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diff --git a/39011.txt b/39011.txt new file mode 100644 index 0000000..98cc190 --- /dev/null +++ b/39011.txt @@ -0,0 +1,10496 @@ +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. + +BY THE SAME AUTHOR. + +Crown 8vo. 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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. 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