of energy of
+potential independent of and out of proportion to the chemical energy
+available by decomposing the substances concerned; even the heat of
+respiration may be superfluous here, for the plant may absorb heat from
+without, and convert it into work.
+
+Tensions often arise in the plant, and do work expressed as
+movements--_e.g._ the springing of elastic Balsam fruits, stamens of
+_Parietaria_, etc.
+
+Osmotic energy not only results in enormous pressures and tensions, but
+causes movements by diffusion and diosmosis, and any given osmotic
+substance which carries this energy with it is not necessarily formed
+always in the same way in the cell--_e.g._ glucose may arise from
+starch, or from carbon-dioxide, or from oil.
+
+Surface-energy is also expressed in the powerful attractions for water
+exhibited in imbibition, swelling, capillarity, absorption, surface
+tensions, etc.
+
+Transpiration induces relatively enormous disturbances of equilibrium,
+and does work in moving water quite independent of chemical energy.
+
+Again, what may be termed excretion-energy, as expressed in the
+separation of a solid body--_e.g._ a crystal--from a solution, may be
+for our purposes regarded separately. Any change in the condition of
+aggregation of a substance in the plant may result in movements and the
+overcoming of resistances.
+
+It will be evident from this short digression--and this is the point I
+wish to emphasise--that in the interval between the securing of a grain
+of starch, representing so much energy won from the external universe,
+and the reconversion of this grain into its equivalent carbon-dioxide
+and water, by respiration, resulting in the loss of the above energy as
+heat, the starch referred to may have undergone numerous transformations
+in the living machinery of the plant, and have played at various times a
+rôle in connection with the most various evolutions of energy.
+
+If we try to picture a possible case, we may take the following. A given
+starch-granule, after being built up in the chlorophyll-corpuscle, is
+decomposed, and yields part of itself as glucose, which passes down into
+other parts of the plant in solution. Part of it is merely re-converted
+into starch, and temporarily stored: another part passes into the arena
+of oxydation-processes, the sum of which constitute respiration, and may
+serve for a time in the molecules of an organic acid: yet another part
+may be converted into a constituent of the cellulose cell-walls; while
+part may be brought into play in the reconstruction of protoplasm.
+
+In this last connection a discovery made by Schulze about 1878, and
+followed up later by Pfeffer, Palladin, and others is of importance.
+Seedlings growing in the dark, or in an atmosphere devoid of
+carbon-dioxide in the light, become surcharged with nitrogenous bodies
+known as amides, formed during the breaking down of the proteids in the
+destructive process preceding and accompanying respiration: if the
+seedlings are allowed free access to light and carbon-dioxide, however,
+the amides disappear. The explanation is that they are combined with
+some of the materials of the carbohydrates, and again built up into the
+material of the living protoplasm.
+
+Returning to our hypothetical starch-grain--or, rather, its parts--we
+have some of it retained as starch, in excess, simply because it is not
+needed at the moment: another portion gives up its energy in
+respiration, and this does work on the spot, or is lost as heat; or in
+the body of an organic acid, or its salt, the part in question may do
+lifting or pressing work by osmosis, or cause diffusion-currents from
+one cell to another. In the constitution of the cell-wall we may have
+part of our starch-grain aiding in imbibition or in the establishment of
+elastic tensions in turgidity: and, finally, parts may be built up into
+the living protoplasmic machinery of the plant.
+
+What is true for the starch-grain is also true for any particle of salt,
+or water, or gas which enters into the metabolism of the living plant,
+regard being paid to the particular case, and circumstances in each
+case.
+
+Enough has been said to show that the plant cannot be properly studied
+merely as the subject of chemical analysis or of physical investigation;
+you might as well expect to understand a watch by assays of the gold,
+silver, steel and diamonds of which its parts are made up, or to learn
+what can be got out of the proper working of a lace machine by
+analysing the silk put into it, and the fabric which comes out, and by
+taking the specific gravity of its parts and testing the physical
+properties of its wheels and levers.
+
+This is not the same thing as denying the value of such knowledge, in
+the case of either the dead machine or the living plant: it is merely
+emphasising the supreme importance of the study of the structure and
+working of the active machinery in both cases.
+
+Nor is it pertinent to remark on the apparent hopelessness of physiology
+being at present able to explain the seemingly infinite complexity of
+the living machinery of protoplasm and its activities. The modern
+locomotive is also a complex affair in its way, but it is profitable to
+investigate it and to know all one can of its working and possibilities,
+for obvious reasons: a little reflection will convince us that it is
+also worth while to investigate that complex machine, the plant--the
+working organism which alone can really enrich a country. Moreover, we
+ought to be encouraged by the satisfactory progress now being made, and
+the splendid practical results which are accruing, rather than dismayed
+by the prospect of unflagging labour which will be required in the
+future.
+
+Enough has perhaps been said to establish the general truth that the
+plant is a complex machine for storing energy and material from outside,
+and we have seen that modern research has at least gone a long way
+towards determining how the living machine works.
+
+It is hardly necessary to point out that important practical
+consequences may result from these phenomena of the accumulation of
+surplus starch or other carbohydrates in the leaves during the day, and
+of their disappearance during the night into the lower parts of the
+plant. For instance, foliage cut for fodder in the morning is far poorer
+in starch than if cut in the evening, and it would be very instructive
+to have experiments made on a large scale to test the result of feeding
+caterpillars or rabbits, for instance, with mulberry, vine, or other
+leaves in the two conditions.
+
+Again, we now see what complications may arise if a parasitic organism
+gains access to the stores of carbohydrates in process of accumulation,
+or attacks and injures the machinery which is building up such
+materials, etc.
+
+
+NOTES TO CHAPTER IV.
+
+     The student who desires to pursue this subject further should
+     read Sachs' _Lectures_, XX. and XXV., and Pfeffer's
+     _Physiology_, pp. 442-566, but he will hardly arrive at the
+     best that has been done without consulting Pfeffer's "Studien
+     zur Energetik der Pflanzen" in the _Abhandl. der Math.-Phys.
+     Classe der Kgl. Sachss. Gesellsch. der Wiss._ (Leipzig, 1892),
+     p. 151; and Kassowitz, _Allgemeine Biologie_ (Vienna, 1899),
+     Bk. I., pp. 1-127.
+
+
+
+
+CHAPTER V.
+
+ROOTS AND ROOT-HAIRS.
+
+     _Older views as to root-hairs--Root-hairs and their
+     development--Surface--Variations--Conditions for maximum
+     formation--Minute structure--Adhesion to particles of
+     soil--Functions._
+
+
+On the roots of most plants are to be found delicate, silky-looking,
+tubular prolongations of some of the superficial cells, known as
+root-hairs. Malpighi (1687) seems to have been the first to observe
+them, and he took them for capillary tubes. Grew (1682) seems to have
+been responsible for the view that the roots act like sponges in taking
+up water.
+
+Simon (1768) was probably the originator of the idea that these
+root-hairs were excretory tubules, a view that became very popular at
+the beginning of this century.
+
+Meyer (1838) was perhaps the first to give a comparative account of
+them, and he supposed them to be delicate prolongations of the
+root-surface to facilitate the absorption of water.
+
+The real importance of these organs, however, has only become apparent
+since Sachs, in 1859, recognised their relations to the particles of
+soil between which they extend and to which they cling.
+
+In 1883 Schwarz made a very thorough study of their biological
+character, and in 1887 Molisch gave us new facts as to their physiology.
+Our knowledge of them has been rendered very much more intimate by the
+researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena,
+and they serve as an excellent study of some of the best results of
+modern physiology.
+
+In the normal case, such as is exemplified by a seedling wheat or bean,
+the root-hairs arise some distance behind the growing tip of the root,
+an obvious adaptation which prevents their being rubbed off by the soil,
+as they would be if developed on parts still actively lengthening. As
+those behind die off, new ones replace them in front, and so we find a
+wave of succession of functionally active root-hairs some little
+distance behind the tip of the root: the same order of events holds for
+each new rootlet as it emerges from the parent root, and so successive
+borings in the soil, made by the diverging root-tips, are thoroughly
+explored by these root-hairs.
+
+Measurements have shown that in various plants the surface of root on 1
+mm. of length is increased by the root-hairs in proportions given in
+the following table:
+
+  ------------+---------------------+-----------------+--------------
+     PLANT.   |  Area of surface    |    Area of      | No. of times
+              | without root-hairs. | root and hairs. |   greater.
+  ------------+---------------------+-----------------+--------------
+  Maize,      |  4.52 sq. mm.       |  25.13 sq. mm.  |     5.5
+  Pea,        |  4.71 sq. mm.       |  58.33 sq. mm.  |    12.4
+  Scindapsus, | 14.02 sq. mm.       | 261.9  sq. mm.  |    18.7
+  ------------+---------------------+-----------------+--------------
+
+--which sufficiently establishes the general proposition that the area
+of the root-surface is enormously increased by these hairs.
+
+But this does not give us any definite idea of the length of the
+cylinders of soil explored by these surfaces, until we find that plants
+such as an ordinary sunflower, hemp, or vegetable-marrow may have roots
+penetrating into a cubic meter of soil, in all directions, and so
+closely that probably no volume so large as a cubic centimeter is left
+unexplored. Clark found by actual measurement that the roots of a large
+gourd, if put end to end, extended over 25 kilometers, and Nobbe gives
+520 meters for the roots of a wheat. Vetches may go nine feet deep, and
+oats more than three feet. The Sal, a tree of the forests of India, has
+roots which penetrate to a depth of 50 to 60 feet.
+
+Some rough notion of the lengths, superficies and penetrating capacities
+of the roots of a large tree may be gathered from the above, but it is
+doubtful whether we can form any adequate ideas as to the millions of
+root-hairs which must be developed along the course of these
+subterranean boring organs.
+
+One of the most striking results of modern enquiry into these matters,
+is the discovery that the number and superficial area of these
+root-hairs, on one and the same plant, may vary to a large extent
+according to the structure, as it were, of the soil, and the degree of
+moisture it is capable of retaining; or, with the same soil, according
+to the amount of water which it receives and holds. Correlations have
+also been observed between the development in length and surface of the
+rootlets themselves.
+
+The following illustrations will suffice to show this:
+
+Six young wheat-plants in soil kept constantly wet, developed roots the
+total length of which measured 365 mm. each, on the average, and almost
+devoid of root-hairs.
+
+Six similar plants in soil only moderately moist, averaged 668 mm., and
+were well furnished (though not densely covered) with root-hairs.
+
+Six similar plants in soil which would be termed dry, averaged 371 mm.,
+but were densely covered with rich crops of root-hairs.
+
+Further researches have shown that the conditions which rule the
+development of the root-system and root-hairs in the soil are very
+complex, and not always easy to trace. The most general statements we
+can make are the following:
+
+There is an optimum degree of moisture in the soil which promotes the
+maximum development of root-hairs. If the soil is too wet they are not
+developed.
+
+These facts are of importance as correlated with the ease or difficulty
+experienced by the roots in obtaining water, and plants such as our
+ordinary agricultural plants show this very distinctly.
+
+Although, as shown in the experiments with wheat, the short roots in dry
+soil were more densely covered with root-hairs than the much longer
+roots in moderately moist soil, subsequent closer investigation shows
+that the total quantity and area of root-hairs is less in the former
+case than in the latter.
+
+The greatest number of root-hairs are developed on roots which are
+growing at their best: too much moisture may prevent the formation of
+root-hairs: too little may induce dense growths of root-hairs locally,
+but the total number is reduced.
+
+Another set of events which exerts influence on the development of
+root-hairs is the composition of the dilute solution--water containing
+dissolved salts--which surrounds them in the soil.
+
+Thus, Schwarz found that when similar oat and wheat plants were grown
+with their roots in solutions of various salts, the results differed as
+follows:
+
+Oats in a 15 per cent. solution of calcium chloride developed no
+root-hairs, though they formed in a 5 per cent. solution, and were very
+numerous in a 0.5 per cent. solution, or in water alone. In a 10 per
+cent. nutritive solution the plants developed no root-hairs, though
+they were abundant in a 1 per cent. solution.
+
+Wheat plants with their roots in a 15 per cent. solution of potassium
+nitrate bore no root-hairs, but they were numerous in a 2 per cent.
+solution of the same salt.
+
+These are extreme cases, for, although the roots were not killed, they
+were strongly inhibited in their growth by the more concentrated
+solutions. However, experiments of this kind at least bring vividly
+before us what variations are possible, and suggest that similar events
+on a smaller scale may occur in a soil which yields large quantities of
+soluble substances, _e.g._ when freshly manured. Obviously these facts
+have a practical significance as regards kind of soil, drainage, season
+(_e.g._ drought or wet), etc.
+
+But there are other factors which rule the development of root-hairs,
+and some experiments by Lesage show that the correlations between the
+development of root-hairs and roots are probably much more complex than
+had been suspected; for he finds that if the lateral rootlets of a Bean,
+in a water culture, are suppressed, the main rootlet develops numerous
+and very long hairs to compensate the loss in surface, a matter of
+obvious importance in the discussion of cases where roots have been
+injured in the soil.
+
+Before proceeding further it is necessary to look a little more closely
+into the structure of a single hair.
+
+It is a tubular prolongation of a single cell of the external covering
+of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10
+mm. in diameter. In special cases the root-hairs of some water plants
+may reach 5 to 18 mm. in length, but of course I am referring to the
+ordinary land plants of agriculture and forestry. This tubular
+prolongation is closed and rounded off at the distal free end, and opens
+at the proximal end into the cell of which it is a protrusion.
+
+The whole structure is bounded by an extremely delicate and elastic wall
+of cellulose, which Frank says is of special composition, almost too
+thin to measure in many cases, but often somewhere near 0.005 to 0.001
+mm. in thickness. This thin membrane is remarkably permeable by water,
+or dilute solutions, as is shown by the rapidity with which a root-hair
+collapses if exposed to evaporation, or with which dense solutions
+abstract water from it, or with which solutions may be seen to penetrate
+it under the microscope.
+
+Overlying the thin cell-wall proper, on the outside, is a thin
+gelatinous layer, a product of alteration of the outermost lamellæ of
+the former.
+
+Closely lining the proper cell-wall on the inside, is an extremely thin
+layer of living protoplasm, and somewhere in this protoplasm is a
+distinct cell-nucleus.
+
+The interior of the tube is filled with cell-sap, and it is the osmotic
+pressure of this cell-sap which keeps the whole living instrument tense
+and rigid, and the thin protoplasmic film close pressed against the
+cellulose cell-wall.
+
+Nothing whatever can pass into the cell-sap, or out from it, without
+traversing both the lining of living protoplasm and the cell-wall.
+
+If we gently pull a living root, of wheat, pea, mustard, etc., from a
+normal soil, we find particles of soil so closely adherent to the
+root-hairs that they cannot all be washed off without tearing the hairs:
+the root-hairs establish relations of contact with these particles, so
+close that they are cemented to the solid surfaces by means of the
+gelatinous layer already referred to. This peculiarity has the following
+consequences. In the first place, the enormous holdfast, ensured by the
+millions of points of adherence, enables the plant to withstand even
+powerful lever actions from above, and provides fixed points against
+which the root-tips can work as they drive deeper into the soil. In the
+second place, the intimate contact of the root-hairs and particles of
+soil, ensures that the films of water held by surface-action on the
+soil-particles and root-hairs shall be in continuity with the water
+saturating the cell-walls of the latter, and therefore with the
+protoplasm and cell-sap in their interior. The importance of this at
+periods when the soil is "dry" will be obvious, when we reflect that no
+soil is ever naturally so dry that surface-films of water are absent
+from the particles.
+
+The fact that the root-hair contains living protoplasm, enables us to
+understand to a certain extent the results of the following
+experiments.
+
+If we have a leafy and healthy plant, with roots, bearing numerous
+root-hairs, properly established in suitably moist soil in the pot, the
+roots cease to absorb water if the temperature of the soil falls below a
+certain minimum, though they recommence to do so if the temperature is
+raised again: this has nothing to do with the temperature of the upper
+parts of the plant, or of the air, and the latter may be so high that
+the plant rapidly droops from loss of water at the leaves, which is not
+being compensated owing to the inactivity of the roots.
+
+Similarly we may have the air so cold, at a time when the soil is warm
+enough to keep the root-hairs actively at work, that the plant becomes
+surcharged with water, which escapes from the leaves like drops of dew.
+The temperatures necessary to cause these disturbances in the action of
+the living root-hairs vary for different plants, and even for different
+varieties of the same species.
+
+Similar arrestation of the functions of the roots may be brought about
+by removing the oxygen from the soil around the root-hairs, and
+replacing it by carbon-dioxide, or the vapour of chloroform. If not kept
+too long in such a condition, the plant recovers rapidly on admitting
+atmospheric oxygen, which is always present in a normal well-drained
+soil both as gas in the capillary interspaces, and dissolved in the
+water on the surfaces of the particles. If the access of oxygen is
+delayed, however, as often happens in rainy seasons and in wet soils,
+the root-hairs are killed, and rot sets in. A good instance of this has
+lately been given by Heinricher in the case of potatoes.
+
+
+NOTES ON CHAPTER V.
+
+     For the further pursuit of this subject the reader should
+     consult Sachs' _Lectures_, II. and XV.; Sorauer, _A Popular
+     Treatise on the Physiology of Plants_, 1895, chapters II. and
+     IV., and Pfeffer's _Physiology_, pp. 149-163. The principal
+     paper on root-hairs referred to in the text is Schwarz, "Die
+     Wurzelhaare der Pflanzen," in _Unters. aus dem bot. Inst. zu
+     Würzburg_, I. Heft 2, 1883, p. 140, where a very exhaustive
+     account of these organs will be found.
+
+
+
+
+CHAPTER VI.
+
+THE FUNCTIONS OF ROOT-HAIRS.
+
+     _Excretions from root-hairs--Osmotic phenomena--Turgescence--
+     Plasmolysis--Control of the protoplasm in absorption, etc.
+     Selective absorption._
+
+
+We see then that the root-hairs are the active living instruments in
+absorbing the water (containing small quantities of dissolved
+substances) of the soil.
+
+If the living root-hairs are so numerous and so active, however, a
+natural inference is that they must exert some influence on the
+composition or arrangement of their environment. All the teachings of
+modern physiology go to show that such a living cell as I have sketched
+cannot carry on its life, brief though it be--the root-hairs are active
+for about four or five days--without forming substances of the nature of
+excreta, and we should expect some of these to pass out to the soil.
+
+Sachs showed, in 1860, that roots growing in contact with polished
+marble corrode the surface of the mineral, and Nobbe, in 1876, showed
+that the roots of seedlings reduce potassium permanganate, a fact which
+Molisch confirmed in 1887. The latter observer also proved that living
+root-hairs secrete substances which colour a solution of guaiacum blue,
+oxidise pyrogallic acid and other organic substances, and rendered it
+probable that they excrete some substance which inverts cane-sugar, and
+in some cases even small quantities of a diastatic enzyme.
+
+Molisch also confirmed an old observation, that roots excrete
+carbon-dioxide; and he and Czapek showed that the root-hairs excrete
+acids more permanent in their nature than carbonic acid, and published a
+method for showing this by means of a dilute solution, slightly
+alkaline, of phenolphthalein.
+
+Molisch declared that the substances secreted by root-hairs may even be
+observed, dissolved in drops which ooze from the surfaces of the
+root-hairs.
+
+That these root-excretions, and particularly the acids, may be of
+service in dissolving and rendering more available various constituents
+of the soil is an obvious suggestion, and it is borne out by Sachs'
+discovery of the corrosion of marble, and by Molisch's observation that
+living roots slowly corrode ivory if continuously kept in contact with
+it.
+
+But a deeper insight into the physiology of these organs was only
+possible when the meaning of the phenomena of osmosis had been rendered
+clearer by the researches of Pfeffer and De Vries in 1877.
+
+De Vries showed that the turgescence of the living cell can be
+diminished, and even reduced to nothing, by placing the cell in contact
+with solutions of substances which attract water from the cell-sap: as
+the turgescence diminishes, the cell contracts, owing to the elasticity
+of the cell-wall, which was previously distended; if the abstraction of
+water continues, the living protoplasmic membrane lining the cell-wall
+contracts away from the latter. He then proved that no injury need
+accrue to the cell by this process of plasmolysis, since the turgescence
+can be restored by washing out the salt with a more dilute solution, or
+with pure water; and the cell may go on living and even growing as
+before. These phenomena can only be produced in cells where the
+protoplasmic lining is intact and alive.
+
+Pfeffer showed that the whole matter depends on the properties of the
+living protoplasmic membrane, which, so long as it is alive, has the
+power of governing the entrance or exit of dissolved substances, but is
+as a rule freely permeable for water. If, then, substances with a
+powerful attraction for water are formed in the cell cavity, and of such
+a nature that the protoplasm does not permit their free diffusion to the
+exterior, they attract water, and hold it fast, and so set up the
+condition of hydrostatic pressure known as turgescence, the limit of
+which depends on the attainment of a state of equilibrium between the
+elastic reaction of the cell-wall and the distending power of the
+absorbed water. When this limit is reached, water begins to filter back
+again through the cell-wall. Numerous researches during the last fifteen
+years have shown that the sap of such a living cell as the root-hair is
+charged with substances of various degrees of osmotic power; bodies like
+sugars, amides, vegetable acids and their salts, being formed by the
+metabolic activity of the protoplasm and accumulated there. Moreover, we
+now know that the salts of the vegetable acids in particular are
+effective, and the researches of Warburg and Palladin in 1886 have
+placed it beyond reasonable doubt that these acids are continually being
+developed and destroyed in the living cell during normal growth and
+respiration, and that great variations as to quantity may be brought
+about by alterations in the conditions of the environment--_e.g._
+temperature, oxygen, etc.
+
+If, now, we bring a solution of some salt, such as potassium nitrate,
+which has a powerful attraction for water, on the outside of the living
+root-hair, the question whether the water remains in the cell, or passes
+out of it, merely depends on whether the substances inside or that
+outside have the most powerful attraction on the water in the sap, since
+the protoplasm allows water to pass freely.
+
+But the protoplasmic lining may affect the whole matter in another way;
+for it may allow the dissolved salt, or other substance, in the solution
+outside or inside the cell to pass through it also, or it may take it
+up and fix it, or break it up or otherwise alter it.
+
+More recent researches, and especially those of Pfeffer, have shown that
+these diosmotic properties of the living protoplasm are of the utmost
+importance in the whole matter of absorption of substances from the
+soil.
+
+Let us suppose the following case. A root-hair, in full vigour, is
+allowed to bathe freely in a dilute solution of various substances, such
+as sugar, potassium nitrate, phosphates, sulphates and carbonates of
+iron, soda, lime, magnesium and others known by experiment to be
+harmless to its life.
+
+Now it turns out to be by no means a foregone conclusion that all or any
+of the substances, even though freely soluble in the water, can pass
+through the protoplasm into the interior of the cell. Some may be
+allowed easy access, others may only be permitted to pass in small
+quantities, and others, again, may be absolutely refused access by the
+delicate living filter, so long as it is vigorously alive. Nor, as
+proved by numerous experimental cultures since De Saussure's time, is
+the entrance of a salt, etc., ruled by its indispensability or otherwise
+in the economy of the plant. And it is important to notice that only
+experiment can prove the point and determine which substances are
+absorbed and which refused by the root-hair.
+
+If we now suppose the protoplasm to give rise to powerfully osmotic
+substances which accumulate in the sap-vacuole, but which are not
+permitted free egress through the protoplasm (and the formation of such
+bodies will occur if the protoplasm is actively respiring), the
+conditions for absorption of water, with or without any dissolved salts,
+which the protoplasm allows to traverse it, are set up.
+
+But the above supposed case is realised, as Pfeffer showed in 1886, when
+he found by a series of beautiful experiments that certain aniline dyes
+can accumulate in living root-hairs, and other living cells, whereas
+others cannot pass the living protoplasm. After accumulating for some
+time, the dye may either remain stored there, or may eventually diffuse
+out.
+
+Pfeffer made another discovery, of equal importance, namely, that under
+the influence of dilute organic acids, such as citric acid, the
+permeability of the living protoplasm may be altered, so that it allows
+substances to pass which could not otherwise have traversed it. De Vries
+had also shown that the condition of the protoplasm affects its power of
+retaining the colouring matter in the sap of the Beet: so long as the
+protoplasm is alive, the crimson sap is retained, even when the cell is
+plasmolysed, but immediately it begins to die the colour escapes through
+it. A similar case exists when the chlorophyll-corpuscles retain their
+colour in living cells known to be charged with acids: so long as the
+protoplasm is alive and normally active the green bodies are protected.
+
+These, and numerous other experiments of the same kind, prove that the
+healthy root-hair is a living instrument for taking up dilute solutions
+out of the soil, and holding them in the sap-cavity for a time. If
+killed, by frost for instance, it loses these powers.
+
+The researches of the last ten years have also shown that a time comes
+when the turgid cell, if an isolated one, and if sufficient supplies of
+water are present, is so tightly distended that the surplus water begins
+to diffuse out again under the pressure proper to the hydrostatic
+conditions set up.
+
+Now we arrive at a very critical point.
+
+When the water, or dilute solution of various substances, begins to
+exude under pressure from the living root-hair, what is to prevent its
+escape into the soil? And if it thus diffuses out, where is the object
+of absorption?
+
+The questions are obviously pertinent, and they may seem the more so in
+that the cells adjoining the root-hair on its inner side are also
+turgid, and possess similar properties to those of the root-hairs. To
+establish a condition of things which shall bring about the inward flow
+of the absorbed water, one of the three following cases is conceivable.
+(1) The cells, as we pass radially into the root, have different
+properties on the wall of the two sides; or (2) they are more and more
+greedy of water owing to some process of extraction of their water by
+tissues in the centre of the root; or (3) these successive series of
+cells possess osmotically more powerful contents at periods coincident
+with the escape of the water from the now osmotically weaker root-hairs.
+
+A little reflection will show that where we have a group of such cells
+as the above, all capable of absorbing water and dilute solutions and of
+becoming turgid, movements of the absorbed water must go on until all
+the cells are in equilibrium, as regards their osmotic pressures.
+
+Now the living rootlet is just such a system, the various cells of which
+are in different conditions of osmotic pressure at any given time: some
+of these cells are old, and their protoplasm is allowing sap to filter
+out under pressure: others are in the height of their vigour, and their
+protoplasm extremely impervious to the highly osmotic sap-constituents
+which it itself is forming actively: others are too young to have
+attained their full turgescence: while others again are in stages
+intermediate between the above.
+
+There is another point of importance, however, to explain some
+peculiarities in the absorption of these dilute solutions of salts,
+etc., by the root-hairs from the soil, and by cells lying deeper in the
+plant from these root-hairs.
+
+It is easy to understand that if a root-hair absorbs a given
+substance--say calcium sulphate, for illustration--and hands it over to
+other cells unchanged, a time must be supposed to arrive when, the sap
+of all the cells being equally charged with calcium sulphate, no more
+could be absorbed: the rate of absorption of this particular substance,
+and the quantity absorbed, up to the hypothetical point of equilibrium
+chosen, would then depend simply on the ease with which its molecules
+traversed the living protoplasmic membrane, and the degree of their
+solubility in the sap.
+
+But now suppose the following new factor to come in. Suppose that
+calcium sulphate undergoes decomposition in some one of the internal
+cells of the system of absorbing cells, or that it is even merely
+crystallised out in such a cell, or in any other way removed from
+solution (_e.g._ by deposition in cell-walls). This alters the state of
+affairs considerably. The separation of the molecules from the
+sap-solution is itself a cause for the flow of more of the solution to
+the cell concerned, and such causes of diffusion are very common in the
+plant.
+
+The importance of this principle consists in that it lies at the base of
+the whole question of selective absorption, application of manures, and
+the rotation of crops; and those who are acquainted with the excellent
+analytical results of De Saussure, Boussingault, Wolff, Trinchinetti,
+Gödechen, etc., and the water-culture experiments of Sachs, Nobbe, and
+others, will understand what an illuminating effect on these points was
+produced by the above generalisation, which we owe especially to
+Pfeffer's splendid researches into the nature of osmotic phenomena.
+
+It will now be clear, I hope, why we regard the living root-hairs as
+instruments--as pieces of living machinery--for the active absorption of
+water, with substances dissolved in it, from the soil; and it will also
+be evident, I think, that no one can form a proper conception of this
+matter of absorption, so important in all agricultural questions, unless
+he pays attention to these biological phenomena. It was hopeless to
+expect to understand these matters merely in the light of chemical
+analyses of plants and soils, and one expression of this hopelessness
+was the belief in the power of roots to select only the substances
+useful to it. We now know that the expression "selective power of roots"
+has a totally different meaning from that implied in the minds of the
+last generation of agriculturalists, and it would be easy to devise
+experiments, with solutions of different strength, where the plant
+should be made to take up relatively large quantities of harmless, but
+useless minerals, etc., and to starve in the midst of plenty of the
+elements proper to its structure, simply because the former are offered
+in a form in which they easily traverse the protoplasm of the
+root-hairs, while the latter are presented in a form unsuitable for
+absorption. That all these matters are of importance in regard to
+manuring and choice of soils, etc., needs no emphasising.
+
+These remarks, of course, do not detract from the value of good
+comparative chemical analyses, when viewed in the light of physiological
+knowledge, as I need hardly say; but they do, and emphatically so,
+attack the position that such analyses alone can explain the problems of
+agriculture.
+
+On the other hand, we must not rest satisfied with the suggestions so
+far put forward to account for the processes referred to, since it is
+impossible to overlook the fact that in their present form they merely
+afford proximate explanations, and are too crudely mechanical for
+finality.
+
+
+NOTES ON CHAPTER VI.
+
+     In addition to the works referred to in the last chapter, the
+     student should consult Pfeffer's _Physiology_, pp. 86-149, and
+     pp. 410-441. With reference to water cultures, Sachs'
+     _Lectures_, XVII., may also be consulted. The standard work on
+     ash constituents of plants is Wolff, _Aschen-analysen_, 1871
+     and 1880, an indispensable book of reference in this
+     connection, though there are others, quoted in Pfeffer, where
+     further literature may also be found.
+
+
+
+
+CHAPTER VII.
+
+THE BIOLOGY OF SOIL.
+
+     _Soil not a dead matrix--Organic materials--The living
+     organisms of the soil--Their activities--Their numbers and
+     importance. Abandonment of the notion that chemical analysis
+     can explain the problem._
+
+
+It is customary to regard the soil, between the particles of which the
+root-hairs of plants are distributed, as if it were merely a dead matrix
+of smaller or larger pieces of rock, such as sand, gravel, stones, etc.,
+and organic remains, such as bits of wood, leaves, bones, etc., with
+water and air in their interstices. As matter of fact, however, soil is
+a much more complex body than was suspected until comparatively recent
+times.
+
+It is, of course, beyond the scope of this book to go into the different
+varieties of soils, their structure or arrangement, and the chemical
+nature of their constituent rocks and the débris mingled with the
+latter. For the same reason I must pass over the curious properties of
+soils in relation to the solutions they yield to water in contact, the
+manner in which they retain some of these solutions and allow others to
+pass easily, and the remarkable double decompositions which go on in
+them. Moreover, I must assume as known the chief physical properties of
+ordinary soils with respect to the phenomena of capillarity, absorption
+of heat, action of frost, and so forth.
+
+But all ideas as to the nature of soil based merely on the study of its
+chemistry and physics are misleading, and it is in just the
+establishment of this truth that modern discoveries in Agricultural and
+Forest Botany have played so important a part.
+
+From the facts that organic débris is found chiefly at the surface of
+the earth, and that the smallest particles are held in suspension by the
+water near the surface, it is comprehensible why such organic remains
+abound in the upper parts of the soil, where the rootlets with their
+absorbing root-hairs are also found, because they must have oxygen. The
+rule is, therefore, that an ordinary soil consists of upper strata, rich
+in organic materials and in oxygen, and a subsoil, poorer in these
+substances.
+
+Among these organic materials are countless myriads of living beings,
+especially fungi and bacteria, which require oxygen and organic
+materials for their subsistence, and it depends on the open or close,
+moderately moist or damp, warm or cold nature of the soil, and on some
+obviously connected factors, how far down these aërobic organisms can
+thrive. As we go deeper down they become fewer and fewer, and gradually
+disappear, and (neglecting certain anaërobic bacteria of putrefaction)
+they are rarely found in marked abundance more than a few inches below
+the surface soil.
+
+These aërobic fungi and bacteria are the great agents of continued
+fertility of a soil, and it is they which, living and multiplying in the
+moist and well-aerated warm interstices of a rich open soil, carry out
+the useful destruction of organic matter, breaking it up into mineral
+and gaseous bodies, which are then dissolved in the water bathing the
+root-hairs or escape into the atmosphere. In this work of destruction
+they are aided by the oxygen of the air and the solar heat: their own
+fermentative action is also accompanied by a marked rise of temperature,
+and the carbon-dioxide and other products of their activity all go to
+complicate the chemical changes going on in the soil around the roots.
+
+Duclaux has calculated that _Aspergillus niger_, a common mould fungus,
+can break down organic substances, such as carbohydrates, at such a rate
+that a metre cube of the fungus would decompose more than 3000 kilogr.
+of starch in a year, and this may serve as an example giving some idea
+of the possibilities in soil.
+
+Analyses of waters containing large quantities of organic matter, as
+they enter such open soils as those referred to, compared with the
+drainage water after passing through the upper strata, show that the
+carbonaceous and nitrogenous materials are broken down to more or less
+completely oxidised simpler compounds, and that the following chief
+changes result. The ammonia and some other nitrogenous bodies remain
+behind in the soil, as also do the phosphoric acid and much of the
+potash; whereas large quantities of nitric and nitrous acids, together
+with much sulphuric acid, chlorides, and calcium salts pass away in the
+drainage. These facts are obviously highly important in agriculture.
+
+Experiments on sewage farms have shown also that the upper soil retains
+most of the bacteria of the sewage. Koch found at Osmont, near Berlin,
+that whereas the different sewage waters contained numbers so enormous
+that each cubic centimeter probably held 38,000,000 germs, the different
+drainage waters held only 87,000 per c.cm.; and the whole process of
+water-filtration through sandy soils depends on these well-known facts.
+
+Recent experiments in connection with soil-filtration, however, bring
+out the further facts that the oxidations which organic matters undergo
+in the soil--and without which they are useless to the higher
+plants--are enormously enfeebled if the upper layers of soil are
+sterilised, so as to deprive them of the myriads of aërobic bacteria,
+fungi and yeasts which they normally contain, and there can no longer be
+any doubt as to the importance of the biology of the soil in connection
+with the preparation of materials suitable for absorption in solution by
+the root-hairs of agricultural and other plants.
+
+The researches of the last ten years have brought to light a long list
+of forms, comprising yeasts, such as Hansen's _Saccharomyces
+apiculatus_, fungi and bacteria which live and grow in the soil, finding
+their water and food supplies in the interstices, and under conditions
+which we now know to be very diverse. They are usually more numerous, in
+species and individuals, in cultivated farm and garden soils than in
+woods, prairies, and untilled lands; but the geological nature of the
+strata, the closeness and otherwise of the soil, its damp or dry
+character and its average temperature (which depends on many things
+besides latitude or altitude) and other factors co-operate to rule their
+distribution and numbers. The fact that cultivated land is so well
+supplied with manures, air, etc., is of great importance in relation to
+their relative abundance there, and it is extremely probable that the
+use of artificial manures lessens their numbers considerably as compared
+with land on which stable and other animal manures are employed.
+
+A list of the soil-bacteria which have been isolated and more or less
+carefully cultivated and examined would comprise about fifty species;
+but it is certain that, as at present classified and named, many more
+species are to be discovered in any ordinary soil.
+
+The fungi are apparently even more numerous than the bacteria, and we
+may rest satisfied for the present with the general statement that the
+life-actions of the myriads of individuals of these organisms in the
+soil completely alter the question of soil-water as understood by the
+last generation of agriculturalists.
+
+But there is another aspect of this question of soil-organisms which has
+grown in importance of late to such an extent that we are more than ever
+justified in regarding the biology of soil as far more vital to the
+interests of the plant than its physical or chemical properties. With
+many of the fungi in the soil the roots of plants have to compete--just
+as plant competes with plant--for water, salts, and other
+food-materials. The toadstools which are so conspicuous in fields and
+forests spring from mycelia which ramify in the ground, and are busily
+breaking down the remains of other organisms, and just such fungi are
+known to store up relatively large quantities of salts of potassium and
+phosphorus--the very salts which are so valuable to crops and occur so
+sparingly in most soils, but which the extensively spread fungus mycelia
+can gradually accumulate. Some of these fungi, moreover, are more active
+in their antagonism, and actually attack and pierce the roots as
+destructive parasites, but I pass these by for the present, as they form
+the subject for further consideration when we come to the diseases of
+plants.
+
+It is obvious that the competition of fungi with root-hairs for mineral
+salts, oxygen, etc., may be at times acute, and it is extremely probable
+that cases of so-called sterility of soil, where a particular soil is
+found unsuitable for a crop, may sometimes be due to this
+over-competition.
+
+The researches of recent years, however, and especially those of Frank,
+Winogradsky, Hellriegel, and Stahl, have brought to light a series of
+relationships between certain of these soil-organisms and the higher
+plants which place the matter of soil-biology in quite new lights.
+
+On the one hand it has been discovered that groups of bacteria are the
+active agents in bringing about the destruction of organic nitrogenous
+matter with the formation of ammonia, in oxidising this ammonia to
+nitrous and to nitric acids, which combine with bases in the soil to
+form the corresponding salts; while, on the other hand, other forms can
+decompose the nitrates and reduce them to nitrites, or set free ammonia
+or even nitrogen from them. Moreover, there are certain species which
+can fix the free nitrogen of the atmosphere, and start the cycle of
+up-building of this inert element into the complex higher compounds we
+term organic. It is impossible to over-estimate the importance of these
+processes of nitrification and denitrification going on in the soil
+about the root-hairs of the higher plants.
+
+But, in addition to this circulation of nitrogen in the soil, it
+turns out that the life-actions of bacteria, and not mere chemical
+decompositions, are largely responsible for the circulation of
+carbon, of iron, of sulphur and other elements formed from the
+decomposition--also by bacterial and fungal agency--of animal and
+vegetable remains in the soil.
+
+Even more startling are the biological relations in the soil between
+the absorbing roots of the higher plants and some of these bacteria and
+fungi, for it has now been established beyond all doubt that certain
+fungi enter the living roots and there flourish not as mere destructive
+parasites, but as messmates not only tolerated by the plant, but even
+indispensable to its welfare. It is probable that nearly half the plants
+of our fields, moors, and forests entertain such fungi in their
+root-tissues. The curious, and long-known nodules on the roots of
+leguminous plants--peas, beans, clover, etc.--are filled with bacteria
+which enable these plants to avail themselves of the free nitrogen of
+the air, and so enrich the soil with nitrogenous substances.
+
+The roots of most forest trees, orchids, and plants of the moorlands,
+meadows and marshes are similarly occupied by fungi, which in some way
+convey salts--probably especially phosphates and potassium compounds--to
+the plant in return for the small tax of organic carbon-compounds it
+exacts from the latter. In some cases at any rate, as Bernard has lately
+shown, the very existence of the plant depends on its seedling roots
+obtaining this advantageous attachment and co-operation (symbiosis) of
+the fungus immediately on germination.
+
+These remarks must suffice to illustrate this part of my subject, and to
+emphasise the statement that the question whether a given plant can be
+grown in a given soil, is by no means one of simply the physical and
+chemical constitution of the latter. The plant will have to run the
+gauntlet of a long series of vicissitudes brought about by the presence
+or absence, relative proportions and vigour, and specific nature of the
+organisms in the soil at its roots, and it is easy to see that many
+cases of disease may be due to the absence of advantageous bacteria or
+fungi, or to circumstances which disfavour their life, as well as to the
+predominance of competing organisms.
+
+It will now be evident that the old points of view must be abandoned,
+and with them, especially, the widely prevalent notion that chemical
+analyses of the plant and soil can explain the real problems of
+agriculture.
+
+It was of course an enormous advance in the science when, thanks to the
+splendid labours of the chemists, at the end of the last century and the
+beginning of this, we obtained that preliminary knowledge of the
+constitution of the air, and of the composition of the water, acids and
+salts, etc., which plants require for their food-materials and
+life-processes. Much was gained by De Saussure's establishment of the
+fact of oxygen respiration, though we now understand by the term
+something very different from, and much more complex than, what he
+understood by it, as, also, much had been gained by the previously
+acquired knowledge of the gas-exchanges in carbon-assimilation: nor must
+we forget the services of those who proved, by laborious analyses,
+continued for long periods, what chemical compounds are found in the
+tissues of plants, and in the soils at their roots and the atmosphere
+which surrounded them. We must also remember many other contributions
+which have been furnished, and are still being furnished by the chemist;
+and I for one hope that his labours will continue to go hand in hand
+with those of the physiologist.
+
+But, when all due honour is paid to the scientific chemist, it must
+still be allowed that his problems are different from the real problems
+of agriculture. To take one set of instances alone. The chemist can
+analyse a given soil or a given manure, and can even go a long way
+towards making them, but his analyses do not tell us what conditions are
+necessary in order that their ingredients may be presented to the roots
+so as to be absorbed and become built up into the plant. Chemistry told
+us that carbon was fixed from the air, but physiological experiments
+determined how this meant the synthesis of certain definite
+carbohydrates--this, too, in the face of the powerful authority of the
+chemist Liebig, who supposed that the vegetable acids were the results
+of the assimilation of carbon. Wolff, De Saussure, and other chemists
+have done yeoman service in showing that different plants, growing in
+the same soil, contain different proportions of mineral substances; but
+it was by means of water-cultures, and other physiological researches,
+such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch
+on root-hairs, that the puzzling question of selective absorption, by
+means of the living root-hairs, came into the arena of our knowledge.
+
+In every case--and, as already said, I am not undervaluing the work
+done--the chemist has left us only on the threshold of the real
+problem. He has stood outside the factory in which the real work we want
+to know about is being carried on, and has told us of so many tons of
+this material being carried in at the gates, and of so many tons of that
+coming out; he has even burnt down the factory, and all its contents and
+machinery, and has then told us how many tons of the various materials
+were there at the time; but this is not what we want, valuable as the
+information is, and still more will be. What we want, and what we expect
+to obtain, is more information regarding what is done with the materials
+in the factory: what machinery they are put into, and how they are put
+in: what stages they go through, and how the stages follow one another:
+what wear and tear has to be endured, and how we can step in and stop
+the working of the machine for our own benefit at the best possible
+time.
+
+The physiologist proceeds empirically, by experimenting with the living
+machinery. He recognises the parts and their structure, and tries to
+find out what they are doing: he knows that the laws of physics and
+chemistry cannot be traversed, but he sees these laws at work under
+special and very complex and peculiar conditions. He therefore, as the
+results of his experiments, sets new questions--or old questions under
+new conditions, if you like--and undoubtedly wants the help of both
+chemist and physicist; or, if it is preferred, the chemist and physicist
+may attack the problems, but they must familiarise themselves with the
+peculiar mechanism of the organism concerned, and cannot hope to attain
+success without experimenting with it. I confess it seems to me as
+reasonable to look upon scientific agriculture as a branch chiefly of
+chemistry as it would be to look upon horse-breeding or pigeon-rearing
+from the same point of view; and why the professed chemist's advice is
+regarded as so comforting and final in the one case and not in the other
+is one of those mysteries which seem inherent in human nature.
+
+The central point in agriculture is the plant: get the most out of
+it--the energy-winning machine which alone can keep the animals and
+everything else connected with the farm going--and all the rest follows.
+The old agriculture has taken a gloomy view of things, and especially on
+account of a large variable which it blames for many ills, namely, the
+season or climate. Perhaps the old agriculture has not sufficiently
+recognised that Nature grows plants in accordance with the fact that
+variation is not peculiar to the weather: if the seasons vary, so do
+fruit and other produce and the plants which yield them; and since man
+cannot hope to control the one variable, possibly relief will be found
+in doing more, within his limits, towards controlling others.
+
+In any case he cannot hope to succeed without study of the physiology of
+the plant.
+
+
+NOTES TO CHAPTER VII.
+
+     An admirable short account of soil in its relation to
+     root-hairs is given in Sachs' _Lectures_, XV.; but for a more
+     exhaustive treatment of the subject of soil the reader is
+     referred to King, _The Soil_ (Wisconsin, 1895), or Warrington,
+     _Lectures on the Physical Properties of Soil_ (Oxford, 1900);
+     Larbalétrier, _L'Agriculture_ (Paris, 1888), chapters II. and
+     III. There is also a very good account in Bailey, _The
+     Principles of Agriculture_ (London, 1898), chapters I.-III.
+
+     With reference to the organisms in soils and the
+     decompositions they bring about, the student should consult
+     Kramer, _Die Bakteriologie in ihren Beziehungen zur
+     Landwirthschaft_ (Wien, 1890), and Lafar, _Technical Mycology_
+     (Engl. edition, 1898), sections V., VIII., and IX.
+
+
+
+
+CHAPTER VIII.
+
+HYBRIDISATION AND SELECTION.
+
+     _The crossing of varieties of wheat, etc.--The essentials of
+     fertilisation--Rimpau's experiments--Hybrids and selected
+     varieties._
+
+
+In the more hopeful view of the case which the new agriculture will have
+to take, it will recognise the physiological truth that since the living
+plant is the important and variable machine which constructs the produce
+looked for, and since that machine will work best in proportion as its
+needs are properly satisfied; therefore in cases where the needs of a
+given type of the machine cannot be efficiently provided for, it will be
+well to select some other type which will take what supplies and
+conditions can be offered. Of course, this is already recognised to a
+certain extent, as is implied in the practices of "rotation of crops,"
+selection of "pedigree wheats" and mixtures of "pasture grasses," and in
+decisions as to the quality of land according to the kinds of weeds
+found on it, and so forth; but I am convinced that the agriculturist of
+the future--and the same applies to the horticulturist, planter and
+forester--will have to concern himself more systematically with the
+working and the variability of the plant, and particularly with what
+Darwin termed Variation under Domestication, than has always been the
+custom in the past. The subject of the plasticity of cultivated plants,
+and especially of hybrids, is in one sense an old one; but much work is
+being done which proves, as such work is apt to do, that very much more
+may be done by well-planned experiments on the selection of new
+varieties raised by hybridising and cultivation.
+
+In illustration of this point, a short summary of some of the results of
+crossing different species of wheat, barley, oats, peas, beet, etc., may
+serve to show what has been gained and what may be hoped for in these
+directions. It should be stated that much has been done and is being
+done in this country as well as abroad, as witness English varieties of
+corn, peas, and potatoes, and the recent experiments on crossing various
+kinds of maize in America.
+
+The hybridiser grows his cereals, etc., in pots until ready for
+crossing, and then takes them into the laboratory, removes the weaker
+spikelets, and takes out the young stamens from the flowers left on the
+plant. The female plant is then ready, and the flowers covered with
+paper caps. The pollen, obtained by a clean wet brush from the plant
+chosen as the father, is then carefully placed in position on the
+stigmas, and the caps replaced. The pollination is repeated
+occasionally, and care taken that no uncrossed flowers develop later. In
+this way a few seeds or grains are got to start with.
+
+This would be the place to introduce an account of the enormous advances
+made by the botanists of the last decade or two in the study of the
+microscopic phenomena of fertilisation. Without going into
+details--which would more than occupy all the space at command--I may
+recall the discoveries of Strasburger and his pupils, and of Guignard,
+which have supplemented the earlier discoveries of De Bary, Cohn, and
+Hofmeister, by establishing the facts that the essential point in
+fertilisation is the fusion of two nuclei, and the bringing together in
+the fused mass of two extremely minute thread-like coiled bodies, the
+so-called chromatosomes or filaments, one of which is derived from the
+male and the other from the female parent. The particulars as to the
+marvellous adaptations to secure the union of these two infinitesimally
+minute threads, their behaviour immediately before and after union, and
+many other points must be passed over, as I have only space to emphasise
+the one crowning discovery that these tiny filaments of nuclear
+substance are the material carriers of all the hereditary properties of
+the parents to the young plant which their union initiates.
+
+It must not be supposed that the above statements are based on any
+meagre foundation of facts. The attraction of the fusing nucleated
+masses had been demonstrated over and over again by Tulasne, De Bary,
+Strasburger and others; but Pfeffer brought the matter to a crisis by
+discovering the attractive (chemotactic) substance emitted in given
+cases, and by collecting the fertilising bodies by its means into
+artificial tubes.
+
+The fusion of the nucleated bodies in the sexual act was observed by
+Strasburger in the living plant a few years ago, and numerous later
+observers have confirmed it. Meanwhile all the stages of approach and
+contact of the essential filaments of the nuclear substance have been
+traced, as also all the stages of the transference of half of each
+filament, male and female, into each of the first two cells of the very
+young embryo-plant.
+
+Moreover, the essentials are found to be the same in the animal kingdom
+also, and the bearing of all these discoveries on the phenomena of
+reproduction, variation, and heredity in living organisms has been and
+is of the highest importance, for they support, control, explain and
+correct so many of the splendid results of Knight, Kölreuter, Sprengel,
+Hildebrand and Hermann Müller, and in every direction throw side-lights
+into the crevices of that magnificent structure, the theory of Natural
+Selection, erected for all time by our countryman, Charles Darwin.
+
+To return now to experiments on crossing. It is found that the first
+products of the crossing appear exactly alike; they may have characters
+intermediate between those of the father and mother, or they may
+resemble one more than the other, but all the seeds of the same cross
+do it in the same way.
+
+On then sowing the seeds of the plants produced from this first cross,
+variations begin to appear. Most of the progeny revert to one or other
+of the parent forms, others show all conceivable combinations of their
+characters, and a few may give rise to entirely new characters. In
+succeeding generations the reversions are preponderant, and, supposing
+no care is taken to prevent it, the whole of the offspring gradually go
+back to the ancestral type.
+
+Some important consequences result, however, if systematic care is
+brought to bear on the matter. This tendency to variation in the second
+generation of crossed plants has often been noted, and it bears out very
+distinctly the conclusions to which Darwin came.
+
+The hybridiser takes advantage of this variation, as others have done,
+to select some forms and rigidly suppress others, in order to obtain
+well-marked varieties of the plants he experiments with. In
+illustration, I may take the following from Rimpau's account of his
+experiments on crossing wheat: By crossing a white English long-eared,
+dense wheat, and celebrated as a heavy cropper, with a red, looser
+German wheat, remarkable for its resistance to winter cold, Rimpau hoped
+to obtain a variety uniting both the above qualities. As regards the
+property of resistance, he failed, and he eventually gave up the
+attempts in face of the advantages offered by the so-called
+_Square-heads_, which then came into the market. His experiments, even
+with the above varieties, are worth noting, however, for they show how
+promising the results of carefully conducted crossing and selection may
+be.
+
+The crossing was done in 1875, in both directions. In 1876 the few
+grains obtained were found to yield plants almost all alike, with the
+long loose ear of the German parent, but the paler colour of the English
+wheat.
+
+In 1877 the plants, obtained by sowing the finest grains, were found to
+consist of pure white, pure red, and of forms which appeared to vary and
+revert in all possible degrees as regards colour, density, and other
+characters intermediate between these.
+
+By carefully separating the closest and densest white wheats from the
+closest and densest red ones, he got in 1878 a large number of each
+coming nearer to the type sown than did the mongrel forms intermingled
+with them: these reversions and intermediate forms were then rigidly
+eliminated, and only the deepest coloured and densest red and white
+forms again sown.
+
+In 1879 these two chosen varieties were constant, so far as concerned
+those selected from the crossing of female English white with male
+German red wheat, and the following year proved the constancy of the red
+variety in the reciprocal cross. In 1886 all four varieties--_i.e._ the
+two reds and the two whites of both the crossings--had become constant.
+
+Still more instructive are the results of the cross between the same
+white English non-bearded wheat and a red German bearded wheat.
+
+The first results of the crossing in 1875 showed the loose ear of the
+German mother, but was paler in colour; while the influence of the
+English father was shown by the absence of beard.
+
+From the reversions and mixtures of the mongrels showing reminiscences
+of the parents in all degrees in 1877, rigid selections and re-sowings
+were made as before, and Rimpau eventually got four very distinct
+varieties, two red and two white, a bearded and a beardless form of
+each, and these were declared fixed and constant in 1879-1882.
+
+Passing over many similar results, and merely noting a very successful
+variety got from a cross between a very early ripening loose red
+American wheat and the dense heavy cropping English Square-head--the
+crossed variety which has proved very suitable for certain light soils
+and dry climates on the Continent, which demand very rapid ripening, and
+are therefore of great physiological and technical interest--I must pass
+on to note the curious result of the successful hybridisation of wheat
+and rye. This cross has been effected several times, and first in this
+country according to reports from Edinburgh (1875), New York (1886), and
+elsewhere, and Rimpau's careful experiments seem to leave no doubt on
+the matter.
+
+First I must remind you that wheat (_Triticum_) differs from rye
+(_Secale_) in several marked characters, such as the breadth and shape
+of the glumes, the number of flowers in the spikelet, etc.; and that the
+cultivated rye differs from cultivated wheats in the characters of the
+straw, in having long ears, and in its flowering glumes remaining widely
+divaricated for some days when in flower.
+
+In 1888 Rimpau removed the young stamens from the German wheat referred
+to, and pollinated the stigmas with pollen from a long-eared rye. Four
+sound grains were obtained, looking like wheat-grains.
+
+The history of one of these grains was as follows: In 1889 it yielded
+ears which were peculiarly narrow and long, and its stalks were also
+much longer than the wheat: the flowers remained exposed, with widely
+open paleae, for several days, and the grains were very peculiar, though
+wheat-like.
+
+Fifteen of the best grains were selected, and in 1890 three of the
+resulting plants proved to be a wheat of the Square-head type and one
+quite sterile. The others retained the elongated, narrow, brownish-red
+ears, the flowering glumes again opening wide for some days. This last
+is a characteristic of rye, but not of wheat.
+
+A long series of natural hybrids of wheat, barley, and oats are also
+described and discussed by Rimpau, as well as artificial crosses--some
+very remarkable--of barleys, but they must be passed over here.
+
+Peas rarely become hybridised naturally. According to Darwin, H. Müller,
+and Focke, the flowers are little visited by insects in our countries,
+though the mechanism points to their adaptation for pollination by large
+bees.
+
+Rimpau confirms Darwin, H. Müller, and Ogle as to the self-fertilisation
+of our cultivated peas. Nevertheless, as is well known, marked varieties
+have been obtained by artificial crossing by Gärtner, Knight, Laxton,
+and others, especially in this country.
+
+At the same time experiments show that while it is very easy to obtain
+artificial hybrids of such plants, and there is no fear of natural
+inter-crossing, the forms are remarkably unstable as yet. Similarly
+unsatisfactory results were obtained with beet. As experiments are still
+going on, however, we may expect to hear more about these and other
+results.
+
+It is probable, from recent experiments by De Vries, Correns, and
+others, that a remarkable regularity, expressed by Mendel in the form of
+a law, obtains in the variations which result from hybridising.
+
+In considering these illustrative cases, it is necessary to thoroughly
+apprehend that two procedures are involved. In the first place we have
+the cross-pollination leading to the formation of the hybrid plant by
+cross-fertilisation. But experience shows that this would lead to very
+uncertain results if the plant-breeder did not supplement them by the
+second and extremely important process of rigid selection--_i.e._ by
+choosing the best of the progeny and breeding from them apart from the
+parent-forms, and gradually intensifying, as it were, the variations in
+certain directions which have been started by the crossing.
+
+It is by selection, careful culture, and repeated selection that so much
+has been done in obtaining the innumerable new varieties of roses,
+sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries,
+melons, tomatoes, early potatoes, etc., brought forward by numerous
+breeders of plants in all countries, as will readily be understood if
+reference be made to the work of Hays and Webber in America; Saunders in
+Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
+
+Nor is it necessary that the new materials for selection to work upon
+should be started by hybridisation. Grafting, change of conditions, and
+even variations so vaguely understood that we term them "spontaneous,"
+may supply the starting-points for changes in the characters of plants,
+so remarkable after intensification by breeding that people find it
+difficult to believe they can have come from one stock.
+
+Here, however, I must conclude, merely remarking that the above sketch
+is a mere outline of the subjects modern agriculture and horticulture
+concern themselves with. There are hundreds of problems connected with
+the germination of seeds, on which valuable recent work has been done by
+Klebs, Green, Horace Brown, and others; with the resistance of seeds
+and seedlings to high and low temperatures, a subject opened out by
+Sachs, Kny, De Vries, Krasan, Just, Höhnel, Dewar, Dyer, and others;
+with the conditions of vegetation which affect the various functions of
+growth, respiration, assimilation, transpiration, and so forth, on which
+I cannot even touch in these pages.
+
+Meanwhile I hope I have succeeded in impressing upon you the grand fact
+that the plant is a living and very complex engine, driven by the
+radiant energy of the sun, and capable of doing work thereby, and this
+just as truly as any heat-engine is driven by chemical energy gained by
+means of the sun's rays, or as a water-mill is driven by power which
+must be referred to the energy of potential in the head of water placed
+in position by the sun's work in evaporation. Fundamentally the whole of
+life and work on our planet is to be referred to the one great source of
+energy which renders possible the establishment of differences of
+potential.
+
+This machine, then, doing work in various ways, adapts itself--or goes
+to the wall--to the conditions of its work among competing organisms or
+opposing circumstances. Curiously enough, while in some cases it suffers
+from the competition, in others it is benefited by its life-actions
+fitting in between those of other organisms, which in their turn
+supplement it. In other words new types of this engine, capable of doing
+the work in various ways, are obtainable; some are good types for the
+conditions afforded, others are bad ones.
+
+Examples of both will occur in the further exposition of the subject.
+
+Man's position in regard to the struggle is that of an intelligent being
+who steps in at certain stages and protects, fosters, and in every way
+favours the agricultural plant--the living machine--and sees that every
+opportunity is given it to do its best work in the best way--from his
+points of view!
+
+
+NOTES TO CHAPTER VIII.
+
+     The foundation of any course of reading on hybridisation
+     and selection should be Darwin's _Effects of Cross and
+     Self-Fertilisation in the Vegetable Kingdom_, which, with his
+     books _On the Origin of Species by means of Natural Selection_
+     and _The Variation of Animals and Plants under Domestication_,
+     will prepare the student for the long course of reading
+     necessary for a full appreciation of what has been done in
+     this department of science.
+
+     From the numerous works which followed these I should select
+     Bailey's _Survival of the Unlike_, London, 1896, and
+     _Evolution of our Native Fruits_, New York, 1898, as
+     especially useful for the reader of this book, to which may
+     also be added _Plant Breeding_, New York, 1896, by the same
+     author, as giving numerous facts and practical directions of
+     value. Further, the "Hybrid Conference Report," _Journ. Roy.
+     Hort. Soc._, 1900, abounds in facts and information. Rimpau,
+     _Landw. Jahrb._, vol. xx., 1891, p. 239. The student who
+     wishes to get towards the root of the matter will hardly be
+     able to dispense with Strasburger's _Neue Untersuchungen über
+     die Befruchtungsvorgang bei den Phanerogamen_, Jena, 1884. An
+     interesting summary of recent work on _Xenia_ and "double
+     fertilisation" will be found in _Bull. No. 22, U.S. Dept. of
+     Agric._, 1900. See also _Nature_, Mar. 15, 1900, p. 470.
+
+     If he wishes to explore the vast region of controversial
+     literature that opens up from these points, and which is far
+     beyond the purpose of this book, he may consult the literature
+     collected in Kassowitz' _Allgemeine Biologie_, Wien, 1899, B.
+     II., and the references in the works quoted; also,
+     Strasburger, "The Periodic Reduction of Chromosomes in Living
+     Organisms," _Ann. Bot._, viii., 1894, p. 281. For "Mendel's
+     Law," see Correns in _Ber. d. deutsch. bot. Gesellsch._, vol.
+     xviii., 1900, p. 158.
+
+
+
+
+_PART II._
+
+DISEASE IN PLANTS.
+
+
+
+
+CHAPTER IX.
+
+PHYTOPATHOLOGY. DERIVATION AND MEANING.
+
+     _History. References in the Bible--Greeks and Romans--
+     Shakespeare--Rouen law--Superstitions--Malpighi and Grew--
+     Hales--Unger--Berkeley--De Bary, etc. Physiology and Biology
+     --Diagnosis--Etiology--Therapeutics. Study of causes._
+
+
+Phytopathology, from Greek words which signify to treat of diseases of
+plants, comprises what is known of the symptoms, course, and causes of
+the diseases which threaten the lives of plants, or bring about injuries
+and abnormalities of structure. As a distinct and systematised branch of
+botany it is a modern study, the history of which only dates from about
+1850, though the subject had been treated more or less disjointedly by
+several authors during the preceding century, and isolated records of
+diseased crops, fruit-trees, etc., exist far back in the history of
+Europe. The existence of mildews and blights on cereals indeed was
+observed and recorded by the writers of the older books of the Bible,
+half a dozen references to such blights being found in the Old
+Testament, as well as others to blasted fig trees, etc., in the New
+Testament. Aristotle, about 350 B.C., noticed the epidemic nature of
+wheat-rust. The Greeks and Romans were so well acquainted with such
+diseases that their philosophers speculated very shrewdly as to causes,
+while the people dedicated such pests to special gods. As regards the
+Middle Ages, we know little beyond the fact that blights and mildews
+existed, but Shakespeare's reference in _King Lear_ (Act III., Sc. 4)
+leaves no doubt as to his acquaintance with mildew in the 17th century,
+and other authorities bear out the same. Even the law took cognisance of
+the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th
+century, however, only meagre notes on the subject occur scattered here
+and there among other matters, and much superstition existed then and
+later regarding these as other diseases.
+
+Malpighi, in 1679, gave excellent figures of leaves rolled by insects
+and of numerous galls, the true nature of which he practically
+discovered by observing the insect piercing the tissues; previous
+observers--Pliny knew that flies emerge from galls, but thought the
+latter grew spontaneously--having nothing but superstitions and
+conjectures to offer. Grew, in 1682, also gave a capital figure and
+description of a leaf mined by "a small flat insect . . . which neither
+ranging in breadth nor striking deep into the leaf, eats so much only
+as lies just before it, and so runs scudding along betwixt the skin and
+the pulp of the leaf, leaving a whitish streak behind it, where the skin
+is now loose, as the measure of its voyage"--a by no means inadequate
+description of the injury and its cause.
+
+During the eighteenth century several academic treatises or
+dissertations dealing with diseases of plants appeared.
+
+But as a rule we only find disjointed notes. Hales (1727-33) discusses
+the rotting of wounds, canker, and a few other matters, but much had to
+be done with the microscope ere any substantial progress could be made.
+
+With the nineteenth century, and the founding of the modern theories of
+nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era
+started. As the discoveries of the microscopists continued to build up
+our knowledge of the anatomy of plants and began to elucidate the
+biology of the fungi and other cryptogams, while the chemists and
+physiologists laid the foundations of our modern science of plant life,
+it gradually became possible to tabulate and classify plant diseases,
+and discuss their symptoms and causes in a more scientific manner. Even
+in 1833, however, Turpin, and a far better observer, Unger, regarded
+parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles
+and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846).
+We may pass over the various treatises of Wiegmann (1839), Meyen (1841),
+Raspail (1846), Kühn (1859), and a number of other works of the period,
+merely referring with emphasis to Berkeley's admirable papers in the
+_Gardener's Chronicle_ (1854) for a summary of what was then known. All
+these works antedate De Bary's _Morphologie und Physiologie der Pilze,
+etc._ (1866), in which he brought together the results of his researches
+during the decade, proving the real nature of parasitic diseases and
+infection as worked out by experiments between 1853 and 1863.
+
+This work put the whole subject of parasitic diseases of plants and
+animals on a new footing, and paved the way for the modern treatment of
+plant pathology as elaborated in the treatises of Frank (1880 and 1895),
+Sorauer (1886), Kirchner (1890), and others, to which the reader is
+referred for further details. I will merely quote the following passage
+from Raspail's _Histoire Naturelle de la Santé et de la Maladie_, 1846
+(vol. ii., p. 176), in illustration of the views entertained by high
+authorities just prior to De Bary's work: "L'insecte qui produit les
+_erineum_, _uredo_, _æcidium_, _xyloma_, _puccinia_, n'est donc plus
+pour nous un insecte inconnu, mais un _acarus_ (grise), un _aphis_
+(puceron) ou un _thrips_, qui produit au printemps une déviation, etc."
+
+And this view, that fungi already well known to mycologists were called
+forth by the punctures of insects, was regarded as not out of harmony
+with the idea that the fungus itself was an abnormal outgrowth of the
+tissues of the host.
+
+The proper study of plant pathology presupposes and involves a knowledge
+of the physiology of plants, of the normal relations of the latter to
+their environment, and of the biology of those animals and plants
+(principally insects and fungi) which are parasitic on them. It is of
+the first importance to understand that a disease is a condition of
+abnormal physiology, and that the boundary lines between health and
+ill-health are vague and difficult to define. As with the study of the
+diseases of man and other animals, so with those of plants, the practice
+resolves itself into the accurate observation and interpretation of
+symptoms (_Diagnosis_) on the one hand, and of causes (_Aetiology_) on
+the other, before any conclusions of value can be drawn as to preventive
+or remedial measures (_Therapeutics_). In plants, however, symptoms of
+disease are apt to exhibit themselves in a very general manner, or at
+any rate it may be that our perceptions of them differentiate symptoms
+due to very different reactions imperfectly, probably because the
+organisation of the plant is less specialised than that of animals. The
+turning yellow and premature falling of leaves, for instance, is a
+frequent symptom of disease; but it may be due to a long series of
+different causes of ill-health--_e.g._ drought, too high or too low a
+temperature, light of insufficient or of excessive intensity, a
+superfluity of water at the roots, the presence in the tissues of
+parasitic fungi, or that of worms or insects at the roots or elsewhere,
+poisonous gases in the air, soil, etc., and so forth. Consequently the
+science of plant pathology is much concerned with the direct action of
+external causes, which are probably less obscure than in the case of
+animals, though by no means always obvious. Such considerations at any
+rate seem to account for the fact that most authorities on plant
+pathology base their classification on the causes of disease, there
+being few noteworthy exceptions.
+
+
+NOTES TO CHAPTER IX.
+
+     The bibliography here quoted will be found in Berkeley,
+     "Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4;
+     Plowright, _British Uredineæ and Ustilagineæ_, 1889; Eriksson
+     and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary,
+     _Comparative Morphology and Biology of the Fungi_, etc., 1887;
+     Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered
+     in the works referred to in them and in the text.
+
+
+
+
+CHAPTER X.
+
+HEALTH AND DISEASE.
+
+     _Variation--Disease--Comparison to a top. Health--Extinction
+     of species--Natural demise. Examples of complex interactions
+     in health--Interference, and tendencies to ill-health._
+
+
+When we come to enquire into the causes of disease, it appears at first
+an obvious and easy plan to subdivide them into groups of factors which
+interfere with the normal physiology of the plant. Scientific experience
+shows, however, that the easy and the obvious are here, as elsewhere in
+nature, only apparent, for disease, like health, is an extremely complex
+phenomenon, involving many reactions and interactions between the plant
+and its environment. If we agree that a living plant in a state of
+health is not a fixed and unaltering thing, but is ever varying and
+undergoing adaptive changes as its life works out its labyrinthine
+course through the vicissitudes of the also ever-varying environment,
+then we cannot escape the conviction that a diseased plant, so long as
+it lives, is also varying in response to the environment. The principal
+difference between the two cases is, that whereas the normal healthy
+plant varies more or less regularly and rhythmically about a mean, the
+diseased one is tending to vary too suddenly or too far in some
+particular directions from the mean; the healthy plant may, for our
+present purposes, be roughly likened to a properly balanced top spinning
+regularly and well, whereas the diseased one is lurching here, or
+wobbling there, to the great danger of its stability. For we must
+recognise at the outset that disease is but variation in directions
+dangerous to the life of the plant. Health consists in variation also,
+but not in such dangerous grooves. That the passage from health to
+disease is gradual and ill-defined in many cases will readily be seen.
+In fact we cannot completely define disease. Mere abnormality of form,
+colour, size, etc., is not necessarily a sign of disease, in the usual
+sense of the word, otherwise the striking variations of our cultivated
+plants would suggest gloomy thoughts indeed, whereas we have reason to
+believe that many cultivated varieties are more healthy--in the sense of
+resisting dangerous exigencies of the environment--than the stocks they
+came from. Strictly speaking, no two buds on a fruit-tree are alike, and
+the shoots they produce vary in position, exposure, number, and vigour
+of leaves, and so forth. The minute variations here referred to are not
+seen by the ordinary observer, but those who bud, graft and multiply by
+cuttings on a large scale know that such bud-variations are important,
+quite apart from more extensive "sports" which occasionally occur.
+
+On the other hand, we have reason to believe that many species have died
+out gradually as the environment altered. These plants died because they
+did not vary sufficiently, or did not vary in the right directions; they
+became diseased with respect to the then prevailing conditions of normal
+physiology or health.
+
+Disease, therefore, may be said to be variation of functions in
+directions, or to extents, which threaten the life of the plant, the
+normal in all cases being the state of the plant characteristic of the
+species.
+
+Even now, however, we have not obtained a complete definition, because,
+since all plants die sooner or later, we have not excluded the natural
+demise of the individual or its parts, and no one would call the
+autumnal fall of leaves, or the withering of an annual after flowering,
+death from disease. Clearly then the idea of disease implies danger of
+premature death, and probably this is as near as we shall get to a
+satisfactory definition. Since this matter is of primary importance for
+our present theme, I will add the following instances for consideration.
+
+A plant in perfect health and in the fullest exercise of all its
+functions, has its roots in a soil which is suitably warmed and aerated,
+contains the right quantities of water which dissolve just the proper
+proportions of all the essential mineral salts, but nothing poisonous,
+while the soil itself has a texture such that the roots and root-hairs
+can extend and do their utmost in absorbing.
+
+The leaves above are exposed to just the right intensity of light, in
+air which is not too dry, and is of suitable temperature and
+composition, containing no poisonous exhalations, etc.; and as the
+foliage is gently moved by the breeze, it manufactures carbohydrates at
+the optimum rate in the chlorophyll, and the so-called "elaborated sap"
+containing the dissolved organic food-supplies is prepared in the
+tissues in maximum quantities and of just the right degrees of
+concentration and quality for use in the buds, stem, roots, etc., for
+which it is destined as they draw on the supplies.
+
+Between these assimilating organs, the leaves, and the absorbing roots,
+we have in the stem the wood, with its vessels adapted in quantity and
+calibre to convey the water containing dissolved salts from the
+absorbing roots to the leaves (to say nothing of other parts) and,
+separated from this wood by the cambium, we find the sieve-tubes and
+cortical tissues in suitable quantity conveying the "elaborated
+sap"--the solutions of organic food-materials from the leaves down to
+the roots, up to the buds, and elsewhere. Joining these cortical and
+wood tissues are adapted series of medullary rays which, apart from
+other connections, bring about the necessary interchanges of water and
+"elaborated sap" with the cambium, the formative tissue which has to be
+fed and served by them, and which by its growth supplies new vessels and
+sieve-tubes, etc., to carry the continually increasing quantities of
+water and food substances as the roots and leaves increase in number and
+area, and thus enables this ideally correlated system to go on working
+at maximum energy.
+
+Now suppose the same plant with its roots in an unsuitable soil--too dry
+or too poor in mineral supplies, for instance--the transpiring leaves
+above cannot obtain sufficient water and salts to supply their needs,
+but we will suppose hypothetically that they still assimilate under the
+same ideal conditions as before. The supplies now coming to the cambium
+are diminished, since the want of water and minerals compels the leaves
+to put aside any excess of carbohydrates (_e.g._ as stored
+starch-grains), and the plastic materials which do pass to the cambium
+so deficient in water cannot be directly utilised, and a starvation
+period sets in. Consequently the cambium forms less wood, and this will
+contain fewer and smaller vessels, and so reduce the conducting
+passages: fewer sieve-tubes also are constructed, and the paths of the
+water current and food supplies narrowed, which of course reacts on the
+tissues everywhere. The reserve substances may slowly be dissolved and
+distributed, however, and considerable quantities be passed in course of
+time into the roots, which, as opportunity offers, gradually employ them
+in making new roots, and if the disturbance has not gone too far and
+the conditions do not become unfavourable, an increased root-supply may
+by its larger absorbing area gradually establish the former state of
+equilibrium of functions. But this at the expense of the plant, which is
+smaller, has fewer leaves and narrower water channels, etc., than a
+plant not thus checked, and it may take a long time to make up for the
+loss of time and stature thus incurred. Indeed if the plant is an annual
+no recovery at all may occur, the reserves passing into fruit and seeds
+instead of slowly supplying the roots as described.
+
+If it be asked, can such a condition of affairs as that described really
+occur, we have only to think of a transplanted specimen with its roots
+maimed and put into unsuitable soil, or of plants in the open with
+feeding roots gnawed by an insect, etc., or of a tree hitherto in
+equilibrium with its fellows in a plantation suddenly set free by
+thinning and so forth.
+
+Now take the case where the roots are maintaining their maximum
+functional activity, but the leaves--owing to want of light, too much
+moisture or too low a temperature of the air--are functionally
+depressed. Here we get a state of over-saturation with water set up, the
+tissues are turgid to bursting point, what supplies do traverse the
+sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and
+the cambium again forms less wood, but the lumina of the vessels are
+larger and the lignification less complete. Growth in length is
+excessive, but more leaves are formed, though they are apt to be
+abnormally thin and may be small. Little or no reserves are stored
+anywhere, and the watery tissues contain dangerously diffusible
+substances which may render them an easy prey to parasitic fungi. Here
+again, however, if the disturbance of equilibrium has not gone too far,
+and if the season permits, the new leaves may come into full activity
+and the situation be saved by transpiration and assimilation gradually
+increasing and restoring the equilibrium. But, as before, the plant has
+suffered, and shows the effect in its weak shoots, retarded flowering,
+and other ways.
+
+Such plight as is here described may actually be attained in greenhouses
+where over-watering is the fault, and even in the open it is not
+uncommon in rainy summers, or in plantations where dominant trees get
+the upper hand and partially shade more slowly growing species, or in
+fields where rank grass is allowed to overwhelm crops of lower stature.
+
+Now it will be evident that either of these typical cases of temporary
+disturbance of functional equilibrium may be carried too far: in the
+first case the plant may wilt and wither, in the second it may rupture
+and rot, to take these eventualities only. And yet it is difficult to
+call these indispositions diseases: they are rather examples of extreme
+departures from the normal standard of health, just on the borderland
+between health and disease. A step further, as it were, and disease
+supervenes: certain tissues die from want of water, and a necrotic area
+is formed, or the cortex bursts and a wound is formed in another way, or
+some fungus gets a hold, and so on. These abnormal states are
+particularly apt to predispose the plant to disease--insects revel in
+such semi-wilted leaves and shoots crammed with reserves, and fungi in
+the water-logged leaves of the second case, while a cold dry wind is
+peculiarly fatal to such tissues.
+
+
+NOTES TO CHAPTER X.
+
+     The reader may consult Hartig, _Diseases of Trees_, Eng. ed.,
+     1894, Introduction; Sorauer, _Pflanzen Krankheiten_, pp. 1-12,
+     and Frank, _Die Krankheiten der Pflanzen_, B. 1, p. 5, for
+     definitions of disease.
+
+
+
+
+CHAPTER XI.
+
+CAUSES OF DISEASE.
+
+     _A. External causes--I. Non-living environment: soil,
+     atmosphere, temperature--II. Living environment: plants,
+     animals--Complex interactions--Predisposing causes--No one
+     factor works alone--Tangled problems of natural selection
+     involved. B. So-called internal causes._
+
+
+It is customary to classify the causes of disease in plants into two
+principal groups--(1) those due to the action of the non-living
+environment--soil, atmosphere, physical conditions such as temperature,
+light, etc.; and (2) those brought about by the activities of living
+organisms--plants and animals of various species. Before passing to
+further subdivisions under these two heads, however, it is necessary to
+observe that no disease can be efficiently caused by an organism alone,
+since its powers for injury as a parasite, or otherwise, are affected by
+its non-living environment as well as by the host-plant. For instance,
+the spores of a parasitic fungus which would infect and rapidly destroy
+a potato plant in moist warm weather may be showered on to such a plant
+with impunity if the air remains dry and cool--or on to a cabbage under
+any circumstances as far as we know.
+
+Again, probably no one factor of the non-living environment ever
+suffices to induce a disease, possibly because no such thing as only one
+change at a time ever occurs. For instance, it is difficult to say, when
+a soil becomes sodden with water, whether the excess of water and
+dissolved matters, the want of air displaced by the water, the lowering
+of the temperature, or the accumulation of foul products, etc., is the
+principal factor in causing the damage which results, and we have to
+determine by the balance of experimental evidence which is the dominant
+factor in all such cases.
+
+The study of aetiology of disease is in fact only a particular case of
+that of aetiology in general. Plants at high altitudes in the Alps
+acquire very different characteristics from the same species in the
+plains. Is this due to the low temperature, the rarer atmosphere, the
+more intense illumination, the changes in moisture, etc., etc.? The
+question is more difficult than it appears at first sight, and we must
+remember that, complex as are the factors working on the host, they are
+equally complex in their actions on a parasite attacking the host,
+whence the resulting disease becomes indeed a tangled problem of natural
+selection.
+
+Finally it remains to say a few words about a numerous class of cases
+where no external cause of disease can be discovered. It was formerly
+the custom to group such cases of "Internal Causes" by themselves, but
+apart from the fact that many of these mysterious diseases have
+subsequently been shown to be due to the action of external agencies,
+the whole question of internal causes resolves itself into one of
+relations between the plant and its surroundings, and it becomes evident
+that no inherited or internal disease can be regarded as explained until
+we know the external causes which have so modified the structure and
+working of the living cells as to make them abnormal in their reactions
+to other parts of the plant. "Internal causes" of disease, therefore, is
+a phrase expressing our ignorance, but somewhat more emphatically than
+usual. If this is clearly understood there seems no reason against its
+employment for the time being in the artificial scheme of classification
+we require. With regard to external causes due to the non-living
+environment, excess or deficiency of materials in the soil, water, or
+atmosphere plays an important part, and--since we may neglect purely
+aquatic plants--it is customary to speak of diseases due to unsuitable
+soils or to injurious atmospheric influences. For instance, any
+deficiency in the supplies of the necessary mineral salts (compounds of
+calcium, magnesium, potassium with sulphuric, nitric and phosphoric
+acids, etc.) leads to pathological changes, as also does the lack of the
+necessary traces of iron. But it is equally true that the presence of
+such ingredients in excess or in combinations unsuited to the plants
+also leads to disaster, as also does the presence of minerals or other
+compounds which poison the root-hairs--_e.g._ products of decomposition,
+soluble salts of copper and other poisons. That these matters are bound
+up with the whole question of manuring and of proper soil-analyses will
+be evident.
+
+Another essential factor is the nature and quantity of organic materials
+in the soil, whether leaf-mould and decomposing vegetable remains,
+stable manures, or other animal matters, all of which affect different
+species very differently, and produce very different results in
+different soils. It is necessary to apprehend in this connection what
+has been stated above: that soil is not a mere dead structureless
+medium, and that the root-hairs of ordinary plants cannot deal with
+large quantities of putrefying organic matter: that a good soil must
+abound in useful bacteria and fungi to render such substances
+available--and in very various ways--and that it must be open and
+aerated, of proper temperature and suitably supplied with water, and so
+forth, or disaster will result. Here, again, then we are brought into
+close contact with all that is known of fermentation, nitrification, and
+the various biological changes going on in soil, and the application of
+such knowledge to the practice of manuring and tillage in all its forms.
+
+In view of the above remarks, the danger of "over-feeding," in this
+sense, has a real meaning for horticulturists, though it must not be
+forgotten that no substance is really a food until it is assimilable
+into the protoplasm: manures, etc., are food-materials, not food. The
+futility of mere chemical analyses to prove what a plant requires is now
+well known, and it is only on the basis of long and carefully conducted
+experiments that we can ever discover what a particular plant in a
+particular soil, situation, and climate requires for healthy
+development. Again, the quantity of water in soil may be too great or
+too small for given species, and this either on the average for the
+year, or during critical periods only; and it is obviously important
+whether the excess or deficiency is due to improper supplies of water,
+the depth or shallowness of the soil, its retentive powers, or the
+nature of the sub-soil and so on, again bringing the whole matter into
+connection with our understanding of the physical constitution and
+structure of soils, and the nature of soil-drainage.
+
+For instance, a common way of killing ferns is to keep the roots and
+soil wet and the air and fronds dry, whereas the natural habitats
+provide for wet and shaded fronds and well-drained soil.
+
+It may be noted here that in most cases where gardeners speak of plants
+being killed under the "drip" of trees--_e.g._ Beech, the injury is due,
+not to the effects of water but to the shade: the loss of light is so
+great that the shaded plants die of inanition because their leaves are
+not able to provide sufficient carbohydrates.
+
+Closely bound up with this is the question of the gases in soils. Apart
+from the disastrous effects of poisons--_e.g._ coal gas escaping from
+pipes under pavements in towns, etc., diseased conditions often result
+from deficiency of oxygen at the root-hairs, due to imperfect aeration
+of soils, brought about by stagnant water, excess of animal matter, and
+so forth.
+
+Unsuitable constitution of the atmosphere is also a fruitful source of
+disease, though its effects are commoner in closed stoves and
+greenhouses than in the open. Nevertheless the continual exhalation of
+sulphurous fumes, chlorine, and other poisonous gases in the
+neighbourhood of manufacturing centres or of large smoky towns,
+volcanoes, etc., play their part in injuring plants; and excessive
+moisture in the form of mist, rain, etc., is also important. All these
+matters bring us at once into the region of physiology, and only an
+intelligent appreciation of what is known about the action of the
+atmosphere on the soil and the plant will save the peasantry of a
+country from a hopeless mysticism but little removed from that of the
+Middle Ages, when blights and other evils were vaguely referred to the
+river-mists, thunder clouds, and easterly winds.
+
+If we summarise the above as the material factors of the environment, we
+may classify another set of external non-living causes of disease as the
+non-material factors. Such are principally the following:
+
+The space at the disposal of plants greatly affects their welfare. The
+crowding of roots in the soil and of foliage in the air, resulting in
+the loss of light to the leaves, involves deficiency of all the
+materials referred to above--minerals, organic materials, gases, and
+water--and no better illustration of the intense struggle for existence
+among these apparently passive and motionless beings, plants, can be
+given than an over-crowded seedbed or plantation. If left to themselves
+such over-stocked areas exhibit to the keen eye of the trained observer
+all the phases of starvation, weakness, wounding, rot, and, so to speak,
+brutal dominance of the stronger over the weaker which it is the object
+of cultivation to prevent. Here, then, we are brought face to face with
+the true significance of thinning and weeding out, pruning, and similar
+processes.
+
+Unsuitable temperature is one of the commonest of all sources of
+disease, for every plant is adapted to certain ranges of temperature,
+and best adapted to a given optimum somewhere between the maximum and
+minimum temperature for each function. Consequently any serious
+departure from the mean may bring about physiological disturbances of
+the nature of disease, and this in very various ways, as exemplified by
+the results of frost, sun-scorching, drought, hail-storms, forest fires,
+and so forth.
+
+As a predisposing factor to disease abnormal temperature effects play a
+great part. Many wound-fungi gain their entrance through frost-cracks,
+bruises due to hailstones, or into tissues chilled below the normal.
+
+No less remarkable are the diseases primarily due to insufficient or
+improper exposure to light, which affects the chlorophyll-apparatus and
+the process of carbon-assimilation and through these the whole
+well-being of the plant. Every plant is adapted to certain ranges of
+light intensity, and most cultivators know how impossible it is to grow
+shade plants in fully exposed situations, and how easily plants which
+live in open sunny situations are "drawn" and killed by shade. It is
+equally important to have the right kind of light, as disastrous
+experiences with greenhouses glazed with glass which cut off certain
+rays of light have taught. Here, again, it is important to notice that
+the optimum intensity or quality of light may differ for different
+functions and organs of the plant, as is shown by many adaptations on
+the part of species growing in natural situations--_e.g._ bud
+protection, orientation of leaves, etc.--and it may be taken as a rule
+that etiolated plants are peculiarly susceptible to other diseases.
+
+As regards other factors of the inorganic environment, disasters which
+come within the scope of our subject may be brought about by many
+agencies, the mechanical effects of snow and hail, wind, avalanches,
+etc., the effects of lightning, and so forth, being a few of them.
+
+
+NOTES TO CHAPTER XI.
+
+     For other detailed classifications of the causes of disease
+     the reader is referred to the works of Sorauer and of Frank
+     referred to in the last chapter. Also Kirchner, _Pflanzen
+     Krankheiten_, Stuttgart, 1890.
+
+     Of more historical importance are the older classifications of
+     Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
+     Chronicle_, 1849-50. These latter are interesting as showing
+     the very different views held by the earlier workers, and
+     comparison of these with the modern views helps to mark the
+     progress of physiology during the half century which has
+     intervened.
+
+
+
+
+CHAPTER XII.
+
+CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
+
+     _Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata
+     --Insects, etc.--Plants as causes of disease--Phanerogams,
+     weeds, etc.--Cryptogams, fungi--Epidemics, etc._
+
+
+Passing now to those causes of disease which are connected with the
+living environment, we may obviously divide them into two groups of
+agents, animals and plants.
+
+Among animals, the various vertebrata, including man, are especially
+responsible for the larger kinds of wounds and wholesale destructive
+processes due to breakage, stripping of leaves and bark, cutting and
+biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
+birds of various kinds stand out prominently as enemies to trees and
+other plants, to which they do immense injury in various ways by their
+horns, teeth, claws, and beaks; and the damage which an ignorant
+gardener or forester can do with his ill-guided footsteps, axe, spade,
+and knife can only be appreciated by one who knows the habits of plants.
+
+It is among the invertebrata, however, especially insects and worms,
+that the most striking agents of disease in plants are to be found, for,
+with the exception of certain rodents--and we may logically include also
+human invasions--vertebrate animals do not often appear in such numbers
+as to bring about the epidemics and scourges only too commonly caused by
+insect pests.
+
+Insects injure plants in very various ways. Some, such as locusts,
+simply devour all before them; others, _e.g._ caterpillars, destroy the
+leaves and bring about all the phenomena of defoliation. Others, again,
+eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and
+so maim the plant that its foliage and assimilation suffer, or its roots
+become too scanty to supply the transpiration current. Many aphides,
+etc., puncture the leaves, suck out the sap, and produce deformations
+and arrest of leaf-surface, as well as actual loss of substance, and
+when numerous such insects induce all the evils of defoliation. Others,
+such as the leaf-miners, tunnel into the leaves, with similar results on
+a smaller scale.
+
+It must be remembered that a single complete defoliation of a herbaceous
+annual, or even of a tuberous plant like the potato, so incapacitates
+the assimilatory machinery of the plant, that no stores can be put aside
+for the seeds, tubers, etc., of another year, or at most so little that
+only feeble plants come up.
+
+In the case of a tree the case is different, and since most large trees
+in full foliage have far more assimilatory surface than is actually
+necessary for immediate needs, a considerable tax can be paid to
+parasites or predatory insects before the stores suffer perceptibly.
+Still, it should be recognised that the injury tells in time, especially
+in seed years.
+
+Many larvae of beetles, moths, etc., bore into the bark and as far as
+the cambium or even into the wood or pith of trees, the local damage
+inducing general injuries in proportion to the number of insects at
+work: moreover, the wounds afford points of entrance for fungi and other
+pests.
+
+Galls and similar excrescences result from the hypertrophy of young
+living tissues pierced by the ovipositors of various insects, and
+irritated by the injected fluid and the presence of the eggs and larvae
+left behind. They may occur on the buds, leaves, stems, or roots, as
+shown by various species of _Cynips_ on oak, _Phylloxera_ on vines,
+etc., in all cases the local damage being relatively small, but the
+general injury to assimilatory, absorptive, and other functions is great
+in proportion to the number of points attacked.
+
+Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or
+internodes of grasses, or the pith of twigs, or into buds, fruits, and
+other organs of plants, and do harm corresponding to the kind and amount
+of tissues injured.
+
+Various species of so-called eelworms--Nematodes--also cause gall-like
+swellings on young roots, or they invade the grains of cereals.
+
+Finally, various slugs and snails cause much injury by devouring young
+leaves and buds and diminishing the assimilatory area.
+
+Plants as agents of disease or injury fall naturally into the two main
+categories of flowering plants (Phanerogams) and Cryptogams, among which
+the fungi are the especially important pests.
+
+Beginning with weeds, we find a large class of injurious agents. Weeds
+damage the plants we value by crowding them out in the struggle for
+existence, as already stated, and when the weed-action is simply due to
+superfluous plants of the same species, we speak of overcrowding. But it
+must not be overlooked that the competition between crowded plants of
+the same species--where every individual is acting as a weed to the
+others--may be more dangerous than between plants and weeds belonging to
+other species and genera, because in the former case they are struggling
+for the same minerals and other necessary food-materials: a matter of
+importance in connection with the rotation of crops.
+
+The question of allowing grass to grow at the foot of fruit trees, as in
+orchards, is a good case in point. Such grass may increase the damp and
+shade, thus favouring fungi at one season, and dry up the moisture of
+the soil to the injury of the fine superficial roots at another, as well
+as exhaust the soil, owing to the competition of the roots for salts
+and other materials. On the other hand, the checking of surface roots by
+competition with the grass has been claimed as advantageous. In this
+connection probably the whole question of the composition of the turf
+arises, as well as that of possible cropping for hay, and manuring.
+
+As regards any particular weed, the cultivator should learn all he can
+respecting its duration, seeding capacity, method of dissemination, the
+depth and spread of its root-system, and any other particulars which
+enable him to judge when and how to attack it. It is only necessary to
+see the victory of such drought-resisting weeds as _Hieracium
+pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may
+win in the struggle for existence with the finer grasses.
+
+Many so-called weeds are, however, partially parasitic, with their roots
+on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much
+damage is done to meadow grasses and herbage by the exhaustive tax which
+these semi-parasites impose.
+
+This is carried still further in the case of such root-parasites as
+_Orobanche_, where the host-plant is burdened with the whole support of
+the pest, because the latter, having no chlorophyll, is entirely
+dependent on the former for all its food.
+
+Even ordinary climbing plants may injure others by shading them, either
+by scrambling over their branches--_e.g._ Bramble, or twisting their
+tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._
+Woodbine, _Convolvulus_, etc. The principal direct injury is in these
+cases owing to the loss of light suffered by the shaded foliage, but
+the weed-action is often increased by the competition of their
+roots--_e.g._ briars; and in the case of woody climbers the gradually
+increased pressure of the woody-coils round the thickening stems
+compresses the cambium and cortex of the support and induces strictures
+and abnormalities which may be fatal in course of time.
+
+Epiphytes, or plants which support themselves wholly on the trunks,
+branches, or leaves of other plants, also injure the latter more
+especially by shading their foliage--_e.g._ tropical Figs, Orchids,
+Aroids, etc.; and similar damage is done by our own Ivy, the main roots
+of which are in the soil, but the numerous adventitious roots of which
+cling to the bark.
+
+When the climber or epiphyte is also parasitic, as in the case of the
+Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance
+stolen from the host by the parasite comes in to supplement any effect
+of shading that the latter may bring about if it is a leafy plant.
+
+Of Cryptogams, apart from a few epiphytic ferns, and the intense
+weed-action of certain Equisetums, the rhizomes and roots of which are
+as troublesome as those of twitch and other phanerogamic weeds, it is
+especially the fungi which act as agents of disease, and which, as we
+now know, are _par excellence_ the causes of epidemics.
+
+The action of fungi may be local or general; and restricted, slow and
+insidious, or virulent and rapidly destructive.
+
+Examples of local action are furnished by _Schinzia_, which forms
+gall-like swellings on the roots of rushes; _Gymnosporangium_, which
+induces excrescences on the stems of junipers, and numerous leaf-fungi
+(_Puccinia_, _Æcidium_, _Septoria_, etc.), which cause yellow, brown, or
+black spots on leaves, as well as by _Ustilago_, which attacks the
+anthers or the ovary of various plants, and so forth. In such cases the
+injury done by a few centres of infection is very slight, but prolonged
+action may bring into play secondary effects such as the gradual
+destruction of the cambium round a branch, when, of course, the effect
+of ringing results; or if the fungus becomes epidemic and myriads of
+leaf-spots are formed, the destruction of foliar tissue, gradual taxing
+of the assimilatory cells, etc., may end in rapid defoliation, and
+renewed attacks soon exhaust the plants and lead to sterility and death,
+as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
+
+It is highly probable that such fungi are particularly exacting owing to
+their exhausting demands for compounds of potassium, phosphoric acid,
+and other bodies.
+
+Examples of virulent and rampant general action are afforded by finger
+and toe in turnips, etc., where the roots are invaded by
+_Plasmodiophora_, which induces hypertrophy and rotting of the roots;
+and by the damping off of seedlings, where the fungus _Pythium_ rapidly
+invades all parts of the seedlings and reduces them to a water-logged,
+putrefying mass; or the potato-disease, which is due to the rapid
+spread of _Phytophthora_ in the leaves and throughout the plant, which
+it blackens and rots in a few days.
+
+Many fungi not in themselves very virulent or aggressive do enormous
+harm owing to the secondary effects they induce. Some of the
+tree-killing hymenomycetes, such as _Agaricus melleus_, for instance,
+penetrate the wood of a pine at the collar, and the result of the large
+flow of resin which results is to so block up the water passages that
+the tree dies off above with all the symptoms of drought. Similarly, the
+_Peziza_ causing the larch disease, having obtained access to the stem
+about a foot or so above the ground, will gradually kill the cambium
+further and further round the stem, and so girdle the tree as
+effectually as if we had cut out the new wood all round. In all such
+cases--and the same applies to the leaf-diseases referred to above--the
+fungus may be compared to an army which is not strong enough to invade
+the whole territory, but which, by striking at the lines of
+communication, cuts off the supplies of water, food, etc., and so brings
+the struggle to an end. Indeed we might compare the cases of fungi which
+attack the root and collar, and so strike at and cut off the water
+supply, to a compact army which at once cuts off the enemy from his
+narrow base; whereas the innumerable units which bring about an epidemic
+attack on the leaves, and so surround the enemy and cut off his food
+supplies all round, is rather like a much larger army which cannot get
+in beyond the natural barriers of the tissues, and so puts a _cordon_
+all round the territory and seizes the multitudes of food-stuffs at the
+frontiers. The end result is similar in both cases, but the methods of
+warfare differ.
+
+Many fungi, however, though they make their presence noticeable by
+conspicuous signs, cannot be said to do much damage to the individual
+plant attacked. The extraordinary malformations induced by parasites
+like _Exoascus_, which live in the ends of twigs of trees and stimulate
+the buds to put out dense tufts of shoots, again densely
+branched--Witches' brooms--are a case in point. Also the curious
+distortions of nettle stems swollen and curved by _Æcidium_, of maize
+stems and leaves attacked by _Ustilago_, and of the inflorescences of
+_Capsella_ by _Cystopus_, etc., are not individually very destructive;
+it is the cumulative effects of numerous attacks, or of large epidemics,
+which tell in the end.
+
+Some very curious effects are due to fungi such as _Æcidium elatinum_,
+which, living in the cortex of firs, stimulate buds to put out shoots
+with erect habit, and with leaves which are radially disposed, annually
+cast, and differently shaped from the normal--characters quite foreign
+to the species of fir in its natural condition.
+
+Equally strange are the shoots of _Euphorbia_ infested with the æcidia
+of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In
+all these cases we must assume a condition of toleration, so to speak,
+on the part of the host, which adapts itself to the altered
+circumstances by marked adaptations in its tissue developments, mode of
+growth and so forth.
+
+This toleration is perhaps most marked in the case of those cereals
+which, though infected by the minute mycelium of _Ustilago_ while still
+a seedling, nevertheless go on growing as apparently healthy green
+plants indistinguishable from the rest, although the fine hyphae of the
+parasite are in the tissues and keeping pace with the growth of the
+shoots just behind the growing points. As the grains of the cereal begin
+to form and swell, however, the hyphae suddenly assume the part of a
+dominant aggressor, consume the endosperm of the enlarging seed, and
+replace the contents of the grain with the well-known black spores known
+as Smut.
+
+
+NOTES TO CHAPTER XII.
+
+     The reader will find a summary of such fungi as are here
+     concerned in Massee, _A Text-Book of Plant Diseases_, 1899, or
+     Prillieux, _Maladies des Plantes Agricoles_.
+
+     For further details the student should consult the works of
+     Frank and Sorauer referred to in the notes to Chapter IX., and
+     Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.
+
+     For experiments on the effects of grass on orchard trees, see
+     _Report of the Woburn Experimental Fruit Farm_, 1900, p. 160.
+
+     For the further study of weeds, the interesting bulletins of
+     the Kansas State Agricultural College, 1895-1898, will show
+     the reader what may be done in the matter of classifying them
+     according to their biological peculiarities.
+
+     In regard to insects, the reader will find the following list
+     embraces the subject: Somerville, _Farm and Garden Insects_,
+     1897; Theobald, _Insect Life_, 1896; Ormerod, _Manual of
+     Injurious Insects_, 1890, and _Handbook of Insects Injurious
+     to Orchards, etc._, 1898.
+
+     The admirable series of publications of the U.S. Department of
+     Agriculture under the editorship of Riley and Howard, and
+     entitled _Insect Life_, 1888-1895, also abounds in
+     information.
+
+     Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880,
+     and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst.
+     Insektenkunde_, 1889.
+
+     For an elementary introduction to the study of fungus
+     diseases, see Marshall Ward, _Diseases of Plants_, Soc. for
+     Promoting Christian Knowledge, London.
+
+
+
+
+CHAPTER XIII.
+
+NATURE OF DISEASE.
+
+     _General and local disease--General death owing to cutting-off
+     supplies, etc.--Disease of organs--Tissue-diseases, e.g.
+     timber--Root-diseases--Leaf-diseases, etc.--Diseases of
+     Respiratory, Assimilatory, and other organs--Physiological and
+     Parasitic diseases--Pathology of the cell--Cuts--Cork--Callus
+     --Irritation--Stimulation by protoplasm--Hypertrophy._
+
+
+On going more deeply into the nature of those changes in plants which we
+term pathological or diseased, it seems evident that we must at the
+outset distinguish between various cases. A plant may be diseased as a
+whole because all or practically all its tissues are in a morbid or
+pathological condition, such as occurs when some fungus invades all the
+parts or organs--_e.g._ seedlings when completely infested by _Pythium_,
+or a unicellular Alga when invaded by a minute parasite; or it may die
+throughout, because some organ with functions essential to its life is
+seriously affected--_e.g._ the roots are rotten and cannot absorb water
+with dissolved minerals and pass it up to the shoot, or all the leaves
+are infested with a parasite and cannot supply the rest of the plant
+with organic food materials, in consequence of which parts not directly
+affected by any malady become starved, dried-up, or poisoned or
+otherwise injured by the results or products of disease elsewhere.
+
+In a large number of cases, however, the disease is purely local, and
+never extends into the rest of the organs or tissues--_e.g._ when an
+insect pierces a leaf at some minute point with its proboscis or its
+ovipositor, killing a few cells and irritating those around so that they
+grow and divide more rapidly than the rest of the leaf tissues and
+produce a swollen hump of tissue, or gall; or when a knife-cut wounds
+the cambium, which forthwith begins to cover up the dead cells with a
+similarly rapid growth of cells, the callus. Numerous minute spots due
+to fungi on leaves, cortex, etc., are further cases in point, the
+mycelium never extending far from the centre of infection.
+
+Many attempts have been made to classify diseases on a basis which
+assumes the essential distinction of the above cases, and we read of
+diseases of the various organs--root-diseases, stem-diseases,
+leaf-diseases, and so forth; or of the various tissues--timber-diseases,
+diseases of the cambium, of the bark, of the parenchyma, and so on.
+Furthermore, attempts have been made to speak of general functional
+disease, of diseases of the respiratory organs, of the absorptive
+organs, and so forth, as opposed to local lesions.
+
+Critical examination, however, shows that no such distinctions can be
+consistently maintained, partly because the organs and functions of
+plants are not so sharply marked off as they are in animals, the
+diseases of which have suggested the above classification, and partly
+because all disease originates in the cells and tissues, and it is a
+matter of detail only that in some cases--_e.g._ severe freezing or
+drought of seedlings, or when some ingredient is wanting in the
+soil--the diseased condition affects practically every cell alike from
+the first, while in others it spreads more or less rapidly from some one
+spot.
+
+Even the distinction into physiological diseases _versus_ parasitic
+diseases cannot be maintained from the standpoint of the nature of the
+disease itself. All disease is physiological in so far as it consists in
+disturbance of normal physiological function, for pathology is merely
+abnormal physiology, no matter how it is brought about. This is not
+saying that no importance is to be attached to the mode in which disease
+is incurred or induced: it is merely insisting on the truth that the
+disease itself consists in the living cell-substance--the
+protoplasm--not working normally as it does in health, and this, whether
+want of water, minerals, or organic food be the cause, or whether the
+presence of some poison or mechanical irritant be the disturbing agent,
+as also whether such want or irritation be due to some defect in soil
+or air, or to the ravages of a fungus or an insect.
+
+This being understood I need not dwell on the common fallacy of
+confounding the fungus, insect, soil or other agent with the disease
+itself, or of making the same blunder in confusing symptoms with
+maladies. In this sense, wheat rust is not a disease: it is a symptom
+which betrays the presence of a disease-inducing fungus, the Rust
+fungus. Similarly, chlorosis is not a disease: it is a symptom of
+imperfect chlorophyll action, and the best proof of the truth of both
+statements is that in both cases the fundamental disease-action is the
+starvation of the cell-protoplasm of carbohydrates and other essential
+food matters--in the one case because the fungus steals the
+carbohydrates as fast as the leaves can make them, in the second because
+the leaf is unable to make them.
+
+The foundation of a knowledge of disease in plants therefore centres in
+the understanding of the pathology of living cells.
+
+If a suitable mass of living cells is neatly cut with a sharp razor the
+first perceptible change is one of colour: the white "flesh" of a potato
+or an apple, for instance, turns brown as the air enters the cut cells,
+and the microscope shows that this browning affects cell-walls and
+contents alike. The cut cells also die forthwith; and the oxygen of the
+air combining with some of their constituents forms the brown colouring
+matter which soaks into the cell-walls. The uninjured cells below them
+grow longer, pushing up the dead débris, and divide across by walls
+parallel to the plane of the wound, and so form series of tabular cells
+with thin walls, which also soon turn brown and die, the cell-walls
+meanwhile undergoing changes which convert them into cork. The living
+cells deeper down are now shut off from the outer world by a skin, of
+several layers, of cork-cells, which prevent the further free access of
+air or moisture. During the period of active cell-division which
+initiates the cork, the temperature of the growing cells rises: a sort
+of fever (wound-fever) is induced, evidently owing to the active
+respiration of the growing cells.
+
+This healing by cork occurs in any tissue of living cells exposed by a
+cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
+applies to any other kind of cutting or tearing injury--such as a prick
+with a needle or the proboscis of an insect, a stripping, or even a
+bruise.
+
+Such healing is prepared for and carried out very thoroughly in the case
+of falling leaves and cast branches, the plane of separation being
+covered by a cicatrix of cork.
+
+If the cell-tissue under the wound is actually growing at the time,
+however, a further process is observed when the wound-cork has been
+formed. The uninjured cells below go on growing outwards more vigorously
+than ever, the pressure of the overlying tissues taken off by the cut
+having been removed, and, lifting up the cork-layer as they do so, they
+rapidly divide into a juicy mass of thin-walled cells which is of a
+cushion-like nature and is termed a _Callus_. This callus is at first a
+homogeneous tissue of cells which are all alike capable of growing and
+dividing, but in course of time it undergoes changes in different parts
+which result in the formation of tracheids, vessels, fibres and other
+tissue-elements, and even organs, just as the embryonic tissues of the
+growing points, cambium, etc., of the healthy plant give origin to new
+growths. Such wound-wood, however, is apt to differ considerably in the
+arrangement, constitution and hardness of its parts as compared with
+normal wood, and its peculiar density and cross-graining are often
+conspicuous.
+
+If instead of a simple tissue, the cut or other wound lays bare a
+complex mass such as wood, the resultant changes are essentially the
+same to start with. The living cells bordering the wound form cork, and
+then those deeper down grow out and form a callus. The exposure of the
+wood however, entails alterations in its non-living elements also. The
+lignified walls of tracheids, fibres, etc., turn brown to a considerable
+depth, and this browning seems to be--like all such discolorations in
+wounds--due to oxidation changes in the tannins and other bodies
+present: the process is probably similar to what occurs in humification
+and in the conversion of sap-wood into heart-wood in trees. Such wood is
+not merely dead, but it is also incapable of conveying water in the
+lumina of its elements, which slowly fill with similarly dark-coloured,
+impervious masses of materials termed "wound-gum," the nature of which
+is obscure, but which slowly undergoes further changes into resin-like
+substances.
+
+The exposure of wood by a wound results also in another mode of stopping
+up the vessels and so hindering the access of air, loss of water, etc.,
+for the living cells of the medullary rays and wood-parenchyma grow into
+the lumina of the larger vessels through the pits, forming _thyloses_,
+again a phenomenon met with in heart-wood. In Conifers the stoppage of
+the lumina is increased by deposition of resin, which also soaks into
+the cell-walls and the wounded wood becomes semi-translucent owing to
+the infiltration.
+
+Every living cell in an active condition is irritable, and one of the
+commonest physiological reactions of growing tissues is that of
+responding to the touch of a resistant body, as is vividly shown by the
+movements of the Sensitive plant, _Dionaea_, etc., and by those of
+tendrils, growing root tips, etc., on careful observation. We have
+reason for stating that if a minute insect, too feeble to pierce the
+cuticle, cling on to one side of the dome-shaped growing point of any
+shoot, the irritation of contact of its claws, hairs, etc., would at
+once cause the protoplasm of the delicate cells to respond by some
+abnormal behaviour; and, as matter of experiment, Darwin showed long ago
+that if a minute piece of glass or other hard body is kept in contact
+with one side of the tip of a root, the growth on the side in contact is
+interfered with. Moreover we know from experiments on heliotropism,
+thermotropism, etc., that even intangible stimuli such as rays of light,
+etc., impinging unsymmetrically on these delicate cells cause
+alterations in their behaviour--_e.g._ arrest or acceleration of growth.
+
+Perhaps the most remarkable class of stimulations, however, is that due
+to the presence of the entire protoplasmic body of one organism in the
+cell of another, each living its own life for the time being, but the
+protoplasm of the host cell showing clearly, by its abnormal behaviour,
+that the presence of the foreign protoplasm is affecting its physiology.
+A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid
+protoplasmic body of which lives in the hypha of _Pilobolus_, causing it
+to swell up like an inflated bladder, in which the parasite then forms
+its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but
+that its protoplasm alters the metabolic physiology of the latter is
+shown by the hypertrophy of the cells, and by the curious fact that it
+stimulates the _Pilobolus_ to form its sexual conjugating cells,
+otherwise rare, an indication of very far-reaching interference with the
+life-actions of the host.
+
+An equally remarkable example is that of _Plasmodiophora_, the amoeboid
+naked protoplasm of which lives and creeps about in the protoplasm of a
+cell of the root of a turnip, to which it gains access through the
+root-hairs. It does not kill the cell, but stimulates its protoplasm to
+increased activity and growth and division, itself dividing also and
+passing new amoebae into each new daughter-cell of the host. Here the
+processes of stimulation, hypertrophy and further division are repeated,
+until hundreds or thousands of the turnip root-cells are infected. The
+externally visible result is the formation of distorted swellings on the
+root (Finger and Toe), most of the cells of which are abnormally large
+and filled with amoeboid _Plasmodiophora_ protoplasm, which finally
+devours the turnip-protoplasm and itself passes over into spores. Here
+we have most convincing proof of the stimulation of protoplasm by other
+protoplasm in direct contact with it; and that the metabolism of the
+host-cells is profoundly altered is shown not only by the abnormal
+growth of the cells, but also by the starvation of the rest of the
+turnip plant as the _Plasmodiophora_ gets the upper hand. We have here,
+in fact, a local intracellular parasitic disease, gradually invading
+large tracts of tissue and eventually inducing general disease resulting
+in death--a state of affairs reminding us of cancer in animals.
+
+Irritation and hypertrophy of cells, however, may be induced by
+parasites which never bring their protoplasm into direct contact with
+that of the host. Many Chytridiaceae penetrate the cells of plants, and
+grow inside them as short tubes, vesicles, etc., the protoplasm of which
+is separated by their own cell-walls from that of the host-cell;
+nevertheless hypertrophy and abnormal cell-divisions and secretions are
+induced, and the effect even extends to neighbouring cells--_e.g._
+_Synchytrium_--showing that some influence is exerted through cells
+themselves not directly affected. This latter point need not surprise us
+now we know that the cells of plant-tissues are connected by fine
+protoplasmic strands passing through the separating cell-walls.
+
+But the invading plant need not actually enter the cells, and may still
+stimulate them through both its own and their own cell-walls to abnormal
+growth. This is well shown by the intercellular mycelium of _Exoacus_
+and _Exobasidium_, and the latter affords an excellent illustration of
+the far-reaching effects of hyphae on the cells (of _Vaccinium_) into
+which they do not penetrate. Not only are the cells stimulated to grow
+larger and divide oftener than normally, thus producing large gall-like
+swellings, but the chlorophyll disappears, the cell sap changes colour
+to red, the numerous compound crystals normally found in the tissues
+diminish in number and are different in shape, large quantities of
+starch are stored up, and even the vascular bundles are altered in
+character. All these changes indicate very profound alterations in the
+physiological working of the protoplasm of the cells of the host, and
+yet the fungus has done its work through both its own cell-walls and
+those of the host.
+
+Even harmless endophytic algae in the intercellular spaces of plants may
+stimulate the cells in their immediate neighbourhood to increased
+growth, _e.g._ _Anabaena_ in the roots of Cycads.
+
+
+NOTES TO CHAPTER XIII.
+
+     With reference to cork-healing and wound-fever the student may
+     consult Shattock "On the Reparative processes which occur in
+     Vegetable Tissues," _Journal of the Linnean Society_, 1882,
+     Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
+     the Aspen," _Journal of Botany_, 1883, Vol. XXI., p. 306. Also
+     Richards, "The Respiration of Wounded Plants," _Annals of
+     Botany_, Vol. X., 1896, p. 531; and "The Evolution of Heat by
+     Wounded Plants," _Ann. of Bot._, Vol. XI., 1897, p. 29.
+
+     For details and figures respecting callus, see Sorauer,
+     _Physiol. of Plants_, p. 175.
+
+     In respect to the irritable movements referred to see Darwin,
+     _The Power of Movements in Plants_, 1880, chapter III. The
+     recent work of Nawaschin, _Beobachtungen ueber den feineren
+     Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI.,
+     1899, p. 404, should be read for details and literature
+     concerning "Finger and Toe."
+
+
+
+
+CHAPTER XIV.
+
+NATURE OF DISEASE (_Continued_).
+
+     _Actions of poisons in small doses--Results of killing a few
+     cells--Malformation--Enzymes--Secretions and excretions--
+     Acids, poisons, etc.--Chemotactic phenomena--Parasitism--
+     Epiphytes and endophytes--Symbiosis--Galls._
+
+
+Physiological research has shown that the respiratory activity of cells
+may be increased by small doses of poisons, and even that growth may be
+accelerated by them--_e.g._ chloroform, ether--and, still more
+remarkable, that fermentative activity may be enhanced by minute doses
+of such powerful mineral poisons as mercuric chloride, iodine salts,
+etc., and that the cells may be gradually accustomed to larger doses
+without injury. Unfertilised eggs of insects have been started into
+growth by treatment with acids and those of frogs with mercury salts,
+and the germination of beans quickened by various poisonous alkaloids.
+In other words, graduated doses of poison may alter the physiological
+activity of living cells, inducing pathological phenomena, while larger
+doses kill them.
+
+Now we know at least one parasitic fungus which poisons the cells of its
+host, and kills them, with similar symptoms to those resulting from
+excessive doses of the above-named toxic agents. _Botrytis_ hyphæ,
+living in the cell-walls of plants, but not entering the cells, excretes
+a poison which kills the protoplasm, and the fungus then feeds on the
+debris. Numerous other fungi form powerful poisons, but we do not know
+whether or how they employ them--_e.g._ Ergot.
+
+It is obvious that if all the young cells of a root-tip or of the apex
+of a shoot, or those of a young leaf, are growing and dividing
+regularly, the killing of one or a few cells at one point on the side of
+the organ must result in irregularities--in malformation--of the adult
+organ. This has been proved experimentally by destroying a few cells
+with a needle. It can also be done by planting a minute mycelium of
+_Botrytis_ laterally on a young organ--_e.g._ a very young lily-bud. The
+fungus adheres to the surface, kills a few epidermis cells, and forms a
+foxy-red spot, which becomes concave as the dead cells lose water and
+dry. Since the rest of the bud goes on growing, however, while this dead
+point remains stationary, the latter gradually becomes the centre of a
+concavity, the growing tissues having grown round it: the bud is
+deformed. Numerous cases of malformed organs are explained in this way;
+a minute insect has bitten or pierced the young tissue, or a fungus has
+killed a minute area, or a drop of acid condensed from fumes in the air
+is the lethal agent, and so forth. And even on a much larger scale we
+see the same kinds of agents at work. Wherever a patch of cells is
+killed whilst those around go on growing, there must result some
+deformation of the resulting organ, since had the injury been withheld
+the number and sizes of the cells now fixed in death would have
+increased and covered a larger area: they now serve to pull over to
+their side the still living and growing cells. The same results follow
+on any lateral wound: the killed spot of tissue serves as a point round
+which the continued growth of other parts of the organ turns. Hence the
+malformation is in these cases a secondary effect, and not, as in simple
+hypertrophy, a direct effect of the action of the cells involved in the
+injury.
+
+There is another class of bodies secreted by fungi, however, which act
+directly on cells, viz. enzymes--that is, soluble bodies which are able
+to dissolve cellulose (_cytases_), starch (_diastases_), proteids
+(proteolytic enzymes), and other substances, by peculiar alterations in
+their constitution. It is by means of its _cytase_ that _Botrytis_
+hyphae pierce the cellulose walls of plants, and no doubt in all cases
+where fungi pierce cell-walls it is by the solvent action of such a
+cytase, and similarly when haustoria penetrate into the cells. It is
+also by means of these starch-dissolving enzymes (diastases) and
+proteolytic enzymes, etc., that the hyphae inside the cells are enabled
+to make use of the starch, proteids, etc., they find there.
+
+All living cells form materials, resulting from the activity of the
+protoplasm, which we may compare with the refuse or by-products formed
+in any great manufacturing industry: these by-products have to be got
+rid of if they are injurious or noisome (_excretions_), and if
+not--_i.e._ if they are capable of further use (_secretions_)--they have
+to be stored away till required. Some of the most prominent of these
+bodies excreted by fungi are, as we have seen, poisonous acids, such as
+oxalic acid, enzymes, and organic poisons, such as those in ergot. But
+similar enzymes, acids, poisons, etc., to those found in fungi are also
+found in the cells of other plants and animals; for only by means of
+their solvent actions can processes like digestion and assimilation of
+the starchy and other materials into the body-substance be accomplished,
+and we have seen that it is a general property of living cells to form
+acids, and other excretions and secretions.
+
+Now we know very little about what may happen when an organism--say a
+fungus--secreting especially one kind of enzyme or poison or other
+active substance, comes into intimate contact with another--say a
+leaf-cell--which secretes predominantly others, but what we do know
+points to the certainty that various complications will occur.
+
+For instance, if certain bacteria which prefer an alkaline medium, and
+yeasts which prefer an acid environment are mixed in a saccharine
+solution, it depends on the reaction of the liquid which organism gains
+the upper hand: if the liquid is acid the yeast may dominate the
+bacteria; if alkaline it may be suppressed by them.
+
+That a parasite may be prevented from successfully attacking a
+particular plant is shown by the failure of _Cuscuta_ to establish its
+haustoria in poisonous plants such as _Euphorbia_, _Aloe_, etc., and it
+has been pointed out that poisonous secretions in the cells of the plant
+protect them against the penetration of fungi. This cannot be taken as
+meaning that any poison protects against any parasite, however, for
+_Euphorbia_ is itself subject to attacks of Uredineae, and _Pangium
+edule_, which contains prussic acid and is extremely poisonous to most
+animals, is eaten with avidity by several insects, while nematode worms
+can live in its tissues. This is no more remarkable, however, than the
+fact that _Fontaria_, a myriapod, secretes prussic acid in its own
+tissues, or than that certain glands of the stomach secrete free
+hydrochloric acid, and _Dolium_ forms sulphuric acid in its glands.
+
+There is yet a further point to notice here. It has been proved that
+certain substances formed in plant-cells, not necessarily nutritive,
+attract the hyphae of parasitic fungi or repel them, according to the
+kind and degree of concentration. So clear has this proof been made that
+it was possible in experiments conducted apart from a host plant, to
+make the hyphae on one side of an artificial membrane--_e.g._
+collodion--penetrate it by placing one of these attractive
+(_chemotropic_) substances in suitable proportions on the other side.
+The hyphae dissolved holes in the membrane by means of enzymes and
+plunged into the attractive substance on the other side.
+
+The foregoing sketch gives us a glimpse into the causes at work in
+parasitism.
+
+Suppose a fungus on the outside of the epidermis of a young organ--say a
+leaf. It may be unable to penetrate into the plant, and finding no
+suitable food outside it dies: or it may be satisfied with the traces of
+organic matter on the epidermis and then lives the life of a saprophyte.
+Or it may be able to establish a hold-fast on the tender epidermal
+surface, but without entering the cells, and irritate the developing
+organ by contact stimulation, inducing slight abnormalities; if in its
+further, purely superficial growth such an epiphyte covers large areas
+of the leaf, and especially if the hyphae are dark coloured--_e.g._
+_Dematium_ and other "Sooty Moulds"--injury may be done to the leaf
+owing to the shading action which deprives the chlorophyll below of its
+full supply of solar energy. Some epiphytes, however, are able to fix
+their hyphae to the epidermis by sending minute peg-like projections
+into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send
+haustoria right through the outer epidermal walls--_e.g._
+_Erysiphe_--and thus supplement mere contact-irritation and shading by
+actual absorption from the external cells. Here the fungus is a
+parasitic epiphyte.
+
+A stage further is attained in those fungi which enter the stomata and
+live in the intercellular spaces--_e.g._ many Uredineae and
+_Phytophthora_--and many such intercellular endophytes increase their
+attack on the cells by piercing their walls with minute (_Cystopus_) or
+large and branched (_Peronospora_) haustoria, or even eventually pierce
+the cells and traverse them bodily (_Pythium_). In all these cases it is
+clear that conflicts must occur between poison and antidote, acid and
+alkali, attractive and repellent substances, enzyme and enzyme, etc., as
+was hinted at above; and the same must take place when the parasite is
+endophytic and intracellular from the first, as in Chytridiaceae, etc.,
+the zoospores of which pierce the outer cell-walls and forthwith grow
+into the cells. There are also fungi which, while able to pierce the
+outer cell-walls, and grow forward in the thickness of the wall itself,
+cannot enter the living cells themselves--_e.g._ _Botrytis_. In the
+example mentioned, the fungus excretes a poison, oxalic acid, which
+soaks into and kills the cells next its point of attack: into these dead
+cells it then extends, and, invigorated by feeding on them, extends into
+other cell-walls and excretes more poison, and so on.
+
+On the basis of the foregoing it seems possible to sketch a general view
+of the nature of parasitism. In order that a fungus may enter the cells
+it must be able to overcome not only the resistance of the cell-walls,
+but that of the living protoplasm also: if it cannot do the latter it
+must remain outside, as a mere epiphyte, or at most an intercellular
+endophyte. If it can do neither it must either content itself with a
+saprophytic existence or fail, so far as that particular host-plant is
+concerned. Its inability to enter may be due to there being no
+chemotropic attraction, or to its incapacity to dissolve the cell-walls,
+or to the existence in the cell of some antagonistic substance which
+neutralises its acid secretions, destroys its enzymes or poisons, or is
+even directly poisonous to it.
+
+Moreover when once inside it does not follow that it can kill the cell.
+The protoplasm of the latter may have been unable to prevent the fungus
+enemy from breaking through its first line of defence--the cell-wall,
+but it may be quite capable of maintaining the fight at close quarters,
+and we see signs of the progress of the struggle in hypertrophy,
+accumulation of stores, and other changes in the invaded cells and their
+contents.
+
+Finally, the invested or invaded cell may so adapt itself to the demands
+of the invader that a sort of arrangement is arrived at by which life in
+common--_Symbiosis_--is established, each organism doing something for
+the other and each taking something from the other. In this latter case,
+which is often realised--_e.g._ lichens, leguminous plants and the
+organisms in their root-nodules, mycorrhiza, etc.--we leave the domain
+of disease, which supervenes indeed if the other symbiont is lacking.
+
+Some interesting facts bearing on the matters here under discussion,
+have been obtained from the study of _Galls_, the curious outgrowths
+found on many plants and due to the action of insects.
+
+A typical gall exhibits three distinct and characteristic layers of
+tissue surrounding the hollow chamber in which the larva of the insect
+lies, viz., an outer layer of soft cells forming a parenchyma covered
+with an epidermis, and frequently also with a layer of cork; an inner
+stratum consisting of very thin-walled delicate cells filled with
+protoplasmic and reserve food-materials on which the larva feeds; and
+between the two a more or less definite layer of thick-walled
+sclerenchyma cells which serve as a protection against accidents to the
+larva as the outer layer shrivels or rots, or if it is exposed to the
+attack of marauders. This layer may be absent from galls which have a
+short life only. Vascular bundles run into the outer layer from the
+leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
+and are frequently of use in the arts on this account: they also contain
+starch, and proteid substances and crystals of calcium oxalate. When the
+larva has consumed the stores of food material and reached the adult
+stage it eats its way out and escapes.
+
+The growth of such a gall is preceded by the laying of an egg on or in
+the embryonic tissue of a leaf, stem, or other young part, and it is
+interesting to note that only organs in the meristematic stage can form
+galls, and that it is by no means necessary that the tissues should be
+wounded. Moreover, the egg as such is incapable of stimulating the plant
+tissues, but when it hatches, the resulting larva, beginning to feed on
+the cells, irritates the tissues and rapid growth and cell-division
+occur, as in the case of other wounds or of fungus attacks. The actual
+wound made by the ovipositor heals up at once. It is evident from
+numerous recent researches that these true galls are not due to any
+poisonous or irritating liquid injected by the parent, but that the
+stimulus to the tissue formation is similar to that exerted by a wound.
+The young gall is in fact a callus enclosing the living larva, and it is
+the continued irritation of the latter which keeps up the stimulation.
+The final shape and constitution of the gall depend on mutual
+reactions--not as yet explained in detail--between the species of plant
+and the species of gall-insect concerned, as may readily be seen from
+the extraordinary variations in size, shape, colouring, hairiness and
+other structural peculiarities of the galls on one species of, for
+instance, the common oak. From what we have learnt about fungus
+parasites, however, there can be little doubt that reactions between the
+cells and the larva of the insect occur, resembling those which take
+place between the cells and the hyphae of the fungus, and this is borne
+out by the study of other hypertrophies due to animals; _e.g._ Nematode
+worms in roots, and the remarkable galls--the simplest known--on
+_Vaucheria_, caused by the entrance into this alga of a species of
+_Notommata_, which induces a different gall on each of the various
+species of its host plants.
+
+It must be concluded that the formation of the _Vaucheria_ gall is
+induced by the mechanical irritation which the Rotifer causes in the
+protoplasm. These galls are comparable to the hypertrophies in
+_Pilobolus_ caused by the presence of _Pleotrachelus_.
+
+Attempts to induce the development of galls artificially by injecting
+formic, acetic and other vegetable acids, poisons and other substances
+into the tissues have, however, failed, and even the substances
+contained in the insect or gall itself only produced negative results.
+Nothing further was obtained than slight callus formations in some
+cases. Nor have experimenters succeeded in obtaining more than slight
+distortions by fixing insects on the growing leaves in such positions
+that they could scratch the epidermis.
+
+We must therefore conclude that very complex interactions between the
+plant and insect are here concerned, among which may be the infiltration
+of some liquid from larva to plant--many of these gall larvae are
+strongly scented, and Kustenmacher says that fluids excreted by the
+larva are absorbed by the gall-tissue apparently as nutriment. This
+would point to the symbiotic character of galls and their guests.
+
+
+NOTES TO CHAPTER XIV.
+
+     With regard to the action of poisons in small doses see
+     further Johannsen, _Das Aether-Verfahren beim Fruhtreiben_,
+     Jena, 1900, and, for _Botrytis_, see Marshall Ward, "A Lily
+     Disease," _Annals of Botany_, Vol. II., 1889, p. 388.
+
+     The subject of enzymes has been exhaustively treated by Green,
+     _The Soluble Ferments and Fermentations_, Cambridge, 1899, to
+     which the reader is referred for literature. I have taken the
+     statements regarding _Fontaria_ and _Dolium_ from Kassowitz,
+     _Allgemeine Biologie_, p. 182. The two most important works on
+     chemotactic phenomena are Pfeffer, "Uber Chemotaktische
+     Bewegungen," etc., _Unters. aus dem Bot. Inst. zu Tubingen_,
+     B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen
+     durch Pilzfaden," _Pringsh. Jahrb. f. Wiss. Bot._, B. XXVIII.,
+     1895, p. 269, and from these the further literature can be
+     traced. As regards the nature of parasitism see Marshall Ward,
+     "On Some Relations between Host and Parasite," etc., being the
+     Croonian Lecture delivered before the Royal Society, _Proc.
+     Roy. Soc._, Vol. 47, p. 393. On Symbiosis, see Marshall Ward,
+     "Symbiosis," _Annals of Botany_, 1899, Vol. XIII., p. 549,
+     where the literature is collected. For a general account of
+     galls the reader may consult Kerner, _The Natural History of
+     Plants_, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,
+     _Alternating Generations, A Biological Study of Oak Galls_,
+     etc., 1894.
+
+
+
+
+CHAPTER XV.
+
+SPREADING OF DISEASE AND EPIDEMICS.
+
+     _Dissemination of fungi by the aid of snails, rabbits, bees,
+     and insects--Man--Distribution in soil, on clothes, through
+     the post, etc.--Worms, wind--Puffing of spores--Creeping of
+     mycelia--Lurking parasites--Spread of insects and other
+     animals--Losses due to epidemics._
+
+
+The dissemination of plant diseases is a subject which has been far too
+much neglected, but our knowledge of it is slowly increasing. The spores
+of fungi such as Rusts and Erysipheae are often carried from plant to
+plant by snails; those of root-destroying and tree-killing Polyporei by
+rabbits, rats, and other mammals which rub their fur against the
+hymenophores. Bees have been shown to carry the spores of _Sclerotinia_
+and infect the stigmas of Bilberries, etc., with them; and flies convey
+the conidia of Ergot from grain to grain. Insects, indeed, of all kinds
+are great disseminators of disease--as witness also the part played by
+mosquitoes in transferring the malaria parasite to man--and beetles,
+bees, flies, etc., of all sorts probably play more active parts in this
+work than has yet been proved, since they not only carry spores attached
+like pollen to their hairy bodies, but in many cases in their alimentary
+canal, to be spread later in the dung.
+
+The part played by man in conveying fungi from plant to plant counts for
+much. Not only do gardeners and farm labourers carry spores on their
+boots and clothes as they pass from infected to non-infected areas, but
+carted soil and manure are frequently infested with spores of Smuts,
+_Fusarium_, _Polyporus_, and the sclerotia or rhizomorphs of
+_Sclerotinia_, _Agaricus melleus_, _Dematophora_, etc. Man also sends
+diseases through the post, and by rail and ship, by spores or mycelia
+attached to seedlings, bulbs, fruits, flowers, etc., as shown in several
+cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every
+time a carpenter saws a piece of fresh timber with the saw which has
+been used previously for cutting wood attacked with dry rot, he risks
+infecting it with the fungus. Similarly in pruning: every cut with a
+knife which the gardener has used on infected branches may infect the
+tree.
+
+Cuttings made with a soil-contaminated knife and stuck into ordinary
+soil in dirty boxes covered with equally dirty glass, present every
+chance for infection by soil organisms; bacteria and fungi obtain access
+to the vessels, and derive plenty of food from the juices, and the
+wonder is not that so many cuttings "damp off," but that any are raised
+at all under ordinary conditions.
+
+That worms bring buried spores to the surface can hardly be doubted
+after Pasteur's experiments with Anthrax, and the principle of Darwin's
+discoveries of the important bearing of the habits of earthworms on this
+subject, and that the soil attached to the feet of ducks and other birds
+teems with small seeds, applies to fungi also. Wind is also responsible
+for distributing fungus-spores over wide areas, as may be easily proved
+by fixing a glass slide smeared with glycerine in the course of a breeze
+passing over an infected area.
+
+But although the fungi are, generally speaking, passive in regard to
+their distribution, such is by no means always the case. Apart from the
+fact that some forms attract insects by means of honey dew (Ergot), or
+by sweet odours (Spermogonia, _Sclerotinia_), the zoospores of
+_Pythium_, _Phytophthora_, etc., are motile, and although they cannot
+move far in the films of water in which they travel, nevertheless in a
+wet potato field, with the wind flapping the leaves one against the
+other, some dissemination of importance must be actively brought about,
+and similarly with the amoebae of _Plasmodiophora_ in the soil.
+
+The shooting of ascospores into the air by certain species of _Peziza_,
+from the discs of which the spores may be seen to puff out in clouds,
+affords further evidence that fungi cannot be regarded as entirely
+passive in respect to distribution of their spores. But when we come to
+certain of the soil fungi--_e.g._ _Agaricus melleus_, _Dematophora_,
+etc.--the active creeping forward by growth in the soil of their
+rhizomorphs and mycelial strands afford examples of active spreading of
+considerable importance in the vineyard and forest, since they pass from
+root to root and from tree to tree and may infect the entire area in
+course of time.
+
+Not the least significant mode of dissemination is that by which what I
+have termed "lurking parasites" are spread: such are fungi which attach
+themselves to the seeds, fruits, tubers, etc., of other plants and so
+obtain all the advantages of being carried and sown with the
+latter--_e.g._ Ustilagineae and Uredineae which adhere to grain,
+_Verticillium_, _Nectria_, etc., in potatoes and other plants.
+
+The spread of diseases due to animals, especially insects, is of course
+more active, in consequence of the motility of the distributing agents.
+This is most marked in the winged species, of which locusts, beetles,
+moths and butterflies, flies and wasps furnish well-known examples; and
+is not inconsiderable in the case of wingless and merely creeping
+species. It is noteworthy that many forms wingless in the parasitic
+stage are winged at certain periods, _e.g._ the females of _Phylloxera_.
+
+That man also spreads insect pests is well known and acted upon, as
+witness the phylloxera laws--which, however, it is to be feared too
+often only illustrate once more the adage concerning the shutting of the
+stable door after the horse has gone.
+
+It would be tedious to attempt anything like a complete account of the
+estimates of loss in different countries, due to the ravages of insects
+and fungi, but the following examples should surely serve to convince
+anyone of the magnitude of these losses and of the economic importance
+of the whole question, and the reader may be referred to the special
+literature for further details.
+
+The coffee leaf-disease of Ceylon, due to the fungus _Hemileia_, is
+estimated to have cost that Colony considerably over £1,000,000 per
+annum for several years. One estimate puts the loss in ten years at from
+£12,000,000 to £15,000,000. The hop-aphis is estimated to have cost Kent
+£2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of
+the United States estimated the annual loss, due to the ravages of
+insects on cotton alone, to amount to £5,000,000; and in 1882 the annual
+loss to the United States due to insects, calculated for all kinds of
+agricultural produce, was put at the appalling figure of from
+£40,000,000 to £60,000,000 sterling. In India, the annual loss due to
+wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000
+rupees, and one insect alone is said to have cost the cotton planters a
+quarter of the crop--valued at seven crores of rupees--in bad years.
+Similarly, in Australia the annual loss from wheat-rust has been put at
+from £2,000,000 to £3,000,000. In 1891 the loss in Prussia alone from
+grain-rusts was officially estimated at over £20,000,000 sterling. Need
+more be said? Even allowing for considerable exaggerations in such
+estimates it is clear that the damage to crops in any country soon
+amounts to sums which even at low rates of interest would easily yield
+incomes capable of supporting the best equipped laboratories and staffs
+for investigations directed to the explanation of the phenomena in
+detail, the sole basis on which intelligent preventive and therapeutic
+measures can be based. But it is far from likely that the estimates are
+exaggerated. The planting and agricultural communities are as a rule
+opposed to the publication of statistics--or at least have been so in
+various countries and at different times--and if we knew the damage done
+to all crops even in our own Empire, the results would probably astonish
+us far more than the above figures have done.
+
+
+NOTES TO CHAPTER XV.
+
+     On the dissemination of fungi, the reader will find Fulton,
+     "Dispersal of the Spores of Fungi by the Agency of Insects,"
+     _Ann. Bot._, Vol. III., 1889, p. 207, and Sturgis, "On Some
+     Aspects of Vegetable Pathology and the Conditions which
+     Influence the Dissemination of Plant Diseases," _Botanical
+     Gazette_, Vol. XXV., 1898, p. 187, both useful papers. Further
+     information will be found in Zopf, _Die Pilze_, Breslau, 1890,
+     pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der
+     Pilze durch Schnecken," in _Zeitschr. f. Pflanzen Krankh._,
+     1896, p. 144. The estimates as to losses due to epidemics are
+     taken from Watt, _Agricultural Ledger_, Calcutta, 1895, p. 71;
+     Balfour, _The Agricultural Pests of India_, London, 1887,
+     pp. 13-15; Eriksson and Henning, _Die Getreideroste_; the
+     publications of the U.S. Department of Agriculture, _The
+     Kew Bulletin_, and elsewhere. The reader will find further
+     examples in Massee, _Text-Book of Plant Diseases_, 1899, pp.
+     47-51. Both these subjects are well worth further attention,
+     and I know of no complete account of them.
+
+
+
+
+CHAPTER XVI.
+
+THE FACTORS OF AN EPIDEMIC.
+
+     _Illustrations afforded by the potato disease--The larch
+     disease--The phylloxera of the vine._
+
+
+When we come to enquire into what circumstances bring about those severe
+and apparently sudden attacks on our crops, orchards, gardens, and
+forests by hosts of some particular parasite, bringing about all the
+dreaded features of an epidemic disease, we soon discover the existence
+of a series of complex problems of intertwined relationships between one
+organism and another, and between both and the non-living environment,
+which fully justify the caution already given against concluding that
+any cause of disease can be a single agent working alone.
+
+The statement of prophecy that a particular insect or fungus need not be
+feared, because it is found to do so little harm in particular cases or
+districts examined, will thus be seen to be a dangerous one: any pest
+may become epidemic if the conditions favour it!
+
+In 1844 and 1845 the potato disease assumed an epidemic character so
+appalling in its effects that it is no exaggeration to say that it
+constituted a national disaster in several countries. It was stated at
+the time that this disease had been known for some time in Belgium, in
+Canada and the United States, in Ireland, in the Isle of Thanet, and in
+other parts of the world. Similar, but less devastating epidemics have
+occurred in various years since. It was generally noticed during such
+epidemics that the plants themselves were full of foliage, surcharged
+with moisture, and of a luxuriant green colour promising abundant
+crops. The now well-known spots, at first pale and then brown and
+fringed with a whitish mould-like growth--the conidiophores of the
+_Phytophthora_--were observed during the dull cloudy and wet weather,
+cooler than usual, when the atmosphere was saturated for days together,
+in July and August. The actual amount of rain does not appear to have
+been excessive, but most observers seem to agree that dull weather with
+moist air had succeeded a warm forcing period of growth. So rapidly did
+the disease run its course that in a few days nearly all the plants were
+a rotting blackened mass in the fields, and the potatoes dug up
+afterwards were either already rotten or soon became so in the stores.
+Further experience has confirmed this, and we now know that the epidemic
+is very apt to appear in any region where potatoes are grown on a large
+scale, in dull moist weather, especially in fields exposed to mists,
+heavy dews, etc., about July and August, when the foliage is full and
+turgid. Similarly on heavy wet soils, unless the season is remarkably
+open and dry; but also on dry light soils in rainy seasons. So evident
+was this that many believed that the mists and dew brought the
+disease--harking back to the superstitions of earlier days. We must
+remember that prior to 1860 the life-history of _Phytophthora_ was not
+known. Since De Bary's proof of the germination of the zoospores and of
+the infection of the leaves, the course of the hyphae in them and in the
+haulms, the origin of the conidia, etc., and the confirmation by
+numerous competent observers of the true fungus nature of this disease,
+we are now in a position to understand the principal factors of the
+various epidemics of potato disease.
+
+It is not merely that the potato-fields afford plenty of food for the
+fungus, and that the dull weather causes the tissues to be surcharged
+with moisture, owing to diminished transpiration, but the mists and
+dew--to say nothing of actual rain and the flapping of wet
+leaves--favour the germination and spread of the zoospores throughout
+the field. Whether the dull light also favours the accumulation of
+sugars in the tissues, and the partial etiolation of the latter implies
+less resistance to the entering hyphae, may be passed over here, but in
+any case it is clear that we have several factors of the non-living
+environment here favouring the parasite and not improving the chances
+of the host, even if they do not directly disfavour it.
+
+As another instance I will take the Larch-disease, which is due to the
+ravages of a Peziza (_Dasyscypha Willkommii_) the hyphae of which obtain
+access by wounds to the sieve-tubes and cambium of the stem, and
+gradually kill them over a larger and larger area and so ring the tree,
+with the symptoms of canker described below.
+
+Now the Larch fungus is also to be found on trees in their Alpine home,
+but there it does very little damage and never becomes epidemic except
+in certain sheltered regions near lakes and in other damp situations.
+How then are we to explain the extensive ravages of the Larch disease
+over the whole of Europe during the latter half of this century? The
+extensive planting, providing large supplies for the fungus, does not
+suffice to explain it, because there are large areas of pure Larch in
+the Alps which do not suffer.
+
+In its mountain home the Larch loses its leaves in September and remains
+quiescent through the intensely cold winter, until May. Then come the
+short spring and rapid passage to summer, and the Larch buds open with
+remarkable celerity when they do begin--_i.e._ when the roots are
+thoroughly awakened to activity. Hence the tender period of young
+foliage is reduced to a minimum, and any agencies which can only injure
+the young leaves and shoots in the tender stage must do their work in a
+few days, or the opportunity is gone, and the tree passes forthwith
+into its summer state.
+
+In the plains, on the contrary, the Larch begins to open at varying
+dates from March to May, and during the tardy spring encounters all
+kinds of vicissitudes in the way of frosts and cold winds following on
+warm days which have started the root-action--for we must bear in mind
+that the roots are more easily awakened after our warmer winters than is
+safe for the tree.
+
+It amounts to this, therefore, that in the plains the long continued
+period of foliation allows insects, frost, winds, etc., some six weeks
+or two months in which to injure the slowly sprouting tender shoots,
+whereas in the mountain heights they have only a fortnight or so in
+which to do such damage. That the lower altitude and longer summer are
+not in themselves inimical to Larch is proved by the splendid growths
+made by the trees first planted a century ago. Then came the epidemic of
+Larch-disease: the fungus, which is merely endemic--_i.e._ obtains a
+livelihood here and there on odd trees, or groups of trees in warmer or
+damper nooks--in the Alps, was favoured by the more numerous points of
+attack afforded to its spores by injuries due to insects--_Coleophora_,
+_Chermes_, etc.--and frost wounds, as well as by the longer periods of
+moist dull weather, and the longer season of foliation. Moreover, as
+time went on almost every consignment of young Larch-trees sent abroad
+was already infected. Here again, then, we find the factors of an
+epidemic consisting in events which favour the reproduction and spread
+of a fungus more than they do the well-being of the host.
+
+As a third illustration I will take the case of an insect epidemic. In
+1863 a disease was observed on vines in the South of France which
+frightened the growers as they realised its destructive effects: the
+roots decayed and the leaves turned yellow and died before the grapes
+ripened, and such vines threw out fewer and feebler shoots the following
+year, and often none at all afterwards. In 1865 the disease was
+evidently becoming epidemic near Bordeaux, and in 1868 it was shown to
+be due to an insect, _Phylloxera_, the female of which lays its eggs on
+the roots, where they hatch. The louse-like offspring sticks its
+proboscis into the tissues as far as the central cylinder. The irritated
+pericycle and cortex then grow and form nodules of soft juicy
+root-tissue at which the insect continues to suck. Rapid reproduction
+results in the majority of the young rootlets being thus attacked, and
+since they cannot form their normal periderm and harden off properly
+they rot, and admit fungi and other evils, in consequence of which the
+vine suffers also in the parts above ground.
+
+Evidence that the general damage is due to the diminished root-action is
+found in the peculiarly dry poor wood formed in the "canes" of diseased
+plants.
+
+By 1877 the epidemic had spread to the northern limits of the French
+vineyards, and by 1888 half the vines in the country were attacked, and
+the yield of wine reduced from half a million hectolitres to 50,000
+only. Meanwhile the disease had spread to Italy, Germany, Madeira,
+Portugal, and even to the Cape, though not in epidemic form as in the
+Bordeaux centre whence it spread.
+
+Now it appears that _Phylloxera_ has long been in the habit of doing
+damage to vines in America, where, however, it attacks the leaves, on
+which it makes pocket-like galls, rather than the roots. Moreover, there
+are species and varieties of American vines which, even when planted in
+Europe, do not suffer at all from this insect at the roots, either
+because the rootlets do not push out at the same season as those of the
+European form, or because they form wood more rapidly and completely, or
+secrete resinous and other matters distasteful to the insect in greater
+quantity and are thus capable of healing the wounds, or in some other
+way they do not respond to the attack or suit the insect. In any case
+the attack on the leaf rather than the root seems to be the exception in
+European vineyards and the rule in American species, and we appear to be
+face to face with a problem of specific predisposition to this
+particular malady. That the resistant properties of the vines of
+America--not all, only particular species and varieties are thus
+"immune"--can be utilised has been proved by European growers; and not
+only so, for Millardet and others have shown that the European vine
+grafted on to these resistant stocks suffer less than when on their own
+roots. It has also been shown that hybrids can be obtained which are
+resistant.
+
+But the most curious point of all is that _Phylloxera_ was itself a
+native of America, and came thence to Europe. It had played its part
+with certain fungi in ruining all the attempts to introduce the European
+vine into America many years ago. A recent authority on the evolution of
+American fruits writes as follows:
+
+"All the most amenable types of grapes had long since perished in the
+struggle for existence, and the types which now persist are necessarily
+those which are, from their very make-up or constitution, almost immune
+from injury, or are least liable to attack . . . the _Phylloxera_ finds
+tough rations on the hard, cord-like roots of any of our eastern species
+of grapes. But an unnaturalised and unsophisticated foreigner, being
+unused to the enemy and undefended, falls a ready victim; or if the
+enemy is transported to a foreign country the same thing occurs."
+
+Further proof that it is in the "constitution" of the European vine that
+the want of resistance to _Phylloxera_ resides, is furnished by the fact
+that in California and the Pacific states the European vine was
+introduced with more success, but is now suffering badly because
+_Phylloxera_ has spread there also. It must not be overlooked, however,
+that we are as yet very ignorant of all that is implied in the word
+"constitution" as used above.
+
+If we enquire further why the _Phylloxera_ epidemic was so much worse
+in the Southern vineyards than in the more Northern ones of Germany, the
+opinion seems to prevail that the warmer climates favour the insect.
+Further, it appears that, in Italy, the vines in loose open soil,
+provided it is equally rich in mineral food-materials and offers no
+disadvantages as regards drainage, suffer less than those in closer
+soils, the reasons alleged being that the young roots can push out more
+rapidly and widely, and so obtain holdfasts with greater distances
+between them.
+
+
+NOTES TO CHAPTER XVI.
+
+     The student may obtain further information on the history of
+     the Potato disease by consulting the following: Berkeley,
+     "Observations, Botanical and Physiological, on the Potato
+     Murrain," _Journal of the Horticultural Society_, Vol. I.,
+     1846, p. 9; De Bary, _Die Gegenwärtig herrschende Kartoffel
+     Krankheit_, etc., Leipzic, 1861; and the pages of the
+     _Gardeners' Chronicle_ from 1860-1900.
+
+     For the Larch disease he should consult Hartig, _Unters. aus
+     der Foist. Botanischen Inst. München_, B. I., 1880; and
+     Willkomm, _Microscop. Feinde des Waldes_, B. II., 1868.
+
+     For _Phylloxera_ the literature is chiefly in the _Comptes
+     Rendus_ and other French publications since 1875, and in the
+     Reports of the U.S. Dept. of Agriculture.
+
+     For a summary of the facts concerning the life-histories of
+     the parasites referred to above, see Frank, _Krankheiten der
+     Pflanzen_, and Marshall Ward, _Diseases of Plants_, p. 59, and
+     _Timber and Some of its Diseases_, London, 1889, chapter X.
+
+     Also Marshall Ward, "On some Relations between Host and
+     Parasite in certain epidemic Diseases of Plants," _Proc. Roy.
+     Soc._, Vol. XLVII., 1890, pp. 393-443; and "Illustrations of
+     the Structure and Life-history of Phytophthora infestans,"
+     _Quart. Journ. Microsc. Soc._, Vol. XXVII., 1887, p. 413; also
+     Marshall Ward, "Researches on the Life-history of Hemileia
+     vastratrix," _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299; and
+     "On the Morphology of Hemileia vastatrix," _Quart. Journ.
+     Microsc. Soc._, 1881, Vol. XXI., p. 1.
+
+
+
+
+CHAPTER XVII.
+
+REMEDIAL MEASURES.
+
+     _Preventible diseases--The principles of therapeutics--Powders
+     and their application--Spraying with liquids--Nature of
+     chemicals employed--Employment of epidemics and natural
+     checks--The struggle for existence._
+
+
+It may be said that in no connection is the proverb "Prevention is
+better than cure" more applicable than with this subject, and
+undoubtedly the best utilitarian argument that can be used in favour of
+a thorough study of the causes of disease is that only by understanding
+these causes is there any hope of avoiding the exposure of crops, garden
+plants, forest trees, etc., to the attacks of preventible diseases.
+Moreover, only an intelligent appreciation of the causes of a disease
+will enable the cultivator to take steps to mitigate their effects when
+once the damage has begun its course. Every cultivator learns by
+experience or by precept that there are some things he must avoid in
+dealing with certain plants, or otherwise they will not succeed; in
+other words they will succumb to diseased conditions and die. It is
+partly owing to the want of systematisation of this knowledge, and its
+extension in other directions, that such extraordinary blunders are made
+in ignorant practice, and trees for instance are planted in low-lying
+frost beds which would succeed in slightly higher situations, or seeds
+subject to damping-off are sown in beds rife with the spores of
+_Peronospora_ or _Pythium_, and so forth.
+
+Many diseases, however, are not preventible in the present state of our
+knowledge, or prevailing conditions are such that the risk must be run
+of endemic diseases gradually becoming epidemic, and thus the natural
+desire for some means of checking the ravages of some pest or another
+has led to innumerable trials to minimise the effects by prophylactic
+measures. The procedure almost invariably followed where parasites are
+concerned, consists in either dusting the plants with some chemical in
+the form of a powder, or spraying it with a liquid, or occasionally in
+enveloping the plant in some gas, in each case poisonous to the insect-
+or fungus-pest. The principal rules to be observed are: (1) the poison
+employed must be sufficiently strong or concentrated to kill the
+parasite, but not sufficiently powerful to injure the host; (2) it must
+be applied at the right period, as suggested by a knowledge of the
+life-history of the fungus or insect in question.
+
+Obviously it is of no use to apply such topical remedies to a parasite
+while it is spending the greater part of its life inside the tissues of
+the host. Further, questions of expense of the materials employed and of
+the labour of applying them help to limit the adoption of such measures.
+
+Among the various kinds of powders employed, finely divided sulphur, or
+a mixture of sulphur and lime, have been used with success in some
+cases--_e.g._ against Hop mildew and other epiphytic Erysipheae, and
+against red spider, aphides, etc., the gaseous sulphur dioxide evolved
+being the efficacious agent. In other cases pyrethrum or tobacco powder,
+wood ashes, etc., have been employed against insects. Such powders are
+applied by hand or by means of bellows, and are very easily manipulated
+in most cases, though, like all such applications, the dangers of
+concentration at particular spots owing to uneven distribution, or of
+dilution and washing off by rain, have to be incurred.
+
+Far more numerous are the various liquids which have been employed for
+washing, spraying, or steeping the affected parts of diseased plants.
+Water alone, or aqueous decoctions or emulsions of various
+kinds--_e.g._, quassia, tobacco, soap, or aloes, have been widely
+employed against insects such as green fly, red spider, etc. In
+greenhouses, where the leaves can be washed by hand or thoroughly
+syringed, and the concentration and time of action thoroughly
+controlled, such liquids are often serviceable, but great practical
+difficulties are apt to interfere with their use in the open field.
+
+The principal liquids employed against fungi have been copper sulphate
+and other metallic compounds (Bordeaux mixture, Eau Céleste, etc.),
+various compounds of arsenic (_e.g._ "Paris green"), potassium sulphite,
+permanganate, etc., and emulsions of carbolic acid, petroleum, and such
+like antiseptics, for the exact composition of which the special
+treatises must be consulted. Some of these, especially Bordeaux mixture,
+have been experimented with on a very large scale, especially in
+America, and various forms of spraying machines have been introduced for
+dealing with large areas.
+
+It is clear that these spraying operations are more particularly adapted
+to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and
+greenhouse plants than to woods and plantations; as a rule they cannot
+be applied to forest trees--though they have been used in orchards--or
+to roots, seeds, and other parts in the soil, and many special forms of
+treatment have been devised for particular cases of these kinds.
+
+One of the oldest of these is the steeping of grain in solutions of
+copper, or in hot water, just before sowing, and the practical
+eradication of Bunt and, partially, of Smut is due to this practice,
+which has lately been adapted to potatoes, the principle being that the
+parasitic germs shall be killed while still adhering to the outside of
+the seeds, tubers, etc., before germination. "Finger and Toe" due to
+_Plasmodiophora_ has been successfully dealt with by the application of
+lime, but we do not know whether the effect is owing to indirect actions
+in the soil, to direct actions on the plasmodia, or to the increased
+production of root-hairs induced by liming.
+
+_Phylloxera_ has been treated by plunging into the soil near the roots
+small blocks of some slowly-soluble medium, such as gelatine,
+impregnated with carbon-bisulphide, the volatile fumes of which kill the
+insect, and even more drastic remedies have been tried along similar
+lines. In America orchard trees infested with insects or fungi have been
+covered one by one with light tents, and the vapours of prussic acid,
+burning sulphur, and other poisons allowed to act inside the tent. In
+all such cases it must be remembered that uncontrolled ignorance of the
+properties of poisons on the part of the operator may lead to disaster,
+and the same applies to the much easier treatment of greenhouses, and
+cases where poisoned food is laid about for insects or vermin.
+
+Attempts, not altogether unsuccessful on the small scale, have also been
+made to introduce epidemic diseases among rats, mice, and locusts and
+other insects, by inoculating some of them with parasitic bacteria or
+fungi (_Empusa_, _Isaria_, etc.), and then allowing them to run loose in
+the hope that they will communicate the disease to their fellows. The
+introduction of lady-birds into districts infested with Coccideae and
+similar pests which they devour, is also recorded as successful, as also
+the importation of birds into forests plagued with caterpillars. It must
+not be over-looked, however, that man's interference with the existing
+balance of events in the natural struggle for existence is occasionally
+disastrous, as witness the results of importing rabbits into Australia,
+goats into the Canary Islands, and sparrows in various countries.
+Darwin's well-known illustration of the inter-relations between clover,
+bees, field-mice, and cats (_Orig. of Species_, 6th ed., 1876, p. 57),
+which shows the astounding probability of the dependence of such a plant
+on the number of cats in the neighbourhood, well illustrates the
+situation.
+
+Mere mention must be made of other special treatments.
+
+Caterpillars and larger animals are often picked by hand or their
+natural enemies--_e.g._ birds, are encouraged in forests. Locusts are
+caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are
+often trapped by laying attractive food such as carrots and overhauling
+the traps daily: similarly with earwigs. Rings of tar round tree stems
+have been employed to prevent caterpillars creeping up them.
+
+American Blight has been treated by rapidly flaming the stems. Syringing
+with hot water has also been employed for vines affected with mildew,
+mealy bug, etc.
+
+With regard to the alleged immunity from devouring insects of certain
+poisonous plants, it has been pointed out that _Pangium edule_, which
+abounds in prussic acid, is infested with a grub, and ivy is
+occasionally eaten by caterpillars.
+
+Another point as regards insect pests is the well-known destructive
+effect of a cold, wet spring on the young larvae. The use of cyanide of
+potassium requires especial care, but has been described as easily
+carried out with success in greenhouses.
+
+It seems probable that lady-birds, the larvae of wasp-flies and
+lace-wings, and ichneumon-flies as well as wrens can keep down aphides.
+
+For an example of the treatment of a complex case of "chlorosis" with
+mineral manures, the reader may consult the _Gardeners' Chronicle_, 1899
+(July), p. 405. Many similar cases have been recorded, but it should not
+be overlooked that very complex inter-relations are here involved.
+
+Charlock has been successfully dealt with by applying 5 lbs. of copper
+sulphate in 25 gallons of water to each acre of land while the weeds are
+young.
+
+In all these cases the guiding idea is derived from accurate knowledge
+of the habits of the insect, fungus, or pest concerned, and obviously
+the procedure must be timed accordingly. It is a particular case of the
+struggle for existence, where man steps in as a third and (so to speak)
+unexpected living agent.
+
+It is clear from our study of the factors of an epidemic that one of the
+primary conditions which favour the spread of any disease is provided
+by growing any crop continuously in "pure culture" over large areas.
+This is sufficiently exemplified by the disastrous spread of such
+diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera,
+Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other
+maladies which have now become historic in agricultural, planting, and
+forest annals. Providing the favourite food-supply in large quantities
+is not the only factor of an epidemic, but it is a most important one in
+that it not only facilitates the growth and reproduction of a pest, but
+affords it every opportunity of spreading rapidly and widely.
+
+Moreover, Nature herself shows us that such pests are kept in check in
+her domain by the struggle for existence entailed by innumerable
+barriers and competitors. As matter of experience also it is found that
+rotation of crops, planting forests of mixed species, and breaking up
+large areas of cultivation into plantations, fields, etc., of different
+species afford natural and often efficient checks to the ravages of
+fungus and insect pests. Over and over again it has been found that a
+fungus or an insect which is merely endemic so long as it is isolated in
+the forest, where its host is separated from other plants of the same
+species by other plants which it cannot attack, becomes epidemic when
+let loose on the continuous acres so beloved of the planter. And the
+same reasoning applies to the success of such pests on open areas from
+which the birds or other enemies of the pest have been driven. True, we
+cannot always trace the tangled skein of inter-relationships between one
+organism and another in Nature: the recognition of the principle of
+natural selection and the struggle for existence is too recent, and our
+studies of natural history as yet too imperfect to lay all the factors
+clear, but no observant and thoughtful man can avoid the truth of the
+general principle here laid down. The history of all great planting
+enterprises teaches us that he who undertakes to cultivate any plant
+continuously in open culture over large areas must run the risk of
+epidemics.
+
+
+NOTES TO CHAPTER XVII.
+
+     The principal literature, now very voluminous, on this subject
+     is contained in the publications of the U.S. Department of
+     Agriculture from 1890 onwards. See especially _Bulletins_,
+     Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The
+     Journal of Mycology_ during the same period. See also Lodeman,
+     _The Spraying of Plants_, London, 1896. A summary of the
+     principal processes will be found in Massee, _Text-Book of
+     Plant Diseases_, pp. 31-47.
+
+     With regard to the history of the subject, which still needs
+     writing, the reader should not overlook Roberts, "On the
+     Therapeutical Action of Sulphur," _St. George's Hospital
+     Reports_, date unknown, but subsequent to the following:
+     Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277,
+     with references. These are, I believe, with the references to
+     steeping of wheat in De Bary, _Unters. über d. Brandpilze_,
+     Berlin, 1853, among the first attempts to utilise such
+     remedies.
+
+     Further facts will be found in the pages of the _Gardeners'
+     Chronicle_, especially since 1890, and in _Zeitsch. f.
+     Pflanzen-krankheiten_ since 1891.
+
+
+
+
+CHAPTER XVIII.
+
+VARIATION AND DISEASE.
+
+     _Predisposition and immunity--Pathological conditions
+     vary--Hardy varieties--"Disease-proof" varieties--Disease
+     dodging--Thick skins--Indian wheats, etc. Cell-contents
+     vary--Citrus, Cinchona, Almonds, etc. Double ideals in
+     selection--Cultivation of pest and host-plant--Variations of
+     fungi--Bacteria--Specialised races--Difficulties--Experiment
+     only will solve the problems._
+
+
+The numerous and often expensive failures in the application of any
+prophylactic treatment, have proved an acute stimulus to the research
+for other ways of combating the ravages of plant diseases. It is a
+matter of every-day experience that particular varieties of cultivated
+plants may suffer less from a given disease than others in the same
+district; also that one and the same species may suffer badly in one
+country and not in another--_e.g._ the Larch in the lowlands of Europe
+as contrasted with the same tree in its Alpine home, and the various
+species of American Vines in Europe.
+
+These matters, in the hands of astute observers, are turning the
+attention of cultivators and experts to new aspects of the question of
+plant diseases, namely, the possible existence of immunity, and the
+breeding of disease-proof varieties; and the existence on the part of
+the host plant of predispositions to disease which may depend on some
+factors in the plant or in the environment over which it is possible to
+exercise control, or which, if known, can be avoided.
+
+The matter is complicated by the recent demonstration of the fact that
+parasites also vary and can adapt themselves to altered conditions, as
+is shown by the history of the coffee-leaf disease (_Hemileia_) in
+Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and
+various experiments with _Coleosporium_ and other Uredineae; but there
+are good grounds for concluding that hybridisation, grafting, and
+selection of varieties may do much towards the establishment of races
+which will resist particular diseases, as shown by Millardet's
+experiments with Vines, and the results obtained by Cobb and others with
+Wheat.
+
+The great difficulty with so-called "disease-proof varieties" is to test
+them under similar conditions in different countries, and for a
+sufficient period of time. A particular race of Wheat may behave very
+differently in Norfolk, Devonshire, and Northumberland, and the recent
+introduction of the purely experimental method in this connection is a
+marked advance. However rough the experiments may of necessity have to
+be, it is only by such means that data can be gradually accumulated.
+
+Having now obtained some insight into the factors concerned in disease,
+let us enquire further into the bearing of variation on these. It is
+evident that pathological conditions may vary; indeed they are
+themselves symptoms of variation, as we have seen. The history of all
+our cultivated plants shows abundantly that many of the variations
+obtained by breeding in our gardens, orchards, fields, etc., involve
+differences of response on the part of the plant to the very agencies
+which induce disease. Every year the florists' catalogues offer new
+"hardy" varieties; but a hardy variety is simply, for our present
+purpose, one which succumbs less readily to frost, cutting winds, cold
+damp weather, and so forth. If anyone doubts that hardy varieties have
+been gradually bred by selection, I refer him to the evidence collected
+by De Candolle, Darwin, Wallace, Bailey and others. When we come to
+enquire into the causes of "hardiness," however, difficulties at once
+beset us. The adaptation may express itself in a difference in the time
+of flowering or leafing, the exigencies of the season being "dodged," as
+it were, in a manner which was impossible with the original stock, as
+appears to have occurred with Peaches in America; or it may be expressed
+in deeper rooting, as is said to be the case in some Apples, or in the
+acquirement of a more deciduous habit, or in actually increased
+resistance to low temperatures. In such cases we cannot trace what
+alterations have occurred in the cells and tissues concerned, though we
+may be sure that some changes do occur.
+
+No experienced cultivator doubts that some varieties of Potato, Wheat,
+Vine, Chrysanthemum, etc., suffer more from epidemic diseases than
+others, and our yearly catalogues furnish us with plenty of promises of
+"disease-proof" varieties. Here also we may imagine several ways in
+which a particular variety may resist or escape the epidemic attacks of
+fungi which in the same neighbourhood decimate other varieties. If we
+could breed a variety of the Larch which opened its buds later than the
+ordinary form in our northern plains, the probability of its escaping
+the Larch-disease would be increased in proportion to the shortness of
+the period of tender foliation described on p. 153. It has been claimed
+for certain varieties of Wheat that increased thickness of the cuticle
+and fewer stomata per square unit of surface have diminished the risk of
+infection by Rust fungi, and for certain varieties of Potato, that the
+thicker periderm of the tuber protects them against fungi in the soil.
+That certain thick-skinned Apples, Tomatoes, and Plums pack and store
+better than those with a more tender epidermis seems proved--that is to
+say, they suffer less from fungi which gain access through bruises and
+other wounds; but it cannot be said that any convincing proof is yet to
+hand explaining in detail why some races of wheat resist Rust, or why
+the roots of American Vines suffer less from _Phylloxera_ than others.
+
+One of the most extraordinary cases known to me in this connection is
+the unconscious selection on the part of native Indian cultivators,
+perfectly ignorant of the principles involved, of spring and autumn
+forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.
+"It has been estimated that Bengal alone possesses as many as 10,000
+recognisable forms of rice." Now there is not the slightest ground for
+doubt that these have been unconsciously bred from the semi-aquatic
+native species during the many centuries of Indian agriculture, and
+nevertheless they have, among other peculiar races, some hill-breeds
+which they cultivate on dry soils and without direct inundation. That is
+to say, they possess tropical and temperate races differing far more
+than our spring and summer wheats.
+
+Something has been gained, then, if we can show that there is nothing
+absurd or hopeless in the search for disease-proof or resistant races,
+and I think this can be done. We must not forget that the ideal usually
+set before himself by a breeder of plants has hitherto been almost
+exclusively some standard of size, form, colouring, and so forth, of the
+flower, or of taste and texture of the fruit, tuber, etc., though
+experiments with _Cinchona_, with brewery yeasts, and other plants
+remind us that variations in other directions have been attended to
+also.
+
+Now it is obvious that in breeding sour limes and sweet oranges the
+cultivator is selecting, and intensifying by selection, very different
+metabolic processes in the cell: he can test the results of these, and
+so the selection proceeds.
+
+The question is, Could he select at the same time those variations in
+cell activity which express themselves in properties of the flower,
+fruit, foliage, etc., he desires, as well as such variations as aid the
+cells in repelling fungi, insects, or exigencies of the non-living
+environment?
+
+That more or less disease-proof varieties could be selected if that
+object alone were kept in view can hardly be doubted; plenty of examples
+exist already which show that the necessary variations to work upon
+exist in just those secretions of protoplasm, etc., which we have seen
+are concerned in repelling or attracting parasites.
+
+The Sweet Almond has lost the power of producing amygdalin and prussic
+acid in its cells; Cinchona plants vary immensely in the quantity of
+quinine formed, and in European hot-houses may even form none at all;
+some varieties of Maize have sugar and dextrine instead of starch in
+their endosperms, or coloured instead of clear sap in the aleurone
+layer, and recent researches prove that they can transmit these
+peculiarities to hybrid offspring; non-poisonous bacteria have
+frequently been got from poisonous species simply by cultivation under
+special conditions, and pigmented forms can be bred into non-pigmented
+races.
+
+But we see that the difficulty of selection is increased in the case
+postulated above, because two ideals are to be worked up to, and they
+may conceivably be incompatible. Not necessarily so, however, for
+breeders have solved such problems before in obtaining early _and_ heavy
+cropping races of potatoes, wheat, etc., sweet _and_ large grapes,
+strawberries, etc., hardy _and_ brilliant flowers, and so forth.
+
+There is, however, another aspect of this question of variability in
+organisms in this connection to be considered. Ever since cultivation
+began man has probably been cultivating not only the crops he desires,
+but also the pests which infest them, and if variation of his chosen
+plants occurs--and no one will deny that--surely variation of the fungi
+and insects which live on them also takes place. That this is so can be
+demonstrated, though, since it is not part of my theme to go into the
+question of peculiarities of species and races of parasites, the subject
+must here be passed over with a few remarks only.
+
+Recent researches have shown not only that fungi vary immensely in form
+and morphological characters according to the amount and kind of
+food-materials put at their disposal, thus bringing the whole question
+of polymorphism into the domain of experimental physiology, but that
+their capacities for infection, spore formation, etc., are also capable
+of variation and are dependent on the quality and quantity of food
+supplies, water, as well as on the temperature, illumination, and other
+factors of the environment. This is true of parasites as well as of
+saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.
+Klebahn found that a _Puccinia_ growing on _Digraphis_ infected
+_Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas
+if sown on _Majanthemum_ it only just infected the plant and then
+remained sterile, while it refused to infect _Paris_ at all. Magnus has
+shown that _Peronospora parasitica_ can only infect meristematic
+tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is
+usually the case, it enters the latter plant by infecting the gall-like
+pustules of hypertrophied tissue induced by that parasite. Numerous
+parasitic fungi can only penetrate particular parts of plants. For
+instance, the _Ustilago_ of wheat can only infect the young seedling,
+and grows for weeks as a barren mycelium, only becoming a dominant
+fungus in the endosperm. Numerous other examples could be given, but
+these suffice to show some of the ways in which the nature of the food
+substratum supplied by the host affects the fungus. It is obvious that
+if the nature of this food changes, the fungus is also affected, and no
+doubt this is the principal reason why Rust-fungi, for instance, vary so
+much in their vigour and reproductive power on different wheats and
+grasses, though the other factors of the environment must also be of
+influence on them as well as on the hosts.
+
+But--and this is the second point--modern research is also showing that
+the various species of Rust-fungi have split up into different varieties
+or specialised races, according to the particular host plants they
+inhabit. For instance there are special varieties or races of the
+particular species known as _Puccinia graminis_, the wheat rust, each of
+which grows well on various kinds of grain and grasses but refuses to
+infect others. Thus, the variety which infects Wheat refuses to infect
+Barley or Oats, while that variety which grows on Rye will not take on
+Wheat and so forth. Now it is important to notice that these specialised
+races are indistinguishable one from another by their visible
+microscopic characters: they are all botanically of the species
+_Puccinia graminis_ which forms its æcida on the Barberry. We must
+therefore conclude that we have here the same phenomenon as that met
+with in culture-races of bacteria which, having been fed for several
+generations on media rich in proteids, refuse to grow on media rich in
+carbohydrates, or when attenuated races are developed by culture under
+special conditions.
+
+Now since such physiological races as I have described are by no means
+confined to _Puccinia_ but are also known in _Melampsora_,
+_Gymnosporangium_ and other fungi, we must conclude from this and from
+what we know of variation in plants and animals generally, that
+variation and adaptation are common among parasites, insects as well as
+fungi.
+
+These considerations will serve to show moreover that the question of
+breeding disease-proof varieties of our cultivated plants is complicated
+by the danger of our breeding at the same time adapted races of their
+pests. It appears at first sight extremely improbable that we should
+escape the danger by breeding from those specimens of our plants which
+have best survived a fungus epidemic. Still, it must not be forgotten
+that "hardy varieties," and races adapted to other exigencies of the
+non-living environment, have been bred by selection--and nevertheless
+this variable non-living environment is always with us. The matter is
+therefore simply and solely one of experiment, and the retort that a
+disease-resisting variety of any particular plant has not yet been
+raised is no more valid than the objection that a true blue primrose has
+not yet been obtained: whether the same remark can be made with regard
+to any hope of a _disease-proof_ plant may be another matter, but in any
+case it must be made more cautiously in the light of our present
+experience.
+
+
+NOTES TO CHAPTER XVIII.
+
+     The reader will find more on this subject in Bailey's
+     _Survival of the Unlike_ and the literature quoted in the
+     notes to Chapter VIII.
+
+     For varieties of Indian Wheats, etc., see Watt, _Agricultural
+     Ledger_, Calcutta, 1895.
+
+     For a discussion on so-called "Disease-proof Wheats" consult
+     Eriksson & Henning, _Die Getreideroste_.
+
+     Magnus' paper is in the _Berichte der Deutschen bot.
+     Gesellsch._, 1894, p. 39.
+
+     Concerning physiological races and adapted varieties of
+     _Puccinia_, etc., see Eriksson, "A General View of the
+     Principal Results of Swedish Research into Grain Rust,"
+     _Botanical Gazette_, vol. 25, 1898, p. 26.
+
+     For an account of Wheat-rust see Marshall Ward, "Illustrations
+     of the Structure and Life-history of _Puccinia graminis_,
+     etc.," _Ann. of Bot._, 1888, Vol. II., p. 215.
+
+
+
+
+CHAPTER XIX.
+
+SYMPTOMS OF DISEASE.
+
+     _Discolorations--Pallor--Etiolation--Laying of Wheat--
+     Chlorosis--Yellowing--Albinism--Variegation--Uprooting,
+     Exposure and Wilting of seedlings._
+
+
+Everybody knows in a general way when the geraniums in the window pots
+are drooping from want of water, or when the young Wheat is sickly, or
+the Pear-trees "blighted," and we have now to see how far we can
+systematise the knowledge that has been gained in course of time
+regarding the signs which sick plants exhibit.
+
+_Pallor._--Under this heading, which includes all cases where the normal
+healthy green colour is replaced by a general sickly yellow or pale hue,
+ultimately resulting in death of the parts if not arrested, we have
+several totally distinct diseases of the chlorophyll apparatus, each
+recognised by the co-existence of other subordinate symptoms. The
+principal varieties of pallor usually met with are the following:
+
+_Etiolation_ is due to insufficient intensity of light, the pale sickly
+yellow organs being unusually watery and deficient in vascular tissue,
+the internodes abnormally long and thin, and the leaves generally
+reduced in size, or, in some plants also "drawn."
+
+Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of
+etiolation purposely induced. The want of light causes the true
+chlorophyll colouring matter to remain in abeyance, and consequently the
+plant as a whole suffers from carbohydrate starvation.
+
+_Laying_ of Wheat and other cereals is a particular case of etiolation.
+The seeds having been sown too thickly, the bases of the haulms, owing
+to the etiolation and consequent lack of carbohydrates, suffer from want
+of stiffening tissues, and the top-heavy plants fall over.
+
+_False etiolation_ depends on a similar abeyance of the chlorophyll, but
+in this case due to too low a temperature. It is often seen in Wheat and
+other monocotyledons when the young leaves unfold in cold weather in
+spring. The symptoms of "drawing" and tenderness are however absent.
+
+Pallor due to too intense illumination must be kept sharply distinct
+from etiolation, the pale green or yellow hue being here due to the
+destruction of the chlorophyll by insolation, and the accessory symptoms
+of "drawing" are wanting.
+
+_Chlorosis_ is a form of pallor where the chlorophyll grains themselves
+are fully developed, but their green pigment remains in abeyance owing
+to a deficiency of iron in the soil, and can often be cured by adding
+traces of a ferrous salt. The distinction between _Icterus_, where the
+organs are only yellow, and _Chlorosis_ proper, where they are nearly
+white cannot always be maintained. In the typical case only those organs
+whose cells are still young can become green on adding iron.
+
+_Yellowing_ or _False Chlorosis_ may be experimentally induced by too
+much carbon-dioxide in the atmosphere. It also often ensues when the
+roots of plants in the open are waterlogged, owing to the stagnant water
+not only driving air from the root-hairs but accumulating dissolved
+substances which poison the plant. Trees frequently thus suffer from
+"wet feet" when their roots have penetrated down to a sodden impervious
+subsoil.
+
+_Yellowing_ accompanied by _Wilting_ is a predominant symptom in most
+cases where transpiration is more active than root-absorption beyond a
+certain limit, as is well known in cases of prolonged drought. It may
+also be caused in evergreens by the foliage transpiring actively in
+bright January weather, for instance, while the ground is frozen and the
+chilled root-hairs cannot absorb.
+
+In other cases similar appearances are traceable to insects devouring
+the roots, _e.g._ wireworms, and the malady is sometimes enhanced by
+their accumulations so fouling the wet soil that the roots die off,
+owing to want of oxygen and to the excess of carbon-dioxide and
+poisonous matters.
+
+Yellowing may also result from the presence of poisonous or acid gases
+in the atmosphere or soil, such as chlorine, hydrochloric acid,
+sulphurous acid, etc., in the neighbourhood of chemical works, or from
+the escape of coal-gas in streets, etc., points of importance in
+connection with the use of fungicides and insecticides.
+
+Yellowness is the prevailing symptom in many cases of fungus attack of
+the roots or collar of the plant, the resulting stoppage of
+transpiration being also sometimes supplemented by rotting of the roots,
+and the consequent deprival of oxygen and accumulation of foul gases. In
+other cases Fungi, and even Bacteria, have been found to have made their
+way into the principal vessels, the lumina of which they stop up, thus
+reducing the transpiration current.
+
+Certain insects may also induce a general yellowing and wilting of
+plants by entering or destroying the tissues concerned in the
+transpiration--_e.g._ _Oniscus_, the Frit Fly, and _Cecidomya_, the
+Hessian Fly, which attack young winter wheat within the sheaths and
+cause the plants to turn yellow and wilt.
+
+_Albinism_ and _Variegation_ are apparently due to causes totally
+different from any yet mentioned. Church's analyses have shown that
+albino leaves contain more water and less organic matter than green ones
+of the same plants, but not necessarily less ash constituents. The
+composition of the ash points to there being more potash and less lime
+in the white organs than in the green ones, and, speaking generally, the
+former are related to the latter much as young leaves are related to
+mature ones.
+
+The whole matter is complicated by the behaviour of certain _variegated_
+plants--_e.g._ Ribbon grass, _Calla_, _Abutilon_, which are usually
+regarded as partial albinos.
+
+Meyen showed long ago that such variegated plants, if grafted on green
+ones, may induce the development of variegated leaves on both scion and
+stock, and Morren and others have not only confirmed this but have also
+shown that variegation may be inherited through the seed. Nevertheless
+some care has to be taken with many of these variegations lest rich
+soil, bright light, and other favourable treatment favour the
+restitution of the green colour. These facts may be interpreted in
+various ways. Some disturbance of physiological functions of the roots,
+due to unfavourable conditions of soil, may be the cause; but Beijerinck
+has lately published some results which show that some of these albino
+diseases can be induced by inoculating normal plants with the juice of
+spotted ones even though such juice has been filtered through porcelain,
+and concludes that a "_contagium fluidum vivum_" of the nature of a
+transmissible enzyme is the agent which disturbs the physiology of the
+infected cells.
+
+Koning, while confirming these results in the main, refers them to a
+micro-organism so small that it traverses the porcelain filter.
+
+_Upheaval of seedlings._--This is a common form of injury, resulting in
+death by drought and exposure, especially in seedling pines, wheat,
+etc., in soils exposed to alternate freezing and thawing during spring
+when there is no snow to protect the plants. The soil freezes during the
+night, and during the thaw next day water accumulates just below the
+surface. The freezing is then repeated, and, partly owing to the
+expansion of the forming ice and partly to the mechanical effect of the
+ice-crystals in the interstices, the surface of the soil is lifted and
+draws the roots with it. During the succeeding thaw the soil particles
+fall away from the lifted root-fibres, and frequent repetition of these
+processes results in such complete exposure of the roots to the full sun
+that the plantlet falls over and wilts.
+
+_Exposure of roots_ is also sometimes effected by winds displacing sandy
+soils liable to shifting in dry weather, and the resulting wilting of
+the plants thus exposed at their roots may be supplemented by damage due
+to the repeated impact of the wind-driven sharp grains of sand, which
+act like a sand-blast and erode the tissues.
+
+In many of the cases given above the principal result is the weakening
+or destruction of the chlorophyll action. This means a loss of
+carbohydrates--sugars, starches, etc.--and in so far a starvation of the
+plant. The injurious effects are quantitative and cumulative: if large
+areas of foliage are concerned, or if the effect lasts a long time, the
+plant suffers from loss of food, and may die. In those cases where the
+effect is due to the cutting off of supplies at the roots, and where the
+yellowing is a secondary symptom, the disease is more general in
+character, and recovery is often impossible, because the loss of water
+cannot be compensated, and the results may be further complicated by the
+gradual penetration of poisonous matter into the cells. It is frequently
+necessary, though sometimes very difficult, to decide which is the
+primary and which secondary (or tertiary, etc.) symptoms in the order of
+their importance, and the diagnosis may be complicated by a number of
+accessory factors which it is impossible to treat generally.
+
+
+NOTES TO CHAPTER XIX.
+
+     The principal cases here described are dealt with in works on
+     plant physiology, and in the works of Sorauer and Frank
+     already referred to.
+
+     As regards damage due to uprooting of seedlings by frost, see
+     Fisher, "Forest Protection" (Engl. ed. of Hess' _Forstchutz_),
+     in Schlich's _Manual of Forestry_, Vol. IV., 1895, pp.
+     439-442.
+
+     On Albinism, see Church, "A Chemical Study of Vegetable
+     Albinism," _Journ. Chem. Soc._, 1879, 1880, 1886.
+
+     Beijerinck's results are contained in his paper, "Ueber ein
+     Contagium vivum fluidum," etc. (with English abstract), in
+     _Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam_, 1898.
+     Koning's paper is in _Zeitschr. f. Pflanzenkrank._, Vol. IX.,
+     1899, p. 65. See also _Nature_, Oct. 11, 1900, p. 576.
+
+
+
+
+CHAPTER XX.
+
+SYMPTOMS OF DISEASE (_Continued_).
+
+     _Spotted leaves--The colours of spots--White, yellow, brown,
+     and black spots on leaves--Parti-coloured spots--The browning,
+     etc., of leaves._
+
+
+_Discoloured spots_ or patches on the herbaceous parts of plants,
+especially leaves, furnish the prominent symptoms in a large class of
+diseases, due to many different causes, and although we cannot maintain
+this group of symptoms sharply apart from the last, as seen from the
+considerations on _albinism_, it is often well marked and of great
+diagnostic value. By far the greater number of spot-diseases are due to
+fungi, but this is by no means always the case. The most generally
+useful method of subdividing the classes, though artificial like all
+such classifications, will be according to the colour of the spots or
+flecks, which, moreover, are usually found on the leaves. It is
+necessary to note, however, that various conditions may modify the
+colour of spots on leaves. Many fungi, for instance, induce different
+coloured spots according to the age of the leaf or other organ attacked,
+or according to the species of host, the weather, etc. Moreover the
+spots due to these parasites are frequently yellow when young and some
+other colour, especially brown or black, when older.
+
+_Scale_ is the name given to the characteristic shield-like insects
+(_Mytilaspis_, _Aspidiotus_, etc.) which attach themselves to branches
+of Apples, Pears, Oranges, Camellias, and numerous other plants, and
+suck the juices. It is the female insect which has the body broadened
+out into the "scale," under which the young are brought up. Enormous
+damage has been done by some forms--_e.g._ the San José scale in the
+United States.
+
+The superficial resemblances of the patches of eggs of some Lepidoptera
+to Aecidia and other fungi may be noted in passing--_e.g._ _Bombyx
+neustria_ on Apple twigs, _Aporia Crataegi_.
+
+_White_ or _greyish spots_ are the common symptom marking the presence
+of many Peronosporeae and Erysipheae in or on leaves, _e.g._
+_Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and
+_Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma
+Ranunculi_, etc.
+
+White spots are also caused by insects such as _Tetranychus_ (red
+spider) on Clover and other plants.
+
+_Yellow_, or _Orange-coloured Spots_. In cases where these occur on
+leaves, and in the case of grasses, etc., on the leaf sheaths as well,
+they commonly indicate the presence of Uredineae, and sections under
+the microscope will show the mycelium in the tissues beneath. Species of
+_Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery
+with spores; _Aecidia_ show the characteristic "cluster cups," and so
+forth. These spots are often slightly pustular, and in some cases
+markedly so.
+
+Other fungi also induce yellow spots on leaves--_e.g._ _Phyllosticta_ on
+Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves,
+_Synchytrium Succisae_ on _Centaurea_, etc.
+
+Yellow spots are also a frequent symptom of the presence of Aphides, of
+Red Spider, etc. Thus the minute golden yellow spots sometimes crowded
+on Oak leaves are due to _Phylloxera_ punctures.
+
+Yellow patches are formed on the large leaves of _Arisarum_ by a species
+of parasitic Alga, _Phyllosiphon_, which lives in the mesophyll. Many
+tropical leaves are spotted yellow by epiphytic Algae--_e.g._
+_Cephaleuros_.
+
+It must be noticed that many fungi produce yellow spots or flecks in the
+earlier stages, which turn brown or black as the fructifications appear,
+_e.g._ _Dilophia graminis_, _Rhytisma acerinum_.
+
+The yellow-spotted leaves of _Farfugium grande_ (_Senecio Kaempferi_)
+are so like those of _Petasites_ attacked with _Aecidium_ in its early
+stages, that an expert might be deceived until the microscopic analysis
+was completed.
+
+_Red spots_, varying from rusty or foxy red to bright crimson, are the
+symptomatic accompaniment of several fungi, the former often
+characterising the teleutospore or aecidium stage of Uredineae--_e.g._
+_Aecidium Grossulariae_--the latter sometimes indicating the presence of
+Chytridiaceae.
+
+Red spots are also caused by _Gloeosporium Fragariae_ on Strawberry
+leaves, _Polystigma rubrum_ on Plums.
+
+Crimson spots on Apple and Pear leaves are also due to _Phytoptus_: they
+turn brown later.
+
+_Brown spots_ or flecks, varying in hue from dull slaty brown to deep
+red browns, are a common symptom of Fungus and Insect diseases, the
+colour often indicating the death of the tissues, rather than any
+special peculiarity of the action of the parasite. Good examples are
+furnished by the Potato-disease, and by _Peronospora viticola_,
+_Sphaerella vitis_ and other disease-fungi of the Grape Vine. The
+teleutospore stage of many Uredineae also occurs in deep brown spots.
+
+Black spots and flecks are exceedingly common symptoms of the presence
+of fungi, _e.g._ _Fusicladium_ on Apples and Pears, and the pycnidial
+and ascus stages of many Ascomycetes--_e.g._ _Phyllachora graminis_. The
+teleutospore stages of species of _Puccinia_, _Phragmidium_, etc., are
+also so deep in colour as to appear almost black.
+
+_Scab_ on Pears is due to the presence of _Fusicladium_, which indurates
+the outer skin of the fruit causing it to crack under pressure from
+within, and to dry up, the deep brown to black patches of fungus
+persisting on the dead surface.
+
+Black spots on grasses and sedges are caused by Ustilagineae, and are
+commonest in the grain, the soot-like powdery spores (Smut) being very
+characteristic. _Ustilago longissima_ induces black streaks on the
+leaves. Many of these fungi cause distortions or pustules on leaves and
+other organs.
+
+Brown and black leaf spots are frequently furnished with concentric
+contours arranged round a paler or other coloured central point--_e.g._
+_Cercospora_ on Beans, _Ascochyta_ on Peas.
+
+Brown spots with bright red margins are formed in young Beans by
+_Gloeosporium_.
+
+Species of _Fumago_, _Herpotrichia_, etc., may cover the entire surface
+of the leaf with sooty patches, or even weave the leaves together as if
+with black spider-webs.
+
+_Mal nero_ of the Vine is a particular case of black spotting and
+streaking of the leaves for which no satisfactory explanation is as yet
+to hand. As with Chestnuts, Walnuts, and other plants containing much
+tannin, the dark spots appear to be due to this substance, but whether
+the predisposing cause is a lack of some ingredients in the soil, or
+some temperature reaction, or fungi at the roots, is as yet unknown. The
+most recent explanation puts the disease down to the action of bacteria,
+but the results obtained by different workers lead to uncertainty.
+
+The "dying back" of leaves, especially of grasses, from the tip, is
+usually accompanied by a succession of colours--yellow, red, brown, to
+black--and is a common symptom of parching from summer drought; and
+spots of similar colours, frequently commencing at the margins of
+leaves, are characteristic symptoms of the injurious action of acid
+gases in the air.
+
+Brown and blackish spots on Pears are caused by a species of _Thrips_.
+
+In many cases the minute spots of Rust-fungi on one and the same leaf
+are bright orange yellow (_uredo_), deep brown, or almost purple-black
+(_teleutospores_), foxy-red brown (older uredospores), or dead slaty
+black where the old teleutospores have died off--_e.g._ _Uromyces Fabae_
+on Beans, _U. Pisi_ on Peas, etc.
+
+_Parti-coloured leaves._--The leaves sometimes start shrivelling with
+red edges, while yellow, red, and finally brown and black blotches
+appear on the lamina, from no known cause--_e.g._ Vines. In other cases
+similar mimicry of the autumnal colouring of leaves results from the
+action of acid gases.
+
+_Burning_ is a common name for all cases where the leaves turn red or
+red-brown in hot, dry weather, and many varieties are distinguished in
+different countries and on different plants, because species react
+dissimilarly. The primary cause is usually want of water--drought.
+
+_Foxy leaves_ are a common sign of drought on hot soils, and the disease
+may usually be recognised by the gradual extension of the drying and
+fox-red colour proceeding from the older to the younger leaves, and from
+base to apex--_e.g._ Hops.
+
+_Coppery leaves._--The leaves of the Hop, etc., may show yellow spots
+and gradually turn red-brown--copper-coloured--as they dry; the damage
+is due to _Tetranychus_, the so-called Red Spider. These cases must of
+course be carefully distinguished from the normal copper-brown of
+certain varieties of Beech, Beet, _Coleus_, etc.
+
+_Silver-leaf._--The leaves of Plum, Apple, and other fruit trees often
+obtain a peculiar silvery appearance in hot summers, the cause of which
+is unknown.
+
+Discolorations in the form of confluent yellow and orange patches, etc.,
+resembling variegations, are not infrequently due to the ravages of Red
+Spider and mites--_e.g._ on Kidney Beans.
+
+_Sun-spots._--Yellow spots, which may turn brown or black according to
+the species of plant affected and the intensity of the action, are often
+caused by the focussing of the solar rays by lens-like thickenings due
+to inequalities in the glass of greenhouses, or by drops of water on
+them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is
+that of a burning glass, and extends throughout the leaf-tissues. Young
+grapes, etc., may also be injured in this way. Water-drops on the glass
+can only act long enough to produce such injuries if the atmosphere is
+saturated. The old idea that a drop on a leaf can thus focus the sun's
+rays into the tissues beneath is not tenable.
+
+Here again we see that the disease-agencies concerned in producing the
+symptoms described in this chapter, agree for the most part in so far
+that the principal effect is generally the disturbance of chlorophyll
+action in the spots or flecks on the leaves, and the rendering useless
+of these areas so far as providing further food-supplies is concerned.
+The effects may be due merely to the shading action of a
+parasite--_e.g._ epiphytic fungi--or to actual destruction of the
+tissues invaded--_e.g._ by endophytic fungi--or the tissues may be
+burnt, poisoned, etc. In so far the results are again quantitative and
+cumulative, and the amount of damage depends on the number and size of
+the spots or other areas affected, and the proportion of foliage
+involved, as well as the length of time the injurious action is at work.
+But, again, it must be remembered that several symptoms may co-exist,
+and matters may be complicated by the spread of the destructive agent,
+or its consequences, to other parts, and in some cases we are quite
+uninformed as to the true nature of the disease.
+
+
+NOTES TO CHAPTER XX.
+
+     Further information regarding these "leaf-diseases" will be
+     found in special works dealing with the fungi and insects
+     which cause them. In addition to works already quoted, the
+     reader may also be referred for Fungi to Massee, _A Textbook
+     of Plant-diseases caused by Cryptogamic Parasites_, London,
+     1899; or Prillieux, _Les Maladies des Plantes Agricoles_,
+     1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional
+     Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol.
+     XIX., 1882, p. 299.
+
+     The question of "Sun-spots" has been dealt with by Jönnson in
+     _Zeitschr. f. Pflanzenkrankh._, 1892, p. 358.
+
+
+
+
+CHAPTER XXI.
+
+ARTIFICIAL WOUNDS.
+
+     _The nature of wounds and of healing processes--Knife wounds--
+     Simple cuts--Stripping--Cuttings--Branch-stumps and pruning--
+     Stool-stumps--Ringing--Bruises._
+
+
+_Wounds._--All the parts of plants are exposed to the danger of wounds,
+from mechanical causes such as wind, falling stones or trees, hail,
+etc., or from the bites of animals such as rabbits, worms, and insects,
+and although such injuries are rarely in themselves dangerous, they open
+the way to other agencies--water, fungi, etc., which may work great
+havoc; or the loss of the destroyed or removed tissues is felt in
+diminished nutrition, restriction of the assimilative area, or in some
+other way.
+
+We have seen that living cells die when cut, bruised, or torn; and that
+the cells next below in a layer of active tissue are stimulated by the
+exposure to increased growth and division, and at once produce a layer
+of cork, the impervious walls of which again protect the living cells
+beneath. This is found to occur in all cell-tissues provided the cells
+are still living, and it matters not whether the wound occurs in the
+mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the
+cortex of a root or stem, or in the fleshy parts of a young fruit, the
+normal effect of the wound is in all cases to call forth an elongation
+of the uninjured cells beneath, in a direction at right angles to the
+plane of the injured surface, which cells then divide by successive
+walls across their axis of growth: the layers of cells thus cut off are
+then converted into cork, by the suberisation of their walls. Further
+changes may then go on beneath the protective layer of wound-cork thus
+produced, and these changes vary according to the nature of the cells
+beneath: the cambium forms new wood, the medullary rays similar rays,
+cortex new cortex, and so on.
+
+_Knife-wounds._--Artificial cuts in stems are easily recognised and soon
+heal up unless disturbed. Several cases, differing in complexity, are to
+be distinguished. The simplest is that of a longitudinal, oblique, or
+horizontal short cut in which the point of the knife severs all the
+tissues of the stem down to the wood. The first effect usually observed
+is that the wound gapes, especially if longitudinal, because the cortex,
+tightly stretched on the wood cylinder, contracts elastically. This
+exposes the living cortex, phloem and cambium to the air, and such
+tissues at once behave as already described above: the cells actually
+cut die, those next below grow out under the released pressure, and
+these give rise to cells which become cork. As the growth and
+cell-division continue in the cells below this thin elastic cork-layer,
+they form a soft herbaceous cushion or _callus_ looking like a thickened
+lip to each margin of the cut. Each lip soon meets its opposite
+neighbour, and the wound is closed over, a slight projection with a
+median axial depression alone appearing on the surface. The depression
+contains the trapped-in callus-cork squeezed more and more in the plane
+of the cut as the two lips of callus press one against the other, and
+sections across the stem and perpendicular to the axis of the cut show
+that this thin cork, like a bit of brown paper, alone intervenes between
+the cambium, phloem and cortex respectively of each lip, as each layer
+attempts to bridge over the interval. If the healing proceeds normally,
+these layers, each pressing against the trapped cork-film, and growing
+more and more in thickness, shear the cork-layer and tear its cells
+asunder, and very soon we find odd cells of the cambium of one lip
+meeting cambium cells of the other, phloem meeting phloem, and cortex
+cortex, and the normal thickening of the now fused layers previously
+separated by the knife goes on as if nothing had happened, the only
+external sign of the wound being a slight ridge-like elevation, and,
+internally, traces of the dead cells and cork trapped here and there
+beneath the ridge. When the conjoined cambium resumes the development
+of a continuous layer of xylem and phloem, no further trace of the
+injury is observable, unless a speck of dead cells remains buried
+beneath the new wood, and indicates the line where the knife point
+killed the former cambium and scored the surface of the wood in making
+the wound.
+
+_Stripping._--Now suppose that, instead of a mere slit with the
+knife-point, a strip of bark is removed down to the wood. Exactly the
+same processes of corking and lip-like callus formation at the edges of
+the wound occur, but of course the occlusion of the bared wood-surface
+by the meeting of the lips occupies a longer time. Moreover, the living
+cells of the medullary rays exposed by the wound on the wood-surface
+also grow out under the released pressure, and form protruding callus
+pads on their own account. In course of time the wood is again
+completely covered by the coming together over its face of these various
+strips of callus, but two important points of difference are found, as
+contrasted with the simpler healing of the slit-wound. In the first
+place the exposed wood dries and turns brown, or it may even begin to
+decay if moisture and putrefactive organisms act on it while exposed to
+the air; and, in the second place, the normal annual layer of wood--or
+layers, as the case may be--formed by the cambium only extends over that
+part of the stem where the cambium is still intact, and is entirely
+wanting over the exposed area. Thus, if it takes two years for the
+cambium to extend across the wound, a layer of wood will be formed all
+round the intact part of the stem, from lip to lip of the cut tissues
+during the first year; then a second annual layer outside this will be
+formed during the second year, but extending further over the edges of
+the wound, and nearly complete, because the cambium has now crept
+further across the wounded surface to meet the opposite lip of cambium;
+and during the third year, when the cambium has once more become
+continuous over the face of the wound, the annual wood layer will be
+complete. But, of course, this last layer covers in the edges of the two
+previously developed incomplete wood-layers as well as the exposed and
+brown, dry, or rotten dead face of the wood. It also covers up the
+trapped-in brown cork and any débris that accumulated in the wound, and
+this "blemish," though buried deeper and deeper in the wood during
+succeeding annual deposits of wood-layers, always remains to remind us
+of the existence of the wound, the date of which can be fixed at any
+future time by counting the annual rings developed subsequently to its
+formation. Obviously, also, the deficiency of wood at this place makes
+itself visible on the outside by a depression.
+
+_Cuttings._--When a cutting of _Pelargonium_, Willow, or other plant is
+made, we have a typical knife-wound, the behaviour of which is very
+instructive in illustration of plant-surgery, and may be most easily
+seen by keeping it in damp air instead of plunging it into sand or
+soil.
+
+All the living cells actually cut or bruised turn brown and die as
+before; those beneath--_e.g._ the living pith, medullary rays, cambium,
+phloem, and cortex, grow out under the released pressure and form a
+callus, the outermost layer of which becomes cork, while those below,
+abundantly supplied with food-materials, proceed to spread, as if
+flowing over the surface of the cut wood, and rapidly occlude the wound.
+Meanwhile new roots are formed adventitiously from the cambium just
+above the plane of section, and push out through the cortex into the
+damp air, and if the cutting had been in soil it would now be capable of
+independent existence. It is important to keep cuttings upright, as the
+roots only spring from the lower end. Such cuttings can be obtained not
+only from stems, but also from roots and even leaves.
+
+Callus-formation is not confined to the basal end of a cutting; it has
+nothing to do with position, but is a reaction to the wound stimuli,
+independent of light, gravitation, etc. As time goes on, however, the
+internal organisation of the erect cutting usually reacts on the callus
+at either end, and roots only rise from the lower one, while shoot-buds
+may form in the upper one, though it is possible to bring about the
+formation of buds from the lower end also.
+
+_Branch stumps._--A more complex example is furnished by a branch cut
+off short some distance--say a foot--from the base, where it springs
+from the trunk. As before, the immediate effect of the section is the
+formation of a callus from the cambium, phloem and cortex, which begins
+to rise as a circular occluding rim round the wood. The transpiration
+current in the trunk, however, is not deflected into the 12 inches or so
+of amputated branch, because there are no leaves to draw the water up
+it, and so the stump dries up and the cortex and cambium die back to the
+base, leaving the dead wood covered with shrivelled cortical tissues
+only. This dead stump gradually rots under the action of wet, fungi, and
+bacteria, and since the pith and heart-wood afford a ready passage of
+the rot-organisms and their products into the heart of the trunk, we
+find in a few years a mere stump of touch-wood and decayed bark, which
+falls out at the insertion like a decayed tooth, leaving a rotten hole
+in the side of the trunk.
+
+If, however, instead of allowing the basal part of the amputated branch
+to protrude as a stump, we cut it off close to the stem, and shave the
+section flush with the normal surface of the latter, the callus formed
+by the cambium, etc., rapidly grows over the surface, and soon forms a
+layer of cambium continuous with that of the rest of the stem. The wound
+heals, in fact, much as if it were a strip-wound, and beyond a slight
+prominence for a year or two no signs are visible from the outside after
+the occlusion. Of course these matters depend on the relative thickness
+of branch and stem, and if much wood is exposed the dangers of rot and a
+resulting hollow in the stem are increased. It is interesting to note
+how much thicker the callus lips are at the sides of the wound than
+above and below, owing to differences in the distribution of the
+nutrient materials.
+
+_Stool-stumps._--When a tree is felled, the stump may, if the section is
+close to the ground and kept moist, begin to form a thick rim-like
+callus round the wood, in which adventitious buds soon make their
+appearance, and grow out into so-called _Stool-shoots_. The products of
+assimilation of these, and the stores accumulated in the stump, often
+suffice to feed the callus sufficiently to enable it to grow over and
+completely occlude the wound, if the wood surface is not too large, or
+so long exposed that rotting processes have meanwhile set in.
+
+_Ringing._--If the strip of cortical tissues and cambium is removed all
+round the stem, exposing the wood in a form of a ring, complications may
+ensue owing to the following circumstances. A well-marked callus appears
+at the upper edge of the wound, because, the transpiration current up
+the young wood not being stopped, plenty of water and salts from the
+soil can reach the leaves; but the nutritive materials supplied by the
+latter are accumulated at the upper lip of the wound owing to the
+stoppage there of their descent in the phloem, cortex, etc. No such
+callus-lip appears at the lower margin of the wound owing to want of
+these supplies. Consequently the occlusion and healing of the ring-wound
+only takes place from above downwards, and if the ring of cortical
+tissues removed is a broad one, the healing may be a long process, or
+may even be indefinitely delayed, a thicker and thicker callus
+projecting over from above. For similar reasons no annual wood layers
+are formed below, but only above the wound, and thus the branch or tree
+may die. The latter contingency is the more likely the further up the
+tree the ringing takes place, owing to the risk of drying up which
+threatens the exposed wood, and to the consequent interruption of the
+transpiration current, and the likelihood that lateral shoots below the
+wound may divert the water to their own leaves. If the ringing occurs
+low down on a stem, and the environment remains damp, the upper thick
+callus may put out new roots; the part above the wound then behaves like
+a cutting. If the ringing is done on a young and vigorous branch of an
+old tree, the lower lip may receive supplies from the leaves of branches
+below the wound, or from shoots which spring from adventitious buds
+close to it, and the wound may heal over normally. Such healing may be
+rendered more certain by keeping the wounded surface moist--_e.g._ by
+means of damp moss, and so encouraging the formation of callus-bridges
+from the medullary rays.
+
+If on ringing a tree or a branch the young wood is removed as well as
+the cambium and cortical layers, the death of the parts above the wound
+is almost certain, owing to the stoppage of the transpiration current:
+the exceptions to this rule depend simply on the existence of other
+channels of communication, such as internal phloems, very thick
+sap-wood, and so forth.
+
+_Bruises._--If a branch or woody stem is struck sharply, with a hammer,
+for instance, the bruised cortex, phloem and cambium are killed by the
+blow, and the general effect is as if these tissues had been removed at
+that spot by the knife, but with the following complications. The
+bruised cortical tissues rapidly dry as they perish, and may adhere to
+the wood below. Consequently the still sound parts bordering on the
+wound are not released from pressure, but, on the contrary, have to
+advance towards each other over the surface of the wood under still
+greater pressures, in part due to the tightening of the whole cortex as
+the dead parts dry and contract, and in part due to the above-mentioned
+adherence of the latter to the wood. It results from this that such
+wounds heal very slowly and badly, and when the killed patch at last
+ruptures, wound-fungi, insects, and other injurious agencies may get in
+and do irreparable damage, as has been found to occur in cases where
+such wounds have been made in striking trees to shake down insects,
+fruit, etc.
+
+
+NOTES TO CHAPTER XXI.
+
+     The essential facts regarding wounds and healing by occlusion
+     are given in Marshall Ward, _Timber and some of its Diseases_,
+     1889, chapters viii. and ix., and in Laslett, _Timber and
+     Timber Trees_, 1894, chapters iv. and v. More detailed
+     treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1.
+     cap. 2, where the special literature is collected. The reader
+     may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894,
+     pp. 225-269.
+
+
+
+
+CHAPTER XXII.
+
+NATURAL WOUNDS.
+
+     _Burrows and excavations. Bark-boring--Wood-boring--Wood
+     fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves--
+     Irregular erosions--Shot holes. Frost cracks--Strangulations--
+     Spiral grooving._
+
+
+Natural wounds are produced in a variety of ways during the life of the
+plant, and, generally speaking, are easily healed over by the normal
+process if the area destroyed is not too large, and the parts remaining
+uninjured are sufficiently provided with foliage, or with supplies of
+food-materials stored up in the roots, rhizomes, medullary rays, etc.,
+to feed a vigorous callus.
+
+The nature of such wounds and the mode of healing are explained by what
+we know of artificial wounds, and it only remains to point out that the
+principal danger of ordinary wounds is not so much the direct traumatic
+action, because the simpler organisation of the plant does not involve
+matters connected with shock, loss of blood, etc., as in animals; the
+danger consists, rather, in their affording access to other injurious
+agents, especially fungi, and the treatment of wounds frequently
+resolves itself into cutting or pruning in order to get clean surfaces
+which can heal readily.
+
+Wounds on leaves imply loss of foliar surface--_i.e._ of chlorophyll
+action--and the remarks on page 193 apply.
+
+_Burrows_ may be taken as comprising all kinds of tunnel-like
+excavations in the various organs of plants, including those cases where
+insects burrow into hollow stems of grasses, etc., as indicated by the
+perforations they make in the outer tissues.
+
+_Bark-boring_ is done by many species of beetles, especially
+_Scolytidae_, which excavate characteristically formed branching
+passages tangentially in the inner bark of Conifers and other trees.
+Some of them also bore down to the surface of the sap wood (_e.g._
+_Tomicus bidentatus_) or even burrow right into the latter (_e.g._ _T.
+lineatum_). It commonly happens that the external apertures show up
+clearly, owing to the brown dust and excrement, sometimes accompanied by
+turpentine, which exude from them. Many of these Bark beetles only
+attack trees which are already injured by fire, lightning, etc.;
+possibly they cannot bore through a cortex which swamps them with sap,
+as a vigorous one might do.
+
+_Wood-boring_ is also done by many of the bark-beetles as well as by
+Longicorns, _e.g._ _Saperda_ in Poplars and Willows, the young shoots
+of which often show characteristic swellings with lateral holes
+indicating the points of exit. From the external apertures comminuted
+wood, like saw-dust, is frequently ejected in quantity and betrays the
+presence of the insects. Certain wood-wasps (_Sirex_) and the larvae of
+moths (_Cossus_) also make large perforations in the wood of Willows and
+other trees, often destroying it completely. In the case of these larger
+borers, whose tunnels may be as broad as the little finger, the foul
+smell as well as abundant "saw-dust" betray the evil.
+
+Excavations in wood are by no means caused only by insects: several of
+the larger Hymenomycetes--_Stereum_, _Thelephora_, _Polyporus_,
+etc.--tunnel the timber in characteristic ways and often after a fashion
+very suggestive of insects. They usually obtain access through
+fractures.
+
+_Tunnels_ in leaves are invariably due to the activity of miners
+belonging to the smaller moths and beetles--_e.g._ _Tinea_, _Orchestes_,
+etc.--the larvae of which eat out the mesophyll but leave the covering
+epidermis or cuticle untouched, and since the insect bores forwards
+only, in an irregular track, and leaves its excrement in the winding
+passage, the effect is very characteristic.
+
+Whitish leaf tunnels in Peas are excavated by _Phytomyza_.
+
+Characteristic foxy-red tunnels are mined in the leaves of Apples by
+_Lyonetia_, _Coleophora_, etc.
+
+_Falling of fruit_, of Apples, Plums, Apricots, etc., before they are
+ripe, is frequently due to insects, of which the various species of
+_Grapholitha_ or _Carpocapsa_ are conspicuous: the fallen fruits show a
+small hole leading by a labyrinth of passages to the "core" or "stone,"
+and in which the grub and its excrement are visible. The cutting off of
+the vascular bundles and disturbance of the water supply only partly
+explain the premature fall.
+
+_Pith-flecks_ are minute brown specks or patches found in the
+wood-layers of many trees, and consist of dead parenchymatous
+thick-walled cells, reminding one of the structure of pith. They are
+explained as due to the borings of minute insects, _Diptera_ or Beetles,
+the larvae of which pierce the cortex and phloem and bore their way into
+the cambium. The latter then occludes the tunnels by filling them up
+with cells, and continuing its wood-forming activity gradually buries
+them deeper and deeper in the wood. Such pith-flecks are common in
+Willow, Birch, Alder, _Sorbus_, etc. It is possible that they may be due
+to other causes also in other trees.
+
+_Erosions_ or _irregular wounds_ on leaves are caused by large numbers
+of grubs and caterpillars and other insects, such as earwigs, as well as
+slugs, snails, and other animals; but it must by no means be assumed
+that all marginal leaf wounds, for instance, are caused by animals,
+since many fungi which rot the tissues, as explained below (p. 208),
+also cause such erosions, the putrescent parts falling out--_e.g._ the
+Potato disease.
+
+_Skeleton leaves_ frequently result from the ravages of caterpillars,
+which leave the coarser ribs and veins untouched, but much finer
+skeletons with the minute veins almost intact may be found on plants
+infested with certain insects--_e.g._ _Selandria_ on Cherries.
+Skeletonised patches on Cherry leaves, often pink or brown-pink, are
+eaten out by this grub.
+
+_Shot-holes_ are perforations in leaves presenting the appearance, from
+their more or less rounded shape, of gunshot wounds. They may be due to
+insects which bore through the young leaves while still folded in the
+bud--_e.g._ Willow Beetle--or which gnaw out the tissue--_e.g._ the
+Beech Miner. Similar but usually more torn and irregular holes are eaten
+out by many caterpillars--_e.g._ the Cabbage Moth.
+
+Shot-holes on Peas may be the work of Thrips.
+
+Leaf perforations are commonly caused by severe hail-storms, the
+hail-stones beating right through the thin mesophyll. Certain chemicals
+used for spraying have also been known to cause shot-holes by killing
+the tissue beneath the standing drops.
+
+There is, however, a class of shot-holes in thin leaves which are due to
+the action of minute fungi, the mycelium of which so rots the tissues in
+a more or less circular area round the point of infection, that, in wet
+weather, the decomposing mass falls out and leaves a round hole--_e.g._
+certain Chytridiaceae, Peronosporeae, _Gloeosporium_, _Exoascus_, etc.
+If dry weather supervenes these holes frequently dry at the edges, and
+the leaves appear as if eaten out.
+
+Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to
+_Phyllosticta_, in Cherry leaves also to _Clasterosporium_, and in
+Potato leaves to _Haltica_.
+
+_Frost-cracks._--The trunks of trees exposed to the north-east, and
+occasionally with other aspects, are apt to show longitudinal ridges
+which realise on a larger scale the features of healed wounds scored
+with a knife. These wounds are due to the outer layers of wood losing
+water from their cell-walls as it congeals to ice in their lumina, more
+rapidly than do the warmer internal parts of the trunk; as this drying
+of the wood causes its shrinkage, especially in the tangential
+direction, the effect of a sudden frost and north-east wind is to rend
+the wood, which splits longitudinally with a loud report, as may often
+be heard in severe winters. Since the cortex and bark are ruptured at
+the same time the total effect resembles that of a deep knife-cut, and
+the same healing processes result on a larger scale when the wood swells
+and closes up the wound again in spring. But this recently-closed lesion
+is evidently a plane of weakness, and if a similarly severe winter
+follows the wound reopens and again heals, and so on, until after a
+succession of years a prominent _Frost-ridge_ results, which may finally
+heal completely if milder winters ensue or the tree be eventually
+protected.
+
+_Strangulations._--We are now in a position to understand the so-called
+strangulations which result when woody climbers, telegraph wires, etc.,
+kill or injure trees by tightly winding round them. If strong wire is
+twisted horizontally round a stem, the growth in thickness of the latter
+causes the trapping of the cortex and cambium, etc., between the wire
+and the wood, and a ringing process is set up in consequence of the
+death of the compressed tissues. A callus then forms above the wound, as
+in the case of true ringing by means of a cut, and eventually bulges
+over the upper side of the wire: in the course of years this overgrowth
+may completely cover in the wire, and, pressing on to the lower lip of
+the wound, may at length fuse with the cambium below. Hereafter the
+thickening rings of wood are continuous over the buried wire. The
+process is obstructed by all the impediments referred to in dealing with
+ringing, and of course the stem thickens more above than below the wire.
+If the sapwood is thin, and the bark is so thick as to put great
+obstacles in the way of the junction of the upper and lower cambiums,
+death may result--the tree is permanently ringed. (See p. 201.)
+
+_Spiral grooves_ are frequently met with where Wood-bine or other woody
+climbers have twined round a young stem or branch, the upper lip of the
+groove always protruding more than the lower. If a kink or a crossing of
+two plants or branches of the twiner results in a complete horizontal
+ring, the results are as in the above cases of ringing and
+strangulation. Naturally grooved walking sticks are often seen.
+
+_Buried letters, etc._--These processes of healing by occlusion enable
+us to understand how letters of the alphabet, cut into the wood of
+trees, come to be buried deep in the timber as successive annual rings
+cover them in more and more. Chains, nails, rope, etc., have frequently
+been found thus buried in wood.
+
+
+NOTES TO CHAPTER XXII.
+
+     In addition to the notes to the last chapter, the reader may
+     be referred to Fisher in Vol. IV. of Schlich's _Manual of
+     Forestry_, Chap. VI., for an account of Hess' excellent work
+     on Boring Beetles, etc.
+
+     The authority on Wood-fungi is Hartig, see especially his
+     _Zersetzungs-erscheinungen des Holzes_, the principal results
+     of which are condensed in his _Diseases of Trees_ already
+     referred to. As regards "Pith-flecks," the reader should
+     consult Frank, _Krankh. der Pflanzen_, B. I., p. 212: the
+     subject needs further investigation.
+
+
+
+
+CHAPTER XXIII.
+
+EXCRESCENCES.
+
+     _Herbaceous excrescences, or galls--Erineum--Intumescences--
+     Corky warts, etc.--Pustules--Frost-blisters--Galls and Cecidia
+     --Root nodules._
+
+
+_Excrescences_, or out-growths of more or less abnormal character from
+the general surface of diseased organs, are very common symptoms, and
+widely recognised. They are due to hypertrophy of the tissues while the
+cells are young and capable of growth, and may be induced by a variety
+of causes, among which the stimulus of insect-punctures and of the
+presence of insect eggs are best known; but that of fungi, though less
+widely recognised, plays an equally important part, and, as we shall
+see, galls and other excrescences may be due to widely different agents.
+
+_Galls_ or _Cecidia_ are protuberances of the most varied shapes,
+colours, and sizes found on herbaceous parts attacked by insects, fungi,
+etc. In the simplest cases the insects only pierce and suck the young
+cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the
+stimulus to hypertrophy starts by the puncture of the embryonic tissue
+of a leaf, root, etc., by the ovipositor of the female insect, which
+then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of
+which appears to intensify the irritating action, or such only occurs
+when the young larva escapes.
+
+Our knowledge of the primary cause of gall-formation amounts to very
+little. Generally speaking, only embryonic or very young cellular tissue
+reacts, and galls on adult leaves and branches have usually been
+initiated long before. The same gall-insect may induce totally different
+galls on different plants, or even on different parts of the same plant,
+and different insects call forth different galls on any one plant. These
+facts point clearly to the co-operation of both plant and insect in the
+gall-formation, and the best hypothesis yet to hand is to the effect
+that a gall is a hypertrophy of cells, the normal nutrition, growth, and
+division of which have been disturbed owing to the action of some poison
+or other irritant derived from the insect, or fungus, or other organism.
+Attempts have been made to reproduce galls by injecting the juices of
+similar galls into the tissue, but as yet without success, and this may
+point to the co-operation of mechanical irritation during the
+hypertrophy in normal gall-formation.
+
+Galls, in the broad sense, are not always preceded by a wound, however.
+Insects on the outside of young tissues may cause such irritations that
+the parts in contact with the animal are arrested in their growth, while
+those further away grow more rapidly--_e.g._ where Mites, etc., cause
+puckers and leaf-rolling. In true galls the hypertrophy may consist
+merely in the enlargement of cells already present, and no new
+cell-divisions and, still less, changes in the nature of the tissues
+result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the
+hairy outgrowths of the epidermis known as _Erineum_. In other cases
+there is not only hypertrophy of existing cells, but new cell-divisions
+are instituted: these cell-divisions may be confined to the direction
+perpendicular to the epidermis, and the tissues grow only in the
+direction of the surface, producing puckerings--_e.g._ the Aphis galls
+on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_,
+_Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_,
+and which must not be confounded with the "pocket plums" and similar
+galls due to Exoasci.
+
+In a third series of cases, cell-divisions occur parallel to the surface
+of the leaf, and galls are formed which grow in thickness, and develop
+the most extraordinary and complicated new tissues--proteid-cells
+surrounding the egg or larva deposited inside, followed by a protective
+layer of sclerenchyma encasing this food layer, and around this again
+softer tissues which may assume the structures and functions of
+respiratory tissues, water-storing tissues, starch reservoirs,
+assimilatory, or protective tissues of various kinds, and over all may
+be a well-marked epidermis, with stomata, or cork with lenticels.
+
+The chief seat of these hypertrophies and--what is more
+remarkable--development of new tissue elements not found elsewhere in
+the leaves, or even in the species, is the mesophyll, and various
+speculations and hypothesis have been founded on these curious
+phenomena.
+
+_Erineum._--The simplest excrescences on plants are certain hair-like
+developments of epidermal cells due to the irritation of species of
+_Phytoptus_, and similar insects which rise in clusters on the surfaces
+of leaves and by their colours, consistence, arrangement in patches,
+spots, etc., so simulate fungi that Persoon was deceived by them and
+gave them the genus name _Erineum_. They occur on most of our trees,
+_e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually
+pale or even white at first, they turn brown as the hair-like outgrowths
+die and lose their sap, but since the latter may be bright
+coloured--yellow, red, purple,--the patches are sometimes very
+conspicuous objects on smooth leaves.
+
+In many cases these hairs exactly resemble in shape and other characters
+the abnormal root-hairs found on roots exposed to the effects of
+poisonous reagents, or of unsuitable food-materials, or the rhizoids
+developed from wounded Algae, etc.
+
+_Intumescences_ are similar trichomatous outgrowths not associated with
+insects or fungi, and due to some disturbance of the balance between
+transpiratory and assimilatory functions of their leaves, as indicated
+by the less localised occurrence and by their non-appearance when the
+plant is under favourable cultural conditions. Structures not unlike
+these have been artificially induced by exposure to particular lights,
+and also by painting spots with dilute corrosive sublimate, indicating
+that poisons may impel the epidermis cells to grow out abnormally.
+
+_Corky warts._--Several forms of disease are known in which the
+pathological condition is expressed by the formation of cork in unwonted
+places and quantities. The _Scab_ or _Scurf_ of Potatoes is a case in
+point. The tissue of the lenticels absorbs water and the outermost cells
+are cut off by cork and die: the cells below them burst the dead
+bark-like masses thus formed, and again cork is formed and cuts off the
+outer masses, and the rough cork warts--_Scab_ or _Scurf_--are the
+result.
+
+The causes predisposing to scab have been variously assigned to
+dampness, want of lime, action of bacteria and fungi--_e.g._
+_Sorosporium_, _Oospora_, _Spongospora_,--the latter making their way
+into the ruptured tissue of the lenticels and irritating the cells to
+further growth.
+
+It seems probable that several different kinds of scab exist in
+Potatoes, as well as in roots--_e.g._ Beets, and the whole subject needs
+further investigation. The scab-like rough scaly bark of Pear trees in
+dry districts may also be mentioned here.
+
+_Cork-wings_ are well known on the young branches of Elms, Maples, etc.,
+some varieties of which have received specific names on this account.
+
+_Corky excrescences_ on leaves occur occasionally in the Gooseberry,
+Holly and other plants, for which no cause has been discovered.
+
+Lenticels are also formed on some leaf-galls, and are remarkable as
+being structures not normal on leaves.
+
+_Pustules._--This term may be employed generally for all slight
+upheavals of the surfaces of herbaceous organs, which subsequently burst
+and give egress to the spores, etc., of the organism causing them, or
+merely fray away at the top if no organism is discoverable. They are
+often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and
+Ustilagineae,--and we may extend the use of the general term also to
+those cases where the _stroma_ of the fungus itself bursts through the
+cortex of older parts and forms the principal part of the
+pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples,
+_Roestelia_ and other Æcidia, forming yellow or orange pustules on
+leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the
+cortex of trees, and _Phoma_ and numerous other Ascomycetes which form
+black cushions. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc.,
+are due to the parasitic Alga _Phyllobium_.
+
+Cylindrical stem swellings are caused by _Calyptospora_: they are due to
+the hypertrophy of the cortex of Bilberry stems permeated by the
+hyphae. _Epichloë_, which clothes the sheaths and halms of grasses with
+its stroma, at first snowy white and later ochre-yellow as the
+perithecia form, is another example.
+
+The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must
+not be confounded with these cases.
+
+_Frost-blisters_ are pustule-like uprisings of the cortex, where the
+living tissues below have formed a callus-like cushion into the cavity
+beneath the dead outer parts of the cortex which were killed by the
+frost; they occur on the stems of young Apples, Pears, etc.
+
+_Galls_ in the narrower sense are tissue outgrowths usually involving
+deeper cell-layers. They are so varied and numerous that classification
+is difficult. For symptomatic purposes we may divide them as follows:
+
+_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or
+_bladders_ in which the whole thickness of the leaf is as it were pushed
+up like a glove-finger at one spot, so that if the upper surface of the
+leaf forms the outside of the gall the lower surface is its lining. Such
+galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms
+(_Tetraneura_), etc. Similar localised extension of the leaf surface,
+compelling it to rise up like a pocket, are caused by fungi--_e.g._
+_Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on
+Bilberries, Rhododendrons, etc.
+
+Another type is that of the _Gall-apple_, so well known on Oaks, where
+the spherical swelling is solid--except for the inner cavity containing
+the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable
+in general characters to the nodules on roots.
+
+Fungus galls with similar external features when young are found on
+Maize (_Ustilago Maydis_), and betray their nature by the black powdery
+spores as they mature.
+
+Bud galls on Willows are due to _Cecidomyia_, which causes several
+internodes to swell out into a greenish barrel-shaped mass, from which
+leaves may spring.
+
+Small irregular excrescences on Willow stems are referred to
+_Phytoptus_, and another species of the same insect induces similar
+swellings on Pines which are not surcharged with resin.
+
+_American Blight_, or Woolly Aphis, on Apples especially, causes the
+tumour-like swellings covered with sticky white fluff, which is a waxy
+excretion of the insect. Galls on _Pilea_, in Java, are due to an
+Alga--_Phytophysa_.
+
+_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._
+_Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied
+"gallflies" of Oaks, and the notorious _Phylloxera_. But similar
+root-galls are produced by Nematode worms, _Heterodora_, on Beets,
+Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus
+_Plasmodiophora_, and it is not always easy to distinguish such cases
+from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_,
+and Leguminoseae where the symbiosis of bacteria or fungi with the
+roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar
+nodules at the collar of young plants of _Adonis_.
+
+_Heterodora javanica_ passes into the cortex of sugar-cane roots through
+fissures, and makes its way to the place where a young rootlet is about
+to emerge; here it sticks its beak into the growing-point and remains
+fixed.
+
+Molliard has shown that in the roots of Melons, _Coleus_, etc.,
+_Heterodora_ causes the cells in immediate contact with its head, and
+which would normally become vessels of the xylem, to swell up into huge
+giant-cells, with their walls curiously folded, and containing large
+supplies of proteids and numerous nuclei, reminding us of the food-layer
+of insect galls and of the tapetal layer of pollen-sacs. While the
+stimulus exerted by the Nematode thus induces hypertrophy and storage
+with food-substances of these cells, those of the next layers undergo
+reticulate thickenings of their walls. Again instances of the evolution
+of new tissue elements by the action of the foreign organism.
+
+So far as galls on leaves are concerned the amount and kind of damage
+done are in proportion to the area of chlorophyll action put out of play
+for the benefit of the plant, and the remarks already made on p. 193
+apply here also. Where buds are destroyed the effects may of course
+extend further, but it rarely happens that leaf-galls are so abundant as
+to maim a tree permanently. Nevertheless we must remember that cases
+like _Phylloxera_ are notorious.
+
+Far more dangerous, however, are the root-galls due to such insects,
+because here the damage is not so local: the water-supplies are cut off,
+and injurious consequences result from the absorption of the products of
+decomposition in the soil.
+
+
+NOTES TO CHAPTER XXIII.
+
+     In addition to the literature on galls quoted in the Notes to
+     Chapter XIV., the reader should consult Dale "On certain
+     Outgrowths (Intumescences) on the green parts of Hibiscus,"
+     _Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.
+     Ass. Rep._, Bradford, 1900.
+
+     The detailed study of the anatomy and histology of Galls has
+     been recently undertaken by Küster, "_Beitrage zur Kenntniss
+     der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the
+     principal references will be found.
+
+     On the root-galls due to Nematodes see Atkinson in _Science
+     Contributions from the Agric. Expt. Station, Alabama_, Vol.
+     I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in
+     _Natural Science_, Vol. VI., 1895, p. 187; and Molliard in
+     _Revue générale de Botanique_, Apl., 1900, p. 157, where the
+     histology is dealt with.
+
+     The nodules of the roots of Leguminoseae are not part of the
+     subject of this work: the literature is collected in _Science
+     Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.
+     Trans._, 1900.
+
+
+
+
+CHAPTER XXIV.
+
+EXCRESCENCES (_continued_).
+
+     _Cankers--Burrs--Sphaeroblasts, and other excrescences of
+     woody tissues--Witches' Brooms._
+
+
+_Cankers_ are irregular excrescences due to the perennial struggle
+between tissues attempting to heal up a wound, and some organism or
+other agent which keeps the lesion open. A canker always originates in a
+wound affecting the cambium, and usually in a small wound such as an
+insect puncture or frost nip; if undisturbed the dead parts would heal
+over by cork and callus, but if recurring frost-cracks break open the
+coverings, or if insects or fungi penetrate the callus and invade the
+cambium, irregularities of growth due to the occluding tissue on the one
+hand, and continued growth of the still unimpaired cambium on the
+opposite side of the injured shoot on the other, result in the canker.
+Frost cankers occur on fruit-trees, Vines, Beeches, etc.
+
+Cankers due to insects are found on Apples, the cortex of which is
+punctured by the woolly Aphis (_Schizoneura_) while the twigs are young,
+and the wound is kept open by the insects nestling in crevices in the
+occlusion tissues. Species of _Coccus_, _Lachnus_, and _Chermes_ also
+produce cankers on forest trees.
+
+Cankers due to fungi usually originate in a wound primarily due to an
+insect puncture or bite, or to frost, the invading fungus hyphae making
+their way into the wounded tissues and gradually extending more and more
+into the cambium and the occluding callus. Among the best known of these
+wound fungi which cause cankers are _Dasyscypha Willkommii_ the peziza
+of Larch disease, _Nectria ditissima_ and _N. cucurbitula_ on Beech and
+Conifers; less common are _Scleroderris_ on Willows, _Aglaospora_ on
+Oaks and some others.
+
+_Peridermium Pini_ and _Aecidium elatinum_ also cause cankers under
+certain conditions, as also does _Gymnosporangium_, but in these cases
+the fungi are more truly parasitic.
+
+In some cases--_e.g._ Ash, Pine, Olives--bacteria are concerned as
+associated organisms in the cankering of trees.
+
+_Burrs_ or _Knauers_ are irregular excrescences, principally woody, with
+gnarled and warted surfaces. They are frequently due to some previous
+injury, such as the crushing or grazing of cortical tissues by
+cart-wheels. The excitation of the tissues thus wounded results in the
+development of shoots from adventitious or dormant buds at the base of
+old tree trunks, or in the starting of the same process where a branch
+has been broken off. The new bud begins to develop a shoot, but soon
+dies at its tip owing to paucity of food-supplies to the weak shoot,
+while new buds at its base repeat the process next year with the same
+result, and each of these again in turn, and so on. The consequence is
+an extremely complex nest of buds, all capable of growing in thickness
+and putting on wood to some extent, but not of growing out in length. In
+course of time this mass may attain dimensions measurable by feet,
+forming huge rounded and extremely hard-knotted burrs, the cross-section
+of which shows the vascular tissues running irregularly in all
+directions, and, owing to the very slow growth, extremely dense and
+hard. The dark spots in such sections--_e.g._ Bird's-eye Maple--are the
+cut bud-axes all fused together, as it were. On old Elms such burrs are
+common at heights on the stem which preclude the assumption of any
+coarse mechanical injury, and similar structures occur on the boles of
+other forest trees suddenly exposed to light by the felling of their
+companions, which suggests that these epicormic shoots result from some
+disturbance due to the action of light.
+
+_Witches' Brooms_ are irregular tufts of twigs often found among the
+branches of trees such as Birches, Hornbeam, etc., where they look like
+crows' nests, and similar structures are to be found on Silver Firs and
+other conifers. In the former case they are due to _Exoascus_, in the
+latter to _Aecidium_, fungi which are perennially parasitic in the
+shoots, and stimulate the twiggy development of a number of buds which
+would normally have remained in abeyance, or not have been formed at
+all, and only do so now in a fashion different from that of normal
+branches.
+
+Rosette-like formations, depending on similar disturbing causes on the
+part of insects, occur in conifers--_e.g._ _Gastropacha Pini_.
+
+Dense tufts of twiggy shoots may be developed on many trees by pruning
+in such a way as to stimulate the shooting out of basal buds which would
+otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that
+injuries such as frost, insect bites, etc., may induce the production of
+such tufts in a tree crown. The dense nests of stool-shoots thrown up
+from felled tree-stumps are of essentially the same nature--partly
+adventitious and partly dormant buds being enabled to grow out because
+they can now be supplied with materials previously carried beyond them
+while the trunk was still there. Suckers, if repeatedly cut down, may
+also behave similarly.
+
+_Wood-nodules_ or _Sphaeroblasts_ are curious marble-like masses of wood
+which protrude with a covering of bark from old trunks of Beeches, etc.,
+and can be readily dug out with a knife. The nodule has arisen by the
+slow growth of the cambium of a dormant bud, the base of which separated
+at an early date from the wood beneath; the cambium then closed in over
+the base and laid on thickening rings all round the axis of the bud
+except at the extreme apex. When the separation occurred the cambium of
+the wood beneath covered over the previous point of junction, and thus
+the woody bud was pushed out with the bark, and now protrudes covered
+with a thin layer of the latter. Similar nodules are occasionally found
+on Apple trees.
+
+
+NOTES TO CHAPTER XXIV.
+
+     For further information on Cankers the student should read
+     Marshall Ward, _Timber and some of its Diseases_, Chapter X.
+     Further, the discussion as to the causes of canker in Frank,
+     _Krankheiten der Pflanzen_, B. I., p. 207, and B. III., pp.
+     167 and 172, and various papers in _Zeitschrift für
+     Pflanzen-krankheiten_.
+
+
+
+
+CHAPTER XXV.
+
+EXUDATIONS AND ROTTING.
+
+     _Tumescence--Rankness--Bursting of fruits, etc.--Root rot--Rot
+     of fruits--Bulb diseases--Flux--Honey-dew--Slime flux--
+     Resinosis--Gummosis--Manna._
+
+
+I put together in one artificial class a varied group of diseases, the
+principal symptom of which is the escape of fluids from the tissues,
+under circumstances which betray an abnormal state of affairs, often
+obvious, but sometimes only to be inferred. In many of these cases
+bacteria abound in the putrefying mass, and some evidence exists for
+connecting these microbes causally with the disease in a few of the more
+thoroughly investigated cases, but in no case has this been sufficiently
+demonstrated; and considering the ease with which bacteria gain access
+_via_ wounds caused by insects and fungi, as well as by other agents,
+the necessity for rigid proof must be insisted upon before we can accept
+such alleged examples of _Bacteriosis_.
+
+_Tumescence._--It occasionally happens that herbaceous parts of plants
+pass into a condition of over-turgescence from excess of water in the
+tissues, an abnormal state which indicates pathological changes
+resulting from various causes, often not evident and therefore regarded
+as internal. Such disease was formerly termed _Oedema_ or _Dropsy_.
+This disease is frequently due to the excessive watering of pot plants
+with large root systems and deficient foliage, in hot-houses with a
+saturated atmosphere: it is, therefore, primarily referable to
+diminished transpiration. It can sometimes be brought about by covering
+potato plants, for instance, with a bell-jar in moist, hot weather; and
+this, and the prevalence of the disease in hot-houses as compared with
+plants grown out of doors, point to the above explanation. Similar
+phenomena do occasionally occur out of doors in hot, moist situations or
+during wet seasons, however, and the watery shoots of rank vegetation
+are merely particular cases of the same class. Moreover, the well-known
+tendency to succulence of sea-side varieties of plants which have thin
+herbaceous leaves when growing inland, points to the action of the
+environment in these matters, excess of salts being no doubt one factor
+in such cases.
+
+_Rankness_ affords another example where superfluity of water is
+concerned, though it does not involve simply this, because the plant may
+also contain excessive quantities of nitrogenous and mineral matters
+taken up by the roots.
+
+Rankness is, in fact, in many respects analogous to etiolation in so far
+as the tissues are soft and surcharged with water, but it differs
+fundamentally in the deep green of the chlorophyll: this may lead to
+abundant assimilation if free access of air and drier conditions can be
+gradually brought about. Any sudden drying, however, may be fatal to the
+tender tissues.
+
+Rankness commonly depends on excess of food materials, especially
+nitrogenous manures, as may be seen in meadows and cornfields where the
+manure heaps have remained on the ground and saturated it to excess as
+compared with the rest of the soil; this may often be observed with
+weeds, etc., in the neighbourhood of farm-buildings. If the period of
+rank growth is accompanied and followed by days of suitably bright
+sunshine and dry air, the increase of vegetative structures usually
+results in increased flowering, heavy crops, or strong wood; but if the
+rankness continues too long, or is accompanied by wet and dull weather,
+the watery tissues are peculiarly susceptible to attacks of fungi and
+insects, and to damage by sudden frosts or chilly winds. Rankness
+affords, in fact, a typical illustration of predisposition to disease.
+
+_Damping off._--When seedlings are too closely crowded in beds kept too
+damp, or in moist weather, they are very apt to rot away, with all the
+symptoms--spreading from a centre, contagious infection, mycelia on and
+in the tissues, etc.--of a fungus attack. The commonest agent concerned
+is one of the species of _Pythium_, the propagation of which is favoured
+by the rank, over-turgid, and etiolated conditions of the plants.
+Species of _Mucor_, _Botrytis_, and other fungi, may also be met with.
+
+_Bursting_ of fleshy fruits, such as Tomatoes, Grapes, etc., is due to
+over-turgescence in rainy weather or excessively moist air. But the
+phenomenon is by no means confined to such organs. Hot-house plants when
+oedematous not infrequently put out watery blisters from the cortex or
+leaves, which rupture; and the stems of fleshy fasciated (_e.g._
+Asparagus) or blanched and forced plants (_e.g._ Celery, Rhubarb) are
+particularly apt to crack here and there from the pressure of the
+turgescent tissues on the strained epidermis. Beets, Turnips, and other
+fleshy roots show the same phenomena in wet seasons. That these ruptures
+and exposures of watery tissues afford dangerous points of entry for
+parasites and moulds will be obvious--_e.g._ _Edelfäule_, a rotten
+condition of the grapes in the Moselle district.
+
+_Root-rot_ is a common disease in damp, sour clay soils after a
+continuance of wet weather--_e.g._ Wheat, especially if root-drawn and
+exposed to thaw water.
+
+In the disease known as Beet-rot, the roots turn black at the tip, where
+the tissues shrivel and become grooved and wrinkled extensively. Inside
+the flesh also blackens and finally rots. In earlier stages, only the
+vascular bundles are brown and blocked with gum-like substances. In
+advanced stages there is much gummy material in the lumina, and even
+large cavities filled with this gum may be found.
+
+The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to
+several fungi, of which _Botrytis_, _Monilia_, _Mucor_, _Penicillium_,
+and _Aspergillus_ are the chief. The fruit may be attacked while still
+on the tree, but very often fungi and bacteria gain access to the
+tissues, through bruises, cracks, etc., formed in the fruit lying in the
+storage baskets or on the shelves.
+
+Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of
+_Botrytis_ or _Sclerotinia_, followed by moulds, yeasts, and bacteria in
+the stores.
+
+_Sour-rot_ in Grapes, and other fleshy fruits which need much sun to
+ripen them, is probably a usual result of continued cold, wet weather at
+the cropping season, setting in when the fruits are beginning to swell.
+
+_Flux._--It is a common event to see fluids of various kinds issuing
+from wounds in trees, or congealing in more or less solid masses about
+them; and owing to the prevailing tendency to compare plant diseases
+with those of animals, we find such expressions as _Gangrene_, _Ulcer_,
+and so forth, applied to these "open sores." In so far as such
+outflowings frequently indicate diseased states of injured tissues which
+are incapable of healing up, the analogy is perhaps a true one; but it
+must be remembered that very different structures and processes in
+detail are concerned. Moreover, liquid excretions more or less
+indicative of diseased states are by no means confined to wounds or
+definitely injured tissues, in which case such terms are wholly
+misapplied.
+
+_Honey-dew._--The leaves, or other organs, of many plants are sticky in
+hot weather, owing to the excretion of a sweet liquid containing sugar,
+the consistency and colour of which vary according to circumstances.
+This honey-dew must not be confounded with the normal viscidity of
+certain insectivorous plants--_e.g._ Sundew--or with the sticky
+secretion on the internodes of species of _Lychnis_, etc., where it
+plays the part of a protection against minute creeping things.
+
+Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of
+these cases the honey-dew is excreted by Aphides, which suck the juices
+of the leaves and pour out the saccharine liquid from their bodies. The
+sweet fluid is in its turn sought after by ants, and also serves as
+nutritive material for various epiphytic fungi--_e.g._ sooty mould,
+_Capnodium_, _Fumago_, and _Antennaria_--which give the leaves and
+honey-dew a brown or black colour. Certain _Coccideae_ also excrete
+honey-dew, especially in the tropics.
+
+At least one case is known where honey-dew is formed as the result of
+the parasitic action of a fungus, namely _Claviceps purpurea_ in its
+conidial stage on the stigmas of cereals, and this may be compared with
+the sweet odorous fluid excreted by the spermogonia of certain
+_Aecidia_. In both cases the sweet fluid attracts insects which
+disperse the spores.
+
+Honey-dew may also be formed without the agency of fungi or insects,
+when hot and dry days are followed by cool nights, with a saturated
+atmosphere, _e.g._ _Caesalpinia_, _Calliandra_ and other trees in the
+tropics, which are called rain trees owing to the numerous drops of
+fluid which drip from the leaves under the abnormally turgescent
+conditions referred to.
+
+_Cuckoo-spit._--The leaves of Willows, Meadow grasses and herbs, etc.,
+are often seen with froth on them, in which is a green insect,
+_Aphrophora_, which sucks the juices from the tissues and excretes the
+frothy watery cuckoo-spit from its body.
+
+_Slime-flux._--The trunks of trees may sometimes be observed to pour out
+a slimy fluid from cracks in the bark, or from old wounds, or branch
+scars. In some cases, _e.g._ in Oaks, the slime has a beery odour and
+white colour, and abounds in yeasts and other fungi to the fermentative
+activity of which the odour and frothiness are due. In other cases the
+slime is red _e.g._--Hornbeam; or brown--_e.g._ Apple and Elm; or
+black--_e.g._ Beech, the colour in such cases being due to the mixture
+of yeasts, bacteria, and fungi with which these slimes abound. The
+phenomenon appears to be due to the exudation of large quantities of sap
+under pressure--root pressure--and is primarily a normal phenomenon
+comparable to the bleeding of cut trees in spring: the fungi, etc., are
+doubtless saprophytes, but their activity is concerned with the
+putrefactive processes going on in the diseased wood, and which may lead
+to rotting of the timber.
+
+The origin of the wounds in the bark and cortex, and which extend into
+the wood and other tissues as the putrefactive and fermentative
+processes increase, appears to be in some cases at least due to
+lightning.
+
+_Resin-flux_ or _Resinosis_.--The stems of Pines and other conifers are
+apt to exude resin from any cut or wound made by insects, or by the
+gnawing of other animals; but in many cases the flow is due to fungi,
+_e.g._ _Peridermium_, the hyphae of which invade the medullary rays and
+resin canals and thus open the way to an outflow through cracks in the
+bark. _Agaricus melleus_ not only invades the resin passages, but
+stimulates the tree to produce abnormal quantities of resin, which flows
+down to the collar and roots, and exudes in great abundance at the
+surface of the soil. Various other plants also exude resin from wounds,
+and in some cases the flux seems to be increased by degeneration of the
+tissues, _e.g._ _Copaifera_.
+
+_Gummosis._--Cherries, Apricots, Acacias, and many other trees are apt
+to produce abnormal quantities of gum, which flows from any wound or
+exudes through cracks in the bark. Degeneration of the wood-cells, and
+especially of the cell-walls of a soft wood formed by abnormal activity
+of the cambium, points to its origin being due, in some cases at any
+rate, to a conversion of the cellulose, and fungi are sometimes found in
+the masses of gum; but beyond the fact that _gummosis_ is a pathological
+phenomenon we know very little of the disease.
+
+With regard to such gumming, it is significant how frequently pruned
+trees--Cherries, Oranges, Lemons, Plums, etc.--suffer.
+
+_Manna flux._--Certain trees, such as the Manna Ash, species of
+Tamarisk, etc., yield manna from wounds, and in some cases the latter
+are due to insects, _e.g._ _Cicada_.
+
+The Potato-disease is best known by the pale whitish fringe, giving an
+almost mealy appearance to the margins of the brown to black patches in
+damp weather. In dry weather the brown patches shrivel and dry, and as
+they are apt to be at the edges and tips of the leaflets, these curl up.
+The young disease spots are yellowish, and the leaves of badly affected
+plants are apt to be sickly yellow throughout.
+
+This Potato-disease due to _Phytophthora_ must be distinguished from the
+curling and puckering, with wilting and browning of the leaves and
+yellow glassy look of the stems, due to the invasion of the vessels by a
+fungus which lurks in the tubers, and gains access thence to the shoots.
+
+In the disease traceable to _Phytophthora_ the stock remains green and
+the leaves plump and plane, and only the brown patches slough out in wet
+or shrivel in dry weather, and are bordered by the pale whitish zone of
+conidiophores.
+
+In the leaf-curl the yellow and flaccid appearance of all the leaves of
+a stalk, or even of the plant, is the striking symptom, and the stem
+soon droops and blackens just above the soil, a white mould appearing
+also at the black spots. Subsequently black spots appear higher up, and
+bacteria gain an entrance. The stolons rot, and eventually the roots and
+the leaves wither. The tubers appear sound, but are small; they are apt
+to rot in the store, the vascular zones turning brown.
+
+This leaf-curl has been ascribed to _Pleospora_, _Polydesmus_,
+_Verticillium_, and other parasites, as well as to excessive manuring
+and other agencies, but it still needs explanation.
+
+Rot of Potato tubers in the soil, or in store, may be brought about by
+very different agents.
+
+If _Phytophthora_ has obtained access, the fungus hyphae spread between
+the cells, starting from the haulm, and cause the flesh to turn
+yellowish and then brown in patches. On the exterior are discoloured
+patches, depressed, with the flesh beneath brown and soft. The mycelium
+spreads mostly in the outer layers, which though they turn deep brown
+remain firm.
+
+Wet rot of potatoes may be due to various fungi, and, in excess of
+water, to putrefactive bacteria (_e.g._ _Clostridium_), which destroy
+the cell-walls. The flesh becomes soft, then soup-like, and finally
+putrefies to a liquid mass with a vile smell of butyric acid, etc., in
+which the starch grains may be seen floating.
+
+Tubers are often found with the cork burst and peeling in shreds, the
+flesh more or less converted into a putrid and stinking pulp, with a
+spotted brown boundary of partly destroyed but firmer tissue between the
+dark utterly rotten and the white and still firm healthy flesh. The
+principal agent in the destruction of the tissues is _Clostridium_, an
+anaerobic bacillus which consumes the cell-walls but leaves the starch
+intact. Hence a thoroughly decomposed tuber consists of a cork bag full
+of starch and foetid liquid. In the dried condition the flesh shows a
+brown marbling; this passes into a soft soupy starchy part, and here and
+there may be violet grey cavities lined with _Spicaria_, _Hypomyces_,
+etc., the white stromata of the latter often appearing externally. The
+excavations are filled with loose starch grains, and bounded by cork and
+cambium formed in the peripheral cells. The cell-walls eventually
+undergo slimy decomposition.
+
+_Spicaria_, _Fusisporium_, various moulds, and bacteria may all be
+associated with wet-rot.
+
+Dry-rot of Potatoes is also due to various fungi and bacteria, but the
+destructive action goes on slowly, owing to there being no more moisture
+than the tissues afford. The flesh becomes excavated here and there,
+owing to the slow destruction of the cell-walls by _Clostridium_: the
+destroyed tissues are brown, and the uninjured starch grains powder them
+all over. Finally the whole shrunken mass has a crumbly consistency.
+
+When the flesh remains white, but assumes a powdery consistency and
+dry-rot, with the cork destroyed here and there, Frank refers the
+damage to _Phellomyces_. Where the dry-rot is due to _Fusarium_ the
+chalk-white stromata may often be detected breaking through the
+periderm; but it must be remembered that the soil-contaminated, broken
+skin of a potato-tuber is a favourable lurking spot for many fungi, and
+_Periola_, _Acrostalagmus_, and others have been detected therein.
+
+Brown spots, depressed into the flesh, sometimes result from the ravages
+of _Tylenchus_, the minute worms being found in the diseased tissues.
+
+In some cases the flesh turns watery and soft, grey, almost glass-like,
+starting at the haulm end, and this may be owing to the invasion of
+_Rhizoctonia_.
+
+
+NOTES TO CHAPTER XXV.
+
+     The rotting of bulbs, roots, etc., has been much discussed
+     during the last few years in the pages of the _Gardeners'
+     Chronicle_, _Zeitschrift für Pflanzenkh._, and elsewhere. The
+     principal references to Bacteriosis--the rot in which bacteria
+     are stated to be the primary agent causing these and similar
+     diseases--may be found in Massee, _Diseases of Plants_, pp.
+     338-342, and more fully in Russell, _Bacteria in their
+     Relation to Vegetable Tissue_, Baltimore, 1892; and in Migula,
+     _Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche
+     Angeblich durch Bakterien verursacht werden_, Semarang, 1892.
+
+     The most convincing accounts, however, are since that date;
+     see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B.
+     III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of
+     Carnations_, Perdue University Agr. Expt. Station, 1896, Vol.
+     VII., p. 17. Woods has lately shown that this disease is due
+     to Aphides only, the bacteria having nothing to do with the
+     disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr.,
+     1900; but it is necessary to bear in mind that actual
+     penetration of the cell-walls from without must be proved, as
+     De Bary proved it for germ-tubes of fungi, before the evidence
+     that Bacteria are truly parasitic in living plants can be
+     called decisive. This is a difficult matter, but until it is
+     settled we do not know whether these organisms are really
+     parasitic in the sense that _Phytophthora_ is, or merely gain
+     access by other means--I have traced them through dead
+     fungus-hyphae--to the vessels, dead cell-walls, etc. The proof
+     of infection _via_ water pores and vessels is given for one
+     species by Harding, "Die Schwarze Faulnis der Kohls," etc.,
+     _Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with
+     literature.
+
+     Concerning the "Damping off" of seedlings, see Marshall Ward,
+     "Observations on the Genus Pythium," _Quart. Journ. Microsc.
+     Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of
+     Cornell University Agric. Expt. Station_, 1895, p. 233.
+
+     On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901,
+     Vol. LXVII., p. 442.
+
+
+
+
+CHAPTER XXVI.
+
+NECROTIC DISEASES.
+
+     _Patches--Frost-patches--Bruising due to hail, shot, etc.--
+     Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost--
+     Fungi--Wound fungi--Defoliation by insects--Defoliation by
+     hand--Staghead._
+
+
+_Necrosis._--This is a general term for cases where the tissues
+gradually turn brown or black in patches which die and dry up, the dead
+area sometimes spreading slowly and invading the usually sharply
+demarcated healthy tissues around. It is a common phenomenon on the more
+slender stems or branches of trees, especially those with a thin cortex,
+and the terms _Brand_ or _Scorching_ sometimes applied signify the
+recognised resemblance between burnt patches and these dead areas of
+necrotic tissue.
+
+Necrosis is often due to frost, which kills the cortex of Pears, Beech,
+etc., in patches of this kind. The dead cortex and cambium stick to the
+wood beneath and contract as they dry. The living cambium and cortex
+around them then begin to push in callus towards the centre of the
+necrotic area; but since this callus is formed under the pressure of the
+cortical tissues it does not form a thick lip or margin to the healing
+wound, as it does in a Canker, but insinuates itself with thinned-off
+edges between the wood and the dead tissue, or at most traps a little of
+the latter in the final closing up of the wound. It is easy to see how
+such an area of Necrosis may become a Canker if the dead tissues split
+or slough off, and fungi or insects obtain access to the callus at the
+margins of the area, setting up the disturbances described on p. 222. As
+matter of fact many Cankers--_e.g._ those of the Larch disease, and
+those due to _Nectria_, or Aphides, etc.--often begin as flattened or
+depressed areas of Necrosis started by frost, and many small necrotic
+patches would eventually become Cankers if not healed up by the callus.
+
+Necrosis may also be due to the bruising of the tissues by large
+hailstones, to gun-shot wounds, or to any form of contusion which kills
+the living cells of cortex and cambium.
+
+Necrosis is a natural and common result of fire, and it frequently
+happens after forest-fires which have run rapidly through the dry
+underwood, fanned by steady winds, that the lower parts of the boles are
+scorched on one side only. The killed cambium and cortex then dry up in
+black necrotic patches, which may eventually heal up by intrusion of
+callus from the uninjured parts.
+
+_Sun-burn_ or _Scorching_.--If thin-barked trees, such as Hornbeam,
+Beech, Firs, etc., which have been growing in partial shade owing to
+dense planting, are suddenly isolated by thinning, the impingement of
+the sun's rays on the south-west side during the hottest part of summer
+days may kill the cambium, and produce necrosis of the cortical tissues,
+and such necrotic patches heal very slowly or not at all, because the
+dead tissues have contracted so tightly on to the wood below that the
+callus cannot readily creep between.
+
+_Sun-cracks_ are due to intense insolation on the south side of trees in
+clear weather in early spring, causing the drying and contraction of the
+wood and its coverings down that side of the tree: the contracted
+tissues consequently split, as in the case of frost-cracks, the healing
+up of which is very similar.
+
+_Dying-back._--All that is true of the necrosis of cortical tissues in
+small patches also applies to cases where the whole of the outer tissues
+of thin twigs and branches die of inanition owing to a premature fall of
+leaves--_e.g._ after a severe attack of some insect or fungus pest. The
+consequent arrest of the transpiration current and the proper supply of
+nutriment to the cambium and cortex explain the phenomena. The younger
+branches of Coffee trees suffering from severe attacks of leaf-disease
+are often denuded of leaves and die back from the causes mentioned, the
+whole of the outer tissues becoming necrotic, and drying up tight on to
+the wood, because other branches with functionally active leaves on them
+divert the transpiration current, and drought and inanition supervene.
+
+Dying-back is frequently also a direct effect of early frosts, which
+kill the thin twigs before the "wood is ripened," as gardeners say.
+
+Dying-back is also a frequent result of direct frost action on thin
+watery shoots or "unripe wood," and is apt to occur every year in
+certain varieties of Roses, for instance, in particular situations, such
+as "frost-beds," or aspects exposed to cutting winds, and so forth. The
+necrosis which results may affect all the tissues, or only the cortex
+and cambium, and the frequent accompaniment of all kinds of saprophytic
+_Ascomycetes_ and moulds or other fungi is in no way causal to the
+phenomenon.
+
+Dying-back may also be caused by fungi, and not necessarily parasites,
+for cases are often observed where saprophytes only are to be found in
+the necrotic tissues of the cortex, having made their way in through
+minute cracks, lenticels, etc.
+
+A simple case is often seen in Chrysanthemums, Roses, etc., chilled and
+wetted to danger point, but not frozen, during the nights of autumn. The
+lowered resistance of the chilled tissues enables fungi like _Botrytis
+cinerea_ to gain a hold, and the peduncles die-back with all the
+symptoms of Necrosis, the fungus gaining power more and more as its
+mycelium spreads in the dead tissues.
+
+Many other cases are known where wound-fungi, such as _Nectria_,
+_Cucurbitaria_, _Phoma_, etc., in themselves incapable of true
+parasitism, gain a hold on the necrotic tissue of a wounded twig, and
+having laboriously accumulated a vigorous mycelium saprophytically,
+extend into other parts. In many of these cases the dying-back of the
+twigs is expedited owing to the mycelium invading the medullary rays and
+wood vessels, and so obstructing the transpiration current. The much
+more rapid spread of the hyphae up into the parts thus killed
+sufficiently indicates the fundamentally saprophytic character of such
+fungi.
+
+Dying-back in all its forms is a common result of defoliation by
+insects, _e.g._ caterpillars, especially if it occurs when the wood is
+depleted of reserve materials, and thus cannot supply the auxiliary buds
+and enable the twigs to clothe themselves with a new flush of foliage, a
+common danger in Conifers.
+
+Any form of defoliation--_e.g._ excessive plucking of tea and mulberry
+leaves, browsing of animals, etc.--exposes the twigs to the dangers of
+dying-back, the accessory phenomena being similar to those already
+described.
+
+_Stag-head._--Old trees, though vigorous and in full foliage throughout
+the crown generally, frequently lose the power of bearing leaves on
+their topmost branches and twigs, which stand out bare and brown, and
+fancifully resemble the antlers of a stag: hence the forester's name
+"stag-head." This "top-dry" condition is frequently due to the removal
+of litter, or to excessive draining, or to the roots having gradually
+penetrated into unsuitable soil. The consequence is that some dry
+summer the drought causes the breakage of the water columns above, and
+the twigs die back.
+
+Tropical trees may also become _stag-headed_ owing to the attacks of
+_Loranthus_ and other parasites, the portions above the point of
+attachment dying back from inanition.
+
+Cases also occur in the tropics where the _stag-head_ condition is due
+to the persistent roosting of frugiferous bats--"flying foxes"--which
+tear the bark and foliage with their claws, and befoul the twigs
+generally.
+
+
+NOTES TO CHAPTER XXVI.
+
+     The principal literature as regards frost is given in the
+     works of Frank, Sorauer, and Hartig already referred to. An
+     excellent summary will be found in Hartig's _Diseases of
+     Trees_, p. 282, and in Fisher "Forest Protection," Vol. IV. or
+     Schlich's _Manual_, p. 423.
+
+
+
+
+CHAPTER XXVII.
+
+MONSTROSITIES AND MALFORMATIONS.
+
+     _Monstrosities--Teratology--Atrophy of organs--Shanking of
+     grapes--Barren fruit trees--Dwarfing--Distortions and
+     malformations--Fasciations--Flattened roots--Torsions--Curling
+     and puckering--Leaf rolling--So-called "spontaneous"
+     teratological changes._
+
+
+_Monstrosities._--In a wide sense this term is applicable to many cases
+here treated under other headings, and signifies any departure from the
+normal standard of size, form, arrangement, or number of parts, and so
+forth, due to arrest of growth, excessive growth of parts, or of the
+whole organs, etc.
+
+Such _teratological_ conditions are however by no means always
+_pathological_: that is to say, they may be variations which do not
+threaten the existence of the plant. In some cases they are clearly due
+to exuberant nutrition, and although they may occasionally predispose to
+disease, in others they show no evidence of doing so. The whole
+practice of horticulture and agriculture abounds in examples of
+teratological sports or varieties which are transmissible by seeds,
+budding and grafting, and other means--_e.g._ double flowers,
+hypertrophied floral organs (cauliflowers), seedless grapes and oranges,
+crested ferns, etc.; and even when such varieties could not live as such
+in a state of nature, there is evidence to show that many of them
+readily revert to the original seed-bearing or single condition, and
+adapt themselves to the altered environment.
+
+Every part of the plant may exhibit teratological changes, and I shall
+for the most part select cases in illustration which indicate approach
+to pathological states, and group with them cases known to be
+pathological in origin.
+
+_Atrophy_ is a common phenomenon denoting dwindling or reductions in
+size of organs due to insufficient nutrition, or arrest of growth from
+various causes.
+
+Atrophy of leaves is a common result of the attacks of parasitic fungi,
+even when the latter induce local hypertrophy--_i.e._ excessive growth
+of particular parts, _e.g._ _Synchytrium_ on Dandelions and Anemones.
+_Puccinia suaveolens_ causes partial atrophy of the leaves of Thistles,
+_Aecidium Euphorbiae_ of those of _Euphorbia_.
+
+The carpels of Anemone are atrophied in plants attacked by _Aecidium_,
+and the whole flower is suppressed in Cherries infested with _Exoascus
+Cerasi_, while other fungi--_e.g._ _Cystopus_, _Exoasci_, etc.--cause
+atrophy of the seeds, and numerous instances of atrophied grain occur in
+plants infested with Ustilagineae.
+
+Atrophy of the grains of cereals is sometimes due to the direct attack
+of animals, _e.g._ eel-worms (_Tylenchus_) eat out the grains of Corn;
+weevils and other beetles (_Curculio_, _Bruchus_, etc.) similarly devour
+the contents of grain and nuts, the flowers of Peas and Apples, and so
+forth, inducing atrophy of the parts left. Still more striking cases are
+afforded by small insects which bore into the halms of cereals, and
+cause atrophy of the whole ear--_e.g._ _Cephus_ in Wheat and Rye. Barley
+occasionally withers after flowering, the grain atrophying from no known
+cause, terms like _consumption_ given to the disease conveying no
+information.
+
+Atrophy of young fruits is commonly due to the flowers not
+setting--_i.e._ some agent has interfered with the normal transference
+of the pollen to the stigma. This may be due to excessive rain washing
+out the pollen (_e.g._ Vine), to a lack of the necessary insects which
+effect pollination, often seen in greenhouse plants; to the stamens
+being barren--_e.g._ certain varieties of Vine--or to the premature
+destruction of the stigmas by frost, as in Cherries, Pears, etc., or by
+insects, as in Apples, or fungi, _e.g._ the infection of bilberries with
+_Sclerotinia_; or even by poisonous gases, as is sometimes seen in
+Wheat, etc., growing near alkali works. Drought is also a common cause
+of atrophy of young Plums.
+
+_Shanking of Grapes_ is a particular case of atrophy and drooping of the
+immature fruits, due to the supplies being cut off by some agency. It
+may arise from very various causes which bring about disease in the
+leaves or roots, and should always be looked upon as a sign of weakness
+in the Vine, the structure of which is affected, _e.g._ poor wood--or
+the functions interfered with, _e.g._ water supplies deficient owing to
+paucity of roots.
+
+Barren Apple, Pear, Plum, and other flowers are often found to have been
+bored through the petals while in bud, and the whole "heart" of the
+flower eaten out by the grubs of _Anthonomus_, leaving the unopened buds
+brown and dead, as if killed by frost or drought, and often erroneously
+supposed to be so.
+
+The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_,
+the mycelium of which pervades the roots and stock, on which the
+sclerotia may be found. Lucerne is similarly killed in Europe by the
+barren mycelium of _Leptosphaeria_, which may be found as a purple mat
+on the roots.
+
+_Dwarfing_ consists in partial atrophy of all the organs, and is a
+common result of starvation in poor, dry, shallow soils, as may often be
+seen in the case of weeds on walls or in stony places. Dwarfs which are
+thus developed in consequence of perennial drought are not, however,
+necessarily diseased, in the more specific sense of the word; their
+organs are reduced in size proportionally throughout in adaptation to
+the conditions, and simply carry out their functions on a smaller scale.
+
+Dwarfing is frequently a consequence of the lack of food materials, or
+of some particular ingredient in the soil, and in such cases is a
+diseased condition of some danger; similar results may ensue in soils
+containing the necessary chemical elements, but in unavailable forms.
+
+Dwarfing may also be brought about by repeated maiming, nipping off the
+buds, pruning, etc., as in the miniature trees of the Japanese; and the
+case of trees continually browsed down by cattle, or of moor plants
+perennially dwarfed by cutting winds, are further illustrations in the
+same category, as are also those of certain alpine and moraine plants,
+whose only chance of survival depends on their adapting themselves to
+the repeated prunings suffered by every young shoot which rises into the
+cutting winds, since there is no question of lack of food-materials in
+these cases.
+
+The practice of the Japanese is to pinch out the growing tips of the
+shoots wherever they wish to prune back, and it is by the judicious use
+of this heading in, and suitable pot-culture, that the dwarfs are made,
+6-20 inches high at from 30-80 years old.
+
+Dwarfing is often brought about by grafting on a slow-growing stock, and
+this method is employed in practice, as are also heading in, pruning of
+roots, and confinement in pots.
+
+Dwarfing may also be due to poor or shrivelled--partially
+atrophied--seeds or such as have had their endosperms or embryos injured
+by insects or fungi, and although it is possible to nurse such dwarfs
+into normal and vigorous plants with good culture, they do not usually
+recover under natural conditions in competition with more vigorous
+plants.
+
+_Distortions_ or _Malformations_ may be defined as abnormalities in the
+form of organs which concern all, or nearly all the parts, and do not
+refer merely to swellings or excrescences on them or excavations, etc.,
+in them.
+
+_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are
+occasionally expanded into broad ribbon-like structures often studded
+with more than the normal number of buds or leaves, etc., such as would
+be found on the usual cylindrical shoots. Such _fasciations_ are due to
+several buds fusing laterally under compression when young and the whole
+mass growing up in common, or, in a few cases, to the unilateral
+overgrowth of one side of the terminal bud. Fasciations appear to depend
+on excessive nutrition in rich soils. They may spread out above in a
+fan-like manner, exaggerating the abnormality, or they may revert to the
+original form. Some cases are more or less fixed by heredity--_e.g._
+_Celosia_. Fasciated stems are frequently curved like a crozier, owing
+to one edge growing more rapidly than the other.
+
+Cauliflowers are really cultivated monstrosities. Fasciated Dandelions,
+_Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved
+Clovers, spiral Teazels, etc., may all, if grown with care, be kept more
+or less constant in the monstrous state. That is to say, the particular
+kinds of variation here manifested can be maintained in proportion as
+the external conditions controlling the variation are maintained. Such
+conditions are chiefly rich supplies of food-stuffs, plenty of water and
+air, suitable temperature and lighting, etc. Mutilations, favouring the
+development of abnormal buds may also induce fasciations.
+
+_Torsions_ or spiral twistings of stems also frequently arise among
+plants grown in rich soils, and are often combined with
+fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that
+the peculiarity is not only transmissible by seed, but may be more or
+less fixed by appropriate culture.
+
+_Contortions_ of stems are often due to the unequal growth on different
+sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on
+Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow,
+_Cystopus_ on inflorescences of _Capsella_, etc.
+
+_Distortions_ of roots may be brought about in various ways by the
+hindrances afforded by stones.
+
+_Spiral roots_ occur occasionally in pot plants.
+
+_Flattened roots_ usually result from compression between rocks, the
+young root having penetrated into a crevice, and been compelled to adapt
+itself later. The distortions of stems by constricting climbers, wire,
+etc., have been described, and fruits--_e.g._ Gourds--are easily
+distorted by means of string tied round them when young.
+
+Distortions of leaves are very common, and are sometimes
+teratological--_i.e._ due to no known cause--_e.g._ the pitcher-like or
+hood-like _cucullate_ leaves of the Lime, Cabbage, _Pelargonium_, etc.,
+and of fused pairs in _Crassula_. Also coherent, bifurcate, crested,
+displaced and twisted leaves occasionally met with, and in some cases
+fixed by cultivation, may be placed in this category.
+
+_Puckers_ must be distinguished from pustules, since they consist in
+local upraisings of the whole tissue, not swellings--_e.g._ the
+yellowish green pockets on Walnut leaves, due to _Phyllereum_.
+
+Puckered leaves in which the area of mesophyll between the venation is
+increased by rising up in an arched or dome-like manner are sometimes
+brought about by excessive moisture in a confined space.
+
+_Leaf-curl_ is a similar deformation caused by fungi, such as _Exoascus_
+on Peaches.
+
+Wrinkling or puckering of leaves is also a common symptom of the work of
+Aphides--_e.g._ Hops.
+
+Characteristic curling and puckering, with yellow and orange tints, of
+the terminal leaves of Apples, Pears, etc., are due to insects of the
+genera _Aphis_, _Psylla_, etc.
+
+Small red and yellow spots with puckerings and curlings of the young
+leaves of Pears, the spots turning darker later on, are due to
+_Phytoptus_.
+
+_Leaf-rolling._--The leaves of Beeches, Poplars, Limes, and many other
+plants, instead of opening out flat, are often rolled in from the
+margins, or from the apex, by various species of _Phytoptus_,
+_Cecidomyia_, or other insects, which puncture or irritate the
+epidermis in the young stages and so arrest its expansion in proportion
+to the other tissues. According as the lower or upper surface is
+attacked the rolling is from the morphologically upper surface
+downwards, or _vice versa_. Very often the mesophyll is somewhat
+thickened where rolled and _Erineum_-like hairs may be developed--_e.g._
+Lime. Many caterpillars also roll leaves, drawing the margins inward to
+form shelters--_e.g._ _Tortrix viridana_, the Oak leaf-roller. Certain
+beetles--_Rhynchitis_--also roll up several leaves to form a shelter in
+which the eggs are laid.
+
+Webs are formed among the mutilated leaves of Apples by the caterpillars
+of _Hyponomeuta_.
+
+It must be borne in mind that instances can be found of teratological
+change of every organ in the plant--_e.g._ stamens transformed into
+carpels or into petals; anthers partly polliniferous and partly
+ovuliferous; ovules producing pollen in their interior, and so on, being
+simply a few startling examples of what may happen. Such abnormalities
+are frequently regarded as evidence of internal causes of disease, and
+this may be true in given cases; in a number of cases investigated,
+however, it has been shown that external agents of very definite nature
+bring about just such deformations as those sometimes cited as examples
+of teratology due to internal causes, and the question is at least an
+open one whether many other cases will not also fall into this category.
+The study of galls has shown that insects can induce the formation of
+not only very extraordinary outgrowths of tissues and organs already in
+existence, but even of new formations and of tissue elements not found
+elsewhere in the plant or even in its allies; and Solms' investigations
+on _Ustilago Treubii_ show that fungi can do the same, and even compel
+new tissues, which the stimulating effects of the hyphae have driven the
+plant to develop, to take part in raising and distributing the spores of
+the fungus--_i.e._ to assume functions for the benefit of the parasite.
+Molliard has given instances of mites whose irritating presence in
+flowers causes them to undergo teratological deformations, and Peyritsch
+has shown that the presence of mites in flowers induces transformations
+of petals into sepals, stamens into petals. Similarly De Bary, Molliard,
+Magnus, Mangin, and Giard have given numerous cases of the
+transformation of floral organs one into another under the irritating
+action of fungi, of which the transformation of normally unisexual
+(female) flowers into hermaphrodite ones, by the production of stamens
+not otherwise found there, are among the most remarkable.
+
+These and similar examples suffice to awaken doubts as to whether any
+teratological change really arises "spontaneously," especially when we
+learn how slight a mechanical irritation of the growing point may induce
+changes in the flower; _e.g._ Sachs showed that a sunflower head is
+profoundly altered by pricking the centre of the torus, and Molliard got
+double flowers by mechanical irritation.
+
+
+NOTES TO CHAPTER XXVII.
+
+     For the details and classification of the multitude of facts,
+     the student is referred to Masters' _Vegetable Teratology_,
+     Ray Society, 1869, and the pages of the _Gardeners' Chronicle_
+     since that date.
+
+     Concerning torsions, etc., the student should read De Vries,
+     "On Biastrepsis in its Relation to Cultivation," _Ann. of
+     Bot._, Vol. XIII., 1899, p. 395, and "Hybridising of
+     Monstrosities," _Hybrid Conference Report_, _Roy. Hort. Soc._,
+     1900, Vol. XXIV., p. 69.
+
+     The reader will find an excellent account of the abnormalities
+     in flowers due to the action of parasitic insects and fungi in
+     Molliard, "Cécidies Florales," _Ann. des Sc. Nat._, Ser.
+     VIII., Bot., T. 1, 1895, p. 67.
+
+
+
+
+CHAPTER XXVIII.
+
+PROLIFERATIONS.
+
+     _Proliferations--Vivipary--Prolepsis--Lammas shoots--Dormant
+     buds--Epicormic shoots--Adventitious buds--Apospory and
+     apogamy._
+
+
+_Proliferation_ consists in the unexpected and abnormal on-growing or
+budding out of parts--stems, tubers, flowers, fruits, etc.--which in the
+ordinary course of events would have ceased to grow further or to bear
+buds or leaf-tufts directly. Thus we do not expect a Strawberry--the
+swollen floral axis--to bear a tuft of leaves terminally above the
+achenes, but it occasionally does so, and similarly Pears may be found
+with a terminal tuft of leaves, Roses with the centre growing out as a
+shoot, Plantains (_Plantago_) with panicles in place of simple spikes,
+and so on.
+
+We regard such cases as _teratological_, because they are exceptional
+for the particular species, and as _pathological_ because they appear to
+be connected with over-feeding in soils with excessive supplies of
+available food-materials; but it should be noted that conditions quite
+comparable to proliferation are normal in the inflorescences of
+Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of
+proliferations come under the head of injurious actions of fungi,
+insects, and other agents.
+
+_Proliferation_ of tubers is sometimes seen in Potatoes still attached
+to the parent plant in wet weather following a drought. The eyes grow
+out into thin stolons, or forthwith into new tubers sessile on the old
+tuber. Similarly in store we sometimes find the eyes transformed
+directly into new tubers, and cases occur where the growth of the eye is
+directed backwards into the softening tuber, and a small potato is
+formed inside the parent one.
+
+Threading is also occasionally met with in the "sets" when ripened too
+rapidly in hot dry soils.
+
+_Vivipary_ is a particular case of proliferation, in a certain sense,
+where the seeds appear to germinate _in situ_, and we have small plants
+springing from the flowers, reminding us of wheat which has sprouted in
+the shocks in damp weather. In reality, however, the grains are here
+replaced by bulbils which sprout before they separate from the
+inflorescence. In varieties of _Poa_, _Polygonum_, _Allium_, _Gagea_,
+etc., this phenomenon is constant in plants growing in damp situations.
+
+_Prolepsis._--It frequently happens that branches or whole plants are
+suddenly defoliated in summer,--_e.g._ by caterpillars or other
+insects--at a time when considerable stores of reserves had already been
+accumulated during the period of active assimilation. In such cases the
+axillary buds, which would normally have passed into a dormant condition
+over the winter had the leaves lived till the autumn-fall, suddenly
+shoot out into _proleptic_ shoots (also termed Lammas shoots), and
+reclothe the tree with foliage. The wood of the year in which this
+occurs may exhibit a double annual ring, and the vigour of the tree is
+likely to suffer in the following season and no fruit be matured.
+
+Proleptic branches may also be due to the shooting out of accessory
+buds--_i.e._ extra buds found in or near the leaf-axils of many plants,
+such as Willow, Maples, _Cercis_, _Robinia_, _Syringa_, _Aristolochia_,
+etc.--which do not normally come to anything, or do so only if a surplus
+of food materials is provided.
+
+_Dormant buds_, or _preventitious buds_, are such as receive no
+sufficient supply of water and food materials to enable them to open
+with the other buds in ordinary years, for in most trees only the upper
+buds on the branches develop into new shoots. The lower buds do not die,
+however, but merely keep pace with the growth in thickness of the parent
+branch, and may be elongated sufficiently each year to raise the minute
+tips level with the bark, their proper cambium only remaining alive but
+not thickening the bud.
+
+When, by the breaking of the branch above the insertion of the dormant
+bud--or by pruning, defoliation by insects, etc.--the transpiration
+current and supplies of food materials are in any way deflected to the
+minute cambium and growing points of the dormant buds, they are
+stimulated to normal growth, and may grow out as _epicormic shoots_ or
+"shoots from the old wood." In many cases such epicormic shoots are
+stimulated to grow out by suddenly exposing an old tree to more
+favourable conditions of root-action and assimilatory activity, owing to
+the felling of competing trees which previously hemmed it in from light
+and air, and restricted the spread and action of its roots in the soil.
+This is often seen in old Elms, Limes, etc.
+
+It is by such means as the above that substitution branches are obtained
+when a leader is broken or cut away.
+
+_Adventitious buds_ are such as are newly formed from callus or other
+tissues in places not normally provided with buds, as is often seen on
+occluding wounds--_e.g._ stool shoots. They may also be developed on
+roots, a fact utilised in propagating _Bouvardias_, Horse-radish, etc.,
+by means of root-cuttings, and the _suckers_ of Plums and other fruit
+trees are shoots springing from adventitious buds on roots.
+
+Adventitious buds are also common on leaves (_e.g._ _Bryophyllum_,
+Ferns, etc.), and are frequently induced on them by wounds--_e.g._
+_Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and
+pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_),
+and in fact practically any wounded tissue with a store of reserve
+materials may be made to develop them: thus they have been found arising
+from the pith of Sea-kale, and are commonly developed from the cut bulb
+scales of Hyacinths.
+
+_Apospory_ and _Apogamy_ are particular cases of the production of
+vegetative buds on the leaves in place of sporangia in Ferns (Apospory),
+and on prothallia in place of Archegonia (Apogamy), in the latter case
+induced by dry conditions and strong illumination.
+
+
+NOTES TO CHAPTER XXVIII.
+
+     In addition to the literature quoted in the notes to Chapter
+     XXVII., the student should consult the works on Forest Botany
+     for the scattered information regarding adventitious buds. A
+     good account may be found in Büsgen, _Bau und Leben unserer
+     Waldbäume_, Jena, 1897.
+
+     For Apospory and Apogamy, see Lang "On Apogamy and the
+     Development of Sporangia upon Fern Prothalli," _Phil. Trans._,
+     vol. 190, 1898, p. 187, where the literature is collected.
+
+
+
+
+CHAPTER XXIX.
+
+GRAFTS.
+
+     _Grafting--Comparison with cuttings--Effects of environment--
+     Relations between scion and stock--Variation in grafts--
+     Grafting and parasitism--Infection--Pollination--Grafts-hybrids
+     --Predisposition of Natural grafts--Root-fusions._
+
+
+Grafting is a process which consists in bringing the cambium of a shoot
+of one plant into direct union with that of another, and is practised in
+various ways, the commonest of which is as follows:
+
+One plant--the _stock_--rooted in the ground, is cut off a short
+distance above the surface of the soil, and a shoot from the second
+plant--the _scion_--cut off obliquely with a sharp knife, is inserted
+into a cleft in the stock, so that the two cambiums (and sometimes the
+cortex and pith of each as well) are in close contact: the scion is then
+tied in position, the wounds covered with grafting wax, and the whole
+left until union of the tissues is completed. This union depends on the
+formation of _callus_ at the cut surfaces, and the intimate union of the
+ingrowing cells from each callus.
+
+The development of the callus follows the course described for wounds,
+cuttings, etc., and the union is exactly comparable to the union of the
+two lips of a healing callus over a wound (see p. 197).
+
+Grafting was known and practised far back in the ages. Virgil was well
+acquainted with the process, and Theophrastus compared it with
+propagation by cuttings.
+
+The scion differs from a cutting, however, in having no roots of its
+own: it is parasitic upon, or rather is in symbiosis with the stock, the
+root and tissues of which intervene between it and the soil.
+Consequently the selective absorption, size and number of vessels, and
+innumerable other physiological and anatomical peculiarities of the
+stock determine what and how much shall go up into the scion, while the
+latter supplies the former with organic materials and rules what and how
+much food, enzymes, and other secretions, etc., it shall receive to
+build up its substance. Surely, then, if such factors as the nature of
+the soil, the water and mineral supplies, the illumination, and the
+various climatic factors of altitude can cause variations on a plant
+direct, these and other factors are still more likely to be effective on
+stock and scion, and each must affect the other.
+
+Nevertheless opinions have differed much as to whether any important
+effect is to be seen, and on no point more than on whether the scion
+can affect the stock, in spite of such examples as _Cytisus Adami_,
+_Garreya_ on _Aucuba_, Sunflower on Jerusalem Artichoke, etc. Recent
+results, especially of experiments with herbaceous plants, show that not
+only can the stock affect the scion (and _vice versa_) directly, but the
+effect of the changes may be invisible on the grafted plant and only
+show itself in the progeny raised from the seed of the grafted plant. In
+other words, variation occurs in grafts either _directly_, as the
+results of the effects of the environment on the graft, or owing to the
+interaction of scion and stock, showing as changes in general nutrition
+in the tissues concerned, etc., owing to special reactions of the
+protoplasm of the uniting cells one on the other, and of the results of
+the further protoplasmic secretions, sortings, and so forth, on the
+cells developed as descendants of these in the further growth of the
+graft: or _indirectly_, in that some of these changes so alter the
+nature of the special protoplasm put aside for reproductive purposes,
+that the resulting embryo in the seed transmits the effects, and they
+show as variations in the seedling. If these results are confirmed they
+should meet all objections that have been urged against the transmission
+of acquired characters.
+
+In fact there are analogies between grafting and parasitism which cannot
+be overlooked, and should not be underestimated, their commonest
+expression appearing in the alterations in stature, habit, period of
+ripening, and so forth. These analogies are easily apprehended when we
+compare parasites like the Mistletoe, _Loranthus_, or even such
+root-parasites as the Broom-rapes and the Rhinanthoideae with grafts;
+but they also exist in the case of many fungus-parasites, and we might
+almost as accurately speak of _grafting_ some fungi on their hosts as of
+_infecting_ the latter with them, especially when it is borne in mind
+that the effect of the scion on the stock is by no means always to the
+benefit of the latter, and that there are reasons for regarding the
+action of some such unions as that of a sort of slow poisoning of the
+stock by the scion. Why do we not here say that the stock has been
+_infected_ by the scion?
+
+The resemblances between pollination and the infection by fungus hyphae
+may also be insisted upon. If we take into account Darwin's remarkable
+experiments showing that in "illegitimate unions" the pollen exerts a
+sort of poisonous action on the stigmas or ovules, it is possible to
+arrange a series of cases starting with perfectly legitimate
+pollinations where the pollen tube feeds as it descends the style on
+materials provided by the cells, and proceeding to cases where the
+pollen is more and more merely just able to penetrate the ovary and
+reach the ovules, to the extreme cases where no union at all is
+possible.
+
+Side by side with such series could be arranged analogous cases where
+fungus spores can enter and infect the cells of the host, and live
+symbiotically with or even in them, or can penetrate only with
+difficulty, or with poisonous effects, and finally cannot infect the
+plant at all.
+
+Less obviously, but nevertheless existing, are gradations in grafting to
+be observed, where one and the same stock may be successfully combined
+with a scion which improves it--or which is improved by it--or the scion
+may unite but acts injuriously on it, or, finally, cannot be induced to
+unite.
+
+But we may go further than this in these comparisons. Just as the
+results of pollination frequently induce far-reaching effects on distant
+tissues--_e.g._ the swelling of Orchid ovaries, and rapid fading of the
+floral organs--so also the effects of hyphae in the tissues may induce
+hypertrophies, deflection of nutrient materials, and the atrophy of
+distant parts--_e.g._ the curious phenomena observed in _Euphorbia_
+attacked by _Uromyces_--and some of the distant actions in grafts may be
+compared similarly.
+
+Going still further, we may compare the effects of cross-breeding or of
+hybridisation, where the _progeny_ show that changes have resulted from
+the mutual interactions and reactions of the commingled protoplasm, with
+Daniel's results, in which he obtains proof of such interactions of the
+commingled protoplasmic cell-contents of grafts in the seedling progeny;
+although there is no probability--we may even say possibility--in this
+latter case that the effects are due to nuclear fusions, but only that
+the germ-plasm of the seed-bearing plant has been affected by the
+changes in the cell-protoplasm which nourishes it when the reproductive
+cells are forming.
+
+In the case of graft-hybrids the matter appears to be somewhat
+different, and we may well suppose, with Strasburger, that the
+commingling of characters observed in flowers, fruits, foliage, etc., on
+shoots borne after grafting are due to the occurrence of nuclear fusions
+during the union of the grafted tissues; though it is by no means
+impossible that what has really happened is profound alterations in the
+nuclear substance (germ-plasm) owing to its being nourished by
+cell-protoplasm (somato-plasm) which has been itself affected by the
+interchanges of substance between scion and stock, and therefore itself
+furnishes a different nutrient medium from the unaltered cytoplasm of
+either.
+
+But even here we can find parallels among the ordinary phenomena of
+plant reproduction. Maize plants with white endosperm containing starch,
+if crossed by pollen from other plants with purple endosperm containing
+sugar, bear seeds with purple endosperm containing sugar, and such
+_Xenia_ may be compared to graft-hybrids in many respects.
+
+I know of no case among fungus infections which could be compared
+directly with these examples, and it is not at all likely that we shall
+meet with any instance of a fungus-hypha handing over nuclear substance
+to an egg-cell, and so affecting the latter that an embryo results. But
+the case is not hypothetically impossible, although the distant
+relationships of the two groups of organisms render it extremely
+improbable among the higher plants. It is by no means so improbable,
+however, that further research may show cases where the egg-cell of a
+lower cryptogam--_e.g._ another fungus--may be affected either directly,
+or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as
+suggested by the extraordinary phenomena of symbiosis.
+
+Some of the variations in grafted plants are found to predispose the
+plant to disease, or the reverse, and cases may be cited where the
+resulting shoots, foliage, or fruits, or seedlings more readily fall a
+prey to, or resist, parasitic fungi and insects than the ungrafted
+plants. Daniel gives instances of such--_e.g._ among other examples,
+Peas grafted on Beans yield seeds which suffer more from Erysipheae than
+the normal seedlings. But the best known cases are those of Vines in
+their relations to _Phylloxera_, already referred to (p. 155).
+
+Several instances are also known where grafted plants show more or less
+resistance to such factors of the environment as low temperatures;
+grafted or budded Roses often suffer much from Erysipheae, and so forth.
+Much research is still needed to determine how far these matters depend
+on real alterations in the nature of the graft, or _are only true for
+the localities in which the experiments have been made_, a point which
+has, I think, been overlooked by all observers.
+
+Grafted plants are apparently very much exposed to injury by slugs,
+insects, and the invasions of parasites during the healing of the callus
+and the fusion process. Here again it must not be overlooked that the
+callus is, so to speak, a tit-bit of luscious, thin-walled, succulent
+tissue; and, like all wounds, the graft affords entrance to parasites
+such as _Nectria_ and Ascomycetes of various kinds, under circumstances
+very favourable to their invasion.
+
+_Natural Grafts._--It is by no means an uncommon event to find the
+branches of Beeches, Limes, and other trees which have been accidentally
+brought into contact during growth, joined where they cross. As they
+press one against the other, they become naturally grafted, by that form
+of the process known as _inarching_: except that in artificial inarching
+the operator cuts off the cortical tissues of the two branches and
+brings their cambial surfaces together, whereas in nature the cambiums
+only come into contact after the destruction by pressure, or slight
+abrasion, of the entrapped intervening tissues. The fusion occurs, in
+fact, exactly as in the burying-in of a nail or wire, referred to on p.
+211.
+
+Natural grafts are very common among the roots of trees, and possibly
+explain some queer cases of the apparent revivification of stumps of
+trees not usually given to forming abundant stool shoots. It is regarded
+as probable in some old forests that the majority of the roots of trees
+of the same species are linked up together by such natural grafts, a
+probability not diminished by the fact that such roots cross at many
+points, and are easily grafted.
+
+
+NOTES TO CHAPTER XXIX.
+
+     The student should read Bailey, _The Nursery Book_, 1896, for
+     details regarding the practice of grafting, and facts in
+     abundance can be obtained from the pages of the _Gardeners'
+     Chronicle_.
+
+     Concerning graft-hybrids and the variations of grafted plants
+     see Jouin, _Can Hybrids be obtained by Grafting?_ and
+     especially Daniel, "La Variation dans la Greffe," in _Ann. des
+     Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the
+     literature there collected. The whole subject is largely
+     controversial, and much work remains to be done.
+
+
+
+
+CHAPTER XXX.
+
+LIFE AND DEATH.
+
+     _Protoplasm--Hypothesis as to its structure and behaviour--
+     Assimilation--Growth--Respiration--Metabolism--Action of the
+     environment--Nuclear protoplasm--Pollination--Grafting--
+     Parasitism--Graft-hybrids--Life--Death--Variation--Disease._
+
+
+We have seen that all the essential phenomena of disease concern only
+the living substance--the protoplasm--of the plant, and that however
+complex the symptoms of disease may be, the occurrence of
+discolorations, lesions, hypertrophies, and so forth are all secondary
+matters subsidiary to the fundamental alterations of structure and
+function constituting the disease. It remains to see if we can adopt any
+hypothesis as to the nature of this physical basis of life--the
+protoplasm--which shall help us to understand still more clearly in what
+must reside those processes which, so long as they proceed harmoniously
+and uninterruptedly, constitute life and health, and which when
+interfered with result in disease and death. The protoplasm of the
+living plant-cell looks like a slimy translucent mass which has been
+superficially compared in appearance to well-boiled sago or clear gum.
+Fifty years of observations and experiments with it have convinced
+physiologists that it is not a mere solution or emulsion, however, or
+even a chemical compound in the ordinary sense of the term, although
+chemical analysis gets little out of it beyond water, proteids,
+carbohydrates and fats, and traces of certain mineral salts; for living
+protoplasm does not respond to the laws of physics and mechanics in
+obeying them, simply as do ordinary solutions and liquids. On the other
+hand, the most delicate chemical manipulation fails us, because when
+killed it is no longer protoplasm. Nor does the microscope advance
+matters far, beyond convincing us that this marvellous material must
+have a structure far more intimate than anything visible to the highest
+magnifying powers at our disposal.
+
+Nevertheless, some information is forthcoming from the comparative
+examination of the protoplasm of numerous different kinds of organisms,
+for we have learnt that certain ingredients and no others are necessary
+for its composition--namely, carbon, hydrogen, oxygen, nitrogen,
+phosphorus, sulphur, calcium[Note: See note at end of chapter.],
+magnesium, potassium--and it is as a rule of no use trying to foist on
+to it any substitute for any one of these. Moreover, these chemical
+elements must be given in certain definite proportions and forms: for
+instance it is of no use to offer the carbon and sulphur in such a form
+as carbon disulphide, or the nitrogen and hydrogen in that of
+hydrocyanic acid, but the carbon must be given to the protoplasm in the
+form of a carbohydrate or in some similar form, the nitrogen as an
+ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so
+forth, and thus water, air, carbohydrates, and the nitrates, sulphates,
+and phosphates of potassium, calcium, and magnesium become the chief
+natural sources of the essential ingredients. Again, we have learnt that
+while there are different forms of protoplasm in the cell, and that
+these react on each other, and go through cycles of arrangement and
+rearrangements, the intimate structure must be of that kind termed
+molecular--beyond the region of vision, just as is the microscopic
+structure of a crystal; but, while like the latter affording evidence of
+order and sequence when properly examined, the structural arrangements
+and changes must be infinitely more complex.
+
+All these, and numerous other results of enquiry, have led to the
+conclusions that we must regard living protoplasm as a complex made up
+of very large molecular units, each containing atom-groupings of the
+elements named; and, partly on account of the large number of atoms they
+contain, and partly due to the vibrations of absorbed heat, these units
+must be extremely labile. Moreover, they are linked up into an
+invisible and intricate meshwork, bathed in a watery liquid held in the
+interstices somewhat as water is held in a sponge. In this imbibed
+liquid are dissolved the substances, consisting of the same elements,
+which are to serve as food, and which are to be taken up into the
+molecular framework and built up into the structure of new molecular
+units--or, as they may be shortly termed, molecules of protoplasm: in
+the bathing liquid are also dispersed the fragments--again containing
+the elements named--which have resulted from the breaking asunder of
+some of the complex protoplasm molecules, and which are partly destined
+to be used up again, partly to be burnt off in respiration, and partly
+to be put aside as metabolic products such as reserves, secretions,
+permanent structure, etc. Among the elements carried into this liquid
+and dissolved in it the free oxygen of the air also plays an important
+part.
+
+As new molecules are formed, by mutual combinations of the
+food-materials selected by molecular attractions, they are taken up into
+the protoplasmic framework, and built in between those already in
+existence, thus distending the whole, and we say that the protoplasm
+_Assimilates_ food-materials and _Grows_. When distended beyond a given
+degree, or disturbed in various other ways, the molecular framework
+breaks, and some of the molecules are shattered, and as they fall to
+pieces certain of their constituent parts containing carbon and hydrogen
+forcibly combine at the moment of liberation with the oxygen in the
+fluid around and are burnt off in the form of carbon-dioxide and water,
+heat being of course evolved. This is the fundamental process of
+_Respiration_.
+
+It is probably the alternation of these processes of _Assimilation_--the
+building up into the protoplasmic structure of new complex labile
+molecules--and _Destruction_--the shattering of such molecules
+with redistribution, oxidation, etc., of their fragments--which
+constitute the fundamental process of life. Different authorities
+attempt to explain the details of these processes in various ways,
+but there is practical agreement on the one point, that life
+consists in the alternate building up of new protoplasm from the
+food-materials--_Assimilation_--and the breaking down of the molecular
+complexes to simpler ones--_Disintegration_, or _Dis-assimilation_, as
+we may call it. During the periods when assimilation prevails, and the
+protoplasm increases in mass, we recognise _Growth_, and since this is
+usually associated with the vigorous imbibition of water, owing to the
+powerful osmotic attractions for that liquid exhibited by some of the
+products, and with consequent further stretching of the invisible
+molecular plexus, the growth may be so evident in increased size, that
+we are accustomed to look upon the visible increase in volume alone as
+growth; but it is essential to understand that growth of the protoplasm
+is always proceeding during life, even when as many older molecules are
+being shattered and dispersed as new ones are being formed by
+assimilation, and when, therefore, no visible permanent enlargement
+occurs. Similarly, during periods when disintegration of the molecules
+prevails, we must not assume that the assimilation of new molecules is
+not occurring and that growth is not proceeding. The two processes are
+always going on during the active life of the protoplasm: in fact life
+consists in the play of these processes, as already said.
+
+That numerous chemical rearrangements of the atom-complexes take place
+outside the protoplasmic molecules--both of those left unemployed in
+assimilation and of those rejected during the destructive
+processes--will be readily understood: many of the bye-products found in
+plants, such as vegetable acids, alkaloids, colouring matters,
+crystalline bodies, etc., etc., are due to these, so to speak,
+fortuitous combinations and re-combinations.
+
+The part played by respiration has often been misunderstood. It consists
+in the burning off of some of the carbon and hydrogen of the shattered
+protoplasm molecules, by means of the oxygen of the air, which finds its
+way into the fluids around the protoplasm, and when it is active every
+act of combustion--which is here an explosion--leads to the shattering
+of more protoplasm molecules, and consequently to more respiratory
+combustion of the products. If the supply of oxygen is limited the
+breaking down of the molecules of protoplasm does not cease, but the
+carbon and hydrogen which would otherwise have been oxidised are now in
+part left to form other compounds in the surrounding liquid, and thus
+incompletely oxidised bodies, such as vegetable acids, alcohols, etc.,
+accumulate. Even in the complete absence of atmospheric oxygen the
+protoplasm may go on breaking down and accumulating various compounds
+containing relatively much carbon and hydrogen--so-called intramolecular
+respiration; but in ordinary plants this process soon comes to an end,
+because the blocking up of the molecular plexus leads to obstruction and
+interferes with the normal assimilation and dis-assimilation, and, if
+prolonged, leads to pathological conditions, and eventually death.
+
+Here, then, we meet with a cause of disease, or of predisposition to
+disease. The deprivation of oxygen interferes with the normal processes
+of building up and breaking down of the protoplasmic molecules, and
+bodies we term poisonous accumulate and may lower the vitality or even
+bring life to an end.
+
+During normal life other products of the disruption of the protoplasm
+molecules are nitrogenous bodies, such as proteids, and these we have
+reason to believe are used up again, acting as the nuclei, so to speak,
+of the new molecules, and so being built up again with fresh
+food-materials into the plexus, to be again set free, and again used up,
+and so on. Others are the carbohydrates, such as cellulose, which pass
+out of the molecule into an insoluble form, and are accumulated outside
+the protoplasm in the form of cellulose membranes, and so forth. It is
+these formed products of metabolism (Metabolites), especially cellulose
+and bodies which result from its subsequent transformation, which
+constitute the main permanent mass of the ordinary plant.
+
+We are now in a position to see how another fundamental cause of disease
+or predisposition to disease exists in the deprivation of the protoplasm
+of any of the elements needed to supply--in the food-materials--the
+place of those which have been permanently put aside in the form of
+cell-walls, or burnt off in respiration, passed out as excretions, or in
+other ways lost.
+
+It is clear that the indispensability of an element must mean that the
+protoplasmic molecule cannot be completed without it: the same
+conclusion is supported by the experimental proof that these elements
+cannot be replaced by chemically similar elements.
+
+It does not follow, however, that the protoplasm molecule must always
+have the same number of atoms of these elements, and grouped always in
+the same atom-complexes before being assimilated; nor that the
+protoplasm molecule, when once built up, always breaks down in exactly
+the same way. On the contrary, while the protoplasm of corresponding
+parts of a daisy and of a rose must contain all the elements named, we
+must believe that the atom groupings are different in the protoplasm
+molecule in each case; and though the molecules of the cell-protoplasm,
+of the nucleus, of the chlorophyll-corpuscles, etc., of one and the
+same plant must have all these elements, the atom groupings and modes of
+building up and breaking down may be very different in each case.
+
+Again, the cell-protoplasm, bathed by the sap taken in by roots from the
+soil or fed directly by that derived from the leaves, must be exposed to
+very different stimuli and modes of nourishment, etc., from those
+incurred by the protoplasm of the nucleus which it encloses: and similar
+conclusions must apply in turn to the protoplasm of the root in the dark
+moist soil and of the leaf in the light dry air, or to that of the
+superficial epidermis cells as contrasted with that of the deeply
+immersed pith, and so on.
+
+It is no doubt in these directions that we must seek for the explanation
+of many life-phenomena at present quite beyond explanation. Thus, it is
+tolerably easy to modify the action of the cell-protoplasm of a plant,
+by exposing it to differences of illumination, temperature, moisture,
+and so forth, within certain limits; at least, since the changes in
+stature, tissue differentiation, cell-secretions, flowering capacity,
+etc., of plants affected by such factors of the environment--_e.g._
+alpine plants brought into the plains--_must_ be due to changes in the
+mode of activity of the protoplasm, we must assume that the above
+factors affect the latter. But it is extremely difficult to reach the
+nuclear-protoplasm directly by such stimuli, as proved by the experience
+that even where we allow the factors to act for a long time, no
+permanent change can be detected in the behaviour of the
+nuclear-protoplasm--the essential material in the reproductive organs
+and reproductive process. At least we must infer that no change has been
+permanently stamped on this nucleo-plasm from such facts as the
+characters of the seedlings of the progeny of the plain-raised plants:
+if they are again sown in an alpine situation they forthwith behave
+again as alpines.
+
+Must we not conclude, then, that this difficulty of reaching the
+nuclear-protoplasm is owing to the fact that it is nourished and
+influenced directly only by the cell-protoplasm? That the
+cell-protoplasm is its environment, and not so directly the outer world?
+We may influence the cell-protoplasm--we may make it work harder or less
+actively, respire vigorously or slowly, build up and break down in
+various different ways, or at different rates, and so forth, _within
+limits_; but it is nevertheless cell-protoplasm of its specific kind,
+with its own range of molecular variations and activities within these
+limits, and it supplies the nuclear-protoplasm with what it wants so
+long as these limits are not exceeded. Consequently, while it is very
+easy to make the cell-protoplasm vary within the limits of its range, it
+is not easy to induce it to vary its effects on the nuclear-protoplasm
+to such an extent or in such a way that the latter is permanently or
+materially altered in constitution.
+
+Nevertheless it would appear that cases do occur where the
+nuclear-protoplasm _is_ reached and affected by external stimuli, as
+evinced by some of the phenomena of hybridisation and of cross-and
+self-fertilisation, because we find the results expressed in the
+mingling of the characters of parents, in strengthened or enfeebled
+progeny, and even in the appearance of unexpected properties, which,
+from the facts of Reproduction, we know must have taken their origin in
+some alteration of the nuclear substance of the embryo.
+
+Here, however, we know in most cases that the principal agent
+which has reached the nuclear-protoplasm, is another portion of
+nuclear-protoplasm. In hybridisation, one which has been fed and
+influenced by cell-protoplasm of a very different plant; in
+cross-fertilisation, one fed and influenced by the cell-protoplasm of a
+different plant of the same species, and in self-fertilisation, one fed
+and influenced by the same cell-protoplasm.
+
+That somewhere, and somehow, such nuclear-protoplasm as induces the
+changes in the characters of hybrids, etc., has been influenced by its
+immediate environment--the cell-protoplasm of the plant--appears to be a
+conclusion from which there is no escape. We may obtain similar evidence
+from the experience of grafting. It is relatively easy to influence the
+cell-protoplasm of a scion by a suitable stock, obviously because the
+latter, while handing on to the former all necessary materials from the
+soil, presents the indispensable elements and compounds in somewhat
+different proportions, dilutions, etc., from those which its own roots
+would have done, and probably mingles with them a certain amount of its
+own peculiar products, as well as affects the modes of working and
+interaction of both by the molecular impetus impressed on them.
+Consequently the cell-protoplasm of the scion, while obtaining from the
+stock all it needs within the limits of its own variations of structure
+and activity, nevertheless builds up and breaks down in ways or at rates
+slightly different from those hitherto normal to it, and perceptible
+variations result when the sequences and correlations of these material
+and mechanical changes have affected a sufficiently large mass for the
+accumulation of visible effects. The limits to grafting suggest not that
+an inappropriate stock does not offer to the protoplasm of the scion the
+right materials, but that it presents them in proportions and in forms
+which are unsuitable for the assimilable powers of the latter, or,
+possibly, mingled with substances poisonous in themselves or capable of
+becoming so in conjunction with bodies in the scion.
+
+What has been said of the action of stock on scion, will also be true,
+_mutatis mutandis_, of the reciprocal action of scion on stock. Here
+again we may have causes for disease, or predisposition to disease.
+
+It occasionally happens, however, that the nuclear protoplasm
+of the stock or scion _is_ affected in grafting, and we infer
+from the difficulty of modifying it in any other way in ordinary
+reproduction than by means of other nuclear protoplasm--_e.g._ in
+hybridisation--that in such cases a fusion of the nuclei of stock and
+scion has occurred during the grafting, and a graft-hybrid has
+resulted--_e.g._ _Cytisus Adami_.
+
+It is not impossible however that the nuclear protoplasm has in such
+graft-hybrids been subsequently modified by the differences in nutrition
+to which it has been subjected, in the modified cell-protoplasm affected
+by the mingling of the juices, etc., of scion and stock; for it is quite
+conceivable that such materials may affect the protoplasm far more
+profoundly than anything derived directly from the environment.
+
+If Daniel's researches are confirmed, however, it appears that in some
+cases, at any rate, the nuclear-protoplasm is so altered by the grafting
+that when the new embryo is developed, after fusion with nuclear
+substance from another plant of the same species, the results are
+apparent only in the progeny, and _the effects of alteration in the
+cell-protoplasm have been transmitted to the nuclear protoplasm of the
+germ-cells_--_i.e._ acquired characters have been transmitted and fixed
+by heredity. Should this prove true the importance of the results can
+hardly be over-estimated. The matter is too problematical for further
+discussion here, but we see that any such action may profoundly affect
+the "constitution" of the resulting plant.
+
+Turning now to the case of fungi or other organisms which obtain access
+to the cell-protoplasm. At the one extreme we have cases where the
+protoplasm of the diseased plant is rapidly and directly poisoned and
+destroyed, as in the killing off of seedlings in "Damping Off": near the
+other extreme we have cases where the foreign protoplasm of the
+parasite, although it gains complete access to that of the host, merely
+stimulates the latter to greater activity and itself works for its own
+ends in conjunction with it--_e.g._ _Plasmodiophora_. In such instances
+we must figure to ourselves the cells of the root of the Crucifer
+handing on food-materials to both masses of protoplasm--that of the
+_Plasmodiophora_ and that of the cell into which it penetrates; and it
+is immaterial whether both obtain the food-materials directly, or, what
+seems more likely, the fungus only at second hand and by the medium of
+the host's protoplasm. In any case, the latter is for a long time at
+least not poisoned or maimed, or in any perceptible way injured by
+excreta from the fungus-protoplasm, although it is evident that each
+must excrete various metabolites which may soak into and be taken up by
+the other: on the contrary the host-protoplasm grows larger, attracts
+more food supplies, makes larger cells, and is evidently stimulated to
+greater activity for the time being, its behaviour reminding us of the
+stimulation of cells by means of slight doses of poison referred to
+previously. We must therefore assume that the general course of building
+up and breaking down of its protoplasm-molecules go on as usual--or
+nearly so--in both the host cell and the invader; and that the
+assimilatory, respiratory, excretory and other functions are carried on
+in the former as in the normal cell, or are but slightly modified to an
+extent which does no immediate injury to its life. But we must further
+assume that the same is also true of the invading protoplasm, and that
+the _Plasmodiophora_ is also supplied with suitable atom-complexes to
+build up its protoplasm molecules, as fast as they are shattered and the
+rejecta burnt off in respiration.
+
+A step further, and we come to instances of _Symbiosis_, where the
+commingled masses of protoplasm of host and invader continue this
+harmonious action during life. Clearly there are resemblances between
+these latter cases and successful grafts, and between both and
+successful sexual unions where the resulting embryo-cell gives rises to
+a vigorous and healthy plant; and the more these resemblances are
+examined in the light of what we know of symbiosis the more they support
+our contention.
+
+Such considerations as the foregoing suggest, then, that life consists
+in the regular and progressive building up and breaking down of the
+complex protoplasm molecules, and is necessarily accompanied by the
+influx of the indispensable food-elements in certain combinations and
+atom-complexes for assimilation, and by the combustion of some of the
+débris of the shattered molecules, which combine with the oxygen in
+respiration and so afford explosions which raise the temperature and
+enhance the lability of existing molecules, and act as stimuli to the
+shattering of further molecules. The results of these rhythmical
+buildings up (assimilation) and shatterings (dis-assimilation) of the
+protoplasm molecules are the growth of the protoplasm, with further
+intercalations of water and new food-supplies, etc., on the one hand,
+and the formation of metabolic products (proteids, cellulose, sugars,
+fats, etc.), some of which are again used up, others respired, others
+deposited as stores, cell-walls, etc., on the other.
+
+That the building-up process depends on the action of molecular forces
+comparable to those by which a growing crystal goes on selecting
+atom-complexes of its particular kind from the solution around seems
+highly probable, and this being the case we can understand how under
+certain circumstances _substitutive_ selections may occur. That is to
+say, just as a crystal will sometimes build up into its structure
+atom-complexes of a kind different from its normal molecules, so, given
+the proper conditions, a protoplasmic molecular unit will build up into
+its structure atom-complexes somewhat different from those it had
+hitherto taken up--_i.e._ assimilated--with consequent modifications of
+its behaviour. If this occurs, the modes of further building up and
+breaking down will be affected by the subsequent action of these
+slightly modified protoplasm units, _and it may well be that the whole
+significance of variation turns on this_. Whether the resulting
+variation makes for the welfare or otherwise of the organism will then
+be decided by the struggle for existence, and the natural selection
+which ensues. Such a view also implies that the energy concerned is
+primarily what is usually termed chemical energy, and that every
+compound entering into the protoplasm carries in a supply of this,
+available in various ways.
+
+_Death_, on the contrary, is the cessation of these rhythmical processes
+of building up and breaking down of the protoplasm molecules. It does
+not imply the cessation of chemical changes of other kinds, but that
+these rhythmical constructions of the complex and labile protoplasm
+molecules breaking down on stimulation to bodies partly re-assimilable,
+partly combustible in respiration, and partly excretory, etc., have
+ceased, and that further chemical changes in the material are
+thenceforth simpler and different in kind and degree, eventually leading
+to total disintegration so that no units are left capable of restoring
+the rhythm.
+
+If these ideas are correct, we may define _Disease_ as dangerous
+disturbances in the regularity, or interference with the completeness or
+range of the molecular activities constituting normal Life--_i.e._
+Health--and it is evident that every degree of transition may be
+realised between the two extremes. Now, if we further assume, as I think
+we must do, that a considerable range or "play" must exist in the
+molecular activities of the protoplasm constituting life, we obtain a
+sort of expression of what we mean by limits of variation. The fact that
+life can go on in a given plant at temperatures between from 1°-5° and
+35°-40° C., or in lights of different intensity, or within considerable
+ranges of water supply, concentration of salts, partial pressure of
+oxygen, etc., implies that the molecular activities of the protoplasm
+are of the normal _kind_ all the time, though they may differ in
+rapidity, and even in _quantitative_ and _qualitative_ respects within
+certain limits; and the meaning of the _optimum_ temperature,
+illumination, oxygen pressure, etc., is, from this point of view, not
+that the molecular activities differ in kind from those nearer the
+minima and maxima, so much as that they are running at the best rates
+for the welfare of the plant--_i.e._ for permanent health.
+
+If we transcend the cardinal points limiting the range of this play,
+however, and we get variations in the _kind_ as well as _rates_ of
+molecular constructions and disruptions, then we pass by imperceptible
+gradations into ill-health--_i.e._ _Disease_.
+
+And similarly in relation to other protoplasm. That of the right kind of
+pollen grain from another plant of its own species, stimulates the
+contents of the ovule to produce a vigorous embryo and healthy seedling:
+that of a similar pollen grain in its own flower either does no positive
+harm, but has a feebler effect, or it may act like a poison. That of
+another pollen grain again may refuse to unite at all; while that of a
+fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the
+style as does the pollen tube and produce death and destruction
+throughout the ovule.
+
+Or again, in Clover, we may have the hypha of a _Botrytis_ with its
+protoplasm unable to do more than penetrate into the cellulose walls
+and diffuse a poison into the adjacent cells, being utterly incapable of
+directly facing, or mingling with the living protoplasm of such cells,
+whereas the protoplasm of another organism--_e.g._ _Rhizobium_--will
+penetrate directly into the cells, live in them for weeks or months
+without injury--nay even with advantage to their life. And hundreds of
+similar cases can be selected.
+
+We may, therefore, conclude that _Variation_ depends fundamentally on
+alterations in the structure or mode of building up and disintegration
+of the protoplasmic molecular unit, brought about either by direct
+modifying action of the inorganic environment--nutrition, temperature,
+oxygen supply, light, etc., etc.--or by the mingling with it of other
+protoplasm, the molecules of which since they have already a slightly
+different composition, configuration, mode of breaking down and building
+up, etc., affect its molecules by supplying them with altered nutritive
+atom-complexes, by competing with them for oxygen, etc., etc. Once these
+molecules are affected, we must assume that long sequences of other
+chemical and molecular changes will be also modified; and although we
+have no conception of _how_ these changes bring about changes in form,
+that they do so is only a conclusion of the same order as that which we
+hold regarding the much simpler changes concerned in the formation of
+crystals.
+
+That such variations may be of every degree as regards profundity,
+permanence, kind, etc., may well be imagined; and there is nothing
+surprising in our being able to induce them more easily by the action of
+external factors _in the readily accessible cell-protoplasm_ than in the
+_less exposed nuclear-protoplasm_; because the latter is only accessible
+through the former, or through the agency of _other nuclear protoplasm
+already modified_. On these and similar phenomena depend the relative
+permanency and transmissibility of the variations. Our measure of the
+latter only begins when the effects referred to have become manifest in
+large masses of cells, because only then do they become appreciable to
+our senses.
+
+Further, variations thus induced may be of advantage to the continued
+life of the plant, or in all degrees disadvantageous or threatening to
+its existence. These latter variations are _Disease_, and if their
+interference with the normal rhythmical play of the building up and
+breaking down of the protoplasm molecules proceeds beyond certain
+limits, life ceases, and we have death supervening on disease.
+
+
+NOTES TO CHAPTER XXX.
+
+     It appears probable that calcium is not always needed by
+     living cells, and may not enter into the composition of
+     protoplasm; on the other hand traces of iron are perhaps
+     necessary.
+
+     The criticisms and summary of facts on which the hypothesis
+     regarding protoplasm here adopted is based are developed at
+     length in Kassowitz, _Allgemeine Biologie_, Wien, 1899, B. I.
+     and II., where the collected literature may be found, and the
+     reader introduced to the huge mass of controversial writings
+     put forward since Darwin and associated with the names of
+     Weismann and others.
+
+     It will probably be noticed that I have employed the term
+     molecular unit of protoplasm, and have not discussed the
+     question of organised structure in the latter: this is because
+     it seems clear to me that living protoplasm as such does not
+     possess "organised structure" in the true sense of that
+     term--it is, rather, busy preparing and making "organised
+     structure," and a molecular constitution would have to be
+     ascribed to all "physiological units" of the nature of
+     micellæ, pangens, ids, etc., as truly as to the structural
+     units of a starch-grain or cell-wall, or even of a crystal. In
+     this connection, the student will find the necessary points of
+     view put forward in Pfeffer, _Physiology_, pp. 37-83.
+
+
+
+
+INDEX.
+
+
+  Absorption by roots, 49.
+
+  Absorption of energy, 23.
+
+  Absorption of light, 27.
+
+  Absorption of water, 50.
+
+  _Abutilon_, 183.
+
+  _Acarus_, 88.
+
+  Accessory buds, 259.
+
+  _Acer_, 214.
+
+  Acid gases, 181, 191.
+
+  Acids, 130, 136.
+
+  Acquired characters, 283.
+
+  _Acrostalagmus_, 238.
+
+  Action of the environment, 271.
+
+  Adaptation, 176.
+
+  Adapted races, 177.
+
+  _Adonis_, 220.
+
+  Adventitious buds, 224, 225, 257, 260.
+
+  _Æcidium_, 88, 114, 116, 187, 188, 189, 217, 223, 225, 232, 247, 252.
+
+  Aeration, 104.
+
+  Aerobic organisms, 57.
+
+  Aetiology, 89, 100.
+
+  _Agaricus melleus_, 115, 143, 145, 234.
+
+  Agents of disease, 113.
+
+  _Aglaospora_, 223.
+
+  Agriculture, 65.
+
+  Agricultural Chemistry, 2.
+
+  _Ajuga_, 217.
+
+  Albinism, 179, 182, 183, 186.
+
+  Alder, 207, 219.
+
+  Aleurone layer, 173.
+
+  Algae, 215.
+
+  _Allium_, 258.
+
+  Almond, 168.
+
+  _Alnus_, 214.
+
+  _Aloe_, 134, 161.
+
+  Alpine plants, 250, 279.
+
+  American blight, 164, 219.
+
+  American vines, 155, 169, 172.
+
+  Amides, 31.
+
+  Amoeba, 144.
+
+  Amount of energy stored, 25.
+
+  Amygdalin, 173.
+
+  _Anabaena_, 128.
+
+  Anaerobic bacteria, 58, 237.
+
+  _Anagallis_, 261.
+
+  Analyses, 65.
+
+  Analyses of waters, 58.
+
+  Anemone, 247.
+
+  Animals, 99, 108, 142, 207.
+
+  _Antennaria_, 232.
+
+  _Anthonomos_, 249.
+
+  Anthrax, 144.
+
+  Antiseptics, 162.
+
+  Ants, 232.
+
+  _Aphis_, 88, 109, 161, 165, 188, 213, 214, 232, 241, 253.
+
+  _Aphrophora_, 233.
+
+  Apogamy, 257, 261.
+
+  _Aporia Crataegi_, 187.
+
+  Apospory, 257, 261.
+
+  Apple, 170, 171, 187, 189, 192, 206, 217, 218, 219, 223, 226, 231,
+      233, 248, 249, 253, 254.
+
+  Apricot, 188, 206.
+
+  Apricots, 234.
+
+  Area of root-surface, 37, 39.
+
+  _Arisarum_, 188.
+
+  _Aristolochia_, 259.
+
+  Aroids, 113.
+
+  Arrest of growth, 246.
+
+  Arsenic, 162.
+
+  Artificial wounds, 194.
+
+  Ascomycetes, 189, 217, 269.
+
+  _Ascochyta_, 190.
+
+  Ash, 182, 223, 225, 251.
+
+  _Asparagus_, 180, 230, 251, 252.
+
+  _Aspergillus_, 231.
+
+  _Aspergillus niger_, 58.
+
+  _Aspidiotus_, 187.
+
+  Assimilation, 8, 21, 133, 271, 275, 277, 285, 286.
+
+  Assimilates, 274.
+
+  Atmosphere, 1, 99.
+
+  Atmospheric influences, 101.
+
+  Atrophy, 246, 247, 266.
+
+  Attractive substances, 136.
+
+  _Aucuba_, 264.
+
+  Autumnal colouring, 191.
+
+  Autumnal fall, 93.
+
+  Avalanches, 106.
+
+
+  Bacteria, 102, 133, 143, 168, 173, 176, 182, 190, 200, 216, 219, 223,
+      227, 231, 236, 237.
+
+  Bacteriosis, 227.
+
+  Barberry, 176.
+
+  Bark boring, 204, 205.
+
+  Bark-beetles, 205.
+
+  Barley, 176, 248.
+
+  Barrenness, 246, 249.
+
+  Bats, 244.
+
+  Bean, 188, 190, 191, 268.
+
+  Beech, 192, 222, 223, 225, 233, 240, 242, 254, 269.
+
+  Beech Miner, 208.
+
+  Bees, 142, 143, 164.
+
+  Beet, 192, 216, 219, 230.
+
+  Beet-rot, 230.
+
+  Beetles, 110, 143, 145, 205, 206, 207, 248, 254.
+
+  Berkeley, 85.
+
+  Bilberries, 116, 142, 217, 218, 248.
+
+  Biology of soil, 56, 102.
+
+  Birch, 207, 218, 224.
+
+  Birds, 108, 144, 164, 166.
+
+  Bird's-eye Maple, 224.
+
+  Black spots on leaves, 186, 189, 191.
+
+  Bladders, 218.
+
+  Blemish, 198.
+
+  Blights, 86, 104, 179.
+
+  Blisters, 230.
+
+  Blue rays, 21.
+
+  _Bombyx_, 187, 218.
+
+  Bordeaux mixture, 162.
+
+  Boring, 204.
+
+  _Botrytis_, 131, 132, 136, 175, 230, 231, 243, 288.
+
+  Boussingault, 5, 10.
+
+  Bouvardia, 260.
+
+  Bramble, 112.
+
+  Branch stumps, 194, 199.
+
+  Brand, 240.
+
+  Breeding, 78.
+
+  Briars, 113.
+
+  Broom-rapes, 265.
+
+  Browning, 122, 186, 235.
+
+  Brown spots, 186, 189, 190, 191.
+
+  Browsing, 244.
+
+  _Bruchus_, 248.
+
+  Bruises, 194, 203, 240, 241.
+
+  Bryony, 112.
+
+  _Bryophyllum_, 260.
+
+  Bud galls, 219.
+
+  Bud variations, 92, 93.
+
+  Bulb diseases, 227.
+
+  Buried objects, 211, 269.
+
+  Burning, 191.
+
+  Burning-glass effect, 192.
+
+  Burrows, 204, 205.
+
+  Burrs, 222, 223, 224.
+
+  Bursting of fruits, 227, 230.
+
+  Butterflies, 145.
+
+  Bye-products, 276.
+
+
+  Cabbage, 253.
+
+  Cabbage moth, 208.
+
+  _Caeoma_, 252.
+
+  _Caesalpinia_, 233.
+
+  Calcium, 272.
+
+  Calcium oxalate, 138.
+
+  _Calla_, 183.
+
+  _Calliandra_, 233.
+
+  Callus, 119, 120, 124, 139, 140, 196, 197, 199, 201, 202, 210, 241,
+      260, 263, 269.
+
+  _Calyptospora_, 116, 217.
+
+  Cambium, 120, 196, 199, 222.
+
+  Camellia, 187.
+
+  Cancer, 127.
+
+  Canker, 87, 222, 223, 241.
+
+  _Capnodium_, 232.
+
+  _Capsella_, 116, 175, 252.
+
+  Carbohydrates, 16, 17, 20, 34, 122, 184, 272, 273, 277.
+
+  Carbolic acid, 162.
+
+  Carbon, 272.
+
+  Carbon assimilation, 8, 10, 28, 106.
+
+  Carbon-bisulphide, 163.
+
+  Cardinal points, 288.
+
+  Carrot, 164.
+
+  _Carpocapsa_, 207.
+
+  Cast branches, 123.
+
+  Castor oil, 172.
+
+  Caterpillars, 109, 164, 207, 208, 244, 254, 259.
+
+  Cats, 164.
+
+  Cattle, 108.
+
+  Cauliflowers, 247, 250.
+
+  Causes of disease, 89, 99, 108, 159, 278, 282.
+
+  _Cecidia_, 212.
+
+  _Cecidomyia_, 182, 213, 214, 218, 219, 254.
+
+  Celery, 180, 230.
+
+  Cell contents, 168.
+
+  Cell-protoplasm, 279, 280, 290.
+
+  Cellulose, 132, 277, 286.
+
+  _Celosia_, 250.
+
+  _Centaurea_, 188.
+
+  _Centhorhynchus_, 219.
+
+  _Cephaleuros_, 188.
+
+  _Cephus_, 248.
+
+  _Cercis_, 259.
+
+  _Cercospora_, 190.
+
+  Cereals, 248.
+
+  Change of conditions, 78.
+
+  Charlock, 165.
+
+  Checks to disease, 166.
+
+  Chemical analysis, 32, 64, 103, 272.
+
+  Chemical antiseptics, 159.
+
+  Chemical energy, 29, 287.
+
+  Chemotactic phenomena, 72, 130, 135, 137.
+
+  _Chermes_, 153, 223.
+
+  Cherry, 208, 209, 231, 234, 235, 247, 248.
+
+  Chestnut, 190.
+
+  Chlorine, 181.
+
+  Chlorophyll, 19, 106, 122.
+
+  Chlorophyll action, 184, 192.
+
+  Chlorophyll corpuscles, 9, 18, 22.
+
+  Chlorosis, 122, 165, 179, 180, 181.
+
+  Chrysanthemum, 243, 252.
+
+  Chytridiaceae, 127, 136, 189, 208.
+
+  _Cicada_, 235.
+
+  Cicatrix, 123.
+
+  _Cinchona_, 168, 172, 173.
+
+  Circulation of carbon, 62.
+
+  Circulation of nitrogen, 62.
+
+  _Citrus_, 168.
+
+  _Clasterosporium_, 188, 209.
+
+  Classification of diseases, 99, 101, 120.
+
+  _Claviceps_, 232.
+
+  Climate, 1.
+
+  Climbing plants, 112, 113, 210.
+
+  _Clostridium_, 236, 237.
+
+  Clothes, 142.
+
+  Clover, 164, 187, 249, 252, 288.
+
+  Cluster-cups, 188.
+
+  Coal gas, 104, 182.
+
+  Coccideae, 164, 232.
+
+  _Coccus_, 223.
+
+  Coffee leaf-disease, 114, 146, 166, 169, 242.
+
+  _Coleophora_, 153, 206.
+
+  _Coleosporium_, 169.
+
+  _Coleus_, 192, 220.
+
+  Competition of fungi, 61.
+
+  Complex interactions, 91, 99.
+
+  Conifers, 125, 205, 223, 225, 234, 258.
+
+  Constitution, 156, 283.
+
+  Consumption, 248.
+
+  Contact irritability, 125, 135.
+
+  _Contagium fluidum vivum_, 183.
+
+  Contortions, 252.
+
+  _Convallaria_, 175.
+
+  _Convolvulus_, 112.
+
+  _Copaifera_, 234.
+
+  Copper sulphate, 162, 165.
+
+  Coppery leaves, 191.
+
+  Cork, 119, 123, 194, 199, 216, 222.
+
+  Cork wings, 217.
+
+  Corky warts, 212.
+
+  Corn, 248.
+
+  Corrosion of marble, 46.
+
+  _Cossus_, 206.
+
+  Cost of epidemics, 146, 147.
+
+  Cotton, 172.
+
+  _Crassula_, 253.
+
+  Creeping of mycelia, 142.
+
+  _Crepis_, 252.
+
+  Crimson spots, 189.
+
+  Cross-breeding, 266.
+
+  Cross-fertilisation, 69, 74, 77, 281.
+
+  Cross-graining, 124.
+
+  Crucifers, 219, 284.
+
+  Cryptogams, 87, 108, 111, 113.
+
+  Cuckoo-spit, 233.
+
+  Cucullate leaves, 253.
+
+  Cucumber, 219.
+
+  _Cucurbitaria_, 217, 243.
+
+  Cultivation of pest and host plant, 168.
+
+  _Curculio_, 248.
+
+  Curling, 235, 246.
+
+  _Cuscuta_, 134.
+
+  Cuts, 119, 143, 194.
+
+  Cuttings, 194, 198, 262, 263.
+
+  Cyanide of potassium, 165.
+
+  Cycads, 128.
+
+  _Cynips_, 110, 213, 219.
+
+  _Cystopus_, 116, 136, 175, 187, 217, 247, 252.
+
+  Cytases, 132.
+
+  _Cytisus Adami_, 264, 283.
+
+
+  Daisy, 278.
+
+  Damping off, 114, 144, 160, 229, 284.
+
+  Dandelion, 247, 252.
+
+  Daniel's researches, 283.
+
+  Dark heat rays, 27.
+
+  Darwin, 72, 125.
+
+  _Dasyscypha Willkommii_, 152, 223.
+
+  Death, 271, 272, 287, 290.
+
+  De Bary, 85, 151.
+
+  Deficiency of iron, 180.
+
+  Defoliation, 109, 240, 244.
+
+  Deformation, 132.
+
+  _Dematium_, 135.
+
+  _Dematophora_, 143, 145.
+
+  Denitrification, 62.
+
+  Derivation of Phytopathology, 85.
+
+  Destruction, 275.
+
+  Development of root-hairs, 40.
+
+  Dextrine, 173.
+
+  Diagnosis, 85, 89.
+
+  Diastases, 132.
+
+  Diffusion, 53.
+
+  Digestion, 133.
+
+  _Digraphis_, 175.
+
+  _Dilophia_, 188.
+
+  _Dionaea_, 125.
+
+  _Dipsacus_, 252.
+
+  _Diptera_, 207.
+
+  Dis-assimilation, 275, 277, 286.
+
+  Discolorations, 179, 186, 192.
+
+  Disease, 64, 91, 271, 272, 277, 287, 288, 290.
+
+  Disease dodging, 168.
+
+  Disease-fungi, 189.
+
+  Disease of organs, 119.
+
+  "Disease-proof" varieties, 168, 169, 171, 173, 177.
+
+  Disease-resisting varieties, 177.
+
+  Diseases of absorptive organs, 121.
+
+  Diseases of assimilatory organs, 119.
+
+  Diseases of bark, 120.
+
+  Diseases of cambium, 120.
+
+  Diseases of parenchyma, 120.
+
+  Diseases of respiratory organs, 119, 121.
+
+  Disintegration, 275.
+
+  Distortions, 140, 246, 251, 252, 253.
+
+  Dissemination of fungi, 142.
+
+  Division, 127.
+
+  Dodder, 113.
+
+  _Dolium_, 134.
+
+  Dormant buds, 224, 225, 257, 259, 260.
+
+  Double flowers, 247, 256.
+
+  Double ideals in selection, 168.
+
+  _Dracaena_, 192.
+
+  Drainage, 103.
+
+  Drawing, 106, 180.
+
+  Drip, 103.
+
+  Drooping, 43, 179.
+
+  Drops of water, 192.
+
+  Dropsy, 228.
+
+  Drought, 121, 183, 190, 191, 245, 248, 249.
+
+  Dry-rot, 143, 237.
+
+  Ducks, 144.
+
+  Dutrochet, 7.
+
+  Dwarfing, 246, 249.
+
+  "Dying back," 190, 240, 242, 243, 244.
+
+
+  Earwigs, 164, 207.
+
+  _Eau Céleste_, 162.
+
+  _Edelfäule_, 230.
+
+  Eelworms, 111, 248.
+
+  Effects of environment, 262.
+
+  Eggs of insects, 187.
+
+  Elaborated sap, 94.
+
+  Elm, 218, 224, 225, 233, 260.
+
+  _Empusa_, 163.
+
+  Endemic diseases, 153, 160, 166.
+
+  Endive, 180.
+
+  Endophytes, 130.
+
+  Endophytic algae, 128.
+
+  Endophytic fungi, 193.
+
+  Energy in plants, 15, 25, 287.
+
+  Engelmann, 20, 27.
+
+  _Entyloma_, 187.
+
+  Enzymes, 10, 130, 132, 136.
+
+  _Epichloë_, 218.
+
+  Epicormic shoots, 224, 257, 260.
+
+  Epidemics, 108, 109, 113, 115, 142, 153, 160, 163, 166.
+
+  Epiphytes, 113, 130, 135, 137.
+
+  Epiphytic algae, 188.
+
+  Epiphytic fungi, 161, 193, 232.
+
+  _Equisetum_, 113.
+
+  Ergot, 131, 142, 144.
+
+  _Erineum_, 88, 212, 214, 215.
+
+  Erosions, 204, 207.
+
+  _Erysipheae_, 135, 142, 161, 187, 268.
+
+  Essentials of fertilisation, 69.
+
+  Estimates of loss, 146.
+
+  Etiolation, 106, 179, 180, 229.
+
+  _Euphorbia_, 116, 134, 247, 266.
+
+  Excavations, 204.
+
+  Excess of food, 229.
+
+  Excess of minerals, 102.
+
+  Excess of water, 100.
+
+  Excessive growth, 246.
+
+  Excessive nutrition, 250.
+
+  Excrescences, 114, 212, 222.
+
+  Excreta, 45, 130, 133.
+
+  _Exobasidium_, 128, 218.
+
+  _Exoascus_, 116, 128, 188, 208, 214, 218, 225, 247, 253.
+
+  Expense of materials, 161.
+
+  Experiments necessary, 168.
+
+  Exposure of roots, 179, 184.
+
+  External causes of disease, 99.
+
+  Extinction of species, 91.
+
+  Exudations, 227.
+
+  Exudation under pressure, 51.
+
+
+  Factors of an epidemic, 149, 165.
+
+  Falling of fruit, 206.
+
+  Falling leaves, 123.
+
+  False chlorosis, 181.
+
+  False etiolation, 180.
+
+  _Farfugium_, 188.
+
+  Fasciations, 230, 246, 251.
+
+  Fats, 272, 286.
+
+  Feeding, 14, 16.
+
+  Fermentation, 58, 102, 130, 233.
+
+  Ferns, 113, 247, 260, 261.
+
+  Fertilisation, 71.
+
+  Field-mice, 164.
+
+  Figs, 113.
+
+  Finger and toe, 114, 127, 163.
+
+  Fire, 240.
+
+  Flaming, 164.
+
+  Flattened roots, 246, 252.
+
+  Fleshiness, 228.
+
+  Flies, 86, 110, 142, 143, 145.
+
+  Flux, 227, 231.
+
+  Flying foxes, 244.
+
+  Focussing of solar rays, 192.
+
+  Foliage, 110.
+
+  _Fontaria_, 134.
+
+  Food, 18.
+
+  Forest-fires, 241.
+
+  Formic-aldehyde, 20.
+
+  Foul products, 100.
+
+  Foxy leaves, 191.
+
+  Freezing, 121, 183.
+
+  Frit fly, 182.
+
+  Frost, 153, 160, 225, 229, 248, 249.
+
+  Frost-beds, 243.
+
+  Frost-blisters, 212, 218.
+
+  Frost canker, 222.
+
+  Frost-cracks, 204, 209, 242.
+
+  Frost-patches, 240.
+
+  Frost-ridge, 209.
+
+  _Fumago_, 190, 232.
+
+  Fumes, 104.
+
+  Functions of roots, 43, 45.
+
+  Functional depression, 96.
+
+  Fungi, 89, 108, 143, 174, 189, 200, 205, 207, 208, 212, 216, 219, 223,
+      229, 231, 233, 238, 240, 241, 243, 248, 251, 255, 258, 265, 267,
+      283, 284, 288.
+
+  Fungus attacks, 139.
+
+  Fungus galls, 219.
+
+  _Fusarium_, 143, 238.
+
+  _Fusicladium_, 189.
+
+  _Fusisporium_, 237.
+
+
+  _Gagea_, 258.
+
+  Gall-apple, 218.
+
+  Gall-flies, 219.
+
+  Gall-insect, 139.
+
+  Gall-like swellings, 128.
+
+  Galls, 86, 110, 120, 130, 138, 212, 214, 218, 255.
+
+  Gangrene, 231.
+
+  _Garreya_, 264.
+
+  Gas, 160.
+
+  Gases in soil, 104.
+
+  _Gastropacha_, 225.
+
+  Gelatine, 163.
+
+  General death, 116.
+
+  General disease, 119, 120.
+
+  Germ-plasm, 267.
+
+  _Gesneria_, 260.
+
+  _Glechoma_, 218.
+
+  _Gloeosporium_, 189, 190, 208.
+
+  _Gloxinia_, 260.
+
+  Goats, 164.
+
+  Gooseberry, 217.
+
+  Graft-hybrids, 262, 267, 271, 283.
+
+  Grafting, 78, 155, 169, 183, 250, 262, 271, 281.
+
+  Grain-rust, 146.
+
+  Grapes, 192, 230, 231.
+
+  _Grapholitha_, 109, 207.
+
+  Grass, 111, 189, 190, 205, 218, 233.
+
+  Green fly, 161.
+
+  Grew, 85.
+
+  Greyish spots, 187.
+
+  Growth, 271, 274, 275, 286.
+
+  Grubs, 110, 207.
+
+  Gumming, 235.
+
+  Gummosis, 227, 234, 235.
+
+  _Gymnosporangium_, 114, 176, 223.
+
+
+  Hail, 106, 240, 241.
+
+  Hales, 85.
+
+  _Haltica_, 209.
+
+  Hardy varieties, 168, 170, 177.
+
+  Haustoria, 134, 135, 136.
+
+  Healing, 194, 196.
+
+  Healing by cork, 123.
+
+  Health, 272, 287.
+
+  Health and disease, 91, 97, 287.
+
+  Heliotropism, 126.
+
+  _Hemileia_, 146, 169.
+
+  Heredity, 72, 283.
+
+  _Herpotrichia_, 135, 190.
+
+  Hessian Fly, 182.
+
+  _Heterodora_, 219, 220.
+
+  _Hieracium_, 112.
+
+  History of Phytopathology, 85.
+
+  Holdfast of roots, 42.
+
+  Hollyhock disease, 143.
+
+  Holly, 217.
+
+  Honey dew, 144, 227, 232, 233.
+
+  Hops, 162, 187, 191, 232, 253.
+
+  Hop-aphis, 146.
+
+  Hop-disease, 166.
+
+  Hop mildew, 161.
+
+  _Hormomyia_, 219.
+
+  Hornbeam, 224, 233, 242.
+
+  Horse-radish, 260.
+
+  Host, 284, 285.
+
+  Hyacinth, 231, 261.
+
+  Hyacinth disease, 143.
+
+  Hybrids, 69, 156, 281.
+
+  Hybridisation, 69, 75, 169, 266, 281.
+
+  Hydrochloric acid, 181.
+
+  Hydrogen, 272.
+
+  Hymenomycetes, 206.
+
+  Hypertrophy, 119, 127, 139, 213, 215, 247, 266.
+
+  _Hypochaeris_, 112.
+
+  _Hypomyces_, 237.
+
+  _Hyponomeuta_, 254.
+
+
+  Ice, 184, 209.
+
+  Ichneumon-flies, 165.
+
+  _Icterus_, 181.
+
+  Illegitimate unions, 265.
+
+  Immunity, 155, 156, 165, 168, 169.
+
+  Impervious subsoil, 181.
+
+  Inarching, 269.
+
+  Increase in dry weight, 23.
+
+  Indian agriculture, 172.
+
+  Indian wheats, 168.
+
+  Indispensability of elements, 278.
+
+  Infection, 262, 265, 267.
+
+  Ingredients of protoplasm, 272.
+
+  Insect bites, 225.
+
+  Insect diseases, 145, 146, 154, 189.
+
+  Insect punctures, 88.
+
+  Insects, 89, 98, 108, 109, 120, 138, 142, 153, 174, 187, 194, 203,
+      205, 206, 207, 208, 212, 223, 229, 241, 244, 248, 251, 254, 255,
+      258, 259, 269.
+
+  Insolation, 180, 242.
+
+  Intercellular endophytes, 136, 137.
+
+  Intercellular mycelium, 128.
+
+  Interference, 91.
+
+  Internal causes of disease, 99, 101.
+
+  Intracellular parasites, 127, 136.
+
+  Intramolecular respiration, 277.
+
+  Intumescences, 212, 215.
+
+  Inulin, 11, 17.
+
+  Invertebrata, 108.
+
+  Irritability, 125, 127.
+
+  Irritation, 119, 139.
+
+  _Isaria_, 163.
+
+  Ivy, 113, 165.
+
+
+  Japanese trees, 250.
+
+  Jerusalem Artichoke, 264.
+
+  _Juncus_, 219.
+
+  Juniper, 114.
+
+
+  Kidney bean, 192.
+
+  Knauers, 223.
+
+  Knife wounds, 194, 195.
+
+
+  Labour, 161.
+
+  Lace-wings, 165.
+
+  _Lachnus_, 223.
+
+  Lady-birds, 164, 165.
+
+  Lammas shoots, 257, 259.
+
+  Larch, 168, 171.
+
+  Larch disease, 115, 149, 152, 166, 171, 223, 241.
+
+  Larvae, 110.
+
+  Lateral wounds, 132.
+
+  Lawns, 112.
+
+  Laying of wheat, 179, 180.
+
+  Leaf-curl, 236, 253.
+
+  Leaf-diseases, 114, 119, 120, 242.
+
+  Leaf-galls, 217, 218.
+
+  Leaf-miner, 86, 109, 204.
+
+  Leaf perforations, 208.
+
+  Leaf rolling, 214, 246, 254.
+
+  Leaf-spots, 114, 190.
+
+  Leguminoseae, 137, 219.
+
+  Lemons, 235.
+
+  Lenticels, 217.
+
+  Lepidoptera, 187.
+
+  _Leptosphaeria_, 249.
+
+  Lichens, 137.
+
+  Liebig, 4.
+
+  Life, 271, 285, 287.
+
+  Life and death, 271.
+
+  Light, 27, 106.
+
+  Lily disease, 143.
+
+  Lime, 163, 215, 218, 232, 253, 254, 260, 269.
+
+  Limes, 172.
+
+  Limits of variation, 287.
+
+  _Linaria_, 252.
+
+  Liquid antiseptics, 160, 161, 162.
+
+  Living environment, 99, 108.
+
+  Local action, 114.
+
+  Local disease, 119, 121.
+
+  Locusts, 109, 145, 163, 164.
+
+  Longicorns, 205.
+
+  _Loranthus_, 113, 245, 265.
+
+  Losses due to epidemics, 142.
+
+  Lowering of temperature, 100.
+
+  Lucerne, 249.
+
+  Lurking parasites, 142, 145.
+
+  Lychnis, 232.
+
+  _Lyonetra_, 206.
+
+  _Lysimachia_, 217.
+
+
+  Machine, plant compared to a, 79.
+
+  Magnesium, 272.
+
+  Maize, 116, 173, 219, 267.
+
+  _Majanthemum_, 175.
+
+  Malformations, 116, 130, 131, 246, 251.
+
+  _Mal nero_, 190.
+
+  Mallow, 252.
+
+  Malpighi, 85.
+
+  Mammals, 142.
+
+  Man and plants, 108, 142, 143.
+
+  Manna, 227, 235.
+
+  Manna Ash, 235.
+
+  Maple, 259.
+
+  Maximum, 288.
+
+  Maximum absorption, 19.
+
+  Maximum assimilation, 19.
+
+  Maximum temperature, 105.
+
+  Mealy bug, 164.
+
+  _Melampsora_, 176.
+
+  Melon, 220.
+
+  Messmates, 63.
+
+  Metabolic products, 274.
+
+  Metabolism, 23, 127, 271.
+
+  Metabolites, 278.
+
+  Metallic compounds, 162.
+
+  Mice, 108, 163.
+
+  Microbes, 227.
+
+  Micro-organisms, 183.
+
+  Mildew, 86, 164.
+
+  Millardet, 169.
+
+  Mineral salts, 101.
+
+  Miniature trees, 250.
+
+  Minimum, 288.
+
+  Minimum temperature, 105.
+
+  Misconceptions, 12.
+
+  Mistletoe, 113, 265.
+
+  Mites, 192, 214, 255.
+
+  Mixed species, 166.
+
+  Molecular structure of protoplasm, 273, 274.
+
+  Mongrel forms, 74.
+
+  _Monilia_, 217, 231.
+
+  Monstrosities, 246.
+
+  Moraine plants, 250.
+
+  Moths, 110, 145, 206.
+
+  Moulds, 230, 231, 237, 243.
+
+  _Mucor_, 230, 231.
+
+  Mulberry, 244.
+
+  Mutilations, 252.
+
+  Mycelial strands, 145.
+
+  Mycelium, 188.
+
+  Mycocecidia, 219.
+
+  Mycorrhiza, 137.
+
+  Myrtaceae, 258.
+
+  _Mytilaspis_, 187.
+
+
+  Natural checks, 159.
+
+  Natural demise, 91, 93.
+
+  Natural Grafts, 269.
+
+  Natural Selection, 72, 99, 167, 286.
+
+  Natural Wounds, 204.
+
+  Nature of soil, 57.
+
+  Necrosis, 240, 241, 243.
+
+  _Nectria_, 145, 217, 223, 241, 243, 269.
+
+  Nematodes, 111, 134, 139, 219, 220.
+
+  Nettle, 116, 252.
+
+  _Neurotus_, 219.
+
+  New formations, 255.
+
+  Nitrate, 273.
+
+  Nitrification, 62, 102.
+
+  Nitrogen, 272.
+
+  Nodosities, 219.
+
+  Nodules on roots, 63, 137.
+
+  Non-living environment, 99.
+
+  _Notommata_, 140.
+
+  Nuclear fusion, 267.
+
+  Nuclear protoplasm, 271, 279, 280, 290.
+
+  Nuclear substance, 71.
+
+  Nucleo-plasm, 280.
+
+  Nuts, 248.
+
+
+  Oak, 110, 188, 215, 218, 219, 223, 233.
+
+  Oak leaf-roller, 254.
+
+  Oat, 176.
+
+  Occlusion, 200, 201, 222, 223.
+
+  Odours, 144.
+
+  Oedema, 228.
+
+  Olive, 223.
+
+  Onion, 231.
+
+  _Oniscus_, 182.
+
+  _Oospora_, 216.
+
+  Optimum temperature, 105, 288.
+
+  Orange, 173, 187, 235, 247.
+
+  Orange-coloured spots, 187.
+
+  Orchard trees, 163.
+
+  _Orchestes_, 206.
+
+  Orchids, 113, 266.
+
+  Organic acids, 50.
+
+  Organisation, 89.
+
+  Organised structure, 13.
+
+  Organisms in soil, 60.
+
+  _Orobanche_, 112.
+
+  Osmosis, 26, 29, 46.
+
+  Osmotic pressures, 18, 41, 52.
+
+  Over-crowding, 104, 111.
+
+  Over-feeding, 102.
+
+  Over-watering, 97.
+
+  Oxalic acid, 134, 136.
+
+  Oxidation, 124.
+
+  Oxygen, 104, 272.
+
+  Oxygen-respiration, 12, 64.
+
+
+  Pallor, 179, 180.
+
+  Palms, 192.
+
+  _Pangium_, 134, 165.
+
+  Parasites, 61, 113, 119, 130, 139, 174, 187, 230, 265, 269, 284.
+
+  Parasitic algae, 188, 217, 219.
+
+  Parasitic bacteria, 163.
+
+  Parasitic diseases, 88, 119, 121.
+
+  Parasitic epiphyte, 136.
+
+  Parasitic fungi, 87, 97.
+
+  Parasitism, 262, 264, 268, 271.
+
+  _Paris_, 175.
+
+  "Paris green," 162.
+
+  Parti-coloured leaves, 191.
+
+  Parti-coloured spots, 186.
+
+  Pasture grasses, 69.
+
+  Pathology, 121, 257.
+
+  Pathology of cell, 119.
+
+  Pathological conditions, 168, 170, 246.
+
+  Pea, 190, 191, 206, 208, 248, 268.
+
+  Peach, 170, 253.
+
+  Pear, 179, 187, 189, 191, 216, 218, 231, 240, 248, 249, 253, 257.
+
+  Pedigree wheats, 69.
+
+  _Pelargonium_, 198, 253.
+
+  Peloria, 252.
+
+  _Penicillium_, 231.
+
+  _Peridermium Pini_, 223, 234.
+
+  _Periola_, 238.
+
+  Permanganate, 162.
+
+  _Peronospora_, 136, 160, 175, 187, 189, 208.
+
+  _Petasites_, 188.
+
+  Petroleum, 162.
+
+  _Peziza_, 115, 144, 152.
+
+  Phanerogams, 108, 111.
+
+  _Phellomyces_, 238.
+
+  _Phoma_, 217, 243.
+
+  Phosphorus, 272.
+
+  Photo-synthesis, 11, 16.
+
+  _Phragmidium_, 189.
+
+  _Phyllachora_, 189.
+
+  _Phyllereum_, 253.
+
+  _Phyllobium_, 217.
+
+  _Phyllosiphon_, 188.
+
+  _Phyllosticta_, 188, 209.
+
+  _Phylloxera_, 110, 145, 149, 154, 155, 163, 166, 172, 188, 219, 220, 268.
+
+  Physiology, 1, 66, 85.
+
+  Physiological diseases, 119, 121.
+
+  _Phytomyza_, 206.
+
+  Phytopathology, 85.
+
+  _Phytophthora_, 115, 136, 144, 150, 151, 235, 236.
+
+  _Phytophysa_, 219.
+
+  _Phytoptus_, 189, 213, 214, 215, 218, 219, 253, 254.
+
+  _Pilea_, 219.
+
+  _Pilobolus_, 126, 140.
+
+  Pines, 183, 223, 234, 251, 252.
+
+  Pine-apple, 258.
+
+  Pith flecks, 204, 207.
+
+  Plant as agent of disease, 99, 108.
+
+  Plant, agricultural chemistry of, 1.
+
+  Plant and its food, 7.
+
+  Plant and its surroundings, 1.
+
+  Plant, a machine, 1, 15.
+
+  Plant, central object of study, 1.
+
+  Plant, physiology, 1.
+
+  _Plantago_, 257.
+
+  Plantain, 112, 257.
+
+  Plants, dying out of, 93.
+
+  Plasmodia, 163.
+
+  _Plasmodiophora_, 114, 126, 127, 144, 163, 219, 284, 285.
+
+  Plasmolysis, 47.
+
+  _Pleospora_, 236.
+
+  _Pleotrachelus_, 126, 140.
+
+  Plum, 171, 189, 192, 209, 214, 206, 231, 235, 248, 249, 260.
+
+  _Poa_, 258.
+
+  Pocket-like galls, 155, 214, 218.
+
+  Pocket-plums, 214.
+
+  Pockets, 253.
+
+  Poison, 102, 130, 136, 163, 216.
+
+  Poisonous gases, 181, 248.
+
+  Pollen grain, 288.
+
+  Pollination, 248, 262, 265, 266, 271.
+
+  _Polydesmus_, 236.
+
+  _Polygonatum_, 175.
+
+  _Polygonum_, 258.
+
+  Polymorphism, 174.
+
+  Polyporei, 142.
+
+  _Polyporus_, 143, 206.
+
+  _Polystigma_, 189.
+
+  Poplar, 188, 206, 215, 218, 254.
+
+  Post and epidemics, 142.
+
+  Potassium, 272.
+
+  Potassium sulphite, 162.
+
+  Potato, 162, 171, 194, 209, 216, 236, 237, 258.
+
+  Potato-disease, 114, 143, 149, 150, 166, 189, 207, 235.
+
+  Powders, antiseptic, 159, 160, 161.
+
+  Predisposition to disease, 98, 99, 105, 168, 169, 229, 262, 268, 277,
+      278, 282.
+
+  Preventible diseases, 159.
+
+  Preventitious buds, 259.
+
+  Prolepsis, 257, 259.
+
+  Proliferations, 257, 258.
+
+  Properties of soil, 57.
+
+  Prophylactic measures, 160.
+
+  Proteids, 132, 138, 272, 277, 286.
+
+  Proteolytic enzymes, 132.
+
+  _Protomyces_, 217.
+
+  Protoplasmic molecules, 276, 278, 286.
+
+  Protoplasm, 33, 41, 271, 272, 274, 276.
+
+  Pruning, 105, 143, 194, 225, 250.
+
+  Prussic acid, 163, 165, 173.
+
+  _Psylla_, 253.
+
+  _Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247, 252.
+
+  Puckers, 214, 235, 246, 253.
+
+  Puffing of spores, 142, 144.
+
+  Punctures, 212.
+
+  Pure culture, 166.
+
+  Purple-black spots, 191.
+
+  Pustules, 188, 190, 212, 217.
+
+  Putrefaction, 234.
+
+  _Pyrethrum_, 161.
+
+  _Pyrus_, 214.
+
+  _Pythium_, 114, 119, 136, 144, 160, 230.
+
+
+  _Quassia_, 161.
+
+  Quinine, 173.
+
+
+  Rabbits, 108, 142, 164, 194.
+
+  Rain trees, 233.
+
+  Rankness, 97, 227, 228.
+
+  Rats, 108, 163.
+
+  Rays of light, 18.
+
+  Red light, 21.
+
+  Red spider, 161, 187, 188, 192.
+
+  Red spots, 188, 253.
+
+  References in Bible, 85.
+
+  Remedial measures, 89.
+
+  Repellent substances, 136.
+
+  Reproduction, 72, 281.
+
+  Reserves, 274.
+
+  Resin, 125.
+
+  Resin-flux, 234.
+
+  Resinosis, 227, 234.
+
+  Resistance to disease, 155, 268.
+
+  Resistant races, 172.
+
+  Respiration, 17, 31, 130, 271, 275, 276, 285, 287.
+
+  Reversions, 73.
+
+  Rhinanthoideae, 265.
+
+  _Rhinanthus_, 112.
+
+  _Rhizobium_, 289.
+
+  _Rhizoctonia_, 238.
+
+  Rhizomorph, 145.
+
+  Rhododendron, 218.
+
+  Rhubarb, 180, 230.
+
+  _Rhynchitis_, 254.
+
+  _Rhytisma_, 188.
+
+  Ribbon grass, 183.
+
+  _Ribes_, 214.
+
+  Rice, 172.
+
+  Rimpau's experiments, 69, 73, 77.
+
+  Ringing, 194, 201, 202, 210.
+
+  Ripened wood, 243.
+
+  _Robinia_, 259.
+
+  Rodents, 109.
+
+  _Roestelia_, 217.
+
+  Rolled leaves, 86.
+
+  Root, 9, 35, 96, 120, 227, 270.
+
+  Root-absorption, 181.
+
+  Root-diseases, 119, 120.
+
+  Root-excretions, 46.
+
+  Root-fusions, 262.
+
+  Root-galls, 221.
+
+  Root-hairs, 34, 102, 163.
+
+  Root-nodules, 212, 219.
+
+  Root-parasites, 112, 265.
+
+  Root-rot, 230.
+
+  Roses, 232, 243, 257, 268, 278.
+
+  Rosettes, 225.
+
+  Rot, 97, 182, 227, 229, 231, 236.
+
+  Rotation of crops, 69, 166.
+
+  Rotifer, 140.
+
+  Rot-organisms, 200.
+
+  Rotting of wounds, 87.
+
+  Rouen law, 85.
+
+  Rushes, 114.
+
+  Rust, 122, 142, 171, 172, 175, 191.
+
+  Rye, 176, 248.
+
+
+  _Saccharomyces_, 60.
+
+  Sachs, 7, 36.
+
+  _Salvia_, 214.
+
+  San José scale, 187.
+
+  Sand-blast action, 184.
+
+  Sandy soils, 184.
+
+  _Saperda_, 205.
+
+  Saprophytes, 135, 137, 175, 234, 243, 244.
+
+  _Scab_, 189, 216.
+
+  _Scale_, 187.
+
+  _Schinzia_, 114.
+
+  _Schizoneura_, 223.
+
+  Scion, 183, 262, 264, 266, 282.
+
+  _Scleroderris_, 223.
+
+  Sclerotia, 143.
+
+  Schwarz, 39.
+
+  _Sclerotinia_, 142, 143, 144, 231, 248, 249, 288.
+
+  Scolytidae, 205.
+
+  Scorching, 240, 241.
+
+  Scurf, 216.
+
+  Sea-kale, 261.
+
+  _Secale_, 76.
+
+  Secretions, 130, 133, 173, 274.
+
+  Sedges, 189.
+
+  Seedless grapes, 247.
+
+  _Selandria_, 208.
+
+  Selection, 69, 74, 78, 169.
+
+  Selective absorption, 53, 65.
+
+  Self-fertilisation, 281.
+
+  Semi-parasites, 112.
+
+  _Senecio_, 188.
+
+  Sensitive plant, 125.
+
+  _Septoria_, 114, 187.
+
+  Sewage waters, 59.
+
+  Sexual act, 72.
+
+  Shaded foliage, 113.
+
+  Shanking, 246, 249.
+
+  Shoots from old wood, 260.
+
+  Shot holes, 204, 208, 209.
+
+  Silver fir, 224.
+
+  Silver leaf, 192.
+
+  _Sirex_, 206.
+
+  Skeleton leaves, 204, 207.
+
+  Slime flux, 227, 233.
+
+  Slime fungus, 219.
+
+  Slugs, 111, 164, 207, 269.
+
+  Smut, 117, 143, 162, 190.
+
+  Snails, 111, 142, 207.
+
+  Snow, 106.
+
+  Soap, as insecticide, 161.
+
+  Soil, 1, 42, 99, 102, 142, 163.
+
+  Soil-bacteria, 60.
+
+  Soil-filtration, 59.
+
+  Soil-organisms, 61, 143.
+
+  Solar energy, 135.
+
+  Somato-plasm, 267.
+
+  Sooty moulds, 135, 190, 232.
+
+  _Sorbus_, 207.
+
+  _Sorosporium_, 216.
+
+  Sour-rot, 231.
+
+  Sparrows, 164.
+
+  Specialised races, 168, 176.
+
+  Specific predisposition, 155.
+
+  Spectrum, 19, 21, 26.
+
+  Spermogonia, 144, 232.
+
+  _Sphaerella_, 189.
+
+  _Sphaerotheca_, 187.
+
+  Sphaeroblasts, 222, 225.
+
+  _Spicaria_, 237.
+
+  Spiral grooving, 204, 210.
+
+  Spiral growth, 252.
+
+  _Spongospora_, 216.
+
+  Spontaneous variations, 78, 246, 255.
+
+  Spores, 144.
+
+  Sports, 93, 247.
+
+  Spots on leaves, 120, 186.
+
+  Spraying, 159, 161, 162.
+
+  Spreading of disease, 142.
+
+  Squirrels, 108.
+
+  Stag-head, 240, 244.
+
+  Starch, 9, 16, 17, 20, 23, 138, 173.
+
+  Statistics of epidemics, 147.
+
+  Steeping, 161.
+
+  Stem diseases, 120.
+
+  _Stereum_, 206.
+
+  Sterility of soil, 61.
+
+  Stimulation, 119.
+
+  Stimuli, 126, 127, 139.
+
+  Stock, 262, 264, 266, 282.
+
+  Stomata, 23.
+
+  Stool-shoots, 201, 225, 269.
+
+  Stool stumps, 194, 201.
+
+  Strangulations, 204, 209.
+
+  Strawberry, 189, 257.
+
+  Stripping, 194, 197.
+
+  Stroma, 217.
+
+  Structure, 274.
+
+  Structure of protoplasm, 271.
+
+  Structure of root-hairs, 40.
+
+  Struggle for existence, 105, 159, 164, 165, 167, 286.
+
+  Study of causes, 85.
+
+  Stumps, 194.
+
+  Subsoil, 57, 103.
+
+  Substitutive selections, 286.
+
+  Suckers, 225, 260.
+
+  Sugar, 11, 17, 20, 173, 286.
+
+  Sugar cane, 172.
+
+  Sugar cane disease, 166.
+
+  Sulphate, 273.
+
+  Sulphur, 161, 163, 272.
+
+  Sulphurous acid, 181.
+
+  Sun-burn, 240, 241.
+
+  Sun-cracks, 240, 242.
+
+  Sundew, 232.
+
+  Sunflower, 256, 264.
+
+  Sun-spots, 192.
+
+  Superstitions, 85.
+
+  Surface energy, 26.
+
+  Surface roots, 112.
+
+  Sweet almond, 173.
+
+  Symbiosis, 63, 130, 137, 219, 263, 265, 268, 285.
+
+  Symptoms of disease, 89, 122, 179, 186.
+
+  _Synchytrium_, 127, 188, 217, 247.
+
+  Synthesis, 65.
+
+  _Syringa_, 259.
+
+  Syringing, 161, 164.
+
+
+  Tamarisk, 235.
+
+  Tannin, 138.
+
+  _Taphrina_, 218.
+
+  Tar, 164.
+
+  Tea, 244.
+
+  Teazel, 252.
+
+  Teleutospore, 189, 191.
+
+  Temperature, 99, 105.
+
+  Tendencies to ill-health, 91.
+
+  Tendrils, 125.
+
+  Teratology, 246, 253, 254, 257.
+
+  _Tetraneura_, 218.
+
+  _Tetranychus_, 187, 192.
+
+  Thawing, 183.
+
+  _Thelephora_, 206.
+
+  Therapeutics, 85, 89, 159.
+
+  Thermotropism, 126.
+
+  _Thesium_, 112.
+
+  Thick-skinned organs, 168, 171.
+
+  Thinning, 96, 105.
+
+  Thistle, 247.
+
+  Thrips, 88, 191, 208.
+
+  Thyloses, 125.
+
+  _Tilia_, 214.
+
+  Timber diseases, 119, 120.
+
+  Timiriazeff, 21.
+
+  _Tinea_, 206.
+
+  Tissue diseases, 119.
+
+  Tobacco, 209.
+
+  Tobacco powder, 161.
+
+  Tomato, 171, 219, 230.
+
+  Top-dry trees, 244.
+
+  Topical remedies, 161.
+
+  _Tomicus_, 205.
+
+  Torsions, 246, 252.
+
+  _Tortrix_, 254.
+
+  Toxic agents, 130.
+
+  Transformation of energy, 25, 28.
+
+  Transformation of organs, 254, 255.
+
+  Transmission of acquired characters, 264, 283, 290.
+
+  Transplanting, 96.
+
+  Transpiration, 181, 228.
+
+  Trees, 109.
+
+  _Trichosphaeria_, 135.
+
+  _Triticum_, 76.
+
+  Tumescence, 227, 228.
+
+  Tunnels, 206.
+
+  Turgescence, 47, 228, 230.
+
+  Turnip, 126, 162, 230.
+
+  Twitch, 113.
+
+  _Tylenchus_, 238, 248.
+
+
+  Ulcer, 231.
+
+  Unger, 85.
+
+  Unsuitable soils, 101.
+
+  Upheaval of seedlings, 179, 183.
+
+  Uredineae, 114, 134, 136, 145, 169, 188, 189.
+
+  _Uredo_, 88, 188, 191.
+
+  Uredospores, 191.
+
+  _Uromyces_, 116, 188, 191, 266.
+
+  _Urocystis_, 220.
+
+  Ustilagineae, 145, 190, 217, 248.
+
+  _Ustilago_, 116, 117, 175, 190, 219, 255.
+
+
+  _Vaccinium_, 128, 288.
+
+  Variability, 174.
+
+  Variation, 67, 72, 91, 92, 168, 174, 176, 246, 262, 263, 264, 271,
+      282, 286, 288, 289.
+
+  Variegation, 179, 182, 183, 192.
+
+  Varieties, 78, 247.
+
+  Varieties of soil, 56.
+
+  _Vaucheria_, 139, 140.
+
+  Vegetable acids, 48.
+
+  Vertebrata, 108.
+
+  _Verticillium_, 145, 236.
+
+  _Viburnum_, 214.
+
+  Vine, 110, 149, 156, 162, 164, 169, 171, 189, 190, 191, 222, 248, 268.
+
+  Vine disease, 143.
+
+  Vivipary, 257, 258.
+
+
+  Walnut, 190, 209, 253.
+
+  Want of air, 100.
+
+  Washing leaves, etc., 161.
+
+  Wasp-flies, 165.
+
+  Wasps, 145.
+
+  Water, 272.
+
+  Water and insects, 161.
+
+  Water-culture, 65.
+
+  Water in soil, 103.
+
+  Waterlogging, 181.
+
+  Weaving of fungi, 190.
+
+  Webs, 190, 254.
+
+  Weeding, 105.
+
+  Weeds, 69, 111, 113, 165, 229, 249.
+
+  Weevils, 248.
+
+  Wet feet, 181.
+
+  Wheat, 169, 171, 172, 176, 179, 180, 182, 183, 230, 248.
+
+  Wheat rust, 86, 122, 146, 166, 169, 176.
+
+  White spots, 186, 187.
+
+  Willow, 206, 207, 219, 223, 233, 259.
+
+  Willow beetle, 208.
+
+  Wilting, 179, 181, 235, 249.
+
+  Wind, 106, 142, 144, 153, 184, 209, 229.
+
+  Wire-worms, 109, 181.
+
+  Witches' brooms, 116, 222, 224.
+
+  Wood, 124.
+
+  Wood-ashes, 161.
+
+  Woodbine, 112, 210.
+
+  Wood-boring, 204, 205.
+
+  Woodlice, 164.
+
+  Wood-nodules, 225.
+
+  Wood-wasps, 206.
+
+  Woolly-aphis, 219, 223.
+
+  Worms, 109, 142, 144, 194, 238.
+
+  Wounds, 108, 139, 194, 204, 207, 213, 260, 263, 269.
+
+  Wound-cork, 195.
+
+  Wound-fever, 123.
+
+  Wound-fungi, 203, 204, 240.
+
+  Wound-gum, 125.
+
+  Wound-wood, 124.
+
+  Wrens, 165.
+
+  Wrinkling, 253.
+
+
+  _Xenia_, 267.
+
+  _Xyloma_, 88.
+
+
+  Yeasts, 134, 172, 231, 233.
+
+  Yellowing, 179, 181, 182, 184.
+
+  Yellow leaves, 89.
+
+  Yellow spots, 186, 187, 188, 253.
+
+
+  Zoospores, 151.
+
+
+GLASGOW: PRINTED AT THE UNIVERSITY PRESS BY ROBERT MACLEHOSE AND CO.
+
+
+
+
+MACMILLAN AND CO.'S WORKS ON BOTANY.
+
+BY THE SAME AUTHOR.
+
+Crown 8vo. Price 6s.
+
+
+Timber and Some of its Diseases.
+
+     By H. MARSHALL WARD, D.Sc., F.R.S., F.L.S., Fellow of Sidney
+     Sussex College, and Professor of Botany in the University of
+     Cambridge. Illustrated.
+
+          _MANCHESTER EXAMINER._--"The subject as a whole is
+          one which is little understood in England, and
+          Professor Marshall Ward's work cannot fail to be
+          useful. The student will be much helped by the
+          numerous illustrations."
+
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+          appearance we hail with great satisfaction. . . . We
+          heartily recommend its perusal to those concerned."
+
+     =The Study of the Biology of Ferns by the Collodion Method.= For
+     Advanced and Collegiate Students. By GEORGE F. ATKINSON,
+     Ph.B., Associate Professor of Cryptogamic Botany in Cornell
+     University. 8vo. 8s. 6d. net.
+
+     =On British Wild Flowers considered in Relation to Insects.= By
+     LORD AVEBURY, F.R.S., D.C.L., LL.D. Illustrated. Cr. 8vo. 4s.
+     6d.
+
+     =Flowers, Fruits, and Leaves.= With illustrations. By LORD
+     AVEBURY, F.R.S., D.C.L., LL.D. Cr. 8vo. 4s. 6d.
+
+     =Lessons with Plants.= By Prof. L. H. BAILEY. Cr. 8vo. 7s. 6d.
+
+     =First Lessons with Plants.= By Prof. L. H. BAILEY. Cr. 8vo. 2s.
+     6d.
+
+     =Botany.= An Elementary Text Book for Schools. By Prof. L. H.
+     BAILEY. Ex. cr. 8vo. 6s.
+
+     =First Lessons in Practical Botany.= By G. T. BETTANY. Pott 8vo.
+     1s.
+
+     =A Course of Practical Instruction in Botany.= By F. O. BOWER,
+     D.Sc., F.L.S., Regius Professor of Botany in the University of
+     Glasgow. Cr. 8vo. 10s. 6d.
+
+     =Practical Botany for Beginners.= By F. O. BOWER, D.Sc., F.L.S.
+     Gl. 8vo. 3s. 6d.
+
+     =Lectures on the Evolution of Plants.= By DOUGLAS HOUGHTON
+     CAMPBELL, Ph.D., Professor of Botany in the Leland Stanford
+     Junr. University. Cr. 8vo. 4s. 6d. net.
+
+     =Botany for Beginners.= By ERNEST EVANS, Burnley Technical
+     School. Globe 8vo. 2s. 6d.
+
+     =The Teaching Botanist.= A Manual of Information upon Botanical
+     Teaching, with an outline for a general course. By WILLIAM F.
+     GANONG, Professor of Botany in Smith College. Cr. 8vo. 5s.
+
+     =Structural Botany; or, Organography on the Basis of
+     Morphology.= To which is added the Principles of Taxonomy and
+     Phytography, and a Glossary of Botanical Terms. By ASA GRAY,
+     LL.D., Fisher Professor of Natural History (Botany) in Harvard
+     University. 8vo. 10s. 6d.
+
+     =Text-Book of the Diseases of Trees.= By Prof. R. HARTIG of the
+     University of Munich. Trans. by Prof. WM. SOMERVILLE, F.R.S.,
+     F.L.S., Professor of Agriculture and Forestry, Durham College
+     of Science, Newcastle-on-Tyne. Introduction by Prof. H.
+     MARSHALL WARD, D.Sc., F.R.S., F.L.S., Professor of Botany at
+     the Royal Indian Engineering College, Cooper's Hill. With
+     numerous illustrations. 8vo. 10s. net.
+
+     =The Student's Flora of the British Isles.= By Sir J. D. HOOKER,
+     M.D., D.C.L., LL.D., F.R.S. Third edition. Globe 8vo. 10s. 6d.
+
+     =A Primer of Botany.= By the same. Pott 8vo. 1s.
+
+     =Timber and Timber Trees, Native and Foreign.= By THOMAS
+     LASLETT. Second edition. Revised by H. MARSHALL WARD, F.R.S.
+     Cr. 8vo. 8s. 6d.
+
+     =The Yew Trees of Great Britain and Ireland.= By JOHN LOWE,
+     M.D., Honorary Physician to His Royal Highness the Prince of
+     Wales; Fellow of the Linnean Society; Fellow of the Botanical
+     Society of Edinburgh. Illustrated. 8vo. 10s. net.
+
+     =The Nature and Work of Plants.= An Introduction to the Study of
+     Botany. By D. T. MACDOUGAL, Ph.D. Crown 8vo. 4s. 6d.
+
+     =Introduction to Study of Seaweeds.= By G. MURRAY. Cr. 8vo. 7s.
+     6d.
+
+     =British Forest Trees and their Sylvicultural Characteristics
+     and Treatment.= By J. NISBET. Cr. 8vo. 6s. net.
+
+     =Lessons in Elementary Botany.= The Part on Systematic Botany
+     based upon material left in manuscript by the late Professor
+     Henslow. By DANIEL OLIVER, F.R.S., F.L.S., formerly Keeper of
+     the Herbarium and Library of the Royal Gardens, Kew, and
+     Professor of Botany in University College, London.
+     Illustrated. Fcap. 8vo. 4s. 6d.
+
+     =First Book of Indian Botany.= By DANIEL OLIVER, F.R.S., F.L.S.
+     Illustrated. Ex. fcap. 8vo. 6s. 6d.
+
+     =Laboratory Practice for Beginners in Botany.= By WILLIAM A.
+     SETCHELL, Ph.D., Professor of Botany in the University of
+     California. Fcap. 8vo. 4s. 6d. net.
+
+     =Economic Plants, Dictionary of Popular Names of=; Their
+     History, Products, and Uses. By J. SMITH. 8vo. 14s.
+
+     =A Text-Book of Botany.= By Dr. E. STRASBURGER, Dr. FRITZ NOLL,
+     Dr. H. SCHENCK, Dr. A. F. W. SCHIMPER. Translated from the
+     German by H. C. PORTER, Ph.D., Assistant Instructor of Botany,
+     University of Pennsylvania. Revised and Edited by A. C.
+     SEWARD. With 594 illustrations, in part coloured. Medium 8vo.
+     18s. net.
+
+     =The Herb of the Field.= By CHARLOTTE M. YONGE. New edition,
+     revised. Cr. 8vo. 5s.
+
+
+MACMILLAN AND CO., LTD., LONDON.
+
+
+
+
+Transcriber's Notes:
+
+
+The word Oedema uses an OE ligature in the original.
+
+The following corrections have been made to the text:
+
+     Page vi: be the better for a real knowledge[original has
+     knowlege]
+
+     Page 55: and[original has and and] are too crudely mechanical
+
+     Page 117: Prillieux[original has Prilleux], _Maladies des
+     Plantes Agricoles_
+
+     Page 128: the intercellular mycelium of _Exoascus_[original
+     has Exoacus]
+
+     Page 134: subject to attacks of Uredineae[original has
+     Uredinae]
+
+     Page 142: carried[original has carrried] from plant to plant
+
+     Page 176: its æcidia[original has æcida] on the Barberry
+
+     Page 182: _e.g._ _Oniscus_[original has Oscinis], the Frit
+     Fly, and _Cecidomyia_[original has Cecidomya]
+
+     Page 182: not necessarily less ash constituents[original has
+     constitutents]
+
+     Page 183: nature of a transmissible enzyme[original has
+     enyzme]
+
+     Page 203: _Krankh. d. Pflanzen_, B. I.[original has 1] cap. 2
+
+     Page 206: leaves of Apples by _Lyonetia_[original has
+     Lyonettia]
+
+     Page 218: _Epichloë_[original has Epichloe], which clothes the
+     sheaths
+
+     Page 219: beetle which attacks Crucifers[original has
+     Crucificers]
+
+     Page 221: on the green parts of Hibiscus,[comma missing in
+     original]
+
+     Page 221: nodules of the roots of Leguminoseae[original has
+     Leguminosae]
+
+     Page 230: _Edelfäule_[original has Edelfaüle], a rotten
+     condition of the grapes
+
+     Page 235: giving an almost mealy[original has meally]
+     appearance
+
+     Page 243: as its mycelium[original has myceliun] spreads
+
+     Page 258: _Prolepsis._[original has Proplesis]--It frequently
+
+     Page 293: Aetiology[original has Ætiology], 89, 100.
+
+     Page 293: _Anthonomus_[original has Anthonomos], 249.
+
+     Page 294: Bird's-eye[original has Birds'-eye] Maple, 224.
+
+     Page 295: _Cercospora_,[original has Cereospora] 190.
+
+     Page 298: _Eau Céleste_[original has Celeste], 162.
+
+     Page 300: _Heterodora_[original has Heterodera], 219, 220.
+
+     Page 300: Holly, 217.[period missing in original]
+
+     Page 300: _Hypomyces_, 237.[original has comma]
+
+     Page 301: _Lyonetia_[original has Lyonetra], 206.
+
+     Page 303: Permanganate[original has Permangate], 162.
+
+     Page 304: Prophylactic[original has Phophylactic] measures,
+     160.
+
+     Page 304: _Phytomyza_, 206.[period missing in original]
+
+     Page 304: _Phyllereum_[original has Phyllereus], 253.
+
+     Page 304: Pine-apple[original has Pine apple], 258.
+
+     Page 305: _Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247,
+     252[original has 252, 247].
+
+     Page 307: Somato-plasm[original has Somatoplasm], 267.
+
+     Page 307: Spermogonia[original has Spermagonia], 144, 232.
+
+     Page 308: _Tomicus_[original has Tornicus], 205.
+
+The following index entries were out of alphabetical order and have been
+moved to the appropriate locations:
+
+     Phylloxera
+     Plants, dying out of
+     Poisonous gases
+     Preventible diseases
+     Prophylactic measures
+     Spermogonia
+
+
+
+
+
+End of the Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
+
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+<pre>
+
+The Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Disease in Plants
+
+Author: H. Marshall Ward
+
+Release Date: February 29, 2012 [EBook #39011]
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DISEASE IN PLANTS ***
+
+
+
+
+Produced by Chris Curnow, Lisa Reigel, and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+</pre>
+
+
+
+<div class="notebox">
+<p>Transcriber's Notes: Variations in spelling and hyphenation have been
+left as in the original. Ellipses match the original.</p>
+
+<p>A few typographical errors have been corrected. A complete list as well
+as other notes <a href="#TN">follows</a> the text.</p>
+
+<p>Click on the page number to see an image of the page.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_i" id="Page_i"></a>[<a href="./images/i.png">i</a>]</span></p>
+<div class="title">
+<h1>DISEASE IN PLANTS</h1>
+</div>
+
+<p><span class="pagenum"><a name="Page_ii" id="Page_ii"></a>[<a href="./images/ii.png">ii</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_iii" id="Page_iii"></a>[<a href="./images/iii.png">iii</a>]</span></p>
+<h1>DISEASE IN PLANTS</h1>
+
+<div class="author">
+<p class="tpother">BY</p>
+
+<p class="tpauthor">H. MARSHALL WARD, Sc.D., F.R.S.</p>
+
+<p class="tpcredentials">FELLOW OF SIDNEY SUSSEX COLLEGE, HONORARY FELLOW OF CHRIST'S COLLEGE<br />
+AND PROFESSOR OF BOTANY IN THE UNIVERSITY OF CAMBRIDGE</p>
+
+<p class="tpcredentials">PRESIDENT OF THE BRITISH MYCOLOGICAL SOCIETY, AND FELLOW OF THE<br />
+LINNEAN AND ROYAL HORTICULTURAL SOCIETIES; HONORARY FELLOW<br />
+OF THE MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY<br />
+AND OF THE BOTANICAL SOCIETY OF EDINBURGH</p>
+</div>
+
+<div class="publisher">
+<p class="tppublisher">London</p>
+
+<p class="tppublisher">MACMILLAN AND CO., <span class="smcap">Limited</span></p>
+
+<p class="tppublisher">NEW YORK: THE MACMILLAN COMPANY</p>
+
+<p class="tppublisher">1901</p>
+
+<p class="tpother"><i>All rights Reserved</i></p>
+</div>
+
+
+<hr class="newchapter" />
+<div class="publisher">
+<p class="tppublisher"><span class="pagenum"><a name="Page_iv" id="Page_iv"></a>[<a href="./images/iv.png">iv</a>]</span>GLASGOW: PRINTED AT THE UNIVERSITY PRESS</p>
+
+<p class="tppublisher">BY ROBERT MACLEHOSE AND CO.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_v" id="Page_v"></a>[<a href="./images/v.png">v</a>]</span></p>
+<h2>PREFACE.</h2>
+
+
+<p>It has often been represented to me that the cultivators of plants,
+among whom are to be included planters and foresters, as well as
+agriculturists and gardeners of every kind, are more particularly
+concerned with, and interested in, the maladies themselves of the plants
+they grow, than in the life-history of the fungi, insects or other
+organisms to which they are due, or in the physiological processes which
+are involved; and although it is impossible to really understand any
+disease unless we also understand the processes by which it is brought
+about, there is room for sympathy with the point of view of the
+cultivator. He says, in effect, "I do not want to know all about the
+biology of the fungus of wheat-rust, or of the <i>phylloxera</i>, nor do I
+want to learn what experts can tell me about the action of bacteria in
+soil, or the process of starch-formation in the leaves: I have neither
+the time nor the means to master these details. What I want is guidance
+as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir
+trees, turnips, etc., <span class="pagenum"><a name="Page_vi" id="Page_vi"></a>[<a href="./images/vi.png">vi</a>]</span>and what I am to do to set things right." Just
+so. With the latter part of this cry one must sympathize, much as a
+doctor does with the wail of the parent who calls him in to cure his
+sick child&mdash;we need not stop to classify or compare the motives of the
+parent and the cultivator, and perhaps I had done better to select a
+breeder of sheep with his flock and a veterinary doctor in the
+illustration, but we will let it pass; and as regards the former part of
+the cry, I do not know that the plant-doctor can expect the cultivator
+to be initiated in the aetiology of the disease any more than the
+physician expects the parent to understand the biology of the typhoid
+bacillus. That both the cultivator and the parent would be the better
+for a real knowledge of the disease in either case must be admitted&mdash;nay
+insisted on, provided the knowledge <i>is</i> real&mdash;but we have to deal with
+facts, and it is a fact that the clients of both doctors are impatient
+of the details of the case.</p>
+
+<p>Now, of course, I am aware that no short cut or "royal road" to science
+exists, and if a man is going to train up trees or other plants, he
+ought to know all about them in health and in sickness, in youth and in
+old age, and he ought to learn everything about the soil they grow in,
+the air that surrounds them, the enemies that beset them, and all the
+multifarious relations of these one to another; but when I look at my
+boy and reflect how much his nurse, his schoolmaster, his tutor, his
+doctor, and his parents <i>ought</i> to know successively <span class="pagenum"><a name="Page_vii" id="Page_vii"></a>[<a href="./images/vii.png">vii</a>]</span>and simultaneously
+about him in sickness and in health, and about his surroundings, etc., I
+begin to wonder whether there is not after all something to be said for
+the cultivator's point of view.</p>
+
+<p>Moreover, the cultivator knows a good deal about his plants which I do
+not know, and although I should much like to know it, his plea of want
+of time rings in my ears and the conviction strikes home that one ought
+to try and meet his views, and tell him something about disease as
+manifested in plants without insisting on his becoming a professional
+mycologist, entomologist, agricultural chemist, and philosopher.</p>
+
+<p>Of course, beyond a certain point, it is his lookout how much the
+information is worth, and its educational value&mdash;a very different
+matter&mdash;is sure to suffer from any restrictions imposed on the treatment
+of the subject; but if the theme of disease in plants, treated from a
+general point of view&mdash;I was about to write "treated in a popular
+manner," but that is impossible until physiology and mycology are more
+widely taught&mdash;enables him to understand better the questions he puts to
+himself, and, still more, if it stimulates him to enquire further into
+the inexhaustible field of science glimpsed at, something may come of
+it.</p>
+
+<p>The purpose of these essays is to treat the subject of disease in plants
+with special reference to the patient itself, and to describe the
+symptoms it exhibits and the course of the malady, with only such
+references to the agents which induce <span class="pagenum"><a name="Page_viii" id="Page_viii"></a>[<a href="./images/viii.png">viii</a>]</span>or cause disease as are necessary
+to an intelligent understanding of the subject, and of the kind of
+treatment called for. Consequently I have avoided any unnecessary
+classification or elaborate descriptions of parasitic fungi or insects,
+histological details of the tissues of plants, chemical and physical
+details regarding the soil, and even matters purely physiological as far
+as possible. Several admirable works on these subjects are already
+available, and must be referred to for further details.</p>
+
+<p>It is, however, quite out of the question to avoid technicalities,
+though I have chosen the simpler course wherever it was found feasible,
+and have tried to so employ the examples selected that the student who
+wishes to go further into the matters dealt with may turn to special
+treatises for further information. For one eminently technical section I
+ought perhaps to apologise, but the temptation to try and set forth, in
+concrete form and suitable for the purposes of this book, some account
+of what is known of the most essential and profound factors concerned in
+the difficult question of the nature of life and death, health and
+disease, was great. Probably my apology should go further, and apply to
+what after all must be failure to explore this mystery to the bottom: my
+only excuse must be that it may stimulate others to go further.</p>
+
+<p>It was an afterthought to add, in Part I., the considerations on the
+factors which influence the <span class="pagenum"><a name="Page_ix" id="Page_ix"></a>[<a href="./images/ix.png">ix</a>]</span>plant regarded as a living machine, so to
+speak, in order that the student may the better apprehend the point of
+view taken of the bearings of the matters discussed in Part II.</p>
+
+<p>With regard to references, it seemed a better plan to give, in the form
+of notes after each chapter, the titles of the principal books and
+papers on which a student may base a further course of reading, than to
+overweight the pages of what is, after all, merely an introductory
+sketch to a huge subject, with detailed quotations from the numerous
+sources of information made use of. I have freely expressed my own
+opinions, but the sources for others are, I hope, as freely given. It
+will, however, be understood that I have not aimed at a complete
+bibliography, and, particularly, I have only given foreign references
+where it seemed that adequate treatment of the subject could not be
+found in English.</p>
+
+<p>My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly
+looked through many of the proofs, and given me the benefit of several
+suggestions: and to my wife for the very material aid she has afforded
+me in the preparation of the index.</p>
+
+<p class="letterauthor1">H. MARSHALL WARD.</p>
+
+<p class="lettertown"><span class="smcap">Cambridge</span>,<br />
+<span style="margin-left: 2em;"><i>November, 1900</i>.</span></p>
+
+<p><span class="pagenum"><a name="Page_x" id="Page_x"></a>[<a href="./images/x.png">x</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_xi" id="Page_xi"></a>[<a href="./images/xi.png">xi</a>]</span></p>
+<h2>CONTENTS.</h2>
+
+
+<table summary="Table of Contents" cellpadding="2" cellspacing="2" border="0">
+<tr>
+  <td class="tdtocpart" colspan="2">PART I.&mdash;SOME FACTORS.</td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER I.</td>
+</tr>
+<tr>
+  <td class="tdtocpage" colspan="2" style="font-size: 70%;">PAGE</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Plant and its Surroundings,</td>
+  <td class="tdtocpage"><a href="#Page_1">1</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER II.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Plant and its Food,</td>
+  <td class="tdtocpage"><a href="#Page_7">7</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER III.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Plant a Living Machine,</td>
+  <td class="tdtocpage"><a href="#Page_15">15</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER IV.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Metabolism,</td>
+  <td class="tdtocpage"><a href="#Page_23">23</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER V.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Roots and Root-Hairs,</td>
+  <td class="tdtocpage"><a href="#Page_35">35</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER VI.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Functions of Root-Hairs,</td>
+  <td class="tdtocpage"><a href="#Page_45">45</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2"><span class="pagenum"><a name="Page_xii" id="Page_xii"></a>[<a href="./images/xii.png">xii</a>]</span>CHAPTER VII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Biology of Soil,</td>
+  <td class="tdtocpage"><a href="#Page_56">56</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER VIII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Hybridisation and Selection,</td>
+  <td class="tdtocpage"><a href="#Page_69">69</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocpart" colspan="2">PART II.&mdash;DISEASE IN PLANTS.</td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER IX.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Phytopathology. Derivation and Meaning,</td>
+  <td class="tdtocpage"><a href="#Page_85">85</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER X.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Health and Disease,</td>
+  <td class="tdtocpage"><a href="#Page_91">91</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XI.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Causes of Disease,</td>
+  <td class="tdtocpage"><a href="#Page_99">99</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc" style="padding-right: 5em;">Causes of Disease. The Living Environment,</td>
+  <td class="tdtocpage"><a href="#Page_108">108</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XIII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Nature of Disease,</td>
+  <td class="tdtocpage"><a href="#Page_119">119</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XIV.</td>
+</tr>
+<tr>
+  <td class="tdtocentry"><span style="font-variant: small-caps;">Nature of Disease</span> (<i>Continued</i>),</td>
+  <td class="tdtocpage"><a href="#Page_130">130</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2"><span class="pagenum"><a name="Page_xiii" id="Page_xiii"></a>[<a href="./images/xiii.png">xiii</a>]</span>CHAPTER XV.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Spreading of Disease and Epidemics,</td>
+  <td class="tdtocpage"><a href="#Page_142">142</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XVI.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">The Factors of an Epidemic,</td>
+  <td class="tdtocpage"><a href="#Page_149">149</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XVII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Remedial Measures,</td>
+  <td class="tdtocpage"><a href="#Page_159">159</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XVIII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Variation and Disease,</td>
+  <td class="tdtocpage"><a href="#Page_168">168</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XIX.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Symptoms of Disease,</td>
+  <td class="tdtocpage"><a href="#Page_179">179</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XX.</td>
+</tr>
+<tr>
+  <td class="tdtocentry"><span style="font-variant: small-caps;">Symptoms of Disease</span> (<i>Continued</i>),</td>
+  <td class="tdtocpage"><a href="#Page_186">186</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXI.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Artificial Wounds,</td>
+  <td class="tdtocpage"><a href="#Page_194">194</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Natural Wounds,</td>
+  <td class="tdtocpage"><a href="#Page_204">204</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXIII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Excrescences,</td>
+  <td class="tdtocpage"><a href="#Page_212">212</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2"><span class="pagenum"><a name="Page_xiv" id="Page_xiv"></a>[<a href="./images/xiv.png">xiv</a>]</span>CHAPTER XXIV.</td>
+</tr>
+<tr>
+  <td class="tdtocentry"><span style="font-variant: small-caps;">Excrescences</span> (<i>Continued</i>),</td>
+  <td class="tdtocpage"><a href="#Page_222">222</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXV.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Exudations and Rotting,</td>
+  <td class="tdtocpage"><a href="#Page_227">227</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXVI.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Necrotic Diseases,</td>
+  <td class="tdtocpage"><a href="#Page_240">240</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXVII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Monstrosities and Malformations,</td>
+  <td class="tdtocpage"><a href="#Page_246">246</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXVIII.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Proliferations,</td>
+  <td class="tdtocpage"><a href="#Page_257">257</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXIX.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Grafts,</td>
+  <td class="tdtocpage"><a href="#Page_262">262</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdtocchapter" colspan="2">CHAPTER XXX.</td>
+</tr>
+<tr>
+  <td class="tdtocentrysc">Life and Death,</td>
+  <td class="tdtocpage"><a href="#Page_271">271</a></td>
+</tr>
+<tr><td>&nbsp;</td></tr>
+<tr>
+  <td class="tdleft">INDEX,</td>
+  <td class="tdtocpage"><a href="#Page_293">293</a></td>
+</tr>
+</table>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_xv" id="Page_xv"></a>[<a href="./images/xv.png">xv</a>]</span></p>
+<h2><i>PART I.</i></h2>
+
+<h3>SOME FACTORS.</h3>
+
+<p><span class="pagenum"><a name="Page_xvi" id="Page_xvi"></a>[<a href="./images/xvi.png">xvi</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_1" id="Page_1"></a>[<a href="./images/1.png">1</a>]</span></p>
+<h2>CHAPTER I.</h2>
+
+<h3>THE PLANT AND ITS SURROUNDINGS.</h3>
+
+<div class="chsub">
+<p>The plant the central object of study&mdash;soil, climate,
+atmosphere, etc., are factors of its environment. Agricultural
+chemistry. The plant a machine. Physiology.</p>
+</div>
+
+
+<p>If I were asked to sum up the most important result of the numerous
+advances made during the past decade in agriculture and forestry, I
+should reply&mdash;the clearer and wider recognition of the fact that the
+plant itself is the centre of the subject, and not the soil, climate,
+season, or other factors of its environment. Until comparatively recent
+times it was the habit of farmers, foresters, planters, and gardeners,
+all the world over, to look upon the plant as a mere item or as a
+mysterious if important one in their calculations, and to regard the
+soil as the chief factor in their studies.</p>
+
+<p>Now all is changing, and the world is gradually awakening more and more
+to the recognition of the truth that the soil and the clouds and the
+<span class="pagenum"><a name="Page_2" id="Page_2"></a>[<a href="./images/2.png">2</a>]</span>atmosphere are merely reservoirs of more or less inert materials, from
+which the living plant draws its supplies, and works them up, by means
+of energy focussed from the sun, into new plant substance.</p>
+
+<p>In other words, the more far-seeing pioneers of scientific agriculture
+and forestry, etc., are recognising that agricultural chemistry is not
+the be-all and end-all of agricultural science; but that, in place of
+the study of the chemical analyses of dead soil, water, air, and
+plant-remains, which has so long held sway, largely owing, I think, to
+the influence of Liebig, the student should have his attention more
+concentrated on the living plant itself and on the physiological actions
+which make up its life. He must regard the living plant as a sort of
+working machine&mdash;infinitely more complex than any machine made by man,
+but a machine nevertheless&mdash;the purpose of which is to store up energy
+from the sun, and so to add to our wealth on this planet, at the expense
+of the extra-terrestrial universe.</p>
+
+<p>It is not, be it noted, that the new study proposes to ignore or abandon
+the old studies: modern physiology owes too much to the physics and
+chemistry on which it is partly based, and to the labours of De
+Saussure, Ingenhousz, Priestley, and others, for that. But it is that
+the new study recognises that the central point, to which all views must
+be focussed, is not the one that it was formerly supposed to be. The
+student is still taught that the chemistry of soils yields <span class="pagenum"><a name="Page_3" id="Page_3"></a>[<a href="./images/3.png">3</a>]</span>valuable
+information, and that lessons of importance are derived from comparisons
+of the analyses of the ashes, etc., of plants; but he is no longer able
+to cherish the hope, however remotely, that such studies solve his most
+important problems.</p>
+
+<p>The scene&mdash;or rather the point to which attention is now directed&mdash;is
+the living, working, energy-accumulating plant itself, and not the dead
+store of materials in the soil. The reason for the change is not far to
+seek: it is due to the enormous strides made in the study of the
+physiology of plants during the last quarter of a century, and the
+subject abounds in examples illustrating the marvellous advances that
+have been made, and at the same time showing how, in the progress of
+researches, made for their own sake&mdash;<i>i.e.</i> in pursuit of satisfaction
+for the intense curiosity of the scientific man&mdash;all kinds of side
+issues turn up which prove to be of value in practice, and suggestive of
+further thinking.</p>
+
+<p>At the beginning of the nineteenth century&mdash;<i>i.e.</i> about 1820&mdash;the best
+thinkers were giving up the old ideas that the environment supplied
+food, as such, to plants, and had recognised that the plant takes up
+substances from without and rearranges these in its own body.</p>
+
+<p>The next twenty years or so form a very dark interval in plant
+physiology, chiefly owing to the influence of the assumption of a
+special "vital force," an assumption which was not allowed merely to
+serve as a hypothesis put forward to stimulate <span class="pagenum"><a name="Page_4" id="Page_4"></a>[<a href="./images/4.png">4</a>]</span>research and suggest
+better ideas, but which gained a hold over men's powers of reasoning to
+an extent which now appears monstrous and phenomenal.</p>
+
+<p>Many errors crept in during this reign of terror, one of the most fatal
+of which was De Candolle's revival of the idea of "spongioles"; and
+another, equally disastrous in many of its effects, was the conception
+of a sort of vegetable food-extract, humus, existing in the soil in a
+form peculiarly suitable for direct use by plants. It was during this
+period that the confusion between the processes of respiration and
+carbon-dioxide assimilation arose, and exerted its effects for evil into
+our own day.</p>
+
+<p>The now astounding statement that oxygen-respiration in plants did not
+occur, laid the foundation of many subsequent difficulties, and so did
+the positive and authoritative views on the uses of minerals to the
+plant. Liebig, in fact, stood in the invidious position of being a high
+authority on purely chemical questions, who was impelled to give
+opinions on matters which can only be solved by physiological
+experiments: his great service was to clear up mistakes as regards the
+chemistry of soils and of plants&mdash;his great mistakes were due to his
+pronouncing on physiological matters; and it may be doubted whether his
+great services to the purely chemical side of subjects connected with
+agricultural matters are the more to be admired, or the disastrous
+influence of his statements on subjects which do not belong to the
+<span class="pagenum"><a name="Page_5" id="Page_5"></a>[<a href="./images/5.png">5</a>]</span>domain of chemistry should be the more deplored. Be that as it may, he
+handed on to succeeding generations some weighty errors as regards
+plant-life, and taught the agriculturist to regard chemical analyses of
+soils and plant ashes with a reverence which obstructed progress for
+some time. As a set-off to this we must place his contributions to the
+destruction of the bugbear vitalism, which was simply preventing
+enquiry, and his services in bringing together and sifting with power
+and originality all that had been then acquired as regards the chemistry
+of the plant, the soil, and the atmosphere.</p>
+
+<p>That Liebig was indispensable in 1840-1850 is one thing; but that his
+influence should extend to the present day is quite another, and his
+inevitable mistakes were almost as powerful for future evil, as his
+clear exposition of the chemistry of his day was productive of immediate
+good.</p>
+
+<p>Boussingault, working at the same time, 1837-1855, but experimentally
+with the living plant, taught us more about these matters than any
+investigator of the time, though it is very probable that the stimulus
+of Liebig's speculations, good and bad, had its effect in impelling
+Boussingault to devote his splendid methods to problems of
+plant-nutrition. Boussingault's contributions to our knowledge of the
+composition of the dead plant cannot be over-estimated; but he did more
+than this, because he so clearly apprehended the necessity for asking
+his questions directly of the living plant, instead of deducing <span class="pagenum"><a name="Page_6" id="Page_6"></a>[<a href="./images/6.png">6</a>]</span>from
+chemical principles what might be supposed to occur in it; and although
+future researches showed that even so careful an investigator solved a
+problem of first importance&mdash;viz. the question of the fixation of free
+nitrogen&mdash;the wrong way, it will be found that so far as he did go his
+conclusions were sound, and well calculated to inspire the confidence
+with which the world received them. As we are here concerned more
+especially with the botany of agriculture, however, it is unnecessary to
+dwell longer on these matters, or on the similar and even more extensive
+experiments, of world-wide reputation, carried on for so many years, and
+still being carried on under the liberal auspices of Sir John Lawes, at
+Rothamsted. Moreover it may be necessary to return to some of these
+points later on.</p>
+
+
+<h4><span class="smcap">Notes to Chapter I.</span></h4>
+
+<div class="chnote">
+<p>The reader will find a further general account of these
+matters in Sachs' <i>Lectures on the Physiology of Plants</i>,
+especially Lectures I. and XII., Engl. ed., Oxford, 1887. He
+may then proceed to Pfeffer's <i>Physiology of Plants</i>, Engl.
+ed., 1899, chapter I., and to the account of the history of
+the subject in Sachs' <i>History of Botany</i>, Oxford, 1890,
+especially pp. 359-375 and 445-524. References to more special
+literature will be found in Pfeffer.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_7" id="Page_7"></a>[<a href="./images/7.png">7</a>]</span></p>
+<h2>CHAPTER II.</h2>
+
+<h3>THE PLANT AND ITS FOOD.</h3>
+
+<div class="chsub">
+<p>The food of plants&mdash;"Vital force"&mdash;Other errors&mdash;Liebig and
+Boussingault&mdash;The botany of agriculture. The synthesis of
+carbohydrates&mdash;The physiology of plant-nutrition. The
+persistence of misconceptions.</p>
+</div>
+
+
+<p>The year 1860 may be regarded as a landmark of importance in the history
+of plant physiology, for it was in that year that Sachs discovered that
+the bringing together of water and carbon-dioxide, in the green
+chlorophyll-corpuscles of the plant exposed to sunlight, results in the
+formation of the grains of starch found in these corpuscles.</p>
+
+<p>Previous to this date Dutrochet (1826-37) had introduced the then crude
+idea of osmosis into physiology; vegetable anatomy had improved, and the
+modern conceptions of the living cell, protoplasm, nucleus, etc., were
+slowly looming; sieve-tubes had been discovered, and the proteids and
+starch in various parts of the plant examined; and the suggestion was
+abroad, replacing Liebig's <span class="pagenum"><a name="Page_8" id="Page_8"></a>[<a href="./images/8.png">8</a>]</span>idea that plant acids were the first
+products of carbon-assimilation, that some substance, of a slimy nature,
+was manufactured in the cells of the leaves and thence distributed as
+the formative material from which the plant constructed its parts. Davy
+and Boussingault had even surmised that a carbohydrate might be the
+first-formed product in assimilation.</p>
+
+<p>There can be little doubt that Sachs' classical proof, by direct
+physiological observation and experiment, first brought forward the
+truth of organic synthesis in the plant in a concrete and convincing
+form.</p>
+
+<p>But it did more than that. It laid the foundation of the modern
+physiology of plant-nutrition on ground already prepared by De Saussure
+and the earlier workers; for, in addition to emphasising the truth of
+organic synthesis&mdash;a truth which had been gradually impressing itself on
+the world for some years&mdash;Sachs' discovery showed clearly the real
+meaning of carbon-assimilation as a process for obtaining combustible
+food, which the plant then proceeds to make use of.</p>
+
+<p>Many points were rapidly cleared up at once, or if not explained were at
+least put into a strong light for further enquiry, and plant-nutrition
+soon ceased to be the mysterious subject for all kinds of wild
+conjectures that it had hitherto been.</p>
+
+<p>The meaning of thin leaves, with numerous stomata and finely ramified or
+divided vascular bundles, became more apparent, as also did the
+significance of the ascending transpiration current; <span class="pagenum"><a name="Page_9" id="Page_9"></a>[<a href="./images/9.png">9</a>]</span>the storage of
+starch-grains in tubers, medullary rays, roots, seeds, etc., obtained
+meanings not understood before; the spread of roots in the soil, and the
+gradually discovered properties of the finer rootlets and of the
+root-hairs, fitted naturally into their places; and, in short, a
+thousand facts, otherwise isolated, became collated into an intelligible
+system, full of suggestions for new work, such as has since gone on and
+is now being pursued with an activity and success never before realised
+in the history of science.</p>
+
+<p>As time went on, while the general truth of Sachs' views was confirmed,
+a number of detailed discoveries were made which seemed to contradict
+them in certain points. It was found that not all leaves form starch,
+for some contain sugar or oil; but Holle and Godlewski proved
+experimentally that this oil may be replaced by starch if the conditions
+of assimilation are slightly modified. More recently Hébert discovered
+that the stalks and leaves of grasses contain a peculiar form of gum,
+which was formerly confounded with starch, a substance not abundant in
+them. Then came Schimper's discovery of starch-forming corpuscles,
+which, if supplied with sugar, are able to form starch-grains in the
+dark, as in tubers, etc., underground; and as subsequent researches have
+proved that the chlorophyll-corpuscles&mdash;which are morphologically the
+same as the starch-forming corpuscles and can be replaced by them&mdash;are
+also able to form starch-grains from sugar, as proved by the experiments
+of Boehm, Acton, Meyer, Laurent, <span class="pagenum"><a name="Page_10" id="Page_10"></a>[<a href="./images/10.png">10</a>]</span>Bokorny, Saposchnikoff, and others, it
+soon became evident that nothing essential needed altering in Sachs'
+view that starch is the first visible product of carbon-dioxide
+assimilation, only it became clearer that the starch-grains are built up
+by the protoplasm from glucose or some similar body, and represent so
+many packets of reserve materials put by for the present because not
+required for the immediate needs of the cell.</p>
+
+<p>Boussingault showed, about thirty years ago, that assimilation soon
+stops in green leaves if cut off from the plant, not because the leaves
+die, but owing to some "maximum capacity" being attained. Sachs had
+shown that the starch passes down to other parts of the plant in
+solution as glucose.</p>
+
+<p>Neither time nor space will permit me to go into the enormous field of
+research and results opened up by these and similar observations made
+between 1860-70. It must suffice to say that they led to the discovery
+and study of the diastatic and other enzymes in the leaves and other
+green parts of plants, and to a clearer understanding of what was
+already known of them in seeds, and this knowledge reacted at once on
+our insight into the processes of transport of reserve materials and
+constructive materials from one part of the plant to another, matters
+which will be referred to later on.</p>
+
+<p>It remains to explain Boussingault's difficulty as regards the cessation
+of assimilation. Recent researches confirm the view that at least three
+<span class="pagenum"><a name="Page_11" id="Page_11"></a>[<a href="./images/11.png">11</a>]</span>causes are at work to bring about the inhibition of the
+carbon-assimilation: first, the chlorophyll-corpuscles become filled to
+excess with starch, which cannot get away because all the passages are
+full and the products are inhibiting the further action of the enzymes
+which should dissolve the solid granules; secondly, the leaf being
+detached from the plant explains why the soluble products cannot get
+away, for this makes a great difference in the rate of exhaustion of the
+leaf; and, thirdly, the same fact involves that the leaf can obtain no
+further supply of salts of potassium, etc., without which elements the
+processes in question cannot go on.</p>
+
+<p>These and numerous other deeper insights into the process of
+assimilation, obviously strengthen the force of Sachs' discovery; though
+it by no means necessarily follows that starch-grains are always the
+resting form of the products of assimilation, and we now know that such
+is often not the case: we now have much deeper glimpses into the initial
+products of carbon-assimilation than Sachs had in 1860, but this
+enhances rather than detracts from the importance of his splendidly
+worked-out discovery. Put more generally, we may now say that the
+process of carbon-dioxide assimilation in green leaves under the
+influence of light is a process of synthesis&mdash;photo-synthesis&mdash;resulting
+in the building up of a carbohydrate such as sugar, inulin or starch
+from the elements carbon, hydrogen and oxygen.</p>
+
+<p>But it must not be supposed that the importance of Sachs' discovery, and
+the rapid consequent <span class="pagenum"><a name="Page_12" id="Page_12"></a>[<a href="./images/12.png">12</a>]</span>extensions of our knowledge, did their work
+forthwith in disabusing men's minds of old and erroneous notions. To say
+nothing of numerous smaller misconceptions which still held their ground
+owing to the stupendous ignorance of plant-physiology which prevailed,
+we find incompetent teachers and text-books were still propagating ideas
+worthy of ancient times. The confusion between oxygen-respiration and
+the gas interchanges in carbon-assimilation was by no means eliminated
+even recently, though it can no longer withstand the deliberate
+onslaughts now made on it. That the roots take up food as such from the
+soil, and that that food is directly employed by the plant for its
+nutrition is even yet implied in daily conversation around us; and
+although matters have advanced so far that everyone now knows that the
+substances at the roots must be in solution, ere they can be received
+into the plant, it sometimes leads to astonishing replies, if we press
+the question very far as to how the absorption takes place, in an
+elementary examination of agricultural students. That manures are foods
+to the plant, that sap circulates, that transpiration is of use to keep
+the plant cool, and wood is a "porous body," etc., are only a few of the
+misconceptions still current, in a decade that has found publishers for
+a work advocating that roots are congealed sap, and that the leaves of
+plants absorb the moisture and dust of the air, and so provide the plant
+with food, and for a paper explaining <span class="pagenum"><a name="Page_13" id="Page_13"></a>[<a href="./images/13.png">13</a>]</span>the action of root-hairs as tubes
+with open pores at their tips. But the gravest misapprehensions current
+among us are due to the crude ideas as to what a plant really is: this,
+I take it, is owing to the difficulty of grasping what physiologists
+mean by organised structure, and leads to regarding the living being
+either as a mere aggregation of chemical compounds, built up by the
+ordinary play of chemical forces, as we know them, acting on dead
+matter, or, as in the days before organic chemistry, as a mysterious
+entity endowed with "vital force," and with properties not amenable to
+scientific investigation. The mistaken notions as to the powers of roots
+to "select" those substances which the plant requires, and to reject
+useless ones was merely an expression of this belief.</p>
+
+<p>The rock on which all are liable to come to grief&mdash;the chemist or
+physicist who requires all his facts in terms of analyses and
+proportions by weight, and therefore takes too mechanical a view of the
+subject, or the man who is not scientifically trained at all, and
+therefore is more liable to go to the other extreme and regard the plant
+as a mysterious something which grows and has poetical associations and
+traditions&mdash;is the great fact of organised structure, and it is the
+recognition of this fact and some of its consequences which has altered
+the whole position of the subject, and brought the study of the plant
+into the domain of physiology. The living plant, its structure and
+organisation, the functions of its <span class="pagenum"><a name="Page_14" id="Page_14"></a>[<a href="./images/14.png">14</a>]</span>mechanism, and its relations to the
+environment, thus forms a subject apart from that which concerns the
+chemical composition of the plant and its environment, and this
+distinction designates, in a word, as it were, the change which has been
+brought about by modern biology.</p>
+
+<p>A point to be emphasised to the utmost where agricultural students are
+concerned is that the essential process of feeding is the same in a
+green plant, a fungus, and an animal; the greatest confusion still
+exists with regard to this matter, owing to misconceptions as to the
+real meaning of the functions of the chlorophyll-corpuscles when
+supplied with carbon-dioxide and water and the energy of the sun's rays.
+The plant does not feed on carbon-dioxide, any more than it feeds on
+oxygen&mdash;it feeds on the organic material after it has been constructed,
+and the chlorophyll-function is merely one mode of obtaining supplies of
+such organic substance.</p>
+
+
+<h4><span class="smcap">Notes to Chapter II.</span></h4>
+
+<div class="chnote">
+<p>In addition to the references in the last chapter, the student
+should consult Sachs' <i>Lectures</i>, XVII.-XIX., and Pfeffer's
+<i>Physiology</i>, pp. 287-329, for the further development of this
+subject. An excellent résumé, with new facts and points of
+view, will be found in Dr. Horace Brown's "Address to the
+Chemical Section," <i>British Association Reports</i>, Dover, 1899;
+and "Chemistry and Physiology of Foliage Leaves" in <i>Trans.
+Chem. Soc.</i>, 1893, p. 604. See also Blackman, "Experimental
+Researches on Vegetable Assimilation and Respiration," <i>Phil.
+Trans.</i>, 1895; and Parkin, "Formation, etc., of Carbohydrates
+in Monocotyledons," <i>Phil. Trans.</i>, 1899.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_15" id="Page_15"></a>[<a href="./images/15.png">15</a>]</span></p>
+<h2>CHAPTER III.</h2>
+
+<h3>THE PLANT A LIVING MACHINE.</h3>
+
+<div class="chsub">
+<p>The plant a machine into which energy and material are
+taken&mdash;Carbon assimilation&mdash;Feeding&mdash;Accumulation and
+transformations in the plant. The action of light&mdash;The
+chlorophyll-function.</p>
+</div>
+
+
+<p>The relations of the plant to the environment can only be understood by
+taking into account the results of modern physiological discoveries.
+These teach us that the living plant is a highly complex machine, the
+details of its organisation and structure being much more numerous and
+much more closely correlated at numerous points, than the parts of any
+other machine known to us.</p>
+
+<p>They also teach us that it is supplied with energy from without, as any
+other machine; and that when so supplied, and properly working, the
+living structure or machinery does work, also as other machines. But
+modern physiology goes further, in that it renders some account of the
+ways by which the external energy is taken into <span class="pagenum"><a name="Page_16" id="Page_16"></a>[<a href="./images/16.png">16</a>]</span>the plant, and there
+applied to do work, or stored up for a time in order that it may be used
+to do work at some future time.</p>
+
+<p>The accumulation of energy thus ensured is associated with corresponding
+changes of material substance, and the principal means for bringing this
+about is recognised in the assimilation of
+carbon-dioxide&mdash;photo-synthesis.</p>
+
+<p>In this process energy enters the chlorophyll-corpuscle in the form of
+the radiant energy of the sun, it is there directed in the mechanism of
+the protoplasm, so as to do work on the molecules of water and
+carbon-dioxide which have also been brought into the machinery; this it
+does, breaking asunder their stable structure into unstable bodies,
+which then re-combine in different ways to form a carbohydrate, such as
+starch, and this starch is temporarily stored as grains, while oxygen
+escapes.</p>
+
+<p>Each starch-grain, therefore, is to be regarded as a packet of matter
+and of potential energy, as it were, capable of yielding up the latter
+at any future time, when put under such circumstances that it must do
+so. Such stores of energy-yielding substance, if I may use the
+much-abused phrase, form the principal food of the plant&mdash;or of an
+animal, if it steps in and takes them&mdash;and we now see that the process
+of carbon-dioxide assimilation, as it has perhaps unfortunately been
+called, is not the same thing as the process of feeding, for the
+<i>feeding</i>&mdash;<i>i.e.</i> the nutrition proper&mdash;of the plant does not begin
+until the <i>food</i> has been thus obtained.</p>
+
+<p><span class="pagenum"><a name="Page_17" id="Page_17"></a>[<a href="./images/17.png">17</a>]</span>We now see what the real position of the plant is, to its environment,
+whether the latter be living or dead. From our point of view, the plant
+serves as a centre for bringing together the substances obtainable from
+the soil, and those derived from the atmosphere, and so focussing and
+directing the radiant energy of the sun upon these substances, that they
+are broken up, and some of their constituents synthesised, with
+absorption of energy, into a body, such as starch, containing more
+energy than did the original substances taken together or separate. It
+matters little whether the actual carbohydrate thus synthesised is
+starch, or sugar or inulin: the point is that energy has been gained
+from outside and bound up with the acquired material for further use.
+But modern physiology has carried matters much further than this, and
+especially in the three following directions.</p>
+
+<p>In the first place, it has shown that much of the energy thus stored
+from without in the plant is again liberated in the process of oxygen
+respiration, and expended partly as appreciable heat and partly as
+driving force for stimulating the machinery of the living plant to
+further activities.</p>
+
+<p>In the second place, part of it is rearranged with the rearrangement of
+the molecules with which the energy is bound up, as it were, so that
+work of various kinds is done <i>in</i> the machinery of the plant: I refer
+to various metabolic and surface-actions resulting from the peculiar
+mode of presentment of the resulting substances, for <span class="pagenum"><a name="Page_18" id="Page_18"></a>[<a href="./images/18.png">18</a>]</span>instance the
+production of osmotic pressures in the cell.</p>
+
+<p>And, thirdly, part of the synthesised substance is worked up into higher
+bodies, by processes which obviously entail the further doing of work on
+the constituents.</p>
+
+<p>The further pursuit of this theme would evidently carry us beyond the
+more immediate subject of this book; but I want to make clear that
+recent researches render it more and more certain that the living plant
+is a complex piece of co-ordinated machinery which brings together
+matter and energy from the external universe, and then gets work out of
+these.</p>
+
+<p>This proposition is the more important because the whole question of the
+enrichment of our planet with new food, new building materials, and new
+fuel, to compensate the daily losses, depends on it, and is of course to
+be referred fundamentally to the acquirement of new supplies of energy
+from the sun. Enormous activity has been displayed by physiologists,
+since 1860, in attempting to solve the question, which of the many
+different rays known to proceed from the sun are absorbed by the
+chlorophyll-corpuscle, and directed to the performance of the work above
+referred to.</p>
+
+<p>The names of Draper, Sachs and Pfeffer stand forth prominently as
+pioneers in this; while those of Lommel, Engelmann, Timiriazeff and
+Langley have been among the most active in making important
+contributions to the subject, and in attempting to answer the further
+questions <span class="pagenum"><a name="Page_19" id="Page_19"></a>[<a href="./images/19.png">19</a>]</span>connected with the mode in which the chlorophyll is concerned
+in utilising the energy of the solar radiations. The point is one of
+supreme importance, because it goes on all fours with modern questions
+as to the rays of light absorbed or dispersed in our atmosphere at
+different seasons of the year, or in special climatic conditions, to say
+nothing of its other scientific aspects. Unfortunately, however, we have
+no satisfactory explanation of the actual rôle played by the chlorophyll
+substance itself, in spite of much industrious work which has been done
+in the subject in this country and elsewhere. As regards the rays
+employed, it was first proved that the most effective belong to the red
+end of the visible spectrum, and that the effect as measured by the
+amounts of oxygen given off, and of starch formed in given periods of
+time, is more or less proportionable to the intensity of the solar
+light. Then it was established that no monochromatic light is so
+powerful as the white light from which it was obtained, though the
+relative numbers expressing the activity in the red and yellow regions
+may stand to those in the blue as something like 12:1. The latest
+results place the maximum assimilation in the red-orange, and this
+coincides with the maximum absorption in the chlorophyll. If we may
+accept the current views as to the distribution of energy in the
+spectrum of solar light, which depends on the complete absorption of all
+the rays by a black body, where they are estimated as heat, we have <span class="pagenum"><a name="Page_20" id="Page_20"></a>[<a href="./images/20.png">20</a>]</span>the
+interesting result that the agricultural or forest plant is adapted to
+catch and retain, broadly speaking, just those particular rays which
+possess most energy.</p>
+
+<p>The probability is increasing that the protoplasmic machinery is the
+really effective mechanism in the process, and we may figure this
+machinery as so holding or presenting the molecules of carbon-dioxide
+and water to the impact of the light-vibrations, that the latter are
+enabled to undo the molecular structure; the atomic combinations thereby
+liberated may then be supposed to form a body like formic-aldehyde,
+which by polymerisation becomes a carbohydrate of the nature of a sugar
+such as glucose, which the protoplasm then builds up into its substance
+and subsequently deposits as starch, and stores temporarily in the form
+of grains or as amorphous material.</p>
+
+<p>This is partly hypothetical, and is largely due to the careful
+deductions of the chemists, but there are very many facts now to hand
+which bear out its probability, especially the recent advances in our
+knowledge of the sugars, and the experimental feeding of leaves and
+plants deprived of starch with such substances as dextrose, levulose,
+maltose, and other sugars, as well as glycerine and other bodies which
+should be convertible into, or yield them, if the theory is true. In
+this last connection, the careful and extensive experiments of Acton, A.
+Meyer, Boehm, and Laurent should be mentioned. It would be interesting
+to enlarge upon Engelmann's beautiful physiological experiments <span class="pagenum"><a name="Page_21" id="Page_21"></a>[<a href="./images/21.png">21</a>]</span>in
+connection with this subject of absorption of solar energy, where the
+maximum accumulation of oxygen-loving bacteria at those parts of a green
+alga which lie in the red-orange of the spectrum, are used as indicators
+of the maximum oxygen evolution (and therefore of the maximum
+carbon-dioxide assimilation), but space will not admit of this. For a
+similar reason I must also pass over the same observer's experiments
+with plants which assimilate in protoplasm behind a red instead of a
+green substance, and which absorb chiefly other rays between the yellow
+and blue, with the remark that they also seem to imply that it is the
+protoplasmic machinery which turns the energy on to the carbon-dioxide
+molecule, the coloured screen being secondary in the matter. Recent
+experiments which show that green plants will not assimilate
+carbon-dioxide in a light which has passed through a solution of
+chlorophyll&mdash;and therefore left its red rays behind; nor behind a screen
+of iodine dissolved in carbon-dioxide&mdash;which lets no visible rays
+between the red and blue pass&mdash;should be noticed, as showing the
+importance of the chlorophyll and the special rays referred to, however;
+and I ought at least to mention Timiriazeff's beautiful proof, published
+in 1890, that if, on the leaf of a plant left in the dark long enough to
+render it free of starch, a bright solar spectrum is steadily projected
+for 3-6 hours, the chlorophyll then removed by alcohol and the
+decolorised leaf placed in iodine, the image of the spectrum is
+reproduced by the different intensities of the starch bands, blue with
+<span class="pagenum"><a name="Page_22" id="Page_22"></a>[<a href="./images/22.png">22</a>]</span>iodine, in the different parts. Here, again, the maximum coloration
+coincides with the maximum absorption in and near the red.</p>
+
+<p>Microscopic observations and photo-chemical experiments alike convince
+us that the chlorophyll-corpuscle is itself a complex piece of
+protoplasmic machinery, working for and with the rest of the plant, and
+there can be little question as to the greater accuracy of our reasoning
+on the whole question I am discussing, since Meyer, Schimper,
+Pringsheim, and others have established the importance of its structural
+peculiarities.</p>
+
+<p>I must now pass on to consider another aspect of the question of
+carbon-assimilation.</p>
+
+
+<h4><span class="smcap">Notes to Chapter III.</span></h4>
+
+<div class="chnote"><p>In addition to the references in the last chapter, the reader
+may be referred to Sachs' <i>Lectures</i>, XXV., and Pfeffer's
+<i>Physiology</i>, pp. 329-356, where the voluminous literature is
+given.</p></div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_23" id="Page_23"></a>[<a href="./images/23.png">23</a>]</span></p>
+<h2>CHAPTER IV.</h2>
+
+<h3>METABOLISM.</h3>
+
+<div class="chsub">
+<p>Quantities of starch formed, and their significance for the
+plant. The absorption of energy&mdash;the conversion of energy in
+the plant. The plant is a complex machine for concentrating
+and storing energy and material from without.</p>
+</div>
+
+
+<p>Sachs measured the increase in dry weight (due to the carbohydrates
+formed in the chlorophyll-corpuscles) per square meter of leaf-surface,
+exposed for a definite period, by drying rapidly at 100° C. equal areas
+of the leaves concerned, and comparing the weights.</p>
+
+<p>Of course the results are not to be pushed too far, in view of the fact
+that some of the starch is continually passing away to be utilised, and
+of the difficulties of comparing the weather, the intensity of light,
+currents of air, hygroscopic conditions of atmosphere, and other
+variable factors which influence the matter. For instance, the stomata
+open and close to different extents according to <span class="pagenum"><a name="Page_24" id="Page_24"></a>[<a href="./images/24.png">24</a>]</span>the conditions of
+light and moisture, and this affects the whole mechanism of
+transpiration especially, and therefore the supplies of water and
+mineral salts. Nevertheless, some interesting and valuable results have
+been obtained in connection with this important subject.</p>
+
+<p>It was found, for instance, that the foliage of a sun-flower or of a
+vegetable-marrow may be forming starch at a rate of considerably over a
+gram per hour in every square meter of leaf-surface exposed on a fine
+day; while in particularly clear and warm sunny weather Sachs obtained
+as much as 24 to 25 grams per square meter per diem.</p>
+
+<p>When one reflects that 200 square meters is not an extravagant estimate
+for the area of leaf-surface exposed on a tree, for a period which even
+in our latitudes may be considerably over 100 days of, say, ten hours'
+light, we need no longer wonder at the rapidity with which wood is
+produced in the stems, and similar estimates (which I have purposely
+kept lower than the estimates for continental and tropical climates) may
+suffice to show how quickly potatoes or the ears of corn, etc., may fill
+up with the starch or other carbohydrates which render them valuable as
+crops. We want more measurements in these connections, moreover, for
+there are several ways in which they are of scientific value and
+practical importance.</p>
+
+<p>It is evident from what has been said that every grain of starch formed
+represents so much <span class="pagenum"><a name="Page_25" id="Page_25"></a>[<a href="./images/25.png">25</a>]</span>energy, packed away for the moment in the
+storehouses of the plant; and we know that&mdash;quite apart, however, from
+intermediate transformations of the energy thus stored&mdash;this energy
+reappears in the kinetic state eventually, when the starch is burned
+off, in presence of oxygen, and transformed into carbon-dioxide and
+water. It matters not how quickly or how gradually this combustion
+occurs, or whether it is accomplished by burning in a fire, or by slow
+and complex stages in respiration or metabolism: the point is that the
+unit of weight of starch yields so many units of heat when its structure
+tumbles down to the original components, carbon-dioxide and water.</p>
+
+<p>Clearly, if we know how many units of heat are yielded by the combustion
+of one gram of starch, we can obtain an estimate of the amount of
+energy, measured in terms of heat, which the foliage gains and stores
+up&mdash;an estimate which will approach the truth in proportion as our
+estimate of the total assimilative activity is correct.</p>
+
+<p>A word of warning is necessary here, however, for those best acquainted
+with physiology recognise that however useful such calculations as the
+above may be, and undoubtedly are, to give a general idea of the fact
+that the energy represented is large, it would be a mistake to suppose
+that such estimates give even an approximate measure of the energy of
+potential which may be got from the carbohydrate, and still less of the
+amount of work that may be got from its employment, according to the way
+it is employed or presented <span class="pagenum"><a name="Page_26" id="Page_26"></a>[<a href="./images/26.png">26</a>]</span>in the plant. To take a single instance
+only. If the carbohydrate is rapidly burned off to carbon-dioxide and
+water, very little is got out of it in the way of work&mdash;most, if not
+all, of the energy set free escapes as heat: whereas if the carbohydrate
+is slowly and gradually oxydised, passing through various stages and
+giving rise to powerfully osmotic bodies in the process, or if it is
+built up into protoplasm, or into the structure of a cell-wall,
+relatively enormous quantities of work may be got out of its
+surface-energy, and heat may be absorbed. Whence it follows that we
+cannot measure the power for physiological work of a body by merely
+obtaining its heat of combustion, any more than we can infer its
+significance in metabolism from its chemical properties.</p>
+
+<p>The general conclusion that the plant stores large quantities of energy
+may of course be arrived at by simply estimating the enormous quantities
+of food-material which we obtain annually from agricultural plants.</p>
+
+<p>Modern physiologists have attempted to proceed further than this,
+however, in their essays to form an estimate of the relations between
+the available energy in the solar rays and that used and stored in the
+plant.</p>
+
+<p>If we reflect on such phenomena as the cool shade of a tree, and the
+deep gloom of a forest, and on experiments which show that an ordinary
+leaf certainly lets very little of the radiant energy of the spectrum
+pass through it, it becomes evident that many of the rays which <span class="pagenum"><a name="Page_27" id="Page_27"></a>[<a href="./images/27.png">27</a>]</span>fall on
+the leaf are absorbed in some form, and it becomes very probable that
+much of the solar energy, other than that we term light, is retained in
+the leaf for other purposes than assimilation&mdash;or, at least, no other
+conclusion seems possible in view of all the facts. Engelmann's
+researches with purple bacteria are almost conclusive on this point, and
+we may regard it as extremely probable that the plant makes other uses
+of rays, perceived by us as heat-rays, as sources of energy. Researches
+on the influences of temperature on assimilation and other functions
+point to the same conclusion; and Pfeffer and Rodemann definitely state
+that heat is converted into work in the osmotic cells. And the study of
+the absorption bands in the spectrum of the living leaf becomes more
+intelligible in the light of these conclusions. Moreover, the fact that
+a plant still carries on processes of metabolism when active
+transpiration has lowered its temperature below that of the surrounding
+air&mdash;and the plant therefore receives heat from the environment&mdash;points
+to similar conclusions.</p>
+
+<p>The importance of the conclusion is immense, for even if the plant had
+no other sources of energy than the darker heat rays of the solar
+spectrum, it is clear that it ought to be able to do work.</p>
+
+<p>The above may suffice for the general establishment of the conclusion
+that the plant absorbs more radiant energy than it employs solely for
+assimilation, and emphasises our deduction that it is a machine for
+storing energy.</p>
+
+<p><span class="pagenum"><a name="Page_28" id="Page_28"></a>[<a href="./images/28.png">28</a>]</span>The question now arises, how is this relatively enormous gain in energy
+employed by the plant? Our answer to the question is not complete, but
+modern discoveries in various directions have supplied clues here and
+there which enable us to sketch in some degree the kinds of changes that
+must go on.</p>
+
+<p>Not the least startling result is that, important as carbon-assimilation
+is as the chief mode of supplying energy, it is not the only means that
+the plant has of obtaining such from the environment, and it is even
+possible&mdash;not to say probable&mdash;that energy from the external universe
+may be conveyed into the body of the plant in forms quite different from
+those perceptible to our eyes as light.</p>
+
+<p>In the most recent survey of this domain, it is pointed out that we may
+distinguish between radiant energy, as not necessarily or obviously
+connected with ponderable matter, and mechanical energy, which is always
+connected in some way with material substance. All mechanical
+performances in the plants depend on transformation of some form of
+these, evident either as actual energy doing mechanical work, or as
+energy of potential ready to do work.</p>
+
+<p>In so far as molecular movements are concerned, we have the special form
+of chemical energy. The evolution of heat, light and electricity by
+plants are instances of radiant energy, and so on.</p>
+
+<p>Many transformations of energy in the plants are due to non-vital
+processes&mdash;<i>e.g.</i> transpiration, <span class="pagenum"><a name="Page_29" id="Page_29"></a>[<a href="./images/29.png">29</a>]</span>warping actions, etc., but we cannot
+always draw sharp lines between the various cases. Nor can we directly
+measure the work done in the living machinery; but from the effects of
+pressures and strains, the lifting of heavy weights, driving of
+root-tips into soil, osmotic phenomena, etc., it is certain that the
+values may be very high.</p>
+
+<p>The following classes of processes in living protoplasm and cells may be
+taken as indicators. First we have transformation of chemical energy,
+without which continued life is impossible: in many cases&mdash;<i>e.g.</i> the
+processes connected with oxygen respiration&mdash;these result in the
+development of heat. Secondly, we have those remarkable manifestations
+of energy known as osmotic processes, which depend on surface actions,
+and with which may be associated other surface effects, such as
+imbibition, secretion, etc., and in connection with which heat may be
+evolved or absorbed. It is true the substances which exhibit the
+properties here referred to may be produced, or placed in position, by
+chemical energy, or they may be absorbed by roots, etc.; but the
+proximate energy exhibited by them is not derived from chemical energy,
+and may be out of all proportion to the chemical energy of the substance
+or substances concerned. Moreover it is significant to note that a
+highly oxydised body may develop much osmotic energy, as well as a
+highly combustible one.</p>
+
+<p>It is of the greatest importance to realise the truth that much work can
+be, and is done in <span class="pagenum"><a name="Page_30" id="Page_30"></a>[<a href="./images/30.png">30</a>]</span>the living plant, by conversions of energy of
+potential independent of and out of proportion to the chemical energy
+available by decomposing the substances concerned; even the heat of
+respiration may be superfluous here, for the plant may absorb heat from
+without, and convert it into work.</p>
+
+<p>Tensions often arise in the plant, and do work expressed as
+movements&mdash;<i>e.g.</i> the springing of elastic Balsam fruits, stamens of
+<i>Parietaria</i>, etc.</p>
+
+<p>Osmotic energy not only results in enormous pressures and tensions, but
+causes movements by diffusion and diosmosis, and any given osmotic
+substance which carries this energy with it is not necessarily formed
+always in the same way in the cell&mdash;<i>e.g.</i> glucose may arise from
+starch, or from carbon-dioxide, or from oil.</p>
+
+<p>Surface-energy is also expressed in the powerful attractions for water
+exhibited in imbibition, swelling, capillarity, absorption, surface
+tensions, etc.</p>
+
+<p>Transpiration induces relatively enormous disturbances of equilibrium,
+and does work in moving water quite independent of chemical energy.</p>
+
+<p>Again, what may be termed excretion-energy, as expressed in the
+separation of a solid body&mdash;<i>e.g.</i> a crystal&mdash;from a solution, may be
+for our purposes regarded separately. Any change in the condition of
+aggregation of a substance in the plant may result in movements and the
+overcoming of resistances.</p>
+
+<p>It will be evident from this short digression&mdash;and this is the point I
+wish to emphasise&mdash;that in <span class="pagenum"><a name="Page_31" id="Page_31"></a>[<a href="./images/31.png">31</a>]</span>the interval between the securing of a grain
+of starch, representing so much energy won from the external universe,
+and the reconversion of this grain into its equivalent carbon-dioxide
+and water, by respiration, resulting in the loss of the above energy as
+heat, the starch referred to may have undergone numerous transformations
+in the living machinery of the plant, and have played at various times a
+rôle in connection with the most various evolutions of energy.</p>
+
+<p>If we try to picture a possible case, we may take the following. A given
+starch-granule, after being built up in the chlorophyll-corpuscle, is
+decomposed, and yields part of itself as glucose, which passes down into
+other parts of the plant in solution. Part of it is merely re-converted
+into starch, and temporarily stored: another part passes into the arena
+of oxydation-processes, the sum of which constitute respiration, and may
+serve for a time in the molecules of an organic acid: yet another part
+may be converted into a constituent of the cellulose cell-walls; while
+part may be brought into play in the reconstruction of protoplasm.</p>
+
+<p>In this last connection a discovery made by Schulze about 1878, and
+followed up later by Pfeffer, Palladin, and others is of importance.
+Seedlings growing in the dark, or in an atmosphere devoid of
+carbon-dioxide in the light, become surcharged with nitrogenous bodies
+known as amides, formed during the breaking down of the proteids in the
+destructive process preceding and <span class="pagenum"><a name="Page_32" id="Page_32"></a>[<a href="./images/32.png">32</a>]</span>accompanying respiration: if the
+seedlings are allowed free access to light and carbon-dioxide, however,
+the amides disappear. The explanation is that they are combined with
+some of the materials of the carbohydrates, and again built up into the
+material of the living protoplasm.</p>
+
+<p>Returning to our hypothetical starch-grain&mdash;or, rather, its parts&mdash;we
+have some of it retained as starch, in excess, simply because it is not
+needed at the moment: another portion gives up its energy in
+respiration, and this does work on the spot, or is lost as heat; or in
+the body of an organic acid, or its salt, the part in question may do
+lifting or pressing work by osmosis, or cause diffusion-currents from
+one cell to another. In the constitution of the cell-wall we may have
+part of our starch-grain aiding in imbibition or in the establishment of
+elastic tensions in turgidity: and, finally, parts may be built up into
+the living protoplasmic machinery of the plant.</p>
+
+<p>What is true for the starch-grain is also true for any particle of salt,
+or water, or gas which enters into the metabolism of the living plant,
+regard being paid to the particular case, and circumstances in each
+case.</p>
+
+<p>Enough has been said to show that the plant cannot be properly studied
+merely as the subject of chemical analysis or of physical investigation;
+you might as well expect to understand a watch by assays of the gold,
+silver, steel and diamonds of which its parts are made up, or to learn
+what can be got out of the proper working of a lace <span class="pagenum"><a name="Page_33" id="Page_33"></a>[<a href="./images/33.png">33</a>]</span>machine by
+analysing the silk put into it, and the fabric which comes out, and by
+taking the specific gravity of its parts and testing the physical
+properties of its wheels and levers.</p>
+
+<p>This is not the same thing as denying the value of such knowledge, in
+the case of either the dead machine or the living plant: it is merely
+emphasising the supreme importance of the study of the structure and
+working of the active machinery in both cases.</p>
+
+<p>Nor is it pertinent to remark on the apparent hopelessness of physiology
+being at present able to explain the seemingly infinite complexity of
+the living machinery of protoplasm and its activities. The modern
+locomotive is also a complex affair in its way, but it is profitable to
+investigate it and to know all one can of its working and possibilities,
+for obvious reasons: a little reflection will convince us that it is
+also worth while to investigate that complex machine, the plant&mdash;the
+working organism which alone can really enrich a country. Moreover, we
+ought to be encouraged by the satisfactory progress now being made, and
+the splendid practical results which are accruing, rather than dismayed
+by the prospect of unflagging labour which will be required in the
+future.</p>
+
+<p>Enough has perhaps been said to establish the general truth that the
+plant is a complex machine for storing energy and material from outside,
+and we have seen that modern research has at least gone a long way
+towards determining how the living machine works.</p>
+
+<p><span class="pagenum"><a name="Page_34" id="Page_34"></a>[<a href="./images/34.png">34</a>]</span>It is hardly necessary to point out that important practical
+consequences may result from these phenomena of the accumulation of
+surplus starch or other carbohydrates in the leaves during the day, and
+of their disappearance during the night into the lower parts of the
+plant. For instance, foliage cut for fodder in the morning is far poorer
+in starch than if cut in the evening, and it would be very instructive
+to have experiments made on a large scale to test the result of feeding
+caterpillars or rabbits, for instance, with mulberry, vine, or other
+leaves in the two conditions.</p>
+
+<p>Again, we now see what complications may arise if a parasitic organism
+gains access to the stores of carbohydrates in process of accumulation,
+or attacks and injures the machinery which is building up such
+materials, etc.</p>
+
+
+<h4><span class="smcap">Notes to Chapter IV.</span></h4>
+
+<div class="chnote">
+<p>The student who desires to pursue this subject further should
+read Sachs' <i>Lectures</i>, XX. and XXV., and Pfeffer's
+<i>Physiology</i>, pp. 442-566, but he will hardly arrive at the
+best that has been done without consulting Pfeffer's "<span lang="de" xml:lang="de">Studien
+zur Energetik der Pflanzen</span>" in the <span lang="de" xml:lang="de"><i>Abhandl. der Math.-Phys.
+Classe der Kgl. Sachss. Gesellsch. der Wiss.</i></span> (Leipzig, 1892),
+p. 151; and Kassowitz, <span lang="de" xml:lang="de"><i>Allgemeine Biologie</i></span> (Vienna, 1899),
+Bk. I., pp. 1-127.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_35" id="Page_35"></a>[<a href="./images/35.png">35</a>]</span></p>
+<h2>CHAPTER V.</h2>
+
+<h3>ROOTS AND ROOT-HAIRS.</h3>
+
+<div class="chsub">
+<p>Older views as to root-hairs&mdash;Root-hairs and their
+development&mdash;Surface&mdash;Variations&mdash;Conditions for maximum
+formation&mdash;Minute structure&mdash;Adhesion to particles of
+soil&mdash;Functions.</p>
+</div>
+
+
+<p>On the roots of most plants are to be found delicate, silky-looking,
+tubular prolongations of some of the superficial cells, known as
+root-hairs. Malpighi (1687) seems to have been the first to observe
+them, and he took them for capillary tubes. Grew (1682) seems to have
+been responsible for the view that the roots act like sponges in taking
+up water.</p>
+
+<p>Simon (1768) was probably the originator of the idea that these
+root-hairs were excretory tubules, a view that became very popular at
+the beginning of this century.</p>
+
+<p>Meyer (1838) was perhaps the first to give a comparative account of
+them, and he supposed <span class="pagenum"><a name="Page_36" id="Page_36"></a>[<a href="./images/36.png">36</a>]</span>them to be delicate prolongations of the
+root-surface to facilitate the absorption of water.</p>
+
+<p>The real importance of these organs, however, has only become apparent
+since Sachs, in 1859, recognised their relations to the particles of
+soil between which they extend and to which they cling.</p>
+
+<p>In 1883 Schwarz made a very thorough study of their biological
+character, and in 1887 Molisch gave us new facts as to their physiology.
+Our knowledge of them has been rendered very much more intimate by the
+researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena,
+and they serve as an excellent study of some of the best results of
+modern physiology.</p>
+
+<p>In the normal case, such as is exemplified by a seedling wheat or bean,
+the root-hairs arise some distance behind the growing tip of the root,
+an obvious adaptation which prevents their being rubbed off by the soil,
+as they would be if developed on parts still actively lengthening. As
+those behind die off, new ones replace them in front, and so we find a
+wave of succession of functionally active root-hairs some little
+distance behind the tip of the root: the same order of events holds for
+each new rootlet as it emerges from the parent root, and so successive
+borings in the soil, made by the diverging root-tips, are thoroughly
+explored by these root-hairs.</p>
+
+<p>Measurements have shown that in various plants the surface of root on 1
+mm. of length is <span class="pagenum"><a name="Page_37" id="Page_37"></a>[<a href="./images/37.png">37</a>]</span>increased by the root-hairs in proportions given in
+the following table:</p>
+
+<div class="centered">
+<table summary="Surface area without and with root hairs" cellpadding="4" cellspacing="0" border="0">
+<tr>
+  <td class="tdcentertbr"><span class="smcap">Plant.</span></td>
+  <td class="tdcentertbr">Area of surface without root-hairs.</td>
+  <td class="tdcentertbr">Area of root and hairs.</td>
+  <td class="tdcentertb">No. of times greater.</td>
+</tr>
+<tr>
+  <td class="tdleftr">Maize,</td>
+  <td class="tdrightr">4.52 sq. mm.</td>
+  <td class="tdrightr">25.13 sq. mm.</td>
+  <td class="tdright">5.5</td>
+</tr>
+<tr>
+  <td class="tdleftr">Pea,</td>
+  <td class="tdrightr">4.71 sq. mm.</td>
+  <td class="tdrightr">58.33 sq. mm.</td>
+  <td class="tdright">12.4</td>
+</tr>
+<tr>
+  <td class="tdleftrb">Scindapsus,</td>
+  <td class="tdrightrb">14.02 sq. mm.</td>
+  <td class="tdrightrb">261.90  sq. mm.</td>
+  <td class="tdrightb">18.7</td>
+</tr>
+</table>
+</div>
+
+<p>&mdash;which sufficiently establishes the general proposition that the area
+of the root-surface is enormously increased by these hairs.</p>
+
+<p>But this does not give us any definite idea of the length of the
+cylinders of soil explored by these surfaces, until we find that plants
+such as an ordinary sunflower, hemp, or vegetable-marrow may have roots
+penetrating into a cubic meter of soil, in all directions, and so
+closely that probably no volume so large as a cubic centimeter is left
+unexplored. Clark found by actual measurement that the roots of a large
+gourd, if put end to end, extended over 25 kilometers, and Nobbe gives
+520 meters for the roots of a wheat. Vetches may go nine feet deep, and
+oats more than three feet. The Sal, a tree of the forests of India, has
+roots which penetrate to a depth of 50 to 60 feet.</p>
+
+<p>Some rough notion of the lengths, superficies and penetrating capacities
+of the roots of a large tree may be gathered from the above, but it is
+doubtful whether we can form any adequate ideas as to the millions of
+root-hairs which must be <span class="pagenum"><a name="Page_38" id="Page_38"></a>[<a href="./images/38.png">38</a>]</span>developed along the course of these
+subterranean boring organs.</p>
+
+<p>One of the most striking results of modern enquiry into these matters,
+is the discovery that the number and superficial area of these
+root-hairs, on one and the same plant, may vary to a large extent
+according to the structure, as it were, of the soil, and the degree of
+moisture it is capable of retaining; or, with the same soil, according
+to the amount of water which it receives and holds. Correlations have
+also been observed between the development in length and surface of the
+rootlets themselves.</p>
+
+<p>The following illustrations will suffice to show this:</p>
+
+<p>Six young wheat-plants in soil kept constantly wet, developed roots the
+total length of which measured 365 mm. each, on the average, and almost
+devoid of root-hairs.</p>
+
+<p>Six similar plants in soil only moderately moist, averaged 668 mm., and
+were well furnished (though not densely covered) with root-hairs.</p>
+
+<p>Six similar plants in soil which would be termed dry, averaged 371 mm.,
+but were densely covered with rich crops of root-hairs.</p>
+
+<p>Further researches have shown that the conditions which rule the
+development of the root-system and root-hairs in the soil are very
+complex, and not always easy to trace. The most general statements we
+can make are the following:</p>
+
+<p>There is an optimum degree of moisture in the soil which promotes the
+maximum development <span class="pagenum"><a name="Page_39" id="Page_39"></a>[<a href="./images/39.png">39</a>]</span>of root-hairs. If the soil is too wet they are not
+developed.</p>
+
+<p>These facts are of importance as correlated with the ease or difficulty
+experienced by the roots in obtaining water, and plants such as our
+ordinary agricultural plants show this very distinctly.</p>
+
+<p>Although, as shown in the experiments with wheat, the short roots in dry
+soil were more densely covered with root-hairs than the much longer
+roots in moderately moist soil, subsequent closer investigation shows
+that the total quantity and area of root-hairs is less in the former
+case than in the latter.</p>
+
+<p>The greatest number of root-hairs are developed on roots which are
+growing at their best: too much moisture may prevent the formation of
+root-hairs: too little may induce dense growths of root-hairs locally,
+but the total number is reduced.</p>
+
+<p>Another set of events which exerts influence on the development of
+root-hairs is the composition of the dilute solution&mdash;water containing
+dissolved salts&mdash;which surrounds them in the soil.</p>
+
+<p>Thus, Schwarz found that when similar oat and wheat plants were grown
+with their roots in solutions of various salts, the results differed as
+follows:</p>
+
+<p>Oats in a 15 per cent. solution of calcium chloride developed no
+root-hairs, though they formed in a 5 per cent. solution, and were very
+numerous in a 0.5 per cent. solution, or in water alone. In a 10 per
+cent. nutritive solution the <span class="pagenum"><a name="Page_40" id="Page_40"></a>[<a href="./images/40.png">40</a>]</span>plants developed no root-hairs, though
+they were abundant in a 1 per cent. solution.</p>
+
+<p>Wheat plants with their roots in a 15 per cent. solution of potassium
+nitrate bore no root-hairs, but they were numerous in a 2 per cent.
+solution of the same salt.</p>
+
+<p>These are extreme cases, for, although the roots were not killed, they
+were strongly inhibited in their growth by the more concentrated
+solutions. However, experiments of this kind at least bring vividly
+before us what variations are possible, and suggest that similar events
+on a smaller scale may occur in a soil which yields large quantities of
+soluble substances, <i>e.g.</i> when freshly manured. Obviously these facts
+have a practical significance as regards kind of soil, drainage, season
+(<i>e.g.</i> drought or wet), etc.</p>
+
+<p>But there are other factors which rule the development of root-hairs,
+and some experiments by Lesage show that the correlations between the
+development of root-hairs and roots are probably much more complex than
+had been suspected; for he finds that if the lateral rootlets of a Bean,
+in a water culture, are suppressed, the main rootlet develops numerous
+and very long hairs to compensate the loss in surface, a matter of
+obvious importance in the discussion of cases where roots have been
+injured in the soil.</p>
+
+<p>Before proceeding further it is necessary to look a little more closely
+into the structure of a single hair.</p>
+
+<p><span class="pagenum"><a name="Page_41" id="Page_41"></a>[<a href="./images/41.png">41</a>]</span>It is a tubular prolongation of a single cell of the external covering
+of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10
+mm. in diameter. In special cases the root-hairs of some water plants
+may reach 5 to 18 mm. in length, but of course I am referring to the
+ordinary land plants of agriculture and forestry. This tubular
+prolongation is closed and rounded off at the distal free end, and opens
+at the proximal end into the cell of which it is a protrusion.</p>
+
+<p>The whole structure is bounded by an extremely delicate and elastic wall
+of cellulose, which Frank says is of special composition, almost too
+thin to measure in many cases, but often somewhere near 0.005 to 0.001
+mm. in thickness. This thin membrane is remarkably permeable by water,
+or dilute solutions, as is shown by the rapidity with which a root-hair
+collapses if exposed to evaporation, or with which dense solutions
+abstract water from it, or with which solutions may be seen to penetrate
+it under the microscope.</p>
+
+<p>Overlying the thin cell-wall proper, on the outside, is a thin
+gelatinous layer, a product of alteration of the outermost lamellæ of
+the former.</p>
+
+<p>Closely lining the proper cell-wall on the inside, is an extremely thin
+layer of living protoplasm, and somewhere in this protoplasm is a
+distinct cell-nucleus.</p>
+
+<p>The interior of the tube is filled with cell-sap, and it is the osmotic
+pressure of this cell-sap which keeps the whole living instrument tense
+and <span class="pagenum"><a name="Page_42" id="Page_42"></a>[<a href="./images/42.png">42</a>]</span>rigid, and the thin protoplasmic film close pressed against the
+cellulose cell-wall.</p>
+
+<p>Nothing whatever can pass into the cell-sap, or out from it, without
+traversing both the lining of living protoplasm and the cell-wall.</p>
+
+<p>If we gently pull a living root, of wheat, pea, mustard, etc., from a
+normal soil, we find particles of soil so closely adherent to the
+root-hairs that they cannot all be washed off without tearing the hairs:
+the root-hairs establish relations of contact with these particles, so
+close that they are cemented to the solid surfaces by means of the
+gelatinous layer already referred to. This peculiarity has the following
+consequences. In the first place, the enormous holdfast, ensured by the
+millions of points of adherence, enables the plant to withstand even
+powerful lever actions from above, and provides fixed points against
+which the root-tips can work as they drive deeper into the soil. In the
+second place, the intimate contact of the root-hairs and particles of
+soil, ensures that the films of water held by surface-action on the
+soil-particles and root-hairs shall be in continuity with the water
+saturating the cell-walls of the latter, and therefore with the
+protoplasm and cell-sap in their interior. The importance of this at
+periods when the soil is "dry" will be obvious, when we reflect that no
+soil is ever naturally so dry that surface-films of water are absent
+from the particles.</p>
+
+<p>The fact that the root-hair contains living protoplasm, enables us to
+understand to a certain extent the results of the following
+experiments.</p>
+
+<p><span class="pagenum"><a name="Page_43" id="Page_43"></a>[<a href="./images/43.png">43</a>]</span>If we have a leafy and healthy plant, with roots, bearing numerous
+root-hairs, properly established in suitably moist soil in the pot, the
+roots cease to absorb water if the temperature of the soil falls below a
+certain minimum, though they recommence to do so if the temperature is
+raised again: this has nothing to do with the temperature of the upper
+parts of the plant, or of the air, and the latter may be so high that
+the plant rapidly droops from loss of water at the leaves, which is not
+being compensated owing to the inactivity of the roots.</p>
+
+<p>Similarly we may have the air so cold, at a time when the soil is warm
+enough to keep the root-hairs actively at work, that the plant becomes
+surcharged with water, which escapes from the leaves like drops of dew.
+The temperatures necessary to cause these disturbances in the action of
+the living root-hairs vary for different plants, and even for different
+varieties of the same species.</p>
+
+<p>Similar arrestation of the functions of the roots may be brought about
+by removing the oxygen from the soil around the root-hairs, and
+replacing it by carbon-dioxide, or the vapour of chloroform. If not kept
+too long in such a condition, the plant recovers rapidly on admitting
+atmospheric oxygen, which is always present in a normal well-drained
+soil both as gas in the capillary interspaces, and dissolved in the
+water on the surfaces of the particles. If the access of oxygen is
+delayed, however, as often happens in rainy seasons and in <span class="pagenum"><a name="Page_44" id="Page_44"></a>[<a href="./images/44.png">44</a>]</span>wet soils,
+the root-hairs are killed, and rot sets in. A good instance of this has
+lately been given by Heinricher in the case of potatoes.</p>
+
+
+<h4><span class="smcap">Notes on Chapter V.</span></h4>
+
+<div class="chnote">
+<p>For the further pursuit of this subject the reader should
+consult Sachs' <i>Lectures</i>, II. and XV.; Sorauer, <i>A Popular
+Treatise on the Physiology of Plants</i>, 1895, chapters II. and
+IV., and Pfeffer's <i>Physiology</i>, pp. 149-163. The principal
+paper on root-hairs referred to in the text is Schwarz, "<span lang="de" xml:lang="de">Die
+Wurzelhaare der Pflanzen</span>," in <span lang="de" xml:lang="de"><i>Unters. aus dem bot. Inst. zu
+Würzburg</i></span>, I. Heft 2, 1883, p. 140, where a very exhaustive
+account of these organs will be found.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_45" id="Page_45"></a>[<a href="./images/45.png">45</a>]</span></p>
+<h2>CHAPTER VI.</h2>
+
+<h3>THE FUNCTIONS OF ROOT-HAIRS.</h3>
+
+<div class="chsub">
+<p>Excretions from root-hairs&mdash;Osmotic
+phenomena&mdash;Turgescence&mdash;Plasmolysis&mdash;Control of the protoplasm
+in absorption, etc. Selective absorption.</p>
+</div>
+
+
+<p>We see then that the root-hairs are the active living instruments in
+absorbing the water (containing small quantities of dissolved
+substances) of the soil.</p>
+
+<p>If the living root-hairs are so numerous and so active, however, a
+natural inference is that they must exert some influence on the
+composition or arrangement of their environment. All the teachings of
+modern physiology go to show that such a living cell as I have sketched
+cannot carry on its life, brief though it be&mdash;the root-hairs are active
+for about four or five days&mdash;without forming substances of the nature of
+excreta, and we should expect some of these to pass out to the soil.</p>
+
+<p>Sachs showed, in 1860, that roots growing in contact with polished
+marble corrode the surface <span class="pagenum"><a name="Page_46" id="Page_46"></a>[<a href="./images/46.png">46</a>]</span>of the mineral, and Nobbe, in 1876, showed
+that the roots of seedlings reduce potassium permanganate, a fact which
+Molisch confirmed in 1887. The latter observer also proved that living
+root-hairs secrete substances which colour a solution of guaiacum blue,
+oxidise pyrogallic acid and other organic substances, and rendered it
+probable that they excrete some substance which inverts cane-sugar, and
+in some cases even small quantities of a diastatic enzyme.</p>
+
+<p>Molisch also confirmed an old observation, that roots excrete
+carbon-dioxide; and he and Czapek showed that the root-hairs excrete
+acids more permanent in their nature than carbonic acid, and published a
+method for showing this by means of a dilute solution, slightly
+alkaline, of phenolphthalein.</p>
+
+<p>Molisch declared that the substances secreted by root-hairs may even be
+observed, dissolved in drops which ooze from the surfaces of the
+root-hairs.</p>
+
+<p>That these root-excretions, and particularly the acids, may be of
+service in dissolving and rendering more available various constituents
+of the soil is an obvious suggestion, and it is borne out by Sachs'
+discovery of the corrosion of marble, and by Molisch's observation that
+living roots slowly corrode ivory if continuously kept in contact with
+it.</p>
+
+<p>But a deeper insight into the physiology of these organs was only
+possible when the meaning of the phenomena of osmosis had been rendered
+<span class="pagenum"><a name="Page_47" id="Page_47"></a>[<a href="./images/47.png">47</a>]</span>clearer by the researches of Pfeffer and De Vries in 1877.</p>
+
+<p>De Vries showed that the turgescence of the living cell can be
+diminished, and even reduced to nothing, by placing the cell in contact
+with solutions of substances which attract water from the cell-sap: as
+the turgescence diminishes, the cell contracts, owing to the elasticity
+of the cell-wall, which was previously distended; if the abstraction of
+water continues, the living protoplasmic membrane lining the cell-wall
+contracts away from the latter. He then proved that no injury need
+accrue to the cell by this process of plasmolysis, since the turgescence
+can be restored by washing out the salt with a more dilute solution, or
+with pure water; and the cell may go on living and even growing as
+before. These phenomena can only be produced in cells where the
+protoplasmic lining is intact and alive.</p>
+
+<p>Pfeffer showed that the whole matter depends on the properties of the
+living protoplasmic membrane, which, so long as it is alive, has the
+power of governing the entrance or exit of dissolved substances, but is
+as a rule freely permeable for water. If, then, substances with a
+powerful attraction for water are formed in the cell cavity, and of such
+a nature that the protoplasm does not permit their free diffusion to the
+exterior, they attract water, and hold it fast, and so set up the
+condition of hydrostatic pressure known as turgescence, the limit of
+which depends on the attainment of a state of equilibrium between the
+<span class="pagenum"><a name="Page_48" id="Page_48"></a>[<a href="./images/48.png">48</a>]</span>elastic reaction of the cell-wall and the distending power of the
+absorbed water. When this limit is reached, water begins to filter back
+again through the cell-wall. Numerous researches during the last fifteen
+years have shown that the sap of such a living cell as the root-hair is
+charged with substances of various degrees of osmotic power; bodies like
+sugars, amides, vegetable acids and their salts, being formed by the
+metabolic activity of the protoplasm and accumulated there. Moreover, we
+now know that the salts of the vegetable acids in particular are
+effective, and the researches of Warburg and Palladin in 1886 have
+placed it beyond reasonable doubt that these acids are continually being
+developed and destroyed in the living cell during normal growth and
+respiration, and that great variations as to quantity may be brought
+about by alterations in the conditions of the environment&mdash;<i>e.g.</i>
+temperature, oxygen, etc.</p>
+
+<p>If, now, we bring a solution of some salt, such as potassium nitrate,
+which has a powerful attraction for water, on the outside of the living
+root-hair, the question whether the water remains in the cell, or passes
+out of it, merely depends on whether the substances inside or that
+outside have the most powerful attraction on the water in the sap, since
+the protoplasm allows water to pass freely.</p>
+
+<p>But the protoplasmic lining may affect the whole matter in another way;
+for it may allow the dissolved salt, or other substance, in the solution
+outside or inside the cell to pass through <span class="pagenum"><a name="Page_49" id="Page_49"></a>[<a href="./images/49.png">49</a>]</span>it also, or it may take it
+up and fix it, or break it up or otherwise alter it.</p>
+
+<p>More recent researches, and especially those of Pfeffer, have shown that
+these diosmotic properties of the living protoplasm are of the utmost
+importance in the whole matter of absorption of substances from the
+soil.</p>
+
+<p>Let us suppose the following case. A root-hair, in full vigour, is
+allowed to bathe freely in a dilute solution of various substances, such
+as sugar, potassium nitrate, phosphates, sulphates and carbonates of
+iron, soda, lime, magnesium and others known by experiment to be
+harmless to its life.</p>
+
+<p>Now it turns out to be by no means a foregone conclusion that all or any
+of the substances, even though freely soluble in the water, can pass
+through the protoplasm into the interior of the cell. Some may be
+allowed easy access, others may only be permitted to pass in small
+quantities, and others, again, may be absolutely refused access by the
+delicate living filter, so long as it is vigorously alive. Nor, as
+proved by numerous experimental cultures since De Saussure's time, is
+the entrance of a salt, etc., ruled by its indispensability or otherwise
+in the economy of the plant. And it is important to notice that only
+experiment can prove the point and determine which substances are
+absorbed and which refused by the root-hair.</p>
+
+<p>If we now suppose the protoplasm to give rise to powerfully osmotic
+substances which accumulate <span class="pagenum"><a name="Page_50" id="Page_50"></a>[<a href="./images/50.png">50</a>]</span>in the sap-vacuole, but which are not
+permitted free egress through the protoplasm (and the formation of such
+bodies will occur if the protoplasm is actively respiring), the
+conditions for absorption of water, with or without any dissolved salts,
+which the protoplasm allows to traverse it, are set up.</p>
+
+<p>But the above supposed case is realised, as Pfeffer showed in 1886, when
+he found by a series of beautiful experiments that certain aniline dyes
+can accumulate in living root-hairs, and other living cells, whereas
+others cannot pass the living protoplasm. After accumulating for some
+time, the dye may either remain stored there, or may eventually diffuse
+out.</p>
+
+<p>Pfeffer made another discovery, of equal importance, namely, that under
+the influence of dilute organic acids, such as citric acid, the
+permeability of the living protoplasm may be altered, so that it allows
+substances to pass which could not otherwise have traversed it. De Vries
+had also shown that the condition of the protoplasm affects its power of
+retaining the colouring matter in the sap of the Beet: so long as the
+protoplasm is alive, the crimson sap is retained, even when the cell is
+plasmolysed, but immediately it begins to die the colour escapes through
+it. A similar case exists when the chlorophyll-corpuscles retain their
+colour in living cells known to be charged with acids: so long as the
+protoplasm is alive and normally active the green bodies are protected.</p>
+
+<p><span class="pagenum"><a name="Page_51" id="Page_51"></a>[<a href="./images/51.png">51</a>]</span>These, and numerous other experiments of the same kind, prove that the
+healthy root-hair is a living instrument for taking up dilute solutions
+out of the soil, and holding them in the sap-cavity for a time. If
+killed, by frost for instance, it loses these powers.</p>
+
+<p>The researches of the last ten years have also shown that a time comes
+when the turgid cell, if an isolated one, and if sufficient supplies of
+water are present, is so tightly distended that the surplus water begins
+to diffuse out again under the pressure proper to the hydrostatic
+conditions set up.</p>
+
+<p>Now we arrive at a very critical point.</p>
+
+<p>When the water, or dilute solution of various substances, begins to
+exude under pressure from the living root-hair, what is to prevent its
+escape into the soil? And if it thus diffuses out, where is the object
+of absorption?</p>
+
+<p>The questions are obviously pertinent, and they may seem the more so in
+that the cells adjoining the root-hair on its inner side are also
+turgid, and possess similar properties to those of the root-hairs. To
+establish a condition of things which shall bring about the inward flow
+of the absorbed water, one of the three following cases is conceivable.
+(1) The cells, as we pass radially into the root, have different
+properties on the wall of the two sides; or (2) they are more and more
+greedy of water owing to some process of extraction of their water by
+tissues in the centre of the root; or (3) these successive series <span class="pagenum"><a name="Page_52" id="Page_52"></a>[<a href="./images/52.png">52</a>]</span>of
+cells possess osmotically more powerful contents at periods coincident
+with the escape of the water from the now osmotically weaker root-hairs.</p>
+
+<p>A little reflection will show that where we have a group of such cells
+as the above, all capable of absorbing water and dilute solutions and of
+becoming turgid, movements of the absorbed water must go on until all
+the cells are in equilibrium, as regards their osmotic pressures.</p>
+
+<p>Now the living rootlet is just such a system, the various cells of which
+are in different conditions of osmotic pressure at any given time: some
+of these cells are old, and their protoplasm is allowing sap to filter
+out under pressure: others are in the height of their vigour, and their
+protoplasm extremely impervious to the highly osmotic sap-constituents
+which it itself is forming actively: others are too young to have
+attained their full turgescence: while others again are in stages
+intermediate between the above.</p>
+
+<p>There is another point of importance, however, to explain some
+peculiarities in the absorption of these dilute solutions of salts,
+etc., by the root-hairs from the soil, and by cells lying deeper in the
+plant from these root-hairs.</p>
+
+<p>It is easy to understand that if a root-hair absorbs a given
+substance&mdash;say calcium sulphate, for illustration&mdash;and hands it over to
+other cells unchanged, a time must be supposed to arrive when, the sap
+of all the cells being equally charged with calcium sulphate, no more
+could be <span class="pagenum"><a name="Page_53" id="Page_53"></a>[<a href="./images/53.png">53</a>]</span>absorbed: the rate of absorption of this particular substance,
+and the quantity absorbed, up to the hypothetical point of equilibrium
+chosen, would then depend simply on the ease with which its molecules
+traversed the living protoplasmic membrane, and the degree of their
+solubility in the sap.</p>
+
+<p>But now suppose the following new factor to come in. Suppose that
+calcium sulphate undergoes decomposition in some one of the internal
+cells of the system of absorbing cells, or that it is even merely
+crystallised out in such a cell, or in any other way removed from
+solution (<i>e.g.</i> by deposition in cell-walls). This alters the state of
+affairs considerably. The separation of the molecules from the
+sap-solution is itself a cause for the flow of more of the solution to
+the cell concerned, and such causes of diffusion are very common in the
+plant.</p>
+
+<p>The importance of this principle consists in that it lies at the base of
+the whole question of selective absorption, application of manures, and
+the rotation of crops; and those who are acquainted with the excellent
+analytical results of De Saussure, Boussingault, Wolff, Trinchinetti,
+Gödechen, etc., and the water-culture experiments of Sachs, Nobbe, and
+others, will understand what an illuminating effect on these points was
+produced by the above generalisation, which we owe especially to
+Pfeffer's splendid researches into the nature of osmotic phenomena.</p>
+
+<p>It will now be clear, I hope, why we regard <span class="pagenum"><a name="Page_54" id="Page_54"></a>[<a href="./images/54.png">54</a>]</span>the living root-hairs as
+instruments&mdash;as pieces of living machinery&mdash;for the active absorption of
+water, with substances dissolved in it, from the soil; and it will also
+be evident, I think, that no one can form a proper conception of this
+matter of absorption, so important in all agricultural questions, unless
+he pays attention to these biological phenomena. It was hopeless to
+expect to understand these matters merely in the light of chemical
+analyses of plants and soils, and one expression of this hopelessness
+was the belief in the power of roots to select only the substances
+useful to it. We now know that the expression "selective power of roots"
+has a totally different meaning from that implied in the minds of the
+last generation of agriculturalists, and it would be easy to devise
+experiments, with solutions of different strength, where the plant
+should be made to take up relatively large quantities of harmless, but
+useless minerals, etc., and to starve in the midst of plenty of the
+elements proper to its structure, simply because the former are offered
+in a form in which they easily traverse the protoplasm of the
+root-hairs, while the latter are presented in a form unsuitable for
+absorption. That all these matters are of importance in regard to
+manuring and choice of soils, etc., needs no emphasising.</p>
+
+<p>These remarks, of course, do not detract from the value of good
+comparative chemical analyses, when viewed in the light of physiological
+knowledge, as I need hardly say; but they do, and <span class="pagenum"><a name="Page_55" id="Page_55"></a>[<a href="./images/55.png">55</a>]</span>emphatically so,
+attack the position that such analyses alone can explain the problems of
+agriculture.</p>
+
+<p>On the other hand, we must not rest satisfied with the suggestions so
+far put forward to account for the processes referred to, since it is
+impossible to overlook the fact that in their present form they merely
+afford proximate explanations, and are too crudely mechanical for
+finality.</p>
+
+
+<h4><span class="smcap">Notes on Chapter VI.</span></h4>
+
+<div class="chnote">
+<p>In addition to the works referred to in the last chapter, the
+student should consult Pfeffer's <i>Physiology</i>, pp. 86-149, and
+pp. 410-441. With reference to water cultures, Sachs'
+<i>Lectures</i>, XVII., may also be consulted. The standard work on
+ash constituents of plants is Wolff, <span lang="de" xml:lang="de"><i>Aschen-analysen</i></span>, 1871
+and 1880, an indispensable book of reference in this
+connection, though there are others, quoted in Pfeffer, where
+further literature may also be found.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_56" id="Page_56"></a>[<a href="./images/56.png">56</a>]</span></p>
+<h2>CHAPTER VII.</h2>
+
+<h3>THE BIOLOGY OF SOIL.</h3>
+
+<div class="chsub">
+<p>Soil not a dead matrix&mdash;Organic materials&mdash;The living
+organisms of the soil&mdash;Their activities&mdash;Their numbers and
+importance. Abandonment of the notion that chemical analysis
+can explain the problem.</p>
+</div>
+
+
+<p>It is customary to regard the soil, between the particles of which the
+root-hairs of plants are distributed, as if it were merely a dead matrix
+of smaller or larger pieces of rock, such as sand, gravel, stones, etc.,
+and organic remains, such as bits of wood, leaves, bones, etc., with
+water and air in their interstices. As matter of fact, however, soil is
+a much more complex body than was suspected until comparatively recent
+times.</p>
+
+<p>It is, of course, beyond the scope of this book to go into the different
+varieties of soils, their structure or arrangement, and the chemical
+nature of their constituent rocks and the débris mingled with the
+latter. For the same reason I must pass over the curious properties of
+soils in relation to <span class="pagenum"><a name="Page_57" id="Page_57"></a>[<a href="./images/57.png">57</a>]</span>the solutions they yield to water in contact, the
+manner in which they retain some of these solutions and allow others to
+pass easily, and the remarkable double decompositions which go on in
+them. Moreover, I must assume as known the chief physical properties of
+ordinary soils with respect to the phenomena of capillarity, absorption
+of heat, action of frost, and so forth.</p>
+
+<p>But all ideas as to the nature of soil based merely on the study of its
+chemistry and physics are misleading, and it is in just the
+establishment of this truth that modern discoveries in Agricultural and
+Forest Botany have played so important a part.</p>
+
+<p>From the facts that organic débris is found chiefly at the surface of
+the earth, and that the smallest particles are held in suspension by the
+water near the surface, it is comprehensible why such organic remains
+abound in the upper parts of the soil, where the rootlets with their
+absorbing root-hairs are also found, because they must have oxygen. The
+rule is, therefore, that an ordinary soil consists of upper strata, rich
+in organic materials and in oxygen, and a subsoil, poorer in these
+substances.</p>
+
+<p>Among these organic materials are countless myriads of living beings,
+especially fungi and bacteria, which require oxygen and organic
+materials for their subsistence, and it depends on the open or close,
+moderately moist or damp, warm or cold nature of the soil, and on some
+obviously connected factors, how far down these aërobic organisms can
+thrive. As we go deeper down they <span class="pagenum"><a name="Page_58" id="Page_58"></a>[<a href="./images/58.png">58</a>]</span>become fewer and fewer, and gradually
+disappear, and (neglecting certain anaërobic bacteria of putrefaction)
+they are rarely found in marked abundance more than a few inches below
+the surface soil.</p>
+
+<p>These aërobic fungi and bacteria are the great agents of continued
+fertility of a soil, and it is they which, living and multiplying in the
+moist and well-aerated warm interstices of a rich open soil, carry out
+the useful destruction of organic matter, breaking it up into mineral
+and gaseous bodies, which are then dissolved in the water bathing the
+root-hairs or escape into the atmosphere. In this work of destruction
+they are aided by the oxygen of the air and the solar heat: their own
+fermentative action is also accompanied by a marked rise of temperature,
+and the carbon-dioxide and other products of their activity all go to
+complicate the chemical changes going on in the soil around the roots.</p>
+
+<p>Duclaux has calculated that <i>Aspergillus niger</i>, a common mould fungus,
+can break down organic substances, such as carbohydrates, at such a rate
+that a metre cube of the fungus would decompose more than 3000 kilogr.
+of starch in a year, and this may serve as an example giving some idea
+of the possibilities in soil.</p>
+
+<p>Analyses of waters containing large quantities of organic matter, as
+they enter such open soils as those referred to, compared with the
+drainage water after passing through the upper strata, show that the
+carbonaceous and nitrogenous materials are broken down to more or less
+completely <span class="pagenum"><a name="Page_59" id="Page_59"></a>[<a href="./images/59.png">59</a>]</span>oxidised simpler compounds, and that the following chief
+changes result. The ammonia and some other nitrogenous bodies remain
+behind in the soil, as also do the phosphoric acid and much of the
+potash; whereas large quantities of nitric and nitrous acids, together
+with much sulphuric acid, chlorides, and calcium salts pass away in the
+drainage. These facts are obviously highly important in agriculture.</p>
+
+<p>Experiments on sewage farms have shown also that the upper soil retains
+most of the bacteria of the sewage. Koch found at Osmont, near Berlin,
+that whereas the different sewage waters contained numbers so enormous
+that each cubic centimeter probably held 38,000,000 germs, the different
+drainage waters held only 87,000 per c.cm.; and the whole process of
+water-filtration through sandy soils depends on these well-known facts.</p>
+
+<p>Recent experiments in connection with soil-filtration, however, bring
+out the further facts that the oxidations which organic matters undergo
+in the soil&mdash;and without which they are useless to the higher
+plants&mdash;are enormously enfeebled if the upper layers of soil are
+sterilised, so as to deprive them of the myriads of aërobic bacteria,
+fungi and yeasts which they normally contain, and there can no longer be
+any doubt as to the importance of the biology of the soil in connection
+with the preparation of materials suitable for absorption in solution by
+the root-hairs of agricultural and other plants.</p>
+
+<p>The researches of the last ten years have <span class="pagenum"><a name="Page_60" id="Page_60"></a>[<a href="./images/60.png">60</a>]</span>brought to light a long list
+of forms, comprising yeasts, such as Hansen's <i>Saccharomyces
+apiculatus</i>, fungi and bacteria which live and grow in the soil, finding
+their water and food supplies in the interstices, and under conditions
+which we now know to be very diverse. They are usually more numerous, in
+species and individuals, in cultivated farm and garden soils than in
+woods, prairies, and untilled lands; but the geological nature of the
+strata, the closeness and otherwise of the soil, its damp or dry
+character and its average temperature (which depends on many things
+besides latitude or altitude) and other factors co-operate to rule their
+distribution and numbers. The fact that cultivated land is so well
+supplied with manures, air, etc., is of great importance in relation to
+their relative abundance there, and it is extremely probable that the
+use of artificial manures lessens their numbers considerably as compared
+with land on which stable and other animal manures are employed.</p>
+
+<p>A list of the soil-bacteria which have been isolated and more or less
+carefully cultivated and examined would comprise about fifty species;
+but it is certain that, as at present classified and named, many more
+species are to be discovered in any ordinary soil.</p>
+
+<p>The fungi are apparently even more numerous than the bacteria, and we
+may rest satisfied for the present with the general statement that the
+life-actions of the myriads of individuals of these organisms in the
+soil completely alter the question <span class="pagenum"><a name="Page_61" id="Page_61"></a>[<a href="./images/61.png">61</a>]</span>of soil-water as understood by the
+last generation of agriculturalists.</p>
+
+<p>But there is another aspect of this question of soil-organisms which has
+grown in importance of late to such an extent that we are more than ever
+justified in regarding the biology of soil as far more vital to the
+interests of the plant than its physical or chemical properties. With
+many of the fungi in the soil the roots of plants have to compete&mdash;just
+as plant competes with plant&mdash;for water, salts, and other
+food-materials. The toadstools which are so conspicuous in fields and
+forests spring from mycelia which ramify in the ground, and are busily
+breaking down the remains of other organisms, and just such fungi are
+known to store up relatively large quantities of salts of potassium and
+phosphorus&mdash;the very salts which are so valuable to crops and occur so
+sparingly in most soils, but which the extensively spread fungus mycelia
+can gradually accumulate. Some of these fungi, moreover, are more active
+in their antagonism, and actually attack and pierce the roots as
+destructive parasites, but I pass these by for the present, as they form
+the subject for further consideration when we come to the diseases of
+plants.</p>
+
+<p>It is obvious that the competition of fungi with root-hairs for mineral
+salts, oxygen, etc., may be at times acute, and it is extremely probable
+that cases of so-called sterility of soil, where a particular soil is
+found unsuitable for a crop, may sometimes be due to this
+over-competition.</p>
+
+<p><span class="pagenum"><a name="Page_62" id="Page_62"></a>[<a href="./images/62.png">62</a>]</span>The researches of recent years, however, and especially those of Frank,
+Winogradsky, Hellriegel, and Stahl, have brought to light a series of
+relationships between certain of these soil-organisms and the higher
+plants which place the matter of soil-biology in quite new lights.</p>
+
+<p>On the one hand it has been discovered that groups of bacteria are the
+active agents in bringing about the destruction of organic nitrogenous
+matter with the formation of ammonia, in oxidising this ammonia to
+nitrous and to nitric acids, which combine with bases in the soil to
+form the corresponding salts; while, on the other hand, other forms can
+decompose the nitrates and reduce them to nitrites, or set free ammonia
+or even nitrogen from them. Moreover, there are certain species which
+can fix the free nitrogen of the atmosphere, and start the cycle of
+up-building of this inert element into the complex higher compounds we
+term organic. It is impossible to over-estimate the importance of these
+processes of nitrification and denitrification going on in the soil
+about the root-hairs of the higher plants.</p>
+
+<p>But, in addition to this circulation of nitrogen in the soil, it turns
+out that the life-actions of bacteria, and not mere chemical
+decompositions, are largely responsible for the circulation of carbon,
+of iron, of sulphur and other elements formed from the
+decomposition&mdash;also by bacterial and fungal agency&mdash;of animal and
+vegetable remains in the soil.</p>
+
+<p>Even more startling are the biological relations <span class="pagenum"><a name="Page_63" id="Page_63"></a>[<a href="./images/63.png">63</a>]</span>in the soil between
+the absorbing roots of the higher plants and some of these bacteria and
+fungi, for it has now been established beyond all doubt that certain
+fungi enter the living roots and there flourish not as mere destructive
+parasites, but as messmates not only tolerated by the plant, but even
+indispensable to its welfare. It is probable that nearly half the plants
+of our fields, moors, and forests entertain such fungi in their
+root-tissues. The curious, and long-known nodules on the roots of
+leguminous plants&mdash;peas, beans, clover, etc.&mdash;are filled with bacteria
+which enable these plants to avail themselves of the free nitrogen of
+the air, and so enrich the soil with nitrogenous substances.</p>
+
+<p>The roots of most forest trees, orchids, and plants of the moorlands,
+meadows and marshes are similarly occupied by fungi, which in some way
+convey salts&mdash;probably especially phosphates and potassium compounds&mdash;to
+the plant in return for the small tax of organic carbon-compounds it
+exacts from the latter. In some cases at any rate, as Bernard has lately
+shown, the very existence of the plant depends on its seedling roots
+obtaining this advantageous attachment and co-operation (symbiosis) of
+the fungus immediately on germination.</p>
+
+<p>These remarks must suffice to illustrate this part of my subject, and to
+emphasise the statement that the question whether a given plant can be
+grown in a given soil, is by no means one of simply the physical and
+chemical constitution of the latter. The plant will have to run the
+gauntlet of a long series of vicissitudes brought <span class="pagenum"><a name="Page_64" id="Page_64"></a>[<a href="./images/64.png">64</a>]</span>about by the presence
+or absence, relative proportions and vigour, and specific nature of the
+organisms in the soil at its roots, and it is easy to see that many
+cases of disease may be due to the absence of advantageous bacteria or
+fungi, or to circumstances which disfavour their life, as well as to the
+predominance of competing organisms.</p>
+
+<p>It will now be evident that the old points of view must be abandoned,
+and with them, especially, the widely prevalent notion that chemical
+analyses of the plant and soil can explain the real problems of
+agriculture.</p>
+
+<p>It was of course an enormous advance in the science when, thanks to the
+splendid labours of the chemists, at the end of the last century and the
+beginning of this, we obtained that preliminary knowledge of the
+constitution of the air, and of the composition of the water, acids and
+salts, etc., which plants require for their food-materials and
+life-processes. Much was gained by De Saussure's establishment of the
+fact of oxygen respiration, though we now understand by the term
+something very different from, and much more complex than, what he
+understood by it, as, also, much had been gained by the previously
+acquired knowledge of the gas-exchanges in carbon-assimilation: nor must
+we forget the services of those who proved, by laborious analyses,
+continued for long periods, what chemical compounds are found in the
+tissues of plants, and in the soils at their roots and the atmosphere
+which surrounded them. We must also remember many other <span class="pagenum"><a name="Page_65" id="Page_65"></a>[<a href="./images/65.png">65</a>]</span>contributions
+which have been furnished, and are still being furnished by the chemist;
+and I for one hope that his labours will continue to go hand in hand
+with those of the physiologist.</p>
+
+<p>But, when all due honour is paid to the scientific chemist, it must
+still be allowed that his problems are different from the real problems
+of agriculture. To take one set of instances alone. The chemist can
+analyse a given soil or a given manure, and can even go a long way
+towards making them, but his analyses do not tell us what conditions are
+necessary in order that their ingredients may be presented to the roots
+so as to be absorbed and become built up into the plant. Chemistry told
+us that carbon was fixed from the air, but physiological experiments
+determined how this meant the synthesis of certain definite
+carbohydrates&mdash;this, too, in the face of the powerful authority of the
+chemist Liebig, who supposed that the vegetable acids were the results
+of the assimilation of carbon. Wolff, De Saussure, and other chemists
+have done yeoman service in showing that different plants, growing in
+the same soil, contain different proportions of mineral substances; but
+it was by means of water-cultures, and other physiological researches,
+such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch
+on root-hairs, that the puzzling question of selective absorption, by
+means of the living root-hairs, came into the arena of our knowledge.</p>
+
+<p>In every case&mdash;and, as already said, I am not undervaluing the work
+done&mdash;the chemist has left <span class="pagenum"><a name="Page_66" id="Page_66"></a>[<a href="./images/66.png">66</a>]</span>us only on the threshold of the real
+problem. He has stood outside the factory in which the real work we want
+to know about is being carried on, and has told us of so many tons of
+this material being carried in at the gates, and of so many tons of that
+coming out; he has even burnt down the factory, and all its contents and
+machinery, and has then told us how many tons of the various materials
+were there at the time; but this is not what we want, valuable as the
+information is, and still more will be. What we want, and what we expect
+to obtain, is more information regarding what is done with the materials
+in the factory: what machinery they are put into, and how they are put
+in: what stages they go through, and how the stages follow one another:
+what wear and tear has to be endured, and how we can step in and stop
+the working of the machine for our own benefit at the best possible
+time.</p>
+
+<p>The physiologist proceeds empirically, by experimenting with the living
+machinery. He recognises the parts and their structure, and tries to
+find out what they are doing: he knows that the laws of physics and
+chemistry cannot be traversed, but he sees these laws at work under
+special and very complex and peculiar conditions. He therefore, as the
+results of his experiments, sets new questions&mdash;or old questions under
+new conditions, if you like&mdash;and undoubtedly wants the help of both
+chemist and physicist; or, if it is preferred, the chemist and physicist
+may attack the problems, but they must familiarise themselves with the
+<span class="pagenum"><a name="Page_67" id="Page_67"></a>[<a href="./images/67.png">67</a>]</span>peculiar mechanism of the organism concerned, and cannot hope to attain
+success without experimenting with it. I confess it seems to me as
+reasonable to look upon scientific agriculture as a branch chiefly of
+chemistry as it would be to look upon horse-breeding or pigeon-rearing
+from the same point of view; and why the professed chemist's advice is
+regarded as so comforting and final in the one case and not in the other
+is one of those mysteries which seem inherent in human nature.</p>
+
+<p>The central point in agriculture is the plant: get the most out of
+it&mdash;the energy-winning machine which alone can keep the animals and
+everything else connected with the farm going&mdash;and all the rest follows.
+The old agriculture has taken a gloomy view of things, and especially on
+account of a large variable which it blames for many ills, namely, the
+season or climate. Perhaps the old agriculture has not sufficiently
+recognised that Nature grows plants in accordance with the fact that
+variation is not peculiar to the weather: if the seasons vary, so do
+fruit and other produce and the plants which yield them; and since man
+cannot hope to control the one variable, possibly relief will be found
+in doing more, within his limits, towards controlling others.</p>
+
+<p>In any case he cannot hope to succeed without study of the physiology of
+the plant.</p>
+
+
+<h4><span class="smcap">Notes to Chapter VII.</span></h4>
+
+<div class="chnote">
+<p>An admirable short account of soil in its relation to
+root-hairs is given in Sachs' <i>Lectures</i>, XV.; but for a more
+exhaustive <span class="pagenum"><a name="Page_68" id="Page_68" style="font-size: 110%;"></a>[<a href="./images/68.png">68</a>]</span>treatment of the subject of soil the reader is
+referred to King, <i>The Soil</i> (Wisconsin, 1895), or Warrington,
+<i>Lectures on the Physical Properties of Soil</i> (Oxford, 1900);
+<span lang="fr" xml:lang="fr">Larbalétrier, <i>L'Agriculture</i></span> (Paris, 1888), chapters II. and
+III. There is also a very good account in Bailey, <i>The
+Principles of Agriculture</i> (London, 1898), chapters I.-III.</p>
+
+<p>With reference to the organisms in soils and the
+decompositions they bring about, the student should consult
+Kramer, <span lang="de" xml:lang="de"><i>Die Bakteriologie in ihren Beziehungen zur
+Landwirthschaft</i></span> (Wien, 1890), and Lafar, <i>Technical Mycology</i>
+(Engl. edition, 1898), sections V., VIII., and IX.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_69" id="Page_69"></a>[<a href="./images/69.png">69</a>]</span></p>
+<h2>CHAPTER VIII.</h2>
+
+<h3>HYBRIDISATION AND SELECTION.</h3>
+
+<div class="chsub">
+<p>The crossing of varieties of wheat, etc.&mdash;The essentials of
+fertilisation&mdash;Rimpau's experiments&mdash;Hybrids and selected
+varieties.</p>
+</div>
+
+
+<p>In the more hopeful view of the case which the new agriculture will have
+to take, it will recognise the physiological truth that since the living
+plant is the important and variable machine which constructs the produce
+looked for, and since that machine will work best in proportion as its
+needs are properly satisfied; therefore in cases where the needs of a
+given type of the machine cannot be efficiently provided for, it will be
+well to select some other type which will take what supplies and
+conditions can be offered. Of course, this is already recognised to a
+certain extent, as is implied in the practices of "rotation of crops,"
+selection of "pedigree wheats" and mixtures of "pasture grasses," and in
+decisions as to the quality of land according to the kinds of weeds
+found on it, and <span class="pagenum"><a name="Page_70" id="Page_70"></a>[<a href="./images/70.png">70</a>]</span>so forth; but I am convinced that the agriculturist of
+the future&mdash;and the same applies to the horticulturist, planter and
+forester&mdash;will have to concern himself more systematically with the
+working and the variability of the plant, and particularly with what
+Darwin termed Variation under Domestication, than has always been the
+custom in the past. The subject of the plasticity of cultivated plants,
+and especially of hybrids, is in one sense an old one; but much work is
+being done which proves, as such work is apt to do, that very much more
+may be done by well-planned experiments on the selection of new
+varieties raised by hybridising and cultivation.</p>
+
+<p>In illustration of this point, a short summary of some of the results of
+crossing different species of wheat, barley, oats, peas, beet, etc., may
+serve to show what has been gained and what may be hoped for in these
+directions. It should be stated that much has been done and is being
+done in this country as well as abroad, as witness English varieties of
+corn, peas, and potatoes, and the recent experiments on crossing various
+kinds of maize in America.</p>
+
+<p>The hybridiser grows his cereals, etc., in pots until ready for
+crossing, and then takes them into the laboratory, removes the weaker
+spikelets, and takes out the young stamens from the flowers left on the
+plant. The female plant is then ready, and the flowers covered with
+paper caps. The pollen, obtained by a clean wet brush from the plant
+chosen as the father, is then carefully placed in <span class="pagenum"><a name="Page_71" id="Page_71"></a>[<a href="./images/71.png">71</a>]</span>position on the
+stigmas, and the caps replaced. The pollination is repeated
+occasionally, and care taken that no uncrossed flowers develop later. In
+this way a few seeds or grains are got to start with.</p>
+
+<p>This would be the place to introduce an account of the enormous advances
+made by the botanists of the last decade or two in the study of the
+microscopic phenomena of fertilisation. Without going into
+details&mdash;which would more than occupy all the space at command&mdash;I may
+recall the discoveries of Strasburger and his pupils, and of Guignard,
+which have supplemented the earlier discoveries of De Bary, Cohn, and
+Hofmeister, by establishing the facts that the essential point in
+fertilisation is the fusion of two nuclei, and the bringing together in
+the fused mass of two extremely minute thread-like coiled bodies, the
+so-called chromatosomes or filaments, one of which is derived from the
+male and the other from the female parent. The particulars as to the
+marvellous adaptations to secure the union of these two infinitesimally
+minute threads, their behaviour immediately before and after union, and
+many other points must be passed over, as I have only space to emphasise
+the one crowning discovery that these tiny filaments of nuclear
+substance are the material carriers of all the hereditary properties of
+the parents to the young plant which their union initiates.</p>
+
+<p>It must not be supposed that the above statements are based on any
+meagre foundation of facts. The attraction of the fusing nucleated
+masses had been demonstrated over and over <span class="pagenum"><a name="Page_72" id="Page_72"></a>[<a href="./images/72.png">72</a>]</span>again by Tulasne, De Bary,
+Strasburger and others; but Pfeffer brought the matter to a crisis by
+discovering the attractive (chemotactic) substance emitted in given
+cases, and by collecting the fertilising bodies by its means into
+artificial tubes.</p>
+
+<p>The fusion of the nucleated bodies in the sexual act was observed by
+Strasburger in the living plant a few years ago, and numerous later
+observers have confirmed it. Meanwhile all the stages of approach and
+contact of the essential filaments of the nuclear substance have been
+traced, as also all the stages of the transference of half of each
+filament, male and female, into each of the first two cells of the very
+young embryo-plant.</p>
+
+<p>Moreover, the essentials are found to be the same in the animal kingdom
+also, and the bearing of all these discoveries on the phenomena of
+reproduction, variation, and heredity in living organisms has been and
+is of the highest importance, for they support, control, explain and
+correct so many of the splendid results of Knight, Kölreuter, Sprengel,
+Hildebrand and Hermann Müller, and in every direction throw side-lights
+into the crevices of that magnificent structure, the theory of Natural
+Selection, erected for all time by our countryman, Charles Darwin.</p>
+
+<p>To return now to experiments on crossing. It is found that the first
+products of the crossing appear exactly alike; they may have characters
+intermediate between those of the father and mother, or they may
+resemble one more than the <span class="pagenum"><a name="Page_73" id="Page_73"></a>[<a href="./images/73.png">73</a>]</span>other, but all the seeds of the same cross
+do it in the same way.</p>
+
+<p>On then sowing the seeds of the plants produced from this first cross,
+variations begin to appear. Most of the progeny revert to one or other
+of the parent forms, others show all conceivable combinations of their
+characters, and a few may give rise to entirely new characters. In
+succeeding generations the reversions are preponderant, and, supposing
+no care is taken to prevent it, the whole of the offspring gradually go
+back to the ancestral type.</p>
+
+<p>Some important consequences result, however, if systematic care is
+brought to bear on the matter. This tendency to variation in the second
+generation of crossed plants has often been noted, and it bears out very
+distinctly the conclusions to which Darwin came.</p>
+
+<p>The hybridiser takes advantage of this variation, as others have done,
+to select some forms and rigidly suppress others, in order to obtain
+well-marked varieties of the plants he experiments with. In
+illustration, I may take the following from Rimpau's account of his
+experiments on crossing wheat: By crossing a white English long-eared,
+dense wheat, and celebrated as a heavy cropper, with a red, looser
+German wheat, remarkable for its resistance to winter cold, Rimpau hoped
+to obtain a variety uniting both the above qualities. As regards the
+property of resistance, he failed, and he eventually gave up the
+attempts in face of the advantages offered by the <span class="pagenum"><a name="Page_74" id="Page_74"></a>[<a href="./images/74.png">74</a>]</span>so-called
+<i>Square-heads</i>, which then came into the market. His experiments, even
+with the above varieties, are worth noting, however, for they show how
+promising the results of carefully conducted crossing and selection may
+be.</p>
+
+<p>The crossing was done in 1875, in both directions. In 1876 the few
+grains obtained were found to yield plants almost all alike, with the
+long loose ear of the German parent, but the paler colour of the English
+wheat.</p>
+
+<p>In 1877 the plants, obtained by sowing the finest grains, were found to
+consist of pure white, pure red, and of forms which appeared to vary and
+revert in all possible degrees as regards colour, density, and other
+characters intermediate between these.</p>
+
+<p>By carefully separating the closest and densest white wheats from the
+closest and densest red ones, he got in 1878 a large number of each
+coming nearer to the type sown than did the mongrel forms intermingled
+with them: these reversions and intermediate forms were then rigidly
+eliminated, and only the deepest coloured and densest red and white
+forms again sown.</p>
+
+<p>In 1879 these two chosen varieties were constant, so far as concerned
+those selected from the crossing of female English white with male
+German red wheat, and the following year proved the constancy of the red
+variety in the reciprocal cross. In 1886 all four varieties&mdash;<i>i.e.</i> the
+two reds and the two whites of both the crossings&mdash;had become constant.</p>
+
+<p><span class="pagenum"><a name="Page_75" id="Page_75"></a>[<a href="./images/75.png">75</a>]</span>Still more instructive are the results of the cross between the same
+white English non-bearded wheat and a red German bearded wheat.</p>
+
+<p>The first results of the crossing in 1875 showed the loose ear of the
+German mother, but was paler in colour; while the influence of the
+English father was shown by the absence of beard.</p>
+
+<p>From the reversions and mixtures of the mongrels showing reminiscences
+of the parents in all degrees in 1877, rigid selections and re-sowings
+were made as before, and Rimpau eventually got four very distinct
+varieties, two red and two white, a bearded and a beardless form of
+each, and these were declared fixed and constant in 1879-1882.</p>
+
+<p>Passing over many similar results, and merely noting a very successful
+variety got from a cross between a very early ripening loose red
+American wheat and the dense heavy cropping English Square-head&mdash;the
+crossed variety which has proved very suitable for certain light soils
+and dry climates on the Continent, which demand very rapid ripening, and
+are therefore of great physiological and technical interest&mdash;I must pass
+on to note the curious result of the successful hybridisation of wheat
+and rye. This cross has been effected several times, and first in this
+country according to reports from Edinburgh (1875), New York (1886), and
+elsewhere, and Rimpau's careful experiments seem to leave no doubt on
+the matter.</p>
+
+<p><span class="pagenum"><a name="Page_76" id="Page_76"></a>[<a href="./images/76.png">76</a>]</span>First I must remind you that wheat (<i>Triticum</i>) differs from rye
+(<i>Secale</i>) in several marked characters, such as the breadth and shape
+of the glumes, the number of flowers in the spikelet, etc.; and that the
+cultivated rye differs from cultivated wheats in the characters of the
+straw, in having long ears, and in its flowering glumes remaining widely
+divaricated for some days when in flower.</p>
+
+<p>In 1888 Rimpau removed the young stamens from the German wheat referred
+to, and pollinated the stigmas with pollen from a long-eared rye. Four
+sound grains were obtained, looking like wheat-grains.</p>
+
+<p>The history of one of these grains was as follows: In 1889 it yielded
+ears which were peculiarly narrow and long, and its stalks were also
+much longer than the wheat: the flowers remained exposed, with widely
+open paleae, for several days, and the grains were very peculiar, though
+wheat-like.</p>
+
+<p>Fifteen of the best grains were selected, and in 1890 three of the
+resulting plants proved to be a wheat of the Square-head type and one
+quite sterile. The others retained the elongated, narrow, brownish-red
+ears, the flowering glumes again opening wide for some days. This last
+is a characteristic of rye, but not of wheat.</p>
+
+<p>A long series of natural hybrids of wheat, barley, and oats are also
+described and discussed by Rimpau, as well as artificial crosses&mdash;some
+very remarkable&mdash;of barleys, but they must be passed over here.</p>
+
+<p><span class="pagenum"><a name="Page_77" id="Page_77"></a>[<a href="./images/77.png">77</a>]</span>Peas rarely become hybridised naturally. According to Darwin, H. Müller,
+and Focke, the flowers are little visited by insects in our countries,
+though the mechanism points to their adaptation for pollination by large
+bees.</p>
+
+<p>Rimpau confirms Darwin, H. Müller, and Ogle as to the self-fertilisation
+of our cultivated peas. Nevertheless, as is well known, marked varieties
+have been obtained by artificial crossing by Gärtner, Knight, Laxton,
+and others, especially in this country.</p>
+
+<p>At the same time experiments show that while it is very easy to obtain
+artificial hybrids of such plants, and there is no fear of natural
+inter-crossing, the forms are remarkably unstable as yet. Similarly
+unsatisfactory results were obtained with beet. As experiments are still
+going on, however, we may expect to hear more about these and other
+results.</p>
+
+<p>It is probable, from recent experiments by De Vries, Correns, and
+others, that a remarkable regularity, expressed by Mendel in the form of
+a law, obtains in the variations which result from hybridising.</p>
+
+<p>In considering these illustrative cases, it is necessary to thoroughly
+apprehend that two procedures are involved. In the first place we have
+the cross-pollination leading to the formation of the hybrid plant by
+cross-fertilisation. But experience shows that this would lead to very
+uncertain results if the plant-breeder did not supplement them by the
+second and extremely <span class="pagenum"><a name="Page_78" id="Page_78"></a>[<a href="./images/78.png">78</a>]</span>important process of rigid selection&mdash;<i>i.e.</i> by
+choosing the best of the progeny and breeding from them apart from the
+parent-forms, and gradually intensifying, as it were, the variations in
+certain directions which have been started by the crossing.</p>
+
+<p>It is by selection, careful culture, and repeated selection that so much
+has been done in obtaining the innumerable new varieties of roses,
+sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries,
+melons, tomatoes, early potatoes, etc., brought forward by numerous
+breeders of plants in all countries, as will readily be understood if
+reference be made to the work of Hays and Webber in America; Saunders in
+Canada; Garton, Sutton, Veitch, Bateson, and others in this country.</p>
+
+<p>Nor is it necessary that the new materials for selection to work upon
+should be started by hybridisation. Grafting, change of conditions, and
+even variations so vaguely understood that we term them "spontaneous,"
+may supply the starting-points for changes in the characters of plants,
+so remarkable after intensification by breeding that people find it
+difficult to believe they can have come from one stock.</p>
+
+<p>Here, however, I must conclude, merely remarking that the above sketch
+is a mere outline of the subjects modern agriculture and horticulture
+concern themselves with. There are hundreds of problems connected with
+the germination of seeds, on which valuable recent work has been done by
+Klebs, Green, Horace Brown, and others; with <span class="pagenum"><a name="Page_79" id="Page_79"></a>[<a href="./images/79.png">79</a>]</span>the resistance of seeds
+and seedlings to high and low temperatures, a subject opened out by
+Sachs, Kny, De Vries, Krasan, Just, Höhnel, Dewar, Dyer, and others;
+with the conditions of vegetation which affect the various functions of
+growth, respiration, assimilation, transpiration, and so forth, on which
+I cannot even touch in these pages.</p>
+
+<p>Meanwhile I hope I have succeeded in impressing upon you the grand fact
+that the plant is a living and very complex engine, driven by the
+radiant energy of the sun, and capable of doing work thereby, and this
+just as truly as any heat-engine is driven by chemical energy gained by
+means of the sun's rays, or as a water-mill is driven by power which
+must be referred to the energy of potential in the head of water placed
+in position by the sun's work in evaporation. Fundamentally the whole of
+life and work on our planet is to be referred to the one great source of
+energy which renders possible the establishment of differences of
+potential.</p>
+
+<p>This machine, then, doing work in various ways, adapts itself&mdash;or goes
+to the wall&mdash;to the conditions of its work among competing organisms or
+opposing circumstances. Curiously enough, while in some cases it suffers
+from the competition, in others it is benefited by its life-actions
+fitting in between those of other organisms, which in their turn
+supplement it. In other words new types of this engine, capable of doing
+the work in various ways, are obtainable; some <span class="pagenum"><a name="Page_80" id="Page_80"></a>[<a href="./images/80.png">80</a>]</span>are good types for the
+conditions afforded, others are bad ones.</p>
+
+<p>Examples of both will occur in the further exposition of the subject.</p>
+
+<p>Man's position in regard to the struggle is that of an intelligent being
+who steps in at certain stages and protects, fosters, and in every way
+favours the agricultural plant&mdash;the living machine&mdash;and sees that every
+opportunity is given it to do its best work in the best way&mdash;from his
+points of view!</p>
+
+
+<h4><span class="smcap"><a name="Chapter_VIII_Notes" id="Chapter_VIII_Notes"></a>Notes To Chapter VIII.</span></h4>
+
+<div class="chnote">
+<p>The foundation of any course of reading on hybridisation and
+selection should be Darwin's <i>Effects of Cross and
+Self-Fertilisation in the Vegetable Kingdom</i>, which, with his
+books <i>On the Origin of Species by means of Natural Selection</i>
+and <i>The Variation of Animals and Plants under Domestication</i>,
+will prepare the student for the long course of reading
+necessary for a full appreciation of what has been done in
+this department of science.</p>
+
+<p>From the numerous works which followed these I should select
+Bailey's <i>Survival of the Unlike</i>, London, 1896, and
+<i>Evolution of our Native Fruits</i>, New York, 1898, as
+especially useful for the reader of this book, to which may
+also be added <i>Plant Breeding</i>, New York, 1896, by the same
+author, as giving numerous facts and practical directions of
+value. Further, the "Hybrid Conference Report," <i>Journ. Roy.
+Hort. Soc.</i>, 1900, abounds in facts and information. Rimpau,
+<span lang="de" xml:lang="de"><i>Landw. Jahrb.</i></span>, vol. xx., 1891, p. 239. The student who
+wishes to get towards the root of the matter will hardly be
+able to dispense with Strasburger's <span lang="de" xml:lang="de"><i>Neue Untersuchungen über
+die Befruchtungsvorgang bei den Phanerogamen</i></span>, Jena, 1884. An
+interesting summary of recent work on <i>Xenia</i> and "double
+fertilisation" will be <span class="pagenum"><a name="Page_81" id="Page_81" style="font-size: 110%;"></a>[<a href="./images/81.png">81</a>]</span>found in <i>Bull. No. 22, U.S. Dept. of
+Agric.</i>, 1900. See also <i>Nature</i>, Mar. 15, 1900, p. 470.</p>
+
+<p>If he wishes to explore the vast region of controversial
+literature that opens up from these points, and which is far
+beyond the purpose of this book, he may consult the literature
+collected in Kassowitz' <span lang="de" xml:lang="de"><i>Allgemeine Biologie</i></span>, Wien, 1899, B.
+II., and the references in the works quoted; also,
+Strasburger, "The Periodic Reduction of Chromosomes in Living
+Organisms," <i>Ann. Bot.</i>, viii., 1894, p. 281. For "Mendel's
+Law," see Correns in <span lang="de" xml:lang="de"><i>Ber. d. deutsch. bot. Gesellsch.</i></span>, vol.
+xviii., 1900, p. 158.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_82" id="Page_82"></a>[<a href="./images/82.png">82</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_83" id="Page_83"></a>[<a href="./images/83.png">83</a>]</span></p>
+<h2><i>PART II.</i></h2>
+
+<h3>DISEASE IN PLANTS.</h3>
+
+<p><span class="pagenum"><a name="Page_84" id="Page_84"></a>[<a href="./images/84.png">84</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_85" id="Page_85"></a>[<a href="./images/85.png">85</a>]</span></p>
+<h2>CHAPTER IX.</h2>
+
+<h3>PHYTOPATHOLOGY. DERIVATION AND MEANING.</h3>
+
+<div class="chsub">
+<p>History. References in the Bible&mdash;Greeks and
+Romans&mdash;Shakespeare&mdash;Rouen law&mdash;Superstitions&mdash;Malpighi and
+Grew&mdash;Hales&mdash;Unger&mdash;Berkeley&mdash;De Bary, etc. Physiology and
+Biology&mdash;Diagnosis&mdash;Etiology&mdash;Therapeutics. Study of causes.</p>
+</div>
+
+
+<p>Phytopathology, from Greek words which signify to treat of diseases of
+plants, comprises what is known of the symptoms, course, and causes of
+the diseases which threaten the lives of plants, or bring about injuries
+and abnormalities of structure. As a distinct and systematised branch of
+botany it is a modern study, the history of which only dates from about
+1850, though the subject had been treated more or less disjointedly by
+several authors during the preceding century, and isolated records of
+diseased crops, fruit-trees, etc., exist far back in the history of
+Europe. The existence of mildews and blights on cereals indeed <span class="pagenum"><a name="Page_86" id="Page_86"></a>[<a href="./images/86.png">86</a>]</span>was
+observed and recorded by the writers of the older books of the Bible,
+half a dozen references to such blights being found in the Old
+Testament, as well as others to blasted fig trees, etc., in the New
+Testament. Aristotle, about 350 <span class="allcapsc">B.C.</span>, noticed the epidemic nature of
+wheat-rust. The Greeks and Romans were so well acquainted with such
+diseases that their philosophers speculated very shrewdly as to causes,
+while the people dedicated such pests to special gods. As regards the
+Middle Ages, we know little beyond the fact that blights and mildews
+existed, but Shakespeare's reference in <i>King Lear</i> (Act <span class="allcapsc">III.</span>, Sc. 4)
+leaves no doubt as to his acquaintance with mildew in the 17th century,
+and other authorities bear out the same. Even the law took cognisance of
+the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th
+century, however, only meagre notes on the subject occur scattered here
+and there among other matters, and much superstition existed then and
+later regarding these as other diseases.</p>
+
+<p>Malpighi, in 1679, gave excellent figures of leaves rolled by insects
+and of numerous galls, the true nature of which he practically
+discovered by observing the insect piercing the tissues; previous
+observers&mdash;Pliny knew that flies emerge from galls, but thought the
+latter grew spontaneously&mdash;having nothing but superstitions and
+conjectures to offer. Grew, in 1682, also gave a capital figure and
+description of a leaf mined by "a small flat insect .&nbsp;.&nbsp;. which neither
+ranging in breadth nor striking deep into the leaf, eats so much only
+<span class="pagenum"><a name="Page_87" id="Page_87"></a>[<a href="./images/87.png">87</a>]</span>as lies just before it, and so runs scudding along betwixt the skin and
+the pulp of the leaf, leaving a whitish streak behind it, where the skin
+is now loose, as the measure of its voyage"&mdash;a by no means inadequate
+description of the injury and its cause.</p>
+
+<p>During the eighteenth century several academic treatises or
+dissertations dealing with diseases of plants appeared.</p>
+
+<p>But as a rule we only find disjointed notes. Hales (1727-33) discusses
+the rotting of wounds, canker, and a few other matters, but much had to
+be done with the microscope ere any substantial progress could be made.</p>
+
+<p>With the nineteenth century, and the founding of the modern theories of
+nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era
+started. As the discoveries of the microscopists continued to build up
+our knowledge of the anatomy of plants and began to elucidate the
+biology of the fungi and other cryptogams, while the chemists and
+physiologists laid the foundations of our modern science of plant life,
+it gradually became possible to tabulate and classify plant diseases,
+and discuss their symptoms and causes in a more scientific manner. Even
+in 1833, however, Turpin, and a far better observer, Unger, regarded
+parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles
+and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846).
+We may pass over the various treatises of Wiegmann (1839), Meyen (1841),
+Raspail <span class="pagenum"><a name="Page_88" id="Page_88"></a>[<a href="./images/88.png">88</a>]</span>(1846), Kühn (1859), and a number of other works of the period,
+merely referring with emphasis to Berkeley's admirable papers in the
+<i>Gardener's Chronicle</i> (1854) for a summary of what was then known. All
+these works antedate De Bary's <i><span lang="de" xml:lang="de">Morphologie und Physiologie der Pilze</span>,
+etc.</i> (1866), in which he brought together the results of his researches
+during the decade, proving the real nature of parasitic diseases and
+infection as worked out by experiments between 1853 and 1863.</p>
+
+<p>This work put the whole subject of parasitic diseases of plants and
+animals on a new footing, and paved the way for the modern treatment of
+plant pathology as elaborated in the treatises of Frank (1880 and 1895),
+Sorauer (1886), Kirchner (1890), and others, to which the reader is
+referred for further details. I will merely quote the following passage
+from Raspail's <span lang="fr" xml:lang="fr"><i>Histoire Naturelle de la Santé et de la Maladie</i></span>, 1846
+(vol. ii., p. 176), in illustration of the views entertained by high
+authorities just prior to De Bary's work: "<span lang="fr" xml:lang="fr">L'insecte qui produit les
+<i>erineum</i>, <i>uredo</i>, <i>æcidium</i>, <i>xyloma</i>, <i>puccinia</i>, n'est donc plus
+pour nous un insecte inconnu, mais un <i>acarus</i> (grise), un <i>aphis</i>
+(puceron) ou un <i>thrips</i>, qui produit au printemps une déviation</span>, etc."</p>
+
+<p>And this view, that fungi already well known to mycologists were called
+forth by the punctures of insects, was regarded as not out of harmony
+with the idea that the fungus itself was an abnormal outgrowth of the
+tissues of the host.</p>
+
+<p><span class="pagenum"><a name="Page_89" id="Page_89"></a>[<a href="./images/89.png">89</a>]</span>The proper study of plant pathology presupposes and involves a knowledge
+of the physiology of plants, of the normal relations of the latter to
+their environment, and of the biology of those animals and plants
+(principally insects and fungi) which are parasitic on them. It is of
+the first importance to understand that a disease is a condition of
+abnormal physiology, and that the boundary lines between health and
+ill-health are vague and difficult to define. As with the study of the
+diseases of man and other animals, so with those of plants, the practice
+resolves itself into the accurate observation and interpretation of
+symptoms (<i>Diagnosis</i>) on the one hand, and of causes (<i>Aetiology</i>) on
+the other, before any conclusions of value can be drawn as to preventive
+or remedial measures (<i>Therapeutics</i>). In plants, however, symptoms of
+disease are apt to exhibit themselves in a very general manner, or at
+any rate it may be that our perceptions of them differentiate symptoms
+due to very different reactions imperfectly, probably because the
+organisation of the plant is less specialised than that of animals. The
+turning yellow and premature falling of leaves, for instance, is a
+frequent symptom of disease; but it may be due to a long series of
+different causes of ill-health&mdash;<i>e.g.</i> drought, too high or too low a
+temperature, light of insufficient or of excessive intensity, a
+superfluity of water at the roots, the presence in the tissues of
+parasitic fungi, or that of worms or insects at the roots or elsewhere,
+poisonous gases in the air, soil, etc., and so <span class="pagenum"><a name="Page_90" id="Page_90"></a>[<a href="./images/90.png">90</a>]</span>forth. Consequently the
+science of plant pathology is much concerned with the direct action of
+external causes, which are probably less obscure than in the case of
+animals, though by no means always obvious. Such considerations at any
+rate seem to account for the fact that most authorities on plant
+pathology base their classification on the causes of disease, there
+being few noteworthy exceptions.</p>
+
+
+<h4><span class="smcap"><a name="Chapter_IX_Notes" id="Chapter_IX_Notes"></a>Notes to Chapter IX.</span></h4>
+
+<div class="chnote">
+<p>The bibliography here quoted will be found in Berkeley,
+"Vegetable Pathology," <i>Gardener's Chronicle</i>, 1854, p. 4;
+Plowright, <i>British Uredineæ and Ustilagineæ</i>, 1889; Eriksson
+and Henning, <span lang="de" xml:lang="de"><i>Die Getreideroste</i></span>, Stockholm, 1896; De Bary,
+<i>Comparative Morphology and Biology of the Fungi</i>, etc., 1887;
+Frank, <span lang="de" xml:lang="de"><i>Die Krankheiten der Pflanzen</i></span>, 1895-96, and scattered
+in the works referred to in them and in the text.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_91" id="Page_91"></a>[<a href="./images/91.png">91</a>]</span></p>
+<h2>CHAPTER X.</h2>
+
+<h3>HEALTH AND DISEASE.</h3>
+
+<div class="chsub">
+<p>Variation&mdash;Disease&mdash;Comparison to a top. Health&mdash;Extinction
+of species&mdash;Natural demise. Examples of complex interactions
+in health&mdash;Interference, and tendencies to ill-health.</p>
+</div>
+
+
+<p>When we come to enquire into the causes of disease, it appears at first
+an obvious and easy plan to subdivide them into groups of factors which
+interfere with the normal physiology of the plant. Scientific experience
+shows, however, that the easy and the obvious are here, as elsewhere in
+nature, only apparent, for disease, like health, is an extremely complex
+phenomenon, involving many reactions and interactions between the plant
+and its environment. If we agree that a living plant in a state of
+health is not a fixed and unaltering thing, but is ever varying and
+undergoing adaptive changes as its life works out its labyrinthine
+course through the vicissitudes of the also ever-varying environment,
+then we cannot <span class="pagenum"><a name="Page_92" id="Page_92"></a>[<a href="./images/92.png">92</a>]</span>escape the conviction that a diseased plant, so long as
+it lives, is also varying in response to the environment. The principal
+difference between the two cases is, that whereas the normal healthy
+plant varies more or less regularly and rhythmically about a mean, the
+diseased one is tending to vary too suddenly or too far in some
+particular directions from the mean; the healthy plant may, for our
+present purposes, be roughly likened to a properly balanced top spinning
+regularly and well, whereas the diseased one is lurching here, or
+wobbling there, to the great danger of its stability. For we must
+recognise at the outset that disease is but variation in directions
+dangerous to the life of the plant. Health consists in variation also,
+but not in such dangerous grooves. That the passage from health to
+disease is gradual and ill-defined in many cases will readily be seen.
+In fact we cannot completely define disease. Mere abnormality of form,
+colour, size, etc., is not necessarily a sign of disease, in the usual
+sense of the word, otherwise the striking variations of our cultivated
+plants would suggest gloomy thoughts indeed, whereas we have reason to
+believe that many cultivated varieties are more healthy&mdash;in the sense of
+resisting dangerous exigencies of the environment&mdash;than the stocks they
+came from. Strictly speaking, no two buds on a fruit-tree are alike, and
+the shoots they produce vary in position, exposure, number, and vigour
+of leaves, and so forth. The minute variations here referred to are <span class="pagenum"><a name="Page_93" id="Page_93"></a>[<a href="./images/93.png">93</a>]</span>not
+seen by the ordinary observer, but those who bud, graft and multiply by
+cuttings on a large scale know that such bud-variations are important,
+quite apart from more extensive "sports" which occasionally occur.</p>
+
+<p>On the other hand, we have reason to believe that many species have died
+out gradually as the environment altered. These plants died because they
+did not vary sufficiently, or did not vary in the right directions; they
+became diseased with respect to the then prevailing conditions of normal
+physiology or health.</p>
+
+<p>Disease, therefore, may be said to be variation of functions in
+directions, or to extents, which threaten the life of the plant, the
+normal in all cases being the state of the plant characteristic of the
+species.</p>
+
+<p>Even now, however, we have not obtained a complete definition, because,
+since all plants die sooner or later, we have not excluded the natural
+demise of the individual or its parts, and no one would call the
+autumnal fall of leaves, or the withering of an annual after flowering,
+death from disease. Clearly then the idea of disease implies danger of
+premature death, and probably this is as near as we shall get to a
+satisfactory definition. Since this matter is of primary importance for
+our present theme, I will add the following instances for consideration.</p>
+
+<p>A plant in perfect health and in the fullest exercise of all its
+functions, has its roots in a soil which is suitably warmed and aerated,
+contains <span class="pagenum"><a name="Page_94" id="Page_94"></a>[<a href="./images/94.png">94</a>]</span>the right quantities of water which dissolve just the proper
+proportions of all the essential mineral salts, but nothing poisonous,
+while the soil itself has a texture such that the roots and root-hairs
+can extend and do their utmost in absorbing.</p>
+
+<p>The leaves above are exposed to just the right intensity of light, in
+air which is not too dry, and is of suitable temperature and
+composition, containing no poisonous exhalations, etc.; and as the
+foliage is gently moved by the breeze, it manufactures carbohydrates at
+the optimum rate in the chlorophyll, and the so-called "elaborated sap"
+containing the dissolved organic food-supplies is prepared in the
+tissues in maximum quantities and of just the right degrees of
+concentration and quality for use in the buds, stem, roots, etc., for
+which it is destined as they draw on the supplies.</p>
+
+<p>Between these assimilating organs, the leaves, and the absorbing roots,
+we have in the stem the wood, with its vessels adapted in quantity and
+calibre to convey the water containing dissolved salts from the
+absorbing roots to the leaves (to say nothing of other parts) and,
+separated from this wood by the cambium, we find the sieve-tubes and
+cortical tissues in suitable quantity conveying the "elaborated
+sap"&mdash;the solutions of organic food-materials from the leaves down to
+the roots, up to the buds, and elsewhere. Joining these cortical and
+wood tissues are adapted series of medullary rays which, apart from
+other connections, bring about the necessary interchanges of water and
+<span class="pagenum"><a name="Page_95" id="Page_95"></a>[<a href="./images/95.png">95</a>]</span>"elaborated sap" with the cambium, the formative tissue which has to be
+fed and served by them, and which by its growth supplies new vessels and
+sieve-tubes, etc., to carry the continually increasing quantities of
+water and food substances as the roots and leaves increase in number and
+area, and thus enables this ideally correlated system to go on working
+at maximum energy.</p>
+
+<p>Now suppose the same plant with its roots in an unsuitable soil&mdash;too dry
+or too poor in mineral supplies, for instance&mdash;the transpiring leaves
+above cannot obtain sufficient water and salts to supply their needs,
+but we will suppose hypothetically that they still assimilate under the
+same ideal conditions as before. The supplies now coming to the cambium
+are diminished, since the want of water and minerals compels the leaves
+to put aside any excess of carbohydrates (<i>e.g.</i> as stored
+starch-grains), and the plastic materials which do pass to the cambium
+so deficient in water cannot be directly utilised, and a starvation
+period sets in. Consequently the cambium forms less wood, and this will
+contain fewer and smaller vessels, and so reduce the conducting
+passages: fewer sieve-tubes also are constructed, and the paths of the
+water current and food supplies narrowed, which of course reacts on the
+tissues everywhere. The reserve substances may slowly be dissolved and
+distributed, however, and considerable quantities be passed in course of
+time into the roots, which, as opportunity offers, gradually employ them
+in making new roots, and if the disturbance has not gone too far and
+the <span class="pagenum"><a name="Page_96" id="Page_96"></a>[<a href="./images/96.png">96</a>]</span>conditions do not become unfavourable, an increased root-supply may
+by its larger absorbing area gradually establish the former state of
+equilibrium of functions. But this at the expense of the plant, which is
+smaller, has fewer leaves and narrower water channels, etc., than a
+plant not thus checked, and it may take a long time to make up for the
+loss of time and stature thus incurred. Indeed if the plant is an annual
+no recovery at all may occur, the reserves passing into fruit and seeds
+instead of slowly supplying the roots as described.</p>
+
+<p>If it be asked, can such a condition of affairs as that described really
+occur, we have only to think of a transplanted specimen with its roots
+maimed and put into unsuitable soil, or of plants in the open with
+feeding roots gnawed by an insect, etc., or of a tree hitherto in
+equilibrium with its fellows in a plantation suddenly set free by
+thinning and so forth.</p>
+
+<p>Now take the case where the roots are maintaining their maximum
+functional activity, but the leaves&mdash;owing to want of light, too much
+moisture or too low a temperature of the air&mdash;are functionally
+depressed. Here we get a state of over-saturation with water set up, the
+tissues are turgid to bursting point, what supplies do traverse the
+sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and
+the cambium again forms less wood, but the lumina of the vessels are
+larger and the lignification less complete. Growth in length is
+excessive, but more leaves are formed, <span class="pagenum"><a name="Page_97" id="Page_97"></a>[<a href="./images/97.png">97</a>]</span>though they are apt to be
+abnormally thin and may be small. Little or no reserves are stored
+anywhere, and the watery tissues contain dangerously diffusible
+substances which may render them an easy prey to parasitic fungi. Here
+again, however, if the disturbance of equilibrium has not gone too far,
+and if the season permits, the new leaves may come into full activity
+and the situation be saved by transpiration and assimilation gradually
+increasing and restoring the equilibrium. But, as before, the plant has
+suffered, and shows the effect in its weak shoots, retarded flowering,
+and other ways.</p>
+
+<p>Such plight as is here described may actually be attained in greenhouses
+where over-watering is the fault, and even in the open it is not
+uncommon in rainy summers, or in plantations where dominant trees get
+the upper hand and partially shade more slowly growing species, or in
+fields where rank grass is allowed to overwhelm crops of lower stature.</p>
+
+<p>Now it will be evident that either of these typical cases of temporary
+disturbance of functional equilibrium may be carried too far: in the
+first case the plant may wilt and wither, in the second it may rupture
+and rot, to take these eventualities only. And yet it is difficult to
+call these indispositions diseases: they are rather examples of extreme
+departures from the normal standard of health, just on the borderland
+between health and disease. A step further, as it were, and disease
+supervenes: certain tissues die from want of water, <span class="pagenum"><a name="Page_98" id="Page_98"></a>[<a href="./images/98.png">98</a>]</span>and a necrotic area
+is formed, or the cortex bursts and a wound is formed in another way, or
+some fungus gets a hold, and so on. These abnormal states are
+particularly apt to predispose the plant to disease&mdash;insects revel in
+such semi-wilted leaves and shoots crammed with reserves, and fungi in
+the water-logged leaves of the second case, while a cold dry wind is
+peculiarly fatal to such tissues.</p>
+
+
+<h4><span class="smcap">Notes to Chapter X.</span></h4>
+
+<div class="chnote">
+<p>The reader may consult Hartig, <i>Diseases of Trees</i>, Eng. ed.,
+1894, Introduction; Sorauer, <span lang="de" xml:lang="de"><i>Pflanzen Krankheiten</i></span>, pp. 1-12,
+and Frank, <span lang="de" xml:lang="de"><i>Die Krankheiten der Pflanzen</i></span>, B. 1, p. 5, for
+definitions of disease.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_99" id="Page_99"></a>[<a href="./images/99.png">99</a>]</span></p>
+<h2>CHAPTER XI.</h2>
+
+<h3>CAUSES OF DISEASE.</h3>
+
+<div class="chsub">
+<p>A. External causes&mdash;I. Non-living environment: soil,
+atmosphere, temperature&mdash;II. Living environment: plants,
+animals&mdash;Complex interactions&mdash;Predisposing causes&mdash;No one
+factor works alone&mdash;Tangled problems of natural selection
+involved. B. So-called internal causes.</p>
+</div>
+
+
+<p>It is customary to classify the causes of disease in plants into two
+principal groups&mdash;(1) those due to the action of the non-living
+environment&mdash;soil, atmosphere, physical conditions such as temperature,
+light, etc.; and (2) those brought about by the activities of living
+organisms&mdash;plants and animals of various species. Before passing to
+further subdivisions under these two heads, however, it is necessary to
+observe that no disease can be efficiently caused by an organism alone,
+since its powers for injury as a parasite, or otherwise, are affected by
+its non-living environment as well as by the host-plant. For instance,
+the <span class="pagenum"><a name="Page_100" id="Page_100"></a>[<a href="./images/100.png">100</a>]</span>spores of a parasitic fungus which would infect and rapidly destroy
+a potato plant in moist warm weather may be showered on to such a plant
+with impunity if the air remains dry and cool&mdash;or on to a cabbage under
+any circumstances as far as we know.</p>
+
+<p>Again, probably no one factor of the non-living environment ever
+suffices to induce a disease, possibly because no such thing as only one
+change at a time ever occurs. For instance, it is difficult to say, when
+a soil becomes sodden with water, whether the excess of water and
+dissolved matters, the want of air displaced by the water, the lowering
+of the temperature, or the accumulation of foul products, etc., is the
+principal factor in causing the damage which results, and we have to
+determine by the balance of experimental evidence which is the dominant
+factor in all such cases.</p>
+
+<p>The study of aetiology of disease is in fact only a particular case of
+that of aetiology in general. Plants at high altitudes in the Alps
+acquire very different characteristics from the same species in the
+plains. Is this due to the low temperature, the rarer atmosphere, the
+more intense illumination, the changes in moisture, etc., etc.? The
+question is more difficult than it appears at first sight, and we must
+remember that, complex as are the factors working on the host, they are
+equally complex in their actions on a parasite attacking the host,
+whence the resulting disease becomes indeed a tangled problem of natural
+selection.</p>
+
+<p><span class="pagenum"><a name="Page_101" id="Page_101"></a>[<a href="./images/101.png">101</a>]</span>Finally it remains to say a few words about a numerous class of cases
+where no external cause of disease can be discovered. It was formerly
+the custom to group such cases of "Internal Causes" by themselves, but
+apart from the fact that many of these mysterious diseases have
+subsequently been shown to be due to the action of external agencies,
+the whole question of internal causes resolves itself into one of
+relations between the plant and its surroundings, and it becomes evident
+that no inherited or internal disease can be regarded as explained until
+we know the external causes which have so modified the structure and
+working of the living cells as to make them abnormal in their reactions
+to other parts of the plant. "Internal causes" of disease, therefore, is
+a phrase expressing our ignorance, but somewhat more emphatically than
+usual. If this is clearly understood there seems no reason against its
+employment for the time being in the artificial scheme of classification
+we require. With regard to external causes due to the non-living
+environment, excess or deficiency of materials in the soil, water, or
+atmosphere plays an important part, and&mdash;since we may neglect purely
+aquatic plants&mdash;it is customary to speak of diseases due to unsuitable
+soils or to injurious atmospheric influences. For instance, any
+deficiency in the supplies of the necessary mineral salts (compounds of
+calcium, magnesium, potassium with sulphuric, nitric and phosphoric
+acids, etc.) leads to pathological changes, as also does the lack of the
+necessary traces of iron. But it is equally <span class="pagenum"><a name="Page_102" id="Page_102"></a>[<a href="./images/102.png">102</a>]</span>true that the presence of
+such ingredients in excess or in combinations unsuited to the plants
+also leads to disaster, as also does the presence of minerals or other
+compounds which poison the root-hairs&mdash;<i>e.g.</i> products of decomposition,
+soluble salts of copper and other poisons. That these matters are bound
+up with the whole question of manuring and of proper soil-analyses will
+be evident.</p>
+
+<p>Another essential factor is the nature and quantity of organic materials
+in the soil, whether leaf-mould and decomposing vegetable remains,
+stable manures, or other animal matters, all of which affect different
+species very differently, and produce very different results in
+different soils. It is necessary to apprehend in this connection what
+has been stated above: that soil is not a mere dead structureless
+medium, and that the root-hairs of ordinary plants cannot deal with
+large quantities of putrefying organic matter: that a good soil must
+abound in useful bacteria and fungi to render such substances
+available&mdash;and in very various ways&mdash;and that it must be open and
+aerated, of proper temperature and suitably supplied with water, and so
+forth, or disaster will result. Here, again, then we are brought into
+close contact with all that is known of fermentation, nitrification, and
+the various biological changes going on in soil, and the application of
+such knowledge to the practice of manuring and tillage in all its forms.</p>
+
+<p>In view of the above remarks, the danger of "over-feeding," in this
+sense, has a real meaning <span class="pagenum"><a name="Page_103" id="Page_103"></a>[<a href="./images/103.png">103</a>]</span>for horticulturists, though it must not be
+forgotten that no substance is really a food until it is assimilable
+into the protoplasm: manures, etc., are food-materials, not food. The
+futility of mere chemical analyses to prove what a plant requires is now
+well known, and it is only on the basis of long and carefully conducted
+experiments that we can ever discover what a particular plant in a
+particular soil, situation, and climate requires for healthy
+development. Again, the quantity of water in soil may be too great or
+too small for given species, and this either on the average for the
+year, or during critical periods only; and it is obviously important
+whether the excess or deficiency is due to improper supplies of water,
+the depth or shallowness of the soil, its retentive powers, or the
+nature of the sub-soil and so on, again bringing the whole matter into
+connection with our understanding of the physical constitution and
+structure of soils, and the nature of soil-drainage.</p>
+
+<p>For instance, a common way of killing ferns is to keep the roots and
+soil wet and the air and fronds dry, whereas the natural habitats
+provide for wet and shaded fronds and well-drained soil.</p>
+
+<p>It may be noted here that in most cases where gardeners speak of plants
+being killed under the "drip" of trees&mdash;<i>e.g.</i> Beech, the injury is due,
+not to the effects of water but to the shade: the loss of light is so
+great that the shaded plants die of inanition because their leaves are
+not able to provide sufficient carbohydrates.</p>
+
+<p><span class="pagenum"><a name="Page_104" id="Page_104"></a>[<a href="./images/104.png">104</a>]</span>Closely bound up with this is the question of the gases in soils. Apart
+from the disastrous effects of poisons&mdash;<i>e.g.</i> coal gas escaping from
+pipes under pavements in towns, etc., diseased conditions often result
+from deficiency of oxygen at the root-hairs, due to imperfect aeration
+of soils, brought about by stagnant water, excess of animal matter, and
+so forth.</p>
+
+<p>Unsuitable constitution of the atmosphere is also a fruitful source of
+disease, though its effects are commoner in closed stoves and
+greenhouses than in the open. Nevertheless the continual exhalation of
+sulphurous fumes, chlorine, and other poisonous gases in the
+neighbourhood of manufacturing centres or of large smoky towns,
+volcanoes, etc., play their part in injuring plants; and excessive
+moisture in the form of mist, rain, etc., is also important. All these
+matters bring us at once into the region of physiology, and only an
+intelligent appreciation of what is known about the action of the
+atmosphere on the soil and the plant will save the peasantry of a
+country from a hopeless mysticism but little removed from that of the
+Middle Ages, when blights and other evils were vaguely referred to the
+river-mists, thunder clouds, and easterly winds.</p>
+
+<p>If we summarise the above as the material factors of the environment, we
+may classify another set of external non-living causes of disease as the
+non-material factors. Such are principally the following:</p>
+
+<p>The space at the disposal of plants greatly <span class="pagenum"><a name="Page_105" id="Page_105"></a>[<a href="./images/105.png">105</a>]</span>affects their welfare. The
+crowding of roots in the soil and of foliage in the air, resulting in
+the loss of light to the leaves, involves deficiency of all the
+materials referred to above&mdash;minerals, organic materials, gases, and
+water&mdash;and no better illustration of the intense struggle for existence
+among these apparently passive and motionless beings, plants, can be
+given than an over-crowded seedbed or plantation. If left to themselves
+such over-stocked areas exhibit to the keen eye of the trained observer
+all the phases of starvation, weakness, wounding, rot, and, so to speak,
+brutal dominance of the stronger over the weaker which it is the object
+of cultivation to prevent. Here, then, we are brought face to face with
+the true significance of thinning and weeding out, pruning, and similar
+processes.</p>
+
+<p>Unsuitable temperature is one of the commonest of all sources of
+disease, for every plant is adapted to certain ranges of temperature,
+and best adapted to a given optimum somewhere between the maximum and
+minimum temperature for each function. Consequently any serious
+departure from the mean may bring about physiological disturbances of
+the nature of disease, and this in very various ways, as exemplified by
+the results of frost, sun-scorching, drought, hail-storms, forest fires,
+and so forth.</p>
+
+<p>As a predisposing factor to disease abnormal temperature effects play a
+great part. Many wound-fungi gain their entrance through frost-cracks,
+bruises due to hailstones, or into tissues chilled below the normal.</p>
+
+<p><span class="pagenum"><a name="Page_106" id="Page_106"></a>[<a href="./images/106.png">106</a>]</span>No less remarkable are the diseases primarily due to insufficient or
+improper exposure to light, which affects the chlorophyll-apparatus and
+the process of carbon-assimilation and through these the whole
+well-being of the plant. Every plant is adapted to certain ranges of
+light intensity, and most cultivators know how impossible it is to grow
+shade plants in fully exposed situations, and how easily plants which
+live in open sunny situations are "drawn" and killed by shade. It is
+equally important to have the right kind of light, as disastrous
+experiences with greenhouses glazed with glass which cut off certain
+rays of light have taught. Here, again, it is important to notice that
+the optimum intensity or quality of light may differ for different
+functions and organs of the plant, as is shown by many adaptations on
+the part of species growing in natural situations&mdash;<i>e.g.</i> bud
+protection, orientation of leaves, etc.&mdash;and it may be taken as a rule
+that etiolated plants are peculiarly susceptible to other diseases.</p>
+
+<p>As regards other factors of the inorganic environment, disasters which
+come within the scope of our subject may be brought about by many
+agencies, the mechanical effects of snow and hail, wind, avalanches,
+etc., the effects of lightning, and so forth, being a few of them.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XI.</span></h4>
+
+<div class="chnote">
+<p>For other detailed classifications of the causes of disease
+the reader is referred to the works of Sorauer and of Frank
+<span class="pagenum"><a name="Page_107" id="Page_107" style="font-size: 110%;"></a>[<a href="./images/107.png">107</a>]</span>referred to in the last chapter. Also Kirchner, <span lang="de" xml:lang="de"><i>Pflanzen
+Krankheiten</i></span>, Stuttgart, 1890.</p>
+
+<p>Of more historical importance are the older classifications of
+Berkeley, <i>Gardeners' Chronicle</i>, 1854, and Re, <i>Gardeners'
+Chronicle</i>, 1849-50. These latter are interesting as showing
+the very different views held by the earlier workers, and
+comparison of these with the modern views helps to mark the
+progress of physiology during the half century which has
+intervened.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_108" id="Page_108"></a>[<a href="./images/108.png">108</a>]</span></p>
+<h2>CHAPTER XII.</h2>
+
+<h3>CAUSES OF DISEASE. THE LIVING ENVIRONMENT.</h3>
+
+<div class="chsub">
+<p>Causes due to animals&mdash;Vertebrata&mdash;Wounds,
+etc.&mdash;Invertebrata&mdash;Insects, etc.&mdash;Plants as causes of
+disease&mdash;Phanerogams, weeds, etc.&mdash;Cryptogams,
+fungi&mdash;Epidemics, etc.</p>
+</div>
+
+
+<p>Passing now to those causes of disease which are connected with the
+living environment, we may obviously divide them into two groups of
+agents, animals and plants.</p>
+
+<p>Among animals, the various vertebrata, including man, are especially
+responsible for the larger kinds of wounds and wholesale destructive
+processes due to breakage, stripping of leaves and bark, cutting and
+biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
+birds of various kinds stand out prominently as enemies to trees and
+other plants, to which they do immense injury in various ways by their
+horns, teeth, claws, and beaks; and the damage which an ignorant
+<span class="pagenum"><a name="Page_109" id="Page_109"></a>[<a href="./images/109.png">109</a>]</span>gardener or forester can do with his ill-guided footsteps, axe, spade,
+and knife can only be appreciated by one who knows the habits of plants.</p>
+
+<p>It is among the invertebrata, however, especially insects and worms,
+that the most striking agents of disease in plants are to be found, for,
+with the exception of certain rodents&mdash;and we may logically include also
+human invasions&mdash;vertebrate animals do not often appear in such numbers
+as to bring about the epidemics and scourges only too commonly caused by
+insect pests.</p>
+
+<p>Insects injure plants in very various ways. Some, such as locusts,
+simply devour all before them; others, <i>e.g.</i> caterpillars, destroy the
+leaves and bring about all the phenomena of defoliation. Others, again,
+eat the buds&mdash;<i>e.g.</i> <i>Grapholitha</i>; or the roots&mdash;<i>e.g.</i> wire-worms, and
+so maim the plant that its foliage and assimilation suffer, or its roots
+become too scanty to supply the transpiration current. Many aphides,
+etc., puncture the leaves, suck out the sap, and produce deformations
+and arrest of leaf-surface, as well as actual loss of substance, and
+when numerous such insects induce all the evils of defoliation. Others,
+such as the leaf-miners, tunnel into the leaves, with similar results on
+a smaller scale.</p>
+
+<p>It must be remembered that a single complete defoliation of a herbaceous
+annual, or even of a tuberous plant like the potato, so incapacitates
+the assimilatory machinery of the plant, that no stores can be put aside
+for the seeds, tubers, etc., of <span class="pagenum"><a name="Page_110" id="Page_110"></a>[<a href="./images/110.png">110</a>]</span>another year, or at most so little that
+only feeble plants come up.</p>
+
+<p>In the case of a tree the case is different, and since most large trees
+in full foliage have far more assimilatory surface than is actually
+necessary for immediate needs, a considerable tax can be paid to
+parasites or predatory insects before the stores suffer perceptibly.
+Still, it should be recognised that the injury tells in time, especially
+in seed years.</p>
+
+<p>Many larvae of beetles, moths, etc., bore into the bark and as far as
+the cambium or even into the wood or pith of trees, the local damage
+inducing general injuries in proportion to the number of insects at
+work: moreover, the wounds afford points of entrance for fungi and other
+pests.</p>
+
+<p>Galls and similar excrescences result from the hypertrophy of young
+living tissues pierced by the ovipositors of various insects, and
+irritated by the injected fluid and the presence of the eggs and larvae
+left behind. They may occur on the buds, leaves, stems, or roots, as
+shown by various species of <i>Cynips</i> on oak, <i>Phylloxera</i> on vines,
+etc., in all cases the local damage being relatively small, but the
+general injury to assimilatory, absorptive, and other functions is great
+in proportion to the number of points attacked.</p>
+
+<p>Many grubs&mdash;larvae of flies, beetles, etc.&mdash;bore into the sheaths or
+internodes of grasses, or the pith of twigs, or into buds, fruits, and
+other organs of plants, and do harm corresponding to the kind and amount
+of tissues injured.</p>
+
+<p><span class="pagenum"><a name="Page_111" id="Page_111"></a>[<a href="./images/111.png">111</a>]</span>Various species of so-called eelworms&mdash;Nematodes&mdash;also cause gall-like
+swellings on young roots, or they invade the grains of cereals.</p>
+
+<p>Finally, various slugs and snails cause much injury by devouring young
+leaves and buds and diminishing the assimilatory area.</p>
+
+<p>Plants as agents of disease or injury fall naturally into the two main
+categories of flowering plants (Phanerogams) and Cryptogams, among which
+the fungi are the especially important pests.</p>
+
+<p>Beginning with weeds, we find a large class of injurious agents. Weeds
+damage the plants we value by crowding them out in the struggle for
+existence, as already stated, and when the weed-action is simply due to
+superfluous plants of the same species, we speak of overcrowding. But it
+must not be overlooked that the competition between crowded plants of
+the same species&mdash;where every individual is acting as a weed to the
+others&mdash;may be more dangerous than between plants and weeds belonging to
+other species and genera, because in the former case they are struggling
+for the same minerals and other necessary food-materials: a matter of
+importance in connection with the rotation of crops.</p>
+
+<p>The question of allowing grass to grow at the foot of fruit trees, as in
+orchards, is a good case in point. Such grass may increase the damp and
+shade, thus favouring fungi at one season, and dry up the moisture of
+the soil to the injury of the fine superficial roots at another, as well
+as exhaust the soil, owing to the competition of the <span class="pagenum"><a name="Page_112" id="Page_112"></a>[<a href="./images/112.png">112</a>]</span>roots for salts
+and other materials. On the other hand, the checking of surface roots by
+competition with the grass has been claimed as advantageous. In this
+connection probably the whole question of the composition of the turf
+arises, as well as that of possible cropping for hay, and manuring.</p>
+
+<p>As regards any particular weed, the cultivator should learn all he can
+respecting its duration, seeding capacity, method of dissemination, the
+depth and spread of its root-system, and any other particulars which
+enable him to judge when and how to attack it. It is only necessary to
+see the victory of such drought-resisting weeds as <i>Hieracium
+pilosella</i>, Plantains, <i>Hypochaeris</i>, on lawns to realise how weeds may
+win in the struggle for existence with the finer grasses.</p>
+
+<p>Many so-called weeds are, however, partially parasitic, with their roots
+on the roots of others&mdash;<i>e.g.</i> <i>Rhinanthus</i>, <i>Thesium</i>, etc., and much
+damage is done to meadow grasses and herbage by the exhaustive tax which
+these semi-parasites impose.</p>
+
+<p>This is carried still further in the case of such root-parasites as
+<i>Orobanche</i>, where the host-plant is burdened with the whole support of
+the pest, because the latter, having no chlorophyll, is entirely
+dependent on the former for all its food.</p>
+
+<p>Even ordinary climbing plants may injure others by shading them, either
+by scrambling over their branches&mdash;<i>e.g.</i> Bramble, or twisting their
+tendrils round the twigs&mdash;<i>e.g.</i> Bryony, or twining round them&mdash;<i>e.g.</i>
+Woodbine, <i>Convolvulus</i>, etc. The principal direct injury is in these
+cases owing to <span class="pagenum"><a name="Page_113" id="Page_113"></a>[<a href="./images/113.png">113</a>]</span>the loss of light suffered by the shaded foliage, but
+the weed-action is often increased by the competition of their
+roots&mdash;<i>e.g.</i> briars; and in the case of woody climbers the gradually
+increased pressure of the woody-coils round the thickening stems
+compresses the cambium and cortex of the support and induces strictures
+and abnormalities which may be fatal in course of time.</p>
+
+<p>Epiphytes, or plants which support themselves wholly on the trunks,
+branches, or leaves of other plants, also injure the latter more
+especially by shading their foliage&mdash;<i>e.g.</i> tropical Figs, Orchids,
+Aroids, etc.; and similar damage is done by our own Ivy, the main roots
+of which are in the soil, but the numerous adventitious roots of which
+cling to the bark.</p>
+
+<p>When the climber or epiphyte is also parasitic, as in the case of the
+Dodder, <i>Loranthus</i>, Mistletoe, etc., the direct loss of substance
+stolen from the host by the parasite comes in to supplement any effect
+of shading that the latter may bring about if it is a leafy plant.</p>
+
+<p>Of Cryptogams, apart from a few epiphytic ferns, and the intense
+weed-action of certain Equisetums, the rhizomes and roots of which are
+as troublesome as those of twitch and other phanerogamic weeds, it is
+especially the fungi which act as agents of disease, and which, as we
+now know, are <i>par excellence</i> the causes of epidemics.</p>
+
+<p>The action of fungi may be local or general; and restricted, slow and
+insidious, or virulent and rapidly destructive.</p>
+
+<p><span class="pagenum"><a name="Page_114" id="Page_114"></a>[<a href="./images/114.png">114</a>]</span>Examples of local action are furnished by <i>Schinzia</i>, which forms
+gall-like swellings on the roots of rushes; <i>Gymnosporangium</i>, which
+induces excrescences on the stems of junipers, and numerous leaf-fungi
+(<i>Puccinia</i>, <i>Æcidium</i>, <i>Septoria</i>, etc.), which cause yellow, brown, or
+black spots on leaves, as well as by <i>Ustilago</i>, which attacks the
+anthers or the ovary of various plants, and so forth. In such cases the
+injury done by a few centres of infection is very slight, but prolonged
+action may bring into play secondary effects such as the gradual
+destruction of the cambium round a branch, when, of course, the effect
+of ringing results; or if the fungus becomes epidemic and myriads of
+leaf-spots are formed, the destruction of foliar tissue, gradual taxing
+of the assimilatory cells, etc., may end in rapid defoliation, and
+renewed attacks soon exhaust the plants and lead to sterility and death,
+as often occurs with Uredineae&mdash;<i>e.g.</i> the coffee leaf-disease.</p>
+
+<p>It is highly probable that such fungi are particularly exacting owing to
+their exhausting demands for compounds of potassium, phosphoric acid,
+and other bodies.</p>
+
+<p>Examples of virulent and rampant general action are afforded by finger
+and toe in turnips, etc., where the roots are invaded by
+<i>Plasmodiophora</i>, which induces hypertrophy and rotting of the roots;
+and by the damping off of seedlings, where the fungus <i>Pythium</i> rapidly
+invades all parts of the seedlings and reduces them to a water-logged,
+putrefying mass; or the potato-disease, which is due <span class="pagenum"><a name="Page_115" id="Page_115"></a>[<a href="./images/115.png">115</a>]</span>to the rapid
+spread of <i>Phytophthora</i> in the leaves and throughout the plant, which
+it blackens and rots in a few days.</p>
+
+<p>Many fungi not in themselves very virulent or aggressive do enormous
+harm owing to the secondary effects they induce. Some of the
+tree-killing hymenomycetes, such as <i>Agaricus melleus</i>, for instance,
+penetrate the wood of a pine at the collar, and the result of the large
+flow of resin which results is to so block up the water passages that
+the tree dies off above with all the symptoms of drought. Similarly, the
+<i>Peziza</i> causing the larch disease, having obtained access to the stem
+about a foot or so above the ground, will gradually kill the cambium
+further and further round the stem, and so girdle the tree as
+effectually as if we had cut out the new wood all round. In all such
+cases&mdash;and the same applies to the leaf-diseases referred to above&mdash;the
+fungus may be compared to an army which is not strong enough to invade
+the whole territory, but which, by striking at the lines of
+communication, cuts off the supplies of water, food, etc., and so brings
+the struggle to an end. Indeed we might compare the cases of fungi which
+attack the root and collar, and so strike at and cut off the water
+supply, to a compact army which at once cuts off the enemy from his
+narrow base; whereas the innumerable units which bring about an epidemic
+attack on the leaves, and so surround the enemy and cut off his food
+supplies all round, is rather like a much larger army which cannot get
+in <span class="pagenum"><a name="Page_116" id="Page_116"></a>[<a href="./images/116.png">116</a>]</span>beyond the natural barriers of the tissues, and so puts a <i>cordon</i>
+all round the territory and seizes the multitudes of food-stuffs at the
+frontiers. The end result is similar in both cases, but the methods of
+warfare differ.</p>
+
+<p>Many fungi, however, though they make their presence noticeable by
+conspicuous signs, cannot be said to do much damage to the individual
+plant attacked. The extraordinary malformations induced by parasites
+like <i>Exoascus</i>, which live in the ends of twigs of trees and stimulate
+the buds to put out dense tufts of shoots, again densely
+branched&mdash;Witches' brooms&mdash;are a case in point. Also the curious
+distortions of nettle stems swollen and curved by <i>Æcidium</i>, of maize
+stems and leaves attacked by <i>Ustilago</i>, and of the inflorescences of
+<i>Capsella</i> by <i>Cystopus</i>, etc., are not individually very destructive;
+it is the cumulative effects of numerous attacks, or of large epidemics,
+which tell in the end.</p>
+
+<p>Some very curious effects are due to fungi such as <i>Æcidium elatinum</i>,
+which, living in the cortex of firs, stimulate buds to put out shoots
+with erect habit, and with leaves which are radially disposed, annually
+cast, and differently shaped from the normal&mdash;characters quite foreign
+to the species of fir in its natural condition.</p>
+
+<p>Equally strange are the shoots of <i>Euphorbia</i> infested with the æcidia
+of <i>Uromyces</i>, those of bilberries affected with <i>Calyptospora</i>, etc. In
+all these cases we must assume a condition of toleration, so to speak,
+on the part of the host, which adapts itself to the altered
+circumstances by marked <span class="pagenum"><a name="Page_117" id="Page_117"></a>[<a href="./images/117.png">117</a>]</span>adaptations in its tissue developments, mode of
+growth and so forth.</p>
+
+<p>This toleration is perhaps most marked in the case of those cereals
+which, though infected by the minute mycelium of <i>Ustilago</i> while still
+a seedling, nevertheless go on growing as apparently healthy green
+plants indistinguishable from the rest, although the fine hyphae of the
+parasite are in the tissues and keeping pace with the growth of the
+shoots just behind the growing points. As the grains of the cereal begin
+to form and swell, however, the hyphae suddenly assume the part of a
+dominant aggressor, consume the endosperm of the enlarging seed, and
+replace the contents of the grain with the well-known black spores known
+as Smut.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XII.</span></h4>
+
+<div class="chnote">
+<p>The reader will find a summary of such fungi as are here
+concerned in Massee, <i>A Text-Book of Plant Diseases</i>, 1899, or
+Prillieux, <span lang="fr" xml:lang="fr"><i>Maladies des Plantes Agricoles</i></span>.</p>
+
+<p>For further details the student should consult the works of
+Frank and Sorauer referred to in the <a href="#Chapter_IX_Notes">notes to Chapter IX.</a>, and
+Tubeuf, <i>The Diseases of Plants</i>, Engl. ed. 1897, pp. 104-539.</p>
+
+<p>For experiments on the effects of grass on orchard trees, see
+<i>Report of the Woburn Experimental Fruit Farm</i>, 1900, p. 160.</p>
+
+<p>For the further study of weeds, the interesting bulletins of
+the Kansas State Agricultural College, 1895-1898, will show
+the reader what may be done in the matter of classifying them
+according to their biological peculiarities.</p>
+
+<p>In regard to insects, the reader will find the following list
+embraces the subject: Somerville, <i>Farm and Garden Insects</i>,
+<span class="pagenum"><a name="Page_118" id="Page_118" style="font-size: 110%;"></a>[<a href="./images/118.png">118</a>]</span>1897; Theobald, <i>Insect Life</i>, 1896; Ormerod, <i>Manual of
+Injurious Insects</i>, 1890, and <i>Handbook of Insects Injurious
+to Orchards, etc.</i>, 1898.</p>
+
+<p>The admirable series of publications of the U.S. Department of
+Agriculture under the editorship of Riley and Howard, and
+entitled <i>Insect Life</i>, 1888-1895, also abounds in
+information.</p>
+
+<p>Further, Taschenberg's <span lang="de" xml:lang="de"><i>Praktische Insektenkunde</i></span>, 1879-1880,
+and Judeich and Nietsche, <span lang="de" xml:lang="de"><i>Lehrbuch der Mitteleurop. Forst.
+Insektenkunde</i></span>, 1889.</p>
+
+<p>For an elementary introduction to the study of fungus
+diseases, see Marshall Ward, <i>Diseases of Plants</i>, Soc. for
+Promoting Christian Knowledge, London.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_119" id="Page_119"></a>[<a href="./images/119.png">119</a>]</span></p>
+<h2>CHAPTER XIII.</h2>
+
+<h3>NATURE OF DISEASE.</h3>
+
+<div class="chsub">
+<p>General and local disease&mdash;General death owing to cutting-off
+supplies, etc.&mdash;Disease of organs&mdash;Tissue-diseases, e.g.
+timber&mdash;Root-diseases&mdash;Leaf-diseases, etc.&mdash;Diseases of
+Respiratory, Assimilatory, and other organs&mdash;Physiological and
+Parasitic diseases&mdash;Pathology of the
+cell&mdash;Cuts&mdash;Cork&mdash;Callus&mdash;Irritation&mdash;Stimulation by
+protoplasm&mdash;Hypertrophy.</p>
+</div>
+
+
+<p>On going more deeply into the nature of those changes in plants which we
+term pathological or diseased, it seems evident that we must at the
+outset distinguish between various cases. A plant may be diseased as a
+whole because all or practically all its tissues are in a morbid or
+pathological condition, such as occurs when some fungus invades all the
+parts or organs&mdash;<i>e.g.</i> seedlings when completely infested by <i>Pythium</i>,
+or a unicellular Alga when invaded by a minute parasite; or it may die
+throughout, because some organ with functions essential to its life is
+<span class="pagenum"><a name="Page_120" id="Page_120"></a>[<a href="./images/120.png">120</a>]</span>seriously affected&mdash;<i>e.g.</i> the roots are rotten and cannot absorb water
+with dissolved minerals and pass it up to the shoot, or all the leaves
+are infested with a parasite and cannot supply the rest of the plant
+with organic food materials, in consequence of which parts not directly
+affected by any malady become starved, dried-up, or poisoned or
+otherwise injured by the results or products of disease elsewhere.</p>
+
+<p>In a large number of cases, however, the disease is purely local, and
+never extends into the rest of the organs or tissues&mdash;<i>e.g.</i> when an
+insect pierces a leaf at some minute point with its proboscis or its
+ovipositor, killing a few cells and irritating those around so that they
+grow and divide more rapidly than the rest of the leaf tissues and
+produce a swollen hump of tissue, or gall; or when a knife-cut wounds
+the cambium, which forthwith begins to cover up the dead cells with a
+similarly rapid growth of cells, the callus. Numerous minute spots due
+to fungi on leaves, cortex, etc., are further cases in point, the
+mycelium never extending far from the centre of infection.</p>
+
+<p>Many attempts have been made to classify diseases on a basis which
+assumes the essential distinction of the above cases, and we read of
+diseases of the various organs&mdash;root-diseases, stem-diseases,
+leaf-diseases, and so forth; or of the various tissues&mdash;timber-diseases,
+diseases of the cambium, of the bark, of the parenchyma, and so on.
+Furthermore, attempts have been made to speak of general functional
+disease, of <span class="pagenum"><a name="Page_121" id="Page_121"></a>[<a href="./images/121.png">121</a>]</span>diseases of the respiratory organs, of the absorptive
+organs, and so forth, as opposed to local lesions.</p>
+
+<p>Critical examination, however, shows that no such distinctions can be
+consistently maintained, partly because the organs and functions of
+plants are not so sharply marked off as they are in animals, the
+diseases of which have suggested the above classification, and partly
+because all disease originates in the cells and tissues, and it is a
+matter of detail only that in some cases&mdash;<i>e.g.</i> severe freezing or
+drought of seedlings, or when some ingredient is wanting in the
+soil&mdash;the diseased condition affects practically every cell alike from
+the first, while in others it spreads more or less rapidly from some one
+spot.</p>
+
+<p>Even the distinction into physiological diseases <i>versus</i> parasitic
+diseases cannot be maintained from the standpoint of the nature of the
+disease itself. All disease is physiological in so far as it consists in
+disturbance of normal physiological function, for pathology is merely
+abnormal physiology, no matter how it is brought about. This is not
+saying that no importance is to be attached to the mode in which disease
+is incurred or induced: it is merely insisting on the truth that the
+disease itself consists in the living cell-substance&mdash;the
+protoplasm&mdash;not working normally as it does in health, and this, whether
+want of water, minerals, or organic food be the cause, or whether the
+presence of some poison or mechanical irritant be the disturbing agent,
+as also whether <span class="pagenum"><a name="Page_122" id="Page_122"></a>[<a href="./images/122.png">122</a>]</span>such want or irritation be due to some defect in soil
+or air, or to the ravages of a fungus or an insect.</p>
+
+<p>This being understood I need not dwell on the common fallacy of
+confounding the fungus, insect, soil or other agent with the disease
+itself, or of making the same blunder in confusing symptoms with
+maladies. In this sense, wheat rust is not a disease: it is a symptom
+which betrays the presence of a disease-inducing fungus, the Rust
+fungus. Similarly, chlorosis is not a disease: it is a symptom of
+imperfect chlorophyll action, and the best proof of the truth of both
+statements is that in both cases the fundamental disease-action is the
+starvation of the cell-protoplasm of carbohydrates and other essential
+food matters&mdash;in the one case because the fungus steals the
+carbohydrates as fast as the leaves can make them, in the second because
+the leaf is unable to make them.</p>
+
+<p>The foundation of a knowledge of disease in plants therefore centres in
+the understanding of the pathology of living cells.</p>
+
+<p>If a suitable mass of living cells is neatly cut with a sharp razor the
+first perceptible change is one of colour: the white "flesh" of a potato
+or an apple, for instance, turns brown as the air enters the cut cells,
+and the microscope shows that this browning affects cell-walls and
+contents alike. The cut cells also die forthwith; and the oxygen of the
+air combining with some of their constituents forms the brown colouring
+matter which soaks into the cell-walls. The uninjured cells below them
+<span class="pagenum"><a name="Page_123" id="Page_123"></a>[<a href="./images/123.png">123</a>]</span>grow longer, pushing up the dead débris, and divide across by walls
+parallel to the plane of the wound, and so form series of tabular cells
+with thin walls, which also soon turn brown and die, the cell-walls
+meanwhile undergoing changes which convert them into cork. The living
+cells deeper down are now shut off from the outer world by a skin, of
+several layers, of cork-cells, which prevent the further free access of
+air or moisture. During the period of active cell-division which
+initiates the cork, the temperature of the growing cells rises: a sort
+of fever (wound-fever) is induced, evidently owing to the active
+respiration of the growing cells.</p>
+
+<p>This healing by cork occurs in any tissue of living cells exposed by a
+cut&mdash;leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
+applies to any other kind of cutting or tearing injury&mdash;such as a prick
+with a needle or the proboscis of an insect, a stripping, or even a
+bruise.</p>
+
+<p>Such healing is prepared for and carried out very thoroughly in the case
+of falling leaves and cast branches, the plane of separation being
+covered by a cicatrix of cork.</p>
+
+<p>If the cell-tissue under the wound is actually growing at the time,
+however, a further process is observed when the wound-cork has been
+formed. The uninjured cells below go on growing outwards more vigorously
+than ever, the pressure of the overlying tissues taken off by the cut
+having been removed, and, lifting up the cork-layer as they do so, they
+rapidly <span class="pagenum"><a name="Page_124" id="Page_124"></a>[<a href="./images/124.png">124</a>]</span>divide into a juicy mass of thin-walled cells which is of a
+cushion-like nature and is termed a <i>Callus</i>. This callus is at first a
+homogeneous tissue of cells which are all alike capable of growing and
+dividing, but in course of time it undergoes changes in different parts
+which result in the formation of tracheids, vessels, fibres and other
+tissue-elements, and even organs, just as the embryonic tissues of the
+growing points, cambium, etc., of the healthy plant give origin to new
+growths. Such wound-wood, however, is apt to differ considerably in the
+arrangement, constitution and hardness of its parts as compared with
+normal wood, and its peculiar density and cross-graining are often
+conspicuous.</p>
+
+<p>If instead of a simple tissue, the cut or other wound lays bare a
+complex mass such as wood, the resultant changes are essentially the
+same to start with. The living cells bordering the wound form cork, and
+then those deeper down grow out and form a callus. The exposure of the
+wood however, entails alterations in its non-living elements also. The
+lignified walls of tracheids, fibres, etc., turn brown to a considerable
+depth, and this browning seems to be&mdash;like all such discolorations in
+wounds&mdash;due to oxidation changes in the tannins and other bodies
+present: the process is probably similar to what occurs in humification
+and in the conversion of sap-wood into heart-wood in trees. Such wood is
+not merely dead, but it is also incapable of conveying water in the
+lumina of its elements, which <span class="pagenum"><a name="Page_125" id="Page_125"></a>[<a href="./images/125.png">125</a>]</span>slowly fill with similarly dark-coloured,
+impervious masses of materials termed "wound-gum," the nature of which
+is obscure, but which slowly undergoes further changes into resin-like
+substances.</p>
+
+<p>The exposure of wood by a wound results also in another mode of stopping
+up the vessels and so hindering the access of air, loss of water, etc.,
+for the living cells of the medullary rays and wood-parenchyma grow into
+the lumina of the larger vessels through the pits, forming <i>thyloses</i>,
+again a phenomenon met with in heart-wood. In Conifers the stoppage of
+the lumina is increased by deposition of resin, which also soaks into
+the cell-walls and the wounded wood becomes semi-translucent owing to
+the infiltration.</p>
+
+<p>Every living cell in an active condition is irritable, and one of the
+commonest physiological reactions of growing tissues is that of
+responding to the touch of a resistant body, as is vividly shown by the
+movements of the Sensitive plant, <i>Dionaea</i>, etc., and by those of
+tendrils, growing root tips, etc., on careful observation. We have
+reason for stating that if a minute insect, too feeble to pierce the
+cuticle, cling on to one side of the dome-shaped growing point of any
+shoot, the irritation of contact of its claws, hairs, etc., would at
+once cause the protoplasm of the delicate cells to respond by some
+abnormal behaviour; and, as matter of experiment, Darwin showed long ago
+that if a minute piece of glass or other hard body is kept in contact
+with one side of the tip of a root, the growth on the side in contact is
+interfered <span class="pagenum"><a name="Page_126" id="Page_126"></a>[<a href="./images/126.png">126</a>]</span>with. Moreover we know from experiments on heliotropism,
+thermotropism, etc., that even intangible stimuli such as rays of light,
+etc., impinging unsymmetrically on these delicate cells cause
+alterations in their behaviour&mdash;<i>e.g.</i> arrest or acceleration of growth.</p>
+
+<p>Perhaps the most remarkable class of stimulations, however, is that due
+to the presence of the entire protoplasmic body of one organism in the
+cell of another, each living its own life for the time being, but the
+protoplasm of the host cell showing clearly, by its abnormal behaviour,
+that the presence of the foreign protoplasm is affecting its physiology.
+A simple example is afforded by Zopfs' <i>Pleotrachelus</i>, the amoeboid
+protoplasmic body of which lives in the hypha of <i>Pilobolus</i>, causing it
+to swell up like an inflated bladder, in which the parasite then forms
+its sporangia. The <i>Pleotrachelus</i> does not kill the <i>Pilobolus</i>, but
+that its protoplasm alters the metabolic physiology of the latter is
+shown by the hypertrophy of the cells, and by the curious fact that it
+stimulates the <i>Pilobolus</i> to form its sexual conjugating cells,
+otherwise rare, an indication of very far-reaching interference with the
+life-actions of the host.</p>
+
+<p>An equally remarkable example is that of <i>Plasmodiophora</i>, the amoeboid
+naked protoplasm of which lives and creeps about in the protoplasm of a
+cell of the root of a turnip, to which it gains access through the
+root-hairs. It does not kill the cell, but stimulates its protoplasm to
+increased activity and growth and division, itself dividing also <span class="pagenum"><a name="Page_127" id="Page_127"></a>[<a href="./images/127.png">127</a>]</span>and
+passing new amoebae into each new daughter-cell of the host. Here the
+processes of stimulation, hypertrophy and further division are repeated,
+until hundreds or thousands of the turnip root-cells are infected. The
+externally visible result is the formation of distorted swellings on the
+root (Finger and Toe), most of the cells of which are abnormally large
+and filled with amoeboid <i>Plasmodiophora</i> protoplasm, which finally
+devours the turnip-protoplasm and itself passes over into spores. Here
+we have most convincing proof of the stimulation of protoplasm by other
+protoplasm in direct contact with it; and that the metabolism of the
+host-cells is profoundly altered is shown not only by the abnormal
+growth of the cells, but also by the starvation of the rest of the
+turnip plant as the <i>Plasmodiophora</i> gets the upper hand. We have here,
+in fact, a local intracellular parasitic disease, gradually invading
+large tracts of tissue and eventually inducing general disease resulting
+in death&mdash;a state of affairs reminding us of cancer in animals.</p>
+
+<p>Irritation and hypertrophy of cells, however, may be induced by
+parasites which never bring their protoplasm into direct contact with
+that of the host. Many Chytridiaceae penetrate the cells of plants, and
+grow inside them as short tubes, vesicles, etc., the protoplasm of which
+is separated by their own cell-walls from that of the host-cell;
+nevertheless hypertrophy and abnormal cell-divisions and secretions are
+induced, and the effect even extends to neighbouring cells&mdash;<i>e.g.</i>
+<i>Synchytrium</i>&mdash;showing <span class="pagenum"><a name="Page_128" id="Page_128"></a>[<a href="./images/128.png">128</a>]</span>that some influence is exerted through cells
+themselves not directly affected. This latter point need not surprise us
+now we know that the cells of plant-tissues are connected by fine
+protoplasmic strands passing through the separating cell-walls.</p>
+
+<p>But the invading plant need not actually enter the cells, and may still
+stimulate them through both its own and their own cell-walls to abnormal
+growth. This is well shown by the intercellular mycelium of <i>Exoacus</i>
+and <i>Exobasidium</i>, and the latter affords an excellent illustration of
+the far-reaching effects of hyphae on the cells (of <i>Vaccinium</i>) into
+which they do not penetrate. Not only are the cells stimulated to grow
+larger and divide oftener than normally, thus producing large gall-like
+swellings, but the chlorophyll disappears, the cell sap changes colour
+to red, the numerous compound crystals normally found in the tissues
+diminish in number and are different in shape, large quantities of
+starch are stored up, and even the vascular bundles are altered in
+character. All these changes indicate very profound alterations in the
+physiological working of the protoplasm of the cells of the host, and
+yet the fungus has done its work through both its own cell-walls and
+those of the host.</p>
+
+<p>Even harmless endophytic algae in the intercellular spaces of plants may
+stimulate the cells in their immediate neighbourhood to increased
+growth, <i>e.g.</i> <i>Anabaena</i> in the roots of Cycads.</p>
+
+<h3><span class="pagenum"><a name="Page_129" id="Page_129" style="font-size: 80%;"></a>[<a href="./images/129.png">129</a>]</span><span class="smcap">Notes to Chapter XIII.</span></h3>
+
+<div class="chnote">
+<p>With reference to cork-healing and wound-fever the student may
+consult Shattock "On the Reparative processes which occur in
+Vegetable Tissues," <i>Journal of the Linnean Society</i>, 1882,
+Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
+the Aspen," <i>Journal of Botany</i>, 1883, Vol. XXI., p. 306. Also
+Richards, "The Respiration of Wounded Plants," <i>Annals of
+Botany</i>, Vol. X., 1896, p. 531; and "The Evolution of Heat by
+Wounded Plants," <i>Ann. of Bot.</i>, Vol. XI., 1897, p. 29.</p>
+
+<p>For details and figures respecting callus, see Sorauer,
+<i>Physiol. of Plants</i>, p. 175.</p>
+
+<p>In respect to the irritable movements referred to see Darwin,
+<i>The Power of Movements in Plants</i>, 1880, chapter III. The
+recent work of Nawaschin, <span lang="de" xml:lang="de"><i>Beobachtungen ueber den feineren
+Bau u. Umwandlungen von Plasmodiophora</i></span>, Flora, Vol. LXXXVI.,
+1899, p. 404, should be read for details and literature
+concerning "Finger and Toe."</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_130" id="Page_130"></a>[<a href="./images/130.png">130</a>]</span></p>
+<h2>CHAPTER XIV.</h2>
+
+<h3>NATURE OF DISEASE (<i>Continued</i>).</h3>
+
+<div class="chsub">
+<p>Actions of poisons in small doses&mdash;Results of killing a few
+cells&mdash;Malformation&mdash;Enzymes&mdash;Secretions and
+excretions&mdash;Acids, poisons, etc.&mdash;Chemotactic
+phenomena&mdash;Parasitism&mdash;Epiphytes and
+endophytes&mdash;Symbiosis&mdash;Galls.</p>
+</div>
+
+
+<p>Physiological research has shown that the respiratory activity of cells
+may be increased by small doses of poisons, and even that growth may be
+accelerated by them&mdash;<i>e.g.</i> chloroform, ether&mdash;and, still more
+remarkable, that fermentative activity may be enhanced by minute doses
+of such powerful mineral poisons as mercuric chloride, iodine salts,
+etc., and that the cells may be gradually accustomed to larger doses
+without injury. Unfertilised eggs of insects have been started into
+growth by treatment with acids and those of frogs with mercury salts,
+and the germination of beans quickened by various poisonous alkaloids.
+In other words, graduated doses of <span class="pagenum"><a name="Page_131" id="Page_131"></a>[<a href="./images/131.png">131</a>]</span>poison may alter the physiological
+activity of living cells, inducing pathological phenomena, while larger
+doses kill them.</p>
+
+<p>Now we know at least one parasitic fungus which poisons the cells of its
+host, and kills them, with similar symptoms to those resulting from
+excessive doses of the above-named toxic agents. <i>Botrytis</i> hyphæ,
+living in the cell-walls of plants, but not entering the cells, excretes
+a poison which kills the protoplasm, and the fungus then feeds on the
+debris. Numerous other fungi form powerful poisons, but we do not know
+whether or how they employ them&mdash;<i>e.g.</i> Ergot.</p>
+
+<p>It is obvious that if all the young cells of a root-tip or of the apex
+of a shoot, or those of a young leaf, are growing and dividing
+regularly, the killing of one or a few cells at one point on the side of
+the organ must result in irregularities&mdash;in malformation&mdash;of the adult
+organ. This has been proved experimentally by destroying a few cells
+with a needle. It can also be done by planting a minute mycelium of
+<i>Botrytis</i> laterally on a young organ&mdash;<i>e.g.</i> a very young lily-bud. The
+fungus adheres to the surface, kills a few epidermis cells, and forms a
+foxy-red spot, which becomes concave as the dead cells lose water and
+dry. Since the rest of the bud goes on growing, however, while this dead
+point remains stationary, the latter gradually becomes the centre of a
+concavity, the growing tissues having grown round it: the bud is
+deformed. Numerous cases of malformed organs are explained in this way;
+a minute insect has <span class="pagenum"><a name="Page_132" id="Page_132"></a>[<a href="./images/132.png">132</a>]</span>bitten or pierced the young tissue, or a fungus has
+killed a minute area, or a drop of acid condensed from fumes in the air
+is the lethal agent, and so forth. And even on a much larger scale we
+see the same kinds of agents at work. Wherever a patch of cells is
+killed whilst those around go on growing, there must result some
+deformation of the resulting organ, since had the injury been withheld
+the number and sizes of the cells now fixed in death would have
+increased and covered a larger area: they now serve to pull over to
+their side the still living and growing cells. The same results follow
+on any lateral wound: the killed spot of tissue serves as a point round
+which the continued growth of other parts of the organ turns. Hence the
+malformation is in these cases a secondary effect, and not, as in simple
+hypertrophy, a direct effect of the action of the cells involved in the
+injury.</p>
+
+<p>There is another class of bodies secreted by fungi, however, which act
+directly on cells, viz. enzymes&mdash;that is, soluble bodies which are able
+to dissolve cellulose (<i>cytases</i>), starch (<i>diastases</i>), proteids
+(proteolytic enzymes), and other substances, by peculiar alterations in
+their constitution. It is by means of its <i>cytase</i> that <i>Botrytis</i>
+hyphae pierce the cellulose walls of plants, and no doubt in all cases
+where fungi pierce cell-walls it is by the solvent action of such a
+cytase, and similarly when haustoria penetrate into the cells. It is
+also by means of these starch-dissolving enzymes (diastases) and
+proteolytic <span class="pagenum"><a name="Page_133" id="Page_133"></a>[<a href="./images/133.png">133</a>]</span>enzymes, etc., that the hyphae inside the cells are enabled
+to make use of the starch, proteids, etc., they find there.</p>
+
+<p>All living cells form materials, resulting from the activity of the
+protoplasm, which we may compare with the refuse or by-products formed
+in any great manufacturing industry: these by-products have to be got
+rid of if they are injurious or noisome (<i>excretions</i>), and if
+not&mdash;<i>i.e.</i> if they are capable of further use (<i>secretions</i>)&mdash;they have
+to be stored away till required. Some of the most prominent of these
+bodies excreted by fungi are, as we have seen, poisonous acids, such as
+oxalic acid, enzymes, and organic poisons, such as those in ergot. But
+similar enzymes, acids, poisons, etc., to those found in fungi are also
+found in the cells of other plants and animals; for only by means of
+their solvent actions can processes like digestion and assimilation of
+the starchy and other materials into the body-substance be accomplished,
+and we have seen that it is a general property of living cells to form
+acids, and other excretions and secretions.</p>
+
+<p>Now we know very little about what may happen when an organism&mdash;say a
+fungus&mdash;secreting especially one kind of enzyme or poison or other
+active substance, comes into intimate contact with another&mdash;say a
+leaf-cell&mdash;which secretes predominantly others, but what we do know
+points to the certainty that various complications will occur.</p>
+
+<p>For instance, if certain bacteria which prefer an <span class="pagenum"><a name="Page_134" id="Page_134"></a>[<a href="./images/134.png">134</a>]</span>alkaline medium, and
+yeasts which prefer an acid environment are mixed in a saccharine
+solution, it depends on the reaction of the liquid which organism gains
+the upper hand: if the liquid is acid the yeast may dominate the
+bacteria; if alkaline it may be suppressed by them.</p>
+
+<p>That a parasite may be prevented from successfully attacking a
+particular plant is shown by the failure of <i>Cuscuta</i> to establish its
+haustoria in poisonous plants such as <i>Euphorbia</i>, <i>Aloe</i>, etc., and it
+has been pointed out that poisonous secretions in the cells of the plant
+protect them against the penetration of fungi. This cannot be taken as
+meaning that any poison protects against any parasite, however, for
+<i>Euphorbia</i> is itself subject to attacks of Uredineae, and <i>Pangium
+edule</i>, which contains prussic acid and is extremely poisonous to most
+animals, is eaten with avidity by several insects, while nematode worms
+can live in its tissues. This is no more remarkable, however, than the
+fact that <i>Fontaria</i>, a myriapod, secretes prussic acid in its own
+tissues, or than that certain glands of the stomach secrete free
+hydrochloric acid, and <i>Dolium</i> forms sulphuric acid in its glands.</p>
+
+<p>There is yet a further point to notice here. It has been proved that
+certain substances formed in plant-cells, not necessarily nutritive,
+attract the hyphae of parasitic fungi or repel them, according to the
+kind and degree of concentration. So clear has this proof been made that
+it was possible in experiments conducted apart from a host plant, <span class="pagenum"><a name="Page_135" id="Page_135"></a>[<a href="./images/135.png">135</a>]</span>to
+make the hyphae on one side of an artificial membrane&mdash;<i>e.g.</i>
+collodion&mdash;penetrate it by placing one of these attractive
+(<i>chemotropic</i>) substances in suitable proportions on the other side.
+The hyphae dissolved holes in the membrane by means of enzymes and
+plunged into the attractive substance on the other side.</p>
+
+<p>The foregoing sketch gives us a glimpse into the causes at work in
+parasitism.</p>
+
+<p>Suppose a fungus on the outside of the epidermis of a young organ&mdash;say a
+leaf. It may be unable to penetrate into the plant, and finding no
+suitable food outside it dies: or it may be satisfied with the traces of
+organic matter on the epidermis and then lives the life of a saprophyte.
+Or it may be able to establish a hold-fast on the tender epidermal
+surface, but without entering the cells, and irritate the developing
+organ by contact stimulation, inducing slight abnormalities; if in its
+further, purely superficial growth such an epiphyte covers large areas
+of the leaf, and especially if the hyphae are dark coloured&mdash;<i>e.g.</i>
+<i>Dematium</i> and other "Sooty Moulds"&mdash;injury may be done to the leaf
+owing to the shading action which deprives the chlorophyll below of its
+full supply of solar energy. Some epiphytes, however, are able to fix
+their hyphae to the epidermis by sending minute peg-like projections
+into the cuticle&mdash;<i>Trichosphaeria</i>, <i>Herpotrichia</i>&mdash;while others send
+haustoria right through the outer epidermal walls&mdash;<i>e.g.</i>
+<i>Erysiphe</i>&mdash;and thus supplement mere contact-irritation and shading by
+<span class="pagenum"><a name="Page_136" id="Page_136"></a>[<a href="./images/136.png">136</a>]</span>actual absorption from the external cells. Here the fungus is a
+parasitic epiphyte.</p>
+
+<p>A stage further is attained in those fungi which enter the stomata and
+live in the intercellular spaces&mdash;<i>e.g.</i> many Uredineae and
+<i>Phytophthora</i>&mdash;and many such intercellular endophytes increase their
+attack on the cells by piercing their walls with minute (<i>Cystopus</i>) or
+large and branched (<i>Peronospora</i>) haustoria, or even eventually pierce
+the cells and traverse them bodily (<i>Pythium</i>). In all these cases it is
+clear that conflicts must occur between poison and antidote, acid and
+alkali, attractive and repellent substances, enzyme and enzyme, etc., as
+was hinted at above; and the same must take place when the parasite is
+endophytic and intracellular from the first, as in Chytridiaceae, etc.,
+the zoospores of which pierce the outer cell-walls and forthwith grow
+into the cells. There are also fungi which, while able to pierce the
+outer cell-walls, and grow forward in the thickness of the wall itself,
+cannot enter the living cells themselves&mdash;<i>e.g.</i> <i>Botrytis</i>. In the
+example mentioned, the fungus excretes a poison, oxalic acid, which
+soaks into and kills the cells next its point of attack: into these dead
+cells it then extends, and, invigorated by feeding on them, extends into
+other cell-walls and excretes more poison, and so on.</p>
+
+<p>On the basis of the foregoing it seems possible to sketch a general view
+of the nature of parasitism. In order that a fungus may enter the cells
+it must be able to overcome not only the resistance of the <span class="pagenum"><a name="Page_137" id="Page_137"></a>[<a href="./images/137.png">137</a>]</span>cell-walls,
+but that of the living protoplasm also: if it cannot do the latter it
+must remain outside, as a mere epiphyte, or at most an intercellular
+endophyte. If it can do neither it must either content itself with a
+saprophytic existence or fail, so far as that particular host-plant is
+concerned. Its inability to enter may be due to there being no
+chemotropic attraction, or to its incapacity to dissolve the cell-walls,
+or to the existence in the cell of some antagonistic substance which
+neutralises its acid secretions, destroys its enzymes or poisons, or is
+even directly poisonous to it.</p>
+
+<p>Moreover when once inside it does not follow that it can kill the cell.
+The protoplasm of the latter may have been unable to prevent the fungus
+enemy from breaking through its first line of defence&mdash;the cell-wall,
+but it may be quite capable of maintaining the fight at close quarters,
+and we see signs of the progress of the struggle in hypertrophy,
+accumulation of stores, and other changes in the invaded cells and their
+contents.</p>
+
+<p>Finally, the invested or invaded cell may so adapt itself to the demands
+of the invader that a sort of arrangement is arrived at by which life in
+common&mdash;<i>Symbiosis</i>&mdash;is established, each organism doing something for
+the other and each taking something from the other. In this latter case,
+which is often realised&mdash;<i>e.g.</i> lichens, leguminous plants and the
+organisms in their root-nodules, mycorrhiza, etc.&mdash;we leave the domain
+of disease, which supervenes indeed if the other symbiont is lacking.</p>
+
+<p><span class="pagenum"><a name="Page_138" id="Page_138"></a>[<a href="./images/138.png">138</a>]</span>Some interesting facts bearing on the matters here under discussion,
+have been obtained from the study of <i>Galls</i>, the curious outgrowths
+found on many plants and due to the action of insects.</p>
+
+<p>A typical gall exhibits three distinct and characteristic layers of
+tissue surrounding the hollow chamber in which the larva of the insect
+lies, viz., an outer layer of soft cells forming a parenchyma covered
+with an epidermis, and frequently also with a layer of cork; an inner
+stratum consisting of very thin-walled delicate cells filled with
+protoplasmic and reserve food-materials on which the larva feeds; and
+between the two a more or less definite layer of thick-walled
+sclerenchyma cells which serve as a protection against accidents to the
+larva as the outer layer shrivels or rots, or if it is exposed to the
+attack of marauders. This layer may be absent from galls which have a
+short life only. Vascular bundles run into the outer layer from the
+leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
+and are frequently of use in the arts on this account: they also contain
+starch, and proteid substances and crystals of calcium oxalate. When the
+larva has consumed the stores of food material and reached the adult
+stage it eats its way out and escapes.</p>
+
+<p>The growth of such a gall is preceded by the laying of an egg on or in
+the embryonic tissue of a leaf, stem, or other young part, and it is
+interesting to note that only organs in the meristematic stage can form
+galls, and that it is <span class="pagenum"><a name="Page_139" id="Page_139"></a>[<a href="./images/139.png">139</a>]</span>by no means necessary that the tissues should be
+wounded. Moreover, the egg as such is incapable of stimulating the plant
+tissues, but when it hatches, the resulting larva, beginning to feed on
+the cells, irritates the tissues and rapid growth and cell-division
+occur, as in the case of other wounds or of fungus attacks. The actual
+wound made by the ovipositor heals up at once. It is evident from
+numerous recent researches that these true galls are not due to any
+poisonous or irritating liquid injected by the parent, but that the
+stimulus to the tissue formation is similar to that exerted by a wound.
+The young gall is in fact a callus enclosing the living larva, and it is
+the continued irritation of the latter which keeps up the stimulation.
+The final shape and constitution of the gall depend on mutual
+reactions&mdash;not as yet explained in detail&mdash;between the species of plant
+and the species of gall-insect concerned, as may readily be seen from
+the extraordinary variations in size, shape, colouring, hairiness and
+other structural peculiarities of the galls on one species of, for
+instance, the common oak. From what we have learnt about fungus
+parasites, however, there can be little doubt that reactions between the
+cells and the larva of the insect occur, resembling those which take
+place between the cells and the hyphae of the fungus, and this is borne
+out by the study of other hypertrophies due to animals; <i>e.g.</i> Nematode
+worms in roots, and the remarkable galls&mdash;the simplest known&mdash;on
+<i>Vaucheria</i>, caused by the entrance into this alga of a species <span class="pagenum"><a name="Page_140" id="Page_140"></a>[<a href="./images/140.png">140</a>]</span>of
+<i>Notommata</i>, which induces a different gall on each of the various
+species of its host plants.</p>
+
+<p>It must be concluded that the formation of the <i>Vaucheria</i> gall is
+induced by the mechanical irritation which the Rotifer causes in the
+protoplasm. These galls are comparable to the hypertrophies in
+<i>Pilobolus</i> caused by the presence of <i>Pleotrachelus</i>.</p>
+
+<p>Attempts to induce the development of galls artificially by injecting
+formic, acetic and other vegetable acids, poisons and other substances
+into the tissues have, however, failed, and even the substances
+contained in the insect or gall itself only produced negative results.
+Nothing further was obtained than slight callus formations in some
+cases. Nor have experimenters succeeded in obtaining more than slight
+distortions by fixing insects on the growing leaves in such positions
+that they could scratch the epidermis.</p>
+
+<p>We must therefore conclude that very complex interactions between the
+plant and insect are here concerned, among which may be the infiltration
+of some liquid from larva to plant&mdash;many of these gall larvae are
+strongly scented, and Kustenmacher says that fluids excreted by the
+larva are absorbed by the gall-tissue apparently as nutriment. This
+would point to the symbiotic character of galls and their guests.</p>
+
+
+<h4><span class="smcap"><a name="Chapter_XIV_Notes" id="Chapter_XIV_Notes"></a>Notes to Chapter XIV.</span></h4>
+
+<div class="chnote">
+<p>With regard to the action of poisons in small doses see
+further Johannsen, <span lang="de" xml:lang="de"><i>Das Aether-Verfahren beim Fruhtreiben</i></span>,
+<span class="pagenum"><a name="Page_141" id="Page_141" style="font-size: 110%;"></a>[<a href="./images/141.png">141</a>]</span>Jena, 1900, and, for <i>Botrytis</i>, see Marshall Ward, "A Lily
+Disease," <i>Annals of Botany</i>, Vol. II., 1889, p. 388.</p>
+
+<p>The subject of enzymes has been exhaustively treated by Green,
+<i>The Soluble Ferments and Fermentations</i>, Cambridge, 1899, to
+which the reader is referred for literature. I have taken the
+statements regarding <i>Fontaria</i> and <i>Dolium</i> from Kassowitz,
+<span lang="de" xml:lang="de"><i>Allgemeine Biologie</i></span>, p. 182. The two most important works on
+chemotactic phenomena are Pfeffer, "<span lang="de" xml:lang="de">Uber Chemotaktische
+Bewegungen</span>," etc., <span lang="de" xml:lang="de"><i>Unters. aus dem Bot. Inst. zu Tubingen</i></span>,
+B. II., p. 582, and Miyoshi, "<span lang="de" xml:lang="de">Die Durchbohrung von Membranen
+durch Pilzfaden</span>," <span lang="de" xml:lang="de"><i>Pringsh. Jahrb. f. Wiss. Bot.</i></span>, B. XXVIII.,
+1895, p. 269, and from these the further literature can be
+traced. As regards the nature of parasitism see Marshall Ward,
+"On Some Relations between Host and Parasite," etc., being the
+Croonian Lecture delivered before the Royal Society, <i>Proc.
+Roy. Soc.</i>, Vol. 47, p. 393. On Symbiosis, see Marshall Ward,
+"Symbiosis," <i>Annals of Botany</i>, 1899, Vol. XIII., p. 549,
+where the literature is collected. For a general account of
+galls the reader may consult Kerner, <i>The Natural History of
+Plants</i>, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,
+<i>Alternating Generations, A Biological Study of Oak Galls</i>,
+etc., 1894.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_142" id="Page_142"></a>[<a href="./images/142.png">142</a>]</span></p>
+<h2>CHAPTER XV.</h2>
+
+<h3>SPREADING OF DISEASE AND EPIDEMICS.</h3>
+
+<div class="chsub">
+<p>Dissemination of fungi by the aid of snails, rabbits, bees,
+and insects&mdash;Man&mdash;Distribution in soil, on clothes, through
+the post, etc.&mdash;Worms, wind&mdash;Puffing of spores&mdash;Creeping of
+mycelia&mdash;Lurking parasites&mdash;Spread of insects and other
+animals&mdash;Losses due to epidemics.</p>
+</div>
+
+
+<p>The dissemination of plant diseases is a subject which has been far too
+much neglected, but our knowledge of it is slowly increasing. The spores
+of fungi such as Rusts and Erysipheae are often carried from plant to
+plant by snails; those of root-destroying and tree-killing Polyporei by
+rabbits, rats, and other mammals which rub their fur against the
+hymenophores. Bees have been shown to carry the spores of <i>Sclerotinia</i>
+and infect the stigmas of Bilberries, etc., with them; and flies convey
+the conidia of Ergot from grain to grain. Insects, indeed, of all kinds
+are great disseminators of disease&mdash;as witness also the part played by
+<span class="pagenum"><a name="Page_143" id="Page_143"></a>[<a href="./images/143.png">143</a>]</span>mosquitoes in transferring the malaria parasite to man&mdash;and beetles,
+bees, flies, etc., of all sorts probably play more active parts in this
+work than has yet been proved, since they not only carry spores attached
+like pollen to their hairy bodies, but in many cases in their alimentary
+canal, to be spread later in the dung.</p>
+
+<p>The part played by man in conveying fungi from plant to plant counts for
+much. Not only do gardeners and farm labourers carry spores on their
+boots and clothes as they pass from infected to non-infected areas, but
+carted soil and manure are frequently infested with spores of Smuts,
+<i>Fusarium</i>, <i>Polyporus</i>, and the sclerotia or rhizomorphs of
+<i>Sclerotinia</i>, <i>Agaricus melleus</i>, <i>Dematophora</i>, etc. Man also sends
+diseases through the post, and by rail and ship, by spores or mycelia
+attached to seedlings, bulbs, fruits, flowers, etc., as shown in several
+cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every
+time a carpenter saws a piece of fresh timber with the saw which has
+been used previously for cutting wood attacked with dry rot, he risks
+infecting it with the fungus. Similarly in pruning: every cut with a
+knife which the gardener has used on infected branches may infect the
+tree.</p>
+
+<p>Cuttings made with a soil-contaminated knife and stuck into ordinary
+soil in dirty boxes covered with equally dirty glass, present every
+chance for infection by soil organisms; bacteria and fungi obtain access
+to the vessels, and derive plenty of food from the juices, and the
+wonder is not that <span class="pagenum"><a name="Page_144" id="Page_144"></a>[<a href="./images/144.png">144</a>]</span>so many cuttings "damp off," but that any are raised
+at all under ordinary conditions.</p>
+
+<p>That worms bring buried spores to the surface can hardly be doubted
+after Pasteur's experiments with Anthrax, and the principle of Darwin's
+discoveries of the important bearing of the habits of earthworms on this
+subject, and that the soil attached to the feet of ducks and other birds
+teems with small seeds, applies to fungi also. Wind is also responsible
+for distributing fungus-spores over wide areas, as may be easily proved
+by fixing a glass slide smeared with glycerine in the course of a breeze
+passing over an infected area.</p>
+
+<p>But although the fungi are, generally speaking, passive in regard to
+their distribution, such is by no means always the case. Apart from the
+fact that some forms attract insects by means of honey dew (Ergot), or
+by sweet odours (Spermogonia, <i>Sclerotinia</i>), the zoospores of
+<i>Pythium</i>, <i>Phytophthora</i>, etc., are motile, and although they cannot
+move far in the films of water in which they travel, nevertheless in a
+wet potato field, with the wind flapping the leaves one against the
+other, some dissemination of importance must be actively brought about,
+and similarly with the amoebae of <i>Plasmodiophora</i> in the soil.</p>
+
+<p>The shooting of ascospores into the air by certain species of <i>Peziza</i>,
+from the discs of which the spores may be seen to puff out in clouds,
+affords further evidence that fungi cannot be regarded as entirely
+passive in respect to distribution of their spores. But when we come to
+<span class="pagenum"><a name="Page_145" id="Page_145"></a>[<a href="./images/145.png">145</a>]</span>certain of the soil fungi&mdash;<i>e.g.</i> <i>Agaricus melleus</i>, <i>Dematophora</i>,
+etc.&mdash;the active creeping forward by growth in the soil of their
+rhizomorphs and mycelial strands afford examples of active spreading of
+considerable importance in the vineyard and forest, since they pass from
+root to root and from tree to tree and may infect the entire area in
+course of time.</p>
+
+<p>Not the least significant mode of dissemination is that by which what I
+have termed "lurking parasites" are spread: such are fungi which attach
+themselves to the seeds, fruits, tubers, etc., of other plants and so
+obtain all the advantages of being carried and sown with the
+latter&mdash;<i>e.g.</i> Ustilagineae and Uredineae which adhere to grain,
+<i>Verticillium</i>, <i>Nectria</i>, etc., in potatoes and other plants.</p>
+
+<p>The spread of diseases due to animals, especially insects, is of course
+more active, in consequence of the motility of the distributing agents.
+This is most marked in the winged species, of which locusts, beetles,
+moths and butterflies, flies and wasps furnish well-known examples; and
+is not inconsiderable in the case of wingless and merely creeping
+species. It is noteworthy that many forms wingless in the parasitic
+stage are winged at certain periods, <i>e.g.</i> the females of <i>Phylloxera</i>.</p>
+
+<p>That man also spreads insect pests is well known and acted upon, as
+witness the phylloxera laws&mdash;which, however, it is to be feared too
+often only illustrate once more the adage concerning the shutting of the
+stable door after the horse has gone.</p>
+
+<p><span class="pagenum"><a name="Page_146" id="Page_146"></a>[<a href="./images/146.png">146</a>]</span>It would be tedious to attempt anything like a complete account of the
+estimates of loss in different countries, due to the ravages of insects
+and fungi, but the following examples should surely serve to convince
+anyone of the magnitude of these losses and of the economic importance
+of the whole question, and the reader may be referred to the special
+literature for further details.</p>
+
+<p>The coffee leaf-disease of Ceylon, due to the fungus <i>Hemileia</i>, is
+estimated to have cost that Colony considerably over £1,000,000 per
+annum for several years. One estimate puts the loss in ten years at from
+£12,000,000 to £15,000,000. The hop-aphis is estimated to have cost Kent
+£2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of
+the United States estimated the annual loss, due to the ravages of
+insects on cotton alone, to amount to £5,000,000; and in 1882 the annual
+loss to the United States due to insects, calculated for all kinds of
+agricultural produce, was put at the appalling figure of from
+£40,000,000 to £60,000,000 sterling. In India, the annual loss due to
+wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000
+rupees, and one insect alone is said to have cost the cotton planters a
+quarter of the crop&mdash;valued at seven crores of rupees&mdash;in bad years.
+Similarly, in Australia the annual loss from wheat-rust has been put at
+from £2,000,000 to £3,000,000. In 1891 the loss in Prussia alone from
+grain-rusts was <span class="pagenum"><a name="Page_147" id="Page_147"></a>[<a href="./images/147.png">147</a>]</span>officially estimated at over £20,000,000 sterling. Need
+more be said? Even allowing for considerable exaggerations in such
+estimates it is clear that the damage to crops in any country soon
+amounts to sums which even at low rates of interest would easily yield
+incomes capable of supporting the best equipped laboratories and staffs
+for investigations directed to the explanation of the phenomena in
+detail, the sole basis on which intelligent preventive and therapeutic
+measures can be based. But it is far from likely that the estimates are
+exaggerated. The planting and agricultural communities are as a rule
+opposed to the publication of statistics&mdash;or at least have been so in
+various countries and at different times&mdash;and if we knew the damage done
+to all crops even in our own Empire, the results would probably astonish
+us far more than the above figures have done.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XV.</span></h4>
+
+<div class="chnote">
+<p>On the dissemination of fungi, the reader will find Fulton,
+"Dispersal of the Spores of Fungi by the Agency of Insects,"
+<i>Ann. Bot.</i>, Vol. III., 1889, p. 207, and Sturgis, "On Some
+Aspects of Vegetable Pathology and the Conditions which
+Influence the Dissemination of Plant Diseases," <i>Botanical
+Gazette</i>, Vol. XXV., 1898, p. 187, both useful papers. Further
+information will be found in Zopf, <span lang="de" xml:lang="de"><i>Die Pilze</i></span>, Breslau, 1890,
+pp. 79-95 and 228, and Wagner, "<span lang="de" xml:lang="de">Ueber die Verbreitung der
+Pilze durch Schnecken</span>," in <span lang="de" xml:lang="de"><i>Zeitschr. f. Pflanzen Krankh.</i></span>,
+1896, p. 144. The estimates as to losses due to epidemics are
+taken from Watt, <i>Agricultural Ledger</i>, Calcutta, 1895, p. 71;
+Balfour, <i>The Agricultural Pests of India</i>, London, 1887, pp.
+13-15; <span class="pagenum"><a name="Page_148" id="Page_148" style="font-size: 110%;"></a>[<a href="./images/148.png">148</a>]</span>Eriksson and Henning, <i>Die Getreideroste</i>; the
+publications of the U.S. Department of Agriculture, <i>The Kew
+Bulletin</i>, and elsewhere. The reader will find further
+examples in Massee, <i>Text-Book of Plant Diseases</i>, 1899, pp.
+47-51. Both these subjects are well worth further attention,
+and I know of no complete account of them.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_149" id="Page_149"></a>[<a href="./images/149.png">149</a>]</span></p>
+<h2>CHAPTER XVI.</h2>
+
+<h3>THE FACTORS OF AN EPIDEMIC.</h3>
+
+<div class="chsub">
+<p>Illustrations afforded by the potato disease&mdash;The larch
+disease&mdash;The phylloxera of the vine.</p>
+</div>
+
+
+<p>When we come to enquire into what circumstances bring about those severe
+and apparently sudden attacks on our crops, orchards, gardens, and
+forests by hosts of some particular parasite, bringing about all the
+dreaded features of an epidemic disease, we soon discover the existence
+of a series of complex problems of intertwined relationships between one
+organism and another, and between both and the non-living environment,
+which fully justify the caution already given against concluding that
+any cause of disease can be a single agent working alone.</p>
+
+<p>The statement of prophecy that a particular insect or fungus need not be
+feared, because it is found to do so little harm in particular cases or
+districts examined, will thus be seen to be a <span class="pagenum"><a name="Page_150" id="Page_150"></a>[<a href="./images/150.png">150</a>]</span>dangerous one: any pest
+may become epidemic if the conditions favour it!</p>
+
+<p>In 1844 and 1845 the potato disease assumed an epidemic character so
+appalling in its effects that it is no exaggeration to say that it
+constituted a national disaster in several countries. It was stated at
+the time that this disease had been known for some time in Belgium, in
+Canada and the United States, in Ireland, in the Isle of Thanet, and in
+other parts of the world. Similar, but less devastating epidemics have
+occurred in various years since. It was generally noticed during such
+epidemics that the plants themselves were full of foliage, surcharged
+with moisture, and of a luxuriant green colour promising abundant crops.
+The now well-known spots, at first pale and then brown and fringed with
+a whitish mould-like growth&mdash;the conidiophores of the
+<i>Phytophthora</i>&mdash;were observed during the dull cloudy and wet weather,
+cooler than usual, when the atmosphere was saturated for days together,
+in July and August. The actual amount of rain does not appear to have
+been excessive, but most observers seem to agree that dull weather with
+moist air had succeeded a warm forcing period of growth. So rapidly did
+the disease run its course that in a few days nearly all the plants were
+a rotting blackened mass in the fields, and the potatoes dug up
+afterwards were either already rotten or soon became so in the stores.
+Further experience has confirmed this, and we now know that the epidemic
+is very apt to appear in any region where <span class="pagenum"><a name="Page_151" id="Page_151"></a>[<a href="./images/151.png">151</a>]</span>potatoes are grown on a large
+scale, in dull moist weather, especially in fields exposed to mists,
+heavy dews, etc., about July and August, when the foliage is full and
+turgid. Similarly on heavy wet soils, unless the season is remarkably
+open and dry; but also on dry light soils in rainy seasons. So evident
+was this that many believed that the mists and dew brought the
+disease&mdash;harking back to the superstitions of earlier days. We must
+remember that prior to 1860 the life-history of <i>Phytophthora</i> was not
+known. Since De Bary's proof of the germination of the zoospores and of
+the infection of the leaves, the course of the hyphae in them and in the
+haulms, the origin of the conidia, etc., and the confirmation by
+numerous competent observers of the true fungus nature of this disease,
+we are now in a position to understand the principal factors of the
+various epidemics of potato disease.</p>
+
+<p>It is not merely that the potato-fields afford plenty of food for the
+fungus, and that the dull weather causes the tissues to be surcharged
+with moisture, owing to diminished transpiration, but the mists and
+dew&mdash;to say nothing of actual rain and the flapping of wet
+leaves&mdash;favour the germination and spread of the zoospores throughout
+the field. Whether the dull light also favours the accumulation of
+sugars in the tissues, and the partial etiolation of the latter implies
+less resistance to the entering hyphae, may be passed over here, but in
+any case it is clear that we have several factors of the non-living
+environment <span class="pagenum"><a name="Page_152" id="Page_152"></a>[<a href="./images/152.png">152</a>]</span>here favouring the parasite and not improving the chances
+of the host, even if they do not directly disfavour it.</p>
+
+<p>As another instance I will take the Larch-disease, which is due to the
+ravages of a Peziza (<span lang="de" xml:lang="de"><i>Dasyscypha Willkommii</i></span>) the hyphae of which obtain
+access by wounds to the sieve-tubes and cambium of the stem, and
+gradually kill them over a larger and larger area and so ring the tree,
+with the symptoms of canker described below.</p>
+
+<p>Now the Larch fungus is also to be found on trees in their Alpine home,
+but there it does very little damage and never becomes epidemic except
+in certain sheltered regions near lakes and in other damp situations.
+How then are we to explain the extensive ravages of the Larch disease
+over the whole of Europe during the latter half of this century? The
+extensive planting, providing large supplies for the fungus, does not
+suffice to explain it, because there are large areas of pure Larch in
+the Alps which do not suffer.</p>
+
+<p>In its mountain home the Larch loses its leaves in September and remains
+quiescent through the intensely cold winter, until May. Then come the
+short spring and rapid passage to summer, and the Larch buds open with
+remarkable celerity when they do begin&mdash;<i>i.e.</i> when the roots are
+thoroughly awakened to activity. Hence the tender period of young
+foliage is reduced to a minimum, and any agencies which can only injure
+the young leaves and shoots in the tender stage must do their work in a
+few days, or the opportunity is <span class="pagenum"><a name="Page_153" id="Page_153"></a>[<a href="./images/153.png">153</a>]</span>gone, and the tree passes forthwith
+into its summer state.</p>
+
+<p>In the plains, on the contrary, the Larch begins to open at varying
+dates from March to May, and during the tardy spring encounters all
+kinds of vicissitudes in the way of frosts and cold winds following on
+warm days which have started the root-action&mdash;for we must bear in mind
+that the roots are more easily awakened after our warmer winters than is
+safe for the tree.</p>
+
+<p>It amounts to this, therefore, that in the plains the long continued
+period of foliation allows insects, frost, winds, etc., some six weeks
+or two months in which to injure the slowly sprouting tender shoots,
+whereas in the mountain heights they have only a fortnight or so in
+which to do such damage. That the lower altitude and longer summer are
+not in themselves inimical to Larch is proved by the splendid growths
+made by the trees first planted a century ago. Then came the epidemic of
+Larch-disease: the fungus, which is merely endemic&mdash;<i>i.e.</i> obtains a
+livelihood here and there on odd trees, or groups of trees in warmer or
+damper nooks&mdash;in the Alps, was favoured by the more numerous points of
+attack afforded to its spores by injuries due to insects&mdash;<i>Coleophora</i>,
+<i>Chermes</i>, etc.&mdash;and frost wounds, as well as by the longer periods of
+moist dull weather, and the longer season of foliation. Moreover, as
+time went on almost every consignment of young Larch-trees sent abroad
+was already infected. Here again, then, we find the factors <span class="pagenum"><a name="Page_154" id="Page_154"></a>[<a href="./images/154.png">154</a>]</span>of an
+epidemic consisting in events which favour the reproduction and spread
+of a fungus more than they do the well-being of the host.</p>
+
+<p>As a third illustration I will take the case of an insect epidemic. In
+1863 a disease was observed on vines in the South of France which
+frightened the growers as they realised its destructive effects: the
+roots decayed and the leaves turned yellow and died before the grapes
+ripened, and such vines threw out fewer and feebler shoots the following
+year, and often none at all afterwards. In 1865 the disease was
+evidently becoming epidemic near Bordeaux, and in 1868 it was shown to
+be due to an insect, <i>Phylloxera</i>, the female of which lays its eggs on
+the roots, where they hatch. The louse-like offspring sticks its
+proboscis into the tissues as far as the central cylinder. The irritated
+pericycle and cortex then grow and form nodules of soft juicy
+root-tissue at which the insect continues to suck. Rapid reproduction
+results in the majority of the young rootlets being thus attacked, and
+since they cannot form their normal periderm and harden off properly
+they rot, and admit fungi and other evils, in consequence of which the
+vine suffers also in the parts above ground.</p>
+
+<p>Evidence that the general damage is due to the diminished root-action is
+found in the peculiarly dry poor wood formed in the "canes" of diseased
+plants.</p>
+
+<p>By 1877 the epidemic had spread to the northern limits of the French
+vineyards, and by 1888 half the vines in the country were attacked, <span class="pagenum"><a name="Page_155" id="Page_155"></a>[<a href="./images/155.png">155</a>]</span>and
+the yield of wine reduced from half a million hectolitres to 50,000
+only. Meanwhile the disease had spread to Italy, Germany, Madeira,
+Portugal, and even to the Cape, though not in epidemic form as in the
+Bordeaux centre whence it spread.</p>
+
+<p>Now it appears that <i>Phylloxera</i> has long been in the habit of doing
+damage to vines in America, where, however, it attacks the leaves, on
+which it makes pocket-like galls, rather than the roots. Moreover, there
+are species and varieties of American vines which, even when planted in
+Europe, do not suffer at all from this insect at the roots, either
+because the rootlets do not push out at the same season as those of the
+European form, or because they form wood more rapidly and completely, or
+secrete resinous and other matters distasteful to the insect in greater
+quantity and are thus capable of healing the wounds, or in some other
+way they do not respond to the attack or suit the insect. In any case
+the attack on the leaf rather than the root seems to be the exception in
+European vineyards and the rule in American species, and we appear to be
+face to face with a problem of specific predisposition to this
+particular malady. That the resistant properties of the vines of
+America&mdash;not all, only particular species and varieties are thus
+"immune"&mdash;can be utilised has been proved by European growers; and not
+only so, for Millardet and others have shown that the European vine
+grafted on to these resistant stocks suffer less than when on their own
+roots. It has <span class="pagenum"><a name="Page_156" id="Page_156"></a>[<a href="./images/156.png">156</a>]</span>also been shown that hybrids can be obtained which are
+resistant.</p>
+
+<p>But the most curious point of all is that <i>Phylloxera</i> was itself a
+native of America, and came thence to Europe. It had played its part
+with certain fungi in ruining all the attempts to introduce the European
+vine into America many years ago. A recent authority on the evolution of
+American fruits writes as follows:</p>
+
+<p>"All the most amenable types of grapes had long since perished in the
+struggle for existence, and the types which now persist are necessarily
+those which are, from their very make-up or constitution, almost immune
+from injury, or are least liable to attack .&nbsp;.&nbsp;. the <i>Phylloxera</i> finds
+tough rations on the hard, cord-like roots of any of our eastern species
+of grapes. But an unnaturalised and unsophisticated foreigner, being
+unused to the enemy and undefended, falls a ready victim; or if the
+enemy is transported to a foreign country the same thing occurs."</p>
+
+<p>Further proof that it is in the "constitution" of the European vine that
+the want of resistance to <i>Phylloxera</i> resides, is furnished by the fact
+that in California and the Pacific states the European vine was
+introduced with more success, but is now suffering badly because
+<i>Phylloxera</i> has spread there also. It must not be overlooked, however,
+that we are as yet very ignorant of all that is implied in the word
+"constitution" as used above.</p>
+
+<p>If we enquire further why the <i>Phylloxera</i> <span class="pagenum"><a name="Page_157" id="Page_157"></a>[<a href="./images/157.png">157</a>]</span>epidemic was so much worse
+in the Southern vineyards than in the more Northern ones of Germany, the
+opinion seems to prevail that the warmer climates favour the insect.
+Further, it appears that, in Italy, the vines in loose open soil,
+provided it is equally rich in mineral food-materials and offers no
+disadvantages as regards drainage, suffer less than those in closer
+soils, the reasons alleged being that the young roots can push out more
+rapidly and widely, and so obtain holdfasts with greater distances
+between them.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XVI.</span></h4>
+
+<div class="chnote">
+<p>The student may obtain further information on the history of
+the Potato disease by consulting the following: Berkeley,
+"Observations, Botanical and Physiological, on the Potato
+Murrain," <i>Journal of the Horticultural Society</i>, Vol. I.,
+1846, p. 9; De Bary, <span lang="de" xml:lang="de"><i>Die Gegenwärtig herrschende Kartoffel
+Krankheit</i></span>, etc., Leipzic, 1861; and the pages of the
+<i>Gardeners' Chronicle</i> from 1860-1900.</p>
+
+<p>For the Larch disease he should consult Hartig, <span lang="de" xml:lang="de"><i>Unters. aus
+der Foist. Botanischen Inst. München</i></span>, B. I., 1880; and
+Willkomm, <span lang="de" xml:lang="de"><i>Microscop. Feinde des Waldes</i></span>, B. II., 1868.</p>
+
+<p>For <i>Phylloxera</i> the literature is chiefly in the <span lang="fr" xml:lang="fr"><i>Comptes
+Rendus</i></span> and other French publications since 1875, and in the
+Reports of the U.S. Dept. of Agriculture.</p>
+
+<p>For a summary of the facts concerning the life-histories of
+the parasites referred to above, see Frank, <span lang="de" xml:lang="de"><i>Krankheiten der
+Pflanzen</i></span>, and Marshall Ward, <i>Diseases of Plants</i>, p. 59, and
+<i>Timber and Some of its Diseases</i>, London, 1889, chapter X.</p>
+
+<p>Also Marshall Ward, "On some Relations between Host and
+Parasite in certain epidemic Diseases of Plants," <i>Proc. Roy.
+Soc.</i>, Vol. XLVII., 1890, pp. 393-443; and "Illustrations <span class="pagenum"><a name="Page_158" id="Page_158" style="font-size: 110%;"></a>[<a href="./images/158.png">158</a>]</span>of
+the Structure and Life-history of Phytophthora infestans,"
+<i>Quart. Journ. Microsc. Soc.</i>, Vol. XXVII., 1887, p. 413; also
+Marshall Ward, "Researches on the Life-history of Hemileia
+vastratrix," <i>Journ. Linn. Soc.</i>, Vol. XIX., 1882, p. 299; and
+"On the Morphology of Hemileia vastatrix," <i>Quart. Journ.
+Microsc. Soc.</i>, 1881, Vol. XXI., p. 1.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_159" id="Page_159"></a>[<a href="./images/159.png">159</a>]</span></p>
+<h2>CHAPTER XVII.</h2>
+
+<h3>REMEDIAL MEASURES.</h3>
+
+<div class="chsub">
+<p>Preventible diseases&mdash;The principles of therapeutics&mdash;Powders
+and their application&mdash;Spraying with liquids&mdash;Nature of
+chemicals employed&mdash;Employment of epidemics and natural
+checks&mdash;The struggle for existence.</p>
+</div>
+
+
+<p>It may be said that in no connection is the proverb "Prevention is
+better than cure" more applicable than with this subject, and
+undoubtedly the best utilitarian argument that can be used in favour of
+a thorough study of the causes of disease is that only by understanding
+these causes is there any hope of avoiding the exposure of crops, garden
+plants, forest trees, etc., to the attacks of preventible diseases.
+Moreover, only an intelligent appreciation of the causes of a disease
+will enable the cultivator to take steps to mitigate their effects when
+once the damage has begun its course. Every cultivator learns by
+experience or by precept that there are some <span class="pagenum"><a name="Page_160" id="Page_160"></a>[<a href="./images/160.png">160</a>]</span>things he must avoid in
+dealing with certain plants, or otherwise they will not succeed; in
+other words they will succumb to diseased conditions and die. It is
+partly owing to the want of systematisation of this knowledge, and its
+extension in other directions, that such extraordinary blunders are made
+in ignorant practice, and trees for instance are planted in low-lying
+frost beds which would succeed in slightly higher situations, or seeds
+subject to damping-off are sown in beds rife with the spores of
+<i>Peronospora</i> or <i>Pythium</i>, and so forth.</p>
+
+<p>Many diseases, however, are not preventible in the present state of our
+knowledge, or prevailing conditions are such that the risk must be run
+of endemic diseases gradually becoming epidemic, and thus the natural
+desire for some means of checking the ravages of some pest or another
+has led to innumerable trials to minimise the effects by prophylactic
+measures. The procedure almost invariably followed where parasites are
+concerned, consists in either dusting the plants with some chemical in
+the form of a powder, or spraying it with a liquid, or occasionally in
+enveloping the plant in some gas, in each case poisonous to the insect-
+or fungus-pest. The principal rules to be observed are: (1) the poison
+employed must be sufficiently strong or concentrated to kill the
+parasite, but not sufficiently powerful to injure the host; (2) it must
+be applied at the right period, as suggested by a knowledge of the
+life-history of the fungus or insect in question.</p>
+
+<p><span class="pagenum"><a name="Page_161" id="Page_161"></a>[<a href="./images/161.png">161</a>]</span>Obviously it is of no use to apply such topical remedies to a parasite
+while it is spending the greater part of its life inside the tissues of
+the host. Further, questions of expense of the materials employed and of
+the labour of applying them help to limit the adoption of such measures.</p>
+
+<p>Among the various kinds of powders employed, finely divided sulphur, or
+a mixture of sulphur and lime, have been used with success in some
+cases&mdash;<i>e.g.</i> against Hop mildew and other epiphytic Erysipheae, and
+against red spider, aphides, etc., the gaseous sulphur dioxide evolved
+being the efficacious agent. In other cases pyrethrum or tobacco powder,
+wood ashes, etc., have been employed against insects. Such powders are
+applied by hand or by means of bellows, and are very easily manipulated
+in most cases, though, like all such applications, the dangers of
+concentration at particular spots owing to uneven distribution, or of
+dilution and washing off by rain, have to be incurred.</p>
+
+<p>Far more numerous are the various liquids which have been employed for
+washing, spraying, or steeping the affected parts of diseased plants.
+Water alone, or aqueous decoctions or emulsions of various
+kinds&mdash;<i>e.g.</i>, quassia, tobacco, soap, or aloes, have been widely
+employed against insects such as green fly, red spider, etc. In
+greenhouses, where the leaves can be washed by hand or thoroughly
+syringed, and the concentration and time of action thoroughly
+controlled, such liquids <span class="pagenum"><a name="Page_162" id="Page_162"></a>[<a href="./images/162.png">162</a>]</span>are often serviceable, but great practical
+difficulties are apt to interfere with their use in the open field.</p>
+
+<p>The principal liquids employed against fungi have been copper sulphate
+and other metallic compounds (Bordeaux mixture, Eau Céleste, etc.),
+various compounds of arsenic (<i>e.g.</i> "Paris green"), potassium sulphite,
+permanganate, etc., and emulsions of carbolic acid, petroleum, and such
+like antiseptics, for the exact composition of which the special
+treatises must be consulted. Some of these, especially Bordeaux mixture,
+have been experimented with on a very large scale, especially in
+America, and various forms of spraying machines have been introduced for
+dealing with large areas.</p>
+
+<p>It is clear that these spraying operations are more particularly adapted
+to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and
+greenhouse plants than to woods and plantations; as a rule they cannot
+be applied to forest trees&mdash;though they have been used in orchards&mdash;or
+to roots, seeds, and other parts in the soil, and many special forms of
+treatment have been devised for particular cases of these kinds.</p>
+
+<p>One of the oldest of these is the steeping of grain in solutions of
+copper, or in hot water, just before sowing, and the practical
+eradication of Bunt and, partially, of Smut is due to this practice,
+which has lately been adapted to potatoes, the principle being that the
+parasitic germs shall <span class="pagenum"><a name="Page_163" id="Page_163"></a>[<a href="./images/163.png">163</a>]</span>be killed while still adhering to the outside of
+the seeds, tubers, etc., before germination. "Finger and Toe" due to
+<i>Plasmodiophora</i> has been successfully dealt with by the application of
+lime, but we do not know whether the effect is owing to indirect actions
+in the soil, to direct actions on the plasmodia, or to the increased
+production of root-hairs induced by liming.</p>
+
+<p><i>Phylloxera</i> has been treated by plunging into the soil near the roots
+small blocks of some slowly-soluble medium, such as gelatine,
+impregnated with carbon-bisulphide, the volatile fumes of which kill the
+insect, and even more drastic remedies have been tried along similar
+lines. In America orchard trees infested with insects or fungi have been
+covered one by one with light tents, and the vapours of prussic acid,
+burning sulphur, and other poisons allowed to act inside the tent. In
+all such cases it must be remembered that uncontrolled ignorance of the
+properties of poisons on the part of the operator may lead to disaster,
+and the same applies to the much easier treatment of greenhouses, and
+cases where poisoned food is laid about for insects or vermin.</p>
+
+<p>Attempts, not altogether unsuccessful on the small scale, have also been
+made to introduce epidemic diseases among rats, mice, and locusts and
+other insects, by inoculating some of them with parasitic bacteria or
+fungi (<i>Empusa</i>, <i>Isaria</i>, etc.), and then allowing them to run loose in
+the hope that they will communicate the disease to their fellows. <span class="pagenum"><a name="Page_164" id="Page_164"></a>[<a href="./images/164.png">164</a>]</span>The
+introduction of lady-birds into districts infested with Coccideae and
+similar pests which they devour, is also recorded as successful, as also
+the importation of birds into forests plagued with caterpillars. It must
+not be over-looked, however, that man's interference with the existing
+balance of events in the natural struggle for existence is occasionally
+disastrous, as witness the results of importing rabbits into Australia,
+goats into the Canary Islands, and sparrows in various countries.
+Darwin's well-known illustration of the inter-relations between clover,
+bees, field-mice, and cats (<i>Orig. of Species</i>, 6th ed., 1876, p. 57),
+which shows the astounding probability of the dependence of such a plant
+on the number of cats in the neighbourhood, well illustrates the
+situation.</p>
+
+<p>Mere mention must be made of other special treatments.</p>
+
+<p>Caterpillars and larger animals are often picked by hand or their
+natural enemies&mdash;<i>e.g.</i> birds, are encouraged in forests. Locusts are
+caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are
+often trapped by laying attractive food such as carrots and overhauling
+the traps daily: similarly with earwigs. Rings of tar round tree stems
+have been employed to prevent caterpillars creeping up them.</p>
+
+<p>American Blight has been treated by rapidly flaming the stems. Syringing
+with hot water has also been employed for vines affected with mildew,
+mealy bug, etc.</p>
+
+<p><span class="pagenum"><a name="Page_165" id="Page_165"></a>[<a href="./images/165.png">165</a>]</span>With regard to the alleged immunity from devouring insects of certain
+poisonous plants, it has been pointed out that <i>Pangium edule</i>, which
+abounds in prussic acid, is infested with a grub, and ivy is
+occasionally eaten by caterpillars.</p>
+
+<p>Another point as regards insect pests is the well-known destructive
+effect of a cold, wet spring on the young larvae. The use of cyanide of
+potassium requires especial care, but has been described as easily
+carried out with success in greenhouses.</p>
+
+<p>It seems probable that lady-birds, the larvae of wasp-flies and
+lace-wings, and ichneumon-flies as well as wrens can keep down aphides.</p>
+
+<p>For an example of the treatment of a complex case of "chlorosis" with
+mineral manures, the reader may consult the <i>Gardeners' Chronicle</i>, 1899
+(July), p. 405. Many similar cases have been recorded, but it should not
+be overlooked that very complex inter-relations are here involved.</p>
+
+<p>Charlock has been successfully dealt with by applying 5 lbs. of copper
+sulphate in 25 gallons of water to each acre of land while the weeds are
+young.</p>
+
+<p>In all these cases the guiding idea is derived from accurate knowledge
+of the habits of the insect, fungus, or pest concerned, and obviously
+the procedure must be timed accordingly. It is a particular case of the
+struggle for existence, where man steps in as a third and (so to speak)
+unexpected living agent.</p>
+
+<p>It is clear from our study of the factors of an epidemic that one of the
+primary conditions which <span class="pagenum"><a name="Page_166" id="Page_166"></a>[<a href="./images/166.png">166</a>]</span>favour the spread of any disease is provided
+by growing any crop continuously in "pure culture" over large areas.
+This is sufficiently exemplified by the disastrous spread of such
+diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera,
+Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other
+maladies which have now become historic in agricultural, planting, and
+forest annals. Providing the favourite food-supply in large quantities
+is not the only factor of an epidemic, but it is a most important one in
+that it not only facilitates the growth and reproduction of a pest, but
+affords it every opportunity of spreading rapidly and widely.</p>
+
+<p>Moreover, Nature herself shows us that such pests are kept in check in
+her domain by the struggle for existence entailed by innumerable
+barriers and competitors. As matter of experience also it is found that
+rotation of crops, planting forests of mixed species, and breaking up
+large areas of cultivation into plantations, fields, etc., of different
+species afford natural and often efficient checks to the ravages of
+fungus and insect pests. Over and over again it has been found that a
+fungus or an insect which is merely endemic so long as it is isolated in
+the forest, where its host is separated from other plants of the same
+species by other plants which it cannot attack, becomes epidemic when
+let loose on the continuous acres so beloved of the planter. And the
+same reasoning applies to the success of such pests on open areas from
+which the birds or other enemies of the pest <span class="pagenum"><a name="Page_167" id="Page_167"></a>[<a href="./images/167.png">167</a>]</span>have been driven. True, we
+cannot always trace the tangled skein of inter-relationships between one
+organism and another in Nature: the recognition of the principle of
+natural selection and the struggle for existence is too recent, and our
+studies of natural history as yet too imperfect to lay all the factors
+clear, but no observant and thoughtful man can avoid the truth of the
+general principle here laid down. The history of all great planting
+enterprises teaches us that he who undertakes to cultivate any plant
+continuously in open culture over large areas must run the risk of
+epidemics.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XVII.</span></h4>
+
+<div class="chnote">
+<p>The principal literature, now very voluminous, on this subject
+is contained in the publications of the U.S. Department of
+Agriculture from 1890 onwards. See especially <i>Bulletins</i>,
+Nos. 3, 6, and 9; <i>Farmers' Bulletin</i>, No. 91, 1899; and <i>The
+Journal of Mycology</i> during the same period. See also Lodeman,
+<i>The Spraying of Plants</i>, London, 1896. A summary of the
+principal processes will be found in Massee, <i>Text-Book of
+Plant Diseases</i>, pp. 31-47.</p>
+
+<p>With regard to the history of the subject, which still needs
+writing, the reader should not overlook Roberts, "On the
+Therapeutical Action of Sulphur," <i>St. George's Hospital
+Reports</i>, date unknown, but subsequent to the following:
+Berkeley, <i>Introduction to Cryptogamic Botany</i>, 1857, p. 277,
+with references. These are, I believe, with the references to
+steeping of wheat in De Bary, <span lang="de" xml:lang="de"><i>Unters. über d. Brandpilze</i></span>,
+Berlin, 1853, among the first attempts to utilise such
+remedies.</p>
+
+<p>Further facts will be found in the pages of the <i>Gardeners'
+Chronicle</i>, especially since 1890, and in <span lang="de" xml:lang="de"><i>Zeitsch. f.
+Pflanzen-krankheiten</i></span> since 1891.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_168" id="Page_168"></a>[<a href="./images/168.png">168</a>]</span></p>
+<h2>CHAPTER XVIII.</h2>
+
+<h3>VARIATION AND DISEASE.</h3>
+
+<div class="chsub">
+<p>Predisposition and immunity&mdash;Pathological conditions
+vary&mdash;Hardy varieties&mdash;"Disease-proof" varieties&mdash;Disease
+dodging&mdash;Thick skins&mdash;Indian wheats, etc. Cell-contents
+vary&mdash;Citrus, Cinchona, Almonds, etc. Double ideals in
+selection&mdash;Cultivation of pest and host-plant&mdash;Variations of
+fungi&mdash;Bacteria&mdash;Specialised races&mdash;Difficulties&mdash;Experiment
+only will solve the problems.</p>
+</div>
+
+
+<p>The numerous and often expensive failures in the application of any
+prophylactic treatment, have proved an acute stimulus to the research
+for other ways of combating the ravages of plant diseases. It is a
+matter of every-day experience that particular varieties of cultivated
+plants may suffer less from a given disease than others in the same
+district; also that one and the same species may suffer badly in one
+country and not in another&mdash;<i>e.g.</i> the Larch in the lowlands of Europe
+as contrasted with the same tree in its Alpine home, <span class="pagenum"><a name="Page_169" id="Page_169"></a>[<a href="./images/169.png">169</a>]</span>and the various
+species of American Vines in Europe.</p>
+
+<p>These matters, in the hands of astute observers, are turning the
+attention of cultivators and experts to new aspects of the question of
+plant diseases, namely, the possible existence of immunity, and the
+breeding of disease-proof varieties; and the existence on the part of
+the host plant of predispositions to disease which may depend on some
+factors in the plant or in the environment over which it is possible to
+exercise control, or which, if known, can be avoided.</p>
+
+<p>The matter is complicated by the recent demonstration of the fact that
+parasites also vary and can adapt themselves to altered conditions, as
+is shown by the history of the coffee-leaf disease (<i>Hemileia</i>) in
+Ceylon, and by Eriksson's results with Wheat-rusts (<i>Puccinia</i>) and
+various experiments with <i>Coleosporium</i> and other Uredineae; but there
+are good grounds for concluding that hybridisation, grafting, and
+selection of varieties may do much towards the establishment of races
+which will resist particular diseases, as shown by Millardet's
+experiments with Vines, and the results obtained by Cobb and others with
+Wheat.</p>
+
+<p>The great difficulty with so-called "disease-proof varieties" is to test
+them under similar conditions in different countries, and for a
+sufficient period of time. A particular race of Wheat may behave very
+differently in Norfolk, Devonshire, and Northumberland, and the recent
+introduction of the purely experimental method in this connection is <span class="pagenum"><a name="Page_170" id="Page_170"></a>[<a href="./images/170.png">170</a>]</span>a
+marked advance. However rough the experiments may of necessity have to
+be, it is only by such means that data can be gradually accumulated.</p>
+
+<p>Having now obtained some insight into the factors concerned in disease,
+let us enquire further into the bearing of variation on these. It is
+evident that pathological conditions may vary; indeed they are
+themselves symptoms of variation, as we have seen. The history of all
+our cultivated plants shows abundantly that many of the variations
+obtained by breeding in our gardens, orchards, fields, etc., involve
+differences of response on the part of the plant to the very agencies
+which induce disease. Every year the florists' catalogues offer new
+"hardy" varieties; but a hardy variety is simply, for our present
+purpose, one which succumbs less readily to frost, cutting winds, cold
+damp weather, and so forth. If anyone doubts that hardy varieties have
+been gradually bred by selection, I refer him to the evidence collected
+by De Candolle, Darwin, Wallace, Bailey and others. When we come to
+enquire into the causes of "hardiness," however, difficulties at once
+beset us. The adaptation may express itself in a difference in the time
+of flowering or leafing, the exigencies of the season being "dodged," as
+it were, in a manner which was impossible with the original stock, as
+appears to have occurred with Peaches in America; or it may be expressed
+in deeper rooting, as is said to be the case in some Apples, or in the
+acquirement of a more deciduous habit, or in actually increased
+resistance to low temperatures. <span class="pagenum"><a name="Page_171" id="Page_171"></a>[<a href="./images/171.png">171</a>]</span>In such cases we cannot trace what
+alterations have occurred in the cells and tissues concerned, though we
+may be sure that some changes do occur.</p>
+
+<p>No experienced cultivator doubts that some varieties of Potato, Wheat,
+Vine, Chrysanthemum, etc., suffer more from epidemic diseases than
+others, and our yearly catalogues furnish us with plenty of promises of
+"disease-proof" varieties. Here also we may imagine several ways in
+which a particular variety may resist or escape the epidemic attacks of
+fungi which in the same neighbourhood decimate other varieties. If we
+could breed a variety of the Larch which opened its buds later than the
+ordinary form in our northern plains, the probability of its escaping
+the Larch-disease would be increased in proportion to the shortness of
+the period of tender foliation described on <a href="#Page_153">p. 153</a>. It has been claimed
+for certain varieties of Wheat that increased thickness of the cuticle
+and fewer stomata per square unit of surface have diminished the risk of
+infection by Rust fungi, and for certain varieties of Potato, that the
+thicker periderm of the tuber protects them against fungi in the soil.
+That certain thick-skinned Apples, Tomatoes, and Plums pack and store
+better than those with a more tender epidermis seems proved&mdash;that is to
+say, they suffer less from fungi which gain access through bruises and
+other wounds; but it cannot be said that any convincing proof is yet to
+hand explaining in detail why some races of wheat <span class="pagenum"><a name="Page_172" id="Page_172"></a>[<a href="./images/172.png">172</a>]</span>resist Rust, or why
+the roots of American Vines suffer less from <i>Phylloxera</i> than others.</p>
+
+<p>One of the most extraordinary cases known to me in this connection is
+the unconscious selection on the part of native Indian cultivators,
+perfectly ignorant of the principles involved, of spring and autumn
+forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.
+"It has been estimated that Bengal alone possesses as many as 10,000
+recognisable forms of rice." Now there is not the slightest ground for
+doubt that these have been unconsciously bred from the semi-aquatic
+native species during the many centuries of Indian agriculture, and
+nevertheless they have, among other peculiar races, some hill-breeds
+which they cultivate on dry soils and without direct inundation. That is
+to say, they possess tropical and temperate races differing far more
+than our spring and summer wheats.</p>
+
+<p>Something has been gained, then, if we can show that there is nothing
+absurd or hopeless in the search for disease-proof or resistant races,
+and I think this can be done. We must not forget that the ideal usually
+set before himself by a breeder of plants has hitherto been almost
+exclusively some standard of size, form, colouring, and so forth, of the
+flower, or of taste and texture of the fruit, tuber, etc., though
+experiments with <i>Cinchona</i>, with brewery yeasts, and other plants
+remind us that variations in other directions have been attended to
+also.</p>
+
+<p>Now it is obvious that in breeding sour limes <span class="pagenum"><a name="Page_173" id="Page_173"></a>[<a href="./images/173.png">173</a>]</span>and sweet oranges the
+cultivator is selecting, and intensifying by selection, very different
+metabolic processes in the cell: he can test the results of these, and
+so the selection proceeds.</p>
+
+<p>The question is, Could he select at the same time those variations in
+cell activity which express themselves in properties of the flower,
+fruit, foliage, etc., he desires, as well as such variations as aid the
+cells in repelling fungi, insects, or exigencies of the non-living
+environment?</p>
+
+<p>That more or less disease-proof varieties could be selected if that
+object alone were kept in view can hardly be doubted; plenty of examples
+exist already which show that the necessary variations to work upon
+exist in just those secretions of protoplasm, etc., which we have seen
+are concerned in repelling or attracting parasites.</p>
+
+<p>The Sweet Almond has lost the power of producing amygdalin and prussic
+acid in its cells; Cinchona plants vary immensely in the quantity of
+quinine formed, and in European hot-houses may even form none at all;
+some varieties of Maize have sugar and dextrine instead of starch in
+their endosperms, or coloured instead of clear sap in the aleurone
+layer, and recent researches prove that they can transmit these
+peculiarities to hybrid offspring; non-poisonous bacteria have
+frequently been got from poisonous species simply by cultivation under
+special conditions, and pigmented forms can be bred into non-pigmented
+races.</p>
+
+<p>But we see that the difficulty of selection is <span class="pagenum"><a name="Page_174" id="Page_174"></a>[<a href="./images/174.png">174</a>]</span>increased in the case
+postulated above, because two ideals are to be worked up to, and they
+may conceivably be incompatible. Not necessarily so, however, for
+breeders have solved such problems before in obtaining early <i>and</i> heavy
+cropping races of potatoes, wheat, etc., sweet <i>and</i> large grapes,
+strawberries, etc., hardy <i>and</i> brilliant flowers, and so forth.</p>
+
+<p>There is, however, another aspect of this question of variability in
+organisms in this connection to be considered. Ever since cultivation
+began man has probably been cultivating not only the crops he desires,
+but also the pests which infest them, and if variation of his chosen
+plants occurs&mdash;and no one will deny that&mdash;surely variation of the fungi
+and insects which live on them also takes place. That this is so can be
+demonstrated, though, since it is not part of my theme to go into the
+question of peculiarities of species and races of parasites, the subject
+must here be passed over with a few remarks only.</p>
+
+<p>Recent researches have shown not only that fungi vary immensely in form
+and morphological characters according to the amount and kind of
+food-materials put at their disposal, thus bringing the whole question
+of polymorphism into the domain of experimental physiology, but that
+their capacities for infection, spore formation, etc., are also capable
+of variation and are dependent on the quality and quantity of food
+supplies, water, as well as on the temperature, illumination, <span class="pagenum"><a name="Page_175" id="Page_175"></a>[<a href="./images/175.png">175</a>]</span>and other
+factors of the environment. This is true of parasites as well as of
+saprophytes. <i>Botrytis</i> forms conidia only in darkness and in moist air.
+Klebahn found that a <i>Puccinia</i> growing on <i>Digraphis</i> infected
+<i>Polygonatum</i> readily and completely, <i>Convallaria</i> imperfectly, whereas
+if sown on <i>Majanthemum</i> it only just infected the plant and then
+remained sterile, while it refused to infect <i>Paris</i> at all. Magnus has
+shown that <i>Peronospora parasitica</i> can only infect meristematic
+tissues, and that when it co-exists with <i>Cystopus</i> on <i>Capsella</i>, as is
+usually the case, it enters the latter plant by infecting the gall-like
+pustules of hypertrophied tissue induced by that parasite. Numerous
+parasitic fungi can only penetrate particular parts of plants. For
+instance, the <i>Ustilago</i> of wheat can only infect the young seedling,
+and grows for weeks as a barren mycelium, only becoming a dominant
+fungus in the endosperm. Numerous other examples could be given, but
+these suffice to show some of the ways in which the nature of the food
+substratum supplied by the host affects the fungus. It is obvious that
+if the nature of this food changes, the fungus is also affected, and no
+doubt this is the principal reason why Rust-fungi, for instance, vary so
+much in their vigour and reproductive power on different wheats and
+grasses, though the other factors of the environment must also be of
+influence on them as well as on the hosts.</p>
+
+<p>But&mdash;and this is the second point&mdash;modern <span class="pagenum"><a name="Page_176" id="Page_176"></a>[<a href="./images/176.png">176</a>]</span>research is also showing that
+the various species of Rust-fungi have split up into different varieties
+or specialised races, according to the particular host plants they
+inhabit. For instance there are special varieties or races of the
+particular species known as <i>Puccinia graminis</i>, the wheat rust, each of
+which grows well on various kinds of grain and grasses but refuses to
+infect others. Thus, the variety which infects Wheat refuses to infect
+Barley or Oats, while that variety which grows on Rye will not take on
+Wheat and so forth. Now it is important to notice that these specialised
+races are indistinguishable one from another by their visible
+microscopic characters: they are all botanically of the species
+<i>Puccinia graminis</i> which forms its æcida on the Barberry. We must
+therefore conclude that we have here the same phenomenon as that met
+with in culture-races of bacteria which, having been fed for several
+generations on media rich in proteids, refuse to grow on media rich in
+carbohydrates, or when attenuated races are developed by culture under
+special conditions.</p>
+
+<p>Now since such physiological races as I have described are by no means
+confined to <i>Puccinia</i> but are also known in <i>Melampsora</i>,
+<i>Gymnosporangium</i> and other fungi, we must conclude from this and from
+what we know of variation in plants and animals generally, that
+variation and adaptation are common among parasites, insects as well as
+fungi.</p>
+
+<p>These considerations will serve to show moreover <span class="pagenum"><a name="Page_177" id="Page_177"></a>[<a href="./images/177.png">177</a>]</span>that the question of
+breeding disease-proof varieties of our cultivated plants is complicated
+by the danger of our breeding at the same time adapted races of their
+pests. It appears at first sight extremely improbable that we should
+escape the danger by breeding from those specimens of our plants which
+have best survived a fungus epidemic. Still, it must not be forgotten
+that "hardy varieties," and races adapted to other exigencies of the
+non-living environment, have been bred by selection&mdash;and nevertheless
+this variable non-living environment is always with us. The matter is
+therefore simply and solely one of experiment, and the retort that a
+disease-resisting variety of any particular plant has not yet been
+raised is no more valid than the objection that a true blue primrose has
+not yet been obtained: whether the same remark can be made with regard
+to any hope of a <i>disease-proof</i> plant may be another matter, but in any
+case it must be made more cautiously in the light of our present
+experience.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XVIII.</span></h4>
+
+<div class="chnote">
+<p>The reader will find more on this subject in Bailey's
+<i>Survival of the Unlike</i> and the literature quoted in the
+<a href="#Chapter_VIII_Notes">notes to Chapter VIII.</a></p>
+
+<p>For varieties of Indian Wheats, etc., see Watt, <i>Agricultural
+Ledger</i>, Calcutta, 1895.</p>
+
+<p>For a discussion on so-called "Disease-proof Wheats" consult
+Eriksson &amp; Henning, <span lang="de" xml:lang="de"><i>Die Getreideroste</i></span>.</p>
+
+<p>Magnus' paper is in the <span lang="de" xml:lang="de"><i>Berichte der Deutschen bot.
+Gesellsch.</i></span>, 1894, p. 39.</p>
+
+<p><span class="pagenum"><a name="Page_178" id="Page_178" style="font-size: 110%;"></a>[<a href="./images/178.png">178</a>]</span>Concerning physiological races and adapted varieties of
+<i>Puccinia</i>, etc., see Eriksson, "A General View of the
+Principal Results of Swedish Research into Grain Rust,"
+<i>Botanical Gazette</i>, vol. 25, 1898, p. 26.</p>
+
+<p>For an account of Wheat-rust see Marshall Ward, "Illustrations
+of the Structure and Life-history of <i>Puccinia graminis</i>,
+etc.," <i>Ann. of Bot.</i>, 1888, Vol. II., p. 215.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_179" id="Page_179"></a>[<a href="./images/179.png">179</a>]</span></p>
+<h2>CHAPTER XIX.</h2>
+
+<h3>SYMPTOMS OF DISEASE.</h3>
+
+<div class="chsub">
+<p>Discolorations&mdash;Pallor&mdash;Etiolation&mdash;Laying of
+Wheat&mdash;Chlorosis&mdash;Yellowing&mdash;Albinism&mdash;Variegation&mdash;Uprooting,
+Exposure and Wilting of seedlings.</p>
+</div>
+
+
+<p>Everybody knows in a general way when the geraniums in the window pots
+are drooping from want of water, or when the young Wheat is sickly, or
+the Pear-trees "blighted," and we have now to see how far we can
+systematise the knowledge that has been gained in course of time
+regarding the signs which sick plants exhibit.</p>
+
+<p><i>Pallor.</i>&mdash;Under this heading, which includes all cases where the normal
+healthy green colour is replaced by a general sickly yellow or pale hue,
+ultimately resulting in death of the parts if not arrested, we have
+several totally distinct diseases of the chlorophyll apparatus, each
+recognised by the co-existence of other subordinate symptoms. The
+principal varieties of pallor usually met with are the following:</p>
+
+<p><span class="pagenum"><a name="Page_180" id="Page_180"></a>[<a href="./images/180.png">180</a>]</span><i>Etiolation</i> is due to insufficient intensity of light, the pale sickly
+yellow organs being unusually watery and deficient in vascular tissue,
+the internodes abnormally long and thin, and the leaves generally
+reduced in size, or, in some plants also "drawn."</p>
+
+<p>Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of
+etiolation purposely induced. The want of light causes the true
+chlorophyll colouring matter to remain in abeyance, and consequently the
+plant as a whole suffers from carbohydrate starvation.</p>
+
+<p><i>Laying</i> of Wheat and other cereals is a particular case of etiolation.
+The seeds having been sown too thickly, the bases of the haulms, owing
+to the etiolation and consequent lack of carbohydrates, suffer from want
+of stiffening tissues, and the top-heavy plants fall over.</p>
+
+<p><i>False etiolation</i> depends on a similar abeyance of the chlorophyll, but
+in this case due to too low a temperature. It is often seen in Wheat and
+other monocotyledons when the young leaves unfold in cold weather in
+spring. The symptoms of "drawing" and tenderness are however absent.</p>
+
+<p>Pallor due to too intense illumination must be kept sharply distinct
+from etiolation, the pale green or yellow hue being here due to the
+destruction of the chlorophyll by insolation, and the accessory symptoms
+of "drawing" are wanting.</p>
+
+<p><i>Chlorosis</i> is a form of pallor where the chlorophyll grains themselves
+are fully developed, but their green pigment remains in abeyance owing
+to a deficiency of iron in the soil, and can often be <span class="pagenum"><a name="Page_181" id="Page_181"></a>[<a href="./images/181.png">181</a>]</span>cured by adding
+traces of a ferrous salt. The distinction between <i>Icterus</i>, where the
+organs are only yellow, and <i>Chlorosis</i> proper, where they are nearly
+white cannot always be maintained. In the typical case only those organs
+whose cells are still young can become green on adding iron.</p>
+
+<p><i>Yellowing</i> or <i>False Chlorosis</i> may be experimentally induced by too
+much carbon-dioxide in the atmosphere. It also often ensues when the
+roots of plants in the open are waterlogged, owing to the stagnant water
+not only driving air from the root-hairs but accumulating dissolved
+substances which poison the plant. Trees frequently thus suffer from
+"wet feet" when their roots have penetrated down to a sodden impervious
+subsoil.</p>
+
+<p><i>Yellowing</i> accompanied by <i>Wilting</i> is a predominant symptom in most
+cases where transpiration is more active than root-absorption beyond a
+certain limit, as is well known in cases of prolonged drought. It may
+also be caused in evergreens by the foliage transpiring actively in
+bright January weather, for instance, while the ground is frozen and the
+chilled root-hairs cannot absorb.</p>
+
+<p>In other cases similar appearances are traceable to insects devouring
+the roots, <i>e.g.</i> wireworms, and the malady is sometimes enhanced by
+their accumulations so fouling the wet soil that the roots die off,
+owing to want of oxygen and to the excess of carbon-dioxide and
+poisonous matters.</p>
+
+<p>Yellowing may also result from the presence of poisonous or acid gases
+in the atmosphere or soil, such as chlorine, hydrochloric acid,
+sulphurous <span class="pagenum"><a name="Page_182" id="Page_182"></a>[<a href="./images/182.png">182</a>]</span>acid, etc., in the neighbourhood of chemical works, or from
+the escape of coal-gas in streets, etc., points of importance in
+connection with the use of fungicides and insecticides.</p>
+
+<p>Yellowness is the prevailing symptom in many cases of fungus attack of
+the roots or collar of the plant, the resulting stoppage of
+transpiration being also sometimes supplemented by rotting of the roots,
+and the consequent deprival of oxygen and accumulation of foul gases. In
+other cases Fungi, and even Bacteria, have been found to have made their
+way into the principal vessels, the lumina of which they stop up, thus
+reducing the transpiration current.</p>
+
+<p>Certain insects may also induce a general yellowing and wilting of
+plants by entering or destroying the tissues concerned in the
+transpiration&mdash;<i>e.g.</i> <i>Oniscus</i>, the Frit Fly, and <i>Cecidomya</i>, the
+Hessian Fly, which attack young winter wheat within the sheaths and
+cause the plants to turn yellow and wilt.</p>
+
+<p><i>Albinism</i> and <i>Variegation</i> are apparently due to causes totally
+different from any yet mentioned. Church's analyses have shown that
+albino leaves contain more water and less organic matter than green ones
+of the same plants, but not necessarily less ash constituents. The
+composition of the ash points to there being more potash and less lime
+in the white organs than in the green ones, and, speaking generally, the
+former are related to the latter much as young leaves are related to
+mature ones.</p>
+
+<p><span class="pagenum"><a name="Page_183" id="Page_183"></a>[<a href="./images/183.png">183</a>]</span>The whole matter is complicated by the behaviour of certain <i>variegated</i>
+plants&mdash;<i>e.g.</i> Ribbon grass, <i>Calla</i>, <i>Abutilon</i>, which are usually
+regarded as partial albinos.</p>
+
+<p>Meyen showed long ago that such variegated plants, if grafted on green
+ones, may induce the development of variegated leaves on both scion and
+stock, and Morren and others have not only confirmed this but have also
+shown that variegation may be inherited through the seed. Nevertheless
+some care has to be taken with many of these variegations lest rich
+soil, bright light, and other favourable treatment favour the
+restitution of the green colour. These facts may be interpreted in
+various ways. Some disturbance of physiological functions of the roots,
+due to unfavourable conditions of soil, may be the cause; but Beijerinck
+has lately published some results which show that some of these albino
+diseases can be induced by inoculating normal plants with the juice of
+spotted ones even though such juice has been filtered through porcelain,
+and concludes that a "<i>contagium fluidum vivum</i>" of the nature of a
+transmissible enzyme is the agent which disturbs the physiology of the
+infected cells.</p>
+
+<p>Koning, while confirming these results in the main, refers them to a
+micro-organism so small that it traverses the porcelain filter.</p>
+
+<p><i>Upheaval of seedlings.</i>&mdash;This is a common form of injury, resulting in
+death by drought and exposure, especially in seedling pines, wheat,
+etc., in soils exposed to alternate freezing and thawing <span class="pagenum"><a name="Page_184" id="Page_184"></a>[<a href="./images/184.png">184</a>]</span>during spring
+when there is no snow to protect the plants. The soil freezes during the
+night, and during the thaw next day water accumulates just below the
+surface. The freezing is then repeated, and, partly owing to the
+expansion of the forming ice and partly to the mechanical effect of the
+ice-crystals in the interstices, the surface of the soil is lifted and
+draws the roots with it. During the succeeding thaw the soil particles
+fall away from the lifted root-fibres, and frequent repetition of these
+processes results in such complete exposure of the roots to the full sun
+that the plantlet falls over and wilts.</p>
+
+<p><i>Exposure of roots</i> is also sometimes effected by winds displacing sandy
+soils liable to shifting in dry weather, and the resulting wilting of
+the plants thus exposed at their roots may be supplemented by damage due
+to the repeated impact of the wind-driven sharp grains of sand, which
+act like a sand-blast and erode the tissues.</p>
+
+<p>In many of the cases given above the principal result is the weakening
+or destruction of the chlorophyll action. This means a loss of
+carbohydrates&mdash;sugars, starches, etc.&mdash;and in so far a starvation of the
+plant. The injurious effects are quantitative and cumulative: if large
+areas of foliage are concerned, or if the effect lasts a long time, the
+plant suffers from loss of food, and may die. In those cases where the
+effect is due to the cutting off of supplies at the roots, and where the
+yellowing is a secondary symptom, the disease is more general in
+character, and recovery is often <span class="pagenum"><a name="Page_185" id="Page_185"></a>[<a href="./images/185.png">185</a>]</span>impossible, because the loss of water
+cannot be compensated, and the results may be further complicated by the
+gradual penetration of poisonous matter into the cells. It is frequently
+necessary, though sometimes very difficult, to decide which is the
+primary and which secondary (or tertiary, etc.) symptoms in the order of
+their importance, and the diagnosis may be complicated by a number of
+accessory factors which it is impossible to treat generally.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XIX.</span></h4>
+
+<div class="chnote">
+<p>The principal cases here described are dealt with in works on
+plant physiology, and in the works of Sorauer and Frank
+already referred to.</p>
+
+<p>As regards damage due to uprooting of seedlings by frost, see
+Fisher, "Forest Protection" (Engl. ed. of Hess' <span lang="de" xml:lang="de"><i>Forstchutz</i></span>),
+in Schlich's <i>Manual of Forestry</i>, Vol. IV., 1895, pp.
+439-442.</p>
+
+<p>On Albinism, see Church, "A Chemical Study of Vegetable
+Albinism," <i>Journ. Chem. Soc.</i>, 1879, 1880, 1886.</p>
+
+<p>Beijerinck's results are contained in his paper, "<span lang="de" xml:lang="de">Ueber ein
+Contagium vivum fluidum</span>," etc. (with English abstract), in
+<span lang="de" xml:lang="de"><i>Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam</i></span>, 1898.
+Koning's paper is in <span lang="de" xml:lang="de"><i>Zeitschr. f. Pflanzenkrank.</i></span>, Vol. IX.,
+1899, p. 65. See also <i>Nature</i>, Oct. 11, 1900, p. 576.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_186" id="Page_186"></a>[<a href="./images/186.png">186</a>]</span></p>
+<h2>CHAPTER XX.</h2>
+
+<h3>SYMPTOMS OF DISEASE (<i>Continued</i>).</h3>
+
+<div class="chsub">
+<p>Spotted leaves&mdash;The colours of spots&mdash;White, yellow, brown,
+and black spots on leaves&mdash;Parti-coloured spots&mdash;The browning,
+etc., of leaves.</p>
+</div>
+
+
+<p><i>Discoloured spots</i> or patches on the herbaceous parts of plants,
+especially leaves, furnish the prominent symptoms in a large class of
+diseases, due to many different causes, and although we cannot maintain
+this group of symptoms sharply apart from the last, as seen from the
+considerations on <i>albinism</i>, it is often well marked and of great
+diagnostic value. By far the greater number of spot-diseases are due to
+fungi, but this is by no means always the case. The most generally
+useful method of subdividing the classes, though artificial like all
+such classifications, will be according to the colour of the spots or
+flecks, which, moreover, are usually found on the leaves. It is
+necessary to note, however, that various conditions may modify the
+colour of spots on leaves. Many <span class="pagenum"><a name="Page_187" id="Page_187"></a>[<a href="./images/187.png">187</a>]</span>fungi, for instance, induce different
+coloured spots according to the age of the leaf or other organ attacked,
+or according to the species of host, the weather, etc. Moreover the
+spots due to these parasites are frequently yellow when young and some
+other colour, especially brown or black, when older.</p>
+
+<p><i>Scale</i> is the name given to the characteristic shield-like insects
+(<i>Mytilaspis</i>, <i>Aspidiotus</i>, etc.) which attach themselves to branches
+of Apples, Pears, Oranges, Camellias, and numerous other plants, and
+suck the juices. It is the female insect which has the body broadened
+out into the "scale," under which the young are brought up. Enormous
+damage has been done by some forms&mdash;<i>e.g.</i> the San José scale in the
+United States.</p>
+
+<p>The superficial resemblances of the patches of eggs of some Lepidoptera
+to Aecidia and other fungi may be noted in passing&mdash;<i>e.g.</i> <i>Bombyx
+neustria</i> on Apple twigs, <i>Aporia Crataegi</i>.</p>
+
+<p><i>White</i> or <i>greyish spots</i> are the common symptom marking the presence
+of many Peronosporeae and Erysipheae in or on leaves, <i>e.g.</i>
+<i>Peronospora Trifoliorum</i>, <i>P. parasitica</i> on Crucifers, etc., and
+<i>Sphaerotheca</i> on Hops; also <i>Septoria piricola</i>, <i>Cystopus</i>, <i>Entyloma
+Ranunculi</i>, etc.</p>
+
+<p>White spots are also caused by insects such as <i>Tetranychus</i> (red
+spider) on Clover and other plants.</p>
+
+<p><i>Yellow</i>, or <i>Orange-coloured Spots</i>. In cases where these occur on
+leaves, and in the case of grasses, etc., on the leaf sheaths as well,
+they <span class="pagenum"><a name="Page_188" id="Page_188"></a>[<a href="./images/188.png">188</a>]</span>commonly indicate the presence of Uredineae, and sections under
+the microscope will show the mycelium in the tissues beneath. Species of
+<i>Uromyces</i>, <i>Puccinia</i>, etc., in the Uredo state have the spots powdery
+with spores; <i>Aecidia</i> show the characteristic "cluster cups," and so
+forth. These spots are often slightly pustular, and in some cases
+markedly so.</p>
+
+<p>Other fungi also induce yellow spots on leaves&mdash;<i>e.g.</i> <i>Phyllosticta</i> on
+Beans, <i>Exoascus</i> on Poplars, <i>Clasterosporium</i> on Apricot leaves,
+<i>Synchytrium Succisae</i> on <i>Centaurea</i>, etc.</p>
+
+<p>Yellow spots are also a frequent symptom of the presence of Aphides, of
+Red Spider, etc. Thus the minute golden yellow spots sometimes crowded
+on Oak leaves are due to <i>Phylloxera</i> punctures.</p>
+
+<p>Yellow patches are formed on the large leaves of <i>Arisarum</i> by a species
+of parasitic Alga, <i>Phyllosiphon</i>, which lives in the mesophyll. Many
+tropical leaves are spotted yellow by epiphytic Algae&mdash;<i>e.g.</i>
+<i>Cephaleuros</i>.</p>
+
+<p>It must be noticed that many fungi produce yellow spots or flecks in the
+earlier stages, which turn brown or black as the fructifications appear,
+<i>e.g.</i> <i>Dilophia graminis</i>, <i>Rhytisma acerinum</i>.</p>
+
+<p>The yellow-spotted leaves of <i>Farfugium grande</i> (<i>Senecio Kaempferi</i>)
+are so like those of <i>Petasites</i> attacked with <i>Aecidium</i> in its early
+stages, that an expert might be deceived until the microscopic analysis
+was completed.</p>
+
+<p><i>Red spots</i>, varying from rusty or foxy red to bright crimson, are the
+symptomatic accompaniment <span class="pagenum"><a name="Page_189" id="Page_189"></a>[<a href="./images/189.png">189</a>]</span>of several fungi, the former often
+characterising the teleutospore or aecidium stage of Uredineae&mdash;<i>e.g.</i>
+<i>Aecidium Grossulariae</i>&mdash;the latter sometimes indicating the presence of
+Chytridiaceae.</p>
+
+<p>Red spots are also caused by <i>Gloeosporium Fragariae</i> on Strawberry
+leaves, <i>Polystigma rubrum</i> on Plums.</p>
+
+<p>Crimson spots on Apple and Pear leaves are also due to <i>Phytoptus</i>: they
+turn brown later.</p>
+
+<p><i>Brown spots</i> or flecks, varying in hue from dull slaty brown to deep
+red browns, are a common symptom of Fungus and Insect diseases, the
+colour often indicating the death of the tissues, rather than any
+special peculiarity of the action of the parasite. Good examples are
+furnished by the Potato-disease, and by <i>Peronospora viticola</i>,
+<i>Sphaerella vitis</i> and other disease-fungi of the Grape Vine. The
+teleutospore stage of many Uredineae also occurs in deep brown spots.</p>
+
+<p>Black spots and flecks are exceedingly common symptoms of the presence
+of fungi, <i>e.g.</i> <i>Fusicladium</i> on Apples and Pears, and the pycnidial
+and ascus stages of many Ascomycetes&mdash;<i>e.g.</i> <i>Phyllachora graminis</i>. The
+teleutospore stages of species of <i>Puccinia</i>, <i>Phragmidium</i>, etc., are
+also so deep in colour as to appear almost black.</p>
+
+<p><i>Scab</i> on Pears is due to the presence of <i>Fusicladium</i>, which indurates
+the outer skin of the fruit causing it to crack under pressure from
+within, and to dry up, the deep brown to black patches of fungus
+persisting on the dead surface.</p>
+
+<p>Black spots on grasses and sedges are caused <span class="pagenum"><a name="Page_190" id="Page_190"></a>[<a href="./images/190.png">190</a>]</span>by Ustilagineae, and are
+commonest in the grain, the soot-like powdery spores (Smut) being very
+characteristic. <i>Ustilago longissima</i> induces black streaks on the
+leaves. Many of these fungi cause distortions or pustules on leaves and
+other organs.</p>
+
+<p>Brown and black leaf spots are frequently furnished with concentric
+contours arranged round a paler or other coloured central point&mdash;<i>e.g.</i>
+<i>Cercospora</i> on Beans, <i>Ascochyta</i> on Peas.</p>
+
+<p>Brown spots with bright red margins are formed in young Beans by
+<i>Gloeosporium</i>.</p>
+
+<p>Species of <i>Fumago</i>, <i>Herpotrichia</i>, etc., may cover the entire surface
+of the leaf with sooty patches, or even weave the leaves together as if
+with black spider-webs.</p>
+
+<p><i>Mal nero</i> of the Vine is a particular case of black spotting and
+streaking of the leaves for which no satisfactory explanation is as yet
+to hand. As with Chestnuts, Walnuts, and other plants containing much
+tannin, the dark spots appear to be due to this substance, but whether
+the predisposing cause is a lack of some ingredients in the soil, or
+some temperature reaction, or fungi at the roots, is as yet unknown. The
+most recent explanation puts the disease down to the action of bacteria,
+but the results obtained by different workers lead to uncertainty.</p>
+
+<p>The "dying back" of leaves, especially of grasses, from the tip, is
+usually accompanied by a succession of colours&mdash;yellow, red, brown, to
+black&mdash;and is a common symptom of parching from summer drought; and
+spots of similar <span class="pagenum"><a name="Page_191" id="Page_191"></a>[<a href="./images/191.png">191</a>]</span>colours, frequently commencing at the margins of
+leaves, are characteristic symptoms of the injurious action of acid
+gases in the air.</p>
+
+<p>Brown and blackish spots on Pears are caused by a species of <i>Thrips</i>.</p>
+
+<p>In many cases the minute spots of Rust-fungi on one and the same leaf
+are bright orange yellow (<i>uredo</i>), deep brown, or almost purple-black
+(<i>teleutospores</i>), foxy-red brown (older uredospores), or dead slaty
+black where the old teleutospores have died off&mdash;<i>e.g.</i> <i>Uromyces Fabae</i>
+on Beans, <i>U. Pisi</i> on Peas, etc.</p>
+
+<p><i>Parti-coloured leaves.</i>&mdash;The leaves sometimes start shrivelling with
+red edges, while yellow, red, and finally brown and black blotches
+appear on the lamina, from no known cause&mdash;<i>e.g.</i> Vines. In other cases
+similar mimicry of the autumnal colouring of leaves results from the
+action of acid gases.</p>
+
+<p><i>Burning</i> is a common name for all cases where the leaves turn red or
+red-brown in hot, dry weather, and many varieties are distinguished in
+different countries and on different plants, because species react
+dissimilarly. The primary cause is usually want of water&mdash;drought.</p>
+
+<p><i>Foxy leaves</i> are a common sign of drought on hot soils, and the disease
+may usually be recognised by the gradual extension of the drying and
+fox-red colour proceeding from the older to the younger leaves, and from
+base to apex&mdash;<i>e.g.</i> Hops.</p>
+
+<p><i>Coppery leaves.</i>&mdash;The leaves of the Hop, etc., may show yellow spots
+and gradually turn <span class="pagenum"><a name="Page_192" id="Page_192"></a>[<a href="./images/192.png">192</a>]</span>red-brown&mdash;copper-coloured&mdash;as they dry; the damage
+is due to <i>Tetranychus</i>, the so-called Red Spider. These cases must of
+course be carefully distinguished from the normal copper-brown of
+certain varieties of Beech, Beet, <i>Coleus</i>, etc.</p>
+
+<p><i>Silver-leaf.</i>&mdash;The leaves of Plum, Apple, and other fruit trees often
+obtain a peculiar silvery appearance in hot summers, the cause of which
+is unknown.</p>
+
+<p>Discolorations in the form of confluent yellow and orange patches, etc.,
+resembling variegations, are not infrequently due to the ravages of Red
+Spider and mites&mdash;<i>e.g.</i> on Kidney Beans.</p>
+
+<p><i>Sun-spots.</i>&mdash;Yellow spots, which may turn brown or black according to
+the species of plant affected and the intensity of the action, are often
+caused by the focussing of the solar rays by lens-like thickenings due
+to inequalities in the glass of greenhouses, or by drops of water on
+them or on other leaves, <i>e.g.</i> Palms, <i>Dracaena</i>, etc. The action is
+that of a burning glass, and extends throughout the leaf-tissues. Young
+grapes, etc., may also be injured in this way. Water-drops on the glass
+can only act long enough to produce such injuries if the atmosphere is
+saturated. The old idea that a drop on a leaf can thus focus the sun's
+rays into the tissues beneath is not tenable.</p>
+
+<p>Here again we see that the disease-agencies concerned in producing the
+symptoms described in this chapter, agree for the most part in so far
+that the principal effect is generally the disturbance of chlorophyll
+action in the spots or flecks on <span class="pagenum"><a name="Page_193" id="Page_193"></a>[<a href="./images/193.png">193</a>]</span>the leaves, and the rendering useless
+of these areas so far as providing further food-supplies is concerned.
+The effects may be due merely to the shading action of a
+parasite&mdash;<i>e.g.</i> epiphytic fungi&mdash;or to actual destruction of the
+tissues invaded&mdash;<i>e.g.</i> by endophytic fungi&mdash;or the tissues may be
+burnt, poisoned, etc. In so far the results are again quantitative and
+cumulative, and the amount of damage depends on the number and size of
+the spots or other areas affected, and the proportion of foliage
+involved, as well as the length of time the injurious action is at work.
+But, again, it must be remembered that several symptoms may co-exist,
+and matters may be complicated by the spread of the destructive agent,
+or its consequences, to other parts, and in some cases we are quite
+uninformed as to the true nature of the disease.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XX.</span></h4>
+
+<div class="chnote">
+<p>Further information regarding these "leaf-diseases" will be
+found in special works dealing with the fungi and insects
+which cause them. In addition to works already quoted, the
+reader may also be referred for Fungi to Massee, <i>A Textbook
+of Plant-diseases caused by Cryptogamic Parasites</i>, London,
+1899; or Prillieux, <span lang="fr" xml:lang="fr"><i>Les Maladies des Plantes Agricoles</i></span>,
+1895. See also Marshall Ward, Coffee-leaf Disease, <i>Sessional
+Papers</i>, XVII., Ceylon, 1881, and <i>Journ. Linn. Soc.</i>, Vol.
+XIX., 1882, p. 299.</p>
+
+<p>The question of "Sun-spots" has been dealt with by Jönnson in
+<span lang="de" xml:lang="de"><i>Zeitschr. f. Pflanzenkrankh.</i></span>, 1892, p. 358.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_194" id="Page_194"></a>[<a href="./images/194.png">194</a>]</span></p>
+<h2>CHAPTER XXI.</h2>
+
+<h3>ARTIFICIAL WOUNDS.</h3>
+
+<div class="chsub">
+<p>The nature of wounds and of healing processes&mdash;Knife
+wounds&mdash;Simple cuts&mdash;Stripping&mdash;Cuttings&mdash;Branch-stumps and
+pruning&mdash;Stool-stumps&mdash;Ringing&mdash;Bruises.</p>
+</div>
+
+
+<p><i>Wounds.</i>&mdash;All the parts of plants are exposed to the danger of wounds,
+from mechanical causes such as wind, falling stones or trees, hail,
+etc., or from the bites of animals such as rabbits, worms, and insects,
+and although such injuries are rarely in themselves dangerous, they open
+the way to other agencies&mdash;water, fungi, etc., which may work great
+havoc; or the loss of the destroyed or removed tissues is felt in
+diminished nutrition, restriction of the assimilative area, or in some
+other way.</p>
+
+<p>We have seen that living cells die when cut, bruised, or torn; and that
+the cells next below in a layer of active tissue are stimulated by the
+exposure to increased growth and division, and at once <span class="pagenum"><a name="Page_195" id="Page_195"></a>[<a href="./images/195.png">195</a>]</span>produce a layer
+of cork, the impervious walls of which again protect the living cells
+beneath. This is found to occur in all cell-tissues provided the cells
+are still living, and it matters not whether the wound occurs in the
+mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the
+cortex of a root or stem, or in the fleshy parts of a young fruit, the
+normal effect of the wound is in all cases to call forth an elongation
+of the uninjured cells beneath, in a direction at right angles to the
+plane of the injured surface, which cells then divide by successive
+walls across their axis of growth: the layers of cells thus cut off are
+then converted into cork, by the suberisation of their walls. Further
+changes may then go on beneath the protective layer of wound-cork thus
+produced, and these changes vary according to the nature of the cells
+beneath: the cambium forms new wood, the medullary rays similar rays,
+cortex new cortex, and so on.</p>
+
+<p><i>Knife-wounds.</i>&mdash;Artificial cuts in stems are easily recognised and soon
+heal up unless disturbed. Several cases, differing in complexity, are to
+be distinguished. The simplest is that of a longitudinal, oblique, or
+horizontal short cut in which the point of the knife severs all the
+tissues of the stem down to the wood. The first effect usually observed
+is that the wound gapes, especially if longitudinal, because the cortex,
+tightly stretched on the wood cylinder, contracts elastically. This
+exposes the living cortex, phloem and cambium to the air, and such
+tissues at once behave as already described above: the cells actually
+cut die, <span class="pagenum"><a name="Page_196" id="Page_196"></a>[<a href="./images/196.png">196</a>]</span>those next below grow out under the released pressure, and
+these give rise to cells which become cork. As the growth and
+cell-division continue in the cells below this thin elastic cork-layer,
+they form a soft herbaceous cushion or <i>callus</i> looking like a thickened
+lip to each margin of the cut. Each lip soon meets its opposite
+neighbour, and the wound is closed over, a slight projection with a
+median axial depression alone appearing on the surface. The depression
+contains the trapped-in callus-cork squeezed more and more in the plane
+of the cut as the two lips of callus press one against the other, and
+sections across the stem and perpendicular to the axis of the cut show
+that this thin cork, like a bit of brown paper, alone intervenes between
+the cambium, phloem and cortex respectively of each lip, as each layer
+attempts to bridge over the interval. If the healing proceeds normally,
+these layers, each pressing against the trapped cork-film, and growing
+more and more in thickness, shear the cork-layer and tear its cells
+asunder, and very soon we find odd cells of the cambium of one lip
+meeting cambium cells of the other, phloem meeting phloem, and cortex
+cortex, and the normal thickening of the now fused layers previously
+separated by the knife goes on as if nothing had happened, the only
+external sign of the wound being a slight ridge-like elevation, and,
+internally, traces of the dead cells and cork trapped here and there
+beneath the ridge. When the conjoined cambium resumes the development
+<span class="pagenum"><a name="Page_197" id="Page_197"></a>[<a href="./images/197.png">197</a>]</span>of a continuous layer of xylem and phloem, no further trace of the
+injury is observable, unless a speck of dead cells remains buried
+beneath the new wood, and indicates the line where the knife point
+killed the former cambium and scored the surface of the wood in making
+the wound.</p>
+
+<p><i>Stripping.</i>&mdash;Now suppose that, instead of a mere slit with the
+knife-point, a strip of bark is removed down to the wood. Exactly the
+same processes of corking and lip-like callus formation at the edges of
+the wound occur, but of course the occlusion of the bared wood-surface
+by the meeting of the lips occupies a longer time. Moreover, the living
+cells of the medullary rays exposed by the wound on the wood-surface
+also grow out under the released pressure, and form protruding callus
+pads on their own account. In course of time the wood is again
+completely covered by the coming together over its face of these various
+strips of callus, but two important points of difference are found, as
+contrasted with the simpler healing of the slit-wound. In the first
+place the exposed wood dries and turns brown, or it may even begin to
+decay if moisture and putrefactive organisms act on it while exposed to
+the air; and, in the second place, the normal annual layer of wood&mdash;or
+layers, as the case may be&mdash;formed by the cambium only extends over that
+part of the stem where the cambium is still intact, and is entirely
+wanting over the exposed area. Thus, if it takes two years for the
+cambium to extend <span class="pagenum"><a name="Page_198" id="Page_198"></a>[<a href="./images/198.png">198</a>]</span>across the wound, a layer of wood will be formed all
+round the intact part of the stem, from lip to lip of the cut tissues
+during the first year; then a second annual layer outside this will be
+formed during the second year, but extending further over the edges of
+the wound, and nearly complete, because the cambium has now crept
+further across the wounded surface to meet the opposite lip of cambium;
+and during the third year, when the cambium has once more become
+continuous over the face of the wound, the annual wood layer will be
+complete. But, of course, this last layer covers in the edges of the two
+previously developed incomplete wood-layers as well as the exposed and
+brown, dry, or rotten dead face of the wood. It also covers up the
+trapped-in brown cork and any débris that accumulated in the wound, and
+this "blemish," though buried deeper and deeper in the wood during
+succeeding annual deposits of wood-layers, always remains to remind us
+of the existence of the wound, the date of which can be fixed at any
+future time by counting the annual rings developed subsequently to its
+formation. Obviously, also, the deficiency of wood at this place makes
+itself visible on the outside by a depression.</p>
+
+<p><i>Cuttings.</i>&mdash;When a cutting of <i>Pelargonium</i>, Willow, or other plant is
+made, we have a typical knife-wound, the behaviour of which is very
+instructive in illustration of plant-surgery, and may be most easily
+seen by keeping it in damp air instead of plunging it into sand or
+soil.</p>
+
+<p><span class="pagenum"><a name="Page_199" id="Page_199"></a>[<a href="./images/199.png">199</a>]</span>All the living cells actually cut or bruised turn brown and die as
+before; those beneath&mdash;<i>e.g.</i> the living pith, medullary rays, cambium,
+phloem, and cortex, grow out under the released pressure and form a
+callus, the outermost layer of which becomes cork, while those below,
+abundantly supplied with food-materials, proceed to spread, as if
+flowing over the surface of the cut wood, and rapidly occlude the wound.
+Meanwhile new roots are formed adventitiously from the cambium just
+above the plane of section, and push out through the cortex into the
+damp air, and if the cutting had been in soil it would now be capable of
+independent existence. It is important to keep cuttings upright, as the
+roots only spring from the lower end. Such cuttings can be obtained not
+only from stems, but also from roots and even leaves.</p>
+
+<p>Callus-formation is not confined to the basal end of a cutting; it has
+nothing to do with position, but is a reaction to the wound stimuli,
+independent of light, gravitation, etc. As time goes on, however, the
+internal organisation of the erect cutting usually reacts on the callus
+at either end, and roots only rise from the lower one, while shoot-buds
+may form in the upper one, though it is possible to bring about the
+formation of buds from the lower end also.</p>
+
+<p><i>Branch stumps.</i>&mdash;A more complex example is furnished by a branch cut
+off short some distance&mdash;say a foot&mdash;from the base, where it springs
+from the trunk. As before, the immediate effect of the <span class="pagenum"><a name="Page_200" id="Page_200"></a>[<a href="./images/200.png">200</a>]</span>section is the
+formation of a callus from the cambium, phloem and cortex, which begins
+to rise as a circular occluding rim round the wood. The transpiration
+current in the trunk, however, is not deflected into the 12 inches or so
+of amputated branch, because there are no leaves to draw the water up
+it, and so the stump dries up and the cortex and cambium die back to the
+base, leaving the dead wood covered with shrivelled cortical tissues
+only. This dead stump gradually rots under the action of wet, fungi, and
+bacteria, and since the pith and heart-wood afford a ready passage of
+the rot-organisms and their products into the heart of the trunk, we
+find in a few years a mere stump of touch-wood and decayed bark, which
+falls out at the insertion like a decayed tooth, leaving a rotten hole
+in the side of the trunk.</p>
+
+<p>If, however, instead of allowing the basal part of the amputated branch
+to protrude as a stump, we cut it off close to the stem, and shave the
+section flush with the normal surface of the latter, the callus formed
+by the cambium, etc., rapidly grows over the surface, and soon forms a
+layer of cambium continuous with that of the rest of the stem. The wound
+heals, in fact, much as if it were a strip-wound, and beyond a slight
+prominence for a year or two no signs are visible from the outside after
+the occlusion. Of course these matters depend on the relative thickness
+of branch and stem, and if much wood is exposed the dangers of rot and a
+resulting hollow in the stem are increased. It is interesting to note
+how much thicker the callus lips are at the <span class="pagenum"><a name="Page_201" id="Page_201"></a>[<a href="./images/201.png">201</a>]</span>sides of the wound than
+above and below, owing to differences in the distribution of the
+nutrient materials.</p>
+
+<p><i>Stool-stumps.</i>&mdash;When a tree is felled, the stump may, if the section is
+close to the ground and kept moist, begin to form a thick rim-like
+callus round the wood, in which adventitious buds soon make their
+appearance, and grow out into so-called <i>Stool-shoots</i>. The products of
+assimilation of these, and the stores accumulated in the stump, often
+suffice to feed the callus sufficiently to enable it to grow over and
+completely occlude the wound, if the wood surface is not too large, or
+so long exposed that rotting processes have meanwhile set in.</p>
+
+<p><i>Ringing.</i>&mdash;If the strip of cortical tissues and cambium is removed all
+round the stem, exposing the wood in a form of a ring, complications may
+ensue owing to the following circumstances. A well-marked callus appears
+at the upper edge of the wound, because, the transpiration current up
+the young wood not being stopped, plenty of water and salts from the
+soil can reach the leaves; but the nutritive materials supplied by the
+latter are accumulated at the upper lip of the wound owing to the
+stoppage there of their descent in the phloem, cortex, etc. No such
+callus-lip appears at the lower margin of the wound owing to want of
+these supplies. Consequently the occlusion and healing of the ring-wound
+only takes place from above downwards, and if the ring of cortical
+tissues removed is a broad one, the healing may be a long process, or
+may even be <span class="pagenum"><a name="Page_202" id="Page_202"></a>[<a href="./images/202.png">202</a>]</span>indefinitely delayed, a thicker and thicker callus
+projecting over from above. For similar reasons no annual wood layers
+are formed below, but only above the wound, and thus the branch or tree
+may die. The latter contingency is the more likely the further up the
+tree the ringing takes place, owing to the risk of drying up which
+threatens the exposed wood, and to the consequent interruption of the
+transpiration current, and the likelihood that lateral shoots below the
+wound may divert the water to their own leaves. If the ringing occurs
+low down on a stem, and the environment remains damp, the upper thick
+callus may put out new roots; the part above the wound then behaves like
+a cutting. If the ringing is done on a young and vigorous branch of an
+old tree, the lower lip may receive supplies from the leaves of branches
+below the wound, or from shoots which spring from adventitious buds
+close to it, and the wound may heal over normally. Such healing may be
+rendered more certain by keeping the wounded surface moist&mdash;<i>e.g.</i> by
+means of damp moss, and so encouraging the formation of callus-bridges
+from the medullary rays.</p>
+
+<p>If on ringing a tree or a branch the young wood is removed as well as
+the cambium and cortical layers, the death of the parts above the wound
+is almost certain, owing to the stoppage of the transpiration current:
+the exceptions to this rule depend simply on the existence of other
+channels of communication, such as internal phloems, very thick
+sap-wood, and so forth.</p>
+
+<p><span class="pagenum"><a name="Page_203" id="Page_203"></a>[<a href="./images/203.png">203</a>]</span><i>Bruises.</i>&mdash;If a branch or woody stem is struck sharply, with a hammer,
+for instance, the bruised cortex, phloem and cambium are killed by the
+blow, and the general effect is as if these tissues had been removed at
+that spot by the knife, but with the following complications. The
+bruised cortical tissues rapidly dry as they perish, and may adhere to
+the wood below. Consequently the still sound parts bordering on the
+wound are not released from pressure, but, on the contrary, have to
+advance towards each other over the surface of the wood under still
+greater pressures, in part due to the tightening of the whole cortex as
+the dead parts dry and contract, and in part due to the above-mentioned
+adherence of the latter to the wood. It results from this that such
+wounds heal very slowly and badly, and when the killed patch at last
+ruptures, wound-fungi, insects, and other injurious agencies may get in
+and do irreparable damage, as has been found to occur in cases where
+such wounds have been made in striking trees to shake down insects,
+fruit, etc.</p>
+
+
+<h4><span class="smcap"><a name="Chapter_XXI_Notes" id="Chapter_XXI_Notes"></a>Notes to Chapter XXI.</span></h4>
+
+<div class="chnote">
+<p>The essential facts regarding wounds and healing by occlusion
+are given in Marshall Ward, <i>Timber and some of its Diseases</i>,
+1889, chapters viii. and ix., and in Laslett, <i>Timber and
+Timber Trees</i>, 1894, chapters iv. and v. More detailed
+treatment will be found in Frank, <span lang="de" xml:lang="de"><i>Krankh. d. Pflanzen</i></span>, B. 1.
+cap. 2, where the special literature is collected. The reader
+may also consult Hartig, <i>Diseases of Trees</i>, Engl. ed. 1894,
+pp. 225-269.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_204" id="Page_204"></a>[<a href="./images/204.png">204</a>]</span></p>
+<h2>CHAPTER XXII.</h2>
+
+<h3>NATURAL WOUNDS.</h3>
+
+<div class="chsub">
+<p>Burrows and excavations. Bark-boring&mdash;Wood-boring&mdash;Wood
+fungi&mdash;Leaf-miners&mdash;Pith flecks&mdash;Erosions. Skeleton
+leaves&mdash;Irregular erosions&mdash;Shot holes. Frost
+cracks&mdash;Strangulations&mdash;Spiral grooving.</p></div>
+
+
+<p>Natural wounds are produced in a variety of ways during the life of the
+plant, and, generally speaking, are easily healed over by the normal
+process if the area destroyed is not too large, and the parts remaining
+uninjured are sufficiently provided with foliage, or with supplies of
+food-materials stored up in the roots, rhizomes, medullary rays, etc.,
+to feed a vigorous callus.</p>
+
+<p>The nature of such wounds and the mode of healing are explained by what
+we know of artificial wounds, and it only remains to point out that the
+principal danger of ordinary wounds is not so much the direct traumatic
+action, because the simpler organisation of the plant does not involve
+matters connected with shock, loss of <span class="pagenum"><a name="Page_205" id="Page_205"></a>[<a href="./images/205.png">205</a>]</span>blood, etc., as in animals; the
+danger consists, rather, in their affording access to other injurious
+agents, especially fungi, and the treatment of wounds frequently
+resolves itself into cutting or pruning in order to get clean surfaces
+which can heal readily.</p>
+
+<p>Wounds on leaves imply loss of foliar surface&mdash;<i>i.e.</i> of chlorophyll
+action&mdash;and the remarks on <a href="#Page_193">page 193</a> apply.</p>
+
+<p><i>Burrows</i> may be taken as comprising all kinds of tunnel-like
+excavations in the various organs of plants, including those cases where
+insects burrow into hollow stems of grasses, etc., as indicated by the
+perforations they make in the outer tissues.</p>
+
+<p><i>Bark-boring</i> is done by many species of beetles, especially
+<i>Scolytidae</i>, which excavate characteristically formed branching
+passages tangentially in the inner bark of Conifers and other trees.
+Some of them also bore down to the surface of the sap wood (<i>e.g.</i>
+<i>Tomicus bidentatus</i>) or even burrow right into the latter (<i>e.g.</i> <i>T.
+lineatum</i>). It commonly happens that the external apertures show up
+clearly, owing to the brown dust and excrement, sometimes accompanied by
+turpentine, which exude from them. Many of these Bark beetles only
+attack trees which are already injured by fire, lightning, etc.;
+possibly they cannot bore through a cortex which swamps them with sap,
+as a vigorous one might do.</p>
+
+<p><i>Wood-boring</i> is also done by many of the bark-beetles as well as by
+Longicorns, <i>e.g.</i> <i>Saperda</i> in <span class="pagenum"><a name="Page_206" id="Page_206"></a>[<a href="./images/206.png">206</a>]</span>Poplars and Willows, the young shoots
+of which often show characteristic swellings with lateral holes
+indicating the points of exit. From the external apertures comminuted
+wood, like saw-dust, is frequently ejected in quantity and betrays the
+presence of the insects. Certain wood-wasps (<i>Sirex</i>) and the larvae of
+moths (<i>Cossus</i>) also make large perforations in the wood of Willows and
+other trees, often destroying it completely. In the case of these larger
+borers, whose tunnels may be as broad as the little finger, the foul
+smell as well as abundant "saw-dust" betray the evil.</p>
+
+<p>Excavations in wood are by no means caused only by insects: several of
+the larger Hymenomycetes&mdash;<i>Stereum</i>, <i>Thelephora</i>, <i>Polyporus</i>,
+etc.&mdash;tunnel the timber in characteristic ways and often after a fashion
+very suggestive of insects. They usually obtain access through
+fractures.</p>
+
+<p><i>Tunnels</i> in leaves are invariably due to the activity of miners
+belonging to the smaller moths and beetles&mdash;<i>e.g.</i> <i>Tinea</i>, <i>Orchestes</i>,
+etc.&mdash;the larvae of which eat out the mesophyll but leave the covering
+epidermis or cuticle untouched, and since the insect bores forwards
+only, in an irregular track, and leaves its excrement in the winding
+passage, the effect is very characteristic.</p>
+
+<p>Whitish leaf tunnels in Peas are excavated by <i>Phytomyza</i>.</p>
+
+<p>Characteristic foxy-red tunnels are mined in the leaves of Apples by
+<i>Lyonetia</i>, <i>Coleophora</i>, etc.</p>
+
+<p><i>Falling of fruit</i>, of Apples, Plums, Apricots, etc., before they are
+ripe, is frequently due to insects, of <span class="pagenum"><a name="Page_207" id="Page_207"></a>[<a href="./images/207.png">207</a>]</span>which the various species of
+<i>Grapholitha</i> or <i>Carpocapsa</i> are conspicuous: the fallen fruits show a
+small hole leading by a labyrinth of passages to the "core" or "stone,"
+and in which the grub and its excrement are visible. The cutting off of
+the vascular bundles and disturbance of the water supply only partly
+explain the premature fall.</p>
+
+<p><i>Pith-flecks</i> are minute brown specks or patches found in the
+wood-layers of many trees, and consist of dead parenchymatous
+thick-walled cells, reminding one of the structure of pith. They are
+explained as due to the borings of minute insects, <i>Diptera</i> or Beetles,
+the larvae of which pierce the cortex and phloem and bore their way into
+the cambium. The latter then occludes the tunnels by filling them up
+with cells, and continuing its wood-forming activity gradually buries
+them deeper and deeper in the wood. Such pith-flecks are common in
+Willow, Birch, Alder, <i>Sorbus</i>, etc. It is possible that they may be due
+to other causes also in other trees.</p>
+
+<p><i>Erosions</i> or <i>irregular wounds</i> on leaves are caused by large numbers
+of grubs and caterpillars and other insects, such as earwigs, as well as
+slugs, snails, and other animals; but it must by no means be assumed
+that all marginal leaf wounds, for instance, are caused by animals,
+since many fungi which rot the tissues, as explained below (<a href="#fungi_shot_hole">p. 208</a>),
+also cause such erosions, the putrescent parts falling out&mdash;<i>e.g.</i> the
+Potato disease.</p>
+
+<p><i>Skeleton leaves</i> frequently result from the <span class="pagenum"><a name="Page_208" id="Page_208"></a>[<a href="./images/208.png">208</a>]</span>ravages of caterpillars,
+which leave the coarser ribs and veins untouched, but much finer
+skeletons with the minute veins almost intact may be found on plants
+infested with certain insects&mdash;<i>e.g.</i> <i>Selandria</i> on Cherries.
+Skeletonised patches on Cherry leaves, often pink or brown-pink, are
+eaten out by this grub.</p>
+
+<p><i>Shot-holes</i> are perforations in leaves presenting the appearance, from
+their more or less rounded shape, of gunshot wounds. They may be due to
+insects which bore through the young leaves while still folded in the
+bud&mdash;<i>e.g.</i> Willow Beetle&mdash;or which gnaw out the tissue&mdash;<i>e.g.</i> the
+Beech Miner. Similar but usually more torn and irregular holes are eaten
+out by many caterpillars&mdash;<i>e.g.</i> the Cabbage Moth.</p>
+
+<p>Shot-holes on Peas may be the work of Thrips.</p>
+
+<p>Leaf perforations are commonly caused by severe hail-storms, the
+hail-stones beating right through the thin mesophyll. Certain chemicals
+used for spraying have also been known to cause shot-holes by killing
+the tissue beneath the standing drops.</p>
+
+<p><a name="fungi_shot_hole" id="fungi_shot_hole"></a>There is, however, a class of shot-holes in thin leaves which are due to
+the action of minute fungi, the mycelium of which so rots the tissues in
+a more or less circular area round the point of infection, that, in wet
+weather, the decomposing mass falls out and leaves a round hole&mdash;<i>e.g.</i>
+certain Chytridiaceae, Peronosporeae, <i>Gloeosporium</i>, <i>Exoascus</i>, etc.
+If dry weather supervenes these holes frequently dry at the edges, and
+the leaves appear as if eaten out.</p>
+
+<p><span class="pagenum"><a name="Page_209" id="Page_209"></a>[<a href="./images/209.png">209</a>]</span>Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to
+<i>Phyllosticta</i>, in Cherry leaves also to <i>Clasterosporium</i>, and in
+Potato leaves to <i>Haltica</i>.</p>
+
+<p><i>Frost-cracks.</i>&mdash;The trunks of trees exposed to the north-east, and
+occasionally with other aspects, are apt to show longitudinal ridges
+which realise on a larger scale the features of healed wounds scored
+with a knife. These wounds are due to the outer layers of wood losing
+water from their cell-walls as it congeals to ice in their lumina, more
+rapidly than do the warmer internal parts of the trunk; as this drying
+of the wood causes its shrinkage, especially in the tangential
+direction, the effect of a sudden frost and north-east wind is to rend
+the wood, which splits longitudinally with a loud report, as may often
+be heard in severe winters. Since the cortex and bark are ruptured at
+the same time the total effect resembles that of a deep knife-cut, and
+the same healing processes result on a larger scale when the wood swells
+and closes up the wound again in spring. But this recently-closed lesion
+is evidently a plane of weakness, and if a similarly severe winter
+follows the wound reopens and again heals, and so on, until after a
+succession of years a prominent <i>Frost-ridge</i> results, which may finally
+heal completely if milder winters ensue or the tree be eventually
+protected.</p>
+
+<p><i>Strangulations.</i>&mdash;We are now in a position to understand the so-called
+strangulations which result when woody climbers, telegraph wires, etc.,
+<span class="pagenum"><a name="Page_210" id="Page_210"></a>[<a href="./images/210.png">210</a>]</span>kill or injure trees by tightly winding round them. If strong wire is
+twisted horizontally round a stem, the growth in thickness of the latter
+causes the trapping of the cortex and cambium, etc., between the wire
+and the wood, and a ringing process is set up in consequence of the
+death of the compressed tissues. A callus then forms above the wound, as
+in the case of true ringing by means of a cut, and eventually bulges
+over the upper side of the wire: in the course of years this overgrowth
+may completely cover in the wire, and, pressing on to the lower lip of
+the wound, may at length fuse with the cambium below. Hereafter the
+thickening rings of wood are continuous over the buried wire. The
+process is obstructed by all the impediments referred to in dealing with
+ringing, and of course the stem thickens more above than below the wire.
+If the sapwood is thin, and the bark is so thick as to put great
+obstacles in the way of the junction of the upper and lower cambiums,
+death may result&mdash;the tree is permanently ringed. (See <a href="#Page_201">p. 201</a>.)</p>
+
+<p><i>Spiral grooves</i> are frequently met with where Wood-bine or other woody
+climbers have twined round a young stem or branch, the upper lip of the
+groove always protruding more than the lower. If a kink or a crossing of
+two plants or branches of the twiner results in a complete horizontal
+ring, the results are as in the above cases of ringing and
+strangulation. Naturally grooved walking sticks are often seen.</p>
+
+<p><span class="pagenum"><a name="Page_211" id="Page_211"></a>[<a href="./images/211.png">211</a>]</span><i>Buried letters, etc.</i>&mdash;These processes of healing by occlusion enable
+us to understand how letters of the alphabet, cut into the wood of
+trees, come to be buried deep in the timber as successive annual rings
+cover them in more and more. Chains, nails, rope, etc., have frequently
+been found thus buried in wood.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXII.</span></h4>
+
+<div class="chnote">
+<p>In addition to the <a href="#Chapter_XXI_Notes">notes</a> to the last chapter, the reader may
+be referred to Fisher in Vol. IV. of Schlich's <i>Manual of
+Forestry</i>, Chap. VI., for an account of Hess' excellent work
+on Boring Beetles, etc.</p>
+
+<p>The authority on Wood-fungi is Hartig, see especially his
+<span lang="de" xml:lang="de"><i>Zersetzungs-erscheinungen des Holzes</i></span>, the principal results
+of which are condensed in his <i>Diseases of Trees</i> already
+referred to. As regards "Pith-flecks," the reader should
+consult Frank, <span lang="de" xml:lang="de"><i>Krankh. der Pflanzen</i></span>, B. I., p. 212: the
+subject needs further investigation.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_212" id="Page_212"></a>[<a href="./images/212.png">212</a>]</span></p>
+<h2>CHAPTER XXIII.</h2>
+
+<h3>EXCRESCENCES.</h3>
+
+<div class="chsub">
+<p>Herbaceous excrescences, or galls&mdash;Erineum&mdash;Intumescences&mdash;Corky warts,
+etc.&mdash;Pustules&mdash;Frost-blisters&mdash;Galls and Cecidia&mdash;Root
+nodules.</p>
+</div>
+
+
+<p><i>Excrescences</i>, or out-growths of more or less abnormal character from
+the general surface of diseased organs, are very common symptoms, and
+widely recognised. They are due to hypertrophy of the tissues while the
+cells are young and capable of growth, and may be induced by a variety
+of causes, among which the stimulus of insect-punctures and of the
+presence of insect eggs are best known; but that of fungi, though less
+widely recognised, plays an equally important part, and, as we shall
+see, galls and other excrescences may be due to widely different agents.</p>
+
+<p><i>Galls</i> or <i>Cecidia</i> are protuberances of the most varied shapes,
+colours, and sizes found on herbaceous parts attacked by insects, fungi,
+etc. In the simplest cases the insects only pierce and <span class="pagenum"><a name="Page_213" id="Page_213"></a>[<a href="./images/213.png">213</a>]</span>suck the young
+cellular tissue&mdash;<i>e.g.</i> <i>Phytoptus</i>, Aphides, etc.&mdash;but in others the
+stimulus to hypertrophy starts by the puncture of the embryonic tissue
+of a leaf, root, etc., by the ovipositor of the female insect, which
+then lays an egg&mdash;<i>e.g.</i> <i>Cynips</i>, <i>Cecidomyia</i>, etc.&mdash;the presence of
+which appears to intensify the irritating action, or such only occurs
+when the young larva escapes.</p>
+
+<p>Our knowledge of the primary cause of gall-formation amounts to very
+little. Generally speaking, only embryonic or very young cellular tissue
+reacts, and galls on adult leaves and branches have usually been
+initiated long before. The same gall-insect may induce totally different
+galls on different plants, or even on different parts of the same plant,
+and different insects call forth different galls on any one plant. These
+facts point clearly to the co-operation of both plant and insect in the
+gall-formation, and the best hypothesis yet to hand is to the effect
+that a gall is a hypertrophy of cells, the normal nutrition, growth, and
+division of which have been disturbed owing to the action of some poison
+or other irritant derived from the insect, or fungus, or other organism.
+Attempts have been made to reproduce galls by injecting the juices of
+similar galls into the tissue, but as yet without success, and this may
+point to the co-operation of mechanical irritation during the
+hypertrophy in normal gall-formation.</p>
+
+<p>Galls, in the broad sense, are not always preceded by a wound, however.
+Insects on the outside of young tissues may cause such irritations <span class="pagenum"><a name="Page_214" id="Page_214"></a>[<a href="./images/214.png">214</a>]</span>that
+the parts in contact with the animal are arrested in their growth, while
+those further away grow more rapidly&mdash;<i>e.g.</i> where Mites, etc., cause
+puckers and leaf-rolling. In true galls the hypertrophy may consist
+merely in the enlargement of cells already present, and no new
+cell-divisions and, still less, changes in the nature of the tissues
+result&mdash;<i>e.g.</i> some pocket galls on <i>Viburnum</i>, <i>Pyrus</i>, etc., and the
+hairy outgrowths of the epidermis known as <i>Erineum</i>. In other cases
+there is not only hypertrophy of existing cells, but new cell-divisions
+are instituted: these cell-divisions may be confined to the direction
+perpendicular to the epidermis, and the tissues grow only in the
+direction of the surface, producing puckerings&mdash;<i>e.g.</i> the Aphis galls
+on <i>Ribes</i>, Phytoptus galls of <i>Salvia</i>, leaf galls on <i>Tilia</i>, <i>Acer</i>,
+<i>Alnus</i>, etc., and the curious galls on Plums due to <i>Cecidomyia Pruni</i>,
+and which must not be confounded with the "pocket plums" and similar
+galls due to Exoasci.</p>
+
+<p>In a third series of cases, cell-divisions occur parallel to the surface
+of the leaf, and galls are formed which grow in thickness, and develop
+the most extraordinary and complicated new tissues&mdash;proteid-cells
+surrounding the egg or larva deposited inside, followed by a protective
+layer of sclerenchyma encasing this food layer, and around this again
+softer tissues which may assume the structures and functions of
+respiratory tissues, water-storing tissues, starch reservoirs,
+assimilatory, or protective tissues of various kinds, <span class="pagenum"><a name="Page_215" id="Page_215"></a>[<a href="./images/215.png">215</a>]</span>and over all may
+be a well-marked epidermis, with stomata, or cork with lenticels.</p>
+
+<p>The chief seat of these hypertrophies and&mdash;what is more
+remarkable&mdash;development of new tissue elements not found elsewhere in
+the leaves, or even in the species, is the mesophyll, and various
+speculations and hypothesis have been founded on these curious
+phenomena.</p>
+
+<p><i>Erineum.</i>&mdash;The simplest excrescences on plants are certain hair-like
+developments of epidermal cells due to the irritation of species of
+<i>Phytoptus</i>, and similar insects which rise in clusters on the surfaces
+of leaves and by their colours, consistence, arrangement in patches,
+spots, etc., so simulate fungi that Persoon was deceived by them and
+gave them the genus name <i>Erineum</i>. They occur on most of our trees,
+<i>e.g.</i> Poplar, Lime, Oak, and are very common in the Tropics. Usually
+pale or even white at first, they turn brown as the hair-like outgrowths
+die and lose their sap, but since the latter may be bright
+coloured&mdash;yellow, red, purple,&mdash;the patches are sometimes very
+conspicuous objects on smooth leaves.</p>
+
+<p>In many cases these hairs exactly resemble in shape and other characters
+the abnormal root-hairs found on roots exposed to the effects of
+poisonous reagents, or of unsuitable food-materials, or the rhizoids
+developed from wounded Algae, etc.</p>
+
+<p><i>Intumescences</i> are similar trichomatous outgrowths not associated with
+insects or fungi, and due to some disturbance of the balance <span class="pagenum"><a name="Page_216" id="Page_216"></a>[<a href="./images/216.png">216</a>]</span>between
+transpiratory and assimilatory functions of their leaves, as indicated
+by the less localised occurrence and by their non-appearance when the
+plant is under favourable cultural conditions. Structures not unlike
+these have been artificially induced by exposure to particular lights,
+and also by painting spots with dilute corrosive sublimate, indicating
+that poisons may impel the epidermis cells to grow out abnormally.</p>
+
+<p><i>Corky warts.</i>&mdash;Several forms of disease are known in which the
+pathological condition is expressed by the formation of cork in unwonted
+places and quantities. The <i>Scab</i> or <i>Scurf</i> of Potatoes is a case in
+point. The tissue of the lenticels absorbs water and the outermost cells
+are cut off by cork and die: the cells below them burst the dead
+bark-like masses thus formed, and again cork is formed and cuts off the
+outer masses, and the rough cork warts&mdash;<i>Scab</i> or <i>Scurf</i>&mdash;are the
+result.</p>
+
+<p>The causes predisposing to scab have been variously assigned to
+dampness, want of lime, action of bacteria and fungi&mdash;<i>e.g.</i>
+<i>Sorosporium</i>, <i>Oospora</i>, <i>Spongospora</i>,&mdash;the latter making their way
+into the ruptured tissue of the lenticels and irritating the cells to
+further growth.</p>
+
+<p>It seems probable that several different kinds of scab exist in
+Potatoes, as well as in roots&mdash;<i>e.g.</i> Beets, and the whole subject needs
+further investigation. The scab-like rough scaly bark of Pear trees in
+dry districts may also be mentioned here.</p>
+
+<p><span class="pagenum"><a name="Page_217" id="Page_217"></a>[<a href="./images/217.png">217</a>]</span><i>Cork-wings</i> are well known on the young branches of Elms, Maples, etc.,
+some varieties of which have received specific names on this account.</p>
+
+<p><i>Corky excrescences</i> on leaves occur occasionally in the Gooseberry,
+Holly and other plants, for which no cause has been discovered.</p>
+
+<p>Lenticels are also formed on some leaf-galls, and are remarkable as
+being structures not normal on leaves.</p>
+
+<p><i>Pustules.</i>&mdash;This term may be employed generally for all slight
+upheavals of the surfaces of herbaceous organs, which subsequently burst
+and give egress to the spores, etc., of the organism causing them, or
+merely fray away at the top if no organism is discoverable. They are
+often due to fungi&mdash;<i>e.g.</i> <i>Synchytrium</i>, <i>Protomyces</i>, <i>Cystopus</i>, and
+Ustilagineae,&mdash;and we may extend the use of the general term also to
+those cases where the <i>stroma</i> of the fungus itself bursts through the
+cortex of older parts and forms the principal part of the
+pustule&mdash;<i>e.g.</i> <i>Monilia</i>, forming white or grey pustules on Apples,
+<i>Roestelia</i> and other Æcidia, forming yellow or orange pustules on
+leaves, etc.; <i>Cucurbitaria</i> and <i>Nectria</i> (red) breaking through the
+cortex of trees, and <i>Phoma</i> and numerous other Ascomycetes which form
+black cushions. <i>Pustules</i> on the leaves of <i>Lysimachia</i>, <i>Ajuga</i>, etc.,
+are due to the parasitic Alga <i>Phyllobium</i>.</p>
+
+<p>Cylindrical stem swellings are caused by <i>Calyptospora</i>: they are due to
+the hypertrophy of the cortex of Bilberry stems permeated by <span class="pagenum"><a name="Page_218" id="Page_218"></a>[<a href="./images/218.png">218</a>]</span>the
+hyphae. <i>Epichloë</i>, which clothes the sheaths and halms of grasses with
+its stroma, at first snowy white and later ochre-yellow as the
+perithecia form, is another example.</p>
+
+<p>The cylindrical layer of eggs of a moth such as <i>Bombyx</i> on a twig must
+not be confounded with these cases.</p>
+
+<p><i>Frost-blisters</i> are pustule-like uprisings of the cortex, where the
+living tissues below have formed a callus-like cushion into the cavity
+beneath the dead outer parts of the cortex which were killed by the
+frost; they occur on the stems of young Apples, Pears, etc.</p>
+
+<p><i>Galls</i> in the narrower sense are tissue outgrowths usually involving
+deeper cell-layers. They are so varied and numerous that classification
+is difficult. For symptomatic purposes we may divide them as follows:</p>
+
+<p><i>Leaf-galls.</i>&mdash;A well-marked type is that of the <i>pocket-galls</i> or
+<i>bladders</i> in which the whole thickness of the leaf is as it were pushed
+up like a glove-finger at one spot, so that if the upper surface of the
+leaf forms the outside of the gall the lower surface is its lining. Such
+galls are common on Limes (<i>Phytoptus</i>), <i>Glechoma</i> (<i>Cecidomyia</i>), Elms
+(<i>Tetraneura</i>), etc. Similar localised extension of the leaf surface,
+compelling it to rise up like a pocket, are caused by fungi&mdash;<i>e.g.</i>
+<i>Taphrina</i> on Poplars, <i>Exoascus</i> on Birches, etc., <i>Exobasidium</i> on
+Bilberries, Rhododendrons, etc.</p>
+
+<p>Another type is that of the <i>Gall-apple</i>, so well known on Oaks, where
+the spherical swelling is <span class="pagenum"><a name="Page_219" id="Page_219"></a>[<a href="./images/219.png">219</a>]</span>solid&mdash;except for the inner cavity containing
+the eggs&mdash;<i>Neurotus</i>, <i>Cynips</i>, <i>Hormomyia</i>, etc. These are comparable
+in general characters to the nodules on roots.</p>
+
+<p>Fungus galls with similar external features when young are found on
+Maize (<i>Ustilago Maydis</i>), and betray their nature by the black powdery
+spores as they mature.</p>
+
+<p>Bud galls on Willows are due to <i>Cecidomyia</i>, which causes several
+internodes to swell out into a greenish barrel-shaped mass, from which
+leaves may spring.</p>
+
+<p>Small irregular excrescences on Willow stems are referred to
+<i>Phytoptus</i>, and another species of the same insect induces similar
+swellings on Pines which are not surcharged with resin.</p>
+
+<p><i>American Blight</i>, or Woolly Aphis, on Apples especially, causes the
+tumour-like swellings covered with sticky white fluff, which is a waxy
+excretion of the insect. Galls on <i>Pilea</i>, in Java, are due to an
+Alga&mdash;<i>Phytophysa</i>.</p>
+
+<p><i>Root-nodules</i> or <i>nodosities</i> are frequently caused by insects&mdash;<i>e.g.</i>
+<i>Centhorhynchus</i>, a beetle which attacks Crucifers, <i>Cynips</i> and allied
+"gallflies" of Oaks, and the notorious <i>Phylloxera</i>. But similar
+root-galls are produced by Nematode worms, <i>Heterodora</i>, on Beets,
+Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus
+<i>Plasmodiophora</i>, and it is not always easy to distinguish such cases
+from the fungus-galls (<i>Mycocecidia</i>) on the roots of Alders, <i>Juncus</i>,
+and Leguminoseae where the symbiosis of bacteria or fungi with <span class="pagenum"><a name="Page_220" id="Page_220"></a>[<a href="./images/220.png">220</a>]</span>the
+roots are of benefit to the plant. <i>Urocystis Leimbachii</i> forms similar
+nodules at the collar of young plants of <i>Adonis</i>.</p>
+
+<p><i>Heterodora javanica</i> passes into the cortex of sugar-cane roots through
+fissures, and makes its way to the place where a young rootlet is about
+to emerge; here it sticks its beak into the growing-point and remains
+fixed.</p>
+
+<p>Molliard has shown that in the roots of Melons, <i>Coleus</i>, etc.,
+<i>Heterodora</i> causes the cells in immediate contact with its head, and
+which would normally become vessels of the xylem, to swell up into huge
+giant-cells, with their walls curiously folded, and containing large
+supplies of proteids and numerous nuclei, reminding us of the food-layer
+of insect galls and of the tapetal layer of pollen-sacs. While the
+stimulus exerted by the Nematode thus induces hypertrophy and storage
+with food-substances of these cells, those of the next layers undergo
+reticulate thickenings of their walls. Again instances of the evolution
+of new tissue elements by the action of the foreign organism.</p>
+
+<p>So far as galls on leaves are concerned the amount and kind of damage
+done are in proportion to the area of chlorophyll action put out of play
+for the benefit of the plant, and the remarks already made on <a href="#Page_193">p. 193</a>
+apply here also. Where buds are destroyed the effects may of course
+extend further, but it rarely happens that leaf-galls are so abundant as
+to maim a tree permanently. Nevertheless we must remember that cases
+like <i>Phylloxera</i> are notorious.</p>
+
+<p><span class="pagenum"><a name="Page_221" id="Page_221"></a>[<a href="./images/221.png">221</a>]</span>Far more dangerous, however, are the root-galls due to such insects,
+because here the damage is not so local: the water-supplies are cut off,
+and injurious consequences result from the absorption of the products of
+decomposition in the soil.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXIII.</span></h4>
+
+<div class="chnote">
+<p>In addition to the literature on galls quoted in the <a href="#Chapter_XIV_Notes">Notes to
+Chapter XIV.</a>, the reader should consult Dale "On certain
+Outgrowths (Intumescences) on the green parts of Hibiscus,"
+<i>Proc. Cambr. Phil. Soc.</i>, Vol. X., 1899, p. 192, and <i>Brit.
+Ass. Rep.</i>, Bradford, 1900.</p>
+
+<p>The detailed study of the anatomy and histology of Galls has
+been recently undertaken by Küster, "<span lang="de" xml:lang="de"><i>Beitrage zur Kenntniss
+der Gallenanatomie</i></span>," Flora, B. 87, 1900, p. 117, where the
+principal references will be found.</p>
+
+<p>On the root-galls due to Nematodes see Atkinson in <i>Science
+Contributions from the Agric. Expt. Station, Alabama</i>, Vol.
+I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in
+<i>Natural Science</i>, Vol. VI., 1895, p. 187; and Molliard in
+<span lang="fr" xml:lang="fr"><i>Revue générale de Botanique</i></span>, Apl., 1900, p. 157, where the
+histology is dealt with.</p>
+
+<p>The nodules of the roots of Leguminoseae are not part of the
+subject of this work: the literature is collected in <i>Science
+Progress</i>, 1895, Vol. III., p. 252, and Dawson, <i>Phil.
+Trans.</i>, 1900.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_222" id="Page_222"></a>[<a href="./images/222.png">222</a>]</span></p>
+<h2>CHAPTER XXIV.</h2>
+
+<h3>EXCRESCENCES (<i>continued</i>).</h3>
+
+<div class="chsub">
+<p>Cankers&mdash;Burrs&mdash;Sphaeroblasts, and other excrescences of
+woody tissues&mdash;Witches' Brooms.</p>
+</div>
+
+
+<p><i>Cankers</i> are irregular excrescences due to the perennial struggle
+between tissues attempting to heal up a wound, and some organism or
+other agent which keeps the lesion open. A canker always originates in a
+wound affecting the cambium, and usually in a small wound such as an
+insect puncture or frost nip; if undisturbed the dead parts would heal
+over by cork and callus, but if recurring frost-cracks break open the
+coverings, or if insects or fungi penetrate the callus and invade the
+cambium, irregularities of growth due to the occluding tissue on the one
+hand, and continued growth of the still unimpaired cambium on the
+opposite side of the injured shoot on the other, result in the canker.
+Frost cankers occur on fruit-trees, Vines, Beeches, etc.</p>
+
+<p><span class="pagenum"><a name="Page_223" id="Page_223"></a>[<a href="./images/223.png">223</a>]</span>Cankers due to insects are found on Apples, the cortex of which is
+punctured by the woolly Aphis (<i>Schizoneura</i>) while the twigs are young,
+and the wound is kept open by the insects nestling in crevices in the
+occlusion tissues. Species of <i>Coccus</i>, <i>Lachnus</i>, and <i>Chermes</i> also
+produce cankers on forest trees.</p>
+
+<p>Cankers due to fungi usually originate in a wound primarily due to an
+insect puncture or bite, or to frost, the invading fungus hyphae making
+their way into the wounded tissues and gradually extending more and more
+into the cambium and the occluding callus. Among the best known of these
+wound fungi which cause cankers are <i>Dasyscypha Willkommii</i> the peziza
+of Larch disease, <i>Nectria ditissima</i> and <i>N. cucurbitula</i> on Beech and
+Conifers; less common are <i>Scleroderris</i> on Willows, <i>Aglaospora</i> on
+Oaks and some others.</p>
+
+<p><i>Peridermium Pini</i> and <i>Aecidium elatinum</i> also cause cankers under
+certain conditions, as also does <i>Gymnosporangium</i>, but in these cases
+the fungi are more truly parasitic.</p>
+
+<p>In some cases&mdash;<i>e.g.</i> Ash, Pine, Olives&mdash;bacteria are concerned as
+associated organisms in the cankering of trees.</p>
+
+<p><i>Burrs</i> or <i>Knauers</i> are irregular excrescences, principally woody, with
+gnarled and warted surfaces. They are frequently due to some previous
+injury, such as the crushing or grazing of cortical tissues by
+cart-wheels. The excitation of the tissues thus wounded results in the
+development <span class="pagenum"><a name="Page_224" id="Page_224"></a>[<a href="./images/224.png">224</a>]</span>of shoots from adventitious or dormant buds at the base of
+old tree trunks, or in the starting of the same process where a branch
+has been broken off. The new bud begins to develop a shoot, but soon
+dies at its tip owing to paucity of food-supplies to the weak shoot,
+while new buds at its base repeat the process next year with the same
+result, and each of these again in turn, and so on. The consequence is
+an extremely complex nest of buds, all capable of growing in thickness
+and putting on wood to some extent, but not of growing out in length. In
+course of time this mass may attain dimensions measurable by feet,
+forming huge rounded and extremely hard-knotted burrs, the cross-section
+of which shows the vascular tissues running irregularly in all
+directions, and, owing to the very slow growth, extremely dense and
+hard. The dark spots in such sections&mdash;<i>e.g.</i> Bird's-eye Maple&mdash;are the
+cut bud-axes all fused together, as it were. On old Elms such burrs are
+common at heights on the stem which preclude the assumption of any
+coarse mechanical injury, and similar structures occur on the boles of
+other forest trees suddenly exposed to light by the felling of their
+companions, which suggests that these epicormic shoots result from some
+disturbance due to the action of light.</p>
+
+<p><i>Witches' Brooms</i> are irregular tufts of twigs often found among the
+branches of trees such as Birches, Hornbeam, etc., where they look like
+crows' nests, and similar structures are to be found on Silver Firs and
+other conifers. In the former case they <span class="pagenum"><a name="Page_225" id="Page_225"></a>[<a href="./images/225.png">225</a>]</span>are due to <i>Exoascus</i>, in the
+latter to <i>Aecidium</i>, fungi which are perennially parasitic in the
+shoots, and stimulate the twiggy development of a number of buds which
+would normally have remained in abeyance, or not have been formed at
+all, and only do so now in a fashion different from that of normal
+branches.</p>
+
+<p>Rosette-like formations, depending on similar disturbing causes on the
+part of insects, occur in conifers&mdash;<i>e.g.</i> <i>Gastropacha Pini</i>.</p>
+
+<p>Dense tufts of twiggy shoots may be developed on many trees by pruning
+in such a way as to stimulate the shooting out of basal buds which would
+otherwise remain dormant, <i>e.g.</i> Elm, Ash, and thus it occurs that
+injuries such as frost, insect bites, etc., may induce the production of
+such tufts in a tree crown. The dense nests of stool-shoots thrown up
+from felled tree-stumps are of essentially the same nature&mdash;partly
+adventitious and partly dormant buds being enabled to grow out because
+they can now be supplied with materials previously carried beyond them
+while the trunk was still there. Suckers, if repeatedly cut down, may
+also behave similarly.</p>
+
+<p><i>Wood-nodules</i> or <i>Sphaeroblasts</i> are curious marble-like masses of wood
+which protrude with a covering of bark from old trunks of Beeches, etc.,
+and can be readily dug out with a knife. The nodule has arisen by the
+slow growth of the cambium of a dormant bud, the base of which separated
+at an early date from the wood beneath; the cambium then closed in over
+the base and laid on <span class="pagenum"><a name="Page_226" id="Page_226"></a>[<a href="./images/226.png">226</a>]</span>thickening rings all round the axis of the bud
+except at the extreme apex. When the separation occurred the cambium of
+the wood beneath covered over the previous point of junction, and thus
+the woody bud was pushed out with the bark, and now protrudes covered
+with a thin layer of the latter. Similar nodules are occasionally found
+on Apple trees.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXIV.</span></h4>
+
+<div class="chnote">
+<p>For further information on Cankers the student should read
+Marshall Ward, <i>Timber and some of its Diseases</i>, Chapter X.
+Further, the discussion as to the causes of canker in Frank,
+<span lang="de" xml:lang="de"><i>Krankheiten der Pflanzen</i></span>, B. I., p. 207, and B. III., pp.
+167 and 172, and various papers in <span lang="de" xml:lang="de"><i>Zeitschrift für
+Pflanzen-krankheiten</i></span>.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_227" id="Page_227"></a>[<a href="./images/227.png">227</a>]</span></p>
+<h2>CHAPTER XXV.</h2>
+
+<h3>EXUDATIONS AND ROTTING.</h3>
+
+<div class="chsub">
+<p>Tumescence&mdash;Rankness&mdash;Bursting of fruits, etc.&mdash;Root rot&mdash;Rot
+of fruits&mdash;Bulb diseases&mdash;Flux&mdash;Honey-dew&mdash;Slime
+flux&mdash;Resinosis&mdash;Gummosis&mdash;Manna.</p>
+</div>
+
+
+<p>I put together in one artificial class a varied group of diseases, the
+principal symptom of which is the escape of fluids from the tissues,
+under circumstances which betray an abnormal state of affairs, often
+obvious, but sometimes only to be inferred. In many of these cases
+bacteria abound in the putrefying mass, and some evidence exists for
+connecting these microbes causally with the disease in a few of the more
+thoroughly investigated cases, but in no case has this been sufficiently
+demonstrated; and considering the ease with which bacteria gain access
+<i>via</i> wounds caused by insects and fungi, as well as by other agents,
+the necessity for rigid proof must be insisted upon before we can accept
+such alleged examples of <i>Bacteriosis</i>.</p>
+
+<p><span class="pagenum"><a name="Page_228" id="Page_228"></a>[<a href="./images/228.png">228</a>]</span><i>Tumescence.</i>&mdash;It occasionally happens that herbaceous parts of plants
+pass into a condition of over-turgescence from excess of water in the
+tissues, an abnormal state which indicates pathological changes
+resulting from various causes, often not evident and therefore regarded
+as internal. Such disease was formerly termed <i>Å’dema</i> or <i>Dropsy</i>.
+This disease is frequently due to the excessive watering of pot plants
+with large root systems and deficient foliage, in hot-houses with a
+saturated atmosphere: it is, therefore, primarily referable to
+diminished transpiration. It can sometimes be brought about by covering
+potato plants, for instance, with a bell-jar in moist, hot weather; and
+this, and the prevalence of the disease in hot-houses as compared with
+plants grown out of doors, point to the above explanation. Similar
+phenomena do occasionally occur out of doors in hot, moist situations or
+during wet seasons, however, and the watery shoots of rank vegetation
+are merely particular cases of the same class. Moreover, the well-known
+tendency to succulence of sea-side varieties of plants which have thin
+herbaceous leaves when growing inland, points to the action of the
+environment in these matters, excess of salts being no doubt one factor
+in such cases.</p>
+
+<p><i>Rankness</i> affords another example where superfluity of water is
+concerned, though it does not involve simply this, because the plant may
+also contain excessive quantities of nitrogenous and mineral matters
+taken up by the roots.</p>
+
+<p><span class="pagenum"><a name="Page_229" id="Page_229"></a>[<a href="./images/229.png">229</a>]</span>Rankness is, in fact, in many respects analogous to etiolation in so far
+as the tissues are soft and surcharged with water, but it differs
+fundamentally in the deep green of the chlorophyll: this may lead to
+abundant assimilation if free access of air and drier conditions can be
+gradually brought about. Any sudden drying, however, may be fatal to the
+tender tissues.</p>
+
+<p>Rankness commonly depends on excess of food materials, especially
+nitrogenous manures, as may be seen in meadows and cornfields where the
+manure heaps have remained on the ground and saturated it to excess as
+compared with the rest of the soil; this may often be observed with
+weeds, etc., in the neighbourhood of farm-buildings. If the period of
+rank growth is accompanied and followed by days of suitably bright
+sunshine and dry air, the increase of vegetative structures usually
+results in increased flowering, heavy crops, or strong wood; but if the
+rankness continues too long, or is accompanied by wet and dull weather,
+the watery tissues are peculiarly susceptible to attacks of fungi and
+insects, and to damage by sudden frosts or chilly winds. Rankness
+affords, in fact, a typical illustration of predisposition to disease.</p>
+
+<p><i>Damping off.</i>&mdash;When seedlings are too closely crowded in beds kept too
+damp, or in moist weather, they are very apt to rot away, with all the
+symptoms&mdash;spreading from a centre, contagious infection, mycelia on and
+in the tissues, etc.&mdash;of a fungus attack. The commonest agent <span class="pagenum"><a name="Page_230" id="Page_230"></a>[<a href="./images/230.png">230</a>]</span>concerned
+is one of the species of <i>Pythium</i>, the propagation of which is favoured
+by the rank, over-turgid, and etiolated conditions of the plants.
+Species of <i>Mucor</i>, <i>Botrytis</i>, and other fungi, may also be met with.</p>
+
+<p><i>Bursting</i> of fleshy fruits, such as Tomatoes, Grapes, etc., is due to
+over-turgescence in rainy weather or excessively moist air. But the
+phenomenon is by no means confined to such organs. Hot-house plants when
+oedematous not infrequently put out watery blisters from the cortex or
+leaves, which rupture; and the stems of fleshy fasciated (<i>e.g.</i>
+Asparagus) or blanched and forced plants (<i>e.g.</i> Celery, Rhubarb) are
+particularly apt to crack here and there from the pressure of the
+turgescent tissues on the strained epidermis. Beets, Turnips, and other
+fleshy roots show the same phenomena in wet seasons. That these ruptures
+and exposures of watery tissues afford dangerous points of entry for
+parasites and moulds will be obvious&mdash;<i>e.g.</i> <i>Edelfäule</i>, a rotten
+condition of the grapes in the Moselle district.</p>
+
+<p><i>Root-rot</i> is a common disease in damp, sour clay soils after a
+continuance of wet weather&mdash;<i>e.g.</i> Wheat, especially if root-drawn and
+exposed to thaw water.</p>
+
+<p>In the disease known as Beet-rot, the roots turn black at the tip, where
+the tissues shrivel and become grooved and wrinkled extensively. Inside
+the flesh also blackens and finally rots. In earlier stages, only the
+vascular bundles are brown and blocked with gum-like substances. <span class="pagenum"><a name="Page_231" id="Page_231"></a>[<a href="./images/231.png">231</a>]</span>In
+advanced stages there is much gummy material in the lumina, and even
+large cavities filled with this gum may be found.</p>
+
+<p>The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to
+several fungi, of which <i>Botrytis</i>, <i>Monilia</i>, <i>Mucor</i>, <i>Penicillium</i>,
+and <i>Aspergillus</i> are the chief. The fruit may be attacked while still
+on the tree, but very often fungi and bacteria gain access to the
+tissues, through bruises, cracks, etc., formed in the fruit lying in the
+storage baskets or on the shelves.</p>
+
+<p>Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of
+<i>Botrytis</i> or <i>Sclerotinia</i>, followed by moulds, yeasts, and bacteria in
+the stores.</p>
+
+<p><i>Sour-rot</i> in Grapes, and other fleshy fruits which need much sun to
+ripen them, is probably a usual result of continued cold, wet weather at
+the cropping season, setting in when the fruits are beginning to swell.</p>
+
+<p><i>Flux.</i>&mdash;It is a common event to see fluids of various kinds issuing
+from wounds in trees, or congealing in more or less solid masses about
+them; and owing to the prevailing tendency to compare plant diseases
+with those of animals, we find such expressions as <i>Gangrene</i>, <i>Ulcer</i>,
+and so forth, applied to these "open sores." In so far as such
+outflowings frequently indicate diseased states of injured tissues which
+are incapable of healing up, the analogy is perhaps a true one; but it
+must be remembered that very different structures and processes in
+detail are concerned. Moreover, <span class="pagenum"><a name="Page_232" id="Page_232"></a>[<a href="./images/232.png">232</a>]</span>liquid excretions more or less
+indicative of diseased states are by no means confined to wounds or
+definitely injured tissues, in which case such terms are wholly
+misapplied.</p>
+
+<p><i>Honey-dew.</i>&mdash;The leaves, or other organs, of many plants are sticky in
+hot weather, owing to the excretion of a sweet liquid containing sugar,
+the consistency and colour of which vary according to circumstances.
+This honey-dew must not be confounded with the normal viscidity of
+certain insectivorous plants&mdash;<i>e.g.</i> Sundew&mdash;or with the sticky
+secretion on the internodes of species of <i>Lychnis</i>, etc., where it
+plays the part of a protection against minute creeping things.</p>
+
+<p>Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of
+these cases the honey-dew is excreted by Aphides, which suck the juices
+of the leaves and pour out the saccharine liquid from their bodies. The
+sweet fluid is in its turn sought after by ants, and also serves as
+nutritive material for various epiphytic fungi&mdash;<i>e.g.</i> sooty mould,
+<i>Capnodium</i>, <i>Fumago</i>, and <i>Antennaria</i>&mdash;which give the leaves and
+honey-dew a brown or black colour. Certain <i>Coccideae</i> also excrete
+honey-dew, especially in the tropics.</p>
+
+<p>At least one case is known where honey-dew is formed as the result of
+the parasitic action of a fungus, namely <i>Claviceps purpurea</i> in its
+conidial stage on the stigmas of cereals, and this may be compared with
+the sweet odorous fluid excreted by the spermogonia of certain
+<i>Aecidia</i>. In both <span class="pagenum"><a name="Page_233" id="Page_233"></a>[<a href="./images/233.png">233</a>]</span>cases the sweet fluid attracts insects which
+disperse the spores.</p>
+
+<p>Honey-dew may also be formed without the agency of fungi or insects,
+when hot and dry days are followed by cool nights, with a saturated
+atmosphere, <i>e.g.</i> <i>Caesalpinia</i>, <i>Calliandra</i> and other trees in the
+tropics, which are called rain trees owing to the numerous drops of
+fluid which drip from the leaves under the abnormally turgescent
+conditions referred to.</p>
+
+<p><i>Cuckoo-spit.</i>&mdash;The leaves of Willows, Meadow grasses and herbs, etc.,
+are often seen with froth on them, in which is a green insect,
+<i>Aphrophora</i>, which sucks the juices from the tissues and excretes the
+frothy watery cuckoo-spit from its body.</p>
+
+<p><i>Slime-flux.</i>&mdash;The trunks of trees may sometimes be observed to pour out
+a slimy fluid from cracks in the bark, or from old wounds, or branch
+scars. In some cases, <i>e.g.</i> in Oaks, the slime has a beery odour and
+white colour, and abounds in yeasts and other fungi to the fermentative
+activity of which the odour and frothiness are due. In other cases the
+slime is red <i>e.g.</i>&mdash;Hornbeam; or brown&mdash;<i>e.g.</i> Apple and Elm; or
+black&mdash;<i>e.g.</i> Beech, the colour in such cases being due to the mixture
+of yeasts, bacteria, and fungi with which these slimes abound. The
+phenomenon appears to be due to the exudation of large quantities of sap
+under pressure&mdash;root pressure&mdash;and is primarily a normal phenomenon
+comparable to the bleeding of cut trees in spring: the fungi, etc., are
+doubtless <span class="pagenum"><a name="Page_234" id="Page_234"></a>[<a href="./images/234.png">234</a>]</span>saprophytes, but their activity is concerned with the
+putrefactive processes going on in the diseased wood, and which may lead
+to rotting of the timber.</p>
+
+<p>The origin of the wounds in the bark and cortex, and which extend into
+the wood and other tissues as the putrefactive and fermentative
+processes increase, appears to be in some cases at least due to
+lightning.</p>
+
+<p><i>Resin-flux</i> or <i>Resinosis</i>.&mdash;The stems of Pines and other conifers are
+apt to exude resin from any cut or wound made by insects, or by the
+gnawing of other animals; but in many cases the flow is due to fungi,
+<i>e.g.</i> <i>Peridermium</i>, the hyphae of which invade the medullary rays and
+resin canals and thus open the way to an outflow through cracks in the
+bark. <i>Agaricus melleus</i> not only invades the resin passages, but
+stimulates the tree to produce abnormal quantities of resin, which flows
+down to the collar and roots, and exudes in great abundance at the
+surface of the soil. Various other plants also exude resin from wounds,
+and in some cases the flux seems to be increased by degeneration of the
+tissues, <i>e.g.</i> <i>Copaifera</i>.</p>
+
+<p><i>Gummosis.</i>&mdash;Cherries, Apricots, Acacias, and many other trees are apt
+to produce abnormal quantities of gum, which flows from any wound or
+exudes through cracks in the bark. Degeneration of the wood-cells, and
+especially of the cell-walls of a soft wood formed by abnormal activity
+of the cambium, points to its origin being due, in some <span class="pagenum"><a name="Page_235" id="Page_235"></a>[<a href="./images/235.png">235</a>]</span>cases at any
+rate, to a conversion of the cellulose, and fungi are sometimes found in
+the masses of gum; but beyond the fact that <i>gummosis</i> is a pathological
+phenomenon we know very little of the disease.</p>
+
+<p>With regard to such gumming, it is significant how frequently pruned
+trees&mdash;Cherries, Oranges, Lemons, Plums, etc.&mdash;suffer.</p>
+
+<p><i>Manna flux.</i>&mdash;Certain trees, such as the Manna Ash, species of
+Tamarisk, etc., yield manna from wounds, and in some cases the latter
+are due to insects, <i>e.g.</i> <i>Cicada</i>.</p>
+
+<p>The Potato-disease is best known by the pale whitish fringe, giving an
+almost mealy appearance to the margins of the brown to black patches in
+damp weather. In dry weather the brown patches shrivel and dry, and as
+they are apt to be at the edges and tips of the leaflets, these curl up.
+The young disease spots are yellowish, and the leaves of badly affected
+plants are apt to be sickly yellow throughout.</p>
+
+<p>This Potato-disease due to <i>Phytophthora</i> must be distinguished from the
+curling and puckering, with wilting and browning of the leaves and
+yellow glassy look of the stems, due to the invasion of the vessels by a
+fungus which lurks in the tubers, and gains access thence to the shoots.</p>
+
+<p>In the disease traceable to <i>Phytophthora</i> the stock remains green and
+the leaves plump and plane, and only the brown patches slough out in wet
+or shrivel in dry weather, and are bordered by the pale whitish zone of
+conidiophores.</p>
+
+<p><span class="pagenum"><a name="Page_236" id="Page_236"></a>[<a href="./images/236.png">236</a>]</span>In the leaf-curl the yellow and flaccid appearance of all the leaves of
+a stalk, or even of the plant, is the striking symptom, and the stem
+soon droops and blackens just above the soil, a white mould appearing
+also at the black spots. Subsequently black spots appear higher up, and
+bacteria gain an entrance. The stolons rot, and eventually the roots and
+the leaves wither. The tubers appear sound, but are small; they are apt
+to rot in the store, the vascular zones turning brown.</p>
+
+<p>This leaf-curl has been ascribed to <i>Pleospora</i>, <i>Polydesmus</i>,
+<i>Verticillium</i>, and other parasites, as well as to excessive manuring
+and other agencies, but it still needs explanation.</p>
+
+<p>Rot of Potato tubers in the soil, or in store, may be brought about by
+very different agents.</p>
+
+<p>If <i>Phytophthora</i> has obtained access, the fungus hyphae spread between
+the cells, starting from the haulm, and cause the flesh to turn
+yellowish and then brown in patches. On the exterior are discoloured
+patches, depressed, with the flesh beneath brown and soft. The mycelium
+spreads mostly in the outer layers, which though they turn deep brown
+remain firm.</p>
+
+<p>Wet rot of potatoes may be due to various fungi, and, in excess of
+water, to putrefactive bacteria (<i>e.g.</i> <i>Clostridium</i>), which destroy
+the cell-walls. The flesh becomes soft, then soup-like, and finally
+putrefies to a liquid mass with a vile smell of butyric acid, etc., in
+which the starch grains may be seen floating.</p>
+
+<p>Tubers are often found with the cork burst and <span class="pagenum"><a name="Page_237" id="Page_237"></a>[<a href="./images/237.png">237</a>]</span>peeling in shreds, the
+flesh more or less converted into a putrid and stinking pulp, with a
+spotted brown boundary of partly destroyed but firmer tissue between the
+dark utterly rotten and the white and still firm healthy flesh. The
+principal agent in the destruction of the tissues is <i>Clostridium</i>, an
+anaerobic bacillus which consumes the cell-walls but leaves the starch
+intact. Hence a thoroughly decomposed tuber consists of a cork bag full
+of starch and foetid liquid. In the dried condition the flesh shows a
+brown marbling; this passes into a soft soupy starchy part, and here and
+there may be violet grey cavities lined with <i>Spicaria</i>, <i>Hypomyces</i>,
+etc., the white stromata of the latter often appearing externally. The
+excavations are filled with loose starch grains, and bounded by cork and
+cambium formed in the peripheral cells. The cell-walls eventually
+undergo slimy decomposition.</p>
+
+<p><i>Spicaria</i>, <i>Fusisporium</i>, various moulds, and bacteria may all be
+associated with wet-rot.</p>
+
+<p>Dry-rot of Potatoes is also due to various fungi and bacteria, but the
+destructive action goes on slowly, owing to there being no more moisture
+than the tissues afford. The flesh becomes excavated here and there,
+owing to the slow destruction of the cell-walls by <i>Clostridium</i>: the
+destroyed tissues are brown, and the uninjured starch grains powder them
+all over. Finally the whole shrunken mass has a crumbly consistency.</p>
+
+<p>When the flesh remains white, but assumes a powdery consistency and
+dry-rot, with the cork <span class="pagenum"><a name="Page_238" id="Page_238"></a>[<a href="./images/238.png">238</a>]</span>destroyed here and there, Frank refers the
+damage to <i>Phellomyces</i>. Where the dry-rot is due to <i>Fusarium</i> the
+chalk-white stromata may often be detected breaking through the
+periderm; but it must be remembered that the soil-contaminated, broken
+skin of a potato-tuber is a favourable lurking spot for many fungi, and
+<i>Periola</i>, <i>Acrostalagmus</i>, and others have been detected therein.</p>
+
+<p>Brown spots, depressed into the flesh, sometimes result from the ravages
+of <i>Tylenchus</i>, the minute worms being found in the diseased tissues.</p>
+
+<p>In some cases the flesh turns watery and soft, grey, almost glass-like,
+starting at the haulm end, and this may be owing to the invasion of
+<i>Rhizoctonia</i>.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXV.</span></h4>
+
+<div class="chnote">
+<p>The rotting of bulbs, roots, etc., has been much discussed
+during the last few years in the pages of the <i>Gardeners'
+Chronicle</i>, <span lang="de" xml:lang="de"><i>Zeitschrift für Pflanzenkh.</i></span>, and elsewhere. The
+principal references to Bacteriosis&mdash;the rot in which bacteria
+are stated to be the primary agent causing these and similar
+diseases&mdash;may be found in Massee, <i>Diseases of Plants</i>, pp.
+338-342, and more fully in Russell, <i>Bacteria in their
+Relation to Vegetable Tissue</i>, Baltimore, 1892; and in Migula,
+<span lang="de" xml:lang="de"><i>Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche
+Angeblich durch Bakterien verursacht werden</i></span>, Semarang, 1892.</p>
+
+<p>The most convincing accounts, however, are since that date;
+see Smith, "Pseudomonas Campestris," <span lang="de" xml:lang="de"><i>Cent. f. Bakt.</i></span>, B.
+III., 1897, p. 284, and Arthur and Bolley, <i>Bacteriosis of
+Carnations</i>, Perdue University Agr. Expt. Station, 1896, Vol.
+VII., p. 17. Woods has lately shown that this disease is due
+to Aphides only, the bacteria having nothing to do with the
+disease primarily, <i>Stigmonose</i>, <i>Bull. 19</i>, U.S. Dept. <span class="pagenum"><a name="Page_239" id="Page_239" style="font-size: 110%;"></a>[<a href="./images/239.png">239</a>]</span>Agr.,
+1900; but it is necessary to bear in mind that actual
+penetration of the cell-walls from without must be proved, as
+De Bary proved it for germ-tubes of fungi, before the evidence
+that Bacteria are truly parasitic in living plants can be
+called decisive. This is a difficult matter, but until it is
+settled we do not know whether these organisms are really
+parasitic in the sense that <i>Phytophthora</i> is, or merely gain
+access by other means&mdash;I have traced them through dead
+fungus-hyphae&mdash;to the vessels, dead cell-walls, etc. The proof
+of infection <i>via</i> water pores and vessels is given for one
+species by Harding, "Die Schwarze Faulnis der Kohls," etc.,
+<i>Cent. f. Bakt.</i>, Abh. II., B. VI., 1900, p. 305, with
+literature.</p>
+
+<p>Concerning the "Damping off" of seedlings, see Marshall Ward,
+"Observations on the Genus Pythium," <i>Quart. Journ. Microsc.
+Soc.</i>, Vol. XXIII., 1883, p. 485, and Atkinson, <i>Bull. 94 of
+Cornell University Agric. Expt. Station</i>, 1895, p. 233.</p>
+
+<p>On Bacteriosis in Turnips, see Potter, <i>Proc. R. S.</i> 1901,
+Vol. LXVII., p. 442.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_240" id="Page_240"></a>[<a href="./images/240.png">240</a>]</span></p>
+<h2>CHAPTER XXVI.</h2>
+
+<h3>NECROTIC DISEASES.</h3>
+
+<div class="chsub">
+<p>Patches&mdash;Frost-patches&mdash;Bruising due to hail, shot,
+etc.&mdash;Fire&mdash;Sun-burn or scorching&mdash;Sun-cracks.
+Dying-back&mdash;Frost&mdash;Fungi&mdash;Wound fungi&mdash;Defoliation by
+insects&mdash;Defoliation by hand&mdash;Staghead.</p>
+</div>
+
+
+<p><i>Necrosis.</i>&mdash;This is a general term for cases where the tissues
+gradually turn brown or black in patches which die and dry up, the dead
+area sometimes spreading slowly and invading the usually sharply
+demarcated healthy tissues around. It is a common phenomenon on the more
+slender stems or branches of trees, especially those with a thin cortex,
+and the terms <i>Brand</i> or <i>Scorching</i> sometimes applied signify the
+recognised resemblance between burnt patches and these dead areas of
+necrotic tissue.</p>
+
+<p>Necrosis is often due to frost, which kills the cortex of Pears, Beech,
+etc., in patches of this kind. The dead cortex and cambium stick to the
+wood beneath and contract as they dry. The living <span class="pagenum"><a name="Page_241" id="Page_241"></a>[<a href="./images/241.png">241</a>]</span>cambium and cortex
+around them then begin to push in callus towards the centre of the
+necrotic area; but since this callus is formed under the pressure of the
+cortical tissues it does not form a thick lip or margin to the healing
+wound, as it does in a Canker, but insinuates itself with thinned-off
+edges between the wood and the dead tissue, or at most traps a little of
+the latter in the final closing up of the wound. It is easy to see how
+such an area of Necrosis may become a Canker if the dead tissues split
+or slough off, and fungi or insects obtain access to the callus at the
+margins of the area, setting up the disturbances described on <a href="#Page_222">p. 222</a>. As
+matter of fact many Cankers&mdash;<i>e.g.</i> those of the Larch disease, and
+those due to <i>Nectria</i>, or Aphides, etc.&mdash;often begin as flattened or
+depressed areas of Necrosis started by frost, and many small necrotic
+patches would eventually become Cankers if not healed up by the callus.</p>
+
+<p>Necrosis may also be due to the bruising of the tissues by large
+hailstones, to gun-shot wounds, or to any form of contusion which kills
+the living cells of cortex and cambium.</p>
+
+<p>Necrosis is a natural and common result of fire, and it frequently
+happens after forest-fires which have run rapidly through the dry
+underwood, fanned by steady winds, that the lower parts of the boles are
+scorched on one side only. The killed cambium and cortex then dry up in
+black necrotic patches, which may eventually heal up by intrusion of
+callus from the uninjured parts.</p>
+
+<p><i>Sun-burn</i> or <i>Scorching</i>.&mdash;If thin-barked trees, <span class="pagenum"><a name="Page_242" id="Page_242"></a>[<a href="./images/242.png">242</a>]</span>such as Hornbeam,
+Beech, Firs, etc., which have been growing in partial shade owing to
+dense planting, are suddenly isolated by thinning, the impingement of
+the sun's rays on the south-west side during the hottest part of summer
+days may kill the cambium, and produce necrosis of the cortical tissues,
+and such necrotic patches heal very slowly or not at all, because the
+dead tissues have contracted so tightly on to the wood below that the
+callus cannot readily creep between.</p>
+
+<p><i>Sun-cracks</i> are due to intense insolation on the south side of trees in
+clear weather in early spring, causing the drying and contraction of the
+wood and its coverings down that side of the tree: the contracted
+tissues consequently split, as in the case of frost-cracks, the healing
+up of which is very similar.</p>
+
+<p><i>Dying-back.</i>&mdash;All that is true of the necrosis of cortical tissues in
+small patches also applies to cases where the whole of the outer tissues
+of thin twigs and branches die of inanition owing to a premature fall of
+leaves&mdash;<i>e.g.</i> after a severe attack of some insect or fungus pest. The
+consequent arrest of the transpiration current and the proper supply of
+nutriment to the cambium and cortex explain the phenomena. The younger
+branches of Coffee trees suffering from severe attacks of leaf-disease
+are often denuded of leaves and die back from the causes mentioned, the
+whole of the outer tissues becoming necrotic, and drying up tight on to
+the wood, because other branches with functionally active leaves on them
+divert the <span class="pagenum"><a name="Page_243" id="Page_243"></a>[<a href="./images/243.png">243</a>]</span>transpiration current, and drought and inanition supervene.</p>
+
+<p>Dying-back is frequently also a direct effect of early frosts, which
+kill the thin twigs before the "wood is ripened," as gardeners say.</p>
+
+<p>Dying-back is also a frequent result of direct frost action on thin
+watery shoots or "unripe wood," and is apt to occur every year in
+certain varieties of Roses, for instance, in particular situations, such
+as "frost-beds," or aspects exposed to cutting winds, and so forth. The
+necrosis which results may affect all the tissues, or only the cortex
+and cambium, and the frequent accompaniment of all kinds of saprophytic
+<i>Ascomycetes</i> and moulds or other fungi is in no way causal to the
+phenomenon.</p>
+
+<p>Dying-back may also be caused by fungi, and not necessarily parasites,
+for cases are often observed where saprophytes only are to be found in
+the necrotic tissues of the cortex, having made their way in through
+minute cracks, lenticels, etc.</p>
+
+<p>A simple case is often seen in Chrysanthemums, Roses, etc., chilled and
+wetted to danger point, but not frozen, during the nights of autumn. The
+lowered resistance of the chilled tissues enables fungi like <i>Botrytis
+cinerea</i> to gain a hold, and the peduncles die-back with all the
+symptoms of Necrosis, the fungus gaining power more and more as its
+mycelium spreads in the dead tissues.</p>
+
+<p>Many other cases are known where wound-fungi, such as <i>Nectria</i>,
+<i>Cucurbitaria</i>, <i>Phoma</i>, etc., in themselves incapable of true
+parasitism, gain a hold <span class="pagenum"><a name="Page_244" id="Page_244"></a>[<a href="./images/244.png">244</a>]</span>on the necrotic tissue of a wounded twig, and
+having laboriously accumulated a vigorous mycelium saprophytically,
+extend into other parts. In many of these cases the dying-back of the
+twigs is expedited owing to the mycelium invading the medullary rays and
+wood vessels, and so obstructing the transpiration current. The much
+more rapid spread of the hyphae up into the parts thus killed
+sufficiently indicates the fundamentally saprophytic character of such
+fungi.</p>
+
+<p>Dying-back in all its forms is a common result of defoliation by
+insects, <i>e.g.</i> caterpillars, especially if it occurs when the wood is
+depleted of reserve materials, and thus cannot supply the auxiliary buds
+and enable the twigs to clothe themselves with a new flush of foliage, a
+common danger in Conifers.</p>
+
+<p>Any form of defoliation&mdash;<i>e.g.</i> excessive plucking of tea and mulberry
+leaves, browsing of animals, etc.&mdash;exposes the twigs to the dangers of
+dying-back, the accessory phenomena being similar to those already
+described.</p>
+
+<p><i>Stag-head.</i>&mdash;Old trees, though vigorous and in full foliage throughout
+the crown generally, frequently lose the power of bearing leaves on
+their topmost branches and twigs, which stand out bare and brown, and
+fancifully resemble the antlers of a stag: hence the forester's name
+"stag-head." This "top-dry" condition is frequently due to the removal
+of litter, or to excessive draining, or to the roots having gradually
+penetrated into unsuitable soil. The consequence is that some dry
+summer <span class="pagenum"><a name="Page_245" id="Page_245"></a>[<a href="./images/245.png">245</a>]</span>the drought causes the breakage of the water columns above, and
+the twigs die back.</p>
+
+<p>Tropical trees may also become <i>stag-headed</i> owing to the attacks of
+<i>Loranthus</i> and other parasites, the portions above the point of
+attachment dying back from inanition.</p>
+
+<p>Cases also occur in the tropics where the <i>stag-head</i> condition is due
+to the persistent roosting of frugiferous bats&mdash;"flying foxes"&mdash;which
+tear the bark and foliage with their claws, and befoul the twigs
+generally.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXVI.</span></h4>
+
+<div class="chnote">
+<p>The principal literature as regards frost is given in the
+works of Frank, Sorauer, and Hartig already referred to. An
+excellent summary will be found in Hartig's <i>Diseases of
+Trees</i>, p. 282, and in Fisher "Forest Protection," Vol. IV. or
+Schlich's <i>Manual</i>, p. 423.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_246" id="Page_246"></a>[<a href="./images/246.png">246</a>]</span></p>
+<h2>CHAPTER XXVII.</h2>
+
+<h3>MONSTROSITIES AND MALFORMATIONS.</h3>
+
+<div class="chsub">
+<p>Monstrosities&mdash;Teratology&mdash;Atrophy of organs&mdash;Shanking of
+grapes&mdash;Barren fruit trees&mdash;Dwarfing&mdash;Distortions and
+malformations&mdash;Fasciations&mdash;Flattened roots&mdash;Torsions&mdash;Curling
+and puckering&mdash;Leaf rolling&mdash;So-called "spontaneous"
+teratological changes.</p>
+</div>
+
+
+<p><i>Monstrosities.</i>&mdash;In a wide sense this term is applicable to many cases
+here treated under other headings, and signifies any departure from the
+normal standard of size, form, arrangement, or number of parts, and so
+forth, due to arrest of growth, excessive growth of parts, or of the
+whole organs, etc.</p>
+
+<p>Such <i>teratological</i> conditions are however by no means always
+<i>pathological</i>: that is to say, they may be variations which do not
+threaten the existence of the plant. In some cases they are clearly due
+to exuberant nutrition, and although they may occasionally predispose to
+disease, in <span class="pagenum"><a name="Page_247" id="Page_247"></a>[<a href="./images/247.png">247</a>]</span>others they show no evidence of doing so. The whole
+practice of horticulture and agriculture abounds in examples of
+teratological sports or varieties which are transmissible by seeds,
+budding and grafting, and other means&mdash;<i>e.g.</i> double flowers,
+hypertrophied floral organs (cauliflowers), seedless grapes and oranges,
+crested ferns, etc.; and even when such varieties could not live as such
+in a state of nature, there is evidence to show that many of them
+readily revert to the original seed-bearing or single condition, and
+adapt themselves to the altered environment.</p>
+
+<p>Every part of the plant may exhibit teratological changes, and I shall
+for the most part select cases in illustration which indicate approach
+to pathological states, and group with them cases known to be
+pathological in origin.</p>
+
+<p><i>Atrophy</i> is a common phenomenon denoting dwindling or reductions in
+size of organs due to insufficient nutrition, or arrest of growth from
+various causes.</p>
+
+<p>Atrophy of leaves is a common result of the attacks of parasitic fungi,
+even when the latter induce local hypertrophy&mdash;<i>i.e.</i> excessive growth
+of particular parts, <i>e.g.</i> <i>Synchytrium</i> on Dandelions and Anemones.
+<i>Puccinia suaveolens</i> causes partial atrophy of the leaves of Thistles,
+<i>Aecidium Euphorbiae</i> of those of <i>Euphorbia</i>.</p>
+
+<p>The carpels of Anemone are atrophied in plants attacked by <i>Aecidium</i>,
+and the whole flower is suppressed in Cherries infested with <i>Exoascus
+Cerasi</i>, while other fungi&mdash;<i>e.g.</i> <i>Cystopus</i>, <i>Exoasci</i>, <span class="pagenum"><a name="Page_248" id="Page_248"></a>[<a href="./images/248.png">248</a>]</span>etc.&mdash;cause
+atrophy of the seeds, and numerous instances of atrophied grain occur in
+plants infested with Ustilagineae.</p>
+
+<p>Atrophy of the grains of cereals is sometimes due to the direct attack
+of animals, <i>e.g.</i> eel-worms (<i>Tylenchus</i>) eat out the grains of Corn;
+weevils and other beetles (<i>Curculio</i>, <i>Bruchus</i>, etc.) similarly devour
+the contents of grain and nuts, the flowers of Peas and Apples, and so
+forth, inducing atrophy of the parts left. Still more striking cases are
+afforded by small insects which bore into the halms of cereals, and
+cause atrophy of the whole ear&mdash;<i>e.g.</i> <i>Cephus</i> in Wheat and Rye. Barley
+occasionally withers after flowering, the grain atrophying from no known
+cause, terms like <i>consumption</i> given to the disease conveying no
+information.</p>
+
+<p>Atrophy of young fruits is commonly due to the flowers not
+setting&mdash;<i>i.e.</i> some agent has interfered with the normal transference
+of the pollen to the stigma. This may be due to excessive rain washing
+out the pollen (<i>e.g.</i> Vine), to a lack of the necessary insects which
+effect pollination, often seen in greenhouse plants; to the stamens
+being barren&mdash;<i>e.g.</i> certain varieties of Vine&mdash;or to the premature
+destruction of the stigmas by frost, as in Cherries, Pears, etc., or by
+insects, as in Apples, or fungi, <i>e.g.</i> the infection of bilberries with
+<i>Sclerotinia</i>; or even by poisonous gases, as is sometimes seen in
+Wheat, etc., growing near alkali works. Drought is also a common cause
+of atrophy of young Plums.</p>
+
+<p><span class="pagenum"><a name="Page_249" id="Page_249"></a>[<a href="./images/249.png">249</a>]</span><i>Shanking of Grapes</i> is a particular case of atrophy and drooping of the
+immature fruits, due to the supplies being cut off by some agency. It
+may arise from very various causes which bring about disease in the
+leaves or roots, and should always be looked upon as a sign of weakness
+in the Vine, the structure of which is affected, <i>e.g.</i> poor wood&mdash;or
+the functions interfered with, <i>e.g.</i> water supplies deficient owing to
+paucity of roots.</p>
+
+<p>Barren Apple, Pear, Plum, and other flowers are often found to have been
+bored through the petals while in bud, and the whole "heart" of the
+flower eaten out by the grubs of <i>Anthonomus</i>, leaving the unopened buds
+brown and dead, as if killed by frost or drought, and often erroneously
+supposed to be so.</p>
+
+<p>The wilting and shrivelling of Clover is sometimes due to <i>Sclerotinia</i>,
+the mycelium of which pervades the roots and stock, on which the
+sclerotia may be found. Lucerne is similarly killed in Europe by the
+barren mycelium of <i>Leptosphaeria</i>, which may be found as a purple mat
+on the roots.</p>
+
+<p><i>Dwarfing</i> consists in partial atrophy of all the organs, and is a
+common result of starvation in poor, dry, shallow soils, as may often be
+seen in the case of weeds on walls or in stony places. Dwarfs which are
+thus developed in consequence of perennial drought are not, however,
+necessarily diseased, in the more specific sense of the word; their
+organs are reduced in size proportionally <span class="pagenum"><a name="Page_250" id="Page_250"></a>[<a href="./images/250.png">250</a>]</span>throughout in adaptation to
+the conditions, and simply carry out their functions on a smaller scale.</p>
+
+<p>Dwarfing is frequently a consequence of the lack of food materials, or
+of some particular ingredient in the soil, and in such cases is a
+diseased condition of some danger; similar results may ensue in soils
+containing the necessary chemical elements, but in unavailable forms.</p>
+
+<p>Dwarfing may also be brought about by repeated maiming, nipping off the
+buds, pruning, etc., as in the miniature trees of the Japanese; and the
+case of trees continually browsed down by cattle, or of moor plants
+perennially dwarfed by cutting winds, are further illustrations in the
+same category, as are also those of certain alpine and moraine plants,
+whose only chance of survival depends on their adapting themselves to
+the repeated prunings suffered by every young shoot which rises into the
+cutting winds, since there is no question of lack of food-materials in
+these cases.</p>
+
+<p>The practice of the Japanese is to pinch out the growing tips of the
+shoots wherever they wish to prune back, and it is by the judicious use
+of this heading in, and suitable pot-culture, that the dwarfs are made,
+6-20 inches high at from 30-80 years old.</p>
+
+<p>Dwarfing is often brought about by grafting on a slow-growing stock, and
+this method is employed in practice, as are also heading in, pruning of
+roots, and confinement in pots.</p>
+
+<p><span class="pagenum"><a name="Page_251" id="Page_251"></a>[<a href="./images/251.png">251</a>]</span>Dwarfing may also be due to poor or shrivelled&mdash;partially
+atrophied&mdash;seeds or such as have had their endosperms or embryos injured
+by insects or fungi, and although it is possible to nurse such dwarfs
+into normal and vigorous plants with good culture, they do not usually
+recover under natural conditions in competition with more vigorous
+plants.</p>
+
+<p><i>Distortions</i> or <i>Malformations</i> may be defined as abnormalities in the
+form of organs which concern all, or nearly all the parts, and do not
+refer merely to swellings or excrescences on them or excavations, etc.,
+in them.</p>
+
+<p><i>Fasciation.</i>&mdash;Shoots of Asparagus, Pine, Ash, and many other plants are
+occasionally expanded into broad ribbon-like structures often studded
+with more than the normal number of buds or leaves, etc., such as would
+be found on the usual cylindrical shoots. Such <i>fasciations</i> are due to
+several buds fusing laterally under compression when young and the whole
+mass growing up in common, or, in a few cases, to the unilateral
+overgrowth of one side of the terminal bud. Fasciations appear to depend
+on excessive nutrition in rich soils. They may spread out above in a
+fan-like manner, exaggerating the abnormality, or they may revert to the
+original form. Some cases are more or less fixed by heredity&mdash;<i>e.g.</i>
+<i>Celosia</i>. Fasciated stems are frequently curved like a crozier, owing
+to one edge growing more rapidly than the other.</p>
+
+<p>Cauliflowers are really cultivated monstrosities. <span class="pagenum"><a name="Page_252" id="Page_252"></a>[<a href="./images/252.png">252</a>]</span>Fasciated Dandelions,
+<i>Crepis</i>, monstrous Chrysanthemums, peloric <i>Linaria</i>, five-leaved
+Clovers, spiral Teazels, etc., may all, if grown with care, be kept more
+or less constant in the monstrous state. That is to say, the particular
+kinds of variation here manifested can be maintained in proportion as
+the external conditions controlling the variation are maintained. Such
+conditions are chiefly rich supplies of food-stuffs, plenty of water and
+air, suitable temperature and lighting, etc. Mutilations, favouring the
+development of abnormal buds may also induce fasciations.</p>
+
+<p><i>Torsions</i> or spiral twistings of stems also frequently arise among
+plants grown in rich soils, and are often combined with
+fasciations&mdash;<i>e.g.</i> Asparagus, <i>Dipsacus</i>; and De Vries has shown that
+the peculiarity is not only transmissible by seed, but may be more or
+less fixed by appropriate culture.</p>
+
+<p><i>Contortions</i> of stems are often due to the unequal growth on different
+sides of the stems owing to the presence of fungi&mdash;<i>e.g.</i> <i>Caeoma</i> on
+Pines, <i>Aecidium</i> on Nettles, also <i>Puccinia</i> on petioles of Mallow,
+<i>Cystopus</i> on inflorescences of <i>Capsella</i>, etc.</p>
+
+<p><i>Distortions</i> of roots may be brought about in various ways by the
+hindrances afforded by stones.</p>
+
+<p><i>Spiral roots</i> occur occasionally in pot plants.</p>
+
+<p><i>Flattened roots</i> usually result from compression between rocks, the
+young root having penetrated into a crevice, and been compelled to adapt
+itself later. The distortions of stems by constricting <span class="pagenum"><a name="Page_253" id="Page_253"></a>[<a href="./images/253.png">253</a>]</span>climbers, wire,
+etc., have been described, and fruits&mdash;<i>e.g.</i> Gourds&mdash;are easily
+distorted by means of string tied round them when young.</p>
+
+<p>Distortions of leaves are very common, and are sometimes
+teratological&mdash;<i>i.e.</i> due to no known cause&mdash;<i>e.g.</i> the pitcher-like or
+hood-like <i>cucullate</i> leaves of the Lime, Cabbage, <i>Pelargonium</i>, etc.,
+and of fused pairs in <i>Crassula</i>. Also coherent, bifurcate, crested,
+displaced and twisted leaves occasionally met with, and in some cases
+fixed by cultivation, may be placed in this category.</p>
+
+<p><i>Puckers</i> must be distinguished from pustules, since they consist in
+local upraisings of the whole tissue, not swellings&mdash;<i>e.g.</i> the
+yellowish green pockets on Walnut leaves, due to <i>Phyllereum</i>.</p>
+
+<p>Puckered leaves in which the area of mesophyll between the venation is
+increased by rising up in an arched or dome-like manner are sometimes
+brought about by excessive moisture in a confined space.</p>
+
+<p><i>Leaf-curl</i> is a similar deformation caused by fungi, such as <i>Exoascus</i>
+on Peaches.</p>
+
+<p>Wrinkling or puckering of leaves is also a common symptom of the work of
+Aphides&mdash;<i>e.g.</i> Hops.</p>
+
+<p>Characteristic curling and puckering, with yellow and orange tints, of
+the terminal leaves of Apples, Pears, etc., are due to insects of the
+genera <i>Aphis</i>, <i>Psylla</i>, etc.</p>
+
+<p>Small red and yellow spots with puckerings and curlings of the young
+leaves of Pears, the spots turning darker later on, are due to
+<i>Phytoptus</i>.</p>
+
+<p><span class="pagenum"><a name="Page_254" id="Page_254"></a>[<a href="./images/254.png">254</a>]</span><i>Leaf-rolling.</i>&mdash;The leaves of Beeches, Poplars, Limes, and many other
+plants, instead of opening out flat, are often rolled in from the
+margins, or from the apex, by various species of <i>Phytoptus</i>,
+<i>Cecidomyia</i>, or other insects, which puncture or irritate the epidermis
+in the young stages and so arrest its expansion in proportion to the
+other tissues. According as the lower or upper surface is attacked the
+rolling is from the morphologically upper surface downwards, or <i>vice
+versa</i>. Very often the mesophyll is somewhat thickened where rolled and
+<i>Erineum</i>-like hairs may be developed&mdash;<i>e.g.</i> Lime. Many caterpillars
+also roll leaves, drawing the margins inward to form shelters&mdash;<i>e.g.</i>
+<i>Tortrix viridana</i>, the Oak leaf-roller. Certain
+beetles&mdash;<i>Rhynchitis</i>&mdash;also roll up several leaves to form a shelter in
+which the eggs are laid.</p>
+
+<p>Webs are formed among the mutilated leaves of Apples by the caterpillars
+of <i>Hyponomeuta</i>.</p>
+
+<p>It must be borne in mind that instances can be found of teratological
+change of every organ in the plant&mdash;<i>e.g.</i> stamens transformed into
+carpels or into petals; anthers partly polliniferous and partly
+ovuliferous; ovules producing pollen in their interior, and so on, being
+simply a few startling examples of what may happen. Such abnormalities
+are frequently regarded as evidence of internal causes of disease, and
+this may be true in given cases; in a number of cases investigated,
+however, it has been shown that external agents of very definite nature
+bring about just such deformations as those sometimes cited as examples
+of teratology <span class="pagenum"><a name="Page_255" id="Page_255"></a>[<a href="./images/255.png">255</a>]</span>due to internal causes, and the question is at least an
+open one whether many other cases will not also fall into this category.
+The study of galls has shown that insects can induce the formation of
+not only very extraordinary outgrowths of tissues and organs already in
+existence, but even of new formations and of tissue elements not found
+elsewhere in the plant or even in its allies; and Solms' investigations
+on <i>Ustilago Treubii</i> show that fungi can do the same, and even compel
+new tissues, which the stimulating effects of the hyphae have driven the
+plant to develop, to take part in raising and distributing the spores of
+the fungus&mdash;<i>i.e.</i> to assume functions for the benefit of the parasite.
+Molliard has given instances of mites whose irritating presence in
+flowers causes them to undergo teratological deformations, and Peyritsch
+has shown that the presence of mites in flowers induces transformations
+of petals into sepals, stamens into petals. Similarly De Bary, Molliard,
+Magnus, Mangin, and Giard have given numerous cases of the
+transformation of floral organs one into another under the irritating
+action of fungi, of which the transformation of normally unisexual
+(female) flowers into hermaphrodite ones, by the production of stamens
+not otherwise found there, are among the most remarkable.</p>
+
+<p>These and similar examples suffice to awaken doubts as to whether any
+teratological change really arises "spontaneously," especially when we
+learn how slight a mechanical irritation of the growing point may induce
+changes in the flower; <i>e.g.</i> Sachs <span class="pagenum"><a name="Page_256" id="Page_256"></a>[<a href="./images/256.png">256</a>]</span>showed that a sunflower head is
+profoundly altered by pricking the centre of the torus, and Molliard got
+double flowers by mechanical irritation.</p>
+
+
+<h4><span class="smcap"><a name="Chapter_XXVII_Notes" id="Chapter_XXVII_Notes"></a>Notes to Chapter XXVII.</span></h4>
+
+<div class="chnote">
+<p>For the details and classification of the multitude of facts,
+the student is referred to Masters' <i>Vegetable Teratology</i>,
+Ray Society, 1869, and the pages of the <i>Gardeners' Chronicle</i>
+since that date.</p>
+
+<p>Concerning torsions, etc., the student should read De Vries,
+"On Biastrepsis in its Relation to Cultivation," <i>Ann. of
+Bot.</i>, Vol. XIII., 1899, p. 395, and "Hybridising of
+Monstrosities," <i>Hybrid Conference Report</i>, <i>Roy. Hort. Soc.</i>,
+1900, Vol. XXIV., p. 69.</p>
+
+<p>The reader will find an excellent account of the abnormalities
+in flowers due to the action of parasitic insects and fungi in
+Molliard, "<span lang="fr" xml:lang="fr">Cécidies Florales</span>," <i>Ann. des Sc. Nat.</i>, Ser.
+VIII., Bot., T. 1, 1895, p. 67.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_257" id="Page_257"></a>[<a href="./images/257.png">257</a>]</span></p>
+<h2>CHAPTER XXVIII.</h2>
+
+<h3>PROLIFERATIONS.</h3>
+
+<div class="chsub">
+<p>Proliferations&mdash;Vivipary&mdash;Prolepsis&mdash;Lammas shoots&mdash;Dormant
+buds&mdash;Epicormic shoots&mdash;Adventitious buds&mdash;Apospory and
+apogamy.</p>
+</div>
+
+
+<p><i>Proliferation</i> consists in the unexpected and abnormal on-growing or
+budding out of parts&mdash;stems, tubers, flowers, fruits, etc.&mdash;which in the
+ordinary course of events would have ceased to grow further or to bear
+buds or leaf-tufts directly. Thus we do not expect a Strawberry&mdash;the
+swollen floral axis&mdash;to bear a tuft of leaves terminally above the
+achenes, but it occasionally does so, and similarly Pears may be found
+with a terminal tuft of leaves, Roses with the centre growing out as a
+shoot, Plantains (<i>Plantago</i>) with panicles in place of simple spikes,
+and so on.</p>
+
+<p>We regard such cases as <i>teratological</i>, because they are exceptional
+for the particular species, and as <i>pathological</i> because they appear to
+be connected with over-feeding in soils with excessive <span class="pagenum"><a name="Page_258" id="Page_258"></a>[<a href="./images/258.png">258</a>]</span>supplies of
+available food-materials; but it should be noted that conditions quite
+comparable to proliferation are normal in the inflorescences of
+Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of
+proliferations come under the head of injurious actions of fungi,
+insects, and other agents.</p>
+
+<p><i>Proliferation</i> of tubers is sometimes seen in Potatoes still attached
+to the parent plant in wet weather following a drought. The eyes grow
+out into thin stolons, or forthwith into new tubers sessile on the old
+tuber. Similarly in store we sometimes find the eyes transformed
+directly into new tubers, and cases occur where the growth of the eye is
+directed backwards into the softening tuber, and a small potato is
+formed inside the parent one.</p>
+
+<p>Threading is also occasionally met with in the "sets" when ripened too
+rapidly in hot dry soils.</p>
+
+<p><i>Vivipary</i> is a particular case of proliferation, in a certain sense,
+where the seeds appear to germinate <i>in situ</i>, and we have small plants
+springing from the flowers, reminding us of wheat which has sprouted in
+the shocks in damp weather. In reality, however, the grains are here
+replaced by bulbils which sprout before they separate from the
+inflorescence. In varieties of <i>Poa</i>, <i>Polygonum</i>, <i>Allium</i>, <i>Gagea</i>,
+etc., this phenomenon is constant in plants growing in damp situations.</p>
+
+<p><i>Prolepsis.</i>&mdash;It frequently happens that branches or whole plants are
+suddenly defoliated in summer,&mdash;<i>e.g.</i> <span class="pagenum"><a name="Page_259" id="Page_259"></a>[<a href="./images/259.png">259</a>]</span>by caterpillars or other
+insects&mdash;at a time when considerable stores of reserves had already been
+accumulated during the period of active assimilation. In such cases the
+axillary buds, which would normally have passed into a dormant condition
+over the winter had the leaves lived till the autumn-fall, suddenly
+shoot out into <i>proleptic</i> shoots (also termed Lammas shoots), and
+reclothe the tree with foliage. The wood of the year in which this
+occurs may exhibit a double annual ring, and the vigour of the tree is
+likely to suffer in the following season and no fruit be matured.</p>
+
+<p>Proleptic branches may also be due to the shooting out of accessory
+buds&mdash;<i>i.e.</i> extra buds found in or near the leaf-axils of many plants,
+such as Willow, Maples, <i>Cercis</i>, <i>Robinia</i>, <i>Syringa</i>, <i>Aristolochia</i>,
+etc.&mdash;which do not normally come to anything, or do so only if a surplus
+of food materials is provided.</p>
+
+<p><i>Dormant buds</i>, or <i>preventitious buds</i>, are such as receive no
+sufficient supply of water and food materials to enable them to open
+with the other buds in ordinary years, for in most trees only the upper
+buds on the branches develop into new shoots. The lower buds do not die,
+however, but merely keep pace with the growth in thickness of the parent
+branch, and may be elongated sufficiently each year to raise the minute
+tips level with the bark, their proper cambium only remaining alive but
+not thickening the bud.</p>
+
+<p><span class="pagenum"><a name="Page_260" id="Page_260"></a>[<a href="./images/260.png">260</a>]</span>When, by the breaking of the branch above the insertion of the dormant
+bud&mdash;or by pruning, defoliation by insects, etc.&mdash;the transpiration
+current and supplies of food materials are in any way deflected to the
+minute cambium and growing points of the dormant buds, they are
+stimulated to normal growth, and may grow out as <i>epicormic shoots</i> or
+"shoots from the old wood." In many cases such epicormic shoots are
+stimulated to grow out by suddenly exposing an old tree to more
+favourable conditions of root-action and assimilatory activity, owing to
+the felling of competing trees which previously hemmed it in from light
+and air, and restricted the spread and action of its roots in the soil.
+This is often seen in old Elms, Limes, etc.</p>
+
+<p>It is by such means as the above that substitution branches are obtained
+when a leader is broken or cut away.</p>
+
+<p><i>Adventitious buds</i> are such as are newly formed from callus or other
+tissues in places not normally provided with buds, as is often seen on
+occluding wounds&mdash;<i>e.g.</i> stool shoots. They may also be developed on
+roots, a fact utilised in propagating <i>Bouvardias</i>, Horse-radish, etc.,
+by means of root-cuttings, and the <i>suckers</i> of Plums and other fruit
+trees are shoots springing from adventitious buds on roots.</p>
+
+<p>Adventitious buds are also common on leaves (<i>e.g.</i> <i>Bryophyllum</i>,
+Ferns, etc.), and are frequently induced on them by wounds&mdash;<i>e.g.</i>
+<i>Gesneria</i>, <i>Gloxinia</i>, etc. Even cut cotyledons may develop <span class="pagenum"><a name="Page_261" id="Page_261"></a>[<a href="./images/261.png">261</a>]</span>them, and
+pieces of leafless inflorescence (Hyacinth), hypocotyl (<i>Anagallis</i>),
+and in fact practically any wounded tissue with a store of reserve
+materials may be made to develop them: thus they have been found arising
+from the pith of Sea-kale, and are commonly developed from the cut bulb
+scales of Hyacinths.</p>
+
+<p><i>Apospory</i> and <i>Apogamy</i> are particular cases of the production of
+vegetative buds on the leaves in place of sporangia in Ferns (Apospory),
+and on prothallia in place of Archegonia (Apogamy), in the latter case
+induced by dry conditions and strong illumination.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXVIII.</span></h4>
+
+<div class="chnote">
+<p>In addition to the literature quoted in the <a href="#Chapter_XXVII_Notes">notes to Chapter
+XXVII.</a>, the student should consult the works on Forest Botany
+for the scattered information regarding adventitious buds. A
+good account may be found in Büsgen, <span lang="de" xml:lang="de"><i>Bau und Leben unserer
+Waldbäume</i></span>, Jena, 1897.</p>
+
+<p>For Apospory and Apogamy, see Lang "On Apogamy and the
+Development of Sporangia upon Fern Prothalli," <i>Phil. Trans.</i>,
+vol. 190, 1898, p. 187, where the literature is collected.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_262" id="Page_262"></a>[<a href="./images/262.png">262</a>]</span></p>
+<h2>CHAPTER XXIX.</h2>
+
+<h3>GRAFTS.</h3>
+
+<div class="chsub">
+<p>Grafting&mdash;Comparison with cuttings&mdash;Effects of
+environment&mdash;Relations between scion and stock&mdash;Variation in
+grafts&mdash;Grafting and
+parasitism&mdash;Infection&mdash;Pollination&mdash;Grafts-hybrids&mdash;Predisposition
+of Natural grafts&mdash;Root-fusions.</p>
+</div>
+
+
+<p>Grafting is a process which consists in bringing the cambium of a shoot
+of one plant into direct union with that of another, and is practised in
+various ways, the commonest of which is as follows:</p>
+
+<p>One plant&mdash;the <i>stock</i>&mdash;rooted in the ground, is cut off a short
+distance above the surface of the soil, and a shoot from the second
+plant&mdash;the <i>scion</i>&mdash;cut off obliquely with a sharp knife, is inserted
+into a cleft in the stock, so that the two cambiums (and sometimes the
+cortex and pith of each as well) are in close contact: the scion is then
+tied in position, the wounds covered with grafting wax, and the whole
+left until union of the tissues is completed. This union depends on the
+<span class="pagenum"><a name="Page_263" id="Page_263"></a>[<a href="./images/263.png">263</a>]</span>formation of <i>callus</i> at the cut surfaces, and the intimate union of the
+ingrowing cells from each callus.</p>
+
+<p>The development of the callus follows the course described for wounds,
+cuttings, etc., and the union is exactly comparable to the union of the
+two lips of a healing callus over a wound (see <a href="#Page_197">p. 197</a>).</p>
+
+<p>Grafting was known and practised far back in the ages. Virgil was well
+acquainted with the process, and Theophrastus compared it with
+propagation by cuttings.</p>
+
+<p>The scion differs from a cutting, however, in having no roots of its
+own: it is parasitic upon, or rather is in symbiosis with the stock, the
+root and tissues of which intervene between it and the soil.
+Consequently the selective absorption, size and number of vessels, and
+innumerable other physiological and anatomical peculiarities of the
+stock determine what and how much shall go up into the scion, while the
+latter supplies the former with organic materials and rules what and how
+much food, enzymes, and other secretions, etc., it shall receive to
+build up its substance. Surely, then, if such factors as the nature of
+the soil, the water and mineral supplies, the illumination, and the
+various climatic factors of altitude can cause variations on a plant
+direct, these and other factors are still more likely to be effective on
+stock and scion, and each must affect the other.</p>
+
+<p>Nevertheless opinions have differed much as to whether any important
+effect is to be seen, and on <span class="pagenum"><a name="Page_264" id="Page_264"></a>[<a href="./images/264.png">264</a>]</span>no point more than on whether the scion
+can affect the stock, in spite of such examples as <i>Cytisus Adami</i>,
+<i>Garreya</i> on <i>Aucuba</i>, Sunflower on Jerusalem Artichoke, etc. Recent
+results, especially of experiments with herbaceous plants, show that not
+only can the stock affect the scion (and <i>vice versa</i>) directly, but the
+effect of the changes may be invisible on the grafted plant and only
+show itself in the progeny raised from the seed of the grafted plant. In
+other words, variation occurs in grafts either <i>directly</i>, as the
+results of the effects of the environment on the graft, or owing to the
+interaction of scion and stock, showing as changes in general nutrition
+in the tissues concerned, etc., owing to special reactions of the
+protoplasm of the uniting cells one on the other, and of the results of
+the further protoplasmic secretions, sortings, and so forth, on the
+cells developed as descendants of these in the further growth of the
+graft: or <i>indirectly</i>, in that some of these changes so alter the
+nature of the special protoplasm put aside for reproductive purposes,
+that the resulting embryo in the seed transmits the effects, and they
+show as variations in the seedling. If these results are confirmed they
+should meet all objections that have been urged against the transmission
+of acquired characters.</p>
+
+<p>In fact there are analogies between grafting and parasitism which cannot
+be overlooked, and should not be underestimated, their commonest
+expression appearing in the alterations in stature, habit, period of
+ripening, and so forth. These analogies <span class="pagenum"><a name="Page_265" id="Page_265"></a>[<a href="./images/265.png">265</a>]</span>are easily apprehended when we
+compare parasites like the Mistletoe, <i>Loranthus</i>, or even such
+root-parasites as the Broom-rapes and the Rhinanthoideae with grafts;
+but they also exist in the case of many fungus-parasites, and we might
+almost as accurately speak of <i>grafting</i> some fungi on their hosts as of
+<i>infecting</i> the latter with them, especially when it is borne in mind
+that the effect of the scion on the stock is by no means always to the
+benefit of the latter, and that there are reasons for regarding the
+action of some such unions as that of a sort of slow poisoning of the
+stock by the scion. Why do we not here say that the stock has been
+<i>infected</i> by the scion?</p>
+
+<p>The resemblances between pollination and the infection by fungus hyphae
+may also be insisted upon. If we take into account Darwin's remarkable
+experiments showing that in "illegitimate unions" the pollen exerts a
+sort of poisonous action on the stigmas or ovules, it is possible to
+arrange a series of cases starting with perfectly legitimate
+pollinations where the pollen tube feeds as it descends the style on
+materials provided by the cells, and proceeding to cases where the
+pollen is more and more merely just able to penetrate the ovary and
+reach the ovules, to the extreme cases where no union at all is
+possible.</p>
+
+<p>Side by side with such series could be arranged analogous cases where
+fungus spores can enter and infect the cells of the host, and live
+symbiotically with or even in them, or can penetrate only with
+<span class="pagenum"><a name="Page_266" id="Page_266"></a>[<a href="./images/266.png">266</a>]</span>difficulty, or with poisonous effects, and finally cannot infect the
+plant at all.</p>
+
+<p>Less obviously, but nevertheless existing, are gradations in grafting to
+be observed, where one and the same stock may be successfully combined
+with a scion which improves it&mdash;or which is improved by it&mdash;or the scion
+may unite but acts injuriously on it, or, finally, cannot be induced to
+unite.</p>
+
+<p>But we may go further than this in these comparisons. Just as the
+results of pollination frequently induce far-reaching effects on distant
+tissues&mdash;<i>e.g.</i> the swelling of Orchid ovaries, and rapid fading of the
+floral organs&mdash;so also the effects of hyphae in the tissues may induce
+hypertrophies, deflection of nutrient materials, and the atrophy of
+distant parts&mdash;<i>e.g.</i> the curious phenomena observed in <i>Euphorbia</i>
+attacked by <i>Uromyces</i>&mdash;and some of the distant actions in grafts may be
+compared similarly.</p>
+
+<p>Going still further, we may compare the effects of cross-breeding or of
+hybridisation, where the <i>progeny</i> show that changes have resulted from
+the mutual interactions and reactions of the commingled protoplasm, with
+Daniel's results, in which he obtains proof of such interactions of the
+commingled protoplasmic cell-contents of grafts in the seedling progeny;
+although there is no probability&mdash;we may even say possibility&mdash;in this
+latter case that the effects are due to nuclear fusions, but only that
+the germ-plasm of the seed-bearing plant has been <span class="pagenum"><a name="Page_267" id="Page_267"></a>[<a href="./images/267.png">267</a>]</span>affected by the
+changes in the cell-protoplasm which nourishes it when the reproductive
+cells are forming.</p>
+
+<p>In the case of graft-hybrids the matter appears to be somewhat
+different, and we may well suppose, with Strasburger, that the
+commingling of characters observed in flowers, fruits, foliage, etc., on
+shoots borne after grafting are due to the occurrence of nuclear fusions
+during the union of the grafted tissues; though it is by no means
+impossible that what has really happened is profound alterations in the
+nuclear substance (germ-plasm) owing to its being nourished by
+cell-protoplasm (somato-plasm) which has been itself affected by the
+interchanges of substance between scion and stock, and therefore itself
+furnishes a different nutrient medium from the unaltered cytoplasm of
+either.</p>
+
+<p>But even here we can find parallels among the ordinary phenomena of
+plant reproduction. Maize plants with white endosperm containing starch,
+if crossed by pollen from other plants with purple endosperm containing
+sugar, bear seeds with purple endosperm containing sugar, and such
+<i>Xenia</i> may be compared to graft-hybrids in many respects.</p>
+
+<p>I know of no case among fungus infections which could be compared
+directly with these examples, and it is not at all likely that we shall
+meet with any instance of a fungus-hypha handing over nuclear substance
+to an egg-cell, and so affecting the latter that an embryo results. <span class="pagenum"><a name="Page_268" id="Page_268"></a>[<a href="./images/268.png">268</a>]</span>But
+the case is not hypothetically impossible, although the distant
+relationships of the two groups of organisms render it extremely
+improbable among the higher plants. It is by no means so improbable,
+however, that further research may show cases where the egg-cell of a
+lower cryptogam&mdash;<i>e.g.</i> another fungus&mdash;may be affected either directly,
+or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as
+suggested by the extraordinary phenomena of symbiosis.</p>
+
+<p>Some of the variations in grafted plants are found to predispose the
+plant to disease, or the reverse, and cases may be cited where the
+resulting shoots, foliage, or fruits, or seedlings more readily fall a
+prey to, or resist, parasitic fungi and insects than the ungrafted
+plants. Daniel gives instances of such&mdash;<i>e.g.</i> among other examples,
+Peas grafted on Beans yield seeds which suffer more from Erysipheae than
+the normal seedlings. But the best known cases are those of Vines in
+their relations to <i>Phylloxera</i>, already referred to (<a href="#Page_155">p. 155</a>).</p>
+
+<p>Several instances are also known where grafted plants show more or less
+resistance to such factors of the environment as low temperatures;
+grafted or budded Roses often suffer much from Erysipheae, and so forth.
+Much research is still needed to determine how far these matters depend
+on real alterations in the nature of the graft, or <i>are only true for
+the localities in which the experiments have been made</i>, a point which
+has, I think, been overlooked by all observers.</p>
+
+<p><span class="pagenum"><a name="Page_269" id="Page_269"></a>[<a href="./images/269.png">269</a>]</span>Grafted plants are apparently very much exposed to injury by slugs,
+insects, and the invasions of parasites during the healing of the callus
+and the fusion process. Here again it must not be overlooked that the
+callus is, so to speak, a tit-bit of luscious, thin-walled, succulent
+tissue; and, like all wounds, the graft affords entrance to parasites
+such as <i>Nectria</i> and Ascomycetes of various kinds, under circumstances
+very favourable to their invasion.</p>
+
+<p><i>Natural Grafts.</i>&mdash;It is by no means an uncommon event to find the
+branches of Beeches, Limes, and other trees which have been accidentally
+brought into contact during growth, joined where they cross. As they
+press one against the other, they become naturally grafted, by that form
+of the process known as <i>inarching</i>: except that in artificial inarching
+the operator cuts off the cortical tissues of the two branches and
+brings their cambial surfaces together, whereas in nature the cambiums
+only come into contact after the destruction by pressure, or slight
+abrasion, of the entrapped intervening tissues. The fusion occurs, in
+fact, exactly as in the burying-in of a nail or wire, referred to on <a href="#Page_211">p.
+211</a>.</p>
+
+<p>Natural grafts are very common among the roots of trees, and possibly
+explain some queer cases of the apparent revivification of stumps of
+trees not usually given to forming abundant stool shoots. It is regarded
+as probable in some old forests that the majority of the roots of trees
+of the same species are linked up together by such <span class="pagenum"><a name="Page_270" id="Page_270"></a>[<a href="./images/270.png">270</a>]</span>natural grafts, a
+probability not diminished by the fact that such roots cross at many
+points, and are easily grafted.</p>
+
+
+<h4><span class="smcap">Notes to Chapter XXIX.</span></h4>
+
+<div class="chnote">
+<p>The student should read Bailey, <i>The Nursery Book</i>, 1896, for
+details regarding the practice of grafting, and facts in
+abundance can be obtained from the pages of the <i>Gardeners'
+Chronicle</i>.</p>
+
+<p>Concerning graft-hybrids and the variations of grafted plants
+see Jouin, <i>Can Hybrids be obtained by Grafting?</i> and
+especially Daniel, "<span lang="fr" xml:lang="fr">La Variation dans la Greffe</span>," in <span lang="fr" xml:lang="fr"><i>Ann. des
+Sc. Naturelles</i></span>, S. VIII., Vol. 8, 1898, p. 1, and the
+literature there collected. The whole subject is largely
+controversial, and much work remains to be done.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_271" id="Page_271"></a>[<a href="./images/271.png">271</a>]</span></p>
+<h2>CHAPTER XXX.</h2>
+
+<h3>LIFE AND DEATH.</h3>
+
+<div class="chsub">
+<p>Protoplasm&mdash;Hypothesis as to its structure and
+behaviour&mdash;Assimilation&mdash;Growth&mdash;Respiration&mdash;Metabolism&mdash;Action
+of the environment&mdash;Nuclear
+protoplasm&mdash;Pollination&mdash;Grafting&mdash;Parasitism&mdash;Graft-hybrids&mdash;Life&mdash;Death&mdash;Variation&mdash;Disease.</p>
+</div>
+
+
+<p>We have seen that all the essential phenomena of disease concern only
+the living substance&mdash;the protoplasm&mdash;of the plant, and that however
+complex the symptoms of disease may be, the occurrence of
+discolorations, lesions, hypertrophies, and so forth are all secondary
+matters subsidiary to the fundamental alterations of structure and
+function constituting the disease. It remains to see if we can adopt any
+hypothesis as to the nature of this physical basis of life&mdash;the
+protoplasm&mdash;which shall help us to understand still more clearly in what
+must reside those processes which, so long as they proceed harmoniously
+and uninterruptedly, constitute life and <span class="pagenum"><a name="Page_272" id="Page_272"></a>[<a href="./images/272.png">272</a>]</span>health, and which when
+interfered with result in disease and death. The protoplasm of the
+living plant-cell looks like a slimy translucent mass which has been
+superficially compared in appearance to well-boiled sago or clear gum.
+Fifty years of observations and experiments with it have convinced
+physiologists that it is not a mere solution or emulsion, however, or
+even a chemical compound in the ordinary sense of the term, although
+chemical analysis gets little out of it beyond water, proteids,
+carbohydrates and fats, and traces of certain mineral salts; for living
+protoplasm does not respond to the laws of physics and mechanics in
+obeying them, simply as do ordinary solutions and liquids. On the other
+hand, the most delicate chemical manipulation fails us, because when
+killed it is no longer protoplasm. Nor does the microscope advance
+matters far, beyond convincing us that this marvellous material must
+have a structure far more intimate than anything visible to the highest
+magnifying powers at our disposal.</p>
+
+<p>Nevertheless, some information is forthcoming from the comparative
+examination of the protoplasm of numerous different kinds of organisms,
+for we have learnt that certain ingredients and no others are necessary
+for its composition&mdash;namely, carbon, hydrogen, oxygen, nitrogen,
+phosphorus, sulphur, calcium[Note: See <a href="#Chapter_XXX_Notes">note</a> at end of chapter.],
+magnesium, potassium&mdash;and it is as a rule of no use trying to foist on
+to it any substitute for any one of these. Moreover, these <span class="pagenum"><a name="Page_273" id="Page_273"></a>[<a href="./images/273.png">273</a>]</span>chemical
+elements must be given in certain definite proportions and forms: for
+instance it is of no use to offer the carbon and sulphur in such a form
+as carbon disulphide, or the nitrogen and hydrogen in that of
+hydrocyanic acid, but the carbon must be given to the protoplasm in the
+form of a carbohydrate or in some similar form, the nitrogen as an
+ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so
+forth, and thus water, air, carbohydrates, and the nitrates, sulphates,
+and phosphates of potassium, calcium, and magnesium become the chief
+natural sources of the essential ingredients. Again, we have learnt that
+while there are different forms of protoplasm in the cell, and that
+these react on each other, and go through cycles of arrangement and
+rearrangements, the intimate structure must be of that kind termed
+molecular&mdash;beyond the region of vision, just as is the microscopic
+structure of a crystal; but, while like the latter affording evidence of
+order and sequence when properly examined, the structural arrangements
+and changes must be infinitely more complex.</p>
+
+<p>All these, and numerous other results of enquiry, have led to the
+conclusions that we must regard living protoplasm as a complex made up
+of very large molecular units, each containing atom-groupings of the
+elements named; and, partly on account of the large number of atoms they
+contain, and partly due to the vibrations of absorbed heat, these units
+must be extremely labile. Moreover, they are linked up into an
+<span class="pagenum"><a name="Page_274" id="Page_274"></a>[<a href="./images/274.png">274</a>]</span>invisible and intricate meshwork, bathed in a watery liquid held in the
+interstices somewhat as water is held in a sponge. In this imbibed
+liquid are dissolved the substances, consisting of the same elements,
+which are to serve as food, and which are to be taken up into the
+molecular framework and built up into the structure of new molecular
+units&mdash;or, as they may be shortly termed, molecules of protoplasm: in
+the bathing liquid are also dispersed the fragments&mdash;again containing
+the elements named&mdash;which have resulted from the breaking asunder of
+some of the complex protoplasm molecules, and which are partly destined
+to be used up again, partly to be burnt off in respiration, and partly
+to be put aside as metabolic products such as reserves, secretions,
+permanent structure, etc. Among the elements carried into this liquid
+and dissolved in it the free oxygen of the air also plays an important
+part.</p>
+
+<p>As new molecules are formed, by mutual combinations of the
+food-materials selected by molecular attractions, they are taken up into
+the protoplasmic framework, and built in between those already in
+existence, thus distending the whole, and we say that the protoplasm
+<i>Assimilates</i> food-materials and <i>Grows</i>. When distended beyond a given
+degree, or disturbed in various other ways, the molecular framework
+breaks, and some of the molecules are shattered, and as they fall to
+pieces certain of their constituent parts containing carbon and hydrogen
+forcibly combine at the moment of liberation with the oxygen in <span class="pagenum"><a name="Page_275" id="Page_275"></a>[<a href="./images/275.png">275</a>]</span>the
+fluid around and are burnt off in the form of carbon-dioxide and water,
+heat being of course evolved. This is the fundamental process of
+<i>Respiration</i>.</p>
+
+<p>It is probably the alternation of these processes of <i>Assimilation</i>&mdash;the
+building up into the protoplasmic structure of new complex labile
+molecules&mdash;and <i>Destruction</i>&mdash;the shattering of such molecules with
+redistribution, oxidation, etc., of their fragments&mdash;which constitute
+the fundamental process of life. Different authorities attempt to
+explain the details of these processes in various ways, but there is
+practical agreement on the one point, that life consists in the
+alternate building up of new protoplasm from the
+food-materials&mdash;<i>Assimilation</i>&mdash;and the breaking down of the molecular
+complexes to simpler ones&mdash;<i>Disintegration</i>, or <i>Dis-assimilation</i>, as
+we may call it. During the periods when assimilation prevails, and the
+protoplasm increases in mass, we recognise <i>Growth</i>, and since this is
+usually associated with the vigorous imbibition of water, owing to the
+powerful osmotic attractions for that liquid exhibited by some of the
+products, and with consequent further stretching of the invisible
+molecular plexus, the growth may be so evident in increased size, that
+we are accustomed to look upon the visible increase in volume alone as
+growth; but it is essential to understand that growth of the protoplasm
+is always proceeding during life, even when as many older molecules are
+being shattered and dispersed as new ones <span class="pagenum"><a name="Page_276" id="Page_276"></a>[<a href="./images/276.png">276</a>]</span>are being formed by
+assimilation, and when, therefore, no visible permanent enlargement
+occurs. Similarly, during periods when disintegration of the molecules
+prevails, we must not assume that the assimilation of new molecules is
+not occurring and that growth is not proceeding. The two processes are
+always going on during the active life of the protoplasm: in fact life
+consists in the play of these processes, as already said.</p>
+
+<p>That numerous chemical rearrangements of the atom-complexes take place
+outside the protoplasmic molecules&mdash;both of those left unemployed in
+assimilation and of those rejected during the destructive
+processes&mdash;will be readily understood: many of the bye-products found in
+plants, such as vegetable acids, alkaloids, colouring matters,
+crystalline bodies, etc., etc., are due to these, so to speak,
+fortuitous combinations and re-combinations.</p>
+
+<p>The part played by respiration has often been misunderstood. It consists
+in the burning off of some of the carbon and hydrogen of the shattered
+protoplasm molecules, by means of the oxygen of the air, which finds its
+way into the fluids around the protoplasm, and when it is active every
+act of combustion&mdash;which is here an explosion&mdash;leads to the shattering
+of more protoplasm molecules, and consequently to more respiratory
+combustion of the products. If the supply of oxygen is limited the
+breaking down of the molecules of protoplasm does not cease, but the
+carbon and hydrogen which would otherwise have been <span class="pagenum"><a name="Page_277" id="Page_277"></a>[<a href="./images/277.png">277</a>]</span>oxidised are now in
+part left to form other compounds in the surrounding liquid, and thus
+incompletely oxidised bodies, such as vegetable acids, alcohols, etc.,
+accumulate. Even in the complete absence of atmospheric oxygen the
+protoplasm may go on breaking down and accumulating various compounds
+containing relatively much carbon and hydrogen&mdash;so-called intramolecular
+respiration; but in ordinary plants this process soon comes to an end,
+because the blocking up of the molecular plexus leads to obstruction and
+interferes with the normal assimilation and dis-assimilation, and, if
+prolonged, leads to pathological conditions, and eventually death.</p>
+
+<p>Here, then, we meet with a cause of disease, or of predisposition to
+disease. The deprivation of oxygen interferes with the normal processes
+of building up and breaking down of the protoplasmic molecules, and
+bodies we term poisonous accumulate and may lower the vitality or even
+bring life to an end.</p>
+
+<p>During normal life other products of the disruption of the protoplasm
+molecules are nitrogenous bodies, such as proteids, and these we have
+reason to believe are used up again, acting as the nuclei, so to speak,
+of the new molecules, and so being built up again with fresh
+food-materials into the plexus, to be again set free, and again used up,
+and so on. Others are the carbohydrates, such as cellulose, which pass
+out of the molecule into an insoluble form, and are accumulated outside
+the protoplasm in the form of cellulose membranes, and so forth. <span class="pagenum"><a name="Page_278" id="Page_278"></a>[<a href="./images/278.png">278</a>]</span>It is
+these formed products of metabolism (Metabolites), especially cellulose
+and bodies which result from its subsequent transformation, which
+constitute the main permanent mass of the ordinary plant.</p>
+
+<p>We are now in a position to see how another fundamental cause of disease
+or predisposition to disease exists in the deprivation of the protoplasm
+of any of the elements needed to supply&mdash;in the food-materials&mdash;the
+place of those which have been permanently put aside in the form of
+cell-walls, or burnt off in respiration, passed out as excretions, or in
+other ways lost.</p>
+
+<p>It is clear that the indispensability of an element must mean that the
+protoplasmic molecule cannot be completed without it: the same
+conclusion is supported by the experimental proof that these elements
+cannot be replaced by chemically similar elements.</p>
+
+<p>It does not follow, however, that the protoplasm molecule must always
+have the same number of atoms of these elements, and grouped always in
+the same atom-complexes before being assimilated; nor that the
+protoplasm molecule, when once built up, always breaks down in exactly
+the same way. On the contrary, while the protoplasm of corresponding
+parts of a daisy and of a rose must contain all the elements named, we
+must believe that the atom groupings are different in the protoplasm
+molecule in each case; and though the molecules of the cell-protoplasm,
+of the nucleus, of the chlorophyll-corpuscles, etc., of one <span class="pagenum"><a name="Page_279" id="Page_279"></a>[<a href="./images/279.png">279</a>]</span>and the
+same plant must have all these elements, the atom groupings and modes of
+building up and breaking down may be very different in each case.</p>
+
+<p>Again, the cell-protoplasm, bathed by the sap taken in by roots from the
+soil or fed directly by that derived from the leaves, must be exposed to
+very different stimuli and modes of nourishment, etc., from those
+incurred by the protoplasm of the nucleus which it encloses: and similar
+conclusions must apply in turn to the protoplasm of the root in the dark
+moist soil and of the leaf in the light dry air, or to that of the
+superficial epidermis cells as contrasted with that of the deeply
+immersed pith, and so on.</p>
+
+<p>It is no doubt in these directions that we must seek for the explanation
+of many life-phenomena at present quite beyond explanation. Thus, it is
+tolerably easy to modify the action of the cell-protoplasm of a plant,
+by exposing it to differences of illumination, temperature, moisture,
+and so forth, within certain limits; at least, since the changes in
+stature, tissue differentiation, cell-secretions, flowering capacity,
+etc., of plants affected by such factors of the environment&mdash;<i>e.g.</i>
+alpine plants brought into the plains&mdash;<i>must</i> be due to changes in the
+mode of activity of the protoplasm, we must assume that the above
+factors affect the latter. But it is extremely difficult to reach the
+nuclear-protoplasm directly by such stimuli, as proved by the experience
+that even where we allow the factors to act for a long time, no
+permanent change can be <span class="pagenum"><a name="Page_280" id="Page_280"></a>[<a href="./images/280.png">280</a>]</span>detected in the behaviour of the
+nuclear-protoplasm&mdash;the essential material in the reproductive organs
+and reproductive process. At least we must infer that no change has been
+permanently stamped on this nucleo-plasm from such facts as the
+characters of the seedlings of the progeny of the plain-raised plants:
+if they are again sown in an alpine situation they forthwith behave
+again as alpines.</p>
+
+<p>Must we not conclude, then, that this difficulty of reaching the
+nuclear-protoplasm is owing to the fact that it is nourished and
+influenced directly only by the cell-protoplasm? That the
+cell-protoplasm is its environment, and not so directly the outer world?
+We may influence the cell-protoplasm&mdash;we may make it work harder or less
+actively, respire vigorously or slowly, build up and break down in
+various different ways, or at different rates, and so forth, <i>within
+limits</i>; but it is nevertheless cell-protoplasm of its specific kind,
+with its own range of molecular variations and activities within these
+limits, and it supplies the nuclear-protoplasm with what it wants so
+long as these limits are not exceeded. Consequently, while it is very
+easy to make the cell-protoplasm vary within the limits of its range, it
+is not easy to induce it to vary its effects on the nuclear-protoplasm
+to such an extent or in such a way that the latter is permanently or
+materially altered in constitution.</p>
+
+<p>Nevertheless it would appear that cases do occur where the
+nuclear-protoplasm <i>is</i> reached and <span class="pagenum"><a name="Page_281" id="Page_281"></a>[<a href="./images/281.png">281</a>]</span>affected by external stimuli, as
+evinced by some of the phenomena of hybridisation and of cross- and
+self-fertilisation, because we find the results expressed in the
+mingling of the characters of parents, in strengthened or enfeebled
+progeny, and even in the appearance of unexpected properties, which,
+from the facts of Reproduction, we know must have taken their origin in
+some alteration of the nuclear substance of the embryo.</p>
+
+<p>Here, however, we know in most cases that the principal agent which has
+reached the nuclear-protoplasm, is another portion of
+nuclear-protoplasm. In hybridisation, one which has been fed and
+influenced by cell-protoplasm of a very different plant; in
+cross-fertilisation, one fed and influenced by the cell-protoplasm of a
+different plant of the same species, and in self-fertilisation, one fed
+and influenced by the same cell-protoplasm.</p>
+
+<p>That somewhere, and somehow, such nuclear-protoplasm as induces the
+changes in the characters of hybrids, etc., has been influenced by its
+immediate environment&mdash;the cell-protoplasm of the plant&mdash;appears to be a
+conclusion from which there is no escape. We may obtain similar evidence
+from the experience of grafting. It is relatively easy to influence the
+cell-protoplasm of a scion by a suitable stock, obviously because the
+latter, while handing on to the former all necessary materials from the
+soil, presents the indispensable elements and compounds in somewhat
+different proportions, dilutions, etc., from those which its <span class="pagenum"><a name="Page_282" id="Page_282"></a>[<a href="./images/282.png">282</a>]</span>own roots
+would have done, and probably mingles with them a certain amount of its
+own peculiar products, as well as affects the modes of working and
+interaction of both by the molecular impetus impressed on them.
+Consequently the cell-protoplasm of the scion, while obtaining from the
+stock all it needs within the limits of its own variations of structure
+and activity, nevertheless builds up and breaks down in ways or at rates
+slightly different from those hitherto normal to it, and perceptible
+variations result when the sequences and correlations of these material
+and mechanical changes have affected a sufficiently large mass for the
+accumulation of visible effects. The limits to grafting suggest not that
+an inappropriate stock does not offer to the protoplasm of the scion the
+right materials, but that it presents them in proportions and in forms
+which are unsuitable for the assimilable powers of the latter, or,
+possibly, mingled with substances poisonous in themselves or capable of
+becoming so in conjunction with bodies in the scion.</p>
+
+<p>What has been said of the action of stock on scion, will also be true,
+<i>mutatis mutandis</i>, of the reciprocal action of scion on stock. Here
+again we may have causes for disease, or predisposition to disease.</p>
+
+<p>It occasionally happens, however, that the nuclear protoplasm of the
+stock or scion <i>is</i> affected in grafting, and we infer from the
+difficulty of modifying it in any other way in ordinary reproduction
+than by means of other nuclear protoplasm&mdash;<i>e.g.</i> <span class="pagenum"><a name="Page_283" id="Page_283"></a>[<a href="./images/283.png">283</a>]</span>in
+hybridisation&mdash;that in such cases a fusion of the nuclei of stock and
+scion has occurred during the grafting, and a graft-hybrid has
+resulted&mdash;<i>e.g.</i> <i>Cytisus Adami</i>.</p>
+
+<p>It is not impossible however that the nuclear protoplasm has in such
+graft-hybrids been subsequently modified by the differences in nutrition
+to which it has been subjected, in the modified cell-protoplasm affected
+by the mingling of the juices, etc., of scion and stock; for it is quite
+conceivable that such materials may affect the protoplasm far more
+profoundly than anything derived directly from the environment.</p>
+
+<p>If Daniel's researches are confirmed, however, it appears that in some
+cases, at any rate, the nuclear-protoplasm is so altered by the grafting
+that when the new embryo is developed, after fusion with nuclear
+substance from another plant of the same species, the results are
+apparent only in the progeny, and <i>the effects of alteration in the
+cell-protoplasm have been transmitted to the nuclear protoplasm of the
+germ-cells</i>&mdash;<i>i.e.</i> acquired characters have been transmitted and fixed
+by heredity. Should this prove true the importance of the results can
+hardly be over-estimated. The matter is too problematical for further
+discussion here, but we see that any such action may profoundly affect
+the "constitution" of the resulting plant.</p>
+
+<p>Turning now to the case of fungi or other organisms which obtain access
+to the cell-protoplasm. At the one extreme we have cases where the
+protoplasm of the diseased plant is rapidly and directly <span class="pagenum"><a name="Page_284" id="Page_284"></a>[<a href="./images/284.png">284</a>]</span>poisoned and
+destroyed, as in the killing off of seedlings in "Damping Off": near the
+other extreme we have cases where the foreign protoplasm of the
+parasite, although it gains complete access to that of the host, merely
+stimulates the latter to greater activity and itself works for its own
+ends in conjunction with it&mdash;<i>e.g.</i> <i>Plasmodiophora</i>. In such instances
+we must figure to ourselves the cells of the root of the Crucifer
+handing on food-materials to both masses of protoplasm&mdash;that of the
+<i>Plasmodiophora</i> and that of the cell into which it penetrates; and it
+is immaterial whether both obtain the food-materials directly, or, what
+seems more likely, the fungus only at second hand and by the medium of
+the host's protoplasm. In any case, the latter is for a long time at
+least not poisoned or maimed, or in any perceptible way injured by
+excreta from the fungus-protoplasm, although it is evident that each
+must excrete various metabolites which may soak into and be taken up by
+the other: on the contrary the host-protoplasm grows larger, attracts
+more food supplies, makes larger cells, and is evidently stimulated to
+greater activity for the time being, its behaviour reminding us of the
+stimulation of cells by means of slight doses of poison referred to
+previously. We must therefore assume that the general course of building
+up and breaking down of its protoplasm-molecules go on as usual&mdash;or
+nearly so&mdash;in both the host cell and the invader; and that the
+assimilatory, respiratory, excretory and other functions are carried on
+in <span class="pagenum"><a name="Page_285" id="Page_285"></a>[<a href="./images/285.png">285</a>]</span>the former as in the normal cell, or are but slightly modified to an
+extent which does no immediate injury to its life. But we must further
+assume that the same is also true of the invading protoplasm, and that
+the <i>Plasmodiophora</i> is also supplied with suitable atom-complexes to
+build up its protoplasm molecules, as fast as they are shattered and the
+rejecta burnt off in respiration.</p>
+
+<p>A step further, and we come to instances of <i>Symbiosis</i>, where the
+commingled masses of protoplasm of host and invader continue this
+harmonious action during life. Clearly there are resemblances between
+these latter cases and successful grafts, and between both and
+successful sexual unions where the resulting embryo-cell gives rises to
+a vigorous and healthy plant; and the more these resemblances are
+examined in the light of what we know of symbiosis the more they support
+our contention.</p>
+
+<p>Such considerations as the foregoing suggest, then, that life consists
+in the regular and progressive building up and breaking down of the
+complex protoplasm molecules, and is necessarily accompanied by the
+influx of the indispensable food-elements in certain combinations and
+atom-complexes for assimilation, and by the combustion of some of the
+débris of the shattered molecules, which combine with the oxygen in
+respiration and so afford explosions which raise the temperature and
+enhance the lability of existing molecules, and act as stimuli to the
+shattering of further molecules. The results of these rhythmical
+buildings <span class="pagenum"><a name="Page_286" id="Page_286"></a>[<a href="./images/286.png">286</a>]</span>up (assimilation) and shatterings (dis-assimilation) of the
+protoplasm molecules are the growth of the protoplasm, with further
+intercalations of water and new food-supplies, etc., on the one hand,
+and the formation of metabolic products (proteids, cellulose, sugars,
+fats, etc.), some of which are again used up, others respired, others
+deposited as stores, cell-walls, etc., on the other.</p>
+
+<p>That the building-up process depends on the action of molecular forces
+comparable to those by which a growing crystal goes on selecting
+atom-complexes of its particular kind from the solution around seems
+highly probable, and this being the case we can understand how under
+certain circumstances <i>substitutive</i> selections may occur. That is to
+say, just as a crystal will sometimes build up into its structure
+atom-complexes of a kind different from its normal molecules, so, given
+the proper conditions, a protoplasmic molecular unit will build up into
+its structure atom-complexes somewhat different from those it had
+hitherto taken up&mdash;<i>i.e.</i> assimilated&mdash;with consequent modifications of
+its behaviour. If this occurs, the modes of further building up and
+breaking down will be affected by the subsequent action of these
+slightly modified protoplasm units, <i>and it may well be that the whole
+significance of variation turns on this</i>. Whether the resulting
+variation makes for the welfare or otherwise of the organism will then
+be decided by the struggle for existence, and the natural selection
+which ensues. Such a view also implies that the <span class="pagenum"><a name="Page_287" id="Page_287"></a>[<a href="./images/287.png">287</a>]</span>energy concerned is
+primarily what is usually termed chemical energy, and that every
+compound entering into the protoplasm carries in a supply of this,
+available in various ways.</p>
+
+<p><i>Death</i>, on the contrary, is the cessation of these rhythmical processes
+of building up and breaking down of the protoplasm molecules. It does
+not imply the cessation of chemical changes of other kinds, but that
+these rhythmical constructions of the complex and labile protoplasm
+molecules breaking down on stimulation to bodies partly re-assimilable,
+partly combustible in respiration, and partly excretory, etc., have
+ceased, and that further chemical changes in the material are
+thenceforth simpler and different in kind and degree, eventually leading
+to total disintegration so that no units are left capable of restoring
+the rhythm.</p>
+
+<p>If these ideas are correct, we may define <i>Disease</i> as dangerous
+disturbances in the regularity, or interference with the completeness or
+range of the molecular activities constituting normal Life&mdash;<i>i.e.</i>
+Health&mdash;and it is evident that every degree of transition may be
+realised between the two extremes. Now, if we further assume, as I think
+we must do, that a considerable range or "play" must exist in the
+molecular activities of the protoplasm constituting life, we obtain a
+sort of expression of what we mean by limits of variation. The fact that
+life can go on in a given plant at temperatures between from 1°-5° and
+35°-40° C., or in lights of different intensity, or within <span class="pagenum"><a name="Page_288" id="Page_288"></a>[<a href="./images/288.png">288</a>]</span>considerable
+ranges of water supply, concentration of salts, partial pressure of
+oxygen, etc., implies that the molecular activities of the protoplasm
+are of the normal <i>kind</i> all the time, though they may differ in
+rapidity, and even in <i>quantitative</i> and <i>qualitative</i> respects within
+certain limits; and the meaning of the <i>optimum</i> temperature,
+illumination, oxygen pressure, etc., is, from this point of view, not
+that the molecular activities differ in kind from those nearer the
+minima and maxima, so much as that they are running at the best rates
+for the welfare of the plant&mdash;<i>i.e.</i> for permanent health.</p>
+
+<p>If we transcend the cardinal points limiting the range of this play,
+however, and we get variations in the <i>kind</i> as well as <i>rates</i> of
+molecular constructions and disruptions, then we pass by imperceptible
+gradations into ill-health&mdash;<i>i.e.</i> <i>Disease</i>.</p>
+
+<p>And similarly in relation to other protoplasm. That of the right kind of
+pollen grain from another plant of its own species, stimulates the
+contents of the ovule to produce a vigorous embryo and healthy seedling:
+that of a similar pollen grain in its own flower either does no positive
+harm, but has a feebler effect, or it may act like a poison. That of
+another pollen grain again may refuse to unite at all; while that of a
+fungus hypha&mdash;<i>e.g.</i> of <i>Sclerotinia</i> on <i>Vaccinium</i>&mdash;may run down the
+style as does the pollen tube and produce death and destruction
+throughout the ovule.</p>
+
+<p>Or again, in Clover, we may have the hypha of a <i>Botrytis</i> with its
+protoplasm unable to do more <span class="pagenum"><a name="Page_289" id="Page_289"></a>[<a href="./images/289.png">289</a>]</span>than penetrate into the cellulose walls
+and diffuse a poison into the adjacent cells, being utterly incapable of
+directly facing, or mingling with the living protoplasm of such cells,
+whereas the protoplasm of another organism&mdash;<i>e.g.</i> <i>Rhizobium</i>&mdash;will
+penetrate directly into the cells, live in them for weeks or months
+without injury&mdash;nay even with advantage to their life. And hundreds of
+similar cases can be selected.</p>
+
+<p>We may, therefore, conclude that <i>Variation</i> depends fundamentally on
+alterations in the structure or mode of building up and disintegration
+of the protoplasmic molecular unit, brought about either by direct
+modifying action of the inorganic environment&mdash;nutrition, temperature,
+oxygen supply, light, etc., etc.&mdash;or by the mingling with it of other
+protoplasm, the molecules of which since they have already a slightly
+different composition, configuration, mode of breaking down and building
+up, etc., affect its molecules by supplying them with altered nutritive
+atom-complexes, by competing with them for oxygen, etc., etc. Once these
+molecules are affected, we must assume that long sequences of other
+chemical and molecular changes will be also modified; and although we
+have no conception of <i>how</i> these changes bring about changes in form,
+that they do so is only a conclusion of the same order as that which we
+hold regarding the much simpler changes concerned in the formation of
+crystals.</p>
+
+<p>That such variations may be of every degree as <span class="pagenum"><a name="Page_290" id="Page_290"></a>[<a href="./images/290.png">290</a>]</span>regards profundity,
+permanence, kind, etc., may well be imagined; and there is nothing
+surprising in our being able to induce them more easily by the action of
+external factors <i>in the readily accessible cell-protoplasm</i> than in the
+<i>less exposed nuclear-protoplasm</i>; because the latter is only accessible
+through the former, or through the agency of <i>other nuclear protoplasm
+already modified</i>. On these and similar phenomena depend the relative
+permanency and transmissibility of the variations. Our measure of the
+latter only begins when the effects referred to have become manifest in
+large masses of cells, because only then do they become appreciable to
+our senses.</p>
+
+<p>Further, variations thus induced may be of advantage to the continued
+life of the plant, or in all degrees disadvantageous or threatening to
+its existence. These latter variations are <i>Disease</i>, and if their
+interference with the normal rhythmical play of the building up and
+breaking down of the protoplasm molecules proceeds beyond certain
+limits, life ceases, and we have death supervening on disease.</p>
+
+
+<h4><span class="smcap"><a name="Chapter_XXX_Notes" id="Chapter_XXX_Notes"></a>Notes to Chapter XXX.</span></h4>
+
+<div class="chnote">
+<p>It appears probable that calcium is not always needed by
+living cells, and may not enter into the composition of
+protoplasm; on the other hand traces of iron are perhaps
+necessary.</p>
+
+<p>The criticisms and summary of facts on which the hypothesis
+regarding protoplasm here adopted is based are developed at
+length in Kassowitz, <span lang="de" xml:lang="de"><i>Allgemeine Biologie</i></span>, <span class="pagenum"><a name="Page_291" id="Page_291" style="font-size: 110%;"></a>[<a href="./images/291.png">291</a>]</span>Wien, 1899, B. I.
+and II., where the collected literature may be found, and the
+reader introduced to the huge mass of controversial writings
+put forward since Darwin and associated with the names of
+Weismann and others.</p>
+
+<p>It will probably be noticed that I have employed the term
+molecular unit of protoplasm, and have not discussed the
+question of organised structure in the latter: this is because
+it seems clear to me that living protoplasm as such does not
+possess "organised structure" in the true sense of that
+term&mdash;it is, rather, busy preparing and making "organised
+structure," and a molecular constitution would have to be
+ascribed to all "physiological units" of the nature of
+micellæ, pangens, ids, etc., as truly as to the structural
+units of a starch-grain or cell-wall, or even of a crystal. In
+this connection, the student will find the necessary points of
+view put forward in Pfeffer, <i>Physiology</i>, pp. 37-83.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_292" id="Page_292"></a>[<a href="./images/292.png">292</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_293" id="Page_293"></a>[<a href="./images/293.png">293</a>]</span></p>
+<h2>INDEX.</h2>
+
+
+<ul class="index">
+<li>Absorption by roots, <a href="#Page_49">49</a>.</li>
+
+<li>Absorption of energy, <a href="#Page_23">23</a>.</li>
+
+<li>Absorption of light, <a href="#Page_27">27</a>.</li>
+
+<li>Absorption of water, <a href="#Page_50">50</a>.</li>
+
+<li><i>Abutilon</i>, <a href="#Page_183">183</a>.</li>
+
+<li><i>Acarus</i>, <a href="#Page_88">88</a>.</li>
+
+<li>Accessory buds, <a href="#Page_259">259</a>.</li>
+
+<li><i>Acer</i>, <a href="#Page_214">214</a>.</li>
+
+<li>Acid gases, <a href="#Page_181">181</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Acids, <a href="#Page_130">130</a>, <a href="#Page_136">136</a>.</li>
+
+<li>Acquired characters, <a href="#Page_283">283</a>.</li>
+
+<li><i>Acrostalagmus</i>, <a href="#Page_238">238</a>.</li>
+
+<li>Action of the environment, <a href="#Page_271">271</a>.</li>
+
+<li>Adaptation, <a href="#Page_176">176</a>.</li>
+
+<li>Adapted races, <a href="#Page_177">177</a>.</li>
+
+<li><i>Adonis</i>, <a href="#Page_220">220</a>.</li>
+
+<li>Adventitious buds, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>, <a href="#Page_257">257</a>, <a href="#Page_260">260</a>.</li>
+
+<li><i>Æcidium</i>, <a href="#Page_88">88</a>, <a href="#Page_114">114</a>, <a href="#Page_116">116</a>, <a href="#Page_187">187</a>, <a href="#Page_188">188</a>, <a href="#Page_189">189</a>, <a href="#Page_217">217</a>, <a href="#Page_223">223</a>, <a href="#Page_225">225</a>, <a href="#Page_232">232</a>, <a href="#Page_247">247</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Aeration, <a href="#Page_104">104</a>.</li>
+
+<li>Aerobic organisms, <a href="#Page_57">57</a>.</li>
+
+<li>Aetiology, <a href="#Page_89">89</a>, <a href="#Page_100">100</a>.</li>
+
+<li><i>Agaricus melleus</i>, <a href="#Page_115">115</a>, <a href="#Page_143">143</a>, <a href="#Page_145">145</a>, <a href="#Page_234">234</a>.</li>
+
+<li>Agents of disease, <a href="#Page_113">113</a>.</li>
+
+<li><i>Aglaospora</i>, <a href="#Page_223">223</a>.</li>
+
+<li>Agriculture, <a href="#Page_65">65</a>.</li>
+
+<li>Agricultural Chemistry, <a href="#Page_2">2</a>.</li>
+
+<li><i>Ajuga</i>, <a href="#Page_217">217</a>.</li>
+
+<li>Albinism, <a href="#Page_179">179</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_186">186</a>.</li>
+
+<li>Alder, <a href="#Page_207">207</a>, <a href="#Page_219">219</a>.</li>
+
+<li>Aleurone layer, <a href="#Page_173">173</a>.</li>
+
+<li>Algae, <a href="#Page_215">215</a>.</li>
+
+<li><i>Allium</i>, <a href="#Page_258">258</a>.</li>
+
+<li>Almond, <a href="#Page_168">168</a>.</li>
+
+<li><i>Alnus</i>, <a href="#Page_214">214</a>.</li>
+
+<li><i>Aloe</i>, <a href="#Page_134">134</a>, <a href="#Page_161">161</a>.</li>
+
+<li>Alpine plants, <a href="#Page_250">250</a>, <a href="#Page_279">279</a>.</li>
+
+<li>American blight, <a href="#Page_164">164</a>, <a href="#Page_219">219</a>.</li>
+
+<li>American vines, <a href="#Page_155">155</a>, <a href="#Page_169">169</a>, <a href="#Page_172">172</a>.</li>
+
+<li>Amides, <a href="#Page_31">31</a>.</li>
+
+<li>Amoeba, <a href="#Page_144">144</a>.</li>
+
+<li>Amount of energy stored, <a href="#Page_25">25</a>.</li>
+
+<li>Amygdalin, <a href="#Page_173">173</a>.</li>
+
+<li><i>Anabaena</i>, <a href="#Page_128">128</a>.</li>
+
+<li>Anaerobic bacteria, <a href="#Page_58">58</a>, <a href="#Page_237">237</a>.</li>
+
+<li><i>Anagallis</i>, <a href="#Page_261">261</a>.</li>
+
+<li>Analyses, <a href="#Page_65">65</a>.</li>
+
+<li>Analyses of waters, <a href="#Page_58">58</a>.</li>
+
+<li>Anemone, <a href="#Page_247">247</a>.</li>
+
+<li>Animals, <a href="#Page_99">99</a>, <a href="#Page_108">108</a>, <a href="#Page_142">142</a>, <a href="#Page_207">207</a>.</li>
+
+<li><i>Antennaria</i>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Anthonomos</i>, <a href="#Page_249">249</a>.</li>
+
+<li>Anthrax, <a href="#Page_144">144</a>.</li>
+
+<li>Antiseptics, <a href="#Page_162">162</a>.</li>
+
+<li><span class="pagenum"><a name="Page_294" id="Page_294"></a>[<a href="./images/294.png">294</a>]</span>Ants, <a href="#Page_232">232</a>.</li>
+
+<li><i>Aphis</i>, <a href="#Page_88">88</a>, <a href="#Page_109">109</a>, <a href="#Page_161">161</a>, <a href="#Page_165">165</a>, <a href="#Page_188">188</a>, <a href="#Page_213">213</a>, <a href="#Page_214">214</a>, <a href="#Page_232">232</a>, <a href="#Page_241">241</a>, <a href="#Page_253">253</a>.</li>
+
+<li><i>Aphrophora</i>, <a href="#Page_233">233</a>.</li>
+
+<li>Apogamy, <a href="#Page_257">257</a>, <a href="#Page_261">261</a>.</li>
+
+<li><i>Aporia Crataegi</i>, <a href="#Page_187">187</a>.</li>
+
+<li>Apospory, <a href="#Page_257">257</a>, <a href="#Page_261">261</a>.</li>
+
+<li>Apple, <a href="#Page_170">170</a>, <a href="#Page_171">171</a>, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>, <a href="#Page_192">192</a>, <a href="#Page_206">206</a>, <a href="#Page_217">217</a>, <a href="#Page_218">218</a>, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>, <a href="#Page_226">226</a>, <a href="#Page_231">231</a>, <a href="#Page_233">233</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_253">253</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Apricot, <a href="#Page_188">188</a>, <a href="#Page_206">206</a>.</li>
+
+<li>Apricots, <a href="#Page_234">234</a>.</li>
+
+<li>Area of root-surface, <a href="#Page_37">37</a>, <a href="#Page_39">39</a>.</li>
+
+<li><i>Arisarum</i>, <a href="#Page_188">188</a>.</li>
+
+<li><i>Aristolochia</i>, <a href="#Page_259">259</a>.</li>
+
+<li>Aroids, <a href="#Page_113">113</a>.</li>
+
+<li>Arrest of growth, <a href="#Page_246">246</a>.</li>
+
+<li>Arsenic, <a href="#Page_162">162</a>.</li>
+
+<li>Artificial wounds, <a href="#Page_194">194</a>.</li>
+
+<li>Ascomycetes, <a href="#Page_189">189</a>, <a href="#Page_217">217</a>, <a href="#Page_269">269</a>.</li>
+
+<li><i>Ascochyta</i>, <a href="#Page_190">190</a>.</li>
+
+<li>Ash, <a href="#Page_182">182</a>, <a href="#Page_223">223</a>, <a href="#Page_225">225</a>, <a href="#Page_251">251</a>.</li>
+
+<li><i>Asparagus</i>, <a href="#Page_180">180</a>, <a href="#Page_230">230</a>, <a href="#Page_251">251</a>, <a href="#Page_252">252</a>.</li>
+
+<li><i>Aspergillus</i>, <a href="#Page_231">231</a>.</li>
+
+<li><i>Aspergillus niger</i>, <a href="#Page_58">58</a>.</li>
+
+<li><i>Aspidiotus</i>, <a href="#Page_187">187</a>.</li>
+
+<li>Assimilation, <a href="#Page_8">8</a>, <a href="#Page_21">21</a>, <a href="#Page_133">133</a>, <a href="#Page_271">271</a>, <a href="#Page_275">275</a>, <a href="#Page_277">277</a>, <a href="#Page_285">285</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Assimilates, <a href="#Page_274">274</a>.</li>
+
+<li>Atmosphere, <a href="#Page_1">1</a>, <a href="#Page_99">99</a>.</li>
+
+<li>Atmospheric influences, <a href="#Page_101">101</a>.</li>
+
+<li>Atrophy, <a href="#Page_246">246</a>, <a href="#Page_247">247</a>, <a href="#Page_266">266</a>.</li>
+
+<li>Attractive substances, <a href="#Page_136">136</a>.</li>
+
+<li><i>Aucuba</i>, <a href="#Page_264">264</a>.</li>
+
+<li>Autumnal colouring, <a href="#Page_191">191</a>.</li>
+
+<li>Autumnal fall, <a href="#Page_93">93</a>.</li>
+
+<li>Avalanches, <a href="#Page_106">106</a>.</li>
+
+
+<li class="newletter">Bacteria, <a href="#Page_102">102</a>, <a href="#Page_133">133</a>, <a href="#Page_143">143</a>, <a href="#Page_168">168</a>, <a href="#Page_173">173</a>, <a href="#Page_176">176</a>, <a href="#Page_182">182</a>, <a href="#Page_190">190</a>, <a href="#Page_200">200</a>, <a href="#Page_216">216</a>, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>, <a href="#Page_227">227</a>, <a href="#Page_231">231</a>, <a href="#Page_236">236</a>, <a href="#Page_237">237</a>.</li>
+
+<li>Bacteriosis, <a href="#Page_227">227</a>.</li>
+
+<li>Barberry, <a href="#Page_176">176</a>.</li>
+
+<li>Bark boring, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</li>
+
+<li>Bark-beetles, <a href="#Page_205">205</a>.</li>
+
+<li>Barley, <a href="#Page_176">176</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Barrenness, <a href="#Page_246">246</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Bats, <a href="#Page_244">244</a>.</li>
+
+<li>Bean, <a href="#Page_188">188</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Beech, <a href="#Page_192">192</a>, <a href="#Page_222">222</a>, <a href="#Page_223">223</a>, <a href="#Page_225">225</a>, <a href="#Page_233">233</a>, <a href="#Page_240">240</a>, <a href="#Page_242">242</a>, <a href="#Page_254">254</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Beech Miner, <a href="#Page_208">208</a>.</li>
+
+<li>Bees, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>, <a href="#Page_164">164</a>.</li>
+
+<li>Beet, <a href="#Page_192">192</a>, <a href="#Page_216">216</a>, <a href="#Page_219">219</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Beet-rot, <a href="#Page_230">230</a>.</li>
+
+<li>Beetles, <a href="#Page_110">110</a>, <a href="#Page_143">143</a>, <a href="#Page_145">145</a>, <a href="#Page_205">205</a>, <a href="#Page_206">206</a>, <a href="#Page_207">207</a>, <a href="#Page_248">248</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Berkeley, <a href="#Page_85">85</a>.</li>
+
+<li>Bilberries, <a href="#Page_116">116</a>, <a href="#Page_142">142</a>, <a href="#Page_217">217</a>, <a href="#Page_218">218</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Biology of soil, <a href="#Page_56">56</a>, <a href="#Page_102">102</a>.</li>
+
+<li>Birch, <a href="#Page_207">207</a>, <a href="#Page_218">218</a>, <a href="#Page_224">224</a>.</li>
+
+<li>Birds, <a href="#Page_108">108</a>, <a href="#Page_144">144</a>, <a href="#Page_164">164</a>, <a href="#Page_166">166</a>.</li>
+
+<li>Bird's-eye Maple, <a href="#Page_224">224</a>.</li>
+
+<li>Black spots on leaves, <a href="#Page_186">186</a>, <a href="#Page_189">189</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Bladders, <a href="#Page_218">218</a>.</li>
+
+<li>Blemish, <a href="#Page_198">198</a>.</li>
+
+<li>Blights, <a href="#Page_86">86</a>, <a href="#Page_104">104</a>, <a href="#Page_179">179</a>.</li>
+
+<li>Blisters, <a href="#Page_230">230</a>.</li>
+
+<li>Blue rays, <a href="#Page_21">21</a>.</li>
+
+<li><i>Bombyx</i>, <a href="#Page_187">187</a>, <a href="#Page_218">218</a>.</li>
+
+<li>Bordeaux mixture, <a href="#Page_162">162</a>.</li>
+
+<li>Boring, <a href="#Page_204">204</a>.</li>
+
+<li><i>Botrytis</i>, <a href="#Page_131">131</a>, <a href="#Page_132">132</a>, <a href="#Page_136">136</a>, <a href="#Page_175">175</a>, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>, <a href="#Page_243">243</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Boussingault, <a href="#Page_5">5</a>, <a href="#Page_10">10</a>.</li>
+
+<li>Bouvardia, <a href="#Page_260">260</a>.</li>
+
+<li><span class="pagenum"><a name="Page_295" id="Page_295"></a>[<a href="./images/295.png">295</a>]</span>Bramble, <a href="#Page_112">112</a>.</li>
+
+<li>Branch stumps, <a href="#Page_194">194</a>, <a href="#Page_199">199</a>.</li>
+
+<li>Brand, <a href="#Page_240">240</a>.</li>
+
+<li>Breeding, <a href="#Page_78">78</a>.</li>
+
+<li>Briars, <a href="#Page_113">113</a>.</li>
+
+<li>Broom-rapes, <a href="#Page_265">265</a>.</li>
+
+<li>Browning, <a href="#Page_122">122</a>, <a href="#Page_186">186</a>, <a href="#Page_235">235</a>.</li>
+
+<li>Brown spots, <a href="#Page_186">186</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Browsing, <a href="#Page_244">244</a>.</li>
+
+<li><i>Bruchus</i>, <a href="#Page_248">248</a>.</li>
+
+<li>Bruises, <a href="#Page_194">194</a>, <a href="#Page_203">203</a>, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>.</li>
+
+<li>Bryony, <a href="#Page_112">112</a>.</li>
+
+<li><i>Bryophyllum</i>, <a href="#Page_260">260</a>.</li>
+
+<li>Bud galls, <a href="#Page_219">219</a>.</li>
+
+<li>Bud variations, <a href="#Page_92">92</a>, <a href="#Page_93">93</a>.</li>
+
+<li>Bulb diseases, <a href="#Page_227">227</a>.</li>
+
+<li>Buried objects, <a href="#Page_211">211</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Burning, <a href="#Page_191">191</a>.</li>
+
+<li>Burning-glass effect, <a href="#Page_192">192</a>.</li>
+
+<li>Burrows, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</li>
+
+<li>Burrs, <a href="#Page_222">222</a>, <a href="#Page_223">223</a>, <a href="#Page_224">224</a>.</li>
+
+<li>Bursting of fruits, <a href="#Page_227">227</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Butterflies, <a href="#Page_145">145</a>.</li>
+
+<li>Bye-products, <a href="#Page_276">276</a>.</li>
+
+
+<li class="newletter">Cabbage, <a href="#Page_253">253</a>.</li>
+
+<li>Cabbage moth, <a href="#Page_208">208</a>.</li>
+
+<li><i>Caeoma</i>, <a href="#Page_252">252</a>.</li>
+
+<li><i>Caesalpinia</i>, <a href="#Page_233">233</a>.</li>
+
+<li>Calcium, <a href="#Page_272">272</a>.</li>
+
+<li>Calcium oxalate, <a href="#Page_138">138</a>.</li>
+
+<li><i>Calla</i>, <a href="#Page_183">183</a>.</li>
+
+<li><i>Calliandra</i>, <a href="#Page_233">233</a>.</li>
+
+<li>Callus, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>, <a href="#Page_124">124</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_196">196</a>, <a href="#Page_197">197</a>, <a href="#Page_199">199</a>, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_210">210</a>, <a href="#Page_241">241</a>, <a href="#Page_260">260</a>, <a href="#Page_263">263</a>, <a href="#Page_269">269</a>.</li>
+
+<li><i>Calyptospora</i>, <a href="#Page_116">116</a>, <a href="#Page_217">217</a>.</li>
+
+<li>Cambium, <a href="#Page_120">120</a>, <a href="#Page_196">196</a>, <a href="#Page_199">199</a>, <a href="#Page_222">222</a>.</li>
+
+<li>Camellia, <a href="#Page_187">187</a>.</li>
+
+<li>Cancer, <a href="#Page_127">127</a>.</li>
+
+<li>Canker, <a href="#Page_87">87</a>, <a href="#Page_222">222</a>, <a href="#Page_223">223</a>, <a href="#Page_241">241</a>.</li>
+
+<li><i>Capnodium</i>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Capsella</i>, <a href="#Page_116">116</a>, <a href="#Page_175">175</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Carbohydrates, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_20">20</a>, <a href="#Page_34">34</a>, <a href="#Page_122">122</a>, <a href="#Page_184">184</a>, <a href="#Page_272">272</a>, <a href="#Page_273">273</a>, <a href="#Page_277">277</a>.</li>
+
+<li>Carbolic acid, <a href="#Page_162">162</a>.</li>
+
+<li>Carbon, <a href="#Page_272">272</a>.</li>
+
+<li>Carbon assimilation, <a href="#Page_8">8</a>, <a href="#Page_10">10</a>, <a href="#Page_28">28</a>, <a href="#Page_106">106</a>.</li>
+
+<li>Carbon-bisulphide, <a href="#Page_163">163</a>.</li>
+
+<li>Cardinal points, <a href="#Page_288">288</a>.</li>
+
+<li>Carrot, <a href="#Page_164">164</a>.</li>
+
+<li><i>Carpocapsa</i>, <a href="#Page_207">207</a>.</li>
+
+<li>Cast branches, <a href="#Page_123">123</a>.</li>
+
+<li>Castor oil, <a href="#Page_172">172</a>.</li>
+
+<li>Caterpillars, <a href="#Page_109">109</a>, <a href="#Page_164">164</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_244">244</a>, <a href="#Page_254">254</a>, <a href="#Page_259">259</a>.</li>
+
+<li>Cats, <a href="#Page_164">164</a>.</li>
+
+<li>Cattle, <a href="#Page_108">108</a>.</li>
+
+<li>Cauliflowers, <a href="#Page_247">247</a>, <a href="#Page_250">250</a>.</li>
+
+<li>Causes of disease, <a href="#Page_89">89</a>, <a href="#Page_99">99</a>, <a href="#Page_108">108</a>, <a href="#Page_159">159</a>, <a href="#Page_278">278</a>, <a href="#Page_282">282</a>.</li>
+
+<li><i>Cecidia</i>, <a href="#Page_212">212</a>.</li>
+
+<li><i>Cecidomyia</i>, <a href="#Page_182">182</a>, <a href="#Page_213">213</a>, <a href="#Page_214">214</a>, <a href="#Page_218">218</a>, <a href="#Page_219">219</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Celery, <a href="#Page_180">180</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Cell contents, <a href="#Page_168">168</a>.</li>
+
+<li>Cell-protoplasm, <a href="#Page_279">279</a>, <a href="#Page_280">280</a>, <a href="#Page_290">290</a>.</li>
+
+<li>Cellulose, <a href="#Page_132">132</a>, <a href="#Page_277">277</a>, <a href="#Page_286">286</a>.</li>
+
+<li><i>Celosia</i>, <a href="#Page_250">250</a>.</li>
+
+<li><i>Centaurea</i>, <a href="#Page_188">188</a>.</li>
+
+<li><i>Centhorhynchus</i>, <a href="#Page_219">219</a>.</li>
+
+<li><i>Cephaleuros</i>, <a href="#Page_188">188</a>.</li>
+
+<li><i>Cephus</i>, <a href="#Page_248">248</a>.</li>
+
+<li><i>Cercis</i>, <a href="#Page_259">259</a>.</li>
+
+<li><i>Cercospora</i>, <a href="#Page_190">190</a>.</li>
+
+<li>Cereals, <a href="#Page_248">248</a>.</li>
+
+<li>Change of conditions, <a href="#Page_78">78</a>.</li>
+
+<li><span class="pagenum"><a name="Page_296" id="Page_296"></a>[<a href="./images/296.png">296</a>]</span>Charlock, <a href="#Page_165">165</a>.</li>
+
+<li>Checks to disease, <a href="#Page_166">166</a>.</li>
+
+<li>Chemical analysis, <a href="#Page_32">32</a>, <a href="#Page_64">64</a>, <a href="#Page_103">103</a>, <a href="#Page_272">272</a>.</li>
+
+<li>Chemical antiseptics, <a href="#Page_159">159</a>.</li>
+
+<li>Chemical energy, <a href="#Page_29">29</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Chemotactic phenomena, <a href="#Page_72">72</a>, <a href="#Page_130">130</a>, <a href="#Page_135">135</a>, <a href="#Page_137">137</a>.</li>
+
+<li><i>Chermes</i>, <a href="#Page_153">153</a>, <a href="#Page_223">223</a>.</li>
+
+<li>Cherry, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_231">231</a>, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>, <a href="#Page_247">247</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Chestnut, <a href="#Page_190">190</a>.</li>
+
+<li>Chlorine, <a href="#Page_181">181</a>.</li>
+
+<li>Chlorophyll, <a href="#Page_19">19</a>, <a href="#Page_106">106</a>, <a href="#Page_122">122</a>.</li>
+
+<li>Chlorophyll action, <a href="#Page_184">184</a>, <a href="#Page_192">192</a>.</li>
+
+<li>Chlorophyll corpuscles, <a href="#Page_9">9</a>, <a href="#Page_18">18</a>, <a href="#Page_22">22</a>.</li>
+
+<li>Chlorosis, <a href="#Page_122">122</a>, <a href="#Page_165">165</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_181">181</a>.</li>
+
+<li>Chrysanthemum, <a href="#Page_243">243</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Chytridiaceae, <a href="#Page_127">127</a>, <a href="#Page_136">136</a>, <a href="#Page_189">189</a>, <a href="#Page_208">208</a>.</li>
+
+<li><i>Cicada</i>, <a href="#Page_235">235</a>.</li>
+
+<li>Cicatrix, <a href="#Page_123">123</a>.</li>
+
+<li><i>Cinchona</i>, <a href="#Page_168">168</a>, <a href="#Page_172">172</a>, <a href="#Page_173">173</a>.</li>
+
+<li>Circulation of carbon, <a href="#Page_62">62</a>.</li>
+
+<li>Circulation of nitrogen, <a href="#Page_62">62</a>.</li>
+
+<li><i>Citrus</i>, <a href="#Page_168">168</a>.</li>
+
+<li><i>Clasterosporium</i>, <a href="#Page_188">188</a>, <a href="#Page_209">209</a>.</li>
+
+<li>Classification of diseases, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>, <a href="#Page_120">120</a>.</li>
+
+<li><i>Claviceps</i>, <a href="#Page_232">232</a>.</li>
+
+<li>Climate, <a href="#Page_1">1</a>.</li>
+
+<li>Climbing plants, <a href="#Page_112">112</a>, <a href="#Page_113">113</a>, <a href="#Page_210">210</a>.</li>
+
+<li><i>Clostridium</i>, <a href="#Page_236">236</a>, <a href="#Page_237">237</a>.</li>
+
+<li>Clothes, <a href="#Page_142">142</a>.</li>
+
+<li>Clover, <a href="#Page_164">164</a>, <a href="#Page_187">187</a>, <a href="#Page_249">249</a>, <a href="#Page_252">252</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Cluster-cups, <a href="#Page_188">188</a>.</li>
+
+<li>Coal gas, <a href="#Page_104">104</a>, <a href="#Page_182">182</a>.</li>
+
+<li>Coccideae, <a href="#Page_164">164</a>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Coccus</i>, <a href="#Page_223">223</a>.</li>
+
+<li>Coffee leaf-disease, <a href="#Page_114">114</a>, <a href="#Page_146">146</a>, <a href="#Page_166">166</a>, <a href="#Page_169">169</a>, <a href="#Page_242">242</a>.</li>
+
+<li><i>Coleophora</i>, <a href="#Page_153">153</a>, <a href="#Page_206">206</a>.</li>
+
+<li><i>Coleosporium</i>, <a href="#Page_169">169</a>.</li>
+
+<li><i>Coleus</i>, <a href="#Page_192">192</a>, <a href="#Page_220">220</a>.</li>
+
+<li>Competition of fungi, <a href="#Page_61">61</a>.</li>
+
+<li>Complex interactions, <a href="#Page_91">91</a>, <a href="#Page_99">99</a>.</li>
+
+<li>Conifers, <a href="#Page_125">125</a>, <a href="#Page_205">205</a>, <a href="#Page_223">223</a>, <a href="#Page_225">225</a>, <a href="#Page_234">234</a>, <a href="#Page_258">258</a>.</li>
+
+<li>Constitution, <a href="#Page_156">156</a>, <a href="#Page_283">283</a>.</li>
+
+<li>Consumption, <a href="#Page_248">248</a>.</li>
+
+<li>Contact irritability, <a href="#Page_125">125</a>, <a href="#Page_135">135</a>.</li>
+
+<li><i>Contagium fluidum vivum</i>, <a href="#Page_183">183</a>.</li>
+
+<li>Contortions, <a href="#Page_252">252</a>.</li>
+
+<li><i>Convallaria</i>, <a href="#Page_175">175</a>.</li>
+
+<li><i>Convolvulus</i>, <a href="#Page_112">112</a>.</li>
+
+<li><i>Copaifera</i>, <a href="#Page_234">234</a>.</li>
+
+<li>Copper sulphate, <a href="#Page_162">162</a>, <a href="#Page_165">165</a>.</li>
+
+<li>Coppery leaves, <a href="#Page_191">191</a>.</li>
+
+<li>Cork, <a href="#Page_119">119</a>, <a href="#Page_123">123</a>, <a href="#Page_194">194</a>, <a href="#Page_199">199</a>, <a href="#Page_216">216</a>, <a href="#Page_222">222</a>.</li>
+
+<li>Cork wings, <a href="#Page_217">217</a>.</li>
+
+<li>Corky warts, <a href="#Page_212">212</a>.</li>
+
+<li>Corn, <a href="#Page_248">248</a>.</li>
+
+<li>Corrosion of marble, <a href="#Page_46">46</a>.</li>
+
+<li><i>Cossus</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Cost of epidemics, <a href="#Page_146">146</a>, <a href="#Page_147">147</a>.</li>
+
+<li>Cotton, <a href="#Page_172">172</a>.</li>
+
+<li><i>Crassula</i>, <a href="#Page_253">253</a>.</li>
+
+<li>Creeping of mycelia, <a href="#Page_142">142</a>.</li>
+
+<li><i>Crepis</i>, <a href="#Page_252">252</a>.</li>
+
+<li>Crimson spots, <a href="#Page_189">189</a>.</li>
+
+<li>Cross-breeding, <a href="#Page_266">266</a>.</li>
+
+<li>Cross-fertilisation, <a href="#Page_69">69</a>, <a href="#Page_74">74</a>, <a href="#Page_77">77</a>, <a href="#Page_281">281</a>.</li>
+
+<li>Cross-graining, <a href="#Page_124">124</a>.</li>
+
+<li>Crucifers, <a href="#Page_219">219</a>, <a href="#Page_284">284</a>.</li>
+
+<li>Cryptogams, <a href="#Page_87">87</a>, <a href="#Page_108">108</a>, <a href="#Page_111">111</a>, <a href="#Page_113">113</a>.</li>
+
+<li>Cuckoo-spit, <a href="#Page_233">233</a>.</li>
+
+<li><span class="pagenum"><a name="Page_297" id="Page_297"></a>[<a href="./images/297.png">297</a>]</span>Cucullate leaves, <a href="#Page_253">253</a>.</li>
+
+<li>Cucumber, <a href="#Page_219">219</a>.</li>
+
+<li><i>Cucurbitaria</i>, <a href="#Page_217">217</a>, <a href="#Page_243">243</a>.</li>
+
+<li>Cultivation of pest and host plant, <a href="#Page_168">168</a>.</li>
+
+<li><i>Curculio</i>, <a href="#Page_248">248</a>.</li>
+
+<li>Curling, <a href="#Page_235">235</a>, <a href="#Page_246">246</a>.</li>
+
+<li><i>Cuscuta</i>, <a href="#Page_134">134</a>.</li>
+
+<li>Cuts, <a href="#Page_119">119</a>, <a href="#Page_143">143</a>, <a href="#Page_194">194</a>.</li>
+
+<li>Cuttings, <a href="#Page_194">194</a>, <a href="#Page_198">198</a>, <a href="#Page_262">262</a>, <a href="#Page_263">263</a>.</li>
+
+<li>Cyanide of potassium, <a href="#Page_165">165</a>.</li>
+
+<li>Cycads, <a href="#Page_128">128</a>.</li>
+
+<li><i>Cynips</i>, <a href="#Page_110">110</a>, <a href="#Page_213">213</a>, <a href="#Page_219">219</a>.</li>
+
+<li><i>Cystopus</i>, <a href="#Page_116">116</a>, <a href="#Page_136">136</a>, <a href="#Page_175">175</a>, <a href="#Page_187">187</a>, <a href="#Page_217">217</a>, <a href="#Page_247">247</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Cytases, <a href="#Page_132">132</a>.</li>
+
+<li><i>Cytisus Adami</i>, <a href="#Page_264">264</a>, <a href="#Page_283">283</a>.</li>
+
+
+<li class="newletter">Daisy, <a href="#Page_278">278</a>.</li>
+
+<li>Damping off, <a href="#Page_114">114</a>, <a href="#Page_144">144</a>, <a href="#Page_160">160</a>, <a href="#Page_229">229</a>, <a href="#Page_284">284</a>.</li>
+
+<li>Dandelion, <a href="#Page_247">247</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Daniel's researches, <a href="#Page_283">283</a>.</li>
+
+<li>Dark heat rays, <a href="#Page_27">27</a>.</li>
+
+<li>Darwin, <a href="#Page_72">72</a>, <a href="#Page_125">125</a>.</li>
+
+<li><i>Dasyscypha Willkommii</i>, <a href="#Page_152">152</a>, <a href="#Page_223">223</a>.</li>
+
+<li>Death, <a href="#Page_271">271</a>, <a href="#Page_272">272</a>, <a href="#Page_287">287</a>, <a href="#Page_290">290</a>.</li>
+
+<li>De Bary, <a href="#Page_85">85</a>, <a href="#Page_151">151</a>.</li>
+
+<li>Deficiency of iron, <a href="#Page_180">180</a>.</li>
+
+<li>Defoliation, <a href="#Page_109">109</a>, <a href="#Page_240">240</a>, <a href="#Page_244">244</a>.</li>
+
+<li>Deformation, <a href="#Page_132">132</a>.</li>
+
+<li><i>Dematium</i>, <a href="#Page_135">135</a>.</li>
+
+<li><i>Dematophora</i>, <a href="#Page_143">143</a>, <a href="#Page_145">145</a>.</li>
+
+<li>Denitrification, <a href="#Page_62">62</a>.</li>
+
+<li>Derivation of Phytopathology, <a href="#Page_85">85</a>.</li>
+
+<li>Destruction, <a href="#Page_275">275</a>.</li>
+
+<li>Development of root-hairs, <a href="#Page_40">40</a>.</li>
+
+<li>Dextrine, <a href="#Page_173">173</a>.</li>
+
+<li>Diagnosis, <a href="#Page_85">85</a>, <a href="#Page_89">89</a>.</li>
+
+<li>Diastases, <a href="#Page_132">132</a>.</li>
+
+<li>Diffusion, <a href="#Page_53">53</a>.</li>
+
+<li>Digestion, <a href="#Page_133">133</a>.</li>
+
+<li><i>Digraphis</i>, <a href="#Page_175">175</a>.</li>
+
+<li><i>Dilophia</i>, <a href="#Page_188">188</a>.</li>
+
+<li><i>Dionaea</i>, <a href="#Page_125">125</a>.</li>
+
+<li><i>Dipsacus</i>, <a href="#Page_252">252</a>.</li>
+
+<li><i>Diptera</i>, <a href="#Page_207">207</a>.</li>
+
+<li>Dis-assimilation, <a href="#Page_275">275</a>, <a href="#Page_277">277</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Discolorations, <a href="#Page_179">179</a>, <a href="#Page_186">186</a>, <a href="#Page_192">192</a>.</li>
+
+<li>Disease, <a href="#Page_64">64</a>, <a href="#Page_91">91</a>, <a href="#Page_271">271</a>, <a href="#Page_272">272</a>, <a href="#Page_277">277</a>, <a href="#Page_287">287</a>, <a href="#Page_288">288</a>, <a href="#Page_290">290</a>.</li>
+
+<li>Disease dodging, <a href="#Page_168">168</a>.</li>
+
+<li>Disease-fungi, <a href="#Page_189">189</a>.</li>
+
+<li>Disease of organs, <a href="#Page_119">119</a>.</li>
+
+<li>"Disease-proof" varieties, <a href="#Page_168">168</a>, <a href="#Page_169">169</a>, <a href="#Page_171">171</a>, <a href="#Page_173">173</a>, <a href="#Page_177">177</a>.</li>
+
+<li>Disease-resisting varieties, <a href="#Page_177">177</a>.</li>
+
+<li>Diseases of absorptive organs, <a href="#Page_121">121</a>.</li>
+
+<li>Diseases of assimilatory organs, <a href="#Page_119">119</a>.</li>
+
+<li>Diseases of bark, <a href="#Page_120">120</a>.</li>
+
+<li>Diseases of cambium, <a href="#Page_120">120</a>.</li>
+
+<li>Diseases of parenchyma, <a href="#Page_120">120</a>.</li>
+
+<li>Diseases of respiratory organs, <a href="#Page_119">119</a>, <a href="#Page_121">121</a>.</li>
+
+<li>Disintegration, <a href="#Page_275">275</a>.</li>
+
+<li>Distortions, <a href="#Page_140">140</a>, <a href="#Page_246">246</a>, <a href="#Page_251">251</a>, <a href="#Page_252">252</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Dissemination of fungi, <a href="#Page_142">142</a>.</li>
+
+<li>Division, <a href="#Page_127">127</a>.</li>
+
+<li>Dodder, <a href="#Page_113">113</a>.</li>
+
+<li><i>Dolium</i>, <a href="#Page_134">134</a>.</li>
+
+<li>Dormant buds, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>, <a href="#Page_257">257</a>, <a href="#Page_259">259</a>, <a href="#Page_260">260</a>.</li>
+
+<li>Double flowers, <a href="#Page_247">247</a>, <a href="#Page_256">256</a>.</li>
+
+<li><span class="pagenum"><a name="Page_298" id="Page_298"></a>[<a href="./images/298.png">298</a>]</span>Double ideals in selection, <a href="#Page_168">168</a>.</li>
+
+<li><i>Dracaena</i>, <a href="#Page_192">192</a>.</li>
+
+<li>Drainage, <a href="#Page_103">103</a>.</li>
+
+<li>Drawing, <a href="#Page_106">106</a>, <a href="#Page_180">180</a>.</li>
+
+<li>Drip, <a href="#Page_103">103</a>.</li>
+
+<li>Drooping, <a href="#Page_43">43</a>, <a href="#Page_179">179</a>.</li>
+
+<li>Drops of water, <a href="#Page_192">192</a>.</li>
+
+<li>Dropsy, <a href="#Page_228">228</a>.</li>
+
+<li>Drought, <a href="#Page_121">121</a>, <a href="#Page_183">183</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_245">245</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Dry-rot, <a href="#Page_143">143</a>, <a href="#Page_237">237</a>.</li>
+
+<li>Ducks, <a href="#Page_144">144</a>.</li>
+
+<li>Dutrochet, <a href="#Page_7">7</a>.</li>
+
+<li>Dwarfing, <a href="#Page_246">246</a>, <a href="#Page_249">249</a>.</li>
+
+<li>"Dying back," <a href="#Page_190">190</a>, <a href="#Page_240">240</a>, <a href="#Page_242">242</a>, <a href="#Page_243">243</a>, <a href="#Page_244">244</a>.</li>
+
+
+<li class="newletter">Earwigs, <a href="#Page_164">164</a>, <a href="#Page_207">207</a>.</li>
+
+<li><i>Eau Céleste</i>, <a href="#Page_162">162</a>.</li>
+
+<li><i>Edelfäule</i>, <a href="#Page_230">230</a>.</li>
+
+<li>Eelworms, <a href="#Page_111">111</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Effects of environment, <a href="#Page_262">262</a>.</li>
+
+<li>Eggs of insects, <a href="#Page_187">187</a>.</li>
+
+<li>Elaborated sap, <a href="#Page_94">94</a>.</li>
+
+<li>Elm, <a href="#Page_218">218</a>, <a href="#Page_224">224</a>, <a href="#Page_225">225</a>, <a href="#Page_233">233</a>, <a href="#Page_260">260</a>.</li>
+
+<li><i>Empusa</i>, <a href="#Page_163">163</a>.</li>
+
+<li>Endemic diseases, <a href="#Page_153">153</a>, <a href="#Page_160">160</a>, <a href="#Page_166">166</a>.</li>
+
+<li>Endive, <a href="#Page_180">180</a>.</li>
+
+<li>Endophytes, <a href="#Page_130">130</a>.</li>
+
+<li>Endophytic algae, <a href="#Page_128">128</a>.</li>
+
+<li>Endophytic fungi, <a href="#Page_193">193</a>.</li>
+
+<li>Energy in plants, <a href="#Page_15">15</a>, <a href="#Page_25">25</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Engelmann, <a href="#Page_20">20</a>, <a href="#Page_27">27</a>.</li>
+
+<li><i>Entyloma</i>, <a href="#Page_187">187</a>.</li>
+
+<li>Enzymes, <a href="#Page_10">10</a>, <a href="#Page_130">130</a>, <a href="#Page_132">132</a>, <a href="#Page_136">136</a>.</li>
+
+<li><i>Epichloë</i>, <a href="#Page_218">218</a>.</li>
+
+<li>Epicormic shoots, <a href="#Page_224">224</a>, <a href="#Page_257">257</a>, <a href="#Page_260">260</a>.</li>
+
+<li>Epidemics, <a href="#Page_108">108</a>, <a href="#Page_109">109</a>, <a href="#Page_113">113</a>, <a href="#Page_115">115</a>, <a href="#Page_142">142</a>, <a href="#Page_153">153</a>, <a href="#Page_160">160</a>, <a href="#Page_163">163</a>, <a href="#Page_166">166</a>.</li>
+
+<li>Epiphytes, <a href="#Page_113">113</a>, <a href="#Page_130">130</a>, <a href="#Page_135">135</a>, <a href="#Page_137">137</a>.</li>
+
+<li>Epiphytic algae, <a href="#Page_188">188</a>.</li>
+
+<li>Epiphytic fungi, <a href="#Page_161">161</a>, <a href="#Page_193">193</a>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Equisetum</i>, <a href="#Page_113">113</a>.</li>
+
+<li>Ergot, <a href="#Page_131">131</a>, <a href="#Page_142">142</a>, <a href="#Page_144">144</a>.</li>
+
+<li><i>Erineum</i>, <a href="#Page_88">88</a>, <a href="#Page_212">212</a>, <a href="#Page_214">214</a>, <a href="#Page_215">215</a>.</li>
+
+<li>Erosions, <a href="#Page_204">204</a>, <a href="#Page_207">207</a>.</li>
+
+<li><i>Erysipheae</i>, <a href="#Page_135">135</a>, <a href="#Page_142">142</a>, <a href="#Page_161">161</a>, <a href="#Page_187">187</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Essentials of fertilisation, <a href="#Page_69">69</a>.</li>
+
+<li>Estimates of loss, <a href="#Page_146">146</a>.</li>
+
+<li>Etiolation, <a href="#Page_106">106</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_229">229</a>.</li>
+
+<li><i>Euphorbia</i>, <a href="#Page_116">116</a>, <a href="#Page_134">134</a>, <a href="#Page_247">247</a>, <a href="#Page_266">266</a>.</li>
+
+<li>Excavations, <a href="#Page_204">204</a>.</li>
+
+<li>Excess of food, <a href="#Page_229">229</a>.</li>
+
+<li>Excess of minerals, <a href="#Page_102">102</a>.</li>
+
+<li>Excess of water, <a href="#Page_100">100</a>.</li>
+
+<li>Excessive growth, <a href="#Page_246">246</a>.</li>
+
+<li>Excessive nutrition, <a href="#Page_250">250</a>.</li>
+
+<li>Excrescences, <a href="#Page_114">114</a>, <a href="#Page_212">212</a>, <a href="#Page_222">222</a>.</li>
+
+<li>Excreta, <a href="#Page_45">45</a>, <a href="#Page_130">130</a>, <a href="#Page_133">133</a>.</li>
+
+<li><i>Exobasidium</i>, <a href="#Page_128">128</a>, <a href="#Page_218">218</a>.</li>
+
+<li><i>Exoascus</i>, <a href="#Page_116">116</a>, <a href="#Page_128">128</a>, <a href="#Page_188">188</a>, <a href="#Page_208">208</a>, <a href="#Page_214">214</a>, <a href="#Page_218">218</a>, <a href="#Page_225">225</a>, <a href="#Page_247">247</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Expense of materials, <a href="#Page_161">161</a>.</li>
+
+<li>Experiments necessary, <a href="#Page_168">168</a>.</li>
+
+<li>Exposure of roots, <a href="#Page_179">179</a>, <a href="#Page_184">184</a>.</li>
+
+<li>External causes of disease, <a href="#Page_99">99</a>.</li>
+
+<li>Extinction of species, <a href="#Page_91">91</a>.</li>
+
+<li>Exudations, <a href="#Page_227">227</a>.</li>
+
+<li>Exudation under pressure, <a href="#Page_51">51</a>.</li>
+
+
+<li class="newletter">Factors of an epidemic, <a href="#Page_149">149</a>, <a href="#Page_165">165</a>.</li>
+
+<li>Falling of fruit, <a href="#Page_206">206</a>.</li>
+
+<li>Falling leaves, <a href="#Page_123">123</a>.</li>
+
+<li>False chlorosis, <a href="#Page_181">181</a>.</li>
+
+<li>False etiolation, <a href="#Page_180">180</a>.</li>
+
+<li><i>Farfugium</i>, <a href="#Page_188">188</a>.</li>
+
+<li>Fasciations, <a href="#Page_230">230</a>, <a href="#Page_246">246</a>, <a href="#Page_251">251</a>.</li>
+
+<li><span class="pagenum"><a name="Page_299" id="Page_299"></a>[<a href="./images/299.png">299</a>]</span>Fats, <a href="#Page_272">272</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Feeding, <a href="#Page_14">14</a>, <a href="#Page_16">16</a>.</li>
+
+<li>Fermentation, <a href="#Page_58">58</a>, <a href="#Page_102">102</a>, <a href="#Page_130">130</a>, <a href="#Page_233">233</a>.</li>
+
+<li>Ferns, <a href="#Page_113">113</a>, <a href="#Page_247">247</a>, <a href="#Page_260">260</a>, <a href="#Page_261">261</a>.</li>
+
+<li>Fertilisation, <a href="#Page_71">71</a>.</li>
+
+<li>Field-mice, <a href="#Page_164">164</a>.</li>
+
+<li>Figs, <a href="#Page_113">113</a>.</li>
+
+<li>Finger and toe, <a href="#Page_114">114</a>, <a href="#Page_127">127</a>, <a href="#Page_163">163</a>.</li>
+
+<li>Fire, <a href="#Page_240">240</a>.</li>
+
+<li>Flaming, <a href="#Page_164">164</a>.</li>
+
+<li>Flattened roots, <a href="#Page_246">246</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Fleshiness, <a href="#Page_228">228</a>.</li>
+
+<li>Flies, <a href="#Page_86">86</a>, <a href="#Page_110">110</a>, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>, <a href="#Page_145">145</a>.</li>
+
+<li>Flux, <a href="#Page_227">227</a>, <a href="#Page_231">231</a>.</li>
+
+<li>Flying foxes, <a href="#Page_244">244</a>.</li>
+
+<li>Focussing of solar rays, <a href="#Page_192">192</a>.</li>
+
+<li>Foliage, <a href="#Page_110">110</a>.</li>
+
+<li><i>Fontaria</i>, <a href="#Page_134">134</a>.</li>
+
+<li>Food, <a href="#Page_18">18</a>.</li>
+
+<li>Forest-fires, <a href="#Page_241">241</a>.</li>
+
+<li>Formic-aldehyde, <a href="#Page_20">20</a>.</li>
+
+<li>Foul products, <a href="#Page_100">100</a>.</li>
+
+<li>Foxy leaves, <a href="#Page_191">191</a>.</li>
+
+<li>Freezing, <a href="#Page_121">121</a>, <a href="#Page_183">183</a>.</li>
+
+<li>Frit fly, <a href="#Page_182">182</a>.</li>
+
+<li>Frost, <a href="#Page_153">153</a>, <a href="#Page_160">160</a>, <a href="#Page_225">225</a>, <a href="#Page_229">229</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Frost-beds, <a href="#Page_243">243</a>.</li>
+
+<li>Frost-blisters, <a href="#Page_212">212</a>, <a href="#Page_218">218</a>.</li>
+
+<li>Frost canker, <a href="#Page_222">222</a>.</li>
+
+<li>Frost-cracks, <a href="#Page_204">204</a>, <a href="#Page_209">209</a>, <a href="#Page_242">242</a>.</li>
+
+<li>Frost-patches, <a href="#Page_240">240</a>.</li>
+
+<li>Frost-ridge, <a href="#Page_209">209</a>.</li>
+
+<li><i>Fumago</i>, <a href="#Page_190">190</a>, <a href="#Page_232">232</a>.</li>
+
+<li>Fumes, <a href="#Page_104">104</a>.</li>
+
+<li>Functions of roots, <a href="#Page_43">43</a>, <a href="#Page_45">45</a>.</li>
+
+<li>Functional depression, <a href="#Page_96">96</a>.</li>
+
+<li>Fungi, <a href="#Page_89">89</a>, <a href="#Page_108">108</a>, <a href="#Page_143">143</a>, <a href="#Page_174">174</a>, <a href="#Page_189">189</a>, <a href="#Page_200">200</a>, <a href="#Page_205">205</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_212">212</a>, <a href="#Page_216">216</a>, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>, <a href="#Page_229">229</a>, <a href="#Page_231">231</a>, <a href="#Page_233">233</a>, <a href="#Page_238">238</a>, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>, <a href="#Page_243">243</a>, <a href="#Page_248">248</a>, <a href="#Page_251">251</a>, <a href="#Page_255">255</a>, <a href="#Page_258">258</a>, <a href="#Page_265">265</a>, <a href="#Page_267">267</a>, <a href="#Page_283">283</a>, <a href="#Page_284">284</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Fungus attacks, <a href="#Page_139">139</a>.</li>
+
+<li>Fungus galls, <a href="#Page_219">219</a>.</li>
+
+<li><i>Fusarium</i>, <a href="#Page_143">143</a>, <a href="#Page_238">238</a>.</li>
+
+<li><i>Fusicladium</i>, <a href="#Page_189">189</a>.</li>
+
+<li><i>Fusisporium</i>, <a href="#Page_237">237</a>.</li>
+
+
+<li class="newletter"><i>Gagea</i>, <a href="#Page_258">258</a>.</li>
+
+<li>Gall-apple, <a href="#Page_218">218</a>.</li>
+
+<li>Gall-flies, <a href="#Page_219">219</a>.</li>
+
+<li>Gall-insect, <a href="#Page_139">139</a>.</li>
+
+<li>Gall-like swellings, <a href="#Page_128">128</a>.</li>
+
+<li>Galls, <a href="#Page_86">86</a>, <a href="#Page_110">110</a>, <a href="#Page_120">120</a>, <a href="#Page_130">130</a>, <a href="#Page_138">138</a>, <a href="#Page_212">212</a>, <a href="#Page_214">214</a>, <a href="#Page_218">218</a>, <a href="#Page_255">255</a>.</li>
+
+<li>Gangrene, <a href="#Page_231">231</a>.</li>
+
+<li><i>Garreya</i>, <a href="#Page_264">264</a>.</li>
+
+<li>Gas, <a href="#Page_160">160</a>.</li>
+
+<li>Gases in soil, <a href="#Page_104">104</a>.</li>
+
+<li><i>Gastropacha</i>, <a href="#Page_225">225</a>.</li>
+
+<li>Gelatine, <a href="#Page_163">163</a>.</li>
+
+<li>General death, <a href="#Page_116">116</a>.</li>
+
+<li>General disease, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>.</li>
+
+<li>Germ-plasm, <a href="#Page_267">267</a>.</li>
+
+<li><i>Gesneria</i>, <a href="#Page_260">260</a>.</li>
+
+<li><i>Glechoma</i>, <a href="#Page_218">218</a>.</li>
+
+<li><i>Gloeosporium</i>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_208">208</a>.</li>
+
+<li><i>Gloxinia</i>, <a href="#Page_260">260</a>.</li>
+
+<li>Goats, <a href="#Page_164">164</a>.</li>
+
+<li>Gooseberry, <a href="#Page_217">217</a>.</li>
+
+<li>Graft-hybrids, <a href="#Page_262">262</a>, <a href="#Page_267">267</a>, <a href="#Page_271">271</a>, <a href="#Page_283">283</a>.</li>
+
+<li>Grafting, <a href="#Page_78">78</a>, <a href="#Page_155">155</a>, <a href="#Page_169">169</a>, <a href="#Page_183">183</a>, <a href="#Page_250">250</a>, <a href="#Page_262">262</a>, <a href="#Page_271">271</a>, <a href="#Page_281">281</a>.</li>
+
+<li>Grain-rust, <a href="#Page_146">146</a>.</li>
+
+<li>Grapes, <a href="#Page_192">192</a>, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>.</li>
+
+<li><i>Grapholitha</i>, <a href="#Page_109">109</a>, <a href="#Page_207">207</a>.</li>
+
+<li>Grass, <a href="#Page_111">111</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_205">205</a>, <a href="#Page_218">218</a>, <a href="#Page_233">233</a>.</li>
+
+<li><span class="pagenum"><a name="Page_300" id="Page_300"></a>[<a href="./images/300.png">300</a>]</span>Green fly, <a href="#Page_161">161</a>.</li>
+
+<li>Grew, <a href="#Page_85">85</a>.</li>
+
+<li>Greyish spots, <a href="#Page_187">187</a>.</li>
+
+<li>Growth, <a href="#Page_271">271</a>, <a href="#Page_274">274</a>, <a href="#Page_275">275</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Grubs, <a href="#Page_110">110</a>, <a href="#Page_207">207</a>.</li>
+
+<li>Gumming, <a href="#Page_235">235</a>.</li>
+
+<li>Gummosis, <a href="#Page_227">227</a>, <a href="#Page_234">234</a>, <a href="#Page_235">235</a>.</li>
+
+<li><i>Gymnosporangium</i>, <a href="#Page_114">114</a>, <a href="#Page_176">176</a>, <a href="#Page_223">223</a>.</li>
+
+
+<li class="newletter">Hail, <a href="#Page_106">106</a>, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>.</li>
+
+<li>Hales, <a href="#Page_85">85</a>.</li>
+
+<li><i>Haltica</i>, <a href="#Page_209">209</a>.</li>
+
+<li>Hardy varieties, <a href="#Page_168">168</a>, <a href="#Page_170">170</a>, <a href="#Page_177">177</a>.</li>
+
+<li>Haustoria, <a href="#Page_134">134</a>, <a href="#Page_135">135</a>, <a href="#Page_136">136</a>.</li>
+
+<li>Healing, <a href="#Page_194">194</a>, <a href="#Page_196">196</a>.</li>
+
+<li>Healing by cork, <a href="#Page_123">123</a>.</li>
+
+<li>Health, <a href="#Page_272">272</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Health and disease, <a href="#Page_91">91</a>, <a href="#Page_97">97</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Heliotropism, <a href="#Page_126">126</a>.</li>
+
+<li><i>Hemileia</i>, <a href="#Page_146">146</a>, <a href="#Page_169">169</a>.</li>
+
+<li>Heredity, <a href="#Page_72">72</a>, <a href="#Page_283">283</a>.</li>
+
+<li><i>Herpotrichia</i>, <a href="#Page_135">135</a>, <a href="#Page_190">190</a>.</li>
+
+<li>Hessian Fly, <a href="#Page_182">182</a>.</li>
+
+<li><i>Heterodora</i>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>.</li>
+
+<li><i>Hieracium</i>, <a href="#Page_112">112</a>.</li>
+
+<li>History of Phytopathology, <a href="#Page_85">85</a>.</li>
+
+<li>Holdfast of roots, <a href="#Page_42">42</a>.</li>
+
+<li>Hollyhock disease, <a href="#Page_143">143</a>.</li>
+
+<li>Holly, <a href="#Page_217">217</a>.</li>
+
+<li>Honey dew, <a href="#Page_144">144</a>, <a href="#Page_227">227</a>, <a href="#Page_232">232</a>, <a href="#Page_233">233</a>.</li>
+
+<li>Hops, <a href="#Page_162">162</a>, <a href="#Page_187">187</a>, <a href="#Page_191">191</a>, <a href="#Page_232">232</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Hop-aphis, <a href="#Page_146">146</a>.</li>
+
+<li>Hop-disease, <a href="#Page_166">166</a>.</li>
+
+<li>Hop mildew, <a href="#Page_161">161</a>.</li>
+
+<li><i>Hormomyia</i>, <a href="#Page_219">219</a>.</li>
+
+<li>Hornbeam, <a href="#Page_224">224</a>, <a href="#Page_233">233</a>, <a href="#Page_242">242</a>.</li>
+
+<li>Horse-radish, <a href="#Page_260">260</a>.</li>
+
+<li>Host, <a href="#Page_284">284</a>, <a href="#Page_285">285</a>.</li>
+
+<li>Hyacinth, <a href="#Page_231">231</a>, <a href="#Page_261">261</a>.</li>
+
+<li>Hyacinth disease, <a href="#Page_143">143</a>.</li>
+
+<li>Hybrids, <a href="#Page_69">69</a>, <a href="#Page_156">156</a>, <a href="#Page_281">281</a>.</li>
+
+<li>Hybridisation, <a href="#Page_69">69</a>, <a href="#Page_75">75</a>, <a href="#Page_169">169</a>, <a href="#Page_266">266</a>, <a href="#Page_281">281</a>.</li>
+
+<li>Hydrochloric acid, <a href="#Page_181">181</a>.</li>
+
+<li>Hydrogen, <a href="#Page_272">272</a>.</li>
+
+<li>Hymenomycetes, <a href="#Page_206">206</a>.</li>
+
+<li>Hypertrophy, <a href="#Page_119">119</a>, <a href="#Page_127">127</a>, <a href="#Page_139">139</a>, <a href="#Page_213">213</a>, <a href="#Page_215">215</a>, <a href="#Page_247">247</a>, <a href="#Page_266">266</a>.</li>
+
+<li><i>Hypochaeris</i>, <a href="#Page_112">112</a>.</li>
+
+<li><i>Hypomyces</i>, <a href="#Page_237">237</a>.</li>
+
+<li><i>Hyponomeuta</i>, <a href="#Page_254">254</a>.</li>
+
+
+<li class="newletter">Ice, <a href="#Page_184">184</a>, <a href="#Page_209">209</a>.</li>
+
+<li>Ichneumon-flies, <a href="#Page_165">165</a>.</li>
+
+<li><i>Icterus</i>, <a href="#Page_181">181</a>.</li>
+
+<li>Illegitimate unions, <a href="#Page_265">265</a>.</li>
+
+<li>Immunity, <a href="#Page_155">155</a>, <a href="#Page_156">156</a>, <a href="#Page_165">165</a>, <a href="#Page_168">168</a>, <a href="#Page_169">169</a>.</li>
+
+<li>Impervious subsoil, <a href="#Page_181">181</a>.</li>
+
+<li>Inarching, <a href="#Page_269">269</a>.</li>
+
+<li>Increase in dry weight, <a href="#Page_23">23</a>.</li>
+
+<li>Indian agriculture, <a href="#Page_172">172</a>.</li>
+
+<li>Indian wheats, <a href="#Page_168">168</a>.</li>
+
+<li>Indispensability of elements, <a href="#Page_278">278</a>.</li>
+
+<li>Infection, <a href="#Page_262">262</a>, <a href="#Page_265">265</a>, <a href="#Page_267">267</a>.</li>
+
+<li>Ingredients of protoplasm, <a href="#Page_272">272</a>.</li>
+
+<li>Insect bites, <a href="#Page_225">225</a>.</li>
+
+<li>Insect diseases, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>, <a href="#Page_154">154</a>, <a href="#Page_189">189</a>.</li>
+
+<li>Insect punctures, <a href="#Page_88">88</a>.</li>
+
+<li>Insects, <a href="#Page_89">89</a>, <a href="#Page_98">98</a>, <a href="#Page_108">108</a>, <a href="#Page_109">109</a>, <a href="#Page_120">120</a>, <a href="#Page_138">138</a>, <a href="#Page_142">142</a>, <a href="#Page_153">153</a>, <a href="#Page_174">174</a>, <a href="#Page_187">187</a>, <a href="#Page_194">194</a>, <a href="#Page_203">203</a>, <a href="#Page_205">205</a>, <a href="#Page_206">206</a>, <a href="#Page_207">207</a>, <a href="#Page_208">208</a>, <a href="#Page_212">212</a>, <a href="#Page_223">223</a>, <a href="#Page_229">229</a>, <a href="#Page_241">241</a>, <a href="#Page_244">244</a>, <a href="#Page_248">248</a>, <a href="#Page_251">251</a>, <a href="#Page_254">254</a>, <a href="#Page_255">255</a>, <a href="#Page_258">258</a>, <a href="#Page_259">259</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Insolation, <a href="#Page_180">180</a>, <a href="#Page_242">242</a>.</li>
+
+<li><span class="pagenum"><a name="Page_301" id="Page_301"></a>[<a href="./images/301.png">301</a>]</span>Intercellular endophytes, <a href="#Page_136">136</a>, <a href="#Page_137">137</a>.</li>
+
+<li>Intercellular mycelium, <a href="#Page_128">128</a>.</li>
+
+<li>Interference, <a href="#Page_91">91</a>.</li>
+
+<li>Internal causes of disease, <a href="#Page_99">99</a>, <a href="#Page_101">101</a>.</li>
+
+<li>Intracellular parasites, <a href="#Page_127">127</a>, <a href="#Page_136">136</a>.</li>
+
+<li>Intramolecular respiration, <a href="#Page_277">277</a>.</li>
+
+<li>Intumescences, <a href="#Page_212">212</a>, <a href="#Page_215">215</a>.</li>
+
+<li>Inulin, <a href="#Page_11">11</a>, <a href="#Page_17">17</a>.</li>
+
+<li>Invertebrata, <a href="#Page_108">108</a>.</li>
+
+<li>Irritability, <a href="#Page_125">125</a>, <a href="#Page_127">127</a>.</li>
+
+<li>Irritation, <a href="#Page_119">119</a>, <a href="#Page_139">139</a>.</li>
+
+<li><i>Isaria</i>, <a href="#Page_163">163</a>.</li>
+
+<li>Ivy, <a href="#Page_113">113</a>, <a href="#Page_165">165</a>.</li>
+
+
+<li class="newletter">Japanese trees, <a href="#Page_250">250</a>.</li>
+
+<li>Jerusalem Artichoke, <a href="#Page_264">264</a>.</li>
+
+<li><i>Juncus</i>, <a href="#Page_219">219</a>.</li>
+
+<li>Juniper, <a href="#Page_114">114</a>.</li>
+
+
+<li class="newletter">Kidney bean, <a href="#Page_192">192</a>.</li>
+
+<li>Knauers, <a href="#Page_223">223</a>.</li>
+
+<li>Knife wounds, <a href="#Page_194">194</a>, <a href="#Page_195">195</a>.</li>
+
+
+<li class="newletter">Labour, <a href="#Page_161">161</a>.</li>
+
+<li>Lace-wings, <a href="#Page_165">165</a>.</li>
+
+<li><i>Lachnus</i>, <a href="#Page_223">223</a>.</li>
+
+<li>Lady-birds, <a href="#Page_164">164</a>, <a href="#Page_165">165</a>.</li>
+
+<li>Lammas shoots, <a href="#Page_257">257</a>, <a href="#Page_259">259</a>.</li>
+
+<li>Larch, <a href="#Page_168">168</a>, <a href="#Page_171">171</a>.</li>
+
+<li>Larch disease, <a href="#Page_115">115</a>, <a href="#Page_149">149</a>, <a href="#Page_152">152</a>, <a href="#Page_166">166</a>, <a href="#Page_171">171</a>, <a href="#Page_223">223</a>, <a href="#Page_241">241</a>.</li>
+
+<li>Larvae, <a href="#Page_110">110</a>.</li>
+
+<li>Lateral wounds, <a href="#Page_132">132</a>.</li>
+
+<li>Lawns, <a href="#Page_112">112</a>.</li>
+
+<li>Laying of wheat, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>.</li>
+
+<li>Leaf-curl, <a href="#Page_236">236</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Leaf-diseases, <a href="#Page_114">114</a>, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>, <a href="#Page_242">242</a>.</li>
+
+<li>Leaf-galls, <a href="#Page_217">217</a>, <a href="#Page_218">218</a>.</li>
+
+<li>Leaf-miner, <a href="#Page_86">86</a>, <a href="#Page_109">109</a>, <a href="#Page_204">204</a>.</li>
+
+<li>Leaf perforations, <a href="#Page_208">208</a>.</li>
+
+<li>Leaf rolling, <a href="#Page_214">214</a>, <a href="#Page_246">246</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Leaf-spots, <a href="#Page_114">114</a>, <a href="#Page_190">190</a>.</li>
+
+<li>Leguminoseae, <a href="#Page_137">137</a>, <a href="#Page_219">219</a>.</li>
+
+<li>Lemons, <a href="#Page_235">235</a>.</li>
+
+<li>Lenticels, <a href="#Page_217">217</a>.</li>
+
+<li>Lepidoptera, <a href="#Page_187">187</a>.</li>
+
+<li><i>Leptosphaeria</i>, <a href="#Page_249">249</a>.</li>
+
+<li>Lichens, <a href="#Page_137">137</a>.</li>
+
+<li>Liebig, <a href="#Page_4">4</a>.</li>
+
+<li>Life, <a href="#Page_271">271</a>, <a href="#Page_285">285</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Life and death, <a href="#Page_271">271</a>.</li>
+
+<li>Light, <a href="#Page_27">27</a>, <a href="#Page_106">106</a>.</li>
+
+<li>Lily disease, <a href="#Page_143">143</a>.</li>
+
+<li>Lime, <a href="#Page_163">163</a>, <a href="#Page_215">215</a>, <a href="#Page_218">218</a>, <a href="#Page_232">232</a>, <a href="#Page_253">253</a>, <a href="#Page_254">254</a>, <a href="#Page_260">260</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Limes, <a href="#Page_172">172</a>.</li>
+
+<li>Limits of variation, <a href="#Page_287">287</a>.</li>
+
+<li><i>Linaria</i>, <a href="#Page_252">252</a>.</li>
+
+<li>Liquid antiseptics, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</li>
+
+<li>Living environment, <a href="#Page_99">99</a>, <a href="#Page_108">108</a>.</li>
+
+<li>Local action, <a href="#Page_114">114</a>.</li>
+
+<li>Local disease, <a href="#Page_119">119</a>, <a href="#Page_121">121</a>.</li>
+
+<li>Locusts, <a href="#Page_109">109</a>, <a href="#Page_145">145</a>, <a href="#Page_163">163</a>, <a href="#Page_164">164</a>.</li>
+
+<li>Longicorns, <a href="#Page_205">205</a>.</li>
+
+<li><i>Loranthus</i>, <a href="#Page_113">113</a>, <a href="#Page_245">245</a>, <a href="#Page_265">265</a>.</li>
+
+<li>Losses due to epidemics, <a href="#Page_142">142</a>.</li>
+
+<li>Lowering of temperature, <a href="#Page_100">100</a>.</li>
+
+<li>Lucerne, <a href="#Page_249">249</a>.</li>
+
+<li>Lurking parasites, <a href="#Page_142">142</a>, <a href="#Page_145">145</a>.</li>
+
+<li>Lychnis, <a href="#Page_232">232</a>.</li>
+
+<li><i>Lyonetra</i>, <a href="#Page_206">206</a>.</li>
+
+<li><i>Lysimachia</i>, <a href="#Page_217">217</a>.</li>
+
+
+<li class="newletter">Machine, plant compared to a, <a href="#Page_79">79</a>.</li>
+
+<li>Magnesium, <a href="#Page_272">272</a>.</li>
+
+<li><span class="pagenum"><a name="Page_302" id="Page_302"></a>[<a href="./images/302.png">302</a>]</span>Maize, <a href="#Page_116">116</a>, <a href="#Page_173">173</a>, <a href="#Page_219">219</a>, <a href="#Page_267">267</a>.</li>
+
+<li><i>Majanthemum</i>, <a href="#Page_175">175</a>.</li>
+
+<li>Malformations, <a href="#Page_116">116</a>, <a href="#Page_130">130</a>, <a href="#Page_131">131</a>, <a href="#Page_246">246</a>, <a href="#Page_251">251</a>.</li>
+
+<li><i>Mal nero</i>, <a href="#Page_190">190</a>.</li>
+
+<li>Mallow, <a href="#Page_252">252</a>.</li>
+
+<li>Malpighi, <a href="#Page_85">85</a>.</li>
+
+<li>Mammals, <a href="#Page_142">142</a>.</li>
+
+<li>Man and plants, <a href="#Page_108">108</a>, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>.</li>
+
+<li>Manna, <a href="#Page_227">227</a>, <a href="#Page_235">235</a>.</li>
+
+<li>Manna Ash, <a href="#Page_235">235</a>.</li>
+
+<li>Maple, <a href="#Page_259">259</a>.</li>
+
+<li>Maximum, <a href="#Page_288">288</a>.</li>
+
+<li>Maximum absorption, <a href="#Page_19">19</a>.</li>
+
+<li>Maximum assimilation, <a href="#Page_19">19</a>.</li>
+
+<li>Maximum temperature, <a href="#Page_105">105</a>.</li>
+
+<li>Mealy bug, <a href="#Page_164">164</a>.</li>
+
+<li><i>Melampsora</i>, <a href="#Page_176">176</a>.</li>
+
+<li>Melon, <a href="#Page_220">220</a>.</li>
+
+<li>Messmates, <a href="#Page_63">63</a>.</li>
+
+<li>Metabolic products, <a href="#Page_274">274</a>.</li>
+
+<li>Metabolism, <a href="#Page_23">23</a>, <a href="#Page_127">127</a>, <a href="#Page_271">271</a>.</li>
+
+<li>Metabolites, <a href="#Page_278">278</a>.</li>
+
+<li>Metallic compounds, <a href="#Page_162">162</a>.</li>
+
+<li>Mice, <a href="#Page_108">108</a>, <a href="#Page_163">163</a>.</li>
+
+<li>Microbes, <a href="#Page_227">227</a>.</li>
+
+<li>Micro-organisms, <a href="#Page_183">183</a>.</li>
+
+<li>Mildew, <a href="#Page_86">86</a>, <a href="#Page_164">164</a>.</li>
+
+<li>Millardet, <a href="#Page_169">169</a>.</li>
+
+<li>Mineral salts, <a href="#Page_101">101</a>.</li>
+
+<li>Miniature trees, <a href="#Page_250">250</a>.</li>
+
+<li>Minimum, <a href="#Page_288">288</a>.</li>
+
+<li>Minimum temperature, <a href="#Page_105">105</a>.</li>
+
+<li>Misconceptions, <a href="#Page_12">12</a>.</li>
+
+<li>Mistletoe, <a href="#Page_113">113</a>, <a href="#Page_265">265</a>.</li>
+
+<li>Mites, <a href="#Page_192">192</a>, <a href="#Page_214">214</a>, <a href="#Page_255">255</a>.</li>
+
+<li>Mixed species, <a href="#Page_166">166</a>.</li>
+
+<li>Molecular structure of protoplasm, <a href="#Page_273">273</a>, <a href="#Page_274">274</a>.</li>
+
+<li>Mongrel forms, <a href="#Page_74">74</a>.</li>
+
+<li><i>Monilia</i>, <a href="#Page_217">217</a>, <a href="#Page_231">231</a>.</li>
+
+<li>Monstrosities, <a href="#Page_246">246</a>.</li>
+
+<li>Moraine plants, <a href="#Page_250">250</a>.</li>
+
+<li>Moths, <a href="#Page_110">110</a>, <a href="#Page_145">145</a>, <a href="#Page_206">206</a>.</li>
+
+<li>Moulds, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>, <a href="#Page_237">237</a>, <a href="#Page_243">243</a>.</li>
+
+<li><i>Mucor</i>, <a href="#Page_230">230</a>, <a href="#Page_231">231</a>.</li>
+
+<li>Mulberry, <a href="#Page_244">244</a>.</li>
+
+<li>Mutilations, <a href="#Page_252">252</a>.</li>
+
+<li>Mycelial strands, <a href="#Page_145">145</a>.</li>
+
+<li>Mycelium, <a href="#Page_188">188</a>.</li>
+
+<li>Mycocecidia, <a href="#Page_219">219</a>.</li>
+
+<li>Mycorrhiza, <a href="#Page_137">137</a>.</li>
+
+<li>Myrtaceae, <a href="#Page_258">258</a>.</li>
+
+<li><i>Mytilaspis</i>, <a href="#Page_187">187</a>.</li>
+
+
+<li class="newletter">Natural checks, <a href="#Page_159">159</a>.</li>
+
+<li>Natural demise, <a href="#Page_91">91</a>, <a href="#Page_93">93</a>.</li>
+
+<li>Natural Grafts, <a href="#Page_269">269</a>.</li>
+
+<li>Natural Selection, <a href="#Page_72">72</a>, <a href="#Page_99">99</a>, <a href="#Page_167">167</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Natural Wounds, <a href="#Page_204">204</a>.</li>
+
+<li>Nature of soil, <a href="#Page_57">57</a>.</li>
+
+<li>Necrosis, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>, <a href="#Page_243">243</a>.</li>
+
+<li><i>Nectria</i>, <a href="#Page_145">145</a>, <a href="#Page_217">217</a>, <a href="#Page_223">223</a>, <a href="#Page_241">241</a>, <a href="#Page_243">243</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Nematodes, <a href="#Page_111">111</a>, <a href="#Page_134">134</a>, <a href="#Page_139">139</a>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>.</li>
+
+<li>Nettle, <a href="#Page_116">116</a>, <a href="#Page_252">252</a>.</li>
+
+<li><i>Neurotus</i>, <a href="#Page_219">219</a>.</li>
+
+<li>New formations, <a href="#Page_255">255</a>.</li>
+
+<li>Nitrate, <a href="#Page_273">273</a>.</li>
+
+<li>Nitrification, <a href="#Page_62">62</a>, <a href="#Page_102">102</a>.</li>
+
+<li>Nitrogen, <a href="#Page_272">272</a>.</li>
+
+<li>Nodosities, <a href="#Page_219">219</a>.</li>
+
+<li>Nodules on roots, <a href="#Page_63">63</a>, <a href="#Page_137">137</a>.</li>
+
+<li>Non-living environment, <a href="#Page_99">99</a>.</li>
+
+<li><i>Notommata</i>, <a href="#Page_140">140</a>.</li>
+
+<li>Nuclear fusion, <a href="#Page_267">267</a>.</li>
+
+<li><span class="pagenum"><a name="Page_303" id="Page_303"></a>[<a href="./images/303.png">303</a>]</span>Nuclear protoplasm, <a href="#Page_271">271</a>, <a href="#Page_279">279</a>, <a href="#Page_280">280</a>, <a href="#Page_290">290</a>.</li>
+
+<li>Nuclear substance, <a href="#Page_71">71</a>.</li>
+
+<li>Nucleo-plasm, <a href="#Page_280">280</a>.</li>
+
+<li>Nuts, <a href="#Page_248">248</a>.</li>
+
+
+<li class="newletter">Oak, <a href="#Page_110">110</a>, <a href="#Page_188">188</a>, <a href="#Page_215">215</a>, <a href="#Page_218">218</a>, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>, <a href="#Page_233">233</a>.</li>
+
+<li>Oak leaf-roller, <a href="#Page_254">254</a>.</li>
+
+<li>Oat, <a href="#Page_176">176</a>.</li>
+
+<li>Occlusion, <a href="#Page_200">200</a>, <a href="#Page_201">201</a>, <a href="#Page_222">222</a>, <a href="#Page_223">223</a>.</li>
+
+<li>Odours, <a href="#Page_144">144</a>.</li>
+
+<li>Å’dema, <a href="#Page_228">228</a>.</li>
+
+<li>Olive, <a href="#Page_223">223</a>.</li>
+
+<li>Onion, <a href="#Page_231">231</a>.</li>
+
+<li><i>Oniscus</i>, <a href="#Page_182">182</a>.</li>
+
+<li><i>Oospora</i>, <a href="#Page_216">216</a>.</li>
+
+<li>Optimum temperature, <a href="#Page_105">105</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Orange, <a href="#Page_173">173</a>, <a href="#Page_187">187</a>, <a href="#Page_235">235</a>, <a href="#Page_247">247</a>.</li>
+
+<li>Orange-coloured spots, <a href="#Page_187">187</a>.</li>
+
+<li>Orchard trees, <a href="#Page_163">163</a>.</li>
+
+<li><i>Orchestes</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Orchids, <a href="#Page_113">113</a>, <a href="#Page_266">266</a>.</li>
+
+<li>Organic acids, <a href="#Page_50">50</a>.</li>
+
+<li>Organisation, <a href="#Page_89">89</a>.</li>
+
+<li>Organised structure, <a href="#Page_13">13</a>.</li>
+
+<li>Organisms in soil, <a href="#Page_60">60</a>.</li>
+
+<li><i>Orobanche</i>, <a href="#Page_112">112</a>.</li>
+
+<li>Osmosis, <a href="#Page_26">26</a>, <a href="#Page_29">29</a>, <a href="#Page_46">46</a>.</li>
+
+<li>Osmotic pressures, <a href="#Page_18">18</a>, <a href="#Page_41">41</a>, <a href="#Page_52">52</a>.</li>
+
+<li>Over-crowding, <a href="#Page_104">104</a>, <a href="#Page_111">111</a>.</li>
+
+<li>Over-feeding, <a href="#Page_102">102</a>.</li>
+
+<li>Over-watering, <a href="#Page_97">97</a>.</li>
+
+<li>Oxalic acid, <a href="#Page_134">134</a>, <a href="#Page_136">136</a>.</li>
+
+<li>Oxidation, <a href="#Page_124">124</a>.</li>
+
+<li>Oxygen, <a href="#Page_104">104</a>, <a href="#Page_272">272</a>.</li>
+
+<li>Oxygen-respiration, <a href="#Page_12">12</a>, <a href="#Page_64">64</a>.</li>
+
+
+<li class="newletter">Pallor, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>.</li>
+
+<li>Palms, <a href="#Page_192">192</a>.</li>
+
+<li><i>Pangium</i>, <a href="#Page_134">134</a>, <a href="#Page_165">165</a>.</li>
+
+<li>Parasites, <a href="#Page_61">61</a>, <a href="#Page_113">113</a>, <a href="#Page_119">119</a>, <a href="#Page_130">130</a>, <a href="#Page_139">139</a>, <a href="#Page_174">174</a>, <a href="#Page_187">187</a>, <a href="#Page_230">230</a>, <a href="#Page_265">265</a>, <a href="#Page_269">269</a>, <a href="#Page_284">284</a>.</li>
+
+<li>Parasitic algae, <a href="#Page_188">188</a>, <a href="#Page_217">217</a>, <a href="#Page_219">219</a>.</li>
+
+<li>Parasitic bacteria, <a href="#Page_163">163</a>.</li>
+
+<li>Parasitic diseases, <a href="#Page_88">88</a>, <a href="#Page_119">119</a>, <a href="#Page_121">121</a>.</li>
+
+<li>Parasitic epiphyte, <a href="#Page_136">136</a>.</li>
+
+<li>Parasitic fungi, <a href="#Page_87">87</a>, <a href="#Page_97">97</a>.</li>
+
+<li>Parasitism, <a href="#Page_262">262</a>, <a href="#Page_264">264</a>, <a href="#Page_268">268</a>, <a href="#Page_271">271</a>.</li>
+
+<li><i>Paris</i>, <a href="#Page_175">175</a>.</li>
+
+<li>"Paris green," <a href="#Page_162">162</a>.</li>
+
+<li>Parti-coloured leaves, <a href="#Page_191">191</a>.</li>
+
+<li>Parti-coloured spots, <a href="#Page_186">186</a>.</li>
+
+<li>Pasture grasses, <a href="#Page_69">69</a>.</li>
+
+<li>Pathology, <a href="#Page_121">121</a>, <a href="#Page_257">257</a>.</li>
+
+<li>Pathology of cell, <a href="#Page_119">119</a>.</li>
+
+<li>Pathological conditions, <a href="#Page_168">168</a>, <a href="#Page_170">170</a>, <a href="#Page_246">246</a>.</li>
+
+<li>Pea, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_206">206</a>, <a href="#Page_208">208</a>, <a href="#Page_248">248</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Peach, <a href="#Page_170">170</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Pear, <a href="#Page_179">179</a>, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>, <a href="#Page_191">191</a>, <a href="#Page_216">216</a>, <a href="#Page_218">218</a>, <a href="#Page_231">231</a>, <a href="#Page_240">240</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_253">253</a>, <a href="#Page_257">257</a>.</li>
+
+<li>Pedigree wheats, <a href="#Page_69">69</a>.</li>
+
+<li><i>Pelargonium</i>, <a href="#Page_198">198</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Peloria, <a href="#Page_252">252</a>.</li>
+
+<li><i>Penicillium</i>, <a href="#Page_231">231</a>.</li>
+
+<li><i>Peridermium Pini</i>, <a href="#Page_223">223</a>, <a href="#Page_234">234</a>.</li>
+
+<li><i>Periola</i>, <a href="#Page_238">238</a>.</li>
+
+<li>Permanganate, <a href="#Page_162">162</a>.</li>
+
+<li><i>Peronospora</i>, <a href="#Page_136">136</a>, <a href="#Page_160">160</a>, <a href="#Page_175">175</a>, <a href="#Page_187">187</a>, <a href="#Page_189">189</a>, <a href="#Page_208">208</a>.</li>
+
+<li><i>Petasites</i>, <a href="#Page_188">188</a>.</li>
+
+<li>Petroleum, <a href="#Page_162">162</a>.</li>
+
+<li><i>Peziza</i>, <a href="#Page_115">115</a>, <a href="#Page_144">144</a>, <a href="#Page_152">152</a>.</li>
+
+<li>Phanerogams, <a href="#Page_108">108</a>, <a href="#Page_111">111</a>.</li>
+
+<li><span class="pagenum"><a name="Page_304" id="Page_304"></a>[<a href="./images/304.png">304</a>]</span><i>Phellomyces</i>, <a href="#Page_238">238</a>.</li>
+
+<li><i>Phoma</i>, <a href="#Page_217">217</a>, <a href="#Page_243">243</a>.</li>
+
+<li>Phosphorus, <a href="#Page_272">272</a>.</li>
+
+<li>Photo-synthesis, <a href="#Page_11">11</a>, <a href="#Page_16">16</a>.</li>
+
+<li><i>Phragmidium</i>, <a href="#Page_189">189</a>.</li>
+
+<li><i>Phyllachora</i>, <a href="#Page_189">189</a>.</li>
+
+<li><i>Phyllereum</i>, <a href="#Page_253">253</a>.</li>
+
+<li><i>Phyllobium</i>, <a href="#Page_217">217</a>.</li>
+
+<li><i>Phyllosiphon</i>, <a href="#Page_188">188</a>.</li>
+
+<li><i>Phyllosticta</i>, <a href="#Page_188">188</a>, <a href="#Page_209">209</a>.</li>
+
+<li><i>Phylloxera</i>, <a href="#Page_110">110</a>, <a href="#Page_145">145</a>, <a href="#Page_149">149</a>, <a href="#Page_154">154</a>, <a href="#Page_155">155</a>, <a href="#Page_163">163</a>, <a href="#Page_166">166</a>, <a href="#Page_172">172</a>, <a href="#Page_188">188</a>, <a href="#Page_219">219</a>, <a href="#Page_220">220</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Physiology, <a href="#Page_1">1</a>, <a href="#Page_66">66</a>, <a href="#Page_85">85</a>.</li>
+
+<li>Physiological diseases, <a href="#Page_119">119</a>, <a href="#Page_121">121</a>.</li>
+
+<li><i>Phytomyza</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Phytopathology, <a href="#Page_85">85</a>.</li>
+
+<li><i>Phytophthora</i>, <a href="#Page_115">115</a>, <a href="#Page_136">136</a>, <a href="#Page_144">144</a>, <a href="#Page_150">150</a>, <a href="#Page_151">151</a>, <a href="#Page_235">235</a>, <a href="#Page_236">236</a>.</li>
+
+<li><i>Phytophysa</i>, <a href="#Page_219">219</a>.</li>
+
+<li><i>Phytoptus</i>, <a href="#Page_189">189</a>, <a href="#Page_213">213</a>, <a href="#Page_214">214</a>, <a href="#Page_215">215</a>, <a href="#Page_218">218</a>, <a href="#Page_219">219</a>, <a href="#Page_253">253</a>, <a href="#Page_254">254</a>.</li>
+
+<li><i>Pilea</i>, <a href="#Page_219">219</a>.</li>
+
+<li><i>Pilobolus</i>, <a href="#Page_126">126</a>, <a href="#Page_140">140</a>.</li>
+
+<li>Pines, <a href="#Page_183">183</a>, <a href="#Page_223">223</a>, <a href="#Page_234">234</a>, <a href="#Page_251">251</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Pine-apple, <a href="#Page_258">258</a>.</li>
+
+<li>Pith flecks, <a href="#Page_204">204</a>, <a href="#Page_207">207</a>.</li>
+
+<li>Plant as agent of disease, <a href="#Page_99">99</a>, <a href="#Page_108">108</a>.</li>
+
+<li>Plant, agricultural chemistry of, <a href="#Page_1">1</a>.</li>
+
+<li>Plant and its food, <a href="#Page_7">7</a>.</li>
+
+<li>Plant and its surroundings, <a href="#Page_1">1</a>.</li>
+
+<li>Plant, a machine, <a href="#Page_1">1</a>, <a href="#Page_15">15</a>.</li>
+
+<li>Plant, central object of study, <a href="#Page_1">1</a>.</li>
+
+<li>Plant, physiology, <a href="#Page_1">1</a>.</li>
+
+<li><i>Plantago</i>, <a href="#Page_257">257</a>.</li>
+
+<li>Plantain, <a href="#Page_112">112</a>, <a href="#Page_257">257</a>.</li>
+
+<li>Plants, dying out of, <a href="#Page_93">93</a>.</li>
+
+<li>Plasmodia, <a href="#Page_163">163</a>.</li>
+
+<li><i>Plasmodiophora</i>, <a href="#Page_114">114</a>, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>, <a href="#Page_144">144</a>, <a href="#Page_163">163</a>, <a href="#Page_219">219</a>, <a href="#Page_284">284</a>, <a href="#Page_285">285</a>.</li>
+
+<li>Plasmolysis, <a href="#Page_47">47</a>.</li>
+
+<li><i>Pleospora</i>, <a href="#Page_236">236</a>.</li>
+
+<li><i>Pleotrachelus</i>, <a href="#Page_126">126</a>, <a href="#Page_140">140</a>.</li>
+
+<li>Plum, <a href="#Page_171">171</a>, <a href="#Page_189">189</a>, <a href="#Page_192">192</a>, <a href="#Page_209">209</a>, <a href="#Page_214">214</a>, <a href="#Page_206">206</a>, <a href="#Page_231">231</a>, <a href="#Page_235">235</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_260">260</a>.</li>
+
+<li><i>Poa</i>, <a href="#Page_258">258</a>.</li>
+
+<li>Pocket-like galls, <a href="#Page_155">155</a>, <a href="#Page_214">214</a>, <a href="#Page_218">218</a>.</li>
+
+<li>Pocket-plums, <a href="#Page_214">214</a>.</li>
+
+<li>Pockets, <a href="#Page_253">253</a>.</li>
+
+<li>Poison, <a href="#Page_102">102</a>, <a href="#Page_130">130</a>, <a href="#Page_136">136</a>, <a href="#Page_163">163</a>, <a href="#Page_216">216</a>.</li>
+
+<li>Poisonous gases, <a href="#Page_181">181</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Pollen grain, <a href="#Page_288">288</a>.</li>
+
+<li>Pollination, <a href="#Page_248">248</a>, <a href="#Page_262">262</a>, <a href="#Page_265">265</a>, <a href="#Page_266">266</a>, <a href="#Page_271">271</a>.</li>
+
+<li><i>Polydesmus</i>, <a href="#Page_236">236</a>.</li>
+
+<li><i>Polygonatum</i>, <a href="#Page_175">175</a>.</li>
+
+<li><i>Polygonum</i>, <a href="#Page_258">258</a>.</li>
+
+<li>Polymorphism, <a href="#Page_174">174</a>.</li>
+
+<li>Polyporei, <a href="#Page_142">142</a>.</li>
+
+<li><i>Polyporus</i>, <a href="#Page_143">143</a>, <a href="#Page_206">206</a>.</li>
+
+<li><i>Polystigma</i>, <a href="#Page_189">189</a>.</li>
+
+<li>Poplar, <a href="#Page_188">188</a>, <a href="#Page_206">206</a>, <a href="#Page_215">215</a>, <a href="#Page_218">218</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Post and epidemics, <a href="#Page_142">142</a>.</li>
+
+<li>Potassium, <a href="#Page_272">272</a>.</li>
+
+<li>Potassium sulphite, <a href="#Page_162">162</a>.</li>
+
+<li>Potato, <a href="#Page_162">162</a>, <a href="#Page_171">171</a>, <a href="#Page_194">194</a>, <a href="#Page_209">209</a>, <a href="#Page_216">216</a>, <a href="#Page_236">236</a>, <a href="#Page_237">237</a>, <a href="#Page_258">258</a>.</li>
+
+<li>Potato-disease, <a href="#Page_114">114</a>, <a href="#Page_143">143</a>, <a href="#Page_149">149</a>, <a href="#Page_150">150</a>, <a href="#Page_166">166</a>, <a href="#Page_189">189</a>, <a href="#Page_207">207</a>, <a href="#Page_235">235</a>.</li>
+
+<li>Powders, antiseptic, <a href="#Page_159">159</a>, <a href="#Page_160">160</a>, <a href="#Page_161">161</a>.</li>
+
+<li>Predisposition to disease, <a href="#Page_98">98</a>, <a href="#Page_99">99</a>, <a href="#Page_105">105</a>, <a href="#Page_168">168</a>, <a href="#Page_169">169</a>, <a href="#Page_229">229</a>, <a href="#Page_262">262</a>, <a href="#Page_268">268</a>, <a href="#Page_277">277</a>, <a href="#Page_278">278</a>, <a href="#Page_282">282</a>.</li>
+
+<li>Preventible diseases, <a href="#Page_159">159</a>.</li>
+
+<li>Preventitious buds, <a href="#Page_259">259</a>.</li>
+
+<li><span class="pagenum"><a name="Page_305" id="Page_305"></a>[<a href="./images/305.png">305</a>]</span>Prolepsis, <a href="#Page_257">257</a>, <a href="#Page_259">259</a>.</li>
+
+<li>Proliferations, <a href="#Page_257">257</a>, <a href="#Page_258">258</a>.</li>
+
+<li>Properties of soil, <a href="#Page_57">57</a>.</li>
+
+<li>Prophylactic measures, <a href="#Page_160">160</a>.</li>
+
+<li>Proteids, <a href="#Page_132">132</a>, <a href="#Page_138">138</a>, <a href="#Page_272">272</a>, <a href="#Page_277">277</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Proteolytic enzymes, <a href="#Page_132">132</a>.</li>
+
+<li><i>Protomyces</i>, <a href="#Page_217">217</a>.</li>
+
+<li>Protoplasmic molecules, <a href="#Page_276">276</a>, <a href="#Page_278">278</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Protoplasm, <a href="#Page_33">33</a>, <a href="#Page_41">41</a>, <a href="#Page_271">271</a>, <a href="#Page_272">272</a>, <a href="#Page_274">274</a>, <a href="#Page_276">276</a>.</li>
+
+<li>Pruning, <a href="#Page_105">105</a>, <a href="#Page_143">143</a>, <a href="#Page_194">194</a>, <a href="#Page_225">225</a>, <a href="#Page_250">250</a>.</li>
+
+<li>Prussic acid, <a href="#Page_163">163</a>, <a href="#Page_165">165</a>, <a href="#Page_173">173</a>.</li>
+
+<li><i>Psylla</i>, <a href="#Page_253">253</a>.</li>
+
+<li><i>Puccinia</i>, <a href="#Page_88">88</a>, <a href="#Page_114">114</a>, <a href="#Page_169">169</a>, <a href="#Page_175">175</a>, <a href="#Page_176">176</a>, <a href="#Page_188">188</a>, <a href="#Page_189">189</a>, <a href="#Page_247">247</a>, <a href="#Page_252">252</a>.</li>
+
+<li>Puckers, <a href="#Page_214">214</a>, <a href="#Page_235">235</a>, <a href="#Page_246">246</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Puffing of spores, <a href="#Page_142">142</a>, <a href="#Page_144">144</a>.</li>
+
+<li>Punctures, <a href="#Page_212">212</a>.</li>
+
+<li>Pure culture, <a href="#Page_166">166</a>.</li>
+
+<li>Purple-black spots, <a href="#Page_191">191</a>.</li>
+
+<li>Pustules, <a href="#Page_188">188</a>, <a href="#Page_190">190</a>, <a href="#Page_212">212</a>, <a href="#Page_217">217</a>.</li>
+
+<li>Putrefaction, <a href="#Page_234">234</a>.</li>
+
+<li><i>Pyrethrum</i>, <a href="#Page_161">161</a>.</li>
+
+<li><i>Pyrus</i>, <a href="#Page_214">214</a>.</li>
+
+<li><i>Pythium</i>, <a href="#Page_114">114</a>, <a href="#Page_119">119</a>, <a href="#Page_136">136</a>, <a href="#Page_144">144</a>, <a href="#Page_160">160</a>, <a href="#Page_230">230</a>.</li>
+
+
+<li class="newletter"><i>Quassia</i>, <a href="#Page_161">161</a>.</li>
+
+<li>Quinine, <a href="#Page_173">173</a>.</li>
+
+
+<li class="newletter">Rabbits, <a href="#Page_108">108</a>, <a href="#Page_142">142</a>, <a href="#Page_164">164</a>, <a href="#Page_194">194</a>.</li>
+
+<li>Rain trees, <a href="#Page_233">233</a>.</li>
+
+<li>Rankness, <a href="#Page_97">97</a>, <a href="#Page_227">227</a>, <a href="#Page_228">228</a>.</li>
+
+<li>Rats, <a href="#Page_108">108</a>, <a href="#Page_163">163</a>.</li>
+
+<li>Rays of light, <a href="#Page_18">18</a>.</li>
+
+<li>Red light, <a href="#Page_21">21</a>.</li>
+
+<li>Red spider, <a href="#Page_161">161</a>, <a href="#Page_187">187</a>, <a href="#Page_188">188</a>, <a href="#Page_192">192</a>.</li>
+
+<li>Red spots, <a href="#Page_188">188</a>, <a href="#Page_253">253</a>.</li>
+
+<li>References in Bible, <a href="#Page_85">85</a>.</li>
+
+<li>Remedial measures, <a href="#Page_89">89</a>.</li>
+
+<li>Repellent substances, <a href="#Page_136">136</a>.</li>
+
+<li>Reproduction, <a href="#Page_72">72</a>, <a href="#Page_281">281</a>.</li>
+
+<li>Reserves, <a href="#Page_274">274</a>.</li>
+
+<li>Resin, <a href="#Page_125">125</a>.</li>
+
+<li>Resin-flux, <a href="#Page_234">234</a>.</li>
+
+<li>Resinosis, <a href="#Page_227">227</a>, <a href="#Page_234">234</a>.</li>
+
+<li>Resistance to disease, <a href="#Page_155">155</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Resistant races, <a href="#Page_172">172</a>.</li>
+
+<li>Respiration, <a href="#Page_17">17</a>, <a href="#Page_31">31</a>, <a href="#Page_130">130</a>, <a href="#Page_271">271</a>, <a href="#Page_275">275</a>, <a href="#Page_276">276</a>, <a href="#Page_285">285</a>, <a href="#Page_287">287</a>.</li>
+
+<li>Reversions, <a href="#Page_73">73</a>.</li>
+
+<li>Rhinanthoideae, <a href="#Page_265">265</a>.</li>
+
+<li><i>Rhinanthus</i>, <a href="#Page_112">112</a>.</li>
+
+<li><i>Rhizobium</i>, <a href="#Page_289">289</a>.</li>
+
+<li><i>Rhizoctonia</i>, <a href="#Page_238">238</a>.</li>
+
+<li>Rhizomorph, <a href="#Page_145">145</a>.</li>
+
+<li>Rhododendron, <a href="#Page_218">218</a>.</li>
+
+<li>Rhubarb, <a href="#Page_180">180</a>, <a href="#Page_230">230</a>.</li>
+
+<li><i>Rhynchitis</i>, <a href="#Page_254">254</a>.</li>
+
+<li><i>Rhytisma</i>, <a href="#Page_188">188</a>.</li>
+
+<li>Ribbon grass, <a href="#Page_183">183</a>.</li>
+
+<li><i>Ribes</i>, <a href="#Page_214">214</a>.</li>
+
+<li>Rice, <a href="#Page_172">172</a>.</li>
+
+<li>Rimpau's experiments, <a href="#Page_69">69</a>, <a href="#Page_73">73</a>, <a href="#Page_77">77</a>.</li>
+
+<li>Ringing, <a href="#Page_194">194</a>, <a href="#Page_201">201</a>, <a href="#Page_202">202</a>, <a href="#Page_210">210</a>.</li>
+
+<li>Ripened wood, <a href="#Page_243">243</a>.</li>
+
+<li><i>Robinia</i>, <a href="#Page_259">259</a>.</li>
+
+<li>Rodents, <a href="#Page_109">109</a>.</li>
+
+<li><i>Roestelia</i>, <a href="#Page_217">217</a>.</li>
+
+<li>Rolled leaves, <a href="#Page_86">86</a>.</li>
+
+<li>Root, <a href="#Page_9">9</a>, <a href="#Page_35">35</a>, <a href="#Page_96">96</a>, <a href="#Page_120">120</a>, <a href="#Page_227">227</a>, <a href="#Page_270">270</a>.</li>
+
+<li>Root-absorption, <a href="#Page_181">181</a>.</li>
+
+<li>Root-diseases, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>.</li>
+
+<li>Root-excretions, <a href="#Page_46">46</a>.</li>
+
+<li>Root-fusions, <a href="#Page_262">262</a>.</li>
+
+<li>Root-galls, <a href="#Page_221">221</a>.</li>
+
+<li><span class="pagenum"><a name="Page_306" id="Page_306"></a>[<a href="./images/306.png">306</a>]</span>Root-hairs, <a href="#Page_34">34</a>, <a href="#Page_102">102</a>, <a href="#Page_163">163</a>.</li>
+
+<li>Root-nodules, <a href="#Page_212">212</a>, <a href="#Page_219">219</a>.</li>
+
+<li>Root-parasites, <a href="#Page_112">112</a>, <a href="#Page_265">265</a>.</li>
+
+<li>Root-rot, <a href="#Page_230">230</a>.</li>
+
+<li>Roses, <a href="#Page_232">232</a>, <a href="#Page_243">243</a>, <a href="#Page_257">257</a>, <a href="#Page_268">268</a>, <a href="#Page_278">278</a>.</li>
+
+<li>Rosettes, <a href="#Page_225">225</a>.</li>
+
+<li>Rot, <a href="#Page_97">97</a>, <a href="#Page_182">182</a>, <a href="#Page_227">227</a>, <a href="#Page_229">229</a>, <a href="#Page_231">231</a>, <a href="#Page_236">236</a>.</li>
+
+<li>Rotation of crops, <a href="#Page_69">69</a>, <a href="#Page_166">166</a>.</li>
+
+<li>Rotifer, <a href="#Page_140">140</a>.</li>
+
+<li>Rot-organisms, <a href="#Page_200">200</a>.</li>
+
+<li>Rotting of wounds, <a href="#Page_87">87</a>.</li>
+
+<li>Rouen law, <a href="#Page_85">85</a>.</li>
+
+<li>Rushes, <a href="#Page_114">114</a>.</li>
+
+<li>Rust, <a href="#Page_122">122</a>, <a href="#Page_142">142</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>, <a href="#Page_175">175</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Rye, <a href="#Page_176">176</a>, <a href="#Page_248">248</a>.</li>
+
+
+<li class="newletter"><i>Saccharomyces</i>, <a href="#Page_60">60</a>.</li>
+
+<li>Sachs, <a href="#Page_7">7</a>, <a href="#Page_36">36</a>.</li>
+
+<li><i>Salvia</i>, <a href="#Page_214">214</a>.</li>
+
+<li>San José scale, <a href="#Page_187">187</a>.</li>
+
+<li>Sand-blast action, <a href="#Page_184">184</a>.</li>
+
+<li>Sandy soils, <a href="#Page_184">184</a>.</li>
+
+<li><i>Saperda</i>, <a href="#Page_205">205</a>.</li>
+
+<li>Saprophytes, <a href="#Page_135">135</a>, <a href="#Page_137">137</a>, <a href="#Page_175">175</a>, <a href="#Page_234">234</a>, <a href="#Page_243">243</a>, <a href="#Page_244">244</a>.</li>
+
+<li><i>Scab</i>, <a href="#Page_189">189</a>, <a href="#Page_216">216</a>.</li>
+
+<li><i>Scale</i>, <a href="#Page_187">187</a>.</li>
+
+<li><i>Schinzia</i>, <a href="#Page_114">114</a>.</li>
+
+<li><i>Schizoneura</i>, <a href="#Page_223">223</a>.</li>
+
+<li>Scion, <a href="#Page_183">183</a>, <a href="#Page_262">262</a>, <a href="#Page_264">264</a>, <a href="#Page_266">266</a>, <a href="#Page_282">282</a>.</li>
+
+<li><i>Scleroderris</i>, <a href="#Page_223">223</a>.</li>
+
+<li>Sclerotia, <a href="#Page_143">143</a>.</li>
+
+<li>Schwarz, <a href="#Page_39">39</a>.</li>
+
+<li><i>Sclerotinia</i>, <a href="#Page_142">142</a>, <a href="#Page_143">143</a>, <a href="#Page_144">144</a>, <a href="#Page_231">231</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Scolytidae, <a href="#Page_205">205</a>.</li>
+
+<li>Scorching, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>.</li>
+
+<li>Scurf, <a href="#Page_216">216</a>.</li>
+
+<li>Sea-kale, <a href="#Page_261">261</a>.</li>
+
+<li><i>Secale</i>, <a href="#Page_76">76</a>.</li>
+
+<li>Secretions, <a href="#Page_130">130</a>, <a href="#Page_133">133</a>, <a href="#Page_173">173</a>, <a href="#Page_274">274</a>.</li>
+
+<li>Sedges, <a href="#Page_189">189</a>.</li>
+
+<li>Seedless grapes, <a href="#Page_247">247</a>.</li>
+
+<li><i>Selandria</i>, <a href="#Page_208">208</a>.</li>
+
+<li>Selection, <a href="#Page_69">69</a>, <a href="#Page_74">74</a>, <a href="#Page_78">78</a>, <a href="#Page_169">169</a>.</li>
+
+<li>Selective absorption, <a href="#Page_53">53</a>, <a href="#Page_65">65</a>.</li>
+
+<li>Self-fertilisation, <a href="#Page_281">281</a>.</li>
+
+<li>Semi-parasites, <a href="#Page_112">112</a>.</li>
+
+<li><i>Senecio</i>, <a href="#Page_188">188</a>.</li>
+
+<li>Sensitive plant, <a href="#Page_125">125</a>.</li>
+
+<li><i>Septoria</i>, <a href="#Page_114">114</a>, <a href="#Page_187">187</a>.</li>
+
+<li>Sewage waters, <a href="#Page_59">59</a>.</li>
+
+<li>Sexual act, <a href="#Page_72">72</a>.</li>
+
+<li>Shaded foliage, <a href="#Page_113">113</a>.</li>
+
+<li>Shanking, <a href="#Page_246">246</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Shoots from old wood, <a href="#Page_260">260</a>.</li>
+
+<li>Shot holes, <a href="#Page_204">204</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>.</li>
+
+<li>Silver fir, <a href="#Page_224">224</a>.</li>
+
+<li>Silver leaf, <a href="#Page_192">192</a>.</li>
+
+<li><i>Sirex</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Skeleton leaves, <a href="#Page_204">204</a>, <a href="#Page_207">207</a>.</li>
+
+<li>Slime flux, <a href="#Page_227">227</a>, <a href="#Page_233">233</a>.</li>
+
+<li>Slime fungus, <a href="#Page_219">219</a>.</li>
+
+<li>Slugs, <a href="#Page_111">111</a>, <a href="#Page_164">164</a>, <a href="#Page_207">207</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Smut, <a href="#Page_117">117</a>, <a href="#Page_143">143</a>, <a href="#Page_162">162</a>, <a href="#Page_190">190</a>.</li>
+
+<li>Snails, <a href="#Page_111">111</a>, <a href="#Page_142">142</a>, <a href="#Page_207">207</a>.</li>
+
+<li>Snow, <a href="#Page_106">106</a>.</li>
+
+<li>Soap, as insecticide, <a href="#Page_161">161</a>.</li>
+
+<li>Soil, <a href="#Page_1">1</a>, <a href="#Page_42">42</a>, <a href="#Page_99">99</a>, <a href="#Page_102">102</a>, <a href="#Page_142">142</a>, <a href="#Page_163">163</a>.</li>
+
+<li>Soil-bacteria, <a href="#Page_60">60</a>.</li>
+
+<li>Soil-filtration, <a href="#Page_59">59</a>.</li>
+
+<li>Soil-organisms, <a href="#Page_61">61</a>, <a href="#Page_143">143</a>.</li>
+
+<li>Solar energy, <a href="#Page_135">135</a>.</li>
+
+<li>Somato-plasm, <a href="#Page_267">267</a>.</li>
+
+<li>Sooty moulds, <a href="#Page_135">135</a>, <a href="#Page_190">190</a>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Sorbus</i>, <a href="#Page_207">207</a>.</li>
+
+<li><i>Sorosporium</i>, <a href="#Page_216">216</a>.</li>
+
+<li><span class="pagenum"><a name="Page_307" id="Page_307"></a>[<a href="./images/307.png">307</a>]</span>Sour-rot, <a href="#Page_231">231</a>.</li>
+
+<li>Sparrows, <a href="#Page_164">164</a>.</li>
+
+<li>Specialised races, <a href="#Page_168">168</a>, <a href="#Page_176">176</a>.</li>
+
+<li>Specific predisposition, <a href="#Page_155">155</a>.</li>
+
+<li>Spectrum, <a href="#Page_19">19</a>, <a href="#Page_21">21</a>, <a href="#Page_26">26</a>.</li>
+
+<li>Spermogonia, <a href="#Page_144">144</a>, <a href="#Page_232">232</a>.</li>
+
+<li><i>Sphaerella</i>, <a href="#Page_189">189</a>.</li>
+
+<li><i>Sphaerotheca</i>, <a href="#Page_187">187</a>.</li>
+
+<li>Sphaeroblasts, <a href="#Page_222">222</a>, <a href="#Page_225">225</a>.</li>
+
+<li><i>Spicaria</i>, <a href="#Page_237">237</a>.</li>
+
+<li>Spiral grooving, <a href="#Page_204">204</a>, <a href="#Page_210">210</a>.</li>
+
+<li>Spiral growth, <a href="#Page_252">252</a>.</li>
+
+<li><i>Spongospora</i>, <a href="#Page_216">216</a>.</li>
+
+<li>Spontaneous variations, <a href="#Page_78">78</a>, <a href="#Page_246">246</a>, <a href="#Page_255">255</a>.</li>
+
+<li>Spores, <a href="#Page_144">144</a>.</li>
+
+<li>Sports, <a href="#Page_93">93</a>, <a href="#Page_247">247</a>.</li>
+
+<li>Spots on leaves, <a href="#Page_120">120</a>, <a href="#Page_186">186</a>.</li>
+
+<li>Spraying, <a href="#Page_159">159</a>, <a href="#Page_161">161</a>, <a href="#Page_162">162</a>.</li>
+
+<li>Spreading of disease, <a href="#Page_142">142</a>.</li>
+
+<li>Squirrels, <a href="#Page_108">108</a>.</li>
+
+<li>Stag-head, <a href="#Page_240">240</a>, <a href="#Page_244">244</a>.</li>
+
+<li>Starch, <a href="#Page_9">9</a>, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_20">20</a>, <a href="#Page_23">23</a>, <a href="#Page_138">138</a>, <a href="#Page_173">173</a>.</li>
+
+<li>Statistics of epidemics, <a href="#Page_147">147</a>.</li>
+
+<li>Steeping, <a href="#Page_161">161</a>.</li>
+
+<li>Stem diseases, <a href="#Page_120">120</a>.</li>
+
+<li><i>Stereum</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Sterility of soil, <a href="#Page_61">61</a>.</li>
+
+<li>Stimulation, <a href="#Page_119">119</a>.</li>
+
+<li>Stimuli, <a href="#Page_126">126</a>, <a href="#Page_127">127</a>, <a href="#Page_139">139</a>.</li>
+
+<li>Stock, <a href="#Page_262">262</a>, <a href="#Page_264">264</a>, <a href="#Page_266">266</a>, <a href="#Page_282">282</a>.</li>
+
+<li>Stomata, <a href="#Page_23">23</a>.</li>
+
+<li>Stool-shoots, <a href="#Page_201">201</a>, <a href="#Page_225">225</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Stool stumps, <a href="#Page_194">194</a>, <a href="#Page_201">201</a>.</li>
+
+<li>Strangulations, <a href="#Page_204">204</a>, <a href="#Page_209">209</a>.</li>
+
+<li>Strawberry, <a href="#Page_189">189</a>, <a href="#Page_257">257</a>.</li>
+
+<li>Stripping, <a href="#Page_194">194</a>, <a href="#Page_197">197</a>.</li>
+
+<li>Stroma, <a href="#Page_217">217</a>.</li>
+
+<li>Structure, <a href="#Page_274">274</a>.</li>
+
+<li>Structure of protoplasm, <a href="#Page_271">271</a>.</li>
+
+<li>Structure of root-hairs, <a href="#Page_40">40</a>.</li>
+
+<li>Struggle for existence, <a href="#Page_105">105</a>, <a href="#Page_159">159</a>, <a href="#Page_164">164</a>, <a href="#Page_165">165</a>, <a href="#Page_167">167</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Study of causes, <a href="#Page_85">85</a>.</li>
+
+<li>Stumps, <a href="#Page_194">194</a>.</li>
+
+<li>Subsoil, <a href="#Page_57">57</a>, <a href="#Page_103">103</a>.</li>
+
+<li>Substitutive selections, <a href="#Page_286">286</a>.</li>
+
+<li>Suckers, <a href="#Page_225">225</a>, <a href="#Page_260">260</a>.</li>
+
+<li>Sugar, <a href="#Page_11">11</a>, <a href="#Page_17">17</a>, <a href="#Page_20">20</a>, <a href="#Page_173">173</a>, <a href="#Page_286">286</a>.</li>
+
+<li>Sugar cane, <a href="#Page_172">172</a>.</li>
+
+<li>Sugar cane disease, <a href="#Page_166">166</a>.</li>
+
+<li>Sulphate, <a href="#Page_273">273</a>.</li>
+
+<li>Sulphur, <a href="#Page_161">161</a>, <a href="#Page_163">163</a>, <a href="#Page_272">272</a>.</li>
+
+<li>Sulphurous acid, <a href="#Page_181">181</a>.</li>
+
+<li>Sun-burn, <a href="#Page_240">240</a>, <a href="#Page_241">241</a>.</li>
+
+<li>Sun-cracks, <a href="#Page_240">240</a>, <a href="#Page_242">242</a>.</li>
+
+<li>Sundew, <a href="#Page_232">232</a>.</li>
+
+<li>Sunflower, <a href="#Page_256">256</a>, <a href="#Page_264">264</a>.</li>
+
+<li>Sun-spots, <a href="#Page_192">192</a>.</li>
+
+<li>Superstitions, <a href="#Page_85">85</a>.</li>
+
+<li>Surface energy, <a href="#Page_26">26</a>.</li>
+
+<li>Surface roots, <a href="#Page_112">112</a>.</li>
+
+<li>Sweet almond, <a href="#Page_173">173</a>.</li>
+
+<li>Symbiosis, <a href="#Page_63">63</a>, <a href="#Page_130">130</a>, <a href="#Page_137">137</a>, <a href="#Page_219">219</a>, <a href="#Page_263">263</a>, <a href="#Page_265">265</a>, <a href="#Page_268">268</a>, <a href="#Page_285">285</a>.</li>
+
+<li>Symptoms of disease, <a href="#Page_89">89</a>, <a href="#Page_122">122</a>, <a href="#Page_179">179</a>, <a href="#Page_186">186</a>.</li>
+
+<li><i>Synchytrium</i>, <a href="#Page_127">127</a>, <a href="#Page_188">188</a>, <a href="#Page_217">217</a>, <a href="#Page_247">247</a>.</li>
+
+<li>Synthesis, <a href="#Page_65">65</a>.</li>
+
+<li><i>Syringa</i>, <a href="#Page_259">259</a>.</li>
+
+<li>Syringing, <a href="#Page_161">161</a>, <a href="#Page_164">164</a>.</li>
+
+
+<li class="newletter">Tamarisk, <a href="#Page_235">235</a>.</li>
+
+<li>Tannin, <a href="#Page_138">138</a>.</li>
+
+<li><i>Taphrina</i>, <a href="#Page_218">218</a>.</li>
+
+<li>Tar, <a href="#Page_164">164</a>.</li>
+
+<li>Tea, <a href="#Page_244">244</a>.</li>
+
+<li><span class="pagenum"><a name="Page_308" id="Page_308"></a>[<a href="./images/308.png">308</a>]</span>Teazel, <a href="#Page_252">252</a>.</li>
+
+<li>Teleutospore, <a href="#Page_189">189</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Temperature, <a href="#Page_99">99</a>, <a href="#Page_105">105</a>.</li>
+
+<li>Tendencies to ill-health, <a href="#Page_91">91</a>.</li>
+
+<li>Tendrils, <a href="#Page_125">125</a>.</li>
+
+<li>Teratology, <a href="#Page_246">246</a>, <a href="#Page_253">253</a>, <a href="#Page_254">254</a>, <a href="#Page_257">257</a>.</li>
+
+<li><i>Tetraneura</i>, <a href="#Page_218">218</a>.</li>
+
+<li><i>Tetranychus</i>, <a href="#Page_187">187</a>, <a href="#Page_192">192</a>.</li>
+
+<li>Thawing, <a href="#Page_183">183</a>.</li>
+
+<li><i>Thelephora</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Therapeutics, <a href="#Page_85">85</a>, <a href="#Page_89">89</a>, <a href="#Page_159">159</a>.</li>
+
+<li>Thermotropism, <a href="#Page_126">126</a>.</li>
+
+<li><i>Thesium</i>, <a href="#Page_112">112</a>.</li>
+
+<li>Thick-skinned organs, <a href="#Page_168">168</a>, <a href="#Page_171">171</a>.</li>
+
+<li>Thinning, <a href="#Page_96">96</a>, <a href="#Page_105">105</a>.</li>
+
+<li>Thistle, <a href="#Page_247">247</a>.</li>
+
+<li>Thrips, <a href="#Page_88">88</a>, <a href="#Page_191">191</a>, <a href="#Page_208">208</a>.</li>
+
+<li>Thyloses, <a href="#Page_125">125</a>.</li>
+
+<li><i>Tilia</i>, <a href="#Page_214">214</a>.</li>
+
+<li>Timber diseases, <a href="#Page_119">119</a>, <a href="#Page_120">120</a>.</li>
+
+<li>Timiriazeff, <a href="#Page_21">21</a>.</li>
+
+<li><i>Tinea</i>, <a href="#Page_206">206</a>.</li>
+
+<li>Tissue diseases, <a href="#Page_119">119</a>.</li>
+
+<li>Tobacco, <a href="#Page_209">209</a>.</li>
+
+<li>Tobacco powder, <a href="#Page_161">161</a>.</li>
+
+<li>Tomato, <a href="#Page_171">171</a>, <a href="#Page_219">219</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Top-dry trees, <a href="#Page_244">244</a>.</li>
+
+<li>Topical remedies, <a href="#Page_161">161</a>.</li>
+
+<li><i>Tomicus</i>, <a href="#Page_205">205</a>.</li>
+
+<li>Torsions, <a href="#Page_246">246</a>, <a href="#Page_252">252</a>.</li>
+
+<li><i>Tortrix</i>, <a href="#Page_254">254</a>.</li>
+
+<li>Toxic agents, <a href="#Page_130">130</a>.</li>
+
+<li>Transformation of energy, <a href="#Page_25">25</a>, <a href="#Page_28">28</a>.</li>
+
+<li>Transformation of organs, <a href="#Page_254">254</a>, <a href="#Page_255">255</a>.</li>
+
+<li>Transmission of acquired characters, <a href="#Page_264">264</a>, <a href="#Page_283">283</a>, <a href="#Page_290">290</a>.</li>
+
+<li>Transplanting, <a href="#Page_96">96</a>.</li>
+
+<li>Transpiration, <a href="#Page_181">181</a>, <a href="#Page_228">228</a>.</li>
+
+<li>Trees, <a href="#Page_109">109</a>.</li>
+
+<li><i>Trichosphaeria</i>, <a href="#Page_135">135</a>.</li>
+
+<li><i>Triticum</i>, <a href="#Page_76">76</a>.</li>
+
+<li>Tumescence, <a href="#Page_227">227</a>, <a href="#Page_228">228</a>.</li>
+
+<li>Tunnels, <a href="#Page_206">206</a>.</li>
+
+<li>Turgescence, <a href="#Page_47">47</a>, <a href="#Page_228">228</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Turnip, <a href="#Page_126">126</a>, <a href="#Page_162">162</a>, <a href="#Page_230">230</a>.</li>
+
+<li>Twitch, <a href="#Page_113">113</a>.</li>
+
+<li><i>Tylenchus</i>, <a href="#Page_238">238</a>, <a href="#Page_248">248</a>.</li>
+
+
+<li class="newletter">Ulcer, <a href="#Page_231">231</a>.</li>
+
+<li>Unger, <a href="#Page_85">85</a>.</li>
+
+<li>Unsuitable soils, <a href="#Page_101">101</a>.</li>
+
+<li>Upheaval of seedlings, <a href="#Page_179">179</a>, <a href="#Page_183">183</a>.</li>
+
+<li>Uredineae, <a href="#Page_114">114</a>, <a href="#Page_134">134</a>, <a href="#Page_136">136</a>, <a href="#Page_145">145</a>, <a href="#Page_169">169</a>, <a href="#Page_188">188</a>, <a href="#Page_189">189</a>.</li>
+
+<li><i>Uredo</i>, <a href="#Page_88">88</a>, <a href="#Page_188">188</a>, <a href="#Page_191">191</a>.</li>
+
+<li>Uredospores, <a href="#Page_191">191</a>.</li>
+
+<li><i>Uromyces</i>, <a href="#Page_116">116</a>, <a href="#Page_188">188</a>, <a href="#Page_191">191</a>, <a href="#Page_266">266</a>.</li>
+
+<li><i>Urocystis</i>, <a href="#Page_220">220</a>.</li>
+
+<li>Ustilagineae, <a href="#Page_145">145</a>, <a href="#Page_190">190</a>, <a href="#Page_217">217</a>, <a href="#Page_248">248</a>.</li>
+
+<li><i>Ustilago</i>, <a href="#Page_116">116</a>, <a href="#Page_117">117</a>, <a href="#Page_175">175</a>, <a href="#Page_190">190</a>, <a href="#Page_219">219</a>, <a href="#Page_255">255</a>.</li>
+
+
+<li class="newletter"><i>Vaccinium</i>, <a href="#Page_128">128</a>, <a href="#Page_288">288</a>.</li>
+
+<li>Variability, <a href="#Page_174">174</a>.</li>
+
+<li>Variation, <a href="#Page_67">67</a>, <a href="#Page_72">72</a>, <a href="#Page_91">91</a>, <a href="#Page_92">92</a>, <a href="#Page_168">168</a>, <a href="#Page_174">174</a>, <a href="#Page_176">176</a>, <a href="#Page_246">246</a>, <a href="#Page_262">262</a>, <a href="#Page_263">263</a>, <a href="#Page_264">264</a>, <a href="#Page_271">271</a>, <a href="#Page_282">282</a>, <a href="#Page_286">286</a>, <a href="#Page_288">288</a>, <a href="#Page_289">289</a>.</li>
+
+<li>Variegation, <a href="#Page_179">179</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_192">192</a>.</li>
+
+<li>Varieties, <a href="#Page_78">78</a>, <a href="#Page_247">247</a>.</li>
+
+<li>Varieties of soil, <a href="#Page_56">56</a>.</li>
+
+<li><i>Vaucheria</i>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>.</li>
+
+<li>Vegetable acids, <a href="#Page_48">48</a>.</li>
+
+<li>Vertebrata, <a href="#Page_108">108</a>.</li>
+
+<li><i>Verticillium</i>, <a href="#Page_145">145</a>, <a href="#Page_236">236</a>.</li>
+
+<li><i>Viburnum</i>, <a href="#Page_214">214</a>.</li>
+
+<li><span class="pagenum"><a name="Page_309" id="Page_309"></a>[<a href="./images/309.png">309</a>]</span>Vine, <a href="#Page_110">110</a>, <a href="#Page_149">149</a>, <a href="#Page_156">156</a>, <a href="#Page_162">162</a>, <a href="#Page_164">164</a>, <a href="#Page_169">169</a>, <a href="#Page_171">171</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_222">222</a>, <a href="#Page_248">248</a>, <a href="#Page_268">268</a>.</li>
+
+<li>Vine disease, <a href="#Page_143">143</a>.</li>
+
+<li>Vivipary, <a href="#Page_257">257</a>, <a href="#Page_258">258</a>.</li>
+
+
+<li class="newletter">Walnut, <a href="#Page_190">190</a>, <a href="#Page_209">209</a>, <a href="#Page_253">253</a>.</li>
+
+<li>Want of air, <a href="#Page_100">100</a>.</li>
+
+<li>Washing leaves, etc., <a href="#Page_161">161</a>.</li>
+
+<li>Wasp-flies, <a href="#Page_165">165</a>.</li>
+
+<li>Wasps, <a href="#Page_145">145</a>.</li>
+
+<li>Water, <a href="#Page_272">272</a>.</li>
+
+<li>Water and insects, <a href="#Page_161">161</a>.</li>
+
+<li>Water-culture, <a href="#Page_65">65</a>.</li>
+
+<li>Water in soil, <a href="#Page_103">103</a>.</li>
+
+<li>Waterlogging, <a href="#Page_181">181</a>.</li>
+
+<li>Weaving of fungi, <a href="#Page_190">190</a>.</li>
+
+<li>Webs, <a href="#Page_190">190</a>, <a href="#Page_254">254</a>.</li>
+
+<li>Weeding, <a href="#Page_105">105</a>.</li>
+
+<li>Weeds, <a href="#Page_69">69</a>, <a href="#Page_111">111</a>, <a href="#Page_113">113</a>, <a href="#Page_165">165</a>, <a href="#Page_229">229</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Weevils, <a href="#Page_248">248</a>.</li>
+
+<li>Wet feet, <a href="#Page_181">181</a>.</li>
+
+<li>Wheat, <a href="#Page_169">169</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>, <a href="#Page_176">176</a>, <a href="#Page_179">179</a>, <a href="#Page_180">180</a>, <a href="#Page_182">182</a>, <a href="#Page_183">183</a>, <a href="#Page_230">230</a>, <a href="#Page_248">248</a>.</li>
+
+<li>Wheat rust, <a href="#Page_86">86</a>, <a href="#Page_122">122</a>, <a href="#Page_146">146</a>, <a href="#Page_166">166</a>, <a href="#Page_169">169</a>, <a href="#Page_176">176</a>.</li>
+
+<li>White spots, <a href="#Page_186">186</a>, <a href="#Page_187">187</a>.</li>
+
+<li>Willow, <a href="#Page_206">206</a>, <a href="#Page_207">207</a>, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>, <a href="#Page_233">233</a>, <a href="#Page_259">259</a>.</li>
+
+<li>Willow beetle, <a href="#Page_208">208</a>.</li>
+
+<li>Wilting, <a href="#Page_179">179</a>, <a href="#Page_181">181</a>, <a href="#Page_235">235</a>, <a href="#Page_249">249</a>.</li>
+
+<li>Wind, <a href="#Page_106">106</a>, <a href="#Page_142">142</a>, <a href="#Page_144">144</a>, <a href="#Page_153">153</a>, <a href="#Page_184">184</a>, <a href="#Page_209">209</a>, <a href="#Page_229">229</a>.</li>
+
+<li>Wire-worms, <a href="#Page_109">109</a>, <a href="#Page_181">181</a>.</li>
+
+<li>Witches' brooms, <a href="#Page_116">116</a>, <a href="#Page_222">222</a>, <a href="#Page_224">224</a>.</li>
+
+<li>Wood, <a href="#Page_124">124</a>.</li>
+
+<li>Wood-ashes, <a href="#Page_161">161</a>.</li>
+
+<li>Woodbine, <a href="#Page_112">112</a>, <a href="#Page_210">210</a>.</li>
+
+<li>Wood-boring, <a href="#Page_204">204</a>, <a href="#Page_205">205</a>.</li>
+
+<li>Woodlice, <a href="#Page_164">164</a>.</li>
+
+<li>Wood-nodules, <a href="#Page_225">225</a>.</li>
+
+<li>Wood-wasps, <a href="#Page_206">206</a>.</li>
+
+<li>Woolly-aphis, <a href="#Page_219">219</a>, <a href="#Page_223">223</a>.</li>
+
+<li>Worms, <a href="#Page_109">109</a>, <a href="#Page_142">142</a>, <a href="#Page_144">144</a>, <a href="#Page_194">194</a>, <a href="#Page_238">238</a>.</li>
+
+<li>Wounds, <a href="#Page_108">108</a>, <a href="#Page_139">139</a>, <a href="#Page_194">194</a>, <a href="#Page_204">204</a>, <a href="#Page_207">207</a>, <a href="#Page_213">213</a>, <a href="#Page_260">260</a>, <a href="#Page_263">263</a>, <a href="#Page_269">269</a>.</li>
+
+<li>Wound-cork, <a href="#Page_195">195</a>.</li>
+
+<li>Wound-fever, <a href="#Page_123">123</a>.</li>
+
+<li>Wound-fungi, <a href="#Page_203">203</a>, <a href="#Page_204">204</a>, <a href="#Page_240">240</a>.</li>
+
+<li>Wound-gum, <a href="#Page_125">125</a>.</li>
+
+<li>Wound-wood, <a href="#Page_124">124</a>.</li>
+
+<li>Wrens, <a href="#Page_165">165</a>.</li>
+
+<li>Wrinkling, <a href="#Page_253">253</a>.</li>
+
+
+<li class="newletter"><i>Xenia</i>, <a href="#Page_267">267</a>.</li>
+
+<li><i>Xyloma</i>, <a href="#Page_88">88</a>.</li>
+
+
+<li class="newletter">Yeasts, <a href="#Page_134">134</a>, <a href="#Page_172">172</a>, <a href="#Page_231">231</a>, <a href="#Page_233">233</a>.</li>
+
+<li>Yellowing, <a href="#Page_179">179</a>, <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_184">184</a>.</li>
+
+<li>Yellow leaves, <a href="#Page_89">89</a>.</li>
+
+<li>Yellow spots, <a href="#Page_186">186</a>, <a href="#Page_187">187</a>, <a href="#Page_188">188</a>, <a href="#Page_253">253</a>.</li>
+
+
+<li class="newletter">Zoospores, <a href="#Page_151">151</a>.</li>
+</ul>
+
+
+<div class="publisher">
+<p class="center">GLASGOW: PRINTED AT THE UNIVERSITY PRESS BY ROBERT MACLEHOSE AND CO.</p>
+</div>
+
+<p><span class="pagenum"><a name="Page_310" id="Page_310"></a>[<a href="./images/310.png">310</a>]</span></p>
+
+
+
+
+<hr class="newchapter" />
+<p><span class="pagenum"><a name="Page_311" id="Page_311"></a>[<a href="./images/311.png">311</a>]</span></p>
+<h2>MACMILLAN AND CO.'S WORKS ON BOTANY.</h2>
+
+
+<hr class="booklist" />
+
+<p class="sectctr">BY THE SAME AUTHOR.</p>
+
+<p class="center">Crown 8vo. Price 6s.</p>
+
+
+<p><b>Timber and Some of its Diseases.</b></p>
+
+<div class="block">
+<p>By <span class="smcap">H. Marshall Ward</span>, D.Sc., F.R.S., F.L.S., Fellow of Sidney
+Sussex College, and Professor of Botany in the University of
+Cambridge. Illustrated.</p>
+</div>
+
+<div class="blockquot">
+<p><i>MANCHESTER EXAMINER.</i>&mdash;"The subject as a whole is one which
+is little understood in England, and Professor Marshall Ward's
+work cannot fail to be useful. The student will be much helped
+by the numerous illustrations."</p>
+
+<p><i>GARDENER'S CHRONICLE.</i>&mdash;"This is a book whose appearance we
+hail with great satisfaction.&nbsp;.&nbsp;.&nbsp;. We heartily recommend its
+perusal to those concerned."</p>
+</div>
+
+<hr class="booklist" />
+
+<p class="hangsec"><b>The Study of the Biology of Ferns by the Collodion Method.</b> For
+Advanced and Collegiate Students. By <span class="smcap">George F. Atkinson</span>,
+Ph.B., Associate Professor of Cryptogamic Botany in Cornell
+University. 8vo. 8s. 6d. net.</p>
+
+<p class="hang"><b>On British Wild Flowers considered in Relation to Insects.</b> By
+<span class="smcap">Lord Avebury</span>, F.R.S., D.C.L., LL.D. Illustrated. Cr. 8vo. 4s.
+6d.</p>
+
+<p class="hang"><b>Flowers, Fruits, and Leaves.</b> With illustrations. By <span class="smcap">Lord
+Avebury</span>, F.R.S., D.C.L., LL.D. Cr. 8vo. 4s. 6d.</p>
+
+<p class="hang"><b>Lessons with Plants.</b> By Prof. <span class="smcap">L. H. Bailey</span>. Cr. 8vo. 7s. 6d.</p>
+
+<p class="hang"><b>First Lessons with Plants.</b> By Prof. <span class="smcap">L. H. Bailey</span>. Cr. 8vo. 2s.
+6d.</p>
+
+<p class="hang"><b>Botany.</b> An Elementary Text Book for Schools. By Prof. <span class="smcap">L. H.
+Bailey</span>. Ex. cr. 8vo. 6s.</p>
+
+<p class="hang"><b>First Lessons in Practical Botany.</b> By <span class="smcap">G. T. Bettany</span>. Pott 8vo.
+1s.</p>
+
+<p class="hang"><b>A Course of Practical Instruction in Botany.</b> By <span class="smcap">F. O. Bower</span>,
+D.Sc., F.L.S., Regius Professor of Botany in the University of
+Glasgow. Cr. 8vo. 10s. 6d.</p>
+
+<p class="hang"><b>Practical Botany for Beginners.</b> By <span class="smcap">F. O. Bower</span>, D.Sc., F.L.S.
+Gl. 8vo. 3s. 6d.</p>
+
+<p class="hang"><b>Lectures on the Evolution of Plants.</b> By <span class="smcap">Douglas Houghton
+Campbell</span>, Ph.D., Professor of Botany in the Leland Stanford
+Junr. University. Cr. 8vo. 4s. 6d. net.</p>
+
+<p class="hang"><b>Botany for Beginners.</b> By <span class="smcap">Ernest Evans</span>, Burnley Technical
+School. Globe 8vo. 2s. 6d.</p>
+
+<p class="hang"><b>The Teaching Botanist.</b> A Manual of Information upon Botanical
+Teaching, with an outline for a general course. By <span class="smcap">William F.
+Ganong</span>, Professor of Botany in Smith College. Cr. 8vo. 5s.</p>
+
+<p class="hang"><b>Structural Botany; or, Organography on the Basis of
+Morphology.</b> To which is added the Principles of Taxonomy and
+Phytography, and a Glossary of Botanical Terms. By <span class="smcap">Asa Gray</span>,
+LL.D., Fisher Professor of Natural History (Botany) in Harvard
+University. 8vo. 10s. 6d.</p>
+
+<p class="hang"><span class="pagenum"><a name="Page_312" id="Page_312"></a>[<a href="./images/312.png">312</a>]</span><b>Text-Book of the Diseases of Trees.</b> By Prof. <span class="smcap">R. Hartig</span> of the
+University of Munich. Trans. by Prof. <span class="smcap">Wm. Somerville</span>, F.R.S.,
+F.L.S., Professor of Agriculture and Forestry, Durham College
+of Science, Newcastle-on-Tyne. Introduction by Prof. <span class="smcap">H.
+Marshall Ward</span>, D.Sc., F.R.S., F.L.S., Professor of Botany at
+the Royal Indian Engineering College, Cooper's Hill. With
+numerous illustrations. 8vo. 10s. net.</p>
+
+<p class="hang"><b>The Student's Flora of the British Isles.</b> By Sir <span class="smcap">J. D. Hooker</span>,
+M.D., D.C.L., LL.D., F.R.S. Third edition. Globe 8vo. 10s. 6d.</p>
+
+<p class="hang"><b>A Primer of Botany.</b> By the same. Pott 8vo. 1s.</p>
+
+<p class="hang"><b>Timber and Timber Trees, Native and Foreign.</b> By <span class="smcap">Thomas
+Laslett</span>. Second edition. Revised by <span class="smcap">H. Marshall Ward</span>, F.R.S.
+Cr. 8vo. 8s. 6d.</p>
+
+<p class="hang"><b>The Yew Trees of Great Britain and Ireland.</b> By <span class="smcap">John Lowe</span>,
+M.D., Honorary Physician to His Royal Highness the Prince of
+Wales; Fellow of the Linnean Society; Fellow of the Botanical
+Society of Edinburgh. Illustrated. 8vo. 10s. net.</p>
+
+<p class="hang"><b>The Nature and Work of Plants.</b> An Introduction to the Study of
+Botany. By <span class="smcap">D. T. Macdougal</span>, Ph.D. Crown 8vo. 4s. 6d.</p>
+
+<p class="hang"><b>Introduction to Study of Seaweeds.</b> By <span class="smcap">G. Murray</span>. Cr. 8vo. 7s.
+6d.</p>
+
+<p class="hang"><b>British Forest Trees and their Sylvicultural Characteristics
+and Treatment.</b> By <span class="smcap">J. Nisbet</span>. Cr. 8vo. 6s. net.</p>
+
+<p class="hang"><b>Lessons in Elementary Botany.</b> The Part on Systematic Botany
+based upon material left in manuscript by the late Professor
+Henslow. By <span class="smcap">Daniel Oliver</span>, F.R.S., F.L.S., formerly Keeper of
+the Herbarium and Library of the Royal Gardens, Kew, and
+Professor of Botany in University College, London.
+Illustrated. Fcap. 8vo. 4s. 6d.</p>
+
+<p class="hang"><b>First Book of Indian Botany.</b> By <span class="smcap">Daniel Oliver</span>, F.R.S., F.L.S.
+Illustrated. Ex. fcap. 8vo. 6s. 6d.</p>
+
+<p class="hang"><b>Laboratory Practice for Beginners in Botany.</b> By <span class="smcap">William A.
+Setchell</span>, Ph.D., Professor of Botany in the University of
+California. Fcap. 8vo. 4s. 6d. net.</p>
+
+<p class="hang"><b>Economic Plants, Dictionary of Popular Names of</b>; Their
+History, Products, and Uses. By <span class="smcap">J. Smith</span>. 8vo. 14s.</p>
+
+<p class="hang"><b>A Text-Book of Botany.</b> By Dr. <span class="smcap">E. Strasburger</span>, Dr. <span class="smcap">Fritz Noll</span>,
+Dr. <span class="smcap">H. Schenck</span>, Dr. <span class="smcap">A. F. W. Schimper</span>. Translated from the
+German by <span class="smcap">H. C. Porter</span>, Ph.D., Assistant Instructor of Botany,
+University of Pennsylvania. Revised and Edited by <span class="smcap">A. C.
+Seward</span>. With 594 illustrations, in part coloured. Medium 8vo.
+18s. net.</p>
+
+<p class="hang"><b>The Herb of the Field.</b> By <span class="smcap">Charlotte M. Yonge</span>. New edition,
+revised. Cr. 8vo. 5s.</p>
+
+<div class="publisher">
+<p class="center">MACMILLAN AND CO., LTD., LONDON.</p>
+</div>
+
+
+
+
+<hr class="newchapter" />
+<div class="notebox">
+<h2><a name="TN" id="TN"></a>Transcriber's Notes:</h2>
+
+
+<p>Pages ii, x, xvi, 82, 84, 292, and 310 are blank in the original.</p>
+
+<p>The following corrections have been made to the text:</p>
+
+<div class="tnblock">
+<p>Page vi: be the better for a real knowledge[original has
+knowlege]</p>
+
+<p>Page 55: and[original has and and] are too crudely mechanical</p>
+
+<p>Page 117: Prillieux[original has Prilleux], <i>Maladies des
+Plantes Agricoles</i></p>
+
+<p>Page 128: the intercellular mycelium of <i>Exoascus</i>[original
+has Exoacus]</p>
+
+<p>Page 134: subject to attacks of Uredineae[original has
+Uredinae]</p>
+
+<p>Page 142: carried[original has carrried] from plant to plant</p>
+
+<p>Page 176: its æcidia[original has æcida] on the Barberry</p>
+
+<p>Page 182: <i>e.g.</i> <i>Oniscus</i>[original has Oscinis], the Frit
+Fly, and <i>Cecidomyia</i>[original has Cecidomya]</p>
+
+<p>Page 182: not necessarily less ash constituents[original has
+constitutents]</p>
+
+<p>Page 183: nature of a transmissible enzyme[original has
+enyzme]</p>
+
+<p>Page 203: <i>Krankh. d. Pflanzen</i>, B. I.[original has 1] cap. 2</p>
+
+<p>Page 206: leaves of Apples by <i>Lyonetia</i>[original has
+Lyonettia]</p>
+
+<p>Page 218: <i>Epichloë</i>[original has Epichloe], which clothes the
+sheaths</p>
+
+<p>Page 219: beetle which attacks Crucifers[original has
+Crucificers]</p>
+
+<p>Page 221: on the green parts of Hibiscus,[comma missing in
+original]</p>
+
+<p>Page 221: nodules of the roots of Leguminoseae[original has
+Leguminosae]</p>
+
+<p>Page 230: <i>Edelfäule</i>[original has Edelfaüle], a rotten
+condition of the grapes</p>
+
+<p>Page 235: giving an almost mealy[original has meally]
+appearance</p>
+
+<p>Page 243: as its mycelium[original has myceliun] spreads</p>
+
+<p>Page 258: <i>Prolepsis.</i>[original has Proplesis]&mdash;It frequently</p>
+
+<p>Page 293: Aetiology[original has Ætiology], 89, 100.</p>
+
+<p>Page 293: <i>Anthonomus</i>[original has Anthonomos], 249.</p>
+
+<p>Page 294: Bird's-eye[original has Birds'-eye] Maple, 224.</p>
+
+<p>Page 295: <i>Cercospora</i>,[original has Cereospora] 190.</p>
+
+<p>Page 298: <i>Eau Céleste</i>[original has Celeste], 162.</p>
+
+<p>Page 300: <i>Heterodora</i>[original has Heterodera], 219, 220.</p>
+
+<p>Page 300: Holly, 217.[period missing in original]</p>
+
+<p>Page 300: <i>Hypomyces</i>, 237.[original has comma]</p>
+
+<p>Page 301: <i>Lyonetia</i>[original has Lyonetra], 206.</p>
+
+<p>Page 303: Å’dema[original has Oedema], 228.</p>
+
+<p>Page 303: Permanganate[original has Permangate], 162.</p>
+
+<p>Page 304: Prophylactic[original has Phophylactic] measures,
+160.</p>
+
+<p>Page 304: <i>Phytomyza</i>, 206.[period missing in original]</p>
+
+<p>Page 304: <i>Phyllereum</i>[original has Phyllereus], 253.</p>
+
+<p>Page 304: Pine-apple[original has Pine apple], 258.</p>
+
+<p>Page 305: <i>Puccinia</i>, 88, 114, 169, 175, 176, 188, 189, 247,
+252[original has 252, 247].</p>
+
+<p>Page 307: Somato-plasm[original has Somatoplasm], 267.</p>
+
+<p>Page 307: Spermogonia[original has Spermagonia], 144, 232.</p>
+
+<p>Page 308: <i>Tomicus</i>[original has Tornicus], 205.</p>
+</div>
+
+<p>The following index entries were out of alphabetical order and have been
+moved to the appropriate locations:</p>
+
+<ul class="list">
+  <li>Phylloxera</li>
+  <li>Plants, dying out of</li>
+  <li>Poisonous gases</li>
+  <li>Preventible diseases</li>
+  <li>Prophylactic measures</li>
+  <li>Spermogonia</li>
+</ul>
+</div>
+
+
+
+
+
+
+
+
+<pre>
+
+
+
+
+
+End of the Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
+
+*** END OF THIS PROJECT GUTENBERG EBOOK DISEASE IN PLANTS ***
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+The Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Disease in Plants
+
+Author: H. Marshall Ward
+
+Release Date: February 29, 2012 [EBook #39011]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK DISEASE IN PLANTS ***
+
+
+
+
+Produced by Chris Curnow, Lisa Reigel, and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+
+Transcriber's Notes: Variations in spelling and hyphenation have been
+left as in the original. Words in italics in the original are surrounded
+by _underscores_. Words in bold in the original are surrounded by =equal
+signs=. Ellipses match the original.
+
+A few typographical errors have been corrected. A complete list as well
+as other notes follows the text.
+
+
+
+
+                           DISEASE IN PLANTS
+
+
+
+
+                           DISEASE IN PLANTS
+
+
+                                   BY
+
+                    H. MARSHALL WARD, Sc.D., F.R.S.
+
+  FELLOW OF SIDNEY SUSSEX COLLEGE, HONORARY FELLOW OF CHRIST'S COLLEGE
+         AND PROFESSOR OF BOTANY IN THE UNIVERSITY OF CAMBRIDGE
+
+    PRESIDENT OF THE BRITISH MYCOLOGICAL SOCIETY, AND FELLOW OF THE
+       LINNEAN AND ROYAL HORTICULTURAL SOCIETIES; HONORARY FELLOW
+          OF THE MANCHESTER LITERARY AND PHILOSOPHICAL SOCIETY
+               AND OF THE BOTANICAL SOCIETY OF EDINBURGH
+
+
+                                 London
+
+                       MACMILLAN AND CO., LIMITED
+
+                    NEW YORK: THE MACMILLAN COMPANY
+
+                                  1901
+
+                         _All rights Reserved_
+
+
+
+
+               GLASGOW: PRINTED AT THE UNIVERSITY PRESS
+
+                     BY ROBERT MACLEHOSE AND CO.
+
+
+
+
+PREFACE.
+
+
+It has often been represented to me that the cultivators of plants,
+among whom are to be included planters and foresters, as well as
+agriculturists and gardeners of every kind, are more particularly
+concerned with, and interested in, the maladies themselves of the plants
+they grow, than in the life-history of the fungi, insects or other
+organisms to which they are due, or in the physiological processes which
+are involved; and although it is impossible to really understand any
+disease unless we also understand the processes by which it is brought
+about, there is room for sympathy with the point of view of the
+cultivator. He says, in effect, "I do not want to know all about the
+biology of the fungus of wheat-rust, or of the _phylloxera_, nor do I
+want to learn what experts can tell me about the action of bacteria in
+soil, or the process of starch-formation in the leaves: I have neither
+the time nor the means to master these details. What I want is guidance
+as to what is wrong with my tomatoes, apple trees, chrysanthemums, fir
+trees, turnips, etc., and what I am to do to set things right." Just
+so. With the latter part of this cry one must sympathize, much as a
+doctor does with the wail of the parent who calls him in to cure his
+sick child--we need not stop to classify or compare the motives of the
+parent and the cultivator, and perhaps I had done better to select a
+breeder of sheep with his flock and a veterinary doctor in the
+illustration, but we will let it pass; and as regards the former part of
+the cry, I do not know that the plant-doctor can expect the cultivator
+to be initiated in the aetiology of the disease any more than the
+physician expects the parent to understand the biology of the typhoid
+bacillus. That both the cultivator and the parent would be the better
+for a real knowledge of the disease in either case must be admitted--nay
+insisted on, provided the knowledge _is_ real--but we have to deal with
+facts, and it is a fact that the clients of both doctors are impatient
+of the details of the case.
+
+Now, of course, I am aware that no short cut or "royal road" to science
+exists, and if a man is going to train up trees or other plants, he
+ought to know all about them in health and in sickness, in youth and in
+old age, and he ought to learn everything about the soil they grow in,
+the air that surrounds them, the enemies that beset them, and all the
+multifarious relations of these one to another; but when I look at my
+boy and reflect how much his nurse, his schoolmaster, his tutor, his
+doctor, and his parents _ought_ to know successively and simultaneously
+about him in sickness and in health, and about his surroundings, etc., I
+begin to wonder whether there is not after all something to be said for
+the cultivator's point of view.
+
+Moreover, the cultivator knows a good deal about his plants which I do
+not know, and although I should much like to know it, his plea of want
+of time rings in my ears and the conviction strikes home that one ought
+to try and meet his views, and tell him something about disease as
+manifested in plants without insisting on his becoming a professional
+mycologist, entomologist, agricultural chemist, and philosopher.
+
+Of course, beyond a certain point, it is his lookout how much the
+information is worth, and its educational value--a very different
+matter--is sure to suffer from any restrictions imposed on the treatment
+of the subject; but if the theme of disease in plants, treated from a
+general point of view--I was about to write "treated in a popular
+manner," but that is impossible until physiology and mycology are more
+widely taught--enables him to understand better the questions he puts to
+himself, and, still more, if it stimulates him to enquire further into
+the inexhaustible field of science glimpsed at, something may come of
+it.
+
+The purpose of these essays is to treat the subject of disease in plants
+with special reference to the patient itself, and to describe the
+symptoms it exhibits and the course of the malady, with only such
+references to the agents which induce or cause disease as are necessary
+to an intelligent understanding of the subject, and of the kind of
+treatment called for. Consequently I have avoided any unnecessary
+classification or elaborate descriptions of parasitic fungi or insects,
+histological details of the tissues of plants, chemical and physical
+details regarding the soil, and even matters purely physiological as far
+as possible. Several admirable works on these subjects are already
+available, and must be referred to for further details.
+
+It is, however, quite out of the question to avoid technicalities,
+though I have chosen the simpler course wherever it was found feasible,
+and have tried to so employ the examples selected that the student who
+wishes to go further into the matters dealt with may turn to special
+treatises for further information. For one eminently technical section I
+ought perhaps to apologise, but the temptation to try and set forth, in
+concrete form and suitable for the purposes of this book, some account
+of what is known of the most essential and profound factors concerned in
+the difficult question of the nature of life and death, health and
+disease, was great. Probably my apology should go further, and apply to
+what after all must be failure to explore this mystery to the bottom: my
+only excuse must be that it may stimulate others to go further.
+
+It was an afterthought to add, in Part I., the considerations on the
+factors which influence the plant regarded as a living machine, so to
+speak, in order that the student may the better apprehend the point of
+view taken of the bearings of the matters discussed in Part II.
+
+With regard to references, it seemed a better plan to give, in the form
+of notes after each chapter, the titles of the principal books and
+papers on which a student may base a further course of reading, than to
+overweight the pages of what is, after all, merely an introductory
+sketch to a huge subject, with detailed quotations from the numerous
+sources of information made use of. I have freely expressed my own
+opinions, but the sources for others are, I hope, as freely given. It
+will, however, be understood that I have not aimed at a complete
+bibliography, and, particularly, I have only given foreign references
+where it seemed that adequate treatment of the subject could not be
+found in English.
+
+My sincere thanks are due to Mr. F. Darwin, F.R.S., who has kindly
+looked through many of the proofs, and given me the benefit of several
+suggestions: and to my wife for the very material aid she has afforded
+me in the preparation of the index.
+
+                                                H. MARSHALL WARD.
+
+     CAMBRIDGE,
+       _November, 1900_.
+
+
+
+
+CONTENTS.
+
+
+  _PART I.--SOME FACTORS._
+
+
+  CHAPTER I.
+                                                    PAGE
+  THE PLANT AND ITS SURROUNDINGS,                      1
+
+
+  CHAPTER II.
+
+  THE PLANT AND ITS FOOD,                              7
+
+
+  CHAPTER III.
+
+  THE PLANT A LIVING MACHINE,                         15
+
+
+  CHAPTER IV.
+
+  METABOLISM,                                         23
+
+
+  CHAPTER V.
+
+  ROOTS AND ROOT-HAIRS,                               35
+
+
+  CHAPTER VI.
+
+  THE FUNCTIONS OF ROOT-HAIRS,                        45
+
+
+  CHAPTER VII.
+
+  THE BIOLOGY OF SOIL,                                56
+
+
+  CHAPTER VIII.
+
+  HYBRIDISATION AND SELECTION,                        69
+
+
+  _PART II.--DISEASE IN PLANTS._
+
+
+  CHAPTER IX.
+
+  PHYTOPATHOLOGY. DERIVATION AND MEANING,             85
+
+
+  CHAPTER X.
+
+  HEALTH AND DISEASE,                                 91
+
+
+  CHAPTER XI.
+
+  CAUSES OF DISEASE,                                  99
+
+
+  CHAPTER XII.
+
+  CAUSES OF DISEASE. THE LIVING ENVIRONMENT,         108
+
+
+  CHAPTER XIII.
+
+  NATURE OF DISEASE,                                 119
+
+
+  CHAPTER XIV.
+
+  NATURE OF DISEASE (_Continued_),                   130
+
+
+  CHAPTER XV.
+
+  SPREADING OF DISEASE AND EPIDEMICS,                142
+
+
+  CHAPTER XVI.
+
+  THE FACTORS OF AN EPIDEMIC,                        149
+
+
+  CHAPTER XVII.
+
+  REMEDIAL MEASURES,                                 159
+
+
+  CHAPTER XVIII.
+
+  VARIATION AND DISEASE,                             168
+
+
+  CHAPTER XIX.
+
+  SYMPTOMS OF DISEASE,                               179
+
+
+  CHAPTER XX.
+
+  SYMPTOMS OF DISEASE (_Continued_),                 186
+
+
+  CHAPTER XXI.
+
+  ARTIFICIAL WOUNDS,                                 194
+
+
+  CHAPTER XXII.
+
+  NATURAL WOUNDS,                                    204
+
+
+  CHAPTER XXIII.
+
+  EXCRESCENCES,                                      212
+
+
+  CHAPTER XXIV.
+
+  EXCRESCENCES (_Continued_),                        222
+
+
+  CHAPTER XXV.
+
+  EXUDATIONS AND ROTTING,                            227
+
+
+  CHAPTER XXVI.
+
+  NECROTIC DISEASES,                                 240
+
+
+  CHAPTER XXVII.
+
+  MONSTROSITIES AND MALFORMATIONS,                   246
+
+
+  CHAPTER XXVIII.
+
+  PROLIFERATIONS,                                    257
+
+
+  CHAPTER XXIX.
+
+  GRAFTS,                                            262
+
+
+  CHAPTER XXX.
+
+  LIFE AND DEATH,                                    271
+
+
+  INDEX,                                             293
+
+
+
+
+_PART I._
+
+SOME FACTORS.
+
+
+
+
+CHAPTER I.
+
+THE PLANT AND ITS SURROUNDINGS.
+
+     _The plant the central object of study--soil, climate,
+     atmosphere, etc., are factors of its environment. Agricultural
+     chemistry. The plant a machine. Physiology._
+
+
+If I were asked to sum up the most important result of the numerous
+advances made during the past decade in agriculture and forestry, I
+should reply--the clearer and wider recognition of the fact that the
+plant itself is the centre of the subject, and not the soil, climate,
+season, or other factors of its environment. Until comparatively recent
+times it was the habit of farmers, foresters, planters, and gardeners,
+all the world over, to look upon the plant as a mere item or as a
+mysterious if important one in their calculations, and to regard the
+soil as the chief factor in their studies.
+
+Now all is changing, and the world is gradually awakening more and more
+to the recognition of the truth that the soil and the clouds and the
+atmosphere are merely reservoirs of more or less inert materials, from
+which the living plant draws its supplies, and works them up, by means
+of energy focussed from the sun, into new plant substance.
+
+In other words, the more far-seeing pioneers of scientific agriculture
+and forestry, etc., are recognising that agricultural chemistry is not
+the be-all and end-all of agricultural science; but that, in place of
+the study of the chemical analyses of dead soil, water, air, and
+plant-remains, which has so long held sway, largely owing, I think, to
+the influence of Liebig, the student should have his attention more
+concentrated on the living plant itself and on the physiological actions
+which make up its life. He must regard the living plant as a sort of
+working machine--infinitely more complex than any machine made by man,
+but a machine nevertheless--the purpose of which is to store up energy
+from the sun, and so to add to our wealth on this planet, at the expense
+of the extra-terrestrial universe.
+
+It is not, be it noted, that the new study proposes to ignore or abandon
+the old studies: modern physiology owes too much to the physics and
+chemistry on which it is partly based, and to the labours of De
+Saussure, Ingenhousz, Priestley, and others, for that. But it is that
+the new study recognises that the central point, to which all views must
+be focussed, is not the one that it was formerly supposed to be. The
+student is still taught that the chemistry of soils yields valuable
+information, and that lessons of importance are derived from comparisons
+of the analyses of the ashes, etc., of plants; but he is no longer able
+to cherish the hope, however remotely, that such studies solve his most
+important problems.
+
+The scene--or rather the point to which attention is now directed--is
+the living, working, energy-accumulating plant itself, and not the dead
+store of materials in the soil. The reason for the change is not far to
+seek: it is due to the enormous strides made in the study of the
+physiology of plants during the last quarter of a century, and the
+subject abounds in examples illustrating the marvellous advances that
+have been made, and at the same time showing how, in the progress of
+researches, made for their own sake--_i.e._ in pursuit of satisfaction
+for the intense curiosity of the scientific man--all kinds of side
+issues turn up which prove to be of value in practice, and suggestive of
+further thinking.
+
+At the beginning of the nineteenth century--_i.e._ about 1820--the best
+thinkers were giving up the old ideas that the environment supplied
+food, as such, to plants, and had recognised that the plant takes up
+substances from without and rearranges these in its own body.
+
+The next twenty years or so form a very dark interval in plant
+physiology, chiefly owing to the influence of the assumption of a
+special "vital force," an assumption which was not allowed merely to
+serve as a hypothesis put forward to stimulate research and suggest
+better ideas, but which gained a hold over men's powers of reasoning to
+an extent which now appears monstrous and phenomenal.
+
+Many errors crept in during this reign of terror, one of the most fatal
+of which was De Candolle's revival of the idea of "spongioles"; and
+another, equally disastrous in many of its effects, was the conception
+of a sort of vegetable food-extract, humus, existing in the soil in a
+form peculiarly suitable for direct use by plants. It was during this
+period that the confusion between the processes of respiration and
+carbon-dioxide assimilation arose, and exerted its effects for evil into
+our own day.
+
+The now astounding statement that oxygen-respiration in plants did not
+occur, laid the foundation of many subsequent difficulties, and so did
+the positive and authoritative views on the uses of minerals to the
+plant. Liebig, in fact, stood in the invidious position of being a high
+authority on purely chemical questions, who was impelled to give
+opinions on matters which can only be solved by physiological
+experiments: his great service was to clear up mistakes as regards the
+chemistry of soils and of plants--his great mistakes were due to his
+pronouncing on physiological matters; and it may be doubted whether his
+great services to the purely chemical side of subjects connected with
+agricultural matters are the more to be admired, or the disastrous
+influence of his statements on subjects which do not belong to the
+domain of chemistry should be the more deplored. Be that as it may, he
+handed on to succeeding generations some weighty errors as regards
+plant-life, and taught the agriculturist to regard chemical analyses of
+soils and plant ashes with a reverence which obstructed progress for
+some time. As a set-off to this we must place his contributions to the
+destruction of the bugbear vitalism, which was simply preventing
+enquiry, and his services in bringing together and sifting with power
+and originality all that had been then acquired as regards the chemistry
+of the plant, the soil, and the atmosphere.
+
+That Liebig was indispensable in 1840-1850 is one thing; but that his
+influence should extend to the present day is quite another, and his
+inevitable mistakes were almost as powerful for future evil, as his
+clear exposition of the chemistry of his day was productive of immediate
+good.
+
+Boussingault, working at the same time, 1837-1855, but experimentally
+with the living plant, taught us more about these matters than
+any investigator of the time, though it is very probable that the
+stimulus of Liebig's speculations, good and bad, had its effect in
+impelling Boussingault to devote his splendid methods to problems of
+plant-nutrition. Boussingault's contributions to our knowledge of the
+composition of the dead plant cannot be over-estimated; but he did more
+than this, because he so clearly apprehended the necessity for asking
+his questions directly of the living plant, instead of deducing from
+chemical principles what might be supposed to occur in it; and although
+future researches showed that even so careful an investigator solved a
+problem of first importance--viz. the question of the fixation of free
+nitrogen--the wrong way, it will be found that so far as he did go his
+conclusions were sound, and well calculated to inspire the confidence
+with which the world received them. As we are here concerned more
+especially with the botany of agriculture, however, it is unnecessary to
+dwell longer on these matters, or on the similar and even more extensive
+experiments, of world-wide reputation, carried on for so many years, and
+still being carried on under the liberal auspices of Sir John Lawes, at
+Rothamsted. Moreover it may be necessary to return to some of these
+points later on.
+
+
+NOTES TO CHAPTER I.
+
+     The reader will find a further general account of these
+     matters in Sachs' _Lectures on the Physiology of Plants_,
+     especially Lectures I. and XII., Engl. ed., Oxford, 1887. He
+     may then proceed to Pfeffer's _Physiology of Plants_, Engl.
+     ed., 1899, chapter I., and to the account of the history of
+     the subject in Sachs' _History of Botany_, Oxford, 1890,
+     especially pp. 359-375 and 445-524. References to more special
+     literature will be found in Pfeffer.
+
+
+
+
+CHAPTER II.
+
+THE PLANT AND ITS FOOD.
+
+     _The food of plants--"Vital force"--Other errors--Liebig and
+     Boussingault--The botany of agriculture. The synthesis of
+     carbohydrates--The physiology of plant-nutrition. The
+     persistence of misconceptions._
+
+
+The year 1860 may be regarded as a landmark of importance in the history
+of plant physiology, for it was in that year that Sachs discovered that
+the bringing together of water and carbon-dioxide, in the green
+chlorophyll-corpuscles of the plant exposed to sunlight, results in the
+formation of the grains of starch found in these corpuscles.
+
+Previous to this date Dutrochet (1826-37) had introduced the then crude
+idea of osmosis into physiology; vegetable anatomy had improved, and the
+modern conceptions of the living cell, protoplasm, nucleus, etc., were
+slowly looming; sieve-tubes had been discovered, and the proteids and
+starch in various parts of the plant examined; and the suggestion was
+abroad, replacing Liebig's idea that plant acids were the first
+products of carbon-assimilation, that some substance, of a slimy nature,
+was manufactured in the cells of the leaves and thence distributed as
+the formative material from which the plant constructed its parts. Davy
+and Boussingault had even surmised that a carbohydrate might be the
+first-formed product in assimilation.
+
+There can be little doubt that Sachs' classical proof, by direct
+physiological observation and experiment, first brought forward the
+truth of organic synthesis in the plant in a concrete and convincing
+form.
+
+But it did more than that. It laid the foundation of the modern
+physiology of plant-nutrition on ground already prepared by De Saussure
+and the earlier workers; for, in addition to emphasising the truth of
+organic synthesis--a truth which had been gradually impressing itself on
+the world for some years--Sachs' discovery showed clearly the real
+meaning of carbon-assimilation as a process for obtaining combustible
+food, which the plant then proceeds to make use of.
+
+Many points were rapidly cleared up at once, or if not explained were at
+least put into a strong light for further enquiry, and plant-nutrition
+soon ceased to be the mysterious subject for all kinds of wild
+conjectures that it had hitherto been.
+
+The meaning of thin leaves, with numerous stomata and finely ramified or
+divided vascular bundles, became more apparent, as also did the
+significance of the ascending transpiration current; the storage of
+starch-grains in tubers, medullary rays, roots, seeds, etc., obtained
+meanings not understood before; the spread of roots in the soil, and the
+gradually discovered properties of the finer rootlets and of the
+root-hairs, fitted naturally into their places; and, in short, a
+thousand facts, otherwise isolated, became collated into an intelligible
+system, full of suggestions for new work, such as has since gone on and
+is now being pursued with an activity and success never before realised
+in the history of science.
+
+As time went on, while the general truth of Sachs' views was confirmed,
+a number of detailed discoveries were made which seemed to contradict
+them in certain points. It was found that not all leaves form starch,
+for some contain sugar or oil; but Holle and Godlewski proved
+experimentally that this oil may be replaced by starch if the conditions
+of assimilation are slightly modified. More recently Hebert discovered
+that the stalks and leaves of grasses contain a peculiar form of gum,
+which was formerly confounded with starch, a substance not abundant in
+them. Then came Schimper's discovery of starch-forming corpuscles,
+which, if supplied with sugar, are able to form starch-grains in the
+dark, as in tubers, etc., underground; and as subsequent researches have
+proved that the chlorophyll-corpuscles--which are morphologically the
+same as the starch-forming corpuscles and can be replaced by them--are
+also able to form starch-grains from sugar, as proved by the experiments
+of Boehm, Acton, Meyer, Laurent, Bokorny, Saposchnikoff, and others, it
+soon became evident that nothing essential needed altering in Sachs'
+view that starch is the first visible product of carbon-dioxide
+assimilation, only it became clearer that the starch-grains are built up
+by the protoplasm from glucose or some similar body, and represent so
+many packets of reserve materials put by for the present because not
+required for the immediate needs of the cell.
+
+Boussingault showed, about thirty years ago, that assimilation soon
+stops in green leaves if cut off from the plant, not because the leaves
+die, but owing to some "maximum capacity" being attained. Sachs had
+shown that the starch passes down to other parts of the plant in
+solution as glucose.
+
+Neither time nor space will permit me to go into the enormous field of
+research and results opened up by these and similar observations made
+between 1860-70. It must suffice to say that they led to the discovery
+and study of the diastatic and other enzymes in the leaves and other
+green parts of plants, and to a clearer understanding of what was
+already known of them in seeds, and this knowledge reacted at once on
+our insight into the processes of transport of reserve materials and
+constructive materials from one part of the plant to another, matters
+which will be referred to later on.
+
+It remains to explain Boussingault's difficulty as regards the cessation
+of assimilation. Recent researches confirm the view that at least three
+causes are at work to bring about the inhibition of the
+carbon-assimilation: first, the chlorophyll-corpuscles become filled to
+excess with starch, which cannot get away because all the passages are
+full and the products are inhibiting the further action of the enzymes
+which should dissolve the solid granules; secondly, the leaf being
+detached from the plant explains why the soluble products cannot get
+away, for this makes a great difference in the rate of exhaustion of the
+leaf; and, thirdly, the same fact involves that the leaf can obtain no
+further supply of salts of potassium, etc., without which elements the
+processes in question cannot go on.
+
+These and numerous other deeper insights into the process of
+assimilation, obviously strengthen the force of Sachs' discovery; though
+it by no means necessarily follows that starch-grains are always the
+resting form of the products of assimilation, and we now know that such
+is often not the case: we now have much deeper glimpses into the initial
+products of carbon-assimilation than Sachs had in 1860, but this
+enhances rather than detracts from the importance of his splendidly
+worked-out discovery. Put more generally, we may now say that the
+process of carbon-dioxide assimilation in green leaves under the
+influence of light is a process of synthesis--photo-synthesis--resulting
+in the building up of a carbohydrate such as sugar, inulin or starch
+from the elements carbon, hydrogen and oxygen.
+
+But it must not be supposed that the importance of Sachs' discovery, and
+the rapid consequent extensions of our knowledge, did their work
+forthwith in disabusing men's minds of old and erroneous notions. To say
+nothing of numerous smaller misconceptions which still held their ground
+owing to the stupendous ignorance of plant-physiology which prevailed,
+we find incompetent teachers and text-books were still propagating ideas
+worthy of ancient times. The confusion between oxygen-respiration and
+the gas interchanges in carbon-assimilation was by no means eliminated
+even recently, though it can no longer withstand the deliberate
+onslaughts now made on it. That the roots take up food as such from the
+soil, and that that food is directly employed by the plant for its
+nutrition is even yet implied in daily conversation around us; and
+although matters have advanced so far that everyone now knows that the
+substances at the roots must be in solution, ere they can be received
+into the plant, it sometimes leads to astonishing replies, if we press
+the question very far as to how the absorption takes place, in an
+elementary examination of agricultural students. That manures are foods
+to the plant, that sap circulates, that transpiration is of use to keep
+the plant cool, and wood is a "porous body," etc., are only a few of the
+misconceptions still current, in a decade that has found publishers for
+a work advocating that roots are congealed sap, and that the leaves of
+plants absorb the moisture and dust of the air, and so provide the plant
+with food, and for a paper explaining the action of root-hairs as tubes
+with open pores at their tips. But the gravest misapprehensions current
+among us are due to the crude ideas as to what a plant really is: this,
+I take it, is owing to the difficulty of grasping what physiologists
+mean by organised structure, and leads to regarding the living being
+either as a mere aggregation of chemical compounds, built up by the
+ordinary play of chemical forces, as we know them, acting on dead
+matter, or, as in the days before organic chemistry, as a mysterious
+entity endowed with "vital force," and with properties not amenable to
+scientific investigation. The mistaken notions as to the powers of roots
+to "select" those substances which the plant requires, and to reject
+useless ones was merely an expression of this belief.
+
+The rock on which all are liable to come to grief--the chemist or
+physicist who requires all his facts in terms of analyses and
+proportions by weight, and therefore takes too mechanical a view of the
+subject, or the man who is not scientifically trained at all, and
+therefore is more liable to go to the other extreme and regard the plant
+as a mysterious something which grows and has poetical associations and
+traditions--is the great fact of organised structure, and it is the
+recognition of this fact and some of its consequences which has altered
+the whole position of the subject, and brought the study of the plant
+into the domain of physiology. The living plant, its structure and
+organisation, the functions of its mechanism, and its relations to the
+environment, thus forms a subject apart from that which concerns the
+chemical composition of the plant and its environment, and this
+distinction designates, in a word, as it were, the change which has been
+brought about by modern biology.
+
+A point to be emphasised to the utmost where agricultural students are
+concerned is that the essential process of feeding is the same in a
+green plant, a fungus, and an animal; the greatest confusion still
+exists with regard to this matter, owing to misconceptions as to the
+real meaning of the functions of the chlorophyll-corpuscles when
+supplied with carbon-dioxide and water and the energy of the sun's rays.
+The plant does not feed on carbon-dioxide, any more than it feeds on
+oxygen--it feeds on the organic material after it has been constructed,
+and the chlorophyll-function is merely one mode of obtaining supplies of
+such organic substance.
+
+
+NOTES TO CHAPTER II.
+
+     In addition to the references in the last chapter, the student
+     should consult Sachs' _Lectures_, XVII.-XIX., and Pfeffer's
+     _Physiology_, pp. 287-329, for the further development of this
+     subject. An excellent resume, with new facts and points of
+     view, will be found in Dr. Horace Brown's "Address to the
+     Chemical Section," _British Association Reports_, Dover, 1899;
+     and "Chemistry and Physiology of Foliage Leaves" in _Trans.
+     Chem. Soc._, 1893, p. 604. See also Blackman, "Experimental
+     Researches on Vegetable Assimilation and Respiration," _Phil.
+     Trans._, 1895; and Parkin, "Formation, etc., of Carbohydrates
+     in Monocotyledons," _Phil. Trans._, 1899.
+
+
+
+
+CHAPTER III.
+
+THE PLANT A LIVING MACHINE.
+
+     _The plant a machine into which energy and material are
+     taken--Carbon assimilation--Feeding--Accumulation and
+     transformations in the plant. The action of light--The
+     chlorophyll-function._
+
+
+The relations of the plant to the environment can only be understood by
+taking into account the results of modern physiological discoveries.
+These teach us that the living plant is a highly complex machine, the
+details of its organisation and structure being much more numerous and
+much more closely correlated at numerous points, than the parts of any
+other machine known to us.
+
+They also teach us that it is supplied with energy from without, as any
+other machine; and that when so supplied, and properly working, the
+living structure or machinery does work, also as other machines. But
+modern physiology goes further, in that it renders some account of the
+ways by which the external energy is taken into the plant, and there
+applied to do work, or stored up for a time in order that it may be used
+to do work at some future time.
+
+The accumulation of energy thus ensured is associated with corresponding
+changes of material substance, and the principal means for bringing this
+about is recognised in the assimilation of carbon-dioxide--photo-synthesis.
+
+In this process energy enters the chlorophyll-corpuscle in the form of
+the radiant energy of the sun, it is there directed in the mechanism of
+the protoplasm, so as to do work on the molecules of water and
+carbon-dioxide which have also been brought into the machinery; this it
+does, breaking asunder their stable structure into unstable bodies,
+which then re-combine in different ways to form a carbohydrate, such as
+starch, and this starch is temporarily stored as grains, while oxygen
+escapes.
+
+Each starch-grain, therefore, is to be regarded as a packet of matter
+and of potential energy, as it were, capable of yielding up the latter
+at any future time, when put under such circumstances that it must do
+so. Such stores of energy-yielding substance, if I may use the
+much-abused phrase, form the principal food of the plant--or of an
+animal, if it steps in and takes them--and we now see that the process
+of carbon-dioxide assimilation, as it has perhaps unfortunately been
+called, is not the same thing as the process of feeding, for the
+_feeding_--_i.e._ the nutrition proper--of the plant does not begin
+until the _food_ has been thus obtained.
+
+We now see what the real position of the plant is, to its environment,
+whether the latter be living or dead. From our point of view, the plant
+serves as a centre for bringing together the substances obtainable from
+the soil, and those derived from the atmosphere, and so focussing and
+directing the radiant energy of the sun upon these substances, that they
+are broken up, and some of their constituents synthesised, with
+absorption of energy, into a body, such as starch, containing more
+energy than did the original substances taken together or separate. It
+matters little whether the actual carbohydrate thus synthesised is
+starch, or sugar or inulin: the point is that energy has been gained
+from outside and bound up with the acquired material for further use.
+But modern physiology has carried matters much further than this, and
+especially in the three following directions.
+
+In the first place, it has shown that much of the energy thus stored
+from without in the plant is again liberated in the process of oxygen
+respiration, and expended partly as appreciable heat and partly as
+driving force for stimulating the machinery of the living plant to
+further activities.
+
+In the second place, part of it is rearranged with the rearrangement of
+the molecules with which the energy is bound up, as it were, so that
+work of various kinds is done _in_ the machinery of the plant: I refer
+to various metabolic and surface-actions resulting from the peculiar
+mode of presentment of the resulting substances, for instance the
+production of osmotic pressures in the cell.
+
+And, thirdly, part of the synthesised substance is worked up into higher
+bodies, by processes which obviously entail the further doing of work on
+the constituents.
+
+The further pursuit of this theme would evidently carry us beyond the
+more immediate subject of this book; but I want to make clear that
+recent researches render it more and more certain that the living plant
+is a complex piece of co-ordinated machinery which brings together
+matter and energy from the external universe, and then gets work out of
+these.
+
+This proposition is the more important because the whole question of the
+enrichment of our planet with new food, new building materials, and new
+fuel, to compensate the daily losses, depends on it, and is of course to
+be referred fundamentally to the acquirement of new supplies of energy
+from the sun. Enormous activity has been displayed by physiologists,
+since 1860, in attempting to solve the question, which of the many
+different rays known to proceed from the sun are absorbed by the
+chlorophyll-corpuscle, and directed to the performance of the work above
+referred to.
+
+The names of Draper, Sachs and Pfeffer stand forth prominently as
+pioneers in this; while those of Lommel, Engelmann, Timiriazeff and
+Langley have been among the most active in making important
+contributions to the subject, and in attempting to answer the further
+questions connected with the mode in which the chlorophyll is concerned
+in utilising the energy of the solar radiations. The point is one of
+supreme importance, because it goes on all fours with modern questions
+as to the rays of light absorbed or dispersed in our atmosphere at
+different seasons of the year, or in special climatic conditions, to say
+nothing of its other scientific aspects. Unfortunately, however, we have
+no satisfactory explanation of the actual role played by the chlorophyll
+substance itself, in spite of much industrious work which has been done
+in the subject in this country and elsewhere. As regards the rays
+employed, it was first proved that the most effective belong to the red
+end of the visible spectrum, and that the effect as measured by the
+amounts of oxygen given off, and of starch formed in given periods of
+time, is more or less proportionable to the intensity of the solar
+light. Then it was established that no monochromatic light is so
+powerful as the white light from which it was obtained, though the
+relative numbers expressing the activity in the red and yellow regions
+may stand to those in the blue as something like 12:1. The latest
+results place the maximum assimilation in the red-orange, and this
+coincides with the maximum absorption in the chlorophyll. If we may
+accept the current views as to the distribution of energy in the
+spectrum of solar light, which depends on the complete absorption of all
+the rays by a black body, where they are estimated as heat, we have the
+interesting result that the agricultural or forest plant is adapted to
+catch and retain, broadly speaking, just those particular rays which
+possess most energy.
+
+The probability is increasing that the protoplasmic machinery is the
+really effective mechanism in the process, and we may figure this
+machinery as so holding or presenting the molecules of carbon-dioxide
+and water to the impact of the light-vibrations, that the latter are
+enabled to undo the molecular structure; the atomic combinations thereby
+liberated may then be supposed to form a body like formic-aldehyde,
+which by polymerisation becomes a carbohydrate of the nature of a sugar
+such as glucose, which the protoplasm then builds up into its substance
+and subsequently deposits as starch, and stores temporarily in the form
+of grains or as amorphous material.
+
+This is partly hypothetical, and is largely due to the careful
+deductions of the chemists, but there are very many facts now to hand
+which bear out its probability, especially the recent advances in our
+knowledge of the sugars, and the experimental feeding of leaves and
+plants deprived of starch with such substances as dextrose, levulose,
+maltose, and other sugars, as well as glycerine and other bodies which
+should be convertible into, or yield them, if the theory is true. In
+this last connection, the careful and extensive experiments of Acton, A.
+Meyer, Boehm, and Laurent should be mentioned. It would be interesting
+to enlarge upon Engelmann's beautiful physiological experiments in
+connection with this subject of absorption of solar energy, where the
+maximum accumulation of oxygen-loving bacteria at those parts of a green
+alga which lie in the red-orange of the spectrum, are used as indicators
+of the maximum oxygen evolution (and therefore of the maximum
+carbon-dioxide assimilation), but space will not admit of this. For a
+similar reason I must also pass over the same observer's experiments
+with plants which assimilate in protoplasm behind a red instead of a
+green substance, and which absorb chiefly other rays between the yellow
+and blue, with the remark that they also seem to imply that it is the
+protoplasmic machinery which turns the energy on to the carbon-dioxide
+molecule, the coloured screen being secondary in the matter. Recent
+experiments which show that green plants will not assimilate
+carbon-dioxide in a light which has passed through a solution of
+chlorophyll--and therefore left its red rays behind; nor behind a screen
+of iodine dissolved in carbon-dioxide--which lets no visible rays
+between the red and blue pass--should be noticed, as showing the
+importance of the chlorophyll and the special rays referred to, however;
+and I ought at least to mention Timiriazeff's beautiful proof, published
+in 1890, that if, on the leaf of a plant left in the dark long enough to
+render it free of starch, a bright solar spectrum is steadily projected
+for 3-6 hours, the chlorophyll then removed by alcohol and the
+decolorised leaf placed in iodine, the image of the spectrum is
+reproduced by the different intensities of the starch bands, blue with
+iodine, in the different parts. Here, again, the maximum coloration
+coincides with the maximum absorption in and near the red.
+
+Microscopic observations and photo-chemical experiments alike convince
+us that the chlorophyll-corpuscle is itself a complex piece of
+protoplasmic machinery, working for and with the rest of the plant, and
+there can be little question as to the greater accuracy of our reasoning
+on the whole question I am discussing, since Meyer, Schimper,
+Pringsheim, and others have established the importance of its structural
+peculiarities.
+
+I must now pass on to consider another aspect of the question of
+carbon-assimilation.
+
+
+NOTES TO CHAPTER III.
+
+     In addition to the references in the last chapter, the reader
+     may be referred to Sachs' _Lectures_, XXV., and Pfeffer's
+     _Physiology_, pp. 329-356, where the voluminous literature is
+     given.
+
+
+
+
+CHAPTER IV.
+
+METABOLISM.
+
+     _Quantities of starch formed, and their significance for the
+     plant. The absorption of energy--the conversion of energy in
+     the plant. The plant is a complex machine for concentrating
+     and storing energy and material from without._
+
+
+Sachs measured the increase in dry weight (due to the carbohydrates
+formed in the chlorophyll-corpuscles) per square meter of leaf-surface,
+exposed for a definite period, by drying rapidly at 100 deg. C. equal areas
+of the leaves concerned, and comparing the weights.
+
+Of course the results are not to be pushed too far, in view of the fact
+that some of the starch is continually passing away to be utilised, and
+of the difficulties of comparing the weather, the intensity of light,
+currents of air, hygroscopic conditions of atmosphere, and other
+variable factors which influence the matter. For instance, the stomata
+open and close to different extents according to the conditions of
+light and moisture, and this affects the whole mechanism of
+transpiration especially, and therefore the supplies of water and
+mineral salts. Nevertheless, some interesting and valuable results have
+been obtained in connection with this important subject.
+
+It was found, for instance, that the foliage of a sun-flower or of a
+vegetable-marrow may be forming starch at a rate of considerably over a
+gram per hour in every square meter of leaf-surface exposed on a fine
+day; while in particularly clear and warm sunny weather Sachs obtained
+as much as 24 to 25 grams per square meter per diem.
+
+When one reflects that 200 square meters is not an extravagant estimate
+for the area of leaf-surface exposed on a tree, for a period which even
+in our latitudes may be considerably over 100 days of, say, ten hours'
+light, we need no longer wonder at the rapidity with which wood is
+produced in the stems, and similar estimates (which I have purposely
+kept lower than the estimates for continental and tropical climates) may
+suffice to show how quickly potatoes or the ears of corn, etc., may fill
+up with the starch or other carbohydrates which render them valuable as
+crops. We want more measurements in these connections, moreover, for
+there are several ways in which they are of scientific value and
+practical importance.
+
+It is evident from what has been said that every grain of starch formed
+represents so much energy, packed away for the moment in the
+storehouses of the plant; and we know that--quite apart, however, from
+intermediate transformations of the energy thus stored--this energy
+reappears in the kinetic state eventually, when the starch is burned
+off, in presence of oxygen, and transformed into carbon-dioxide and
+water. It matters not how quickly or how gradually this combustion
+occurs, or whether it is accomplished by burning in a fire, or by slow
+and complex stages in respiration or metabolism: the point is that the
+unit of weight of starch yields so many units of heat when its structure
+tumbles down to the original components, carbon-dioxide and water.
+
+Clearly, if we know how many units of heat are yielded by the combustion
+of one gram of starch, we can obtain an estimate of the amount of
+energy, measured in terms of heat, which the foliage gains and stores
+up--an estimate which will approach the truth in proportion as our
+estimate of the total assimilative activity is correct.
+
+A word of warning is necessary here, however, for those best acquainted
+with physiology recognise that however useful such calculations as the
+above may be, and undoubtedly are, to give a general idea of the fact
+that the energy represented is large, it would be a mistake to suppose
+that such estimates give even an approximate measure of the energy of
+potential which may be got from the carbohydrate, and still less of the
+amount of work that may be got from its employment, according to the way
+it is employed or presented in the plant. To take a single instance
+only. If the carbohydrate is rapidly burned off to carbon-dioxide and
+water, very little is got out of it in the way of work--most, if not
+all, of the energy set free escapes as heat: whereas if the carbohydrate
+is slowly and gradually oxydised, passing through various stages and
+giving rise to powerfully osmotic bodies in the process, or if it is
+built up into protoplasm, or into the structure of a cell-wall,
+relatively enormous quantities of work may be got out of its
+surface-energy, and heat may be absorbed. Whence it follows that we
+cannot measure the power for physiological work of a body by merely
+obtaining its heat of combustion, any more than we can infer its
+significance in metabolism from its chemical properties.
+
+The general conclusion that the plant stores large quantities of energy
+may of course be arrived at by simply estimating the enormous quantities
+of food-material which we obtain annually from agricultural plants.
+
+Modern physiologists have attempted to proceed further than this,
+however, in their essays to form an estimate of the relations between
+the available energy in the solar rays and that used and stored in the
+plant.
+
+If we reflect on such phenomena as the cool shade of a tree, and the
+deep gloom of a forest, and on experiments which show that an ordinary
+leaf certainly lets very little of the radiant energy of the spectrum
+pass through it, it becomes evident that many of the rays which fall on
+the leaf are absorbed in some form, and it becomes very probable that
+much of the solar energy, other than that we term light, is retained in
+the leaf for other purposes than assimilation--or, at least, no other
+conclusion seems possible in view of all the facts. Engelmann's
+researches with purple bacteria are almost conclusive on this point, and
+we may regard it as extremely probable that the plant makes other uses
+of rays, perceived by us as heat-rays, as sources of energy. Researches
+on the influences of temperature on assimilation and other functions
+point to the same conclusion; and Pfeffer and Rodemann definitely state
+that heat is converted into work in the osmotic cells. And the study of
+the absorption bands in the spectrum of the living leaf becomes more
+intelligible in the light of these conclusions. Moreover, the fact that
+a plant still carries on processes of metabolism when active
+transpiration has lowered its temperature below that of the surrounding
+air--and the plant therefore receives heat from the environment--points
+to similar conclusions.
+
+The importance of the conclusion is immense, for even if the plant had
+no other sources of energy than the darker heat rays of the solar
+spectrum, it is clear that it ought to be able to do work.
+
+The above may suffice for the general establishment of the conclusion
+that the plant absorbs more radiant energy than it employs solely for
+assimilation, and emphasises our deduction that it is a machine for
+storing energy.
+
+The question now arises, how is this relatively enormous gain in energy
+employed by the plant? Our answer to the question is not complete, but
+modern discoveries in various directions have supplied clues here and
+there which enable us to sketch in some degree the kinds of changes that
+must go on.
+
+Not the least startling result is that, important as carbon-assimilation
+is as the chief mode of supplying energy, it is not the only means that
+the plant has of obtaining such from the environment, and it is even
+possible--not to say probable--that energy from the external universe
+may be conveyed into the body of the plant in forms quite different from
+those perceptible to our eyes as light.
+
+In the most recent survey of this domain, it is pointed out that we may
+distinguish between radiant energy, as not necessarily or obviously
+connected with ponderable matter, and mechanical energy, which is always
+connected in some way with material substance. All mechanical
+performances in the plants depend on transformation of some form of
+these, evident either as actual energy doing mechanical work, or as
+energy of potential ready to do work.
+
+In so far as molecular movements are concerned, we have the special form
+of chemical energy. The evolution of heat, light and electricity by
+plants are instances of radiant energy, and so on.
+
+Many transformations of energy in the plants are due to non-vital
+processes--_e.g._ transpiration, warping actions, etc., but we cannot
+always draw sharp lines between the various cases. Nor can we directly
+measure the work done in the living machinery; but from the effects of
+pressures and strains, the lifting of heavy weights, driving of
+root-tips into soil, osmotic phenomena, etc., it is certain that the
+values may be very high.
+
+The following classes of processes in living protoplasm and cells may be
+taken as indicators. First we have transformation of chemical energy,
+without which continued life is impossible: in many cases--_e.g._ the
+processes connected with oxygen respiration--these result in the
+development of heat. Secondly, we have those remarkable manifestations
+of energy known as osmotic processes, which depend on surface actions,
+and with which may be associated other surface effects, such as
+imbibition, secretion, etc., and in connection with which heat may be
+evolved or absorbed. It is true the substances which exhibit the
+properties here referred to may be produced, or placed in position, by
+chemical energy, or they may be absorbed by roots, etc.; but the
+proximate energy exhibited by them is not derived from chemical energy,
+and may be out of all proportion to the chemical energy of the substance
+or substances concerned. Moreover it is significant to note that a
+highly oxydised body may develop much osmotic energy, as well as a
+highly combustible one.
+
+It is of the greatest importance to realise the truth that much work can
+be, and is done in the living plant, by conversions of energy of
+potential independent of and out of proportion to the chemical energy
+available by decomposing the substances concerned; even the heat of
+respiration may be superfluous here, for the plant may absorb heat from
+without, and convert it into work.
+
+Tensions often arise in the plant, and do work expressed as
+movements--_e.g._ the springing of elastic Balsam fruits, stamens of
+_Parietaria_, etc.
+
+Osmotic energy not only results in enormous pressures and tensions, but
+causes movements by diffusion and diosmosis, and any given osmotic
+substance which carries this energy with it is not necessarily formed
+always in the same way in the cell--_e.g._ glucose may arise from
+starch, or from carbon-dioxide, or from oil.
+
+Surface-energy is also expressed in the powerful attractions for water
+exhibited in imbibition, swelling, capillarity, absorption, surface
+tensions, etc.
+
+Transpiration induces relatively enormous disturbances of equilibrium,
+and does work in moving water quite independent of chemical energy.
+
+Again, what may be termed excretion-energy, as expressed in the
+separation of a solid body--_e.g._ a crystal--from a solution, may be
+for our purposes regarded separately. Any change in the condition of
+aggregation of a substance in the plant may result in movements and the
+overcoming of resistances.
+
+It will be evident from this short digression--and this is the point I
+wish to emphasise--that in the interval between the securing of a grain
+of starch, representing so much energy won from the external universe,
+and the reconversion of this grain into its equivalent carbon-dioxide
+and water, by respiration, resulting in the loss of the above energy as
+heat, the starch referred to may have undergone numerous transformations
+in the living machinery of the plant, and have played at various times a
+role in connection with the most various evolutions of energy.
+
+If we try to picture a possible case, we may take the following. A given
+starch-granule, after being built up in the chlorophyll-corpuscle, is
+decomposed, and yields part of itself as glucose, which passes down into
+other parts of the plant in solution. Part of it is merely re-converted
+into starch, and temporarily stored: another part passes into the arena
+of oxydation-processes, the sum of which constitute respiration, and may
+serve for a time in the molecules of an organic acid: yet another part
+may be converted into a constituent of the cellulose cell-walls; while
+part may be brought into play in the reconstruction of protoplasm.
+
+In this last connection a discovery made by Schulze about 1878, and
+followed up later by Pfeffer, Palladin, and others is of importance.
+Seedlings growing in the dark, or in an atmosphere devoid of
+carbon-dioxide in the light, become surcharged with nitrogenous bodies
+known as amides, formed during the breaking down of the proteids in the
+destructive process preceding and accompanying respiration: if the
+seedlings are allowed free access to light and carbon-dioxide, however,
+the amides disappear. The explanation is that they are combined with
+some of the materials of the carbohydrates, and again built up into the
+material of the living protoplasm.
+
+Returning to our hypothetical starch-grain--or, rather, its parts--we
+have some of it retained as starch, in excess, simply because it is not
+needed at the moment: another portion gives up its energy in
+respiration, and this does work on the spot, or is lost as heat; or in
+the body of an organic acid, or its salt, the part in question may do
+lifting or pressing work by osmosis, or cause diffusion-currents from
+one cell to another. In the constitution of the cell-wall we may have
+part of our starch-grain aiding in imbibition or in the establishment of
+elastic tensions in turgidity: and, finally, parts may be built up into
+the living protoplasmic machinery of the plant.
+
+What is true for the starch-grain is also true for any particle of salt,
+or water, or gas which enters into the metabolism of the living plant,
+regard being paid to the particular case, and circumstances in each
+case.
+
+Enough has been said to show that the plant cannot be properly studied
+merely as the subject of chemical analysis or of physical investigation;
+you might as well expect to understand a watch by assays of the gold,
+silver, steel and diamonds of which its parts are made up, or to learn
+what can be got out of the proper working of a lace machine by
+analysing the silk put into it, and the fabric which comes out, and by
+taking the specific gravity of its parts and testing the physical
+properties of its wheels and levers.
+
+This is not the same thing as denying the value of such knowledge, in
+the case of either the dead machine or the living plant: it is merely
+emphasising the supreme importance of the study of the structure and
+working of the active machinery in both cases.
+
+Nor is it pertinent to remark on the apparent hopelessness of physiology
+being at present able to explain the seemingly infinite complexity of
+the living machinery of protoplasm and its activities. The modern
+locomotive is also a complex affair in its way, but it is profitable to
+investigate it and to know all one can of its working and possibilities,
+for obvious reasons: a little reflection will convince us that it is
+also worth while to investigate that complex machine, the plant--the
+working organism which alone can really enrich a country. Moreover, we
+ought to be encouraged by the satisfactory progress now being made, and
+the splendid practical results which are accruing, rather than dismayed
+by the prospect of unflagging labour which will be required in the
+future.
+
+Enough has perhaps been said to establish the general truth that the
+plant is a complex machine for storing energy and material from outside,
+and we have seen that modern research has at least gone a long way
+towards determining how the living machine works.
+
+It is hardly necessary to point out that important practical
+consequences may result from these phenomena of the accumulation of
+surplus starch or other carbohydrates in the leaves during the day, and
+of their disappearance during the night into the lower parts of the
+plant. For instance, foliage cut for fodder in the morning is far poorer
+in starch than if cut in the evening, and it would be very instructive
+to have experiments made on a large scale to test the result of feeding
+caterpillars or rabbits, for instance, with mulberry, vine, or other
+leaves in the two conditions.
+
+Again, we now see what complications may arise if a parasitic organism
+gains access to the stores of carbohydrates in process of accumulation,
+or attacks and injures the machinery which is building up such
+materials, etc.
+
+
+NOTES TO CHAPTER IV.
+
+     The student who desires to pursue this subject further should
+     read Sachs' _Lectures_, XX. and XXV., and Pfeffer's
+     _Physiology_, pp. 442-566, but he will hardly arrive at the
+     best that has been done without consulting Pfeffer's "Studien
+     zur Energetik der Pflanzen" in the _Abhandl. der Math.-Phys.
+     Classe der Kgl. Sachss. Gesellsch. der Wiss._ (Leipzig, 1892),
+     p. 151; and Kassowitz, _Allgemeine Biologie_ (Vienna, 1899),
+     Bk. I., pp. 1-127.
+
+
+
+
+CHAPTER V.
+
+ROOTS AND ROOT-HAIRS.
+
+     _Older views as to root-hairs--Root-hairs and their
+     development--Surface--Variations--Conditions for maximum
+     formation--Minute structure--Adhesion to particles of
+     soil--Functions._
+
+
+On the roots of most plants are to be found delicate, silky-looking,
+tubular prolongations of some of the superficial cells, known as
+root-hairs. Malpighi (1687) seems to have been the first to observe
+them, and he took them for capillary tubes. Grew (1682) seems to have
+been responsible for the view that the roots act like sponges in taking
+up water.
+
+Simon (1768) was probably the originator of the idea that these
+root-hairs were excretory tubules, a view that became very popular at
+the beginning of this century.
+
+Meyer (1838) was perhaps the first to give a comparative account of
+them, and he supposed them to be delicate prolongations of the
+root-surface to facilitate the absorption of water.
+
+The real importance of these organs, however, has only become apparent
+since Sachs, in 1859, recognised their relations to the particles of
+soil between which they extend and to which they cling.
+
+In 1883 Schwarz made a very thorough study of their biological
+character, and in 1887 Molisch gave us new facts as to their physiology.
+Our knowledge of them has been rendered very much more intimate by the
+researches of Pfeffer and De Vries on osmotic and plasmolytic phenomena,
+and they serve as an excellent study of some of the best results of
+modern physiology.
+
+In the normal case, such as is exemplified by a seedling wheat or bean,
+the root-hairs arise some distance behind the growing tip of the root,
+an obvious adaptation which prevents their being rubbed off by the soil,
+as they would be if developed on parts still actively lengthening. As
+those behind die off, new ones replace them in front, and so we find a
+wave of succession of functionally active root-hairs some little
+distance behind the tip of the root: the same order of events holds for
+each new rootlet as it emerges from the parent root, and so successive
+borings in the soil, made by the diverging root-tips, are thoroughly
+explored by these root-hairs.
+
+Measurements have shown that in various plants the surface of root on 1
+mm. of length is increased by the root-hairs in proportions given in
+the following table:
+
+  ------------+---------------------+-----------------+--------------
+     PLANT.   |  Area of surface    |    Area of      | No. of times
+              | without root-hairs. | root and hairs. |   greater.
+  ------------+---------------------+-----------------+--------------
+  Maize,      |  4.52 sq. mm.       |  25.13 sq. mm.  |     5.5
+  Pea,        |  4.71 sq. mm.       |  58.33 sq. mm.  |    12.4
+  Scindapsus, | 14.02 sq. mm.       | 261.9  sq. mm.  |    18.7
+  ------------+---------------------+-----------------+--------------
+
+--which sufficiently establishes the general proposition that the area
+of the root-surface is enormously increased by these hairs.
+
+But this does not give us any definite idea of the length of the
+cylinders of soil explored by these surfaces, until we find that plants
+such as an ordinary sunflower, hemp, or vegetable-marrow may have roots
+penetrating into a cubic meter of soil, in all directions, and so
+closely that probably no volume so large as a cubic centimeter is left
+unexplored. Clark found by actual measurement that the roots of a large
+gourd, if put end to end, extended over 25 kilometers, and Nobbe gives
+520 meters for the roots of a wheat. Vetches may go nine feet deep, and
+oats more than three feet. The Sal, a tree of the forests of India, has
+roots which penetrate to a depth of 50 to 60 feet.
+
+Some rough notion of the lengths, superficies and penetrating capacities
+of the roots of a large tree may be gathered from the above, but it is
+doubtful whether we can form any adequate ideas as to the millions of
+root-hairs which must be developed along the course of these
+subterranean boring organs.
+
+One of the most striking results of modern enquiry into these matters,
+is the discovery that the number and superficial area of these
+root-hairs, on one and the same plant, may vary to a large extent
+according to the structure, as it were, of the soil, and the degree of
+moisture it is capable of retaining; or, with the same soil, according
+to the amount of water which it receives and holds. Correlations have
+also been observed between the development in length and surface of the
+rootlets themselves.
+
+The following illustrations will suffice to show this:
+
+Six young wheat-plants in soil kept constantly wet, developed roots the
+total length of which measured 365 mm. each, on the average, and almost
+devoid of root-hairs.
+
+Six similar plants in soil only moderately moist, averaged 668 mm., and
+were well furnished (though not densely covered) with root-hairs.
+
+Six similar plants in soil which would be termed dry, averaged 371 mm.,
+but were densely covered with rich crops of root-hairs.
+
+Further researches have shown that the conditions which rule the
+development of the root-system and root-hairs in the soil are very
+complex, and not always easy to trace. The most general statements we
+can make are the following:
+
+There is an optimum degree of moisture in the soil which promotes the
+maximum development of root-hairs. If the soil is too wet they are not
+developed.
+
+These facts are of importance as correlated with the ease or difficulty
+experienced by the roots in obtaining water, and plants such as our
+ordinary agricultural plants show this very distinctly.
+
+Although, as shown in the experiments with wheat, the short roots in dry
+soil were more densely covered with root-hairs than the much longer
+roots in moderately moist soil, subsequent closer investigation shows
+that the total quantity and area of root-hairs is less in the former
+case than in the latter.
+
+The greatest number of root-hairs are developed on roots which are
+growing at their best: too much moisture may prevent the formation of
+root-hairs: too little may induce dense growths of root-hairs locally,
+but the total number is reduced.
+
+Another set of events which exerts influence on the development of
+root-hairs is the composition of the dilute solution--water containing
+dissolved salts--which surrounds them in the soil.
+
+Thus, Schwarz found that when similar oat and wheat plants were grown
+with their roots in solutions of various salts, the results differed as
+follows:
+
+Oats in a 15 per cent. solution of calcium chloride developed no
+root-hairs, though they formed in a 5 per cent. solution, and were very
+numerous in a 0.5 per cent. solution, or in water alone. In a 10 per
+cent. nutritive solution the plants developed no root-hairs, though
+they were abundant in a 1 per cent. solution.
+
+Wheat plants with their roots in a 15 per cent. solution of potassium
+nitrate bore no root-hairs, but they were numerous in a 2 per cent.
+solution of the same salt.
+
+These are extreme cases, for, although the roots were not killed, they
+were strongly inhibited in their growth by the more concentrated
+solutions. However, experiments of this kind at least bring vividly
+before us what variations are possible, and suggest that similar events
+on a smaller scale may occur in a soil which yields large quantities of
+soluble substances, _e.g._ when freshly manured. Obviously these facts
+have a practical significance as regards kind of soil, drainage, season
+(_e.g._ drought or wet), etc.
+
+But there are other factors which rule the development of root-hairs,
+and some experiments by Lesage show that the correlations between the
+development of root-hairs and roots are probably much more complex than
+had been suspected; for he finds that if the lateral rootlets of a Bean,
+in a water culture, are suppressed, the main rootlet develops numerous
+and very long hairs to compensate the loss in surface, a matter of
+obvious importance in the discussion of cases where roots have been
+injured in the soil.
+
+Before proceeding further it is necessary to look a little more closely
+into the structure of a single hair.
+
+It is a tubular prolongation of a single cell of the external covering
+of the young root, usually about 1 to 3 mm. in length, and 0.01 to 0.10
+mm. in diameter. In special cases the root-hairs of some water plants
+may reach 5 to 18 mm. in length, but of course I am referring to the
+ordinary land plants of agriculture and forestry. This tubular
+prolongation is closed and rounded off at the distal free end, and opens
+at the proximal end into the cell of which it is a protrusion.
+
+The whole structure is bounded by an extremely delicate and elastic wall
+of cellulose, which Frank says is of special composition, almost too
+thin to measure in many cases, but often somewhere near 0.005 to 0.001
+mm. in thickness. This thin membrane is remarkably permeable by water,
+or dilute solutions, as is shown by the rapidity with which a root-hair
+collapses if exposed to evaporation, or with which dense solutions
+abstract water from it, or with which solutions may be seen to penetrate
+it under the microscope.
+
+Overlying the thin cell-wall proper, on the outside, is a thin
+gelatinous layer, a product of alteration of the outermost lamellae of
+the former.
+
+Closely lining the proper cell-wall on the inside, is an extremely thin
+layer of living protoplasm, and somewhere in this protoplasm is a
+distinct cell-nucleus.
+
+The interior of the tube is filled with cell-sap, and it is the osmotic
+pressure of this cell-sap which keeps the whole living instrument tense
+and rigid, and the thin protoplasmic film close pressed against the
+cellulose cell-wall.
+
+Nothing whatever can pass into the cell-sap, or out from it, without
+traversing both the lining of living protoplasm and the cell-wall.
+
+If we gently pull a living root, of wheat, pea, mustard, etc., from a
+normal soil, we find particles of soil so closely adherent to the
+root-hairs that they cannot all be washed off without tearing the hairs:
+the root-hairs establish relations of contact with these particles, so
+close that they are cemented to the solid surfaces by means of the
+gelatinous layer already referred to. This peculiarity has the following
+consequences. In the first place, the enormous holdfast, ensured by the
+millions of points of adherence, enables the plant to withstand even
+powerful lever actions from above, and provides fixed points against
+which the root-tips can work as they drive deeper into the soil. In the
+second place, the intimate contact of the root-hairs and particles of
+soil, ensures that the films of water held by surface-action on the
+soil-particles and root-hairs shall be in continuity with the water
+saturating the cell-walls of the latter, and therefore with the
+protoplasm and cell-sap in their interior. The importance of this at
+periods when the soil is "dry" will be obvious, when we reflect that no
+soil is ever naturally so dry that surface-films of water are absent
+from the particles.
+
+The fact that the root-hair contains living protoplasm, enables us to
+understand to a certain extent the results of the following
+experiments.
+
+If we have a leafy and healthy plant, with roots, bearing numerous
+root-hairs, properly established in suitably moist soil in the pot, the
+roots cease to absorb water if the temperature of the soil falls below a
+certain minimum, though they recommence to do so if the temperature is
+raised again: this has nothing to do with the temperature of the upper
+parts of the plant, or of the air, and the latter may be so high that
+the plant rapidly droops from loss of water at the leaves, which is not
+being compensated owing to the inactivity of the roots.
+
+Similarly we may have the air so cold, at a time when the soil is warm
+enough to keep the root-hairs actively at work, that the plant becomes
+surcharged with water, which escapes from the leaves like drops of dew.
+The temperatures necessary to cause these disturbances in the action of
+the living root-hairs vary for different plants, and even for different
+varieties of the same species.
+
+Similar arrestation of the functions of the roots may be brought about
+by removing the oxygen from the soil around the root-hairs, and
+replacing it by carbon-dioxide, or the vapour of chloroform. If not kept
+too long in such a condition, the plant recovers rapidly on admitting
+atmospheric oxygen, which is always present in a normal well-drained
+soil both as gas in the capillary interspaces, and dissolved in the
+water on the surfaces of the particles. If the access of oxygen is
+delayed, however, as often happens in rainy seasons and in wet soils,
+the root-hairs are killed, and rot sets in. A good instance of this has
+lately been given by Heinricher in the case of potatoes.
+
+
+NOTES ON CHAPTER V.
+
+     For the further pursuit of this subject the reader should
+     consult Sachs' _Lectures_, II. and XV.; Sorauer, _A Popular
+     Treatise on the Physiology of Plants_, 1895, chapters II. and
+     IV., and Pfeffer's _Physiology_, pp. 149-163. The principal
+     paper on root-hairs referred to in the text is Schwarz, "Die
+     Wurzelhaare der Pflanzen," in _Unters. aus dem bot. Inst. zu
+     Wuerzburg_, I. Heft 2, 1883, p. 140, where a very exhaustive
+     account of these organs will be found.
+
+
+
+
+CHAPTER VI.
+
+THE FUNCTIONS OF ROOT-HAIRS.
+
+     _Excretions from root-hairs--Osmotic phenomena--Turgescence--
+     Plasmolysis--Control of the protoplasm in absorption, etc.
+     Selective absorption._
+
+
+We see then that the root-hairs are the active living instruments in
+absorbing the water (containing small quantities of dissolved
+substances) of the soil.
+
+If the living root-hairs are so numerous and so active, however, a
+natural inference is that they must exert some influence on the
+composition or arrangement of their environment. All the teachings of
+modern physiology go to show that such a living cell as I have sketched
+cannot carry on its life, brief though it be--the root-hairs are active
+for about four or five days--without forming substances of the nature of
+excreta, and we should expect some of these to pass out to the soil.
+
+Sachs showed, in 1860, that roots growing in contact with polished
+marble corrode the surface of the mineral, and Nobbe, in 1876, showed
+that the roots of seedlings reduce potassium permanganate, a fact which
+Molisch confirmed in 1887. The latter observer also proved that living
+root-hairs secrete substances which colour a solution of guaiacum blue,
+oxidise pyrogallic acid and other organic substances, and rendered it
+probable that they excrete some substance which inverts cane-sugar, and
+in some cases even small quantities of a diastatic enzyme.
+
+Molisch also confirmed an old observation, that roots excrete
+carbon-dioxide; and he and Czapek showed that the root-hairs excrete
+acids more permanent in their nature than carbonic acid, and published a
+method for showing this by means of a dilute solution, slightly
+alkaline, of phenolphthalein.
+
+Molisch declared that the substances secreted by root-hairs may even be
+observed, dissolved in drops which ooze from the surfaces of the
+root-hairs.
+
+That these root-excretions, and particularly the acids, may be of
+service in dissolving and rendering more available various constituents
+of the soil is an obvious suggestion, and it is borne out by Sachs'
+discovery of the corrosion of marble, and by Molisch's observation that
+living roots slowly corrode ivory if continuously kept in contact with
+it.
+
+But a deeper insight into the physiology of these organs was only
+possible when the meaning of the phenomena of osmosis had been rendered
+clearer by the researches of Pfeffer and De Vries in 1877.
+
+De Vries showed that the turgescence of the living cell can be
+diminished, and even reduced to nothing, by placing the cell in contact
+with solutions of substances which attract water from the cell-sap: as
+the turgescence diminishes, the cell contracts, owing to the elasticity
+of the cell-wall, which was previously distended; if the abstraction of
+water continues, the living protoplasmic membrane lining the cell-wall
+contracts away from the latter. He then proved that no injury need
+accrue to the cell by this process of plasmolysis, since the turgescence
+can be restored by washing out the salt with a more dilute solution, or
+with pure water; and the cell may go on living and even growing as
+before. These phenomena can only be produced in cells where the
+protoplasmic lining is intact and alive.
+
+Pfeffer showed that the whole matter depends on the properties of the
+living protoplasmic membrane, which, so long as it is alive, has the
+power of governing the entrance or exit of dissolved substances, but is
+as a rule freely permeable for water. If, then, substances with a
+powerful attraction for water are formed in the cell cavity, and of such
+a nature that the protoplasm does not permit their free diffusion to the
+exterior, they attract water, and hold it fast, and so set up the
+condition of hydrostatic pressure known as turgescence, the limit of
+which depends on the attainment of a state of equilibrium between the
+elastic reaction of the cell-wall and the distending power of the
+absorbed water. When this limit is reached, water begins to filter back
+again through the cell-wall. Numerous researches during the last fifteen
+years have shown that the sap of such a living cell as the root-hair is
+charged with substances of various degrees of osmotic power; bodies like
+sugars, amides, vegetable acids and their salts, being formed by the
+metabolic activity of the protoplasm and accumulated there. Moreover, we
+now know that the salts of the vegetable acids in particular are
+effective, and the researches of Warburg and Palladin in 1886 have
+placed it beyond reasonable doubt that these acids are continually being
+developed and destroyed in the living cell during normal growth and
+respiration, and that great variations as to quantity may be brought
+about by alterations in the conditions of the environment--_e.g._
+temperature, oxygen, etc.
+
+If, now, we bring a solution of some salt, such as potassium nitrate,
+which has a powerful attraction for water, on the outside of the living
+root-hair, the question whether the water remains in the cell, or passes
+out of it, merely depends on whether the substances inside or that
+outside have the most powerful attraction on the water in the sap, since
+the protoplasm allows water to pass freely.
+
+But the protoplasmic lining may affect the whole matter in another way;
+for it may allow the dissolved salt, or other substance, in the solution
+outside or inside the cell to pass through it also, or it may take it
+up and fix it, or break it up or otherwise alter it.
+
+More recent researches, and especially those of Pfeffer, have shown that
+these diosmotic properties of the living protoplasm are of the utmost
+importance in the whole matter of absorption of substances from the
+soil.
+
+Let us suppose the following case. A root-hair, in full vigour, is
+allowed to bathe freely in a dilute solution of various substances, such
+as sugar, potassium nitrate, phosphates, sulphates and carbonates of
+iron, soda, lime, magnesium and others known by experiment to be
+harmless to its life.
+
+Now it turns out to be by no means a foregone conclusion that all or any
+of the substances, even though freely soluble in the water, can pass
+through the protoplasm into the interior of the cell. Some may be
+allowed easy access, others may only be permitted to pass in small
+quantities, and others, again, may be absolutely refused access by the
+delicate living filter, so long as it is vigorously alive. Nor, as
+proved by numerous experimental cultures since De Saussure's time, is
+the entrance of a salt, etc., ruled by its indispensability or otherwise
+in the economy of the plant. And it is important to notice that only
+experiment can prove the point and determine which substances are
+absorbed and which refused by the root-hair.
+
+If we now suppose the protoplasm to give rise to powerfully osmotic
+substances which accumulate in the sap-vacuole, but which are not
+permitted free egress through the protoplasm (and the formation of such
+bodies will occur if the protoplasm is actively respiring), the
+conditions for absorption of water, with or without any dissolved salts,
+which the protoplasm allows to traverse it, are set up.
+
+But the above supposed case is realised, as Pfeffer showed in 1886, when
+he found by a series of beautiful experiments that certain aniline dyes
+can accumulate in living root-hairs, and other living cells, whereas
+others cannot pass the living protoplasm. After accumulating for some
+time, the dye may either remain stored there, or may eventually diffuse
+out.
+
+Pfeffer made another discovery, of equal importance, namely, that under
+the influence of dilute organic acids, such as citric acid, the
+permeability of the living protoplasm may be altered, so that it allows
+substances to pass which could not otherwise have traversed it. De Vries
+had also shown that the condition of the protoplasm affects its power of
+retaining the colouring matter in the sap of the Beet: so long as the
+protoplasm is alive, the crimson sap is retained, even when the cell is
+plasmolysed, but immediately it begins to die the colour escapes through
+it. A similar case exists when the chlorophyll-corpuscles retain their
+colour in living cells known to be charged with acids: so long as the
+protoplasm is alive and normally active the green bodies are protected.
+
+These, and numerous other experiments of the same kind, prove that the
+healthy root-hair is a living instrument for taking up dilute solutions
+out of the soil, and holding them in the sap-cavity for a time. If
+killed, by frost for instance, it loses these powers.
+
+The researches of the last ten years have also shown that a time comes
+when the turgid cell, if an isolated one, and if sufficient supplies of
+water are present, is so tightly distended that the surplus water begins
+to diffuse out again under the pressure proper to the hydrostatic
+conditions set up.
+
+Now we arrive at a very critical point.
+
+When the water, or dilute solution of various substances, begins to
+exude under pressure from the living root-hair, what is to prevent its
+escape into the soil? And if it thus diffuses out, where is the object
+of absorption?
+
+The questions are obviously pertinent, and they may seem the more so in
+that the cells adjoining the root-hair on its inner side are also
+turgid, and possess similar properties to those of the root-hairs. To
+establish a condition of things which shall bring about the inward flow
+of the absorbed water, one of the three following cases is conceivable.
+(1) The cells, as we pass radially into the root, have different
+properties on the wall of the two sides; or (2) they are more and more
+greedy of water owing to some process of extraction of their water by
+tissues in the centre of the root; or (3) these successive series of
+cells possess osmotically more powerful contents at periods coincident
+with the escape of the water from the now osmotically weaker root-hairs.
+
+A little reflection will show that where we have a group of such cells
+as the above, all capable of absorbing water and dilute solutions and of
+becoming turgid, movements of the absorbed water must go on until all
+the cells are in equilibrium, as regards their osmotic pressures.
+
+Now the living rootlet is just such a system, the various cells of which
+are in different conditions of osmotic pressure at any given time: some
+of these cells are old, and their protoplasm is allowing sap to filter
+out under pressure: others are in the height of their vigour, and their
+protoplasm extremely impervious to the highly osmotic sap-constituents
+which it itself is forming actively: others are too young to have
+attained their full turgescence: while others again are in stages
+intermediate between the above.
+
+There is another point of importance, however, to explain some
+peculiarities in the absorption of these dilute solutions of salts,
+etc., by the root-hairs from the soil, and by cells lying deeper in the
+plant from these root-hairs.
+
+It is easy to understand that if a root-hair absorbs a given
+substance--say calcium sulphate, for illustration--and hands it over to
+other cells unchanged, a time must be supposed to arrive when, the sap
+of all the cells being equally charged with calcium sulphate, no more
+could be absorbed: the rate of absorption of this particular substance,
+and the quantity absorbed, up to the hypothetical point of equilibrium
+chosen, would then depend simply on the ease with which its molecules
+traversed the living protoplasmic membrane, and the degree of their
+solubility in the sap.
+
+But now suppose the following new factor to come in. Suppose that
+calcium sulphate undergoes decomposition in some one of the internal
+cells of the system of absorbing cells, or that it is even merely
+crystallised out in such a cell, or in any other way removed from
+solution (_e.g._ by deposition in cell-walls). This alters the state of
+affairs considerably. The separation of the molecules from the
+sap-solution is itself a cause for the flow of more of the solution to
+the cell concerned, and such causes of diffusion are very common in the
+plant.
+
+The importance of this principle consists in that it lies at the base of
+the whole question of selective absorption, application of manures, and
+the rotation of crops; and those who are acquainted with the excellent
+analytical results of De Saussure, Boussingault, Wolff, Trinchinetti,
+Goedechen, etc., and the water-culture experiments of Sachs, Nobbe, and
+others, will understand what an illuminating effect on these points was
+produced by the above generalisation, which we owe especially to
+Pfeffer's splendid researches into the nature of osmotic phenomena.
+
+It will now be clear, I hope, why we regard the living root-hairs as
+instruments--as pieces of living machinery--for the active absorption of
+water, with substances dissolved in it, from the soil; and it will also
+be evident, I think, that no one can form a proper conception of this
+matter of absorption, so important in all agricultural questions, unless
+he pays attention to these biological phenomena. It was hopeless to
+expect to understand these matters merely in the light of chemical
+analyses of plants and soils, and one expression of this hopelessness
+was the belief in the power of roots to select only the substances
+useful to it. We now know that the expression "selective power of roots"
+has a totally different meaning from that implied in the minds of the
+last generation of agriculturalists, and it would be easy to devise
+experiments, with solutions of different strength, where the plant
+should be made to take up relatively large quantities of harmless, but
+useless minerals, etc., and to starve in the midst of plenty of the
+elements proper to its structure, simply because the former are offered
+in a form in which they easily traverse the protoplasm of the
+root-hairs, while the latter are presented in a form unsuitable for
+absorption. That all these matters are of importance in regard to
+manuring and choice of soils, etc., needs no emphasising.
+
+These remarks, of course, do not detract from the value of good
+comparative chemical analyses, when viewed in the light of physiological
+knowledge, as I need hardly say; but they do, and emphatically so,
+attack the position that such analyses alone can explain the problems of
+agriculture.
+
+On the other hand, we must not rest satisfied with the suggestions so
+far put forward to account for the processes referred to, since it is
+impossible to overlook the fact that in their present form they merely
+afford proximate explanations, and are too crudely mechanical for
+finality.
+
+
+NOTES ON CHAPTER VI.
+
+     In addition to the works referred to in the last chapter, the
+     student should consult Pfeffer's _Physiology_, pp. 86-149, and
+     pp. 410-441. With reference to water cultures, Sachs'
+     _Lectures_, XVII., may also be consulted. The standard work on
+     ash constituents of plants is Wolff, _Aschen-analysen_, 1871
+     and 1880, an indispensable book of reference in this
+     connection, though there are others, quoted in Pfeffer, where
+     further literature may also be found.
+
+
+
+
+CHAPTER VII.
+
+THE BIOLOGY OF SOIL.
+
+     _Soil not a dead matrix--Organic materials--The living
+     organisms of the soil--Their activities--Their numbers and
+     importance. Abandonment of the notion that chemical analysis
+     can explain the problem._
+
+
+It is customary to regard the soil, between the particles of which the
+root-hairs of plants are distributed, as if it were merely a dead matrix
+of smaller or larger pieces of rock, such as sand, gravel, stones, etc.,
+and organic remains, such as bits of wood, leaves, bones, etc., with
+water and air in their interstices. As matter of fact, however, soil is
+a much more complex body than was suspected until comparatively recent
+times.
+
+It is, of course, beyond the scope of this book to go into the different
+varieties of soils, their structure or arrangement, and the chemical
+nature of their constituent rocks and the debris mingled with the
+latter. For the same reason I must pass over the curious properties of
+soils in relation to the solutions they yield to water in contact, the
+manner in which they retain some of these solutions and allow others to
+pass easily, and the remarkable double decompositions which go on in
+them. Moreover, I must assume as known the chief physical properties of
+ordinary soils with respect to the phenomena of capillarity, absorption
+of heat, action of frost, and so forth.
+
+But all ideas as to the nature of soil based merely on the study of its
+chemistry and physics are misleading, and it is in just the
+establishment of this truth that modern discoveries in Agricultural and
+Forest Botany have played so important a part.
+
+From the facts that organic debris is found chiefly at the surface of
+the earth, and that the smallest particles are held in suspension by the
+water near the surface, it is comprehensible why such organic remains
+abound in the upper parts of the soil, where the rootlets with their
+absorbing root-hairs are also found, because they must have oxygen. The
+rule is, therefore, that an ordinary soil consists of upper strata, rich
+in organic materials and in oxygen, and a subsoil, poorer in these
+substances.
+
+Among these organic materials are countless myriads of living beings,
+especially fungi and bacteria, which require oxygen and organic
+materials for their subsistence, and it depends on the open or close,
+moderately moist or damp, warm or cold nature of the soil, and on some
+obviously connected factors, how far down these aerobic organisms can
+thrive. As we go deeper down they become fewer and fewer, and gradually
+disappear, and (neglecting certain anaerobic bacteria of putrefaction)
+they are rarely found in marked abundance more than a few inches below
+the surface soil.
+
+These aerobic fungi and bacteria are the great agents of continued
+fertility of a soil, and it is they which, living and multiplying in the
+moist and well-aerated warm interstices of a rich open soil, carry out
+the useful destruction of organic matter, breaking it up into mineral
+and gaseous bodies, which are then dissolved in the water bathing the
+root-hairs or escape into the atmosphere. In this work of destruction
+they are aided by the oxygen of the air and the solar heat: their own
+fermentative action is also accompanied by a marked rise of temperature,
+and the carbon-dioxide and other products of their activity all go to
+complicate the chemical changes going on in the soil around the roots.
+
+Duclaux has calculated that _Aspergillus niger_, a common mould fungus,
+can break down organic substances, such as carbohydrates, at such a rate
+that a metre cube of the fungus would decompose more than 3000 kilogr.
+of starch in a year, and this may serve as an example giving some idea
+of the possibilities in soil.
+
+Analyses of waters containing large quantities of organic matter, as
+they enter such open soils as those referred to, compared with the
+drainage water after passing through the upper strata, show that the
+carbonaceous and nitrogenous materials are broken down to more or less
+completely oxidised simpler compounds, and that the following chief
+changes result. The ammonia and some other nitrogenous bodies remain
+behind in the soil, as also do the phosphoric acid and much of the
+potash; whereas large quantities of nitric and nitrous acids, together
+with much sulphuric acid, chlorides, and calcium salts pass away in the
+drainage. These facts are obviously highly important in agriculture.
+
+Experiments on sewage farms have shown also that the upper soil retains
+most of the bacteria of the sewage. Koch found at Osmont, near Berlin,
+that whereas the different sewage waters contained numbers so enormous
+that each cubic centimeter probably held 38,000,000 germs, the different
+drainage waters held only 87,000 per c.cm.; and the whole process of
+water-filtration through sandy soils depends on these well-known facts.
+
+Recent experiments in connection with soil-filtration, however, bring
+out the further facts that the oxidations which organic matters undergo
+in the soil--and without which they are useless to the higher
+plants--are enormously enfeebled if the upper layers of soil are
+sterilised, so as to deprive them of the myriads of aerobic bacteria,
+fungi and yeasts which they normally contain, and there can no longer be
+any doubt as to the importance of the biology of the soil in connection
+with the preparation of materials suitable for absorption in solution by
+the root-hairs of agricultural and other plants.
+
+The researches of the last ten years have brought to light a long list
+of forms, comprising yeasts, such as Hansen's _Saccharomyces
+apiculatus_, fungi and bacteria which live and grow in the soil, finding
+their water and food supplies in the interstices, and under conditions
+which we now know to be very diverse. They are usually more numerous, in
+species and individuals, in cultivated farm and garden soils than in
+woods, prairies, and untilled lands; but the geological nature of the
+strata, the closeness and otherwise of the soil, its damp or dry
+character and its average temperature (which depends on many things
+besides latitude or altitude) and other factors co-operate to rule their
+distribution and numbers. The fact that cultivated land is so well
+supplied with manures, air, etc., is of great importance in relation to
+their relative abundance there, and it is extremely probable that the
+use of artificial manures lessens their numbers considerably as compared
+with land on which stable and other animal manures are employed.
+
+A list of the soil-bacteria which have been isolated and more or less
+carefully cultivated and examined would comprise about fifty species;
+but it is certain that, as at present classified and named, many more
+species are to be discovered in any ordinary soil.
+
+The fungi are apparently even more numerous than the bacteria, and we
+may rest satisfied for the present with the general statement that the
+life-actions of the myriads of individuals of these organisms in the
+soil completely alter the question of soil-water as understood by the
+last generation of agriculturalists.
+
+But there is another aspect of this question of soil-organisms which has
+grown in importance of late to such an extent that we are more than ever
+justified in regarding the biology of soil as far more vital to the
+interests of the plant than its physical or chemical properties. With
+many of the fungi in the soil the roots of plants have to compete--just
+as plant competes with plant--for water, salts, and other
+food-materials. The toadstools which are so conspicuous in fields and
+forests spring from mycelia which ramify in the ground, and are busily
+breaking down the remains of other organisms, and just such fungi are
+known to store up relatively large quantities of salts of potassium and
+phosphorus--the very salts which are so valuable to crops and occur so
+sparingly in most soils, but which the extensively spread fungus mycelia
+can gradually accumulate. Some of these fungi, moreover, are more active
+in their antagonism, and actually attack and pierce the roots as
+destructive parasites, but I pass these by for the present, as they form
+the subject for further consideration when we come to the diseases of
+plants.
+
+It is obvious that the competition of fungi with root-hairs for mineral
+salts, oxygen, etc., may be at times acute, and it is extremely probable
+that cases of so-called sterility of soil, where a particular soil is
+found unsuitable for a crop, may sometimes be due to this
+over-competition.
+
+The researches of recent years, however, and especially those of Frank,
+Winogradsky, Hellriegel, and Stahl, have brought to light a series of
+relationships between certain of these soil-organisms and the higher
+plants which place the matter of soil-biology in quite new lights.
+
+On the one hand it has been discovered that groups of bacteria are the
+active agents in bringing about the destruction of organic nitrogenous
+matter with the formation of ammonia, in oxidising this ammonia to
+nitrous and to nitric acids, which combine with bases in the soil to
+form the corresponding salts; while, on the other hand, other forms can
+decompose the nitrates and reduce them to nitrites, or set free ammonia
+or even nitrogen from them. Moreover, there are certain species which
+can fix the free nitrogen of the atmosphere, and start the cycle of
+up-building of this inert element into the complex higher compounds we
+term organic. It is impossible to over-estimate the importance of these
+processes of nitrification and denitrification going on in the soil
+about the root-hairs of the higher plants.
+
+But, in addition to this circulation of nitrogen in the soil, it
+turns out that the life-actions of bacteria, and not mere chemical
+decompositions, are largely responsible for the circulation of
+carbon, of iron, of sulphur and other elements formed from the
+decomposition--also by bacterial and fungal agency--of animal and
+vegetable remains in the soil.
+
+Even more startling are the biological relations in the soil between
+the absorbing roots of the higher plants and some of these bacteria and
+fungi, for it has now been established beyond all doubt that certain
+fungi enter the living roots and there flourish not as mere destructive
+parasites, but as messmates not only tolerated by the plant, but even
+indispensable to its welfare. It is probable that nearly half the plants
+of our fields, moors, and forests entertain such fungi in their
+root-tissues. The curious, and long-known nodules on the roots of
+leguminous plants--peas, beans, clover, etc.--are filled with bacteria
+which enable these plants to avail themselves of the free nitrogen of
+the air, and so enrich the soil with nitrogenous substances.
+
+The roots of most forest trees, orchids, and plants of the moorlands,
+meadows and marshes are similarly occupied by fungi, which in some way
+convey salts--probably especially phosphates and potassium compounds--to
+the plant in return for the small tax of organic carbon-compounds it
+exacts from the latter. In some cases at any rate, as Bernard has lately
+shown, the very existence of the plant depends on its seedling roots
+obtaining this advantageous attachment and co-operation (symbiosis) of
+the fungus immediately on germination.
+
+These remarks must suffice to illustrate this part of my subject, and to
+emphasise the statement that the question whether a given plant can be
+grown in a given soil, is by no means one of simply the physical and
+chemical constitution of the latter. The plant will have to run the
+gauntlet of a long series of vicissitudes brought about by the presence
+or absence, relative proportions and vigour, and specific nature of the
+organisms in the soil at its roots, and it is easy to see that many
+cases of disease may be due to the absence of advantageous bacteria or
+fungi, or to circumstances which disfavour their life, as well as to the
+predominance of competing organisms.
+
+It will now be evident that the old points of view must be abandoned,
+and with them, especially, the widely prevalent notion that chemical
+analyses of the plant and soil can explain the real problems of
+agriculture.
+
+It was of course an enormous advance in the science when, thanks to the
+splendid labours of the chemists, at the end of the last century and the
+beginning of this, we obtained that preliminary knowledge of the
+constitution of the air, and of the composition of the water, acids and
+salts, etc., which plants require for their food-materials and
+life-processes. Much was gained by De Saussure's establishment of the
+fact of oxygen respiration, though we now understand by the term
+something very different from, and much more complex than, what he
+understood by it, as, also, much had been gained by the previously
+acquired knowledge of the gas-exchanges in carbon-assimilation: nor must
+we forget the services of those who proved, by laborious analyses,
+continued for long periods, what chemical compounds are found in the
+tissues of plants, and in the soils at their roots and the atmosphere
+which surrounded them. We must also remember many other contributions
+which have been furnished, and are still being furnished by the chemist;
+and I for one hope that his labours will continue to go hand in hand
+with those of the physiologist.
+
+But, when all due honour is paid to the scientific chemist, it must
+still be allowed that his problems are different from the real problems
+of agriculture. To take one set of instances alone. The chemist can
+analyse a given soil or a given manure, and can even go a long way
+towards making them, but his analyses do not tell us what conditions are
+necessary in order that their ingredients may be presented to the roots
+so as to be absorbed and become built up into the plant. Chemistry told
+us that carbon was fixed from the air, but physiological experiments
+determined how this meant the synthesis of certain definite
+carbohydrates--this, too, in the face of the powerful authority of the
+chemist Liebig, who supposed that the vegetable acids were the results
+of the assimilation of carbon. Wolff, De Saussure, and other chemists
+have done yeoman service in showing that different plants, growing in
+the same soil, contain different proportions of mineral substances; but
+it was by means of water-cultures, and other physiological researches,
+such as those of Pfeffer on osmotic phenomena and of Schwarz and Molisch
+on root-hairs, that the puzzling question of selective absorption, by
+means of the living root-hairs, came into the arena of our knowledge.
+
+In every case--and, as already said, I am not undervaluing the work
+done--the chemist has left us only on the threshold of the real
+problem. He has stood outside the factory in which the real work we want
+to know about is being carried on, and has told us of so many tons of
+this material being carried in at the gates, and of so many tons of that
+coming out; he has even burnt down the factory, and all its contents and
+machinery, and has then told us how many tons of the various materials
+were there at the time; but this is not what we want, valuable as the
+information is, and still more will be. What we want, and what we expect
+to obtain, is more information regarding what is done with the materials
+in the factory: what machinery they are put into, and how they are put
+in: what stages they go through, and how the stages follow one another:
+what wear and tear has to be endured, and how we can step in and stop
+the working of the machine for our own benefit at the best possible
+time.
+
+The physiologist proceeds empirically, by experimenting with the living
+machinery. He recognises the parts and their structure, and tries to
+find out what they are doing: he knows that the laws of physics and
+chemistry cannot be traversed, but he sees these laws at work under
+special and very complex and peculiar conditions. He therefore, as the
+results of his experiments, sets new questions--or old questions under
+new conditions, if you like--and undoubtedly wants the help of both
+chemist and physicist; or, if it is preferred, the chemist and physicist
+may attack the problems, but they must familiarise themselves with the
+peculiar mechanism of the organism concerned, and cannot hope to attain
+success without experimenting with it. I confess it seems to me as
+reasonable to look upon scientific agriculture as a branch chiefly of
+chemistry as it would be to look upon horse-breeding or pigeon-rearing
+from the same point of view; and why the professed chemist's advice is
+regarded as so comforting and final in the one case and not in the other
+is one of those mysteries which seem inherent in human nature.
+
+The central point in agriculture is the plant: get the most out of
+it--the energy-winning machine which alone can keep the animals and
+everything else connected with the farm going--and all the rest follows.
+The old agriculture has taken a gloomy view of things, and especially on
+account of a large variable which it blames for many ills, namely, the
+season or climate. Perhaps the old agriculture has not sufficiently
+recognised that Nature grows plants in accordance with the fact that
+variation is not peculiar to the weather: if the seasons vary, so do
+fruit and other produce and the plants which yield them; and since man
+cannot hope to control the one variable, possibly relief will be found
+in doing more, within his limits, towards controlling others.
+
+In any case he cannot hope to succeed without study of the physiology of
+the plant.
+
+
+NOTES TO CHAPTER VII.
+
+     An admirable short account of soil in its relation to
+     root-hairs is given in Sachs' _Lectures_, XV.; but for a more
+     exhaustive treatment of the subject of soil the reader is
+     referred to King, _The Soil_ (Wisconsin, 1895), or Warrington,
+     _Lectures on the Physical Properties of Soil_ (Oxford, 1900);
+     Larbaletrier, _L'Agriculture_ (Paris, 1888), chapters II. and
+     III. There is also a very good account in Bailey, _The
+     Principles of Agriculture_ (London, 1898), chapters I.-III.
+
+     With reference to the organisms in soils and the
+     decompositions they bring about, the student should consult
+     Kramer, _Die Bakteriologie in ihren Beziehungen zur
+     Landwirthschaft_ (Wien, 1890), and Lafar, _Technical Mycology_
+     (Engl. edition, 1898), sections V., VIII., and IX.
+
+
+
+
+CHAPTER VIII.
+
+HYBRIDISATION AND SELECTION.
+
+     _The crossing of varieties of wheat, etc.--The essentials of
+     fertilisation--Rimpau's experiments--Hybrids and selected
+     varieties._
+
+
+In the more hopeful view of the case which the new agriculture will have
+to take, it will recognise the physiological truth that since the living
+plant is the important and variable machine which constructs the produce
+looked for, and since that machine will work best in proportion as its
+needs are properly satisfied; therefore in cases where the needs of a
+given type of the machine cannot be efficiently provided for, it will be
+well to select some other type which will take what supplies and
+conditions can be offered. Of course, this is already recognised to a
+certain extent, as is implied in the practices of "rotation of crops,"
+selection of "pedigree wheats" and mixtures of "pasture grasses," and in
+decisions as to the quality of land according to the kinds of weeds
+found on it, and so forth; but I am convinced that the agriculturist of
+the future--and the same applies to the horticulturist, planter and
+forester--will have to concern himself more systematically with the
+working and the variability of the plant, and particularly with what
+Darwin termed Variation under Domestication, than has always been the
+custom in the past. The subject of the plasticity of cultivated plants,
+and especially of hybrids, is in one sense an old one; but much work is
+being done which proves, as such work is apt to do, that very much more
+may be done by well-planned experiments on the selection of new
+varieties raised by hybridising and cultivation.
+
+In illustration of this point, a short summary of some of the results of
+crossing different species of wheat, barley, oats, peas, beet, etc., may
+serve to show what has been gained and what may be hoped for in these
+directions. It should be stated that much has been done and is being
+done in this country as well as abroad, as witness English varieties of
+corn, peas, and potatoes, and the recent experiments on crossing various
+kinds of maize in America.
+
+The hybridiser grows his cereals, etc., in pots until ready for
+crossing, and then takes them into the laboratory, removes the weaker
+spikelets, and takes out the young stamens from the flowers left on the
+plant. The female plant is then ready, and the flowers covered with
+paper caps. The pollen, obtained by a clean wet brush from the plant
+chosen as the father, is then carefully placed in position on the
+stigmas, and the caps replaced. The pollination is repeated
+occasionally, and care taken that no uncrossed flowers develop later. In
+this way a few seeds or grains are got to start with.
+
+This would be the place to introduce an account of the enormous advances
+made by the botanists of the last decade or two in the study of the
+microscopic phenomena of fertilisation. Without going into
+details--which would more than occupy all the space at command--I may
+recall the discoveries of Strasburger and his pupils, and of Guignard,
+which have supplemented the earlier discoveries of De Bary, Cohn, and
+Hofmeister, by establishing the facts that the essential point in
+fertilisation is the fusion of two nuclei, and the bringing together in
+the fused mass of two extremely minute thread-like coiled bodies, the
+so-called chromatosomes or filaments, one of which is derived from the
+male and the other from the female parent. The particulars as to the
+marvellous adaptations to secure the union of these two infinitesimally
+minute threads, their behaviour immediately before and after union, and
+many other points must be passed over, as I have only space to emphasise
+the one crowning discovery that these tiny filaments of nuclear
+substance are the material carriers of all the hereditary properties of
+the parents to the young plant which their union initiates.
+
+It must not be supposed that the above statements are based on any
+meagre foundation of facts. The attraction of the fusing nucleated
+masses had been demonstrated over and over again by Tulasne, De Bary,
+Strasburger and others; but Pfeffer brought the matter to a crisis by
+discovering the attractive (chemotactic) substance emitted in given
+cases, and by collecting the fertilising bodies by its means into
+artificial tubes.
+
+The fusion of the nucleated bodies in the sexual act was observed by
+Strasburger in the living plant a few years ago, and numerous later
+observers have confirmed it. Meanwhile all the stages of approach and
+contact of the essential filaments of the nuclear substance have been
+traced, as also all the stages of the transference of half of each
+filament, male and female, into each of the first two cells of the very
+young embryo-plant.
+
+Moreover, the essentials are found to be the same in the animal kingdom
+also, and the bearing of all these discoveries on the phenomena of
+reproduction, variation, and heredity in living organisms has been and
+is of the highest importance, for they support, control, explain and
+correct so many of the splendid results of Knight, Koelreuter, Sprengel,
+Hildebrand and Hermann Mueller, and in every direction throw side-lights
+into the crevices of that magnificent structure, the theory of Natural
+Selection, erected for all time by our countryman, Charles Darwin.
+
+To return now to experiments on crossing. It is found that the first
+products of the crossing appear exactly alike; they may have characters
+intermediate between those of the father and mother, or they may
+resemble one more than the other, but all the seeds of the same cross
+do it in the same way.
+
+On then sowing the seeds of the plants produced from this first cross,
+variations begin to appear. Most of the progeny revert to one or other
+of the parent forms, others show all conceivable combinations of their
+characters, and a few may give rise to entirely new characters. In
+succeeding generations the reversions are preponderant, and, supposing
+no care is taken to prevent it, the whole of the offspring gradually go
+back to the ancestral type.
+
+Some important consequences result, however, if systematic care is
+brought to bear on the matter. This tendency to variation in the second
+generation of crossed plants has often been noted, and it bears out very
+distinctly the conclusions to which Darwin came.
+
+The hybridiser takes advantage of this variation, as others have done,
+to select some forms and rigidly suppress others, in order to obtain
+well-marked varieties of the plants he experiments with. In
+illustration, I may take the following from Rimpau's account of his
+experiments on crossing wheat: By crossing a white English long-eared,
+dense wheat, and celebrated as a heavy cropper, with a red, looser
+German wheat, remarkable for its resistance to winter cold, Rimpau hoped
+to obtain a variety uniting both the above qualities. As regards the
+property of resistance, he failed, and he eventually gave up the
+attempts in face of the advantages offered by the so-called
+_Square-heads_, which then came into the market. His experiments, even
+with the above varieties, are worth noting, however, for they show how
+promising the results of carefully conducted crossing and selection may
+be.
+
+The crossing was done in 1875, in both directions. In 1876 the few
+grains obtained were found to yield plants almost all alike, with the
+long loose ear of the German parent, but the paler colour of the English
+wheat.
+
+In 1877 the plants, obtained by sowing the finest grains, were found to
+consist of pure white, pure red, and of forms which appeared to vary and
+revert in all possible degrees as regards colour, density, and other
+characters intermediate between these.
+
+By carefully separating the closest and densest white wheats from the
+closest and densest red ones, he got in 1878 a large number of each
+coming nearer to the type sown than did the mongrel forms intermingled
+with them: these reversions and intermediate forms were then rigidly
+eliminated, and only the deepest coloured and densest red and white
+forms again sown.
+
+In 1879 these two chosen varieties were constant, so far as concerned
+those selected from the crossing of female English white with male
+German red wheat, and the following year proved the constancy of the red
+variety in the reciprocal cross. In 1886 all four varieties--_i.e._ the
+two reds and the two whites of both the crossings--had become constant.
+
+Still more instructive are the results of the cross between the same
+white English non-bearded wheat and a red German bearded wheat.
+
+The first results of the crossing in 1875 showed the loose ear of the
+German mother, but was paler in colour; while the influence of the
+English father was shown by the absence of beard.
+
+From the reversions and mixtures of the mongrels showing reminiscences
+of the parents in all degrees in 1877, rigid selections and re-sowings
+were made as before, and Rimpau eventually got four very distinct
+varieties, two red and two white, a bearded and a beardless form of
+each, and these were declared fixed and constant in 1879-1882.
+
+Passing over many similar results, and merely noting a very successful
+variety got from a cross between a very early ripening loose red
+American wheat and the dense heavy cropping English Square-head--the
+crossed variety which has proved very suitable for certain light soils
+and dry climates on the Continent, which demand very rapid ripening, and
+are therefore of great physiological and technical interest--I must pass
+on to note the curious result of the successful hybridisation of wheat
+and rye. This cross has been effected several times, and first in this
+country according to reports from Edinburgh (1875), New York (1886), and
+elsewhere, and Rimpau's careful experiments seem to leave no doubt on
+the matter.
+
+First I must remind you that wheat (_Triticum_) differs from rye
+(_Secale_) in several marked characters, such as the breadth and shape
+of the glumes, the number of flowers in the spikelet, etc.; and that the
+cultivated rye differs from cultivated wheats in the characters of the
+straw, in having long ears, and in its flowering glumes remaining widely
+divaricated for some days when in flower.
+
+In 1888 Rimpau removed the young stamens from the German wheat referred
+to, and pollinated the stigmas with pollen from a long-eared rye. Four
+sound grains were obtained, looking like wheat-grains.
+
+The history of one of these grains was as follows: In 1889 it yielded
+ears which were peculiarly narrow and long, and its stalks were also
+much longer than the wheat: the flowers remained exposed, with widely
+open paleae, for several days, and the grains were very peculiar, though
+wheat-like.
+
+Fifteen of the best grains were selected, and in 1890 three of the
+resulting plants proved to be a wheat of the Square-head type and one
+quite sterile. The others retained the elongated, narrow, brownish-red
+ears, the flowering glumes again opening wide for some days. This last
+is a characteristic of rye, but not of wheat.
+
+A long series of natural hybrids of wheat, barley, and oats are also
+described and discussed by Rimpau, as well as artificial crosses--some
+very remarkable--of barleys, but they must be passed over here.
+
+Peas rarely become hybridised naturally. According to Darwin, H. Mueller,
+and Focke, the flowers are little visited by insects in our countries,
+though the mechanism points to their adaptation for pollination by large
+bees.
+
+Rimpau confirms Darwin, H. Mueller, and Ogle as to the self-fertilisation
+of our cultivated peas. Nevertheless, as is well known, marked varieties
+have been obtained by artificial crossing by Gaertner, Knight, Laxton,
+and others, especially in this country.
+
+At the same time experiments show that while it is very easy to obtain
+artificial hybrids of such plants, and there is no fear of natural
+inter-crossing, the forms are remarkably unstable as yet. Similarly
+unsatisfactory results were obtained with beet. As experiments are still
+going on, however, we may expect to hear more about these and other
+results.
+
+It is probable, from recent experiments by De Vries, Correns, and
+others, that a remarkable regularity, expressed by Mendel in the form of
+a law, obtains in the variations which result from hybridising.
+
+In considering these illustrative cases, it is necessary to thoroughly
+apprehend that two procedures are involved. In the first place we have
+the cross-pollination leading to the formation of the hybrid plant by
+cross-fertilisation. But experience shows that this would lead to very
+uncertain results if the plant-breeder did not supplement them by the
+second and extremely important process of rigid selection--_i.e._ by
+choosing the best of the progeny and breeding from them apart from the
+parent-forms, and gradually intensifying, as it were, the variations in
+certain directions which have been started by the crossing.
+
+It is by selection, careful culture, and repeated selection that so much
+has been done in obtaining the innumerable new varieties of roses,
+sweet-peas, orchids, orchard fruits, cereals, grapes, strawberries,
+melons, tomatoes, early potatoes, etc., brought forward by numerous
+breeders of plants in all countries, as will readily be understood if
+reference be made to the work of Hays and Webber in America; Saunders in
+Canada; Garton, Sutton, Veitch, Bateson, and others in this country.
+
+Nor is it necessary that the new materials for selection to work upon
+should be started by hybridisation. Grafting, change of conditions, and
+even variations so vaguely understood that we term them "spontaneous,"
+may supply the starting-points for changes in the characters of plants,
+so remarkable after intensification by breeding that people find it
+difficult to believe they can have come from one stock.
+
+Here, however, I must conclude, merely remarking that the above sketch
+is a mere outline of the subjects modern agriculture and horticulture
+concern themselves with. There are hundreds of problems connected with
+the germination of seeds, on which valuable recent work has been done by
+Klebs, Green, Horace Brown, and others; with the resistance of seeds
+and seedlings to high and low temperatures, a subject opened out by
+Sachs, Kny, De Vries, Krasan, Just, Hoehnel, Dewar, Dyer, and others;
+with the conditions of vegetation which affect the various functions of
+growth, respiration, assimilation, transpiration, and so forth, on which
+I cannot even touch in these pages.
+
+Meanwhile I hope I have succeeded in impressing upon you the grand fact
+that the plant is a living and very complex engine, driven by the
+radiant energy of the sun, and capable of doing work thereby, and this
+just as truly as any heat-engine is driven by chemical energy gained by
+means of the sun's rays, or as a water-mill is driven by power which
+must be referred to the energy of potential in the head of water placed
+in position by the sun's work in evaporation. Fundamentally the whole of
+life and work on our planet is to be referred to the one great source of
+energy which renders possible the establishment of differences of
+potential.
+
+This machine, then, doing work in various ways, adapts itself--or goes
+to the wall--to the conditions of its work among competing organisms or
+opposing circumstances. Curiously enough, while in some cases it suffers
+from the competition, in others it is benefited by its life-actions
+fitting in between those of other organisms, which in their turn
+supplement it. In other words new types of this engine, capable of doing
+the work in various ways, are obtainable; some are good types for the
+conditions afforded, others are bad ones.
+
+Examples of both will occur in the further exposition of the subject.
+
+Man's position in regard to the struggle is that of an intelligent being
+who steps in at certain stages and protects, fosters, and in every way
+favours the agricultural plant--the living machine--and sees that every
+opportunity is given it to do its best work in the best way--from his
+points of view!
+
+
+NOTES TO CHAPTER VIII.
+
+     The foundation of any course of reading on hybridisation
+     and selection should be Darwin's _Effects of Cross and
+     Self-Fertilisation in the Vegetable Kingdom_, which, with his
+     books _On the Origin of Species by means of Natural Selection_
+     and _The Variation of Animals and Plants under Domestication_,
+     will prepare the student for the long course of reading
+     necessary for a full appreciation of what has been done in
+     this department of science.
+
+     From the numerous works which followed these I should select
+     Bailey's _Survival of the Unlike_, London, 1896, and
+     _Evolution of our Native Fruits_, New York, 1898, as
+     especially useful for the reader of this book, to which may
+     also be added _Plant Breeding_, New York, 1896, by the same
+     author, as giving numerous facts and practical directions of
+     value. Further, the "Hybrid Conference Report," _Journ. Roy.
+     Hort. Soc._, 1900, abounds in facts and information. Rimpau,
+     _Landw. Jahrb._, vol. xx., 1891, p. 239. The student who
+     wishes to get towards the root of the matter will hardly be
+     able to dispense with Strasburger's _Neue Untersuchungen ueber
+     die Befruchtungsvorgang bei den Phanerogamen_, Jena, 1884. An
+     interesting summary of recent work on _Xenia_ and "double
+     fertilisation" will be found in _Bull. No. 22, U.S. Dept. of
+     Agric._, 1900. See also _Nature_, Mar. 15, 1900, p. 470.
+
+     If he wishes to explore the vast region of controversial
+     literature that opens up from these points, and which is far
+     beyond the purpose of this book, he may consult the literature
+     collected in Kassowitz' _Allgemeine Biologie_, Wien, 1899, B.
+     II., and the references in the works quoted; also,
+     Strasburger, "The Periodic Reduction of Chromosomes in Living
+     Organisms," _Ann. Bot._, viii., 1894, p. 281. For "Mendel's
+     Law," see Correns in _Ber. d. deutsch. bot. Gesellsch._, vol.
+     xviii., 1900, p. 158.
+
+
+
+
+_PART II._
+
+DISEASE IN PLANTS.
+
+
+
+
+CHAPTER IX.
+
+PHYTOPATHOLOGY. DERIVATION AND MEANING.
+
+     _History. References in the Bible--Greeks and Romans--
+     Shakespeare--Rouen law--Superstitions--Malpighi and Grew--
+     Hales--Unger--Berkeley--De Bary, etc. Physiology and Biology
+     --Diagnosis--Etiology--Therapeutics. Study of causes._
+
+
+Phytopathology, from Greek words which signify to treat of diseases of
+plants, comprises what is known of the symptoms, course, and causes of
+the diseases which threaten the lives of plants, or bring about injuries
+and abnormalities of structure. As a distinct and systematised branch of
+botany it is a modern study, the history of which only dates from about
+1850, though the subject had been treated more or less disjointedly by
+several authors during the preceding century, and isolated records of
+diseased crops, fruit-trees, etc., exist far back in the history of
+Europe. The existence of mildews and blights on cereals indeed was
+observed and recorded by the writers of the older books of the Bible,
+half a dozen references to such blights being found in the Old
+Testament, as well as others to blasted fig trees, etc., in the New
+Testament. Aristotle, about 350 B.C., noticed the epidemic nature of
+wheat-rust. The Greeks and Romans were so well acquainted with such
+diseases that their philosophers speculated very shrewdly as to causes,
+while the people dedicated such pests to special gods. As regards the
+Middle Ages, we know little beyond the fact that blights and mildews
+existed, but Shakespeare's reference in _King Lear_ (Act III., Sc. 4)
+leaves no doubt as to his acquaintance with mildew in the 17th century,
+and other authorities bear out the same. Even the law took cognisance of
+the danger of wheat-rust in 1660 in Rouen (Loverdo). Prior to the 18th
+century, however, only meagre notes on the subject occur scattered here
+and there among other matters, and much superstition existed then and
+later regarding these as other diseases.
+
+Malpighi, in 1679, gave excellent figures of leaves rolled by insects
+and of numerous galls, the true nature of which he practically
+discovered by observing the insect piercing the tissues; previous
+observers--Pliny knew that flies emerge from galls, but thought the
+latter grew spontaneously--having nothing but superstitions and
+conjectures to offer. Grew, in 1682, also gave a capital figure and
+description of a leaf mined by "a small flat insect . . . which neither
+ranging in breadth nor striking deep into the leaf, eats so much only
+as lies just before it, and so runs scudding along betwixt the skin and
+the pulp of the leaf, leaving a whitish streak behind it, where the skin
+is now loose, as the measure of its voyage"--a by no means inadequate
+description of the injury and its cause.
+
+During the eighteenth century several academic treatises or
+dissertations dealing with diseases of plants appeared.
+
+But as a rule we only find disjointed notes. Hales (1727-33) discusses
+the rotting of wounds, canker, and a few other matters, but much had to
+be done with the microscope ere any substantial progress could be made.
+
+With the nineteenth century, and the founding of the modern theories of
+nutrition by Ingenhousz, Priestley, and De Saussure, we find a new era
+started. As the discoveries of the microscopists continued to build up
+our knowledge of the anatomy of plants and began to elucidate the
+biology of the fungi and other cryptogams, while the chemists and
+physiologists laid the foundations of our modern science of plant life,
+it gradually became possible to tabulate and classify plant diseases,
+and discuss their symptoms and causes in a more scientific manner. Even
+in 1833, however, Turpin, and a far better observer, Unger, regarded
+parasitic fungi as due to diseased outgrowths of chlorophyll-corpuscles
+and parenchyma cells, views shared by Meyen (1837) and Schleiden (1846).
+We may pass over the various treatises of Wiegmann (1839), Meyen (1841),
+Raspail (1846), Kuehn (1859), and a number of other works of the period,
+merely referring with emphasis to Berkeley's admirable papers in the
+_Gardener's Chronicle_ (1854) for a summary of what was then known. All
+these works antedate De Bary's _Morphologie und Physiologie der Pilze,
+etc._ (1866), in which he brought together the results of his researches
+during the decade, proving the real nature of parasitic diseases and
+infection as worked out by experiments between 1853 and 1863.
+
+This work put the whole subject of parasitic diseases of plants and
+animals on a new footing, and paved the way for the modern treatment of
+plant pathology as elaborated in the treatises of Frank (1880 and 1895),
+Sorauer (1886), Kirchner (1890), and others, to which the reader is
+referred for further details. I will merely quote the following passage
+from Raspail's _Histoire Naturelle de la Sante et de la Maladie_, 1846
+(vol. ii., p. 176), in illustration of the views entertained by high
+authorities just prior to De Bary's work: "L'insecte qui produit les
+_erineum_, _uredo_, _aecidium_, _xyloma_, _puccinia_, n'est donc plus
+pour nous un insecte inconnu, mais un _acarus_ (grise), un _aphis_
+(puceron) ou un _thrips_, qui produit au printemps une deviation, etc."
+
+And this view, that fungi already well known to mycologists were called
+forth by the punctures of insects, was regarded as not out of harmony
+with the idea that the fungus itself was an abnormal outgrowth of the
+tissues of the host.
+
+The proper study of plant pathology presupposes and involves a knowledge
+of the physiology of plants, of the normal relations of the latter to
+their environment, and of the biology of those animals and plants
+(principally insects and fungi) which are parasitic on them. It is of
+the first importance to understand that a disease is a condition of
+abnormal physiology, and that the boundary lines between health and
+ill-health are vague and difficult to define. As with the study of the
+diseases of man and other animals, so with those of plants, the practice
+resolves itself into the accurate observation and interpretation of
+symptoms (_Diagnosis_) on the one hand, and of causes (_Aetiology_) on
+the other, before any conclusions of value can be drawn as to preventive
+or remedial measures (_Therapeutics_). In plants, however, symptoms of
+disease are apt to exhibit themselves in a very general manner, or at
+any rate it may be that our perceptions of them differentiate symptoms
+due to very different reactions imperfectly, probably because the
+organisation of the plant is less specialised than that of animals. The
+turning yellow and premature falling of leaves, for instance, is a
+frequent symptom of disease; but it may be due to a long series of
+different causes of ill-health--_e.g._ drought, too high or too low a
+temperature, light of insufficient or of excessive intensity, a
+superfluity of water at the roots, the presence in the tissues of
+parasitic fungi, or that of worms or insects at the roots or elsewhere,
+poisonous gases in the air, soil, etc., and so forth. Consequently the
+science of plant pathology is much concerned with the direct action of
+external causes, which are probably less obscure than in the case of
+animals, though by no means always obvious. Such considerations at any
+rate seem to account for the fact that most authorities on plant
+pathology base their classification on the causes of disease, there
+being few noteworthy exceptions.
+
+
+NOTES TO CHAPTER IX.
+
+     The bibliography here quoted will be found in Berkeley,
+     "Vegetable Pathology," _Gardener's Chronicle_, 1854, p. 4;
+     Plowright, _British Uredineae and Ustilagineae_, 1889; Eriksson
+     and Henning, _Die Getreideroste_, Stockholm, 1896; De Bary,
+     _Comparative Morphology and Biology of the Fungi_, etc., 1887;
+     Frank, _Die Krankheiten der Pflanzen_, 1895-96, and scattered
+     in the works referred to in them and in the text.
+
+
+
+
+CHAPTER X.
+
+HEALTH AND DISEASE.
+
+     _Variation--Disease--Comparison to a top. Health--Extinction
+     of species--Natural demise. Examples of complex interactions
+     in health--Interference, and tendencies to ill-health._
+
+
+When we come to enquire into the causes of disease, it appears at first
+an obvious and easy plan to subdivide them into groups of factors which
+interfere with the normal physiology of the plant. Scientific experience
+shows, however, that the easy and the obvious are here, as elsewhere in
+nature, only apparent, for disease, like health, is an extremely complex
+phenomenon, involving many reactions and interactions between the plant
+and its environment. If we agree that a living plant in a state of
+health is not a fixed and unaltering thing, but is ever varying and
+undergoing adaptive changes as its life works out its labyrinthine
+course through the vicissitudes of the also ever-varying environment,
+then we cannot escape the conviction that a diseased plant, so long as
+it lives, is also varying in response to the environment. The principal
+difference between the two cases is, that whereas the normal healthy
+plant varies more or less regularly and rhythmically about a mean, the
+diseased one is tending to vary too suddenly or too far in some
+particular directions from the mean; the healthy plant may, for our
+present purposes, be roughly likened to a properly balanced top spinning
+regularly and well, whereas the diseased one is lurching here, or
+wobbling there, to the great danger of its stability. For we must
+recognise at the outset that disease is but variation in directions
+dangerous to the life of the plant. Health consists in variation also,
+but not in such dangerous grooves. That the passage from health to
+disease is gradual and ill-defined in many cases will readily be seen.
+In fact we cannot completely define disease. Mere abnormality of form,
+colour, size, etc., is not necessarily a sign of disease, in the usual
+sense of the word, otherwise the striking variations of our cultivated
+plants would suggest gloomy thoughts indeed, whereas we have reason to
+believe that many cultivated varieties are more healthy--in the sense of
+resisting dangerous exigencies of the environment--than the stocks they
+came from. Strictly speaking, no two buds on a fruit-tree are alike, and
+the shoots they produce vary in position, exposure, number, and vigour
+of leaves, and so forth. The minute variations here referred to are not
+seen by the ordinary observer, but those who bud, graft and multiply by
+cuttings on a large scale know that such bud-variations are important,
+quite apart from more extensive "sports" which occasionally occur.
+
+On the other hand, we have reason to believe that many species have died
+out gradually as the environment altered. These plants died because they
+did not vary sufficiently, or did not vary in the right directions; they
+became diseased with respect to the then prevailing conditions of normal
+physiology or health.
+
+Disease, therefore, may be said to be variation of functions in
+directions, or to extents, which threaten the life of the plant, the
+normal in all cases being the state of the plant characteristic of the
+species.
+
+Even now, however, we have not obtained a complete definition, because,
+since all plants die sooner or later, we have not excluded the natural
+demise of the individual or its parts, and no one would call the
+autumnal fall of leaves, or the withering of an annual after flowering,
+death from disease. Clearly then the idea of disease implies danger of
+premature death, and probably this is as near as we shall get to a
+satisfactory definition. Since this matter is of primary importance for
+our present theme, I will add the following instances for consideration.
+
+A plant in perfect health and in the fullest exercise of all its
+functions, has its roots in a soil which is suitably warmed and aerated,
+contains the right quantities of water which dissolve just the proper
+proportions of all the essential mineral salts, but nothing poisonous,
+while the soil itself has a texture such that the roots and root-hairs
+can extend and do their utmost in absorbing.
+
+The leaves above are exposed to just the right intensity of light, in
+air which is not too dry, and is of suitable temperature and
+composition, containing no poisonous exhalations, etc.; and as the
+foliage is gently moved by the breeze, it manufactures carbohydrates at
+the optimum rate in the chlorophyll, and the so-called "elaborated sap"
+containing the dissolved organic food-supplies is prepared in the
+tissues in maximum quantities and of just the right degrees of
+concentration and quality for use in the buds, stem, roots, etc., for
+which it is destined as they draw on the supplies.
+
+Between these assimilating organs, the leaves, and the absorbing roots,
+we have in the stem the wood, with its vessels adapted in quantity and
+calibre to convey the water containing dissolved salts from the
+absorbing roots to the leaves (to say nothing of other parts) and,
+separated from this wood by the cambium, we find the sieve-tubes and
+cortical tissues in suitable quantity conveying the "elaborated
+sap"--the solutions of organic food-materials from the leaves down to
+the roots, up to the buds, and elsewhere. Joining these cortical and
+wood tissues are adapted series of medullary rays which, apart from
+other connections, bring about the necessary interchanges of water and
+"elaborated sap" with the cambium, the formative tissue which has to be
+fed and served by them, and which by its growth supplies new vessels and
+sieve-tubes, etc., to carry the continually increasing quantities of
+water and food substances as the roots and leaves increase in number and
+area, and thus enables this ideally correlated system to go on working
+at maximum energy.
+
+Now suppose the same plant with its roots in an unsuitable soil--too dry
+or too poor in mineral supplies, for instance--the transpiring leaves
+above cannot obtain sufficient water and salts to supply their needs,
+but we will suppose hypothetically that they still assimilate under the
+same ideal conditions as before. The supplies now coming to the cambium
+are diminished, since the want of water and minerals compels the leaves
+to put aside any excess of carbohydrates (_e.g._ as stored
+starch-grains), and the plastic materials which do pass to the cambium
+so deficient in water cannot be directly utilised, and a starvation
+period sets in. Consequently the cambium forms less wood, and this will
+contain fewer and smaller vessels, and so reduce the conducting
+passages: fewer sieve-tubes also are constructed, and the paths of the
+water current and food supplies narrowed, which of course reacts on the
+tissues everywhere. The reserve substances may slowly be dissolved and
+distributed, however, and considerable quantities be passed in course of
+time into the roots, which, as opportunity offers, gradually employ them
+in making new roots, and if the disturbance has not gone too far and
+the conditions do not become unfavourable, an increased root-supply may
+by its larger absorbing area gradually establish the former state of
+equilibrium of functions. But this at the expense of the plant, which is
+smaller, has fewer leaves and narrower water channels, etc., than a
+plant not thus checked, and it may take a long time to make up for the
+loss of time and stature thus incurred. Indeed if the plant is an annual
+no recovery at all may occur, the reserves passing into fruit and seeds
+instead of slowly supplying the roots as described.
+
+If it be asked, can such a condition of affairs as that described really
+occur, we have only to think of a transplanted specimen with its roots
+maimed and put into unsuitable soil, or of plants in the open with
+feeding roots gnawed by an insect, etc., or of a tree hitherto in
+equilibrium with its fellows in a plantation suddenly set free by
+thinning and so forth.
+
+Now take the case where the roots are maintaining their maximum
+functional activity, but the leaves--owing to want of light, too much
+moisture or too low a temperature of the air--are functionally
+depressed. Here we get a state of over-saturation with water set up, the
+tissues are turgid to bursting point, what supplies do traverse the
+sieve-tubes, cortex, etc., do so slowly and are excessively diluted, and
+the cambium again forms less wood, but the lumina of the vessels are
+larger and the lignification less complete. Growth in length is
+excessive, but more leaves are formed, though they are apt to be
+abnormally thin and may be small. Little or no reserves are stored
+anywhere, and the watery tissues contain dangerously diffusible
+substances which may render them an easy prey to parasitic fungi. Here
+again, however, if the disturbance of equilibrium has not gone too far,
+and if the season permits, the new leaves may come into full activity
+and the situation be saved by transpiration and assimilation gradually
+increasing and restoring the equilibrium. But, as before, the plant has
+suffered, and shows the effect in its weak shoots, retarded flowering,
+and other ways.
+
+Such plight as is here described may actually be attained in greenhouses
+where over-watering is the fault, and even in the open it is not
+uncommon in rainy summers, or in plantations where dominant trees get
+the upper hand and partially shade more slowly growing species, or in
+fields where rank grass is allowed to overwhelm crops of lower stature.
+
+Now it will be evident that either of these typical cases of temporary
+disturbance of functional equilibrium may be carried too far: in the
+first case the plant may wilt and wither, in the second it may rupture
+and rot, to take these eventualities only. And yet it is difficult to
+call these indispositions diseases: they are rather examples of extreme
+departures from the normal standard of health, just on the borderland
+between health and disease. A step further, as it were, and disease
+supervenes: certain tissues die from want of water, and a necrotic area
+is formed, or the cortex bursts and a wound is formed in another way, or
+some fungus gets a hold, and so on. These abnormal states are
+particularly apt to predispose the plant to disease--insects revel in
+such semi-wilted leaves and shoots crammed with reserves, and fungi in
+the water-logged leaves of the second case, while a cold dry wind is
+peculiarly fatal to such tissues.
+
+
+NOTES TO CHAPTER X.
+
+     The reader may consult Hartig, _Diseases of Trees_, Eng. ed.,
+     1894, Introduction; Sorauer, _Pflanzen Krankheiten_, pp. 1-12,
+     and Frank, _Die Krankheiten der Pflanzen_, B. 1, p. 5, for
+     definitions of disease.
+
+
+
+
+CHAPTER XI.
+
+CAUSES OF DISEASE.
+
+     _A. External causes--I. Non-living environment: soil,
+     atmosphere, temperature--II. Living environment: plants,
+     animals--Complex interactions--Predisposing causes--No one
+     factor works alone--Tangled problems of natural selection
+     involved. B. So-called internal causes._
+
+
+It is customary to classify the causes of disease in plants into two
+principal groups--(1) those due to the action of the non-living
+environment--soil, atmosphere, physical conditions such as temperature,
+light, etc.; and (2) those brought about by the activities of living
+organisms--plants and animals of various species. Before passing to
+further subdivisions under these two heads, however, it is necessary to
+observe that no disease can be efficiently caused by an organism alone,
+since its powers for injury as a parasite, or otherwise, are affected by
+its non-living environment as well as by the host-plant. For instance,
+the spores of a parasitic fungus which would infect and rapidly destroy
+a potato plant in moist warm weather may be showered on to such a plant
+with impunity if the air remains dry and cool--or on to a cabbage under
+any circumstances as far as we know.
+
+Again, probably no one factor of the non-living environment ever
+suffices to induce a disease, possibly because no such thing as only one
+change at a time ever occurs. For instance, it is difficult to say, when
+a soil becomes sodden with water, whether the excess of water and
+dissolved matters, the want of air displaced by the water, the lowering
+of the temperature, or the accumulation of foul products, etc., is the
+principal factor in causing the damage which results, and we have to
+determine by the balance of experimental evidence which is the dominant
+factor in all such cases.
+
+The study of aetiology of disease is in fact only a particular case of
+that of aetiology in general. Plants at high altitudes in the Alps
+acquire very different characteristics from the same species in the
+plains. Is this due to the low temperature, the rarer atmosphere, the
+more intense illumination, the changes in moisture, etc., etc.? The
+question is more difficult than it appears at first sight, and we must
+remember that, complex as are the factors working on the host, they are
+equally complex in their actions on a parasite attacking the host,
+whence the resulting disease becomes indeed a tangled problem of natural
+selection.
+
+Finally it remains to say a few words about a numerous class of cases
+where no external cause of disease can be discovered. It was formerly
+the custom to group such cases of "Internal Causes" by themselves, but
+apart from the fact that many of these mysterious diseases have
+subsequently been shown to be due to the action of external agencies,
+the whole question of internal causes resolves itself into one of
+relations between the plant and its surroundings, and it becomes evident
+that no inherited or internal disease can be regarded as explained until
+we know the external causes which have so modified the structure and
+working of the living cells as to make them abnormal in their reactions
+to other parts of the plant. "Internal causes" of disease, therefore, is
+a phrase expressing our ignorance, but somewhat more emphatically than
+usual. If this is clearly understood there seems no reason against its
+employment for the time being in the artificial scheme of classification
+we require. With regard to external causes due to the non-living
+environment, excess or deficiency of materials in the soil, water, or
+atmosphere plays an important part, and--since we may neglect purely
+aquatic plants--it is customary to speak of diseases due to unsuitable
+soils or to injurious atmospheric influences. For instance, any
+deficiency in the supplies of the necessary mineral salts (compounds of
+calcium, magnesium, potassium with sulphuric, nitric and phosphoric
+acids, etc.) leads to pathological changes, as also does the lack of the
+necessary traces of iron. But it is equally true that the presence of
+such ingredients in excess or in combinations unsuited to the plants
+also leads to disaster, as also does the presence of minerals or other
+compounds which poison the root-hairs--_e.g._ products of decomposition,
+soluble salts of copper and other poisons. That these matters are bound
+up with the whole question of manuring and of proper soil-analyses will
+be evident.
+
+Another essential factor is the nature and quantity of organic materials
+in the soil, whether leaf-mould and decomposing vegetable remains,
+stable manures, or other animal matters, all of which affect different
+species very differently, and produce very different results in
+different soils. It is necessary to apprehend in this connection what
+has been stated above: that soil is not a mere dead structureless
+medium, and that the root-hairs of ordinary plants cannot deal with
+large quantities of putrefying organic matter: that a good soil must
+abound in useful bacteria and fungi to render such substances
+available--and in very various ways--and that it must be open and
+aerated, of proper temperature and suitably supplied with water, and so
+forth, or disaster will result. Here, again, then we are brought into
+close contact with all that is known of fermentation, nitrification, and
+the various biological changes going on in soil, and the application of
+such knowledge to the practice of manuring and tillage in all its forms.
+
+In view of the above remarks, the danger of "over-feeding," in this
+sense, has a real meaning for horticulturists, though it must not be
+forgotten that no substance is really a food until it is assimilable
+into the protoplasm: manures, etc., are food-materials, not food. The
+futility of mere chemical analyses to prove what a plant requires is now
+well known, and it is only on the basis of long and carefully conducted
+experiments that we can ever discover what a particular plant in a
+particular soil, situation, and climate requires for healthy
+development. Again, the quantity of water in soil may be too great or
+too small for given species, and this either on the average for the
+year, or during critical periods only; and it is obviously important
+whether the excess or deficiency is due to improper supplies of water,
+the depth or shallowness of the soil, its retentive powers, or the
+nature of the sub-soil and so on, again bringing the whole matter into
+connection with our understanding of the physical constitution and
+structure of soils, and the nature of soil-drainage.
+
+For instance, a common way of killing ferns is to keep the roots and
+soil wet and the air and fronds dry, whereas the natural habitats
+provide for wet and shaded fronds and well-drained soil.
+
+It may be noted here that in most cases where gardeners speak of plants
+being killed under the "drip" of trees--_e.g._ Beech, the injury is due,
+not to the effects of water but to the shade: the loss of light is so
+great that the shaded plants die of inanition because their leaves are
+not able to provide sufficient carbohydrates.
+
+Closely bound up with this is the question of the gases in soils. Apart
+from the disastrous effects of poisons--_e.g._ coal gas escaping from
+pipes under pavements in towns, etc., diseased conditions often result
+from deficiency of oxygen at the root-hairs, due to imperfect aeration
+of soils, brought about by stagnant water, excess of animal matter, and
+so forth.
+
+Unsuitable constitution of the atmosphere is also a fruitful source of
+disease, though its effects are commoner in closed stoves and
+greenhouses than in the open. Nevertheless the continual exhalation of
+sulphurous fumes, chlorine, and other poisonous gases in the
+neighbourhood of manufacturing centres or of large smoky towns,
+volcanoes, etc., play their part in injuring plants; and excessive
+moisture in the form of mist, rain, etc., is also important. All these
+matters bring us at once into the region of physiology, and only an
+intelligent appreciation of what is known about the action of the
+atmosphere on the soil and the plant will save the peasantry of a
+country from a hopeless mysticism but little removed from that of the
+Middle Ages, when blights and other evils were vaguely referred to the
+river-mists, thunder clouds, and easterly winds.
+
+If we summarise the above as the material factors of the environment, we
+may classify another set of external non-living causes of disease as the
+non-material factors. Such are principally the following:
+
+The space at the disposal of plants greatly affects their welfare. The
+crowding of roots in the soil and of foliage in the air, resulting in
+the loss of light to the leaves, involves deficiency of all the
+materials referred to above--minerals, organic materials, gases, and
+water--and no better illustration of the intense struggle for existence
+among these apparently passive and motionless beings, plants, can be
+given than an over-crowded seedbed or plantation. If left to themselves
+such over-stocked areas exhibit to the keen eye of the trained observer
+all the phases of starvation, weakness, wounding, rot, and, so to speak,
+brutal dominance of the stronger over the weaker which it is the object
+of cultivation to prevent. Here, then, we are brought face to face with
+the true significance of thinning and weeding out, pruning, and similar
+processes.
+
+Unsuitable temperature is one of the commonest of all sources of
+disease, for every plant is adapted to certain ranges of temperature,
+and best adapted to a given optimum somewhere between the maximum and
+minimum temperature for each function. Consequently any serious
+departure from the mean may bring about physiological disturbances of
+the nature of disease, and this in very various ways, as exemplified by
+the results of frost, sun-scorching, drought, hail-storms, forest fires,
+and so forth.
+
+As a predisposing factor to disease abnormal temperature effects play a
+great part. Many wound-fungi gain their entrance through frost-cracks,
+bruises due to hailstones, or into tissues chilled below the normal.
+
+No less remarkable are the diseases primarily due to insufficient or
+improper exposure to light, which affects the chlorophyll-apparatus and
+the process of carbon-assimilation and through these the whole
+well-being of the plant. Every plant is adapted to certain ranges of
+light intensity, and most cultivators know how impossible it is to grow
+shade plants in fully exposed situations, and how easily plants which
+live in open sunny situations are "drawn" and killed by shade. It is
+equally important to have the right kind of light, as disastrous
+experiences with greenhouses glazed with glass which cut off certain
+rays of light have taught. Here, again, it is important to notice that
+the optimum intensity or quality of light may differ for different
+functions and organs of the plant, as is shown by many adaptations on
+the part of species growing in natural situations--_e.g._ bud
+protection, orientation of leaves, etc.--and it may be taken as a rule
+that etiolated plants are peculiarly susceptible to other diseases.
+
+As regards other factors of the inorganic environment, disasters which
+come within the scope of our subject may be brought about by many
+agencies, the mechanical effects of snow and hail, wind, avalanches,
+etc., the effects of lightning, and so forth, being a few of them.
+
+
+NOTES TO CHAPTER XI.
+
+     For other detailed classifications of the causes of disease
+     the reader is referred to the works of Sorauer and of Frank
+     referred to in the last chapter. Also Kirchner, _Pflanzen
+     Krankheiten_, Stuttgart, 1890.
+
+     Of more historical importance are the older classifications of
+     Berkeley, _Gardeners' Chronicle_, 1854, and Re, _Gardeners'
+     Chronicle_, 1849-50. These latter are interesting as showing
+     the very different views held by the earlier workers, and
+     comparison of these with the modern views helps to mark the
+     progress of physiology during the half century which has
+     intervened.
+
+
+
+
+CHAPTER XII.
+
+CAUSES OF DISEASE. THE LIVING ENVIRONMENT.
+
+     _Causes due to animals--Vertebrata--Wounds, etc.--Invertebrata
+     --Insects, etc.--Plants as causes of disease--Phanerogams,
+     weeds, etc.--Cryptogams, fungi--Epidemics, etc._
+
+
+Passing now to those causes of disease which are connected with the
+living environment, we may obviously divide them into two groups of
+agents, animals and plants.
+
+Among animals, the various vertebrata, including man, are especially
+responsible for the larger kinds of wounds and wholesale destructive
+processes due to breakage, stripping of leaves and bark, cutting and
+biting, and so forth. Cattle, rabbits, rats and mice, squirrels and
+birds of various kinds stand out prominently as enemies to trees and
+other plants, to which they do immense injury in various ways by their
+horns, teeth, claws, and beaks; and the damage which an ignorant
+gardener or forester can do with his ill-guided footsteps, axe, spade,
+and knife can only be appreciated by one who knows the habits of plants.
+
+It is among the invertebrata, however, especially insects and worms,
+that the most striking agents of disease in plants are to be found, for,
+with the exception of certain rodents--and we may logically include also
+human invasions--vertebrate animals do not often appear in such numbers
+as to bring about the epidemics and scourges only too commonly caused by
+insect pests.
+
+Insects injure plants in very various ways. Some, such as locusts,
+simply devour all before them; others, _e.g._ caterpillars, destroy the
+leaves and bring about all the phenomena of defoliation. Others, again,
+eat the buds--_e.g._ _Grapholitha_; or the roots--_e.g._ wire-worms, and
+so maim the plant that its foliage and assimilation suffer, or its roots
+become too scanty to supply the transpiration current. Many aphides,
+etc., puncture the leaves, suck out the sap, and produce deformations
+and arrest of leaf-surface, as well as actual loss of substance, and
+when numerous such insects induce all the evils of defoliation. Others,
+such as the leaf-miners, tunnel into the leaves, with similar results on
+a smaller scale.
+
+It must be remembered that a single complete defoliation of a herbaceous
+annual, or even of a tuberous plant like the potato, so incapacitates
+the assimilatory machinery of the plant, that no stores can be put aside
+for the seeds, tubers, etc., of another year, or at most so little that
+only feeble plants come up.
+
+In the case of a tree the case is different, and since most large trees
+in full foliage have far more assimilatory surface than is actually
+necessary for immediate needs, a considerable tax can be paid to
+parasites or predatory insects before the stores suffer perceptibly.
+Still, it should be recognised that the injury tells in time, especially
+in seed years.
+
+Many larvae of beetles, moths, etc., bore into the bark and as far as
+the cambium or even into the wood or pith of trees, the local damage
+inducing general injuries in proportion to the number of insects at
+work: moreover, the wounds afford points of entrance for fungi and other
+pests.
+
+Galls and similar excrescences result from the hypertrophy of young
+living tissues pierced by the ovipositors of various insects, and
+irritated by the injected fluid and the presence of the eggs and larvae
+left behind. They may occur on the buds, leaves, stems, or roots, as
+shown by various species of _Cynips_ on oak, _Phylloxera_ on vines,
+etc., in all cases the local damage being relatively small, but the
+general injury to assimilatory, absorptive, and other functions is great
+in proportion to the number of points attacked.
+
+Many grubs--larvae of flies, beetles, etc.--bore into the sheaths or
+internodes of grasses, or the pith of twigs, or into buds, fruits, and
+other organs of plants, and do harm corresponding to the kind and amount
+of tissues injured.
+
+Various species of so-called eelworms--Nematodes--also cause gall-like
+swellings on young roots, or they invade the grains of cereals.
+
+Finally, various slugs and snails cause much injury by devouring young
+leaves and buds and diminishing the assimilatory area.
+
+Plants as agents of disease or injury fall naturally into the two main
+categories of flowering plants (Phanerogams) and Cryptogams, among which
+the fungi are the especially important pests.
+
+Beginning with weeds, we find a large class of injurious agents. Weeds
+damage the plants we value by crowding them out in the struggle for
+existence, as already stated, and when the weed-action is simply due to
+superfluous plants of the same species, we speak of overcrowding. But it
+must not be overlooked that the competition between crowded plants of
+the same species--where every individual is acting as a weed to the
+others--may be more dangerous than between plants and weeds belonging to
+other species and genera, because in the former case they are struggling
+for the same minerals and other necessary food-materials: a matter of
+importance in connection with the rotation of crops.
+
+The question of allowing grass to grow at the foot of fruit trees, as in
+orchards, is a good case in point. Such grass may increase the damp and
+shade, thus favouring fungi at one season, and dry up the moisture of
+the soil to the injury of the fine superficial roots at another, as well
+as exhaust the soil, owing to the competition of the roots for salts
+and other materials. On the other hand, the checking of surface roots by
+competition with the grass has been claimed as advantageous. In this
+connection probably the whole question of the composition of the turf
+arises, as well as that of possible cropping for hay, and manuring.
+
+As regards any particular weed, the cultivator should learn all he can
+respecting its duration, seeding capacity, method of dissemination, the
+depth and spread of its root-system, and any other particulars which
+enable him to judge when and how to attack it. It is only necessary to
+see the victory of such drought-resisting weeds as _Hieracium
+pilosella_, Plantains, _Hypochaeris_, on lawns to realise how weeds may
+win in the struggle for existence with the finer grasses.
+
+Many so-called weeds are, however, partially parasitic, with their roots
+on the roots of others--_e.g._ _Rhinanthus_, _Thesium_, etc., and much
+damage is done to meadow grasses and herbage by the exhaustive tax which
+these semi-parasites impose.
+
+This is carried still further in the case of such root-parasites as
+_Orobanche_, where the host-plant is burdened with the whole support of
+the pest, because the latter, having no chlorophyll, is entirely
+dependent on the former for all its food.
+
+Even ordinary climbing plants may injure others by shading them, either
+by scrambling over their branches--_e.g._ Bramble, or twisting their
+tendrils round the twigs--_e.g._ Bryony, or twining round them--_e.g._
+Woodbine, _Convolvulus_, etc. The principal direct injury is in these
+cases owing to the loss of light suffered by the shaded foliage, but
+the weed-action is often increased by the competition of their
+roots--_e.g._ briars; and in the case of woody climbers the gradually
+increased pressure of the woody-coils round the thickening stems
+compresses the cambium and cortex of the support and induces strictures
+and abnormalities which may be fatal in course of time.
+
+Epiphytes, or plants which support themselves wholly on the trunks,
+branches, or leaves of other plants, also injure the latter more
+especially by shading their foliage--_e.g._ tropical Figs, Orchids,
+Aroids, etc.; and similar damage is done by our own Ivy, the main roots
+of which are in the soil, but the numerous adventitious roots of which
+cling to the bark.
+
+When the climber or epiphyte is also parasitic, as in the case of the
+Dodder, _Loranthus_, Mistletoe, etc., the direct loss of substance
+stolen from the host by the parasite comes in to supplement any effect
+of shading that the latter may bring about if it is a leafy plant.
+
+Of Cryptogams, apart from a few epiphytic ferns, and the intense
+weed-action of certain Equisetums, the rhizomes and roots of which are
+as troublesome as those of twitch and other phanerogamic weeds, it is
+especially the fungi which act as agents of disease, and which, as we
+now know, are _par excellence_ the causes of epidemics.
+
+The action of fungi may be local or general; and restricted, slow and
+insidious, or virulent and rapidly destructive.
+
+Examples of local action are furnished by _Schinzia_, which forms
+gall-like swellings on the roots of rushes; _Gymnosporangium_, which
+induces excrescences on the stems of junipers, and numerous leaf-fungi
+(_Puccinia_, _AEcidium_, _Septoria_, etc.), which cause yellow, brown, or
+black spots on leaves, as well as by _Ustilago_, which attacks the
+anthers or the ovary of various plants, and so forth. In such cases the
+injury done by a few centres of infection is very slight, but prolonged
+action may bring into play secondary effects such as the gradual
+destruction of the cambium round a branch, when, of course, the effect
+of ringing results; or if the fungus becomes epidemic and myriads of
+leaf-spots are formed, the destruction of foliar tissue, gradual taxing
+of the assimilatory cells, etc., may end in rapid defoliation, and
+renewed attacks soon exhaust the plants and lead to sterility and death,
+as often occurs with Uredineae--_e.g._ the coffee leaf-disease.
+
+It is highly probable that such fungi are particularly exacting owing to
+their exhausting demands for compounds of potassium, phosphoric acid,
+and other bodies.
+
+Examples of virulent and rampant general action are afforded by finger
+and toe in turnips, etc., where the roots are invaded by
+_Plasmodiophora_, which induces hypertrophy and rotting of the roots;
+and by the damping off of seedlings, where the fungus _Pythium_ rapidly
+invades all parts of the seedlings and reduces them to a water-logged,
+putrefying mass; or the potato-disease, which is due to the rapid
+spread of _Phytophthora_ in the leaves and throughout the plant, which
+it blackens and rots in a few days.
+
+Many fungi not in themselves very virulent or aggressive do enormous
+harm owing to the secondary effects they induce. Some of the
+tree-killing hymenomycetes, such as _Agaricus melleus_, for instance,
+penetrate the wood of a pine at the collar, and the result of the large
+flow of resin which results is to so block up the water passages that
+the tree dies off above with all the symptoms of drought. Similarly, the
+_Peziza_ causing the larch disease, having obtained access to the stem
+about a foot or so above the ground, will gradually kill the cambium
+further and further round the stem, and so girdle the tree as
+effectually as if we had cut out the new wood all round. In all such
+cases--and the same applies to the leaf-diseases referred to above--the
+fungus may be compared to an army which is not strong enough to invade
+the whole territory, but which, by striking at the lines of
+communication, cuts off the supplies of water, food, etc., and so brings
+the struggle to an end. Indeed we might compare the cases of fungi which
+attack the root and collar, and so strike at and cut off the water
+supply, to a compact army which at once cuts off the enemy from his
+narrow base; whereas the innumerable units which bring about an epidemic
+attack on the leaves, and so surround the enemy and cut off his food
+supplies all round, is rather like a much larger army which cannot get
+in beyond the natural barriers of the tissues, and so puts a _cordon_
+all round the territory and seizes the multitudes of food-stuffs at the
+frontiers. The end result is similar in both cases, but the methods of
+warfare differ.
+
+Many fungi, however, though they make their presence noticeable by
+conspicuous signs, cannot be said to do much damage to the individual
+plant attacked. The extraordinary malformations induced by parasites
+like _Exoascus_, which live in the ends of twigs of trees and stimulate
+the buds to put out dense tufts of shoots, again densely
+branched--Witches' brooms--are a case in point. Also the curious
+distortions of nettle stems swollen and curved by _AEcidium_, of maize
+stems and leaves attacked by _Ustilago_, and of the inflorescences of
+_Capsella_ by _Cystopus_, etc., are not individually very destructive;
+it is the cumulative effects of numerous attacks, or of large epidemics,
+which tell in the end.
+
+Some very curious effects are due to fungi such as _AEcidium elatinum_,
+which, living in the cortex of firs, stimulate buds to put out shoots
+with erect habit, and with leaves which are radially disposed, annually
+cast, and differently shaped from the normal--characters quite foreign
+to the species of fir in its natural condition.
+
+Equally strange are the shoots of _Euphorbia_ infested with the aecidia
+of _Uromyces_, those of bilberries affected with _Calyptospora_, etc. In
+all these cases we must assume a condition of toleration, so to speak,
+on the part of the host, which adapts itself to the altered
+circumstances by marked adaptations in its tissue developments, mode of
+growth and so forth.
+
+This toleration is perhaps most marked in the case of those cereals
+which, though infected by the minute mycelium of _Ustilago_ while still
+a seedling, nevertheless go on growing as apparently healthy green
+plants indistinguishable from the rest, although the fine hyphae of the
+parasite are in the tissues and keeping pace with the growth of the
+shoots just behind the growing points. As the grains of the cereal begin
+to form and swell, however, the hyphae suddenly assume the part of a
+dominant aggressor, consume the endosperm of the enlarging seed, and
+replace the contents of the grain with the well-known black spores known
+as Smut.
+
+
+NOTES TO CHAPTER XII.
+
+     The reader will find a summary of such fungi as are here
+     concerned in Massee, _A Text-Book of Plant Diseases_, 1899, or
+     Prillieux, _Maladies des Plantes Agricoles_.
+
+     For further details the student should consult the works of
+     Frank and Sorauer referred to in the notes to Chapter IX., and
+     Tubeuf, _The Diseases of Plants_, Engl. ed. 1897, pp. 104-539.
+
+     For experiments on the effects of grass on orchard trees, see
+     _Report of the Woburn Experimental Fruit Farm_, 1900, p. 160.
+
+     For the further study of weeds, the interesting bulletins of
+     the Kansas State Agricultural College, 1895-1898, will show
+     the reader what may be done in the matter of classifying them
+     according to their biological peculiarities.
+
+     In regard to insects, the reader will find the following list
+     embraces the subject: Somerville, _Farm and Garden Insects_,
+     1897; Theobald, _Insect Life_, 1896; Ormerod, _Manual of
+     Injurious Insects_, 1890, and _Handbook of Insects Injurious
+     to Orchards, etc._, 1898.
+
+     The admirable series of publications of the U.S. Department of
+     Agriculture under the editorship of Riley and Howard, and
+     entitled _Insect Life_, 1888-1895, also abounds in
+     information.
+
+     Further, Taschenberg's _Praktische Insektenkunde_, 1879-1880,
+     and Judeich and Nietsche, _Lehrbuch der Mitteleurop. Forst.
+     Insektenkunde_, 1889.
+
+     For an elementary introduction to the study of fungus
+     diseases, see Marshall Ward, _Diseases of Plants_, Soc. for
+     Promoting Christian Knowledge, London.
+
+
+
+
+CHAPTER XIII.
+
+NATURE OF DISEASE.
+
+     _General and local disease--General death owing to cutting-off
+     supplies, etc.--Disease of organs--Tissue-diseases, e.g.
+     timber--Root-diseases--Leaf-diseases, etc.--Diseases of
+     Respiratory, Assimilatory, and other organs--Physiological and
+     Parasitic diseases--Pathology of the cell--Cuts--Cork--Callus
+     --Irritation--Stimulation by protoplasm--Hypertrophy._
+
+
+On going more deeply into the nature of those changes in plants which we
+term pathological or diseased, it seems evident that we must at the
+outset distinguish between various cases. A plant may be diseased as a
+whole because all or practically all its tissues are in a morbid or
+pathological condition, such as occurs when some fungus invades all the
+parts or organs--_e.g._ seedlings when completely infested by _Pythium_,
+or a unicellular Alga when invaded by a minute parasite; or it may die
+throughout, because some organ with functions essential to its life is
+seriously affected--_e.g._ the roots are rotten and cannot absorb water
+with dissolved minerals and pass it up to the shoot, or all the leaves
+are infested with a parasite and cannot supply the rest of the plant
+with organic food materials, in consequence of which parts not directly
+affected by any malady become starved, dried-up, or poisoned or
+otherwise injured by the results or products of disease elsewhere.
+
+In a large number of cases, however, the disease is purely local, and
+never extends into the rest of the organs or tissues--_e.g._ when an
+insect pierces a leaf at some minute point with its proboscis or its
+ovipositor, killing a few cells and irritating those around so that they
+grow and divide more rapidly than the rest of the leaf tissues and
+produce a swollen hump of tissue, or gall; or when a knife-cut wounds
+the cambium, which forthwith begins to cover up the dead cells with a
+similarly rapid growth of cells, the callus. Numerous minute spots due
+to fungi on leaves, cortex, etc., are further cases in point, the
+mycelium never extending far from the centre of infection.
+
+Many attempts have been made to classify diseases on a basis which
+assumes the essential distinction of the above cases, and we read of
+diseases of the various organs--root-diseases, stem-diseases,
+leaf-diseases, and so forth; or of the various tissues--timber-diseases,
+diseases of the cambium, of the bark, of the parenchyma, and so on.
+Furthermore, attempts have been made to speak of general functional
+disease, of diseases of the respiratory organs, of the absorptive
+organs, and so forth, as opposed to local lesions.
+
+Critical examination, however, shows that no such distinctions can be
+consistently maintained, partly because the organs and functions of
+plants are not so sharply marked off as they are in animals, the
+diseases of which have suggested the above classification, and partly
+because all disease originates in the cells and tissues, and it is a
+matter of detail only that in some cases--_e.g._ severe freezing or
+drought of seedlings, or when some ingredient is wanting in the
+soil--the diseased condition affects practically every cell alike from
+the first, while in others it spreads more or less rapidly from some one
+spot.
+
+Even the distinction into physiological diseases _versus_ parasitic
+diseases cannot be maintained from the standpoint of the nature of the
+disease itself. All disease is physiological in so far as it consists in
+disturbance of normal physiological function, for pathology is merely
+abnormal physiology, no matter how it is brought about. This is not
+saying that no importance is to be attached to the mode in which disease
+is incurred or induced: it is merely insisting on the truth that the
+disease itself consists in the living cell-substance--the
+protoplasm--not working normally as it does in health, and this, whether
+want of water, minerals, or organic food be the cause, or whether the
+presence of some poison or mechanical irritant be the disturbing agent,
+as also whether such want or irritation be due to some defect in soil
+or air, or to the ravages of a fungus or an insect.
+
+This being understood I need not dwell on the common fallacy of
+confounding the fungus, insect, soil or other agent with the disease
+itself, or of making the same blunder in confusing symptoms with
+maladies. In this sense, wheat rust is not a disease: it is a symptom
+which betrays the presence of a disease-inducing fungus, the Rust
+fungus. Similarly, chlorosis is not a disease: it is a symptom of
+imperfect chlorophyll action, and the best proof of the truth of both
+statements is that in both cases the fundamental disease-action is the
+starvation of the cell-protoplasm of carbohydrates and other essential
+food matters--in the one case because the fungus steals the
+carbohydrates as fast as the leaves can make them, in the second because
+the leaf is unable to make them.
+
+The foundation of a knowledge of disease in plants therefore centres in
+the understanding of the pathology of living cells.
+
+If a suitable mass of living cells is neatly cut with a sharp razor the
+first perceptible change is one of colour: the white "flesh" of a potato
+or an apple, for instance, turns brown as the air enters the cut cells,
+and the microscope shows that this browning affects cell-walls and
+contents alike. The cut cells also die forthwith; and the oxygen of the
+air combining with some of their constituents forms the brown colouring
+matter which soaks into the cell-walls. The uninjured cells below them
+grow longer, pushing up the dead debris, and divide across by walls
+parallel to the plane of the wound, and so form series of tabular cells
+with thin walls, which also soon turn brown and die, the cell-walls
+meanwhile undergoing changes which convert them into cork. The living
+cells deeper down are now shut off from the outer world by a skin, of
+several layers, of cork-cells, which prevent the further free access of
+air or moisture. During the period of active cell-division which
+initiates the cork, the temperature of the growing cells rises: a sort
+of fever (wound-fever) is induced, evidently owing to the active
+respiration of the growing cells.
+
+This healing by cork occurs in any tissue of living cells exposed by a
+cut--leaf-tissue, young stem or root, fruit, cambium, etc.; and the same
+applies to any other kind of cutting or tearing injury--such as a prick
+with a needle or the proboscis of an insect, a stripping, or even a
+bruise.
+
+Such healing is prepared for and carried out very thoroughly in the case
+of falling leaves and cast branches, the plane of separation being
+covered by a cicatrix of cork.
+
+If the cell-tissue under the wound is actually growing at the time,
+however, a further process is observed when the wound-cork has been
+formed. The uninjured cells below go on growing outwards more vigorously
+than ever, the pressure of the overlying tissues taken off by the cut
+having been removed, and, lifting up the cork-layer as they do so, they
+rapidly divide into a juicy mass of thin-walled cells which is of a
+cushion-like nature and is termed a _Callus_. This callus is at first a
+homogeneous tissue of cells which are all alike capable of growing and
+dividing, but in course of time it undergoes changes in different parts
+which result in the formation of tracheids, vessels, fibres and other
+tissue-elements, and even organs, just as the embryonic tissues of the
+growing points, cambium, etc., of the healthy plant give origin to new
+growths. Such wound-wood, however, is apt to differ considerably in the
+arrangement, constitution and hardness of its parts as compared with
+normal wood, and its peculiar density and cross-graining are often
+conspicuous.
+
+If instead of a simple tissue, the cut or other wound lays bare a
+complex mass such as wood, the resultant changes are essentially the
+same to start with. The living cells bordering the wound form cork, and
+then those deeper down grow out and form a callus. The exposure of the
+wood however, entails alterations in its non-living elements also. The
+lignified walls of tracheids, fibres, etc., turn brown to a considerable
+depth, and this browning seems to be--like all such discolorations in
+wounds--due to oxidation changes in the tannins and other bodies
+present: the process is probably similar to what occurs in humification
+and in the conversion of sap-wood into heart-wood in trees. Such wood is
+not merely dead, but it is also incapable of conveying water in the
+lumina of its elements, which slowly fill with similarly dark-coloured,
+impervious masses of materials termed "wound-gum," the nature of which
+is obscure, but which slowly undergoes further changes into resin-like
+substances.
+
+The exposure of wood by a wound results also in another mode of stopping
+up the vessels and so hindering the access of air, loss of water, etc.,
+for the living cells of the medullary rays and wood-parenchyma grow into
+the lumina of the larger vessels through the pits, forming _thyloses_,
+again a phenomenon met with in heart-wood. In Conifers the stoppage of
+the lumina is increased by deposition of resin, which also soaks into
+the cell-walls and the wounded wood becomes semi-translucent owing to
+the infiltration.
+
+Every living cell in an active condition is irritable, and one of the
+commonest physiological reactions of growing tissues is that of
+responding to the touch of a resistant body, as is vividly shown by the
+movements of the Sensitive plant, _Dionaea_, etc., and by those of
+tendrils, growing root tips, etc., on careful observation. We have
+reason for stating that if a minute insect, too feeble to pierce the
+cuticle, cling on to one side of the dome-shaped growing point of any
+shoot, the irritation of contact of its claws, hairs, etc., would at
+once cause the protoplasm of the delicate cells to respond by some
+abnormal behaviour; and, as matter of experiment, Darwin showed long ago
+that if a minute piece of glass or other hard body is kept in contact
+with one side of the tip of a root, the growth on the side in contact is
+interfered with. Moreover we know from experiments on heliotropism,
+thermotropism, etc., that even intangible stimuli such as rays of light,
+etc., impinging unsymmetrically on these delicate cells cause
+alterations in their behaviour--_e.g._ arrest or acceleration of growth.
+
+Perhaps the most remarkable class of stimulations, however, is that due
+to the presence of the entire protoplasmic body of one organism in the
+cell of another, each living its own life for the time being, but the
+protoplasm of the host cell showing clearly, by its abnormal behaviour,
+that the presence of the foreign protoplasm is affecting its physiology.
+A simple example is afforded by Zopfs' _Pleotrachelus_, the amoeboid
+protoplasmic body of which lives in the hypha of _Pilobolus_, causing it
+to swell up like an inflated bladder, in which the parasite then forms
+its sporangia. The _Pleotrachelus_ does not kill the _Pilobolus_, but
+that its protoplasm alters the metabolic physiology of the latter is
+shown by the hypertrophy of the cells, and by the curious fact that it
+stimulates the _Pilobolus_ to form its sexual conjugating cells,
+otherwise rare, an indication of very far-reaching interference with the
+life-actions of the host.
+
+An equally remarkable example is that of _Plasmodiophora_, the amoeboid
+naked protoplasm of which lives and creeps about in the protoplasm of a
+cell of the root of a turnip, to which it gains access through the
+root-hairs. It does not kill the cell, but stimulates its protoplasm to
+increased activity and growth and division, itself dividing also and
+passing new amoebae into each new daughter-cell of the host. Here the
+processes of stimulation, hypertrophy and further division are repeated,
+until hundreds or thousands of the turnip root-cells are infected. The
+externally visible result is the formation of distorted swellings on the
+root (Finger and Toe), most of the cells of which are abnormally large
+and filled with amoeboid _Plasmodiophora_ protoplasm, which finally
+devours the turnip-protoplasm and itself passes over into spores. Here
+we have most convincing proof of the stimulation of protoplasm by other
+protoplasm in direct contact with it; and that the metabolism of the
+host-cells is profoundly altered is shown not only by the abnormal
+growth of the cells, but also by the starvation of the rest of the
+turnip plant as the _Plasmodiophora_ gets the upper hand. We have here,
+in fact, a local intracellular parasitic disease, gradually invading
+large tracts of tissue and eventually inducing general disease resulting
+in death--a state of affairs reminding us of cancer in animals.
+
+Irritation and hypertrophy of cells, however, may be induced by
+parasites which never bring their protoplasm into direct contact with
+that of the host. Many Chytridiaceae penetrate the cells of plants, and
+grow inside them as short tubes, vesicles, etc., the protoplasm of which
+is separated by their own cell-walls from that of the host-cell;
+nevertheless hypertrophy and abnormal cell-divisions and secretions are
+induced, and the effect even extends to neighbouring cells--_e.g._
+_Synchytrium_--showing that some influence is exerted through cells
+themselves not directly affected. This latter point need not surprise us
+now we know that the cells of plant-tissues are connected by fine
+protoplasmic strands passing through the separating cell-walls.
+
+But the invading plant need not actually enter the cells, and may still
+stimulate them through both its own and their own cell-walls to abnormal
+growth. This is well shown by the intercellular mycelium of _Exoacus_
+and _Exobasidium_, and the latter affords an excellent illustration of
+the far-reaching effects of hyphae on the cells (of _Vaccinium_) into
+which they do not penetrate. Not only are the cells stimulated to grow
+larger and divide oftener than normally, thus producing large gall-like
+swellings, but the chlorophyll disappears, the cell sap changes colour
+to red, the numerous compound crystals normally found in the tissues
+diminish in number and are different in shape, large quantities of
+starch are stored up, and even the vascular bundles are altered in
+character. All these changes indicate very profound alterations in the
+physiological working of the protoplasm of the cells of the host, and
+yet the fungus has done its work through both its own cell-walls and
+those of the host.
+
+Even harmless endophytic algae in the intercellular spaces of plants may
+stimulate the cells in their immediate neighbourhood to increased
+growth, _e.g._ _Anabaena_ in the roots of Cycads.
+
+
+NOTES TO CHAPTER XIII.
+
+     With reference to cork-healing and wound-fever the student may
+     consult Shattock "On the Reparative processes which occur in
+     Vegetable Tissues," _Journal of the Linnean Society_, 1882,
+     Vol. XIX., p. 1; and Shattock "On the Fall of Branchlets in
+     the Aspen," _Journal of Botany_, 1883, Vol. XXI., p. 306. Also
+     Richards, "The Respiration of Wounded Plants," _Annals of
+     Botany_, Vol. X., 1896, p. 531; and "The Evolution of Heat by
+     Wounded Plants," _Ann. of Bot._, Vol. XI., 1897, p. 29.
+
+     For details and figures respecting callus, see Sorauer,
+     _Physiol. of Plants_, p. 175.
+
+     In respect to the irritable movements referred to see Darwin,
+     _The Power of Movements in Plants_, 1880, chapter III. The
+     recent work of Nawaschin, _Beobachtungen ueber den feineren
+     Bau u. Umwandlungen von Plasmodiophora_, Flora, Vol. LXXXVI.,
+     1899, p. 404, should be read for details and literature
+     concerning "Finger and Toe."
+
+
+
+
+CHAPTER XIV.
+
+NATURE OF DISEASE (_Continued_).
+
+     _Actions of poisons in small doses--Results of killing a few
+     cells--Malformation--Enzymes--Secretions and excretions--
+     Acids, poisons, etc.--Chemotactic phenomena--Parasitism--
+     Epiphytes and endophytes--Symbiosis--Galls._
+
+
+Physiological research has shown that the respiratory activity of cells
+may be increased by small doses of poisons, and even that growth may be
+accelerated by them--_e.g._ chloroform, ether--and, still more
+remarkable, that fermentative activity may be enhanced by minute doses
+of such powerful mineral poisons as mercuric chloride, iodine salts,
+etc., and that the cells may be gradually accustomed to larger doses
+without injury. Unfertilised eggs of insects have been started into
+growth by treatment with acids and those of frogs with mercury salts,
+and the germination of beans quickened by various poisonous alkaloids.
+In other words, graduated doses of poison may alter the physiological
+activity of living cells, inducing pathological phenomena, while larger
+doses kill them.
+
+Now we know at least one parasitic fungus which poisons the cells of its
+host, and kills them, with similar symptoms to those resulting from
+excessive doses of the above-named toxic agents. _Botrytis_ hyphae,
+living in the cell-walls of plants, but not entering the cells, excretes
+a poison which kills the protoplasm, and the fungus then feeds on the
+debris. Numerous other fungi form powerful poisons, but we do not know
+whether or how they employ them--_e.g._ Ergot.
+
+It is obvious that if all the young cells of a root-tip or of the apex
+of a shoot, or those of a young leaf, are growing and dividing
+regularly, the killing of one or a few cells at one point on the side of
+the organ must result in irregularities--in malformation--of the adult
+organ. This has been proved experimentally by destroying a few cells
+with a needle. It can also be done by planting a minute mycelium of
+_Botrytis_ laterally on a young organ--_e.g._ a very young lily-bud. The
+fungus adheres to the surface, kills a few epidermis cells, and forms a
+foxy-red spot, which becomes concave as the dead cells lose water and
+dry. Since the rest of the bud goes on growing, however, while this dead
+point remains stationary, the latter gradually becomes the centre of a
+concavity, the growing tissues having grown round it: the bud is
+deformed. Numerous cases of malformed organs are explained in this way;
+a minute insect has bitten or pierced the young tissue, or a fungus has
+killed a minute area, or a drop of acid condensed from fumes in the air
+is the lethal agent, and so forth. And even on a much larger scale we
+see the same kinds of agents at work. Wherever a patch of cells is
+killed whilst those around go on growing, there must result some
+deformation of the resulting organ, since had the injury been withheld
+the number and sizes of the cells now fixed in death would have
+increased and covered a larger area: they now serve to pull over to
+their side the still living and growing cells. The same results follow
+on any lateral wound: the killed spot of tissue serves as a point round
+which the continued growth of other parts of the organ turns. Hence the
+malformation is in these cases a secondary effect, and not, as in simple
+hypertrophy, a direct effect of the action of the cells involved in the
+injury.
+
+There is another class of bodies secreted by fungi, however, which act
+directly on cells, viz. enzymes--that is, soluble bodies which are able
+to dissolve cellulose (_cytases_), starch (_diastases_), proteids
+(proteolytic enzymes), and other substances, by peculiar alterations in
+their constitution. It is by means of its _cytase_ that _Botrytis_
+hyphae pierce the cellulose walls of plants, and no doubt in all cases
+where fungi pierce cell-walls it is by the solvent action of such a
+cytase, and similarly when haustoria penetrate into the cells. It is
+also by means of these starch-dissolving enzymes (diastases) and
+proteolytic enzymes, etc., that the hyphae inside the cells are enabled
+to make use of the starch, proteids, etc., they find there.
+
+All living cells form materials, resulting from the activity of the
+protoplasm, which we may compare with the refuse or by-products formed
+in any great manufacturing industry: these by-products have to be got
+rid of if they are injurious or noisome (_excretions_), and if
+not--_i.e._ if they are capable of further use (_secretions_)--they have
+to be stored away till required. Some of the most prominent of these
+bodies excreted by fungi are, as we have seen, poisonous acids, such as
+oxalic acid, enzymes, and organic poisons, such as those in ergot. But
+similar enzymes, acids, poisons, etc., to those found in fungi are also
+found in the cells of other plants and animals; for only by means of
+their solvent actions can processes like digestion and assimilation of
+the starchy and other materials into the body-substance be accomplished,
+and we have seen that it is a general property of living cells to form
+acids, and other excretions and secretions.
+
+Now we know very little about what may happen when an organism--say a
+fungus--secreting especially one kind of enzyme or poison or other
+active substance, comes into intimate contact with another--say a
+leaf-cell--which secretes predominantly others, but what we do know
+points to the certainty that various complications will occur.
+
+For instance, if certain bacteria which prefer an alkaline medium, and
+yeasts which prefer an acid environment are mixed in a saccharine
+solution, it depends on the reaction of the liquid which organism gains
+the upper hand: if the liquid is acid the yeast may dominate the
+bacteria; if alkaline it may be suppressed by them.
+
+That a parasite may be prevented from successfully attacking a
+particular plant is shown by the failure of _Cuscuta_ to establish its
+haustoria in poisonous plants such as _Euphorbia_, _Aloe_, etc., and it
+has been pointed out that poisonous secretions in the cells of the plant
+protect them against the penetration of fungi. This cannot be taken as
+meaning that any poison protects against any parasite, however, for
+_Euphorbia_ is itself subject to attacks of Uredineae, and _Pangium
+edule_, which contains prussic acid and is extremely poisonous to most
+animals, is eaten with avidity by several insects, while nematode worms
+can live in its tissues. This is no more remarkable, however, than the
+fact that _Fontaria_, a myriapod, secretes prussic acid in its own
+tissues, or than that certain glands of the stomach secrete free
+hydrochloric acid, and _Dolium_ forms sulphuric acid in its glands.
+
+There is yet a further point to notice here. It has been proved that
+certain substances formed in plant-cells, not necessarily nutritive,
+attract the hyphae of parasitic fungi or repel them, according to the
+kind and degree of concentration. So clear has this proof been made that
+it was possible in experiments conducted apart from a host plant, to
+make the hyphae on one side of an artificial membrane--_e.g._
+collodion--penetrate it by placing one of these attractive
+(_chemotropic_) substances in suitable proportions on the other side.
+The hyphae dissolved holes in the membrane by means of enzymes and
+plunged into the attractive substance on the other side.
+
+The foregoing sketch gives us a glimpse into the causes at work in
+parasitism.
+
+Suppose a fungus on the outside of the epidermis of a young organ--say a
+leaf. It may be unable to penetrate into the plant, and finding no
+suitable food outside it dies: or it may be satisfied with the traces of
+organic matter on the epidermis and then lives the life of a saprophyte.
+Or it may be able to establish a hold-fast on the tender epidermal
+surface, but without entering the cells, and irritate the developing
+organ by contact stimulation, inducing slight abnormalities; if in its
+further, purely superficial growth such an epiphyte covers large areas
+of the leaf, and especially if the hyphae are dark coloured--_e.g._
+_Dematium_ and other "Sooty Moulds"--injury may be done to the leaf
+owing to the shading action which deprives the chlorophyll below of its
+full supply of solar energy. Some epiphytes, however, are able to fix
+their hyphae to the epidermis by sending minute peg-like projections
+into the cuticle--_Trichosphaeria_, _Herpotrichia_--while others send
+haustoria right through the outer epidermal walls--_e.g._
+_Erysiphe_--and thus supplement mere contact-irritation and shading by
+actual absorption from the external cells. Here the fungus is a
+parasitic epiphyte.
+
+A stage further is attained in those fungi which enter the stomata and
+live in the intercellular spaces--_e.g._ many Uredineae and
+_Phytophthora_--and many such intercellular endophytes increase their
+attack on the cells by piercing their walls with minute (_Cystopus_) or
+large and branched (_Peronospora_) haustoria, or even eventually pierce
+the cells and traverse them bodily (_Pythium_). In all these cases it is
+clear that conflicts must occur between poison and antidote, acid and
+alkali, attractive and repellent substances, enzyme and enzyme, etc., as
+was hinted at above; and the same must take place when the parasite is
+endophytic and intracellular from the first, as in Chytridiaceae, etc.,
+the zoospores of which pierce the outer cell-walls and forthwith grow
+into the cells. There are also fungi which, while able to pierce the
+outer cell-walls, and grow forward in the thickness of the wall itself,
+cannot enter the living cells themselves--_e.g._ _Botrytis_. In the
+example mentioned, the fungus excretes a poison, oxalic acid, which
+soaks into and kills the cells next its point of attack: into these dead
+cells it then extends, and, invigorated by feeding on them, extends into
+other cell-walls and excretes more poison, and so on.
+
+On the basis of the foregoing it seems possible to sketch a general view
+of the nature of parasitism. In order that a fungus may enter the cells
+it must be able to overcome not only the resistance of the cell-walls,
+but that of the living protoplasm also: if it cannot do the latter it
+must remain outside, as a mere epiphyte, or at most an intercellular
+endophyte. If it can do neither it must either content itself with a
+saprophytic existence or fail, so far as that particular host-plant is
+concerned. Its inability to enter may be due to there being no
+chemotropic attraction, or to its incapacity to dissolve the cell-walls,
+or to the existence in the cell of some antagonistic substance which
+neutralises its acid secretions, destroys its enzymes or poisons, or is
+even directly poisonous to it.
+
+Moreover when once inside it does not follow that it can kill the cell.
+The protoplasm of the latter may have been unable to prevent the fungus
+enemy from breaking through its first line of defence--the cell-wall,
+but it may be quite capable of maintaining the fight at close quarters,
+and we see signs of the progress of the struggle in hypertrophy,
+accumulation of stores, and other changes in the invaded cells and their
+contents.
+
+Finally, the invested or invaded cell may so adapt itself to the demands
+of the invader that a sort of arrangement is arrived at by which life in
+common--_Symbiosis_--is established, each organism doing something for
+the other and each taking something from the other. In this latter case,
+which is often realised--_e.g._ lichens, leguminous plants and the
+organisms in their root-nodules, mycorrhiza, etc.--we leave the domain
+of disease, which supervenes indeed if the other symbiont is lacking.
+
+Some interesting facts bearing on the matters here under discussion,
+have been obtained from the study of _Galls_, the curious outgrowths
+found on many plants and due to the action of insects.
+
+A typical gall exhibits three distinct and characteristic layers of
+tissue surrounding the hollow chamber in which the larva of the insect
+lies, viz., an outer layer of soft cells forming a parenchyma covered
+with an epidermis, and frequently also with a layer of cork; an inner
+stratum consisting of very thin-walled delicate cells filled with
+protoplasmic and reserve food-materials on which the larva feeds; and
+between the two a more or less definite layer of thick-walled
+sclerenchyma cells which serve as a protection against accidents to the
+larva as the outer layer shrivels or rots, or if it is exposed to the
+attack of marauders. This layer may be absent from galls which have a
+short life only. Vascular bundles run into the outer layer from the
+leaf-veins or the stele of the shoot, etc. Such galls abound in tannin,
+and are frequently of use in the arts on this account: they also contain
+starch, and proteid substances and crystals of calcium oxalate. When the
+larva has consumed the stores of food material and reached the adult
+stage it eats its way out and escapes.
+
+The growth of such a gall is preceded by the laying of an egg on or in
+the embryonic tissue of a leaf, stem, or other young part, and it is
+interesting to note that only organs in the meristematic stage can form
+galls, and that it is by no means necessary that the tissues should be
+wounded. Moreover, the egg as such is incapable of stimulating the plant
+tissues, but when it hatches, the resulting larva, beginning to feed on
+the cells, irritates the tissues and rapid growth and cell-division
+occur, as in the case of other wounds or of fungus attacks. The actual
+wound made by the ovipositor heals up at once. It is evident from
+numerous recent researches that these true galls are not due to any
+poisonous or irritating liquid injected by the parent, but that the
+stimulus to the tissue formation is similar to that exerted by a wound.
+The young gall is in fact a callus enclosing the living larva, and it is
+the continued irritation of the latter which keeps up the stimulation.
+The final shape and constitution of the gall depend on mutual
+reactions--not as yet explained in detail--between the species of plant
+and the species of gall-insect concerned, as may readily be seen from
+the extraordinary variations in size, shape, colouring, hairiness and
+other structural peculiarities of the galls on one species of, for
+instance, the common oak. From what we have learnt about fungus
+parasites, however, there can be little doubt that reactions between the
+cells and the larva of the insect occur, resembling those which take
+place between the cells and the hyphae of the fungus, and this is borne
+out by the study of other hypertrophies due to animals; _e.g._ Nematode
+worms in roots, and the remarkable galls--the simplest known--on
+_Vaucheria_, caused by the entrance into this alga of a species of
+_Notommata_, which induces a different gall on each of the various
+species of its host plants.
+
+It must be concluded that the formation of the _Vaucheria_ gall is
+induced by the mechanical irritation which the Rotifer causes in the
+protoplasm. These galls are comparable to the hypertrophies in
+_Pilobolus_ caused by the presence of _Pleotrachelus_.
+
+Attempts to induce the development of galls artificially by injecting
+formic, acetic and other vegetable acids, poisons and other substances
+into the tissues have, however, failed, and even the substances
+contained in the insect or gall itself only produced negative results.
+Nothing further was obtained than slight callus formations in some
+cases. Nor have experimenters succeeded in obtaining more than slight
+distortions by fixing insects on the growing leaves in such positions
+that they could scratch the epidermis.
+
+We must therefore conclude that very complex interactions between the
+plant and insect are here concerned, among which may be the infiltration
+of some liquid from larva to plant--many of these gall larvae are
+strongly scented, and Kustenmacher says that fluids excreted by the
+larva are absorbed by the gall-tissue apparently as nutriment. This
+would point to the symbiotic character of galls and their guests.
+
+
+NOTES TO CHAPTER XIV.
+
+     With regard to the action of poisons in small doses see
+     further Johannsen, _Das Aether-Verfahren beim Fruhtreiben_,
+     Jena, 1900, and, for _Botrytis_, see Marshall Ward, "A Lily
+     Disease," _Annals of Botany_, Vol. II., 1889, p. 388.
+
+     The subject of enzymes has been exhaustively treated by Green,
+     _The Soluble Ferments and Fermentations_, Cambridge, 1899, to
+     which the reader is referred for literature. I have taken the
+     statements regarding _Fontaria_ and _Dolium_ from Kassowitz,
+     _Allgemeine Biologie_, p. 182. The two most important works on
+     chemotactic phenomena are Pfeffer, "Uber Chemotaktische
+     Bewegungen," etc., _Unters. aus dem Bot. Inst. zu Tubingen_,
+     B. II., p. 582, and Miyoshi, "Die Durchbohrung von Membranen
+     durch Pilzfaden," _Pringsh. Jahrb. f. Wiss. Bot._, B. XXVIII.,
+     1895, p. 269, and from these the further literature can be
+     traced. As regards the nature of parasitism see Marshall Ward,
+     "On Some Relations between Host and Parasite," etc., being the
+     Croonian Lecture delivered before the Royal Society, _Proc.
+     Roy. Soc._, Vol. 47, p. 393. On Symbiosis, see Marshall Ward,
+     "Symbiosis," _Annals of Botany_, 1899, Vol. XIII., p. 549,
+     where the literature is collected. For a general account of
+     galls the reader may consult Kerner, _The Natural History of
+     Plants_, Eng. ed., 1895, Vol. II., pp. 527-554, and Adler,
+     _Alternating Generations, A Biological Study of Oak Galls_,
+     etc., 1894.
+
+
+
+
+CHAPTER XV.
+
+SPREADING OF DISEASE AND EPIDEMICS.
+
+     _Dissemination of fungi by the aid of snails, rabbits, bees,
+     and insects--Man--Distribution in soil, on clothes, through
+     the post, etc.--Worms, wind--Puffing of spores--Creeping of
+     mycelia--Lurking parasites--Spread of insects and other
+     animals--Losses due to epidemics._
+
+
+The dissemination of plant diseases is a subject which has been far too
+much neglected, but our knowledge of it is slowly increasing. The spores
+of fungi such as Rusts and Erysipheae are often carried from plant to
+plant by snails; those of root-destroying and tree-killing Polyporei by
+rabbits, rats, and other mammals which rub their fur against the
+hymenophores. Bees have been shown to carry the spores of _Sclerotinia_
+and infect the stigmas of Bilberries, etc., with them; and flies convey
+the conidia of Ergot from grain to grain. Insects, indeed, of all kinds
+are great disseminators of disease--as witness also the part played by
+mosquitoes in transferring the malaria parasite to man--and beetles,
+bees, flies, etc., of all sorts probably play more active parts in this
+work than has yet been proved, since they not only carry spores attached
+like pollen to their hairy bodies, but in many cases in their alimentary
+canal, to be spread later in the dung.
+
+The part played by man in conveying fungi from plant to plant counts for
+much. Not only do gardeners and farm labourers carry spores on their
+boots and clothes as they pass from infected to non-infected areas, but
+carted soil and manure are frequently infested with spores of Smuts,
+_Fusarium_, _Polyporus_, and the sclerotia or rhizomorphs of
+_Sclerotinia_, _Agaricus melleus_, _Dematophora_, etc. Man also sends
+diseases through the post, and by rail and ship, by spores or mycelia
+attached to seedlings, bulbs, fruits, flowers, etc., as shown in several
+cases of potato, vine, hollyhock, lily, and hyacinth diseases. Every
+time a carpenter saws a piece of fresh timber with the saw which has
+been used previously for cutting wood attacked with dry rot, he risks
+infecting it with the fungus. Similarly in pruning: every cut with a
+knife which the gardener has used on infected branches may infect the
+tree.
+
+Cuttings made with a soil-contaminated knife and stuck into ordinary
+soil in dirty boxes covered with equally dirty glass, present every
+chance for infection by soil organisms; bacteria and fungi obtain access
+to the vessels, and derive plenty of food from the juices, and the
+wonder is not that so many cuttings "damp off," but that any are raised
+at all under ordinary conditions.
+
+That worms bring buried spores to the surface can hardly be doubted
+after Pasteur's experiments with Anthrax, and the principle of Darwin's
+discoveries of the important bearing of the habits of earthworms on this
+subject, and that the soil attached to the feet of ducks and other birds
+teems with small seeds, applies to fungi also. Wind is also responsible
+for distributing fungus-spores over wide areas, as may be easily proved
+by fixing a glass slide smeared with glycerine in the course of a breeze
+passing over an infected area.
+
+But although the fungi are, generally speaking, passive in regard to
+their distribution, such is by no means always the case. Apart from the
+fact that some forms attract insects by means of honey dew (Ergot), or
+by sweet odours (Spermogonia, _Sclerotinia_), the zoospores of
+_Pythium_, _Phytophthora_, etc., are motile, and although they cannot
+move far in the films of water in which they travel, nevertheless in a
+wet potato field, with the wind flapping the leaves one against the
+other, some dissemination of importance must be actively brought about,
+and similarly with the amoebae of _Plasmodiophora_ in the soil.
+
+The shooting of ascospores into the air by certain species of _Peziza_,
+from the discs of which the spores may be seen to puff out in clouds,
+affords further evidence that fungi cannot be regarded as entirely
+passive in respect to distribution of their spores. But when we come to
+certain of the soil fungi--_e.g._ _Agaricus melleus_, _Dematophora_,
+etc.--the active creeping forward by growth in the soil of their
+rhizomorphs and mycelial strands afford examples of active spreading of
+considerable importance in the vineyard and forest, since they pass from
+root to root and from tree to tree and may infect the entire area in
+course of time.
+
+Not the least significant mode of dissemination is that by which what I
+have termed "lurking parasites" are spread: such are fungi which attach
+themselves to the seeds, fruits, tubers, etc., of other plants and so
+obtain all the advantages of being carried and sown with the
+latter--_e.g._ Ustilagineae and Uredineae which adhere to grain,
+_Verticillium_, _Nectria_, etc., in potatoes and other plants.
+
+The spread of diseases due to animals, especially insects, is of course
+more active, in consequence of the motility of the distributing agents.
+This is most marked in the winged species, of which locusts, beetles,
+moths and butterflies, flies and wasps furnish well-known examples; and
+is not inconsiderable in the case of wingless and merely creeping
+species. It is noteworthy that many forms wingless in the parasitic
+stage are winged at certain periods, _e.g._ the females of _Phylloxera_.
+
+That man also spreads insect pests is well known and acted upon, as
+witness the phylloxera laws--which, however, it is to be feared too
+often only illustrate once more the adage concerning the shutting of the
+stable door after the horse has gone.
+
+It would be tedious to attempt anything like a complete account of the
+estimates of loss in different countries, due to the ravages of insects
+and fungi, but the following examples should surely serve to convince
+anyone of the magnitude of these losses and of the economic importance
+of the whole question, and the reader may be referred to the special
+literature for further details.
+
+The coffee leaf-disease of Ceylon, due to the fungus _Hemileia_, is
+estimated to have cost that Colony considerably over L1,000,000 per
+annum for several years. One estimate puts the loss in ten years at from
+L12,000,000 to L15,000,000. The hop-aphis is estimated to have cost Kent
+L2,700,000 in the year 1882. In 1874 the Agricultural Commissioner of
+the United States estimated the annual loss, due to the ravages of
+insects on cotton alone, to amount to L5,000,000; and in 1882 the annual
+loss to the United States due to insects, calculated for all kinds of
+agricultural produce, was put at the appalling figure of from
+L40,000,000 to L60,000,000 sterling. In India, the annual loss due to
+wheat-rust alone has recently been estimated at 4,000,000 to 20,000,000
+rupees, and one insect alone is said to have cost the cotton planters a
+quarter of the crop--valued at seven crores of rupees--in bad years.
+Similarly, in Australia the annual loss from wheat-rust has been put at
+from L2,000,000 to L3,000,000. In 1891 the loss in Prussia alone from
+grain-rusts was officially estimated at over L20,000,000 sterling. Need
+more be said? Even allowing for considerable exaggerations in such
+estimates it is clear that the damage to crops in any country soon
+amounts to sums which even at low rates of interest would easily yield
+incomes capable of supporting the best equipped laboratories and staffs
+for investigations directed to the explanation of the phenomena in
+detail, the sole basis on which intelligent preventive and therapeutic
+measures can be based. But it is far from likely that the estimates are
+exaggerated. The planting and agricultural communities are as a rule
+opposed to the publication of statistics--or at least have been so in
+various countries and at different times--and if we knew the damage done
+to all crops even in our own Empire, the results would probably astonish
+us far more than the above figures have done.
+
+
+NOTES TO CHAPTER XV.
+
+     On the dissemination of fungi, the reader will find Fulton,
+     "Dispersal of the Spores of Fungi by the Agency of Insects,"
+     _Ann. Bot._, Vol. III., 1889, p. 207, and Sturgis, "On Some
+     Aspects of Vegetable Pathology and the Conditions which
+     Influence the Dissemination of Plant Diseases," _Botanical
+     Gazette_, Vol. XXV., 1898, p. 187, both useful papers. Further
+     information will be found in Zopf, _Die Pilze_, Breslau, 1890,
+     pp. 79-95 and 228, and Wagner, "Ueber die Verbreitung der
+     Pilze durch Schnecken," in _Zeitschr. f. Pflanzen Krankh._,
+     1896, p. 144. The estimates as to losses due to epidemics are
+     taken from Watt, _Agricultural Ledger_, Calcutta, 1895, p. 71;
+     Balfour, _The Agricultural Pests of India_, London, 1887,
+     pp. 13-15; Eriksson and Henning, _Die Getreideroste_; the
+     publications of the U.S. Department of Agriculture, _The
+     Kew Bulletin_, and elsewhere. The reader will find further
+     examples in Massee, _Text-Book of Plant Diseases_, 1899, pp.
+     47-51. Both these subjects are well worth further attention,
+     and I know of no complete account of them.
+
+
+
+
+CHAPTER XVI.
+
+THE FACTORS OF AN EPIDEMIC.
+
+     _Illustrations afforded by the potato disease--The larch
+     disease--The phylloxera of the vine._
+
+
+When we come to enquire into what circumstances bring about those severe
+and apparently sudden attacks on our crops, orchards, gardens, and
+forests by hosts of some particular parasite, bringing about all the
+dreaded features of an epidemic disease, we soon discover the existence
+of a series of complex problems of intertwined relationships between one
+organism and another, and between both and the non-living environment,
+which fully justify the caution already given against concluding that
+any cause of disease can be a single agent working alone.
+
+The statement of prophecy that a particular insect or fungus need not be
+feared, because it is found to do so little harm in particular cases or
+districts examined, will thus be seen to be a dangerous one: any pest
+may become epidemic if the conditions favour it!
+
+In 1844 and 1845 the potato disease assumed an epidemic character so
+appalling in its effects that it is no exaggeration to say that it
+constituted a national disaster in several countries. It was stated at
+the time that this disease had been known for some time in Belgium, in
+Canada and the United States, in Ireland, in the Isle of Thanet, and in
+other parts of the world. Similar, but less devastating epidemics have
+occurred in various years since. It was generally noticed during such
+epidemics that the plants themselves were full of foliage, surcharged
+with moisture, and of a luxuriant green colour promising abundant
+crops. The now well-known spots, at first pale and then brown and
+fringed with a whitish mould-like growth--the conidiophores of the
+_Phytophthora_--were observed during the dull cloudy and wet weather,
+cooler than usual, when the atmosphere was saturated for days together,
+in July and August. The actual amount of rain does not appear to have
+been excessive, but most observers seem to agree that dull weather with
+moist air had succeeded a warm forcing period of growth. So rapidly did
+the disease run its course that in a few days nearly all the plants were
+a rotting blackened mass in the fields, and the potatoes dug up
+afterwards were either already rotten or soon became so in the stores.
+Further experience has confirmed this, and we now know that the epidemic
+is very apt to appear in any region where potatoes are grown on a large
+scale, in dull moist weather, especially in fields exposed to mists,
+heavy dews, etc., about July and August, when the foliage is full and
+turgid. Similarly on heavy wet soils, unless the season is remarkably
+open and dry; but also on dry light soils in rainy seasons. So evident
+was this that many believed that the mists and dew brought the
+disease--harking back to the superstitions of earlier days. We must
+remember that prior to 1860 the life-history of _Phytophthora_ was not
+known. Since De Bary's proof of the germination of the zoospores and of
+the infection of the leaves, the course of the hyphae in them and in the
+haulms, the origin of the conidia, etc., and the confirmation by
+numerous competent observers of the true fungus nature of this disease,
+we are now in a position to understand the principal factors of the
+various epidemics of potato disease.
+
+It is not merely that the potato-fields afford plenty of food for the
+fungus, and that the dull weather causes the tissues to be surcharged
+with moisture, owing to diminished transpiration, but the mists and
+dew--to say nothing of actual rain and the flapping of wet
+leaves--favour the germination and spread of the zoospores throughout
+the field. Whether the dull light also favours the accumulation of
+sugars in the tissues, and the partial etiolation of the latter implies
+less resistance to the entering hyphae, may be passed over here, but in
+any case it is clear that we have several factors of the non-living
+environment here favouring the parasite and not improving the chances
+of the host, even if they do not directly disfavour it.
+
+As another instance I will take the Larch-disease, which is due to the
+ravages of a Peziza (_Dasyscypha Willkommii_) the hyphae of which obtain
+access by wounds to the sieve-tubes and cambium of the stem, and
+gradually kill them over a larger and larger area and so ring the tree,
+with the symptoms of canker described below.
+
+Now the Larch fungus is also to be found on trees in their Alpine home,
+but there it does very little damage and never becomes epidemic except
+in certain sheltered regions near lakes and in other damp situations.
+How then are we to explain the extensive ravages of the Larch disease
+over the whole of Europe during the latter half of this century? The
+extensive planting, providing large supplies for the fungus, does not
+suffice to explain it, because there are large areas of pure Larch in
+the Alps which do not suffer.
+
+In its mountain home the Larch loses its leaves in September and remains
+quiescent through the intensely cold winter, until May. Then come the
+short spring and rapid passage to summer, and the Larch buds open with
+remarkable celerity when they do begin--_i.e._ when the roots are
+thoroughly awakened to activity. Hence the tender period of young
+foliage is reduced to a minimum, and any agencies which can only injure
+the young leaves and shoots in the tender stage must do their work in a
+few days, or the opportunity is gone, and the tree passes forthwith
+into its summer state.
+
+In the plains, on the contrary, the Larch begins to open at varying
+dates from March to May, and during the tardy spring encounters all
+kinds of vicissitudes in the way of frosts and cold winds following on
+warm days which have started the root-action--for we must bear in mind
+that the roots are more easily awakened after our warmer winters than is
+safe for the tree.
+
+It amounts to this, therefore, that in the plains the long continued
+period of foliation allows insects, frost, winds, etc., some six weeks
+or two months in which to injure the slowly sprouting tender shoots,
+whereas in the mountain heights they have only a fortnight or so in
+which to do such damage. That the lower altitude and longer summer are
+not in themselves inimical to Larch is proved by the splendid growths
+made by the trees first planted a century ago. Then came the epidemic of
+Larch-disease: the fungus, which is merely endemic--_i.e._ obtains a
+livelihood here and there on odd trees, or groups of trees in warmer or
+damper nooks--in the Alps, was favoured by the more numerous points of
+attack afforded to its spores by injuries due to insects--_Coleophora_,
+_Chermes_, etc.--and frost wounds, as well as by the longer periods of
+moist dull weather, and the longer season of foliation. Moreover, as
+time went on almost every consignment of young Larch-trees sent abroad
+was already infected. Here again, then, we find the factors of an
+epidemic consisting in events which favour the reproduction and spread
+of a fungus more than they do the well-being of the host.
+
+As a third illustration I will take the case of an insect epidemic. In
+1863 a disease was observed on vines in the South of France which
+frightened the growers as they realised its destructive effects: the
+roots decayed and the leaves turned yellow and died before the grapes
+ripened, and such vines threw out fewer and feebler shoots the following
+year, and often none at all afterwards. In 1865 the disease was
+evidently becoming epidemic near Bordeaux, and in 1868 it was shown to
+be due to an insect, _Phylloxera_, the female of which lays its eggs on
+the roots, where they hatch. The louse-like offspring sticks its
+proboscis into the tissues as far as the central cylinder. The irritated
+pericycle and cortex then grow and form nodules of soft juicy
+root-tissue at which the insect continues to suck. Rapid reproduction
+results in the majority of the young rootlets being thus attacked, and
+since they cannot form their normal periderm and harden off properly
+they rot, and admit fungi and other evils, in consequence of which the
+vine suffers also in the parts above ground.
+
+Evidence that the general damage is due to the diminished root-action is
+found in the peculiarly dry poor wood formed in the "canes" of diseased
+plants.
+
+By 1877 the epidemic had spread to the northern limits of the French
+vineyards, and by 1888 half the vines in the country were attacked, and
+the yield of wine reduced from half a million hectolitres to 50,000
+only. Meanwhile the disease had spread to Italy, Germany, Madeira,
+Portugal, and even to the Cape, though not in epidemic form as in the
+Bordeaux centre whence it spread.
+
+Now it appears that _Phylloxera_ has long been in the habit of doing
+damage to vines in America, where, however, it attacks the leaves, on
+which it makes pocket-like galls, rather than the roots. Moreover, there
+are species and varieties of American vines which, even when planted in
+Europe, do not suffer at all from this insect at the roots, either
+because the rootlets do not push out at the same season as those of the
+European form, or because they form wood more rapidly and completely, or
+secrete resinous and other matters distasteful to the insect in greater
+quantity and are thus capable of healing the wounds, or in some other
+way they do not respond to the attack or suit the insect. In any case
+the attack on the leaf rather than the root seems to be the exception in
+European vineyards and the rule in American species, and we appear to be
+face to face with a problem of specific predisposition to this
+particular malady. That the resistant properties of the vines of
+America--not all, only particular species and varieties are thus
+"immune"--can be utilised has been proved by European growers; and not
+only so, for Millardet and others have shown that the European vine
+grafted on to these resistant stocks suffer less than when on their own
+roots. It has also been shown that hybrids can be obtained which are
+resistant.
+
+But the most curious point of all is that _Phylloxera_ was itself a
+native of America, and came thence to Europe. It had played its part
+with certain fungi in ruining all the attempts to introduce the European
+vine into America many years ago. A recent authority on the evolution of
+American fruits writes as follows:
+
+"All the most amenable types of grapes had long since perished in the
+struggle for existence, and the types which now persist are necessarily
+those which are, from their very make-up or constitution, almost immune
+from injury, or are least liable to attack . . . the _Phylloxera_ finds
+tough rations on the hard, cord-like roots of any of our eastern species
+of grapes. But an unnaturalised and unsophisticated foreigner, being
+unused to the enemy and undefended, falls a ready victim; or if the
+enemy is transported to a foreign country the same thing occurs."
+
+Further proof that it is in the "constitution" of the European vine that
+the want of resistance to _Phylloxera_ resides, is furnished by the fact
+that in California and the Pacific states the European vine was
+introduced with more success, but is now suffering badly because
+_Phylloxera_ has spread there also. It must not be overlooked, however,
+that we are as yet very ignorant of all that is implied in the word
+"constitution" as used above.
+
+If we enquire further why the _Phylloxera_ epidemic was so much worse
+in the Southern vineyards than in the more Northern ones of Germany, the
+opinion seems to prevail that the warmer climates favour the insect.
+Further, it appears that, in Italy, the vines in loose open soil,
+provided it is equally rich in mineral food-materials and offers no
+disadvantages as regards drainage, suffer less than those in closer
+soils, the reasons alleged being that the young roots can push out more
+rapidly and widely, and so obtain holdfasts with greater distances
+between them.
+
+
+NOTES TO CHAPTER XVI.
+
+     The student may obtain further information on the history of
+     the Potato disease by consulting the following: Berkeley,
+     "Observations, Botanical and Physiological, on the Potato
+     Murrain," _Journal of the Horticultural Society_, Vol. I.,
+     1846, p. 9; De Bary, _Die Gegenwaertig herrschende Kartoffel
+     Krankheit_, etc., Leipzic, 1861; and the pages of the
+     _Gardeners' Chronicle_ from 1860-1900.
+
+     For the Larch disease he should consult Hartig, _Unters. aus
+     der Foist. Botanischen Inst. Muenchen_, B. I., 1880; and
+     Willkomm, _Microscop. Feinde des Waldes_, B. II., 1868.
+
+     For _Phylloxera_ the literature is chiefly in the _Comptes
+     Rendus_ and other French publications since 1875, and in the
+     Reports of the U.S. Dept. of Agriculture.
+
+     For a summary of the facts concerning the life-histories of
+     the parasites referred to above, see Frank, _Krankheiten der
+     Pflanzen_, and Marshall Ward, _Diseases of Plants_, p. 59, and
+     _Timber and Some of its Diseases_, London, 1889, chapter X.
+
+     Also Marshall Ward, "On some Relations between Host and
+     Parasite in certain epidemic Diseases of Plants," _Proc. Roy.
+     Soc._, Vol. XLVII., 1890, pp. 393-443; and "Illustrations of
+     the Structure and Life-history of Phytophthora infestans,"
+     _Quart. Journ. Microsc. Soc._, Vol. XXVII., 1887, p. 413; also
+     Marshall Ward, "Researches on the Life-history of Hemileia
+     vastratrix," _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299; and
+     "On the Morphology of Hemileia vastatrix," _Quart. Journ.
+     Microsc. Soc._, 1881, Vol. XXI., p. 1.
+
+
+
+
+CHAPTER XVII.
+
+REMEDIAL MEASURES.
+
+     _Preventible diseases--The principles of therapeutics--Powders
+     and their application--Spraying with liquids--Nature of
+     chemicals employed--Employment of epidemics and natural
+     checks--The struggle for existence._
+
+
+It may be said that in no connection is the proverb "Prevention is
+better than cure" more applicable than with this subject, and
+undoubtedly the best utilitarian argument that can be used in favour of
+a thorough study of the causes of disease is that only by understanding
+these causes is there any hope of avoiding the exposure of crops, garden
+plants, forest trees, etc., to the attacks of preventible diseases.
+Moreover, only an intelligent appreciation of the causes of a disease
+will enable the cultivator to take steps to mitigate their effects when
+once the damage has begun its course. Every cultivator learns by
+experience or by precept that there are some things he must avoid in
+dealing with certain plants, or otherwise they will not succeed; in
+other words they will succumb to diseased conditions and die. It is
+partly owing to the want of systematisation of this knowledge, and its
+extension in other directions, that such extraordinary blunders are made
+in ignorant practice, and trees for instance are planted in low-lying
+frost beds which would succeed in slightly higher situations, or seeds
+subject to damping-off are sown in beds rife with the spores of
+_Peronospora_ or _Pythium_, and so forth.
+
+Many diseases, however, are not preventible in the present state of our
+knowledge, or prevailing conditions are such that the risk must be run
+of endemic diseases gradually becoming epidemic, and thus the natural
+desire for some means of checking the ravages of some pest or another
+has led to innumerable trials to minimise the effects by prophylactic
+measures. The procedure almost invariably followed where parasites are
+concerned, consists in either dusting the plants with some chemical in
+the form of a powder, or spraying it with a liquid, or occasionally in
+enveloping the plant in some gas, in each case poisonous to the insect-
+or fungus-pest. The principal rules to be observed are: (1) the poison
+employed must be sufficiently strong or concentrated to kill the
+parasite, but not sufficiently powerful to injure the host; (2) it must
+be applied at the right period, as suggested by a knowledge of the
+life-history of the fungus or insect in question.
+
+Obviously it is of no use to apply such topical remedies to a parasite
+while it is spending the greater part of its life inside the tissues of
+the host. Further, questions of expense of the materials employed and of
+the labour of applying them help to limit the adoption of such measures.
+
+Among the various kinds of powders employed, finely divided sulphur, or
+a mixture of sulphur and lime, have been used with success in some
+cases--_e.g._ against Hop mildew and other epiphytic Erysipheae, and
+against red spider, aphides, etc., the gaseous sulphur dioxide evolved
+being the efficacious agent. In other cases pyrethrum or tobacco powder,
+wood ashes, etc., have been employed against insects. Such powders are
+applied by hand or by means of bellows, and are very easily manipulated
+in most cases, though, like all such applications, the dangers of
+concentration at particular spots owing to uneven distribution, or of
+dilution and washing off by rain, have to be incurred.
+
+Far more numerous are the various liquids which have been employed for
+washing, spraying, or steeping the affected parts of diseased plants.
+Water alone, or aqueous decoctions or emulsions of various
+kinds--_e.g._, quassia, tobacco, soap, or aloes, have been widely
+employed against insects such as green fly, red spider, etc. In
+greenhouses, where the leaves can be washed by hand or thoroughly
+syringed, and the concentration and time of action thoroughly
+controlled, such liquids are often serviceable, but great practical
+difficulties are apt to interfere with their use in the open field.
+
+The principal liquids employed against fungi have been copper sulphate
+and other metallic compounds (Bordeaux mixture, Eau Celeste, etc.),
+various compounds of arsenic (_e.g._ "Paris green"), potassium sulphite,
+permanganate, etc., and emulsions of carbolic acid, petroleum, and such
+like antiseptics, for the exact composition of which the special
+treatises must be consulted. Some of these, especially Bordeaux mixture,
+have been experimented with on a very large scale, especially in
+America, and various forms of spraying machines have been introduced for
+dealing with large areas.
+
+It is clear that these spraying operations are more particularly adapted
+to field crops such as Turnips, Hops, Vines, Potatoes, and to garden and
+greenhouse plants than to woods and plantations; as a rule they cannot
+be applied to forest trees--though they have been used in orchards--or
+to roots, seeds, and other parts in the soil, and many special forms of
+treatment have been devised for particular cases of these kinds.
+
+One of the oldest of these is the steeping of grain in solutions of
+copper, or in hot water, just before sowing, and the practical
+eradication of Bunt and, partially, of Smut is due to this practice,
+which has lately been adapted to potatoes, the principle being that the
+parasitic germs shall be killed while still adhering to the outside of
+the seeds, tubers, etc., before germination. "Finger and Toe" due to
+_Plasmodiophora_ has been successfully dealt with by the application of
+lime, but we do not know whether the effect is owing to indirect actions
+in the soil, to direct actions on the plasmodia, or to the increased
+production of root-hairs induced by liming.
+
+_Phylloxera_ has been treated by plunging into the soil near the roots
+small blocks of some slowly-soluble medium, such as gelatine,
+impregnated with carbon-bisulphide, the volatile fumes of which kill the
+insect, and even more drastic remedies have been tried along similar
+lines. In America orchard trees infested with insects or fungi have been
+covered one by one with light tents, and the vapours of prussic acid,
+burning sulphur, and other poisons allowed to act inside the tent. In
+all such cases it must be remembered that uncontrolled ignorance of the
+properties of poisons on the part of the operator may lead to disaster,
+and the same applies to the much easier treatment of greenhouses, and
+cases where poisoned food is laid about for insects or vermin.
+
+Attempts, not altogether unsuccessful on the small scale, have also been
+made to introduce epidemic diseases among rats, mice, and locusts and
+other insects, by inoculating some of them with parasitic bacteria or
+fungi (_Empusa_, _Isaria_, etc.), and then allowing them to run loose in
+the hope that they will communicate the disease to their fellows. The
+introduction of lady-birds into districts infested with Coccideae and
+similar pests which they devour, is also recorded as successful, as also
+the importation of birds into forests plagued with caterpillars. It must
+not be over-looked, however, that man's interference with the existing
+balance of events in the natural struggle for existence is occasionally
+disastrous, as witness the results of importing rabbits into Australia,
+goats into the Canary Islands, and sparrows in various countries.
+Darwin's well-known illustration of the inter-relations between clover,
+bees, field-mice, and cats (_Orig. of Species_, 6th ed., 1876, p. 57),
+which shows the astounding probability of the dependence of such a plant
+on the number of cats in the neighbourhood, well illustrates the
+situation.
+
+Mere mention must be made of other special treatments.
+
+Caterpillars and larger animals are often picked by hand or their
+natural enemies--_e.g._ birds, are encouraged in forests. Locusts are
+caught in nets, trenches, etc., and buried. Woodlice, slugs, etc., are
+often trapped by laying attractive food such as carrots and overhauling
+the traps daily: similarly with earwigs. Rings of tar round tree stems
+have been employed to prevent caterpillars creeping up them.
+
+American Blight has been treated by rapidly flaming the stems. Syringing
+with hot water has also been employed for vines affected with mildew,
+mealy bug, etc.
+
+With regard to the alleged immunity from devouring insects of certain
+poisonous plants, it has been pointed out that _Pangium edule_, which
+abounds in prussic acid, is infested with a grub, and ivy is
+occasionally eaten by caterpillars.
+
+Another point as regards insect pests is the well-known destructive
+effect of a cold, wet spring on the young larvae. The use of cyanide of
+potassium requires especial care, but has been described as easily
+carried out with success in greenhouses.
+
+It seems probable that lady-birds, the larvae of wasp-flies and
+lace-wings, and ichneumon-flies as well as wrens can keep down aphides.
+
+For an example of the treatment of a complex case of "chlorosis" with
+mineral manures, the reader may consult the _Gardeners' Chronicle_, 1899
+(July), p. 405. Many similar cases have been recorded, but it should not
+be overlooked that very complex inter-relations are here involved.
+
+Charlock has been successfully dealt with by applying 5 lbs. of copper
+sulphate in 25 gallons of water to each acre of land while the weeds are
+young.
+
+In all these cases the guiding idea is derived from accurate knowledge
+of the habits of the insect, fungus, or pest concerned, and obviously
+the procedure must be timed accordingly. It is a particular case of the
+struggle for existence, where man steps in as a third and (so to speak)
+unexpected living agent.
+
+It is clear from our study of the factors of an epidemic that one of the
+primary conditions which favour the spread of any disease is provided
+by growing any crop continuously in "pure culture" over large areas.
+This is sufficiently exemplified by the disastrous spread of such
+diseases as Wheat-rust, Larch-disease, Potato-disease, Phylloxera,
+Hop-disease, Sugar-cane disease, Coffee-leaf disease, and numerous other
+maladies which have now become historic in agricultural, planting, and
+forest annals. Providing the favourite food-supply in large quantities
+is not the only factor of an epidemic, but it is a most important one in
+that it not only facilitates the growth and reproduction of a pest, but
+affords it every opportunity of spreading rapidly and widely.
+
+Moreover, Nature herself shows us that such pests are kept in check in
+her domain by the struggle for existence entailed by innumerable
+barriers and competitors. As matter of experience also it is found that
+rotation of crops, planting forests of mixed species, and breaking up
+large areas of cultivation into plantations, fields, etc., of different
+species afford natural and often efficient checks to the ravages of
+fungus and insect pests. Over and over again it has been found that a
+fungus or an insect which is merely endemic so long as it is isolated in
+the forest, where its host is separated from other plants of the same
+species by other plants which it cannot attack, becomes epidemic when
+let loose on the continuous acres so beloved of the planter. And the
+same reasoning applies to the success of such pests on open areas from
+which the birds or other enemies of the pest have been driven. True, we
+cannot always trace the tangled skein of inter-relationships between one
+organism and another in Nature: the recognition of the principle of
+natural selection and the struggle for existence is too recent, and our
+studies of natural history as yet too imperfect to lay all the factors
+clear, but no observant and thoughtful man can avoid the truth of the
+general principle here laid down. The history of all great planting
+enterprises teaches us that he who undertakes to cultivate any plant
+continuously in open culture over large areas must run the risk of
+epidemics.
+
+
+NOTES TO CHAPTER XVII.
+
+     The principal literature, now very voluminous, on this subject
+     is contained in the publications of the U.S. Department of
+     Agriculture from 1890 onwards. See especially _Bulletins_,
+     Nos. 3, 6, and 9; _Farmers' Bulletin_, No. 91, 1899; and _The
+     Journal of Mycology_ during the same period. See also Lodeman,
+     _The Spraying of Plants_, London, 1896. A summary of the
+     principal processes will be found in Massee, _Text-Book of
+     Plant Diseases_, pp. 31-47.
+
+     With regard to the history of the subject, which still needs
+     writing, the reader should not overlook Roberts, "On the
+     Therapeutical Action of Sulphur," _St. George's Hospital
+     Reports_, date unknown, but subsequent to the following:
+     Berkeley, _Introduction to Cryptogamic Botany_, 1857, p. 277,
+     with references. These are, I believe, with the references to
+     steeping of wheat in De Bary, _Unters. ueber d. Brandpilze_,
+     Berlin, 1853, among the first attempts to utilise such
+     remedies.
+
+     Further facts will be found in the pages of the _Gardeners'
+     Chronicle_, especially since 1890, and in _Zeitsch. f.
+     Pflanzen-krankheiten_ since 1891.
+
+
+
+
+CHAPTER XVIII.
+
+VARIATION AND DISEASE.
+
+     _Predisposition and immunity--Pathological conditions
+     vary--Hardy varieties--"Disease-proof" varieties--Disease
+     dodging--Thick skins--Indian wheats, etc. Cell-contents
+     vary--Citrus, Cinchona, Almonds, etc. Double ideals in
+     selection--Cultivation of pest and host-plant--Variations of
+     fungi--Bacteria--Specialised races--Difficulties--Experiment
+     only will solve the problems._
+
+
+The numerous and often expensive failures in the application of any
+prophylactic treatment, have proved an acute stimulus to the research
+for other ways of combating the ravages of plant diseases. It is a
+matter of every-day experience that particular varieties of cultivated
+plants may suffer less from a given disease than others in the same
+district; also that one and the same species may suffer badly in one
+country and not in another--_e.g._ the Larch in the lowlands of Europe
+as contrasted with the same tree in its Alpine home, and the various
+species of American Vines in Europe.
+
+These matters, in the hands of astute observers, are turning the
+attention of cultivators and experts to new aspects of the question of
+plant diseases, namely, the possible existence of immunity, and the
+breeding of disease-proof varieties; and the existence on the part of
+the host plant of predispositions to disease which may depend on some
+factors in the plant or in the environment over which it is possible to
+exercise control, or which, if known, can be avoided.
+
+The matter is complicated by the recent demonstration of the fact that
+parasites also vary and can adapt themselves to altered conditions, as
+is shown by the history of the coffee-leaf disease (_Hemileia_) in
+Ceylon, and by Eriksson's results with Wheat-rusts (_Puccinia_) and
+various experiments with _Coleosporium_ and other Uredineae; but there
+are good grounds for concluding that hybridisation, grafting, and
+selection of varieties may do much towards the establishment of races
+which will resist particular diseases, as shown by Millardet's
+experiments with Vines, and the results obtained by Cobb and others with
+Wheat.
+
+The great difficulty with so-called "disease-proof varieties" is to test
+them under similar conditions in different countries, and for a
+sufficient period of time. A particular race of Wheat may behave very
+differently in Norfolk, Devonshire, and Northumberland, and the recent
+introduction of the purely experimental method in this connection is a
+marked advance. However rough the experiments may of necessity have to
+be, it is only by such means that data can be gradually accumulated.
+
+Having now obtained some insight into the factors concerned in disease,
+let us enquire further into the bearing of variation on these. It is
+evident that pathological conditions may vary; indeed they are
+themselves symptoms of variation, as we have seen. The history of all
+our cultivated plants shows abundantly that many of the variations
+obtained by breeding in our gardens, orchards, fields, etc., involve
+differences of response on the part of the plant to the very agencies
+which induce disease. Every year the florists' catalogues offer new
+"hardy" varieties; but a hardy variety is simply, for our present
+purpose, one which succumbs less readily to frost, cutting winds, cold
+damp weather, and so forth. If anyone doubts that hardy varieties have
+been gradually bred by selection, I refer him to the evidence collected
+by De Candolle, Darwin, Wallace, Bailey and others. When we come to
+enquire into the causes of "hardiness," however, difficulties at once
+beset us. The adaptation may express itself in a difference in the time
+of flowering or leafing, the exigencies of the season being "dodged," as
+it were, in a manner which was impossible with the original stock, as
+appears to have occurred with Peaches in America; or it may be expressed
+in deeper rooting, as is said to be the case in some Apples, or in the
+acquirement of a more deciduous habit, or in actually increased
+resistance to low temperatures. In such cases we cannot trace what
+alterations have occurred in the cells and tissues concerned, though we
+may be sure that some changes do occur.
+
+No experienced cultivator doubts that some varieties of Potato, Wheat,
+Vine, Chrysanthemum, etc., suffer more from epidemic diseases than
+others, and our yearly catalogues furnish us with plenty of promises of
+"disease-proof" varieties. Here also we may imagine several ways in
+which a particular variety may resist or escape the epidemic attacks of
+fungi which in the same neighbourhood decimate other varieties. If we
+could breed a variety of the Larch which opened its buds later than the
+ordinary form in our northern plains, the probability of its escaping
+the Larch-disease would be increased in proportion to the shortness of
+the period of tender foliation described on p. 153. It has been claimed
+for certain varieties of Wheat that increased thickness of the cuticle
+and fewer stomata per square unit of surface have diminished the risk of
+infection by Rust fungi, and for certain varieties of Potato, that the
+thicker periderm of the tuber protects them against fungi in the soil.
+That certain thick-skinned Apples, Tomatoes, and Plums pack and store
+better than those with a more tender epidermis seems proved--that is to
+say, they suffer less from fungi which gain access through bruises and
+other wounds; but it cannot be said that any convincing proof is yet to
+hand explaining in detail why some races of wheat resist Rust, or why
+the roots of American Vines suffer less from _Phylloxera_ than others.
+
+One of the most extraordinary cases known to me in this connection is
+the unconscious selection on the part of native Indian cultivators,
+perfectly ignorant of the principles involved, of spring and autumn
+forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops.
+"It has been estimated that Bengal alone possesses as many as 10,000
+recognisable forms of rice." Now there is not the slightest ground for
+doubt that these have been unconsciously bred from the semi-aquatic
+native species during the many centuries of Indian agriculture, and
+nevertheless they have, among other peculiar races, some hill-breeds
+which they cultivate on dry soils and without direct inundation. That is
+to say, they possess tropical and temperate races differing far more
+than our spring and summer wheats.
+
+Something has been gained, then, if we can show that there is nothing
+absurd or hopeless in the search for disease-proof or resistant races,
+and I think this can be done. We must not forget that the ideal usually
+set before himself by a breeder of plants has hitherto been almost
+exclusively some standard of size, form, colouring, and so forth, of the
+flower, or of taste and texture of the fruit, tuber, etc., though
+experiments with _Cinchona_, with brewery yeasts, and other plants
+remind us that variations in other directions have been attended to
+also.
+
+Now it is obvious that in breeding sour limes and sweet oranges the
+cultivator is selecting, and intensifying by selection, very different
+metabolic processes in the cell: he can test the results of these, and
+so the selection proceeds.
+
+The question is, Could he select at the same time those variations in
+cell activity which express themselves in properties of the flower,
+fruit, foliage, etc., he desires, as well as such variations as aid the
+cells in repelling fungi, insects, or exigencies of the non-living
+environment?
+
+That more or less disease-proof varieties could be selected if that
+object alone were kept in view can hardly be doubted; plenty of examples
+exist already which show that the necessary variations to work upon
+exist in just those secretions of protoplasm, etc., which we have seen
+are concerned in repelling or attracting parasites.
+
+The Sweet Almond has lost the power of producing amygdalin and prussic
+acid in its cells; Cinchona plants vary immensely in the quantity of
+quinine formed, and in European hot-houses may even form none at all;
+some varieties of Maize have sugar and dextrine instead of starch in
+their endosperms, or coloured instead of clear sap in the aleurone
+layer, and recent researches prove that they can transmit these
+peculiarities to hybrid offspring; non-poisonous bacteria have
+frequently been got from poisonous species simply by cultivation under
+special conditions, and pigmented forms can be bred into non-pigmented
+races.
+
+But we see that the difficulty of selection is increased in the case
+postulated above, because two ideals are to be worked up to, and they
+may conceivably be incompatible. Not necessarily so, however, for
+breeders have solved such problems before in obtaining early _and_ heavy
+cropping races of potatoes, wheat, etc., sweet _and_ large grapes,
+strawberries, etc., hardy _and_ brilliant flowers, and so forth.
+
+There is, however, another aspect of this question of variability in
+organisms in this connection to be considered. Ever since cultivation
+began man has probably been cultivating not only the crops he desires,
+but also the pests which infest them, and if variation of his chosen
+plants occurs--and no one will deny that--surely variation of the fungi
+and insects which live on them also takes place. That this is so can be
+demonstrated, though, since it is not part of my theme to go into the
+question of peculiarities of species and races of parasites, the subject
+must here be passed over with a few remarks only.
+
+Recent researches have shown not only that fungi vary immensely in form
+and morphological characters according to the amount and kind of
+food-materials put at their disposal, thus bringing the whole question
+of polymorphism into the domain of experimental physiology, but that
+their capacities for infection, spore formation, etc., are also capable
+of variation and are dependent on the quality and quantity of food
+supplies, water, as well as on the temperature, illumination, and other
+factors of the environment. This is true of parasites as well as of
+saprophytes. _Botrytis_ forms conidia only in darkness and in moist air.
+Klebahn found that a _Puccinia_ growing on _Digraphis_ infected
+_Polygonatum_ readily and completely, _Convallaria_ imperfectly, whereas
+if sown on _Majanthemum_ it only just infected the plant and then
+remained sterile, while it refused to infect _Paris_ at all. Magnus has
+shown that _Peronospora parasitica_ can only infect meristematic
+tissues, and that when it co-exists with _Cystopus_ on _Capsella_, as is
+usually the case, it enters the latter plant by infecting the gall-like
+pustules of hypertrophied tissue induced by that parasite. Numerous
+parasitic fungi can only penetrate particular parts of plants. For
+instance, the _Ustilago_ of wheat can only infect the young seedling,
+and grows for weeks as a barren mycelium, only becoming a dominant
+fungus in the endosperm. Numerous other examples could be given, but
+these suffice to show some of the ways in which the nature of the food
+substratum supplied by the host affects the fungus. It is obvious that
+if the nature of this food changes, the fungus is also affected, and no
+doubt this is the principal reason why Rust-fungi, for instance, vary so
+much in their vigour and reproductive power on different wheats and
+grasses, though the other factors of the environment must also be of
+influence on them as well as on the hosts.
+
+But--and this is the second point--modern research is also showing that
+the various species of Rust-fungi have split up into different varieties
+or specialised races, according to the particular host plants they
+inhabit. For instance there are special varieties or races of the
+particular species known as _Puccinia graminis_, the wheat rust, each of
+which grows well on various kinds of grain and grasses but refuses to
+infect others. Thus, the variety which infects Wheat refuses to infect
+Barley or Oats, while that variety which grows on Rye will not take on
+Wheat and so forth. Now it is important to notice that these specialised
+races are indistinguishable one from another by their visible
+microscopic characters: they are all botanically of the species
+_Puccinia graminis_ which forms its aecida on the Barberry. We must
+therefore conclude that we have here the same phenomenon as that met
+with in culture-races of bacteria which, having been fed for several
+generations on media rich in proteids, refuse to grow on media rich in
+carbohydrates, or when attenuated races are developed by culture under
+special conditions.
+
+Now since such physiological races as I have described are by no means
+confined to _Puccinia_ but are also known in _Melampsora_,
+_Gymnosporangium_ and other fungi, we must conclude from this and from
+what we know of variation in plants and animals generally, that
+variation and adaptation are common among parasites, insects as well as
+fungi.
+
+These considerations will serve to show moreover that the question of
+breeding disease-proof varieties of our cultivated plants is complicated
+by the danger of our breeding at the same time adapted races of their
+pests. It appears at first sight extremely improbable that we should
+escape the danger by breeding from those specimens of our plants which
+have best survived a fungus epidemic. Still, it must not be forgotten
+that "hardy varieties," and races adapted to other exigencies of the
+non-living environment, have been bred by selection--and nevertheless
+this variable non-living environment is always with us. The matter is
+therefore simply and solely one of experiment, and the retort that a
+disease-resisting variety of any particular plant has not yet been
+raised is no more valid than the objection that a true blue primrose has
+not yet been obtained: whether the same remark can be made with regard
+to any hope of a _disease-proof_ plant may be another matter, but in any
+case it must be made more cautiously in the light of our present
+experience.
+
+
+NOTES TO CHAPTER XVIII.
+
+     The reader will find more on this subject in Bailey's
+     _Survival of the Unlike_ and the literature quoted in the
+     notes to Chapter VIII.
+
+     For varieties of Indian Wheats, etc., see Watt, _Agricultural
+     Ledger_, Calcutta, 1895.
+
+     For a discussion on so-called "Disease-proof Wheats" consult
+     Eriksson & Henning, _Die Getreideroste_.
+
+     Magnus' paper is in the _Berichte der Deutschen bot.
+     Gesellsch._, 1894, p. 39.
+
+     Concerning physiological races and adapted varieties of
+     _Puccinia_, etc., see Eriksson, "A General View of the
+     Principal Results of Swedish Research into Grain Rust,"
+     _Botanical Gazette_, vol. 25, 1898, p. 26.
+
+     For an account of Wheat-rust see Marshall Ward, "Illustrations
+     of the Structure and Life-history of _Puccinia graminis_,
+     etc.," _Ann. of Bot._, 1888, Vol. II., p. 215.
+
+
+
+
+CHAPTER XIX.
+
+SYMPTOMS OF DISEASE.
+
+     _Discolorations--Pallor--Etiolation--Laying of Wheat--
+     Chlorosis--Yellowing--Albinism--Variegation--Uprooting,
+     Exposure and Wilting of seedlings._
+
+
+Everybody knows in a general way when the geraniums in the window pots
+are drooping from want of water, or when the young Wheat is sickly, or
+the Pear-trees "blighted," and we have now to see how far we can
+systematise the knowledge that has been gained in course of time
+regarding the signs which sick plants exhibit.
+
+_Pallor._--Under this heading, which includes all cases where the normal
+healthy green colour is replaced by a general sickly yellow or pale hue,
+ultimately resulting in death of the parts if not arrested, we have
+several totally distinct diseases of the chlorophyll apparatus, each
+recognised by the co-existence of other subordinate symptoms. The
+principal varieties of pallor usually met with are the following:
+
+_Etiolation_ is due to insufficient intensity of light, the pale sickly
+yellow organs being unusually watery and deficient in vascular tissue,
+the internodes abnormally long and thin, and the leaves generally
+reduced in size, or, in some plants also "drawn."
+
+Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of
+etiolation purposely induced. The want of light causes the true
+chlorophyll colouring matter to remain in abeyance, and consequently the
+plant as a whole suffers from carbohydrate starvation.
+
+_Laying_ of Wheat and other cereals is a particular case of etiolation.
+The seeds having been sown too thickly, the bases of the haulms, owing
+to the etiolation and consequent lack of carbohydrates, suffer from want
+of stiffening tissues, and the top-heavy plants fall over.
+
+_False etiolation_ depends on a similar abeyance of the chlorophyll, but
+in this case due to too low a temperature. It is often seen in Wheat and
+other monocotyledons when the young leaves unfold in cold weather in
+spring. The symptoms of "drawing" and tenderness are however absent.
+
+Pallor due to too intense illumination must be kept sharply distinct
+from etiolation, the pale green or yellow hue being here due to the
+destruction of the chlorophyll by insolation, and the accessory symptoms
+of "drawing" are wanting.
+
+_Chlorosis_ is a form of pallor where the chlorophyll grains themselves
+are fully developed, but their green pigment remains in abeyance owing
+to a deficiency of iron in the soil, and can often be cured by adding
+traces of a ferrous salt. The distinction between _Icterus_, where the
+organs are only yellow, and _Chlorosis_ proper, where they are nearly
+white cannot always be maintained. In the typical case only those organs
+whose cells are still young can become green on adding iron.
+
+_Yellowing_ or _False Chlorosis_ may be experimentally induced by too
+much carbon-dioxide in the atmosphere. It also often ensues when the
+roots of plants in the open are waterlogged, owing to the stagnant water
+not only driving air from the root-hairs but accumulating dissolved
+substances which poison the plant. Trees frequently thus suffer from
+"wet feet" when their roots have penetrated down to a sodden impervious
+subsoil.
+
+_Yellowing_ accompanied by _Wilting_ is a predominant symptom in most
+cases where transpiration is more active than root-absorption beyond a
+certain limit, as is well known in cases of prolonged drought. It may
+also be caused in evergreens by the foliage transpiring actively in
+bright January weather, for instance, while the ground is frozen and the
+chilled root-hairs cannot absorb.
+
+In other cases similar appearances are traceable to insects devouring
+the roots, _e.g._ wireworms, and the malady is sometimes enhanced by
+their accumulations so fouling the wet soil that the roots die off,
+owing to want of oxygen and to the excess of carbon-dioxide and
+poisonous matters.
+
+Yellowing may also result from the presence of poisonous or acid gases
+in the atmosphere or soil, such as chlorine, hydrochloric acid,
+sulphurous acid, etc., in the neighbourhood of chemical works, or from
+the escape of coal-gas in streets, etc., points of importance in
+connection with the use of fungicides and insecticides.
+
+Yellowness is the prevailing symptom in many cases of fungus attack of
+the roots or collar of the plant, the resulting stoppage of
+transpiration being also sometimes supplemented by rotting of the roots,
+and the consequent deprival of oxygen and accumulation of foul gases. In
+other cases Fungi, and even Bacteria, have been found to have made their
+way into the principal vessels, the lumina of which they stop up, thus
+reducing the transpiration current.
+
+Certain insects may also induce a general yellowing and wilting of
+plants by entering or destroying the tissues concerned in the
+transpiration--_e.g._ _Oniscus_, the Frit Fly, and _Cecidomya_, the
+Hessian Fly, which attack young winter wheat within the sheaths and
+cause the plants to turn yellow and wilt.
+
+_Albinism_ and _Variegation_ are apparently due to causes totally
+different from any yet mentioned. Church's analyses have shown that
+albino leaves contain more water and less organic matter than green ones
+of the same plants, but not necessarily less ash constituents. The
+composition of the ash points to there being more potash and less lime
+in the white organs than in the green ones, and, speaking generally, the
+former are related to the latter much as young leaves are related to
+mature ones.
+
+The whole matter is complicated by the behaviour of certain _variegated_
+plants--_e.g._ Ribbon grass, _Calla_, _Abutilon_, which are usually
+regarded as partial albinos.
+
+Meyen showed long ago that such variegated plants, if grafted on green
+ones, may induce the development of variegated leaves on both scion and
+stock, and Morren and others have not only confirmed this but have also
+shown that variegation may be inherited through the seed. Nevertheless
+some care has to be taken with many of these variegations lest rich
+soil, bright light, and other favourable treatment favour the
+restitution of the green colour. These facts may be interpreted in
+various ways. Some disturbance of physiological functions of the roots,
+due to unfavourable conditions of soil, may be the cause; but Beijerinck
+has lately published some results which show that some of these albino
+diseases can be induced by inoculating normal plants with the juice of
+spotted ones even though such juice has been filtered through porcelain,
+and concludes that a "_contagium fluidum vivum_" of the nature of a
+transmissible enzyme is the agent which disturbs the physiology of the
+infected cells.
+
+Koning, while confirming these results in the main, refers them to a
+micro-organism so small that it traverses the porcelain filter.
+
+_Upheaval of seedlings._--This is a common form of injury, resulting in
+death by drought and exposure, especially in seedling pines, wheat,
+etc., in soils exposed to alternate freezing and thawing during spring
+when there is no snow to protect the plants. The soil freezes during the
+night, and during the thaw next day water accumulates just below the
+surface. The freezing is then repeated, and, partly owing to the
+expansion of the forming ice and partly to the mechanical effect of the
+ice-crystals in the interstices, the surface of the soil is lifted and
+draws the roots with it. During the succeeding thaw the soil particles
+fall away from the lifted root-fibres, and frequent repetition of these
+processes results in such complete exposure of the roots to the full sun
+that the plantlet falls over and wilts.
+
+_Exposure of roots_ is also sometimes effected by winds displacing sandy
+soils liable to shifting in dry weather, and the resulting wilting of
+the plants thus exposed at their roots may be supplemented by damage due
+to the repeated impact of the wind-driven sharp grains of sand, which
+act like a sand-blast and erode the tissues.
+
+In many of the cases given above the principal result is the weakening
+or destruction of the chlorophyll action. This means a loss of
+carbohydrates--sugars, starches, etc.--and in so far a starvation of the
+plant. The injurious effects are quantitative and cumulative: if large
+areas of foliage are concerned, or if the effect lasts a long time, the
+plant suffers from loss of food, and may die. In those cases where the
+effect is due to the cutting off of supplies at the roots, and where the
+yellowing is a secondary symptom, the disease is more general in
+character, and recovery is often impossible, because the loss of water
+cannot be compensated, and the results may be further complicated by the
+gradual penetration of poisonous matter into the cells. It is frequently
+necessary, though sometimes very difficult, to decide which is the
+primary and which secondary (or tertiary, etc.) symptoms in the order of
+their importance, and the diagnosis may be complicated by a number of
+accessory factors which it is impossible to treat generally.
+
+
+NOTES TO CHAPTER XIX.
+
+     The principal cases here described are dealt with in works on
+     plant physiology, and in the works of Sorauer and Frank
+     already referred to.
+
+     As regards damage due to uprooting of seedlings by frost, see
+     Fisher, "Forest Protection" (Engl. ed. of Hess' _Forstchutz_),
+     in Schlich's _Manual of Forestry_, Vol. IV., 1895, pp.
+     439-442.
+
+     On Albinism, see Church, "A Chemical Study of Vegetable
+     Albinism," _Journ. Chem. Soc._, 1879, 1880, 1886.
+
+     Beijerinck's results are contained in his paper, "Ueber ein
+     Contagium vivum fluidum," etc. (with English abstract), in
+     _Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam_, 1898.
+     Koning's paper is in _Zeitschr. f. Pflanzenkrank._, Vol. IX.,
+     1899, p. 65. See also _Nature_, Oct. 11, 1900, p. 576.
+
+
+
+
+CHAPTER XX.
+
+SYMPTOMS OF DISEASE (_Continued_).
+
+     _Spotted leaves--The colours of spots--White, yellow, brown,
+     and black spots on leaves--Parti-coloured spots--The browning,
+     etc., of leaves._
+
+
+_Discoloured spots_ or patches on the herbaceous parts of plants,
+especially leaves, furnish the prominent symptoms in a large class of
+diseases, due to many different causes, and although we cannot maintain
+this group of symptoms sharply apart from the last, as seen from the
+considerations on _albinism_, it is often well marked and of great
+diagnostic value. By far the greater number of spot-diseases are due to
+fungi, but this is by no means always the case. The most generally
+useful method of subdividing the classes, though artificial like all
+such classifications, will be according to the colour of the spots or
+flecks, which, moreover, are usually found on the leaves. It is
+necessary to note, however, that various conditions may modify the
+colour of spots on leaves. Many fungi, for instance, induce different
+coloured spots according to the age of the leaf or other organ attacked,
+or according to the species of host, the weather, etc. Moreover the
+spots due to these parasites are frequently yellow when young and some
+other colour, especially brown or black, when older.
+
+_Scale_ is the name given to the characteristic shield-like insects
+(_Mytilaspis_, _Aspidiotus_, etc.) which attach themselves to branches
+of Apples, Pears, Oranges, Camellias, and numerous other plants, and
+suck the juices. It is the female insect which has the body broadened
+out into the "scale," under which the young are brought up. Enormous
+damage has been done by some forms--_e.g._ the San Jose scale in the
+United States.
+
+The superficial resemblances of the patches of eggs of some Lepidoptera
+to Aecidia and other fungi may be noted in passing--_e.g._ _Bombyx
+neustria_ on Apple twigs, _Aporia Crataegi_.
+
+_White_ or _greyish spots_ are the common symptom marking the presence
+of many Peronosporeae and Erysipheae in or on leaves, _e.g._
+_Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and
+_Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma
+Ranunculi_, etc.
+
+White spots are also caused by insects such as _Tetranychus_ (red
+spider) on Clover and other plants.
+
+_Yellow_, or _Orange-coloured Spots_. In cases where these occur on
+leaves, and in the case of grasses, etc., on the leaf sheaths as well,
+they commonly indicate the presence of Uredineae, and sections under
+the microscope will show the mycelium in the tissues beneath. Species of
+_Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery
+with spores; _Aecidia_ show the characteristic "cluster cups," and so
+forth. These spots are often slightly pustular, and in some cases
+markedly so.
+
+Other fungi also induce yellow spots on leaves--_e.g._ _Phyllosticta_ on
+Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves,
+_Synchytrium Succisae_ on _Centaurea_, etc.
+
+Yellow spots are also a frequent symptom of the presence of Aphides, of
+Red Spider, etc. Thus the minute golden yellow spots sometimes crowded
+on Oak leaves are due to _Phylloxera_ punctures.
+
+Yellow patches are formed on the large leaves of _Arisarum_ by a species
+of parasitic Alga, _Phyllosiphon_, which lives in the mesophyll. Many
+tropical leaves are spotted yellow by epiphytic Algae--_e.g._
+_Cephaleuros_.
+
+It must be noticed that many fungi produce yellow spots or flecks in the
+earlier stages, which turn brown or black as the fructifications appear,
+_e.g._ _Dilophia graminis_, _Rhytisma acerinum_.
+
+The yellow-spotted leaves of _Farfugium grande_ (_Senecio Kaempferi_)
+are so like those of _Petasites_ attacked with _Aecidium_ in its early
+stages, that an expert might be deceived until the microscopic analysis
+was completed.
+
+_Red spots_, varying from rusty or foxy red to bright crimson, are the
+symptomatic accompaniment of several fungi, the former often
+characterising the teleutospore or aecidium stage of Uredineae--_e.g._
+_Aecidium Grossulariae_--the latter sometimes indicating the presence of
+Chytridiaceae.
+
+Red spots are also caused by _Gloeosporium Fragariae_ on Strawberry
+leaves, _Polystigma rubrum_ on Plums.
+
+Crimson spots on Apple and Pear leaves are also due to _Phytoptus_: they
+turn brown later.
+
+_Brown spots_ or flecks, varying in hue from dull slaty brown to deep
+red browns, are a common symptom of Fungus and Insect diseases, the
+colour often indicating the death of the tissues, rather than any
+special peculiarity of the action of the parasite. Good examples are
+furnished by the Potato-disease, and by _Peronospora viticola_,
+_Sphaerella vitis_ and other disease-fungi of the Grape Vine. The
+teleutospore stage of many Uredineae also occurs in deep brown spots.
+
+Black spots and flecks are exceedingly common symptoms of the presence
+of fungi, _e.g._ _Fusicladium_ on Apples and Pears, and the pycnidial
+and ascus stages of many Ascomycetes--_e.g._ _Phyllachora graminis_. The
+teleutospore stages of species of _Puccinia_, _Phragmidium_, etc., are
+also so deep in colour as to appear almost black.
+
+_Scab_ on Pears is due to the presence of _Fusicladium_, which indurates
+the outer skin of the fruit causing it to crack under pressure from
+within, and to dry up, the deep brown to black patches of fungus
+persisting on the dead surface.
+
+Black spots on grasses and sedges are caused by Ustilagineae, and are
+commonest in the grain, the soot-like powdery spores (Smut) being very
+characteristic. _Ustilago longissima_ induces black streaks on the
+leaves. Many of these fungi cause distortions or pustules on leaves and
+other organs.
+
+Brown and black leaf spots are frequently furnished with concentric
+contours arranged round a paler or other coloured central point--_e.g._
+_Cercospora_ on Beans, _Ascochyta_ on Peas.
+
+Brown spots with bright red margins are formed in young Beans by
+_Gloeosporium_.
+
+Species of _Fumago_, _Herpotrichia_, etc., may cover the entire surface
+of the leaf with sooty patches, or even weave the leaves together as if
+with black spider-webs.
+
+_Mal nero_ of the Vine is a particular case of black spotting and
+streaking of the leaves for which no satisfactory explanation is as yet
+to hand. As with Chestnuts, Walnuts, and other plants containing much
+tannin, the dark spots appear to be due to this substance, but whether
+the predisposing cause is a lack of some ingredients in the soil, or
+some temperature reaction, or fungi at the roots, is as yet unknown. The
+most recent explanation puts the disease down to the action of bacteria,
+but the results obtained by different workers lead to uncertainty.
+
+The "dying back" of leaves, especially of grasses, from the tip, is
+usually accompanied by a succession of colours--yellow, red, brown, to
+black--and is a common symptom of parching from summer drought; and
+spots of similar colours, frequently commencing at the margins of
+leaves, are characteristic symptoms of the injurious action of acid
+gases in the air.
+
+Brown and blackish spots on Pears are caused by a species of _Thrips_.
+
+In many cases the minute spots of Rust-fungi on one and the same leaf
+are bright orange yellow (_uredo_), deep brown, or almost purple-black
+(_teleutospores_), foxy-red brown (older uredospores), or dead slaty
+black where the old teleutospores have died off--_e.g._ _Uromyces Fabae_
+on Beans, _U. Pisi_ on Peas, etc.
+
+_Parti-coloured leaves._--The leaves sometimes start shrivelling with
+red edges, while yellow, red, and finally brown and black blotches
+appear on the lamina, from no known cause--_e.g._ Vines. In other cases
+similar mimicry of the autumnal colouring of leaves results from the
+action of acid gases.
+
+_Burning_ is a common name for all cases where the leaves turn red or
+red-brown in hot, dry weather, and many varieties are distinguished in
+different countries and on different plants, because species react
+dissimilarly. The primary cause is usually want of water--drought.
+
+_Foxy leaves_ are a common sign of drought on hot soils, and the disease
+may usually be recognised by the gradual extension of the drying and
+fox-red colour proceeding from the older to the younger leaves, and from
+base to apex--_e.g._ Hops.
+
+_Coppery leaves._--The leaves of the Hop, etc., may show yellow spots
+and gradually turn red-brown--copper-coloured--as they dry; the damage
+is due to _Tetranychus_, the so-called Red Spider. These cases must of
+course be carefully distinguished from the normal copper-brown of
+certain varieties of Beech, Beet, _Coleus_, etc.
+
+_Silver-leaf._--The leaves of Plum, Apple, and other fruit trees often
+obtain a peculiar silvery appearance in hot summers, the cause of which
+is unknown.
+
+Discolorations in the form of confluent yellow and orange patches, etc.,
+resembling variegations, are not infrequently due to the ravages of Red
+Spider and mites--_e.g._ on Kidney Beans.
+
+_Sun-spots._--Yellow spots, which may turn brown or black according to
+the species of plant affected and the intensity of the action, are often
+caused by the focussing of the solar rays by lens-like thickenings due
+to inequalities in the glass of greenhouses, or by drops of water on
+them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is
+that of a burning glass, and extends throughout the leaf-tissues. Young
+grapes, etc., may also be injured in this way. Water-drops on the glass
+can only act long enough to produce such injuries if the atmosphere is
+saturated. The old idea that a drop on a leaf can thus focus the sun's
+rays into the tissues beneath is not tenable.
+
+Here again we see that the disease-agencies concerned in producing the
+symptoms described in this chapter, agree for the most part in so far
+that the principal effect is generally the disturbance of chlorophyll
+action in the spots or flecks on the leaves, and the rendering useless
+of these areas so far as providing further food-supplies is concerned.
+The effects may be due merely to the shading action of a
+parasite--_e.g._ epiphytic fungi--or to actual destruction of the
+tissues invaded--_e.g._ by endophytic fungi--or the tissues may be
+burnt, poisoned, etc. In so far the results are again quantitative and
+cumulative, and the amount of damage depends on the number and size of
+the spots or other areas affected, and the proportion of foliage
+involved, as well as the length of time the injurious action is at work.
+But, again, it must be remembered that several symptoms may co-exist,
+and matters may be complicated by the spread of the destructive agent,
+or its consequences, to other parts, and in some cases we are quite
+uninformed as to the true nature of the disease.
+
+
+NOTES TO CHAPTER XX.
+
+     Further information regarding these "leaf-diseases" will be
+     found in special works dealing with the fungi and insects
+     which cause them. In addition to works already quoted, the
+     reader may also be referred for Fungi to Massee, _A Textbook
+     of Plant-diseases caused by Cryptogamic Parasites_, London,
+     1899; or Prillieux, _Les Maladies des Plantes Agricoles_,
+     1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional
+     Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol.
+     XIX., 1882, p. 299.
+
+     The question of "Sun-spots" has been dealt with by Joennson in
+     _Zeitschr. f. Pflanzenkrankh._, 1892, p. 358.
+
+
+
+
+CHAPTER XXI.
+
+ARTIFICIAL WOUNDS.
+
+     _The nature of wounds and of healing processes--Knife wounds--
+     Simple cuts--Stripping--Cuttings--Branch-stumps and pruning--
+     Stool-stumps--Ringing--Bruises._
+
+
+_Wounds._--All the parts of plants are exposed to the danger of wounds,
+from mechanical causes such as wind, falling stones or trees, hail,
+etc., or from the bites of animals such as rabbits, worms, and insects,
+and although such injuries are rarely in themselves dangerous, they open
+the way to other agencies--water, fungi, etc., which may work great
+havoc; or the loss of the destroyed or removed tissues is felt in
+diminished nutrition, restriction of the assimilative area, or in some
+other way.
+
+We have seen that living cells die when cut, bruised, or torn; and that
+the cells next below in a layer of active tissue are stimulated by the
+exposure to increased growth and division, and at once produce a layer
+of cork, the impervious walls of which again protect the living cells
+beneath. This is found to occur in all cell-tissues provided the cells
+are still living, and it matters not whether the wound occurs in the
+mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the
+cortex of a root or stem, or in the fleshy parts of a young fruit, the
+normal effect of the wound is in all cases to call forth an elongation
+of the uninjured cells beneath, in a direction at right angles to the
+plane of the injured surface, which cells then divide by successive
+walls across their axis of growth: the layers of cells thus cut off are
+then converted into cork, by the suberisation of their walls. Further
+changes may then go on beneath the protective layer of wound-cork thus
+produced, and these changes vary according to the nature of the cells
+beneath: the cambium forms new wood, the medullary rays similar rays,
+cortex new cortex, and so on.
+
+_Knife-wounds._--Artificial cuts in stems are easily recognised and soon
+heal up unless disturbed. Several cases, differing in complexity, are to
+be distinguished. The simplest is that of a longitudinal, oblique, or
+horizontal short cut in which the point of the knife severs all the
+tissues of the stem down to the wood. The first effect usually observed
+is that the wound gapes, especially if longitudinal, because the cortex,
+tightly stretched on the wood cylinder, contracts elastically. This
+exposes the living cortex, phloem and cambium to the air, and such
+tissues at once behave as already described above: the cells actually
+cut die, those next below grow out under the released pressure, and
+these give rise to cells which become cork. As the growth and
+cell-division continue in the cells below this thin elastic cork-layer,
+they form a soft herbaceous cushion or _callus_ looking like a thickened
+lip to each margin of the cut. Each lip soon meets its opposite
+neighbour, and the wound is closed over, a slight projection with a
+median axial depression alone appearing on the surface. The depression
+contains the trapped-in callus-cork squeezed more and more in the plane
+of the cut as the two lips of callus press one against the other, and
+sections across the stem and perpendicular to the axis of the cut show
+that this thin cork, like a bit of brown paper, alone intervenes between
+the cambium, phloem and cortex respectively of each lip, as each layer
+attempts to bridge over the interval. If the healing proceeds normally,
+these layers, each pressing against the trapped cork-film, and growing
+more and more in thickness, shear the cork-layer and tear its cells
+asunder, and very soon we find odd cells of the cambium of one lip
+meeting cambium cells of the other, phloem meeting phloem, and cortex
+cortex, and the normal thickening of the now fused layers previously
+separated by the knife goes on as if nothing had happened, the only
+external sign of the wound being a slight ridge-like elevation, and,
+internally, traces of the dead cells and cork trapped here and there
+beneath the ridge. When the conjoined cambium resumes the development
+of a continuous layer of xylem and phloem, no further trace of the
+injury is observable, unless a speck of dead cells remains buried
+beneath the new wood, and indicates the line where the knife point
+killed the former cambium and scored the surface of the wood in making
+the wound.
+
+_Stripping._--Now suppose that, instead of a mere slit with the
+knife-point, a strip of bark is removed down to the wood. Exactly the
+same processes of corking and lip-like callus formation at the edges of
+the wound occur, but of course the occlusion of the bared wood-surface
+by the meeting of the lips occupies a longer time. Moreover, the living
+cells of the medullary rays exposed by the wound on the wood-surface
+also grow out under the released pressure, and form protruding callus
+pads on their own account. In course of time the wood is again
+completely covered by the coming together over its face of these various
+strips of callus, but two important points of difference are found, as
+contrasted with the simpler healing of the slit-wound. In the first
+place the exposed wood dries and turns brown, or it may even begin to
+decay if moisture and putrefactive organisms act on it while exposed to
+the air; and, in the second place, the normal annual layer of wood--or
+layers, as the case may be--formed by the cambium only extends over that
+part of the stem where the cambium is still intact, and is entirely
+wanting over the exposed area. Thus, if it takes two years for the
+cambium to extend across the wound, a layer of wood will be formed all
+round the intact part of the stem, from lip to lip of the cut tissues
+during the first year; then a second annual layer outside this will be
+formed during the second year, but extending further over the edges of
+the wound, and nearly complete, because the cambium has now crept
+further across the wounded surface to meet the opposite lip of cambium;
+and during the third year, when the cambium has once more become
+continuous over the face of the wound, the annual wood layer will be
+complete. But, of course, this last layer covers in the edges of the two
+previously developed incomplete wood-layers as well as the exposed and
+brown, dry, or rotten dead face of the wood. It also covers up the
+trapped-in brown cork and any debris that accumulated in the wound, and
+this "blemish," though buried deeper and deeper in the wood during
+succeeding annual deposits of wood-layers, always remains to remind us
+of the existence of the wound, the date of which can be fixed at any
+future time by counting the annual rings developed subsequently to its
+formation. Obviously, also, the deficiency of wood at this place makes
+itself visible on the outside by a depression.
+
+_Cuttings._--When a cutting of _Pelargonium_, Willow, or other plant is
+made, we have a typical knife-wound, the behaviour of which is very
+instructive in illustration of plant-surgery, and may be most easily
+seen by keeping it in damp air instead of plunging it into sand or
+soil.
+
+All the living cells actually cut or bruised turn brown and die as
+before; those beneath--_e.g._ the living pith, medullary rays, cambium,
+phloem, and cortex, grow out under the released pressure and form a
+callus, the outermost layer of which becomes cork, while those below,
+abundantly supplied with food-materials, proceed to spread, as if
+flowing over the surface of the cut wood, and rapidly occlude the wound.
+Meanwhile new roots are formed adventitiously from the cambium just
+above the plane of section, and push out through the cortex into the
+damp air, and if the cutting had been in soil it would now be capable of
+independent existence. It is important to keep cuttings upright, as the
+roots only spring from the lower end. Such cuttings can be obtained not
+only from stems, but also from roots and even leaves.
+
+Callus-formation is not confined to the basal end of a cutting; it has
+nothing to do with position, but is a reaction to the wound stimuli,
+independent of light, gravitation, etc. As time goes on, however, the
+internal organisation of the erect cutting usually reacts on the callus
+at either end, and roots only rise from the lower one, while shoot-buds
+may form in the upper one, though it is possible to bring about the
+formation of buds from the lower end also.
+
+_Branch stumps._--A more complex example is furnished by a branch cut
+off short some distance--say a foot--from the base, where it springs
+from the trunk. As before, the immediate effect of the section is the
+formation of a callus from the cambium, phloem and cortex, which begins
+to rise as a circular occluding rim round the wood. The transpiration
+current in the trunk, however, is not deflected into the 12 inches or so
+of amputated branch, because there are no leaves to draw the water up
+it, and so the stump dries up and the cortex and cambium die back to the
+base, leaving the dead wood covered with shrivelled cortical tissues
+only. This dead stump gradually rots under the action of wet, fungi, and
+bacteria, and since the pith and heart-wood afford a ready passage of
+the rot-organisms and their products into the heart of the trunk, we
+find in a few years a mere stump of touch-wood and decayed bark, which
+falls out at the insertion like a decayed tooth, leaving a rotten hole
+in the side of the trunk.
+
+If, however, instead of allowing the basal part of the amputated branch
+to protrude as a stump, we cut it off close to the stem, and shave the
+section flush with the normal surface of the latter, the callus formed
+by the cambium, etc., rapidly grows over the surface, and soon forms a
+layer of cambium continuous with that of the rest of the stem. The wound
+heals, in fact, much as if it were a strip-wound, and beyond a slight
+prominence for a year or two no signs are visible from the outside after
+the occlusion. Of course these matters depend on the relative thickness
+of branch and stem, and if much wood is exposed the dangers of rot and a
+resulting hollow in the stem are increased. It is interesting to note
+how much thicker the callus lips are at the sides of the wound than
+above and below, owing to differences in the distribution of the
+nutrient materials.
+
+_Stool-stumps._--When a tree is felled, the stump may, if the section is
+close to the ground and kept moist, begin to form a thick rim-like
+callus round the wood, in which adventitious buds soon make their
+appearance, and grow out into so-called _Stool-shoots_. The products of
+assimilation of these, and the stores accumulated in the stump, often
+suffice to feed the callus sufficiently to enable it to grow over and
+completely occlude the wound, if the wood surface is not too large, or
+so long exposed that rotting processes have meanwhile set in.
+
+_Ringing._--If the strip of cortical tissues and cambium is removed all
+round the stem, exposing the wood in a form of a ring, complications may
+ensue owing to the following circumstances. A well-marked callus appears
+at the upper edge of the wound, because, the transpiration current up
+the young wood not being stopped, plenty of water and salts from the
+soil can reach the leaves; but the nutritive materials supplied by the
+latter are accumulated at the upper lip of the wound owing to the
+stoppage there of their descent in the phloem, cortex, etc. No such
+callus-lip appears at the lower margin of the wound owing to want of
+these supplies. Consequently the occlusion and healing of the ring-wound
+only takes place from above downwards, and if the ring of cortical
+tissues removed is a broad one, the healing may be a long process, or
+may even be indefinitely delayed, a thicker and thicker callus
+projecting over from above. For similar reasons no annual wood layers
+are formed below, but only above the wound, and thus the branch or tree
+may die. The latter contingency is the more likely the further up the
+tree the ringing takes place, owing to the risk of drying up which
+threatens the exposed wood, and to the consequent interruption of the
+transpiration current, and the likelihood that lateral shoots below the
+wound may divert the water to their own leaves. If the ringing occurs
+low down on a stem, and the environment remains damp, the upper thick
+callus may put out new roots; the part above the wound then behaves like
+a cutting. If the ringing is done on a young and vigorous branch of an
+old tree, the lower lip may receive supplies from the leaves of branches
+below the wound, or from shoots which spring from adventitious buds
+close to it, and the wound may heal over normally. Such healing may be
+rendered more certain by keeping the wounded surface moist--_e.g._ by
+means of damp moss, and so encouraging the formation of callus-bridges
+from the medullary rays.
+
+If on ringing a tree or a branch the young wood is removed as well as
+the cambium and cortical layers, the death of the parts above the wound
+is almost certain, owing to the stoppage of the transpiration current:
+the exceptions to this rule depend simply on the existence of other
+channels of communication, such as internal phloems, very thick
+sap-wood, and so forth.
+
+_Bruises._--If a branch or woody stem is struck sharply, with a hammer,
+for instance, the bruised cortex, phloem and cambium are killed by the
+blow, and the general effect is as if these tissues had been removed at
+that spot by the knife, but with the following complications. The
+bruised cortical tissues rapidly dry as they perish, and may adhere to
+the wood below. Consequently the still sound parts bordering on the
+wound are not released from pressure, but, on the contrary, have to
+advance towards each other over the surface of the wood under still
+greater pressures, in part due to the tightening of the whole cortex as
+the dead parts dry and contract, and in part due to the above-mentioned
+adherence of the latter to the wood. It results from this that such
+wounds heal very slowly and badly, and when the killed patch at last
+ruptures, wound-fungi, insects, and other injurious agencies may get in
+and do irreparable damage, as has been found to occur in cases where
+such wounds have been made in striking trees to shake down insects,
+fruit, etc.
+
+
+NOTES TO CHAPTER XXI.
+
+     The essential facts regarding wounds and healing by occlusion
+     are given in Marshall Ward, _Timber and some of its Diseases_,
+     1889, chapters viii. and ix., and in Laslett, _Timber and
+     Timber Trees_, 1894, chapters iv. and v. More detailed
+     treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1.
+     cap. 2, where the special literature is collected. The reader
+     may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894,
+     pp. 225-269.
+
+
+
+
+CHAPTER XXII.
+
+NATURAL WOUNDS.
+
+     _Burrows and excavations. Bark-boring--Wood-boring--Wood
+     fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves--
+     Irregular erosions--Shot holes. Frost cracks--Strangulations--
+     Spiral grooving._
+
+
+Natural wounds are produced in a variety of ways during the life of the
+plant, and, generally speaking, are easily healed over by the normal
+process if the area destroyed is not too large, and the parts remaining
+uninjured are sufficiently provided with foliage, or with supplies of
+food-materials stored up in the roots, rhizomes, medullary rays, etc.,
+to feed a vigorous callus.
+
+The nature of such wounds and the mode of healing are explained by what
+we know of artificial wounds, and it only remains to point out that the
+principal danger of ordinary wounds is not so much the direct traumatic
+action, because the simpler organisation of the plant does not involve
+matters connected with shock, loss of blood, etc., as in animals; the
+danger consists, rather, in their affording access to other injurious
+agents, especially fungi, and the treatment of wounds frequently
+resolves itself into cutting or pruning in order to get clean surfaces
+which can heal readily.
+
+Wounds on leaves imply loss of foliar surface--_i.e._ of chlorophyll
+action--and the remarks on page 193 apply.
+
+_Burrows_ may be taken as comprising all kinds of tunnel-like
+excavations in the various organs of plants, including those cases where
+insects burrow into hollow stems of grasses, etc., as indicated by the
+perforations they make in the outer tissues.
+
+_Bark-boring_ is done by many species of beetles, especially
+_Scolytidae_, which excavate characteristically formed branching
+passages tangentially in the inner bark of Conifers and other trees.
+Some of them also bore down to the surface of the sap wood (_e.g._
+_Tomicus bidentatus_) or even burrow right into the latter (_e.g._ _T.
+lineatum_). It commonly happens that the external apertures show up
+clearly, owing to the brown dust and excrement, sometimes accompanied by
+turpentine, which exude from them. Many of these Bark beetles only
+attack trees which are already injured by fire, lightning, etc.;
+possibly they cannot bore through a cortex which swamps them with sap,
+as a vigorous one might do.
+
+_Wood-boring_ is also done by many of the bark-beetles as well as by
+Longicorns, _e.g._ _Saperda_ in Poplars and Willows, the young shoots
+of which often show characteristic swellings with lateral holes
+indicating the points of exit. From the external apertures comminuted
+wood, like saw-dust, is frequently ejected in quantity and betrays the
+presence of the insects. Certain wood-wasps (_Sirex_) and the larvae of
+moths (_Cossus_) also make large perforations in the wood of Willows and
+other trees, often destroying it completely. In the case of these larger
+borers, whose tunnels may be as broad as the little finger, the foul
+smell as well as abundant "saw-dust" betray the evil.
+
+Excavations in wood are by no means caused only by insects: several of
+the larger Hymenomycetes--_Stereum_, _Thelephora_, _Polyporus_,
+etc.--tunnel the timber in characteristic ways and often after a fashion
+very suggestive of insects. They usually obtain access through
+fractures.
+
+_Tunnels_ in leaves are invariably due to the activity of miners
+belonging to the smaller moths and beetles--_e.g._ _Tinea_, _Orchestes_,
+etc.--the larvae of which eat out the mesophyll but leave the covering
+epidermis or cuticle untouched, and since the insect bores forwards
+only, in an irregular track, and leaves its excrement in the winding
+passage, the effect is very characteristic.
+
+Whitish leaf tunnels in Peas are excavated by _Phytomyza_.
+
+Characteristic foxy-red tunnels are mined in the leaves of Apples by
+_Lyonetia_, _Coleophora_, etc.
+
+_Falling of fruit_, of Apples, Plums, Apricots, etc., before they are
+ripe, is frequently due to insects, of which the various species of
+_Grapholitha_ or _Carpocapsa_ are conspicuous: the fallen fruits show a
+small hole leading by a labyrinth of passages to the "core" or "stone,"
+and in which the grub and its excrement are visible. The cutting off of
+the vascular bundles and disturbance of the water supply only partly
+explain the premature fall.
+
+_Pith-flecks_ are minute brown specks or patches found in the
+wood-layers of many trees, and consist of dead parenchymatous
+thick-walled cells, reminding one of the structure of pith. They are
+explained as due to the borings of minute insects, _Diptera_ or Beetles,
+the larvae of which pierce the cortex and phloem and bore their way into
+the cambium. The latter then occludes the tunnels by filling them up
+with cells, and continuing its wood-forming activity gradually buries
+them deeper and deeper in the wood. Such pith-flecks are common in
+Willow, Birch, Alder, _Sorbus_, etc. It is possible that they may be due
+to other causes also in other trees.
+
+_Erosions_ or _irregular wounds_ on leaves are caused by large numbers
+of grubs and caterpillars and other insects, such as earwigs, as well as
+slugs, snails, and other animals; but it must by no means be assumed
+that all marginal leaf wounds, for instance, are caused by animals,
+since many fungi which rot the tissues, as explained below (p. 208),
+also cause such erosions, the putrescent parts falling out--_e.g._ the
+Potato disease.
+
+_Skeleton leaves_ frequently result from the ravages of caterpillars,
+which leave the coarser ribs and veins untouched, but much finer
+skeletons with the minute veins almost intact may be found on plants
+infested with certain insects--_e.g._ _Selandria_ on Cherries.
+Skeletonised patches on Cherry leaves, often pink or brown-pink, are
+eaten out by this grub.
+
+_Shot-holes_ are perforations in leaves presenting the appearance, from
+their more or less rounded shape, of gunshot wounds. They may be due to
+insects which bore through the young leaves while still folded in the
+bud--_e.g._ Willow Beetle--or which gnaw out the tissue--_e.g._ the
+Beech Miner. Similar but usually more torn and irregular holes are eaten
+out by many caterpillars--_e.g._ the Cabbage Moth.
+
+Shot-holes on Peas may be the work of Thrips.
+
+Leaf perforations are commonly caused by severe hail-storms, the
+hail-stones beating right through the thin mesophyll. Certain chemicals
+used for spraying have also been known to cause shot-holes by killing
+the tissue beneath the standing drops.
+
+There is, however, a class of shot-holes in thin leaves which are due to
+the action of minute fungi, the mycelium of which so rots the tissues in
+a more or less circular area round the point of infection, that, in wet
+weather, the decomposing mass falls out and leaves a round hole--_e.g._
+certain Chytridiaceae, Peronosporeae, _Gloeosporium_, _Exoascus_, etc.
+If dry weather supervenes these holes frequently dry at the edges, and
+the leaves appear as if eaten out.
+
+Shot-holes in Cherry, Walnut, Tobacco, and Plum leaves are due to
+_Phyllosticta_, in Cherry leaves also to _Clasterosporium_, and in
+Potato leaves to _Haltica_.
+
+_Frost-cracks._--The trunks of trees exposed to the north-east, and
+occasionally with other aspects, are apt to show longitudinal ridges
+which realise on a larger scale the features of healed wounds scored
+with a knife. These wounds are due to the outer layers of wood losing
+water from their cell-walls as it congeals to ice in their lumina, more
+rapidly than do the warmer internal parts of the trunk; as this drying
+of the wood causes its shrinkage, especially in the tangential
+direction, the effect of a sudden frost and north-east wind is to rend
+the wood, which splits longitudinally with a loud report, as may often
+be heard in severe winters. Since the cortex and bark are ruptured at
+the same time the total effect resembles that of a deep knife-cut, and
+the same healing processes result on a larger scale when the wood swells
+and closes up the wound again in spring. But this recently-closed lesion
+is evidently a plane of weakness, and if a similarly severe winter
+follows the wound reopens and again heals, and so on, until after a
+succession of years a prominent _Frost-ridge_ results, which may finally
+heal completely if milder winters ensue or the tree be eventually
+protected.
+
+_Strangulations._--We are now in a position to understand the so-called
+strangulations which result when woody climbers, telegraph wires, etc.,
+kill or injure trees by tightly winding round them. If strong wire is
+twisted horizontally round a stem, the growth in thickness of the latter
+causes the trapping of the cortex and cambium, etc., between the wire
+and the wood, and a ringing process is set up in consequence of the
+death of the compressed tissues. A callus then forms above the wound, as
+in the case of true ringing by means of a cut, and eventually bulges
+over the upper side of the wire: in the course of years this overgrowth
+may completely cover in the wire, and, pressing on to the lower lip of
+the wound, may at length fuse with the cambium below. Hereafter the
+thickening rings of wood are continuous over the buried wire. The
+process is obstructed by all the impediments referred to in dealing with
+ringing, and of course the stem thickens more above than below the wire.
+If the sapwood is thin, and the bark is so thick as to put great
+obstacles in the way of the junction of the upper and lower cambiums,
+death may result--the tree is permanently ringed. (See p. 201.)
+
+_Spiral grooves_ are frequently met with where Wood-bine or other woody
+climbers have twined round a young stem or branch, the upper lip of the
+groove always protruding more than the lower. If a kink or a crossing of
+two plants or branches of the twiner results in a complete horizontal
+ring, the results are as in the above cases of ringing and
+strangulation. Naturally grooved walking sticks are often seen.
+
+_Buried letters, etc._--These processes of healing by occlusion enable
+us to understand how letters of the alphabet, cut into the wood of
+trees, come to be buried deep in the timber as successive annual rings
+cover them in more and more. Chains, nails, rope, etc., have frequently
+been found thus buried in wood.
+
+
+NOTES TO CHAPTER XXII.
+
+     In addition to the notes to the last chapter, the reader may
+     be referred to Fisher in Vol. IV. of Schlich's _Manual of
+     Forestry_, Chap. VI., for an account of Hess' excellent work
+     on Boring Beetles, etc.
+
+     The authority on Wood-fungi is Hartig, see especially his
+     _Zersetzungs-erscheinungen des Holzes_, the principal results
+     of which are condensed in his _Diseases of Trees_ already
+     referred to. As regards "Pith-flecks," the reader should
+     consult Frank, _Krankh. der Pflanzen_, B. I., p. 212: the
+     subject needs further investigation.
+
+
+
+
+CHAPTER XXIII.
+
+EXCRESCENCES.
+
+     _Herbaceous excrescences, or galls--Erineum--Intumescences--
+     Corky warts, etc.--Pustules--Frost-blisters--Galls and Cecidia
+     --Root nodules._
+
+
+_Excrescences_, or out-growths of more or less abnormal character from
+the general surface of diseased organs, are very common symptoms, and
+widely recognised. They are due to hypertrophy of the tissues while the
+cells are young and capable of growth, and may be induced by a variety
+of causes, among which the stimulus of insect-punctures and of the
+presence of insect eggs are best known; but that of fungi, though less
+widely recognised, plays an equally important part, and, as we shall
+see, galls and other excrescences may be due to widely different agents.
+
+_Galls_ or _Cecidia_ are protuberances of the most varied shapes,
+colours, and sizes found on herbaceous parts attacked by insects, fungi,
+etc. In the simplest cases the insects only pierce and suck the young
+cellular tissue--_e.g._ _Phytoptus_, Aphides, etc.--but in others the
+stimulus to hypertrophy starts by the puncture of the embryonic tissue
+of a leaf, root, etc., by the ovipositor of the female insect, which
+then lays an egg--_e.g._ _Cynips_, _Cecidomyia_, etc.--the presence of
+which appears to intensify the irritating action, or such only occurs
+when the young larva escapes.
+
+Our knowledge of the primary cause of gall-formation amounts to very
+little. Generally speaking, only embryonic or very young cellular tissue
+reacts, and galls on adult leaves and branches have usually been
+initiated long before. The same gall-insect may induce totally different
+galls on different plants, or even on different parts of the same plant,
+and different insects call forth different galls on any one plant. These
+facts point clearly to the co-operation of both plant and insect in the
+gall-formation, and the best hypothesis yet to hand is to the effect
+that a gall is a hypertrophy of cells, the normal nutrition, growth, and
+division of which have been disturbed owing to the action of some poison
+or other irritant derived from the insect, or fungus, or other organism.
+Attempts have been made to reproduce galls by injecting the juices of
+similar galls into the tissue, but as yet without success, and this may
+point to the co-operation of mechanical irritation during the
+hypertrophy in normal gall-formation.
+
+Galls, in the broad sense, are not always preceded by a wound, however.
+Insects on the outside of young tissues may cause such irritations that
+the parts in contact with the animal are arrested in their growth, while
+those further away grow more rapidly--_e.g._ where Mites, etc., cause
+puckers and leaf-rolling. In true galls the hypertrophy may consist
+merely in the enlargement of cells already present, and no new
+cell-divisions and, still less, changes in the nature of the tissues
+result--_e.g._ some pocket galls on _Viburnum_, _Pyrus_, etc., and the
+hairy outgrowths of the epidermis known as _Erineum_. In other cases
+there is not only hypertrophy of existing cells, but new cell-divisions
+are instituted: these cell-divisions may be confined to the direction
+perpendicular to the epidermis, and the tissues grow only in the
+direction of the surface, producing puckerings--_e.g._ the Aphis galls
+on _Ribes_, Phytoptus galls of _Salvia_, leaf galls on _Tilia_, _Acer_,
+_Alnus_, etc., and the curious galls on Plums due to _Cecidomyia Pruni_,
+and which must not be confounded with the "pocket plums" and similar
+galls due to Exoasci.
+
+In a third series of cases, cell-divisions occur parallel to the surface
+of the leaf, and galls are formed which grow in thickness, and develop
+the most extraordinary and complicated new tissues--proteid-cells
+surrounding the egg or larva deposited inside, followed by a protective
+layer of sclerenchyma encasing this food layer, and around this again
+softer tissues which may assume the structures and functions of
+respiratory tissues, water-storing tissues, starch reservoirs,
+assimilatory, or protective tissues of various kinds, and over all may
+be a well-marked epidermis, with stomata, or cork with lenticels.
+
+The chief seat of these hypertrophies and--what is more
+remarkable--development of new tissue elements not found elsewhere in
+the leaves, or even in the species, is the mesophyll, and various
+speculations and hypothesis have been founded on these curious
+phenomena.
+
+_Erineum._--The simplest excrescences on plants are certain hair-like
+developments of epidermal cells due to the irritation of species of
+_Phytoptus_, and similar insects which rise in clusters on the surfaces
+of leaves and by their colours, consistence, arrangement in patches,
+spots, etc., so simulate fungi that Persoon was deceived by them and
+gave them the genus name _Erineum_. They occur on most of our trees,
+_e.g._ Poplar, Lime, Oak, and are very common in the Tropics. Usually
+pale or even white at first, they turn brown as the hair-like outgrowths
+die and lose their sap, but since the latter may be bright
+coloured--yellow, red, purple,--the patches are sometimes very
+conspicuous objects on smooth leaves.
+
+In many cases these hairs exactly resemble in shape and other characters
+the abnormal root-hairs found on roots exposed to the effects of
+poisonous reagents, or of unsuitable food-materials, or the rhizoids
+developed from wounded Algae, etc.
+
+_Intumescences_ are similar trichomatous outgrowths not associated with
+insects or fungi, and due to some disturbance of the balance between
+transpiratory and assimilatory functions of their leaves, as indicated
+by the less localised occurrence and by their non-appearance when the
+plant is under favourable cultural conditions. Structures not unlike
+these have been artificially induced by exposure to particular lights,
+and also by painting spots with dilute corrosive sublimate, indicating
+that poisons may impel the epidermis cells to grow out abnormally.
+
+_Corky warts._--Several forms of disease are known in which the
+pathological condition is expressed by the formation of cork in unwonted
+places and quantities. The _Scab_ or _Scurf_ of Potatoes is a case in
+point. The tissue of the lenticels absorbs water and the outermost cells
+are cut off by cork and die: the cells below them burst the dead
+bark-like masses thus formed, and again cork is formed and cuts off the
+outer masses, and the rough cork warts--_Scab_ or _Scurf_--are the
+result.
+
+The causes predisposing to scab have been variously assigned to
+dampness, want of lime, action of bacteria and fungi--_e.g._
+_Sorosporium_, _Oospora_, _Spongospora_,--the latter making their way
+into the ruptured tissue of the lenticels and irritating the cells to
+further growth.
+
+It seems probable that several different kinds of scab exist in
+Potatoes, as well as in roots--_e.g._ Beets, and the whole subject needs
+further investigation. The scab-like rough scaly bark of Pear trees in
+dry districts may also be mentioned here.
+
+_Cork-wings_ are well known on the young branches of Elms, Maples, etc.,
+some varieties of which have received specific names on this account.
+
+_Corky excrescences_ on leaves occur occasionally in the Gooseberry,
+Holly and other plants, for which no cause has been discovered.
+
+Lenticels are also formed on some leaf-galls, and are remarkable as
+being structures not normal on leaves.
+
+_Pustules._--This term may be employed generally for all slight
+upheavals of the surfaces of herbaceous organs, which subsequently burst
+and give egress to the spores, etc., of the organism causing them, or
+merely fray away at the top if no organism is discoverable. They are
+often due to fungi--_e.g._ _Synchytrium_, _Protomyces_, _Cystopus_, and
+Ustilagineae,--and we may extend the use of the general term also to
+those cases where the _stroma_ of the fungus itself bursts through the
+cortex of older parts and forms the principal part of the
+pustule--_e.g._ _Monilia_, forming white or grey pustules on Apples,
+_Roestelia_ and other AEcidia, forming yellow or orange pustules on
+leaves, etc.; _Cucurbitaria_ and _Nectria_ (red) breaking through the
+cortex of trees, and _Phoma_ and numerous other Ascomycetes which form
+black cushions. _Pustules_ on the leaves of _Lysimachia_, _Ajuga_, etc.,
+are due to the parasitic Alga _Phyllobium_.
+
+Cylindrical stem swellings are caused by _Calyptospora_: they are due to
+the hypertrophy of the cortex of Bilberry stems permeated by the
+hyphae. _Epichloe_, which clothes the sheaths and halms of grasses with
+its stroma, at first snowy white and later ochre-yellow as the
+perithecia form, is another example.
+
+The cylindrical layer of eggs of a moth such as _Bombyx_ on a twig must
+not be confounded with these cases.
+
+_Frost-blisters_ are pustule-like uprisings of the cortex, where the
+living tissues below have formed a callus-like cushion into the cavity
+beneath the dead outer parts of the cortex which were killed by the
+frost; they occur on the stems of young Apples, Pears, etc.
+
+_Galls_ in the narrower sense are tissue outgrowths usually involving
+deeper cell-layers. They are so varied and numerous that classification
+is difficult. For symptomatic purposes we may divide them as follows:
+
+_Leaf-galls._--A well-marked type is that of the _pocket-galls_ or
+_bladders_ in which the whole thickness of the leaf is as it were pushed
+up like a glove-finger at one spot, so that if the upper surface of the
+leaf forms the outside of the gall the lower surface is its lining. Such
+galls are common on Limes (_Phytoptus_), _Glechoma_ (_Cecidomyia_), Elms
+(_Tetraneura_), etc. Similar localised extension of the leaf surface,
+compelling it to rise up like a pocket, are caused by fungi--_e.g._
+_Taphrina_ on Poplars, _Exoascus_ on Birches, etc., _Exobasidium_ on
+Bilberries, Rhododendrons, etc.
+
+Another type is that of the _Gall-apple_, so well known on Oaks, where
+the spherical swelling is solid--except for the inner cavity containing
+the eggs--_Neurotus_, _Cynips_, _Hormomyia_, etc. These are comparable
+in general characters to the nodules on roots.
+
+Fungus galls with similar external features when young are found on
+Maize (_Ustilago Maydis_), and betray their nature by the black powdery
+spores as they mature.
+
+Bud galls on Willows are due to _Cecidomyia_, which causes several
+internodes to swell out into a greenish barrel-shaped mass, from which
+leaves may spring.
+
+Small irregular excrescences on Willow stems are referred to
+_Phytoptus_, and another species of the same insect induces similar
+swellings on Pines which are not surcharged with resin.
+
+_American Blight_, or Woolly Aphis, on Apples especially, causes the
+tumour-like swellings covered with sticky white fluff, which is a waxy
+excretion of the insect. Galls on _Pilea_, in Java, are due to an
+Alga--_Phytophysa_.
+
+_Root-nodules_ or _nodosities_ are frequently caused by insects--_e.g._
+_Centhorhynchus_, a beetle which attacks Crucifers, _Cynips_ and allied
+"gallflies" of Oaks, and the notorious _Phylloxera_. But similar
+root-galls are produced by Nematode worms, _Heterodora_, on Beets,
+Tomatoes, Cucumbers and numerous other plants, and by the Slime fungus
+_Plasmodiophora_, and it is not always easy to distinguish such cases
+from the fungus-galls (_Mycocecidia_) on the roots of Alders, _Juncus_,
+and Leguminoseae where the symbiosis of bacteria or fungi with the
+roots are of benefit to the plant. _Urocystis Leimbachii_ forms similar
+nodules at the collar of young plants of _Adonis_.
+
+_Heterodora javanica_ passes into the cortex of sugar-cane roots through
+fissures, and makes its way to the place where a young rootlet is about
+to emerge; here it sticks its beak into the growing-point and remains
+fixed.
+
+Molliard has shown that in the roots of Melons, _Coleus_, etc.,
+_Heterodora_ causes the cells in immediate contact with its head, and
+which would normally become vessels of the xylem, to swell up into huge
+giant-cells, with their walls curiously folded, and containing large
+supplies of proteids and numerous nuclei, reminding us of the food-layer
+of insect galls and of the tapetal layer of pollen-sacs. While the
+stimulus exerted by the Nematode thus induces hypertrophy and storage
+with food-substances of these cells, those of the next layers undergo
+reticulate thickenings of their walls. Again instances of the evolution
+of new tissue elements by the action of the foreign organism.
+
+So far as galls on leaves are concerned the amount and kind of damage
+done are in proportion to the area of chlorophyll action put out of play
+for the benefit of the plant, and the remarks already made on p. 193
+apply here also. Where buds are destroyed the effects may of course
+extend further, but it rarely happens that leaf-galls are so abundant as
+to maim a tree permanently. Nevertheless we must remember that cases
+like _Phylloxera_ are notorious.
+
+Far more dangerous, however, are the root-galls due to such insects,
+because here the damage is not so local: the water-supplies are cut off,
+and injurious consequences result from the absorption of the products of
+decomposition in the soil.
+
+
+NOTES TO CHAPTER XXIII.
+
+     In addition to the literature on galls quoted in the Notes to
+     Chapter XIV., the reader should consult Dale "On certain
+     Outgrowths (Intumescences) on the green parts of Hibiscus,"
+     _Proc. Cambr. Phil. Soc._, Vol. X., 1899, p. 192, and _Brit.
+     Ass. Rep._, Bradford, 1900.
+
+     The detailed study of the anatomy and histology of Galls has
+     been recently undertaken by Kuester, "_Beitrage zur Kenntniss
+     der Gallenanatomie_," Flora, B. 87, 1900, p. 117, where the
+     principal references will be found.
+
+     On the root-galls due to Nematodes see Atkinson in _Science
+     Contributions from the Agric. Expt. Station, Alabama_, Vol.
+     I., p. 1, 1889; Percival, "An Eel-worm disease of Hops" in
+     _Natural Science_, Vol. VI., 1895, p. 187; and Molliard in
+     _Revue generale de Botanique_, Apl., 1900, p. 157, where the
+     histology is dealt with.
+
+     The nodules of the roots of Leguminoseae are not part of the
+     subject of this work: the literature is collected in _Science
+     Progress_, 1895, Vol. III., p. 252, and Dawson, _Phil.
+     Trans._, 1900.
+
+
+
+
+CHAPTER XXIV.
+
+EXCRESCENCES (_continued_).
+
+     _Cankers--Burrs--Sphaeroblasts, and other excrescences of
+     woody tissues--Witches' Brooms._
+
+
+_Cankers_ are irregular excrescences due to the perennial struggle
+between tissues attempting to heal up a wound, and some organism or
+other agent which keeps the lesion open. A canker always originates in a
+wound affecting the cambium, and usually in a small wound such as an
+insect puncture or frost nip; if undisturbed the dead parts would heal
+over by cork and callus, but if recurring frost-cracks break open the
+coverings, or if insects or fungi penetrate the callus and invade the
+cambium, irregularities of growth due to the occluding tissue on the one
+hand, and continued growth of the still unimpaired cambium on the
+opposite side of the injured shoot on the other, result in the canker.
+Frost cankers occur on fruit-trees, Vines, Beeches, etc.
+
+Cankers due to insects are found on Apples, the cortex of which is
+punctured by the woolly Aphis (_Schizoneura_) while the twigs are young,
+and the wound is kept open by the insects nestling in crevices in the
+occlusion tissues. Species of _Coccus_, _Lachnus_, and _Chermes_ also
+produce cankers on forest trees.
+
+Cankers due to fungi usually originate in a wound primarily due to an
+insect puncture or bite, or to frost, the invading fungus hyphae making
+their way into the wounded tissues and gradually extending more and more
+into the cambium and the occluding callus. Among the best known of these
+wound fungi which cause cankers are _Dasyscypha Willkommii_ the peziza
+of Larch disease, _Nectria ditissima_ and _N. cucurbitula_ on Beech and
+Conifers; less common are _Scleroderris_ on Willows, _Aglaospora_ on
+Oaks and some others.
+
+_Peridermium Pini_ and _Aecidium elatinum_ also cause cankers under
+certain conditions, as also does _Gymnosporangium_, but in these cases
+the fungi are more truly parasitic.
+
+In some cases--_e.g._ Ash, Pine, Olives--bacteria are concerned as
+associated organisms in the cankering of trees.
+
+_Burrs_ or _Knauers_ are irregular excrescences, principally woody, with
+gnarled and warted surfaces. They are frequently due to some previous
+injury, such as the crushing or grazing of cortical tissues by
+cart-wheels. The excitation of the tissues thus wounded results in the
+development of shoots from adventitious or dormant buds at the base of
+old tree trunks, or in the starting of the same process where a branch
+has been broken off. The new bud begins to develop a shoot, but soon
+dies at its tip owing to paucity of food-supplies to the weak shoot,
+while new buds at its base repeat the process next year with the same
+result, and each of these again in turn, and so on. The consequence is
+an extremely complex nest of buds, all capable of growing in thickness
+and putting on wood to some extent, but not of growing out in length. In
+course of time this mass may attain dimensions measurable by feet,
+forming huge rounded and extremely hard-knotted burrs, the cross-section
+of which shows the vascular tissues running irregularly in all
+directions, and, owing to the very slow growth, extremely dense and
+hard. The dark spots in such sections--_e.g._ Bird's-eye Maple--are the
+cut bud-axes all fused together, as it were. On old Elms such burrs are
+common at heights on the stem which preclude the assumption of any
+coarse mechanical injury, and similar structures occur on the boles of
+other forest trees suddenly exposed to light by the felling of their
+companions, which suggests that these epicormic shoots result from some
+disturbance due to the action of light.
+
+_Witches' Brooms_ are irregular tufts of twigs often found among the
+branches of trees such as Birches, Hornbeam, etc., where they look like
+crows' nests, and similar structures are to be found on Silver Firs and
+other conifers. In the former case they are due to _Exoascus_, in the
+latter to _Aecidium_, fungi which are perennially parasitic in the
+shoots, and stimulate the twiggy development of a number of buds which
+would normally have remained in abeyance, or not have been formed at
+all, and only do so now in a fashion different from that of normal
+branches.
+
+Rosette-like formations, depending on similar disturbing causes on the
+part of insects, occur in conifers--_e.g._ _Gastropacha Pini_.
+
+Dense tufts of twiggy shoots may be developed on many trees by pruning
+in such a way as to stimulate the shooting out of basal buds which would
+otherwise remain dormant, _e.g._ Elm, Ash, and thus it occurs that
+injuries such as frost, insect bites, etc., may induce the production of
+such tufts in a tree crown. The dense nests of stool-shoots thrown up
+from felled tree-stumps are of essentially the same nature--partly
+adventitious and partly dormant buds being enabled to grow out because
+they can now be supplied with materials previously carried beyond them
+while the trunk was still there. Suckers, if repeatedly cut down, may
+also behave similarly.
+
+_Wood-nodules_ or _Sphaeroblasts_ are curious marble-like masses of wood
+which protrude with a covering of bark from old trunks of Beeches, etc.,
+and can be readily dug out with a knife. The nodule has arisen by the
+slow growth of the cambium of a dormant bud, the base of which separated
+at an early date from the wood beneath; the cambium then closed in over
+the base and laid on thickening rings all round the axis of the bud
+except at the extreme apex. When the separation occurred the cambium of
+the wood beneath covered over the previous point of junction, and thus
+the woody bud was pushed out with the bark, and now protrudes covered
+with a thin layer of the latter. Similar nodules are occasionally found
+on Apple trees.
+
+
+NOTES TO CHAPTER XXIV.
+
+     For further information on Cankers the student should read
+     Marshall Ward, _Timber and some of its Diseases_, Chapter X.
+     Further, the discussion as to the causes of canker in Frank,
+     _Krankheiten der Pflanzen_, B. I., p. 207, and B. III., pp.
+     167 and 172, and various papers in _Zeitschrift fuer
+     Pflanzen-krankheiten_.
+
+
+
+
+CHAPTER XXV.
+
+EXUDATIONS AND ROTTING.
+
+     _Tumescence--Rankness--Bursting of fruits, etc.--Root rot--Rot
+     of fruits--Bulb diseases--Flux--Honey-dew--Slime flux--
+     Resinosis--Gummosis--Manna._
+
+
+I put together in one artificial class a varied group of diseases, the
+principal symptom of which is the escape of fluids from the tissues,
+under circumstances which betray an abnormal state of affairs, often
+obvious, but sometimes only to be inferred. In many of these cases
+bacteria abound in the putrefying mass, and some evidence exists for
+connecting these microbes causally with the disease in a few of the more
+thoroughly investigated cases, but in no case has this been sufficiently
+demonstrated; and considering the ease with which bacteria gain access
+_via_ wounds caused by insects and fungi, as well as by other agents,
+the necessity for rigid proof must be insisted upon before we can accept
+such alleged examples of _Bacteriosis_.
+
+_Tumescence._--It occasionally happens that herbaceous parts of plants
+pass into a condition of over-turgescence from excess of water in the
+tissues, an abnormal state which indicates pathological changes
+resulting from various causes, often not evident and therefore regarded
+as internal. Such disease was formerly termed _Oedema_ or _Dropsy_.
+This disease is frequently due to the excessive watering of pot plants
+with large root systems and deficient foliage, in hot-houses with a
+saturated atmosphere: it is, therefore, primarily referable to
+diminished transpiration. It can sometimes be brought about by covering
+potato plants, for instance, with a bell-jar in moist, hot weather; and
+this, and the prevalence of the disease in hot-houses as compared with
+plants grown out of doors, point to the above explanation. Similar
+phenomena do occasionally occur out of doors in hot, moist situations or
+during wet seasons, however, and the watery shoots of rank vegetation
+are merely particular cases of the same class. Moreover, the well-known
+tendency to succulence of sea-side varieties of plants which have thin
+herbaceous leaves when growing inland, points to the action of the
+environment in these matters, excess of salts being no doubt one factor
+in such cases.
+
+_Rankness_ affords another example where superfluity of water is
+concerned, though it does not involve simply this, because the plant may
+also contain excessive quantities of nitrogenous and mineral matters
+taken up by the roots.
+
+Rankness is, in fact, in many respects analogous to etiolation in so far
+as the tissues are soft and surcharged with water, but it differs
+fundamentally in the deep green of the chlorophyll: this may lead to
+abundant assimilation if free access of air and drier conditions can be
+gradually brought about. Any sudden drying, however, may be fatal to the
+tender tissues.
+
+Rankness commonly depends on excess of food materials, especially
+nitrogenous manures, as may be seen in meadows and cornfields where the
+manure heaps have remained on the ground and saturated it to excess as
+compared with the rest of the soil; this may often be observed with
+weeds, etc., in the neighbourhood of farm-buildings. If the period of
+rank growth is accompanied and followed by days of suitably bright
+sunshine and dry air, the increase of vegetative structures usually
+results in increased flowering, heavy crops, or strong wood; but if the
+rankness continues too long, or is accompanied by wet and dull weather,
+the watery tissues are peculiarly susceptible to attacks of fungi and
+insects, and to damage by sudden frosts or chilly winds. Rankness
+affords, in fact, a typical illustration of predisposition to disease.
+
+_Damping off._--When seedlings are too closely crowded in beds kept too
+damp, or in moist weather, they are very apt to rot away, with all the
+symptoms--spreading from a centre, contagious infection, mycelia on and
+in the tissues, etc.--of a fungus attack. The commonest agent concerned
+is one of the species of _Pythium_, the propagation of which is favoured
+by the rank, over-turgid, and etiolated conditions of the plants.
+Species of _Mucor_, _Botrytis_, and other fungi, may also be met with.
+
+_Bursting_ of fleshy fruits, such as Tomatoes, Grapes, etc., is due to
+over-turgescence in rainy weather or excessively moist air. But the
+phenomenon is by no means confined to such organs. Hot-house plants when
+oedematous not infrequently put out watery blisters from the cortex or
+leaves, which rupture; and the stems of fleshy fasciated (_e.g._
+Asparagus) or blanched and forced plants (_e.g._ Celery, Rhubarb) are
+particularly apt to crack here and there from the pressure of the
+turgescent tissues on the strained epidermis. Beets, Turnips, and other
+fleshy roots show the same phenomena in wet seasons. That these ruptures
+and exposures of watery tissues afford dangerous points of entry for
+parasites and moulds will be obvious--_e.g._ _Edelfaeule_, a rotten
+condition of the grapes in the Moselle district.
+
+_Root-rot_ is a common disease in damp, sour clay soils after a
+continuance of wet weather--_e.g._ Wheat, especially if root-drawn and
+exposed to thaw water.
+
+In the disease known as Beet-rot, the roots turn black at the tip, where
+the tissues shrivel and become grooved and wrinkled extensively. Inside
+the flesh also blackens and finally rots. In earlier stages, only the
+vascular bundles are brown and blocked with gum-like substances. In
+advanced stages there is much gummy material in the lumina, and even
+large cavities filled with this gum may be found.
+
+The rot of Cherries, Pears, Apples, Plums, etc., in store may be due to
+several fungi, of which _Botrytis_, _Monilia_, _Mucor_, _Penicillium_,
+and _Aspergillus_ are the chief. The fruit may be attacked while still
+on the tree, but very often fungi and bacteria gain access to the
+tissues, through bruises, cracks, etc., formed in the fruit lying in the
+storage baskets or on the shelves.
+
+Rot in Onions, Hyacinth bulbs, etc., is frequently due to the access of
+_Botrytis_ or _Sclerotinia_, followed by moulds, yeasts, and bacteria in
+the stores.
+
+_Sour-rot_ in Grapes, and other fleshy fruits which need much sun to
+ripen them, is probably a usual result of continued cold, wet weather at
+the cropping season, setting in when the fruits are beginning to swell.
+
+_Flux._--It is a common event to see fluids of various kinds issuing
+from wounds in trees, or congealing in more or less solid masses about
+them; and owing to the prevailing tendency to compare plant diseases
+with those of animals, we find such expressions as _Gangrene_, _Ulcer_,
+and so forth, applied to these "open sores." In so far as such
+outflowings frequently indicate diseased states of injured tissues which
+are incapable of healing up, the analogy is perhaps a true one; but it
+must be remembered that very different structures and processes in
+detail are concerned. Moreover, liquid excretions more or less
+indicative of diseased states are by no means confined to wounds or
+definitely injured tissues, in which case such terms are wholly
+misapplied.
+
+_Honey-dew._--The leaves, or other organs, of many plants are sticky in
+hot weather, owing to the excretion of a sweet liquid containing sugar,
+the consistency and colour of which vary according to circumstances.
+This honey-dew must not be confounded with the normal viscidity of
+certain insectivorous plants--_e.g._ Sundew--or with the sticky
+secretion on the internodes of species of _Lychnis_, etc., where it
+plays the part of a protection against minute creeping things.
+
+Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of
+these cases the honey-dew is excreted by Aphides, which suck the juices
+of the leaves and pour out the saccharine liquid from their bodies. The
+sweet fluid is in its turn sought after by ants, and also serves as
+nutritive material for various epiphytic fungi--_e.g._ sooty mould,
+_Capnodium_, _Fumago_, and _Antennaria_--which give the leaves and
+honey-dew a brown or black colour. Certain _Coccideae_ also excrete
+honey-dew, especially in the tropics.
+
+At least one case is known where honey-dew is formed as the result of
+the parasitic action of a fungus, namely _Claviceps purpurea_ in its
+conidial stage on the stigmas of cereals, and this may be compared with
+the sweet odorous fluid excreted by the spermogonia of certain
+_Aecidia_. In both cases the sweet fluid attracts insects which
+disperse the spores.
+
+Honey-dew may also be formed without the agency of fungi or insects,
+when hot and dry days are followed by cool nights, with a saturated
+atmosphere, _e.g._ _Caesalpinia_, _Calliandra_ and other trees in the
+tropics, which are called rain trees owing to the numerous drops of
+fluid which drip from the leaves under the abnormally turgescent
+conditions referred to.
+
+_Cuckoo-spit._--The leaves of Willows, Meadow grasses and herbs, etc.,
+are often seen with froth on them, in which is a green insect,
+_Aphrophora_, which sucks the juices from the tissues and excretes the
+frothy watery cuckoo-spit from its body.
+
+_Slime-flux._--The trunks of trees may sometimes be observed to pour out
+a slimy fluid from cracks in the bark, or from old wounds, or branch
+scars. In some cases, _e.g._ in Oaks, the slime has a beery odour and
+white colour, and abounds in yeasts and other fungi to the fermentative
+activity of which the odour and frothiness are due. In other cases the
+slime is red _e.g._--Hornbeam; or brown--_e.g._ Apple and Elm; or
+black--_e.g._ Beech, the colour in such cases being due to the mixture
+of yeasts, bacteria, and fungi with which these slimes abound. The
+phenomenon appears to be due to the exudation of large quantities of sap
+under pressure--root pressure--and is primarily a normal phenomenon
+comparable to the bleeding of cut trees in spring: the fungi, etc., are
+doubtless saprophytes, but their activity is concerned with the
+putrefactive processes going on in the diseased wood, and which may lead
+to rotting of the timber.
+
+The origin of the wounds in the bark and cortex, and which extend into
+the wood and other tissues as the putrefactive and fermentative
+processes increase, appears to be in some cases at least due to
+lightning.
+
+_Resin-flux_ or _Resinosis_.--The stems of Pines and other conifers are
+apt to exude resin from any cut or wound made by insects, or by the
+gnawing of other animals; but in many cases the flow is due to fungi,
+_e.g._ _Peridermium_, the hyphae of which invade the medullary rays and
+resin canals and thus open the way to an outflow through cracks in the
+bark. _Agaricus melleus_ not only invades the resin passages, but
+stimulates the tree to produce abnormal quantities of resin, which flows
+down to the collar and roots, and exudes in great abundance at the
+surface of the soil. Various other plants also exude resin from wounds,
+and in some cases the flux seems to be increased by degeneration of the
+tissues, _e.g._ _Copaifera_.
+
+_Gummosis._--Cherries, Apricots, Acacias, and many other trees are apt
+to produce abnormal quantities of gum, which flows from any wound or
+exudes through cracks in the bark. Degeneration of the wood-cells, and
+especially of the cell-walls of a soft wood formed by abnormal activity
+of the cambium, points to its origin being due, in some cases at any
+rate, to a conversion of the cellulose, and fungi are sometimes found in
+the masses of gum; but beyond the fact that _gummosis_ is a pathological
+phenomenon we know very little of the disease.
+
+With regard to such gumming, it is significant how frequently pruned
+trees--Cherries, Oranges, Lemons, Plums, etc.--suffer.
+
+_Manna flux._--Certain trees, such as the Manna Ash, species of
+Tamarisk, etc., yield manna from wounds, and in some cases the latter
+are due to insects, _e.g._ _Cicada_.
+
+The Potato-disease is best known by the pale whitish fringe, giving an
+almost mealy appearance to the margins of the brown to black patches in
+damp weather. In dry weather the brown patches shrivel and dry, and as
+they are apt to be at the edges and tips of the leaflets, these curl up.
+The young disease spots are yellowish, and the leaves of badly affected
+plants are apt to be sickly yellow throughout.
+
+This Potato-disease due to _Phytophthora_ must be distinguished from the
+curling and puckering, with wilting and browning of the leaves and
+yellow glassy look of the stems, due to the invasion of the vessels by a
+fungus which lurks in the tubers, and gains access thence to the shoots.
+
+In the disease traceable to _Phytophthora_ the stock remains green and
+the leaves plump and plane, and only the brown patches slough out in wet
+or shrivel in dry weather, and are bordered by the pale whitish zone of
+conidiophores.
+
+In the leaf-curl the yellow and flaccid appearance of all the leaves of
+a stalk, or even of the plant, is the striking symptom, and the stem
+soon droops and blackens just above the soil, a white mould appearing
+also at the black spots. Subsequently black spots appear higher up, and
+bacteria gain an entrance. The stolons rot, and eventually the roots and
+the leaves wither. The tubers appear sound, but are small; they are apt
+to rot in the store, the vascular zones turning brown.
+
+This leaf-curl has been ascribed to _Pleospora_, _Polydesmus_,
+_Verticillium_, and other parasites, as well as to excessive manuring
+and other agencies, but it still needs explanation.
+
+Rot of Potato tubers in the soil, or in store, may be brought about by
+very different agents.
+
+If _Phytophthora_ has obtained access, the fungus hyphae spread between
+the cells, starting from the haulm, and cause the flesh to turn
+yellowish and then brown in patches. On the exterior are discoloured
+patches, depressed, with the flesh beneath brown and soft. The mycelium
+spreads mostly in the outer layers, which though they turn deep brown
+remain firm.
+
+Wet rot of potatoes may be due to various fungi, and, in excess of
+water, to putrefactive bacteria (_e.g._ _Clostridium_), which destroy
+the cell-walls. The flesh becomes soft, then soup-like, and finally
+putrefies to a liquid mass with a vile smell of butyric acid, etc., in
+which the starch grains may be seen floating.
+
+Tubers are often found with the cork burst and peeling in shreds, the
+flesh more or less converted into a putrid and stinking pulp, with a
+spotted brown boundary of partly destroyed but firmer tissue between the
+dark utterly rotten and the white and still firm healthy flesh. The
+principal agent in the destruction of the tissues is _Clostridium_, an
+anaerobic bacillus which consumes the cell-walls but leaves the starch
+intact. Hence a thoroughly decomposed tuber consists of a cork bag full
+of starch and foetid liquid. In the dried condition the flesh shows a
+brown marbling; this passes into a soft soupy starchy part, and here and
+there may be violet grey cavities lined with _Spicaria_, _Hypomyces_,
+etc., the white stromata of the latter often appearing externally. The
+excavations are filled with loose starch grains, and bounded by cork and
+cambium formed in the peripheral cells. The cell-walls eventually
+undergo slimy decomposition.
+
+_Spicaria_, _Fusisporium_, various moulds, and bacteria may all be
+associated with wet-rot.
+
+Dry-rot of Potatoes is also due to various fungi and bacteria, but the
+destructive action goes on slowly, owing to there being no more moisture
+than the tissues afford. The flesh becomes excavated here and there,
+owing to the slow destruction of the cell-walls by _Clostridium_: the
+destroyed tissues are brown, and the uninjured starch grains powder them
+all over. Finally the whole shrunken mass has a crumbly consistency.
+
+When the flesh remains white, but assumes a powdery consistency and
+dry-rot, with the cork destroyed here and there, Frank refers the
+damage to _Phellomyces_. Where the dry-rot is due to _Fusarium_ the
+chalk-white stromata may often be detected breaking through the
+periderm; but it must be remembered that the soil-contaminated, broken
+skin of a potato-tuber is a favourable lurking spot for many fungi, and
+_Periola_, _Acrostalagmus_, and others have been detected therein.
+
+Brown spots, depressed into the flesh, sometimes result from the ravages
+of _Tylenchus_, the minute worms being found in the diseased tissues.
+
+In some cases the flesh turns watery and soft, grey, almost glass-like,
+starting at the haulm end, and this may be owing to the invasion of
+_Rhizoctonia_.
+
+
+NOTES TO CHAPTER XXV.
+
+     The rotting of bulbs, roots, etc., has been much discussed
+     during the last few years in the pages of the _Gardeners'
+     Chronicle_, _Zeitschrift fuer Pflanzenkh._, and elsewhere. The
+     principal references to Bacteriosis--the rot in which bacteria
+     are stated to be the primary agent causing these and similar
+     diseases--may be found in Massee, _Diseases of Plants_, pp.
+     338-342, and more fully in Russell, _Bacteria in their
+     Relation to Vegetable Tissue_, Baltimore, 1892; and in Migula,
+     _Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche
+     Angeblich durch Bakterien verursacht werden_, Semarang, 1892.
+
+     The most convincing accounts, however, are since that date;
+     see Smith, "Pseudomonas Campestris," _Cent. f. Bakt._, B.
+     III., 1897, p. 284, and Arthur and Bolley, _Bacteriosis of
+     Carnations_, Perdue University Agr. Expt. Station, 1896, Vol.
+     VII., p. 17. Woods has lately shown that this disease is due
+     to Aphides only, the bacteria having nothing to do with the
+     disease primarily, _Stigmonose_, _Bull. 19_, U.S. Dept. Agr.,
+     1900; but it is necessary to bear in mind that actual
+     penetration of the cell-walls from without must be proved, as
+     De Bary proved it for germ-tubes of fungi, before the evidence
+     that Bacteria are truly parasitic in living plants can be
+     called decisive. This is a difficult matter, but until it is
+     settled we do not know whether these organisms are really
+     parasitic in the sense that _Phytophthora_ is, or merely gain
+     access by other means--I have traced them through dead
+     fungus-hyphae--to the vessels, dead cell-walls, etc. The proof
+     of infection _via_ water pores and vessels is given for one
+     species by Harding, "Die Schwarze Faulnis der Kohls," etc.,
+     _Cent. f. Bakt._, Abh. II., B. VI., 1900, p. 305, with
+     literature.
+
+     Concerning the "Damping off" of seedlings, see Marshall Ward,
+     "Observations on the Genus Pythium," _Quart. Journ. Microsc.
+     Soc._, Vol. XXIII., 1883, p. 485, and Atkinson, _Bull. 94 of
+     Cornell University Agric. Expt. Station_, 1895, p. 233.
+
+     On Bacteriosis in Turnips, see Potter, _Proc. R. S._ 1901,
+     Vol. LXVII., p. 442.
+
+
+
+
+CHAPTER XXVI.
+
+NECROTIC DISEASES.
+
+     _Patches--Frost-patches--Bruising due to hail, shot, etc.--
+     Fire--Sun-burn or scorching--Sun-cracks. Dying-back--Frost--
+     Fungi--Wound fungi--Defoliation by insects--Defoliation by
+     hand--Staghead._
+
+
+_Necrosis._--This is a general term for cases where the tissues
+gradually turn brown or black in patches which die and dry up, the dead
+area sometimes spreading slowly and invading the usually sharply
+demarcated healthy tissues around. It is a common phenomenon on the more
+slender stems or branches of trees, especially those with a thin cortex,
+and the terms _Brand_ or _Scorching_ sometimes applied signify the
+recognised resemblance between burnt patches and these dead areas of
+necrotic tissue.
+
+Necrosis is often due to frost, which kills the cortex of Pears, Beech,
+etc., in patches of this kind. The dead cortex and cambium stick to the
+wood beneath and contract as they dry. The living cambium and cortex
+around them then begin to push in callus towards the centre of the
+necrotic area; but since this callus is formed under the pressure of the
+cortical tissues it does not form a thick lip or margin to the healing
+wound, as it does in a Canker, but insinuates itself with thinned-off
+edges between the wood and the dead tissue, or at most traps a little of
+the latter in the final closing up of the wound. It is easy to see how
+such an area of Necrosis may become a Canker if the dead tissues split
+or slough off, and fungi or insects obtain access to the callus at the
+margins of the area, setting up the disturbances described on p. 222. As
+matter of fact many Cankers--_e.g._ those of the Larch disease, and
+those due to _Nectria_, or Aphides, etc.--often begin as flattened or
+depressed areas of Necrosis started by frost, and many small necrotic
+patches would eventually become Cankers if not healed up by the callus.
+
+Necrosis may also be due to the bruising of the tissues by large
+hailstones, to gun-shot wounds, or to any form of contusion which kills
+the living cells of cortex and cambium.
+
+Necrosis is a natural and common result of fire, and it frequently
+happens after forest-fires which have run rapidly through the dry
+underwood, fanned by steady winds, that the lower parts of the boles are
+scorched on one side only. The killed cambium and cortex then dry up in
+black necrotic patches, which may eventually heal up by intrusion of
+callus from the uninjured parts.
+
+_Sun-burn_ or _Scorching_.--If thin-barked trees, such as Hornbeam,
+Beech, Firs, etc., which have been growing in partial shade owing to
+dense planting, are suddenly isolated by thinning, the impingement of
+the sun's rays on the south-west side during the hottest part of summer
+days may kill the cambium, and produce necrosis of the cortical tissues,
+and such necrotic patches heal very slowly or not at all, because the
+dead tissues have contracted so tightly on to the wood below that the
+callus cannot readily creep between.
+
+_Sun-cracks_ are due to intense insolation on the south side of trees in
+clear weather in early spring, causing the drying and contraction of the
+wood and its coverings down that side of the tree: the contracted
+tissues consequently split, as in the case of frost-cracks, the healing
+up of which is very similar.
+
+_Dying-back._--All that is true of the necrosis of cortical tissues in
+small patches also applies to cases where the whole of the outer tissues
+of thin twigs and branches die of inanition owing to a premature fall of
+leaves--_e.g._ after a severe attack of some insect or fungus pest. The
+consequent arrest of the transpiration current and the proper supply of
+nutriment to the cambium and cortex explain the phenomena. The younger
+branches of Coffee trees suffering from severe attacks of leaf-disease
+are often denuded of leaves and die back from the causes mentioned, the
+whole of the outer tissues becoming necrotic, and drying up tight on to
+the wood, because other branches with functionally active leaves on them
+divert the transpiration current, and drought and inanition supervene.
+
+Dying-back is frequently also a direct effect of early frosts, which
+kill the thin twigs before the "wood is ripened," as gardeners say.
+
+Dying-back is also a frequent result of direct frost action on thin
+watery shoots or "unripe wood," and is apt to occur every year in
+certain varieties of Roses, for instance, in particular situations, such
+as "frost-beds," or aspects exposed to cutting winds, and so forth. The
+necrosis which results may affect all the tissues, or only the cortex
+and cambium, and the frequent accompaniment of all kinds of saprophytic
+_Ascomycetes_ and moulds or other fungi is in no way causal to the
+phenomenon.
+
+Dying-back may also be caused by fungi, and not necessarily parasites,
+for cases are often observed where saprophytes only are to be found in
+the necrotic tissues of the cortex, having made their way in through
+minute cracks, lenticels, etc.
+
+A simple case is often seen in Chrysanthemums, Roses, etc., chilled and
+wetted to danger point, but not frozen, during the nights of autumn. The
+lowered resistance of the chilled tissues enables fungi like _Botrytis
+cinerea_ to gain a hold, and the peduncles die-back with all the
+symptoms of Necrosis, the fungus gaining power more and more as its
+mycelium spreads in the dead tissues.
+
+Many other cases are known where wound-fungi, such as _Nectria_,
+_Cucurbitaria_, _Phoma_, etc., in themselves incapable of true
+parasitism, gain a hold on the necrotic tissue of a wounded twig, and
+having laboriously accumulated a vigorous mycelium saprophytically,
+extend into other parts. In many of these cases the dying-back of the
+twigs is expedited owing to the mycelium invading the medullary rays and
+wood vessels, and so obstructing the transpiration current. The much
+more rapid spread of the hyphae up into the parts thus killed
+sufficiently indicates the fundamentally saprophytic character of such
+fungi.
+
+Dying-back in all its forms is a common result of defoliation by
+insects, _e.g._ caterpillars, especially if it occurs when the wood is
+depleted of reserve materials, and thus cannot supply the auxiliary buds
+and enable the twigs to clothe themselves with a new flush of foliage, a
+common danger in Conifers.
+
+Any form of defoliation--_e.g._ excessive plucking of tea and mulberry
+leaves, browsing of animals, etc.--exposes the twigs to the dangers of
+dying-back, the accessory phenomena being similar to those already
+described.
+
+_Stag-head._--Old trees, though vigorous and in full foliage throughout
+the crown generally, frequently lose the power of bearing leaves on
+their topmost branches and twigs, which stand out bare and brown, and
+fancifully resemble the antlers of a stag: hence the forester's name
+"stag-head." This "top-dry" condition is frequently due to the removal
+of litter, or to excessive draining, or to the roots having gradually
+penetrated into unsuitable soil. The consequence is that some dry
+summer the drought causes the breakage of the water columns above, and
+the twigs die back.
+
+Tropical trees may also become _stag-headed_ owing to the attacks of
+_Loranthus_ and other parasites, the portions above the point of
+attachment dying back from inanition.
+
+Cases also occur in the tropics where the _stag-head_ condition is due
+to the persistent roosting of frugiferous bats--"flying foxes"--which
+tear the bark and foliage with their claws, and befoul the twigs
+generally.
+
+
+NOTES TO CHAPTER XXVI.
+
+     The principal literature as regards frost is given in the
+     works of Frank, Sorauer, and Hartig already referred to. An
+     excellent summary will be found in Hartig's _Diseases of
+     Trees_, p. 282, and in Fisher "Forest Protection," Vol. IV. or
+     Schlich's _Manual_, p. 423.
+
+
+
+
+CHAPTER XXVII.
+
+MONSTROSITIES AND MALFORMATIONS.
+
+     _Monstrosities--Teratology--Atrophy of organs--Shanking of
+     grapes--Barren fruit trees--Dwarfing--Distortions and
+     malformations--Fasciations--Flattened roots--Torsions--Curling
+     and puckering--Leaf rolling--So-called "spontaneous"
+     teratological changes._
+
+
+_Monstrosities._--In a wide sense this term is applicable to many cases
+here treated under other headings, and signifies any departure from the
+normal standard of size, form, arrangement, or number of parts, and so
+forth, due to arrest of growth, excessive growth of parts, or of the
+whole organs, etc.
+
+Such _teratological_ conditions are however by no means always
+_pathological_: that is to say, they may be variations which do not
+threaten the existence of the plant. In some cases they are clearly due
+to exuberant nutrition, and although they may occasionally predispose to
+disease, in others they show no evidence of doing so. The whole
+practice of horticulture and agriculture abounds in examples of
+teratological sports or varieties which are transmissible by seeds,
+budding and grafting, and other means--_e.g._ double flowers,
+hypertrophied floral organs (cauliflowers), seedless grapes and oranges,
+crested ferns, etc.; and even when such varieties could not live as such
+in a state of nature, there is evidence to show that many of them
+readily revert to the original seed-bearing or single condition, and
+adapt themselves to the altered environment.
+
+Every part of the plant may exhibit teratological changes, and I shall
+for the most part select cases in illustration which indicate approach
+to pathological states, and group with them cases known to be
+pathological in origin.
+
+_Atrophy_ is a common phenomenon denoting dwindling or reductions in
+size of organs due to insufficient nutrition, or arrest of growth from
+various causes.
+
+Atrophy of leaves is a common result of the attacks of parasitic fungi,
+even when the latter induce local hypertrophy--_i.e._ excessive growth
+of particular parts, _e.g._ _Synchytrium_ on Dandelions and Anemones.
+_Puccinia suaveolens_ causes partial atrophy of the leaves of Thistles,
+_Aecidium Euphorbiae_ of those of _Euphorbia_.
+
+The carpels of Anemone are atrophied in plants attacked by _Aecidium_,
+and the whole flower is suppressed in Cherries infested with _Exoascus
+Cerasi_, while other fungi--_e.g._ _Cystopus_, _Exoasci_, etc.--cause
+atrophy of the seeds, and numerous instances of atrophied grain occur in
+plants infested with Ustilagineae.
+
+Atrophy of the grains of cereals is sometimes due to the direct attack
+of animals, _e.g._ eel-worms (_Tylenchus_) eat out the grains of Corn;
+weevils and other beetles (_Curculio_, _Bruchus_, etc.) similarly devour
+the contents of grain and nuts, the flowers of Peas and Apples, and so
+forth, inducing atrophy of the parts left. Still more striking cases are
+afforded by small insects which bore into the halms of cereals, and
+cause atrophy of the whole ear--_e.g._ _Cephus_ in Wheat and Rye. Barley
+occasionally withers after flowering, the grain atrophying from no known
+cause, terms like _consumption_ given to the disease conveying no
+information.
+
+Atrophy of young fruits is commonly due to the flowers not
+setting--_i.e._ some agent has interfered with the normal transference
+of the pollen to the stigma. This may be due to excessive rain washing
+out the pollen (_e.g._ Vine), to a lack of the necessary insects which
+effect pollination, often seen in greenhouse plants; to the stamens
+being barren--_e.g._ certain varieties of Vine--or to the premature
+destruction of the stigmas by frost, as in Cherries, Pears, etc., or by
+insects, as in Apples, or fungi, _e.g._ the infection of bilberries with
+_Sclerotinia_; or even by poisonous gases, as is sometimes seen in
+Wheat, etc., growing near alkali works. Drought is also a common cause
+of atrophy of young Plums.
+
+_Shanking of Grapes_ is a particular case of atrophy and drooping of the
+immature fruits, due to the supplies being cut off by some agency. It
+may arise from very various causes which bring about disease in the
+leaves or roots, and should always be looked upon as a sign of weakness
+in the Vine, the structure of which is affected, _e.g._ poor wood--or
+the functions interfered with, _e.g._ water supplies deficient owing to
+paucity of roots.
+
+Barren Apple, Pear, Plum, and other flowers are often found to have been
+bored through the petals while in bud, and the whole "heart" of the
+flower eaten out by the grubs of _Anthonomus_, leaving the unopened buds
+brown and dead, as if killed by frost or drought, and often erroneously
+supposed to be so.
+
+The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_,
+the mycelium of which pervades the roots and stock, on which the
+sclerotia may be found. Lucerne is similarly killed in Europe by the
+barren mycelium of _Leptosphaeria_, which may be found as a purple mat
+on the roots.
+
+_Dwarfing_ consists in partial atrophy of all the organs, and is a
+common result of starvation in poor, dry, shallow soils, as may often be
+seen in the case of weeds on walls or in stony places. Dwarfs which are
+thus developed in consequence of perennial drought are not, however,
+necessarily diseased, in the more specific sense of the word; their
+organs are reduced in size proportionally throughout in adaptation to
+the conditions, and simply carry out their functions on a smaller scale.
+
+Dwarfing is frequently a consequence of the lack of food materials, or
+of some particular ingredient in the soil, and in such cases is a
+diseased condition of some danger; similar results may ensue in soils
+containing the necessary chemical elements, but in unavailable forms.
+
+Dwarfing may also be brought about by repeated maiming, nipping off the
+buds, pruning, etc., as in the miniature trees of the Japanese; and the
+case of trees continually browsed down by cattle, or of moor plants
+perennially dwarfed by cutting winds, are further illustrations in the
+same category, as are also those of certain alpine and moraine plants,
+whose only chance of survival depends on their adapting themselves to
+the repeated prunings suffered by every young shoot which rises into the
+cutting winds, since there is no question of lack of food-materials in
+these cases.
+
+The practice of the Japanese is to pinch out the growing tips of the
+shoots wherever they wish to prune back, and it is by the judicious use
+of this heading in, and suitable pot-culture, that the dwarfs are made,
+6-20 inches high at from 30-80 years old.
+
+Dwarfing is often brought about by grafting on a slow-growing stock, and
+this method is employed in practice, as are also heading in, pruning of
+roots, and confinement in pots.
+
+Dwarfing may also be due to poor or shrivelled--partially
+atrophied--seeds or such as have had their endosperms or embryos injured
+by insects or fungi, and although it is possible to nurse such dwarfs
+into normal and vigorous plants with good culture, they do not usually
+recover under natural conditions in competition with more vigorous
+plants.
+
+_Distortions_ or _Malformations_ may be defined as abnormalities in the
+form of organs which concern all, or nearly all the parts, and do not
+refer merely to swellings or excrescences on them or excavations, etc.,
+in them.
+
+_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are
+occasionally expanded into broad ribbon-like structures often studded
+with more than the normal number of buds or leaves, etc., such as would
+be found on the usual cylindrical shoots. Such _fasciations_ are due to
+several buds fusing laterally under compression when young and the whole
+mass growing up in common, or, in a few cases, to the unilateral
+overgrowth of one side of the terminal bud. Fasciations appear to depend
+on excessive nutrition in rich soils. They may spread out above in a
+fan-like manner, exaggerating the abnormality, or they may revert to the
+original form. Some cases are more or less fixed by heredity--_e.g._
+_Celosia_. Fasciated stems are frequently curved like a crozier, owing
+to one edge growing more rapidly than the other.
+
+Cauliflowers are really cultivated monstrosities. Fasciated Dandelions,
+_Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved
+Clovers, spiral Teazels, etc., may all, if grown with care, be kept more
+or less constant in the monstrous state. That is to say, the particular
+kinds of variation here manifested can be maintained in proportion as
+the external conditions controlling the variation are maintained. Such
+conditions are chiefly rich supplies of food-stuffs, plenty of water and
+air, suitable temperature and lighting, etc. Mutilations, favouring the
+development of abnormal buds may also induce fasciations.
+
+_Torsions_ or spiral twistings of stems also frequently arise among
+plants grown in rich soils, and are often combined with
+fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that
+the peculiarity is not only transmissible by seed, but may be more or
+less fixed by appropriate culture.
+
+_Contortions_ of stems are often due to the unequal growth on different
+sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on
+Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow,
+_Cystopus_ on inflorescences of _Capsella_, etc.
+
+_Distortions_ of roots may be brought about in various ways by the
+hindrances afforded by stones.
+
+_Spiral roots_ occur occasionally in pot plants.
+
+_Flattened roots_ usually result from compression between rocks, the
+young root having penetrated into a crevice, and been compelled to adapt
+itself later. The distortions of stems by constricting climbers, wire,
+etc., have been described, and fruits--_e.g._ Gourds--are easily
+distorted by means of string tied round them when young.
+
+Distortions of leaves are very common, and are sometimes
+teratological--_i.e._ due to no known cause--_e.g._ the pitcher-like or
+hood-like _cucullate_ leaves of the Lime, Cabbage, _Pelargonium_, etc.,
+and of fused pairs in _Crassula_. Also coherent, bifurcate, crested,
+displaced and twisted leaves occasionally met with, and in some cases
+fixed by cultivation, may be placed in this category.
+
+_Puckers_ must be distinguished from pustules, since they consist in
+local upraisings of the whole tissue, not swellings--_e.g._ the
+yellowish green pockets on Walnut leaves, due to _Phyllereum_.
+
+Puckered leaves in which the area of mesophyll between the venation is
+increased by rising up in an arched or dome-like manner are sometimes
+brought about by excessive moisture in a confined space.
+
+_Leaf-curl_ is a similar deformation caused by fungi, such as _Exoascus_
+on Peaches.
+
+Wrinkling or puckering of leaves is also a common symptom of the work of
+Aphides--_e.g._ Hops.
+
+Characteristic curling and puckering, with yellow and orange tints, of
+the terminal leaves of Apples, Pears, etc., are due to insects of the
+genera _Aphis_, _Psylla_, etc.
+
+Small red and yellow spots with puckerings and curlings of the young
+leaves of Pears, the spots turning darker later on, are due to
+_Phytoptus_.
+
+_Leaf-rolling._--The leaves of Beeches, Poplars, Limes, and many other
+plants, instead of opening out flat, are often rolled in from the
+margins, or from the apex, by various species of _Phytoptus_,
+_Cecidomyia_, or other insects, which puncture or irritate the
+epidermis in the young stages and so arrest its expansion in proportion
+to the other tissues. According as the lower or upper surface is
+attacked the rolling is from the morphologically upper surface
+downwards, or _vice versa_. Very often the mesophyll is somewhat
+thickened where rolled and _Erineum_-like hairs may be developed--_e.g._
+Lime. Many caterpillars also roll leaves, drawing the margins inward to
+form shelters--_e.g._ _Tortrix viridana_, the Oak leaf-roller. Certain
+beetles--_Rhynchitis_--also roll up several leaves to form a shelter in
+which the eggs are laid.
+
+Webs are formed among the mutilated leaves of Apples by the caterpillars
+of _Hyponomeuta_.
+
+It must be borne in mind that instances can be found of teratological
+change of every organ in the plant--_e.g._ stamens transformed into
+carpels or into petals; anthers partly polliniferous and partly
+ovuliferous; ovules producing pollen in their interior, and so on, being
+simply a few startling examples of what may happen. Such abnormalities
+are frequently regarded as evidence of internal causes of disease, and
+this may be true in given cases; in a number of cases investigated,
+however, it has been shown that external agents of very definite nature
+bring about just such deformations as those sometimes cited as examples
+of teratology due to internal causes, and the question is at least an
+open one whether many other cases will not also fall into this category.
+The study of galls has shown that insects can induce the formation of
+not only very extraordinary outgrowths of tissues and organs already in
+existence, but even of new formations and of tissue elements not found
+elsewhere in the plant or even in its allies; and Solms' investigations
+on _Ustilago Treubii_ show that fungi can do the same, and even compel
+new tissues, which the stimulating effects of the hyphae have driven the
+plant to develop, to take part in raising and distributing the spores of
+the fungus--_i.e._ to assume functions for the benefit of the parasite.
+Molliard has given instances of mites whose irritating presence in
+flowers causes them to undergo teratological deformations, and Peyritsch
+has shown that the presence of mites in flowers induces transformations
+of petals into sepals, stamens into petals. Similarly De Bary, Molliard,
+Magnus, Mangin, and Giard have given numerous cases of the
+transformation of floral organs one into another under the irritating
+action of fungi, of which the transformation of normally unisexual
+(female) flowers into hermaphrodite ones, by the production of stamens
+not otherwise found there, are among the most remarkable.
+
+These and similar examples suffice to awaken doubts as to whether any
+teratological change really arises "spontaneously," especially when we
+learn how slight a mechanical irritation of the growing point may induce
+changes in the flower; _e.g._ Sachs showed that a sunflower head is
+profoundly altered by pricking the centre of the torus, and Molliard got
+double flowers by mechanical irritation.
+
+
+NOTES TO CHAPTER XXVII.
+
+     For the details and classification of the multitude of facts,
+     the student is referred to Masters' _Vegetable Teratology_,
+     Ray Society, 1869, and the pages of the _Gardeners' Chronicle_
+     since that date.
+
+     Concerning torsions, etc., the student should read De Vries,
+     "On Biastrepsis in its Relation to Cultivation," _Ann. of
+     Bot._, Vol. XIII., 1899, p. 395, and "Hybridising of
+     Monstrosities," _Hybrid Conference Report_, _Roy. Hort. Soc._,
+     1900, Vol. XXIV., p. 69.
+
+     The reader will find an excellent account of the abnormalities
+     in flowers due to the action of parasitic insects and fungi in
+     Molliard, "Cecidies Florales," _Ann. des Sc. Nat._, Ser.
+     VIII., Bot., T. 1, 1895, p. 67.
+
+
+
+
+CHAPTER XXVIII.
+
+PROLIFERATIONS.
+
+     _Proliferations--Vivipary--Prolepsis--Lammas shoots--Dormant
+     buds--Epicormic shoots--Adventitious buds--Apospory and
+     apogamy._
+
+
+_Proliferation_ consists in the unexpected and abnormal on-growing or
+budding out of parts--stems, tubers, flowers, fruits, etc.--which in the
+ordinary course of events would have ceased to grow further or to bear
+buds or leaf-tufts directly. Thus we do not expect a Strawberry--the
+swollen floral axis--to bear a tuft of leaves terminally above the
+achenes, but it occasionally does so, and similarly Pears may be found
+with a terminal tuft of leaves, Roses with the centre growing out as a
+shoot, Plantains (_Plantago_) with panicles in place of simple spikes,
+and so on.
+
+We regard such cases as _teratological_, because they are exceptional
+for the particular species, and as _pathological_ because they appear to
+be connected with over-feeding in soils with excessive supplies of
+available food-materials; but it should be noted that conditions quite
+comparable to proliferation are normal in the inflorescences of
+Pine-apples, some Myrtaceae, Conifers, etc., and that many instances of
+proliferations come under the head of injurious actions of fungi,
+insects, and other agents.
+
+_Proliferation_ of tubers is sometimes seen in Potatoes still attached
+to the parent plant in wet weather following a drought. The eyes grow
+out into thin stolons, or forthwith into new tubers sessile on the old
+tuber. Similarly in store we sometimes find the eyes transformed
+directly into new tubers, and cases occur where the growth of the eye is
+directed backwards into the softening tuber, and a small potato is
+formed inside the parent one.
+
+Threading is also occasionally met with in the "sets" when ripened too
+rapidly in hot dry soils.
+
+_Vivipary_ is a particular case of proliferation, in a certain sense,
+where the seeds appear to germinate _in situ_, and we have small plants
+springing from the flowers, reminding us of wheat which has sprouted in
+the shocks in damp weather. In reality, however, the grains are here
+replaced by bulbils which sprout before they separate from the
+inflorescence. In varieties of _Poa_, _Polygonum_, _Allium_, _Gagea_,
+etc., this phenomenon is constant in plants growing in damp situations.
+
+_Prolepsis._--It frequently happens that branches or whole plants are
+suddenly defoliated in summer,--_e.g._ by caterpillars or other
+insects--at a time when considerable stores of reserves had already been
+accumulated during the period of active assimilation. In such cases the
+axillary buds, which would normally have passed into a dormant condition
+over the winter had the leaves lived till the autumn-fall, suddenly
+shoot out into _proleptic_ shoots (also termed Lammas shoots), and
+reclothe the tree with foliage. The wood of the year in which this
+occurs may exhibit a double annual ring, and the vigour of the tree is
+likely to suffer in the following season and no fruit be matured.
+
+Proleptic branches may also be due to the shooting out of accessory
+buds--_i.e._ extra buds found in or near the leaf-axils of many plants,
+such as Willow, Maples, _Cercis_, _Robinia_, _Syringa_, _Aristolochia_,
+etc.--which do not normally come to anything, or do so only if a surplus
+of food materials is provided.
+
+_Dormant buds_, or _preventitious buds_, are such as receive no
+sufficient supply of water and food materials to enable them to open
+with the other buds in ordinary years, for in most trees only the upper
+buds on the branches develop into new shoots. The lower buds do not die,
+however, but merely keep pace with the growth in thickness of the parent
+branch, and may be elongated sufficiently each year to raise the minute
+tips level with the bark, their proper cambium only remaining alive but
+not thickening the bud.
+
+When, by the breaking of the branch above the insertion of the dormant
+bud--or by pruning, defoliation by insects, etc.--the transpiration
+current and supplies of food materials are in any way deflected to the
+minute cambium and growing points of the dormant buds, they are
+stimulated to normal growth, and may grow out as _epicormic shoots_ or
+"shoots from the old wood." In many cases such epicormic shoots are
+stimulated to grow out by suddenly exposing an old tree to more
+favourable conditions of root-action and assimilatory activity, owing to
+the felling of competing trees which previously hemmed it in from light
+and air, and restricted the spread and action of its roots in the soil.
+This is often seen in old Elms, Limes, etc.
+
+It is by such means as the above that substitution branches are obtained
+when a leader is broken or cut away.
+
+_Adventitious buds_ are such as are newly formed from callus or other
+tissues in places not normally provided with buds, as is often seen on
+occluding wounds--_e.g._ stool shoots. They may also be developed on
+roots, a fact utilised in propagating _Bouvardias_, Horse-radish, etc.,
+by means of root-cuttings, and the _suckers_ of Plums and other fruit
+trees are shoots springing from adventitious buds on roots.
+
+Adventitious buds are also common on leaves (_e.g._ _Bryophyllum_,
+Ferns, etc.), and are frequently induced on them by wounds--_e.g._
+_Gesneria_, _Gloxinia_, etc. Even cut cotyledons may develop them, and
+pieces of leafless inflorescence (Hyacinth), hypocotyl (_Anagallis_),
+and in fact practically any wounded tissue with a store of reserve
+materials may be made to develop them: thus they have been found arising
+from the pith of Sea-kale, and are commonly developed from the cut bulb
+scales of Hyacinths.
+
+_Apospory_ and _Apogamy_ are particular cases of the production of
+vegetative buds on the leaves in place of sporangia in Ferns (Apospory),
+and on prothallia in place of Archegonia (Apogamy), in the latter case
+induced by dry conditions and strong illumination.
+
+
+NOTES TO CHAPTER XXVIII.
+
+     In addition to the literature quoted in the notes to Chapter
+     XXVII., the student should consult the works on Forest Botany
+     for the scattered information regarding adventitious buds. A
+     good account may be found in Buesgen, _Bau und Leben unserer
+     Waldbaeume_, Jena, 1897.
+
+     For Apospory and Apogamy, see Lang "On Apogamy and the
+     Development of Sporangia upon Fern Prothalli," _Phil. Trans._,
+     vol. 190, 1898, p. 187, where the literature is collected.
+
+
+
+
+CHAPTER XXIX.
+
+GRAFTS.
+
+     _Grafting--Comparison with cuttings--Effects of environment--
+     Relations between scion and stock--Variation in grafts--
+     Grafting and parasitism--Infection--Pollination--Grafts-hybrids
+     --Predisposition of Natural grafts--Root-fusions._
+
+
+Grafting is a process which consists in bringing the cambium of a shoot
+of one plant into direct union with that of another, and is practised in
+various ways, the commonest of which is as follows:
+
+One plant--the _stock_--rooted in the ground, is cut off a short
+distance above the surface of the soil, and a shoot from the second
+plant--the _scion_--cut off obliquely with a sharp knife, is inserted
+into a cleft in the stock, so that the two cambiums (and sometimes the
+cortex and pith of each as well) are in close contact: the scion is then
+tied in position, the wounds covered with grafting wax, and the whole
+left until union of the tissues is completed. This union depends on the
+formation of _callus_ at the cut surfaces, and the intimate union of the
+ingrowing cells from each callus.
+
+The development of the callus follows the course described for wounds,
+cuttings, etc., and the union is exactly comparable to the union of the
+two lips of a healing callus over a wound (see p. 197).
+
+Grafting was known and practised far back in the ages. Virgil was well
+acquainted with the process, and Theophrastus compared it with
+propagation by cuttings.
+
+The scion differs from a cutting, however, in having no roots of its
+own: it is parasitic upon, or rather is in symbiosis with the stock, the
+root and tissues of which intervene between it and the soil.
+Consequently the selective absorption, size and number of vessels, and
+innumerable other physiological and anatomical peculiarities of the
+stock determine what and how much shall go up into the scion, while the
+latter supplies the former with organic materials and rules what and how
+much food, enzymes, and other secretions, etc., it shall receive to
+build up its substance. Surely, then, if such factors as the nature of
+the soil, the water and mineral supplies, the illumination, and the
+various climatic factors of altitude can cause variations on a plant
+direct, these and other factors are still more likely to be effective on
+stock and scion, and each must affect the other.
+
+Nevertheless opinions have differed much as to whether any important
+effect is to be seen, and on no point more than on whether the scion
+can affect the stock, in spite of such examples as _Cytisus Adami_,
+_Garreya_ on _Aucuba_, Sunflower on Jerusalem Artichoke, etc. Recent
+results, especially of experiments with herbaceous plants, show that not
+only can the stock affect the scion (and _vice versa_) directly, but the
+effect of the changes may be invisible on the grafted plant and only
+show itself in the progeny raised from the seed of the grafted plant. In
+other words, variation occurs in grafts either _directly_, as the
+results of the effects of the environment on the graft, or owing to the
+interaction of scion and stock, showing as changes in general nutrition
+in the tissues concerned, etc., owing to special reactions of the
+protoplasm of the uniting cells one on the other, and of the results of
+the further protoplasmic secretions, sortings, and so forth, on the
+cells developed as descendants of these in the further growth of the
+graft: or _indirectly_, in that some of these changes so alter the
+nature of the special protoplasm put aside for reproductive purposes,
+that the resulting embryo in the seed transmits the effects, and they
+show as variations in the seedling. If these results are confirmed they
+should meet all objections that have been urged against the transmission
+of acquired characters.
+
+In fact there are analogies between grafting and parasitism which cannot
+be overlooked, and should not be underestimated, their commonest
+expression appearing in the alterations in stature, habit, period of
+ripening, and so forth. These analogies are easily apprehended when we
+compare parasites like the Mistletoe, _Loranthus_, or even such
+root-parasites as the Broom-rapes and the Rhinanthoideae with grafts;
+but they also exist in the case of many fungus-parasites, and we might
+almost as accurately speak of _grafting_ some fungi on their hosts as of
+_infecting_ the latter with them, especially when it is borne in mind
+that the effect of the scion on the stock is by no means always to the
+benefit of the latter, and that there are reasons for regarding the
+action of some such unions as that of a sort of slow poisoning of the
+stock by the scion. Why do we not here say that the stock has been
+_infected_ by the scion?
+
+The resemblances between pollination and the infection by fungus hyphae
+may also be insisted upon. If we take into account Darwin's remarkable
+experiments showing that in "illegitimate unions" the pollen exerts a
+sort of poisonous action on the stigmas or ovules, it is possible to
+arrange a series of cases starting with perfectly legitimate
+pollinations where the pollen tube feeds as it descends the style on
+materials provided by the cells, and proceeding to cases where the
+pollen is more and more merely just able to penetrate the ovary and
+reach the ovules, to the extreme cases where no union at all is
+possible.
+
+Side by side with such series could be arranged analogous cases where
+fungus spores can enter and infect the cells of the host, and live
+symbiotically with or even in them, or can penetrate only with
+difficulty, or with poisonous effects, and finally cannot infect the
+plant at all.
+
+Less obviously, but nevertheless existing, are gradations in grafting to
+be observed, where one and the same stock may be successfully combined
+with a scion which improves it--or which is improved by it--or the scion
+may unite but acts injuriously on it, or, finally, cannot be induced to
+unite.
+
+But we may go further than this in these comparisons. Just as the
+results of pollination frequently induce far-reaching effects on distant
+tissues--_e.g._ the swelling of Orchid ovaries, and rapid fading of the
+floral organs--so also the effects of hyphae in the tissues may induce
+hypertrophies, deflection of nutrient materials, and the atrophy of
+distant parts--_e.g._ the curious phenomena observed in _Euphorbia_
+attacked by _Uromyces_--and some of the distant actions in grafts may be
+compared similarly.
+
+Going still further, we may compare the effects of cross-breeding or of
+hybridisation, where the _progeny_ show that changes have resulted from
+the mutual interactions and reactions of the commingled protoplasm, with
+Daniel's results, in which he obtains proof of such interactions of the
+commingled protoplasmic cell-contents of grafts in the seedling progeny;
+although there is no probability--we may even say possibility--in this
+latter case that the effects are due to nuclear fusions, but only that
+the germ-plasm of the seed-bearing plant has been affected by the
+changes in the cell-protoplasm which nourishes it when the reproductive
+cells are forming.
+
+In the case of graft-hybrids the matter appears to be somewhat
+different, and we may well suppose, with Strasburger, that the
+commingling of characters observed in flowers, fruits, foliage, etc., on
+shoots borne after grafting are due to the occurrence of nuclear fusions
+during the union of the grafted tissues; though it is by no means
+impossible that what has really happened is profound alterations in the
+nuclear substance (germ-plasm) owing to its being nourished by
+cell-protoplasm (somato-plasm) which has been itself affected by the
+interchanges of substance between scion and stock, and therefore itself
+furnishes a different nutrient medium from the unaltered cytoplasm of
+either.
+
+But even here we can find parallels among the ordinary phenomena of
+plant reproduction. Maize plants with white endosperm containing starch,
+if crossed by pollen from other plants with purple endosperm containing
+sugar, bear seeds with purple endosperm containing sugar, and such
+_Xenia_ may be compared to graft-hybrids in many respects.
+
+I know of no case among fungus infections which could be compared
+directly with these examples, and it is not at all likely that we shall
+meet with any instance of a fungus-hypha handing over nuclear substance
+to an egg-cell, and so affecting the latter that an embryo results. But
+the case is not hypothetically impossible, although the distant
+relationships of the two groups of organisms render it extremely
+improbable among the higher plants. It is by no means so improbable,
+however, that further research may show cases where the egg-cell of a
+lower cryptogam--_e.g._ another fungus--may be affected either directly,
+or indirectly, by the protoplasm of a parasitic or symbiotic hypha, as
+suggested by the extraordinary phenomena of symbiosis.
+
+Some of the variations in grafted plants are found to predispose the
+plant to disease, or the reverse, and cases may be cited where the
+resulting shoots, foliage, or fruits, or seedlings more readily fall a
+prey to, or resist, parasitic fungi and insects than the ungrafted
+plants. Daniel gives instances of such--_e.g._ among other examples,
+Peas grafted on Beans yield seeds which suffer more from Erysipheae than
+the normal seedlings. But the best known cases are those of Vines in
+their relations to _Phylloxera_, already referred to (p. 155).
+
+Several instances are also known where grafted plants show more or less
+resistance to such factors of the environment as low temperatures;
+grafted or budded Roses often suffer much from Erysipheae, and so forth.
+Much research is still needed to determine how far these matters depend
+on real alterations in the nature of the graft, or _are only true for
+the localities in which the experiments have been made_, a point which
+has, I think, been overlooked by all observers.
+
+Grafted plants are apparently very much exposed to injury by slugs,
+insects, and the invasions of parasites during the healing of the callus
+and the fusion process. Here again it must not be overlooked that the
+callus is, so to speak, a tit-bit of luscious, thin-walled, succulent
+tissue; and, like all wounds, the graft affords entrance to parasites
+such as _Nectria_ and Ascomycetes of various kinds, under circumstances
+very favourable to their invasion.
+
+_Natural Grafts._--It is by no means an uncommon event to find the
+branches of Beeches, Limes, and other trees which have been accidentally
+brought into contact during growth, joined where they cross. As they
+press one against the other, they become naturally grafted, by that form
+of the process known as _inarching_: except that in artificial inarching
+the operator cuts off the cortical tissues of the two branches and
+brings their cambial surfaces together, whereas in nature the cambiums
+only come into contact after the destruction by pressure, or slight
+abrasion, of the entrapped intervening tissues. The fusion occurs, in
+fact, exactly as in the burying-in of a nail or wire, referred to on p.
+211.
+
+Natural grafts are very common among the roots of trees, and possibly
+explain some queer cases of the apparent revivification of stumps of
+trees not usually given to forming abundant stool shoots. It is regarded
+as probable in some old forests that the majority of the roots of trees
+of the same species are linked up together by such natural grafts, a
+probability not diminished by the fact that such roots cross at many
+points, and are easily grafted.
+
+
+NOTES TO CHAPTER XXIX.
+
+     The student should read Bailey, _The Nursery Book_, 1896, for
+     details regarding the practice of grafting, and facts in
+     abundance can be obtained from the pages of the _Gardeners'
+     Chronicle_.
+
+     Concerning graft-hybrids and the variations of grafted plants
+     see Jouin, _Can Hybrids be obtained by Grafting?_ and
+     especially Daniel, "La Variation dans la Greffe," in _Ann. des
+     Sc. Naturelles_, S. VIII., Vol. 8, 1898, p. 1, and the
+     literature there collected. The whole subject is largely
+     controversial, and much work remains to be done.
+
+
+
+
+CHAPTER XXX.
+
+LIFE AND DEATH.
+
+     _Protoplasm--Hypothesis as to its structure and behaviour--
+     Assimilation--Growth--Respiration--Metabolism--Action of the
+     environment--Nuclear protoplasm--Pollination--Grafting--
+     Parasitism--Graft-hybrids--Life--Death--Variation--Disease._
+
+
+We have seen that all the essential phenomena of disease concern only
+the living substance--the protoplasm--of the plant, and that however
+complex the symptoms of disease may be, the occurrence of
+discolorations, lesions, hypertrophies, and so forth are all secondary
+matters subsidiary to the fundamental alterations of structure and
+function constituting the disease. It remains to see if we can adopt any
+hypothesis as to the nature of this physical basis of life--the
+protoplasm--which shall help us to understand still more clearly in what
+must reside those processes which, so long as they proceed harmoniously
+and uninterruptedly, constitute life and health, and which when
+interfered with result in disease and death. The protoplasm of the
+living plant-cell looks like a slimy translucent mass which has been
+superficially compared in appearance to well-boiled sago or clear gum.
+Fifty years of observations and experiments with it have convinced
+physiologists that it is not a mere solution or emulsion, however, or
+even a chemical compound in the ordinary sense of the term, although
+chemical analysis gets little out of it beyond water, proteids,
+carbohydrates and fats, and traces of certain mineral salts; for living
+protoplasm does not respond to the laws of physics and mechanics in
+obeying them, simply as do ordinary solutions and liquids. On the other
+hand, the most delicate chemical manipulation fails us, because when
+killed it is no longer protoplasm. Nor does the microscope advance
+matters far, beyond convincing us that this marvellous material must
+have a structure far more intimate than anything visible to the highest
+magnifying powers at our disposal.
+
+Nevertheless, some information is forthcoming from the comparative
+examination of the protoplasm of numerous different kinds of organisms,
+for we have learnt that certain ingredients and no others are necessary
+for its composition--namely, carbon, hydrogen, oxygen, nitrogen,
+phosphorus, sulphur, calcium[Note: See note at end of chapter.],
+magnesium, potassium--and it is as a rule of no use trying to foist on
+to it any substitute for any one of these. Moreover, these chemical
+elements must be given in certain definite proportions and forms: for
+instance it is of no use to offer the carbon and sulphur in such a form
+as carbon disulphide, or the nitrogen and hydrogen in that of
+hydrocyanic acid, but the carbon must be given to the protoplasm in the
+form of a carbohydrate or in some similar form, the nitrogen as an
+ammonium salt, nitrate or proteid, the sulphur as a sulphate, and so
+forth, and thus water, air, carbohydrates, and the nitrates, sulphates,
+and phosphates of potassium, calcium, and magnesium become the chief
+natural sources of the essential ingredients. Again, we have learnt that
+while there are different forms of protoplasm in the cell, and that
+these react on each other, and go through cycles of arrangement and
+rearrangements, the intimate structure must be of that kind termed
+molecular--beyond the region of vision, just as is the microscopic
+structure of a crystal; but, while like the latter affording evidence of
+order and sequence when properly examined, the structural arrangements
+and changes must be infinitely more complex.
+
+All these, and numerous other results of enquiry, have led to the
+conclusions that we must regard living protoplasm as a complex made up
+of very large molecular units, each containing atom-groupings of the
+elements named; and, partly on account of the large number of atoms they
+contain, and partly due to the vibrations of absorbed heat, these units
+must be extremely labile. Moreover, they are linked up into an
+invisible and intricate meshwork, bathed in a watery liquid held in the
+interstices somewhat as water is held in a sponge. In this imbibed
+liquid are dissolved the substances, consisting of the same elements,
+which are to serve as food, and which are to be taken up into the
+molecular framework and built up into the structure of new molecular
+units--or, as they may be shortly termed, molecules of protoplasm: in
+the bathing liquid are also dispersed the fragments--again containing
+the elements named--which have resulted from the breaking asunder of
+some of the complex protoplasm molecules, and which are partly destined
+to be used up again, partly to be burnt off in respiration, and partly
+to be put aside as metabolic products such as reserves, secretions,
+permanent structure, etc. Among the elements carried into this liquid
+and dissolved in it the free oxygen of the air also plays an important
+part.
+
+As new molecules are formed, by mutual combinations of the
+food-materials selected by molecular attractions, they are taken up into
+the protoplasmic framework, and built in between those already in
+existence, thus distending the whole, and we say that the protoplasm
+_Assimilates_ food-materials and _Grows_. When distended beyond a given
+degree, or disturbed in various other ways, the molecular framework
+breaks, and some of the molecules are shattered, and as they fall to
+pieces certain of their constituent parts containing carbon and hydrogen
+forcibly combine at the moment of liberation with the oxygen in the
+fluid around and are burnt off in the form of carbon-dioxide and water,
+heat being of course evolved. This is the fundamental process of
+_Respiration_.
+
+It is probably the alternation of these processes of _Assimilation_--the
+building up into the protoplasmic structure of new complex labile
+molecules--and _Destruction_--the shattering of such molecules
+with redistribution, oxidation, etc., of their fragments--which
+constitute the fundamental process of life. Different authorities
+attempt to explain the details of these processes in various ways,
+but there is practical agreement on the one point, that life
+consists in the alternate building up of new protoplasm from the
+food-materials--_Assimilation_--and the breaking down of the molecular
+complexes to simpler ones--_Disintegration_, or _Dis-assimilation_, as
+we may call it. During the periods when assimilation prevails, and the
+protoplasm increases in mass, we recognise _Growth_, and since this is
+usually associated with the vigorous imbibition of water, owing to the
+powerful osmotic attractions for that liquid exhibited by some of the
+products, and with consequent further stretching of the invisible
+molecular plexus, the growth may be so evident in increased size, that
+we are accustomed to look upon the visible increase in volume alone as
+growth; but it is essential to understand that growth of the protoplasm
+is always proceeding during life, even when as many older molecules are
+being shattered and dispersed as new ones are being formed by
+assimilation, and when, therefore, no visible permanent enlargement
+occurs. Similarly, during periods when disintegration of the molecules
+prevails, we must not assume that the assimilation of new molecules is
+not occurring and that growth is not proceeding. The two processes are
+always going on during the active life of the protoplasm: in fact life
+consists in the play of these processes, as already said.
+
+That numerous chemical rearrangements of the atom-complexes take place
+outside the protoplasmic molecules--both of those left unemployed in
+assimilation and of those rejected during the destructive
+processes--will be readily understood: many of the bye-products found in
+plants, such as vegetable acids, alkaloids, colouring matters,
+crystalline bodies, etc., etc., are due to these, so to speak,
+fortuitous combinations and re-combinations.
+
+The part played by respiration has often been misunderstood. It consists
+in the burning off of some of the carbon and hydrogen of the shattered
+protoplasm molecules, by means of the oxygen of the air, which finds its
+way into the fluids around the protoplasm, and when it is active every
+act of combustion--which is here an explosion--leads to the shattering
+of more protoplasm molecules, and consequently to more respiratory
+combustion of the products. If the supply of oxygen is limited the
+breaking down of the molecules of protoplasm does not cease, but the
+carbon and hydrogen which would otherwise have been oxidised are now in
+part left to form other compounds in the surrounding liquid, and thus
+incompletely oxidised bodies, such as vegetable acids, alcohols, etc.,
+accumulate. Even in the complete absence of atmospheric oxygen the
+protoplasm may go on breaking down and accumulating various compounds
+containing relatively much carbon and hydrogen--so-called intramolecular
+respiration; but in ordinary plants this process soon comes to an end,
+because the blocking up of the molecular plexus leads to obstruction and
+interferes with the normal assimilation and dis-assimilation, and, if
+prolonged, leads to pathological conditions, and eventually death.
+
+Here, then, we meet with a cause of disease, or of predisposition to
+disease. The deprivation of oxygen interferes with the normal processes
+of building up and breaking down of the protoplasmic molecules, and
+bodies we term poisonous accumulate and may lower the vitality or even
+bring life to an end.
+
+During normal life other products of the disruption of the protoplasm
+molecules are nitrogenous bodies, such as proteids, and these we have
+reason to believe are used up again, acting as the nuclei, so to speak,
+of the new molecules, and so being built up again with fresh
+food-materials into the plexus, to be again set free, and again used up,
+and so on. Others are the carbohydrates, such as cellulose, which pass
+out of the molecule into an insoluble form, and are accumulated outside
+the protoplasm in the form of cellulose membranes, and so forth. It is
+these formed products of metabolism (Metabolites), especially cellulose
+and bodies which result from its subsequent transformation, which
+constitute the main permanent mass of the ordinary plant.
+
+We are now in a position to see how another fundamental cause of disease
+or predisposition to disease exists in the deprivation of the protoplasm
+of any of the elements needed to supply--in the food-materials--the
+place of those which have been permanently put aside in the form of
+cell-walls, or burnt off in respiration, passed out as excretions, or in
+other ways lost.
+
+It is clear that the indispensability of an element must mean that the
+protoplasmic molecule cannot be completed without it: the same
+conclusion is supported by the experimental proof that these elements
+cannot be replaced by chemically similar elements.
+
+It does not follow, however, that the protoplasm molecule must always
+have the same number of atoms of these elements, and grouped always in
+the same atom-complexes before being assimilated; nor that the
+protoplasm molecule, when once built up, always breaks down in exactly
+the same way. On the contrary, while the protoplasm of corresponding
+parts of a daisy and of a rose must contain all the elements named, we
+must believe that the atom groupings are different in the protoplasm
+molecule in each case; and though the molecules of the cell-protoplasm,
+of the nucleus, of the chlorophyll-corpuscles, etc., of one and the
+same plant must have all these elements, the atom groupings and modes of
+building up and breaking down may be very different in each case.
+
+Again, the cell-protoplasm, bathed by the sap taken in by roots from the
+soil or fed directly by that derived from the leaves, must be exposed to
+very different stimuli and modes of nourishment, etc., from those
+incurred by the protoplasm of the nucleus which it encloses: and similar
+conclusions must apply in turn to the protoplasm of the root in the dark
+moist soil and of the leaf in the light dry air, or to that of the
+superficial epidermis cells as contrasted with that of the deeply
+immersed pith, and so on.
+
+It is no doubt in these directions that we must seek for the explanation
+of many life-phenomena at present quite beyond explanation. Thus, it is
+tolerably easy to modify the action of the cell-protoplasm of a plant,
+by exposing it to differences of illumination, temperature, moisture,
+and so forth, within certain limits; at least, since the changes in
+stature, tissue differentiation, cell-secretions, flowering capacity,
+etc., of plants affected by such factors of the environment--_e.g._
+alpine plants brought into the plains--_must_ be due to changes in the
+mode of activity of the protoplasm, we must assume that the above
+factors affect the latter. But it is extremely difficult to reach the
+nuclear-protoplasm directly by such stimuli, as proved by the experience
+that even where we allow the factors to act for a long time, no
+permanent change can be detected in the behaviour of the
+nuclear-protoplasm--the essential material in the reproductive organs
+and reproductive process. At least we must infer that no change has been
+permanently stamped on this nucleo-plasm from such facts as the
+characters of the seedlings of the progeny of the plain-raised plants:
+if they are again sown in an alpine situation they forthwith behave
+again as alpines.
+
+Must we not conclude, then, that this difficulty of reaching the
+nuclear-protoplasm is owing to the fact that it is nourished and
+influenced directly only by the cell-protoplasm? That the
+cell-protoplasm is its environment, and not so directly the outer world?
+We may influence the cell-protoplasm--we may make it work harder or less
+actively, respire vigorously or slowly, build up and break down in
+various different ways, or at different rates, and so forth, _within
+limits_; but it is nevertheless cell-protoplasm of its specific kind,
+with its own range of molecular variations and activities within these
+limits, and it supplies the nuclear-protoplasm with what it wants so
+long as these limits are not exceeded. Consequently, while it is very
+easy to make the cell-protoplasm vary within the limits of its range, it
+is not easy to induce it to vary its effects on the nuclear-protoplasm
+to such an extent or in such a way that the latter is permanently or
+materially altered in constitution.
+
+Nevertheless it would appear that cases do occur where the
+nuclear-protoplasm _is_ reached and affected by external stimuli, as
+evinced by some of the phenomena of hybridisation and of cross-and
+self-fertilisation, because we find the results expressed in the
+mingling of the characters of parents, in strengthened or enfeebled
+progeny, and even in the appearance of unexpected properties, which,
+from the facts of Reproduction, we know must have taken their origin in
+some alteration of the nuclear substance of the embryo.
+
+Here, however, we know in most cases that the principal agent
+which has reached the nuclear-protoplasm, is another portion of
+nuclear-protoplasm. In hybridisation, one which has been fed and
+influenced by cell-protoplasm of a very different plant; in
+cross-fertilisation, one fed and influenced by the cell-protoplasm of a
+different plant of the same species, and in self-fertilisation, one fed
+and influenced by the same cell-protoplasm.
+
+That somewhere, and somehow, such nuclear-protoplasm as induces the
+changes in the characters of hybrids, etc., has been influenced by its
+immediate environment--the cell-protoplasm of the plant--appears to be a
+conclusion from which there is no escape. We may obtain similar evidence
+from the experience of grafting. It is relatively easy to influence the
+cell-protoplasm of a scion by a suitable stock, obviously because the
+latter, while handing on to the former all necessary materials from the
+soil, presents the indispensable elements and compounds in somewhat
+different proportions, dilutions, etc., from those which its own roots
+would have done, and probably mingles with them a certain amount of its
+own peculiar products, as well as affects the modes of working and
+interaction of both by the molecular impetus impressed on them.
+Consequently the cell-protoplasm of the scion, while obtaining from the
+stock all it needs within the limits of its own variations of structure
+and activity, nevertheless builds up and breaks down in ways or at rates
+slightly different from those hitherto normal to it, and perceptible
+variations result when the sequences and correlations of these material
+and mechanical changes have affected a sufficiently large mass for the
+accumulation of visible effects. The limits to grafting suggest not that
+an inappropriate stock does not offer to the protoplasm of the scion the
+right materials, but that it presents them in proportions and in forms
+which are unsuitable for the assimilable powers of the latter, or,
+possibly, mingled with substances poisonous in themselves or capable of
+becoming so in conjunction with bodies in the scion.
+
+What has been said of the action of stock on scion, will also be true,
+_mutatis mutandis_, of the reciprocal action of scion on stock. Here
+again we may have causes for disease, or predisposition to disease.
+
+It occasionally happens, however, that the nuclear protoplasm
+of the stock or scion _is_ affected in grafting, and we infer
+from the difficulty of modifying it in any other way in ordinary
+reproduction than by means of other nuclear protoplasm--_e.g._ in
+hybridisation--that in such cases a fusion of the nuclei of stock and
+scion has occurred during the grafting, and a graft-hybrid has
+resulted--_e.g._ _Cytisus Adami_.
+
+It is not impossible however that the nuclear protoplasm has in such
+graft-hybrids been subsequently modified by the differences in nutrition
+to which it has been subjected, in the modified cell-protoplasm affected
+by the mingling of the juices, etc., of scion and stock; for it is quite
+conceivable that such materials may affect the protoplasm far more
+profoundly than anything derived directly from the environment.
+
+If Daniel's researches are confirmed, however, it appears that in some
+cases, at any rate, the nuclear-protoplasm is so altered by the grafting
+that when the new embryo is developed, after fusion with nuclear
+substance from another plant of the same species, the results are
+apparent only in the progeny, and _the effects of alteration in the
+cell-protoplasm have been transmitted to the nuclear protoplasm of the
+germ-cells_--_i.e._ acquired characters have been transmitted and fixed
+by heredity. Should this prove true the importance of the results can
+hardly be over-estimated. The matter is too problematical for further
+discussion here, but we see that any such action may profoundly affect
+the "constitution" of the resulting plant.
+
+Turning now to the case of fungi or other organisms which obtain access
+to the cell-protoplasm. At the one extreme we have cases where the
+protoplasm of the diseased plant is rapidly and directly poisoned and
+destroyed, as in the killing off of seedlings in "Damping Off": near the
+other extreme we have cases where the foreign protoplasm of the
+parasite, although it gains complete access to that of the host, merely
+stimulates the latter to greater activity and itself works for its own
+ends in conjunction with it--_e.g._ _Plasmodiophora_. In such instances
+we must figure to ourselves the cells of the root of the Crucifer
+handing on food-materials to both masses of protoplasm--that of the
+_Plasmodiophora_ and that of the cell into which it penetrates; and it
+is immaterial whether both obtain the food-materials directly, or, what
+seems more likely, the fungus only at second hand and by the medium of
+the host's protoplasm. In any case, the latter is for a long time at
+least not poisoned or maimed, or in any perceptible way injured by
+excreta from the fungus-protoplasm, although it is evident that each
+must excrete various metabolites which may soak into and be taken up by
+the other: on the contrary the host-protoplasm grows larger, attracts
+more food supplies, makes larger cells, and is evidently stimulated to
+greater activity for the time being, its behaviour reminding us of the
+stimulation of cells by means of slight doses of poison referred to
+previously. We must therefore assume that the general course of building
+up and breaking down of its protoplasm-molecules go on as usual--or
+nearly so--in both the host cell and the invader; and that the
+assimilatory, respiratory, excretory and other functions are carried on
+in the former as in the normal cell, or are but slightly modified to an
+extent which does no immediate injury to its life. But we must further
+assume that the same is also true of the invading protoplasm, and that
+the _Plasmodiophora_ is also supplied with suitable atom-complexes to
+build up its protoplasm molecules, as fast as they are shattered and the
+rejecta burnt off in respiration.
+
+A step further, and we come to instances of _Symbiosis_, where the
+commingled masses of protoplasm of host and invader continue this
+harmonious action during life. Clearly there are resemblances between
+these latter cases and successful grafts, and between both and
+successful sexual unions where the resulting embryo-cell gives rises to
+a vigorous and healthy plant; and the more these resemblances are
+examined in the light of what we know of symbiosis the more they support
+our contention.
+
+Such considerations as the foregoing suggest, then, that life consists
+in the regular and progressive building up and breaking down of the
+complex protoplasm molecules, and is necessarily accompanied by the
+influx of the indispensable food-elements in certain combinations and
+atom-complexes for assimilation, and by the combustion of some of the
+debris of the shattered molecules, which combine with the oxygen in
+respiration and so afford explosions which raise the temperature and
+enhance the lability of existing molecules, and act as stimuli to the
+shattering of further molecules. The results of these rhythmical
+buildings up (assimilation) and shatterings (dis-assimilation) of the
+protoplasm molecules are the growth of the protoplasm, with further
+intercalations of water and new food-supplies, etc., on the one hand,
+and the formation of metabolic products (proteids, cellulose, sugars,
+fats, etc.), some of which are again used up, others respired, others
+deposited as stores, cell-walls, etc., on the other.
+
+That the building-up process depends on the action of molecular forces
+comparable to those by which a growing crystal goes on selecting
+atom-complexes of its particular kind from the solution around seems
+highly probable, and this being the case we can understand how under
+certain circumstances _substitutive_ selections may occur. That is to
+say, just as a crystal will sometimes build up into its structure
+atom-complexes of a kind different from its normal molecules, so, given
+the proper conditions, a protoplasmic molecular unit will build up into
+its structure atom-complexes somewhat different from those it had
+hitherto taken up--_i.e._ assimilated--with consequent modifications of
+its behaviour. If this occurs, the modes of further building up and
+breaking down will be affected by the subsequent action of these
+slightly modified protoplasm units, _and it may well be that the whole
+significance of variation turns on this_. Whether the resulting
+variation makes for the welfare or otherwise of the organism will then
+be decided by the struggle for existence, and the natural selection
+which ensues. Such a view also implies that the energy concerned is
+primarily what is usually termed chemical energy, and that every
+compound entering into the protoplasm carries in a supply of this,
+available in various ways.
+
+_Death_, on the contrary, is the cessation of these rhythmical processes
+of building up and breaking down of the protoplasm molecules. It does
+not imply the cessation of chemical changes of other kinds, but that
+these rhythmical constructions of the complex and labile protoplasm
+molecules breaking down on stimulation to bodies partly re-assimilable,
+partly combustible in respiration, and partly excretory, etc., have
+ceased, and that further chemical changes in the material are
+thenceforth simpler and different in kind and degree, eventually leading
+to total disintegration so that no units are left capable of restoring
+the rhythm.
+
+If these ideas are correct, we may define _Disease_ as dangerous
+disturbances in the regularity, or interference with the completeness or
+range of the molecular activities constituting normal Life--_i.e._
+Health--and it is evident that every degree of transition may be
+realised between the two extremes. Now, if we further assume, as I think
+we must do, that a considerable range or "play" must exist in the
+molecular activities of the protoplasm constituting life, we obtain a
+sort of expression of what we mean by limits of variation. The fact that
+life can go on in a given plant at temperatures between from 1 deg.-5 deg. and
+35 deg.-40 deg. C., or in lights of different intensity, or within considerable
+ranges of water supply, concentration of salts, partial pressure of
+oxygen, etc., implies that the molecular activities of the protoplasm
+are of the normal _kind_ all the time, though they may differ in
+rapidity, and even in _quantitative_ and _qualitative_ respects within
+certain limits; and the meaning of the _optimum_ temperature,
+illumination, oxygen pressure, etc., is, from this point of view, not
+that the molecular activities differ in kind from those nearer the
+minima and maxima, so much as that they are running at the best rates
+for the welfare of the plant--_i.e._ for permanent health.
+
+If we transcend the cardinal points limiting the range of this play,
+however, and we get variations in the _kind_ as well as _rates_ of
+molecular constructions and disruptions, then we pass by imperceptible
+gradations into ill-health--_i.e._ _Disease_.
+
+And similarly in relation to other protoplasm. That of the right kind of
+pollen grain from another plant of its own species, stimulates the
+contents of the ovule to produce a vigorous embryo and healthy seedling:
+that of a similar pollen grain in its own flower either does no positive
+harm, but has a feebler effect, or it may act like a poison. That of
+another pollen grain again may refuse to unite at all; while that of a
+fungus hypha--_e.g._ of _Sclerotinia_ on _Vaccinium_--may run down the
+style as does the pollen tube and produce death and destruction
+throughout the ovule.
+
+Or again, in Clover, we may have the hypha of a _Botrytis_ with its
+protoplasm unable to do more than penetrate into the cellulose walls
+and diffuse a poison into the adjacent cells, being utterly incapable of
+directly facing, or mingling with the living protoplasm of such cells,
+whereas the protoplasm of another organism--_e.g._ _Rhizobium_--will
+penetrate directly into the cells, live in them for weeks or months
+without injury--nay even with advantage to their life. And hundreds of
+similar cases can be selected.
+
+We may, therefore, conclude that _Variation_ depends fundamentally on
+alterations in the structure or mode of building up and disintegration
+of the protoplasmic molecular unit, brought about either by direct
+modifying action of the inorganic environment--nutrition, temperature,
+oxygen supply, light, etc., etc.--or by the mingling with it of other
+protoplasm, the molecules of which since they have already a slightly
+different composition, configuration, mode of breaking down and building
+up, etc., affect its molecules by supplying them with altered nutritive
+atom-complexes, by competing with them for oxygen, etc., etc. Once these
+molecules are affected, we must assume that long sequences of other
+chemical and molecular changes will be also modified; and although we
+have no conception of _how_ these changes bring about changes in form,
+that they do so is only a conclusion of the same order as that which we
+hold regarding the much simpler changes concerned in the formation of
+crystals.
+
+That such variations may be of every degree as regards profundity,
+permanence, kind, etc., may well be imagined; and there is nothing
+surprising in our being able to induce them more easily by the action of
+external factors _in the readily accessible cell-protoplasm_ than in the
+_less exposed nuclear-protoplasm_; because the latter is only accessible
+through the former, or through the agency of _other nuclear protoplasm
+already modified_. On these and similar phenomena depend the relative
+permanency and transmissibility of the variations. Our measure of the
+latter only begins when the effects referred to have become manifest in
+large masses of cells, because only then do they become appreciable to
+our senses.
+
+Further, variations thus induced may be of advantage to the continued
+life of the plant, or in all degrees disadvantageous or threatening to
+its existence. These latter variations are _Disease_, and if their
+interference with the normal rhythmical play of the building up and
+breaking down of the protoplasm molecules proceeds beyond certain
+limits, life ceases, and we have death supervening on disease.
+
+
+NOTES TO CHAPTER XXX.
+
+     It appears probable that calcium is not always needed by
+     living cells, and may not enter into the composition of
+     protoplasm; on the other hand traces of iron are perhaps
+     necessary.
+
+     The criticisms and summary of facts on which the hypothesis
+     regarding protoplasm here adopted is based are developed at
+     length in Kassowitz, _Allgemeine Biologie_, Wien, 1899, B. I.
+     and II., where the collected literature may be found, and the
+     reader introduced to the huge mass of controversial writings
+     put forward since Darwin and associated with the names of
+     Weismann and others.
+
+     It will probably be noticed that I have employed the term
+     molecular unit of protoplasm, and have not discussed the
+     question of organised structure in the latter: this is because
+     it seems clear to me that living protoplasm as such does not
+     possess "organised structure" in the true sense of that
+     term--it is, rather, busy preparing and making "organised
+     structure," and a molecular constitution would have to be
+     ascribed to all "physiological units" of the nature of
+     micellae, pangens, ids, etc., as truly as to the structural
+     units of a starch-grain or cell-wall, or even of a crystal. In
+     this connection, the student will find the necessary points of
+     view put forward in Pfeffer, _Physiology_, pp. 37-83.
+
+
+
+
+INDEX.
+
+
+  Absorption by roots, 49.
+
+  Absorption of energy, 23.
+
+  Absorption of light, 27.
+
+  Absorption of water, 50.
+
+  _Abutilon_, 183.
+
+  _Acarus_, 88.
+
+  Accessory buds, 259.
+
+  _Acer_, 214.
+
+  Acid gases, 181, 191.
+
+  Acids, 130, 136.
+
+  Acquired characters, 283.
+
+  _Acrostalagmus_, 238.
+
+  Action of the environment, 271.
+
+  Adaptation, 176.
+
+  Adapted races, 177.
+
+  _Adonis_, 220.
+
+  Adventitious buds, 224, 225, 257, 260.
+
+  _AEcidium_, 88, 114, 116, 187, 188, 189, 217, 223, 225, 232, 247, 252.
+
+  Aeration, 104.
+
+  Aerobic organisms, 57.
+
+  Aetiology, 89, 100.
+
+  _Agaricus melleus_, 115, 143, 145, 234.
+
+  Agents of disease, 113.
+
+  _Aglaospora_, 223.
+
+  Agriculture, 65.
+
+  Agricultural Chemistry, 2.
+
+  _Ajuga_, 217.
+
+  Albinism, 179, 182, 183, 186.
+
+  Alder, 207, 219.
+
+  Aleurone layer, 173.
+
+  Algae, 215.
+
+  _Allium_, 258.
+
+  Almond, 168.
+
+  _Alnus_, 214.
+
+  _Aloe_, 134, 161.
+
+  Alpine plants, 250, 279.
+
+  American blight, 164, 219.
+
+  American vines, 155, 169, 172.
+
+  Amides, 31.
+
+  Amoeba, 144.
+
+  Amount of energy stored, 25.
+
+  Amygdalin, 173.
+
+  _Anabaena_, 128.
+
+  Anaerobic bacteria, 58, 237.
+
+  _Anagallis_, 261.
+
+  Analyses, 65.
+
+  Analyses of waters, 58.
+
+  Anemone, 247.
+
+  Animals, 99, 108, 142, 207.
+
+  _Antennaria_, 232.
+
+  _Anthonomos_, 249.
+
+  Anthrax, 144.
+
+  Antiseptics, 162.
+
+  Ants, 232.
+
+  _Aphis_, 88, 109, 161, 165, 188, 213, 214, 232, 241, 253.
+
+  _Aphrophora_, 233.
+
+  Apogamy, 257, 261.
+
+  _Aporia Crataegi_, 187.
+
+  Apospory, 257, 261.
+
+  Apple, 170, 171, 187, 189, 192, 206, 217, 218, 219, 223, 226, 231,
+      233, 248, 249, 253, 254.
+
+  Apricot, 188, 206.
+
+  Apricots, 234.
+
+  Area of root-surface, 37, 39.
+
+  _Arisarum_, 188.
+
+  _Aristolochia_, 259.
+
+  Aroids, 113.
+
+  Arrest of growth, 246.
+
+  Arsenic, 162.
+
+  Artificial wounds, 194.
+
+  Ascomycetes, 189, 217, 269.
+
+  _Ascochyta_, 190.
+
+  Ash, 182, 223, 225, 251.
+
+  _Asparagus_, 180, 230, 251, 252.
+
+  _Aspergillus_, 231.
+
+  _Aspergillus niger_, 58.
+
+  _Aspidiotus_, 187.
+
+  Assimilation, 8, 21, 133, 271, 275, 277, 285, 286.
+
+  Assimilates, 274.
+
+  Atmosphere, 1, 99.
+
+  Atmospheric influences, 101.
+
+  Atrophy, 246, 247, 266.
+
+  Attractive substances, 136.
+
+  _Aucuba_, 264.
+
+  Autumnal colouring, 191.
+
+  Autumnal fall, 93.
+
+  Avalanches, 106.
+
+
+  Bacteria, 102, 133, 143, 168, 173, 176, 182, 190, 200, 216, 219, 223,
+      227, 231, 236, 237.
+
+  Bacteriosis, 227.
+
+  Barberry, 176.
+
+  Bark boring, 204, 205.
+
+  Bark-beetles, 205.
+
+  Barley, 176, 248.
+
+  Barrenness, 246, 249.
+
+  Bats, 244.
+
+  Bean, 188, 190, 191, 268.
+
+  Beech, 192, 222, 223, 225, 233, 240, 242, 254, 269.
+
+  Beech Miner, 208.
+
+  Bees, 142, 143, 164.
+
+  Beet, 192, 216, 219, 230.
+
+  Beet-rot, 230.
+
+  Beetles, 110, 143, 145, 205, 206, 207, 248, 254.
+
+  Berkeley, 85.
+
+  Bilberries, 116, 142, 217, 218, 248.
+
+  Biology of soil, 56, 102.
+
+  Birch, 207, 218, 224.
+
+  Birds, 108, 144, 164, 166.
+
+  Bird's-eye Maple, 224.
+
+  Black spots on leaves, 186, 189, 191.
+
+  Bladders, 218.
+
+  Blemish, 198.
+
+  Blights, 86, 104, 179.
+
+  Blisters, 230.
+
+  Blue rays, 21.
+
+  _Bombyx_, 187, 218.
+
+  Bordeaux mixture, 162.
+
+  Boring, 204.
+
+  _Botrytis_, 131, 132, 136, 175, 230, 231, 243, 288.
+
+  Boussingault, 5, 10.
+
+  Bouvardia, 260.
+
+  Bramble, 112.
+
+  Branch stumps, 194, 199.
+
+  Brand, 240.
+
+  Breeding, 78.
+
+  Briars, 113.
+
+  Broom-rapes, 265.
+
+  Browning, 122, 186, 235.
+
+  Brown spots, 186, 189, 190, 191.
+
+  Browsing, 244.
+
+  _Bruchus_, 248.
+
+  Bruises, 194, 203, 240, 241.
+
+  Bryony, 112.
+
+  _Bryophyllum_, 260.
+
+  Bud galls, 219.
+
+  Bud variations, 92, 93.
+
+  Bulb diseases, 227.
+
+  Buried objects, 211, 269.
+
+  Burning, 191.
+
+  Burning-glass effect, 192.
+
+  Burrows, 204, 205.
+
+  Burrs, 222, 223, 224.
+
+  Bursting of fruits, 227, 230.
+
+  Butterflies, 145.
+
+  Bye-products, 276.
+
+
+  Cabbage, 253.
+
+  Cabbage moth, 208.
+
+  _Caeoma_, 252.
+
+  _Caesalpinia_, 233.
+
+  Calcium, 272.
+
+  Calcium oxalate, 138.
+
+  _Calla_, 183.
+
+  _Calliandra_, 233.
+
+  Callus, 119, 120, 124, 139, 140, 196, 197, 199, 201, 202, 210, 241,
+      260, 263, 269.
+
+  _Calyptospora_, 116, 217.
+
+  Cambium, 120, 196, 199, 222.
+
+  Camellia, 187.
+
+  Cancer, 127.
+
+  Canker, 87, 222, 223, 241.
+
+  _Capnodium_, 232.
+
+  _Capsella_, 116, 175, 252.
+
+  Carbohydrates, 16, 17, 20, 34, 122, 184, 272, 273, 277.
+
+  Carbolic acid, 162.
+
+  Carbon, 272.
+
+  Carbon assimilation, 8, 10, 28, 106.
+
+  Carbon-bisulphide, 163.
+
+  Cardinal points, 288.
+
+  Carrot, 164.
+
+  _Carpocapsa_, 207.
+
+  Cast branches, 123.
+
+  Castor oil, 172.
+
+  Caterpillars, 109, 164, 207, 208, 244, 254, 259.
+
+  Cats, 164.
+
+  Cattle, 108.
+
+  Cauliflowers, 247, 250.
+
+  Causes of disease, 89, 99, 108, 159, 278, 282.
+
+  _Cecidia_, 212.
+
+  _Cecidomyia_, 182, 213, 214, 218, 219, 254.
+
+  Celery, 180, 230.
+
+  Cell contents, 168.
+
+  Cell-protoplasm, 279, 280, 290.
+
+  Cellulose, 132, 277, 286.
+
+  _Celosia_, 250.
+
+  _Centaurea_, 188.
+
+  _Centhorhynchus_, 219.
+
+  _Cephaleuros_, 188.
+
+  _Cephus_, 248.
+
+  _Cercis_, 259.
+
+  _Cercospora_, 190.
+
+  Cereals, 248.
+
+  Change of conditions, 78.
+
+  Charlock, 165.
+
+  Checks to disease, 166.
+
+  Chemical analysis, 32, 64, 103, 272.
+
+  Chemical antiseptics, 159.
+
+  Chemical energy, 29, 287.
+
+  Chemotactic phenomena, 72, 130, 135, 137.
+
+  _Chermes_, 153, 223.
+
+  Cherry, 208, 209, 231, 234, 235, 247, 248.
+
+  Chestnut, 190.
+
+  Chlorine, 181.
+
+  Chlorophyll, 19, 106, 122.
+
+  Chlorophyll action, 184, 192.
+
+  Chlorophyll corpuscles, 9, 18, 22.
+
+  Chlorosis, 122, 165, 179, 180, 181.
+
+  Chrysanthemum, 243, 252.
+
+  Chytridiaceae, 127, 136, 189, 208.
+
+  _Cicada_, 235.
+
+  Cicatrix, 123.
+
+  _Cinchona_, 168, 172, 173.
+
+  Circulation of carbon, 62.
+
+  Circulation of nitrogen, 62.
+
+  _Citrus_, 168.
+
+  _Clasterosporium_, 188, 209.
+
+  Classification of diseases, 99, 101, 120.
+
+  _Claviceps_, 232.
+
+  Climate, 1.
+
+  Climbing plants, 112, 113, 210.
+
+  _Clostridium_, 236, 237.
+
+  Clothes, 142.
+
+  Clover, 164, 187, 249, 252, 288.
+
+  Cluster-cups, 188.
+
+  Coal gas, 104, 182.
+
+  Coccideae, 164, 232.
+
+  _Coccus_, 223.
+
+  Coffee leaf-disease, 114, 146, 166, 169, 242.
+
+  _Coleophora_, 153, 206.
+
+  _Coleosporium_, 169.
+
+  _Coleus_, 192, 220.
+
+  Competition of fungi, 61.
+
+  Complex interactions, 91, 99.
+
+  Conifers, 125, 205, 223, 225, 234, 258.
+
+  Constitution, 156, 283.
+
+  Consumption, 248.
+
+  Contact irritability, 125, 135.
+
+  _Contagium fluidum vivum_, 183.
+
+  Contortions, 252.
+
+  _Convallaria_, 175.
+
+  _Convolvulus_, 112.
+
+  _Copaifera_, 234.
+
+  Copper sulphate, 162, 165.
+
+  Coppery leaves, 191.
+
+  Cork, 119, 123, 194, 199, 216, 222.
+
+  Cork wings, 217.
+
+  Corky warts, 212.
+
+  Corn, 248.
+
+  Corrosion of marble, 46.
+
+  _Cossus_, 206.
+
+  Cost of epidemics, 146, 147.
+
+  Cotton, 172.
+
+  _Crassula_, 253.
+
+  Creeping of mycelia, 142.
+
+  _Crepis_, 252.
+
+  Crimson spots, 189.
+
+  Cross-breeding, 266.
+
+  Cross-fertilisation, 69, 74, 77, 281.
+
+  Cross-graining, 124.
+
+  Crucifers, 219, 284.
+
+  Cryptogams, 87, 108, 111, 113.
+
+  Cuckoo-spit, 233.
+
+  Cucullate leaves, 253.
+
+  Cucumber, 219.
+
+  _Cucurbitaria_, 217, 243.
+
+  Cultivation of pest and host plant, 168.
+
+  _Curculio_, 248.
+
+  Curling, 235, 246.
+
+  _Cuscuta_, 134.
+
+  Cuts, 119, 143, 194.
+
+  Cuttings, 194, 198, 262, 263.
+
+  Cyanide of potassium, 165.
+
+  Cycads, 128.
+
+  _Cynips_, 110, 213, 219.
+
+  _Cystopus_, 116, 136, 175, 187, 217, 247, 252.
+
+  Cytases, 132.
+
+  _Cytisus Adami_, 264, 283.
+
+
+  Daisy, 278.
+
+  Damping off, 114, 144, 160, 229, 284.
+
+  Dandelion, 247, 252.
+
+  Daniel's researches, 283.
+
+  Dark heat rays, 27.
+
+  Darwin, 72, 125.
+
+  _Dasyscypha Willkommii_, 152, 223.
+
+  Death, 271, 272, 287, 290.
+
+  De Bary, 85, 151.
+
+  Deficiency of iron, 180.
+
+  Defoliation, 109, 240, 244.
+
+  Deformation, 132.
+
+  _Dematium_, 135.
+
+  _Dematophora_, 143, 145.
+
+  Denitrification, 62.
+
+  Derivation of Phytopathology, 85.
+
+  Destruction, 275.
+
+  Development of root-hairs, 40.
+
+  Dextrine, 173.
+
+  Diagnosis, 85, 89.
+
+  Diastases, 132.
+
+  Diffusion, 53.
+
+  Digestion, 133.
+
+  _Digraphis_, 175.
+
+  _Dilophia_, 188.
+
+  _Dionaea_, 125.
+
+  _Dipsacus_, 252.
+
+  _Diptera_, 207.
+
+  Dis-assimilation, 275, 277, 286.
+
+  Discolorations, 179, 186, 192.
+
+  Disease, 64, 91, 271, 272, 277, 287, 288, 290.
+
+  Disease dodging, 168.
+
+  Disease-fungi, 189.
+
+  Disease of organs, 119.
+
+  "Disease-proof" varieties, 168, 169, 171, 173, 177.
+
+  Disease-resisting varieties, 177.
+
+  Diseases of absorptive organs, 121.
+
+  Diseases of assimilatory organs, 119.
+
+  Diseases of bark, 120.
+
+  Diseases of cambium, 120.
+
+  Diseases of parenchyma, 120.
+
+  Diseases of respiratory organs, 119, 121.
+
+  Disintegration, 275.
+
+  Distortions, 140, 246, 251, 252, 253.
+
+  Dissemination of fungi, 142.
+
+  Division, 127.
+
+  Dodder, 113.
+
+  _Dolium_, 134.
+
+  Dormant buds, 224, 225, 257, 259, 260.
+
+  Double flowers, 247, 256.
+
+  Double ideals in selection, 168.
+
+  _Dracaena_, 192.
+
+  Drainage, 103.
+
+  Drawing, 106, 180.
+
+  Drip, 103.
+
+  Drooping, 43, 179.
+
+  Drops of water, 192.
+
+  Dropsy, 228.
+
+  Drought, 121, 183, 190, 191, 245, 248, 249.
+
+  Dry-rot, 143, 237.
+
+  Ducks, 144.
+
+  Dutrochet, 7.
+
+  Dwarfing, 246, 249.
+
+  "Dying back," 190, 240, 242, 243, 244.
+
+
+  Earwigs, 164, 207.
+
+  _Eau Celeste_, 162.
+
+  _Edelfaeule_, 230.
+
+  Eelworms, 111, 248.
+
+  Effects of environment, 262.
+
+  Eggs of insects, 187.
+
+  Elaborated sap, 94.
+
+  Elm, 218, 224, 225, 233, 260.
+
+  _Empusa_, 163.
+
+  Endemic diseases, 153, 160, 166.
+
+  Endive, 180.
+
+  Endophytes, 130.
+
+  Endophytic algae, 128.
+
+  Endophytic fungi, 193.
+
+  Energy in plants, 15, 25, 287.
+
+  Engelmann, 20, 27.
+
+  _Entyloma_, 187.
+
+  Enzymes, 10, 130, 132, 136.
+
+  _Epichloe_, 218.
+
+  Epicormic shoots, 224, 257, 260.
+
+  Epidemics, 108, 109, 113, 115, 142, 153, 160, 163, 166.
+
+  Epiphytes, 113, 130, 135, 137.
+
+  Epiphytic algae, 188.
+
+  Epiphytic fungi, 161, 193, 232.
+
+  _Equisetum_, 113.
+
+  Ergot, 131, 142, 144.
+
+  _Erineum_, 88, 212, 214, 215.
+
+  Erosions, 204, 207.
+
+  _Erysipheae_, 135, 142, 161, 187, 268.
+
+  Essentials of fertilisation, 69.
+
+  Estimates of loss, 146.
+
+  Etiolation, 106, 179, 180, 229.
+
+  _Euphorbia_, 116, 134, 247, 266.
+
+  Excavations, 204.
+
+  Excess of food, 229.
+
+  Excess of minerals, 102.
+
+  Excess of water, 100.
+
+  Excessive growth, 246.
+
+  Excessive nutrition, 250.
+
+  Excrescences, 114, 212, 222.
+
+  Excreta, 45, 130, 133.
+
+  _Exobasidium_, 128, 218.
+
+  _Exoascus_, 116, 128, 188, 208, 214, 218, 225, 247, 253.
+
+  Expense of materials, 161.
+
+  Experiments necessary, 168.
+
+  Exposure of roots, 179, 184.
+
+  External causes of disease, 99.
+
+  Extinction of species, 91.
+
+  Exudations, 227.
+
+  Exudation under pressure, 51.
+
+
+  Factors of an epidemic, 149, 165.
+
+  Falling of fruit, 206.
+
+  Falling leaves, 123.
+
+  False chlorosis, 181.
+
+  False etiolation, 180.
+
+  _Farfugium_, 188.
+
+  Fasciations, 230, 246, 251.
+
+  Fats, 272, 286.
+
+  Feeding, 14, 16.
+
+  Fermentation, 58, 102, 130, 233.
+
+  Ferns, 113, 247, 260, 261.
+
+  Fertilisation, 71.
+
+  Field-mice, 164.
+
+  Figs, 113.
+
+  Finger and toe, 114, 127, 163.
+
+  Fire, 240.
+
+  Flaming, 164.
+
+  Flattened roots, 246, 252.
+
+  Fleshiness, 228.
+
+  Flies, 86, 110, 142, 143, 145.
+
+  Flux, 227, 231.
+
+  Flying foxes, 244.
+
+  Focussing of solar rays, 192.
+
+  Foliage, 110.
+
+  _Fontaria_, 134.
+
+  Food, 18.
+
+  Forest-fires, 241.
+
+  Formic-aldehyde, 20.
+
+  Foul products, 100.
+
+  Foxy leaves, 191.
+
+  Freezing, 121, 183.
+
+  Frit fly, 182.
+
+  Frost, 153, 160, 225, 229, 248, 249.
+
+  Frost-beds, 243.
+
+  Frost-blisters, 212, 218.
+
+  Frost canker, 222.
+
+  Frost-cracks, 204, 209, 242.
+
+  Frost-patches, 240.
+
+  Frost-ridge, 209.
+
+  _Fumago_, 190, 232.
+
+  Fumes, 104.
+
+  Functions of roots, 43, 45.
+
+  Functional depression, 96.
+
+  Fungi, 89, 108, 143, 174, 189, 200, 205, 207, 208, 212, 216, 219, 223,
+      229, 231, 233, 238, 240, 241, 243, 248, 251, 255, 258, 265, 267,
+      283, 284, 288.
+
+  Fungus attacks, 139.
+
+  Fungus galls, 219.
+
+  _Fusarium_, 143, 238.
+
+  _Fusicladium_, 189.
+
+  _Fusisporium_, 237.
+
+
+  _Gagea_, 258.
+
+  Gall-apple, 218.
+
+  Gall-flies, 219.
+
+  Gall-insect, 139.
+
+  Gall-like swellings, 128.
+
+  Galls, 86, 110, 120, 130, 138, 212, 214, 218, 255.
+
+  Gangrene, 231.
+
+  _Garreya_, 264.
+
+  Gas, 160.
+
+  Gases in soil, 104.
+
+  _Gastropacha_, 225.
+
+  Gelatine, 163.
+
+  General death, 116.
+
+  General disease, 119, 120.
+
+  Germ-plasm, 267.
+
+  _Gesneria_, 260.
+
+  _Glechoma_, 218.
+
+  _Gloeosporium_, 189, 190, 208.
+
+  _Gloxinia_, 260.
+
+  Goats, 164.
+
+  Gooseberry, 217.
+
+  Graft-hybrids, 262, 267, 271, 283.
+
+  Grafting, 78, 155, 169, 183, 250, 262, 271, 281.
+
+  Grain-rust, 146.
+
+  Grapes, 192, 230, 231.
+
+  _Grapholitha_, 109, 207.
+
+  Grass, 111, 189, 190, 205, 218, 233.
+
+  Green fly, 161.
+
+  Grew, 85.
+
+  Greyish spots, 187.
+
+  Growth, 271, 274, 275, 286.
+
+  Grubs, 110, 207.
+
+  Gumming, 235.
+
+  Gummosis, 227, 234, 235.
+
+  _Gymnosporangium_, 114, 176, 223.
+
+
+  Hail, 106, 240, 241.
+
+  Hales, 85.
+
+  _Haltica_, 209.
+
+  Hardy varieties, 168, 170, 177.
+
+  Haustoria, 134, 135, 136.
+
+  Healing, 194, 196.
+
+  Healing by cork, 123.
+
+  Health, 272, 287.
+
+  Health and disease, 91, 97, 287.
+
+  Heliotropism, 126.
+
+  _Hemileia_, 146, 169.
+
+  Heredity, 72, 283.
+
+  _Herpotrichia_, 135, 190.
+
+  Hessian Fly, 182.
+
+  _Heterodora_, 219, 220.
+
+  _Hieracium_, 112.
+
+  History of Phytopathology, 85.
+
+  Holdfast of roots, 42.
+
+  Hollyhock disease, 143.
+
+  Holly, 217.
+
+  Honey dew, 144, 227, 232, 233.
+
+  Hops, 162, 187, 191, 232, 253.
+
+  Hop-aphis, 146.
+
+  Hop-disease, 166.
+
+  Hop mildew, 161.
+
+  _Hormomyia_, 219.
+
+  Hornbeam, 224, 233, 242.
+
+  Horse-radish, 260.
+
+  Host, 284, 285.
+
+  Hyacinth, 231, 261.
+
+  Hyacinth disease, 143.
+
+  Hybrids, 69, 156, 281.
+
+  Hybridisation, 69, 75, 169, 266, 281.
+
+  Hydrochloric acid, 181.
+
+  Hydrogen, 272.
+
+  Hymenomycetes, 206.
+
+  Hypertrophy, 119, 127, 139, 213, 215, 247, 266.
+
+  _Hypochaeris_, 112.
+
+  _Hypomyces_, 237.
+
+  _Hyponomeuta_, 254.
+
+
+  Ice, 184, 209.
+
+  Ichneumon-flies, 165.
+
+  _Icterus_, 181.
+
+  Illegitimate unions, 265.
+
+  Immunity, 155, 156, 165, 168, 169.
+
+  Impervious subsoil, 181.
+
+  Inarching, 269.
+
+  Increase in dry weight, 23.
+
+  Indian agriculture, 172.
+
+  Indian wheats, 168.
+
+  Indispensability of elements, 278.
+
+  Infection, 262, 265, 267.
+
+  Ingredients of protoplasm, 272.
+
+  Insect bites, 225.
+
+  Insect diseases, 145, 146, 154, 189.
+
+  Insect punctures, 88.
+
+  Insects, 89, 98, 108, 109, 120, 138, 142, 153, 174, 187, 194, 203,
+      205, 206, 207, 208, 212, 223, 229, 241, 244, 248, 251, 254, 255,
+      258, 259, 269.
+
+  Insolation, 180, 242.
+
+  Intercellular endophytes, 136, 137.
+
+  Intercellular mycelium, 128.
+
+  Interference, 91.
+
+  Internal causes of disease, 99, 101.
+
+  Intracellular parasites, 127, 136.
+
+  Intramolecular respiration, 277.
+
+  Intumescences, 212, 215.
+
+  Inulin, 11, 17.
+
+  Invertebrata, 108.
+
+  Irritability, 125, 127.
+
+  Irritation, 119, 139.
+
+  _Isaria_, 163.
+
+  Ivy, 113, 165.
+
+
+  Japanese trees, 250.
+
+  Jerusalem Artichoke, 264.
+
+  _Juncus_, 219.
+
+  Juniper, 114.
+
+
+  Kidney bean, 192.
+
+  Knauers, 223.
+
+  Knife wounds, 194, 195.
+
+
+  Labour, 161.
+
+  Lace-wings, 165.
+
+  _Lachnus_, 223.
+
+  Lady-birds, 164, 165.
+
+  Lammas shoots, 257, 259.
+
+  Larch, 168, 171.
+
+  Larch disease, 115, 149, 152, 166, 171, 223, 241.
+
+  Larvae, 110.
+
+  Lateral wounds, 132.
+
+  Lawns, 112.
+
+  Laying of wheat, 179, 180.
+
+  Leaf-curl, 236, 253.
+
+  Leaf-diseases, 114, 119, 120, 242.
+
+  Leaf-galls, 217, 218.
+
+  Leaf-miner, 86, 109, 204.
+
+  Leaf perforations, 208.
+
+  Leaf rolling, 214, 246, 254.
+
+  Leaf-spots, 114, 190.
+
+  Leguminoseae, 137, 219.
+
+  Lemons, 235.
+
+  Lenticels, 217.
+
+  Lepidoptera, 187.
+
+  _Leptosphaeria_, 249.
+
+  Lichens, 137.
+
+  Liebig, 4.
+
+  Life, 271, 285, 287.
+
+  Life and death, 271.
+
+  Light, 27, 106.
+
+  Lily disease, 143.
+
+  Lime, 163, 215, 218, 232, 253, 254, 260, 269.
+
+  Limes, 172.
+
+  Limits of variation, 287.
+
+  _Linaria_, 252.
+
+  Liquid antiseptics, 160, 161, 162.
+
+  Living environment, 99, 108.
+
+  Local action, 114.
+
+  Local disease, 119, 121.
+
+  Locusts, 109, 145, 163, 164.
+
+  Longicorns, 205.
+
+  _Loranthus_, 113, 245, 265.
+
+  Losses due to epidemics, 142.
+
+  Lowering of temperature, 100.
+
+  Lucerne, 249.
+
+  Lurking parasites, 142, 145.
+
+  Lychnis, 232.
+
+  _Lyonetra_, 206.
+
+  _Lysimachia_, 217.
+
+
+  Machine, plant compared to a, 79.
+
+  Magnesium, 272.
+
+  Maize, 116, 173, 219, 267.
+
+  _Majanthemum_, 175.
+
+  Malformations, 116, 130, 131, 246, 251.
+
+  _Mal nero_, 190.
+
+  Mallow, 252.
+
+  Malpighi, 85.
+
+  Mammals, 142.
+
+  Man and plants, 108, 142, 143.
+
+  Manna, 227, 235.
+
+  Manna Ash, 235.
+
+  Maple, 259.
+
+  Maximum, 288.
+
+  Maximum absorption, 19.
+
+  Maximum assimilation, 19.
+
+  Maximum temperature, 105.
+
+  Mealy bug, 164.
+
+  _Melampsora_, 176.
+
+  Melon, 220.
+
+  Messmates, 63.
+
+  Metabolic products, 274.
+
+  Metabolism, 23, 127, 271.
+
+  Metabolites, 278.
+
+  Metallic compounds, 162.
+
+  Mice, 108, 163.
+
+  Microbes, 227.
+
+  Micro-organisms, 183.
+
+  Mildew, 86, 164.
+
+  Millardet, 169.
+
+  Mineral salts, 101.
+
+  Miniature trees, 250.
+
+  Minimum, 288.
+
+  Minimum temperature, 105.
+
+  Misconceptions, 12.
+
+  Mistletoe, 113, 265.
+
+  Mites, 192, 214, 255.
+
+  Mixed species, 166.
+
+  Molecular structure of protoplasm, 273, 274.
+
+  Mongrel forms, 74.
+
+  _Monilia_, 217, 231.
+
+  Monstrosities, 246.
+
+  Moraine plants, 250.
+
+  Moths, 110, 145, 206.
+
+  Moulds, 230, 231, 237, 243.
+
+  _Mucor_, 230, 231.
+
+  Mulberry, 244.
+
+  Mutilations, 252.
+
+  Mycelial strands, 145.
+
+  Mycelium, 188.
+
+  Mycocecidia, 219.
+
+  Mycorrhiza, 137.
+
+  Myrtaceae, 258.
+
+  _Mytilaspis_, 187.
+
+
+  Natural checks, 159.
+
+  Natural demise, 91, 93.
+
+  Natural Grafts, 269.
+
+  Natural Selection, 72, 99, 167, 286.
+
+  Natural Wounds, 204.
+
+  Nature of soil, 57.
+
+  Necrosis, 240, 241, 243.
+
+  _Nectria_, 145, 217, 223, 241, 243, 269.
+
+  Nematodes, 111, 134, 139, 219, 220.
+
+  Nettle, 116, 252.
+
+  _Neurotus_, 219.
+
+  New formations, 255.
+
+  Nitrate, 273.
+
+  Nitrification, 62, 102.
+
+  Nitrogen, 272.
+
+  Nodosities, 219.
+
+  Nodules on roots, 63, 137.
+
+  Non-living environment, 99.
+
+  _Notommata_, 140.
+
+  Nuclear fusion, 267.
+
+  Nuclear protoplasm, 271, 279, 280, 290.
+
+  Nuclear substance, 71.
+
+  Nucleo-plasm, 280.
+
+  Nuts, 248.
+
+
+  Oak, 110, 188, 215, 218, 219, 223, 233.
+
+  Oak leaf-roller, 254.
+
+  Oat, 176.
+
+  Occlusion, 200, 201, 222, 223.
+
+  Odours, 144.
+
+  Oedema, 228.
+
+  Olive, 223.
+
+  Onion, 231.
+
+  _Oniscus_, 182.
+
+  _Oospora_, 216.
+
+  Optimum temperature, 105, 288.
+
+  Orange, 173, 187, 235, 247.
+
+  Orange-coloured spots, 187.
+
+  Orchard trees, 163.
+
+  _Orchestes_, 206.
+
+  Orchids, 113, 266.
+
+  Organic acids, 50.
+
+  Organisation, 89.
+
+  Organised structure, 13.
+
+  Organisms in soil, 60.
+
+  _Orobanche_, 112.
+
+  Osmosis, 26, 29, 46.
+
+  Osmotic pressures, 18, 41, 52.
+
+  Over-crowding, 104, 111.
+
+  Over-feeding, 102.
+
+  Over-watering, 97.
+
+  Oxalic acid, 134, 136.
+
+  Oxidation, 124.
+
+  Oxygen, 104, 272.
+
+  Oxygen-respiration, 12, 64.
+
+
+  Pallor, 179, 180.
+
+  Palms, 192.
+
+  _Pangium_, 134, 165.
+
+  Parasites, 61, 113, 119, 130, 139, 174, 187, 230, 265, 269, 284.
+
+  Parasitic algae, 188, 217, 219.
+
+  Parasitic bacteria, 163.
+
+  Parasitic diseases, 88, 119, 121.
+
+  Parasitic epiphyte, 136.
+
+  Parasitic fungi, 87, 97.
+
+  Parasitism, 262, 264, 268, 271.
+
+  _Paris_, 175.
+
+  "Paris green," 162.
+
+  Parti-coloured leaves, 191.
+
+  Parti-coloured spots, 186.
+
+  Pasture grasses, 69.
+
+  Pathology, 121, 257.
+
+  Pathology of cell, 119.
+
+  Pathological conditions, 168, 170, 246.
+
+  Pea, 190, 191, 206, 208, 248, 268.
+
+  Peach, 170, 253.
+
+  Pear, 179, 187, 189, 191, 216, 218, 231, 240, 248, 249, 253, 257.
+
+  Pedigree wheats, 69.
+
+  _Pelargonium_, 198, 253.
+
+  Peloria, 252.
+
+  _Penicillium_, 231.
+
+  _Peridermium Pini_, 223, 234.
+
+  _Periola_, 238.
+
+  Permanganate, 162.
+
+  _Peronospora_, 136, 160, 175, 187, 189, 208.
+
+  _Petasites_, 188.
+
+  Petroleum, 162.
+
+  _Peziza_, 115, 144, 152.
+
+  Phanerogams, 108, 111.
+
+  _Phellomyces_, 238.
+
+  _Phoma_, 217, 243.
+
+  Phosphorus, 272.
+
+  Photo-synthesis, 11, 16.
+
+  _Phragmidium_, 189.
+
+  _Phyllachora_, 189.
+
+  _Phyllereum_, 253.
+
+  _Phyllobium_, 217.
+
+  _Phyllosiphon_, 188.
+
+  _Phyllosticta_, 188, 209.
+
+  _Phylloxera_, 110, 145, 149, 154, 155, 163, 166, 172, 188, 219, 220, 268.
+
+  Physiology, 1, 66, 85.
+
+  Physiological diseases, 119, 121.
+
+  _Phytomyza_, 206.
+
+  Phytopathology, 85.
+
+  _Phytophthora_, 115, 136, 144, 150, 151, 235, 236.
+
+  _Phytophysa_, 219.
+
+  _Phytoptus_, 189, 213, 214, 215, 218, 219, 253, 254.
+
+  _Pilea_, 219.
+
+  _Pilobolus_, 126, 140.
+
+  Pines, 183, 223, 234, 251, 252.
+
+  Pine-apple, 258.
+
+  Pith flecks, 204, 207.
+
+  Plant as agent of disease, 99, 108.
+
+  Plant, agricultural chemistry of, 1.
+
+  Plant and its food, 7.
+
+  Plant and its surroundings, 1.
+
+  Plant, a machine, 1, 15.
+
+  Plant, central object of study, 1.
+
+  Plant, physiology, 1.
+
+  _Plantago_, 257.
+
+  Plantain, 112, 257.
+
+  Plants, dying out of, 93.
+
+  Plasmodia, 163.
+
+  _Plasmodiophora_, 114, 126, 127, 144, 163, 219, 284, 285.
+
+  Plasmolysis, 47.
+
+  _Pleospora_, 236.
+
+  _Pleotrachelus_, 126, 140.
+
+  Plum, 171, 189, 192, 209, 214, 206, 231, 235, 248, 249, 260.
+
+  _Poa_, 258.
+
+  Pocket-like galls, 155, 214, 218.
+
+  Pocket-plums, 214.
+
+  Pockets, 253.
+
+  Poison, 102, 130, 136, 163, 216.
+
+  Poisonous gases, 181, 248.
+
+  Pollen grain, 288.
+
+  Pollination, 248, 262, 265, 266, 271.
+
+  _Polydesmus_, 236.
+
+  _Polygonatum_, 175.
+
+  _Polygonum_, 258.
+
+  Polymorphism, 174.
+
+  Polyporei, 142.
+
+  _Polyporus_, 143, 206.
+
+  _Polystigma_, 189.
+
+  Poplar, 188, 206, 215, 218, 254.
+
+  Post and epidemics, 142.
+
+  Potassium, 272.
+
+  Potassium sulphite, 162.
+
+  Potato, 162, 171, 194, 209, 216, 236, 237, 258.
+
+  Potato-disease, 114, 143, 149, 150, 166, 189, 207, 235.
+
+  Powders, antiseptic, 159, 160, 161.
+
+  Predisposition to disease, 98, 99, 105, 168, 169, 229, 262, 268, 277,
+      278, 282.
+
+  Preventible diseases, 159.
+
+  Preventitious buds, 259.
+
+  Prolepsis, 257, 259.
+
+  Proliferations, 257, 258.
+
+  Properties of soil, 57.
+
+  Prophylactic measures, 160.
+
+  Proteids, 132, 138, 272, 277, 286.
+
+  Proteolytic enzymes, 132.
+
+  _Protomyces_, 217.
+
+  Protoplasmic molecules, 276, 278, 286.
+
+  Protoplasm, 33, 41, 271, 272, 274, 276.
+
+  Pruning, 105, 143, 194, 225, 250.
+
+  Prussic acid, 163, 165, 173.
+
+  _Psylla_, 253.
+
+  _Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247, 252.
+
+  Puckers, 214, 235, 246, 253.
+
+  Puffing of spores, 142, 144.
+
+  Punctures, 212.
+
+  Pure culture, 166.
+
+  Purple-black spots, 191.
+
+  Pustules, 188, 190, 212, 217.
+
+  Putrefaction, 234.
+
+  _Pyrethrum_, 161.
+
+  _Pyrus_, 214.
+
+  _Pythium_, 114, 119, 136, 144, 160, 230.
+
+
+  _Quassia_, 161.
+
+  Quinine, 173.
+
+
+  Rabbits, 108, 142, 164, 194.
+
+  Rain trees, 233.
+
+  Rankness, 97, 227, 228.
+
+  Rats, 108, 163.
+
+  Rays of light, 18.
+
+  Red light, 21.
+
+  Red spider, 161, 187, 188, 192.
+
+  Red spots, 188, 253.
+
+  References in Bible, 85.
+
+  Remedial measures, 89.
+
+  Repellent substances, 136.
+
+  Reproduction, 72, 281.
+
+  Reserves, 274.
+
+  Resin, 125.
+
+  Resin-flux, 234.
+
+  Resinosis, 227, 234.
+
+  Resistance to disease, 155, 268.
+
+  Resistant races, 172.
+
+  Respiration, 17, 31, 130, 271, 275, 276, 285, 287.
+
+  Reversions, 73.
+
+  Rhinanthoideae, 265.
+
+  _Rhinanthus_, 112.
+
+  _Rhizobium_, 289.
+
+  _Rhizoctonia_, 238.
+
+  Rhizomorph, 145.
+
+  Rhododendron, 218.
+
+  Rhubarb, 180, 230.
+
+  _Rhynchitis_, 254.
+
+  _Rhytisma_, 188.
+
+  Ribbon grass, 183.
+
+  _Ribes_, 214.
+
+  Rice, 172.
+
+  Rimpau's experiments, 69, 73, 77.
+
+  Ringing, 194, 201, 202, 210.
+
+  Ripened wood, 243.
+
+  _Robinia_, 259.
+
+  Rodents, 109.
+
+  _Roestelia_, 217.
+
+  Rolled leaves, 86.
+
+  Root, 9, 35, 96, 120, 227, 270.
+
+  Root-absorption, 181.
+
+  Root-diseases, 119, 120.
+
+  Root-excretions, 46.
+
+  Root-fusions, 262.
+
+  Root-galls, 221.
+
+  Root-hairs, 34, 102, 163.
+
+  Root-nodules, 212, 219.
+
+  Root-parasites, 112, 265.
+
+  Root-rot, 230.
+
+  Roses, 232, 243, 257, 268, 278.
+
+  Rosettes, 225.
+
+  Rot, 97, 182, 227, 229, 231, 236.
+
+  Rotation of crops, 69, 166.
+
+  Rotifer, 140.
+
+  Rot-organisms, 200.
+
+  Rotting of wounds, 87.
+
+  Rouen law, 85.
+
+  Rushes, 114.
+
+  Rust, 122, 142, 171, 172, 175, 191.
+
+  Rye, 176, 248.
+
+
+  _Saccharomyces_, 60.
+
+  Sachs, 7, 36.
+
+  _Salvia_, 214.
+
+  San Jose scale, 187.
+
+  Sand-blast action, 184.
+
+  Sandy soils, 184.
+
+  _Saperda_, 205.
+
+  Saprophytes, 135, 137, 175, 234, 243, 244.
+
+  _Scab_, 189, 216.
+
+  _Scale_, 187.
+
+  _Schinzia_, 114.
+
+  _Schizoneura_, 223.
+
+  Scion, 183, 262, 264, 266, 282.
+
+  _Scleroderris_, 223.
+
+  Sclerotia, 143.
+
+  Schwarz, 39.
+
+  _Sclerotinia_, 142, 143, 144, 231, 248, 249, 288.
+
+  Scolytidae, 205.
+
+  Scorching, 240, 241.
+
+  Scurf, 216.
+
+  Sea-kale, 261.
+
+  _Secale_, 76.
+
+  Secretions, 130, 133, 173, 274.
+
+  Sedges, 189.
+
+  Seedless grapes, 247.
+
+  _Selandria_, 208.
+
+  Selection, 69, 74, 78, 169.
+
+  Selective absorption, 53, 65.
+
+  Self-fertilisation, 281.
+
+  Semi-parasites, 112.
+
+  _Senecio_, 188.
+
+  Sensitive plant, 125.
+
+  _Septoria_, 114, 187.
+
+  Sewage waters, 59.
+
+  Sexual act, 72.
+
+  Shaded foliage, 113.
+
+  Shanking, 246, 249.
+
+  Shoots from old wood, 260.
+
+  Shot holes, 204, 208, 209.
+
+  Silver fir, 224.
+
+  Silver leaf, 192.
+
+  _Sirex_, 206.
+
+  Skeleton leaves, 204, 207.
+
+  Slime flux, 227, 233.
+
+  Slime fungus, 219.
+
+  Slugs, 111, 164, 207, 269.
+
+  Smut, 117, 143, 162, 190.
+
+  Snails, 111, 142, 207.
+
+  Snow, 106.
+
+  Soap, as insecticide, 161.
+
+  Soil, 1, 42, 99, 102, 142, 163.
+
+  Soil-bacteria, 60.
+
+  Soil-filtration, 59.
+
+  Soil-organisms, 61, 143.
+
+  Solar energy, 135.
+
+  Somato-plasm, 267.
+
+  Sooty moulds, 135, 190, 232.
+
+  _Sorbus_, 207.
+
+  _Sorosporium_, 216.
+
+  Sour-rot, 231.
+
+  Sparrows, 164.
+
+  Specialised races, 168, 176.
+
+  Specific predisposition, 155.
+
+  Spectrum, 19, 21, 26.
+
+  Spermogonia, 144, 232.
+
+  _Sphaerella_, 189.
+
+  _Sphaerotheca_, 187.
+
+  Sphaeroblasts, 222, 225.
+
+  _Spicaria_, 237.
+
+  Spiral grooving, 204, 210.
+
+  Spiral growth, 252.
+
+  _Spongospora_, 216.
+
+  Spontaneous variations, 78, 246, 255.
+
+  Spores, 144.
+
+  Sports, 93, 247.
+
+  Spots on leaves, 120, 186.
+
+  Spraying, 159, 161, 162.
+
+  Spreading of disease, 142.
+
+  Squirrels, 108.
+
+  Stag-head, 240, 244.
+
+  Starch, 9, 16, 17, 20, 23, 138, 173.
+
+  Statistics of epidemics, 147.
+
+  Steeping, 161.
+
+  Stem diseases, 120.
+
+  _Stereum_, 206.
+
+  Sterility of soil, 61.
+
+  Stimulation, 119.
+
+  Stimuli, 126, 127, 139.
+
+  Stock, 262, 264, 266, 282.
+
+  Stomata, 23.
+
+  Stool-shoots, 201, 225, 269.
+
+  Stool stumps, 194, 201.
+
+  Strangulations, 204, 209.
+
+  Strawberry, 189, 257.
+
+  Stripping, 194, 197.
+
+  Stroma, 217.
+
+  Structure, 274.
+
+  Structure of protoplasm, 271.
+
+  Structure of root-hairs, 40.
+
+  Struggle for existence, 105, 159, 164, 165, 167, 286.
+
+  Study of causes, 85.
+
+  Stumps, 194.
+
+  Subsoil, 57, 103.
+
+  Substitutive selections, 286.
+
+  Suckers, 225, 260.
+
+  Sugar, 11, 17, 20, 173, 286.
+
+  Sugar cane, 172.
+
+  Sugar cane disease, 166.
+
+  Sulphate, 273.
+
+  Sulphur, 161, 163, 272.
+
+  Sulphurous acid, 181.
+
+  Sun-burn, 240, 241.
+
+  Sun-cracks, 240, 242.
+
+  Sundew, 232.
+
+  Sunflower, 256, 264.
+
+  Sun-spots, 192.
+
+  Superstitions, 85.
+
+  Surface energy, 26.
+
+  Surface roots, 112.
+
+  Sweet almond, 173.
+
+  Symbiosis, 63, 130, 137, 219, 263, 265, 268, 285.
+
+  Symptoms of disease, 89, 122, 179, 186.
+
+  _Synchytrium_, 127, 188, 217, 247.
+
+  Synthesis, 65.
+
+  _Syringa_, 259.
+
+  Syringing, 161, 164.
+
+
+  Tamarisk, 235.
+
+  Tannin, 138.
+
+  _Taphrina_, 218.
+
+  Tar, 164.
+
+  Tea, 244.
+
+  Teazel, 252.
+
+  Teleutospore, 189, 191.
+
+  Temperature, 99, 105.
+
+  Tendencies to ill-health, 91.
+
+  Tendrils, 125.
+
+  Teratology, 246, 253, 254, 257.
+
+  _Tetraneura_, 218.
+
+  _Tetranychus_, 187, 192.
+
+  Thawing, 183.
+
+  _Thelephora_, 206.
+
+  Therapeutics, 85, 89, 159.
+
+  Thermotropism, 126.
+
+  _Thesium_, 112.
+
+  Thick-skinned organs, 168, 171.
+
+  Thinning, 96, 105.
+
+  Thistle, 247.
+
+  Thrips, 88, 191, 208.
+
+  Thyloses, 125.
+
+  _Tilia_, 214.
+
+  Timber diseases, 119, 120.
+
+  Timiriazeff, 21.
+
+  _Tinea_, 206.
+
+  Tissue diseases, 119.
+
+  Tobacco, 209.
+
+  Tobacco powder, 161.
+
+  Tomato, 171, 219, 230.
+
+  Top-dry trees, 244.
+
+  Topical remedies, 161.
+
+  _Tomicus_, 205.
+
+  Torsions, 246, 252.
+
+  _Tortrix_, 254.
+
+  Toxic agents, 130.
+
+  Transformation of energy, 25, 28.
+
+  Transformation of organs, 254, 255.
+
+  Transmission of acquired characters, 264, 283, 290.
+
+  Transplanting, 96.
+
+  Transpiration, 181, 228.
+
+  Trees, 109.
+
+  _Trichosphaeria_, 135.
+
+  _Triticum_, 76.
+
+  Tumescence, 227, 228.
+
+  Tunnels, 206.
+
+  Turgescence, 47, 228, 230.
+
+  Turnip, 126, 162, 230.
+
+  Twitch, 113.
+
+  _Tylenchus_, 238, 248.
+
+
+  Ulcer, 231.
+
+  Unger, 85.
+
+  Unsuitable soils, 101.
+
+  Upheaval of seedlings, 179, 183.
+
+  Uredineae, 114, 134, 136, 145, 169, 188, 189.
+
+  _Uredo_, 88, 188, 191.
+
+  Uredospores, 191.
+
+  _Uromyces_, 116, 188, 191, 266.
+
+  _Urocystis_, 220.
+
+  Ustilagineae, 145, 190, 217, 248.
+
+  _Ustilago_, 116, 117, 175, 190, 219, 255.
+
+
+  _Vaccinium_, 128, 288.
+
+  Variability, 174.
+
+  Variation, 67, 72, 91, 92, 168, 174, 176, 246, 262, 263, 264, 271,
+      282, 286, 288, 289.
+
+  Variegation, 179, 182, 183, 192.
+
+  Varieties, 78, 247.
+
+  Varieties of soil, 56.
+
+  _Vaucheria_, 139, 140.
+
+  Vegetable acids, 48.
+
+  Vertebrata, 108.
+
+  _Verticillium_, 145, 236.
+
+  _Viburnum_, 214.
+
+  Vine, 110, 149, 156, 162, 164, 169, 171, 189, 190, 191, 222, 248, 268.
+
+  Vine disease, 143.
+
+  Vivipary, 257, 258.
+
+
+  Walnut, 190, 209, 253.
+
+  Want of air, 100.
+
+  Washing leaves, etc., 161.
+
+  Wasp-flies, 165.
+
+  Wasps, 145.
+
+  Water, 272.
+
+  Water and insects, 161.
+
+  Water-culture, 65.
+
+  Water in soil, 103.
+
+  Waterlogging, 181.
+
+  Weaving of fungi, 190.
+
+  Webs, 190, 254.
+
+  Weeding, 105.
+
+  Weeds, 69, 111, 113, 165, 229, 249.
+
+  Weevils, 248.
+
+  Wet feet, 181.
+
+  Wheat, 169, 171, 172, 176, 179, 180, 182, 183, 230, 248.
+
+  Wheat rust, 86, 122, 146, 166, 169, 176.
+
+  White spots, 186, 187.
+
+  Willow, 206, 207, 219, 223, 233, 259.
+
+  Willow beetle, 208.
+
+  Wilting, 179, 181, 235, 249.
+
+  Wind, 106, 142, 144, 153, 184, 209, 229.
+
+  Wire-worms, 109, 181.
+
+  Witches' brooms, 116, 222, 224.
+
+  Wood, 124.
+
+  Wood-ashes, 161.
+
+  Woodbine, 112, 210.
+
+  Wood-boring, 204, 205.
+
+  Woodlice, 164.
+
+  Wood-nodules, 225.
+
+  Wood-wasps, 206.
+
+  Woolly-aphis, 219, 223.
+
+  Worms, 109, 142, 144, 194, 238.
+
+  Wounds, 108, 139, 194, 204, 207, 213, 260, 263, 269.
+
+  Wound-cork, 195.
+
+  Wound-fever, 123.
+
+  Wound-fungi, 203, 204, 240.
+
+  Wound-gum, 125.
+
+  Wound-wood, 124.
+
+  Wrens, 165.
+
+  Wrinkling, 253.
+
+
+  _Xenia_, 267.
+
+  _Xyloma_, 88.
+
+
+  Yeasts, 134, 172, 231, 233.
+
+  Yellowing, 179, 181, 182, 184.
+
+  Yellow leaves, 89.
+
+  Yellow spots, 186, 187, 188, 253.
+
+
+  Zoospores, 151.
+
+
+GLASGOW: PRINTED AT THE UNIVERSITY PRESS BY ROBERT MACLEHOSE AND CO.
+
+
+
+
+MACMILLAN AND CO.'S WORKS ON BOTANY.
+
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+
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+
+
+Timber and Some of its Diseases.
+
+     By H. MARSHALL WARD, D.Sc., F.R.S., F.L.S., Fellow of Sidney
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+
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+
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+
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+     LORD AVEBURY, F.R.S., D.C.L., LL.D. Illustrated. Cr. 8vo. 4s.
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+
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+
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+
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+     of Science, Newcastle-on-Tyne. Introduction by Prof. H.
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+     the Royal Indian Engineering College, Cooper's Hill. With
+     numerous illustrations. 8vo. 10s. net.
+
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+
+     =A Primer of Botany.= By the same. Pott 8vo. 1s.
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+
+     =The Yew Trees of Great Britain and Ireland.= By JOHN LOWE,
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+
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+
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+     based upon material left in manuscript by the late Professor
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+     the Herbarium and Library of the Royal Gardens, Kew, and
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+     revised. Cr. 8vo. 5s.
+
+
+MACMILLAN AND CO., LTD., LONDON.
+
+
+
+
+Transcriber's Notes:
+
+
+The word Oedema uses an OE ligature in the original.
+
+The following corrections have been made to the text:
+
+     Page vi: be the better for a real knowledge[original has
+     knowlege]
+
+     Page 55: and[original has and and] are too crudely mechanical
+
+     Page 117: Prillieux[original has Prilleux], _Maladies des
+     Plantes Agricoles_
+
+     Page 128: the intercellular mycelium of _Exoascus_[original
+     has Exoacus]
+
+     Page 134: subject to attacks of Uredineae[original has
+     Uredinae]
+
+     Page 142: carried[original has carrried] from plant to plant
+
+     Page 176: its aecidia[original has aecida] on the Barberry
+
+     Page 182: _e.g._ _Oniscus_[original has Oscinis], the Frit
+     Fly, and _Cecidomyia_[original has Cecidomya]
+
+     Page 182: not necessarily less ash constituents[original has
+     constitutents]
+
+     Page 183: nature of a transmissible enzyme[original has
+     enyzme]
+
+     Page 203: _Krankh. d. Pflanzen_, B. I.[original has 1] cap. 2
+
+     Page 206: leaves of Apples by _Lyonetia_[original has
+     Lyonettia]
+
+     Page 218: _Epichloe_[original has Epichloe], which clothes the
+     sheaths
+
+     Page 219: beetle which attacks Crucifers[original has
+     Crucificers]
+
+     Page 221: on the green parts of Hibiscus,[comma missing in
+     original]
+
+     Page 221: nodules of the roots of Leguminoseae[original has
+     Leguminosae]
+
+     Page 230: _Edelfaeule_[original has Edelfauele], a rotten
+     condition of the grapes
+
+     Page 235: giving an almost mealy[original has meally]
+     appearance
+
+     Page 243: as its mycelium[original has myceliun] spreads
+
+     Page 258: _Prolepsis._[original has Proplesis]--It frequently
+
+     Page 293: Aetiology[original has AEtiology], 89, 100.
+
+     Page 293: _Anthonomus_[original has Anthonomos], 249.
+
+     Page 294: Bird's-eye[original has Birds'-eye] Maple, 224.
+
+     Page 295: _Cercospora_,[original has Cereospora] 190.
+
+     Page 298: _Eau Celeste_[original has Celeste], 162.
+
+     Page 300: _Heterodora_[original has Heterodera], 219, 220.
+
+     Page 300: Holly, 217.[period missing in original]
+
+     Page 300: _Hypomyces_, 237.[original has comma]
+
+     Page 301: _Lyonetia_[original has Lyonetra], 206.
+
+     Page 303: Permanganate[original has Permangate], 162.
+
+     Page 304: Prophylactic[original has Phophylactic] measures,
+     160.
+
+     Page 304: _Phytomyza_, 206.[period missing in original]
+
+     Page 304: _Phyllereum_[original has Phyllereus], 253.
+
+     Page 304: Pine-apple[original has Pine apple], 258.
+
+     Page 305: _Puccinia_, 88, 114, 169, 175, 176, 188, 189, 247,
+     252[original has 252, 247].
+
+     Page 307: Somato-plasm[original has Somatoplasm], 267.
+
+     Page 307: Spermogonia[original has Spermagonia], 144, 232.
+
+     Page 308: _Tomicus_[original has Tornicus], 205.
+
+The following index entries were out of alphabetical order and have been
+moved to the appropriate locations:
+
+     Phylloxera
+     Plants, dying out of
+     Poisonous gases
+     Preventible diseases
+     Prophylactic measures
+     Spermogonia
+
+
+
+
+
+End of the Project Gutenberg EBook of Disease in Plants, by H. Marshall Ward
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