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+The Project Gutenberg EBook of The Power of Movement in Plants,
+by Charles Darwin
+(#22 in our series by Charles Darwin)
+
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+Title: The Power of Movement in Plants
+
+Author: Charles Darwin
+
+Release Date: May, 2004  [EBook #5605]
+[Most recently updated: August 14, 2002]
+
+Edition: 11
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THE PROJECT GUTENBERG EBOOK, THE POWER OF MOVEMENT IN PLANTS ***
+
+
+
+
+This eBook was produced by Sue Asscher.
+
+
+
+[page i.]
+
+THE
+
+POWER OF MOVEMENT
+
+IN
+
+PLANTS.
+
+[page ii.]
+
+[page iii.]
+
+
+
+THE
+
+POWER OF MOVEMENT
+
+IN
+
+PLANTS.
+
+
+
+ BY CHARLES DARWIN, LL.D., F.R.S.
+
+
+
+ASSISTED BY
+
+FRANCIS DARWIN.
+
+
+
+
+[page iv.]
+
+[page v.]
+
+
+
+CONTENTS.
+
+-----
+
+INTRODUCTION...Page 1-9.
+
+CHAPTER I.
+
+THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
+
+Brassica oleracea, circumnutation of the radicle, of the arched hypocotyl
+whilst still buried beneath the ground, whilst rising above the ground and
+straightening itself, and when erect--Circumnutation of the cotyledons--
+Rate of movement--Analogous observations on various organs in species of
+Githago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of several
+Leguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula,
+Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus,
+Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena,
+Nephrodium, and Selaginella...10-66
+
+
+CHAPTER II.
+
+GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
+
+Generality of the circumnutating movement--Radicles, their circumnutation
+of service--Manner in which they penetrate the ground--Manner in which
+hypocotyls and other organs break through the ground by being arched--
+Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons-
+-Circumnutation of hypocotyls and epicotyls whilst still buried and arched-
+-Their power of straightening themselves--Bursting of the seed-coats--
+Inherited effect of the arching process in hypo-
+[page vi.]
+gean hypocotyls--Circumnutation of hypocotyls and epicotyls when erect--
+Circumnutation of cotyledons--Pulvini or joints of cotyledons, duration of
+their activity, rudimentary in Oxalis corniculata, their development--
+Sensitiveness of cotyledons to light and consequent disturbance of their
+periodic movements--Sensitiveness of cotyledons to contact...Page 67-128
+
+
+CHAPTER III.
+
+SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.
+
+Manner in which radicles bend when they encounter an obstacle in the soil--
+Vicia faba, tips of radicles highly sensitive to contact and other
+irritants--Effects of too high a temperature--Power of discriminating
+between objects attached on opposite sides--Tips of secondary radicles
+sensitive--Pisum, tips of radicles sensitive--Effects of such sensitiveness
+in overcoming geotropism--Secondary radicles--Phaseolus, tips of radicles
+hardly sensitive to contact, but highly sensitive to caustic and to the
+removal of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus,
+tip not sensitive to slight contact, highly sensitive to caustic--Quercus,
+tip highly sensitive to contact--Power of discrimination--Zea, tip highly
+sensitive, secondary radicles--Sensitiveness of radicles to moist air--
+Summary of chapter...129-200
+
+
+CHAPTER IV.
+
+THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.
+
+Circumnutation of stems: concluding remarks on--Circumnutation of stolons:
+aid thus afforded in winding amongst the stems of surrounding plants--
+Circumnutation of flower-stems--Circumnutation of Dicotyledonous leaves--
+Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sink
+at night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concluding
+remarks on the circumnutation of leaves; generally rise in the evening and
+sink in the morning...201-262
+[page vii.]
+
+
+CHAPTER V.
+
+MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC
+MOVEMENTS.
+
+Circumnutation modified through innate causes or through the action of
+external conditions--Innate causes--Climbing plants; similarity of their
+movements with those of ordinary plants; increased amplitude; occasional
+points of difference--Epinastic growth of young leaves--Hyponastic growth
+of the hypocotyls and epicotyls of seedlings--Hooked tips of climbing and
+other plants due to modified circumnutation--Ampelopsis tricuspidata--
+Smithia Pfundii--Straightening of the tip due to hyponasty--Epinastic
+growth and circumnutation of the flower-peduncles of Trifolium repens and
+Oxalis carnosa...Page 263-279
+
+
+CHAPTER VI.
+
+MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP
+OF COTYLEDONS.
+
+Preliminary sketch of the sleep or nyctitropic movements of leaves--
+Presence of pulvini--The lessening of radiation the final cause of
+nyctitropic movements--Manner of trying experiments on leaves of Oxalis,
+Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of
+Mimosa--Concluding remarks on radiation from leaves--Small differences in
+the conditions make a great difference in the result - Description of the
+nyctitropic position and movements of the cotyledons of various plants--
+List of species--Concluding remarks--Independence of the nyctitropic
+movements of the leaves and cotyledons of the same species--Reasons for
+believing that the movements have been acquired for a special
+purpose...280-316
+
+
+CHAPTER VII.
+
+MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
+
+Conditions necessary for these movements--List of Genera and Families,
+which include sleeping plants--Description of the movements in the several
+Genera--Oxalis: leaflets folded at
+[page viii.]
+night--Averrhoa: rapid movements of the leaflets--Porlieria: leaflets close
+when plant kept very dry--Tropaeolum: leaves do not sleep unless well
+illuminated during day--Lupinus: various modes of sleeping--Melilotus:
+singular movements of terminal leaflet--Trifolium--Desmodium: rudimentary
+lateral leaflets, movements of, not developed on young plants, state of
+their pulvini--Cassia: complex movements of the leaflets--Bauhinia: leaves
+folded at night--Mimosa pudica: compounded movements of leaves, effect of
+darkness--Mimosa albida, reduced leaflets of--Schrankia: downward movement
+of the pinnae--Marsilea: the only cryptogam known to sleep--Concluding
+remarks and summary--Nyctitropism consists of modified circumnutation,
+regulated by the alternations of light and darkness--Shape of first true
+leaves...Page 317-417
+
+
+CHAPTER VIII.
+
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
+
+Distinction between heliotropism and the effects of light on the
+periodicity of the movements of leaves--Heliotropic movements of Beta,
+Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in
+Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements
+of tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of the
+pods--Heliotropism and apheliotropism modified forms of circumnutation--
+Steps by which one movement is converted into the other--
+Transversal-heliotropismus or diaheliotropism influenced by epinasty, the
+weight of the part and apogeotropism--Apogeotropism overcome during the
+middle of the day by diaheliotropism--Effects of the weight of the blades
+of cotyledons--So called diurnal sleep--Chlorophyll injured by intense
+light--Movements to avoid intense light...418-448
+
+
+CHAPTER IX.
+
+SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
+
+Uses of heliotropism--Insectivorous and climbing plants not heliotropic--
+Same organ heliotropic at one age and not at another--Extraordinary
+sensitiveness of some plants to light--The effects
+[page ix.]
+of light do not correspond with its intensity--Effects of previous
+illumination--Time required for the action of light--After-effects of
+light--Apogeotropism acts as soon as light fails--Accuracy with which
+plants bend to the light--This dependent on the illumination of one whole
+side of the part--Localised sensitiveness to light and its transmitted
+effects--Cotyledons of Phalaris, manner of bending--Results of the
+exclusion of light from their tips--Effects transmitted beneath the surface
+of the ground--Lateral illumination of the tip determines the direction of
+the curvature of the base--Cotyledons of Avena, curvature of basal part due
+to the illumination of upper part--Similar results with the hypocotyls of
+Brassica and Beta--Radicles of Sinapis apheliotropic, due to the
+sensitiveness of their tips--Concluding remarks and summary of chapter--
+Means by which circumnutation has been converted into heliotropism or
+apheliotropism...Page 449-492
+
+
+CHAPTER X.
+
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
+
+Means of observation--Apogeotropism--Cytisus--Verbena--Beta--Gradual
+conversion of the movement of circumnutation into apogeotropism in Rubus,
+Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded by
+heliotropism--Effected by the aid of joints or pulvini--Movements of
+flower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism--
+Movements of radicles--Burying of seed-capsules--Use of process--Trifolium
+subterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion...493-522
+
+
+CHAPTER XI.
+
+LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
+
+General considerations--Vicia faba, effects of amputating the tips of the
+radicles--Regeneration of the tips--Effects of a short exposure of the tips
+to geotropic action and their subsequent amputation--Effects of amputating
+the tips obliquely--Effects of cauterising the tips--Effects of grease on
+the tips--Pisum
+[page x.]
+sativum, tips of radicles cauterised transversely, and on their upper and
+lower sides--Phaseolus, cauterisation and grease on the tips--Gossypium--
+Cucurbita, tips cauterised transversely, and on their upper and lower
+sides--Zea, tips cauterised--Concluding remarks and summary of chapter--
+Advantages of the sensibility to geotropism being localised in the tips of
+the radicles...Page 523-545
+
+
+CHAPTER XII.
+
+SUMMARY AND CONCLUDING REMARKS.
+
+Nature of the circumnutating movement--History of a germinating seed--The
+radicle first protrudes and circumnutates--Its tip highly sensitive--
+Emergence of the hypocotyl or of the epicotyl from the ground under the
+form of an arch--Its circumnutation and that of the cotyledons--The
+seedling throws up a leaf-bearing stem--The circumnutation of all the parts
+or organs--Modified circumnutation--Epinasty and hyponasty--Movements of
+climbing plants--Nyctitropic movements--Movements excited by light and
+gravitation--Localised sensitiveness--Resemblance between the movements of
+plants and animals--The tip of the radicle acts like a brain...546-573
+
+
+INDEX...574-593
+
+[page 1]
+
+
+
+
+THE MOVEMENTS OF PLANTS.
+
+
+
+INTRODUCTION.
+
+THE chief object of the present work is to describe and connect together
+several large classes of movement, common to almost all plants. The most
+widely prevalent movement is essentially of the same nature as that of the
+stem of a climbing plant, which bends successively to all points of the
+compass, so that the tip revolves. This movement has been called by Sachs
+"revolving nutation;" but we have found it much more convenient to use the
+terms circumnutation and circumnutate. As we shall have to say much about
+this movement, it will be useful here briefly to describe its nature. If we
+observe a circumnutating stem, which happens at the time to be bent, we
+will say towards the north, it will be found gradually to bend more and
+more easterly, until it faces the east; and so onwards to the south, then
+to the west, and back again to the north. If the movement had been quite
+regular, the apex would have described a circle, or rather, as the stem is
+always growing upwards, a circular spiral. But it generally describes
+irregular elliptical or oval figures; for the apex, after pointing in any
+one direction, commonly moves back to the opposite side, not, however,
+returning along the same line. Afterwards other irregular ellipses or ovals
+are successively described, with their longer
+[page 2]
+axes directed to different points of the compass. Whilst describing such
+figures, the apex often travels in a zigzag line, or makes small
+subordinate loops or triangles. In the case of leaves the ellipses are
+generally narrow.
+
+Until recently the cause of all such bending movements was believed to be
+due to the increased growth of the side which becomes for a time convex;
+that this side does temporarily grow more quickly than the concave side has
+been well established; but De Vries has lately shown that such increased
+growth follows a previously increased state of turgescence on the convex
+side.* In the case of parts provided with a so-called joint, cushion or
+pulvinus, which consists of an aggregate of small cells that have ceased to
+increase in size from a very early age, we meet with similar movements; and
+here, as Pfeffer has shown** and as we shall see in the course of this
+work, the increased turgescence of the cells on opposite sides is not
+followed by increased growth. Wiesner denies in certain cases the accuracy
+of De Vries' conclusion about turgescence, and maintains*** that the
+increased extensibility of the cell-walls is the more important element.
+That such extensibility must accompany increased turgescence in order that
+the part may bend is manifest, and this has been insisted on by several
+botanists; but in the case of unicellular plants it can hardly fail to be
+the more important element. On the whole we may at present conclude that
+in-
+
+* Sachs first showed ('Lehrbuch,' etc., 4th edit. p. 452) the intimate
+connection between turgescence and growth. For De Vries' interesting essay,
+'Wachsthumskr�mmungen mehrzelliger Organe,' see 'Bot. Zeitung,' Dec. 19,
+1879, p. 830.
+
+** 'Die Periodischen Bewegungen der Blattorgane,' 1875.
+
+*** 'Untersuchungen �ber den Heliotropismus,' Sitzb. der K. Akad. der
+Wissenschaft. (Vienna), Jan. 1880.
+
+[page 3]
+creased growth, first on one side and then on another, is a secondary
+effect, and that the increased turgescence of the cells, together with the
+extensibility of their walls, is the primary cause of the movement of
+circumnutation.*
+
+In the course of the present volume it will be shown that apparently every
+growing part of every plant is continually circumnutating, though often on
+a small scale. Even the stems of seedlings before they have broken through
+the ground, as well as their buried radicles, circumnutate, as far as the
+pressure of the surrounding earth permits. In this universally present
+movement we have the basis or groundwork for the acquirement, according to
+the requirements of the plant, of the most diversified movements. Thus, the
+great sweeps made by the stems of twining plants, and by the tendrils of
+other climbers, result from a mere increase in the amplitude of the
+ordinary movement of circumnutation. The position which young leaves and
+other organs ultimately assume is acquired by the circumnutating movement
+being increased in some one direction. the leaves of various plants are
+said to sleep at night, and it will be seen that their blades then assume a
+vertical position through modified circumnutation, in order to protect
+their upper surfaces from being chilled through radiation.  The movements
+of various organs to the light, which are so general throughout the
+vegetable kingdom, and occasionally from the light, or transversely with
+respect to it, are all modified
+
+* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts in
+W�rzburg,' B. II. pp. 142, 143, 1878) on this intricate subject.
+Hofmeister's observations ('Jahreschrifte des Vereins f�r Vaterl.
+Naturkunde in W�rtemberg,' 1874, p. 211) on the curious movements of
+Spirogyra, a plant consisting of a single row of cells, are valuable in
+relation to this subject.
+
+[page 4]
+forms of circumnutation; as again are the equally prevalent movements of
+stems, etc., towards the zenith, and of roots towards the centre of the
+earth. In accordance with these conclusions, a considerable difficulty in
+the way of evolution is in part removed, for it might have been asked, how
+did all these diversified movements for the most different purposes first
+arise? As the case stands, we know that there is always movement in
+progress, and its amplitude, or direction, or both, have only to be
+modified for the good of the plant in relation with internal or external
+stimuli.
+
+Besides describing the several modified forms of circumnutation, some other
+subjects will be discussed. The two which have interested us most are,
+firstly, the fact that with some seedling plants the uppermost part alone
+is sensitive to light, and transmits an influence to the lower part,
+causing it to bend. If therefore the upper part be wholly protected from
+light, the lower part may be exposed for hours to it, and yet does not
+become in the least bent, although this would have occurred quickly if the
+upper part had been excited by light. Secondly, with the radicles of
+seedlings, the tip is sensitive to various stimuli, especially to very
+slight pressure, and when thus excited, transmits an influence to the upper
+part, causing it to bend from the pressed side. On the other hand, if the
+tip is subjected to the vapour of water proceeding from one side, the upper
+part of the radicle bends towards this side. Again it is the tip, as stated
+by Ciesielski, though denied by others, which is sensitive to the
+attraction of gravity, and by transmission causes the adjoining parts of
+the radicle to bend towards the centre of the earth. These several cases of
+the effects of contact, other irritants, vapour, light, and the
+[page 5]
+attraction of gravity being transmitted from the excited part for some
+little distance along the organ in question, have an important bearing on
+the theory of all such movements.
+
+[Terminology.--A brief explanation of some terms which will be used, must
+here be given. With seedlings, the stem which supports the cotyledons (i.e.
+the organs which represent the first leaves) has been called by many
+botanists the hypocotyledonous stem, but for brevity sake we will speak of
+it merely as the hypocotyl: the stem immediately above the cotyledons will
+be called the epicotyl or plumule. The radicle can be distinguished from
+the hypocotyl only by the presence of root-hairs and the nature of its
+covering. The meaning of the word circumnutation has already been
+explained. Authors speak of positive and negative heliotropism,*--that is,
+the bending of an organ to or from the light; but it is much more
+convenient to confine the word heliotropism to bending towards the light,
+and to designate as apheliotropism bending from the light. There is another
+reason for this change, for writers, as we have observed, occasionally drop
+the adjectives positive and negative, and thus introduce confusion into
+their discussions. Diaheliotropism may express a position more or less
+transverse to the light and induced by it. In like manner positive
+geotropism, or bending towards the centre of the earth, will be called by
+us geotropism; apogeotropism will mean bending in opposition to gravity or
+from the centre of the earth; and diageotropism, a position more or less
+transverse to the radius of the earth. The words heliotropism and
+geotropism properly mean the act of moving in relation to the light or the
+earth; but in the same manner as gravitation, though defined as "the act of
+tending to the centre," is often used to express the cause of a body
+falling, so it will be found convenient occasionally to employ heliotropism
+and geotropism, etc., as the cause of the movements in question.
+
+The term epinasty is now often used in Germany, and implies that the upper
+surface of an organ grows more quickly than the
+
+* The highly useful terms of Heliotropism and Geotropism were first used by
+Dr. A. B. Frank: see his remarkable 'Beitr�ge zur Pflanzenphysiologie,'
+1868.
+[page 6]
+lower surface, and thus causes it to bend downwards. Hyponasty is the
+reverse, and implies increased growth along the lower surface, causing the
+part to bend upwards.*
+
+Methods of Observation.--The movements, sometimes very small and sometimes
+considerable in extent, of the various organs observed by us, were traced
+in the manner which after many trials we found to be best, and which must
+be described. Plants growing in pots were protected wholly from the light,
+or had light admitted from above, or on one side as the case might require,
+and were covered above by a large horizontal sheet of glass, and with
+another vertical sheet on one side. A glass filament, not thicker than a
+horsehair, and from a quarter to three-quarters of an inch in length, was
+affixed to the part to be observed by means of shellac dissolved in
+alcohol. The solution was allowed to evaporate, until it became so thick
+that it set hard in two or three seconds, and it never injured the tissues,
+even the tips of tender radicles, to which it was applied. To the end of
+the glass filament an excessively minute bead of black sealing-wax was
+cemented, below or behind which a bit of card with a black dot was fixed to
+a stick driven into the ground. The weight of the filament was so slight
+that even small leaves were not perceptibly pressed down. another method of
+observation, when much magnification of the movement was not required, will
+presently be described. The bead and the dot on the card were viewed
+through the horizontal or vertical glass-plate (according to the position
+of the object), and when one exactly covered the other, a dot was made on
+the glass-plate with a sharply pointed stick dipped in thick Indian-ink.
+Other dots were made at short intervals of time and these were afterwards
+joined by straight lines. The figures thus traced were therefore angular;
+but if dots had been made every 1 or 2 minutes, the lines would have been
+more curvilinear, as occurred when radicles were allowed to trace their own
+courses on smoked glass-plates. To make the dots accurately was the sole
+difficulty, and required some practice. Nor could this be done quite
+accurately, when the movement was much magnified, such as 30 times and
+upwards; yet even in this case the general course may be trusted. To test
+the accuracy of the above method of observation, a filament was fixed to an
+
+* These terms are used in the sense given them by De Vries, 'W�rzburg
+Arbeiten,' Heft ii 1872, p. 252.
+
+[page 7]
+inanimate object which was made to slide along a straight edge and dots
+were repeatedly made on a glass-plate; when these were joined, the result
+ought to have been a perfectly straight line, and the line was very nearly
+straight. It may be added that when the dot on the card was placed
+half-an-inch below or behind the bead of sealing-wax, and when the
+glass-plate (supposing it to have been properly curved) stood at a distance
+of 7 inches in front (a common distance), then the tracing represented the
+movement of the bead magnified 15 times.
+
+Whenever a great increase of the movement was not required, another, and in
+some respects better, method of observation was followed. This consisted in
+fixing two minute triangles of thin paper, about 1/20 inch in height, to
+the two ends of the attached glass filament; and when their tips were
+brought into a line so that they covered one another, dots were made as
+before on the glass-plate. If we suppose the glass-plate to stand at a
+distance of seven inches from the end of the shoot bearing the filament,
+the dots when joined, will give nearly the same figure as if a filament
+seven inches long, dipped in ink, had been fixed to the moving shoot, and
+had inscribed its own course on the plate. The movement is thus
+considerably magnified; for instance, if a shoot one inch in length were
+bending, and the glass-plate stood at the distance of seven inches, the
+movement would be magnified eight times. It would, however, have been very
+difficult to have ascertained in each case how great a length of the shoot
+was bending; and this is indispensable for ascertaining the degree to which
+the movement is magnified.
+
+After dots had been made on the glass-plates by either of the above
+methods, they were copied on tracing paper and joined by ruled lines, with
+arrows showing the direction of the movement. The nocturnal courses are
+represented by straight broken lines. the first dot is always made larger
+than the others, so as to catch the eye, as may be seen in the diagrams.
+The figures on the glass-plates were often drawn on too large a scale to be
+reproduced on the pages of this volume, and the proportion in which they
+have been reduced is always given.* Whenever it could be approximately told
+how much the movement had been magnified, this is stated. We have perhaps
+
+* We are much indebted to Mr. Cooper for the care with which he has reduced
+and engraved our diagrams.
+
+[page 8]
+introduced a superfluous number of diagrams; but they take up less space
+than a full description of the movements. Almost all the sketches of plants
+asleep, etc., were carefully drawn for us by Mr. George Darwin.
+
+As shoots, leaves, etc., in circumnutating bend more and more, first in one
+direction and then in another, they were necessarily viewed at different
+times more or less obliquely; and as the dots were made on a flat surface,
+the apparent amount of movement is exaggerated according to the degree of
+obliquity of the point of view. It would, therefore, have been a much
+better plan to have used hemispherical glasses, if we had possessed them of
+all sizes, and if the bending part of the shoot had been distinctly hinged
+and could have been placed so as to have formed one of the radii of the
+sphere. But even in this case it would have been necessary afterwards to
+have projected the figures on paper; so that complete accuracy could not
+have been attained. From the distortion of our figures, owing to the above
+causes, they are of no use to any one who wishes to know the exact amount
+of movement, or the exact course pursued; but they serve excellently for
+ascertaining whether or not the part moved at all, as well as the general
+character of the movement.]
+
+In the following chapters, the movements of a considerable number of plants
+are described; and the species have been arranged according to the system
+adopted by Hooker in Le Maout and Decaisne's 'Descriptive Botany.' No one
+who is not investigating the present subject need read all the details,
+which, however, we have thought it advisable to give. To save the reader
+trouble, the conclusions and most of the more important parts have been
+printed in larger type than the other parts. He may, if he thinks fit, read
+the last chapter first, as it includes a summary of the whole volume; and
+he will thus see what points interest him, and on which he requires the
+full evidence.
+
+Finally, we must have the pleasure of returning our
+[page 9]
+sincere thanks to Sir Joseph Hooker and to Mr. W. Thiselton Dyer for their
+great kindness, in not only sending us plants from Kew, but in procuring
+others from several sources when they were required for our observations;
+also, for naming many species, and giving us information on various points.
+[page 10]
+
+
+
+CHAPTER I.
+
+THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
+
+Brassica oleracea, circumnutation of the radicle, of the arched hypocotyl
+whilst still buried beneath the ground, whilst rising above the ground and
+straightening itself, and when erect--Circumnutation of the cotyledons--
+Rate of movement--Analogous observations on various organs in species of
+Githago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of several
+Leguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula,
+Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus,
+Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena,
+Nephrodium, and Selaginella.
+
+THE following chapter is devoted to the circumnutating movements of the
+radicles, hypocotyls, and cotyledons of seedling plants; and, when the
+cotyledons do not rise above the ground, to the movements of the epicotyl.
+But in a future chapter we shall have to recur to the movements of certain
+cotyledons which sleep at night.
+
+[Brassica oleracea (Cruciferae)'.--Fuller details will be given with
+respect to the movements in this case than in any other, as space and time
+will thus ultimately be saved.
+
+Radicle.--A seed with the radicle projecting .05 inch was fastened with
+shellac to a little plate of zinc, so that the radicle stood up vertically;
+and a fine glass filament was then fixed near its base, that is, close to
+the seed-coats. The seed was surrounded by little bits of wet sponge, and
+the movement of the bead at the end of the filament was traced (Fig. 1)
+during sixty hours. In this time the radicle increased in length from .05
+to .11 inch. Had the filament been attached at first close to the apex of
+the radicle, and if it could have remained there all the time, the movement
+exhibited would have
+[page 11]
+been much greater, for at the close of our observations the tip, instead of
+standing vertically upwards, had become bowed downwards through geotropism,
+so as almost to touch the zinc plate. As far as we could roughly ascertain
+by measurements made with compasses on other seeds, the tip alone, for a
+length of only 2/100 to 3/100 of an inch, is acted on by geotropism. But
+the tracing shows that the basal part of the radicle continued to
+circumnutate irregularly during the whole time. The actual extreme amount
+of movement of the bead at the end of the filament was nearly .05 inch, but
+to what extent the movement of the radicle was magnified by the filament,
+which was nearly 3/4 inch in length, it was impossible to estimate.
+
+Fig. 1. Brassica oleracea: circumnutation of radicle, traced on horizontal
+glass, from 9 A.M. Jan. 31st to 9 P.M. Feb. 2nd. Movement of bead at end of
+filament magnified about 40 times.
+
+Another seed was treated and observed in the same manner, but the radicle
+in this case protruded .1 inch, and was not
+Fig. 2. Brassica oleracea: circumnutating and geotropic movement of
+radicle, traced on horizontal glass during 46 hours.
+
+fastened so as to project quite vertically upwards. The filament was
+affixed close to its base. The tracing (Fig. 2, reduced by half) shows the
+movement from 9 A.M. Jan. 31st to 7 A.M. Feb. 2nd; but it continued to move
+during the whole of the
+[page 12]
+2nd in the same general direction, and in a similar zigzag manner. From the
+radicle not being quite perpendicular when the filament was affixed
+geotropism came into play at once; but the irregular zigzag course shows
+that there was growth (probably preceded by turgescence), sometimes on one
+and sometimes on another side. Occasionally the bead remained stationary
+for about an hour, and then probably growth occurred on the side opposite
+to that which caused the geotropic curvature. In the case previously
+described the basal part of the very short radicle from being turned
+vertically upwards, was at first very little affected by geotropism.
+Filaments were affixed in two other instances to rather longer radicles
+protruding obliquely from seeds which had been turned upside down; and in
+these cases the lines traced on the horizontal glasses were only slightly
+zigzag, and the movement was always in the same general direction, through
+the action of geotropism. All these observations are liable to several
+causes of error, but we believe, from what will hereafter be shown with
+respect to the movements of the radicles of other plants, that they may be
+largely trusted.
+
+Hypocotyl.--The hypocotyl protrudes through the seed-coats as a rectangular
+projection, which grows rapidly into an arch like the letter U turned
+upside down; the cotyledons being still enclosed within the seed. In
+whatever position the seed may be embedded in the earth or otherwise fixed,
+both legs of the arch bend upwards through apogeotropism, and thus rise
+vertically above the ground. As soon as this has taken place, or even
+earlier, the inner or concave surface of the arch grows more quickly than
+the upper or convex surface; and this tends to separate the two legs and
+aids in drawing the cotyledons out of the buried seed-coats. By the growth
+of the whole arch the cotyledons are ultimately dragged from beneath the
+ground, even from a considerable depth; and now the hypocotyl quickly
+straightens itself by the increased growth of the concave side.
+
+Even whilst the arched or doubled hypocotyl is still beneath the ground, it
+circumnutates as much as the pressure of the surrounding soil will permit;
+but this was difficult to observe, because as soon as the arch is freed
+from lateral pressure the two legs begin to separate, even at a very early
+age, before the arch would naturally have reached the surface. Seeds were
+allowed to germinate on the surface of damp earth, and after they had fixed
+themselves by their radicles, and after the, as yet, only
+[page 13]
+slightly arched hypocotyl had become nearly vertical, a glass filament was
+affixed on two occasions near to the base of the basal leg (i.e. the one in
+connection with the radicle), and its movements were traced in darkness on
+a horizontal glass. The result was that long lines were formed running in
+nearly the plane of the vertical arch, due to the early separation of the
+two legs now freed from pressure; but as the lines were zigzag, showing
+lateral movement, the arch must have been circumnutating, whilst it was
+straightening itself by growth along its inner or concave surface.
+
+A somewhat different method of observation was next followed:
+Fig. 3. Brassica oleracea: circumnutating movement of buried and arched
+hypocotyl (dimly illuminated from above), traced on horizontal glass during
+45 hours. Movement of bead of filament magnified about 25 times, and here
+reduced to one-half of original scale.
+
+as soon as the earth with seeds in a pot began to crack, the surface was
+removed in parts to the depth of .2 inch; and a filament was fixed to the
+basal leg of a buried and arched hypocotyl, just above the summit of the
+radicle. The cotyledons were still almost completely enclosed within the
+much-cracked seed-coats; and these were again covered up with damp adhesive
+soil pressed pretty firmly down. The movement of the filament was traced
+(Fig. 3) from 11 A.M. Feb. 5th till 8 A.M. Feb. 7th. By this latter period
+the cotyledons had been dragged from beneath the pressed-down earth, but
+the upper part of the hypocotyl still formed nearly a right angle with the
+lower part. The tracing shows that the arched hypocotyl tends at this early
+[page 14]
+age to circumnutate irregularly. On the first day the greater movement
+(from right to left in the figure) was not in the plane of the vertical and
+arched hypocotyl, but at right angles to it, or in the plane of the two
+cotyledons, which were still in close contact. The basal leg of the arch at
+the time when the filament was affixed to it, was already bowed
+considerably backwards, or from the cotyledons; had the filament been
+affixed before this bowing occurred, the chief movement would have been at
+right angles to that shown in the figure. A filament was attached to
+another buried hypocotyl of the same age, and it moved in a similar general
+manner, but the line traced was not so complex. This hypocotyl became
+almost straight, and the cotyledons were dragged from beneath the ground on
+the evening of the second day.
+
+Fig. 4. Brassica oleracea: circumnutating movement of buried and arched
+hypocotyl, with the two legs of the arch tied together, traced on
+horizontal glass during 33 � hours. Movement of the bead of filament
+magnified about 26 times, and here reduced to one-half original scale.
+
+Before the above observations were made, some arched hypocotyls buried at
+the depth of a quarter of an inch were uncovered; and in order to prevent
+the two legs of the arch from beginning to separate at once, they were tied
+together with fine silk. This was done partly because we wished to
+ascertain how long the hypocotyl, in its arched condition, would continue
+to move, and whether the movement when not masked and disturbed by the
+straightening process, indicated circumnutation. Firstly a filament was
+fixed to the basal leg of an arched hypocotyl close above the summit of the
+radicle. The cotyledons were still partially enclosed within the
+seed-coats. The movement was traced (Fig. 4) from 9.20 A.M. on Dec.
+[page 15]
+23rd to 6.45 A.M. on Dec. 25th. No doubt the natural movement was much
+disturbed by the two legs having been tied together; but we see that it was
+distinctly zigzag, first in one direction and then in an almost opposite
+one. After 3 P.M. on the 24th the arched hypocotyl sometimes remained
+stationary for a considerable time, and when moving, moved far slower than
+before. Therefore, on the morning of the 25th, the glass filament was
+removed from the base of the basal leg, and was fixed horizontally on the
+summit of the arch, which, from the legs having been tied, had grown broad
+and almost flat. The movement was now traced during 23 hours (Fig. 5), and
+we
+
+Fig. 5. Brassica oleracea: circumnutating movement of the crown of a buried
+and arched hypocotyl, with the two legs tied together, traced on a
+horizontal glass during 23 hours. Movement of the bead of the filament
+magnified about 58 times, and here reduced to one-half original scale.
+
+see that the course was still zigzag, which indicates a tendency to
+circumnutation. The base of the basal leg by this time had almost
+completely ceased to move.
+
+As soon as the cotyledons have been naturally dragged from beneath the
+ground, and the hypocotyl has straightened itself by growth along the inner
+or concave surface, there is nothing to interfere with the free movements
+of the parts; and the circumnutation now becomes much more regular and
+clearly displayed, as shown in the following cases:--A seedling was placed
+in front and near a north-east window with a line joining the
+[page 16]
+two cotyledons parallel to the window. It was thus left the whole day so as
+to accommodate itself to the light. On the following morning a filament was
+fixed to the midrib of the larger and taller cotyledon (which enfolds the
+other and smaller one, whilst still within the seed), and a mark being
+placed close behind, the movement of the whole plant, that is, of the
+hypocotyl and cotyledon, was traced greatly magnified on a vertical glass.
+At first the plant bent so much towards the light that it was useless to
+attempt to trace the movement; but at 10 A.M. heliotropism almost wholly
+ceased and the first dot was
+
+Fig. 6. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons during 10 hours 45 minutes. Figure here reduced to one-half
+original scale.
+
+made on the glass. The last was made at 8.45 P.M.; seventeen dots being
+altogether made in this interval of 10 h. 45 m. (see Fig. 6). It should be
+noticed that when I looked shortly after 4 P.M. the bead was pointing off
+the glass, but it came on again at 5.30 P.M., and the course during this
+interval of 1 h. 30 m. has been filled up by imagination, but cannot be far
+from correct. The bead moved seven times from side to side, and thus
+described 3 � ellipses in 10 3/4 h.; each being completed on an average in
+3 h. 4 m.
+
+On the previous day another seedling had been observed under similar
+conditions, excepting that the plant was so
+[page 17]
+placed that a line joining the two cotyledons pointed towards the window;
+and the filament was attached to the smaller cotyledon on the side furthest
+from the window. Moreover the plant was now for the first time placed in
+this position. The cotyledons bowed themselves greatly towards the light
+from 8 to 10.50 A.M., when the first dot was made (Fig. 7). During the
+
+Fig. 7. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons, from 10.50 A.M. to 8 A.M. on the following morning. Tracing
+made on a vertical glass.
+
+next 12 hours the bead swept obliquely up and down 8 times and described 4
+figures representing ellipses; so that it travelled at nearly the same rate
+as in the previous case. during the night it moved upwards, owing to the
+sleep-movement of the cotyledons, and continued to move in the same
+direction till 9 A.M. on the following morning; but this latter movement
+would not have occurred with seedlings under their natural conditions fully
+exposed to the light.
+
+By 9.25 A.M. on this second day the same cotyledon had
+[page 18]
+begun to fall, and a dot was made on a fresh glass. The movement was traced
+until 5.30 P.M. as shown in (Fig. 8), which is given, because the course
+followed was much more irregular than on the two previous occasions. During
+these 8 hours the bead changed its course greatly 10 times. The upward
+movement of the cotyledon during the afternoon and early part of the night
+is here plainly shown.
+
+Fig. 8. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons during 8 hours. Figure here reduced to one-third of the original
+scale, as traced on a vertical glass.
+
+As the filaments were fixed in the three last cases to one of the
+cotyledons, and as the hypocotyl was left free, the tracings show the
+movement of both organs conjoined; and we now wished to ascertain whether
+both circumnutated. Filaments were therefore fixed horizontally to two
+hypocotyls close beneath the petioles of their cotyledons. These seedlings
+had stood for two days in the same position before a north-east window. In
+the morning, up to about 11 A.M., they moved in zigzag lines towards the
+light; and at night they again became almost upright through apogeotropism.
+After about 11 A.M. they moved a little back from the light, often crossing
+and recrossing their former path in zigzag lines. the sky on this day
+varied much in brightness, and these observations merely proved that the
+hypocotyls were continually moving in a manner resembling circumnutation.
+On a previous day which was uniformly cloudy, a hypocotyl was firmly
+secured to a little stick, and a filament was fixed to the larger of the
+two cotyledons, and its movement was traced on a vertical glass. It fell
+greatly from 8.52 A.M., when the first dot was made, till 10.55 A.M.; it
+then rose greatly until 12.17 P.M. Afterwards it fell a little and made a
+loop, but by 2.22 P.M. it had risen a little and continued rising till 9.23
+P.M., when it made another loop, and at 10.30 P.M. was again rising. These
+observations show that the cotyledons move
+[page 19]
+vertically up and down all day long, and as there was some slight lateral
+movement, they circumnutated.
+
+Fig. 9. Brassica oleracea: circumnutation of hypocotyl, in darkness, traced
+on a horizontal glass, by means of a filament with a bead fixed across its
+summit, between 9.15 A.M. and 8.30 A.M. on the following morning. Figure
+here reduced to one-half of original scale.
+
+The cabbage was one of the first plants, the seedlings of which were
+observed by us, and we did not then know how far the circumnutation of the
+different parts was affected by light. Young seedlings were therefore kept
+in complete darkness except for a minute or two during each observation,
+when they were illuminated by a small wax taper held almost vertically
+above them. During the first day the hypocotyl of one changed its course 13
+times (see Fig. 9); and it deserves notice that the longer axes of the
+figures described often cross one another at right or nearly right angles.
+Another seedling was observed in the same manner, but it was much older,
+for it had formed a true leaf a quarter of an inch in length, and the
+hypocotyl was 1 3/8 inch in height. The figure traced was a very complex
+one, though the movement was not so great in extent as in the last case.
+
+The hypocotyl of another seedling of the same age was secured to a little
+stick, and a filament having been fixed to the midrib of one of the
+cotyledons, the movement of the bead was traced during 14 h. 15 m. (see
+Fig. 10) in darkness. It should be noted that the chief movement of the
+cotyledons, namely, up and down, would be shown on a horizontal glass-plate
+only by the lines in the direction of the midrib (that is,
+[page 20]
+up and down, as Fig. 10 here stands) being a little lengthened or
+shortened; whereas any lateral movement would be well exhibited. The
+present tracing shows that the cotyledon did thus move laterally (that is,
+from side to side in the tracing) 12 times in the 14 h. 15 m. of
+observation. Therefore the cotyledons certainly circumnutated, though the
+chief movement was up and down in a vertical plane.
+
+Fig 10. Brassica oleracea: circumnutation of a cotyledon, the hypocotyl
+having been secured to a stick, traced on a horizontal glass, in darkness,
+from 8.15 A.M. to 10.30 P.M. Movement of the bead of the filament magnified
+13 times.
+
+Rate of Movement.--The movements of the hypocotyls and cotyledons of
+seedling cabbages of different ages have now been sufficiently illustrated.
+With respect to the rate, seedlings were placed under the microscope with
+the stage removed, and with a micrometer eye-piece so adjusted that each
+division equalled 1/500 inch; the plants were illuminated by light passing
+through a solution of bichromate of potassium so as to eliminate
+heliotropism. Under these circumstances it was interesting to observe how
+rapidly the circumnutating apex of a cotyledon passed across the divisions
+of the micrometer. Whilst travelling in any direction the apex generally
+oscillated backwards and forwards to the extent of 1/500 and sometimes of
+nearly 1/250 of an inch. These oscillations were quite different from the
+trembling caused by any disturbance in the same room or by the shutting of
+a distant door. The first seedling observed was nearly two inches in height
+and had been etiolated by having been grown in darkness. The tip of the
+cotyledon passed across 10 divisions of the micrometer, that is, 1/50 of an
+inch, in 6 m. 40 s. Short glass filaments were then fixed vertically to the
+hypocotyls of several seedlings so as to project a little above the
+cotyledons, thus exaggerating the rate of movement; but only a few of the
+observations thus made are worth giving. The most remarkable fact was the
+oscillatory movement above described, and the difference of rate at which
+the point crossed the divisions of the micrometer, after short intervals of
+time. For instance, a tall not-etiolated seedling had been kept for 14 h.
+in darkness; it was exposed before a north-east window for only
+[page 21]
+two or three minutes whilst a glass filament was fixed vertically to the
+hypocotyl; it was then again placed in darkness for half an hour and
+afterwards observed by light passing through bichromate of potassium.  The
+point, oscillating as usual, crossed five divisions of the micrometer (i.e.
+1/100 inch) in 1 m. 30 s. The seedling was then left in darkness for an
+hour, and now it required 3 m. 6 s. to cross one division, that is, 15 m.
+30 s. to have crossed five divisions. Another seedling, after being
+occasionally observed in the back part of a northern room with a very dull
+light, and left in complete darkness for intervals of half an hour, crossed
+five divisions in 5 m. in the direction of the window, so that we concluded
+that the movement was heliotropic. But this was probably not the case, for
+it was placed close to a north-east window and left there for 25 m., after
+which time, instead of moving still more quickly towards the light, as
+might have been expected, it travelled only at the rate of 12 m. 30 s. for
+five divisions. It was then again left in complete darkness for 1 h., and
+the point now travelled in the same direction as before, but at the rate of
+3 m. 18 s. for five divisions.
+
+We shall have to recur to the cotyledons of the cabbage in a future
+chapter, when we treat of their sleep-movements. The circumnutation, also,
+of the leaves of fully-developed plants will hereafter be described.
+
+Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on a
+horizontal glass, by means of a filament fixed transversely across its
+summit, from 8.15 A.M. to 12.15 P.M. on the following day. Movement of bead
+of filament magnified about 13 times, here reduced to one-half the original
+scale.
+
+Githago segetum (Caryophylleae).--A young seedling was dimly illuminated
+from above, and the circumnutation of the hypo-
+[page 22]
+cotyl was observed during 28 h., as shown in Fig. 11. It moved in all
+directions; the lines from right and to left in the figure being parallel
+to the blades of the cotyledons. The actual distance travelled from side to
+side by the summit of the hypocotyl was about .2 of an inch; but it was
+impossible to be accurate on this head, as the more obliquely the plant was
+viewed, after it had moved for some time, the more the distances were
+exaggerated.
+
+We endeavoured to observe the circumnutation of the cotyledons, but as they
+close together unless kept exposed to a moderately bright light, and as the
+hypocotyl is extremely heliotropic, the necessary arrangements were too
+troublesome. We shall recur to the nocturnal or sleep-movements of the
+cotyledons in a future chapter.
+
+Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontal
+glass, from 10.30 A.M. to 9.30 A.M. on following morning, by means of a
+filament fixed across its summit. Movement of bead of filament magnified
+about twice; seedling illuminated from above.
+
+Gossypium (var. Nankin cotton) (Malvaceae).--The circumnutation of a
+hypocotyl was observed in the hot-house, but the movement was so much
+exaggerated that the bead twice passed for a time out of view. It was,
+however, manifest that two somewhat irregular ellipses were nearly
+completed in 9 h. Another seedling, 1 � in. in height, was then observed
+during 23 h.; but the observations were not made at sufficiently short
+intervals, as shown by the few dots in Fig. 12, and the tracing was not now
+sufficiently enlarged. Nevertheless there could be no doubt about the
+circumnutation of the hypocotyl, which described in 12 h. a figure
+representing three irregular ellipses of unequal sizes.
+
+ The cotyledons are in constant movement up and down during the whole day,
+and as they offer the unusual case of moving downwards late in the evening
+and in the early part of the night, many observations were made on them. A
+filament was fixed along the middle of one, and its movement traced on a
+vertical glass; but the tracing is not given, as the hypocotyl was not
+secured, so that it was impossible to distinguish clearly between its
+movement and that of the cotyledon. The cotyledons rose from 10.30 A.M. to
+about 3 P.M.; they then sank till 10 P.M., rising, however, greatly in the
+latter part of the night.
+[page 23]
+The angles above the horizon at which the cotyledons of another seedling
+stood at different hours is recorded in the following short table: --
+
+Oct. 20   2.50 P.M...25o above horizon.
+Oct. 20   4.20 P.M...22o above horizon.
+Oct. 20   5.20 P.M...15o above horizon.
+Oct. 20 10.40 P.M...8o above horizon.
+Oct. 21   8.40 A.M...28o above horizon.
+Oct. 21 11.15 A.M...35o above horizon.
+Oct. 21   9.11 P.M...10o below horizon.
+
+The position of the two cotyledons was roughly sketched at various hours
+with the same general result.
+
+In the following summer, the hypocotyl of a fourth seedling was secured to
+a little stick, and a glass filament with triangles of paper having been
+fixed to one of the cotyledons, its movements were traced on a vertical
+glass under a double skylight in the house. The first dot was made at 4.20
+P.M. June 20th; and the cotyledon fell till 10.15 P.M. in a nearly straight
+line. Just past midnight it was found a little lower and somewhat to one
+side. By the early morning, at 3.45 A.M., it had risen greatly, but by 6.20
+A.M. had fallen a little. During the whole of this day (21st) it fell in a
+slightly zigzag line, but its normal course was disturbed by the want of
+sufficient illumination, for during the night it rose only a little, and
+travelled irregularly during the whole of the following day and night of
+June 22nd. The ascending and descending lines traced during the three days
+did not coincide, so that the movement was one of circumnutation. This
+seedling was then taken back to the hot-house, and after five days was
+inspected at 10 P.M., when the cotyledons were found hanging so nearly
+vertically down, that they might justly be said to have been asleep. On the
+following morning they had resumed their usual horizontal position.
+
+Oxalis rosea (Oxalideae).--The hypocotyl was secured to a little stick, and
+an extremely thin glass filament, with two triangles of paper, was attached
+to one of the cotyledons, which was .15 inch in length. In this and the
+following species the end of the petiole, where united to the blade, is
+developed into a pulvinus. The apex of the cotyledon stood only 5 inches
+from the vertical glass, so that its movement was not greatly exaggerated
+as long as it remained nearly horizontal; but in the course of the day it
+both rose considerably above and fell beneath a horizontal position, and
+then of course the movement was much exaggerated.
+[page 24]
+In Fig. 13 its course is shown from 6.45 A.M. on June 17th, to 7.40 A.M. on
+the following morning; and we see that during the daytime, in the course of
+11 h. 15 m., it travelled thrice down and twice up. After 5.45 P.M. it
+moved rapidly downwards, and in an hour or two depended vertically; it thus
+remained all night asleep. This position could not be represented on the
+vertical glass nor in the figure here given. By 6.40 A.M. on the following
+morning (18th) both cotyledons had risen greatly, and they continued to
+rise until 8 A.M., when they stood almost horizontally. Their movement was
+traced during the whole of this day and until the next morning; but a
+tracing is not given, as it was closely similar to Fig. 13, excepting that
+the lines were more zigzag. The cotyledons moved 7 times, either upwards or
+downwards; and at about 4 P.M. the great nocturnal sinking movement
+commenced.
+
+Fig. 13. Oxalis rosea: circumnutation of cotyledons, the hypocotyl being
+secured to a stick; illuminated from above. Figure here given one-half of
+original scale.
+
+Another seedling was observed in a similar manner during nearly 24 h., but
+with the difference that the hypocotyl was left free. The movement also was
+less magnified. Between 8.12 A.M. and 5 P.M. on the 18th, the apex of the
+cotyledon moved 7 times upwards or downwards (Fig. 14). The nocturnal
+sinking movement, which is merely a great increase of one of the diurnal
+oscillations, commenced about 4 P.M.
+
+Oxalis Valdiviana.--This species is interesting, as the coty-
+[page 25]
+ledons rise perpendicularly upwards at night so as to come into close
+contact, instead of sinking vertically downwards, as in the case of O.
+rosea. A glass filament was fixed to a cotyledon, .17 of an inch in length,
+and the hypocotyl was left free. On
+
+Fig. 14. Oxalis rosea: conjoint circumnutation of the cotyledons and
+hypocotyl, traced from 8.12 A.M. on June 18th to 7.30 A.M. 19th. The apex
+of the cotyledon stood only 3 3/4 inches from the vertical glass. Figure
+here given one-half of original scale.
+
+Fig. 15. Oxalis Valdiviana: conjoint circumnutation of a cotyledon and the
+hypocotyl, traced on vertical glass, during 24 hours. Figure here given
+one-half of original scale; seedling illuminated from above.
+
+the first day the seedling was placed too far from the vertical glass; so
+that the tracing was enormously exaggerated and the movement could not be
+traced when the cotyledon either rose or sank much; but it was clearly seen
+that the cotyledons rose thrice and fell twice between 8.15 A.M. and 4.15
+P.M. Early on the following morning (June 19th) the apex of a cotyledon was
+[page 26]
+placed only 1 7/8 inch from the vertical glass. At 6.40 A.M. it stood
+horizontally; it then fell till 8.35, and then rose. Altogether in the
+course of 12 h. it rose thrice and fell thrice, as may be seen in Fig. 15.
+The great nocturnal rise of the cotyledons usually commences about 4 or 5
+P.M., and on the following morning they are expanded or stand horizontally
+at about 6.30 A.M. In the present instance, however, the great nocturnal
+rise did not commence till 7 P.M.; but this was due to the hypocotyl having
+from some unknown cause temporarily bent to the left side, as is shown in
+the tracing. To ascertain positively that the hypocotyl circumnutated, a
+mark was placed at 8.15 P.M. behind the two now closed and vertical
+cotyledons; and the movement of a glass filament fixed upright to the top
+of the hypocotyl was traced until 10.40 P.M. During this time it moved from
+side to side, as well as backwards and forwards, plainly showing
+circumnutation; but the movement was small in extent. Therefore Fig. 15
+represents fairly well the movements of the cotyledons alone, with the
+exception of the one great afternoon curvature to the left.
+
+Oxalis corniculata (var. cuprea).--The cotyledons rise at night to a
+variable degree above the horizon, generally about 45o: those on some
+seedlings between 2 and 5 days old were found to be in continued movement
+all day long; but the movements were more simple than in the last two
+species. This may have partly resulted from their not being sufficiently
+illuminated whilst being observed, as was shown by their not beginning to
+rise until very late in the evening.
+
+Oxalis (Biophytum) sensitiva.--The cotyledons are highly remarkable from
+the amplitude and rapidity of their movements during the day. The angles at
+which they stood above or beneath the horizon were measured at short
+intervals of time; and we regret that their course was not traced during
+the whole day. We will give only a few of the measurements, which were made
+whilst the seedlings were exposed to a temperature of 22 1/2o to 24 �
+decrees C. One cotyledon rose 70o in 11 m.; another, on a distinct
+seedling, fell 80o in 12 m. Immediately before this latter fall the same
+cotyledon had risen from a vertically downward to a vertically upward
+position in 1 h. 48 m., and had therefore passed through 180o in under 2 h.
+We have met with no other instance of a circumnutating movement of such
+great amplitude as 180o; nor of such rapidity of movement as the passage
+through 80o in 12 m. The cotyledons of this plant sleep at night by rising
+[page 27]
+vertically and coming into close contact. This upward movement differs from
+one of the great diurnal oscillations above described only by the position
+being permanent during the night and by its periodicity, as it always
+commences late in the evening.
+
+Tropaeolum minus (?) (var. Tom Thumb) (Tropaeoleae).--The cotyledons are
+hypogean, or never rise above the ground. By removing the soil a buried
+epicotyl or plumule was found, with its summit arched abruptly downwards,
+like the arched hypocotyl of the cabbage previously described. A glass
+filament with a bead at its end was affixed to the basal half or leg, just
+above the hypogean cotyledons, which were again almost surrounded by loose
+earth. The tracing (Fig. 16) shows the course of the bead during 11 h.
+After the last dot given in the figure, the bead moved to a great distance,
+and finally off the glass, in the direction indicated by the broken line.
+This great movement, due to increased growth along the concave surface of
+the arch, was caused by the basal leg bending backwards from the upper
+part, that is in a direction opposite to the dependent tip, in the same
+manner as occurred with the hypocotyl of the cabbage. Another buried and
+arched epicotyl was observed in the same manner, excepting that the two
+legs of the arch were tied together with fine silk for the sake of
+preventing the great movement just mentioned. It moved, however, in the
+evening in the same direction as before, but the line followed was not so
+straight. During the morning the tied arch moved in an irregularly
+circular, strongly zigzag course, and to a greater distance than in the
+previous case, as was shown in a tracing, magnified 18 times. The movements
+of a young plant bearing a few leaves and of a mature plant, will hereafter
+be described.
+
+Fig. 16. Tropaeolum minus (?): circumnutation of buried and arched
+epicotyl, traced on a horizontal glass, from 9.20 A.M. to 8.15 P.M.
+Movement of bead of filament magnified 27 times.
+[page 28]
+
+Citrus aurantium (Orange) (Aurantiaceae).--The cotyledons are hypogean. The
+circumnutation of an epicotyl, which at the close of our observations was
+.59 of an inch (15 mm.) in height above the ground, is shown in the annexed
+figure (Fig. 17), as observed during a period of 44 h. 40 m.
+
+Fig. 17. Citrus aurantium: circumnutation of epicotyl with a filament fixed
+transversely near its apex, traced on a horizontal glass, from 12.13 P.M.
+on Feb. 20th to 8.55 A.M. on 22nd. The movement of the bead of the filament
+was at first magnified 21 times, or 10 1/2, in figure here given, and
+afterwards 36 times, or 18 as here given; seedling illuminated from above.
+
+Aesculus hippocastanum (Hippocastaneae).--Germinating seeds were placed in
+a tin box, kept moist internally, with a sloping bank of damp argillaceous
+sand, on which four smoked glass-plates rested, inclined at angles of 70o
+and 65o with the horizon. The tips of the radicles were placed so as just
+to touch the upper end of the glass-plates, and, as they grew downwards
+they pressed lightly, owing to geotropism, on the smoked surfaces, and left
+tracks of their course. In the middle part of each track the glass was
+swept clean, but the margins were much blurred and irregular. Copies of two
+of these tracks (all four being nearly alike) were made on tracing paper
+placed over the glass-plates after they had been varnished; and they are as
+exact as possible considering the nature of the margins (Fig. 18). They
+suffice to show that there was some lateral, almost serpentine movement,
+and that the tips in their downward course pressed with unequal force on
+the plates, as
+[page 29]
+the tracks varied in breadth. The more perfectly serpentine tracks made by
+the radicles of Phaseolus multiflorus and Vicia faba (presently to be
+described), render it almost certain that the radicles of the present plant
+circumnutated.
+
+Fig. 18. Aesculus hippocastanum: outlines of tracks left on inclined
+glass-plates by tips of radicles. In A the plate was inclined at 70o with
+the horizon, and the radicle was 1.9 inch in length, and .23 inch in
+diameter at base. In B the plate was inclined 65o with the horizon, and the
+radicle was a trifle larger.
+
+Phaseolus multiflorus (Leguminosae).--Four smoked glass-plates were
+arranged in the same manner as described under Aesculus, and the tracks
+left by the tips of four radicles of the present plant, whilst growing
+downwards, were photographed as transparent objects. Three of them are here
+exactly copied (Fig. 19). Their serpentine courses show that the tips moved
+regularly from side to side; they also pressed alternately with greater or
+less force on the plates, sometimes rising up and leaving them altogether
+for a very short distance; but this was better seen on the original plates
+than in the copies. These radicles therefore were continually moving in all
+directions--that is, they circumnutated. The distance between the extreme
+right and left positions of the radicle A, in its lateral movement, was 2
+mm., as ascertained by measurement with an eye-piece micrometer.
+
+Fig. 19. Phaseolus multiflorus: tracks left on inclined smoked glass-plates
+by tips of radicles in growing downwards. A and C, plates inclined at 60o,
+B inclined at 68o with the horizon.
+
+Vicia faba (Common Bean) (Leguminosae).--Radicle.--Some beans were allowed
+to germinate on bare sand, and after one had protruded its radicle to a
+length of .2 of an inch, it was turned upside down, so that the radicle,
+which was kept in damp air, now stood upright. A filament, nearly an inch
+in length, was affixed obliquely near its tip; and the movement of the
+terminal bead was traced from 8.30 A.M. to 10.30 P.M., as shown in Fig. 18.
+The radicle at first changed its course twice
+[page 30]
+abruptly, then made a small loop and then a larger zigzag curve. During the
+night and till 11 A.M. on the following
+
+Fig. 20. Vicia faba: circumnutation of a radicle, at first pointing
+vertically upwards, kept in darkness, traced on a horizontal glass, during
+14 hours. Movement of bead of filament magnified 23 times, here reduced to
+one-half of original scale.
+
+morning, the bead moved to a great distance in a nearly straight line, in
+the direction indicated by the broken line in the figure. This resulted
+from the tip bending quickly downwards, as it had now become much declined,
+and had thus gained a position highly favourable for the action of
+geotropism.
+Fig. 21. Vicia faba: tracks left on inclined smoked glass-plates, by tips
+of radicles in growing downwards. Plate C was inclined at 63o, plates A and
+D at 71o, plate B at 75o, and plate E at a few degrees beneath the horizon.
+[page 31]
+
+We next experimented on nearly a score of radicles by allowing them to grow
+downwards over inclined plates of smoked glass, in exactly the same manner
+as with Aesculus and Phaseolus. Some of the plates were inclined only a few
+degrees beneath the horizon, but most of them between 60o and 75o. In the
+latter cases the radicles in growing downwards were deflected only a little
+from the direction which they had followed whilst germinating in sawdust,
+and they pressed lightly on the glass-plates (Fig. 21). Five of the most
+distinct tracks are here copied, and they are all slightly sinuous, showing
+circumnutation. Moreover, a close examination of almost every one of the
+tracks clearly showed that the tips in their downward course had
+alternately pressed with greater or less force on the plates, and had
+sometimes risen up so as nearly to leave them for short intervals. The
+distance between the extreme right and left positions of the radicle A was
+0.7 mm., ascertained in the same manner as in the case of Phaseolus.
+
+Epicotyl.--At the point where the radicle had protruded from a bean laid on
+its side, a flattened solid lump projected .1 of an inch, in the same
+horizontal plane with the bean. This protuberance consisted of the convex
+summit of the arched epicotyl; and as it became developed the two legs of
+the arch curved themselves laterally upwards, owing to apogeotropism, at
+such a rate that the arch stood highly inclined after 14 h., and vertically
+in 48 h. A filament was fixed to the crown of the protuberance before any
+arch was visible, but the basal half grew so quickly that on the second
+morning the end of the filament was bowed greatly downwards. It was
+therefore removed and fixed lower down. The line traced during these two
+days extended in the same general direction, and was in parts nearly
+straight, and in others plainly zigzag, thus giving some evidence of
+circumnutation.
+
+As the arched epicotyl, in whatever position it may be placed, bends
+quickly upwards through apogeotropism, and as the two legs tend at a very
+early age to separate from one another, as soon as they are relieved from
+the pressure of the surrounding earth, it was difficult to ascertain
+positively whether the epicotyl, whilst remaining arched, circumnutated.
+Therefore some rather deeply buried beans were uncovered, and the two legs
+of the arches were tied together, as had been done with the epicotyl of
+Tropaeolum and the hypocotyl of the Cabbage. The movements of the tied
+arches were traced in the usual manner on
+[page 32]
+two occasions during three days. But the tracings made under such unnatural
+conditions are not worth giving; and it need only be said that the lines
+were decidedly zigzag, and that small loops were occasionally formed. We
+may therefore conclude that the epicotyl circumnutates whilst still arched
+and before it has grown tall enough to break through the surface of the
+ground.
+
+In order to observe the movements of the epicotyl at a somewhat more
+advanced age, a filament was fixed near the base of one which was no longer
+arched, for its upper half now formed a right angle with the lower half.
+This bean had germinated on bare damp sand, and the epicotyl began to
+straighten itself much sooner than would have occurred if it had been
+properly planted. The course pursued during 50 h. (from 9 A.M. Dec. 26th,
+to 11 A.M. 28th) is here shown (Fig. 22); and we see
+Fig. 22. Vicia faba: circumnutation of young epicotyl, traced in darkness
+during 50 hours on a horizontal glass. Movement of bead of filament
+magnified 20 times, here reduced to one-half of original scale.
+
+that the epicotyl circumnutated during the whole time. Its basal part grew
+so much during the 50 h. that the filament at the end of our observations
+was attached at the height of .4 inch above the upper surface of the bean,
+instead of close to it. If the bean had been properly planted, this part of
+the epicotyl would still have been beneath the soil.
+
+Late in the evening of the 28th, some hours after the above observations
+were completed, the epicotyl had grown much straighter, for the upper part
+now formed a widely open angle with the lower part. A filament was fixed to
+the upright basal part, higher up than before, close beneath the lowest
+scale-like process or homologue of a leaf; and its movement was traced
+[page 33]
+during 38 h. (Fig. 23). We here again have plain evidence of continued
+circumnutation. Had the bean been properly planted, the part of the
+epicotyl to which the filament was attached, the
+
+Fig. 23. Vicia faba: circumnutation of the same epicotyl as in Fig. 22, a
+little more advanced in age, traced under similar conditions as before,
+from 8.40 A.M. Dec. 28th, to 10.50 A.M. 30th. Movement of bead here
+magnified 20 times.
+
+movement of which is here shown, would probably have just risen above the
+surface of the ground.
+
+Lathyrus nissolia (Leguminosae).--This plant was selected for observation
+from being an abnormal form with grass-like leaves.
+
+Fig. 24. Lathyrus nissolia: circumnutation of stem of young seedling,
+traced in darkness on a horizontal glass, from 6.45 A.M. Nov. 22nd, to 7
+A.M. 23rd. Movement of end of leaf magnified about 12 times, here reduced
+to one-half of original scale.
+
+The cotyledons are hypogean, and the epicotyl breaks through the ground in
+an arched form. The movements of a stem, 1.2 inch in height, consisting of
+three internodes, the lower one almost wholly subterranean, and the upper
+one bearing a short,
+[page 34]
+narrow leaf, is shown during 24 h., in Fig. 24. No glass filament was
+employed, but a mark was placed beneath the apex of the leaf. The actual
+length of the longer of the two ellipses described by the stem was about
+.14 of an inch. On the previous day the chief line of movement was nearly
+at right angles to that shown in the present figure, and it was more
+simple.
+
+Cassia tora* (Leguminosae).--A seedling was placed before a
+
+Fig. 25. Cassia tora: conjoint circumnutation of cotyledons and hypocotyl,
+traced on vertical glass, from 7.10 A.M. Sept. 25th to 7.30 A.M. 26th.
+Figure here given reduced to one-half of original scale.
+
+* Seeds of this plant, which grew near the sea-side, were sent to us by
+Fritz M�ller from S. Brazil. The seedlings did not flourish or flower well
+with us; they were sent to Kew, and were pronounced not to be
+distinguishable from C. tora.
+[page 35]
+
+north-east window; it bent very little towards it, as the hypocotyl which
+was left free was rather old, and therefore not highly heliotropic. A
+filament had been fixed to the midrib of one of the cotyledons, and the
+movement of the whole seedling was traced during two days. The
+circumnutation of the hypocotyl is quite insignificant compared with that
+of the cotyledons. These rise up vertically at night and come into close
+contact; so that they may be said to sleep. This seedling was so old that a
+very small true leaf had been developed, which at night was completely
+hidden by the closed cotyledons. On Sept. 24th, between 8 A.M. and 5 P.M.,
+the cotyledons moved five times up and five times down; they therefore
+described five irregular ellipses in the course of the 9 h. The great
+nocturnal rise commenced about 4.30 P.M.
+
+On the following morning (Sept. 25th) the movement of the same cotyledon
+was again traced in the same manner during 24 h.; and a copy of the tracing
+is here given (Fig. 25). The morning was cold, and the window had been
+accidentally left open for a short time, which must have chilled the plant;
+and this probably prevented it from moving quite as freely as on the
+previous day; for it rose only four and sank only four times during the
+day, one of the oscillations being very small. At 7.10 A.M., when the first
+dot was made, the cotyledons were not fully open or awake; they continued
+to open till about 9 A.M., by which time they had sunk a little beneath the
+horizon: by 9.30 A.M. they had risen, and then they oscillated up and down;
+but the upward and downward lines never quite coincided. At about 4.30 P.M.
+the great nocturnal rise commenced. At 7 A.M. on the following morning
+(Sept. 26th) they occupied nearly the same level as on the previous
+morning, as shown in the diagram: they then began to open or sink in the
+usual manner. The diagram leads to the belief that the great periodical
+daily rise and fall does not differ essentially, excepting in amplitude,
+from the oscillations during the middle of the day.
+
+Lotus Jacoboeus (Leguminosae).--The cotyledons of this plant, after the few
+first days of their life, rise so as to stand almost, though rarely quite,
+vertically at night. They continue to act in this manner for a long time
+even after the development of some of the true leaves. With seedlings, 3
+inches in height, and bearing five or six leaves, they rose at night about
+45o. They continued to act thus for about an additional fortnight.
+Subsequently they remained horizontal at night, though still green
+[page 36]
+and at last dropped off. Their rising at night so as to stand almost
+vertically appears to depend largely on temperature; for when the seedlings
+were kept in a cool house, though they still continued to grow, the
+cotyledons did not become vertical at night. It is remarkable that the
+cotyledons do not generally rise at night to any conspicuous extent during
+the first four or five days after germination; but the period was extremely
+variable with seedlings kept under the same conditions; and many were
+observed. Glass filaments with minute triangles of paper were fixed to the
+cotyledons (1 � mm. in breadth) of two seedlings, only 24 h. old, and the
+hypocotyl was secured to a stick; their movements greatly magnified were
+traced, and they certainly circumnutated the whole time on a small scale,
+but they did not exhibit any distinct nocturnal and diurnal movement. The
+hypocotyls, when left free, circumnutated over a large space.
+
+Another and much older seedling, bearing a half-developed leaf, had its
+movements traced in a similar manner during the three first days and nights
+of June; but seedlings at this age appear to be very sensitive to a
+deficiency of light; they were observed under a rather dim skylight, at a
+temperature of between 16o to 17 1/2o C.' and apparently, in consequence of
+these conditions, the great daily movement of the cotyledons ceased on the
+third day. During the first two days they began rising in the early
+afternoon in a nearly straight line, until between 6 and 7 P.M., when they
+stood vertically. During the latter part of the night, or more probably in
+the early morning, they began to fall or open, so that by 6.45 A.M. they
+stood fully expanded and horizontal. They continued to fall slowly for some
+time, and during the second day described a single small ellipse, between 9
+A.M. and 2 P.M., in addition to the great diurnal movement. The course
+pursued during the whole 24 h. was far less complex than in the foregoing
+case of Cassia. On the third morning they fell very much, and then
+circumnutated on a small scale round the same spot; by 8.20 P.M. they
+showed no tendency to rise at night. Nor did the cotyledons of any of the
+many other seedlings in the same pot rise; and so it was on the following
+night of June 5th. The pot was then taken back into the hot-house, where it
+was exposed to the sun, and on the succeeding night all the cotyledons rose
+again to a high angle, but did not stand quite vertically. On each of the
+above days the line representing the great nocturnal
+[page 37]
+rise did not coincide with that of the great diurnal fall, so that narrow
+ellipses were described, as is the usual rule with circumnutating organs.
+The cotyledons are provided with a pulvinus, and its development will
+hereafter be described.
+
+Mimosa pudica (Leguminosae).--The cotyledons rise up vertically at night,
+so as to close together. Two seedlings were observed in the greenhouse
+(temp. 16o to 17o C. or 63o to 65o F.). Their hypocotyls were secured to
+sticks, and glass filaments bearing little triangles of paper were affixed
+to the cotyledons of both. Their movements were traced on a vertical glass
+during 24 h. on November 13th. The pot had stood for some time in the same
+position, and they were chiefly illuminated through the glass-roof. The
+cotyledons of one of these seedlings moved downward in the morning till
+11.30 A.M., and then rose, moving rapidly in the evening until they stood
+vertically, so that in this case there was simply a single great daily fall
+and rise. The other seedling behaved rather differently, for it fell in the
+morning until 11.30 A.M., and then rose, but after 12.10 P.M. again fell;
+and the great evening rise did not begin until 1.22 P.M. On the following
+morning this cotyledon had fallen greatly from its vertical position by
+8.15 A.M. Two other seedlings (one seven and the other eight days old) had
+been previously observed under unfavourable circumstances, for they had
+been brought into a room and placed before a north-east window, where the
+temperature was between only 56o and 57o F. They had, moreover, to be
+protected from lateral light, and perhaps were not sufficiently
+illuminated. Under these circumstances the cotyledons moved simply
+downwards from 7 A.M. till 2 P.M., after which hour and during a large part
+of the night they continued to rise. Between 7 and 8 A.M. on the following
+morning they fell again; but on this second and likewise on the third day
+the movements became irregular, and between 3 and 10.30 P.M. they
+circumnutated to a small extent about the same spot; but they did not rise
+at night. Nevertheless, on the following night they rose as usual.
+
+Cytisus fragrans (Leguminosae).--Only a few observations were made on this
+plant. The hypocotyl circumnutated to a considerable extent, but in a
+simple manner--namely, for two hours in one direction, and then much more
+slowly back again in a zigzag course, almost parallel to the first line,
+and beyond the starting-point. It moved in the same direction all night,
+but next morning began to return. The cotyledons continually
+[page 38]
+move both up and down and laterally; but they do not rise up at night in a
+conspicuous manner.
+
+Lupinus luteus (Leguminosae).--Seedlings of this plant were observed
+because the cotyledons are so thick (about .08 of an inch) that it seemed
+unlikely that they would move. Our observations were not very successful,
+as the seedlings are strongly heliotropic, and their circumnutation could
+not be accurately observed near a north-east window, although they had been
+kept during the previous day in the same position. A seedling was then
+placed in darkness with the hypocotyl secured to a stick; both cotyledons
+rose a little at first, and then fell during the rest of the day; in the
+evening between 5 and 6 P.M. they moved very slowly; during the night one
+continued to fall and the other rose, though only a little. The tracing was
+not much magnified, and as the lines were plainly zigzag, the cotyledons
+must have moved a little laterally, that is, they must have circumnutated.
+
+The hypocotyl is rather thick, about .12 of inch; nevertheless it
+circumnutated in a complex course, though to a small extent. The movement
+of an old seedling with two true leaves partially developed, was observed
+in the dark. As the movement was magnified about 100 times it is not
+trustworthy and is not given; but there could be no doubt that the
+hypocotyl moved in all directions during the day, changing its course 19
+times. The extreme actual distance from side to side through which the
+upper part of the hypocotyl passed in the course of 14 � hours was only
+1/60 of an inch; it sometimes travelled at the rate of 1/50 of an inch in
+an hour.
+
+Cucurbita ovifera (Cucurbitaceae).--Radicle: a seed which had
+
+Fig. 26. Cucurbita ovifera: course followed by a radicle in bending
+geotropically downwards, traced on a horizontal glass, between 11.25 A.M.
+and 10.25 P.M.; the direction during the night is indicated by the broken
+line. Movement of bead magnified 14 times.
+
+germinated on damp sand was fixed so that the slightly curved radicle,
+which was only .07 inch in length, stood almost vertically
+[page 39]
+upwards, in which position geotropism would act at first with little power.
+A filament was attached near to its base, and projected at about an angle
+of 45o above the horizon. The general course followed during the 11 hours
+of observation and during the following night is shown in the accompanying
+diagram (Fig. 26), and was plainly due to geotropism; but it was also clear
+that the radicle circumnutated. By the next morning the tip had curved so
+much downwards that the filament, instead of projecting at 45o above the
+horizon, was nearly horizontal. Another germinating seed was turned upside
+down and covered with damp sand; and a filament was fastened to the radicle
+so as to project at an angle of about 50o above the horizon; this radicle
+was .35 of an inch in length and a little curved. The course pursued was
+mainly governed, as in the last case, by geotropism, but the line traced
+during 12 hours and magnified as before was more strongly zigzag, again
+showing circumnutation.
+
+Four radicles were allowed to grow downwards over plates of smoked glass,
+inclined at 70o to the horizon, under the
+
+Fig. 27. Cucurbita ovifera: tracks left by tips of radicles in growing
+downwards over smoked glass-plates, inclined at 70o to the horizon.
+
+Fig. 28. Cucurbita ovifera: circumnutation of arched hypocotyl at a very
+early age, traced in darkness on a horizontal glass, from 8 A.M. to 10.20
+A.M. on the following day. The movement of the bead magnified 20 times,
+here reduced to one-half of original scale.
+
+same conditions as in the cases of Aesculus, Phaseolus, and Vicia.
+Facsimiles are here given (Fig. 27) of two of these tracks; and a third
+short one was almost as plainly serpentine as that at A. It was also
+manifest by a greater or less amount of soot having been swept off the
+glasses, that the tips had
+[page 40]
+pressed alternately with greater and less force on them. There must,
+therefore, have been movement in at least two planes at right angles to one
+another. These radicles were so delicate that they rarely had the power to
+sweep the glasses quite clean. One of them had developed some lateral or
+secondary rootlets, which projected a few degrees beneath the horizon; and
+it is an important fact that three of them left distinctly serpentine
+tracks on the smoked surface, showing beyond doubt that they had
+circumnutated like the main or primary radicle. But the tracks were so
+slight that they could not be traced and copied after the smoked surface
+had been varnished.
+
+Fig. 29. Cucurbita ovifera: circumnutation of straight and vertical
+hypocotyl, with filament fastened transversely across its upper end, traced
+in darkness on a horizontal glass, from 8.30 A.M. to 8.30 P.M. The movement
+of the terminal bead originally magnified about 18 times, here only 4 �
+times.
+
+Hypocotyl.--A seed lying on damp sand was firmly fixed by two crossed wires
+and by its own growing radicle. The cotyledons were still enclosed within
+the seed-coats; and the short hypocotyl, between the summit of the radicle
+and the cotyledons, was as yet only slightly arched. A filament (.85 of
+inch in length) was attached at an angle of 35o above the horizon to the
+side of the arch adjoining the cotyledons. This part would ultimately form
+the upper end of the hypocotyl, after it had grown straight and vertical.
+Had the seed been properly planted, the hypocotyl at this stage of growth
+would have been deeply buried beneath the surface. The course followed by
+the bead of the filament is shown in Fig. 28. The chief lines of movement
+from left to right in the figure were parallel to the plane of the two
+united cotyledons and of the flattened seed; and this movement would aid in
+dragging them out of the seed-coats, which are held down by a special
+structure hereafter to be described. The movement at right angles to the
+above lines was due to the arched hypocotyl becoming more arched as it
+increased in height. The foregoing observations apply to the leg of the
+arch next to the cotyledons, but
+[page 41]
+the other leg adjoining the radicle likewise circumnutated at an equally
+early age.
+
+The movement of the same hypocotyl after it had become straight and
+vertical, but with the cotyledons only partially expanded, is shown in Fig.
+29. The course pursued during 12 h. apparently represents four and a half
+ellipses or ovals, with the longer axis of the first at nearly right angles
+to that of the others. The longer axes of all were oblique to a line
+joining the opposite cotyledons. The actual extreme distance from side to
+side over which the summit of the tall hypocotyl passed in the course of 12
+h. was .28 of an inch. The original figure was traced on a large scale, and
+from the obliquity of the line of view the outer parts of the diagram are
+much exaggerated.
+
+Cotyledons.--On two occasions the movements of the cotyledons were traced
+on a vertical glass, and as the ascending and descending lines did not
+quite coincide, very narrow ellipses were formed; they therefore
+circumnutated. Whilst young they rise vertically up at night, but their
+tips always remain reflexed; on the following morning they sink down again.
+With a seedling kept in complete darkness they moved in the same manner,
+for they sank from 8.45 A.M. to 4.30 P.M.; they then began to rise and
+remained close together until 10 P.M., when they were last observed. At 7
+A.M. on the following morning they were as much expanded as at any hour on
+the previous day. The cotyledons of another young seedling, exposed to the
+light, were fully open for the first time on a certain day, but were found
+completely closed at 7 A.M. on the following morning. They soon began to
+expand again, and continued doing so till about 5 P.M.; they then began to
+rise, and by 10.30 P.M. stood vertically and were almost closed. At 7 A.M.
+on the third morning they were nearly vertical, and again expanded during
+the day; on the fourth morning they were not closed, yet they opened a
+little in the course of the day and rose a little on the following night.
+By this time a minute true leaf had become developed. Another seedling,
+still older, bearing a well-developed leaf, had a sharp rigid filament
+affixed to one of its cotyledons (85 mm. in length), which recorded its own
+movements on a revolving drum with smoked paper. The observations were made
+in the hot-house, where the plant had lived, so that there was no change in
+temperature or light. The record commenced at 11 A.M. on February 18th; and
+from this hour till 3 P.M. the
+[page 42]
+cotyledon fell; it then rose rapidly till 9 P.M., then very gradually till
+3 A.M. February 19th, after which hour it sank gradually till 4.30 P.M.;
+but the downward movement was interrupted by one slight rise or oscillation
+about 1.30 P.M. After 4.30 P.M. (19th) the cotyledon rose till 1 A.M. (in
+the night of February 20th) and then sank very gradually till 9.30 A.M.,
+when our observations ceased. The amount of movement was greater on the
+18th than on the 19th or on the morning of the 20th.
+
+Cucurbita aurantia.--An arched hypocotyl was found buried a little beneath
+the surface of the soil; and in order to prevent it straightening itself
+quickly, when relieved from the surrounding pressure of the soil, the two
+legs of the arch were tied together. The seed was then lightly covered with
+loose damp earth. A filament with a bead at the end was affixed to the
+basal leg, the movements of which were observed during two days in the
+usual manner. On the first day the arch moved in a zigzag line towards the
+side of the basal leg. On the next day, by which time the dependent
+cotyledons had been dragged above the surface of the soil, the tied arch
+changed its course greatly nine times in the course of 14 � h. It swept a
+large, extremely irregular, circular figure, returning at night to nearly
+the same spot whence it had started early in the morning. The line was so
+strongly zigzag that it apparently represented five ellipses, with their
+longer axes pointing in various directions. With respect to the periodical
+movements of the cotyledons, those of several young seedlings formed
+together at 4 P.M. an angle of about 60o, and at 10 P.M. their lower parts
+stood vertically and were in contact; their tips, however, as is usual in
+the genus, were permanently reflexed. These cotyledons, at 7 A.M. on the
+following morning, were again well expanded.
+
+Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbitaceae).--A
+seedling opened its cotyledons, the movements of which were alone observed,
+slightly on June 27th and closed them at night: next day, at noon (28th),
+they included an angle of 53o, and at 10 P.M. they were in close contact,
+so that each had risen 26 1/2o. At noon, on the 29th, they included an
+angle of 118o, and at 10 P.M. an angle of 54o, so each had risen 32o. On the
+following day they were still more open, and the nocturnal rise was
+greater, but the angles were not measured. Two other seedlings were
+observed, and behaved during three days in a closely similar manner. The
+cotyledons, therefore,
+[page 43]
+open more and more on each succeeding day, and rise each night about 30o;
+consequently during the first two nights of their life they stand
+vertically and come into contact.
+
+Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1 � inch in
+length, apex only 4 3/4 inches from the vertical glass, on which its
+movements were traced from 7.35 A.M. July 11th to 9.5 A.M. on the 14th.
+Figure here given reduced to one-third of original scale.
+
+In order to ascertain more accurately the nature of these movements, the
+hypocotyl of a seedling, with its cotyledons well expanded, was secured to
+a little stick, and a filament with triangles of paper was affixed to one
+of the cotyledons. The observations were made under a rather dim skylight,
+and the temperature during the whole time was between 17 1/2o to 18o C. (63o
+to 65o F.). Had the temperature been higher and the light brighter, the
+movements would probably have been greater. On July 11th (see Fig. 30), the
+cotyledon fell from 7.35 A.M. till 10 A.M.; it then rose (rapidly after 4
+P.M.) till it stood quite vertically at 8.40 P.M. During the early morning
+of the next day (12th) it fell, and continued to fall till 8 A.M., after
+which hour it rose, then fell, and again rose, so that by 10.35 P.M. it
+stood much higher than it did in the morning, but was not vertical as on
+the preceding night. During the following early morning and whole day
+(13th) it fell and circumnutated, but had not risen when observed late in
+the evening; and this was probably due to the deficiency of heat or light,
+or of both. We thus see that the cotyledons became more widely open at noon
+on each succeeding day; and that they rose considerably each night, though
+not acquiring a vertical position, except during the first two nights.
+
+Cucumis dudaim (Cucurbitaceae).--Two seedlings had opened
+[page 44]
+their cotyledons for the first time during the day,--one to the extent of
+90o and the other rather more; they remained in nearly the same position
+until 10.40 P.M.; but by 7 A.M. on the following morning the one which had
+been previously open to the extent of 90o had its cotyledons vertical and
+completely shut; the other seedling had them nearly shut. Later in the
+morning they opened in the ordinary manner. It appears therefore that the
+cotyledons of this plant close and open at somewhat different periods from
+those of the foregoing species of the allied genera of Cucurbita and
+Lagenaria.
+
+Fig. 31. Opuntia basilaris: conjoint circumnutation of hypocotyl and
+cotyledon; filament fixed longitudinally to cotyledon, and movement traced
+during 66 h. on horizontal glass. Movement of the terminal bead magnified
+about 30 times, here reduced to one-third scale. Seedling kept in
+hot-house, feebly illuminated from above.
+
+Opuntia basilaris (Cacteae).--A seedling was carefully observed, because,
+considering its appearance and the nature of the mature plant, it seemed
+very unlikely that either the hypocotyl or cotyledons would circumnutate to
+an appreciable extent. The cotyledons were well developed, being .9 of an
+inch in length, .22 in breadth, and .15 in thickness. The almost
+cylindrical hypocotyl, now bearing a minute spinous bud on its summit, was
+only .45 of an inch in height, and .19 in diameter. The tracing (Fig. 31)
+shows the combined movement of the hypocotyl and of one of the cotyledons,
+from 4.45 P.M. on May 28th to 11 A.M. on the 31st. On the 29th a nearly
+perfect ellipse was completed. On the 30th the hypocotyl moved, from some
+unknown cause, in the same general direction in a zigzag line; but between
+4.30 and 10 P.M. almost completed a second small ellipse. The cotyledons
+move only a little up and down: thus at 10.15 P.M. they stood only 10o
+higher than at noon. The chief seat of movement therefore, at least when
+the cotyledons are rather old as in the present case, lies in the
+hypocotyl. The ellipse described on the 29th had its longer axis directed
+at nearly right angles to a line joining the two cotyledons. The actual
+amount of movement of the bead at the end of the
+[page 45]
+filament was, as far as could be ascertained, about .14 of an inch.
+
+Fig. 32. Helianthus annuus: circumnutation of hypocotyl, with filament
+fixed across its summit, traced on a horizontal glass in darkness, from
+8.45 A.M. to 10.45 P.M., and for an hour on following morning. Movement of
+bead magnified 21 times, here reduced to one-half of original scale.
+
+Helianthus annuus (Compositae).--The upper part of the hypocotyl moved
+during the day-time in the course shown in the annexed figure (Fig. 32). As
+the line runs in various directions, crossing itself several times, the
+movement may be considered as one of circumnutation. The extreme actual
+distance travelled was at least .1 of an inch. The movements of the
+cotyledons of two seedlings were observed; one facing a north-east window,
+and the other so feebly illuminated from above us as to be almost in
+darkness. They continued to sink till about noon, when they began to rise;
+but between 5 and 7 or 8 P.M. they either sank a little, or moved
+laterally, and then again began to rise. At 7 A.M. on the following morning
+those on the plant before the north-east window had opened so little that
+they stood at an angle of 73o above the horizon, and were not observed any
+longer. Those on the seedling which had been kept in almost complete
+darkness, sank during the whole day, without rising about mid-day, but rose
+during the night. On the third and fourth days they continued sinking
+without any alternate ascending movement; and this, no doubt, was due to
+the absence of light.
+
+Primula Sinensis (Primulaceae).--A seedling was placed with the two
+cotyledons parallel to a north-east window on a day when the light was
+nearly uniform, and a filament was affixed to one of them.  From
+observations subsequently made on another seedling with the stem secured to
+a stick, the greater part of the movement shown in the annexed figure (Fig.
+33), must have been that of the hypocotyl, though the cotyledons certainly
+move up and down to a certain extent both during the day and night. The
+movements of the same seedling were traced
+[page 46]
+on the following day with nearly the same result; and there can be no doubt
+about the circumnutation of the hypocotyl.
+
+Fig. 33. Primula Sinensis: conjoint circumnutation of hypocotyl and
+cotyledon, traced on vertical glass, from 8.40 A.M. to 10.45 P.M. Movements
+of bead magnified about 26 times.
+
+Cyclamen Persicum (Primulaceae).--This plant is generally supposed to
+produce only a single cotyledon, but Dr. H. Gressner* has shown that a
+second one is developed after a long interval of time. The hypocotyl is
+converted into a globular corm, even before the first cotyledon has broken
+through the ground with its blade closely enfolded and with its petiole in
+the form of an arch, like the arched hypocotyl or epicotyl of any ordinary
+dicotyledonous plant. A glass filament was affixed to a cotyledon, .55 of
+an inch in height, the petiole of which had straightened itself and stood
+nearly vertical, but with the blade not as yet fully expanded. Its
+movements were traced during 24 � h. on a horizontal glass, magnified 50
+times; and in this interval it described two irregular small circles; it
+therefore circumnutates, though on an extremely small scale.
+
+Fig. 34. Stapelia sarpedon: circumnutation of hypocotyl, illuminated from
+above, traced on horizontal glass, from 6.45 A.M. June 26th to 8.45 A.M.
+28th. Temp. 23-24o C. Movement of bead magnified 21 times.
+
+Stapelia sarpedon (Asclepiadeae).--This plant, when mature, resembles a
+cactus. The flattened hypocotyl is fleshy, enlarged in the upper part, and
+bears two rudimentary cotyledons. It breaks through the ground in an arched
+form, with the rudimentary cotyledons closed or in contact. A filament was
+affixed almost
+
+* 'Bot. Zeitung,' 1874, p. 837.
+[page 47]
+
+vertically to the hypocotyl of a seedling half an inch high; and its
+movements were traced during 50 h. on a horizontal glass (Fig. 34). From
+some unknown cause it bowed itself to one side, and as this was effected by
+a zigzag course, it probably circumnutated; but with hardly any other
+seedling observed by us was this movement so obscurely shown.
+
+Ipomoea caerulea vel Pharbitis nil (Convolvulaceae).--Seedlings of this
+plant were observed because it is a twiner, the upper internodes of which
+circumnutate conspicuously; but like other twining plants, the first few
+internodes which rise above the ground are stiff enough to support
+themselves, and therefore do not circumnutate in any plainly recognisable
+manner.* In this particular instance the fifth internode (including the
+hypocotyl) was the first which plainly circumnutated and twined round a
+stick. We therefore wished to learn whether circumnutation could be
+observed in the hypocotyl if carefully observed in our usual manner. Two
+seedlings were kept in the dark with filaments fixed to the upper part of
+their hypocotyls; but from circumstances not worth explaining their
+movements were traced for only a short time. One moved thrice forwards and
+twice backwards in nearly opposite directions, in the course of 3 h. 15 m.;
+and the other twice forwards and twice backwards in 2 h. 22 m. The
+hypocotyl therefore circumnutated at a remarkably rapid rate. It may here
+be added that a filament was affixed transversely to the summit of the
+second internode above the cotyledons of a little plant 3 � inches in
+height; and its movements were traced on a horizontal glass. It
+circumnutated, and the actual distance travelled from side to side was a
+quarter of an inch, which was too small an amount to be perceived without
+the aid of marks.
+
+The movements of the cotyledons are interesting from their complexity and
+rapidity, and in some other respects. The hypocotyl (2 inches high) of a
+vigorous seedling was secured to a stick, and a filament with triangles of
+paper was affixed to one of the cotyledons. The plant was kept all day in
+the hot-house, and at 4.20 P.M. (June 20th) was placed under a skylight in
+the house, and observed occasionally during the evening and night. It fell
+in a slightly zigzag line to a moderate extent from 4.20 P.M. till 10.15
+P.M. When looked at shortly after midnight (12.30 P.M.) it had risen a very
+little, and considerably by
+
+* 'Movements and Habits of Climbing Plants,' p. 33, 1875.
+[page 48]
+
+3.45 A.M. When again looked at, at 6.10 A.M. (21st), it had fallen largely.
+A new tracing was now begun (see Fig. 35), and soon afterwards, at 6.42
+A.M., the cotyledon had risen a little. During the forenoon it was observed
+about every hour; but between 12.30 and 6 P.M. every half-hour. If the
+observations had been made at these short intervals during the whole day,
+the figure would have been too intricate to have been copied. As it was,
+the cotyledon moved up and down in the course of 16 h. 20 m. (i.e. between
+6.10 A.M. and 10.30 P.M.) thirteen times.
+
+Fig 35. Ipomoea caerulea: circumnutation of cotyledon, traced on vertical
+glass, from 6.10 A.M. June 21st to 6.45 A.M. 22nd. Cotyledon with petiole
+1.6 inch in length, apex of blade 4.1 inch from the vertical glass; so
+movement not greatly magnified; temp. 20o C.
+
+The cotyledons of this seedling sank downwards during both evenings and the
+early part of the night, but rose during the latter part. As this is an
+unusual movement, the cotyledons of twelve other seedlings were observed;
+they stood almost or quite horizontally at mid-day, and at 10 P.M. were all
+declined at various angles. The most usual angle was between 30o and 35o;
+but three stood at about 50o and one at even 70o beneath the horizon. The
+blades of all these cotyledons had attained almost their full size, viz.
+from 1 to 1 � inches in length, measured along their midribs. It is a
+remarkable fact that whilst young--that is, when less than half an inch in
+length, measured in the same manner--they do not sink
+[page 49]
+downwards in the evening. Therefore their weight, which is considerable
+when almost fully developed, probably came into play in originally
+determining the downward movement. The periodicity of this movement is much
+influenced by the degree of light to which the seedlings have been exposed
+during the day; for three kept in an obscure place began to sink about
+noon, instead of late in the evening; and those of another seedling were
+almost paralysed by having been similarly kept during two whole days. The
+cotyledons of several other species of Ipomoea likewise sink downwards late
+in the evening.
+
+Cerinthe major (Boragineae).--The circumnutation of the hypocotyl of a
+young seedling with the cotyledons hardly
+
+Fig. 36. Cerinthe major: circumnutation of hypocotyl, with filament fixed
+across its summit, illuminated from above, traced on horizontal glass, from
+9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead magnified 30
+times, here reduced to one-third of original scale.
+
+expanded, is shown in the annexed figure (Fig. 36), which apparently
+represents four or five irregular ellipses, described in the course of a
+little over 12 hours. Two older seedlings were similarly observed,
+excepting that one of them was kept in the dark; their hypocotyls also
+circumnutated, but in a more simple manner. The cotyledons on a seedling
+exposed to the light fell from the early morning until a little after noon,
+and then continued to rise until 10.30 P.M. or later. The cotyledons of
+this same seedling acted in the same general manner during the two
+following days. It had previously been tried in the dark, and after being
+thus kept for only 1 h. 40 m. the cotyledons began at 4.30 P.M. to sink,
+instead of continuing to rise till late at night.
+[page 50]
+
+Nolana prostrata (Nolaneae).--The movements were not traced, but a pot with
+seedlings, which had been kept in the dark for an hour, was placed under
+the microscope, with the micrometer eye-piece so adjusted that each
+division equalled 1/500th of an inch. The apex of one of the cotyledons
+crossed rather obliquely four divisions in 13 minutes; it was also sinking,
+as shown by getting out of focus. The seedlings were again placed in
+darkness for another hour, and the apex now crossed two divisions in 6 m.
+18 s.; that is, at very nearly the same rate as before. After another
+interval of an hour in darkness, it crossed two divisions in 4 m. 15 s.,
+therefore at a quicker rate. In the afternoon, after a longer interval in
+the dark, the apex was motionless, but after a time it recommenced moving,
+though slowly; perhaps the room was too cold. Judging from previous cases,
+there can hardly be a doubt that this seedling was circumnutating.
+
+Fig. 37. Solanum lycopersicum: circumnutation of hypocotyl, with filament
+fixed across its summit, traced on horizontal glass, from 10 A.M. to 5 P.M.
+Oct. 24th. Illuminated obliquely from above. Movement of bead magnified
+about 35 times, here reduced to one-third of original scale.
+
+Solanum lycopersicum (Solaneae).--The movements of the hypocotyls of two
+seedling tomatoes were observed during seven hours, and there could be no
+doubt that both circumnutated. They were illuminated from above, but by an
+accident a little light entered on one side, and in the accompanying figure
+(Fig. 37) it may be seen that the hypocotyl moved to this side (the upper
+one in the figure), making small loops and zigzagging in its course. The
+movements of the cotyledons were also traced both on vertical and
+horizontal glasses; their angles with the horizon were likewise measured at
+various hours. They fell from 8.30 A.M. (October 17th) to about noon; then
+moved laterally in a zigzag line, and at about 4 P.M. began to rise; they
+continued to do so until 10.30 P.M., by which hour they stood vertically
+and were asleep. At what hour of the night or early morning they began to
+fall was not ascertained. Owing to the lateral movement shortly after
+mid-day, the descending and ascending lines did not coincide, and irregular
+ellipses were described during each 24 h. The regular periodicity of these
+movements is destroyed, as we shall hereafter see, if the seedlings are
+kept in the dark.
+[page 51]
+
+Solanum palinacanthum.--Several arched hypocotyls rising nearly .2 of an
+inch above the ground, but with the cotyledons still buried beneath the
+surface, were observed, and the tracings showed that they circumnutated.
+Moreover, in several cases little open circular spaces or cracks in the
+argillaceous sand which surrounded the arched hypocotyls were visible, and
+these appeared to have been made by the hypocotyls having bent first to one
+and then to another side whilst growing upwards. In two instances the
+vertical arches were observed to move to a considerable distance backwards
+from the point where the cotyledons lay buried; this movement, which has
+been noticed in some other cases, and which seems to aid in extracting the
+cotyledons from the buried seed-coats, is due to the commencement of the
+straightening of the hypocotyl. In order to prevent this latter movement,
+the two legs of an arch, the
+
+Fig. 38. Solanum palinacanthum: circumnutation of an arched hypocotyl, just
+emerging from the ground, with the two legs tied together, traced in
+darkness on a horizontal glass, from 9.20 A.M. Dec. 17th to 8.30 A.M. 19th.
+Movement of bead magnified 13 times; but the filament, which was affixed
+obliquely to the crown of the arch, was of unusual length.
+
+summit of which was on a level with the surface of the soil, were tied
+together; the earth having been previously removed to a little depth all
+round. The movement of the arch during 47 hours under these unnatural
+circumstances is exhibited in the annexed figure.
+
+The cotyledons of some seedlings in the hot-house were horizontal about
+noon on December 13th; and at 10 P.M. had risen to an angle of 27o above
+the horizon; at 7 A.M. on the following
+[page 52]
+morning, before it was light, they had risen to 59o above the horizon; in
+the afternoon of the same day they were found again horizontal.
+
+Beta vulgaris (Chenopodeae).--The seedlings are excessively sensitive to
+light, so that although on the first day they were uncovered only during
+two or three minutes at each observation, they all moved steadily towards
+the side of the room whence the light proceeded, and the tracings consisted
+only of slightly zigzag lines directed towards the light. On the next day
+the plants were placed in a completely darkened room, and at each
+observation were illuminated as much as possible from vertically above by a
+small wax taper. The annexed figure (Fig. 39) shows the movement of the
+hypocotyl during 9 h. under these circumstances. A second seedling was
+similarly observed at the same time, and the tracing had the same peculiar
+character, due to the hypocotyl often moving and returning in nearly
+parallel lines. The movement of a third hypocotyl differed greatly.
+
+Fig. 39. Beta vulgaris: circumnutation of hypocotyl, with filament fixed
+obliquely across its summit, traced in darkness on horizontal glass, from
+8.25 A.M. to 5.30 P.M. Nov. 4th. Movement of bead magnified 23 times, here
+reduced to one-third of original scale.
+
+We endeavoured to trace the movements of the cotyledons, and for this
+purpose some seedlings were kept in the dark, but they moved in an abnormal
+manner; they continued rising from 8.45 A.M. to 2 P.M., then moved
+laterally, and from 3 to 6 P.M. descended; whereas cotyledons which have
+been exposed all the day to the light rise in the evening so as to stand
+vertically at night; but this statement applies only to young seedlings.
+For instance, six seedlings in the greenhouse had their cotyledons
+partially open for the first time on the morning of November 15th, and at
+8.45 P.M. all were completely closed, so that they might properly be said
+to be asleep. Again, on the morning of November 27th, the cotyledons of
+four other seedlings, which were surrounded by a collar of brown paper so
+that they received light only from above, were open to the extent of 39o;
+at 10 P.M. they were completely closed; next morning (November 28th) at
+6.45 A.M. whilst it was still dark, two of them
+[page 53]
+were partially open and all opened in the course of the morning; but at
+10.20 P.M. all four (not to mention nine others which had been open in the
+morning and six others on another occasion) were again completely closed.
+On the morning of the 29th they were open, but at night only one of the
+four was closed, and this only partially; the three others had their
+cotyledons much more raised than during the day. On the night of the 30th
+the cotyledons of the four were only slightly raised.
+
+Ricinus Borboniensis (Euphorbiaceae).--Seeds were purchased under the above
+name--probably a variety of the common castor-oil plant. As soon as an
+arched hypocotyl had risen clear above the ground, a filament was attached
+to the upper leg bearing the cotyledons which were still buried beneath the
+surface, and the movement of the bead was traced on a horizontal glass
+during a period of 34 h. The lines traced were strongly zigzag, and as the
+bead twice returned nearly parallel to its former course in two different
+directions, there could be no doubt that the arched hypocotyl
+circumnutated. At the close of the 34 h. the upper part began to rise and
+straighten itself, dragging the cotyledons out of the ground, so that the
+movements of the bead could no longer be traced on the glass.
+
+Quercus (American sp.) (Cupuliferae).--Acorns of an American oak which had
+germinated at Kew were planted in a pot in the greenhouse. This
+transplantation checked their growth; but after a time one grew to a height
+of five inches, measured to the tips of the small partially unfolded leaves
+on the summit, and now looked vigorous. It consisted of six very thin
+internodes of unequal lengths. Considering these circumstances and the
+nature of the plant, we hardly expected that it would circumnutate; but the
+annexed figure (Fig. 40) shows that it did so in a conspicuous manner,
+changing its course many times and travelling in all directions during the
+48 h. of observation. The figure seems to represent 5 or 6 irregular ovals
+or ellipses. The actual amount of movement from side to side (excluding one
+great bend to the left) was about .2 of an inch; but this was difficult to
+estimate, as owing to the rapid growth of the stem, the attached filament
+was much further from the mark beneath at the close than at the
+commencement of the observations. It deserves notice that the pot was
+placed in a north-east room within a deep box, the top of which was not at
+first covered up, so that the inside facing
+[page 54]
+the windows was a little more illuminated than the opposite side; and
+during the first morning the stem travelled to a greater distance in this
+direction (to the left in the figure) than it did afterwards when the box
+was completely protected from light.
+
+Fig. 40. Quercus (American sp.): circumnutation of young stem, traced on
+horizontal glass, from 12.50 P.M. Feb. 22nd to 12.50 P.M. 24th. Movement of
+bead greatly magnified at first, but slightly towards the close of the
+observations--about 10 times on an average.
+
+Quercus robur.--Observations were made only on the movements of the
+radicles from germinating acorns, which were allowed to grow downwards in
+the manner previously described, over plates of smoked glass, inclined at
+angles between 65o and 69o to the horizon. In four cases the tracks left
+were almost straight, but the tips had pressed sometimes with more and
+sometimes with less force on the glass, as shown by the varying thickness
+of the tracks and by little bridges of soot left across them. In the fifth
+case the track was slightly serpentine, that is, the tip had moved a little
+from side to side. In the sixth case (Fig. 41, A) it was plainly
+serpentine, and the tip had pressed almost equably on the glass in its
+whole course. In the seventh case (B) the tip had moved both laterally and
+had pressed
+[page 55]
+alternately with unequal force on the glass; so that it had moved a little
+in two planes at right angles to one another. In the eighth and last case
+(C) it had moved very little laterally, but had alternately left the glass
+and come into contact with it again. There can be no doubt that in the last
+four cases the radicle of the oak circumnutated whilst growing downwards.
+
+Fig. 41. Quercus robur: tracks left on inclined smoked glass-plates by tips
+of radicles in growing downwards. Plates A and C inclined at 65o and plate
+B at 68o to the horizon.
+
+Corylus avellana (Corylaceae).--The epicotyl breaks through the ground in
+an arched form; but in the specimen which was first examined, the apex had
+become decayed, and the epicotyl grew to some distance through the soil, in
+a tortuous, almost horizontal direction, like a root. In consequence of
+this injury it had emitted near the hypogean cotyledons two secondary
+shoots, and it was remarkable that both of these were arched, like the
+normal epicotyl in ordinary cases. The soil was removed from around one of
+these arched secondary shoots, and a glass filament was affixed to the
+basal leg. The whole was kept damp beneath a metal-box with a glass lid,
+and was thus illuminated only from above. Owing apparently to the lateral
+pressure of the earth being removed, the terminal and bowed-down part of
+the shoot began at once to move upwards, so that after 24 h. it formed a
+right angle with the lower part. This lower part, to which the filament was
+attached, also straightened itself, and moved a little backwards from the
+upper part. Consequently a long line was traced on the horizontal glass;
+and
+[page 56]
+this was in parts straight and in parts decidedly zigzag, indicating
+circumnutation.
+
+On the following day the other secondary shoot was observed; it was a
+little more advanced in age, for the upper part, instead of depending
+vertically downwards, stood at an angle of 45o above the horizon. The tip
+of the shoot projected obliquely .4 of an inch above the ground, but by the
+close of our observations, which lasted 47 h., it had grown, chiefly
+towards its base, to a height of .85 of an inch. The filament was fixed
+transversely to the basal and almost upright half of the shoot, close
+beneath the lowest scale-like appendage. The circumnutating course pursued
+is shown in the accompanying figure (Fig. 42). The actual distance
+traversed from side to side was about .04 of an inch.
+
+Fig. 42. Corylus avellana: circumnutation of a young shoot emitted from the
+epicotyl, the apex of which had been injured, traced on a horizontal glass,
+from 9 A.M. Feb. 2nd to 8 A.M. 4th. Movement of bead magnified about 27
+times.
+
+Pinus pinaster (Coniferae).--A young hypocotyl, with the tips of the
+cotyledons still enclosed within the seed-coats, was at first only .35 of
+an inch in height; but the upper part grew so rapidly that at the end of
+our observations it was .6 in height,
+
+Fig. 43. Pinus pinaster: circumnutation of hypocotyl, with filament fixed
+across its summit, traced on horizontal glass, from 10 A.M. March 21st to 9
+A.M. 23rd. Seedling kept in darkness. Movement of bead magnified about 35
+times.
+[page 57]
+
+and by this time the filament was attached some way down the little stem.
+From some unknown cause, the hypocotyl moved far towards the left, but
+there could be no doubt (Fig. 43) that it circumnutated. Another hypocotyl
+was similarly observed, and it likewise moved in a strongly zigzag line to
+the same side. This lateral movement was not caused by the attachment of
+the glass filaments, nor by the action of light; for no light was allowed
+to enter when each observation was made, except from vertically above.
+
+The hypocotyl of a seedling was secured to a little stick; it bore nine in
+appearance distinct cotyledons, arranged in a circle. The movements of two
+nearly opposite ones were observed. The tip of one was painted white, with
+a mark placed below, and the figure described (Fig. 44, A) shows that it
+made an irregular
+
+Fig. 44. Pinus pinaster: circumnutation of two opposite cotyledons, traced
+on horizontal glass in darkness, from 8.45 A.M. to 8.35 P.M. Nov. 25th.
+Movement of tip in A magnified about 22 times, here reduced to one-half of
+original scale.
+
+circle in the course of about 8 h. during the night it travelled to a
+considerable distance in the direction indicated by the broken line. A
+glass filament was attached longitudinally to the other cotyledon, and this
+nearly completed (Fig, 44, B) an irregular circular figure in about 12
+hours. During the night it also moved to a considerable distance, in the
+direction indicated by the broken line. The cotyledons therefore
+circumnutate independently of the movement of the hypocotyl. Although they
+moved much during the night, they did not approach each other so as to
+stand more vertically than during the day.
+[page 58]
+
+Cycas pectinata (Cycadeae).--The large seeds of this plant in germinating
+first protrude a single leaf, which breaks through the ground with the
+petiole bowed into an arch and with the leaflets involuted. A leaf in this
+condition, which at the close of our observations was 2 � inches in height,
+had its movements traced in a warm greenhouse by means of a glass filament
+bearing paper triangles attached across its tip. The tracing (Fig. 45)
+shows how large, complex, and rapid were the circum-
+
+Fig. 45. Cycas pectinata: circumnutation of young leaf whilst emerging from
+the ground, feebly illuminated from above, traced on vertical glass, from 5
+P.M. May 28th to 11 A.M. 31st. Movement magnified 7 times, here reduced to
+two-thirds of original scale.
+
+nutating movements. The extreme distance from side to side which it passed
+over amounted to between .6 and .7 of an inch.
+
+Canna Warscewiczii (Cannaceae).--A seedling with the plumule projecting one
+inch above the ground was observed, but not under fair conditions, as it
+was brought out of the hot-house and kept in a room not sufficiently warm.
+Nevertheless the tracing (Fig. 46) shows that it made two or three
+incomplete irregular circles or ellipses in the course of 48 hours. The
+plumule is straight; and this was the first instance observed
+[page 59]
+by us of the part that first breaks through the ground not being arched.
+
+Fig. 46. Canna Warscewiczii: circumnutation of plumule with filament
+affixed obliquely to outer sheath-like leaf, traced in darkness on
+horizontal glass from 8.45 A.M. Nov. 9th to 8.10 A.M. 11th. Movement of
+bead magnified 6 times.
+
+Allium cepa (Liliaceae).--The narrow green leaf, which protrudes from the
+seed of the common onion as a cotyledon,* breaks through the ground in the
+form of an arch, in the same manner as the hypocotyl or epicotyl of a
+dicotyledonous plant. Long after the arch has risen above the surface the
+apex remains within the seed-coats, evidently absorbing the still abundant
+contents. The summit or crown of the arch, when it first protrudes from the
+seed and is still buried beneath the ground, is simply rounded; but before
+it reaches the surface it is developed into a conical protuberance of a
+white colour (owing to the absence of chlorophyll), whilst the adjoining
+parts are green, with the epidermis apparently rather thicker and tougher
+than elsewhere. We may therefore conclude that this conical protuberance is
+a special adaptation for breaking through the ground,** and answers the
+same end as the knife-like white crest on the summit of the straight
+cotyledon of the Gramineae.
+
+* This is the expression used by Sachs in his 'Text-book of Botany.'
+
+** Haberlandt has briefly described ('Die
+Schutzeinrichtungen...Keimpflanze,' 1877, p. 77) this curious structure and
+the purpose which it subserves. He states that good figures of the
+cotyledon of the onion have been given by Tittmann and by Sachs in his
+'Experimental Physiologie,' p. 93.
+[page 60]
+
+After a time the apex is drawn out of the empty seed-coats, and rises up,
+forming a right angle, or more commonly a still larger angle with the lower
+part, and occasionally the whole becomes nearly straight. The conical
+protuberance, which originally formed the crown of the arch, is now seated
+on one side, and appears like a joint or knee, which from acquiring
+chlorophyll becomes green, and increases in size. In rarely or never
+becoming perfectly straight, these cotyledons differ remarkably from the
+ultimate condition of the arched hypocotyls or epicotyls of dicotyledons.
+It is, also, a singular circumstance that the attenuated extremity of the
+upper bent portion invariably withers and dies.
+
+A filament, 1.7 inch in length, was affixed nearly upright beneath the knee
+to the basal and vertical portion of a cotyledon; and its movements were
+traced during 14 h. in the usual manner. The tracing here given (Fig. 47)
+indicates circumnutation. The movement of the upper part above the knee of
+the same cotyledon, which projected at about an angle of 45o above the
+horizon, was observed at the same time. A filament was not affixed to it,
+but a mark was placed beneath the apex, which was almost white from
+beginning to wither, and its movements were thus traced. The figure
+described resembled pretty closely that above given; and this shows that
+the chief seat of movement is in the lower or basal part of the cotyledon.
+
+Fig. 47. Allium cepa: circumnutation of basal half of arched cotyledon,
+traced in darkness on horizontal glass, from 8.15 A.M. to 10 P.M. Oct.
+31st. Movement of bead magnified about 17 times.
+
+Asparagus officinalis (Asparageae).--The tip of a straight plumule or
+cotyledon (for we do not know which it should be called) was found at a
+depth of .1 inch beneath the surface, and the earth was then removed all
+round to the dept of .3 inch. a glass filament was affixed obliquely to it,
+and the movement of the bead, magnified 17 times, was traced in darkness.
+During the first 1 h. 15 m. the plumule moved to the right, and during the
+next two hours it returned in a roughly parallel but strongly zigzag
+course. From some unknown cause it had grown up through the soil in an
+inclined direction, and now through apogeotropism it moved during nearly 24
+h. in
+[page 61]
+the same general direction, but in a slightly zigzag manner, until it
+became upright. On the following morning it changed its course completely.
+There can therefore hardly be a doubt that the plumule circumnutates,
+whilst buried beneath the ground, as much as the pressure of the
+surrounding earth will permit. The surface of the soil in the pot was now
+covered with a thin layer of very fine argillaceous sand, which was kept
+damp; and after the tapering seedlings had grown a few tenths of an inch in
+height, each was found surrounded by a little open space or circular crack;
+and this could be accounted for only by their having circumnutated and thus
+pushed away the sand on all sides; for there was no vestige of a crack in
+any other part.
+
+In order to prove that there was circumnutation, the move-
+
+Fig. 48. Asparagus officinalis: circumnutation of plumules with tips
+whitened and marks placed beneath, traced on a horizontal glass. A, young
+plumule; movement traced from 8.30 A.M. Nov. 30th to 7.15 A.M. next
+morning; magnified about 35 times. B, older plumule; movement traced from
+10.15 A.M. to 8.10 P.M. Nov. 29th; magnified 9 times, but here reduced to
+one-half of original scale.
+
+ments of five seedlings, varying in height from .3 inch to 2 inches, were
+traced. They were placed within a box and illuminated from above; but in
+all five cases the longer axes of the figures described were directed to
+nearly the same point; so that more light seemed to have come through the
+glass roof of the greenhouse on one side than on any other. All five
+tracings resembled each other to a certain extent, and it will suffice to
+give two of them. In A (Fig. 48) the seedling was only .45 of an
+[page 62]
+inch in height, and consisted of a single internode bearing a bud on its
+summit. The apex described between 8.30 A.M. and 10.20 P.M. (i.e. during
+nearly 14 hours) a figure which would probably have consisted of 3 �
+ellipses, had not the stem been drawn to one side until 1 P.M., after which
+hour it moved backwards. On the following morning it was not far distant
+from the point whence it had first started. The actual amount of movement
+of the apex from side to side was very small, viz. about 1/18th of an inch.
+The seedling of which the movements are shown in Fig. 48, B, was 1 3/4 inch
+in height, and consisted of three internodes besides the bud on the summit.
+The figure, which was described during 10 h., apparently represents two
+irregular and unequal ellipses or circles. The actual amount of movement of
+the apex, in the line not influenced by the light, was .11 of an inch, and
+in that thus influenced .37 of an inch. With a seedling 2 inches in height
+it was obvious, even without the aid of any tracing, that the uppermost
+part of the stem bent successively to all points of the compass, like the
+stem of a twining plant. A little increase in the power of circumnutating
+and in the flexibility of the stem, would convert the common asparagus into
+a twining plant, as has occurred with one species in this genus, namely, A.
+scandens.
+
+Phalaris Canariensis (Gramineae).--With the Gramineae the part which first
+rises above the ground has been called by some authors the pileole; and
+various views have been expressed on its homological nature. It is
+considered by some great authorities to be a cotyledon, which term we will
+use without venturing to express any opinion on the subject.* It consists
+in the present case of a slightly flattened reddish sheath, terminating
+upwards in a sharp white edge; it encloses a true green leaf, which
+protrudes from the sheath through a slit-like orifice, close beneath and at
+right angles to the sharp edge on the summit. The sheath is not arched when
+it breaks through the ground.
+
+The movements of three rather old seedlings, about 1 � inch in height,
+shortly before the protrusion of the leaves, were first traced. They were
+illuminated exclusively from above; for, as will hereafter be shown, they
+are excessively sensitive to the
+* We are indebted to the Rev. G. Henslow for an abstract of the views which
+have been held on this subject, together with references.
+[page 63]
+
+action of light; and if any enters even temporarily on one side, they
+merely bend to this side in slightly zigzag lines. Of the three tracings
+one alone (Fig. 49) is here given. Had the observations been more frequent
+during the 12 h. two oval figures would have been described with their
+longer axes at right angles to one another. The actual amount of movement
+of the apex from side to side was about .3 of an inch. The figures
+described by the other two seedlings resembled to a certain extent the one
+here given.
+
+Fig. 49. Phalaris Canariensis: circumnutation of a cotyledon, with a mark
+placed below the apex, traced on a horizontal glass, from 8.35 A.M. Nov.
+26th to 8.45 A.M. 27th. Movement of apex magnified 7 times, here reduced to
+one-half scale.
+
+A seedling which had just broken through the ground and projected only
+1/20th of an inch above the surface, was next observed in the same manner
+as before. It was necessary to clear away the earth all round the seedling
+to a little depth in order to place a mark beneath the apex. The figure
+(Fig. 50) shows that the apex moved to one side, but changed its course ten
+times in the course of the ten hours of observation; so that there can be
+no doubt about its circumnutation. The cause of the general movement in one
+direction could hardly be attributed to the entrance of lateral light, as
+this was carefully guarded against; and we suppose it was in some manner
+connected with the removal of the earth round the little seedling.
+
+Fig. 50. Phalaris Canariensis: circumnutation of a very young cotyledon,
+with a mark placed below the apex, traced on a horizontal glass, from 11.37
+A.M. to 9.30 P.M. Dec. 13th. Movement of apex greatly magnified, here
+reduced to one-fourth of original scale.
+
+Lastly, the soil in the same pot was searched with the aid of a lens, and
+the white knife-like apex of a seedling was found on an exact level with
+that of the surrounding surface. The soil was removed all round the apex to
+the depth of a quarter of an inch, the seed itself remaining covered. The
+pot, protected from lateral light, was placed under the micro-
+[page 64]
+scope with a micrometer eye-piece, so arranged that each division equalled
+1/500th of an inch. After an interval of 30 m. the apex was observed, and
+it was seen to cross a little obliquely two divisions of the micrometer in
+9 m. 15 s.; and after a few minutes it crossed the same space in 8 m. 50s.
+The seedling was again observed after an interval of three-quarters of an
+hour, and now the apex crossed rather obliquely two divisions in 10 m. We
+may therefore conclude that it was travelling at about the rate of 1/50th
+of an inch in 45 minutes. We may also conclude from these and the previous
+observations, that the seedlings of Phalaris in breaking through the
+surface of the soil circumnutate as much as the surrounding pressure will
+permit. This fact accounts (as in the case before given of the asparagus)
+for a circular, narrow, open space or crack being distinctly visible round
+several seedlings which had risen through very fine argillaceous sand, kept
+uniformly damp.
+
+Fig. 51. Zea mays: circumnutation of cotyledon, traced on horizontal glass,
+from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead magnified on an
+average about 25 times.
+
+Zea mays (Gramineae).--A glass filament was fixed obliquely to the summit
+of a cotyledon, rising .2 of an inch above the ground; but by the third
+morning it had grown to exactly thrice this height, so that the distance of
+the bead from the mark below was greatly increased, consequently the
+tracing (Fig. 51) was much more magnified on the first than on the second
+day. The upper part of the cotyledon changed its course by at least as much
+as a rectangle six times on each of the two days. The plant was illuminated
+by an obscure light from vertically above. This was a necessary precaution,
+as on the previous day we had traced the movements of cotyledons placed in
+a deep box, the inner side of which was feebly illuminated on one side from
+a distant north-east window, and at each observation by a wax taper held
+for a minute or two on the same side; and the result was that the
+cotyledons travelled all day long to this side, though making in their
+course some conspicuous flexures, from which fact alone we might have
+[page 65]
+concluded that they were circumnutating; but we thought it advisable to
+make the tracing above given.
+
+Radicles.--Glass filaments were fixed to two short radicles, placed so as
+to stand almost upright, and whilst bending downwards through geotropism
+their courses were strongly zigzag; from this latter circumstance
+circumnutation might have been inferred, had not their tips become slightly
+withered after the first 24 h., though they were watered and the air kept
+very damp. Nine radicles were next arranged in the manner formerly
+described, so that in growing downwards they left tracks on smoked
+glass-plates, inclined at various angles between 45o and 80o beneath the
+horizon. Almost every one of these tracks offered evidence in their greater
+or less breadth in different parts, or in little bridges of soot being
+left, that the apex had come alternately into more and less close contact
+with the glass. In the accompanying figure (Fig. 52) we have an accurate
+copy of one such track. In two instances alone (and in these the plates
+were highly inclined) there was some evidence of slight lateral movement.
+We presume therefore that the friction of the apex on the smoked surface,
+little as this could have been, sufficed to check the movement from side to
+side of these delicate radicles.
+
+Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip of
+radicle in growing downwards.
+
+Avena sativa (Gramineae).--A cotyledon, 1 � inch in height, was placed in
+front of a north-east window, and the movement of the apex was traced on a
+horizontal glass during two days. It moved towards the light in a slightly
+zigzag line from 9 to 11.30 A.M. on October 15th; it then moved a little
+backwards and zigzagged much until 5 P.M., after which hour, and curing the
+night, it continued to move towards the window. On the following morning
+the same movement was continued in a nearly straight line until 12.40 P.M.,
+when the sky remained until 2.35 extraordinarily dark from thunder-clouds.
+During this interval of 1 h. 55 m., whilst the light was obscure, it was
+interesting to observe how circumnutation overcame heliotropism, for the
+apex, instead of continuing to move towards the window in a slightly zigzag
+line, reversed its course four times, making two small narrow ellipses. A
+diagram of this case will be given in the chapter on Heliotropism.
+[page 66]
+
+A filament was next fixed to a cotyledon only 1/4 of an inch in height,
+which was illuminated exclusively from above, and as it was kept in a warm
+greenhouse, it grew rapidly; and now there could be no doubt about its
+circumnutation, for it described a figure of 8 as well as two small
+ellipses in 5 � hours.
+
+Nephrodium molle (Filices).--A seedling fern of this species came up by
+chance in a flowerpot near its parent. The frond, as yet only slightly
+lobed, was only .16 of an inch in length and .2 in breadth, and was
+supported on a rachis as fine as a hair and .23 of an inch in height. A
+very thin glass filament, which projected for a length of .36 of an inch,
+was fixed to the end of the frond. The movement was so highly magnified
+that the figure (Fig. 53) cannot be fully trusted; but the frond was
+constantly moving in a complex manner, and the bead greatly changed its
+course eighteen times in the 12 hours of observation. Within half an hour
+it often returned in a line almost parallel to its former course. The
+greatest amount of movement occurred between 4 and 6 P.M. The
+circumnutation of this plant is interesting, because the species in the
+genus Lygodium are well known to circumnutate conspicuously and to twine
+round any neighbouring object.
+
+Fig. 53. Nephrodium molle: circumnutation of very young frond, traced in
+darkness on horizontal glass, from 9 A.M. to 9 P.M. Oct. 30th. Movement of
+bead magnified 48 times.
+
+Selaginella Kraussii (?) (Lycopodiaceae).--A very young plant, only .4 of
+an inch in height, had sprung up in a pot in the hot-house. An extremely
+fine glass filament was fixed to the end of the frond-like stem, and the
+movement of the bead traced on a horizontal glass. It changed its course
+several times, as shown in Fig. 54, whilst observed during 13 h. 15 m., and
+returned at night to a point not far distant from that whence it had
+started in the morning. There can be no doubt that this little plant
+circumnutated.
+
+Fig. 54. Selaginella Kraussii (?): circumnutation of young plant, kept in
+darkness, traced from 8.45 A.M. to 10 P.M. Oct. 31st.
+[page 67]
+
+
+
+CHAPTER II.
+
+GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
+
+Generality of the circumnutating movement--Radicles, their circumnutation
+of service--Manner in which they penetrate the ground--Manner in which
+hypocotyls and other organs break through the ground by being arched--
+Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons-
+-Circumnutation of hypocotyls and epicotyls whilst still buried and arched-
+-Their power of straightening themselves--Bursting of the seed-coats--
+Inherited effect of the arching process in hypogean hypocotyls--
+Circumnutation of hypocotyls and epicotyls when erect--Circumnutation of
+cotyledons--Pulvini or joints of cotyledons, duration of their activity,
+rudimentary in Oxalis corniculata, their development--Sensitiveness of
+cotyledons to light and consequent disturbance of their periodic movements-
+-Sensitiveness of cotyledons to contact.
+
+THE circumnutating movements of the several parts or organs of a
+considerable number of seedling plants have been described in the last
+chapter. A list is here appended of the Families, Cohorts, Sub-classes,
+etc., to which they belong, arranged and numbered according to the
+classification adopted by Hooker.* Any one who will consider this list will
+see that the young plants selected for observation, fairly represent the
+whole vegetable series excepting the lowest cryptogams, and the movements
+of some of the latter when mature will hereafter be described. As all the
+seedlings which were observed, including Conifers, Cycads and Ferns, which
+belong to the most ancient
+
+* As given in the 'General System of Botany,' by Le Maout and Decaisne,
+1873.
+[page 68]
+
+types amongst plants, were continually circumnutating, we may infer that
+this kind of movement is common to every seedling species.
+
+SUB-KINGDOM I.--Phaenogamous Plants.
+
+Class I.--DICOTYLEDONS.
+
+Sub-class I.--Angiosperms.
+Family.                           Cohort.
+14.   Cruciferae.  II. PARIETALES.
+26.   Caryophylleae.  IV. CARYOPHYLLALES.
+36.   Malvaceae.  VI  MALVALES.
+41.   Oxalideae.  VII.  GERANIALES.
+49.   Tropaeoleae.  DITTO
+52.   Aurantiaceae.  DITTO
+70.   Hippocastaneae.  X.  SAPINDALES.
+75.   Leguminosae.  XI.  ROSALES.
+106.  Cucurbitaceae.  XII.  PASSIFLORALES.
+109.  Cacteae.  XIV.  FICOIDALES.
+122.  Compositae.  XVII.  ASTRALES.
+135.  Primulaceae.  XX.  PRIMULALES.
+145.  Asclepiadeae.  XXII.  GENTIANALES.
+151.  Convolvulaceae.  XXIII.  POLEMONIALES.
+154.  Boragineae.  DITTO
+156.  Nolaneae.  DITTO
+157.  Solaneae.  XXIV.  SOLANALES.
+181.  Chenopodieae.  XXVII.  CHENOPODIALES.
+202.  Euphorbiaceae.  XXXII.  EUPHORBIALES.
+211.  Cupuliferae.  XXXVI.  QUERNALES.
+212.  Corylaceae.  DITTO
+
+Sub-class II.--Gymnosperms.
+223.  Coniferae.
+224.  Cycadeae.
+
+Class II.--MONOCOTYLEDONS.
+2.    Cannaceae.  II.  AMOMALES.
+34.   Liliaceae.  XI.  LILIALES.
+41.   Asparageae.  DITTO
+55.   Gramineae.  XV.  GLUMALES.
+
+SUB-KINGDOM II.--Cryptogamic Plants.
+
+1.    Filices. I.   FILICALES.
+6.    Lycopodiaceae.  DITTO
+[page 69]
+
+Radicles.--In all the germinating seeds observed by us, the first change is
+the protrusion of the radicle, which immediately bends downwards and
+endeavours to penetrate the ground. In order to effect this, it is almost
+necessary that the seed should be pressed down so as to offer some
+resistance, unless indeed the soil is extremely loose; for otherwise the
+seed is lifted up, instead of the radicle penetrating the surface. But
+seeds often get covered by earth thrown up by burrowing quadrupeds or
+scratching birds, by the castings of earth-worms, by heaps of excrement,
+the decaying branches of trees, etc., and will thus be pressed down; and
+they must often fall into cracks when the ground is dry, or into holes.
+Even with seeds lying on the bare surface, the first developed root-hairs,
+by becoming attached to stones or other objects on the surface, are able to
+hold down the upper part of the radicle, whilst the tip penetrates the
+ground. Sachs has shown* how well and closely root-hairs adapt themselves
+by growth to the most irregular particles in the soil, and become firmly
+attached to them. This attachment seems to be effected by the softening or
+liquefaction of the outer surface of the wall of the hair and its
+subsequent consolidation, as will be on some future occasion more fully
+described. This intimate union plays an important part, according to Sachs,
+in the absorption of water and of the inorganic matter dissolved in it. The
+mechanical aid afforded by the root-hairs in penetrating the ground is
+probably only a secondary service.
+
+The tip of the radicle, as soon as it protrudes from the seed-coats, begins
+to circumnutate, and the whole
+
+* 'Physiologie V�g�tale,' 1868, pp. 199, 205.
+[page 70]
+
+growing part continues to do so, probably for as long as growth continues.
+This movement of the radicle has been described in Brassica, Aesculus,
+Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability of its
+occurrence was inferred by Sachs,* from radicles placed vertically upwards
+being acted on by geotropism (which we likewise found to be the case), for
+if they had remained absolutely perpendicular, the attraction of gravity
+could not have caused them to bend to any one side. Circumnutation was
+observed in the above specified cases, either by means of extremely fine
+filaments of glass affixed to the radicles in the manner previously
+described, or by their being allowed to grow downwards over inclined smoked
+glass-plates, on which they left their tracks. In the latter cases the
+serpentine course (see Figs. 19, 21, 27, 41) showed unequivocally that the
+apex had continually moved from side to side. This lateral movement was
+small in extent, being in the case of Phaseolus at most about 1 mm. from a
+medial line to both sides. But there was also movement in a vertical plane
+at right angles to the inclined glass-plates. This was shown by the tracks
+often being alternately a little broader and narrower, due to the radicles
+having alternately pressed with greater and less force on the plates.
+Occasionally little bridges of soot were left across the tracks, showing
+that the apex had at these spots been lifted up. This latter fact was
+especially apt to occur
+* 'Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in
+W�rzburg,' Heft iii. 1873, p. 460. This memoir, besides its intrinsic and
+great interest, deserves to be studied as a model of careful investigation,
+and we shall have occasion to refer to it repeatedly. Dr. Frank had
+previously remarked ('Beitr�ge zur Pflanzenphysiologie, 1868, p. 81) on the
+fact of radicles placed vertically upwards being acted on by geotropism,
+and he explained it by the supposition that their growth was not equal on
+all sides.
+
+[page 71]
+when the radicle instead of travelling straight down the glass made a
+semicircular bend; but Fig. 52 shows that this may occur when the track is
+rectilinear. The apex by thus rising, was in one instance able to surmount
+a bristle cemented across an inclined glass-plate; but slips of wood only
+1/40 of an inch in thickness always caused the radicles to bend
+rectangularly to one side, so that the apex did not rise to this small
+height in opposition to geotropism.
+
+In those cases in which radicles with attached filaments were placed so as
+to stand up almost vertically, they curved downwards through the action of
+geotropism, circumnutating at the same time, and their courses were
+consequently zigzag. Sometimes, however, they made great circular sweeps,
+the lines being likewise zigzag.
+
+Radicles closely surrounded by earth, even when this is thoroughly soaked
+and softened, may perhaps be quite prevented from circumnutating. Yet we
+should remember that the circumnutating sheath-like cotyledons of Phalaris,
+the hypocotyls of Solanum, and the epicotyls of Asparagus formed round
+themselves little circular cracks or furrows in a superficial layer of damp
+argillaceous sand. They were also able, as well as the hypocotyls of
+Brassica, to form straight furrows in damp sand, whilst circumnutating and
+bending towards a lateral light. In a future chapter it will be shown that
+the rocking or circumnutating movement of the flower-heads of Trifolium
+subterraneum aids them in burying themselves. It is therefore probable that
+the circumnutation of the tip of the radicle aids it slightly in
+penetrating the ground; and it may be observed in several of the previously
+given diagrams, that the movement is more strongly pronounced in radicles
+when they first
+[page 72]
+protrude from the seed than at a rather later period; but whether this is
+an accidental or an adaptive coincidence we do not pretend to decide.
+Nevertheless, when young radicles of Phaseolus multiflorus were fixed
+vertically close over damp sand, in the expectation that as soon as they
+reached it they would form circular furrows, this did not occur,--a fact
+which may be accounted for, as we believe, by the furrow being filled up as
+soon as formed by the rapid increase of thickness in the apex of the
+radicle. Whether or not a radicle, when surrounded by softened earth, is
+aided in forming a passage for itself by circumnutating, this movement can
+hardly fail to be of high importance, by guiding the radicle along a line
+of least resistance, as will be seen in the next chapter when we treat of
+the sensibility of the tip to contact. If, however, a radicle in its
+downward growth breaks obliquely into any crevice, or a hole left by a
+decayed root, or one made by the larva of an insect, and more especially by
+worms, the circumnutating movement of the tip will materially aid it in
+following such open passage; and we have observed that roots commonly run
+down the old burrows of worms.*
+
+ When a radicle is placed in a horizontal or inclined position, the
+terminal growing part, as is well known, bends down towards the centre of
+the earth; and Sachs* has shown that whilst thus bending, the growth of the
+lower surface is greatly retarded, whilst that
+
+* See, also, Prof. Hensen's statements ('Zeitschrift f�r Wissen, Zool.,' B.
+xxviii. p. 354, 1877) to the same effect. He goes so far as to believe that
+roots are able to penetrate the ground to a great depth only by means of
+the burrows made by worms.
+
+* 'Arbeiten des bot. Inst. W�rzburg,' vol. i. 1873, p. 461. See also p. 397
+for the length of the growing part, and p. 451 on the force of geotropism.
+[page 73]
+
+of the upper surface continues at the normal rate, or may be even somewhat
+increased. He has further shown by attaching a thread, running over a
+pulley, to a horizontal radicle of large size, namely that of the common
+bean, that it was able to pull up a weight of only one gramme, or 15.4
+grains. We may therefore conclude that geotropism does not give a radicle
+force sufficient to penetrate the ground, but merely tells it (if such an
+expression may be used) which course to pursue. Before we knew of Sachs'
+more precise observations we covered a flat surface of damp sand with the
+thinnest tin-foil which we could procure (.02 to .03 mm., or .00012 to
+.00079 of an inch in thickness), and placed a radicle close above, in such
+a position that it grew almost perpendicularly downwards. When the apex
+came into contact with the polished level surface it turned at right angles
+and glided over it without leaving any impression; yet the tin-foil was so
+flexible, that a little stick of soft wood, pointed to the same degree as
+the end of the radicle and gently loaded with a weight of only a quarter of
+an ounce (120 grains) plainly indented the tin-foil.
+
+Radicles are able to penetrate the ground by the force due to their
+longitudinal and transverse growth; the seeds themselves being held down by
+the weight of the superincumbent soil. In the case of the bean the apex,
+protected by the root-cap, is sharp, and the growing part, from 8 to 10 mm.
+in length, is much more rigid, as Sachs has proved, than the part
+immediately above, which has ceased to increase in length. We endeavoured
+to ascertain the downward pressure of the growing part, by placing
+germinating beans between two small metal plates, the upper one of which
+was loaded with a known weight; and the
+[page 74]
+radicle was then allowed to grow into a narrow hole in wood, 2 or 3 tenths
+of an inch in depth, and closed at the bottom. The wood was so cut that the
+short space of radicle between the mouth of the hole and the bean could not
+bend laterally on three sides; but it was impossible to protect the fourth
+side, close to the bean. Consequently, as long as the radicle continued to
+increase in length and remained straight, the weighted bean would be lifted
+up after the tip had reached the bottom of the shallow hole. Beans thus
+arranged, surrounded by damp sand, lifted up a quarter of a pound in 24 h.
+after the tip of the radicle had entered the hole. With a greater weight
+the radicles themselves always became bent on the one unguarded side; but
+this probably would not have occurred if they had been closely surrounded
+on all sides by compact earth. There was, however, a possible, but not
+probable, source of error in these trials, for it was not ascertained
+whether the beans themselves go on swelling for several days after they
+have germinated, and after having been treated in the manner in which ours
+had been; namely, being first left for 24 h. in water, then allowed to
+germinate in very damp air, afterwards placed over the hole and almost
+surrounded by damp sand in a closed box.
+
+Fig. 55. Outline of piece of stick (reduced to one-half natural size) with
+a hole through which the radicle of a bean grew. Thickness of stick at
+narrow end .08 inch, at broad end .16; depth of hole .1 inch.
+We succeeded better in ascertaining the force exerted transversely by these
+radicles. Two were so placed as to penetrate small holes made in little
+sticks, one of which was cut into the shape here exactly copied (Fig. 55).
+The short end of the stick beyond the hole was purposely split, but not the
+opposite
+[page 75]
+end. As the wood was highly elastic, the split or fissure closed
+immediately after being made. After six days the stick and bean were dug
+out of the damp sand, and the radicle was found to be much enlarged above
+and beneath the hole. The fissure which was at first quite closed, was now
+open to a width of 4 mm.; as soon as the radicle was extracted, it
+immediately closed to a width of 2 mm. The stick was then suspended
+horizontally by a fine wire passing through the hole lately filled by the
+radicle, and a little saucer was suspended beneath to receive the weights;
+and it required 8 lbs. 8 ozs. to open the fissure to the width of 4 mm.--
+that is, the width before the root was extracted. But the part of the
+radicle (only .1 of an inch in length) which was embedded in the hole,
+probably exerted a greater transverse strain even than 8 lbs. 8 ozs., for
+it had split the solid wood for a length of rather more than a quarter of
+an inch (exactly .275 inch), and this fissure is shown in Fig. 55. A second
+stick was tried in the same manner with almost exactly the same result.
+
+Fig. 56. Wooden pincers, kept closed by a spiral brass spring, with a hole
+(.14 inch in diameter and .6 inch in depth) bored through the narrow closed
+part, through which a radicle of a bean was allowed to grow. Temp. 50o -
+60o F.
+
+We then followed a better plan. Holes were bored near the narrow end of two
+wooden clips or pincers (Fig. 56), kept closed by brass spiral springs. Two
+radicles in damp sand were allowed to grow through these holes. The
+[page 76]
+pincers rested on glass-plates to lessen the friction from the sand. The
+holes were a little larger (viz..14 inch) and considerably deeper (viz..6
+inch) than in the trials with the sticks; so that a greater length of a
+rather thicker radicle exerted a transverse strain. After 13 days they were
+taken up. The distance of two dots (see the figure) on the longer ends of
+the pincers was now carefully measured; the radicles were then extracted
+from the holes, and the pincers of course closed. They were then suspended
+horizontally in the same manner as were the bits of sticks, and a weight of
+1500 grams (or 3 pounds 4 ounces) was necessary with one of the pincers to
+open them to the same extent as had been effected by the transverse growth
+of the radicle. As soon as this radicle had slightly opened the pincers, it
+had grown into a flattened form and had escaped a little beyond the hole;
+its diameter in one direction being 4.2 mm., and at rightangles 3.5 mm. If
+this escape and flattening could have been prevented, the radicle would
+probably have exerted a greater strain than the 3 pounds 4 ounces. With the
+other pincers the radicle escaped still further out of the hole; and the
+weight required to open them to the same extent as had been effected by the
+radicle, was only 600 grams.
+
+With these facts before us, there seems little difficulty in understanding
+how a radicle penetrates the ground. The apex is pointed and is protected
+by the root-cap; the terminal growing part is rigid, and increases in
+length with a force equal, as far as our observations can be trusted, to
+the pressure of at least a quarter of a pound, probably with a much greater
+force when prevented from bending to any side by the surrounding earth.
+Whilst thus increasing in length it increases in thickness, pushing away
+the damp
+[page 77]
+earth on all sides, with a force of above 8 pounds in one case, of 3 pounds
+in another case. It was impossible to decide whether the actual apex
+exerts, relatively to its diameter, the same transverse strain as the parts
+a little higher up; but there seems no reason to doubt that this would be
+the case. The growing part therefore does not act like a nail when hammered
+into a board, but more like a wedge of wood, which whilst slowly driven
+into a crevice continually expands at the same time by the absorption of
+water; and a wedge thus acting will split even a mass of rock.
+
+Manner in which Hypocotyls, Epicotyls, etc., rise up and break through the
+ground.--After the radicle has penetrated the ground and fixed the seed,
+the hypocotyls of all the dicotyledonous seedlings observed by us, which
+lift their cotyledons above the surface, break through the ground in the
+form of an arch. When the cotyledons are hypogean, that is, remain buried
+in the soil, the hypocotyl is hardly developed, and the epicotyl or plumule
+rises in like manner as an arch through the ground. In all, or at least in
+most of such cases, the downwardly bent apex remains for a time enclosed
+within the seed-coats. With Corylus avellena the cotyledons are hypogean,
+and the epicotyl is arched; but in the particular case described in the
+last chapter its apex had been injured, and it grew laterally through the
+soil like a root; and in consequence of this it had emitted two secondary
+shoots, which likewise broke through the ground as arches.
+
+Cyclamen does not produce any distinct stem, and only a single cotyledon
+appears at first;* its petiole
+
+* This is the conclusion arrived at by Dr. H. Gressner ('Bot. Zeitung,'
+1874, p. 837), who maintains that what has been considered by other
+botanists as the first true leaf is really the second cotyledon, which is
+greatly delayed in its development.
+[page 78]
+
+breaks through the ground as an arch (Fig. 57). Abronia has only a single
+fully developed cotyledon, but in this case it is the hypocotyl which first
+emerges and is arched. Abronia umbellata, however, presents this
+peculiarity, that the enfolded blade of the one developed cotyledon (with
+the enclosed endosperm) whilst still beneath the surface has its apex
+upturned and parallel to the descending leg of the arched hypocotyl; but it
+is dragged out of the ground by the continued growth of the hypocotyl, with
+the apex pointing downward. With Cycas pectinata the cotyledons are
+hypogean, and a true leaf first breaks through the ground with its petiole
+forming an arch.
+
+Fig. 57. Cyclamen Persicum: seedling, figure enlarged: c, blade of
+cotyledon, not yet expanded, with arched petiole beginning to straighten
+itself; h, hypocotyl developed into a corm; r, secondary radicles.
+
+Fig. 58. Acanthus mollis: seedling with the hypogean cotyledon on the near
+side removed and the radicles cut off; a, blade of first leaf beginning to
+expand, with petiole still partially arched; b, second and opposite leaf,
+as yet very imperfectly developed; c, hypogean cotyledon on the opposite
+side.
+
+In the genus Acanthus the cotyledons are likewise hypogean. In A. mollis, a
+single leaf first breaks through the ground with its petiole arched, and
+with the opposite leaf much less developed, short, straight, of a yellowish
+colour, and with the petiole at first not half as thick as that of the
+other. The undeveloped leaf is protected by standing beneath its arched
+fellow; and it is an instruc-
+[page 79]
+tive fact that it is not arched, as it has not to force for itself a
+passage through the ground. In the accompanying sketch (Fig. 58) the
+petiole of the first leaf has already partially straightened itself, and
+the blade is beginning to unfold. The small second leaf ultimately grows to
+an equal size with the first, but this process is effected at very
+different rates in different individuals: in one instance the second leaf
+did not appear fully above the ground until six weeks after the first leaf.
+As the leaves in the whole family of the Acanthaceae stand either opposite
+one another or in whorls, and as these are of equal size, the great
+inequality between the first two leaves is a singular fact. We can see how
+this inequality of development and the arching of the petiole could have
+been gradually acquired, if they were beneficial to the seedlings by
+favouring their emergence; for with A. candelabrum, spinosus, and
+latifolius there was a great variability in the inequality between the two
+first leaves and in the arching of their petioles. In one seedling of A.
+candelabrum the first leaf was arched and nine times as long as the second,
+which latter consisted of a mere little, yellowish-white, straight, hairy
+style. In other seedlings the difference in length between the two leaves
+was as 3 to 2, or as 4 to 3, or as only .76 to .62 inch. In these latter
+cases the first and taller leaf was not properly arched. Lastly, in another
+seedling there was not the least difference in size between the two first
+leaves, and both of them had their petioles straight; their laminae were
+enfolded and pressed against each other, forming a lance or wedge, by which
+means they had broken through the ground. Therefore in different
+individuals of this same species of Acanthus the first pair of leaves
+breaks through the ground by two widely different methods; and if
+[page 80]
+either had proved decidedly advantageous or disadvantageous, one of them no
+doubt would soon have prevailed.
+
+Asa Gray has described* the peculiar manner of germination of three widely
+different plants, in which the hypocotyl is hardly at all developed. These
+were therefore observed by us in relation to our present subject.
+
+Delphinium nudicaule.--The elongated petioles of the two cotyledons are
+confluent (as are sometimes their blades at the base), and they break
+through the ground as an arch. They thus resemble in a most deceptive
+manner a hypocotyl. At first they are solid, but after a time become
+tubular; and the basal part beneath the ground is enlarged into a hollow
+chamber, within which the young leaves are developed without any prominent
+plumule. Externally root-hairs are formed on the confluent petioles, either
+a little above, or on a level with, the plumule. The first leaf at an early
+period of its growth and whilst within the chamber is quite straight, but
+the petiole soon becomes arched; and the swelling of this part (and
+probably of the blade) splits open one side of the chamber, and the leaf
+then emerges. The slit was found in one case to be 3.2 mm. in length, and
+it is seated on the line of confluence of the two petioles. The leaf when
+it first escapes from the chamber is buried beneath the ground, and now an
+upper part of the petiole near the blade becomes arched in the usual
+manner. The second leaf comes out of the slit either straight or somewhat
+arched, but afterwards the upper part of the petiole,--certainly in some,
+and we believe in all cases,--arches itself whilst forcing a passage
+through the soil.
+
+* 'Botanical Text-Book,' 1879, p. 22.
+[page 81]
+
+Megarrhiza Californica.--The cotyledons of this Gourd never free themselves
+from the seed-coats and are hypogean. Their petioles are completely
+confluent, forming a tube which terminates downwards in a little solid
+point, consisting of a minute radicle and hypocotyl, with the likewise
+minute plumule enclosed within the base of the tube. This structure was
+well exhibited in an abnormal specimen, in which one of the two cotyledons
+failed to produce a petiole, whilst the other produced one consisting of an
+open semicylinder ending in a sharp point, formed of the parts just
+described. As soon as the confluent petioles protrude from the seed they
+bend down, as they are strongly geotropic, and penetrate the ground. The
+seed itself retains its original position, either on the surface or buried
+at some depth, as the case may be. If, however, the point of the confluent
+petioles meets with some obstacle in the soil, as appears to have occurred
+with the seedlings described and figured by Asa Gray,* the cotyledons are
+lifted up above the ground. The petioles are clothed with root-hairs like
+those on a true radicle, and they likewise resemble radicles in becoming
+brown when immersed in a solution of permanganate of potassium. Our seeds
+were subjected to a high temperature, and in the course of three or four
+days the petioles penetrated the soil perpendicularly to a depth of from 2
+to 2 � inches; and not until then did the true radicle begin to grow. In
+one specimen which was closely observed, the petioles in 7 days after their
+first protrusion attained a length of 2 � inches, and the radicle by this
+time had also become well developed. The plumule, still enclosed within the
+tube, was now
+
+* 'American Journal of Science,' vol. xiv. 1877, p. 21.
+[page 82]
+
+.3 inch in length, and was quite straight; but from having increased in
+thickness it had just begun to split open the lower part of the petioles on
+one side, along the line of their confluence. By the following morning the
+upper part of the plumule had arched itself into a right angle, and the
+convex side or elbow had thus been forced out through the slit. Here then
+the arching of the plumule plays the same part as in the case of the
+petioles of the Delphinium. As the plumule continued to grow, the tip
+became more arched, and in the course of six days it emerged through the 2
+� inches of superincumbent soil, still retaining its arched form. After
+reaching the surface it straightened itself in the usual manner. In the
+accompanying figure (Fig. 58, A) we have a sketch of a seedling in this
+advanced state of development; the surface of the ground being represented
+by the line G...........G.
+
+Fig. 58, A. Megarrhiza Californica: sketch of seedling, copied from Asa
+Gray, reduced to one-half scale: c, cotyledons within seed-coats; p, the
+two confluent petioles; h and r, hypocotyl and radicle; p1, plumule;
+G..........G, surface of soil.
+
+The germination of the seeds in their native Californian home proceeds in a
+rather different manner, as we infer from an interesting letter from Mr.
+Rattan, sent to us by Prof. Asa Gray. The petioles protrude from the seeds
+soon after the autumnal rains, and penetrate the ground, generally in a
+vertical direction, to a depth of from 4 to even 6 inches. they were found
+in this state by Mr. Rattan during the Christmas vacation, with the plu-
+[page 83]
+mules still enclosed within the tubes; and he remarks that if the plumules
+had been at once developed and had reached the surface (as occurred with
+our seeds which were exposed to a high temperature), they would surely have
+been killed by the frost. As it is, they lie dormant at some depth beneath
+the surface, and are thus protected from the cold; and the root-hairs on
+the petioles would supply them with sufficient moisture. We shall hereafter
+see that many seedlings are protected from frost, but by a widely different
+process, namely, by being drawn beneath the surface by the contraction of
+their radicles. We may, however, believe that the extraordinary manner of
+germination of Megarrhiza has another and secondary advantage. The radicle
+begins in a few weeks to enlarge into a little tuber, which then abounds
+with starch and is only slightly bitter. It would therefore be very liable
+to be devoured by animals, were it not protected by being buried whilst
+young and tender, at a depth of some inches beneath the surface. Ultimately
+it grows to a huge size.
+
+Ipomoea leptophylla.--In most of the species of this genus the hypocotyl is
+well developed, and breaks through the ground as an arch. But the seeds of
+the present species in germinating behave like those of Megarrhiza,
+excepting that the elongated petioles of the cotyledons are not confluent.
+After they have protruded from the seed, they are united at their lower
+ends with the undeveloped hypocotyl and undeveloped radicle, which together
+form a point only about .1 inch in length. They are at first highly
+geotropic, and penetrate the ground to a depth of rather above half an
+inch. The radicle then begins to grow. On four occasions after the petioles
+had grown for a short distance vertically downwards, they
+[page 84]
+were placed in a horizontal position in damp air in the dark, and in the
+course of 4 hours they again became curved vertically downwards, having
+passed through 90o in this time. But their sensitiveness to geotropism
+lasts for only 2 or 3 days; and the terminal part alone, for a length of
+between .2 and .4 inch, is thus sensitive. Although the petioles of our
+specimens did not penetrate the ground to a greater depth than about �
+inch, yet they continued for some time to grow rapidly, and finally
+attained the great length of about 3 inches. The upper part is
+apogeotropic, and therefore grows vertically upwards, excepting a short
+portion close to the blades, which at an early period bends downwards and
+becomes arched, and thus breaks through the ground. Afterwards this portion
+straightens itself, and the cotyledons then free themselves from the
+seed-coats. Thus we here have in different parts of the same organ widely
+different kinds of movement and of sensitiveness; for the basal part is
+geotropic, the upper part apogeotropic, and a portion near the blades
+temporarily and spontaneously arches itself. The plumule is not developed
+for some little time; and as it rises between the bases of the parallel and
+closely approximate petioles of the cotyledons, which in breaking through
+the ground have formed an almost open passage, it does not require to be
+arched and is consequently always straight. Whether the plumule remains
+buried and dormant for a time in its native country, and is thus protected
+from the cold of winter, we do not know. The radicle, like that of the
+Megarrhiza, grows into a tuber-like mass, which ultimately attains a great
+size. So it is with Ipomoea pandurata, the germination of which, as Asa
+Gray informs us, resembles that of I. leptophylla.
+
+The following case is interesting in connection with
+[page 85]
+the root-like nature of the petioles. The radicle of a seedling was cut
+off, as it was completely decayed, and the two now separated cotyledons
+were planted. They emitted roots from their bases, and continued green and
+healthy for two months. The blades of both then withered, and on removing
+the earth the bases of the petioles (instead of the radicle) were found
+enlarged into little tubers. Whether these would have had the power of
+producing two independent plants in the following summer, we do not know.
+
+In Quercus virens, according to Dr. Engelmann,* both the cotyledons and
+their petioles are confluent. The latter grow to a length "of an inch or
+even more;" and, if we understand rightly, penetrate the ground, so that
+they must be geotropic. The nutriment within the cotyledons is then quickly
+transferred to the hypocotyl or radicle, which thus becomes developed into
+a fusiform tuber. The fact of tubers being formed by the foregoing three
+widely distinct plants, makes us believe that their protection from animals
+at an early age and whilst tender, is one at least of the advantages gained
+by the remarkable elongation of the petioles of the cotyledons, together
+with their power of penetrating the ground like roots under the guidance of
+geotropism.
+
+The following cases may be here given, as they bear on our present subject,
+though not relating to seedlings. The flower-stem of the parasitic Lathraea
+squamaria, which is destitute of true leaves, breaks through the ground as
+an arch;** so does the flower-
+
+* 'Transact. St. Louis Acad. Science,' vol. iv. p. 190.
+
+** The passage of the flower-stem of the Lathraea through the ground cannot
+fail to be greatly facilitated by the extraordinary quantity of water
+secreted at this period of the year by the subter-
+[[page 86]]
+ranean scale-like leaves; not that there is any reason to suppose that the
+secretion is a special adaptation for this purpose: it probably follows
+from the great quantity of sap absorbed in the early spring by the
+parasitic roots. After a long period without any rain, the earth had become
+light-coloured and very dry, but it was dark-coloured and damp, even in
+parts quite wet, for a distance of at least six inches all round each
+flower-stem. The water is secreted by glands (described by Cohn, 'Bericht.
+Bot. Sect. der Schlesischen Gesell.,' 1876, p. 113) which line the
+longitudinal channels running through each scale-like leaf. A large plant
+was dug up, washed so as to remove the earth, left for some time to drain,
+and then placed in the evening on a dry glass-plate, covered with a
+bell-glass, and by next morning it had secreted a large pool of water. The
+plate was wiped dry, and in the course of the succeeding 7 or 8 hours
+another little pool was secreted, and after 16 additional hours several
+large drops. A smaller plant was washed and placed in a large jar, which
+was left inclined for an hour, by which time no more water drained off. The
+jar was then placed upright and closed: after 23 hours two drachms of water
+were collected from the bottom, and a little more after 25 additional
+hours. The flower-stems were now cut off, for they do not secrete, and the
+subterranean part of the plant was found to weigh 106.8 grams (1611
+grains), and the water secreted during the 48 hours weighed 11.9 grams (183
+grains),--that is, one-ninth of the whole weight of the plant, excluding
+the flower-stems. We should remember that plants in a state of nature would
+probably secrete in 48 hours much more than the above large amount, for
+their roots would continue all the time absorbing sap from the plant on
+which they were parasitic.
+[page 86]
+
+stem of the parasitic and leafless Monotropa hypopitys. With Helleborus
+niger, the flower-stems, which rise up independently of the leaves,
+likewise break through the ground as arches. This is also the case with the
+greatly elongated flower-stems, as well as with the petioles of Epimedium
+pinnatum. So it is with the petioles of Ranunculus ficaria, when they have
+to break through the ground, but when they arise from the summit of the
+bulb above ground, they are from the first quite straight; and this is a
+fact which deserves notice. The rachis of the bracken fern (Pteris
+aquilina), and of some, probably many, other ferns, likewise rises above
+ground under the form of an arch. No doubt other analogous instances could
+be found by careful search. In all ordinary cases of bulbs, rhizomes,
+[page 87]
+root-stocks, etc., buried beneath the ground, the surface is broken by a
+cone formed by the young imbricated leaves, the combined growth of which
+gives them force sufficient for the purpose.
+
+With germinating monocotyledonous seeds, of which, however, we did not
+observe a large number, the plumules, for instance, those of Asparagus and
+Canna, are straight whilst breaking through the ground. With the Gramineae,
+the sheath-like cotyledons are likewise straight; they, however, terminate
+in a sharp crest, which is white and somewhat indurated; and this structure
+obviously facilitates their emergence from the soil: the first true leaves
+escape from the sheath through a slit beneath the chisel-like apex and at
+right angles to it. In the case of the onion (Allium cepa) we again meet
+with an arch; the leaf-like cotyledon being abruptly bowed, when it breaks
+through the ground, with the apex still enclosed within the seed-coats. The
+crown of the arch, as previously described, is developed into a white
+conical protuberance, which we may safely believe to be a special
+adaptation for this office.
+
+The fact of so many organs of different kinds--hypocotyls and epicotyls,
+the petioles of some cotyledons and of some first leaves, the cotyledons of
+the onion, the rachis of some ferns, and some flower-stems--being all
+arched whilst they break through the ground, shows how just are Dr.
+Haberlandt's* remarks on the importance of the arch to seedling plants. He
+attributes its chief importance to the upper, young, and more tender parts
+of the hypocotyl
+
+* 'Die Schutzeinrichtungen in der Entwickelung der Keimpflanze,' 1877. We
+have learned much from this interesting essay, though our observations lead
+us to differ on some points from the author.
+[page 88]
+
+or epicotyl, being thus saved from abrasion and pressure whilst breaking
+through the ground. But we think that some importance may be attributed to
+the increased force gained by the hypocotyl, epicotyl, or other organ by
+being at first arched; for both legs of the arch increase in length, and
+both have points of resistance as long as the tip remains enclosed within
+the seed-coats; and thus the crown of the arch is pushed up through the
+earth with twice as much force as that which a straight hypocotyl, etc.,
+could exert. As soon, however, as the upper end has freed itself, all the
+work has to be done by the basal leg. In the case of the epicotyl of the
+common bean, the basal leg (the apex having freed itself from the
+seed-coats) grew upwards with a force sufficient to lift a thin plate of
+zinc, loaded with 12 ounces. Two more ounces were added, and the 14 ounces
+were lifted up to a very little height, and then the epicotyl yielded and
+bent to one side.
+
+With respect to the primary cause of the arching process, we long thought
+in the case of many seedlings that this might be attributed to the manner
+in which the hypocotyl or epicotyl was packed and curved within the
+seed-coats; and that the arched shape thus acquired was merely retained
+until the parts in question reached the surface of the ground. But it is
+doubtful whether this is the whole of the truth in any case. For instance,
+with the common bean, the epicotyl or plumule is bowed into an arch whilst
+breaking through the seed-coats, as shown in Fig. 59 (p. 92). The plumule
+first protrudes as a solid knob (e in A), which after twenty-four hours'
+growth is seen (e in B) to be the crown of an arch. Nevertheless, with
+several beans which germinated in damp air, and had otherwise been treated
+in an unnatural manner, little
+[page 89]
+plumules were developed in the axils of the petioles of both cotyledons,
+and these were as perfectly arched as the normal plumule; yet they had not
+been subjected to any confinement or pressure, for the seed-coats were
+completely ruptured, and they grew in the open air. This proves that the
+plumule has an innate or spontaneous tendency to arch itself.
+
+In some other cases the hypocotyl or epicotyl protrudes from the seed at
+first only slightly bowed; but the bowing afterwards increases
+independently of any constraint. The arch is thus made narrow, with the two
+legs, which are sometimes much elongated, parallel and close together, and
+thus it becomes well fitted for breaking through the ground.
+
+With many kinds of plants, the radicle, whilst still enclosed within the
+seed and likewise after its first protrusion, lies in a straight line with
+the future hypocotyl and with the longitudinal axis of the cotyledons. This
+is the case with Cucurbita ovifera: nevertheless, in whatever position the
+seeds were buried, the hypocotyl always came up arched in one particular
+direction. Seeds were planted in friable peat at a depth of about an inch
+in a vertical position, with the end from which the radicle protrudes
+downwards. Therefore all the parts occupied the same relative positions
+which they would ultimately hold after the seedlings had risen clear above
+the surface. Notwithstanding this fact, the hypocotyl arched itself; and as
+the arch grew upwards through the peat, the buried seeds were turned either
+upside down, or were laid horizontally, being afterwards dragged above the
+ground. Ultimately the hypocotyl straightened itself in the usual manner;
+and now after all these movements the several parts occupied the same
+position relatively to one another and to the centre of the earth, which
+they
+[page 90]
+had done when the seeds were first buried. But it may be argued in this and
+other such cases that, as the hypocotyl grows up through the soil, the seed
+will almost certainly be tilted to one side; and then from the resistance
+which it must offer during its further elevation, the upper part of the
+hypocotyl will be doubled down and thus become arched. This view seems the
+more probable, because with Ranunculus ficaria only the petioles of the
+leaves which forced a passage through the earth were arched; and not those
+which arose from the summits of the bulbs above the ground. Nevertheless,
+this explanation does not apply to the Cucurbita, for when germinating
+seeds were suspended in damp air in various positions by pins passing
+through the cotyledons, fixed to the inside of the lids of jars, in which
+case the hypocotyls were not subjected to any friction or constraint, yet
+the upper part became spontaneously arched. This fact, moreover, proves
+that it is not the weight of the cotyledons which causes the arching. Seeds
+of Helianthus annuus and of two species of Ipomoea (those of 'I. bona nox'
+being for the genus large and heavy) were pinned in the same manner, and
+the hypocotyls became spontaneously arched; the radicles, which had been
+vertically dependent, assumed in consequence a horizontal position. In the
+case of Ipomoea leptophylla it is the petioles of the cotyledons which
+become arched whilst rising through the ground; and this occurred
+spontaneously when the seeds were fixed to the lids of jars.
+
+It may, however, be suggested with some degree of probability that the
+arching was aboriginally caused by mechanical compulsion, owing to the
+confinement of the parts in question within the seed-coats, or to friction
+whilst they were being dragged upwards. But
+[page 91]
+if this is so, we must admit from the cases just given, that a tendency in
+the upper part of the several specified organs to bend downwards and thus
+to become arched, has now become with many plants firmly inherited. The
+arching, to whatever cause it may be due, is the result of modified
+circumnutation, through increased growth along the convex side of the part;
+such growth being only temporary, for the part always straightens itself
+subsequently by increased growth along the concave side, as will hereafter
+be described.
+
+It is a curious fact that the hypocotyls of some plants, which are but
+little developed and which never raise their cotyledons above the ground,
+nevertheless inherit a slight tendency to arch themselves, although this
+movement is not of the least use to them. We refer to a movement observed
+by Sachs in the hypocotyls of the bean and some other Leguminosae, and
+which is shown in the accompanying figure (Fig. 59), copied from his
+Essay.* The hypocotyl and radicle at first grow perpendicularly downwards,
+as at A, and then bend, often in the course of 24 hours, into the position
+shown at B. As we shall hereafter often have to recur to this movement, we
+will, for brevity sake, call it "Sachs' curvature." At first sight it might
+be thought that the altered position of the radicle in B was wholly due to
+the outgrowth of the epicotyl (e), the petiole (p) serving as a hinge; and
+it is probable that this is partly the cause; but the hypocotyl and upper
+part of the radicle themselves become slightly curved.
+
+The above movement in the bean was repeatedly seen by us; but our
+observations were made chiefly on Phaseolus multiflorus, the cotyledons of
+which are like-
+
+* 'Arbeiten des bot. Instit. W�rzburg,' vol. i. 1873, p. 403.
+[page 92]
+
+wise hypogean. Some seedlings with well-developed radicles were first
+immersed in a solution of permanganate of potassium; and, judging from the
+changes of colour (though these were not very clearly defined), the
+hypocotyl is about .3 inch in length. Straight, thin, black lines of this
+length were now drawn from the bases of the short petioles along the
+hypocotyls
+
+Fig. 59. Vicia faba: germinating seeds, suspended in damp air: A, with
+radicle growing perpendicularly downwards; B, the same bean after 24 hours
+and after the radicle has curved itself; r. radicle; h, short hypocotyl; e,
+epicotyl appearing as a knob in A and as an arch in B; p, petiole of the
+cotyledon, the latter enclosed within the seed-coats.
+
+of 23 germinating seeds, which were pinned to the lids of jars, generally
+with the hilum downwards, and with their radicles pointing to the centre of
+the earth. After an interval of from 24 to 48 hours the black lines on the
+hypocotyls of 16 out of the 23 seedlings became distinctly curved, but in
+very various degrees (namely, with radii between 20 and
+[page 93]
+80 mm. on Sachs' cyclometer) in the same relative direction as shown at B
+in Fig. 59. As geotropism will obviously tend to check this curvature,
+seven seeds were allowed to germinate with proper precautions for their
+growth in a klinostat,* by which means geotropism was eliminated. The
+position of the hypocotyls was observed during four successive days, and
+they continued to bend towards the hilum and lower surface of the seed. On
+the fourth day they were deflected by an average angle of 63o from a line
+perpendicular to the lower surface, and were therefore considerably more
+curved than the hypocotyl and radicle in the bean at B (Fig. 59), though in
+the same relative direction.
+
+It will, we presume, be admitted that all leguminous plants with hypogean
+cotyledons are descended from forms which once raised their cotyledons
+above the ground in the ordinary manner; and in doing so, it is certain
+that their hypocotyls would have been abruptly arched, as in the case of
+every other dicotyledonous plant. This is especially clear in the case of
+Phaseolus, for out of five species, the seedlings of which we observed,
+namely, P. multiflorus, caracalla, vulgaris, Hernandesii and Roxburghii
+(inhabitants of the Old and New Worlds), the three last-named species have
+well-developed hypocotyls which break through the ground as arches. Now, if
+we imagine a seedling of the common bean or of P. multiflorus, to behave as
+its progenitors once did, the hypocotyl (h, Fig. 59), in whatever position
+the seed may have been buried, would become so much arched that the upper
+part would be doubled down parallel to the lower part; and
+
+* An instrument devised by Sachs, consisting essentially of a slowly
+revolving horizontal axis, on which the plant under observation is
+supported: see 'W�rzburg Arbeiten,' 1879, p. 209.
+[page 94]
+
+this is exactly the kind of curvature which actually occurs in these two
+plants, though to a much less degree. Therefore we can hardly doubt that
+their short hypocotyls have retained by inheritance a tendency to curve
+themselves in the same manner as they did at a former period, when this
+movement was highly important to them for breaking through the ground,
+though now rendered useless by the cotyledons being hypogean. Rudimentary
+structures are in most cases highly variable, and we might expect that
+rudimentary or obsolete actions would be equally so; and Sachs' curvature
+varies extremely in amount, and sometimes altogether fails. This is the
+sole instance known to us of the inheritance, though in a feeble degree, of
+movements which have become superfluous from changes which the species has
+undergone.
+
+Rudimentary Cotyledons.--A few remarks on this subject may be here
+interpolated. It is well known that some dicotyledonous plants produce only
+a single cotyledon; for instance, certain species of Ranunculus, Corydalis,
+Chaerophyllum; and we will here endeavour to show that the loss of one or
+both cotyledons is apparently due to a store of nutriment being laid up in
+some other part, as in the hypocotyl or one of the two cotyledons, or one
+of the secondary radicles.
+
+Fig. 60. Citrus aurantium: two young seedlings: c, larger cotyledon; c',
+smaller cotyledon; h, thickened hypocotyl; r, radicle. In A the epicotyl is
+still arched, in B it has become erect.
+[page 95]
+
+With the orange (Citrus aurantium) the cotyledons are hypogean, and one is
+larger than the other, as may be seen in A (Fig. 60). In B the inequality
+is rather greater, and the stem has grown between the points of insertion
+of the two petioles, so that they do not stand opposite to one another; in
+another case the separation amounted to one-fifth of an inch. The smaller
+cotyledon of one seedling was extremely thin, and not half the length of
+the larger one, so that it was clearly becoming rudimentary,* In all these
+seedlings the hypocotyl was enlarged or swollen.
+
+Fig. 61. Abronia umbellata: seedling twice natural size: c cotyledon; c',
+rudimentary cotyledon; h, enlarged hypocotyl, with a heel or projection
+(h') at the lower end; r, radicle.
+
+With Abronia umbellata one of the cotyledons is quite rudimentary, as may
+be seen (c') in Fig. 61. In this specimen it consisted of a little green
+flap, 1/84th inch in length, destitute of a petiole and covered with glands
+like those on the fully developed cotyledon (c). At first it stood opposite
+to the larger cotyledon; but as the petiole of the latter increased in
+length and grew in the same line with the hypocotyl (h), the rudiment
+appeared in older seedlings as if seated some way down the hypocotyl. With
+Abronia arenaria there is a similar rudiment, which in one
+
+* In Pachira aquatica, as described by Mr. R. I. Lynch ('Journal Linn. Soc.
+Bot.' vol. xvii. 1878, p. 147), one of the hypogean cotyledons is of
+immense size; the other is small and soon falls off; the pair do not always
+stand opposite. In another and very different water-plant, 'Trapa natans',
+one of the cotyledons, filled with farinaceous matter, is much larger than
+the other, which is scarcely visible, as is stated by Aug. de Candolle,
+'Physiologie Veg.' tom. ii. p. 834, 1832.
+[page 96]
+
+specimen was only 1/100th and in another 1/60th inch in length; it
+ultimately appeared as if seated halfway down the hypocotyl. In both these
+species the hypocotyl is so much enlarged, especially at a very early age,
+that it might almost be called a corm. The lower end forms a heel or
+projection, the use of which will hereafter be described.
+
+ In Cyclamen Persicum the hypocotyl, even whilst still within the seed, is
+enlarged into a regular corm,* and only a single cotyledon is at first
+developed (see former Fig. 57). With Ranunculus ficaria two cotyledons are
+never produced, and here one of the secondary radicles is developed at an
+early age into a so-called bulb.** Again, certain species of Chaerophyllum
+and Corydalis produce only a single cotyledon;*** in the former the
+hypocotyl, and in the latter the radicle is enlarged, according to Irmisch,
+into a bulb.
+
+In the several foregoing cases one of the cotyledons is delayed in its
+development, or reduced in size, or rendered rudimentary, or quite aborted;
+but in other cases both cotyledons are represented by mere rudiments. With
+Opuntia basilaris this is not the case, for both cotyledons are thick and
+large, and the hypocotyl shows at first no signs of enlargement; but
+afterwards, when the cotyledons have withered and disarticulated
+themselves, it becomes thickened, and from its tapering form, together with
+its smooth, tough, brown skin, appears, when ultimately drawn down to some
+depth into the soil, like a root. On the other
+
+* Dr. H. Gressner, 'Bot. Zeitung,' 1874, p. 824.
+
+** Irmisch, 'Beitr�ge zur Morphologie der Pflanzen,' 1854, pp. 11, 12;
+'Bot. Zeitung,' 1874, p. 805.
+
+*** Delpino, 'Rivista Botanica,' 1877, p. 21. It is evident from Vaucher's
+account ('Hist. Phys. des Plantes d'Europe,' tom. i. 1841, p. 149) of the
+germination of the seeds of several species of Corydalis, that the bulb or
+tubercule begins to be formed at an extremely early age.
+[page 97]
+
+hand, with several other Cacteae, the hypocotyl is from the first much
+enlarged, and both cotyledons are almost or quite rudimentary. Thus with
+Cereus Landbeckii two little triangular projections, representing the
+cotyledons, are narrower than the hypocotyl, which is pear-shaped, with the
+point downwards. In Rhipsalis cassytha the cotyledons are represented by
+mere points on the enlarged hypocotyl. In Echinocactus viridescens the
+hypocotyl is globular, with two little prominences on its summit. In
+Pilocereus Houlletii the hypocotyl, much swollen in the upper part, is
+merely notched on the summit; and each side of the notch evidently
+represents a cotyledon. Stapelia sarpedon, a member of the very distinct
+family of the Asclepiadeae, is fleshy like a cactus; and here again the
+upper part of the flattened hypocotyl is much thickened and bears two
+minute cotyledons, which, measured internally, were only .15 inch in
+length, and in breadth not equal to one-fourth of the diameter of the
+hypocotyl in its narrow axis; yet these minute cotyledons are probably not
+quite useless, for when the hypocotyl breaks through the ground in the form
+of an arch, they are closed or pressed against one another, and thus
+protect the plumule. They afterwards open.
+
+From the several cases now given, which refer to widely distinct plants, we
+may infer that there is some close connection between the reduced size of
+one or both cotyledons and the formation, by the enlargement of the
+hypocotyl or of the radicle, of a so-called bulb. But it may be asked, did
+the cotyledons first tend to abort, or did a bulb first begin to be formed?
+As all dicotyledons naturally produce two well-developed cotyledons, whilst
+the thickness of the hypocotyl and of the radicle differs much in different
+plants, it seems probable that these latter organs first became from
+[page 98]
+some cause thickened--in several instances apparently in correlation with
+the fleshy nature of the mature plant--so as to contain a store of
+nutriment sufficient for the seedling, and then that one or both
+cotyledons, from being superfluous, decreased in size. It is not surprising
+that one cotyledon alone should sometimes have been thus affected, for with
+certain plants, for instance the cabbage, the cotyledons are at first of
+unequal size, owing apparently to the manner in which they are packed
+within the seed. It does not, however, follow from the above connection,
+that whenever a bulb is formed at an early age, one or both cotyledons will
+necessarily become superfluous, and consequently more or less rudimentary.
+Finally, these cases offer a good illustration of the principle of
+compensation or balancement of growth, or, as Goethe expresses it, "in
+order to spend on one side, Nature is forced to economise on the other
+side."
+
+Circumnutation and other movements of Hypocotyls and Epicotyls, whilst
+still arched and buried beneath the ground, and whilst breaking through
+it.--According to the position in which a seed may chance to have been
+buried, the arched hypocotyl or epicotyl will begin to protrude in a
+horizontal, a more or less inclined, or in a vertical plane. Except when
+already standing vertically upwards, both legs of the arch are acted on
+from the earliest period by apogeotropism. Consequently they both bend
+upwards until the arch becomes vertical. During the whole of this process,
+even before the arch has broken through the ground, it is continually
+trying to circumnutate to a slight extent; as it likewise does if it
+happens at first to stand vertically up,--all which cases have been
+observed and described, more or less fully, in the last chapter. After the
+arch has grown to some
+[page 99]
+height upwards the basal part ceases to circumnutate, whilst the upper part
+continues to do so.
+
+That an arched hypocotyl or epicotyl, with the two legs fixed in the
+ground, should be able to circumnutate, seemed to us, until we had read
+Prof. Wiesner's observations, an inexplicable fact. He has shown* in the
+case of certain seedlings, whose tips are bent downwards (or which nutate),
+that whilst the posterior side of the upper or dependent portion grows
+quickest, the anterior and opposite side of the basal portion of the same
+internode grows quickest; these two portions being separated by an
+indifferent zone, where the growth is equal on all sides. There may be even
+more than one indifferent zone in the same internode; and the opposite
+sides of the parts above and below each such zone grow quickest. This
+peculiar manner of growth is called by Wiesner "undulatory nutation."
+Circumnutation depends on one side of an organ growing quickest (probably
+preceded by increased turgescence), and then another side, generally almost
+the opposite one, growing quickest. Now if we look at an arch like this
+[upside down U] and suppose the whole of one side--we will say the whole
+convex side of both legs--to increase in length, this would not cause the
+arch to bend to either side. But if the outer side or surface of the left
+leg were to increase in length the arch would be pushed over to the right,
+and this would be aided by the inner side of the right leg increasing in
+length. If afterwards the process were reversed, the arch would be pushed
+over to the opposite or left side, and so on alternately,--that is, it
+would circumnutate. As an arched hypo-
+
+* 'Die undulirende Nutation der Internodien,' Akad. der Wissench. (Vienna),
+Jan. 17th, 1878. Also published separately, see p. 32.
+[page 100]
+
+cotyl, with the two legs fixed in the ground, certainly circumnutates, and
+as it consists of a single internode, we may conclude that it grows in the
+manner described by Wiesner. It may be added, that the crown of the arch
+does not grow, or grows very slowly, for it does not increase much in
+breadth, whilst the arch itself increases greatly in height.
+
+The circumnutating movements of arched hypocotyls and epicotyls can hardly
+fail to aid them in breaking through the ground, if this be damp and soft;
+though no doubt their emergence depends mainly on the force exerted by
+their longitudinal growth. Although the arch circumnutates only to a slight
+extent and probably with little force, yet it is able to move the soil near
+the surface, though it may not be able to do so at a moderate depth. A pot
+with seeds of Solanum palinacanthum, the tall arched hypocotyls of which
+had emerged and were growing rather slowly, was covered with fine
+argillaceous sand kept damp, and this at first closely surrounded the bases
+of the arches; but soon a narrow open crack was formed round each of them,
+which could be accounted for only by their having pushed away the sand on
+all sides; for no such cracks surrounded some little sticks and pins which
+had been driven into the sand. It has already been stated that the
+cotyledons of Phalaris and Avena, the plumules of Asparagus and the
+hypocotyls of Brassica, were likewise able to displace the same kind of
+sand, either whilst simply circumnutating or whilst bending towards a
+lateral light.
+
+As long as an arched hypocotyl or epicotyl remains buried beneath the
+ground, the two legs cannot separate from one another, except to a slight
+extent from the yielding of the soil; but as soon as the arch rises above
+the ground, or at an earlier period if
+[page 101]
+the pressure of the surrounding earth be artificially removed, the arch
+immediately begins to straighten itself. This no doubt is due to growth
+along the whole inner surface of both legs of the arch; such growth being
+checked or prevented, as long as the two legs of the arch are firmly
+pressed together. When the earth is removed all round an arch and the two
+legs are tied together at their bases, the growth on the under side of the
+crown causes it after a time to become much flatter and broader than
+naturally occurs. The straightening process consists of a modified form of
+circumnutation, for the lines described during this process (as with the
+hypocotyl of Brassica, and the epicotyls of Vicia and Corylus) were often
+plainly zigzag and sometimes looped. After hypocotyls or epicotyls have
+emerged from the ground, they quickly become perfectly straight. No trace
+is left of their former abrupt curvature, excepting in the case of Allium
+cepa, in which the cotyledon rarely becomes quite straight, owing to the
+protuberance developed on the crown of the arch.
+
+The increased growth along the inner surface of the arch which renders it
+straight, apparently begins in the basal leg or that which is united to the
+radicle; for this leg, as we often observed, is first bowed backwards from
+the other leg. This movement facilitates the withdrawal of the tip of the
+epicotyl or of the cotyledons, as the case may be, from within the
+seed-coats and from the ground. But the cotyledons often emerge from the
+ground still tightly enclosed within the seed-coats, which apparently serve
+to protect them. The seed-coats are afterwards ruptured and cast off by the
+swelling of the closely conjoined cotyledons, and not by any movement or
+their separation from one another.
+
+Nevertheless, in some few cases, especially with the
+[page 102]
+Cucurbitaceae, the seed-coats are ruptured by a curious contrivance,
+described by M. Flahault.* A heel or peg is developed on one side of the
+summit of the radicle or base of the hypocotyl; and this holds down the
+lower half of the seed-coats (the radicle being fixed into the ground)
+whilst the continued growth of the arched hypocotyl forced upwards the
+upper half, and tears asunder the seed-coats at one end, and the cotyledons
+are then easily withdrawn.
+
+Fig. 62. Cucurbita ovifera: germinating seed, showing the heel or peg
+projecting on one side from summit of radicle and holding down lower tip of
+seed-coats, which have been partially ruptured by the growth of the arched
+hypocotyl.
+
+The accompanying figure (Fig. 62) will render this description
+intelligible. Forty-one seeds of Cucurbita ovifera were laid on friable
+peat and were covered by a layer about an inch in thickness, not much
+pressed down, so that the cotyledons in being dragged up were subjected to
+very little friction, yet forty of them came up naked, the seed-coats being
+left buried in the peat. This was certainly due to the action of the peg,
+for when it was prevented from acting, the cotyledons, as we shall
+presently see, were lifted up still enclosed in their seed-coats. They
+were, however, cast off in the course of two or three days by the swelling
+of the cotyledons. Until this occurs light is excluded, and the cotyledons
+cannot decompose carbonic acid; but no one probably would have thought that
+the advantage thus gained by a little earlier cast-
+
+* 'Bull. Soc. Bot. de France,' tom. xxiv. 1877, p. 201.
+[page 103]
+
+ing off of the seed-coats would be sufficient to account for the
+development of the peg. Yet according to M. Flahault, seedlings which have
+been prevented from casting their seed-coats whilst beneath the ground, are
+inferior to those which have emerged with their cotyledons naked and ready
+to act.
+
+The peg is developed with extraordinary rapidity; for it could only just be
+distinguished in two seedlings, having radicles .35 inch in length, but
+after an interval of only 24 hours was well developed in both. It is
+formed, according to Flahault, by the enlargement of the layers of the
+cortical parenchyma at the base of the hypocotyl. If, however, we judge by
+the effects of a solution of permanganate of potassium, it is developed on
+the exact line of junction between the hypocotyl and radicle; for the flat
+lower surface, as well as the edges, were coloured brown like the radicle;
+whilst the upper slightly inclined surface was left uncoloured like the
+hypocotyl, excepting indeed in one out of 33 immersed seedlings in which a
+large part of the upper surface was coloured brown. Secondary roots
+sometimes spring from the lower surface of the peg, which thus seems in all
+respects to partake of the nature of the radicle. The peg is always
+developed on the side which becomes concave by the arching of the
+hypocotyl; and it would be of no service if it were formed on any other
+side. It is also always developed with the flat lower side, which, as just
+stated, forms a part of the radicle, at right angles to it, and in a
+horizontal plane. This fact was clearly shown by burying some of the thin
+flat seeds in the same position as in Fig. 62, excepting that they were not
+laid on their flat broad sides, but with one edge downwards. Nine seeds
+were thus planted, and the peg was developed in the
+[page 104]
+same position, relatively to the radicle, as in the figure; consequently it
+did not rest on the flat tip of the lower half of the seed-coats, but was
+inserted like a wedge between the two tips. As the arched hypocotyl grew
+upwards it tended to draw up the whole seed, and the peg necessarily rubbed
+against both tips, but did not hold either down. The result was, that the
+cotyledons of five out of the nine seeds thus placed were raised above the
+ground still enclosed within their seed-coats. Four seeds were buried with
+the end from which the radicle protrudes pointing vertically downwards, and
+owing to the peg being always developed in the same position, its apex
+alone came into contact with, and rubbed against the tip on one side; the
+result was, that the cotyledons of all four emerged still within their
+seed-coats. These cases show us how the peg acts in co-ordination with the
+position which the flat, thin, broad seeds would almost always occupy when
+naturally sown. When the tip of the lower half of the seed-coats was cut
+off, Flahault found (as we did likewise) that the peg could not act, since
+it had nothing to press on, and the cotyledons were raised above the ground
+with their seed-coats not cast off. Lastly, nature shows us the use of the
+peg; for in the one Cucurbitaceous genus known to us, in which the
+cotyledons are hypogean and do not cast their seed-coats, namely,
+Megarrhiza, there is no vestige of a peg. This structure seems to be
+present in most of the other genera in the family, judging from Flahault's
+statements' we found it well-developed and properly acting in Trichosanthes
+anguina, in which we hardly expected to find it, as the cotyledons are
+somewhat thick and fleshy. Few cases can be advanced of a structure better
+adapted for a special purpose than the present one.
+[page 105]
+
+With Mimosa pudica the radicle protrudes from a small hole in the sharp
+edge of the seed; and on its summit, where united with the hypocotyl, a
+transverse ridge is developed at an early age, which clearly aids in
+splitting the tough seed-coats; but it does not aid in casting them off, as
+this is subsequently effected by the swelling of the cotyledons after they
+have been raised above the ground. The ridge or heel therefore acts rather
+differently from that of Cucurbita. Its lower surface and the edges were
+coloured brown by the permanganate of potassium, but not the upper surface.
+It is a singular fact that after the ridge has done its work and has
+escaped from the seed-coats, it is developed into a frill all round the
+summit of the radicle.*
+
+At the base of the enlarged hypocotyl of Abronia umbellata, where it blends
+into the radicle, there is a projection or heel which varies in shape, but
+its outline is too angular in our former figure (Fig. 61). The radicle
+first protrudes from a small hole at one end of the tough, leathery, winged
+fruit. At this period the upper part of the radicle is packed within the
+fruit parallel to the hypocotyl, and the single cotyledon is doubled back
+parallel to the latter. The swelling of these three parts, and especially
+the rapid development of the thick heel between the hypocotyl and radicle
+at the point where they are doubled, ruptures the tough fruit at the upper
+end and allows the arched hypocotyl to emerge; and this seems to be the
+function of the heel. A seed was cut out of the fruit and
+
+* Our attention was called to this case by a brief statement by Nobbe in
+his 'Handbuch der Samenkunde,' 1876, p. 215, where a figure is also given
+of a seedling of Martynia with a heel or ridge at the junction of the
+radicle and hypocotyl. This seed possesses a very hard and tough coat, and
+would be likely to require aid in bursting and freeing the cotyledons.
+[page 106]
+
+allowed to germinate in damp air, and now a thin flat disc was developed
+all round the base of the hypocotyl and grew to an extraordinary breadth,
+like the frill described under Mimosa, but somewhat broader. Flahault says
+that with Mirabilis, a member of the same family with Abronia, a heel or
+collar is developed all round the base of the hypocotyl, but more on one
+side than on the other; and that it frees the cotyledons from their
+seed-coats. We observed only old seeds, and these were ruptured by the
+absorption of moisture, independently of any aid from the heel and before
+the protrusion of the radicle; but it does not follow from our experience
+that fresh and tough fruits would behave in a like manner.
+
+In concluding this section of the present chapter it may be convenient to
+summarise, under the form of an illustration, the usual movements of the
+hypocotyls and epicotyls of seedlings, whilst breaking through the ground
+and immediately afterwards. We may suppose a man to be thrown down on his
+hands and knees, and at the same time to one side, by a load of hay falling
+on him. He would first endeavour to get his arched back upright, wriggling
+at the same time in all directions to free himself a little from the
+surrounding pressure; and this may represent the combined effects of
+apogeotropism and circumnutation, when a seed is so buried that the arched
+hypocotyl or epicotyl protrudes at first in a horizontal or inclined plane.
+The man, still wriggling, would then raise his arched back as high as he
+could; and this may represent the growth and continued circumnutation of an
+arched hypocotyl or epicotyl, before it has reached the surface of the
+ground. As soon as the man felt himself at all free, he would raise the
+upper part of his body, whilst still on
+[page 107]
+his knees and still wriggling; and this may represent the bowing backwards
+of the basal leg of the arch, which in most cases aids in the withdrawal of
+the cotyledons from the buried and ruptured seed-coats, and the subsequent
+straightening of the whole hypocotyl or epicotyl--circumnutation still
+continuing.
+
+Circumnutation of Hypocotyls and Epicotyls, when erect.--The hypocotyls,
+epicotyls, and first shoots of the many seedlings observed by us, after
+they had become straight and erect, circumnutated continuously. The
+diversified figures described by them, often during two successive days,
+have been shown in the woodcuts in the last chapter. It should be
+recollected that the dots were joined by straight lines, so that the
+figures are angular; but if the observations had been made every few
+minutes the lines would have been more or less curvilinear, and irregular
+ellipses or ovals, or perhaps occasionally circles, would have been formed.
+The direction of the longer axes of the ellipses made during the same day
+or on successive days generally changed completely, so as to stand at right
+angles to one another. The number of irregular ellipses or circles made
+within a given time differs much with different species. Thus with Brassica
+oleracea, Cerinthe major, and Cucurbita ovifera about four such figures
+were completed in 12 h.; whereas with Solanum palinacanthum and Opuntia
+basilaris, scarcely more than one. The figures likewise differ greatly in
+size; thus they were very small and in some degree doubtful in Stapelia,
+and large in Brassica, etc. The ellipses described by Lathyrus nissolia and
+Brassica were narrow, whilst those made by the Oak were broad. The figures
+are often complicated by small loops and zigzag lines.
+
+As most seedling plants before the development of true leaves are of low,
+sometimes very low stature,
+[page 108]
+the extreme amount of movement from side to side of their circumnutating
+stems was small; that of the hypocotyl of Githago segetum was about .2 of
+an inch, and that of Cucurbita ovifera about .28. A very young shoot of
+Lathyrus nissolia moved about .14, that of an American oak .2, that of the
+common nut only .04, and a rather tall shoot of the Asparagus .11 of an
+inch. The extreme amount of movement of the sheath-like cotyledon of
+Phalaris Canariensis was .3 of an inch; but it did not move very quickly,
+the tip crossing on one occasion five divisions of the micrometer, that is,
+1/100th of an inch, in 22 m. 5 s. A seedling Nolana prostrata travelled the
+same distance in 10 m. 38 s. Seedling cabbages circumnutate much more
+quickly, for the tip of a cotyledon crossed 1/100th of an inch on the
+micrometer in 3 m. 20 s.; and this rapid movement, accompanied by incessant
+oscillations, was a wonderful spectacle when beheld under the microscope.
+
+The absence of light, for at least a day, does not interfere in the least
+with the circumnutation of the hypocotyls, epicotyls, or young shoots of
+the various dicotyledonous seedlings observed by us; nor with that of the
+young shoots of some monocotyledons. The circumnutation was indeed much
+plainer in darkness than in light, for if the light was at all lateral the
+stem bent towards it in a more or less zigzag course.
+
+Finally, the hypocotyls of many seedlings are drawn during the winter into
+the ground, or even beneath it so that they disappear. This remarkable
+process, which apparently serves for their protection, has been fully
+described by De Vries.* He shows that
+
+* 'Bot. Zeitung,' 1879, p. 649. See also Winkler in 'Verhandl. des Bot.
+Vereins der P. Brandenburg,' Jahrg. xvi. p. 16, as quoted by Haberlandt,
+'Schutzeinrichungen der Keimpflanze,' 1877, p. 52.
+[page 109]
+
+it is effected by the contraction of the parenchyma-cells of the root. But
+the hypocotyl itself in some cases contracts greatly, and although at first
+smooth becomes covered with zigzag ridges, as we observed with Githago
+segetum. How much of the drawing down and burying of the hypocotyl of
+Opuntia basilaris was due to the contraction of this part and how much to
+that of the radicle, we did not observe.
+
+Circumnutation of Cotyledons.--With all the dicotyledonous seedlings
+described in the last chapter, the cotyledons were in constant movement,
+chiefly in a vertical plane, and commonly once up and once down in the
+course of the 24 hours. But there were many exceptions to such simplicity
+of movement; thus the cotyledons of Ipomoea caerulea moved 13 times either
+upwards or downwards in the course of 16 h.. 18 m. Those of Oxalis rosea
+moved in the same manner 7 times in the course of 24 h.; and those of
+Cassia tora described 5 irregular ellipses in 9 h. The cotyledons of some
+individuals of Mimosa pudica and of Lotus Jacobaeus moved only once up and
+down in 24 h., whilst those of others performed within the same period an
+additional small oscillation. Thus with different species, and with
+different individuals of the same species, there were many gradations from
+a single diurnal movement to oscillations as complex as those of the
+Ipomoea and Cassia. The opposite cotyledons on the same seedling move to a
+certain extent independently of one another. This was conspicuous with
+those of Oxalis sensitiva, in which one cotyledon might be seen during the
+daytime rising up until it stood vertically, whilst the opposite one was
+sinking down.
+
+Although the movements of cotyledons were generally in nearly the same
+vertical plane, yet their upward and downward courses never exactly coin-
+[page 110]
+cided; so that ellipses, more or less narrow, were described, and the
+cotyledons may safely be said to have circumnutated. Nor could this fact be
+accounted for by the mere increase in length of the cotyledons through
+growth, for this by itself would not induce any lateral movement. That
+there was lateral movement in some instances, as with the cotyledons of the
+cabbage, was evident; for these, besides moving up and down, changed their
+course from right to left 12 times in 14 h. 15 m. With Solanum lycopersicum
+the cotyledons, after falling in the forenoon, zigzagged from side to side
+between 12 and 4 P.M., and then commenced rising. The cotyledons of Lupinus
+luteus are so thick (about .08 of an inch) and fleshy,* that they seemed
+little likely to move, and were therefore observed with especial interest;
+they certainly moved largely up and down, and as the line traced was zigzag
+there was some lateral movement. The nine cotyledons of a seedling Pinus
+pinaster plainly circumnutated; and the figures described approached more
+nearly to irregular circles than to irregular ovals or ellipses. The
+sheath-like cotyledons of the Gramineae circumnutate, that is, move to all
+sides, as plainly as do the hypocotyls or epicotyls of any dicotyledonous
+plants. Lastly, the very young fronds of a Fern and of a Selaginella
+circumnutated.
+
+In a large majority of the cases which were carefully observed, the
+cotyledons sink a little downwards in the forenoon, and rise a little in
+the afternoon or evening. They thus stand rather more highly inclined
+during the night than during the mid-day, at which
+
+* The cotyledons, though bright green, resemble to a certain extent
+hypogean ones; see the interesting discussion by Haberlandt ('Die
+Schutzeinrichtungen,' etc., 1877, p. 95), on the gradations in the
+Leguminosae between suba�rial and subterranean cotyledons.
+[page 111]
+
+time they are expanded almost horizontally. The circumnutating movement is
+thus at least partially periodic, no doubt in connection, as we shall
+hereafter see, with the daily alternations of light and darkness. The
+cotyledons of several plants move up so much at night as to stand nearly or
+quite vertically; and in this latter case they come into close contact with
+one another. On the other hand, the cotyledons of a few plants sink almost
+or quite vertically down at night; and in this latter case they clasp the
+upper part of the hypocotyl. In the same genus Oxalis the cotyledons of
+certain species stand vertically up, and those of other species vertically
+down, at night. In all such cases the cotyledons may be said to sleep, for
+they act in the same manner as do the leaves of many sleeping plants. This
+is a movement for a special purpose, and will therefore be considered in a
+future chapter devoted to this subject.
+
+In order to gain some rude notion of the proportional number of cases in
+which the cotyledons of dicotyledonous plants (hypogean ones being of
+course excluded) changed their position in a conspicuous manner at night,
+one or more species in several genera were cursorily observed, besides
+those described in the last chapter. Altogether 153 genera, included in as
+many families as could be procured, were thus observed by us. The
+cotyledons were looked at in the middle of the day and again at night; and
+those were noted as sleeping which stood either vertically or at an angle
+of at least 60o above or beneath the horizon. Of such genera there were 26;
+and in 21 of them the cotyledons of some of the species rose, and in only 6
+sank at night; and some of these latter cases are rather doubtful from
+causes to be explained in the chapter on the sleep of cotyledons. When
+[page 112]
+cotyledons which at noon were nearly horizontal, stood at night at more
+than 20o and less than 60o above the horizon, they were recorded as
+"plainly raised;" and of such genera there were 38. We did not meet with
+any distinct instances of cotyledons periodically sinking only a few
+degrees at night, although no doubt such occur. We have now accounted for
+64 genera out of the 153, and there remain 89 in which the cotyledons did
+not change their position at night by as much as 20o--that is, in a
+conspicuous manner which could easily be detected by the unaided eye and by
+memory; but it must not be inferred from this statement that these
+cotyledons did not move at all, for in several cases a rise of a few
+degrees was recorded, when they were carefully observed. The number 89
+might have been a little increased, for the cotyledons remained almost
+horizontal at night in some species in a few genera, for instance,
+Trifolium and Geranium, which are included amongst the sleepers, such
+genera might therefore have been added to the 89. Again, one species of
+Oxalis generally raised its cotyledons at night more than 20o and less than
+60o above the horizon; so that this genus might have been included under
+two heads. But as several species in the same genus were not often
+observed, such double entries have been avoided.
+
+In a future chapter it will be shown that the leaves of many plants which
+do not sleep, rise a few degrees in the evening and during the early part
+of the night; and it will be convenient to defer until then the
+consideration of the periodicity of the movements of cotyledons.
+
+On the Pulvini or Joints of Cotyledons.--With several of the seedlings
+described in this and the last chapter, the summit of the petiole is
+developed into a pulvinus,
+[page 113]
+cushion, or joint (as this organ has been variously called), like that with
+which many leaves are provided. It consists of a mass of small cells
+usually of a pale colour from the absence of chlorophyll, and with its
+outline more or less convex, as shown in the annexed figure. In the case of
+Oxalis sensitiva two-thirds of the petiole, and in that of Mimosa pudica,
+apparently the whole of the short sub-petioles of the leaflets have been
+converted into pulvini. With pulvinated leaves (i.e. those provided with a
+pulvinus) their periodical movements depend, according to Pfeffer,* on the
+cells of the pulvinus alternately expanding more quickly on one side than
+on the other; whereas the similar movements of leaves not provided with
+pulvini, depend on their growth being alternately more rapid on one side
+than on the other.** As long as a leaf provided with a pulvinus is young
+and continues to grow, its movement depends on both these causes
+combined;*** and if the view now held by many botanists be sound, namely,
+that growth is always preceded by the expansion of the growing cells, then
+the difference between the movements induced by the aid of pulvini and
+
+Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit of
+the petiole of a cotyledon, drawn with the camera lucida, magnified 75
+times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement of blade
+of cotyledon.
+
+* 'Die Periodische Bewegungen der Blattorgane,' 1875.
+
+** Batalin, 'Flora,' Oct. 1st, 1873
+
+*** Pfeffer, ibid. p. 5.
+[page 114]
+
+without such aid, is reduced to the expansion of the cells not being
+followed by growth in the first case, and being so followed in the second
+case.
+
+Dots were made with Indian ink along the midrib of both pulvinated
+cotyledons of a rather old seedling of Oxalis Valdiviana; their distances
+were repeatedly measured with an eye-piece micrometer during 8 3/4 days,
+and they did not exhibit the least trace of increase. It is therefore
+almost certain that the pulvinus itself was not then growing. Nevertheless,
+during this whole time and for ten days afterwards, these cotyledons rose
+vertically every night. In the case of some seedlings raised from seeds
+purchased under the name of Oxalis floribunda, the cotyledons continued for
+a long time to move vertically down at night, and the movement apparently
+depended exclusively on the pulvini, for their petioles were of nearly the
+same length in young, and in old seedlings which had produced true leaves.
+With some species of Cassia, on the other hand, it was obvious without any
+measurement that the pulvinated cotyledons continued to increase greatly in
+length during some weeks; so that here the expansion of the cells of the
+pulvini and the growth of the petiole were probably combined in causing
+their prolonged periodic movements. It was equally evident that the
+cotyledons of many plants, not provided with pulvini, increased rapidly in
+length; and their periodic movements no doubt were exclusively due to
+growth.
+
+In accordance with the view that the periodic movements of all cotyledons
+depend primarily on the expansion of the cells, whether or not followed by
+growth, we can understand the fact that there is but little difference in
+the kind or form of movement in the two sets of cases. This may be seen by
+com-
+[page 115]
+paring the diagrams given in the last chapter. Thus the movements of the
+cotyledons of Brassica oleracea and of Ipomoea caerulea, which are not
+provided with pulvini, are as complex as those of Oxalis and Cassia which
+are thus provided. The pulvinated cotyledons of some individuals of Mimosa
+pudica and Lotus Jacobaeus made only a single oscillation, whilst those of
+other individuals moved twice up and down in the course of 24 hours; so it
+was occasionally with the cotyledons of Cucurbita ovifera, which are
+destitute of a pulvinus. The movements of pulvinated cotyledons are
+generally larger in extent than those without a pulvinus; nevertheless some
+of the latter moved through an angle of 90o. There is, however, one
+important difference in the two sets of cases; the nocturnal movements of
+cotyledons without pulvini, for instance, those in the Cruciferae,
+Cucurbitaceae, Githago, and Beta, never last even for a week, to any
+conspicuous degree. Pulvinated cotyledons, on the other hand, continue to
+rise at night for a much longer period, even for more than a month, as we
+shall now show. But the period no doubt depends largely on the temperature
+to which the seedlings are exposed and their consequent rate of
+development.
+
+[Oxalis Valdiviana.--Some cotyledons which had lately opened and were
+horizontal on March 6th at noon, stood at night vertically up; on the 13th
+the first true leaf was formed, and was embraced at night by the
+cotyledons; on April 9th, after an interval of 35 days, six leaves were
+developed, and yet the cotyledons rose almost vertically at night. The
+cotyledons of another seedling, which when first observed had already
+produced a leaf, stood vertically at night and continued to do so for 11
+additional days. After 16 days from the first observation two leaves were
+developed, and the cotyledons were still greatly raised at night. After 21
+days the cotyledons during the day were deflected beneath the horizon, but
+at night were raised 45o
+[page 116]
+above it. After 24 days from the first observation (begun after a true leaf
+had been developed) the cotyledons ceased to rise at night.
+
+Oxalis (Biophytum) sensitiva.--The cotyledons of several seedlings, 45 days
+after their first expansion, stood nearly vertical at night, and closely
+embraced either one or two true leaves which by this time had been formed.
+These seedlings had been kept in a very warm house, and their development
+had been rapid.
+
+Oxalis corniculata.--The cotyledons do not stand vertical at night, but
+generally rise to an angle of about 45o above the horizon. They continued
+thus to act for 23 days after their first expansion, by which time two
+leaves had been formed; even after 29 days they still rose moderately above
+their horizontal or downwardly deflected diurnal position.
+
+Mimosa pudica.--The cotyledons were expanded for the first time on Nov.
+2nd, and stood vertical at night. On the 15th the first leaf was formed,
+and at night the cotyledons were vertical. On the 28th they behaved in the
+same manner. On Dec. 15th, that is after 44 days, the cotyledons were still
+considerably raised at night; but those of another seedling, only one day
+older, were raised very little.
+
+Mimosa albida.--A seedling was observed during only 12 days, by which time
+a leaf had been formed, and the cotyledons were then quite vertical at
+night.
+
+Trifolium subterraneum.--A seedling, 8 days old, had its cotyledons
+horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an interval of two
+months, by which time the first and second true leaves had been developed,
+the cotyledons still performed the same movement. They had now increased
+greatly in size, and had become oval; and their petioles were actually .8
+of an inch in length!
+
+Trifolium strictum.--After 17 days the cotyledons still rose at night, but
+were not afterwards observed.
+
+Lotus Jacoboeus.--The cotyledons of some seedlings having well-developed
+leaves rose to an angle of about 45o at night; and even after 3 or 4 whorls
+of leaves had been formed, the cotyledons rose at night considerably above
+their diurnal horizontal position.
+
+Cassia mimosoides.--The cotyledons of this Indian species, 14 days after
+their first expansion, and when a leaf had been formed, stood during the
+day horizontal, and at night vertical.
+
+Cassia sp? (a large S. Brazilian tree raised from seeds sent us
+[page 117]
+by F. M�ller).--The cotyledons, after 16 days from their first expansion,
+had increased greatly in size with two leaves just formed. They stood
+horizontally during the day and vertically at night, but were not
+afterwards observed.
+
+Cassia neglecta (likewise a S. Brazilian species).--A seedling, 34 days
+after the first expansion of its cotyledons, was between 3 and 4 inches in
+height, with 3 well-developed leaves; and the cotyledons, which during the
+day were nearly horizontal, at night stood vertical, closely embracing the
+young stem. The cotyledons of another seedling of the same age, 5 inches in
+height, with 4 well-developed leaves, behaved at night in exactly the same
+manner.]
+
+It is known* that there is no difference in structure between the upper and
+lower halves of the pulvini of leaves, sufficient to account for their
+upward or downward movements. In this respect cotyledons offer an unusually
+good opportunity for comparing the structure of the two halves; for the
+cotyledons of Oxalis Valdiviana rise vertically at night, whilst those of
+O. rosea sink vertically; yet when sections of their pulvini were made, no
+clear difference could be detected between the corresponding halves of this
+organ in the two species which move so differently. With O. rosea, however,
+there were rather more cells in the lower than in the upper half, but this
+was likewise the case in one specimen of O. Valdiviana. the cotyledons of
+both species (3 � mm. in length) were examined in the morning whilst
+extended horizontally, and the upper surface of the pulvinus of O. rosea
+was then wrinkled transversely, showing that it was in a state of
+compression, and this might have been expected, as the cotyledons sink at
+night; with O. Valdiviana it was the lower surface which was wrinkled, and
+its cotyledons rise at night.
+
+Trifolium is a natural genus, and the leaves of all
+
+* Pfeffer, 'Die Period. Bewegungen,' 1875, p. 157.
+[page 118]
+
+the species seen by us are pulvinated; so it is with the cotyledons of T.
+subterraneum and strictum, which stand vertically at night; whereas those
+of T. resupinatum exhibit not a trace of a pulvinus, nor of any nocturnal
+movement. This was ascertained by measuring the distance between the tips
+of the cotyledons of four seedlings at mid-day and at night. In this
+species, however, as in the others, the first-formed leaf, which is simple
+or not trifoliate, rises up and sleeps like the terminal leaflet on a
+mature plant.
+
+In another natural genus, Oxalis, the cotyledons of O. Valdiviana, rosea,
+floribunda, articulata, and sensitiva are pulvinated, and all move at night
+into an upward or downward vertical position. In these several species the
+pulvinus is seated close to the blade of the cotyledon, as is the usual
+rule with most plants. Oxalis corniculata (var. Atro-purpurea) differs in
+several respects; the cotyledons rise at night to a very variable amount,
+rarely more than 45o; and in one lot of seedlings (purchased under the name
+of O. tropaeoloides, but certainly belonging to the above variety) they
+rose only from 5o to 15o above the horizon. The pulvinus is developed
+imperfectly and to an extremely variable degree, so that apparently it is
+tending towards abortion. No such case has hitherto, we believe, been
+described. It is coloured green from its cells containing chlorophyll; and
+it is seated nearly in the middle of the petiole, instead of at the upper
+end as in all the other species. The nocturnal movement is effected partly
+by its aid, and partly by the growth of the upper part of the petiole as in
+the case of plants destitute of a pulvinus. From these several reasons and
+from our having partially traced the development of the pulvinus from an
+early age, the case seems worth describing in some detail.
+[page 119]
+
+[When the cotyledons of O. corniculata were dissected out of a seed from
+which they would soon have naturally emerged, no trace of a pulvinus could
+be detected; and all the cells forming the short petiole, 7 in number in a
+longitudinal row, were of nearly equal size. In seedlings one or two days
+old, the pulvinus was so indistinct that we thought at first that it did
+not exist; but in the middle of the petiole an ill-defined transverse zone
+of cells could be seen, which were much shorter than those both above and
+below, although of the same breadth with them. They presented the
+appearance of having been just formed by the transverse division of longer
+cells; and there can be little doubt that this had occurred, for the cells
+in the petiole which had
+
+Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of the
+cotyledons of two rather old seedlings, viewed as transparent objects.
+Magnified 50 times.
+
+been dissected out of the seed averaged in length 7 divisions of the
+micrometer (each division equalling .003 mm.), and were a little longer
+than those forming a well-developed pulvinus, which varied between 4 and 6
+of these same divisions. After a few additional days the ill-defined zone
+of cells becomes distinct, and although it does not extend across the whole
+width of the petiole, and although the cells are of a green colour from
+containing chlorophyll, yet they certainly constitute a pulvinus, which as
+we shall presently see, acts as one. These small cells were arranged in
+longitudinal rows, and varied from 4 to 7 in number; and the cells
+themselves varied in length in different parts of the
+[page 120]
+same pulvinus and in different individuals. In the accompanying figures, A
+and B (Fig. 64), we have views of the epidermis* in the middle part of the
+petioles of two seedlings, in which the pulvinus was for this species well
+developed. They offer a striking contrast with the pulvinus of O. rosea
+(see former Fig. 63), or of O. Valdiviana. With the seedlings, falsely
+called O. tropaeoloides, the cotyledons of which rise very little at night,
+the small cells were still fewer in number and in parts formed a single
+transverse row, and in other parts short longitudinal rows of only two or
+three. Nevertheless they sufficed to attract the eye, when the whole
+petiole was viewed as a transparent object beneath the microscope. In these
+seedlings there could hardly be a doubt that the pulvinus was becoming
+rudimentary and tending to disappear; and this accounts for its great
+variability in structure and function.
+
+In the following Table some measurements of the cells in fairly
+well-developed pulvini of O. corniculata are given:--
+
+Seedling 1 day old, with cotyledon 2.3 mm. in length.
+Divisions of  Micrometer.**
+Average length of cells of
+pulvinus..................................................6 to 7
+Length of longest cell below the
+pulvinus.....................................  13
+Length of longest cell above the
+pulvinus......................................  20
+
+Seedling 5 days old, cotyledon 3.1 mm. in length, with the pulvinus quite
+distinct.
+Average length of cells of
+pulvinus..................................................   6
+Length of longest cell below the
+pulvinus.....................................  22
+Length of longest cell above the
+pulvinus......................................  40
+
+Seedling 8 days old, cotyledon 5 mm. in length, with a true leaf formed but
+not yet expanded.
+Average length of cells of
+pulvinus..................................................   9
+Length of longest cell below the
+pulvinus.....................................  44
+Length of longest cell above the
+pulvinus......................................  70
+
+Seedling 13 days old, cotyledon 4.5 mm. in length, with a small true leaf
+fully developed. Average length of cells of
+pulvinus..................................................   7
+Length of longest cell below the
+pulvinus..................................... 30
+Length of longest cell above the
+pulvinus......................................  60
+
+______________________________________
+
+* Longitudinal sections show that the forms of the epidermic cells may be
+taken as a fair representation of those constituting the pulvinus.
+
+** Each division equalled .003 mm.
+[page 121]
+
+We here see that the cells of the pulvinus increase but little in length
+with advancing age, in comparison with those of the petiole both above and
+below it; but they continue to grow in width, and keep equal in this
+respect with the other cells of the petiole. The rate of growth, however,
+varies in all parts of the cotyledons, as may be observed in the
+measurements of the 8-days' old seedling.
+
+The cotyledons of seedlings only a day old rise at night considerably,
+sometimes as much as afterwards; but there was much variation in this
+respect. As the pulvinus is so indistinct at first, the movement probably
+does not then depend on the expansion of its cells, but on periodically
+unequal growth in the petiole. By the comparison of seedlings of different
+known ages, it was evident that the chief seat of growth of the petiole was
+in the upper part between the pulvinus and the blade; and this agrees with
+the fact (shown in the measurements above given) that the cells grow to a
+greater length in the upper than in the lower part. With a seedling 11 days
+old, the nocturnal rise was found to depend largely on the action of the
+pulvinus, for the petiole at night was curved upwards at this point; and
+during the day, whilst the petiole was horizontal, the lower surface of the
+pulvinus was wrinkled with the upper surface tense. Although the cotyledons
+at an advanced age do not rise at night to a higher inclination than whilst
+young, yet they have to pass through a larger angle (in one instance
+amounting to 63o) to gain their nocturnal position, as they are generally
+deflected beneath the horizon during the day. Even with the 11-days' old
+seedling the movement did not depend exclusively on the pulvinus, for the
+blade where joined to the petiole was curved upwards, and this must be
+attributed to unequal growth. Therefore the periodic movements of the
+cotyledons of 'O. corniculata' depend on two distinct but conjoint actions,
+namely, the expansion of the cells of the pulvinus and on the growth of the
+upper part of the petiole, including the base of the blade.
+
+Lotus Jacoboeus.--The seedlings of this plant present a case parallel to
+that of Oxalis corniculata in some respects, and in others unique, as far
+as we have seen. The cotyledons during the first 4 or 5 days of their life
+do not exhibit any plain nocturnal movement; but afterwards they stand
+vertically or almost vertically up at night. There is, however, some degree
+of variability in this respect, apparently dependent on the season and on
+the degree to which they have been illuminated during
+[page 122]
+the day. With older seedlings, having cotyledons 4 mm. in length, which
+rise considerably at night, there is a well-developed pulvinus close to the
+blade, colourless, and rather narrower than the rest of the petiole, from
+which it is abruptly separated. It is formed of a mass of small cells of an
+average length of .021 mm.; whereas the cells in the lower part of the
+petiole are about .06 mm., and those in the blade from .034 to .04 mm. in
+length. The epidermic cells in the lower part of the petiole project
+conically, and thus differ in shape from those over the pulvinus.
+
+Turning now to very young seedlings, the cotyledons of which do not rise at
+night and are only from 2 to 2 � mm. in length, their petioles do not
+exhibit any defined zone of small cells, destitute of chlorophyll and
+differing in shape exteriorly from the lower ones. Nevertheless, the cells
+at the place where a pulvinus will afterwards be developed are smaller
+(being on an average .015 mm. in length) than those in the lower parts of
+the same petiole, which gradually become larger in proceeding downwards,
+the largest being .030 mm. in length. At this early age the cells of the
+blade are about .027 mm. in length. We thus see that the pulvinus is formed
+by the cells in the uppermost part of the petiole, continuing for only a
+short time to increase in length, then being arrested in their growth,
+accompanied by the loss of their chlorophyll grains; whilst the cells in
+the lower part of the petiole continue for a long time to increase in
+length, those of the epidermis becoming more conical. The singular fact of
+the cotyledons of this plant not sleeping at first is therefore due to the
+pulvinus not being developed at an early age.]
+
+We learn from these two cases of Lotus and Oxalis, that the development of
+a pulvinus follows from the growth of the cells over a small defined space
+of the petiole being almost arrested at an early age. With Lotus Jacobaeus
+the cells at first increase a little in length; in Oxalis corniculata they
+decrease a little, owing to self-division. A mass of such small cells
+forming a pulvinus, might therefore be either acquired or lost without any
+special difficulty, by different species in the same natural genus: and we
+know that
+[page 123]
+with seedlings of Trifolium, Lotus, and Oxalis some of the species have a
+well-developed pulvinus, and others have none, or one in a rudimentary
+condition. As the movements caused by the alternate turgescence of the
+cells in the two halves of a pulvinus, must be largely determined by the
+extensibility and subsequent contraction of their walls, we can perhaps
+understand why a large number of small cells will be more efficient than a
+small number of large cells occupying the same space. As a pulvinus is
+formed by the arrestment of the growth of its cells, movements dependent on
+their action may be long-continued without any increase in length of the
+part thus provided; and such long-continued movements seem to be one chief
+end gained by the development of a pulvinus. Long-continued movement would
+be impossible in any part, without an inordinate increase in its length, if
+the turgescence of the cells was always followed by growth.
+
+Disturbance of the Periodic Movements of Cotyledons by Light.--The
+hypocotyls and cotyledons of most seedling plants are, as is well known,
+extremely heliotropic; but cotyledons, besides being heliotropic, are
+affected paratonically (to use Sachs' expression) by light; that is, their
+daily periodic movements are greatly and quickly disturbed by changes in
+its intensity or by its absence. It is not that they cease to circumnutate
+in darkness, for in all the many cases observed by us they continued to do
+so; but the normal order of their movements in relation to the alternations
+of day and night is much disturbed or quite annulled. This holds good with
+species the cotyledons of which rise or sink so much at night that they may
+be said to sleep, as well as with others which rise only a little. But
+different species are affected in very different degrees by changes in the
+light.
+[page 124]
+
+[For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum,
+Cerinthe major, and Lupinus luteus, when placed in darkness, moved down
+during the afternoon and early night, instead of rising as they would have
+done if they had been exposed to the light. All the individuals of the
+Solanum did not behave in the same manner, for the cotyledons of one
+circumnutated about the same spot between 2.30 and 10 P.M. The cotyledons
+of a seedling of Oxalis corniculata, which was feebly illuminated from
+above, moved downwards during the first morning in the normal manner, but
+on the second morning it moved upwards. The cotyledons of Lotus Jacoboeus
+were not affected by 4 h. of complete darkness, but when placed under a
+double skylight and thus feebly illuminated, they quite lost their
+periodical movements on the third morning. On the other hand, the
+cotyledons of Cucurbita ovifera moved in the normal manner during a whole
+day in darkness.
+
+Seedlings of Githago segetum were feebly illuminated from above in the
+morning before their cotyledons had expanded, and they remained closed for
+the next 40 h. Other seedlings were placed in the dark after their
+cotyledons had opened in the morning and these did not begin to close until
+about 4 h. had elapsed. The cotyledons of Oxalis rosea sank vertically
+downwards after being left for 1 h. 20 m. in darkness; but those of some
+other species of Oxalis were not affected by several hours of darkness. The
+cotyledons of several species of Cassia are eminently susceptible to
+changes in the degree of light to which they are exposed: thus seedlings of
+an unnamed S. Brazilian species (a large and beautiful tree) were brought
+out of the hot-house and placed on a table in the middle of a room with two
+north-east and one north-west window, so that they were fairly well
+illuminated, though of course less so than in the hot-house, the day being
+moderately bright; and after 36 m. the cotyledons which had been horizontal
+rose up vertically and closed together as when asleep; after thus remaining
+on the table for 1 h. 13 m. they began to open. The cotyledons of young
+seedlings of another Brazilian species and of C. neglecta, treated in the
+same manner, behaved similarly, excepting that they did not rise up quite
+so much: they again became horizontal after about an hour.
+
+Here is a more interesting case: seedlings of Cassia tora in two pots,
+which had stood for some time on the table in the room just described, had
+their cotyledons horizontal. One pot was now exposed for 2 h. to dull
+sunshine, and the cotyledons
+[page 125]
+remained horizontal; it was then brought back to the table, and after 50 m.
+the cotyledons had risen 68o above the horizon. The other pot was placed
+during the same 2 h. behind a screen in the room, where the light was very
+obscure, and the cotyledons rose 63o above the horizon; the pot was then
+replaced on the table, and after 50 m. the cotyledons had fallen 33o. These
+two pots with seedlings of the same age stood close together, and were
+exposed to exactly the same amount of light, yet the cotyledons in the one
+pot were rising, whilst those in the other pot were at the same time
+sinking. This fact illustrates in a striking manner that their movements
+are not governed by the actual amount, but by a change in the intensity or
+degree of the light. A similar experiment was tried with two sets of
+seedlings, both exposed to a dull light, but different in degree, and the
+result was the same. The movements of the cotyledons of this Cassia are,
+however, determined (as in many other cases) largely by habit or
+inheritance, independently of light; for seedlings which had been
+moderately illuminated during the day, were kept all night and on the
+following morning in complete darkness; yet the cotyledons were partially
+open in the morning and remained open in the dark for about 6 h. The
+cotyledons in another pot, similarly treated on another occasion, were open
+at 7 A.M. and remained open in the dark for 4 h. 30 m., after which time
+they began to close. Yet these same seedlings, when brought in the middle
+of the day from a moderately bright into only a moderately dull light
+raised, as we have seen, their cotyledons high above the horizon.
+
+Sensitiveness of Cotyledons to contact.--This subject does not possess much
+interest, as it is not known that sensitiveness of this kind is of any
+service to seedling plants. We have observed cases in only four genera,
+though we have vainly observed the cotyledons of many others. The genus
+cassia seems to be pre-eminent in this respect: thus, the cotyledons of C.
+tora, when extended horizontally, were both lightly tapped with a very thin
+twig for 3 m. and in the course of a few minutes they formed together an
+angle of 90o, so that each had risen 45o. A single cotyledon of another
+seedling was tapped in a like manner for 1 m., and it rose 27o in 9 m.; and
+after eight additional minutes it had risen 10o more; the opposite
+cotyledon, which was not tapped, hardly moved at all. The cotyledons in all
+these cases became horizontal again in less than half an hour. The pulvinus
+is the most sensitive part, for on slightly pricking three cotyledons with
+a
+[page 126]
+pin in this part, they rose up vertically; but the blade was found also to
+be sensitive, care having been taken that the pulvinus was not touched.
+Drops of water placed quietly on these cotyledons produced no effect, but
+an extremely fine stream of water, ejected from a syringe, caused them to
+move upwards. When a pot of seedlings was rapidly hit with a stick and thus
+jarred, the cotyledons rose slightly. When a minute drop of nitric acid was
+placed on both pulvini of a seedling, the cotyledons rose so quickly that
+they could easily be seen to move, and almost immediately afterwards they
+began to fall; but the pulvini had been killed and became brown.
+
+The cotyledons of an unnamed species of Cassia (a large tree from S.
+Brazil) rose 31o in the course of 26 m. after the pulvini and the blades
+had both been rubbed during 1 m. with a twig; but when the blade alone was
+similarly rubbed the cotyledons rose only 8o. The remarkably long and
+narrow cotyledons, of a third unnamed species from S. Brazil, did not move
+when their blades were rubbed on six occasions with a pointed stick for 30
+s. or for 1 m.; but when the pulvinus was rubbed and slightly pricked with
+a pin, the cotyledons rose in the course of a few minutes through an angle
+of 60o. Several cotyledons of C. neglecta (likewise from S. Brazil) rose in
+from 5 m. to 15 m. to various angles between 16o and 34o, after being
+rubbed during 1 m. with a twig. Their sensitiveness is retained to a
+somewhat advanced age, for the cotyledons of a little plant of C. neglecta,
+34 days old and bearing three true leaves, rose when lightly pinched
+between the finger and thumb. Some seedlings were exposed for 30 m. to a
+wind (temp. 50o F.) sufficiently strong to keep the cotyledons vibrating,
+but this to our surprise did not cause any movement. The cotyledons of four
+seedlings of the Indian C. glauca were either rubbed with a thin twig for 2
+m. or were lightly pinched: one rose 34o; a second only 6o; a third 13o; and
+a fourth 17o. A cotyledon of C. florida similarly treated rose 9o; one of
+C. corymbosa rose 7 1/2o, and one of the very distinct C. mimosoides only
+6o. Those of C. pubescens did not appear to be in the least sensitive; nor
+were those of C. nodosa, but these latter are rather thick and fleshy, and
+do not rise at night or go to sleep.
+
+Smithia sensitiva.--This plant belongs to a distinct sub-order of the
+Leguminosae from Cassia. Both cotyledons of an oldish seedling, with the
+first true leaf partially unfolded, were rubbed for 1 m. with a fine twig,
+and in 5 m. each rose 32o; they
+[page 127]
+remained in this position for 15 m., but when looked at again 40 m. after
+the rubbing, each had fallen 14o. Both cotyledons of another and younger
+seedling were lightly rubbed in the same manner for 1 m., and after an
+interval of 32 m. each had risen 30o. They were hardly at all sensitive to
+a fine jet of water. The cotyledons of S. Pfundii, an African water plant,
+are thick and fleshy; they are not sensitive and do not go to sleep.
+
+Mimosa pudica and albida.--The blades of several cotyledons of both these
+plants were rubbed or slightly scratched with a needle during 1 m. or 2 m.;
+but they did not move in the least. When, however, the pulvini of six
+cotyledons of M. pudica were thus scratched, two of them were slightly
+raised. In these two cases perhaps the pulvinus was accidentally pricked,
+for on pricking the pulvinus of another cotyledon it rose a little. It thus
+appears that the cotyledons of Mimosa are less sensitive than those of the
+previously mentioned plants.*
+
+Oxalis sensitiva.--The blades and pulvini of two cotyledons, standing
+horizontally, were rubbed or rather tickled for 30 s. with a fine split
+bristle, and in 10 m. each had risen 48o; when looked at again in 35 m.
+after being rubbed they had risen 4o more; after 30 additional minutes they
+were again horizontal. On hitting a pot rapidly with a stick for 1 m., the
+cotyledons of two seedlings were considerably raised in the course of 11 m.
+A pot was carried a little distance on a tray and thus jolted; and the
+cotyledons of four seedlings were all raised in 10 m.; after 17 m. one had
+risen 56o, a second 45o, a third almost 90o, and a fourth 90o. After an
+additional interval of 40 m. three of them had re-expanded to a
+considerable extent. These observations were made before we were aware at
+what an extraordinarily rapid rate the cotyledons circumnutate, and are
+therefore liable to error. Nevertheless it is extremely improbable that the
+cotyledons in the eight cases given, should all have been rising at the
+time when they were irritated. The cotyledons of Oxalis Valdiviana and
+rosea were rubbed and did not exhibit any sensitiveness.]
+
+Finally, there seems to exist some relation between
+
+* The sole notice which we have met with on the sensitiveness of
+cotyledons, relates to Mimosa; for Aug. P. De Candolle says ('Phys. V�g.,'
+1832, tom. ii. p. 865), "les cotyledons du M. pudica tendent � se raprocher
+par leurs faces sup�rieures lorsqu'on les irrite."
+[page 128]
+
+the habit of cotyledons rising vertically at night or going to sleep, and
+their sensitiveness, especially that of their pulvini, to a touch; for all
+the above-named plants sleep at night. On the other hand, there are many
+plants the cotyledons of which sleep, and are not in the least sensitive.
+As the cotyledons of several species of Cassia are easily affected both by
+slightly diminished light and by contact, we thought that these two kinds
+of sensitiveness might be connected; but this is not necessarily the case,
+for the cotyledons of Oxalis sensitiva did not rise when kept on one
+occasion for 1 � h., and on a second occasion for nearly 4 h., in a dark
+closet. Some other cotyledons, as those of Githago segetum, are much
+affected by a feeble light, but do not move when scratched by a needle.
+That with the same plant there is some relation between the sensitiveness
+of its cotyledons and leaves seems highly probable, for the above described
+Smithia and Oxalis have been called sensitiva, owing to their leaves being
+sensitive; and though the leaves of the several species of Cassia are not
+sensitive to a touch, yet if a branch be shaken or syringed with water,
+they partially assume their nocturnal dependent position. But the relation
+between the sensitiveness to contact of the cotyledons and of the leaves of
+the same plant is not very close, as may be inferred from the cotyledons of
+Mimosa pudica being only slightly sensitive, whilst the leaves are well
+known to be so in the highest degree. Again, the leaves of Neptunia
+oleracea are very sensitive to a touch, whilst the cotyledons do not appear
+to be so in any degree.
+[page 129]
+
+
+
+
+
+CHAPTER III.
+
+SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.
+
+Manner in which radicles bend when they encounter an obstacle in the soil--
+Vicia faba, tips of radicles highly sensitive to contact and other
+irritants--Effects of too high a temperature--Power of discriminating
+between objects attached on opposite sides--Tips of secondary radicles
+sensitive--Pisum, tips of radicles sensitive--Effects of such sensitiveness
+in overcoming geotropism--Secondary radicles--Phaseolus, tips of radicles
+hardly sensitive to contact, but highly sensitive to caustic and to the
+removal of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus,
+tip not sensitive to slight contact, highly sensitive to caustic--Quercus,
+tip highly sensitive to contact--Power of discrimination--Zea, tip highly
+sensitive, secondary radicles--Sensitiveness of radicles to moist air--
+Summary of chapter.
+
+IN order to see how the radicles of seedlings would pass over stones,
+roots, and other obstacles, which they must incessantly encounter in the
+soil, germinating beans (Vicia faba) were so placed that the tips of the
+radicles came into contact, almost rectangularly or at a high angle, with
+underlying plates of glass. In other cases the beans were turned about
+whilst their radicles were growing, so that they descended nearly
+vertically on their own smooth, almost flat, broad upper surfaces. The
+delicate root-cap, when it first touched any directly opposing surface, was
+a little flattened transversely; the flattening soon became oblique, and in
+a few hours quite disappeared, the apex now pointing at right angles, or at
+nearly right angles, to its former course. The radicle then seemed to glide
+in its new direction over the surface which had opposed
+[page 130]
+it, pressing on it with very little force. How far such abrupt changes in
+its former course are aided by the circumnutation of the tip must be left
+doubtful. Thin slips of wood were cemented on more or less steeply inclined
+glass-plates, at right angles to the radicles which were gliding down them.
+Straight lines had been painted along the growing terminal part of some of
+these radicles, before they met the opposing slip of wood; and the lines
+became sensibly curved in 2 h. after the apex had come into contact with
+the slips. In one case of a radicle, which was growing rather slowly, the
+root-cap, after encountering a rough slip of wood at right angles, was at
+first slightly flattened transversely: after an interval of 2 h. 30 m. the
+flattening became oblique; and after an additional 3 hours the flattening
+had wholly disappeared, and the apex now pointed at right angles to its
+former course. It then continued to grow in its new direction alongside the
+slip of wood, until it came to the end of it, round which it bent
+rectangularly. Soon afterwards when coming to the edge of the plate of
+glass, it was again bent at a large angle, and descended perpendicularly
+into the damp sand.
+
+When, as in the above cases, radicles encountered an obstacle at right
+angles to their course, the terminal growing part became curved for a
+length of between .3 and .4 of an inch (8-10 mm.), measured from the apex.
+This was well shown by the black lines which had been previously painted on
+them. The first and most obvious explanation of the curvature is, that it
+results merely from the mechanical resistance to the growth of the radicle
+in its original direction. Nevertheless, this explanation did not seem to
+us satisfactory. The radicles did not present the appearance of having been
+subjected to a sufficient pressure to account for
+[page 131]
+their curvature; and Sachs has shown* that the growing part is more rigid
+than the part immediately above which has ceased to grow, so that the
+latter might have been expected to yield and become curved as soon as the
+apex encountered an unyielding object; whereas it was the stiff growing
+part which became curved. Moreover, an object which yields with the
+greatest ease will deflect a radicle: thus, as we have seen, when the apex
+of the radicle of the bean encountered the polished surface of extremely
+thin tin-foil laid on soft sand, no impression was left on it, yet the
+radicle became deflected at right angles. A second explanation occurred to
+us, namely, that even the gentlest pressure might check the growth of the
+apex, and in this case growth could continue only on one side, and thus the
+radicle would assume a rectangular form; but this view leaves wholly
+unexplained the curvature of the upper part, extending for a length of 8-10
+mm.
+
+We were therefore led to suspect that the apex was sensitive to contact,
+and that an effect was transmitted from it to the upper part of the
+radicle, which was thus excited to bend away from the touching object. As a
+little loop of fine thread hung on a tendril or on the petiole of a
+leaf-climbing plant, causes it to bend, we thought that any small hard
+object affixed to the tip of a radicle, freely suspended and growing in
+damp air, might cause it to bend, if it were sensitive, and yet would not
+offer any mechanical resistance to its growth. Full details will be given
+of the experiments which were tried, as the result proved remarkable. The
+fact of the apex of a radicle being sensitive to contact has never been
+observed, though, as we shall
+
+* 'Arbeiten Bot. Inst. W�rzburg,' Heft iii. 1873, p. 398.
+
+[page 132]
+hereafter see, Sachs discovered that the radicle a little above the apex is
+sensitive, and bends like a tendril towards the touching object. But when
+one side of the apex is pressed by any object, the growing part bends away
+from the object; and this seems a beautiful adaptation for avoiding
+obstacles in the soil, and, as we shall see, for following the lines of
+least resistance. Many organs, when touched, bend in one fixed direction,
+such as the stamens of Berberis, the lobes of Dionaea, etc.; and many
+organs, such as tendrils, whether modified leaves or flower-peduncles, and
+some few stems, bend towards a touching object; but no case, we believe, is
+known of an organ bending away from a touching object.
+
+Sensitiveness of the Apex of the Radicle of Vicia faba.--Common beans,
+after being soaked in water for 24 h., were pinned with the hilum downwards
+(in the manner followed by Sachs), inside the cork lids of glass-vessels,
+which were half filled with water; the sides and the cork were well
+moistened, and light was excluded. As soon as the beans had protruded
+radicles, some to a length of less than a tenth of an inch, and others to a
+length of several tenths, little squares or oblongs of card were affixed to
+the short sloping sides of their conical tips. The squares therefore
+adhered obliquely with reference to the longitudinal axis of the radicle;
+and this is a very necessary precaution, for if the bits of card
+accidentally became displaced, or were drawn by the viscid matter employed
+so as to adhere parallel to the side of the radicle, although only a little
+way above the conical apex, the radicle did not bend in the peculiar manner
+which we are here considering. Squares of about the 1/20th of an inch (i.e.
+about 1 � mm.), or oblong bits of nearly the same size, were found to
+[page 133]
+be the most convenient and effective. We employed at first ordinary thin
+card, such as visiting cards, or bits of very thin glass, and various other
+objects; but afterwards sand-paper was chiefly employed, for it was almost
+as stiff as thin card, and the roughened surface favoured its adhesion. At
+first we generally used very thick gum-water; and this of course, under the
+circumstances, never dried in the least; on the contrary, it sometimes
+seemed to absorb vapour, so that the bits of card became separated by a
+layer of fluid from the tip. When there was no such absorption and the card
+was not displaced, it acted well and caused the radicle to bend to the
+opposite side. I should state that thick gum-water by itself induces no
+action. In most cases the bits of card were touched with an extremely small
+quantity of a solution of shellac in spirits of wine, which had been left
+to evaporate until it was thick; it then set hard in a few seconds, and
+fixed the bits of card well. When small drops of the shellac were placed on
+the tips without any card, they set into hard little beads, and these acted
+like any other hard object, causing the radicles to bend to the opposite
+side. Even extremely minute beads of the shellac occasionally acted in a
+slight degree, as will hereafter be described. But that it was the cards
+which chiefly acted in our many trials, was proved by coating one side of
+the tip with a little bit of goldbeaters' skin (which by itself hardly
+acts), and then fixing a bit of card to the skin with shellac which never
+came into contact with the radicle: nevertheless the radicle bent away from
+the attached card in the ordinary manner.
+
+Some preliminary trials were made, presently to be described, by which the
+proper temperature was determined, and then the following experiments were
+made. It should be premised that the beans were
+[page 134]
+always fixed to the cork-lids, for the convenience of manipulation, with
+the edge from which the radicle and plumule protrudes, outwards; and it
+must be remembered that owing to what we have called Sachs' curvature, the
+radicles, instead of growing perpendicularly downwards, often bend
+somewhat, even as much
+
+Fig. 65. Vicia faba: A, radicle beginning to bend from the attached little
+square of card; B, bent at a rectangle; C, bent into a circle or loop, with
+the tip beginning to bend downwards through the action of geotropism.
+
+as about 45o inwards, or under the suspended bean. Therefore when a square
+of card was fixed to the apex in front, the bowing induced by it coincided
+with Sachs' curvature, and could be distinguished from it only by being
+more strongly pronounced or by occurring more quickly. To avoid this source
+of doubt, the squares
+[page 135]
+were fixed either behind, causing a curvature in direct opposition to that
+of Sachs', or more commonly to the right or left sides. For the sake of
+brevity, we will speak of the bits of card, etc., as fixed in front, or
+behind, or laterally. As the chief curvature of the radicle is at a little
+distance from the apex, and as the extreme terminal and basal portions are
+nearly straight, it is possible to estimate in a rough manner the amount of
+curvature by an angle; and when it is said that the radicle became
+deflected at any angle from the perpendicular, this implies that the apex
+was turned upwards by so many degrees from the downward direction which it
+would naturally have followed, and to the side opposite to that to which
+the card was affixed. That the reader may have a clear idea of the kind of
+movement excited by the bits of attached card, we append here accurate
+sketches of three germinating beans thus treated, and selected out of
+several specimens to show the gradations in the degrees of curvature. We
+will now give in detail a series of experiments, and afterwards a summary
+of the results.
+
+[In the first 12 trials, little squares or oblongs of sanded card, 1.8 mm.
+in length, and 1.5 or only 0.9 mm. in breadth (i.e. .071 of an inch in
+length and .059 or .035 of an inch in breadth) were fixed with shellac to
+the tips of the radicles. In the subsequent trials the little squares were
+only occasionally measured, but were of about the same size.
+
+(1.) A young radicle, 4 mm. in length, had a card fixed behind: after 9 h.
+deflected in the plane in which the bean is flattened, 50o from the
+perpendicular and from the card, and in opposition to Sachs' curvature: no
+change next morning, 23 h. from the time of attachment.
+
+(2.) Radicle 5.5 mm. in length, card fixed behind: after 9 h. deflected in
+the plane of the bean 20o from the perpendicular and from the card, and in
+opposition to Sachs' curvature: after 23 h. no change.
+[page 136]
+
+(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected in
+the plane of the bean 40o from the perpendicular and from the card, and in
+opposition to Sachs' curvature. The tip of the radicle more curved than the
+upper part, but in the same plane. After 23 h. the extreme tip was slightly
+bent towards the card; the general curvature of the radicle remaining the
+same.
+
+(4.) Radicle 9 mm. long, card fixed behind and a little laterally: after 9
+h. deflected in the plane of the bean only about 7o or 8o from the
+perpendicular and from the card, in opposition to Sachs' curvature. There
+was in addition a slight lateral curvature directed partly from the card.
+After 23 h. no change.
+
+(5.) Radicle 8 mm. long, card affixed almost laterally: after 9 h.
+deflected 30o from the perpendicular, in the plane of the bean and in
+opposition to Sachs' curvature; also deflected in a plane at right angles
+to the above one, 20o from the perpendicular: after 23 h. no change.
+
+(6.) Radicle 9 mm. long, card affixed in front: after 9 h. deflected in the
+plane of the bean about 40o from the vertical, away from the card and in
+the direction of Sachs' curvature. Here therefore we have no evidence of
+the card being the cause of the deflection, except that a radicle never
+moves spontaneously, as far as we have seen, as much as 40o in the course
+of 9 h. After 23 h. no change.
+
+(7.) Radicle 7 mm. long, card affixed to the back: after 9 h. the terminal
+part of the radicle deflected in the plane of the bean 20o from the
+vertical, away from the card and in opposition to Sachs' curvature. After
+22 h. 30 m. this part of the radicle had become straight.
+
+(8.) Radicle 12 mm. long, card affixed almost laterally: after 9 h.
+deflected laterally in a plane at right angles to that of the bean between
+40o and 50o from the vertical and from the card. In the plane of the bean
+itself the deflection amounted to 8o or 9o from the vertical and from the
+card, in opposition to Sachs' curvature. After 22 h. 30 m. the extreme tip
+had become slightly curved towards the card.
+
+(9.) Card fixed laterally: after 11 h. 30 m. no effect, the radicle being
+still almost vertical.
+
+(10.) Card fixed almost laterally: after 11 h. 30 m. deflected 90o from the
+vertical and from the card, in a plane intermediate between that of the
+bean itself and one at right
+[page 137]
+angles to it. Radicle consequently partially deflected from Sachs'
+curvature.
+
+(11.) Tip of radicle protected with goldbeaters' skin, with a square of
+card of the usual dimensions affixed with shellac: after 11 h. greatly
+deflected in the plane of the bean, in the direction of Sachs' curvature,
+but to a much greater degree and in less time than ever occurs
+spontaneously.
+
+(12.) Tip of radicle protected as in last case: after 11 h. no effect, but
+after 24 h. 40 m. radicle clearly deflected from the card. This slow action
+was probably due to a portion of the goldbeaters' skin having curled round
+and lightly touched the opposite side of the tip and thus irritated it.
+
+(13.) A radicle of considerable length had a small square of card fixed
+with shellac to its apex laterally: after only 7 h. 15 m. a length of .4 of
+an inch from the apex, measured along the middle, was considerably curved
+from the side bearing the card.
+
+(14.) Case like the last in all respects, except that a length of only .25
+of an inch of the radicle was thus deflected.
+
+(15.) A small square of card fixed with shellac to the apex of a young
+radicle; after 9 h. 15 m. deflected through 90o from the perpendicular and
+from the card. After 24 h. deflection much decreased, and after an
+additional day, reduced to 23o from the perpendicular.
+
+(16.) Square of card fixed with shellac behind the apex of a radicle, which
+from its position having been changed during growth had become very
+crooked; but the terminal portion was straight, and this became deflected
+to about 45o from the perpendicular and from the card, in opposition to
+Sachs' curvature.
+
+(17.) Square of card affixed with shellac: after 8 h. radicle curved at
+right angles from the perpendicular and from the card. After 15 additional
+hours curvature much decreased.
+
+(18.) Square of card affixed with shellac: after 8 h. no effect; after 23
+h. 3 m. from time of affixing, radicle much curved from the square. (19.)
+Square of card affixed with shellac: after 24 h. no effect, but the radicle
+had not grown well and seemed sickly.
+
+(20.) Square of card affixed with shellac: after 24 h. no effect.
+
+(21, 22.) Squares of card affixed with shellac: after 24 h. radicles of
+both curved at about 45o from the perpendicular and from the cards.
+
+(23.) Square of card fixed with shellac to young radicle: after
+[page 138]
+9 h. very slightly curved from the card; after 24 h. tip curved towards
+card. Refixed new square laterally, after 9 h. distinctly curved from the
+card, and after 24 h. curved at right angles from the perpendicular and
+from the card.
+
+(24.) A rather large oblong piece of card fixed with shellac to apex: after
+24 h. no effect, but the card was found not to be touching the apex. A
+small square was now refixed with shellac; after 16 h. slight deflection
+from the perpendicular and from the card. After an additional day the
+radicle became almost straight.
+
+(25.) Square of card fixed laterally to apex of young radicle; after 9 h.
+deflection from the perpendicular considerable; after 24 h. deflection
+reduced. Refixed a fresh square with shellac: after 24 h. deflection about
+40o from the perpendicular and from the card.
+
+(26.) A very small square of card fixed with shellac to apex of young
+radicle: after 9 h. the deflection from the perpendicular and from the card
+amounted to nearly a right angle; after 24 h. deflection much reduced;
+after an additional 24 h. radicle almost straight.
+
+(27.) Square of card fixed with shellac to apex of young radicle: after 9
+h. deflection from the card and from the perpendicular a right angle; next
+morning quite straight. Refixed a square laterally with shellac; after 9 h.
+a little deflection, which after 24 h. increased to nearly 20o from the
+perpendicular and from the card.
+
+(28.) Square of card fixed with shellac; after 9 h. some deflection; next
+morning the card dropped off; refixed it with shellac; it again became
+loose and was refixed; and now on the third trial the radicle was deflected
+after 14 h. at right angles from the card.
+
+(29.) A small square of card was first fixed with thick gum-water to the
+apex. It produced a slight effect but soon fell off. A similar square was
+now affixed laterally with shellac: after 9 h. the radicle was deflected
+nearly 45o from the perpendicular and from the card. After 36 additional
+hours angle of deflection reduced to about 30o.
+
+(30.) A very small piece, less than 1/20th of an inch square, of thin
+tin-foil fixed with shellac to the apex of a young radicle; after 24 h. no
+effect. Tin-foil removed, and a small square of sanded card fixed with
+shellac; after 9 h. deflection at nearly right angles from the
+perpendicular and from the card. Next
+[page 139]
+morning deflection reduced to about 40o from the perpendicular.
+
+(31.) A splinter of thin glass gummed to apex, after 9 h. no effect, but it
+was then found not to be touching the apex of the radicle. Next morning a
+square of card was fixed with shellac to it, and after 9 h. radicle greatly
+deflected from the card. After two additional days the deflection had
+decreased and was only 35o from the perpendicular.
+
+(32.) Small square of sanded card, attached with thick gum-water laterally
+to the apex of a long straight radicle: after 9 h. greatly deflected from
+the perpendicular and from the card. Curvature extended for a length of .22
+of an inch from the apex. After 3 additional hours terminal portion
+deflected at right angles from the perpendicular. Next morning the curved
+portion was .36 in length.
+
+(33.) Square of card gummed to apex: after 15 h. deflected at nearly 90o
+from the perpendicular and from the card.
+
+(34.) Small oblong of sanded card gummed to apex: after 15 h. deflected 90o
+from the perpendicular and from the card: in the course of the three
+following days the terminal portion became much contorted and ultimately
+coiled into a helix.
+
+(35.) Square of card gummed to apex: after 9 h. deflected from card: after
+24 h. from time of attachment greatly deflected obliquely and partly in
+opposition to Sachs' curvature.
+
+(36.) Small piece of card, rather less than 1/20th of an inch square,
+gummed to apex: in 9 h. considerably deflected from card and in opposition
+to Sachs' curvature; after 24 h. greatly deflected in the same direction.
+After an additional day the extreme tip was curved towards the card.
+
+(37.) Square of card, gummed to apex in front, caused after 8 h. 30 m.
+hardly any effect; refixed fresh square laterally, after 15 h. deflected
+almost 90o from the perpendicular and from the card. After 2 additional
+days deflection much reduced.
+
+(38.) Square of card gummed to apex: after 9 h. much deflection, which
+after 24 h. from time of fixing increased to nearly 90o. After an
+additional day terminal portion was curled into a loop, and on the
+following day into a helix.
+
+(39.) Small oblong piece of card gummed to apex, nearly in front, but a
+little to one side; in 9 h. slightly deflected in the direction of Sachs'
+curvature, but rather obliquely, and to side opposite to card. Next day
+more curved in the same direction, and after 2 additional days coiled into
+a ring.
+[page 140]
+
+(40.) Square of card gummed to apex: after 9 h. slightly curved from card;
+next morning radicle straight, and apex had grown beyond the card. Refixed
+another square laterally with shellac; in 9 h. deflected laterally, but
+also in the direction of Sachs' curvature. After 2 additional days'
+curvature considerably increased in the same direction.
+
+(41.) Little square of tin-foil fixed with gum to one side of apex of a
+young and short radicle: after 15 h. no effect, but tin-foil had become
+displaced. A little square of card was now gummed to one side of apex,
+which after 8 h. 40 m. was slightly deflected; in 24 h. from the time of
+attachment deflected at 90o from the perpendicular and from the card; after
+9 additional hours became hooked, with the apex pointing to the zenith. In
+3 days from the time of attachment the terminal portion of the radicle
+formed a ring or circle.
+
+(42.) A little square of thick letter-paper gummed to the apex of a
+radicle, which after 9 h. was deflected from it. In 24 h. from time when
+the paper was affixed the deflection much increased, and after 2 additional
+days it amounted to 50o from the perpendicular and from the paper.
+
+(43.) A narrow chip of a quill was fixed with shellac to the apex of a
+radicle. After 9 h. no effect; after 24 h. moderate deflection, but now the
+quill had ceased to touch the apex. Removed quill and gummed a little
+square of card to apex, which after 8 h. caused slight deflection. On the
+fourth day from the first attachment of any object, the extreme tip was
+curved towards the card.
+
+(44.) A rather long and narrow splinter of extremely thin glass, fixed with
+shellac to apex, it caused in 9 h. slight deflection, which disappeared in
+24 h.; the splinter was then found not touching the apex. It was twice
+refixed, with nearly similar results, that is, it caused slight deflection,
+which soon disappeared. On the fourth day from the time of first attachment
+the tip was bent towards the splinter.]
+
+From these experiments it is clear that the apex of the radicle of the bean
+is sensitive to contact, and that it causes the upper part to bend away
+from the touching object. But before giving a summary of the results, it
+will be convenient briefly to give a few other observations. Bits of very
+thin glass and little squares
+[page 141]
+of common card were affixed with thick gum-water to the tips of the
+radicles of seven beans, as a preliminary trial. Six of these were plainly
+acted on, and in two cases the radicles became coiled up into complete
+loops. One radicle was curved into a semi-circle in so short a period as 6
+h. 10 m. The seventh radicle which was not affected was apparently sickly,
+as it became brown on the following day; so that it formed no real
+exception. Some of these trials were made in the early spring during cold
+weather in a sitting-room, and others in a greenhouse, but the temperature
+was not recorded. These six striking cases almost convinced us that the
+apex was sensitive, but of course we determined to make many more trials.
+As we had noticed that the radicles grew much more quickly when subjected
+to considerable heat, and as we imagined that heat would increase their
+sensitiveness, vessels with germinating beans suspended in damp air were
+placed on a chimney-piece, where they were subjected during the greater
+part of the day to a temperature of between 69o and 72o F.; some, however,
+were placed in the hot-house where the temperature was rather higher. Above
+two dozen beans were thus tried; and when a square of glass or card did not
+act, it was removed, and a fresh one affixed, this being often done thrice
+to the same radicle. Therefore between five and six dozen trials were
+altogether made. But there was moderately distinct deflection from the
+perpendicular and from the attached object in only one radicle out of this
+large number of cases. In five other cases there was very slight and
+doubtful deflection. We were astonished at this result, and concluded that
+we had made some inexplicable mistake in the first six experiments. But
+before finally relinquishing the subject, we resolved to make one
+[page 142]
+other trial for it occurred to us that sensitiveness is easily affected by
+external conditions, and that radicles growing naturally in the earth in
+the early spring would not be subjected to a temperature nearly so high as
+70o F. We therefore allowed the radicles of 12 beans to grow at a
+temperature of between 55o and 60o F. The result was that in every one of
+these cases (included in the above-described experiments) the radicle was
+deflected in the course of a few hours from the attached object. All the
+above recorded successful trials, and some others presently to be given,
+were made in a sitting-room at the temperatures just specified. It
+therefore appears that a temperature of about, or rather above, 70o F.
+destroys the sensitiveness of the radicles, either directly, or indirectly
+through abnormally accelerated growth; and this curious fact probably
+explains why Sachs, who expressly states that his beans were kept at a high
+temperature, failed to detect the sensitiveness of the apex of the radicle.
+
+But other causes interfere with this sensibility. Eighteen radicles were
+tried with little squares of sanded card, some affixed with shellac and
+some with gum-water, during the few last days of 1878, and few first days
+of the next year. They were kept in a room at the proper temperature during
+the day, but were probably too cold at night, as there was a hard frost at
+the time. The radicles looked healthy but grew very slowly. The result was
+that only 6 out of the 18 were deflected from the attached cards, and this
+only to a slight degree and at a very slow rate. These radicles therefore
+presented a striking contrast with the 44 above described. On March 6th and
+7th, when the temperature of the room varied between 53o and 59o F., eleven
+germinating beans were tried in the
+[page 143]
+same manner, and now every one of the radicles became curved away from the
+cards, though one was only slightly deflected. Some horticulturists believe
+that certain kinds of seeds will not germinate properly in the middle of
+the winter, although kept at a right temperature. If there really is any
+proper period for the germination of the bean, the feeble degree of
+sensibility of the above radicles may have resulted from the trial having
+been made in the middle of the winter, and not simply from the nights being
+too cold. Lastly, the radicles of four beans, which from some innate cause
+germinated later than all the others of the same lot, and which grew slowly
+though appearing healthy, were similarly tried, and even after 24 h. they
+were hardly at all deflected from the attached cards. We may therefore
+infer that any cause which renders the growth of the radicles either slower
+or more rapid than the normal rate, lessens or annuls the sensibility of
+their tips to contact. It deserves particular attention that when the
+attached objects failed to act, there was no bending of any kind, excepting
+Sachs' curvature. The force of our evidence would have been greatly
+weakened if occasionally, though rarely, the radicles had become curved in
+any direction independently of the attached objects. In the foregoing
+numbered paragraphs, however, it may be observed that the extreme tip
+sometimes becomes, after a considerable interval of time, abruptly curved
+towards the bit of card; but this is a totally distinct phenomenon, as will
+presently be explained.
+
+Summary of the Results of the foregoing Experiments on the Radicles of
+Vicia faba.--Altogether little squares (about 1/20th of an inch), generally
+of sanded paper as stiff as thin card (between .15 and .20 mm. in
+thickness), sometimes of ordinary card, or little frag-
+[page 144]
+ments of very thin glass etc., were affixed at different times to one side
+of the conical tips of 55 radicles. The 11 last-mentioned cases, but not
+the preliminary ones, are here included. The squares, etc., were most
+commonly affixed with shellac, but in 19 cases with thick gum-water. When
+the latter was used, the squares were sometimes found, as previously
+stated, to be separated from the apex by a layer of thick fluid, so that
+there was no contact, and consequently no bending of the radicle; and such
+few cases were not recorded. But in every instance in which shellac was
+employed, unless the square fell off very soon, the result was recorded. In
+several instances when the squares became displaced, so as to stand
+parallel to the radicle, or were separated by fluid from the apex, or soon
+fell off, fresh squares were attached, and these cases (described under the
+numbered paragraphs) are here included. Out of 55 radicles experimented on
+under the proper temperature, 52 became bent, generally to a considerable
+extent from the perpendicular, and away from the side to which the object
+was attached. Of the three failures, one can be accounted for, as the
+radicle became sickly on the following day; and a second was observed only
+during 11 h. 30 m. As in several cases the terminal growing part of the
+radicle continued for some time to bend from the attached object, it formed
+itself into a hook, with the apex pointing to the zenith, or even into a
+ring, and occasionally into a spire or helix. It is remarkable that these
+latter cases occurred more frequently when objects were attached with thick
+gum-water, which never became dry, than when shellac was employed. The
+curvature was often well-marked in from 7 h. to 11 h.; and in one instance
+a semicircle was formed in 6 h. 10 m, from the time
+[page 145]
+of attachment. But in order to see the phenomenon as well displayed as in
+the above described cases, it is indispensable that the bits of card, etc.,
+should be made to adhere closely to one side of the conical apex; that
+healthy radicles should be selected and kept at not too high or too low a
+temperature, and apparently that the trials should not be made in the
+middle of the winter.
+
+In ten instances, radicles which had curved away from a square of card or
+other object attached to their tips, straightened themselves to a certain
+extent, or even completely, in the course of from one to two days from the
+time of attachment. This was more especially apt to occur when the
+curvature was slight. But in one instance (No. 27) a radicle which in 9 h.
+had been deflected about 90o from the perpendicular, became quite straight
+in 24 h. from the period of attachment. With No. 26, the radicle was almost
+straight in 48 h. We at first attributed the straightening process to the
+radicles becoming accustomed to a slight stimulus, in the same manner as a
+tendril or sensitive petiole becomes accustomed to a very light loop of
+thread, and unbends itself though the loop remains still suspended; but
+Sachs states* that radicles of the bean placed horizontally in damp air
+after curving downwards through geotropism, straighten themselves a little
+by growth along their lower or concave sides. Why this should occur is not
+clear: but perhaps it likewise occurred in the above ten cases. There is
+another occasional movement which must not be passed over: the tip of the
+radicle, for a length of from 2 to 3 mm., was found in six instances,
+
+* 'Arbeiten Bot. Instit., W�rzburg,' Heft iii. p. 456.
+[page 146]
+
+after an interval of about 24 or more hours, bent towards the bit of still
+attached card,--that is, in a direction exactly opposite to the previously
+induced curvature of the whole growing part for a length of from 7 to 8 mm.
+This occurred chiefly when the first curvature was small, and when an
+object had been affixed more than once to the apex of the same radicle. The
+attachment of a bit of card by shellac to one side of the tender apex may
+sometimes mechanically prevent its growth; or the application of thick
+gum-water more than once to the same side may injure it; and then checked
+growth on this side with continued growth on the opposite and unaffected
+side would account for the reversed curvature of the apex.
+
+Various trials were made for ascertaining, as far as we could, the nature
+and degree of irritation to which the apex must be subjected, in order that
+the terminal growing part should bend away, as if to avoid the cause of
+irritation. We have seen in the numbered experiments, that a little square
+of rather thick letter-paper gummed to the apex induced, though slowly,
+considerable deflection. Judging from several cases in which various
+objects had been affixed with gum, and had soon become separated from the
+apex by a layer of fluid, as well as from some trials in which drops of
+thick gum-water alone had been applied, this fluid never causes bending. We
+have also seen in the numbered experiments that narrow splinters of quill
+and of very thin glass, affixed with shellac, caused only a slight degree
+of deflection, and this may perhaps have been due to the shellac itself.
+Little squares of goldbeaters' skin, which is excessively thin, were
+damped, and thus made to adhere to one side of the tips of two radicles;
+one of these, after 24 h., produced no effect; nor did the
+[page 147]
+other in 8 h., within which time squares of card usually act; but after 24
+h. there was slight deflection.
+
+An oval bead, or rather cake, of dried shellac, 1.01 mm. in length and 0.63
+in breadth, caused a radicle to become deflected at nearly right angles in
+the course of only 6 h.; but after 23 h. it had nearly straightened itself.
+A very small quantity of dissolved shellac was spread over a bit of card,
+and the tips of 9 radicles were touched laterally with it; only two of them
+became slightly deflected to the side opposite to that bearing the speck of
+dried shellac, and they afterwards straightened themselves. These specks
+were removed, and both together weighed less than 1/100th of a grain; so
+that a weight of rather less than 1/200th of a grain (0.32 mg.) sufficed to
+excite movement in two out of the nine radicles. Here then we have
+apparently reached nearly the minimum weight which will act.
+
+A moderately thick bristle (which on measurement was found rather
+flattened, being 0.33 mm. in one diameter, and 0.20 mm. in the other) was
+cut into lengths of about 1/20th of an inch. These after being touched with
+thick gum-water, were placed on the tips of eleven radicles. Three of them
+were affected; one being deflected in 8 h. 15 m. to an angle of about 90o
+from the perpendicular; a second to the same amount when looked at after 9
+h.; but after 24 h. from the time of first attachment the deflection had
+decreased to only 19o; the third was only slightly deflected after 9 h.,
+and the bit of bristle was then found not touching the apex; it was
+replaced, and after 15 additional hours the deflection amounted to 26o from
+the perpendicular. The remaining eight radicles were not at all acted on by
+the bits of bristle, so that we here appear to have nearly reached the
+minimum
+[page 148]
+of size of an object which will act on the radicle of the bean. But it is
+remarkable that when the bits of bristle did act, that they should have
+acted so quickly and efficiently.
+
+As the apex of a radicle in penetrating the ground must be pressed on all
+sides, we wished to learn whether it could distinguish between harder or
+more resisting, and softer substances. A square of the sanded paper, almost
+as stiff as card, and a square of extremely thin paper (too thin for
+writing on), of exactly the same size (about 1/20th of an inch), were fixed
+with shellac on opposite sides of the apices of 12 suspended radicles. The
+sanded card was between 0.15 and 0.20 mm. (or between 0.0059 and 0.0079 of
+an inch), and the thin paper only 0.045 mm. (or 0.00176 of an inch) in
+thickness. In 8 out of the 12 cases there could be no doubt that the
+radicle was deflected from the side to which the card-like paper was
+attached, and towards the opposite side, bearing the very thin paper. This
+occurred in some instances in 9 h., but in others not until 24 h. had
+elapsed. Moreover, some of the four failures can hardly be considered as
+really failures: thus, in one of them, in which the radicle remained quite
+straight, the square of thin paper was found, when both were removed from
+the apex, to have been so thickly coated with shellac that it was almost as
+stiff as the card: in the second case, the radicle was bent upwards into a
+semicircle, but the deflection was not directly from the side bearing the
+card, and this was explained by the two squares having become cemented
+laterally together, forming a sort of stiff gable, from which the radicle
+was deflected: in the third case, the square of card had been fixed by
+mistake in front, and though there was deflection from it, this might have
+been due to Sachs' curvature:
+[page 149]
+in the fourth case alone no reason could be assigned why the radicle had
+not been at all deflected. These experiments suffice to prove that the apex
+of the radicle possesses the extraordinary power of discriminating between
+thin card and very thin paper, and is deflected from the side pressed by
+the more resisting or harder substance.
+
+Some trials were next made by irritating the tips without any object being
+left in contact with them. Nine radicles, suspended over water, had their
+tips rubbed, each six times with a needle, with sufficient force to shake
+the whole bean; the temperature was favourable, viz. about 63o F. In 7 out
+of these cases no effect whatever was produced; in the eighth case the
+radicle became slightly deflected from, and in the ninth case slightly
+deflected towards, the rubbed side; but these two latter opposed curvatures
+were probably accidental, as radicles do not always grow perfectly straight
+downwards. The tips of two other radicles were rubbed in the same manner
+for 15 seconds with a little round twig, two others for 30 seconds, and two
+others for 1 minute, but without any effect being produced. We may
+therefore conclude from these 15 trials that the radicles are not sensitive
+to temporary contact, but are acted on only by prolonged, though very
+slight, pressure.
+
+We then tried the effects of cutting off a very thin slice parallel to one
+of the sloping sides of the apex, as we thought that the wound would cause
+prolonged irritation, which might induce bending towards the opposite side,
+as in the case of an attached object. Two preliminary trials were made:
+firstly, slices were cut from the radicles of 6 beans suspended in damp
+air, with a pair of scissors, which, though sharp, probably caused
+considerable crushing, and no curva-
+[page 150]
+ture followed. Secondly, thin slices were cut with a razor obliquely off
+the tips of three radicles similarly suspended; and after 44 h. two were
+found plainly bent from the sliced surface; and the third, the whole apex
+of which had been cut off obliquely by accident, was curled upwards over
+the bean, but it was not clearly ascertained whether the curvature had been
+at first directed from the cut surface. These results led us to pursue the
+experiment, and 18 radicles, which had grown vertically downwards in damp
+air, had one side of their conical tips sliced off with a razor. The tips
+were allowed just to enter the water in the jars, and they were exposed to
+a temperature 14o - 16o C. (57o - 61o F.). The observations were made at
+different times. Three were examined 12 h. after being sliced, and were all
+slightly curved from the cut surface; and the curvature increased
+considerably after an additional 12 h. Eight were examined after 19 h.;
+four after 22 h. 30 m.; and three after 25 h. The final result was that out
+of the 18 radicles thus tried, 13 were plainly bent from the cut surface
+after the above intervals of time; and one other became so after an
+additional interval of 13 h. 30 m. So that only 4 out of the 18 radicles
+were not acted on. To these 18 cases the 3 previously mentioned ones should
+be added. It may, therefore, be concluded that a thin slice removed by a
+razor from one side of the conical apex of the radicle causes irritation,
+like that from an attached object, and induces curvature from the injured
+surface.
+
+Lastly, dry caustic (nitrate of silver) was employed to irritate one side
+of the apex. If one side of the apex or of the whole terminal growing part
+of a radicle, is by any means killed or badly injured, the other side
+continues to grow; and this causes the part
+[page 151]
+to bend over towards the injured side.* But in the following experiments we
+endeavoured, generally with success, to irritate the tips on one side,
+without badly injuring them. This was effected by first drying the tip as
+far as possible with blotting-paper, though it still remained somewhat
+damp, and then touching it once with quite dry caustic. Seventeen radicles
+were thus treated, and were suspended in moist air over water at a
+temperature of 58o F. They were examined after an interval of 21 h. or 24h.
+The tips of two were found blackened equally all round, so that they could
+tell nothing and were rejected, 15 being left. Of these, 10 were curved
+from the side which had been touched, where there was a minute brown or
+blackish mark. Five of these radicles, three of which were already slightly
+deflected, were allowed to enter the water in the jar, and were re-examined
+after an additional interval of 27 h. (i.e. in 48 h. after the application
+of the caustic), and now four of them had become hooked, being bent from
+the discoloured side, with their points directed to the zenith; the fifth
+remained unaffected and straight. Thus 11 radicles out of the 15 were acted
+on. But the curvature of the four just described was so plain, that they
+alone would have sufficed to show that the radicles of the bean bend away
+from that side of the apex which has been slightly irritated by caustic.
+
+The Power of an Irritant on the apex of the Radicle
+
+* Ciesielski found this to be the case ('Untersuchungen �ber die
+Abwartskr�mmung der Wurzel,' 1871, p. 28) after burning with heated
+platinum one side of a radicle. So did we when we painted longitudinally
+half of the whole length of 7 radicles, suspended over water, with a thick
+layer of grease, which is very injurious or even fatal to growing parts;
+for after 48 hours five of these radicles were curved towards the greased
+side, two remaining straight.
+[page 152]
+
+of the Bean, compared with that of Geotropism.--We know that when a little
+square of card or other object is fixed to one side of the tip of a
+vertically dependent radicle, the growing part bends from it often into a
+semicircle, in opposition to geotropism, which force is conquered by the
+effect of the irritation from the attached object. Radicles were therefore
+extended horizontally in damp air, kept at the proper low temperature for
+full sensitiveness, and squares of card were affixed with shellac on the
+lower sides of their tips, so that if the squares acted, the terminal
+growing part would curve upwards. Firstly, eight beans were so placed that
+their short, young, horizontally extended radicles would be simultaneously
+acted on both by geotropism and by Sachs' curvature, if the latter came
+into play; and they all eight became bowed downwards to the centre of the
+earth in 20 h., excepting one which was only slightly acted on. Two of them
+were a little bowed downwards in only 5 h.! Therefore the cards, affixed to
+the lower sides of their tips, seemed to produce no effect; and geotropism
+easily conquered the effects of the irritation thus caused. Secondly, 5
+oldish radicles, 1 � inch in length, and therefore less sensitive than the
+above-mentioned young ones, were similarly placed and similarly treated.
+From what has been seen on many other occasions, it may be safely inferred
+that if they had been suspended vertically they would have bent away from
+the cards; and if they had been extended horizontally, without cards
+attached to them, they would have quickly bent vertically downwards through
+geotropism; but the result was that two of these radicles were still
+horizontal after 23 h.; two were curved only slightly, and the fifth as
+much as 40o beneath the horizon. Thirdly, 5 beans were fastened
+[page 153]
+with their flat surfaces parallel to the cork-lid, so that Sachs' curvature
+would not tend to make the horizontally extended radicles turn either
+upwards or downwards, and little squares of card were affixed as before, to
+the lower sides of their tips. The result was that all five radicles were
+bent down, or towards the centre of the earth, after only 8 h. 20 m. At the
+same time and within the same jars, 3 radicles of the same age, with
+squares affixed to one side, were suspended vertically; and after 8 h. 20
+m. they were considerably deflected from the cards, and therefore curved
+upwards in opposition to geotropism. In these latter cases the irritation
+from the squares had over-powered geotropism; whilst in the former cases,
+in which the radicles were extended horizontally, geotropism had
+overpowered the irritation. Thus within the same jars, some of the radicles
+were curving upwards and others downwards at the same time--these opposite
+movements depending on whether the radicles, when the squares were first
+attached to them, projected vertically down, or were extended horizontally.
+This difference in their behaviour seems at first inexplicable, but can, we
+believe, be simply explained by the difference between the initial power of
+the two forces under the above circumstances, combined with the well-known
+principle of the after-effects of a stimulus. When a young and sensitive
+radicle is extended horizontally, with a square attached to the lower side
+of the tip, geotropism acts on it at right angles, and, as we have seen, is
+then evidently more efficient than the irritation from the square; and the
+power of geotropism will be strengthened at each successive period by its
+previous action--that is, by its after-effects. On the other hand, when a
+square is affixed to a vertically dependent radicle, and the apex begins to
+[page 154]
+curve upwards, this movement will be opposed by geotropism acting only at a
+very oblique angle, and the irritation from the card will be strengthened
+by its previous action. We may therefore conclude that the initial power of
+an irritant on the apex of the radicle of the bean, is less than that of
+geotropism when acting at right angles, but greater than that of geotropism
+when acting obliquely on it.
+
+Sensitiveness of the tips of the Secondary Radicles of the Bean to
+contact.--All the previous observations relate to the main or primary
+radicle. Some beans suspended to cork-lids, with their radicles dipping
+into water, had developed secondary or lateral radicles, which were
+afterwards kept in very damp air, at the proper low temperature for full
+sensitiveness. They projected, as usual, almost horizontally, with only a
+slight downward curvature, and retained this position during several days.
+Sachs has shown* that these secondary roots are acted on in a peculiar
+manner by geotropism, so that if displaced they reassume their former
+sub-horizontal position, and do not bend vertically downwards like the
+primary radicle. Minute squares of the stiff sanded paper were affixed by
+means of shellac (but in some instances with thick gum-water) to the tips
+of 39 secondary radicles of different ages, generally the uppermost ones.
+Most of the squares were fixed to the lower sides of the apex, so that if
+they acted the radicle would bend upwards; but some were fixed laterally,
+and a few on the upper side. Owing to the extreme tenuity of these
+radicles, it was very difficult to attach the square to the actual apex.
+Whether owing to this or some other circumstance, only nine of the squares
+induced any
+* 'Arbeiten Bot. Inst., W�rzburg,' Heft iv. 1874, p. 605-617.
+[page 155]
+
+curvature. The curvature amounted in some cases to about 45o above the
+horizon, in others to 90o, and then the tip pointed to the zenith. In one
+instance a distinct upward curvature was observed in 8 h. 15 m., but
+usually not until 24 h. had elapsed. Although only 9 out of 39 radicles
+were affected, yet the curvature was so distinct in several of them, that
+there could be no doubt that the tip is sensitive to slight contact, and
+that the growing part bends away from the touching object. It is possible
+that some secondary radicles are more sensitive than others; for Sachs has
+proved* the interesting fact that each individual secondary radicle
+possesses its own peculiar constitution.
+
+Sensitiveness to contact of the Primary Radicle, a little above the apex,
+in the Bean (Vicia faba) and Pea (Pisum sativum).--The sensitiveness of the
+apex of the radicle in the previously described cases, and the consequent
+curvature of the upper part from the touching object or other source of
+irritation, is the more remarkable, because Sachs** has shown that pressure
+at the distance of a few millimeters above the apex causes the radicle to
+bend, like a tendril, towards the touching object. By fixing pins so that
+they pressed against the radicles of beans suspended vertically in damp
+air, we saw this kind of curvature; but rubbing the part with a twig or
+needle for a few minutes produced no effect. Haberlandt remarks,*** that
+these radicles in breaking through the seed-coats often rub and press
+against the ruptured edges, and consequently bend round them. As little
+squares of the card-like paper affixed with shellac to the tips were highly
+efficient in causing the radicles to bend away from them, similar pieces
+(of about 1/20th
+
+* 'Arbeiten Bot. Instit., W�rzburg,' Heft, iv. 1874, p. 620.
+
+** Ibid. Heft iii. 1873, p. 437.
+
+*** 'Die Schutzeinrichtungen der Keimpflanze,' 1877, p. 25.
+[page 156]
+
+inch square, or rather less) were attached in the same manner to one side
+of the radicle at a distance of 3 or 4 mm. above the apex. In our first
+trial on 15 radicles no effect was produced. In a second trial on the same
+number, three became abruptly curved (but only one strongly) towards the
+card within 24 h. From these cases we may infer that the pressure from a
+bit of card affixed with shellac to one side above the apex, is hardly a
+sufficient irritant; but that it occasionally causes the radicle to bend
+like a tendril towards this side.
+
+We next tried the effect of rubbing several radicles at a distance of 4 mm.
+from the apex for a few seconds with lunar caustic (nitrate of silver); and
+although the radicles had been wiped dry and the stick of caustic was dry,
+yet the part rubbed was much injured and a slight permanent depression was
+left. In such cases the opposite side continues to grow, and the radicle
+necessarily becomes bent towards the injured side. But when a point 4 mm.
+from the apex was momentarily touched with dry caustic, it was only faintly
+discoloured, and no permanent injury was caused. This was shown by several
+radicles thus treated straightening themselves after one or two days; yet
+at first they became curved towards the touched side, as if they had been
+there subjected to slight continued pressure. These cases deserve notice,
+because when one side of the apex was just touched with caustic, the
+radicle, as we have seen, curved itself in an opposite direction, that is,
+away from the touched side.
+
+The radicle of the common pea at a point a little above the apex is rather
+more sensitive to continued pressure than that of the bean, and bends
+towards the pressed side.* We experimented on a variety (York-
+
+* Sachs, 'Arbeiten Bot. Institut., W�rzburg,' Heft iii. p. 438.
+[page 157]
+
+shire Hero) which has a much wrinkled tough skin, too large for the
+included cotyledons; so that out of 30 peas which had been soaked for 24 h.
+and allowed to germinate on damp sand, the radicles of three were unable to
+escape, and were crumpled up in a strange manner within the skin; four
+other radicles were abruptly bent round the edges of the ruptured skin
+against which they had pressed. Such abnormalities would probably never, or
+very rarely, occur with forms developed in a state of nature and subjected
+to natural selection. One of the four radicles just mentioned in doubling
+backwards came into contact with the pin by which the pea was fixed to the
+cork-lid; and now it bent at right angles round the pin, in a direction
+quite different from that of the first curvature due to contact with the
+ruptured skin; and it thus afforded a good illustration of the tendril-like
+sensitiveness of the radicle a little above the apex.
+
+Little squares of the card-like paper were next affixed to radicles of the
+pea at 4 mm. above the apex, in the same manner as with the bean.
+Twenty-eight radicles suspended vertically over water were thus treated on
+different occasions, and 13 of them became curved towards the cards. The
+greatest degree of curvature amounted to 62o from the perpendicular; but so
+large an angle was only once formed. On one occasion a slight curvature was
+perceptible after 5 h. 45 m., and it was generally well-marked after 14 h.
+There can therefore be no doubt that with the pea, irritation from a bit of
+card attached to one side of the radicle above the apex suffices to induce
+curvature.
+
+Squares of card were attached to one side of the tips of 11 radicles within
+the same jars in which the above trials were made, and five of them became
+plainly, and one slightly, curved away from this side. Other
+[page 158]
+analogous cases will be immediately described. The fact is here mentioned
+because it was a striking spectacle, showing the difference in the
+sensitiveness of the radicle in different parts, to behold in the same jar
+one set of radicles curved away from the squares on their tips, and another
+set curved towards the squares attached a little higher up. Moreover, the
+kind of curvature in the two cases is different. The squares attached above
+the apex cause the radicle to bend abruptly, the part above and beneath
+remaining nearly straight; so that here there is little or no transmitted
+effect. On the other hand, the squares attached to the apex affect the
+radicle for a length of from about 4 to even 8 mm., inducing in most cases
+a symmetrical curvature; so that here some influence is transmitted from
+the apex for this distance along the radicle.
+
+Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of the
+Radicle.--Little squares of the same card-like paper were affixed (April
+24th) with shellac to one side of the apex of 10 vertically suspended
+radicles: the temperature of the water in the bottom of the jars was 60o -
+61o F. Most of these radicles were acted on in 8 h. 30 m.; and eight of
+them became in the course of 24 h. conspicuously, and the remaining two
+slightly, deflected from the perpendicular and from the side bearing the
+attached squares. Thus all were acted on; but it will suffice to describe
+two conspicuous cases. In one the terminal portion of the radicle was bent
+at right angles (A, Fig. 66) after 24h.; and in the other (B) it had by
+this time become hooked, with the apex pointing to the zenith. The two bits
+of card here used were .07 inch in length and .04 inch in breadth. Two
+other radicles, which after 8 h. 30 m. were moderately deflected, became
+straight again after 24h. Another
+[page 159]
+trial was made in the same manner with 15 radicles; but from circumstances,
+not worth explaining, they were only once and briefly examined after the
+short
+
+Fig. 66. Pisum sativum: deflection produced within 24 hours in the growth
+of vertically dependent radicles, by little squares of card affixed with
+shellac to one side of apex: A, bent at right angles; B, hooked.
+
+interval of 5 h. 30 m.; and we merely record in our notes "almost all bent
+slightly from the perpendicular, and away from the squares; the deflection
+amounting in one or two instances to nearly a rectangle." These two sets of
+cases, especially the first one, prove that the apex of the radicle is
+sensitive to slight contact and that the upper part bends from the touching
+object. Nevertheless, on June 1st and 4th, 8 other radicles were tried in
+the same manner at a temperature of 58o - 60o F., and after 24 h. only 1
+was decidedly bent from the card, 4 slightly, 2 doubtfully, and 1 not in
+the least. The amount of curvature was unaccountably small; but all the
+radicles which were at all bent, were bent away from the cards.
+
+We now tried the effects of widely different temperatures on the
+sensitiveness of these radicles with squares
+[page 160]
+of card attached to their tips. Firstly, 13 peas, most of them having very
+short and young radicles, were placed in an ice-box, in which the
+temperature rose during three days from 44o to 47o F. They grew slowly, but
+10 out of the 13 became in the course of the three days very slightly
+curved from the squares; the other 3 were not affected; so that this
+temperature was too low for any high degree of sensitiveness or for much
+movement. Jars with 13 other radicles were next placed on a chimney-piece,
+where they were subjected to a temperature of between 68o and 72o F., and
+after 24 h., 4 were conspicuously curved from the cards, 2 slightly, and 7
+not at all; so that this temperature was rather too high. Lastly 12
+radicles were subjected to a temperature varying between 72o and 85o F.,
+and none of them were in the least affected by the squares. The above
+several trials, especially the first recorded one, indicate that the most
+favourable temperature for the sensitiveness of the radicle of the pea is
+about 60o F.
+
+The tips of 6 vertically dependent radicles were touched once with dry
+caustic, in the manner described under Vicia faba. After 24 h. four of them
+were bent from the side bearing a minute black mark; and the curvature
+increased in one case after 38 h., and in another case after 48 h., until
+the terminal part projected almost horizontally. The two remaining radicles
+were not affected.
+
+With radicles of the bean, when extended horizontally in damp air,
+geotropism always conquered the effects of the irritation caused by squares
+of card attached to the lower sides of their tips. A similar experiment was
+tried on 13 radicles of the pea; the squares being attached with shellac,
+and the temperature between 58o - 60o F. The result was somewhat different;
+for
+[page 161]
+these radicles are either less strongly acted on by geotropism, or, what is
+more probable, are more sensitive to contact. After a time geotropism
+always prevailed, but its action was often delayed; and in three instances
+there was a most curious struggle between geotropism and the irritation
+caused by the cards. Four of the 13 radicles were a little curved downwards
+within 6 or 8 h., always reckoning from the time when the squares were
+first attached, and after 23 h. three of them pointed vertically downwards,
+and the fourth at an angle of 45o beneath the horizon. These four radicles
+therefore did not seem
+
+Fig. 67. Pisum sativum: a radicle extended horizontally in damp air with a
+little square of card affixed to the lower side of its tip, causing it to
+bend upwards in opposition to geotropism. The deflection of the radicle
+after 21 hours is shown at A, and of the same radicle after 45 hours at B,
+now forming a loop.
+
+to have been at all affected by the attached squares. Four others were not
+acted on by geotropism within the first 6 or 8 h., but after 23 h. were
+much bowed down. Two others remained almost horizontal for 23 h., but
+afterwards were acted on. So that in these latter six cases the action of
+geotropism was much delayed. The eleventh radicle was slightly curved down
+after 8 h., but when looked at again after 23 h. the terminal portion was
+curved upwards; if it had
+[page 162]
+been longer observed, the tip no doubt would have been found again curved
+down, and it would have formed a loop as in the following case. The twelfth
+radicle after 6 h. was slightly curved downwards; but when looked at again
+after 21 h., this curvature had disappeared and the apex pointed upwards;
+after 30 h. the radicle formed a hook, as shown at A (Fig. 67); which hook
+after 45 h. was converted into a loop (B). The thirteenth radicle after 6
+h. was slightly curved downwards, but within 21 h. had curved considerably
+up, and then down again at an angle of 45o beneath the horizon, afterwards
+becoming perpendicular. In these three last cases geotropism and the
+irritation caused by the attached squares alternately prevailed in a highly
+remarkable manner; geotropism being ultimately victorious.
+
+Similar experiments were not always quite so successful as in the above
+cases. Thus 6 radicles, horizontally extended with attached squares, were
+tried on June 8th at a proper temperature, and after 7 h. 30 m. none were
+in the least curved upwards and none were distinctly geotropic; whereas of
+6 radicles without any attached squares, which served as standards of
+comparison or controls, 3 became slightly and 3 almost rectangularly
+geotropic within the 7 h. 30 m.; but after 23 h. the two lots were equally
+geotropic. On July 10th another trial was made with 6 horizontally extended
+radicles, with squares attached in the same manner beneath their tips; and
+after 7 h. 30 m., 4 were slightly geotropic, 1 remained horizontal, and 1
+was curved upwards in opposition to gravity or geotropism. This latter
+radicle after 48 h. formed a loop, like that at B (Fig. 67).
+
+An analogous trial was now made, but instead of attaching squares of card
+to the lower sides of the
+[page 163]
+tips, these were touched with dry caustic. The details of the experiment
+will be given in the chapter on Geotropism, and it will suffice here to say
+that 10 peas, with radicles extended horizontally and not cauterised, were
+laid on and under damp friable peat; these, which served as standards or
+controls, as well as 10 others which had been touched on the upper side
+with the caustic, all became strongly geotropic in 24 h. Nine radicles,
+similarly placed, had their tips touched on the lower side with the
+caustic; and after 24 h., 3 were slightly geotropic, 2 remained horizontal,
+and 4 were bowed upwards in opposition to gravity and to geotropism. This
+upward curvature was distinctly visible in 8 h. 45m. after the lower sides
+of the tips had been cauterised.
+
+Little squares of card were affixed with shellac on two occasions to the
+tips of 22 young and short secondary radicles, which had been emitted from
+the primary radicle whilst growing in water, but were now suspended in damp
+air. Besides the difficulty of attaching the squares to such finely pointed
+objects as were these radicles, the temperature was too high,--varying on
+the first occasion from 72o to 77o F., and on the second being almost
+steadily 78o F.; and this probably lessened the sensitiveness of the tips.
+The result was that after an interval of 8 h. 30 m., 6 of the 22 radicles
+were bowed upwards (one of them greatly) in opposition to gravity, and 2
+laterally; the remaining 14 were not affected. Considering the unfavourable
+circumstances, and bearing in mind the case of the bean, the evidence
+appears sufficient to show that the tips of the secondary radicles of the
+pea are sensitive to slight contact.
+
+Phaseolus multiflorus: Sensitiveness of the apex of the Radicle.--
+Fifty-nine radicles were tried with squares
+[page 164]
+of various sizes of the same card-like paper, also with bits of thin glass
+and rough cinders, affixed with shellac to one side of the apex. Rather
+large drops of the dissolved shellac were also placed on them and allowed
+to set into hard beads. The specimens were subjected to various
+temperatures between 60o and 72o F., more commonly at about the latter. But
+out of this considerable number of trials only 5 radicles were plainly
+bent, and 8 others slightly or even doubtfully, from the attached objects;
+the remaining 46 not being at all affected. It is therefore clear that the
+tips of the radicles of this Phaseolus are much less sensitive to contact
+than are those of the bean or pea. We thought that they might be sensitive
+to harder pressure, but after several trials we could not devise any method
+for pressing harder on one side of the apex than on the other, without at
+the same time offering mechanical resistance to its growth. We therefore
+tried other irritants.
+
+The tips of 13 radicles, dried with blotting-paper, were thrice touched or
+just rubbed on one side with dry nitrate of silver. They were rubbed
+thrice, because we supposed from the foregoing trials, that the tips were
+not highly sensitive. After 24 h. the tips were found greatly blackened; 6
+were blackened equally all round, so that no curvature to any one side
+could be expected; 6 were much blackened on one side for a length of about
+1/10th of an inch, and this length became curved at right angles towards
+the blackened surface, the curvature afterwards increasing in several
+instances until little hooks were formed. It was manifest that the
+blackened side was so much injured that it could not grow, whilst the
+opposite side continued to grow. One alone out of these 13 radicles became
+curved from the blackened side, the
+[page 165]
+curvature extending for some little distance above the apex.
+
+After the experience thus gained, the tips of six almost dry radicles were
+once touched with the dry caustic on one side; and after an interval of 10
+m. were allowed to enter water, which was kept at a temperature of 65o -
+67o F. The result was that after an interval of 8 h. a minute blackish
+speck could just be distinguished on one side of the apex of five of these
+radicles, all of which became curved towards the opposite side--in two
+cases at about an angle of 45o--in two other cases at nearly a rectangle--
+and in the fifth case at above a rectangle, so that the apex was a little
+hooked; in this latter case the black mark was rather larger than in the
+others. After 24 h. from the application of the caustic, the curvature of
+three of these radicles (including the hooked one) had diminished; in the
+fourth it remained the same, and in the fifth it had increased, the tip
+being now hooked. It has been said that after 8 h. black specks could be
+seen on one side of the apex of five of the six radicles; on the sixth the
+speck, which was extremely minute, was on the actual apex and therefore
+central; and this radicle alone did not become curved. It was therefore
+again touched on one side with caustic, and after 15 h. 30 m. was found
+curved from the perpendicular and from the blackened side at an angle of
+34o, which increased in nine additional hours to 54o.
+
+It is therefore certain that the apex of the radicle of this Phaseolus is
+extremely sensitive to caustic, more so than that of the bean, though the
+latter is far more sensitive to pressure. In the experiments just given,
+the curvature from the slightly cauterised side of the tip, extended along
+the radicle for a length of nearly 10 mm.; whereas in the first set
+[page 166]
+of experiments, when the tips of several were greatly blackened and injured
+on one side, so that their growth was arrested, a length of less than 3 mm.
+became curved towards the much blackened side, owing to the continued
+growth of the opposite side. This difference in the results is interesting,
+for it shows that too strong an irritant does not induce any transmitted
+effect, and does not cause the adjoining, upper and growing part of the
+radicle to bend. We have analogous cases with Drosera, for a strong
+solution of carbonate of ammonia when absorbed by the glands, or too great
+heat suddenly applied to them, or crushing them, does not cause the basal
+part of the tentacles to bend, whilst a weak solution of the carbonate, or
+a moderate heat, or slight pressure always induced such bending. Similar
+results were observed with Dionaea and Pinguicula.
+
+The effect of cutting off with a razor a thin slice from one side of the
+conical apex of 14 young and short radicles was next tried. Six of them
+after being operated on were suspended in damp air; the tips of the other
+eight, similarly suspended, were allowed to enter water at a temperature of
+about 65o F. It was recorded in each case which side of the apex had been
+sliced off, and when they were afterwards examined the direction of the
+curvature was noted, before the record was consulted. Of the six radicles
+in damp air, three had their tips curved after an interval of 10 h. 15 m.
+directly away from the sliced surface, whilst the other three were not
+affected and remained straight; nevertheless, one of them after 13
+additional hours became slightly curved from the sliced surface. Of the
+eight radicles with their tips immersed in water, seven were plainly curved
+away from the sliced surfaces after 10 h. 15 m.; and with
+[page 167]
+respect to the eighth which remained quite straight, too thick a slice had
+been accidentally removed, so that it hardly formed a real exception to the
+general result. When the seven radicles were looked at again, after an
+interval of 23 h. from the time of slicing, two had become distorted; four
+were deflected at an angle of about 70o from the perpendicular and from the
+cut surface; and one was deflected at nearly 90o, so that it projected
+almost horizontally, but with the extreme tip now beginning to bend
+downwards through the action of geotropism. It is therefore manifest that a
+thin slice cut off one side of the conical apex, causes the upper growing
+part of the radicle of this Phaseolus to bend, through the transmitted
+effects of the irritation, away from the sliced surface.
+
+Tropaeolum majus: Sensitiveness of the apex of the Radicle to contact.--
+Little squares of card were attached with shellac to one side of the tips
+of 19 radicles, some of which were subjected to 78o F., and others to a
+much lower temperature. Only 3 became plainly curved from the squares, 5
+slightly, 4 doubtfully, and 7 not at all. These seeds were, as we believed,
+old, so we procured a fresh lot, and now the results were widely different.
+Twenty-three were tried in the same manner; five of the squares produced no
+effect, but three of these cases were no real exceptions, for in two of
+them the squares had slipped and were parallel to the apex, and in the
+third the shellac was in excess and had spread equally all round the apex.
+One radicle was deflected only slightly from the perpendicular and from the
+card; whilst seventeen were plainly deflected. The angles in several of
+these latter cases varied between 40o and 65o from the perpendicular; and
+in two of them it amounted after 15 h. or 16 h. to about 90o. In one
+instance a loop
+[page 168]
+was nearly completed in 16 h. There can, therefore, be no doubt that the
+apex is highly sensitive to slight contact, and that the upper part of the
+radicle bends away from the touching object.
+
+Gossypium herbaceum: Sensitiveness of the apex of the Radicle.--Radicles
+were experimented on in the same manner as before, but they proved
+ill-fitted for our purpose, as they soon became unhealthy when suspended in
+damp air. Of 38 radicles thus suspended, at temperatures varying from 66o
+to 69o F., with squares of card attached to their tips, 9 were plainly and
+7 slightly or even doubtfully deflected from the squares and from the
+perpendicular; 22 not being affected. We thought that perhaps the above
+temperature was not high enough, so 19 radicles with attached squares,
+likewise suspended in damp air, were subjected to a temperature of from 74o
+to 79o F., but not one of them was acted on, and they soon became
+unhealthy. Lastly, 19 radicles were suspended in water at a temperature
+from 70o to 75o F., with bits of glass or squares of the card attached to
+their tips by means of Canada-balsam or asphalte, which adhered rather
+better than shellac beneath the water. The radicles did not keep healthy
+for long. The result was that 6 were plainly and 2 doubtfully deflected
+from the attached objects and the perpendicular; 11 not being affected. The
+evidence consequently is hardly conclusive, though from the two sets of
+cases tried under a moderate temperature, it is probable that the radicles
+are sensitive to contact; and would be more so under favourable conditions.
+
+Fifteen radicles which had germinated in friable peat were suspended
+vertically over water. Seven of them served as controls, and they remained
+quite straight during 24 h. The tips of the other eight radicles
+[page 169]
+were just touched with dry caustic on one side. After only 5 h. 10 m. five
+of them were slightly curved from the perpendicular and from the side
+bearing the little blackish marks. After 8 h. 40 m., 4 out of these 5 were
+deflected at angles between 15o and 65o from the perpendicular. On the
+other hand, one which had been slightly curved after 5 h. 10 m., now became
+straight. After 24 h. the curvature in two cases had considerably
+increased; also in four other cases, but these latter radicles had now
+become so contorted, some being turned upwards, that it could no longer be
+ascertained whether they were still curved from the cauterised side. The
+control specimens exhibited no such irregular growth, and the two sets
+presented a striking contrast. Out of the 8 radicles which had been touched
+with caustic, two alone were not affected, and the marks left on their tips
+by the caustic were extremely minute. These marks in all cases were oval or
+elongated; they were measured in three instances, and found to be of nearly
+the same size, viz. 2/3 of a mm. in length. Bearing this fact in mind, it
+should be observed that the length of the curved part of the radicle, which
+had become deflected from the cauterised side in the course of 8 h. 40 m.
+was found to be in three cases 6, 7, and 9 mm.
+
+Cucurbita ovifera: Sensitiveness of the apex of the Radicle.--The tips
+proved ill-fitted for the attachment of cards, as they are extremely fine
+and flexible. Moreover, owing to the hypocotyls being soon developed and
+becoming arched, the whole radicle is quickly displaced and confusion is
+thus caused. A large number of trials were made, but without any definite
+result, excepting on two occasions, when out of 23 radicles 10 were
+deflected from the attached squares
+[page 170]
+of card, and 13 were not acted on. Rather large squares, though difficult
+to affix, seemed more efficient than very small ones.
+
+We were much more successful with caustic; but in our first trial, 15
+radicles were too much cauterised, and only two became curved from the
+blackened side; the others being either killed on one side, or blackened
+equally all round. In our next trial the dried tips of 11 radicles were
+touched momentarily with dry caustic, and after a few minutes were immersed
+in water. The elongated marks thus caused were never black, only brown, and
+about � mm. in length, or even less. In 4 h. 30 m. after the cauterisation,
+6 of them were plainly curved from the side with the brown mark, 4
+slightly, and 1 not at all. The latter proved unhealthy, and never grew;
+and the marks on 2 of the 4 slightly curved radicles were excessively
+minute, one being distinguishable only with the aid of a lens. Of 10
+control specimens tried in the same jars at the same time, not one was in
+the least curved. In 8 h. 40 m. after the cauterisation, 5 of the radicles
+out of the 10 (the one unhealthy one being omitted) were deflected at about
+90o, and 3 at about 45o from the perpendicular and from the side bearing
+the brown mark. After 24 h. all 10 radicles had increased immensely in
+length; in 5 of them the curvature was nearly the same, in 2 it had
+increased, and in 3 it had decreased. The contrast presented by the 10
+controls, after both the 8 h. 40 m. and the 24 h. intervals, was very
+great; for they had continued to grow vertically downwards, excepting two
+which, from some unknown cause, had become somewhat tortuous.
+
+In the chapter on Geotropism we shall see that 10 radicles of this plant
+were extended horizontally on and beneath damp friable peat, under which
+conditions
+[page 171]
+they grow better and more naturally than in damp air; and their tips were
+slightly cauterised on the lower side, brown marks about � mm. in length
+being thus caused. Uncauterised specimens similarly placed became much bent
+downwards through geotropism in the course of 5 or 6 hours. After 8 h. only
+3 of the cauterised ones were bowed downwards, and this in a slight degree;
+4 remained horizontal; and 3 were curved upwards in opposition to
+geotropism and from the side bearing the brown mark. Ten other specimens
+had their tips cauterised at the same time and in the same degree, on the
+upper side; and this, if it produced any effect, would tend to increase the
+power of geotropism; and all these radicles were strongly bowed downwards
+after 8 h. From the several foregoing facts, there can be no doubt that the
+cauterisation of the tip of the radicle of this Cucurbita on one side, if
+done lightly enough, causes the whole growing part to bend to the opposite
+side.
+Raphanus sativus: Sensitiveness of the apex of the Radicle.--We here
+encountered many difficulties in our trials, both with squares of card and
+with caustic; for when seeds were pinned to a cork-lid, many of the
+radicles, to which nothing had been done, grew irregularly, often curving
+upwards, as if attracted by the damp surface above; and when they were
+immersed in water they likewise often grew irregularly. We did not
+therefore dare to trust our experiments with attached squares of card;
+nevertheless some of them seemed to indicate that the tips were sensitive
+to contact. Our trials with caustic generally failed from the difficulty of
+not injuring too greatly the extremely fine tips. Out of 7 radicles thus
+tried, one became bowed after 22 h. at an angle of 60o, a second at 40o,
+[page 172]
+and a third very slightly from the perpendicular and from the cauterised
+side.
+
+Aesculus hippocastanum: Sensitiveness of the apex of the Radicle.--Bits of
+glass and squares of card were affixed with shellac or gum-water to the
+tips of 12 radicles of the horse-chestnut; and when these objects fell off,
+they were refixed; but not in a single instance was any curvature thus
+caused. These massive radicles, one of which was above 2 inches in length
+and .3 inch in diameter at its base, seemed insensible to so slight a
+stimulus as any small attached object. Nevertheless, when the apex
+encountered an obstacle in its downward course, the growing part became so
+uniformly and symmetrically curved, that its appearance indicated not mere
+mechanical bending, but increased growth along the whole convex side, due
+to the irritation of the apex.
+
+That this is the correct view may be inferred from the effects of the more
+powerful stimulus of caustic. The bending from the cauterised side occurred
+much slower than in the previously described species, and it will perhaps
+be worth while to give our trials in detail.
+
+[The seeds germinated in sawdust, and one side of the tips of the radicles
+were slightly rubbed once with dry nitrate of silver; and after a few
+minutes were allowed to dip into water. They were subjected to a rather
+varying temperature, generally between 52o and 58o F. A few cases have not
+been thought worth recording, in which the whole tip was blackened, or in
+which the seedling soon became unhealthy.
+
+(1.) The radicle was slightly deflected from the cauterised side in one day
+(i.e. 24 h.); in three days it stood at 60o from the perpendicular; in four
+days at 90o; on the fifth day it was curved up about 40o above the horizon;
+so that it had passed through an angle of 130o in the five days, and this
+was the greatest amount of curvature observed.
+
+(2.) In two days radicle slightly deflected; after seven days
+[page 173]
+deflected 69o from the perpendicular and from the cauterised side; after
+eight days the angle amounted to nearly 90o.
+
+(3.) After one day slight deflection, but the cauterised mark was so faint
+that the same side was again touched with caustic. In four days from the
+first touch deflection amounted to 78o, which in an additional day
+increased to 90o.
+
+(4.) After two days slight deflection, which during the next three days
+certainly increased but never became great; the radicle did not grow well
+and died on the eighth day.
+
+(5.) After two days very slight deflection; but this on the fourth day
+amounted to 56o from the perpendicular and from the cauterised side.
+
+(6.) After three days doubtfully, but after four days certainly deflected
+from the cauterised side. On the fifth day deflection amounted to 45o from
+the perpendicular, and this on the seventh day increased to about 90o.
+
+(7.) After two days slightly deflected; on the third day the deflection
+amounted to 25o from the perpendicular, and this did not afterwards
+increase.
+
+(8.) After one day deflection distinct; on the third day it amounted to
+44o, and on the fourth day to 72o from the perpendicular and the cauterised
+side.
+
+(9.) After two days deflection slight, yet distinct; on the third day the
+tip was again touched on the same side with caustic and thus killed.
+
+(10.) After one day slight deflection, which after six days increased to
+50o from the perpendicular and the cauterised side.
+
+(11.) After one day decided deflection, which after six days increased to
+62o from the perpendicular and from the cauterised side.
+
+(12.) After one day slight deflection, which on the second day amounted to
+35o, on the fourth day to 50o, and the sixth day to 63o from the
+perpendicular and the cauterised side.
+
+(13.) Whole tip blackened, but more on one side than the other; on the
+fourth day slightly, and on the sixth day greatly deflected from the more
+blackened side; the deflection on the ninth day amounted to 90o from the
+perpendicular.
+
+(14.) Whole tip blackened in the same manner as in the last case: on the
+second day decided deflection from the more blackened side, which increased
+on the seventh day to nearly 90o; on the following day the radicle appeared
+unhealthy.
+
+(15.) Here we had the anomalous case of a radicle bending
+[page 174]
+slightly towards the cauterised side on the first day, and continuing to do
+so for the next three days, when the deflection amounted to about 90o from
+the perpendicular. The cause appeared to lie in the tendril-like
+sensitiveness of the upper part of the radicle, against which the point of
+a large triangular flap of the seed-coats pressed with considerable force;
+and this irritation apparently conquered that from the cauterised apex.]
+
+These several cases show beyond doubt that the irritation of one side of
+the apex, excites the upper part of the radicle to bend slowly towards the
+opposite side. This fact was well exhibited in one lot of five seeds pinned
+to the cork-lid of a jar; for when after 6 days the lid was turned upside
+down and viewed from directly above, the little black marks made by the
+caustic were now all distinctly visible on the upper sides of the tips of
+the laterally bowed radicles. A thin slice was shaved off with a razor from
+one side of the tips of 22 radicles, in the manner described under the
+common bean; but this kind of irritation did not prove very effective. Only
+7 out of the 22 radicles became moderately deflected in from 3 to 5 days
+from the sliced surface, and several of the others grew irregularly. The
+evidence, therefore, is far from conclusive.
+
+Quercus robur: Sensitiveness of the apex of the Radicle.--The tips of the
+radicles of the common oak are fully as sensitive to slight contact as are
+those of any plant examined by us. They remained healthy in damp air for 10
+days, but grew slowly. Squares of the card-like paper were fixed with
+shellac to the tips of 15 radicles, and ten of these became conspicuously
+bowed from the perpendicular and from the squares; two slightly, and three
+not at all. But two of the latter were not real exceptions, as they were at
+first very short, and hardly grew afterwards. Some of the more
+[page 175]
+remarkable cases are worth describing. The radicles were examined on each
+successive morning, at nearly the same hour, that is, after intervals of 24
+h.
+
+[No. 1. This radicle suffered from a series of accidents, and acted in an
+anomalous manner, for the apex appeared at first insensible and afterwards
+sensitive to contact. The first square was attached on Oct 19th; on the
+21st the radicle was not at all curved, and the square was accidentally
+knocked off; it was refixed on the 22nd, and the radicle became slightly
+curved from the square, but the curvature disappeared on the 23rd, when the
+square was removed and refixed. No curvature ensued, and the square was
+again accidentally knocked off, and refixed. On the morning of the 27th it
+was washed off by having reached the water in the bottom of the jar. The
+square was refixed, and on the 29th, that is, ten days after the first
+square had been attached, and two days after the attachment of the last
+square, the radicle had grown to the great length of 3.2 inches, and now
+the terminal growing part had become bent away from the square into a hook
+(see Fig. 68).
+
+Fig. 68. Quercus robur: radicle with square of card attached to one side of
+apex, causing it to become hooked. Drawing one-half natural scale.
+
+No. 2. Square attached on the 19th; on the 20th radicle slightly deflected
+from it and from the perpendicular; on the 21st deflected at nearly right
+angles; it remained during the next two days in this position, but on the
+25th the upward curvature was lessened through the action of geotropism,
+and still more so on the 26th.
+
+No. 3. Square attached on the 19th; on the 21st a trace of curvature from
+the square, which amounted on the 22nd to about 40o, and on the 23rd to 53o
+from the perpendicular.
+
+No. 4. Square attached on the 21st; on the 22nd trace of curvature from the
+square; on the 23rd completely hooked with the point turned up to the
+zenith. Three days afterwards (i.e. 26th) the curvature had wholly
+disappeared and the apex pointed perpendicularly downwards.
+
+No. 5. Square attached on the 21st; on the 22nd decided
+[page 176]
+though slight curvature from the square; on the 23rd the tip had curved up
+above the horizon, and on the 24th was hooked with the apex pointing almost
+to the zenith, as in Fig. 68.
+
+No. 6. Square attached on the 21st; on the 22nd slightly curved from the
+square; 23rd more curved; 25th considerably curved; 27th all curvature
+lost, and the radicle was now directed perpendicularly downwards.
+
+No. 7. Square attached on the 21st; on the 22nd a trace of curvature from
+the square, which increased next day, and on the 24th amounted to a right
+angle.
+
+It is, therefore, manifest that the apex of the radicle of the oak is
+highly sensitive to contact, and retains its sensitiveness during several
+days. The movement thus induced was, however, slower than in any of the
+previous cases, with the exception of that of Aesculus. As with the bean,
+the terminal growing part, after bending, sometimes straightened itself
+through the action of geotropism, although the object still remained
+attached to the tip.
+
+The same remarkable experiment was next tried, as in the case of the bean;
+namely, little squares of exactly the same size of the card-like sanded
+paper and of very thin paper (the thicknesses of which have been given
+under Vicia faba) were attached with shellac on opposite sides (as
+accurately as could be done) of the tips of 13 radicles, suspended in damp
+air, at a temperature of 65o - 66o F. The result was striking, for 9 out of
+these 13 radicles became plainly, and 1 very slightly, curved from the
+thick paper towards the side bearing the thin paper. In two of these cases
+the apex became completely hooked after two days; in four cases the
+deflection from the perpendicular and from the side bearing the thick
+paper, amounted in from two to four days to angles of 90o, 72o, 60o, and
+49o, but in two other cases to only 18o and 15o. It should, however, be
+stated that in the
+[page 177]
+case in which the deflection was 49o, the two squares had accidentally come
+into contact on one side of the apex, and thus formed a lateral gable; and
+the deflection was directed in part from this gable and in part from the
+thick paper. In three cases alone the radicles were not affected by the
+difference in thickness of the squares of paper attached to their tips, and
+consequently did not bend away from the side bearing the stiffer paper.
+
+Zea mays: Sensitiveness of the apex of the Radicle to contact.--A large
+number of trials were made on this plant, as it was the only monocotyledon
+on which we experimented. An abstract of the results will suffice. In the
+first place, 22 germinating seeds were pinned to cork-lids without any
+object being attached to their radicles, some being exposed to a
+temperature of 65o - 66o F., and others to between 74o and 79o; and none of
+them became curved, though some were a little inclined to one side. A few
+were selected, which from having germinated on sand were crooked, but when
+suspended in damp air the terminal part grew straight downwards. This fact
+having been ascertained, little squares of the card-like paper were affixed
+with shellac, on several occasions, to the tips of 68 radicles. Of these
+the terminal growing part of 39 became within 24 h. conspicuously curved
+away from the attached squares and from the perpendicular; 13 out of the 39
+forming hooks with their points directed towards the zenith, and 8 forming
+loops. Moreover, 7 other radicles out of the 68, were slightly and two
+doubtfully deflected from the cards. There remain 20 which were not
+affected; but 10 of these ought not to be counted; for one was diseased,
+two had their tips quite surrounded by shellac, and the squares on 7 had
+slipped so as to stand parallel to the apex, instead of obliquely
+[page 178]
+on it. There were therefore only 10 out of the 68 which certainly were not
+acted on. Some of the radicles which were experimented on were young and
+short, most of them of moderate length, and two or three exceeded three
+inches in length. The curvature in the above cases occurred within 24 h.,
+but it was often conspicuous within a much shorter period. For instance,
+the terminal growing part of one radicle was bent upwards into a rectangle
+in 8 h. 15 m., and of another in 9 h. On one occasion a hook was formed in
+9 h. Six of the radicles in a jar containing nine seeds, which stood on a
+sand-bath, raised to a temperature varying from 76o to 82o F., became
+hooked, and a seventh formed a complete loop, when first looked at after 15
+hours.
+
+The accompanying figures of four germinating seeds (Fig. 69) show, firstly,
+a radicle (A) the apex of which has become so much bent away from the
+attached square as to form a hook. Secondly (B), a hook converted through
+the continued irritation of the card, aided perhaps by geotropism, into an
+almost complete circle or loop. The tip in the act of forming a loop
+generally rubs against the upper part of the radicle, and pushes off the
+attached square; the loop then contracts or closes, but never disappears;
+and the apex afterwards grows vertically downwards, being no longer
+irritated by any attached object. This frequently occurred, and is
+represented at C. The jar above mentioned with the six hooked radicles and
+another jar were kept for two additional days, for the sake of observing
+how the hooks would be modified. Most of them became converted into simple
+loops, like that figured at C; but in one case the apex did not rub against
+the upper part of the radicle and thus remove the card; and it consequently
+made, owing
+[page 179]
+to the continued irritation from the card, two complete loops, that is, a
+helix of two spires; which afterwards became pressed closely together. Then
+geotropism prevailed and caused the apex to grow perpendicularly downwards.
+In another case, shown at (D), the apex
+
+Fig. 69. Zea mays: radicles excited to bend away from the little squares of
+card attached to one side of their tips.
+
+in making a second turn or spire, passed through the first loop, which was
+at first widely open, and in doing so knocked off the card; it then grew
+perpendicularly downwards, and thus tied itself into a knot, which soon
+became tight!
+
+Secondary Radicles of Zea.--A short time after the first radicle has
+appeared, others protrude from the
+[page 180]
+seed, but not laterally from the primary one. Ten of these secondary
+radicles, which were directed obliquely downwards, were experimented on
+with very small squares of card attached with shellac to the lower sides of
+their tips. If therefore the squares acted, the radicles would bend upwards
+in opposition to gravity. The jar stood (protected from light) on a
+sand-bath, which varied between 76o and 82o F. After only 5 h. one appeared
+to be a little deflected from the square, and after 20 h. formed a loop.
+Four others were considerably curved from the squares after 20 h., and
+three of them became hooked, with their tips pointing to the zenith,--one
+after 29 h. and the two others after 44 h. By this latter time a sixth
+radicle had become bent at a right angle from the side bearing the square.
+Thus altogether six out of the ten secondary radicles were acted on, four
+not being affected. There can, therefore, be no doubt that the tips of
+these secondary radicles are sensitive to slight contact, and that when
+thus excited they cause the upper part to bend from the touching object;
+but generally, as it appears, not in so short a time as in the case of the
+first-formed radicle.
+
+SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR.
+
+Sachs made the interesting discovery, a few years ago, that the radicles of
+many seedling plants bend towards an adjoining damp surface.* We shall here
+endeavour to show that this peculiar form of sensitiveness resides in their
+tips. The movement is directly the reverse of that excited by the irritants
+hitherto considered, which cause the growing part of the
+
+* 'Arbeiten des Bot. Institut., in W�rzburg,' vol. i. 1872, p. 209.
+[page 181]
+
+radicle to bend away from the source of irritation. In our experiments we
+followed Sachs' plan, and sieves with seeds germinating in damp sawdust
+were suspended so that the bottom was generally inclined at 40o with the
+horizon. If the radicles had been acted on solely by geotropism, they would
+have grown out of the bottom of the sieve perpendicularly downwards; but as
+they were attracted by the adjoining damp surface they bent towards it and
+were deflected 50o from the perpendicular. For the sake of ascertaining
+whether the tip or the whole growing part of the radicle was sensitive to
+the moist air, a length of from 1 to 2 mm. was coated in a certain number
+of cases with a mixture of olive-oil and lamp-black. This mixture was made
+in order to give consistence to the oil, so that a thick layer could be
+applied, which would exclude, at least to a large extent, the moist air,
+and would be easily visible. A greater number of experiments than those
+which were actually tried would have been necessary, had not it been
+clearly established that the tip of the radicle is the part which is
+sensitive to various other irritants.
+
+[Phaseolus multiflorus.--Twenty-nine radicles, to which nothing had been
+done, growing out of a sieve, were observed at the same time with those
+which had their tips greased, and for an equal length of time. Of the 29,
+24 curved themselves so as to come into close contact with the bottom of
+the sieve. The place of chief curvature was generally at a distance of 5 or
+6 mm. from the apex. Eight radicles had their tips greased for a length of
+2 mm., and two others for a length of 1 � mm.; they were kept at a
+temperature of 15o - 16o C. After intervals of from 19 h. to 24 h. all were
+still vertically or almost vertically dependent, for some of them had moved
+towards the adjoining damp surface by about 10o. They had therefore not
+been acted on, or only slightly acted on, by the damper air on one side,
+although the whole upper part was freely exposed. After 48 h. three of
+these radicles became
+[page 182]
+considerably curved towards the sieve; and the absence of curvature in some
+of the others might perhaps be accounted for by their not having grown very
+well. But it should be observed that during the first 19 h. to 24 h. all
+grew well; two of them having increased 2 and 3 mm. in length in 11 h.;
+five others increased 5 to 8 mm. in 19 h.; and two, which had been at first
+4 and 6 mm. in length, increased in 24 h. to 15 and 20 mm.
+
+The tips of 10 radicles, which likewise grew well, were coated with the
+grease for a length of only 1 mm., and now the result was somewhat
+different; for of these 4 curved themselves to the sieve in from 21 h. to
+24h., whilst 6 did not do so. Five of the latter were observed for an
+additional day, and now all excepting one became curved to the sieve.
+
+The tips of 5 radicles were cauterised with nitrate of silver, and about 1
+mm. in length was thus destroyed. They were observed for periods varying
+between 11 h. and 24h., and were found to have grown well. One of them had
+curved until it came into contact with the sieve; another was curving
+towards it; whilst the remaining three were still vertically dependent. Of
+7 not cauterised radicles observed at the same time, all had come into
+contact with the sieve.
+
+The tips of 11 radicles were protected by moistened gold-beaters' skin,
+which adheres closely, for a length varying from 1 � to 2 � mm. After 22 h.
+to 24 h., 6 of these radicles were clearly bent towards or had come into
+contact with the sieve; 2 were slightly curved in this direction, and 3 not
+at all. All had grown well. Of 14 control specimens observed at the same
+time, all excepting one had closely approached the sieve. It appears from
+these cases that a cap of goldbeaters' skin checks, though only to a slight
+degree, the bending of the radicles to an adjoining damp surface. Whether
+an extremely thin sheet of this substance when moistened allows moisture
+from the air to pass through it, we do not know. One case indicated that
+the caps were sometimes more efficient than appears from the above results;
+for a radicle, which after 23 h. had only slightly approached the sieve,
+had its cap (1 � mm. in length) removed, and during the next 15 � h. it
+curved itself abruptly towards the source of moisture, the chief seat of
+curvature being at a distance of 2 to 3 mm. from the apex.
+
+Vicia faba.--The tips of 13 radicles were coated with the grease for a
+length of 2 mm.; and it should be remembered that with these radicles the
+seat of chief curvature is about
+[page 183]
+4 or 5 mm. from the apex. Four of them were examined after 22h., three
+after 26 h., and six after 36 h., and none had been attracted towards the
+damp lower surface of the sieve. In another trial 7 radicles were similarly
+treated, and 5 of them still pointed perpendicularly downwards after 11 h.,
+whilst 2 were a little curved towards the sieve; by an accident they were
+not subsequently observed. In both these trials the radicles grew well; 7
+of them, which were at first from 4 to 11 mm. in length, were after 11 h.
+between 7 and 16 mm.; 3 which were at first from 6 to 8 mm. after 26 h.
+were 11.5 to 18 mm. in length; and lastly, 4 radicles which were at first 5
+to 8 mm. after 46 h. were 18 to 23 mm. in length. The control or ungreased
+radicles were not invariably attracted towards the bottom of the sieve. But
+on one occasion 12 out of 13, which were observed for periods between 22 h.
+and 36 h., were thus attracted. On two other occasions taken together, 38
+out of 40 were similarly attracted. On another occasion only 7 out of 14
+behaved in this manner, but after two more days the proportion of the
+curved increased to 17 out of 23. On a last occasion only 11 out of 20 were
+thus attracted. If we add up these numbers, we find that 78 out of 96 of
+the control specimens curved themselves towards the bottom of the sieve. Of
+the specimens with greased tips, 2 alone out of the 20 (but 7 of these were
+not observed for a sufficiently long time) thus curved themselves. We can,
+therefore, hardly doubt that the tip for a length of 2 mm. is the part
+which is sensitive to a moist atmosphere, and causes the upper part to bend
+towards its source.
+
+The tips of 15 radicles were cauterised with nitrate of silver, and they
+grew as well as those above described with greased tips. After an interval
+of 24 h., 9 of them were not at all curved towards the bottom of the sieve;
+2 were curved towards it at angles of 20o and 12o from their former
+vertical position, and 4 had come into close contact with it. Thus the
+destruction of the tip for a length of about 1 mm. prevented the curvature
+of the greater number of these radicles to the adjoining damp surface. Of
+24 control specimens, 23 were bent to the sieve, and on a second occasion
+15 out of 16 were similarly curved in a greater or less degree. These
+control trials are included in those given in the foregoing paragraph.
+
+Avena sativa.--The tips of 13 radicles, which projected between 2 and 4 mm.
+from the bottom of the sieve, many of
+[page 184]
+them not quite perpendicularly downwards, were coated with the black grease
+for a length of from 1 to 1 � mm. The sieves were inclined at 30o with the
+horizon. The greater number of these radicles were examined after 22 h.,
+and a few after 25 h., and within these intervals they had grown so quickly
+as to have nearly doubled their lengths. With the ungreased radicles the
+chief seat of curvature is at a distance of not less than between 3.5 and
+5.5 mm., and not more than between 7 and 10 mm. from the apex. Out of the
+13 radicles with greased tips, 4 had not moved at all towards the sieve; 6
+were deflected towards it and from the perpendicular by angles varying
+between 10o and 35o; and 3 had come into close contact with it. It appears,
+therefore, at first sight that greasing the tips of these radicles had
+checked but little their bending to the adjoining damp surface. But the
+inspection of the sieves on two occasions produced a widely different
+impression on the mind; for it was impossible to behold the radicles with
+the black greased tips projecting from the bottom, and all those with
+ungreased tips, at least 40 to 50 in number, clinging closely to it, and
+feel any doubt that the greasing had produced a great effect. On close
+examination only a single ungreased radicle could be found which had not
+become curved towards the sieve. It is probable that if the tips had been
+protected by grease for a length of 2 mm. instead of from 1 to 1 � mm.,
+they would not have been affected by the moist air and none would have
+become curved.
+
+Triticum vulgare.--Analogous trials were made on 8 radicles of the common
+wheat; and greasing their tips produced much less effect than in the case
+of the oats. After 22 h., 5 of them had come into contact with the bottom
+of the sieve; 2 had moved towards it 10o and 15o, and one alone remained
+perpendicular. Not one of the very numerous ungreased radicles failed to
+come into close contact with the sieve. These trials were made on Nov.
+28th, when the temperature was only 4.8o C. at 10 A.M. We should hardly
+have thought this case worth notice, had it not been for the following
+circumstance. In the beginning of October, when the temperature was
+considerably higher, viz., 12o to 13o C., we found that only a few of the
+ungreased radicles became bent towards the sieve; and this indicates that
+sensitiveness to moisture in the air is increased by a low temperature, as
+we have seen with the radicles of Vicia faba relatively to objects attached
+to their tips. But in the present instance it is possible that a difference
+in the dryness
+[page 185]
+of the air may have caused the difference in the results at the two
+periods.]
+
+Finally, the facts just given with respect to Phaseolus multiflorus, Vicia
+faba, and Avena sativa show, as it seems to us, that a layer of grease
+spread for a length of 1 � to 2 mm. over the tip of the radicle, or the
+destruction of the tip by caustic, greatly lessens or quite annuls in the
+upper and exposed part the power of bending towards a neighbouring source
+of moisture. We should bear in mind that the part which bends most, lies at
+some little distance above the greased or cauterised tip; and that the
+rapid growth of this part, proves that it has not been injured by the tips
+having been thus treated. In those cases in which the radicles with greased
+tips became curved, it is possible that the layer of grease was not
+sufficiently thick wholly to exclude moisture, or that a sufficient length
+was not thus protected, or, in the case of the caustic, not destroyed. When
+radicles with greased tips are left to grow for several days in damp air,
+the grease is drawn out into the finest reticulated threads and dots, with
+narrow portions of the surface left clean. Such portions would, it is
+probable, be able to absorb moisture, and thus we can account for several
+of the radicles with greased tips having become curved towards the sieve
+after an interval of one or two days. On the whole, we may infer that
+sensitiveness to a difference in the amount of moisture in the air on the
+two sides of a radicle resides in the tip, which transmits some influence
+to the upper part, causing it to bend towards the source of moisture.
+Consequently, the movement is the reverse of that caused by objects
+attached to one side of the tip, or by a thin slice being cut off, or by
+being slightly cauterised. In a future chapter it will be shown that
+sensitiveness to the attraction of
+[page 186]
+gravity likewise resides in the tip; so that it is the tip which excites
+the adjoining parts of a horizontally extended radicle to bend towards the
+centre of the earth.
+
+SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY THE DESTRUCTION OR
+INJURY OF THE TERMINAL PART OF THE PRIMARY RADICLE.
+
+Sachs has shown that the lateral or secondary radicles of the bean, and
+probably of other plants, are acted on by geotropism in so peculiar a
+manner, that they grow out horizontally or a little inclined downwards; and
+he has further shown* the interesting fact, that if the end of the primary
+radicle be cut off, one of the nearest secondary radicles changes its
+nature and grows perpendicularly downwards, thus replacing the primary
+radicle. We repeated this experiment, and planted beans with amputated
+radicles in friable peat, and saw the result described by Sachs; but
+generally two or three of the secondary radicles grew perpendicularly
+downwards. We also modified the experiment, by pinching young radicles a
+little way above their tips, between the arms of a U-shaped piece of thick
+leaden wire. The part pinched was thus flattened, and was afterwards
+prevented from growing thicker. Five radicles had their ends cut off, and
+served as controls or standards. Eight were pinched; of these 2 were
+pinched too severely and their ends died and dropped off; 2 were not
+pinched enough and were not sensibly affected; the remaining 4 were pinched
+sufficiently to check the growth of the terminal part, but did not appear
+otherwise injured. When the U-shaped wires were removed, after an
+
+* 'Arbeiten Bot. Institut., W�rzburg,' Heft iv. 1874, p. 622.
+[page 187]
+
+interval of 15 days, the part beneath the wire was found to be very thin
+and easily broken, whilst the part above was thickened. Now in these four
+cases, one or more of the secondary radicles, arising from the thickened
+part just above the wire, had grown perpendicularly downwards. In the best
+case the primary radicle (the part below the wire being 1 � inch in length)
+was somewhat distorted, and was not half as long as three adjoining
+secondary radicles, which had grown vertically, or almost vertically,
+downwards. Some of these secondary radicles adhered together or had become
+confluent. We learn from these four cases that it is not necessary, in
+order that a secondary radicle should assume the nature of a primary one,
+that the latter should be actually amputated; it is sufficient that the
+flow of sap into it should be checked, and consequently should be directed
+into the adjoining secondary radicles; for this seems to be the most
+obvious result of the primary radicle being pinched between the arms of a
+U-shaped wire.
+
+This change in the nature of secondary radicles is clearly analogous, as
+Sachs has remarked, to that which occurs with the shoots of trees, when the
+leading one is destroyed and is afterwards replaced by one or more of the
+lateral shoots; for these now grow upright instead of sub-horizontally. But
+in this latter case the lateral shoots are rendered apogeotropic, whereas
+with radicles the lateral ones are rendered geotropic. We are naturally led
+to suspect that the same cause acts with shoots as with roots, namely, an
+increased flow of sap into the lateral ones. We made some trials with Abies
+communis and pectinata, by pinching with wire the leading and all the
+lateral shoots excepting one. But we believe that they were too old when
+experimented on; and some were pinched too severely, and
+[page 188]
+some not enough. Only one case succeeded, namely, with the spruce-fir. The
+leading shoot was not killed, but its growth was checked; at its base there
+were three lateral shoots in a whorl, two of which were pinched, one being
+thus killed; the third was left untouched. These lateral shoots, when
+operated on (July 14th) stood at an angle of 8o above the horizon; by Sept.
+8th the unpinched one had risen 35o; by Oct. 4th it had risen 46o, and by
+Jan. 26th 48o, and it had now become a little curved inwards. Part of this
+rise of 48o may be attributed to ordinary growth, for the pinched shoot
+rose 12o within the same period. It thus follows that the unpinched shoot
+stood, on Jan. 26th, 56o above the horizon, or 34o from the vertical; and
+it was thus obviously almost ready to replace the slowly growing, pinched,
+leading shoot. Nevertheless, we feel some doubt about this experiment, for
+we have since observed with spruce-firs growing rather unhealthily, that
+the lateral shoots near the summit sometimes become highly inclined, whilst
+the leading shoot remains apparently sound.
+
+A widely different agency not rarely causes shoots which naturally would
+have brown out horizontally to grow up vertically. The lateral branches of
+the Silver Fir (A. pectinata) are often affected by a fungus, Aecidium
+elatinum, which causes the branch to enlarge into an oval knob formed of
+hard wood, in one of which we counted 24 rings of growth. According to De
+Bary*, when the mycelium penetrates a bud beginning to elongate, the shoot
+developed from it grows vertically upwards. Such upright shoots after-
+
+* See his valuable article in 'Bot. Zeitung,' 1867, p. 257, on these
+monstrous growths, which are called in German "Hexenbesen," or
+"witch-brooms."
+[page 189]
+
+wards produce lateral and horizontal branches; and they then present a
+curious appearance, as if a young fir-tree had grown out of a ball of clay
+surrounding the branch. These upright shoots have manifestly changed their
+nature and become apogeotropic; for if they had not been affected by the
+Aecidium, they would have grown out horizontally like all the other twigs
+on the same branches. This change can hardly be due to an increased flow of
+sap into the part; but the presence of the mycelium will have greatly
+disturbed its natural constitution.
+
+According to Mr. Meehan,* the stems of three species of Euphorbia and of
+Portulaca oleracea are "normally prostrate or procumbent;" but when they
+are attacked by an Aecidium, they "assume an erect habit." Dr. Stahl
+informs us that he knows of several analogous cases; and these seem to be
+closely related to that of the Abies. The rhizomes of Sparganium ramosum
+grow out horizontally in the soil to a considerable length, or are
+diageotropic; but F. Elfving found that when they were cultivated in water
+their tips turned upwards, and they became apogeotropic. The same result
+followed when the stem of the plant was bent until it cracked or was merely
+much bowed.**
+
+No explanation has hitherto been attempted of such cases as the foregoing,-
+-namely, of secondary radicles growing vertically downwards, and of lateral
+shoots growing vertically upwards, after the amputation of
+
+* 'Proc. Acad. Nat. Sc. Philadelphia,' June 16th, 1874, and July 23rd,
+1875.
+** See F. Elfving's interesting paper in 'Arbeiten Bot. Institut., in
+W�rzburg,' vol. ii. 1880, p. 489. Carl Kraus (Triesdorf) had previously
+observed ('Flora,' 1878, p. 324) that the underground shoots of Triticum
+repens bend vertically up when the parts above ground are removed, and when
+the rhizomes are kept partly immersed in water.
+[page 190]
+
+the primary radicle or of the leading shoot. The following considerations
+give us, as we believe, the clue. Firstly, any cause which disturbs the
+constitution* is apt to induce reversion; such as the crossing of two
+distinct races, or a change of conditions, as when domestic animals become
+feral. But the case which most concerns us, is the frequent appearance of
+peloric flowers on the summit of a stem, or in the centre of the
+inflorescence,--parts which, it is believed, receive the most sap; for when
+an irregular flower becomes perfectly regular or peloric, this may be
+attributed, at least partly, to reversion to a primitive and normal type.
+Even the position of a seed at the end of the capsule sometimes gives to
+the seedling developed from it a tendency to revert. Secondly, reversions
+often occur by means of buds, independently of reproduction by seed; so
+that a bud may revert to the character of a former state many
+bud-generations ago. In the case of animals, reversions may occur in the
+individual with advancing age. Thirdly and lastly, radicles when they first
+protrude from the seed are always geotropic, and plumules or shoots almost
+always apogeotropic. If then any cause, such as an increased flow of sap or
+the presence of mycelium, disturbs the constitution of a lateral shoot or
+of a secondary radicle, it is apt to revert to its primordial state; and it
+becomes either apogeotropic or geotropic, as the case may be, and
+consequently grows either vertically upwards or downwards. It is indeed
+pos-
+
+* The facts on which the following conclusions are founded are given in
+'The Variation of Animals and Plants under Domestication,' 2nd edit. 1875.
+On the causes leading to reversion see chap. xii. vol. ii. and p. 59, chap.
+xiv. On peloric flowers, chap. xiii. p. 32; and see p. 337 on their
+position on the plant. With respect to seeds, p. 340. On reversion by means
+of buds, p. 438, chap. xi. vol. i.
+[page 191]
+
+sible, or even probable, that this tendency to reversion may have been
+increased, as it is manifestly of service to the plant.
+
+SUMMARY OF CHAPTER.
+
+A part or organ may be called sensitive, when its irritation excites
+movement in an adjoining part. Now it has been shown in this chapter, that
+the tip of the radicle of the bean is in this sense sensitive to the
+contact of any small object attached to one side by shellac or gum-water;
+also to a slight touch with dry caustic, and to a thin slice cut off one
+side. The radicles of the pea were tried with attached objects and caustic,
+both of which acted. With Phaseolus multiflorus the tip was hardly
+sensitive to small squares of attached card, but was sensitive to caustic
+and to slicing. The radicles of Tropaeolum were highly sensitive to
+contact; and so, as far as we could judge, were those of Gossypium
+herbaceum, and they were certainly sensitive to caustic. The tips of the
+radicles of Cucurbita ovifera were likewise highly sensitive to caustic,
+though only moderately so to contact. Raphanus sativus offered a somewhat
+doubtful case. With Aesculus the tips were quite indifferent to bodies
+attached to them, though sensitive to caustic. Those of Quercus robur and
+Zea mays were highly sensitive to contact, as were the radicles of the
+latter to caustic. In several of these cases the difference in
+sensitiveness of the tip to contact and to caustic was, as we believe,
+merely apparent; for with Gossypium, Raphanus, and Cucurbita, the tip was
+so fine and flexible that it was very difficult to attach any object to one
+of its sides. With the radicles of Aesculus, the tips were not at all
+sensitive to small bodies attached to them; but it does not follow from
+this
+[page 192]
+fact that they would not have been sensitive to somewhat greater continued
+pressure, if this could have been applied.
+
+The peculiar form of sensitiveness which we are here considering, is
+confined to the tip of the radicle for a length of from 1 mm. to 1.5 mm.
+When this part is irritated by contact with any object, by caustic, or by a
+thin slice being cut off, the upper adjoining part of the radicle, for a
+length of from 6 or 7 to even 12 mm., is excited to bend away from the side
+which has been irritated. Some influence must therefore be transmitted from
+the tip along the radicle for this length. The curvature thus caused is
+generally symmetrical. The part which bends most apparently coincides with
+that of the most rapid growth. The tip and the basal part grow very slowly
+and they bend very little.
+
+Considering the widely separated position in the vegetable series of the
+several above-named genera, we may conclude that the tips of the radicles
+of all, or almost all, plants are similarly sensitive, and transmit an
+influence causing the upper part to bend. With respect to the tips of the
+secondary radicles, those of Vicia faba, Pisum sativum, and Zea mays were
+alone observed, and they were found similarly sensitive.
+
+In order that these movements should be properly displayed, it appears
+necessary that the radicles should grow at their normal rate. If subjected
+to a high temperature and made to grow rapidly, the tips seem either to
+lose their sensitiveness, or the upper part to lose the power of bending.
+So it appears to be if they grow very slowly from not being vigorous, or
+from being kept at too low a temperature; also when they are forced to
+germinate in the middle of the winter.
+[page 193]
+
+The curvature of the radicle sometimes occurs within from 6 to 8 hours
+after the tip has been irritated, and almost always within 24 h., excepting
+in the case of the massive radicles of Aesculus. The curvature often
+amounts to a rectangle,--that is, the terminal part bends upwards until the
+tip, which is but little curved, projects almost horizontally. Occasionally
+the tip, from the continued irritation of the attached object, continues to
+bend up until it forms a hook with the point directed towards the zenith,
+or a loop, or even a spire. After a time the radicle apparently becomes
+accustomed to the irritation, as occurs in the case of tendrils, for it
+again grows downwards, although the bit of card or other object may remain
+attached to the tip. It is evident that a small object attached to the free
+point of a vertically suspended radicle can offer no mechanical resistance
+to its growth as a whole, for the object is carried downwards as the
+radicle elongates, or upwards as the radicle curves upwards. Nor can the
+growth of the tip itself be mechanically checked by an object attached to
+it by gum-water, which remains all the time perfectly soft. The weight of
+the object, though quite insignificant, is opposed to the upward curvature.
+We may therefore conclude that it is the irritation due to contact which
+excites the movement. The contact, however, must be prolonged, for the tips
+of 15 radicles were rubbed for a short time, and this did not cause them to
+bend. Here then we have a case of specialised sensibility, like that of the
+glands of Drosera; for these are exquisitely sensitive to the slightest
+pressure if prolonged, but not to two or three rough touches.
+
+When the tip of a radicle is lightly touched on one side with dry nitrate
+of silver, the injury caused is
+[page 194]
+very slight, and the adjoining upper part bends away from the cauterised
+point, with more certainty in most cases than from an object attached on
+one side. Here it obviously is not the mere touch, but the effect produced
+by the caustic, which induces the tip to transmit some influence to the
+adjoining part, causing it to bend away. If one side of the tip is badly
+injured or killed by the caustic, it ceases to grow, whilst the opposite
+side continues growing; and the result is that the tip itself bends towards
+the injured side and often becomes completely hooked; and it is remarkable
+that in this case the adjoining upper part does not bend. The stimulus is
+too powerful or the shock too great for the proper influence to be
+transmitted from the tip. We have strictly analogous cases with Drosera,
+Dionaea and Pinguicula, with which plants a too powerful stimulus does not
+excite the tentacles to become incurved, or the lobes to close, or the
+margin to be folded inwards.
+
+With respect to the degree of sensitiveness of the apex to contact under
+favourable conditions, we have seen that with Vicia faba a little square of
+writing-paper affixed with shellac sufficed to cause movement; as did on
+one occasion a square of merely damped goldbeaters' skin, but it acted very
+slowly. Short bits of moderately thick bristle (of which measurements have
+been given) affixed with gum-water acted in only three out of eleven
+trials, and beads of dried shellac under 1/200th of a grain in weight acted
+only twice in nine cases; so that here we have nearly reached the minimum
+of necessary irritation. The apex, therefore, is much less sensitive to
+pressure than the glands of Drosera, for these are affected by far thinner
+objects than bits of bristle, and by a very much less weight than 1/200th
+of a grain.
+[page 195]
+But the most interesting evidence of the delicate sensitiveness of the tip
+of the radicle, was afforded by its power of discriminating between
+equal-sized squares of card-like and very thin paper, when these were
+attached on opposite sides, as was observed with the radicles of the bean
+and oak.
+
+When radicles of the bean are extended horizontally with squares of card
+attached to the lower sides of their tips, the irritation thus caused was
+always conquered by geotropism, which then acts under the most favourable
+conditions at right angles to the radicle. But when objects were attached
+to the radicles of any of the above-named genera, suspended vertically, the
+irritation conquered geotropism, which latter power at first acted
+obliquely on the radicle; so that the immediate irritation from the
+attached object, aided by its after-effects, prevailed and caused the
+radicle to bend upwards, until sometimes the point was directed to the
+zenith. We must, however, assume that the after-effects of the irritation
+of the tip by an attached object come into play, only after movement has
+been excited. The tips of the radicles of the pea seem to be more sensitive
+to contact than those of the bean, for when they were extended horizontally
+with squares of card adhering to their lower sides, a most curious struggle
+occasionally arose, sometimes one and sometimes the other force prevailing,
+but ultimately geotropism was always victorious; nevertheless, in two
+instances the terminal part became so much curved upwards that loops were
+subsequently formed. With the pea, therefore, the irritation from an
+attached object, and from geotropism when acting at right angles to the
+radicle, are nearly balanced forces. Closely similar results were observed
+with the horizontally extended radicles of Cucurbita ovifera,
+[page 196]
+when their tips were slightly cauterised on the lower side.
+
+Finally, the several co-ordinated movements by which radicles are enabled
+to perform their proper functions are admirably perfect. In whatever
+direction the primary radicle first protrudes from the seed, geotropism
+guides it perpendicularly downwards; and the capacity to be acted on by the
+attraction of gravity resides in the tip. But Sachs has proved* that the
+secondary radicles, or those emitted by the primary one, are acted on by
+geotropism in such a manner that they tend to bend only obliquely
+downwards. If they had been acted on like the primary radicle, all the
+radicles would have penetrated the ground in a close bundle. We have seen
+that if the end of the primary radicle is cut off or injured, the adjoining
+secondary radicles become geotropic and grow vertically downwards. This
+power must often be of great service to the plant, when the primary radicle
+has been destroyed by the larvae of insects, burrowing animals, or any
+other accident. The tertiary radicles, or those emitted by the secondary
+ones, are not influenced, at least in the case of the bean, by geotropism;
+so they grow out freely in all directions. From this manner of growth of
+the various kinds of radicles, they are distributed, together with their
+absorbent hairs, throughout the surrounding soil, as Sachs has remarked, in
+the most advantageous manner; for the whole soil is thus closely searched.
+
+ Geotropism, as was shown in the last chapter, excites the primary radicle
+to bend downwards with very little force, quite insufficient to penetrate
+the ground. Such penetration is effected by the pointed
+
+* 'Arbeiten Bot. Institut, W�rzburg,' Heft iv. 1874, pp. 605-631.
+[page 197]
+
+apex (protected by the root-cap) being pressed down by the longitudinal
+expansion or growth of the terminal rigid portion, aided by its transverse
+expansion, both of which forces act powerfully. It is, however,
+indispensable that the seeds should be at first held down in some manner.
+When they lie on the bare surface they are held down by the attachment of
+the root-hairs to any adjoining objects; and this apparently is effected by
+the conversion of their outer surfaces into a cement. But many seeds get
+covered up by various accidents, or they fall into crevices or holes. With
+some seeds their own weight suffices. The circumnutating movement of the
+terminal growing part both of the primary and secondary radicles is so
+feeble that it can aid them very little in penetrating the ground,
+excepting when the superficial layer is very soft and damp. But it must aid
+them materially when they happen to break obliquely into cracks, or into
+burrows made by earth-worms or larvae. This movement, moreover, combined
+with the sensitiveness of the tip to contact, can hardly fail to be of the
+highest importance; for as the tip is always endeavouring to bend to all
+sides it will press on all sides, and will thus be able to discriminate
+between the harder and softer adjoining surfaces, in the same manner as it
+discriminated between the attached squares of card-like and thin paper.
+Consequently it will tend to bend from the harder soil, and will thus
+follow the lines of least resistance. So it will be if it meets with a
+stone or the root of another plant in the soil, as must incessantly occur.
+If the tip were not sensitive, and if it did not excite the upper part of
+the root to bend away, whenever it encountered at right angles some
+obstacle in the ground, it would be liable
+[page 198]
+to be doubled up into a contorted mass. But we have seen with radicles
+growing down inclined plates of glass, that as soon as the tip merely
+touched a slip of wood cemented across the plate, the whole terminal
+growing part curved away, so that the tip soon stood at right angles to its
+former direction; and thus it would be with an obstacle encountered in the
+ground, as far as the pressure of the surrounding soil would permit. We can
+also understand why thick and strong radicles, like those of Aesculus,
+should be endowed with less sensitiveness than more delicate ones; for the
+former would be able by the force of their growth to overcome any slight
+obstacle.
+
+After a radicle, which has been deflected by some stone or root from its
+natural downward course, reaches the edge of the obstacle, geotropism will
+direct it to grow again straight downward; but we know that geotropism acts
+with very little force, and here another excellent adaptation, as Sachs has
+remarked,* comes into play. For the upper part of the radicle, a little
+above the apex, is, as we have seen, likewise sensitive; and this
+sensitiveness causes the radicle to bend like a tendril towards the
+touching object, so that as it rubs over the edge of an obstacle, it will
+bend downwards; and the curvature thus induced is abrupt, in which respect
+it differs from that caused by the irritation of one side of the tip. This
+downward bending coincides with that due to geotropism, and both will cause
+the root to resume its original course.
+
+As radicles perceive an excess of moisture in the air on one side and bend
+towards this side, we may infer that they will act in the same manner with
+respect to moisture in the earth. The sensitiveness to moisture
+
+* 'Arbeiten Bot. Inst., W�rzburg,' Heft iii. p. 456.
+[page 199]
+
+resides in the tip, which determines the bending of the upper part. This
+capacity perhaps partly accounts for the extent to which drain-pipes often
+become choked with roots.
+
+Considering the several facts given in this chapter, we see that the course
+followed by a root through the soil is governed by extraordinarily complex
+and diversified agencies,--by geotropism acting in a different manner on
+the primary, secondary, and tertiary radicles,--by sensitiveness to
+contact, different in kind in the apex and in the part immediately above
+the apex, and apparently by sensitiveness to the varying dampness of
+different parts of the soil. These several stimuli to movement are all more
+powerful than geotropism, when this acts obliquely on a radicle, which has
+been deflected from its perpendicular downward course. The roots, moreover,
+of most plants are excited by light to bend either to or from it; but as
+roots are not naturally exposed to the light it is doubtful whether this
+sensitiveness, which is perhaps only the indirect result of the radicles
+being highly sensitive to other stimuli, is of any service to the plant.
+The direction which the apex takes at each successive period of the growth
+of a root, ultimately determines its whole course; it is therefore highly
+important that the apex should pursue from the first the most advantageous
+direction; and we can thus understand why sensitiveness to geotropism, to
+contact and to moisture, all reside in the tip, and why the tip determines
+the upper growing part to bend either from or to the exciting cause. A
+radicle may be compared with a burrowing animal such as a mole, which
+wishes to penetrate perpendicularly down into the ground. By continually
+moving his head from side to side, or circumnutating, he will feel any
+stone
+[page 200]
+or other obstacle, as well as any difference in the hardness of the soil,
+and he will turn from that side; if the earth is damper on one than on the
+other side he will turn thitherward as a better hunting-ground.
+Nevertheless, after each interruption, guided by the sense of gravity, he
+will be able to recover his downward course and to burrow to a greater
+depth.
+[page 201]
+
+
+
+CHAPTER IV.
+
+THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.
+
+Circumnutation of stems: concluding remarks on--Circumnutation of stolons:
+aid thus afforded in winding amongst the stems of surrounding plants--
+Circumnutation of flower-stems--Circumnutation of Dicotyledonous leaves--
+Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sink
+at night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concluding
+remarks on the circumnutation of leaves; generally rise in the evening and
+sink in the morning.
+
+WE have seen in the first chapter that the stems of all seedlings, whether
+hypocotyls or epicotyls, as well as the cotyledons and the radicles, are
+continually circumnutating--that is they grow first on one side and then on
+another, such growth being probably preceded by increased turgescence of
+the cells. As it was unlikely that plants should change their manner of
+growth with advancing age, it seemed probable that the various organs of
+all plants at all ages, as long as they continued to grow, would be found
+to circumnutate, though perhaps to an extremely small extent. As it was
+important for us to discover whether this was the case, we determined to
+observe carefully a certain number of plants which were growing vigorously,
+and which were not known to move in any manner. We commenced with stems.
+Observations of this kind are tedious, and it appeared to us that it would
+be sufficient to observe the stems in about a score of genera, belonging to
+widely distinct families and inhabitants of various countries. Several
+plants
+[page 202]
+were selected which, from being woody, or for other reasons, seemed the
+least likely to circumnutate. The observations and the diagrams were made
+in the manner described in the Introduction. Plants in pots were subjected
+to a proper temperature, and whilst being observed, were kept either in
+darkness or were feebly illuminated from above. They are arranged in the
+order adopted by Hooker in Le Maout and Decaisne's 'System of Botany.' The
+number of the family to which each genus belongs is appended, as this
+serves to show the place of each in the series.
+
+[(1.) Iberis umbellata (Cruciferae, Fam. 14).--The movement of the stem of
+a young plant, 4 inches in height, consisting of four internodes (the
+hypocotyl included) besides a large bud
+
+Fig. 70. Iberis umbellata: circumnutation of stem of young plant, traced
+from 8.30 A.M. Sept. 13th to same hour on following morning. Distance of
+summit of stem beneath the horizontal glass 7.6 inches. Diagram reduced to
+half of original size. Movement as here shown magnified between 4 and 5
+times.
+
+on the summit, was traced, as here shown, during 24 h. (Fig. 70). As far as
+we could judge the uppermost inch alone of the stem circumnutated, and this
+in a simple manner. The movement was slow, and the rate very unequal at
+different times. In part of its course an irregular ellipse, or rather
+triangle, was completed in 6 h. 30 m.
+
+ (2.) Brassica oleracea (Cruciferae).--A very young plant, bearing three
+leaves, of which the longest was only three-quarters of an inch in length,
+was placed under a microscope, furnished with an eye-piece micrometer, and
+the tip of the largest leaf was
+[page 203]
+found to be in constant movement. It crossed five divisions of the
+micrometer, that is, 1/100th of an inch, in 6 m. 20 s. There could hardly
+be a doubt that it was the stem which chiefly moved, for the tip did not
+get quickly out of focus; and this would have occurred had the movement
+been confined to the leaf, which moves up or down in nearly the same
+vertical plane.
+
+(3.) Linum usitatissimum (Lineae, Fam. 39).--The stems of this plant,
+shortly before the flowering period, are stated by Fritz M�ller ('Jenaische
+Zeitschrift,' B. v. p. 137) to revolve, or circumnutate.
+
+(4.) Pelargonium zonale (Geraniaceae, Fam. 47).--A young plant, 7 � inches
+in height, was observed in the usual manner; but, in order to see the bead
+at the end of the glass filament
+
+Fig. 71. Pelargonium zonale: circumnutation of stem of young plant, feebly
+illuminated from above. Movement of bead magnified about 11 times; traced
+on a horizontal glass from noon on March 9th to 8 A.M. on the 11th.
+
+and at the same time the mark beneath, it was necessary to cut off three
+leaves on one side. We do not know whether it was owing to this cause, or
+to the plant having previously become bent to one side through
+heliotropism, but from the morning of the 7th of March to 10.30 P.M. on the
+8th, the stem moved a considerable distance in a zigzag line in the same
+general direction. During the night of the 8th it moved to some distance at
+right angles to its former course, and next morning (9th) stood for a time
+almost still. At noon on the 9th a new tracing was begun (see Fig. 71),
+which was continued till 8 A.M. on the 11th. Between noon on the 9th and 5
+P.M. on the 10th (i.e. in the course of 29 h.), the stem described a
+circle. This plant therefore circumnutates, but at a very slow rate, and to
+a small extent.
+
+ (5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb); (Geraniaceae,
+Fam. 47).--The species of this genus climb by the
+[page 204]
+aid of their sensitive petioles, but some of them also twine round
+supports; but even these latter species do not begin to circumnutate in a
+conspicuous manner whilst young. The
+
+Fig. 72. Tropaeolum majus (?): circumnutation of stem of young plant,
+traced on a horizontal glass from 9 A.M. Dec. 26th to 10 A.M. on 27th.
+Movement of bead magnified about 5 times, and here reduced to half of
+original scale.
+
+variety here treated of has a rather thick stem, and is so dwarf that
+apparently it does not climb in any manner. We therefore wished to
+ascertain whether the stem of a young plant, consisting of two internodes,
+together 3.2 inches in height, circumnutated. It was observed during 25 h.,
+and we see in Fig. 72 that the stem moved in a zigzag course, indicating
+circumnutation.
+
+Fig. 73. Trifolium resupinatum: circumnutation of stem, traced on vertical
+glass from 9.30 A.M. to 4.30 P.M. Nov. 3rd. Tracing not greatly magnified,
+reduced to half of original size. Plant feebly illuminated from above.
+
+ (6.) Trifolium resupinatum (Leguminosae, Fam. 75).--When we treat of the
+sleep of plants, we shall see that the stems in several Leguminous genera,
+for instance, those of Hedysarum, Mimosa, Melilotus, etc., which are not
+climbers, circumnutate in a conspicuous manner. We will here give only a
+single instance (Fig. 73), showing the circumnutation of the stem of a
+large plant of a clover, Trifolium resupinatum. In the course of 7 h. the
+stem changed
+[page 205]
+its course greatly eight times and completed three irregular circles or
+ellipses. It therefore circumnutated rapidly. Some of the lines run at
+right angles to one another.
+
+Fig. 74. Rubus (hybrid): circumnutation of stem, traced on horizontal
+glass, from 4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magnified,
+reduced to half of original size. Plant illuminated feebly from above.
+
+ (7.) Rubus idaeus (hybrid) (Rosaceae, Fam. 76).--As we happened to have a
+young plant, 11 inches in height and growing vigorously, which had been
+raised from a cross between the raspberry (Rubus idaeus) and a North
+American Rubus, it was observed in the usual manner. During the morning of
+March 14th the stem almost completed a circle, and then moved far to the
+right. At 4 P.M. it reversed its course, and now a fresh tracing was begun,
+which was continued during 40 � h., and is given in Fig. 74. We here have
+well-marked circumnutation.
+
+(8.) Deutzia gracilis (Saxifrageae, Fam. 77).--A shoot on a bush about 18
+inches in height was observed. The bead changed its course greatly eleven
+times in the course of 10 h. 30 m. (Fig. 75), and there could be no doubt
+about the circumnutation of the stem.
+
+Fig. 75. Deutzia gracilis: circumnutation of stem, kept in darkness, traced
+on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead
+originally magnified about 20 times, here reduced to half scale.
+
+(9.) Fuchsia (greenhouse var., with large flowers, probably a hybrid)
+(Onagrarieae, Fam. 100).--A young plant, 15 inches in height, was observed
+during nearly 48 h. The
+[page 206]
+accompanying figure (Fig. 76) gives the necessary particulars, and shows
+that the stem circumnutated, though rather slowly.
+
+Fig. 76. Fuchsia (garden var.): circumnutation of stem, kept in darkness,
+traced on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement
+of bead originally magnified about 40 times, here reduced to half scale.
+
+(10.) Cereus speciocissimus (garden var., sometimes called Phyllocactus
+multiflorus) (Cacteae, Fam. 109).--This plant, which was growing vigorously
+from having been removed a few days before from the greenhouse to the
+hot-house, was observed with especial interest, as it seemed so little
+probable that the stem would circumnutate. The branches are flat, or
+flabelliform; but some of them are triangular in section, with the three
+sides hollowed out. A branch of this latter shape, 9 inches in length and 1
+� in diameter, was chosen for observation, as less likely to circumnutate
+than a flabelliform branch. The movement of the bead at the end of the
+glass filament, affixed to the summit of the branch, was traced (A, Fig.
+77) from 9.23 A.M. to 4.30 P.M. on Nov. 23rd, during which time it changed
+its course greatly six times. On the 24th another tracing was made (see B),
+and the bead on this day changed its course oftener, making in 8 h. what
+may be considered as four ellipses, with their longer axes differently
+directed. The position of the stem and its commencing course on the
+following morning are likewise shown. There can be no doubt that this
+branch, though appearing quite rigid, circumnutated; but the
+[page 207]
+extreme amount of movement during the time was very small, probably rather
+less than the 1/20th of an inch.
+
+Fig 77. Cereus speciocissimus: circumnutation of stem, illuminated from
+above, traced on a horizontal glass, in A from 9 A.M. to 4.30 P.M. on Nov.
+23rd; and in B from 8.30 A.M. on the 24th to 8 A.M. on the 25th. Movement
+of the bead in B magnified about 38 times.
+
+(11.) Hedera helix (Araliaceae, Fam. 114).--The stem is known to be
+apheliotropic, and several seedlings growing in a pot in the greenhouse
+became bent in the middle of the summer at right angles from the light. On
+Sept. 2nd some of these stems were tied up so as to stand vertically, and
+were placed before a north-east window; but to our surprise they were now
+decidedly heliotropic, for during 4 days they curved themselves towards the
+light, and their course being traced on a horizontal glass, was strongly
+zigzag. During the 6 succeeding days they circumnutated over the same small
+space at a slow rate, but there could be no doubt about their
+circumnutation. The plants were kept exactly in the same place before the
+window, and after an interval of 15 days the stems were again observed
+during 2 days and their movements traced, and
+[page 208]
+they were found to be still circumnutating, but on a yet smaller scale.
+
+(12.) Gazania ringens (Compositae, Fam. 122).--The circumnutation of the
+stem of a young plant, 7 inches in height, as measured to the tip of the
+highest leaf, was traced during 33 h., and is shown in the accompanying
+figure (Fig. 78). Two
+
+Fig. 78. Gazania ringens: circumnutation of stem traced from 9 A.M. March
+21st to 6 P.M. on 22nd; plant kept in darkness. Movement of bead at the
+close of the observations magnified 34 times, here reduced to half the
+original scale.
+
+main lines may be observed running at nearly right angles to two other main
+lines; but these are interrupted by small loops.
+
+(13.) Azalea Indica (Ericineae, Fam. 128).--A bush 21 inches in height was
+selected for observation, and the circumnutation of its leading shoot was
+traced during 26 h. 40 m., as shown in the following figure (Fig. 79).
+
+ (14.) Plumbago Capensis (Plumbagineae, Fam. 134).--A small lateral branch
+which projected from a tall freely growing bush, at an angle of 35o above
+the horizon, was selected for observation. For the first 11 h. it moved to
+a considerable distance in a nearly straight line to one side, owing
+probably to its having been previously deflected by the light whilst
+standing in the greenhouse. At 7.20 P.M. on March 7th a fresh tracing was
+begun and continued for the next 43 h. 40 m. (see Fig. 80). During the
+first 2 h. it followed nearly the same direction as before, and then
+changed it a little; during the night it moved at nearly right angles to
+its previous course. Next
+[page 209]
+day (8th) it zigzagged greatly, and on the 9th moved irregularly round and
+round a small circular space. By 3 P.M. on the 9th the figure had become so
+complicated that no more dots could be made; but the shoot continued during
+the evening of the 9th, the whole of the 10th, and the morning of the 11th
+to
+
+Fig. 79. Azalea Indica: circumnutation of stem, illuminated from above,
+traced on horizontal glass, from 9.30 A.M. March 9th to 12.10 P.M. on the
+10th. But on the morning of the 10th only four dots were made between 8.30
+A.M. and 12.10 P.M., both hours included, so that the circumnutation is not
+fairly represented in this part of the diagram. Movement of the bead here
+magnified about 30 times.
+
+Fig. 80. Plumbago Capensis: circumnutation of tip of a lateral branch,
+traced on horizontal glass, from 7.20 P.M. on March 7th to 3 P.M. on the
+9th. Movement of bead magnified 13 times. Plant feebly illuminated from
+above.
+
+circumnutate over the same small space, which was only about the 1/26th of
+an inch (.97 mm.) in diameter. Although this branch circumnutated to a very
+small extent, yet it changed its course frequently. The movements ought to
+have been more magnified.
+
+(15.) Aloysia citriodora (Verbenaceae, Fam. 173).--The following figure
+(Fig. 81) gives the movements of a shoot during
+[page 210]
+31 h. 40 m., and shows that it circumnutated. The bush was 15 inches in
+height.
+
+Fig. 81. Aloysia citriodora: circumnutation of stem, traced from 8.20 A.M.
+on March 22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement magnified
+about 40 times.
+
+(16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.)
+(Verbenaceae).--A shoot 8 inches in height had been laid horizontally, for
+the sake of observing its apogeotropism, and the terminal portion had grown
+vertically upwards for a length of 1 � inch. A glass filament, with a bead
+at the end, was fixed
+
+Fig. 82. Verbena melindres: circumnutation of stem in darkness, traced on
+vertical glass, from 5.30 P.M. on June 5th to 11 A.M. June 7th. Movement of
+bead magnified 9 times.
+
+upright to the tip, and its movements were traced during 41 h. 30 m. on a
+vertical glass (Fig. 82). Under these circumstances the lateral movements
+were chiefly shown; but as the lines from side to side are not on the same
+level, the shoot
+[page 211]
+must have moved in a plane at right angles to that of the lateral movement,
+that is, it must have circumnutated. On the next day (6th) the shoot moved
+in the course of 16 h. four times to the right, and four times to the left;
+and this apparently represents the formation of four ellipses, so that each
+was completed in 4 h.
+(17.) Ceratophyllum demersum (Ceratophylleae, Fam. 220).--An interesting
+account of the movements of the stem of this water-plant has been published
+by M. E. Rodier.* The movements are confined to the young internodes,
+becoming less and less lower down the stem; and they are extraordinary from
+their amplitude. The stems sometimes moved through an angle of above 200o
+in 6 h., and in one instance through 220o in 3 h. They generally bent from
+right to left in the morning, and in an opposite direction in the
+afternoon; but the movement was sometimes temporarily reversed or quite
+arrested. It was not affected by light. It does not appear that M. Rodier
+made any diagram on a horizontal plane representing the actual course
+pursued by the apex, but he speaks of the "branches executing round their
+axes of growth a movement of torsion." From the particulars above given,
+and remembering in the case of twining plants and of tendrils, how
+difficult it is not to mistake their bending to all points of the compass
+for true torsion, we are led to believe that the stems of this
+Ceratophyllum circumnutate, probably in the shape of narrow ellipses, each
+completed in about 26 h. The following statement, however, seems to
+indicate something different from ordinary circumnutation, but we cannot
+fully understand it. M. Rodier says: "Il est alors facile de voir que le
+mouvement de flexion se produit d'abord dans les m�rithalles sup�rieurs,
+qu'il se propage ensuite, en s'amoindrissant du haut en bas; tandis qu'au
+contraire le movement de redressement commence par la partie inf�rieur pour
+se terminer a la partie sup�rieure qui, quelquefois, peu de temps avant de
+se relever tout � fait, forme avec l'axe un angle tr�s aigu."
+
+ (18.) Coniferae.--Dr. Maxwell Masters states ('Journal Linn. Soc.,' Dec.
+2nd, 1879) that the leading shoots of many Coniferae during the season of
+their active growth exhibit very remarkable movements of revolving
+nutation, that is, they circumnutate. We may feel sure that the lateral
+shoots whilst growing would exhibit the same movement if carefully
+observed.
+
+* 'Comptes Rendus,' April 30th, 1877. Also a second notice published
+separately in Bourdeaux, Nov. 12th, 1877.
+[page 212]
+
+(19.) Lilium auratum (Fam. Liliaceae).--The circumnutation
+
+Fig. 83. Lilium auratum: circumnutation of a stem in darkness, traced on a
+horizontal glass, from 8 A.M. on March 14th to 8.35 A.M. on 16th. But it
+should be noted that our observations were interrupted between 6 P.M. on
+the 14th and 12.15 P.M. on the 15th, and the movements during this interval
+of 18 h. 15 m. are represented by a long broken line. Diagram reduced to
+half original scale.
+
+of the stem of a plant 24 inches in height is represented in the above
+figure (Fig. 83).
+
+Fig. 84. Cyperus alternifolius: circumnutation of stem, illuminated from
+above, traced on horizontal glass, from 9.45 A.M. March 9th to 9 P.M. on
+10th. The stem grew so rapidly whilst being observed, that it was not
+possible to estimate how much its movements were magnified in the tracing.
+
+ (20.) Cyperus alternifolius (Fam. Cyperaceae.)--A glass
+[page 213]
+filament, with a bead at the end, was fixed across the summit of a young
+stem 10 inches in height, close beneath the crown of elongated leaves. On
+March 8th, between 12.20 and 7.20 P.M. the stem described an ellipse, open
+at one end. On the following day a new tracing was begun (Fig. 84), which
+plainly shows that the stem completed three irregular figures in the course
+of 35 h. 15 m.]
+
+Concluding Remarks on the Circumnutation of Stems.--Any one who will
+inspect the diagrams now given, and will bear in mind the widely separated
+position of the plants described in the series,--remembering that we have
+good grounds for the belief that the hypocotyls and epicotyls of all
+seedlings circumnutate,--not forgetting the number of plants distributed in
+the most distinct families which climb by a similar movement,--will
+probably admit that the growing stems of all plants, if carefully observed,
+would be found to circumnutate to a greater or less extent. When we treat
+of the sleep and other movements of plants, many other cases of
+circumnutating stems will be incidentally given. In looking at the
+diagrams, we should remember that the stems were always growing, so that in
+each case the circumnutating apex as it rose will have described a spire of
+some kind. The dots were made on the glasses generally at intervals of an
+hour, or hour and a half, and were then joined by straight lines. If they
+had been made at intervals of 2 or 3 minutes, the lines would have been
+more curvilinear, as in the case of the tracks left on the smoked
+glass-plates by the tips of the circumnutating radicles of seedling plants.
+The diagrams generally approach in form to a succession of more or less
+irregular ellipses or ovals, with their longer axes directed to different
+points of the compass during the same day or on succeeding days. The stems
+there-
+[page 214]
+fore, sooner or later, bend to all sides; but after a stem has bent in any
+one direction, it commonly bends back at first in nearly, though not quite,
+the opposite direction; and this gives the tendency to the formation of
+ellipses, which are generally narrow, but not so narrow as those described
+by stolons and leaves. On the other hand, the figures sometimes approach in
+shape to circles. Whatever the figure may be, the course pursued is often
+interrupted by zigzags, small triangles, loops, or ellipses. A stem may
+describe a single large ellipse one day, and two on the next. With
+different plants the complexity, rate, and amount of movement differ much.
+The stems, for instance, of Iberis and Azalea described only a single large
+ellipse in 24 h.; whereas those of the Deutzia made four or five deep
+zigzags or narrow ellipses in 11 � h., and those of the Trifolium three
+triangular or quadrilateral figures in 7 h.
+
+CIRCUMNUTATION OF STOLONS OR RUNNERS.
+
+Stolons consist of much elongated, flexible branches, which run along the
+surface of the ground and form roots at a distance from the parent-plant.
+They are therefore of the same homological nature as stems; and the three
+following cases may be added to the twenty previously given cases.
+
+[Fragaria (cultivated garden var.): Rosaceae.--A plant growing in a pot had
+emitted a long stolon; this was supported by a stick, so that it projected
+for the length of several inches horizontally. A glass filament bearing two
+minute triangles of paper was affixed to the terminal bud, which was a
+little upturned; and its movements were traced during 21 h., as shown in
+Fig. 85. In the course of the first 12 h. it moved twice up and twice down
+in somewhat zigzag lines, and no doubt travelled in the same manner during
+the night. On the following
+[page 215]
+morning after an interval of 20 h. the apex stood a little higher than it
+did at first, and this shows that the stolon had not been
+Fig. 85. Fragaria: circumnutation of stolon, kept in darkness, traced on
+vertical glass, from 10.45 A.M. May 18th to 7.45 A.M. on 19th.
+
+acted on within this time by geotropism;* nor had its own weight caused it
+to bend downwards.
+
+On the following morning (19th) the glass filament was detached and refixed
+close behind the bud, as it appeared possible that the circumnutation of
+the terminal bud and of the adjoining part of the stolon might be
+different. The movement was now traced during two consecutive days (Fig.
+86). During the first day the filament travelled in the course of 14 h. 30
+m. five times up and four times down, besides some lateral movement. On the
+20th the course was even more complicated, and can hardly be followed in
+the figure; but the filament moved in 16 h. at least five times up and five
+times down, with very little
+
+* Dr. A. B. Frank states ('Die Naturliche wagerechte Richtung von
+Pflanzentheilen,' 1870, p. 20) that the stolons of this plant are acted on
+by geotropism, but only after a considerable interval of time.
+[page 216]
+
+lateral deflection. The first and last dots made on this second day, viz.,
+at 7 A.M. and 11 P.M., were close together, showing that the stolon had not
+fallen or risen. Nevertheless, by comparing its position on the morning of
+the 19th and 21st, it is obvious that the stolon had sunk; and this may be
+attributed to slow bending down either from its own weight or from
+geotropism.
+
+Fig. 86. Fragaria: circumnutation of the same stolon as in the last figure,
+observed in the same manner, and traced from 8 A.M. May 19th to 8 A.M.
+21st.
+
+During a part of the 20th an orthogonal tracing was made by applying a cube
+of wood to the vertical glass and bringing the apex of the stolon at
+successive periods into a line with one edge; a dot being made each time on
+the glass. This tracing therefore represented very nearly the actual amount
+of movement of the apex; and in the course of 9 h. the distance of the
+extreme dots from one another was .45 inch. By the same method it was
+ascertained that the apex moved between 7 A.M. on the 20th and 8 A.M. on
+the 21st a distance of .82 inch.
+
+A younger and shorter stolon was supported so that it projected at about
+45o above the horizon, and its movement was traced by the same orthogonal
+method. On the first day the apex soon rose above the field of vision. By
+the next morning it had sunk, and the course pursued was now traced during
+14 h. 30 m. (Fig. 87). The amount of movement was almost the same,
+[page 217]
+from side to side as up and down; and differed in this respect remarkably
+from the movement in the previous cases. During the latter part of the day,
+viz., between 3 and 10.30 P.M., the
+
+Fig. 87. Fragaria: circumnutation of another and younger stolon, traced
+from 8 A.M. to 10.30 P.M. Figure reduced to one-half of original scale.
+
+actual distance travelled by the apex amounted to 1.15 inch; and in the
+course of the whole day to at least 2.67 inches. This is an amount of
+movement almost comparable with that of some climbing plants. The same
+stolon was observed on the following day, and now it moved in a somewhat
+less complex manner, in a plane not far from vertical. The extreme amount
+of actual movement was 1.55 inch in one direction, and .6 inch in another
+direction at right angles. During neither of these days did the stolon bend
+downwards through geotropism or its own weight.
+
+Four stolons still attached to the plant were laid on damp sand in the back
+of a room, with their tips facing the north-east windows. They were thus
+placed because De Vries says* that they are apheliotropic when exposed to
+the light of the sun; but we could not perceive any effect from the above
+feeble degree of illumination. We may add that on another occasion, late in
+the summer, some stolons, placed upright before a south-west window
+
+* 'Arbeiten Bot Inst., W�rzburg,' 1872, p. 434.
+[page 218]
+
+on a cloudy day, became distinctly curved towards the light, and were
+therefore heliotropic. Close in front of the tips of the prostrate stolons,
+a crowd of very thin sticks and the dried haulms of grasses were driven
+into the sand, to represent the crowded stems of surrounding plants in a
+state of nature. This was done for the sake of observing how the growing
+stolons would pass through them. They did so easily in the course of 6
+days, and their circumnutation apparently facilitated their passage. When
+the tips encountered sticks so close together that they could not pass
+between them, they rose up and passed over them. The sticks and haulms were
+removed after the passage of the four stolons, two of which were found to
+have assumed a permanently sinuous shape, and two were still straight. But
+to this subject we shall recur under Saxifraga.
+
+Saxifraga sarmentosa (Saxifrageae).--A plant in a suspended pot had emitted
+long branched stolons, which depended like
+
+Fig. 88. Saxifraga sarmentosa: circumnutation of an inclined stolon, traced
+in darkness on a horizontal glass, from 7.45 A.M. April 18th to 9 A.M. on
+19th. Movement of end of stolon magnified 2.2 times.
+
+threads on all sides. Two were tied up so as to stand vertically, and their
+upper ends became gradually bent downwards, but so slowly in the course of
+several days, that the bending was probably due to their weight and not to
+geotropism. A glass filament with little triangles of paper was fixed to
+the end of one of these stolons, which was 17 � inches in length, and had
+already become much bent down, but still projected at a considerable angle
+above the horizon. It moved only slightly three times from side to side and
+then upwards; on the following day
+[page 219]
+the movement was even less. As this stolon was so long we thought that its
+growth was nearly completed, so we tried another which was thicker and
+shorter, viz., 10 1/4 inches in length. It moved greatly, chiefly upwards,
+and changed its course five times in the course of the day. During the
+night it curved so much upwards in opposition to gravity, that the movement
+could no longer be traced on the vertical glass, and a horizontal one had
+to be used. The movement was followed during the next 25 h., as shown in
+Fig. 88. Three irregular ellipses, with their longer axes somewhat
+differently directed, were almost completed in the first 15 h. The extreme
+actual amount of movement of the tip during the 25 h. was .75 inch.
+Several stolons were laid on a flat surface of damp sand, in the same
+manner as with those of the strawberry. The friction of the sand did not
+interfere with their circumnutation; nor could we detect any evidence of
+their being sensitive to contact. In order to see how in a state of nature
+they would act, when encountering a stone or other obstacle on the ground,
+short pieces of smoked glass, an inch in height, were stuck upright into
+the sand in front of two thin lateral branches. Their tips scratched the
+smoked surface in various directions; one made three upward and two
+downward lines, besides a nearly horizontal one; the other curled quite
+away from the glass; but ultimately both surmounted the glass and pursued
+their original course. The apex of a third thick stolon swept up the glass
+in a curved line, recoiled and again came into contact with it; it then
+moved to the right, and after ascending, descended vertically; ultimately
+it passed round one end of the glass instead of over it.
+
+Many long pins were next driven rather close together into the sand, so as
+to form a crowd in front of the same two thin lateral branches; but these
+easily wound their way through the crowd. A thick stolon was much delayed
+in its passage; at one place it was forced to turn at right angles to its
+former course; at another place it could not pass through the pins, and the
+hinder part became bowed; it then curved upwards and passed through an
+opening between the upper part of some pins which happened to diverge; it
+then descended and finally emerged through the crowd. This stolon was
+rendered permanently sinuous to a slight degree, and was thicker where
+sinuous than elsewhere, apparently from its longitudinal growth having been
+checked.
+
+Cotyledon umbilicus (Crassulaceae).--A plant growing in a pan
+[page 220]
+of damp moss had emitted 2 stolons, 22 and 20 inches in length. One of
+these was supported, so that a length of 4 � inches projected in a straight
+and horizontal line, and the movement of the apex was traced. The first dot
+was made at 9.10 A.M.;
+
+Fig. 89. Cotyledon umbilicus: circumnutation of stolon, traced from 11.15
+A.M. Aug. 25th to 11 A.M. 27th. Plant illuminated from above. The terminal
+internode was .25 inch in length, the penultimate 2.25 and the third 3.0
+inches in length. Apex of stolon stood at a distance of 5.75 inches from
+the vertical glass; but it was not possible to ascertain how much the
+tracing was magnified, as it was not known how great a length of the
+internode circumnutated.
+
+the terminal portion soon began to bend downwards and continued to do so
+until noon. Therefore a straight line, very nearly as long as the whole
+figure here given (Fig. 89), was first traced on the glass; but the upper
+part of this line has not been copied in the diagram. The curvature
+occurred in the middle
+[page 221]
+of the penultimate internode; and its chief seat was at the distance of 1
+1/4 inch from the apex; it appeared due to the weight of the terminal
+portion, acting on the more flexible part of the internode, and not to
+geotropism. The apex after thus sinking down from 9.10 A.M. to noon, moved
+a little to the left; it then rose up and circumnutated in a nearly
+vertical plane until 10.35 P.M. On the following day (26th) it was ob-
+
+Fig. 90. Cotyledon umbilicus: circumnutation and downward movement of
+another stolon, traced on vertical glass, from 9.11 A.M. Aug. 25th to 11
+A.M. 27th. Apex close to glass, so that figure but little magnified, and
+here reduced to two-thirds of original size.
+
+served from 6.40 A.M. to 5.20 P.M., and within this time it moved twice up
+and twice down. On the morning of the 27th the apex stood as high as it did
+at 11.30 A.M. on the 25th. Nor did it sink down during the 28th, but
+continued to circumnutate about the same place.
+
+Another stolon, which resembled the last in almost every
+[page 222]
+respect, was observed during the same two days, but only two inches of the
+terminal portion was allowed to project freely and horizontally. On the
+25th it continued from 9.10 A.M. to 1.30 P.M. to bend straight downwards,
+apparently owing to its weight (Fig. 90); but after this hour until 10.35
+P.M. it zigzagged. This fact deserves notice, for we here probably see the
+combined effects of the bending down from weight and of circumnutation. The
+stolon, however, did not circumnutate when it first began to bend down, as
+may be observed in the present diagram, and as was still more evident in
+the last case, when a longer portion of the stolon was left unsupported. On
+the following day (26th) the stolon moved twice up and twice down, but
+still continued to fall; in the evening and during the night it travelled
+from some unknown cause in an oblique direction.]
+
+We see from these three cases that stolons or runners circumnutate in a
+very complex manner. The lines generally extend in a vertical plane, and
+this may probably be attributed to the effect of the weight of the
+unsupported end of the stolon; but there is always some, and occasionally a
+considerable, amount of lateral movement. The circumnutation is so great in
+amplitude that it may almost be compared with that of climbing plants. That
+the stolons are thus aided in passing over obstacles and in winding between
+the stems of the surrounding plants, the observations above given render
+almost certain. If they had not circumnutated, their tips would have been
+liable to have been doubled up, as often as they met with obstacles in
+their path; but as it is, they easily avoid them. This must be a
+considerable advantage to the plant in spreading from its parent-stock; but
+we are far from supposing that the power has been gained by the stolons for
+this purpose, for circumnutation seems to be of universal occurrence with
+all growing parts; but it is not improbable that the amplitude of the
+movement may have been specially increased for this purpose.
+[page 223]
+
+CIRCUMNUTATION OF FLOWER-STEMS.
+
+We did not think it necessary to make any special observations on the
+circumnutation of flower-stems, these being axial in their nature, like
+stems or stolons; but some were incidentally made whilst attending to other
+subjects, and these we will here briefly give. A few observations have also
+been made by other botanists. These taken together suffice to render it
+probable that all peduncles and sub-peduncles circumnutate whilst growing.
+
+[Oxalis carnosa.--The peduncle which springs from the thick and woody stem
+of this plant bears three or four sub-peduncles.
+
+Fig. 91. Oxalis carnosa: flower-stem, feebly illuminated from above, its
+circumnutation traced from 9 A.M. April 13th to 9 A.M. 15th. Summit of
+flower 8 inches beneath the horizontal glass. Movement probably magnified
+about 6 times.
+
+A filament with little triangles of paper was fixed within the calyx of a
+flower which stood upright. Its movements were observed for 48 h.; during
+the first half of this time the flower was fully expanded, and during the
+second half withered. The figure here given (Fig. 91) represents 8 or 9
+ellipses. Although the main peduncle circumnutated, and described one large
+and
+[page 224]
+two smaller ellipses in the course of 24 h., yet the chief seat of movement
+lies in the sub-peduncles, which ultimately bend vertically downwards, as
+will be described in a future chapter. The peduncles of Oxalis acetosella
+likewise bend downwards, and afterwards, when the pods are nearly mature,
+upwards; and this is effected by a circumnutating movement.
+
+It may be seen in the above figure that the flower-stem of O. carnosa
+circumnutated during two days about the same spot. On the other hand, the
+flower-stem of O. sensitiva undergoes a strongly marked, daily, periodical
+change of position, when kept at a proper temperature. In the middle of the
+day it stands vertically up, or at a high angle; in the afternoon it sinks,
+and in the evening projects horizontally, or almost horizontally, rising
+again during the night. This movement continues from the period when the
+flowers are in bud to when, as we believe, the pods are mature: and it
+ought perhaps to have been included amongst the so-called sleep-movements
+of plants. A tracing was not made, but the angles were measured at
+successive periods during one whole day; and these showed that the movement
+was not continuous, but that the peduncle oscillated up and down. We may
+therefore conclude that it circumnutated. At the base of the peduncle there
+is a mass of small cells, forming a well-developed pulvinus, which is
+exteriorly coloured purple and hairy. In no other genus, as far as we know,
+is the peduncle furnished with a pulvinus. The peduncle of O. Ortegesii
+behaved differently from that of O. sensitiva, for it stood at a less angle
+above the horizon in the middle of the day, then in the morning or evening.
+By 10.20 P.M. it had risen greatly. During the middle of the day it
+oscillated much up and down.
+
+Trifolium subterraneum.--A filament was fixed vertically to the uppermost
+part of the peduncle of a young and upright flower-head (the stem of the
+plant having been secured to a stick); and its movements were traced during
+36 h. Within this time it described (see Fig. 92) a figure which represents
+four ellipses; but during the latter part of the time the peduncle began to
+bend downwards, and after 10.30 P.M. on the 24th it curved so rapidly down,
+that by 6.45 A.M. on the 25th it stood only 19o above the horizon. It went
+on circumnutating in nearly the same position for two days. Even after the
+flower-heads have buried themselves in the ground they continue, as will
+hereafter be shown, to circumnutate. It will also be seen in the next
+chapter that the sub-peduncles of the separate flowers of
+[page 225]
+Trifolium repens circumnutate in a complicated course during several days.
+I may add that the gynophore of Arachis hypogoea,
+
+Fig. 92. Trifolium subterraneum: main flower-peduncle, illuminated from
+above, circumnutation traced on horizontal glass, from 8.40 A.M. July 23rd
+to 10.30 P.M. 24th.
+
+which looks exactly like a peduncle, circumnutates whilst growing
+vertically downwards, in order to bury the young pod in the ground.
+
+The movements of the flowers of Cyclamen Persicum were not observed; but
+the peduncle, whilst the pod is forming, increases much in length, and bows
+itself down by a circumnutating movement. A young peduncle of Maurandia
+semperflorens, 1 � inch in length, was carefully observed during a whole
+day, and it made 4 � narrow, vertical, irregular and short ellipses, each
+at an average rate of about 2 h. 25 m. An adjoining peduncle described
+during the same time similar, though fewer, ellipses.* According to Sachs**
+the flower-stems, whilst growing,
+
+* 'The Movements and Habits of Climbing Plants,' 2nd edit., 1875, p. 68.
+
+** 'Text-Book of Botany,' 1875,
+[[page 226]]
+p. 766. Linnaeus and Treviranus (according to Pfeffer, 'Die Periodischen
+Bewegungen,' etc., p. 162) state that the flower-stalks of many plants
+occupy different positions by night and day, and we shall see in the
+chapter on the Sleep of Plants that this implies circumnutation.
+[page 226]
+
+of many plants, for instance, those of Brassica napus, revolve or
+circumnutate; those of Allium porrum bend from side to side, and, if this
+movement had been traced on a horizontal glass, no doubt ellipses would
+have been formed. Fritz M�ller has described* the spontaneous revolving
+movements of the flower-stems of an Alisma, which he compares with those of
+a climbing plant.
+
+We made no observations on the movements of the different parts of flowers.
+Morren, however, has observed** in the stamens of Sparmannia and Cereus a
+"fremissement spontan�," which, it may be suspected, is a circumnutating
+movement. The circumnutation of the gynostemium of Stylidium, as described
+by Gad,*** is highly remarkable, and apparently aids in the fertilisation
+of the flowers. The gynostemium, whilst spontaneously moving, comes into
+contact with the viscid labellum, to which it adheres, until freed by the
+increasing tension of the parts or by being touched.]
+
+We have now seen that the flower-stems of plants belonging to such widely
+different families as the Cruciferae, Oxalidae, Leguminosae, Primulaceae,
+Scrophularineae, Alismaceae, and Liliaceae, circumnutate; and that there
+are indications of this movement in many other families. With these facts
+before us, bearing also in mind that the tendrils of not a few plants
+consist of modified peduncles, we may admit without much doubt that all
+growing flower-stems circumnutate.
+
+CIRCUMNUTATION OF LEAVES: DICOTYLEDONS.
+
+Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries,
+Batalin, Millardet, etc., have ob-
+
+* 'Jenaische Zeitsch.,' B. v. p. 133.
+
+**  'N. Mem. de l'Acad. R. de Bruxelles,' tom. xiv. 1841, p. 3.
+
+*** 'Sitzungbericht des bot. Vereins der P. Brandenburg,' xxi. p. 84.
+[page 227]
+served, and some of them with the greatest care, the periodical movements
+of leaves; but their attention has been chiefly, though not exclusively,
+directed to those which move largely and are commonly said to sleep at
+night. From considerations hereafter to be given, plants of this nature are
+here excluded, and will be treated of separately. As we wished to ascertain
+whether all young and growing leaves circumnutated, we thought that it
+would be sufficient if we observed between 30 and 40 genera, widely
+distributed throughout the vegetable series, selecting some unusual forms
+and others on woody plants. All the plants were healthy and grew in pots.
+They were illuminated from above, but the light perhaps was not always
+sufficiently bright, as many of them were observed under a skylight of
+ground-glass. Except in a few specified cases, a fine glass filament with
+two minute triangles of paper was fixed to the leaves, and their movements
+were traced on a vertical glass (when not stated to the contrary) in the
+manner already described. I may repeat that the broken lines represent the
+nocturnal course. The stem was always secured to a stick, close to the base
+of the leaf under observation. The arrangement of the species, with the
+number of the Family appended, is the same as in the case of stems.
+
+Fig. 93. Sarracenia purpurea: circumnutation of young pitcher, traced from
+8 A.M. July 3rd to 10.15 A.M. 4th. Temp. 17o - 18o C. Apex of pitcher 20
+inches from glass, so movement greatly magnified.
+
+(1.) Sarracenia purpurea (Sarraceneae, Fam. 11).--A young leaf, or pitcher,
+8 � inches in height, with the bladder swollen but with the hood not as yet
+open, had a filament fixed transversely
+[page 228]
+across its apex; it was observed for 48 h., and during the whole of this
+time it circumnutated in a nearly similar manner, but to a very small
+extent. The tracing given (Fig. 93) relates only to the movement during the
+first 26 h.
+
+(2) Glaucium luteum (Papaveraceae, Fam. 12).--A young plant, bearing only 8
+leaves, had a filament attached to the youngest leaf but one, which was 3
+inches in length, including the petiole. The circumnutating movement was
+traced during 47 h. On both days the leaf descended from before 7 A.M.
+until about 11 A.M., and then ascended slightly during the rest of the day
+and the early part of the night. During the latter part of the night it
+fell greatly. It did not ascend so much during the second as during the
+first day, and it descended considerably lower on the second night than on
+the first. This difference was probably due to the illumination from above
+having been insufficient during the two days of observation. Its course
+during the two days is shown in Fig. 94.
+
+Fig. 94. Glaucium luteum: circumnutation of young leaf, traced from 9.30
+A.M. June 14th to 8.30 A.M. 16th. Tracing not much magnified, as apex of
+leaf stood only 5 � inches from the glass.
+
+(3.) Crambe maritima (Cruciferae, Fam. 14).--A leaf 9 � inches in length on
+a plant not growing vigorously was first observed. Its apex was in constant
+movement, but this could hardly be traced, from being so small in extent.
+The apex, however, certainly changed its course at least 6 times in the
+course of 14 h. A more vigorous young plant, bearing only 4 leaves, was
+then selected, and a filament was affixed to the midrib of the third leaf
+from the base, which, with the petiole, was 5 inches in length. The leaf
+stood up almost vertically, but the tip
+[page 229]
+was deflected, so that the filament projected almost horizontally, and its
+movements were traced during 48 h. on a vertical glass as shown in the
+accompanying figure (Fig. 95). We here plainly see that the leaf was
+continually circumnutating; but the proper periodicity of its movements was
+disturbed by its being only dimly illuminated from above through a double
+skylight. We infer that this was the case, because two leaves on plants
+growing out of doors, had their angles above the horizon measured in the
+middle of the day and at 9 to about 10 P.M. on successive nights, and they
+were found at this latter hour to have risen by an average angle of 9o
+above their mid-day position: on the following morning they fell to their
+former position. Now it may be observed in the diagram that the leaf rose
+during the second night, so that it stood at 6.40 A.M. higher than at 10.20
+P.M. on the preceding night; and this may be attributed to the leaf
+adjusting itself to the dim light, coming exclusively from above.
+
+Fig. 95. Crambe maritima: circumnutation of leaf, disturbed by being
+insufficiently illuminated from above, traced from 7.50 A.M. June 23rd to 8
+A.M. 25th. Apex of leaf 15 1/4 inches from the vertical glass, so that the
+tracing was much magnified, but is here reduced to one-fourth of original
+scale.
+
+(4.) Brassica oleracea (Cruciferae).--Hofmeister and Batalin* state that
+the leaves of the cabbage rise at night, and fall by day. We covered a
+young plant, bearing 8 leaves, under a large bell-glass, placing it in the
+same position with respect to the
+
+* 'Flora,' 1873, p. 437.
+[page 230]
+
+light in which it had long remained, and a filament was fixed at the
+distance of .4 of an inch from the apex of a young leaf nearly 4 inches in
+length. Its movements were then traced during three days, but the tracing
+is not worth giving. The leaf fell during the whole morning, and rose in
+the evening and during the early part of the night. The ascending and
+descending lines did not coincide, so that an irregular ellipse was formed
+each 24 h. The basal part of the midrib did not move, as was ascertained by
+measuring at successive periods the angle which it formed with the horizon,
+so that the movement was confined to the terminal portion of the leaf,
+which moved through an angle of 11o in the course of 24 h., and the
+distance travelled by the apex, up and down, was between .8 and .9 of an
+inch.
+
+In order to ascertain the effect of darkness, a filament was fixed to a
+leaf 5 � inches in length, borne by a plant which after forming a head had
+produced a stem. The leaf was inclined 44o above the horizon, and its
+movements were traced on a vertical glass every hour by the aid of a taper.
+During the first day the leaf rose from 8 A.M. to 10.40 P.M. in a slightly
+zigzag course, the actual distance travelled by the apex being .67 of an
+inch. During the night the leaf fell, whereas it ought to have risen; and
+by 7 A.M. on the following morning it had fallen .23 of an inch, and it
+continued falling until 9.40 A.M. It then rose until 10.50 P.M., but the
+rise was interrupted by one considerable oscillation, that is, by a fall
+and re-ascent. During the second night it again fell, but only to a very
+short distance, and on the following morning re-ascended to a very short
+distance. Thus the normal course of the leaf was greatly disturbed, or
+rather completely inverted, by the absence of light; and the movements were
+likewise greatly diminished in amplitude.
+
+We may add that, according to Mr. A. Stephen Wilson,* the young leaves of
+the Swedish turnip, which is a hybrid between B. oleracea and rapa, draw
+together in the evening so much "that the horizontal breadth diminishes
+about 30 per cent. of the daylight breadth." Therefore the leaves must rise
+considerably at night.
+
+(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).--The
+
+* 'Trans. Bot. Soc. Edinburgh,' vol. xiii. p. 32. With respect to the
+origin of the Swedish turnip, see Darwin, 'Animals and Plants under
+Domestication,' 2nd edit. vol. i. p. 344.
+[page 231]
+
+terminal shoot of a young plant, growing very vigorously, was selected for
+observation. The young leaves at first stand up vertically and close
+together, but they soon bend outwards and downwards, so as to become
+horizontal, and often at the same time a little to one side. A filament was
+fixed to the tip of a young leaf whilst still highly inclined, and the
+first dot was made on the vertical glass at 8.30 A.M. June 13th, but it
+curved downwards so quickly that by 6.40 A.M. on the following morning it
+stood only a little above the horizon. In Fig. 96
+
+Fig. 96. Dianthus caryophyllus: circumnutation of young leaf, traced from
+10.15 P.M. June 13th to 10.35 P.M. 16th. Apex of leaf stood, at the close
+of our observations, 8 3/4 inches from the vertical glass, so tracing not
+greatly magnified. The leaf was 5 1/4 inches long. Temp. 15 1/2o - 17 1/2o
+C.
+
+the long, slightly zigzag line representing this rapid downward course,
+which was somewhat inclined to the left, is not given; but the figure shows
+the highly tortuous and zigzag course, together with some loops, pursued
+during the next 2 � days. As the leaf continued to move all the time to the
+left, it is evident that the zigzag line represents many circumnutations.
+
+(6.) Camellia Japonica (Camelliaceae, Fam. 32).--A youngish leaf, which
+together with its petiole was 2 3/4 inches in length and which arose from a
+side branch on a tall bush, had a filament attached to its apex. This leaf
+sloped downwards at an angle of 40o beneath the horizon. As it was thick
+and rigid, and its
+[page 232]
+petiole very short, much movement could not be expected. Nevertheless, the
+apex changed its course completely seven times in the course of 11 � h.,
+but moved to only a very small distance. On the next day the movement of
+the apex was traced during 26 h. 20 m. (as shown in Fig. 97), and was
+nearly of the same nature, but rather less complex. The movement seems to
+be periodical, for on both days the leaf circumnutated in the forenoon,
+fell in the afternoon (on the first day until between 3 and 4 P.M., and on
+the second day until 6 P.M.), and then rose, falling again during the night
+or early morning.
+
+Fig. 97. Camellia Japonica: circumnutation of leaf, traced from 6.40 A.M.
+June 14th to 6.50 A.M. 15th. Apex of leaf 12 inches from the vertical
+glass, so figure considerably magnified. Temp. 16o - 16 1/2o C.
+
+In the chapter on the Sleep of Plants we shall see that the leaves in
+several Malvaceous genera sink
+
+Fig. 98. Pelargonium zonale: circumnutation and downward movement of young
+leaf, traced from 9.30 A.M. June 14th to 6.30 P.M. 16th. Apex of leaf 9 1.4
+inches from the vertical glass, so figure moderately magnified. Temp. 15o -
+16 1/2o C.
+
+at night; and as they often do not then occupy a vertical position,
+especially if they have not been well illuminated during
+[page 233]
+the day, it is doubtful whether some of these cases ought not to have been
+included in the present chapter.
+
+(7.) Pelargonium zonale (Geraniaceae, Fam. 47).--A young leaf, 1 1/4 inch
+in breadth, with its petiole 1 inch long, borne on a young plant, was
+observed in the usual manner during 61 h.; and its course is shown in the
+preceding figure (Fig. 98). During the first day and night the leaf moved
+downwards, but circumnutated between 10 A.M. and 4.30 P.M. On the second
+day it sank and rose again, but between 10 A.M. and 6 P.M. it circumnutated
+on an extremely small scale. On the third day the circumnutation was more
+plainly marked.
+
+(8.) Cissus discolor (Ampelideae, Fam. 67).--A leaf, not nearly full-grown,
+the third from the apex of a shoot on a cut-down plant, was observed during
+31 h. 30 m. (see Fig. 99). The day was cold (15o - 16o C.), and if the
+plant had been observed in the hot-house, the circumnutation, though plain
+enough as it was, would probably have been far more conspicuous.
+
+Fig. 99. Cissus discolor: circumnutation of leaf, traced from 10.35 A.M.
+May 28th to 6 P.M. 29th. Apex of leaf 8 3/4 inches from the vertical glass.
+
+(9.) Vicia faba (Leguminosae, Fam. 75).--A young leaf, 3.1 inches in
+length, measured from base of petiole to end of leaflets, had a filament
+affixed to the midrib of one of the two terminal leaflets, and its
+movements were traced during 51 � h. The filament fell all morning (July
+2nd) till 3 P.M., and then rose greatly till 10.35 P.M.; but the rise this
+day was so great, compared with that which subsequently occurred, that it
+was probably due in part to the plant being illuminated from above. The
+latter part of the course on July 2nd is alone given in the following
+figure (Fig. 100). On the next day (July 3rd) the leaf again fell in the
+morning, then circumnutated in a conspicuous manner, and rose till late at
+night; but the movement was not traced after 7.15 P.M., as by that time the
+filament pointed towards the upper edge of the glass. During the latter
+part of the night or early morning it again fell in the same manner as
+before.
+[page 234]
+
+As the evening rise and the early morning fall were unusually large, the
+angle of the petiole above the horizon was measured at the two periods, and
+the leaf was found to have risen 19o
+
+Fig. 100. Vicia faba: circumnutation of leaf, traced from 7.15 P.M. July
+2nd to 10.15 A.M. 4th. Apex of the two terminal leaflets 7 1/4 inches from
+the vertical glass. Figure here reduced to two-thirds of original scale.
+Temp. 17o - 18o C.
+
+between 12.20 P.M. and 10.45 P.M., and to have fallen 23o 30 seconds
+between the latter hour and 10.20 A.M. on the following morning.
+
+The main petiole was now secured to a stick close to the base
+[page 235]
+of the two terminal leaflets, which were 1.4 inch in length; and the
+movements of one of them were traced during 48 h. (see Fig. 101). The
+course pursued is closely analogous to that of the whole leaf. The zigzag
+line between 8.30 A.M. and 3.30 P.M. on the second day represents 5 very
+small ellipses, with their
+Fig 101. Vicia faba: circumnutation of one of the two terminal leaflets,
+the main petiole having been secured, traced from 10.40 A.M. July 4th to
+10.30 A.M. 6th. Apex of leaflet 6 5/8 inches from the vertical glass.
+Tracing here reduced to one-half of original scale. Temp. 16o - 18o C.
+
+longer axes differently directed. From these observations it follows that
+both the whole leaf and the terminal leaflets undergo a well-marked daily
+periodical movement, rising in the evening and falling during the latter
+part of the night or early morning; whilst in the middle of the day they
+generally circumnutate round the same small space.
+[page 236]
+
+(10.) Acacia retinoides (Leguminosae).--The movement of a young phyllode, 2
+3/8 inches in length, and inclined at a considerable angle above the
+horizon, was traced during 45 h. 30 m.; but in the figure here given (Fig.
+102), its circumnutation is shown during only 21 h. 30 m. During part of
+this time (viz., 14 h. 30 m.) the phyllode described a figure representing
+5 or 6 small ellipses. The actual amount of movement in a vertical
+direction was .3 inch. The phyllode rose considerably between 1.30 P.M. and
+4 P.M., but there was no evidence on either day of a regular periodic
+movement.
+
+Fig. 102. Acacia retinoides: circumnutation of a young phyllode, traced
+from 10.45 A.M. July 18th to 8.15 A.M. 19th. Apex of phyllode 9 inches from
+the vertical glass; temp. 16 1/2o - 17 1/2o C.
+
+(11.) Lupinus speciosus (Leguminosae).--Plants were raised from seed
+purchased under this name. This is one of the species in this large genus,
+the leaves of which do not sleep at night. The petioles rise direct from
+the ground, and are from 5 to 7 inches in length. A filament was fixed to
+the midrib of one of the longer leaflets, and the movement of the whole
+leaf was traced, as shown in Fig. 103. In the course of 6 h. 30 m. the
+filament went four times up and three times down. A new tracing was then
+begun (not here given), and during 12 � h. the leaf moved eight times up
+and seven times down; so that it described 7 � ellipses in this time, and
+this is an extraordinary rate of movement. The summit of the petiole was
+then secured to a stick, and the separate leaflets were found to be
+continually circumnutating.
+
+Fig. 103. Lupinus speciosus: circumnutation of leaf, traced on vertical
+glass, from 10.15 A.M. to 5.45 P.M.; i.e., during 6 h. 30 m.
+[page 237]
+
+(12.) Echeveria stolonifera (Crassulaceae, Fam. 84).--The older leaves of
+this plant are so thick and fleshy, and the young ones so short and broad,
+that it seemed very improbable that any circumnutation could be detected. A
+filament was fixed to a young upwardly inclined leaf, .75 inch in length
+and .28 in breadth, which stood on the outside of a terminal rosette of
+leaves, produced by a plant growing very vigorously. Its movement was
+traced during 3 days, as here shown (Fig. 104). The course was chiefly in
+an upward direction, and this may be attributed to the elongation of the
+leaf through growth; but we see that the lines are strongly zigzag, and
+that occasionally there was distinct circumnutation, though on a very small
+scale.
+
+Fig. 104. Echeveria stolonifera: circumnutation of leaf, traced from 8.20
+A.M. June 25th to 8.45 A.M. 28th. Apex of leaf 12 1/4 inches from the
+glass, so that the movement was much magnified; temp. 23o - 24 1/2o C.
+(13.) Bryophyllum (vel Calanchae) calycinum (Crassulaceae).--Duval-Jouve
+('Bull. Soc. Bot. de France,' Feb. 14th, 1868) measured the distance
+between the tips of the upper pair of leaves on this plant, with the result
+shown in the following Table. It should be noted that the measurements on
+Dec. 2nd were made on a different pair of leaves: --
+
+               8 A.M.      2 P.M.      7 P.M.
+Nov. 16. . . . . . . . . . . . . . . . . . .15 mm.. . . . . .25 mm. . . ..
+. . .(?)
+  "     19. . . . . . . . . . . . . . . . . . .48  " . . . . . . .  60 ". .
+. . . . .  48 mm.
+Dec.   2. . . . . . . . . . . . . . . . . . .22  ". . . . . . . . 43 ". . .
+. . . . .28  "
+
+We see from this Table that the leaves stood considerably further apart at
+2 P.M. than at either 8 A.M. or 7 P.M.; and this shows that they rise a
+little in the evening and fall or open in the forenoon.
+
+(14.) Drosera rotundifolia (Droseraceae, Fam. 85).--The movements of a
+young leaf, having a long petiole but with its tentacles (or gland-bearing
+hairs) as yet unfolded, were traced during 47 h. 15 m. The figure (Fig.
+105) shows that it circumnutated largely, chiefly in a vertical direction,
+making two ellipses each
+[page 238]
+day. On both days the leaf began to descend after 12 or 1 o'clock, and
+continued to do so all night, though to a very unequal distance on the two
+occasions. We therefore thought that the movement was periodic; but on
+observing three other leaves during several successive days and nights, we
+found this to be an error; and the case is given merely as a caution. On
+the third morning the above leaf occupied almost exactly the same position
+as on the first morning; and the tentacles by this time had unfolded
+sufficiently to project at right angles to the blade or disc.
+
+Fig. 105. Drosera rotundifolia: circumnutation of young leaf, with filament
+fixed to back of blade, traced from 9.15 A.M. June 7th to 8.30 A.M. June
+9th. Figure here reduced to one-half original scale.
+
+The leaves as they grow older generally sink more and more downwards. The
+movement of an oldish leaf, the glands of which were still secreting
+freely, was traced for 24 h., during which time it continued to sink a
+little in a slightly zigzag line. On the following morning, at 7 A.M., a
+drop of a solution of carbonate of ammonia (2 gr. to 1 oz. of water) was
+placed on the disc, and this blackened the glands and induced inflection of
+many of the tentacles. The weight of the drop caused the leaf at first to
+sink a little; but immediately afterwards it began to rise in a somewhat
+zigzag course, and continued to do so till 3 P.M. It then circumnutated
+about the same spot on a very small scale for 21 h.; and during the next 21
+h. it sank in a zigzag line to nearly the same level which it had held when
+the ammonia was first administered. By this time the tentacles had
+re-expanded, and the glands had recovered their proper colour. We thus
+learn that an old leaf
+[page 239]
+circumnutates on a small scale, at least whilst absorbing carbonate of
+ammonia; for it is probable that this absorption may stimulate growth and
+thus re-excite circumnutation. Whether the rising of the glass filament
+which was attached to the back of the leaf, resulted from its margin
+becoming slightly inflected (as generally occurs), or from the rising of
+the petiole, was not ascertained.
+
+In order to learn whether the tentacles or gland-bearing hairs
+circumnutate, the back of a young leaf, with the innermost tentacles as yet
+incurved, was firmly cemented with shellac to a flat stick driven into
+compact damp argillaceous sand. The plant was placed under a microscope
+with the stage removed and with an eye-piece micrometer, of which each
+division equalled 1/500 of an inch. It should be stated that as the leaves
+grow older the tentacles of the exterior rows bend outwards and downwards,
+so as ultimately to become deflected considerably beneath the horizon. A
+tentacle in the second row from the margin was selected for observation,
+and was found to be moving outwards at a rate of 1/500 of an inch in 20 m.,
+or 1/100 of inch in 1 h. 40 m.; but as it likewise moved from side to side
+to an extent of above 1/500 of inch, the movement was probably one of
+modified circumnutation. A tentacle on an old leaf was next observed in the
+same manner. In 15 m. after being placed under the microscope it had moved
+about 1/1000 of an inch. During the next 7 � h. it was looked at
+repeatedly, and during this whole time it moved only another 1/1000 of an
+inch; and this small movement may have been due to the settling of the damp
+sand (on which the plant rested), though the sand had been firmly pressed
+down. We may therefore conclude that the tentacles when old do not
+circumnutate; yet this tentacle was so sensitive, that in 23 seconds after
+its gland had been merely touched with a bit of raw meat, it began to curl
+inwards. This fact is of some importance, as it apparently shows that the
+inflection of the tentacles from the stimulus of absorbed animal matter
+(and no doubt from that of contact with any object) is not due to modified
+circumnutation.
+
+(15.) Dionoea muscipula (Droseraceae).--It should be premised that the
+leaves at an early stage of their development have the two lobes pressed
+closely together. These are at first directed back towards the centre of
+the plant; but they gradually rise up and soon stand at right angles to the
+petiole, and ultimately in nearly a straight line with it. A young leaf,
+which with the
+[page 240]
+petiole was only 1.2 inch in length, had a filament fixed externally along
+the midrib of the still closed lobes, which projected at right angles to
+the petiole. In the evening this leaf completed an ellipse in the course of
+2 h. On the following day (Sept. 25th) its movements were traced during 22
+h.; and we see in Fig. 106 that it moved in the same general direction, due
+to the straightening of the leaf, but in an extremely zigzag line. This
+line represents several drawn-out or modified ellipses. There can therefore
+be no doubt that this young leaf circumnutated.
+
+Fig. 106. Dionaea muscipula: circumnutation of a young and expanding leaf,
+traced on a horizontal glass in darkness, from noon Sept. 24th to 10 A.M.
+25th. Apex of leaf 13 � inches from the glass, so tracing considerably
+magnified.
+
+A rather old, horizontally extended leaf, with a filament attached along
+the under side of the midrib, was next observed during 7 h. It hardly
+moved, but when one of its sensitive hairs was touched, the blades closed,
+though not very quickly. A new dot was now made on the glass, but in the
+course of 14 h. 20 m. there was hardly any change in the position of the
+filament. We may therefore infer that an old and only moderately sensitive
+leaf does not circumnutate plainly; but we shall soon see that it by no
+means follows that such a leaf is absolutely motionless. We may further
+infer that the stimulus from a touch does not re-excite plain
+circumnutation.
+
+Another full-grown leaf had a filament attached externally along one side
+of the midrib and parallel to it, so that the filament would move if the
+lobes closed. It should be first stated that, although a touch on one of
+the sensitive hairs of a vigorous leaf causes it to close quickly, often
+almost instantly, yet when a bit of damp meat or some solution of carbonate
+of ammonia is placed on the lobes, they close so slowly that generally 24
+h. is required for the completion of the act. The above leaf was first
+observed for 2 h. 30 m., and did not circumnutate, but it ought to have
+been observed for a
+[page 241]
+longer period; although, as we have seen, a young leaf completed a fairly
+large ellipse in 2 h. A drop of an infusion of raw meat was then placed on
+the leaf, and within 2 h. the glass filament rose a little; and this
+implies that the lobes had begun to close, and perhaps the petiole to rise.
+It continued to rise with extreme slowness for the next 8 h. 30 m. The
+position of the pot was then (7.15 P.M., Sept. 24th) slightly changed and
+an additional drop of the infusion given, and a new tracing was begun (Fig.
+107). By 10.50 P.M. the filament had risen only a little more, and it fell
+during the night. On the following morning the lobes were closing more
+quickly, and by 5 P.M. it was evident to the eye that they had closed
+considerably; by 8.48. P.M. this was still plainer, and by 10.45 P.M. the
+marginal spikes were interlocked. The leaf fell a little during the night,
+and next morning (25th) at 7 A.M. the lobes were completely shut. The
+course pursued, as may be seen in the figure, was
+
+Fig. 107. Dionoea muscipula: closure of the lobes and circumnutation of a
+full-grown leaf, whilst absorbing an infusion of raw meat, traced in
+darkness, from 7.15 P.M. Sept. 24th to 9 A.M. 26th. Apex of leaf 8 � inches
+from the vertical glass. Figure here reduced to two-thirds of original
+scale.
+
+strongly zigzag, and this indicates that the closing of the lobes was
+combined with the circumnutation of the whole leaf; and there cannot be
+much doubt, considering how motionless the leaf was during 2 h. 30 m.
+before it received the infusion, that the absorption of the animal matter
+had excited it to circumnutate. The leaf was occasionally observed for the
+next four days, but was kept in rather too cool a place; nevertheless, it
+continued to circumnutate to a small extent, and the lobes remained closed.
+
+It is sometimes stated in botanical works that the lobes close or sleep at
+night; but this is an error. To test the statement, very long glass
+filaments were fixed inside the two lobes of three leaves, and the
+distances between their tips were measured in the middle of the day and at
+night; but no difference could be detected.
+
+The previous observations relate to the movements of the whole leaf, but
+the lobes move independently of the petiole, and
+[page 242]
+seem to be continually opening and shutting to a very small extent. A
+nearly full-grown leaf (afterwards proved to be highly sensitive to
+contact) stood almost horizontally, so that by driving a long thin pin
+through the foliaceous petiole close to the blade, it was rendered
+motionless. The plant, with a little triangle of paper attached to one of
+the marginal spikes, was placed under a microscope with an eye-piece
+micrometer, each division of which equalled 1/500 of an inch. The apex of
+the paper-triangle was now seen to be in constant slight movement; for in 4
+h. it crossed nine divisions, or 9/500 of an inch, and after ten additional
+hours it moved back and had crossed 5/500 in an opposite direction. The
+plant was kept in rather too cool a place, and on the following day it
+moved rather less, namely, 1/500 in 3 h., and 2/500 in an opposite
+direction during the next 6 h. The two lobes, therefore, seem to be
+constantly closing or opening, though to a very small distance; for we must
+remember that the little triangle of paper affixed to the marginal spike
+increased its length, and thus exaggerated somewhat the movement. Similar
+observations, with the important difference that the petiole was left free
+and the plant kept under a high temperature, were made on a leaf, which was
+healthy, but so old that it did not close when its sensitive hairs were
+repeatedly touched, though judging from other cases it would have slowly
+closed if it had been stimulated by animal matter. The apex of the triangle
+was in almost, though not quite, constant movement, sometimes in one
+direction and sometimes in an opposite one; and it thrice crossed five
+divisions of the micrometer (i.e. 1/100 of an inch) in 30 m. This movement
+on so small a scale is hardly comparable with ordinary circumnutation; but
+it may perhaps be compared with the zigzag lines and little loops, by which
+the larger ellipses made by other plants are often interrupted.
+
+In the first chapter of this volume, the remarkable oscillatory movements
+of the circumnutating hypocotyl of the cabbage have been described. The
+leaves of Dionaea present the same phenomenon, which is a wonderful one, as
+viewed under a low power (2-inch object-glass), with an eye-piece
+micrometer of which each division (1/500 of an inch) appeared as a rather
+wide space. The young unexpanded leaf, of which the circumnutating
+movements were traced (Fig. 106), had a glass filament fixed
+perpendicularly to it; and the movement of the apex was observed in the
+hot-house (temp. 84o to 86o F.), with light admitted only from above, and
+with any lateral currents of air
+[page 243]
+excluded. The apex sometimes crossed one or two divisions of the micrometer
+at an imperceptibly slow rate, but generally it moved onwards by rapid
+starts or jerks of 2/1000 or 3/1000, and in one instance of 4/1000 of an
+inch. After each jerk forwards, the apex drew itself backwards with
+comparative slowness for part of the distance which had just been gained;
+and then after a very short time made another jerk forwards. Four
+conspicuous jerks forwards, with slower retreats, were seen on one occasion
+to occur in exactly one minute, besides some minor oscillations. As far as
+we could judge, the advancing and retreating lines did not coincide, and if
+so, extremely minute ellipses were each time described. Sometimes the apex
+remained quite motionless for a short period. Its general course during the
+several hours of observation was in two opposite directions, so that the
+leaf was probably circumnutating.
+
+An older leaf with the lobes fully expanded, and which was afterwards
+proved to be highly sensitive to contact, was next observed in a similar
+manner, except that the plant was exposed to a lower temperature in a room.
+The apex oscillated forwards and backwards in the same manner as before;
+but the jerks forward were less in extent, viz. about 1/1000 inch; and
+there were longer motionless periods. As it appeared possible that the
+movements might be due to currents of air, a wax taper was held close to
+the leaf during one of the motionless periods, but no oscillations were
+thus caused. After 10 m., however, vigorous oscillations commenced, perhaps
+owing to the plant having been warmed and thus stimulated. The candle was
+then removed and before long the oscillations ceased; nevertheless, when
+looked at again after an interval of 1 h. 30 m., it was again oscillating.
+The plant was taken back into the hot-house, and on the following morning
+was seen to be oscillating, though not very vigorously. Another old but
+healthy leaf, which was not in the least sensitive to a touch, was likewise
+observed during two days in the hot-house, and the attached filament made
+many little jerks forwards of about 2/1000 or only 1/1000 of an inch.
+
+Finally, to ascertain whether the lobes independently of the petiole
+oscillated, the petiole of an old leaf was cemented close to the blade with
+shellac to the top of a little stick driven into the soil. But before this
+was done the leaf was observed, and found to be vigorously oscillating or
+jerking; and after it had been cemented to the stick, the oscillations of
+about 2/1000 of an inch still continued. On the following day a little
+infusion
+[page 244]
+of raw meat was placed on the leaf, which caused the lobes to close
+together very slowly in the course of two days; and the oscillations
+continued during this whole time and for the next two days. After nine
+additional days the leaf began to open and the margins were a little
+everted, and now the apex of the glass filament remained for long periods
+motionless, and then moved backwards and forwards for a distance of about
+1/1000 of an inch slowly, without any jerks. Nevertheless, after warming
+the leaf with a taper held close to it, the jerking movement recommenced.
+
+This same leaf had been observed 2 � months previously, and was then found
+to be oscillating or jerking. We may therefore infer that this kind of
+movement goes on night and day for a very long period; and it is common to
+young unexpanded leaves and to leaves so old as to have lost their
+sensitiveness to a touch, but which were still capable of absorbing
+nitrogenous matter. The phenomenon when well displayed, as in the young
+leaf just described, is a very interesting one. It often brought before our
+minds the idea of effort, or of a small animal struggling to escape from
+some constraint.
+
+(16.) Eucalyptus resinifera (Myrtaceae, Fam. 94).--A young leaf, two inches
+in length together with the petiole, produced by a lateral shoot from a
+cut-down tree, was observed in the usual manner. The blade had not as yet
+assumed its vertical position. On June 7th only a few observations were
+made, and the tracing merely showed that the leaf had moved three times
+upwards and three downwards. On the following day it was observed more
+frequently; and two tracings were made (see A and B, Fig. 108), as a single
+one would have been too complicated. The apex changed its course 13 times
+in the course of 16 h., chiefly up and down, but with some lateral
+movement. The actual amount of movement in any one direction was small.
+
+Fig. 108. Eucalyptus resinifera: circumnutation of a leaf, traced, A, from
+6.40 A.M. to 1 P.M. June 8th; B, from 1 P.M. 8th to 8.30 A.M. 9th. Apex of
+leaf 14 � inches from the horizontal glass, so figures considerably
+magnified.
+
+(17.) Dahlia (garden var.) (Compositae, Fam. 122).--A fine young
+[page 245]
+leaf 5 3/4 inches in length, produced by a young plant 2 feet high, growing
+vigorously in a large pot, was directed at an angle of about 45o beneath
+the horizon. On June 18th the leaf descended from 10 A.M. till 11.35 A.M.
+(see Fig. 109); it then ascended greatly till 6 P.M., this ascent being
+probably due to the light
+
+Fig. 109. Dahlia: circumnutation of leaf, traced from 10 A.M. June 18th to
+8.10 A.M. 20th, but with a break of 1 h. 40 m. on the morning of the 19th,
+as, owing to the glass filament pointing too much to one side, the pot had
+to be slightly moved; therefore the relative position of the two tracings
+is somewhat arbitrary. The figure here given is reduced to one-fifth of the
+original scale. Apex of leaf 9 inches from the glass in the line of its
+inclination, and 4 3/4 in a horizontal line.
+coming only from above. It zigzagged between 6 P.M. and 10.35 P.M., and
+ascended a little during the night. It should be remarked that the vertical
+distances in the lower part of the diagram are much exaggerated, as the
+leaf was at first deflected beneath the horizon, and after it had sunk
+downwards, the filament pointed in a very oblique line towards the glass.
+Next
+[page 246]
+day the leaf descended from 8.20 A.M. till 7.15 P.M., then zigzagged and
+ascended greatly during the night. On the morning of the 20th the leaf was
+probably beginning to descend, though the short line in the diagram is
+horizontal. The actual distances travelled by the apex of the leaf were
+considerable, but could not be calculated with safety. From the course
+pursued on the second day, when the plant had accommodated itself to the
+light from above, there cannot be much doubt that the leaves undergo a
+daily periodic movement, sinking during the day and rising at night.
+
+(18.) Mutisia clematis (Compositae).--The leaves terminate in tendrils and
+circumnutate like those of other tendril-bearers; but this plant is here
+mentioned, on account of an erroneous statement* which has been published,
+namely, that the leaves sink at night and rise during the day. The leaves
+which behaved in this manner had been kept for some days in a northern room
+and had not been sufficiently illuminated. A plant therefore was left
+undisturbed in the hot-house, and three leaves had their angles measured at
+noon and at 10 P.M. All three were inclined a little beneath the horizon at
+noon, but one stood at night 2o, the second 21o, and the third 10o higher
+than in the middle of the day; so that instead of sinking they rise a
+little at night.
+
+(19.) Cyclamen Persicum (Primulaceae, Fam. 135).--A young leaf, 1.8 of an
+inch in length, petiole included, produced by an old root-stock, was
+observed during three days in the usual manner (Fig. 110). On the first day
+the leaf fell more than afterwards, apparently from adjusting itself to the
+light from above. On all three days it fell from the early morning to about
+7 P.M., and from that hour rose during the night, the course being slightly
+zigzag. The movement therefore is strictly periodic. It should be noted
+that the leaf would have sunk each evening a little lower down than it did,
+had not the glass filament rested between 5 and 6 P.M. on the rim of the
+pot. The amount of movement was considerable; for if we assume that the
+whole leaf to the base of the petiole became bent, the tracing would be
+magnified rather less than five times, and this would give to the apex a
+rise and fall of half an inch, with some lateral movement. This amount,
+however, would not attract attention without the aid of a tracing or
+measurement of some kind.
+
+* 'The Movements and Habits of Climbing Plants,' 1875, p. 118.
+[page 247]
+
+ (20.) Allamanda Schottii (Apocyneae, Fam. 144).--The young leaves of this
+shrub are elongated, with the blade bowed so much
+
+Fig. 110. Cyclamen Persicum: circumnutation of leaf, traced from 6.45 A.M.
+June 2nd to 6.40 A.M. 5th. Apex of leaf 7 inches from the vertical glass.
+
+downwards as almost to form a semicircle. The chord--that is, a line drawn
+from the apex of the blade to the base of the petiole--of a young leaf, 4
+3/4 inches in length, stood at 2.50 P.M. on
+[page 248]
+Dec. 5th at an angle of 13o beneath the horizon, but by 9.30 P.M. the blade
+had straightened itself so much, which implies the raising of the apex,
+that the chord now stood at 37o above the horizon, and had therefore risen
+50o. On the next day similar angular measurements of the same leaf were
+made; and at noon the chord stood 36o beneath the horizon, and 9.30 P.M. 3
+1/2o above it, so had risen 39 1/2o. The chief cause of the rising movement
+lies in the straightening of the blade, but the short petiole rises between
+4o and 5o. On the third night the chord stood at 35o above the horizon, and
+if the leaf occupied the same position at noon, as on the previous day, it
+had risen 71o. With older leaves no such change of curvature could be
+detected. The plant was then brought into the house and kept in a
+north-east room, but at night there was no change in the curvature of the
+young leaves; so that previous exposure to a strong light is apparently
+requisite for the periodical change of curvature in the blade, and for the
+slight rising of the petiole.
+
+(21.) Wigandia (Hydroleaceae, Fam. 149).--Professor Pfeffer informs us that
+the leaves of this plant rise in the evening; but as we do not know whether
+or not the rising is great, this species ought perhaps to be classed
+amongst sleeping plants.
+
+Fig. 111. Petunia violacea: downward movement and circumnutation of a very
+young leaf, traced from 10 A.M. June 2nd to 9.20 A.M. June 6th. N.B.--At
+6.40 A.M. on the 5th it was necessary to move the pot a little, and a new
+tracing was begun at the point where two dots are not joined in the
+diagram. Apex of leaf 7 inches from the vertical glass. Temp. generally 17
+1/2o C.
+[page 249]
+
+(22.) Petunia violacea (Solaneae, Fam. 157).--A very young leaf, only 3/4
+inch in length, highly inclined upwards, was observed for four days. During
+the whole of this time it bent outwards and downwards, so as to become more
+and more nearly horizontal. The strongly marked zigzag line in the figure
+on p. 248 (Fig. 111), shows that this was effected by modified
+circumnutation; and during the latter part of the time there was much
+ordinary circumnutation on a small scale. The movement in the diagram is
+magnified between 10 and 11 times. It exhibits a clear trace of
+periodicity, as the leaf rose a little each evening; but this upward
+tendency appeared to be almost conquered by the leaf striving to become
+more and more horizontal as it grew older. The angles which two older
+leaves formed together, were measured in the evening and about noon on 3
+successive days, and each night the angle decreased a little, though
+irregularly.
+
+Fig. 112. Acanthus mollis: circumnutation of young leaf, traced from 9.20
+A.M. June 14th to 8.30 A.M. 16th. Apex of leaf 11 inches from the vertical
+glass, so movement considerably magnified. Figure here reduced to one-half
+of original scale. Temp. 15o - 16 1/2o C.
+
+(23.) Acanthus mollis (Acanthaceae, Fam. 168).--The younger of two leaves,
+2 1/4 inches in length, petiole included, produced by a seedling plant, was
+observed during 47 h. Early on each of the three mornings, the apex of the
+leaf fell; and it continued to fall till 3 P.M., on the two afternoons when
+observed. After 3 P.M. it rose considerably, and continued to rise on the
+second night until the early morning. But on the first night it fell
+instead of rising, and we have little doubt that this was owing to the leaf
+being very young and becoming through epinastic growth more and more
+horizontal; for it may be seen in the diagram (Fig. 112), that the leaf
+stood on a higher level on the first than on the second day. The leaves of
+an allied species ('A. spinosus') certainly rose every night; and the rise
+between noon and 10.15 P.M., when measured on one occasion, was 10o. This
+rise was chiefly
+[page 250]
+or exclusively due to the straightening of the blade, and not to the
+movement of the petiole. We may therefore conclude that the leaves of
+Acanthus circumnutate periodically, falling in the morning and rising in
+the afternoon and night.
+
+(24.) Cannabis sativa (Cannabineae, Fam. 195).--We have here the rare case
+of leaves moving downwards in the evening, but not to a sufficient degree
+to be called sleep.* In the early morning, or in the latter part of the
+night, they move upwards. For instance, all the young leaves near the
+summits of several stems stood almost horizontally at 8 A.M. May 29th and
+at 10.30 P.M. were considerably declined. On a subsequent day two leaves
+stood at 2 P.M. at 21o and 12o beneath the horizon, and at 10 P.M. at 38o
+beneath it. Two other leaves on a younger plant were horizontal at 2 P.M.,
+and at 10 P.M. had sunk to 36o beneath the horizon. With respect to this
+downward movement of the leaves, Kraus believes that it is due to their
+epinastic growth. He adds, that the leaves are relaxed during the day, and
+tense at night, both in sunny and rainy weather.
+
+(25.) Pinus pinaster (Coniferae, Fam. 223).--The leaves on the summits of
+the terminal shoots stand at first in a bundle almost upright, but they
+soon diverge and ultimately become almost horizontal. The movements of a
+young leaf, nearly one inch in length, on the summit of a seedling plant
+only 3 inches high, were traced from the early morning of June 2nd to the
+evening of the 7th. During these five days the leaf diverged, and its apex
+descended at first in an almost straight line; but during the two latter
+days it zigzagged so much that it was evidently circumnutating. The same
+little plant, when grown to a height of 5 inches, was again observed during
+four days. A filament was fixed transversely to the apex of a leaf, one
+inch in length, and which had already diverged considerably from its
+originally upright position. It continued to diverge (see A, Fig. 113), and
+to descend from 11.45 A.M. July 31st to 6.40 A.M. Aug. 1st. On August 1st
+it circumnutated about the same small space, and again descended at night.
+Next morning the pot was moved nearly one inch to the right, and a new
+tracing was begun (B). From this time, viz., 7 A.M. August 2nd to 8.20 A.M.
+on the 4th,
+
+* We were led to observe this plant by Dr. Carl Kraus' paper, 'Beitr�ge zur
+Kentniss der Bewegungen Wachsender Laubbl�tter,' Flora, 1879, p. 66. We
+regret that we cannot fully understand parts of this paper.
+[page 251]
+
+the leaf manifestly circumnutated. It does not appear from the diagram that
+the leaves move periodically, for the descending course during the first
+two nights, was clearly due to epinastic
+
+Fig. 113. Pinus pinaster: circumnutation of young leaf, traced from 11.45
+A.M. July 31st to 8.20 A.M. Aug. 4th. At 7 A.M. Aug. 2nd the pot was moved
+an inch to one side, so that the tracing consists of two figures. Apex of
+leaf 14 � inches from the vertical glass, so movements much magnified.
+
+growth, and at the close of our observations the leaf was not nearly so
+horizontal as it would ultimately become.
+
+Pinus austriaca.--Two leaves, 3 inches in length, but not
+[page 252]
+quite fully grown, produced by a lateral shoot, on a young tree 3 feet in
+height, were observed during 29 h. (July 31st), in the same manner as the
+leaves of the previous species. Both these leaves certainly circumnutated,
+making within the above period two, or two and a half, small, irregular
+ellipses.
+
+(26.) Cycas pectinata (Cycadeae, Fam. 224).--A young leaf, 11 � inches in
+length, of which the leaflets had only recently become uncurled, was
+observed during 47 h. 30 m. The main petiole was secured to a stick at the
+base of the two terminal leaflets. To one of the latter, 3 3/4 inches in
+length, a filament was fixed; the leaflet was much bowed downward, but as
+the terminal part was upturned, the filament projected almost horizontally.
+The leaflet moved (see Fig. 114) largely and periodically, for it fell
+until about 7 P.M. and rose during the night, falling again next morning
+after 6.40 A.M. The descending lines are in a marked manner zigzag, and so
+probably would have been the ascending lines, if they had been traced
+throughout the night.
+
+Fig. 114. Cycas pectinata: circumnutation of one of the terminal leaflets,
+traced from 8.30 A.M. June 22nd to 8 A.M. June 24th. Apex of leaflet 7 3/4
+inches from the vertical glass, so tracing not greatly magnified, and here
+reduced to one-third of original scale; temp. 19o - 21o C.
+
+CIRCUMNUTATION OF LEAVES: MONOCOTYLEDONS.
+
+(27.) Canna Warscewiczii (Cannaceae, Fam. 2).--The movements of a young
+leaf, 8 inches in length and 3 � in breadth, produced by a vigorous young
+plant, were observed during 45 h. 50 m., as shown in Fig. 115. The pot was
+slided about an inch to the right on the morning of the 11th, as a single
+figure would have been too complicated; but the two figures are continuous
+in time. The movement is periodical, as the leaf descended from the early
+morning until about 5 P.M., and ascended during the rest of the evening and
+[page 253]
+part of the night. On the evening of the 11th it circumnutated on a small
+scale for some time about the same spot.
+
+Fig. 115. Canna Warscewiczii: circumnutation of leaf, traced (A) from 11.30
+A.M. June 10th to 6.40 A.M. 11th; and (B) from 6.40 A.M. 11th to 8.40 A.M.
+12th. Apex of leaf 9 inches from the vertical glass.
+
+(28.) Iris pseudo-acorus (Irideae, Fam. 10).--The movements of a young
+leaf, rising 13 inches above the water in which the plant grew, were traced
+as shown in the figure (Fig. 116), during 27 h. 30 m. It manifestly
+circumnutated, though only to a small extent. On the second morning,
+between 6.40 A.M. and 2 P.M. (at which latter hour the figure here given
+ends), the apex changed its course five times. During the next 8 h. 40 m.
+it zigzagged much, and descended as far as the lowest dot in the figure,
+making in its course two very small ellipses; but if these lines had been
+added to the diagram it would have been too complex.
+
+Fig. 116. Iris pseudo-acorus: circumnutation of leaf, traced from 10.30
+A.M. May 28th to 2 P.M. 29th. Tracing continued to 11 P.M., but not here
+copied. Apex of leaf 12 inches beneath the horizontal glass, so figure
+considerably magnified. Temp. 15o - 16o C.
+(29.) Crinum Capense (Amaryllideae, Fam. 11).--The leaves of this plant are
+remarkable for their great length and narrowness: one was measured and
+found to be 53 inches long and only 1.4 broad at the base. Whilst quite
+young they stand up almost vertically to the height of about a foot;
+afterwards
+[page 254]
+their tips begin to bend over, and subsequently hang vertically down, and
+thus continue to grow. A rather young leaf was selected, of which the
+dependent tapering point was as yet only 5 � inches in length, the upright
+basal part being 20 inches high, though this part would ultimately become
+shorter by being more bent over. A large bell-glass was placed over the
+plant, with a black dot on one side; and by bringing the dependent apex of
+the leaf into a line with this dot, the accompanying figure (Fig. 117) was
+traced on the other side of the bell, during 2 � days. During the first day
+(22nd) the tip travelled laterally far to the left, perhaps in consequence
+of the plant having been
+
+Fig. 117. Crinum Capense: circumnutation of dependent tip of young leaf,
+traced on a bell-glass, from 10.30 P.M. May 22nd to 10.15 A.M. 25th. Figure
+not greatly magnified.
+
+disturbed; and the last dot made at 10.30 P.M. on this day is alone here
+given. As we see in the figure, there can be no doubt that the apex of this
+leaf circumnutated.
+
+A glass filament with little triangles of paper was at the same time fixed
+obliquely across the tip of a still younger leaf, which stood vertically up
+and was as yet straight. Its movements were traced from 3 P.M. May 22nd to
+10.15 A.M. 25th. The leaf was growing rapidly, so that the apex ascended
+greatly during this period; as it zigzagged much it was clearly
+circumnutating, and it apparently tended to form one ellipse each day. The
+lines traced during the night were much more vertical than those traced
+during the day; and this indicates that the tracing would have exhibited a
+nocturnal rise and a diurnal fall, if the leaf had not grown so quickly.
+The movement of this same leaf after an interval of six days (May 31st), by
+which time the tip had curved outwards into a horizontal position,
+[page 255]
+and had thus made the first step towards becoming dependent, was traced
+orthogonically by the aid of a cube of wood (in the manner before
+explained); and it was thus ascertained that the actual distance travelled
+by the apex, and due to circumnutation, was 3 1/8 inches in the course of
+20 � h. During the next 24 h. it travelled 2 � inches. The circumnutating
+movement, therefore, of this young leaf was strongly marked.
+
+(30.) Pancratium littorale (Amaryllideae).--The movements, much magnified,
+of a leaf, 9 inches in length and inclined at about 45o above the horizon,
+were traced during two days. On the first day it changed its course
+completely, upwards and downwards and laterally, 9 times in 12 h.; and the
+figure traced apparently represented five ellipses. On the second day it
+was observed seldomer, and was therefore not seen to change its course so
+often, viz., only 6 times, but in the same complex manner as before. The
+movements were small in extent, but there could be no doubt about the
+circumnutation of the leaf.
+
+(31.) Imatophyllum vel Clivia (sp.?) (Amaryllideae).--A long glass filament
+was fixed to a leaf, and the angle formed by it with the horizon was
+measured occasionally during three successive days. It fell each morning
+until between 3 and 4 P.M., and rose at night. The smallest angle at any
+time above the horizon was 48o, and the largest 50o; so that it rose only 2o
+at night; but as this was observed each day, and as similar observations
+were nightly made on another leaf on a distinct plant, there can be no
+doubt that the leaves move periodically, though to a very small extent. The
+position of the apex when it stood highest was .8 of an inch above its
+lowest point.
+
+(32.) Pistia stratiotes (Aroideae, Fam. 30).--Hofmeister remarks that the
+leaves of this floating water-plant are more highly inclined at night than
+by day.* We therefore fastened a fine glass filament to the midrib of a
+moderately young leaf, and on Sept. 19th measured the angle which it formed
+with the horizon 14 times between 9 A.M. and 11.50 P.M. The temperature of
+the hot-house varied during the two days of observation between 18 1/2o and
+23 1/2o C. At 9 A.M. the filament stood at 32o above the horizon; at 3.34
+P.M. at 10o and at 11.50 P.M. at 55o; these two latter angles being the
+highest and the lowest observed during the day, showing a difference of
+45o. The rising did not become strongly marked until between
+
+* 'Die Lehre von der Pflanzenzelle,' 1867, p. 327.
+[page 256]
+
+5 and 6 P.M. On the next day the leaf stood at only 10o above the horizon
+at 8.25 A.M., and it remained at about 15o till past 3 P.M.; at 5.40 P.M.
+it was 23o, and at 9.30 P.M. 58o; so that the rise was more sudden this
+evening than on the previous one, and the difference in the angle amounted
+to 48o. The movement is obviously periodical, and as the leaf stood on the
+first night at 55o, and on the second night at 58o above the horizon, it
+appeared very steeply inclined. This case, as we shall see in a future
+chapter, ought perhaps to have been included under the head of sleeping
+plants.
+
+(33.) Pontederia (sp.?) (from the highlands of St. Catharina,
+
+Fig. 118. Pontederia (sp.?): circumnutation of leaf, traced from 4.50 P.M.
+July 2nd to 10.15 A.M. 4th. Apex of leaf 16 � inches from the vertical
+glass, so tracing greatly magnified. Temp. about 17o C., and therefore
+rather too low.
+
+Brazil) (Pontederiaceae, Fam. 46).--A filament was fixed across the apex of
+a moderately young leaf, 7 � inches in height, and its movements were
+traced during 42 � h. (see Fig. 118). On the first evening, when the
+tracing was begun, and during the night, the leaf descended considerably.
+On the next morning it ascended in a strongly marked zigzag line, and
+descended again in the evening and during the night. The movement,
+therefore, seems to be periodic, but some doubt is thrown on this
+conclusion, because another leaf, 8 inches in height, appearing older and
+standing more highly inclined, behaved differently. During the first 12 h.
+it circumnutated over a
+[page 257]
+small space, but during the night and the whole following day it ascended
+in the same general direction; the ascent being effected by repeated up and
+down well-pronounced oscillations.
+
+CRYPTOGAMS.
+
+(34.) Nephrodium molle (Filices, Fam. 1).--A filament was fixed near the
+apex of a young frond of this Fern, 17 inches in height, which was not as
+yet fully uncurled; and its movements were traced during 24 h. We see in
+Fig. 119 that it
+
+Fig. 119. Nephrodium molle: circumnutation of rachis, traced from 9.15 A.M.
+May 28th to 9 A.M. 29th. Figure here given two-thirds of original scale.
+
+plainly circumnutated. The movement was not greatly magnified as the frond
+was placed near to the vertical glass, and would probably have been greater
+and more rapid had the day been warmer. For the plant was brought out of a
+warm greenhouse and observed under a skylight, where the temperature was
+between 15o and 16o C. We have seen in Chap. I. that a frond of this Fern,
+as yet only slightly lobed and with a rachis only .23 inch in height,
+plainly circumnutated.*
+
+* Mr. Loomis and Prof. Asa Gray have described ('Botanical Gazette,' 1880,
+pp. 27, 43), an extremely curious case of movement in the fronds, but only
+in the fruiting fronds, of Asplenium trichomanes. They move almost as
+rapidly as the little leaflets of Desmodium gyrans, alternately backwards
+and forwards through from 20 to 40 degrees, in a plane at right angles to
+that of the frond. The apex of the frond describes "a long and very narrow
+ellipse," so that it circumnutates. But the movement differs from ordinary
+[[page 258]]
+circumnutation as it occurs only when the plant is exposed to the light;
+even artificial light "is sufficient to excite motion for a few minutes."
+[page 258]
+
+In the chapter on the Sleep of Plants the conspicuous circumnutation of
+Marsilea quadrifoliata (Marsileaceae, Fam. 4) will be described.
+
+It has also been shown in Chap. I. that a very young Selaginella
+(Lycopodiaceae, Fam. 6), only .4 inch in height, plainly circumnutated; we
+may therefore conclude that older plants, whilst growing, would do the
+same.
+
+Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9 A.M.
+Oct 25th to 8 A.M. 27th.
+
+(35.) Lunularia vulgaris (Hepaticae, Fam. 11, Muscales).--The earth in an
+old flower-pot was coated with this plant, bearing gemmae. A highly
+inclined frond, which projected .3 inch above the soil and was .4 inch in
+breadth, was selected for observation. A glass hair of extreme tenuity, .75
+inch in length, with its end whitened, was cemented with shellac to the
+frond at right angles to its breadth; and a white stick with a minute black
+spot was driven into the soil close behind the end of the hair. The white
+end could be accurately brought into a line with the black spot, and dots
+could thus be successively made on the vertical glass-plate in front. Any
+movement of the frond would of course be exhibited and increased by the
+long glass hair; and the black spot was placed so close behind the end of
+the hair, relatively to the distance of the glass-plate in front, that the
+movement of the end was magnified about 40 times. Nevertheless, we are
+convinced that our tracing gives a fairly faithful representation of the
+movements of the frond. In the intervals between each observation, the
+plant was covered by a small bell-glass. The frond, as already stated,
+[page 259]
+was highly inclined, and the pot stood in front of a north-east window.
+During the five first days the frond moved downwards or became less
+inclined; and the long line which was traced was strongly zigzag, with
+loops occasionally formed or nearly formed; and this indicated
+circumnutation. Whether the sinking was due to epinastic growth, or
+apheliotropism, we do not know. As the sinking was slight on the fifth day,
+a new tracing was begun on the sixth day (Oct. 25th), and was continued for
+47 h.; it is here given (Fig. 120). Another tracing was made on the next
+day (27th) and the frond was found to be still circumnutating, for during
+14 h. 30 m. it changed its course completely (besides minor changes) 10
+times. It was casually observed for two more days, and was seen to be
+continually moving.
+
+The lowest members of the vegetable series, the Thallogens, apparently
+circumnutate. If an Oscillaria be watched under the microscope, it may be
+seen to describe circles about every 40 seconds. After it has bent to one
+side, the tip first begins to bend back to the opposite side and then the
+whole filament curves over in the same direction. Hofmeister* has given a
+minute account of the curious, but less regular though constant, movements
+of Spirogyra: during 2 � h. the filament moved 4 times to the left and 3
+times to the right, and he refers to a movement at right angles to the
+above. The tip moved at the rate of about 0.1 mm. in five minutes. He
+compares the movement with the nutation of the higher plants.** We shall
+hereafter see that heliotropic movements result from modified
+circumnutation, and as unicellular Moulds bend to the light we may infer
+that they also circumnutate.]
+
+CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES.
+
+The circumnutating movements of young leaves in 33 genera, belonging to 25
+families, widely distributed
+
+* 'Ueber die Bewegungen der Faden der Spirogyra princeps: Jahreshefte des
+Vereins f�r vaterl�ndische Naturkunde in W�rttemberg,' 1874, p. 211.
+
+** Zukal also remarks (as quoted in 'Journal R. Microscop. Soc.,' 1880,
+vol. iii. p. 320) that the movements of Spirulina, a member of the
+Oscillatorieae, are closely analogous "to the well-known rotation of
+growing shoots and tendrils."
+[page 260]
+
+amongst ordinary and gymnospermous Dicotyledons and amongst Monocotyledons,
+together with several Cryptogams, have now been described. It would,
+therefore, not be rash to assume that the growing leaves of all plants
+circumnutate, as we have seen reason to conclude is the case with
+cotyledons. The seat of movement generally lies in the petiole, but
+sometimes both in the petiole and blade, or in the blade alone. The extent
+of the movement differed much in different plants; but the distance passed
+over was never great, except with Pistia, which ought perhaps to have been
+included amongst sleeping plants. The angular movement of the leaves was
+only occasionally measured; it commonly varied from only 2o (and probably
+even less in some instances) to about 10o; but it amounted to 23o in the
+common bean. The movement is chiefly in a vertical plane, but as the
+ascending and descending lines never coincided, there was always some
+lateral movement, and thus irregular ellipses were formed. The movement,
+therefore, deserves to be called one of circumnutation; for all
+circumnutating organs tend to describe ellipses,--that is, growth on one
+side is succeeded by growth on nearly but not quite the opposite side. The
+ellipses, or the zigzag lines representing drawn-out ellipses, are
+generally very narrow; yet with the Camellia, their minor axes were half as
+long, and with the Eucalyptus more than half as long as their major axes.
+In the case of Cissus, parts of the figure more nearly represented circles
+than ellipses. The amount of lateral movement is therefore sometimes
+considerable. Moreover, the longer axes of the successively formed ellipses
+(as with the Bean, Cissus, and Sea-kale), and in several instances the
+zigzag lines representing ellipses, were extended in very different
+directions during the same day or on
+[page 261]
+the next day. The course followed was curvilinear or straight, or slightly
+or strongly zigzag, and little loops or triangles were often formed. A
+single large irregular ellipse may be described on one day, and two smaller
+ones by the same plant on the next day. With Drosera two, and with Lupinus,
+Eucalyptus and Pancratium, several were formed each day.
+
+The oscillatory and jerking movements of the leaves of Dionaea, which
+resemble those of the hypocotyl of the cabbage, are highly remarkable, as
+seen under the microscope. They continue night and day for some months, and
+are displayed by young unexpanded leaves, and by old ones which have lost
+their sensibility to a touch, but which, after absorbing animal matter,
+close their lobes. We shall hereafter meet with the same kind of movement
+in the joints of certain Gramineae, and it is probably common to many
+plants while circumnutating. It is, therefore, a strange fact that no such
+movement could be detected in the tentacles of Drosera rotundifolia, though
+a member of the same family with Dionaea; yet the tentacle which was
+observed was so sensitive, that it began to curl inwards in 23 seconds
+after being touched by a bit of raw meat.
+
+One of the most interesting facts with respect to the circumnutation of
+leaves is the periodicity of their movements; for they often, or even
+generally, rise a little in the evening and early part of the night, and
+sink again on the following morning. Exactly the same phenomenon was
+observed in the case of cotyledons. The leaves in 16 genera out of the 33
+which were observed behaved in this manner, as did probably 2 others. Nor
+must it be supposed that in the remaining 15 genera there was no
+periodicity in their movements; for 6 of them were observed during too
+short a period for any judgment to be formed on this head,
+[page 262]
+and 3 were so young that their epinastic growth, which serves to bring them
+down into a horizontal position, overpowered every other kind of movement.
+In only one genus, Cannabis, did the leaves sink in the evening, and Kraus
+attributes this movement to the prepotency of their epinastic growth. That
+the periodicity is determined by the daily alternations of light and
+darkness there can hardly be a doubt, as will hereafter be shown.
+Insectivorous plants are very little affected, as far as their movements
+are concerned, by light; and hence probably it is that their leaves, at
+least in the cases of Sarracenia, Drosera, and Dionaea, do not move
+periodically. The upward movement in the evening is at first slow, and with
+different plants begins at very different hours;--with Glaucium as early as
+11 A.M., commonly between 3 and 5 P.M., but sometimes as late as 7 P.M. It
+should be observed that none of the leaves described in this chapter
+(except, as we believe, those of Lupinus speciosus) possess a pulvinus; for
+the periodical movements of leaves thus provided have generally been
+amplified into so-called sleep-movements, with which we are not here
+concerned. The fact of leaves and cotyledons frequently, or even generally,
+rising a little in the evening and sinking in the morning, is of interest
+as giving the foundation from which the specialised sleep-movements of many
+leaves and cotyledons, not provided with a pulvinus, have been developed.
+the above periodicity should be kept in mind, by any one considering the
+problem of the horizontal position of leaves and cotyledons during the day,
+whilst illuminated from above.
+[page 263]
+
+CHAPTER V.
+
+MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC
+MOVEMENTS.
+
+Circumnutation modified through innate causes or through the action of
+external conditions--Innate causes--Climbing plants; similarity of their
+movements with those of ordinary plants; increased amplitude; occasional
+points of difference--Epinastic growth of young leaves--Hyponastic growth
+of the hypocotyls and epicotyls of seedlings--Hooked tips of climbing and
+other plants due to modified circumnutation--Ampelopsis tricuspidata--
+Smithia Pfundii--Straightening of the tip due to hyponasty--Epinastic
+growth and circumnutation of the flower-peduncles of Trifolium repens and
+Oxalis carnosa.
+
+THE radicles, hypocotyls and epicotyls of seedling plants, even before they
+emerge from the ground, and afterwards the cotyledons, are all continually
+circumnutating. So it is with the stems, stolons, flower-peduncles, and
+leaves of older plants. We may, therefore, infer with a considerable degree
+of safety that all the growing parts of all plants circumnutate. Although
+this movement, in its ordinary or unmodified state, appears in some cases
+to be of service to plants, either directly or indirectly--for instance,
+the circumnutation of the radicle in penetrating the ground, or that of the
+arched hypocotyl and epicotyl in breaking through the surface--yet
+circumnutation is so general, or rather so universal a phenomenon, that we
+cannot suppose it to have been gained for any special purpose. We must
+believe that it follows in some unknown way from the manner in which
+vegetable tissues grow.
+[page 264]
+
+We shall now consider the many cases in which circumnutation has been
+modified for various special purposes; that is, a movement already in
+progress is temporarily increased in some one direction, and temporarily
+diminished or quite arrested in other directions. These cases may be
+divided in two sub-classes; in one of which the modification depends on
+innate or constitutional causes, and is independent of external conditions,
+excepting in so far that the proper ones for growth must be present. In the
+second sub-class the modification depends to a large extent on external
+agencies, such as the daily alternations of light and darkness, or light
+alone, temperature, or the attraction of gravity. The first small sub-class
+will be considered in the present chapter, and the second sub-class in the
+remainder of this volume.
+
+THE CIRCUMNUTATION OF CLIMBING PLANTS.
+
+The simplest case of modified circumnutation is that offered by climbing
+plants, with the exception of those which climb by the aid of motionless
+hooks or of rootlets: for the modification consists chiefly in the greatly
+increased amplitude of the movement. This would follow either from greatly
+increased growth over a small length, or more probably from moderately
+increased growth spread over a considerable length of the moving organ,
+preceded by turgescence, and acting successively on all sides. The
+circumnutation of climbers is more regular than that of ordinary plants;
+but in almost every other respect there is a close similarity between their
+movements, namely, in their tendency to describe ellipses directed
+successively to all points of the compass--in their courses being often
+interrupted by zigzag lines, triangles, loops, or small
+[page 265]
+ellipses--in the rate of movement, and in different species revolving once
+or several times within the same length of time. In the same internode, the
+movements cease first in the lower part and then slowly upwards. In both
+sets of cases the movement may be modified in a closely analogous manner by
+geotropism and by heliotropism; though few climbing plants are heliotropic.
+Other points of similarity might be pointed out.
+
+That the movements of climbing plants consist of ordinary circumnutation,
+modified by being increased in amplitude, is well exhibited whilst the
+plants are very young; for at this early age they move like other
+seedlings, but as they grow older their movements gradually increase
+without undergoing any other change. That this power is innate, and is not
+excited by any external agencies, beyond those necessary for growth and
+vigour, is obvious. No one doubts that this power has been gained for the
+sake of enabling climbing plants to ascend to a height, and thus to reach
+the light. This is effected by two very different methods; first, by
+twining spirally round a support, but to do so their stems must be long and
+flexible; and, secondly, in the case of leaf-climbers and tendril-bearers,
+by bringing these organs into contact with a support, which is then seized
+by the aid of their sensitiveness. It may be here remarked that these
+latter movements have no relation, as far as we can judge, with
+circumnutation. In other cases the tips of tendrils, after having been
+brought into contact with a support, become developed into little discs
+which adhere firmly to it.
+
+We have said that the circumnutation of climbing plants differs from that
+of ordinary plants chiefly by its greater amplitude. But most leaves
+circumnutate
+[page 266]
+in an almost vertical plane, and therefore describe very narrow ellipses,
+whereas the many kinds of tendrils which consist of metamorphosed leaves,
+make much broader ellipses or nearly circular figures; and thus they have a
+far better chance of catching hold of a support on any side. The movements
+of climbing plants have also been modified in some few other special ways.
+Thus the circumnutating stems of Solnanum dulcamara can twine round a
+support only when this is as thin and flexible as a string or thread. The
+twining stems of several British plants cannot twine round a support when
+it is more than a few inches in thickness; whilst in tropical forests some
+can embrace thick trunks;* and this great difference in power depends on
+some unknown difference in their manner of circumnutation. The most
+remarkable special modification of this movement which we have observed is
+in the tendrils of Echinocystis lobata; these are usually inclined at about
+45o above the horizon, but they stiffen and straighten themselves so as to
+stand upright in a part of their circular course, namely, when they
+approach and have to pass over the summit or the shoot from which they
+arise. If they had not possessed and exercised this curious power, they
+would infallibly have struck against the summit of the shoot and been
+arrested in their course. As soon as one of these tendrils with its three
+branches begins to stiffen itself and rise up vertically, the revolving
+motion becomes more rapid; and as soon as it has passed over the point of
+difficulty, its motion coinciding with that from its own weight, causes it
+to fall into its previously inclined position so quickly, that the apex can
+be seen travelling like the hand of a gigantic clock.
+
+* 'The Movements and Habits of Climbing Plants,' p. 36.
+[page 267]
+
+A large number of ordinary leaves and leaflets and a few flower-peduncles
+are provided with pulvini; but this is not the case with a single tendril
+at present known. The cause of this difference probably lies in the fact,
+that the chief service of a pulvinus is to prolong the movement of the part
+thus provided after growth has ceased; and as tendrils or other
+climbing-organs are of use only whilst the plant is increasing in height or
+growing, a pulvinus which served to prolong their movements would be
+useless.
+
+It was shown in the last chapter that the stolons or runners of certain
+plants circumnutate largely, and that this movement apparently aids them in
+finding a passage between the crowded stems of adjoining plants. If it
+could be proved that their movements had been modified and increased for
+this special purpose, they ought to have been included in the present
+chapter; but as the amplitude of their revolutions is not so conspicuously
+different from that of ordinary plants, as in the case of climbers, we have
+no evidence on this head. We encounter the same doubt in the case of some
+plants which bury their pods in the ground. This burying process is
+certainly favoured by the circumnutation of the flower-peduncle; but we do
+not know whether it has been increased for this special purpose.
+
+EPINASTY--HYPONASTY.
+
+The term epinasty is used by De Vries* to express greater longitudinal
+growth along the upper than
+
+* 'Arbeiten des Bot. Inst., in W�rzburg,' Heft ii. 1872, p. 223. De Vries
+has slightly modified (p. 252) the meaning of the above two terms as first
+used by Schimper, and they have been adopted in this sense by Sachs.
+[page 268]
+
+along the lower side of a part, which is thus caused to bend downwards; and
+hyponasty is used for the reversed process, by which the part is made to
+bend upwards. These actions come into play so frequently that the use of
+the above two terms is highly convenient. The movements thus induced result
+from a modified form of circumnutation; for, as we shall immediately see,
+an organ under the influence of epinasty does not generally move in a
+straight line downwards, or under that of hyponasty upwards, but oscillates
+up and down with some lateral movement: it moves, however, in a
+preponderant manner in one direction. This shows that there is some growth
+on all sides of the part, but more on the upper side in the case of
+epinasty, and more on the lower side in that of hyponasty, than on the
+other sides. At the same time there may be in addition, as De Vries
+insists, increased growth on one side due to geotropism, and on another
+side due to heliotropism; and thus the effects of epinasty or of hyponasty
+may be either increased or lessened.
+
+He who likes, may speak of ordinary circumnutation as being combined with
+epinasty, hyponasty, the effects of gravitation, light, etc.; but it seems
+to us, from reasons hereafter to be given, to be more correct to say that
+circumnutation is modified by these several agencies. We will therefore
+speak of circumnutation, which is always in progress, as modified by
+epinasty, hyponasty, geotropism, or other agencies, whether internal or
+external.
+
+[One of the commonest and simplest cases of epinasty is that offered by
+leaves, which at an early age are crowded together round the buds, and
+diverge as they grow older. Sachs first remarked that this was due to
+increased growth along the upper side of the petiole and blade; and De
+Vries has now shown in more detail that the movement is thus caused, aided
+slightly by
+[page 269]
+the weight of the leaf, and resisted as he believes by apogeotropism, at
+least after the leaf has somewhat diverged. In our observations on the
+circumnutation of leaves, some were selected which were rather too young,
+so that they continued to diverge or sink downwards whilst their movements
+were being traced. This may be seen in the diagrams (Figs. 98 and 112, pp.
+232 and 249) representing the circumnutation of the young leaves of
+Acanthus mollis and Pelargonium zonale. Similar cases were observed with
+Drosera. The movements of a young leaf, only 3/4 inch in length, of Petunia
+violacea were traced during four days, and offers a better instance (Fig.
+111, p. 248) as it diverged during the whole of this time in a curiously
+zigzag line with some of the angles sharply acute, and during the latter
+days plainly circumnutated. Some young leaves of about the same age on a
+plant of this Petunia, which had been laid horizontally, and on another
+plant which was left upright, both being kept in complete darkness,
+diverged in the same manner for 48 h., and apparently were not affected by
+apogeotropism; though their stems were in a state of high tension, for when
+freed from the sticks to which they had been tied, they instantly curled
+upwards.
+
+The leaves, whilst very young, on the leading shoots of the Carnation
+(Dianthus caryophyllus) are highly inclined or vertical; and if the plant
+is growing vigorously they diverge so quickly that they become almost
+horizontal in a day. But they move downwards in a rather oblique line and
+continue for some time afterwards to move in the same direction, in
+connection, we presume, with their spiral arrangement on the stem. The
+course pursued by a young leaf whilst thus obliquely descending was traced,
+and the line was distinctly yet not strongly zigzag; the larger angles
+formed by the successive lines amounting only to 135o, 154o, and 163o. The
+subsequent lateral movement (shown in Fig. 96, p. 231) was strongly zigzag
+with occasional circumnutations. The divergence and sinking of the young
+leaves of this plant seem to be very little affected by geotropism or
+heliotropism; for a plant, the leaves of which were growing rather slowly
+(as ascertained by measurement) was laid horizontally, and the opposite
+young leaves diverged from one another symmetrically in the usual manner,
+without any upturning in the direction of gravitation or towards the light.
+
+The needle-like leaves of Pinus pinaster form a bundle whilst young;
+afterwards they slowly diverge, so that those on the upright shoots become
+horizontal. The movements of one such
+[page 270]
+young leaf was traced during 4 � days, and the tracing here given (Fig.
+121) shows that it descended at first in a nearly straight line, but
+afterwards zigzagged, making one or two little loops. The diverging and
+descending movements of a rather older leaf were also traced (see former
+Fig. 113, p. 251): it descended during the first day and night in a
+somewhat zigzag line; it then circumnutated round a small space and again
+descended. By this time the leaf had nearly assumed its final position, and
+now plainly circumnutated. As in the case of the Carnation, the leaves,
+whilst very young, do not seem to be much affected by geotropism or
+heliotropism, for those on a young plant laid horizontally, and those on
+another plant left upright, both kept in the dark, continued to diverge in
+the usual manner without bending to either side.
+
+Fig. 121. Pinus pinaster: epinastic downward movement of a young leaf,
+produced by a young plant in a pot, traced on a vertical glass under a
+skylight, from 6.45 A.M. June 2nd to 10.40 P.M. 6th.
+
+With Coboea scandens, the young leaves, as they successively diverge from
+the leading shoot which is bent to one side, rise up so as to project
+vertically, and they retain this position for some time whilst the tendril
+is revolving. The diverging and ascending movements of the petiole of one
+such a leaf, were traced on a vertical glass under a skylight; and the
+course pursued was in most parts nearly straight, but there were two
+[page 271]
+well-marked zigzags (one of them forming an angle of 112o), and this
+indicates circumnutation.
+
+The still closed lobes of a young leaf of Dionaea projected at right angles
+to the petiole, and were in the act of slowly rising. A glass filament was
+attached to the under side of the midrib, and its movements were traced on
+a vertical glass. It circumnutated once in the evening, and on the next day
+rose, as already described (see Fig. 106, p. 240), by a number of acutely
+zigzag lines, closely approaching in character to ellipses. This movement
+no doubt was due to epinasty, aided by apogeotropism, for the closed lobes
+of a very young leaf on a plant which had been placed horizontally, moved
+into nearly the same line with the petiole, as if the plant had stood
+upright; but at the same time the lobes curved laterally upwards, and thus
+occupied an unnatural position, obliquely to the plane of the foliaceous
+petiole.
+
+As the hypocotyls and epicotyls of some plants protrude from the seed-coats
+in an arched form, it is doubtful whether the arching of these parts, which
+is invariably present when they break through the ground, ought always to
+be attributed to epinasty; but when they are at first straight and
+afterwards become arched, as often happens, the arching is certainly due to
+epinasty. As long as the arch is surrounded by compact earth it must retain
+its form; but as soon as it rises above the surface, or even before this
+period if artificially freed from the surrounding pressure, it begins to
+straighten itself, and this no doubt is mainly due to hyponasty. The
+movement of the upper and lower half of the arch, and of the crown, was
+occasionally traced; and the course was more or less zigzag, showing
+modified circumnutation.
+
+With not a few plants, especially climbers, the summit of the shoot is
+hooked, so that the apex points vertically downwards. In seven genera of
+twining plants* the hooking, or as it has been called by Sachs, the
+nutation of the tip, is mainly due to an exaggerated form of
+circumnutation. That is, the growth is so great along one side that it
+bends the shoot completely over to the opposite side, thus forming a hook;
+the longitudinal line or zone of growth then travels a little laterally
+round the shoot, and the hook points in a slightly different direction, and
+so onwards until the hook is completely reversed. Ultimately it
+
+* 'The Movements and Habits of Climbing Plants,' 2nd edit. p. 13.
+[page 272]
+
+comes back to the point whence it started. This was ascertained by painting
+narrow lines with Indian ink along the convex surface of several hooks, and
+the line was found slowly to become at first lateral, then to appear along
+the concave surface, and ultimately back again on the convex surface. In
+the case of Lonicera brachypoda the hooked terminal part of the revolving
+shoot straightens itself periodically, but is never reversed; that is, the
+periodically increased growth of the concave side of the hook is sufficient
+only to straighten it, and not to bend it over to the opposite side. The
+hooking of the tip is of service to twining plants by aiding them to catch
+hold of a support, and afterwards by enabling this part to embrace the
+support much more closely than it could otherwise have done at first, thus
+preventing it, as we often observed, from being blown away by a strong
+wind. Whether the advantage thus gained by twining plants accounts for
+their summits being so frequently hooked, we do not know, as this structure
+is not very rare with plants which do not climb, and with some climbers
+(for instance, Vitis, Ampelopsis, Cissus, etc.) to whom it does not afford
+any assistance in climbing.
+
+With respect to those cases in which the tip remains always bent or hooked
+towards the same side, as in the genera just named, the most obvious
+explanation is that the bending is due to continued growth in excess along
+the convex side. Wiesner, however, maintains* that in all cases the hooking
+of the tip is the result of its plasticity and weight,--a conclusion which
+from what we have already seen with several climbing plants is certainly
+erroneous. Nevertheless, we fully admit that the weight of the part, as
+well as geotropism, etc., sometimes come into play.
+
+Ampelopsis tricuspidata.--This plant climbs by the aid of adhesive
+tendrils, and the hooked tips of the shoots do not appear to be of any
+service to it. The hooking depends chiefly, as far as we could ascertain,
+on the tip being affected by epinasty and geotropism; the lower and older
+parts continually straightening themselves through hyponasty and
+apogeotropism. We believe that the weight of the apex is an unimportant
+element, because on horizontal or inclined shoots the hook is often
+extended horizontally or even faces upwards. Moreover shoots frequently
+form loops instead of hooks; and in this case the
+
+* 'Sitzb. der k. Akad. der Wissensch.,' Vienna, Jan. 1880, p. 16.
+[page 273]
+
+Fig. 122. Ampelopsis tricuspidata: hyponastic movement of hooked tip of
+leading shoot, traced from 8.10 A.M. July 13th to 8 A.M. 15th. Apex of
+shoot 5 � inches from the vertical glass. Plant illuminated through a
+skylight. Temp. 17 1/2o - 19o C. Diagram reduced to one-third of original
+scale.
+
+extreme part, instead of hanging vertically down as would follow if weight
+was the efficient cause, extends horizontally or even points upwards. A
+shoot, which terminated in a rather open hook, was fastened in a highly
+inclined downward position, so that the concave side faced upwards, and the
+result was that the apex at first curved upwards. This apparently was due
+to epinasty and not to apogeotropism, for the apex, soon after passing the
+perpendicular, curved so rapidly downwards that we could not doubt that the
+movement was at least aided by geotropism. In the course of a few hours the
+hook was thus converted into a loop with the apex of the shoot pointing
+straight downwards. The longer axis of the loop was at first horizontal,
+but afterwards became vertical. During this same time the basal part of the
+hook (and subsequently of the loop) curved itself slowly upwards; and this
+must have been wholly due to apogeotropism in opposition to hyponasty. The
+loop was then fastened upside down, so that its basal half would be
+simultaneously acted on by hyponasty (if present) and by apogeotropism; and
+now it curved itself so greatly upwards in the course of only 4 h. that
+there could hardly be a doubt that both forces were acting
+[page 274]
+together. At the same time the loop became open and was thus reconverted
+into a hook, and this apparently was effected by the geotropic movement of
+the apex in opposition to epinasty. In the case of Ampelopsis hederacea,
+weight plays, as far as we could judge, a more important part in the
+hooking of the tip.
+
+In order to ascertain whether the shoots of A. tricuspidata in
+straightening themselves under the combined action of hyponasty and
+apogeotropism moved in a simple straight course, or whether they
+circumnutated, glass filaments were fixed to the crowns of four hooked tips
+standing in their natural position; and the movements of the filaments were
+traced on a vertical glass. All four tracings resembled each other in a
+general manner; but we will give only one (see Fig. 122, p. 273). The
+filament rose at first, which shows that the hook was straightening itself;
+it then zigzagged, moving a little to the left between 9.25 A.M. and 9 P.M.
+From this latter hour on the 13th to 10.50 A.M. on the following morning
+(14th) the hook continued to straighten itself, and then zigzagged a short
+distance to the right. But from 1 P.M. to 10.40 P.M. on the 14th the
+movement
+
+Fig. 123. Smithia Pfundii: hyponastic movement of the curved summit of a
+stem, whilst straightening itself, traced from 9 A.M. July 10th to 3 P.M.
+13th. Apex 9 � inches from the vertical glass. Diagram reduced to one-fifth
+of original scale. Plant illuminated through skylight; temp. 17 1/2o - 19o
+C.
+[page 275]
+
+was reversed and the shoot became more hooked. During the night, after
+10.40 P.M. to 8.15 A.M. on the 15th, the hook again opened or straightened
+itself. By this time the glass filament had become so highly inclined that
+its movements could no longer be traced with accuracy; and by 1.30 P.M. on
+this same day, the crown of the former arch or hook had become perfectly
+straight and vertical. There can therefore be no doubt that the
+straightening of the hooked shoot of this plant is effected by the
+circumnutation of the arched portion--that is, by growth alternating
+between the upper and lower surface, but preponderant on the lower surface,
+with some little lateral movement.
+
+We were enabled to trace the movement of another straightening shoot for a
+longer period (owing to its slower growth and to its having been placed
+further from the vertical glass), namely, from the early morning on July
+13th to late in the evening of the 16th. During the whole daytime of the
+14th, the hook straightened itself very little, but zigzagged and plainly
+circumnutated about nearly the same spot. By the 16th it had become nearly
+straight, and the tracing was no longer accurate, yet it was manifest that
+there was still a considerable amount of movement both up and down and
+laterally; for the crown whilst continuing to straighten itself
+occasionally became for a short time more curved, causing the filament to
+descend twice during the day.
+
+Smithia Pfundii.--The stiff terminal shoots of this Leguminous water-plant
+from Africa project so as to make a rectangle with the stem below; but this
+occurs only when the plants are growing vigorously, for when kept in a cool
+place, the summits of the stems become straight, as they likewise did at
+the close of the growing season. The direction of the rectangularly bent
+part is independent of the chief source of light. But from observing the
+effects of placing plants in the dark, in which case several shoots became
+in two or three days upright or nearly upright, and when brought back into
+the light again became rectangularly curved, we believe that the bending is
+in part due to apheliotropism, apparently somewhat opposed by
+apogeotropism. On the other hand, from observing the effects of tying a
+shoot downwards, so that the rectangle faced upwards, we are led to believe
+that the curvature is partly due to epinasty. As the rectangularly bent
+portion of an upright stem grows older, the lower part straightens itself;
+and this is effected through hyponasty. He who has read Sachs' recent Essay
+on the vertical
+[page 276]
+and inclined positions of the parts of plants* will see how difficult a
+subject this is, and will feel no surprise at our expressing ourselves
+doubtfully in this and other such cases.
+
+A plant, 20 inches in height, was secured to a stick close beneath the
+curved summit, which formed rather less than a rectangle with the stem
+below. The shoot pointed away from the observer; and a glass filament
+pointing towards the vertical glass on which the tracing was made, was
+fixed to the convex surface of the curved portion. Therefore the descending
+lines in the figure represent the straightening of the curved portion as it
+grew older. The tracing (Fig. 123, p. 274) was begun at 9 A.M. on July
+10th; the filament at first moved but little in a zigzag line, but at 2
+P.M. it began rising and continued to do so till 9 P.M.; and this proves
+that the terminal portion was being more bent downwards. After 9 P.M. on
+the 10th an opposite movement commenced, and the curved portion began to
+straighten itself, and this continued till 11.10 A.M. on the 12th, but was
+interrupted by some small oscillations and zigzags, showing movement in
+different directions. After 11.10 A.M. on the 12th this part of the stem,
+still considerably curved, circumnutated in a conspicuous manner until
+nearly 3 P.M. on the 13th; but during all this time a downward movement of
+the filament prevailed, caused by the continued straightening of the stem.
+By the afternoon of the 13th, the summit, which had originally been
+deflected more than a right angle from the perpendicular, had grown so
+nearly straight that the tracing could no longer be continued on the
+vertical glass. There can therefore be no doubt that the straightening of
+the abruptly curved portion of the growing stem of this plant, which
+appears to be wholly due to hyponasty, is the result of modified
+circumnutation. We will only add that a filament was fixed in a different
+manner across the curved summit of another plant, and the same general kind
+of movement was observed.
+
+Trifolium repens.--In many, but not in all the species of Trifolium, as the
+separate little flowers wither, the sub-peduncles bend downwards, so as to
+depend parallel to the upper part of the main peduncle. In Tr. subterraneum
+the main peduncle curves downwards for the sake of burying its capsules,
+and in this species the sub-peduncles of the separate flowers bend
+
+* 'Ueber Orthotrope und Plagiotrope Pflanzentheile;' 'Arbeiten des Bot.
+Inst., in W�rzburg,' Heft ii. 1879, p. 226.
+[page 277]
+
+Fig. 124. Trifolium repens: circumnutating and epinastic movements of the
+sub-peduncle of a single flower, traced on a vertical glass under a
+skylight, in A from 11.30 A.M. Aug. 27th to 7 A.M. 30th; in B from 7 A.M.
+Aug. 30th to a little after 6 P.M. Sept. 8th.
+[page 278]
+
+upwards, so as to occupy the same position relatively to the upper part of
+the main peduncle as in Tr. repens. This fact alone would render it
+probable that the movements of the sub-peduncles in Tr. repens were
+independent of geotropism. Nevertheless, to make sure, some flower-heads
+were tied to little sticks upside down and others in a horizontal position;
+their sub-peduncles, however, all quickly curved upwards through the action
+of heliotropism. We therefore protected some flower-heads, similarly
+secured to sticks, from the light, and although some of them rotted, many
+of their sub-peduncles turned very slowly from their reversed or from their
+horizontal positions, so as to stand in the normal manner parallel to the
+upper part of the main peduncle. These facts show that the movement is
+independent of geotropism or apheliotropism; it must there[fore] be
+attributed to epinasty, which however is checked, at least as long as the
+flowers are young, by heliotropism. Most of the above flowers were never
+fertilised owing to the exclusion of bees; they consequently withered very
+slowly, and the movements of the sub-peduncles were in like manner much
+retarded.
+
+To ascertain the nature of the movement of the sub-peduncle, whilst bending
+downwards, a filament was fixed across the summit of the calyx of a not
+fully expanded and almost upright flower, nearly in the centre of the head.
+The main peduncle was secured to a stick close beneath the head. In order
+to see the marks on the glass filament, a few flowers had to be cut away on
+the lower side of the head. The flower under observation at first diverged
+a little from its upright position, so as to occupy the open space caused
+by the removal of the adjoining flowers. This required two days, after
+which time a new tracing was begun (Fig. 124). In A we see the complex
+circumnutating course pursued from 11.30 A.M. Aug. 26th to 7 A.M. on the
+30th. The pot was then moved a very little to the right, and the tracing
+(B) was continued without interruption from 7 A.M. Aug. 30th to after 6
+P.M. Sept. 8th. It should be observed that on most of these days, only a
+single dot was made each morning at the same hour. Whenever the flower was
+observed carefully, as on Aug. 30th and Sept. 5th and 6th, it was found to
+be circumnutating over a small space. At last, on Sept. 7th, it began to
+bend downwards, and continued to do so until after 6 P.M. on the 8th, and
+indeed until the morning of the 9th, when its movements could no longer be
+traced on the vertical glass. It was carefully observed during the whole of
+the 8th, and by
+[page 279]
+10.30 P.M. it had descended to a point lower down by two-thirds of the
+length of the figure as here given; but from want of space the tracing has
+been copied in B, only to a little after 6 P.M. On the morning of the 9th
+the flower was withered, and the sub-peduncle now stood at an angle of 57o
+beneath the horizon. If the flower had been fertilised it would have
+withered much sooner, and have moved much more quickly. We thus see that
+the sub-peduncle oscillated up and down, or circumnutated, during its whole
+downward epinastic course.
+
+The sub-peduncles of the fertilised and withered flowers of Oxalis carnosa
+likewise bend downwards through epinasty, as will be shown in a future
+chapter; and their downward course is strongly zigzag, indicating
+circumnutation.]
+
+The number of instances in which various organs move through epinasty or
+hyponasty, often in combination with other forces, for the most diversified
+purposes, seems to be inexhaustibly great; and from the several cases which
+have been here given, we may safely infer that such movements are due to
+modified circumnutation.
+[page 280]
+
+
+CHAPTER VI.
+
+MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP
+OF COTYLEDONS.
+
+Preliminary sketch of the sleep or nyctitropic movements of leaves--
+Presence of pulvini--The lessening of radiation the final cause of
+nyctitropic movements--Manner of trying experiments on leaves of Oxalis,
+Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of
+Mimosa--Concluding remarks on radiation from leaves--Small differences in
+the conditions make a great difference in the result - Description of the
+nyctitropic position and movements of the cotyledons of various plants--
+List of species--Concluding remarks--Independence of the nyctitropic
+movements of the leaves and cotyledons of the same species--Reasons for
+believing that the movements have been acquired for a special purpose.
+
+The so-called sleep of leaves is so conspicuous a phenomenon that it was
+observed as early as the time of Pliny;* and since Linnaeus published his
+famous Essay, 'Somnus Plantarum,' it has been the subject of several
+memoirs. Many flowers close at night, and these are likewise said to sleep;
+but we are not here concerned with their movements, for although effected
+by the same mechanism as in the case of young leaves, namely, unequal
+growth on the opposite sides (as first proved by Pfeffer), yet they differ
+essentially in being excited chiefly by changes of temperature instead of
+light; and in being effected, as far as we can judge, for a different
+purpose. Hardly any one supposes that there is any real analogy
+
+* Pfeffer has given a clear and interesting sketch of the history of this
+subject in his 'Die Periodischen Bewegungen der Blattorgane,' 1875, P. 163.
+[page 281]
+
+between the sleep of animals and that of plants,* whether of leaves or
+flowers. It seems therefore, advisable to give a distinct name to the
+so-called sleep-movements of plants. These have also generally been
+confounded, under the term "periodic," with the slight daily rise and fall
+of leaves, as described in the fourth chapter; and this makes it all the
+more desirable to give some distinct name to sleep-movements. Nyctitropism
+and nyctitropic, i.e. night-turning, may be applied both to leaves and
+flowers, and will be occasionally used by us; but it would be best to
+confine the term to leaves. The leaves of some few plants move either
+upwards or downwards when the sun shines intensely on them, and this
+movement has sometimes been called diurnal sleep; but we believe it to be
+of an essentially different nature from the nocturnal movement, and it will
+be briefly considered in a future chapter.
+
+The sleep or nyctitropism of leaves is a large subject, and we think that
+the most convenient plan will be first to give a brief account of the
+position which leaves assume at night, and of the advantages apparently
+thus gained. Afterwards the more remarkable cases will be described in
+detail, with respect to cotyledons in the present chapter, and to leaves in
+the next chapter. Finally, it will be shown that these movements result
+from circumnutation, much modified and regulated by the alternations of day
+and night, or light and darkness; but that they are also to a certain
+extent inherited.
+
+Leaves, when they go to sleep, move either upwards or downwards, or in the
+case of the leaflets of com-
+
+* Ch. Royer must, however, be excepted; see 'Annales des Sc. Nat.' (5th
+series), Bot. vol. ix. 1868, p. 378.
+
+[page 282]
+pound leaves, forwards, that is, towards the apex of the leaf, or
+backwards, that is, towards its base; or, again, they may rotate on their
+own axes without moving either upwards or downwards. But in almost every
+case the plane of the blade is so placed as to stand nearly or quite
+vertically at night. Therefore the apex, or the base, or either lateral
+edge, may be directed towards the zenith. Moreover, the upper surface of
+each leaf, and more especially of each leaflet, is often brought into close
+contact with that of the opposite one; and this is sometimes effected by
+singularly complicated movements. This fact suggests that the upper surface
+requires more protection than the lower one. For instance, the terminal
+leaflet in Trifolium, after turning up at night so as to stand vertically,
+often continues to bend over until the upper surface is directed downwards
+whilst the lower surface is fully exposed to the sky; and an arched roof is
+thus formed over the two lateral leaflets, which have their upper surfaces
+pressed closely together. Here we have the unusual case of one of the
+leaflets not standing vertically, or almost vertically, at night.
+
+Considering that leaves in assuming their nyctitropic positions often move
+through an angle of 90o; that the movement is rapid in the evening; that in
+some cases, as we shall see in the next chapter, it is extraordinarily
+complicated; that with certain seedlings, old enough to bear true leaves,
+the cotyledons move vertically upwards at night, whilst at the same time
+the leaflets move vertically downwards; and that in the same genus the
+leaves or cotyledons of some species move upwards, whilst those of other
+species move downwards;--from these and other such facts, it is hardly
+possible to doubt that plants must derive some
+[page 283]
+great advantage from such remarkable powers of movement.
+
+The nyctitropic movements of leaves and cotyledons are effected in two
+ways,* firstly, by means of pulvini which become, as Pfeffer has shown,
+alternately more turgescent on opposite sides; and secondly, by increased
+growth along one side of the petiole or midrib, and then on the opposite
+side, as was first proved by Batalin.** But as it has been shown by De
+Vries*** that in these latter cases increased growth is preceded by the
+increased turgescence of the cells, the difference between the above two
+means of movement is much diminished, and consists chiefly in the
+turgescence of the cells of a fully developed pulvinus, not being followed
+by growth. When the movements of leaves or cotyledons, furnished with a
+pulvinus and destitute of one, are compared, they are seen to be closely
+similar, and are apparently effected for the same purpose. Therefore, with
+our object in view, it does not appear advisable to separate the above two
+sets of cases into two distinct classes. There is, however, one important
+distinction between them, namely, that movements effected by growth on the
+alternate sides, are confined to young growing leaves, whilst those
+effected by means of a pulvinus last for a long time. We have already seen
+well-marked instances of this latter fact with cotyledons, and so it is
+with leaves, as has been observed by Pfeffer and by ourselves. The long
+endurance of the nyctitropic movements when effected by the aid of pulvini
+indicates, in addition to the evidence already advanced, the functional
+import-
+
+* This distinction was first pointed out (according to Pfeffer, 'Die
+Periodischen Bewegungen der Blattorgane,' 1875, p. 161) by Dassen in 1837.
+
+** 'Flora,' 1873, p. 433.
+
+*** 'Bot. Zeitung,' 1879, Dec. 19th, p. 830.
+
+[page 284]
+ance of such movements to the plant. There is another difference between
+the two sets of cases, namely, that there is never, or very rarely, any
+torsion of the leaves, excepting when a pulvinus is present;* but this
+statement applies only to periodic and nyctitropic movements as may be
+inferred from other cases given by Frank.**
+The fact that the leaves of many plants place themselves at night in widely
+different positions from what they hold during the day, but with the one
+point in common, that their upper surfaces avoid facing the zenith, often
+with the additional fact that they come into close contact with opposite
+leaves or leaflets, clearly indicates, as it seems to us, that the object
+gained is the protection of the upper surfaces from being chilled at night
+by radiation. There is nothing improbable in the upper surface needing
+protection more than the lower, as the two differ in function and
+structure. All gardeners know that plants suffer from radiation. It is this
+and not cold winds which the peasants of Southern Europe fear for their
+olives.*** Seedlings are often protected from radiation by a very thin
+covering of straw; and fruit-trees on walls by a few fir-branches, or even
+by a fishing-net, suspended over them. There is a variety of the
+gooseberry,**** the flowers of which from being produced before the leaves,
+are not protected by them from radiation, and consequently often fail to
+yield fruit. An excellent observer***** has remarked
+
+* Pfeffer, 'Die Period. Beweg. der Blattorgane.' 1875, p. 159.
+
+** 'Die Nat. Wagerechte Richtung von Pflanzentheilen,' 1870, p. 52
+
+*** Martins in 'Bull. Soc. Bot. de France,' tom. xix. 1872. Wells, in his
+famous 'Essay on Dew,' remarks that an exposed thermometer rises as soon as
+even a fleecy cloud, high in the sky, passes over the zenith.
+
+**** 'Loudon's Gardener's Mag.,' vol. iv. 1828, p. 112.
+
+***** Mr. Rivers in 'Gardener's Chron.,' 1866, p. 732.
+[page 285]
+
+that one variety of the cherry has the petals of its flowers much curled
+backwards, and after a severe frost all the stigmas were killed; whilst at
+the same time, in another variety with incurved petals, the stigmas were
+not in the least injured.
+
+This view that the sleep of leaves saves them from being chilled at night
+by radiation, would no doubt have occurred to Linnaeus, had the principle
+of radiation been then discovered; for he suggests in many parts of his
+'Somnus Plantarum' that the position of the leaves at night protects the
+young stems and buds, and often the young inflorescence, against cold
+winds. We are far from doubting that an additional advantage may be thus
+gained; and we have observed with several plants, for instance, Desmodium
+gyrans, that whilst the blade of the leaf sinks vertically down at night,
+the petiole rises, so that the blade has to move through a greater angle in
+order to assume its vertical position than would otherwise have been
+necessary; but with the result that all the leaves on the same plant are
+crowded together as if for mutual protection.
+
+We doubted at first whether radiation would affect in any important manner
+objects so thin as are many cotyledons and leaves, and more especially
+affect differently their upper and lower surfaces; for although the
+temperature of their upper surfaces would undoubtedly fall when freely
+exposed to a clear sky, yet we thought that they would so quickly acquire
+by conduction the temperature of the surrounding air, that it could hardly
+make any sensible difference to them, whether they stood horizontally and
+radiated into the open sky, or vertically and radiated chiefly in a lateral
+direction towards neighbouring plants and other objects. We endeavoured,
+therefore, to ascertain something on this head by preventing the leaves
+[page 286]
+of several plants from going to sleep, and by exposing to a clear sky when
+the temperature was beneath the freezing-point, these, as well as the other
+leaves on the same plants which had already assumed their nocturnal
+vertical position. Our experiments show that leaves thus compelled to
+remain horizontal at night, suffered much more injury from frost than those
+which were allowed to assume their normal vertical position. It may,
+however, be said that conclusions drawn from such observations are not
+applicable to sleeping plants, the inhabitants of countries where frosts do
+not occur. But in every country, and at all seasons, leaves must be exposed
+to nocturnal chills through radiation, which might be in some degree
+injurious to them, and which they would escape by assuming a vertical
+position.
+
+In our experiments, leaves were prevented from assuming their nyctitropic
+position, generally by being fastened with the finest entomological pins
+(which did not sensibly injure them) to thin sheets of cork supported on
+sticks. But in some instances they were fastened down by narrow strips of
+card, and in others by their petioles being passed through slits in the
+cork. The leaves were at first fastened close to the cork, for as this is a
+bad conductor, and as the leaves were not exposed for long periods, we
+thought that the cork, which had been kept in the house, would very
+slightly warm them; so that if they were injured by the frost in a greater
+degree than the free vertical leaves, the evidence would be so much the
+stronger that the horizontal position was injurious. But we found that when
+there was any slight difference in the result, which could be detected only
+occasionally, the leaves which had been fastened closely down suffered
+rather more than those fastened with very long and
+[page 287]
+thin pins, so as to stand from � to 3/4 inch above the cork. This
+difference in the result, which is in itself curious as showing what a very
+slight difference in the conditions influences the amount of injury
+inflicted, may be attributed, as we believe, to the surrounding warmer air
+not circulating freely beneath the closely pinned leaves and thus slightly
+warming them. This conclusion is supported by some analogous facts
+hereafter to be given.
+
+We will now describe in detail the experiments which were tried. These were
+troublesome from our not being able to predict how much cold the leaves of
+the several species could endure. Many plants had every leaf killed, both
+those which were secured in a horizontal position and those which were
+allowed to sleep--that is, to rise up or sink down vertically. Others again
+had not a single leaf in the least injured, and these had to be re-exposed
+either for a longer time or to a lower temperature.
+
+[Oxalis acetosella.--A very large pot, thickly covered with between 300 and
+400 leaves, had been kept all winter in the greenhouse. Seven leaves were
+pinned horizontally open, and were exposed on March 16th for 2 h. to a
+clear sky, the temperature on the surrounding grass being -4o C. (24o to 25o
+F.). Next morning all seven leaves were found quite killed, so were many of
+the free ones which had previously gone to sleep, and about 100 of them,
+either dead or browned and injured were picked off. Some leaves showed that
+they had been slightly injured by not expanding during the whole of the
+next day, though they afterwards recovered. As all the leaves which were
+pinned open were killed, and only about a third or fourth of the others
+were either killed or injured, we had some little evidence that those which
+were prevented from assuming their vertically dependent position suffered
+most.
+
+The following night (17th) was clear and almost equally cold (-3o to -4o C.
+on the grass), and the pot was again exposed, but this time for only 30 m.
+Eight leaves had been pinned out,
+[page 288]
+and in the morning two of them were dead, whilst not a single other leaf on
+the many plants was even injured.
+
+On the 23rd the pot was exposed for 1 h. 30 m., the temperature on the
+grass being only -2o C., and not one leaf was injured: the pinned open
+leaves, however, all stood from � to 3/4 of an inch above the cork.
+
+On the 24th the pot was again placed on the ground and exposed to a clear
+sky for between 35 m. and 40 m. By a mistake the thermometer was left on an
+adjoining sun-dial 3 feet high, instead of being placed on the grass; it
+recorded 25o to 26o F. (-3.3o to -3.8o C.), but when looked at after 1 h.
+had fallen to 22o F. (-5.5o C.); so that the pot was perhaps exposed to
+rather a lower temperature than on the two first occasions. Eight leaves
+had been pinned out, some close to the cork and some above it, and on the
+following morning five of them (i.e. 63 per cent.) were found killed. By
+counting a portion of the leaves we estimated that about 250 had been
+allowed to go to sleep, and of these about 20 were killed (i.e. only 8 per
+cent.), and about 30 injured.
+
+Considering these cases, there can be no doubt that the leaves of this
+Oxalis, when allowed to assume their normal vertically dependent position
+at night, suffer much less from frost than those (23 in number) which had
+their upper surfaces exposed to the zenith.
+
+Oxalis carnosa.--A plant of this Chilian species was exposed for 30 m. to a
+clear sky, the thermometer on the grass standing at -2o C., with some of
+its leaves pinned open, and not one leaf on the whole bushy plant was in
+the least injured. On the 16th of March another plant was similarly exposed
+for 30 m., when the temperature on the grass was only a little lower, viz.,
+-3o to -4o C. Six of the leaves had been pinned open, and next morning five
+of them were found much browned. The plant was a large one, and none of the
+free leaves, which were asleep and depended vertically, were browned,
+excepting four very young ones. But three other leaves, though not browned,
+were in a rather flaccid condition, and retained their nocturnal position
+during the whole of the following day. In this case it was obvious that the
+leaves which were exposed horizontally to the zenith suffered most. This
+same pot was afterwards exposed for 35 - 40 m. on a slightly colder night,
+and every leaf, both the pinned open and the free ones, was killed. It may
+be added that two pots of O. corniculata (var. Atro-
+[page 289]
+purpurea) were exposed for 2 h. and 3 h. to a clear sky with the temp. on
+grass -2o C., and none of the leaves, whether free or pinned open, were at
+all injured.
+
+Arachis hypogoea.--Some plants in a pot were exposed at night for 30 m. to
+a clear sky, the temperature on the surrounding grass being -2o C., and on
+two nights afterwards they were again exposed to the same temperature, but
+this time during 1 h. 30 m. On neither occasion was a single leaf, whether
+pinned open or free, injured; and this surprised us much, considering its
+native tropical African home. Two plants were next exposed (March 16th) for
+30 m. to a clear sky, the temperature of the surrounding grass being now
+lower, viz., between -3o and -4o C., and all four pinned-open leaves were
+killed and blackened. These two plants bore 22 other and free leaves
+(excluding some very young bud-like ones) and only two of these were killed
+and three somewhat injured; that is, 23 per cent. were either killed or
+injured, whereas all four pinned-open leaves were utterly killed.
+
+On another night two pots with several plants were exposed for between 35
+m. and 40 m. to a clear sky, and perhaps to a rather lower temperature, for
+a thermometer on a dial, 3 feet high, close by stood at -3.3o to -3.8o C.
+In one pot three leaves were pinned open, and all were badly injured; of
+the 44 free leaves, 26 were injured, that is, 59 per cent. In the other pot
+3 leaves were pinned open and all were killed; four other leaves were
+prevented from sleeping by narrow strips of stiff paper gummed across them,
+and all were killed; of 24 free leaves, 10 were killed, 2 much injured, and
+12 unhurt; that is, 50 per cent. of the free leaves were either killed or
+much injured. Taking the two pots together, we may say that rather more
+than half of the free leaves, which were asleep, were either killed or
+injured, whilst all the ten horizontally extended leaves, which had been
+prevented from going to sleep, were either killed or much injured.
+
+Cassia floribunda.--A bush was exposed at night for 40 m. to a clear sky,
+the temperature on the surrounding grass being -2o C., and not a leaf was
+injured.* It was again exposed on
+
+* Cassia laevigata was exposed to a clear sky for 35 m., and C. calliantha
+(a Guiana species) for 60 m., the temperature on the surrounding grass
+being -2o C., and neither was in the least injured. But when C. laevigata
+was exposed for 1 h., the temp. on the surrounding grass being between -3o
+and -4o C., every leaf was killed.
+[page 290]
+
+another night for 1 h., when the temperature of the grass was -4o C.; and
+now all the leaves on a large bush, whether pinned flat open or free, were
+killed, blackened, and shrivelled, with the exception of those on one small
+branch, low down, which was very slightly protected by the leaves on the
+branches above. Another tall bush, with four of its large compound leaves
+pinned out horizontally, was afterwards exposed (temp. of surrounding grass
+exactly the same, viz., -4o C.), but only for 30 m. On the following
+morning every single leaflet on these four leaves was dead, with both their
+upper and lower surfaces completely blackened. Of the many free leaves on
+the bush, only seven were blackened, and of these only a single one (which
+was a younger and more tender leaf than any of the pinned ones) had both
+surfaces of the leaflets blackened. The contrast in this latter respect was
+well shown by a free leaf, which stood between two pinned-open ones; for
+these latter had the lower surfaces of their leaflets as black as ink,
+whilst the intermediate free leaf, though badly injured, still retained a
+plain tinge of green on the lower surface of the leaflets. This bush
+exhibited in a striking manner the evil effects of the leaves not being
+allowed to assume at night their normal dependent position; for had they
+all been prevented from doing so, assuredly every single leaf on the bush
+would have been utterly killed by this exposure of only 30 m. The leaves
+whilst sinking downwards in the evening twist round, so that the upper
+surface is turned inwards, and is thus better protected than the outwardly
+turned lower surface. Nevertheless, it was always the upper surface which
+was more blackened than the lower, whenever any difference could be
+perceived between them; but whether this was due to the cells near the
+upper surface being more tender, or merely to their containing more
+chlorophyll, we do not know.
+
+Melilotus officinalis.--A large pot with many plants, which had been kept
+during the winter in the greenhouse, was exposed during 5 h. at night to a
+slight frost and clear sky. Four leaves had been pinned out, and these died
+after a few days; but so did many of the free leaves. Therefore nothing
+certain could be inferred from this trial, though it indicated that the
+horizontally extended leaves suffered most. Another large pot with many
+plants was next exposed for 1 h., the temperature on the surrounding grass
+being lower, viz., -3o to -4o C. Ten leaves had been pinned out, and the
+result was striking, for on the following morning all these were found much
+injured or
+[page 291]
+killed, and none of the many free leaves on the several plants were at all
+injured, with the doubtful exception of two or three very young ones.
+
+Melilotus Italica.--Six leaves were pinned out horizontally, three with
+their upper and three with their lower surfaces turned to the zenith. The
+plants were exposed for 5 h. to a clear sky, the temperature on ground
+being about -1o C. Next morning the six pinned-open leaves seemed more
+injured even than the younger and more tender free ones on the same
+branches. The exposure, however, had been too long, for after an interval
+of some days many of the free leaves seemed in almost as bad a condition as
+the pinned-out ones. It was not possible to decide whether the leaves with
+their upper or those with their lower surfaces turned to the zenith had
+suffered most.
+
+Melilotus suaveolens.--Some plants with 8 leaves pinned out were exposed to
+a clear sky during 2 h., the temperature on the surrounding grass being -2o
+C. Next morning 6 out of these 8 leaves were in a flaccid condition. There
+were about 150 free leaves on the plant, and none of these were injured,
+except 2 or 3 very young ones. But after two days, the plants having been
+brought back into the greenhouse, the 6 pinned-out leaves all recovered.
+
+Melilotus Taurica.--Several plants were exposed for 5 h. during two nights
+to a clear sky and slight frost, accompanied by some wind; and 5 leaves
+which had been pinned out suffered more than those both above and below on
+the same branches which had gone to sleep. Another pot, which had likewise
+been kept in the greenhouse, was exposed for 35 - 40 m. to a clear sky, the
+temperature of the surrounding grass being between -3o and -4o C. Nine
+leaves had been pinned out, and all of these were killed. On the same
+plants there were 210 free leaves, which had been allowed to go to sleep,
+and of these about 80 were killed, i.e. only 38 per cent.
+
+Melilotus Petitpierreana.--The plants were exposed to a clear sky for 35 -
+40 m.: temperature on surrounding grass -3o to -4o C. Six leaves had been
+pinned out so as to stand about � inch above the cork, and four had been
+pinned close to it. These 10 leaves were all killed, but the closely pinned
+ones suffered most, as 4 of the 6 which stood above the cork still retained
+small patches of a green colour. A considerable number, but not nearly all,
+of the free leaves, were killed or much injured, whereas all the pinned out
+ones were killed.
+[page 292]
+
+Melilotus macrorrhiza.--The plants were exposed in the same manner as in
+the last case. Six leaves had been pinned out horizontally, and five of
+them were killed, that is, 83 percent. We estimated that there were 200
+free leaves on the plants, and of these about 50 were killed and 20 badly
+injured, so that about 35 per cent of the free leaves were killed or
+injured.
+
+Lotus aristata.--Six plants were exposed for nearly 5 h. to a clear sky;
+temperature on surrounding grass -1.5o C. Four leaves had been pinned out
+horizontally, and 2 of these suffered more than those above or below on the
+same branches, which had been allowed to go to sleep. It is rather a
+remarkable fact that some plants of Lotus Jacoboeus, an inhabitant of so
+hot a country as the Cape Verde Islands, were exposed one night to a clear
+sky, with the temperature of the surrounding grass -2o C., and on a second
+night for 30 m. with the temperature of the grass between -3o and -4o C.,
+and not a single leaf, either the pinned-out or free ones, was in the least
+injured.
+
+Marsilea quadrifoliata.--A large plant of this species--the only
+Cryptogamic plant known to sleep--with some leaves pinned open, was exposed
+for 1 h. 35 m. to a clear sky, the temperature on the surrounding ground
+being -2o C., and not a single leaf was injured. After an interval of some
+days the plant was again exposed for 1 h. to a clear sky, with the
+temperature on the surrounding ground lower, viz., -4o C. Six leaves had
+been pinned out horizontally, and all of them were utterly killed. The
+plant had emitted long trailing stems, and these had been wrapped round
+with a blanket, so as to protect them from the frozen ground and from
+radiation; but a very large number of leaves were left freely exposed,
+which had gone to sleep, and of these only 12 were killed. After another
+interval, the plant, with 9 leaves pinned out, was again exposed for 1 h.,
+the temperature on the ground being again -4o C. Six of the leaves were
+killed, and one which did not at first appear injured afterwards became
+streaked with brown. The trailing branches, which rested on the frozen
+ground, had one-half or three-quarters of their leaves killed, but of the
+many other leaves on the plant, which alone could be fairly compared with
+the pinned-out ones, none appeared at first sight to have been killed, but
+on careful search 12 were found in this state. After another interval, the
+plant with 9 leaves pinned out, was exposed for 35 - 40 m. to a clear sky
+and to nearly the same, or perhaps a rather lower, temperature (for the
+thermometer by an accident had been left on a
+[page 293]
+sun-dial close by), and 8 of these leaves were killed. Of the free leaves
+(those on the trailing branches not being considered), a good many were
+killed, but their number, compared with the uninjured ones, was small.
+Finally, taking the three trials together, 24 leaves, extended
+horizontally, were exposed to the zenith and to unobstructed radiation, and
+of these 20 were killed and 1 injured; whilst a relatively very small
+proportion of the leaves, which had been allowed to go to sleep with their
+leaflets vertically dependent, were killed or injured.
+
+The cotyledons of several plants were prepared for trial, but the weather
+was mild and we succeeded only in a single instance in having seedlings of
+the proper age on nights which were clear and cold. The cotyledons of 6
+seedlings of Mimosa pudica were fastened open on cork and were thus exposed
+for 1 h. 45 m. to a clear sky, with the temperature on the surrounding
+ground at 29o F.; of these, 3 were killed. Two other seedlings, after their
+cotyledons had risen up and had closed together, were bent over and
+fastened so that they stood horizontally, with the lower surface of one
+cotyledon fully exposed to the zenith, and both were killed. Therefore of
+the 8 seedlings thus tried 5, or more than half, were killed. Seven other
+seedlings with their cotyledons in their normal nocturnal position, viz.,
+vertical and closed, were exposed at the same time, and of these only 2
+were killed.* Hence it appears, as far as these few trials tell anything,
+that the vertical position at night of the cotyledons of Mimosa pudica
+protects them to a certain degree from the evil effects of radiation and
+cold.]
+
+Concluding Remarks on the Radiation from Leaves at Night.--We exposed on
+two occasions during the summer to a clear sky several pinned-open leaflets
+of Trifolium pratense, which naturally rise at night, and of Oxalis
+purpurea, which naturally sink at night (the plants growing out of doors),
+and looked at
+
+* We were surprised that young seedlings of so tropical a plant as Mimosa
+pudica were able to resist, as well as they did, exposure for 1 hr. 45 m.
+to a clear sky, the temperature on the surrounding ground being 29o F. It
+may be added that seedlings of the Indian 'Cassia pubescens' were exposed
+for 1 h. 30 m. to a clear sky, with the temp. on the surrounding ground at
+-2o C., and they were not in the least injured.
+[page 294]
+
+them early on several successive mornings, after they had assumed their
+diurnal positions. The difference in the amount of dew on the pinned-open
+leaflets and on those which had gone to sleep was generally conspicuous;
+the latter being sometimes absolutely dry, whilst the leaflets which had
+been horizontal were coated with large beads of dew. This shows how much
+cooler the leaflets fully exposed to the zenith must have become, than
+those which stood almost vertically, either upwards or downwards, during
+the night.
+
+From the several cases above given, there can be no doubt that the position
+of the leaves at night affects their temperature through radiation to such
+a degree, that when exposed to a clear sky during a frost, it is a question
+of life and death. We may therefore admit as highly probable, seeing that
+their nocturnal position is so well adapted to lessen radiation, that the
+object gained by their often complicated sleep movements, is to lessen the
+degree to which they are chilled at night. It should be kept in mind that
+it is especially the upper surface which is thus protected, as it is never
+directed towards the zenith, and is often brought into close contact with
+the upper surface of an opposite leaf or leaflet.
+
+We failed to obtain sufficient evidence, whether the better protection of
+the upper surface has been gained from its being more easily injured than
+the lower surface, or from its injury being a greater evil to the plant.
+That there is some difference in constitution between the two surfaces is
+shown by the following cases. Cassia floribunda was exposed to a clear sky
+on a sharp frosty night, and several leaflets which had assumed their
+nocturnal dependent position with their lower surfaces turned outwards so
+as to be
+[page 295]
+exposed obliquely to the zenith, nevertheless had these lower surfaces less
+blackened than the upper surfaces which were turned inwards and were in
+close contact with those of the opposite leaflets. Again, a pot full of
+plants of Trifolium resupinatum, which had been kept in a warm room for
+three days, was turned out of doors (Sept. 21st) on a clear and almost
+frosty night. Next morning ten of the terminal leaflets were examined as
+opaque objects under the microscope. These leaflets, in going to sleep,
+either turn vertically upwards, or more commonly bend a little over the
+lateral leaflets, so that their lower surfaces are more exposed to the
+zenith than their upper surfaces. Nevertheless, six of these ten leaflets
+were distinctly yellower on the upper than on the lower and more exposed
+surface. In the remaining four, the result was not so plain, but certainly
+whatever difference there was leaned to the side of the upper surface
+having suffered most.
+
+It has been stated that some of the leaflets experimented on were fastened
+close to the cork, and others at a height of from � to 3/4 of an inch above
+it; and that whenever, after exposure to a frost, any difference could be
+detected in their states, the closely pinned ones had suffered most. We
+attributed this difference to the air, not cooled by radiation, having been
+prevented from circulating freely beneath the closely pinned leaflets. That
+there was really a difference in the temperature of leaves treated in these
+two different methods, was plainly shown on one occasion; for after the
+exposure of a pot with plants of Melilotus dentata for 2 h. to a clear sky
+(the temperature on the surrounding grass being -2o C.), it was manifest
+that more dew had congealed into hoar-frost on the closely pinned leaflets,
+than on those which stood horizontally
+[page 296]
+a little above the cork. Again, the tips of some few leaflets, which had
+been pinned close to the cork, projected a little beyond the edge, so that
+the air could circulate freely round them. This occurred with six leaflets
+of Oxalis acetosella, and their tips certainly suffered rather less then
+the rest of the same leaflets; for on the following morning they were still
+slightly green. The same result followed, even still more clearly, in two
+cases with leaflets of Melilotus officinalis which projected a little
+beyond the cork; and in two other cases some leaflets which were pinned
+close to the cork were injured, whilst other free leaflets on the same
+leaves, which had not space to rotate and assume their proper vertical
+position, were not at all injured.
+
+Another analogous fact deserves notice: we observed on several occasions
+that a greater number of free leaves were injured on the branches which had
+been kept motionless by some of their leaves having been pinned to the
+corks, than on the other branches. This was conspicuously the case with
+those of Melilotus Petitpierreana, but the injured leaves in this instance
+were not actually counted. With Arachis hypogaea, a young plant with 7
+stems bore 22 free leaves, and of these 5 were injured by the frost, all of
+which were on two stems, bearing four leaves pinned to the cork-supports.
+With Oxalis carnosa, 7 free leaves were injured, and every one of them
+belonged to a cluster of leaves, some of which had been pinned to the cork.
+We could account for these cases only by supposing that the branches which
+were quite free had been slightly waved about by the wind, and that their
+leaves had thus been a little warmed by the surrounding warmer air. If we
+hold our hands motionless before a hot fire, and then wave them about, we
+[page 297]
+immediately feel relief; and this is evidently an analogous, though
+reversed, case. These several facts--in relation to leaves pinned close to
+or a little above the cork-supports--to their tips projecting beyond it--
+and to the leaves on branches kept motionless--seem to us curious, as
+showing how a difference, apparently trifling, may determine the greater or
+less injury of the leaves. We may even infer as probable that the less or
+greater destruction during a frost of the leaves on a plant which does not
+sleep, may often depend on the greater or less degree of flexibility of
+their petioles and of the branches which bear them.
+
+NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.
+
+We now come to the descriptive part of our work, and will begin with
+cotyledons, passing on to leaves in the next chapter. We have met with only
+two brief notices of cotyledons sleeping. Hofmeister,* after stating that
+the cotyledons of all the observed seedlings of the Caryophylleae (Alsineae
+and Sileneae) bend upwards at night (but to what angle he does not state),
+remarks that those of Stellaria media rise up so as to touch one another;
+they may therefore safely be said to sleep. Secondly, according to Ramey**,
+the cotyledons of Mimosa pudica and of Clianthus Dampieri rise up almost
+vertically at night and approach each other closely. It has been shown in a
+previous chapter that the cotyledons of a large number of plants bend a
+little upwards at night, and we here have to meet the difficult question at
+what inclination may they be said to sleep? According to the view which we
+maintain, no movement deserves to be called
+
+* 'Die Lehre von der Pflanzenzelle,' 1867, p. 327.
+
+** 'Adansonia,' March 10th, 1869.
+
+[page 298]
+nyctitropic, unless it has been acquired for the sake of lessening
+radiation; but this could be discovered only by a long series of
+experiments, showing that the leaves of each species suffered from this
+cause, if prevented from sleeping. We must therefore take an arbitrary
+limit. If a cotyledon or leaf is inclined at 60o above or beneath the
+horizon, it exposes to the zenith about one-half of its area; consequently
+the intensity of its radiation will be lessened by about half, compared
+with what it would have been if the cotyledon or leaf had remained
+horizontal. This degree of diminution certainly would make a great
+difference to a plant having a tender constitution. We will therefore speak
+of a cotyledon and hereafter of a leaf as sleeping, only when it rises at
+night to an angle of about 60o, or to a still higher angle, above the
+horizon, or sinks beneath it to the same amount. Not but that a lesser
+diminution of radiation may be advantageous to a plant, as in the case of
+Datura stramonium, the cotyledons of which rose from 31o at noon to 55o at
+night above the horizon. The Swedish turnip may profit by the area of its
+leaves being reduced at night by about 30 per cent., as estimated by Mr. A.
+S. Wilson; though in this case the angle through which the leaves rose was
+not observed. On the other hand, when the angular rise of cotyledons or of
+leaves is small, such as less than 30o, the diminution of radiation is so
+slight that it probably is of no significance to the plant in relation to
+radiation. For instance, the cotyledons of Geranium Ibericum rose at night
+to 27o above the horizon, and this would lessen radiation by only 11 per
+cent.: those of Linum Berendieri rose to 33o, and this would lessen
+radiation by 16 per cent.
+
+There are, however, some other sources of doubt with
+[page 299]
+respect to the sleep of cotyledons. In certain cases, the cotyledons whilst
+young diverge during the day to only a very moderate extent, so that a
+small rise at night, which we know occurs with the cotyledons of many
+plants, would necessarily cause them to assume a vertical or nearly
+vertical position at night; and in this case it would be rash to infer that
+the movement was effected for any special purpose. On this account we
+hesitated long whether we should introduce several Cucurbitaceous plants
+into the following list; but from reasons, presently to be given, we
+thought that they had better be at least temporarily included. This same
+source of doubt applies in some few other cases; for at the commencement of
+our observations we did not always attend sufficiently to whether the
+cotyledons stood nearly horizontally in the middle of the day. With several
+seedlings, the cotyledons assume a highly inclined position at night during
+so short a period of their life, that a doubt naturally arises whether this
+can be of any service to the plant. Nevertheless, in most of the cases
+given in the following list, the cotyledons may be as certainly said to
+sleep as may the leaves of any plant. In two cases, namely with the cabbage
+and radish, the cotyledons of which rise almost vertically during the few
+first nights of their life, it was ascertained by placing young seedlings
+in the klinostat, that the upward movement was not due to apogeotropism.
+
+The names of the plants, the cotyledons of which stand at night at an angle
+of at least 60o with the horizon, are arranged in the appended list on the
+same system as previously followed. The numbers of the Families, and with
+the Leguminosae the numbers of the Tribes, have been added to show how
+widely the plants in question are distributed throughout the
+[page 300]
+dicotyledonous series. A few remarks will have to be made about many of the
+plants in the list. In doing so, it will be convenient not to follow
+strictly any systematic order, but to treat of the Oxalidae and the
+Leguminosae at the close; for in these two Families the cotyledons are
+generally provided with a pulvinus, and their movements endure for a much
+longer time than those of the other plants in the list.
+
+List of Seedling Plants, the cotyledons of which rise or sink at night to
+an angle of at least 60o above or beneath the horizon.
+
+Brassica oleracea. Cruciferae (Fam. 14).
+-- napus (as we are informed by Prof. Pfeffer). Raphanus sativus.
+Cruciferae.
+Githago segetum. Caryophylleae (Fam. 26).
+Stellaria media (according to Hofmeister, as quoted). Caryophylleae.
+Anoda Wrightii. Malvaceae (Fam. 36).
+Gossypium (var. Nankin cotton). Malvaceae.
+Oxalis rosea. Oxalidae (Fam. 41).
+-- floribunda.
+-- articulata.
+-- Valdiviana.
+-- sensitiva.
+Geranium rotundifolium. Geraniaceae (Fam. 47).
+Trifolium subterraneum. Leguminosae (Fam. 75, Tribe 3).
+-- strictum.
+-- leucanthemum.
+Lotus ornithopopoides. Leguminosae (Tribe 4).
+-- peregrinus.
+-- Jacobaeus.
+Clianthus Dampieri. Leguminosae (Tribe 5)--according to M. Ramey.
+Smithia sensitiva. Leguminosae (Tribe 6).
+Haematoxylon Campechianum. Leguminosae (Tribe 13)--according to Mr. R. I.
+Lynch.
+Cassia mimosoides. Leguminosae (Tribe 14).
+-- glauca.
+-- florida.
+-- corymbosa.
+-- pubescens.
+-- tora.
+-- neglecta.
+-- 3 other Brazilian unnamed species.
+Bauhinia (sp.?. Leguminosae (Tribe 15).
+Neptunia oleracea. Leguminosae (Tribe 20).
+Mimosa pudica. Leguminosae (Tribe 21).
+-- albida.
+Cucurbita ovifera. Cucurbitaceae (Fam. 106).
+-- aurantia.
+Lagenaria vulgaris. Cucurbitaceae.
+Cucumis dudaim. Cucurbitaceae.
+Apium petroselinum. Umbelliferae (Fam. 113).
+-- graveolens.
+Lactuca scariola. Compositae (Fam. 122).
+Helianthus annuus (?). Compositae.
+Ipomoea caerulea. Convolvulaceae (Fam. 151).
+-- purpurea.
+-- bona-nox.
+-- coccinea.
+[page 301]
+List of Seedling Plants (continued).
+Solanum lycopersicum. Solaneae (Fam. 157.)
+Mimulus, (sp. ?) Scrophularineae (Fam. 159)--from information given us by
+Prof. Pfeffer.
+Mirabilis jalapa. Nyctagineae (Fam. 177).
+Mirabilis longiflora.
+Beta vulgaris. Polygoneae (Fam. 179).
+Amaranthus caudatus. Amaranthaceae (Fam. 180).
+Cannabis sativa (?). Cannabineae (Fam. 195).
+
+Brassica oleracea (Cruciferae).--It was shown in the first chapter that the
+cotyledons of the common cabbage rise in the evening and stand vertically
+up at night with their petioles in contact. But as the two cotyledons are
+of unequal height, they frequently interfere a little with each other's
+movements, the shorter one often not standing quite vertically. They awake
+early in the morning; thus at 6.45 A.M. on Nov. 27th, whilst if was still
+dark, the cotyledons, which had been vertical and in contact on the
+previous evening, were reflexed, and thus presented a very different
+appearance. It should be borne in mind that seedlings in germinating at the
+proper season, would not be subjected to darkness at this hour in the
+morning. The above amount of movement of the cotyledons is only temporary,
+lasting with plants kept in a warm greenhouse from four to six days; how
+long it would last with seedlings growing out of doors we do not know.
+
+Raphanus sativus.--In the middle of the day the blades of the cotyledons of
+10 seedlings stood at right angles to their hypocotyls, with their petioles
+a little divergent; at night the blades stood vertically, with their bases
+in contact and with their petioles parallel. Next morning, at 6.45 A.M.,
+whilst it was still dark, the blades were horizontal. On the following
+night they were much raised, but hardly stood sufficiently vertical to be
+said to be asleep, and so it was in a still less degree on the third night.
+Therefore the cotyledons of this plant (kept in the greenhouse) go to sleep
+for even a shorter time than those of the cabbage. Similar observations
+were made, but only during a single day and night, on 13 other seedlings
+likewise raised in the greenhouse, with the same result.
+
+The petioles of the cotyledons of 11 young seedlings of Sinapis nigra were
+slightly divergent at noon, and the blades stood at right angles to the
+hypocotyls; at night the petioles were in close contact, and the blades
+considerably raised, with their bases in contact, but only a few stood
+sufficiently upright to be called asleep. On the following morning,
+[page 302]
+the petioles diverged before it was light. The hypocotyl is slightly
+sensitive, so that if rubbed with a needle it bends towards the rubbed
+side. In the case of Lepidium sativum, the petioles of the cotyledons of
+young seedlings diverge during the day and converge so as to touch each
+other during the night, by which means the bases of the tripartite blades
+are brought into contact; but the blades are so little raised that they
+cannot be said to sleep. The cotyledons of several other cruciferous plants
+were observed, but they did not rise sufficiently during the night to be
+said to sleep.
+
+Githago segetum (Caryophylleae).--On the first day after the cotyledons had
+burst through the seed-coats, they stood at noon at an angle of 75o above
+the horizon; at night they moved upwards, each through an angle of 15o so
+as to stand quite vertical and in contact with one another. On the second
+day they stood at noon at 59o above the horizon, and again at night were
+completely closed, each having risen 31o. On the fourth day the cotyledons
+did not quite close at night. The first and succeeding pairs of young true
+leaves behaved in exactly the same manner. We think that the movement in
+this case may be called nyctitropic, though the angle passed through was
+small. The cotyledons are very sensitive to light and will not expand if
+exposed to an extremely dim one.
+
+Anoda Wrightii (Malvaceae).--The cotyledons whilst moderately young, and
+only from .2 to .3 inch in diameter, sink in the evening from their mid-day
+horizontal position to about 35o beneath the horizon. But when the same
+seedlings were older and had produced small true leaves, the almost
+orbicular cotyledons, now .55 inch in diameter, moved vertically downwards
+at night. This fact made us suspect that their sinking might be due merely
+to their weight; but they were not in the least flaccid, and when lifted up
+sprang back through elasticity into their former dependent position. A pot
+with some old seedlings was turned upside down in the afternoon, before the
+nocturnal fall had commenced, and at night they assumed in opposition to
+their own weight (and to any geotropic action) an upwardly directed
+vertical position. When pots were thus reversed, after the evening fall had
+already commenced, the sinking movement appeared to be somewhat disturbed;
+but all their movements were occasionally variable without any apparent
+cause. This latter fact, as well as that of the young cotyledons not
+sinking nearly so much as the older ones, deserves notice.
+[page 303]
+Although the movement of the cotyledons endured for a long time, no
+pulvinus was exteriorly visible; but their growth continued for a long
+time. The cotyledons appear to be only slightly heliotropic, though the
+hypocotyl is strongly so.
+
+Gossypium arboreum (?) (var. Nankin cotton) (Malvaceae).--The cotyledons
+behave in nearly the same manner as those of the Anoda. On June 15th the
+cotyledons of two seedlings were .65 inch in length (measured along the
+midrib) and stood horizontally at noon; at 10 P.M. they occupied the same
+position and had not fallen at all. On June 23rd, the cotyledons of one of
+these seedlings were 1.1 inch in length, and by 10 P.M. they had fallen
+from a horizontal position to 62o beneath the horizon. The cotyledons of
+the other seedling were 1.3 inch in length, and a minute true leaf had been
+formed; they had fallen at 10 P.M. to 70o beneath the horizon. On June
+25th, the true leaf of this latter seedling was .9 inch in length, and the
+cotyledons occupied nearly the same position at night. By July 9th the
+cotyledons appeared very old and showed signs of withering; but they stood
+at noon almost horizontally, and at 10 P.M. hung down vertically.
+
+Gossypium herbaceum.--It is remarkable that the cotyledons of this species
+behave differently from those of the last. They were observed during 6
+weeks from their first development until they had grown to a very large
+size (still appearing fresh and green), viz. 2 � inches in breadth. At this
+age a true leaf had been formed, which with its petiole was 2 inches long.
+During the whole of these 6 weeks the cotyledons did not sink at night; yet
+when old their weight was considerable and they were borne by much
+elongated petioles. Seedlings raised from some seed sent us from Naples,
+behaved in the same manner; as did those of a kind cultivated in Alabama
+and of the Sea-island cotton. To what species these three latter forms
+belong we do not know. We could not make out in the case of the Naples
+cotton, that the position of the cotyledons at night was influenced by the
+soil being more or less dry; care being taken that they were not rendered
+flaccid by being too dry. The weight of the large cotyledons of the Alabama
+and Sea-island kinds caused them to hang somewhat downwards, when the pots
+in which they grew were left for a time upside down. It should, however, be
+observed that these three kinds were raised in the middle of the winter,
+which sometimes greatly interferes with the proper nyctitropic movements of
+leaves and cotyledons.
+[page 304]
+
+Cucurbitaceae.--The cotyledons of Cucurbita aurantia and ovifera, and of
+Lagenaria vulgaris, stand from the 1st to the 3rd day of their life at
+about 60o above the horizon, and at night rise up so as to become vertical
+and in close contact with one another. With Cucumis dudaim they stood at
+noon at 45o above the horizon, and closed at night. The tips of the
+cotyledons of all these species are, however, reflexed, so that this part
+is fully exposed to the zenith at night; and this fact is opposed to the
+belief that the movement is of the same nature as that of sleeping plants.
+After the first two or three days the cotyledons diverge more during the
+day and cease to close at night. Those of Trichosanthes anguina are
+somewhat thick and fleshy, and did not rise at night; and they could
+perhaps hardly be expected to do so. On the other hand, those of
+Acanthosicyos horrida* present nothing in their appearance opposed to their
+moving at night in the same manner as the preceding species; yet they did
+not rise up in any plain manner. This fact leads to the belief that the
+nocturnal movements of the above-named species has been acquired for some
+special purpose, which may be to protect the young plumule from radiation,
+by the close contact of the whole basal portion of the two cotyledons.
+
+Geranium rotundifolium (Geraniaceae).--A single seedling came up
+accidentally in a pot, and its cotyledons were observed to bend
+perpendicularly downwards during several successive nights, having been
+horizontal at noon. It grew into a fine plant but died before flowering: it
+was sent to Kew and pronounced to be certainly a Geranium, and in all
+probability the above-named species. This case is remarkable because the
+cotyledons of G. cinereum, Endressii, Ibericum, Richardsoni, and
+subcaulescens were observed during some weeks in the winter, and they did
+not sink, whilst those of G. Ibericum rose 27o at night.
+
+Apium petroselinum (Umbelliferae).--A seedling had its cotyledons (Nov.
+22nd) almost fully expanded during the day; by 8.30 P.M. they had risen
+considerably, and at 10.30 P.M. were almost closed, their tips being only
+8/100 of an inch apart. On the following morning (23rd) the tips were
+58/100 of an inch apart,
+
+* This plant, from Dammara Land in S. Africa, is remarkable from being the
+one known member of the Family which is not a climber; it has been
+described in 'Transact. Linn. Soc.,' xxvii. p. 30.
+[page 305]
+
+or more than seven times as much. On the next night the cotyledons occupied
+nearly the same position as before. On the morning of the 24th they stood
+horizontally, and at night were 60o above the horizon; and so it was on the
+night of the 25th. But four days afterwards (on the 29th), when the
+seedlings were a week old, the cotyledons had ceased to rise at night to
+any plain degree.
+
+Apium graveolens.--The cotyledons at noon were horizontal, and at 10 P.M.
+stood at an angle of 61o above the horizon.
+
+Lactuca scariola (Compositae).--The cotyledons whilst young stood
+sub-horizontally during the day, and at night rose so as to be almost
+vertical, and some were quite vertical and closed; but this movement ceased
+when they had grown old and large, after an interval of 11 days.
+
+Helianthus annuus (Compositae).--This case is rather doubtful; the
+cotyledons rise at night, and on one occasion they stood at 73o above the
+horizon, so that they might then be said to have been asleep.
+
+Ipomoea caerulea vel Pharbitis nil (Convolvulaceae).--The cotyledons behave
+in nearly the same manner as those of the Anoda and Nankin cotton, and like
+them grow to a large size. Whilst young and small, so that their blades
+were from .5 to .6 of an inch in length, measured along the middle to the
+base of the central notch, they remained horizontal both during the middle
+of the day and at night. As they increased in size they began to sink more
+and more in the evening and early night; and when they had grown to a
+length (measured in the above manner) of from 1 to 1.25 inch, they sank
+between 55o and 70o beneath the horizon. They acted, however, in this
+manner only when they had been well illuminated during the day.
+Nevertheless, the cotyledons have little or no power of bending towards a
+lateral light, although the hypocotyl is strongly heliotropic. They are not
+provided with a pulvinus, but continue to grow for a long time.
+
+Ipomoea purpurea (vel Pharbitis hispida).--The cotyledons behave in all
+respects like those of I. caerulea. A seedling with cotyledons .75 inch in
+length (measured as before) and 1.65 inch in breadth, having a small true
+leaf developed, was placed at 5.30 P.M. on a klinostat in a darkened box,
+so that neither weight nor geotropism could act on them. At 10 P.M. one
+cotyledon stood at 77o and the other at 82o beneath the horizon. Before
+being placed in the klinostat they stood at 15o and 29o
+[page 306]
+beneath the horizon. The nocturnal position depends chiefly on the
+curvature of the petiole close to the blade, but the whole petiole becomes
+slightly curved downwards. It deserves notice that seedlings of this and
+the last-named species were raised at the end of February and another lot
+in the middle of March, and the cotyledons in neither case exhibited any
+nyctitropic movement.
+
+Ipomoea bona-nox.--The cotyledons after a few days grow to an enormous
+size, those on a young seedling being 3 1/4 inches in breadth. They were
+extended horizontally at noon, and at 10 P.M. stood at 63o beneath the
+horizon. five days afterwards they were 4 � inches in breadth, and at night
+one stood at 64o and the other 48o beneath the horizon. Though the blades
+are thin, yet from their great size and from the petioles being long, we
+imagined that their depression at night might be determined by their
+weight; but when the pot was laid horizontally, they became curved towards
+the hypocotyl, which movement could not have been in the least aided by
+their weight, at the same time they were somewhat twisted upwards through
+apogeotropism. Nevertheless, the weight of the cotyledons is so far
+influential, that when on another night the pot was turned upside down,
+they were unable to rise and thus to assume their proper nocturnal
+position.
+
+Ipomoea coccinea.--The cotyledons whilst young do not sink at night, but
+when grown a little older, but still only .4 inch in length (measured as
+before) and .82 in breadth, they became greatly depressed. In one case they
+were horizontal at noon, and at 10 P.M. one of them stood at 64o and the
+other at 47o beneath the horizon. The blades are thin, and the petioles,
+which become much curved down at night, are short, so that here weight can
+hardly have produced any effect. With all the above species of Ipomoea,
+when the two cotyledons on the same seedling were unequally depressed at
+night, this seemed to depend on the position which they had held during the
+day with reference to the light.
+
+Solanum lycopersicum (Solaneae).--The cotyledons rise so much at night as
+to come nearly in contact. Those of 'S. palinacanthum' were horizontal at
+noon, and by 10 P.M. had risen only 27o 30 minutes; but on the following
+morning before it was light they stood at 59o above the horizon, and in the
+afternoon of the same day were again horizontal. The behaviour of the
+cotyledons of this latter species seems, therefore, to be anomalous.
+[page 307]
+
+Mirabilis jalapa and longiflora (Nyctagineae).--The cotyledons, which are
+of unequal size, stand horizontally during the middle of the day, and at
+night rise up vertically and come into close contact with one another. But
+this movement with M. longiflora lasted for only the three first nights.
+
+Beta vulgaris (Polygoneae).--A large number of seedlings were observed on
+three occasions. During the day the cotyledons sometimes stood
+sub-horizontally, but more commonly at an angle of about 50o above the
+horizon, and for the first two or three nights they rose up vertically so
+as to be completely closed. During the succeeding one or two nights they
+rose only a little, and afterwards hardly at all.
+
+Amaranthus caudatus (Amaranthaceae).--At noon the cotyledons of many
+seedlings, which had just germinated, stood at about 45o above the horizon,
+and at 10.15 P.M. some were nearly and the others quite closed. On the
+following morning they were again well expanded or open.
+
+Cannabis sativa (Cannabineae).--We are very doubtful whether this plant
+ought to be here included. The cotyledons of a large number of seedlings,
+after being well illuminated during the day, were curved downwards at
+night, so that the tips of some pointed directly to the ground, but the
+basal part did not appear to be at all depressed. On the following morning
+they were again flat and horizontal. the cotyledons of many other seedlings
+were at the same time not in any way affected. Therefore this case seems
+very different from that of ordinary sleep, and probably comes under the
+head of epinasty, as is the case with the leaves of this plant according to
+Kraus. The cotyledons are heliotropic, and so is the hypocotyl in a still
+stronger degree.
+
+Oxalis.--We now come to cotyledons provided with a pulvinus, all of which
+are remarkable from the continuance of the nocturnal movements during
+several days or even weeks, and apparently after growth has ceased. The
+cotyledons of O. rosea, floribunda and articulata sink vertically down at
+night and clasp the upper part of the hypocotyl. Those of O. Valdiviana and
+sensitiva, on the contrary, rise vertically up, so that their upper
+surfaces come into close contact; and after the young leaves are developed
+these are clasped by the cotyledons. As in the daytime they stand
+horizontally, or are even a little deflected beneath the horizon, they move
+in the evening through an angle of at least 90o. Their complicated
+circumnutating movements during the day have
+[page 308]
+been described in the first chapter. The experiment was a superfluous one,
+but pots with seedlings of O. rosea and floribunda were turned upside down,
+as soon as the cotyledons began to show any signs of sleep, and this made
+no difference in their movements.
+
+Leguminosae.--It may be seen in our list that the cotyledons of several
+species in nine genera, widely distributed throughout the Family, sleep at
+night; and this probably is the case with many others. The cotyledons of
+all these species are provided with a pulvinus; and the movement in all is
+continued during many days or weeks. In Cassia the cotyledons of the ten
+species in the list rise up vertically at night and come into close contact
+with one another. We observed that those of C. florida opened in the
+morning rather later than those of C. glauca and pubescens. The movement is
+exactly the same in C. mimosoides as in the other species, though its
+subsequently developed leaves sleep in a different manner. The cotyledons
+of an eleventh species, namely, C. nodosa, are thick and fleshy, and do not
+rise up at night. The circumnutation of the cotyledons during the day of C.
+tora has been described in the first chapter. Although the cotyledons of
+Smithia sensitiva rose from a horizontal position in the middle of the day
+to a vertical one at night, those of S. Pfundii, which are thick and
+fleshy, did not sleep. When Mimosa pudica and albida have been kept at a
+sufficiently high temperature during the day, the cotyledons come into
+close contact at night; otherwise they merely rise up almost vertically.
+The circumnutation of those of M. pudica has been described. The cotyledons
+of a Bauhinia from St. Catharina in Brazil stood during the day at an angle
+of about 50o above the horizon, and at night rose to 77o; but it is
+probable that they would have closed completely, if the seedlings had been
+kept in a warmer place.
+
+Lotus.--In three species of Lotus the cotyledons were observed to sleep.
+Those of L. Jacoboeus present the singular case of not rising at night in
+any conspicuous manner for the first 5 or 6 days of their life, and the
+pulvinus is not well developed at this period. Afterwards the sleeping
+movement is well displayed, though to a variable degree, and is long
+continued. We shall hereafter meet with a nearly parallel case with the
+leaves of Sida rhombifolia. The cotyledons of L. Gebelii are only slightly
+raised at night, and differ much in this respect from the three species in
+our list.
+[page 309]
+
+Trifolium.--The germination of 21 species was observed. In most of them the
+cotyledons rise hardly at all, or only slightly, at night; but those of T.
+glomeratum, striatum and incarnactum rose from 45o to 55o above the
+horizon. With T. subterraneum, leucanthemum and strictum, they stood up
+vertically; and with T. strictum the rising movement is accompanied, as we
+shall see, by another movement, which makes us believe that the rising is
+truly nyctitropic. We did not carefully examine the cotyledons of all the
+species for a pulvinus, but this organ was distinctly present in those of
+T. subterraneum and strictum; whilst there was no trace of a pulvinus in
+some species, for instance, in T. resupinatum, the cotyledons of which do
+not rise at night.
+
+Trifolium subterraneum.--The blades of the cotyledons on the first day
+after germination (Nov. 21st) were not fully expanded, being inclined at
+about 35o above the horizon; at night they rose to about 75o. Two days
+afterwards the blades at noon were horizontal, with the petioles highly
+inclined upwards; and it is remarkable that the nocturnal movement is
+almost wholly confined to the blades, being effected by the pulvinus at
+their bases; whilst the petioles retain day and night nearly the same
+inclination. On this night (Nov. 23rd), and for some few succeeding nights,
+the blades rose from a horizontal into a vertical position, and then became
+bowed inwards at about an average angle of 10o; so that they had passed
+through an angle of 100o. Their tips now almost touched one another, their
+bases being slightly divergent. The two blades thus formed a highly
+inclined roof over the axis of the seedling. This movement is the same as
+that of the terminal leaflet of the tripartite leaves of many species of
+Trifolium. After an interval of 8 days (Nov. 29th) the blades were
+horizontal during the day, and vertical at night, and now they were no
+longer bowed inwards. They continued to move in the same manner for the
+following two months, by which time they had increased greatly in size,
+their petioles being no less than .8 of an inch in length, and two true
+leaves had by this time been developed.
+
+Trifolium strictum.--On the first day after germination the cotyledons,
+which are provided with a pulvinus, stood at noon horizontally, and at
+night rose to only about 45o above the horizon. Four days afterwards the
+seedlings were again observed at night, and now the blades stood vertically
+and were in contact, excepting the tips, which were much deflexed, so that
+they faced the zenith. At this age the petioles are curved
+[page 310]
+upwards, and at night, when the bases of the blades are in contact, the two
+petioles together form a vertical ring surrounding the plumule. The
+cotyledons continued to act in nearly the same manner for 8 or 10 days from
+the period of germination; but the petioles had by this time become
+straight and had increased much in length. After from 12 to 14 days the
+first simple true leaf was formed, and during the ensuing fortnight a
+remarkable movement was repeatedly observed. At I. (Fig. 125) we have a
+sketch, made in the middle of the day, of a seedling about a fortnight old.
+The two cotyledons, of which Rc is the right and Lc the left one, stand
+directly opposite one another,
+
+Fig. 125. Trifolium strictum: diurnal and nocturnal positions of the two
+cotyledons and of the first leaf. I. Seedling viewed obliquely from above,
+during the day: Rc, right cotyledon; Lc, left cotyledon; F, first true
+leaf. II. A rather younger seedling, viewed at night: Rc, right cotyledon
+raised, but its position not otherwise changed; Lc, left cotyledon raised
+and laterally twisted; F, first leaf raised and twisted so as to face the
+left twisted cotyledon. III. Same seedling viewed at night from the
+opposite side. The back of the first leaf, F, is here shown instead of the
+front, as in II.
+
+and the first true leaf (F) projects at right angles to them. At night (see
+II. and III.) the right cotyledon (Rc) is greatly raised, but is not
+otherwise changed in position. The left cotyledon (Lc) is likewise raised,
+but it is also twisted so that its blade, instead of exactly facing the
+opposite one, now stands at nearly right angles to it. This nocturnal
+twisting movement is effected not by means of the pulvinus, but by the
+twisting of the whole length of the petiole, as could be seen by the curved
+line of its upper concave surface. At the same time the true leaf (F) rises
+up, so as to stand vertically, or it even passes the vertical and is
+inclined a little inwards. It also twists a little, by which means the
+upper surface of its blade fronts, and almost comes into contact with, the
+upper surface of the twisted
+[page 311]
+left cotyledon. This seems to be the object gained by these singular
+movements. Altogether 20 seedlings were examined on successive nights, and
+in 19 of them it was the left cotyledon alone which became twisted, with
+the true leaf always so twisted that its upper surface approached closely
+and fronted that of the left cotyledon. In only one instance was the right
+cotyledon twisted, with the true leaf twisted towards it; but this seedling
+was in an abnormal condition, as the left cotyledon did not rise up
+properly at night. This whole case is remarkable, as with the cotyledons of
+no other plant have we seen any nocturnal movement except vertically
+upwards or downwards. It is the more remarkable, because we shall meet with
+an analogous case in the leaves of the allied genus Melilotus, in which the
+terminal leaflet rotates at night so as to present one edge to the zenith
+and at the same time bends to one side, so that its upper surface comes
+into contact with that of one of the two now vertical lateral leaflets.]
+
+Concluding Remarks on the Nyctitropic Movements of Cotyledons.--The sleep
+of cotyledons (though this is a subject which has been little attended to),
+seems to be a more common phenomenon than that of leaves. We observed the
+position of the cotyledons during the day and night in 153 genera, widely
+distributed throughout the dicotyledonous series, but otherwise selected
+almost by hazard; and one or more species in 26 of these genera placed
+their cotyledons at night so as to stand vertically or almost vertically,
+having generally moved through an angle of at least 60o. If we lay on one
+side the Leguminosae, the cotyledons of which are particularly liable to
+sleep, 140 genera remain; and out of these, the cotyledons of at least one
+species in 19 genera slept. Now if we were to select by hazard 140 genera,
+excluding the Leguminosae, and observed their leaves at night, assuredly
+not nearly so many as 19 would be found to include sleeping species. We
+here refer exclusively to the plants observed by ourselves.
+[page 312]
+
+In our entire list of seedlings, there are 30 genera, belonging to 16
+Families, the cotyledons of which in some of the species rise or sink in
+the evening or early night, so as to stand at least 60o above or beneath
+the horizon. In a large majority of the genera, namely, 24, the movement is
+a rising one; so that the same direction prevails in these nyctitropic
+movements as in the lesser periodic ones described in the second chapter.
+The cotyledons move downwards during the early part of the night in only 6
+of the genera; and in one of them, Cannabis, the curving down of the tip is
+probably due to epinasty, as Kraus believes to be the case with the leaves.
+The downward movement to the amount of 90o is very decided in Oxalis
+Valdiviana and sensitiva, and in Geranium rotundifolium. It is a remarkable
+fact that with Anoda Wrightii, one species of Gossypium and at least 3
+species of Ipomoea, the cotyledons whilst young and light sink at night
+very little or not at all; although this movement becomes well pronounced
+as soon as they have grown large and heavy. Although the downward movement
+cannot be attributed to the weight of the cotyledons in the several cases
+which were investigated, namely, in those of the Anoda, Ipomoea purpurea
+and bona-nox, nor in that of I. coccinea, yet bearing in mind that
+cotyledons are continually circumnutating, a slight cause might at first
+have determined whether the great nocturnal movement should be upwards or
+downwards. We may therefore suspect that in some aboriginal member of the
+groups in question, the weight of the cotyledons first determined the
+downward direction. The fact of the cotyledons of these species not sinking
+down much whilst they are young and tender, seems opposed to the belief
+that the greater movement when they are
+[page 313]
+grown older, has been acquired for the sake of protecting them from
+radiation at night; but then we should remember that there are many plants,
+the leaves of which sleep, whilst the cotyledons do not; and if in some
+cases the leaves are protected from cold at night whilst the cotyledons are
+not protected, so in other cases it may be of more importance to the
+species that the nearly full-grown cotyledons should be better protected
+than the young ones.
+
+In all the species of Oxalis observed by us, the cotyledons are provided
+with pulvini; but this organ has become more or less rudimentary in O.
+corniculata, and the amount of upward movement of its cotyledons at night
+is very variable, but is never enough to be called sleep. We omitted to
+ascertain whether the cotyledons of Geranium rotundifolium possess pulvini.
+In the Leguminosae all the cotyledons which sleep, as far as we have seen,
+are provided with pulvini. But with Lotus Jacobaeus, these are not fully
+developed during the first few days of the life of the seedling, and the
+cotyledons do not then rise much at night. With Trifolium strictum the
+blades of the cotyledons rise at night by the aid of their pulvini; whilst
+the petiole of one cotyledon twists half-round at the same time,
+independently of its pulvinus.
+
+As a general rule, cotyledons which are provided with pulvini continue to
+rise or sink at night during a much longer period than those destitute of
+this organ. In this latter case the movement no doubt depends on
+alternately greater growth on the upper and lower side of the petiole, or
+of the blade, or of both, preceded probably by the increased turgescence of
+the growing cells. Such movements generally last for a very short period--
+for instance, with Brassica and Githago for 4 or 5 nights, with Beta for 2
+or 3, and with
+[page 314]
+Raphanus for only a single night. There are, however, some strong
+exceptions to this rule, as the cotyledons of Gossypium, Anoda and Ipomoea
+do not possess pulvini, yet continue to move and to grow for a long time.
+We thought at first that when the movement lasted for only 2 or 3 nights,
+it could hardly be of any service to the plant, and hardly deserved to be
+called sleep; but as many quickly-growing leaves sleep for only a few
+nights, and as cotyledons are rapidly developed and soon complete their
+growth, this doubt now seems to us not well-founded, more especially as
+these movements are in many instances so strongly pronounced. We may here
+mention another point of similarity between sleeping leaves and cotyledons,
+namely, that some of the latter (for instance, those of Cassia and Githago)
+are easily affected by the absence of light; and they then either close, or
+if closed do not open; whereas others (as with the cotyledons of Oxalis)
+are very little affected by light. In the next chapter it will be shown
+that the nyctitropic movements both of cotyledons and leaves consist of a
+modified form of circumnutation.
+
+As in the Leguminosae and Oxalidae, the leaves and the cotyledons of the
+same species generally sleep, the idea at first naturally occurred to us,
+that the sleep of the cotyledons was merely an early development of a habit
+proper to a more advanced stage of life. But no such explanation can be
+admitted, although there seems to be some connection, as might have been
+expected, between the two sets of cases. For the leaves of many plants
+sleep, whilst their cotyledons do not do so--of which fact Desmodium gyrans
+offers a good instance, as likewise do three species of Nicotiana observed
+by us; also Sida rhombifolia, Abutilon Darwinii, and Chenopodium album. On
+the other
+[page 315]
+hand, the cotyledons of some plants sleep and not the leaves, as with the
+species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis, named
+in our list. Still more striking is the fact that, in the same genus, the
+leaves of several or of all the species may sleep, but the cotyledons of
+only some of them, as occurs with Trifolium, Lotus, Gossypium, and
+partially with Oxalis. Again, when both the cotyledons and the leaves of
+the same plant sleep, their movements may be of a widely dissimilar nature:
+thus with Cassia the cotyledons rise vertically up at night, whilst their
+leaves sink down and twist round so as to turn their lower surfaces
+outwards. With seedlings of Oxalis Valdiviana, having 2 or 3 well-developed
+leaves, it was a curious spectacle to behold at night each leaflet folded
+inwards and hanging perpendicularly downwards, whilst at the same time and
+on the same plant the cotyledons stood vertically upwards.
+
+These several facts, showing the independence of the nocturnal movements of
+the leaves and cotyledons on the same plant, and on plants belonging to the
+same genus, lead to the belief that the cotyledons have acquired their
+power of movement for some special purpose. Other facts lead to the same
+conclusion, such as the presence of pulvini, by the aid of which the
+nocturnal movement is continued during some weeks. In Oxalis the cotyledons
+of some species move vertically upwards, and of others vertically downwards
+at night; but this great difference within the same natural genus is not so
+surprising as it may at first appear, seeing that the cotyledons of all the
+species are continually oscillating up and down during the day, so that a
+small cause might determine whether they should rise or sink at night.
+Again, the peculiar nocturnal movement of the left-hand coty-
+[page 316]
+ledon of Trifolium strictum, in combination with that of the first true
+leaf. Lastly, the wide distribution in the dicotyledonous series of plants
+with cotyledons which sleep. Reflecting on these several facts, our
+conclusion seems justified, that the nyctitropic movements of cotyledons,
+by which the blade is made to stand either vertically or almost vertically
+upwards or downwards at night, has been acquired, at least in most cases,
+for some special purpose; nor can we doubt that this purpose is the
+protection of the upper surface of the blade, and perhaps of the central
+bud or plumule, from radiation at night.
+[page 317]
+
+
+CHAPTER VII.
+
+MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
+
+Conditions necessary for these movements--List of Genera and Families,
+which include sleeping plants--Description of the movements in the several
+Genera--Oxalis: leaflets folded at night--Averrhoa: rapid movements of the
+leaflets--Porlieria: leaflets close when plant kept very dry--Tropaeolum:
+leaves do not sleep unless well illuminated during day--Lupinus: various
+modes of sleeping--Melilotus: singular movements of terminal leaflet--
+Trifolium--Desmodium: rudimentary lateral leaflets, movements of, not
+developed on young plants, state of their pulvini--Cassia: complex
+movements of the leaflets--Bauhinia: leaves folded at night--Mimosa pudica:
+compounded movements of leaves, effect of darkness--Mimosa albida, reduced
+leaflets of--Schrankia: downward movement of the pinnae--Marsilea: the only
+cryptogam known to sleep--Concluding remarks and summary--Nyctitropism
+consists of modified circumnutation, regulated by the alternations of light
+and darkness--Shape of first true leaves.
+
+WE now come to the nyctitropic or sleep movements of leaves. It should be
+remembered that we confine this term to leaves which place their blades at
+night either in a vertical position or not more than 30o from the
+vertical,--that is, at least 60o above or beneath the horizon. In some few
+cases this is effected by the rotation of the blade, the petiole not being
+either raised or lowered to any considerable extent. The limit of 30o from
+the vertical is obviously an arbitrary one, and has been selected for
+reasons previously assigned, namely, that when the blade approaches the
+perpendicular as nearly as this, only half as much of the surface is
+exposed at night to the
+[page 318]
+zenith and to free radiation as when the blade is horizontal. Nevertheless,
+in a few instances, leaves which seem to be prevented by their structure
+from moving to so great an extent as 60o above or beneath the horizon, have
+been included amongst sleeping plants.
+
+It should be premised that the nyctitropic movements of leaves are easily
+affected by the conditions to which the plants have been subjected. If the
+ground is kept too dry, the movements are much delayed or fail: according
+to Dassen,* even if the air is very dry the leaves of Impatiens and Malva
+are rendered motionless. Carl Kraus has also lately insisted** on the great
+influence which the quantity of water absorbed has on the periodic
+movements of leaves; and he believes that this cause chiefly determines the
+variable amount of sinking of the leaves of Polygonum convolvulus at night;
+and if so, their movements are not in our sense strictly nyctitropic.
+Plants in order to sleep must have been exposed to a proper temperature:
+Erythrina crista-galli, out of doors and nailed against a wall, seemed in
+fairly good health, but the leaflets did not sleep, whilst those on another
+plant kept in a warm greenhouse were all vertically dependent at night. In
+a kitchen-garden the leaflets of Phaseolus vulgaris did not sleep during
+the early part of the summer. Ch. Royer says,*** referring I suppose to the
+native plants in France, that they do not sleep when the temperature is
+below 5o C. or 41o F. In the case of several sleeping plants, viz., species
+of
+
+* Dassen,'Tijdschrift vor. Naturlijke Gesch. en Physiologie,' 1837, vol.
+iv. p. 106. See also Ch. Royer on the importance of a proper state of
+turgescence of the cells, in 'Annal. des Sc. Nat. Bot.' (5th series), ix.
+1868, p. 345.
+
+** 'Beitr�ge zur Kentniss der Bewegungen,' etc., in 'Flora,' 1879, pp. 42,
+43, 67, etc.
+
+*** 'Annal. des Sc. Nat. Bot.' (5th Series), ix. 1868, p. 366.
+[page 319]
+
+Tropaeolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia, and probably other
+genera, it is indispensable that the leaves should be well illuminated
+during the day in order that they may assume at night a vertical position;
+and it was probably owing to this cause that seedlings of Chenopodium album
+and Siegesbeckia orientalis, raised by us during the middle of the winter,
+though kept at a proper temperature, did not sleep. Lastly, violent
+agitation by a strong wind, during a few minutes, of the leaves of Maranta
+arundinacea (which previously had not been disturbed in the hot-house),
+prevented their sleeping during the two next nights.
+
+We will now give our observations on sleeping plants, made in the manner
+described in the Introduction. The stem of the plant was always secured
+(when not stated to the contrary) close to the base of the leaf, the
+movements of which were being observed, so as to prevent the stem from
+circumnutating. As the tracings were made on a vertical glass in front of
+the plant, it was obviously impossible to trace its course as soon as the
+leaf became in the evening greatly inclined either upwards or downwards; it
+must therefore be understood that the broken lines in the diagrams, which
+represent the evening and nocturnal courses, ought always to be prolonged
+to a much greater distance, either upwards or downwards, than appears in
+them. The conclusions which may be deduced from our observations will be
+given near the end of this chapter.
+
+In the following list all the genera which include sleeping plants are
+given, as far as known to us. The same arrangement is followed as in former
+cases, and the number of the Family is appended. This list possesses some
+interest, as it shows that the habit of
+[page 320]
+sleeping is common to some few plants throughout the whole vascular series.
+The greater number of the genera in the list have been observed by
+ourselves with more or less care; but several are given on the authority of
+others (whose names are appended in the list), and about these we have
+nothing more to say. No doubt the list is very imperfect, and several
+genera might have been added from the 'Somnus Plantarum' by Linnaeus; but
+we could not judge in some of his cases, whether the blades occupied at
+night a nearly vertical position. He refers to some plants as sleeping, for
+instance, Lathyrus odoratus and Vicia faba, in which we could observe no
+movement deserving to be called sleep, and as no one can doubt the accuracy
+of Linnaeus, we are left in doubt.
+
+[List of Genera, including species the leaves of which sleep.
+
+CLASS I. DICOTYLEDONS.
+
+Sub-class I. ANGIOSPERMS.
+
+Genus                      Family.
+
+Githago                      Caryophylleae (26).
+Stellaria (Batalin).                "
+Portulaca (Ch.Royer).               Portulaceae (27).
+Sida                        Malvaceae (36).
+Abutilon.                       "
+Malva (Linnaeus and Pfeffer).             "
+Hibiscus (Linnaeus).                 "
+Anoda.                        "
+Gossypium.                      "
+Ayenia (Linnaeus).                Sterculaceae (37).
+Triumfetta (Linnaeus).              Tiliaceae (38).
+Linum (Batalin).                 Lineae (39).
+Oxalis.                      Oxalidae (41).
+Averrhoa.                       "
+Porlieria.                    Zygophylleae (45).
+Guiacum.                        "
+Impatiens (Linnaeus, Pfeffer, Batalin).           Balsamineae (48).
+Tropaeolum.                    Tropaeoleae (49).
+Crotolaria (Thiselton Dyer).           Leguminosae (75) Tribe II.
+Lupinus.                        "       "
+Cytisus.                         "       "
+Trigonella.                       "     Tr. III.
+Medicago.                                "
+Melilotus.                        "       "
+Trifolium.                        "       "
+Securigera.                       "     Tr. IV.
+Lotus.                          "       "
+Psoralea.                       "      Tr. V.
+Amorpha (Cuchartre).                   "       "
+Daelea.                         "       "
+Indigofera.                      "        "
+Tephrosia.                      "         "
+Wistaria.                       "         "
+Robinia.                        "        "
+Sphaerophysa.                     "        "
+Colutea.                        "        "
+Astragalus.                      "        "
+Glycyrrhiza.                      "        "
+Coronilla.                       "      Tr. VI.
+Hedysarum.                       "        "
+[page 321]
+
+List of Genera (continued).
+
+CLASS I. DICOTYLEDONS.
+
+Sub-class I. ANGIOSPERMS.
+
+Genus                      Family.
+Onobrychis.         Leguminosae (75) Tr. VI.
+Smithia.                        "        "
+Arachis.                        "        "
+Desmodium.                       "        "
+Urania.                        "        "
+Vicia.                         "      Tr. VII.
+Centrosema.                      "      Tr. VIII.
+Amphicarpaea.                     "        "
+Glycine.                        "        "
+Erythrina.                       "        "
+Apios.                         "        "
+Phaseolus.                       "        "
+Sophora.                        "       Tr. X.
+Caesalpinia.                      "       Tr. XIII.
+Haematoxylon.                     "        "
+Gleditschia (Duchartre).                "        "
+Poinciana.                       "        "
+Cassia.                        "       Tr. XIV.
+Bauhinia.                       "       Tr. XV.
+Tamarindus.                      "       Tr. XVI.
+Adenanthera.                      "       Tr. XX.
+Prosopis.                       "        "
+Neptunia.                       "        "
+Mimosa.                        "        "
+Schrankia.                       "        "
+Acacia.                        "       Tr. XXII.
+Albizzia.                       "       Tr. XXIII.
+Melaleuca (Bouch�).           Myrtaceae (94).
+
+Sub-class I. ANGIOSPERMS (continued).
+
+Genus                      Family.
+Aenothera (Linnaeus).          Omagrarieae (100).
+Passiflora.               Passifloracea (105).
+Siegesbeckia.              Compositae (122).
+Ipomoea.                 Convolvulacea (151).
+Nicotiana.                Solaneae (157).
+Mirabilis.                Nyctagineae (177).
+Polygonum (Batalin).           Polygoneae (179).
+Amaranthus.               Amaranthaceae (180).
+Chenopodium.               Chenopodieae (181).
+Pimelia (Bouch�).            Thymeteae (188).
+Euphorbia.                Euphorbiaceae (202)
+Phyllanthus (Pfeffer).            "
+
+Sub-class II. GYMNOSPERMS.
+Aies (Chatin).
+
+CLASS II. MONOCOTYLEDONS.
+
+Thalia.                   Cannaceae (21).
+Maranta.                      "
+Colocasia.                 Aroideae (30).
+Strephium.                 Gramineae (55).
+
+CLASS III. ACOTYLEDONS.
+
+Marsilea.                  Marsileaceae (4).
+
+Githago segetum (Caryophylleae).--The first leaves produced by young
+seedlings, rise up and close together at night. On a rather older seedling,
+two young leaves stood at noon at 55o above the horizon, and at night at
+86o, so each had risen 31o. The angle, however, was less in some cases.
+Similar observations were occasionally made on young leaves (for the older
+ones moved very little) produced by nearly full-grown plants. Batalin says
+('Flora,' Oct. 1st, 1873, p. 437) that the young leaves of Stellaria close
+up so completely at night that they form together great buds.
+
+Sida (Malvaceae).--the nyctitropic movements of the leaves in this genus
+are remarkable in some respects. Batalin informs
+[page 322]
+us (see also 'Flora,' Oct. 1st, 1873, p. 437) that those of S. napaea fall
+at night, but to what angle he cannot remember. The leaves of S.
+rhombifolia and retusa, on the other hand, rise up vertically, and are
+pressed against the stem. We have therefore here within the same genus,
+directly opposite movements. Again, the leaves of S. rhombifolia are
+furnished with a pulvinus, formed of a mass of small cells destitute of
+chlorophyll, and with their longer axes perpendicular to the axis of the
+petiole. As measured along this latter line, these cells are only 1/5th of
+the length of those of the petiole; but instead of being abruptly separated
+from them (as is usual with the pulvinus in most plants), they graduate
+into the larger cells of the petiole. On the other hand, S. napaea,
+according to Batalin, does not possess a pulvinus; and he informs us that a
+gradation may be traced in the several species of the genus between these
+two states of the petiole. Sida rhombifolia presents another peculiarity,
+of which we have seen no other instance with leaves that sleep: for those
+on very young plants, though they rise somewhat in the evening, do not go
+to sleep, as we observed
+
+Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep)
+movements of a leaf on a young plant, 9 � inches high; filament fixed to
+midrib of nearly full-grown leaf, 2 3/8 inches in length; movement traced
+under a sky-light. Apex of leaf 5 5/8 inches from the vertical glass, so
+diagram not greatly enlarged.
+[page 323]
+
+on several occasions; whilst those on rather older plants sleep in a
+conspicuous manner. For instance a leaf (.85 of an inch in length) on a
+very young seedling 2 inches high, stood at noon 9o above the horizon, and
+at 10 P.M. at 28o, so it had risen only 19o; another leaf (1.4 inch in
+length) on a seedling of the same height, stood at the same two periods at
+7o and 32o, and therefore had risen 25o. These leaves, which moved so
+little, had a fairly well-developed pulvinus. After an interval of some
+weeks, when the same seedlings were 2 � and 3 inches in height, some of the
+young leaves stood up at night quite vertically, and others were highly
+inclined; and so it was with bushes which were fully grown and were
+flowering.
+
+The movement of a leaf was traced from 9.15 A.M. on May 28th to 8.30 A.M.
+on the 30th. The temperature was too low (15o - 16o C.), and the
+illumination hardly sufficient; consequently the leaves did not become
+quite so highly inclined at night, as they had done previously and as they
+did subsequently in the hot-house: but the movements did not appear
+otherwise disturbed. On the first day the leaf sank till 5.15 P.M.; it then
+rose rapidly and greatly till 10.5 P.M., and only a little higher during
+the rest of the night (Fig. 126). Early on the next day (29th) it fell in a
+slightly zigzag line rapidly until 9 A.M., by which time it had reached
+nearly the same place as on the previous morning. During the remainder of
+the day it fell slowly, and zigzagged laterally. The evening rise began
+after 4 P.M. in the same manner as before, and on the second morning it
+again fell rapidly. The ascending and descending lines do not coincide, as
+may be seen in the diagram. On the 30th a new tracing was made (not here
+given) on a rather enlarged scale, as the apex of the leaf now stood 9
+inches from the vertical glass. In order to observe more carefully the
+course pursued at the time when the diurnal fall changes into the nocturnal
+rise, dots were made every half-hour between 4 P.M. and 10.30 P.M. This
+rendered the lateral zigzagging movement during the evening more
+conspicuous than in the diagram given, but it was of the same nature as
+there shown. The impression forced on our minds was that the leaf was
+expending superfluous movement, so that the great nocturnal rise might not
+occur at too early an hour.
+
+Abutilon Darwinii (Malvaceae).--The leaves on some very young plants stood
+almost horizontally during the day, and hung down vertically at night. Very
+fine plants kept in a
+[page 324]
+large hall, lighted only from the roof, did not sleep at night for in order
+to do so the leaves must be well illuminated during the day. The cotyledons
+do not sleep. Linnaeus says that the leaves of his Sida abutilon sink
+perpendicularly down at night, though the petioles rise. Prof. Pfeffer
+informs us that the leaves of a Malva, allied to M. sylvestris, rise
+greatly at night; and this genus, as well as that of Hibiscus, are included
+by Linnaeus in his list of sleeping plants.
+
+Anoda Wrightii (Malvaceae).--The leaves, produced by very young plants,
+when grown to a moderate size, sink at night either almost vertically down
+or to an angle of about 45o beneath the horizon; for there is a
+considerable degree of variability in the amount of sinking at night, which
+depends in part on the degree to which they have been illuminated during
+the day. But the leaves, whilst quite young, do not sink down at night, and
+this is a very unusual circumstance. The summit of the petiole, where it
+joins the blade, is developed into a pulvinus, and this is present in very
+young leaves which do not sleep; though it is not so well defined as in
+older leaves.
+
+Gossypium (var. Nankin cotton, Malvaceae).--Some young leaves, between 1
+and 2 inches in length, borne by two seedlings 6 and 7 � inches in height,
+stood horizontally, or were raised a little above the horizon at noon on
+July 8th and 9th; but by 10 P.M. they had sunk down to between 68o and 90o
+beneath the horizon. When the same plants had grown to double the above
+height, their leaves stood at night almost or quite vertically dependent.
+The leaves on some large plants of G. maritimum and Brazilense, which were
+kept in a very badly lighted hot-house, only occasionally sank much
+downwards at night, and hardly enough to be called sleep.
+
+Oxalis (Oxalidae).--In most of the species in this large genus the three
+leaflets sink vertically down at night; but as their sub-petioles are short
+the blades could not assume this position from the want of space, unless
+they were in some manner rendered narrower; and this is effected by their
+becoming more or less folded (Fig. 127). The angle formed by the two halves
+of the same leaflet was found to vary in different individuals of several
+species between 92o and 150o; in three of the best folded leaflets of O.
+fragrans it was 76o, 74o, and 54o. The angle is often different in the three
+leaflets of the same leaf. As the leaflets sink down at night and become
+folded, their lower surfaces are brought near together (see B), or even
+into
+[page 325]
+close contact; and from this circumstance it might be thought that the
+object of the folding was the protection of their lower surfaces. If this
+had been the case, it would have formed a strongly marked exception to the
+rule, that when there is any difference in the degree of protection from
+radiation of the two surfaces of the leaves, it is always the upper surface
+which is the best protected. But that the folding of the leaflets, and
+consequent mutual approximation of their lower surfaces, serves merely to
+allow them to sink down vertically, may be
+
+Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B, diagram
+of leaf asleep, also seen from vertically above.
+
+inferred from the fact that when the leaflets do not radiate from the
+summit of a common petiole, or, again, when there is plenty of room from
+the sub-petioles not being very short, the leaflets sink down without
+becoming folded. This occurs with the leaflets of O. sensitiva, Plumierii,
+and bupleurifolia.
+
+There is no use in giving a long list of the many species which sleep in
+the above described manner. This holds good with species having rather
+fleshy leaves, like those of O. carnosa, or large leaves like those of O.
+Ortegesii, or four leaflets like those of O. variabilis. There are,
+however, some species which show no signs of sleep, viz., O. pentaphylla,
+enneaphylla, hirta, and rubella. We will now describe the nature of the
+movements in some of the species.
+
+Oxalis acetosella.--The movement of a leaflet, together with that of the
+main petiole, are shown in the following diagram (Fig. 128), traced between
+11 A.M. on October 4th and 7.45 A.M. on the 5th. After 5.30 P.M. on the 4th
+the leaflet sank rapidly, and at 7 P.M. depended vertically. for some time
+before it assumed this latter position, its movements could, of course, no
+longer be traced on the vertical glass, and the broken line in the diagram
+ought to be extended much further
+[page 326]
+down in this and all other cases. By 6.45 A.M. on the following morning it
+had risen considerably, and continued to rise for the next hour; but,
+judging from other observations, it would soon have begun to fall again.
+Between 11 A.M. and 5.30 P.M. the leaflet moved at least four times up and
+four times down before the great nocturnal fall commenced; it reached its
+highest point at noon. Similar observations were made on two other
+leaflets, with nearly the same results. Sachs and Pfeffer have also
+described briefly* the autonomous movements of the leaves of this plant.
+
+Fig 128. Oxalis acetosella: circumnutation and nyctitropic movements of a
+nearly full-grown leaf, with filament attached to the midrib of one of the
+leaflets; traced on vertical glass during 20 h. 45m.
+
+On another occasion the petiole of a leaf was secured to a little stick
+close beneath the leaflets, and a filament tipped with a bead of
+sealing-wax was affixed to the mid-rib of one of them, and a mark was
+placed close behind. At 7 P.M., when the leaflets were asleep, the filament
+depended vertically down, and the movements of the bead were then traced
+till 10.40 P.M., as shown in the following diagram (Fig. 129). We here see
+that the leaflet moved a little from side to side, as well as a little up
+and down, whilst asleep.
+
+* Sachs in 'Flora,' 1863, p. 470, etc; Pfeffer, 'Die Period. Bewegungen,'
+etc., 1875, p. 53.
+[page 327]
+
+Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep; traced
+on vertical glass during 3 h. 40 m.
+
+Oxalis Valdiviana.--The leaves resemble those of the last species, and the
+movements of two leaflets (the main petioles of both having been secured)
+were traced during two days; but the tracings are not given, as they
+resembled that of O. acetosella, with the exception that the up and down
+oscillations were not so frequent during the day, and there was more
+lateral movement, so that broader ellipses were described. The leaves awoke
+early in the morning, for by 6.45 A.M. on June 12th and 13th they had not
+only risen to their full height, but had already begun to fall, that is,
+they were circumnutating. We have seen in the last chapter that the
+cotyledons, instead of sinking, rise up vertically at night.
+
+Oxalis Ortegesii.--The large leaves of this plant sleep like those of the
+previous species. The main petioles are long, and that of a young leaf rose
+20o between noon and 10 P.M., whilst the petiole of an older leaf rose only
+13o. Owing to this rising of the petioles, and the vertical sinking of the
+large leaflets, the leaves become crowded together at night, and the whole
+plant then exposes a much smaller surface to radiation than during the day.
+
+Oxalis Plumierii.--In this species the three leaflets do not surround the
+summit of the petiole, but the terminal leaflet projects in the line of the
+petiole, with a lateral leaflet on each side. They all sleep by bending
+vertically downwards, but do not become at all folded. The petiole is
+rather long, and, one having been secured to a stick, the movement of the
+terminal leaflet was traced during 45 h. on a vertical glass. It moved in a
+very simple manner, sinking rapidly after 5 P.M., and rising rapidly early
+next morning. During the middle of the day it moved slowly and a little
+laterally. Consequently the ascending and descending lines did not
+coincide, and a single great ellipse was formed each day. There was no
+other evidence of circumnutation, and this fact is of interest, as we shall
+hereafter see.
+
+Oxalis sensitiva.--The leaflets, as in the last species, bend vertically
+down at night, without becoming folded. The much elongated main petiole
+rises considerably in the evening, but in
+[page 328]
+some very young plants the rise did not commence until late at night. We
+have seen that the cotyledons, instead of sinking like the leaflets, rise
+up vertically at night.
+
+Oxalis bupleurifolia.--This species is rendered remarkable by the petioles
+being foliaceous, like the phyllodes of many Acacias. The leaflets are
+small, of a paler green and more tender consistence than the foliaceous
+petioles. The leaflet which was observed was .55 inch in length, and was
+borne by a petiole 2 inches long and .3 inch broad. It may be suspected
+that the leaflets are on the road to abortion or obliteration, as has
+actually occurred with those of another Brazilian species, O. rusciformis.
+Nevertheless, in the present species the nyctitropic movements are
+perfectly performed. The foliaceous petiole was first observed during 48
+h., and found to be in continued circumnutation, as shown in the
+accompanying figure (Fig. 130). It rose during the day and early part of
+the night, and fell during the remainder of the night and early morning;
+but the movement was not sufficient to be called sleep. The ascending and
+descending lines did not coincide, so that an ellipse was formed each day.
+There was but little zigzagging; if the filament had been fixed
+longitudinally, we should probably have seen that there was more lateral
+movement than appears in the diagram.
+
+Fig. 130. Oxalis bupleurifolia: circumnutation of foliaceous petiole,
+filament fixed obliquely across end of petiole; movements traced on
+vertical glass from 9 A.M. June 26th to 8.50 A.M. 28th. Apex of leaflet 4 �
+inches from the glass, so movement not much magnified. Plant 9 inches high,
+illuminated from above. Temp. 23 1/2o - 24 1/2o C.
+
+A terminal leaflet on another leaf was next observed (the petiole being
+secured), and its movements are shown in Fig. 131. During the day the
+leaflets are extended horizontally, and at night depend vertically; and as
+the petiole rises during the day the leaflets have to bend down in the
+evening
+[page 329]
+more than 90o, so as to assume at night their vertical position. On the
+first day the leaflet simply moved up and down; on the
+
+Fig. 131. Oxalis bupleurifolia: circumnutation and nyctitropic movement of
+terminal leaflet, with filament affixed along the midrib; traced on a
+vertical glass from 9 A.M. on June 26th to 8.45 A.M. 28th. Conditions the
+same as in the last case.
+
+second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after
+which hour the great evening fall commenced.
+[page 330]
+
+Averrhoa bilimbi (Oxalidae).--It has long been known,* firstly, that the
+leaflets in this genus sleep; secondly, that they move spontaneously during
+the day; and thirdly, that they are sensitive to a touch; but in none of
+these respects do they differ essentially from the species of Oxalis. They
+differ, however, as Mr. R. I. Lynch** has lately shown, in their
+spontaneous movements being strongly marked. In the case of A. bilimbi, it
+is a wonderful spectacle to behold on a warm sunny day the leaflets one
+after the other sinking rapidly downwards, and again ascending slowly.
+Their movements rival those of Desmodium gyrans. At night the leaflets hang
+vertically down; and now
+
+Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced.
+
+they are motionless, but this may be due to the opposite ones being pressed
+together (Fig. 132). The main petiole is in constant movement during the
+day, but no careful observations were made on it. The following diagrams
+are graphic representations of the variations in the angle, which a given
+leaflet makes with the vertical. The observations were made as follows. The
+plant growing in a pot was kept in a high temperature, the petiole of the
+leaf to be observed pointing straight at the observer, being separated from
+him by a vertical pane of glass. The petiole was secured so that the basal
+joint, or pulvinus, of one of the lateral leaflets was at the centre of a
+graduated arc placed close behind the leaflet. A fine glass filament was
+fixed to the leaf, so as to project like a continuation of the
+
+* Dr. Bruce, 'Philosophical Trans.,' 1785, p. 356.
+
+** 'Journal Linn. Soc.,' vol. xvi. 1877, p. 231.
+[page 331]
+
+midrib. This filament acted as an index; and as the leaf rose and fell,
+rotating about its basal joint, its angular movement
+
+Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its
+evening descent, when going to sleep. Temp. 78o - 81o F.
+
+could be recorded by reading off at short intervals of time the position of
+the glass filament on the graduated arc. In order
+[page 332]
+to avoid errors of parallax, all readings were made by looking through a
+small ring painted on the vertical glass, in a line with the joint of the
+leaflet and the centre of the graduated arc. In the following diagrams the
+ordinates represent the angles which the leaflet made with the vertical at
+successive instants.* It follows that a fall in the curve represents an
+actual dropping of the leaf, and that the zero line represents a vertically
+dependent position. Fig. 133 represents the nature of the movements which
+occur in the evening, as soon as the leaflets begin to assume their
+nocturnal position. At 4.55 P.M. the leaflet formed an angle of 85o with
+the vertical, or was only 5o below the horizontal; but in order that the
+diagram might get into our page, the leaflet is represented falling from
+75o instead of 85o. Shortly after 6 P.M. it hung vertically down, and had
+attained its nocturnal position. Between 6.10 and 6.35 P.M. it performed a
+number of minute oscillations of about 2o each, occupying periods of 4 or 5
+m. The complete state of rest of the leaflet which ultimately followed is
+not shown in the diagram. It is manifest that each oscillation consists of
+a gradual rise, followed by a sudden fall. Each time the leaflet fell, it
+approached nearer to the nocturnal position than it did on the previous
+fall. The amplitude of the oscillations diminished, while the periods of
+oscillation became shorter.
+
+In bright sunshine the leaflets assume a highly inclined dependent
+position. A leaflet in diffused light was observed rising for 25 m. A blind
+was then pulled up so that the plant was brightly illuminated (BR in Fig.
+134), and within a minute it began to fall, and ultimately fell 47o, as
+shown in the diagram. This descent was performed by six descending steps,
+precisely similar to those by which the nocturnal fall is effected. The
+plant was then again shaded (SH), and a long slow rise occurred until
+another series of falls commenced at BR', when the sun was again admitted.
+In this experiment cool air was allowed to enter by the windows being
+opened at the same time that the blinds were pulled up, so that in spite of
+the sun shining on the plant the temperature was not raised.
+
+The effect of an increase of temperature in diffused light is
+
+* In all the diagrams 1 mm. in the horizontal direction represents one
+minute of time. Each mm. in the vertical direction represents one degree of
+angular movement. In Figs. 133 and 134 the temperature is represented
+(along the ordinates) in the scale of 1 mm. to each 0.1 degree C. In Fig.
+135 each mm. equals 0.2o F.
+[page 333]
+
+shown in Fig. 135. The temperature began to rise at 11.35 A.M. (in
+consequence of the fire being lighted), but by 12.42 a marked fall had
+occurred. It may be seen in the diagram that when the temperature was
+highest there were rapid oscillations
+
+Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a change
+from bright illumination to shade; temperature (broken line) remaining
+nearly the same.
+
+of small amplitude, the mean position of the leaflet being at the time
+nearer the vertical. When the temperature began to fall, the oscillations
+became slower and larger, and the mean position of the leaf again
+approached the horizontal. The rate of oscillation was sometimes quicker
+than is represented in the above diagram. Thus, when the temperature was
+between 31o and
+[page 334]
+
+Fig. 135. Averrhoa bilimbi: angular movement of leaflet during a change of
+temperature; light remaining the same. The broken line shows the change of
+temperature.
+[page 335]
+
+32o C., 14 oscillations of a few degrees occurred in 19 m. On the other
+hand, an oscillation may be much slower; thus a leaflet was observed
+(temperature 25o C.) to rise during 40 m. before it fell and completed its
+oscillation.
+
+Fig. 136. Porlieria hygrometrica: circumnutation and nyctitropic movements
+of petiole of leaf, traced from 9.35 A.M. July 7th to about midnight on the
+8th. Apex of leaf 7 � inches from the vertical glass. Temp. 19 1/2o - 20
+1/2o C.
+
+Porlieria hygrometrica (Zygophylleae).--The leaves of this plant (Chilian
+form) are from 1 to 1 � inch in length, and bear as many as 16 or 17 small
+leaflets on each side, which do not stand opposite one another. They are
+articulated to the petiole, and the petiole to the branch by a pulvinus. We
+must premise that apparently two forms are confounded under the same name:
+the leaves on a bush from Chili, which was sent to us from Kew, bore many
+leaflets, whilst those on plants in the Botanic Garden at W�rzburg bore
+only 8 or 9 pairs; and the whole character of the bushes appeared somewhat
+different. We shall also see that they differ in a remarkable physiological
+peculiarity. On the Chilian plant the petioles of the younger leaves on
+upright branches, stood horizontally during the day, and at night sank down
+vertically so as to depend parallel and close to the branch beneath. The
+petioles of rather older leaves did not become at night vertically
+depressed, but only highly inclined. In one instance we found a branch
+which had grown perpendicularly downwards, and the petioles on it moved in
+the same direction relatively to the branch as just stated, and therefore
+moved upwards. On horizontal branches the younger petioles likewise move at
+night in the same direction as before, that is, towards the branch, and are
+consequently then extended horizontally; but it is remarkable that the
+older petioles on the
+[page 336]
+same branch, though moving a little in the same direction, also bend
+downwards; they thus occupy a somewhat different position, relatively to
+the centre of the earth and to the branch, from that of the petioles on the
+upright branches. With respect to the leaflets, they move at night towards
+the apex of the petiole until their midribs stand nearly parallel to it;
+and they then lie neatly imbricated one over the other. Thus half of the
+upper surface of each leaflet is in close contact with half of the lower
+surface of the one next in advance; and all the leaflets, excepting the
+basal ones, have the whole of their upper surfaces and half of their lower
+surfaces well protected. Those on the opposite sides of the same petiole do
+not come into close contact at night, as occurs with the leaflets of so
+many Leguminosae but are separated by an open furrow; nor could they
+exactly coincide, as they stand alternately with respect to one another.
+
+The circumnutation of the petiole of a leaf 3/4 of an inch in length, on an
+upright branch, was observed during 36h., and is shown in the preceding
+diagram (Fig. 136). On the first morning, the leaf fell a little and then
+rose until 1 P.M., and this was probably due to its being now illuminated
+through a skylight from above; it then circumnutated on a very small scale
+round the same spot until about 4 P.M., when the great evening fall
+commenced. During the latter part of the night or very early on the next
+morning the leaf rose again. On the second day it fell during the morning
+till 1 P.M., and this no doubt is its normal habit. From 1 to 4 P.M. it
+rose in a zigzag line, and soon afterwards the great evening fall
+commenced. It thus completed a double oscillation during the 24 h.
+
+The specific name given to this plant by Ruiz and Pavon, indicates that in
+its native arid home it is affected in some manner by the dryness or
+dampness of the atmosphere.* In the Botanic Garden at W�rzburg, there was a
+plant in a pot out of doors which was daily watered, and another in the
+open ground which was never watered. After some hot and dry weather there
+was a great difference in the state of the leaflets on these two plants;
+those on the unwatered plant in the open ground remaining half,
+
+* 'Systema Veg. Florae Peruvianae et Chilensis,' tom. i. p. 95, 1798. We
+cannot understand the account given by the authors of the behaviour of this
+plant in its native home. There is much about its power of foretelling
+changes in the weather; and it appears as if the brightness of the sky
+largely determined the opening and closing of the leaflets.
+[page 337]
+
+or even quite, closed during the day. But twigs cut from this bush, with
+their ends standing in water, or wholly immersed in it, or kept in damp air
+under a bell-glass, opened their leaves though exposed to a blazing sun;
+whilst those on the plant in the ground remained closed. The leaves on this
+same plant, after some heavy rain, remained open for two days; they then
+became half closed during two days, and after an additional day were quite
+closed. This plant was now copiously watered, and on the following morning
+the leaflets were fully expanded. The other plant growing in a pot, after
+having been exposed to heavy rain, was placed before a window in the
+Laboratory, with its leaflets open, and they remained so during the daytime
+for 48 h.; but after an additional day were half closed. The plant was then
+watered, and the leaflets on the two following days remained open. On the
+third day they were again half closed, but on being again watered remained
+open during the two next days. From these several facts we may conclude
+that the plant soon feels the want of water; and that as soon as this
+occurs, it partially or quite closes its leaflets, which in their then
+imbricated condition expose a small surface to evaporation. It is therefore
+probable that this sleep-like movement, which occurs only when the ground
+is dry, is an adaptation against the loss of moisture.
+
+A bush about 4 feet in height, a native of Chili, which was thickly covered
+with leaves, behaved very differently, for during the day it never closed
+its leaflets. On July 6th the earth in the small pot in which it grew
+appeared extremely dry, and it was given a very little water. After 21 and
+22 days (on the 27th and 28th), during the whole of which time the plant
+did not receive a drop of water, the leaves began to droop, but they showed
+no signs of closing during the day. It appeared almost incredible that any
+plant, except a fleshy one, could have kept alive in soil so dry, which
+resembled the dust on a road. On the 29th, when the bush was shaken, some
+leaves fell off, and the remaining ones were unable to sleep at night. It
+was therefore moderately watered, as well as syringed, late in the evening.
+On the next morning (30th) the bush looked as fresh as ever, and at night
+the leaves went to sleep. It may be added that a small branch while growing
+on the bush was enclosed, by means of a curtain of bladder, during 13 days
+in a large bottle half full of quicklime, so that the air within must have
+been intensely dry; yet the leaves on this branch did not suffer in the
+[page 338]
+least, and did not close at all during the hottest days. Another trial was
+made with the same bush on August 2nd and 6th (the soil appearing at this
+latter date extremely dry), for it was exposed out of doors during the
+whole day to the wind, but the leaflets showed no signs of closing. The
+Chilian form therefore differs widely from the one at W�rzburg, in not
+closing its leaflets when suffering from the want of water; and it can live
+for a surprisingly long time without water.
+
+Tropaeolum majus (?) (cultivated var.) (Tropaeoleae).--Several plants in
+pots stood in the greenhouse, and the blades of the leaves which faced the
+front-lights were during the day highly inclined and at night vertical;
+whilst the leaves on the back of the pots, though of course illuminated
+through the roof, did not become vertical at night. We thought, at first,
+that this difference in their positions was in some manner due to
+heliotropism, for the leaves are highly heliotropic. The true explanation,
+however, is that unless they are well illuminated during at least a part of
+the day they do not sleep at night; and a little difference in the degree
+of illumination determines whether or not they shall become vertical at
+night. We have observed no other so well-marked a case as this, of the
+influence of previous illumination on nyctitropic movements. The leaves
+present also another peculiarity in their habit of rising or awaking in the
+morning, being more strongly fixed or inherited than that of sinking or
+sleeping at night. The movements are caused by the bending of an upper part
+of the petiole, between � and 1 inch in length; but the part close to the
+blade, for about 1/4 of an inch in length, does not bend and always remains
+at right angles to the blade. The bending portion does not present any
+external or internal difference in structure from the rest of the petiole.
+We will now give the experiments on which the above conclusions are
+founded.
+
+A large pot with several plants was brought on the morning of Sept. 3rd out
+of the greenhouse and placed before a north-east window, in the same
+position as before with respect to the light, as far as that was possible.
+On the front of the plants, 24 leaves were marked with thread, some of
+which had their blades horizontal, but the greater number were inclined at
+about 45o, beneath the horizon; at night all these, without exception,
+became vertical. Early on the following morning (4th) they reassumed their
+former positions, and at night again became vertical. On the 5th the
+shutters were opened at 6.15 A.M., and
+[page 339]
+by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and had
+acquired their diurnal position, they were placed in a dark cupboard. They
+were looked at twice during the day and thrice in the evening, the last
+time at 10.30 P.M., and not one had become vertical. At 8 A.M. on the
+following morning (6th) they still retained the same diurnal position, and
+were now replaced before the north-east window. At night all the leaves
+which had faced the light had their petioles curved and their blades
+vertical; whereas none of the leaves on the back of the plants, although
+they had been moderately illuminated by the diffused light of the room,
+were vertical. They were now at night placed in the same dark cupboard; at
+9 A.M. on the next morning (7th) all those which had been asleep had
+reassumed their diurnal position. The pot was then placed for 3 h. in the
+sunshine, so as to stimulate the plants; at noon they were placed before
+the same north-east window, and at night the leaves slept in the usual
+manner and awoke on the following morning. At noon on this day (8th) the
+plants, after having been left before the north-east window for 5 h. 45 m.
+and thus illuminated (though not brightly, as the sky was cloudy during the
+whole time), were replaced in the dark cupboard, and at 3 P.M. the position
+of the leaves was very little, if at all, altered, so that they are not
+quickly affected by darkness; but by 10.15 P.M. all the leaves which had
+faced the north-east sky during the 5 h. 45 m. of illumination stood
+vertical, whereas those on the back of the plant retained their diurnal
+position. On the following morning (9th) the leaves awoke as on the two
+former occasions in the dark, and they were kept in the dark during the
+whole day; at night a very few of them became vertical, and this was the
+one instance in which we observed any inherited tendency or habit in this
+plant to sleep at the proper time. That it was real sleep was shown by
+these same leaves reassuming their diurnal position on the following
+morning (10th) whilst still kept in the dark.
+
+The pot was then (9.45 A.M. 10th) replaced, after having been kept for 36
+h. in darkness, before the north-east window; and at night the blades of
+all the leaves (excepting a few on the back of the plants) became
+conspicuously vertical.
+At 6.45 A.M. (11th) after the plants had been illuminated on the same side
+as before during only 25 m., the pot was turned round, so that the leaves
+which had faced the light now faced the interior of the room, and not one
+of these went to sleep at night;
+[page 340]
+whilst some, but not many, of those which had formerly stood facing the
+back of the room and which had never before been well illuminated or gone
+to sleep, now assumed a vertical position at night. On the next day (12th)
+the plant was turned round into its original position, so that the same
+leaves faced the light as formerly, and these now went to sleep in the
+usual manner. We will only add that with some young seedlings kept in the
+greenhouse, the blades of the first pair of true leaves (the cotyledons
+being hypogean) stood during the day almost horizontally and at night
+almost vertically.
+
+A few observations were subsequently made on the circumnutation of three
+leaves, whilst facing a north-east window; but the tracings are not given,
+as the leaves moved somewhat towards the light. It was, however, manifest
+that they rose and fell more than once during the daytime, the ascending
+and descending lines being in parts extremely zigzag. The nocturnal fall
+commenced about 7 P.M., and the leaves had risen considerably by 6.45 A.M.
+on the following morning.
+
+Leguminosae.--This Family includes many more genera with sleeping species
+than all the other families put together. The number of the tribes to which
+each genus belongs, according to Bentham and Hooker's arrangement, has been
+added.
+
+Crotolaria (sp.?) (Tribe 2).--This plant is monophyllous, and we are
+informed by Mr. T. Thiselton Dyer that the leaves rise up vertically at
+night and press against the stem.
+
+Lupinus (Tribe 2).--The palmate or digitate leaves of the species in this
+large genus sleep in three different manners. One of the simplest, is that
+all the leaflets become steeply inclined downwards at night, having been
+during the day extended horizontally. This is shown in the accompanying
+figures (Fig. 137), of a leaf of L. pilosus, as seen during the day from
+vertically above, and of another leaf asleep with the leaflets inclined
+downwards. As in this position they are crowded together, and as they do
+not become folded like those in the genus Oxalis, they cannot occupy a
+vertically dependent position; but they are often inclined at an angle of
+50o beneath the horizon. In this species, whilst the leaflets are sinking,
+the petioles rise up, in two instances when the angles were measured to the
+extent of 23o. The leaflets of L. sub-carnosus and arboreus, which were
+horizontal during the day, sank down at night in nearly the same manner;
+the former to an angle of 38o and the latter of 36o, beneath the horizon;
+but their petioles
+[page 341]
+did not move in any plainly perceptible degree. It is, however, quite
+possible, as we shall presently see, that if a large number of plants of
+the three foregoing and of the following species
+
+Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in daytime;
+B, leaf asleep, seen laterally at night.
+
+were to be observed at all seasons, some of the leaves would be found to
+sleep in a different manner.
+
+In the two following species the leaflets, instead of moving downwards,
+rise at night. With L. Hartwegii some stood at noon at a mean angle of 36o
+above the horizon, and at night at 51o, thus forming together a hollow cone
+with moderately steep sides. The petiole of one leaf rose 14o and of a
+second 11o at night. With L. luteus a leaflet rose from 47o at noon to 65o
+above the horizon at night, and another on a distinct leaf rose from 45o to
+69o. The petioles, however, sink at night to a small extent, viz., in three
+instances by 2o, 6o, and 9o 30 seconds. Owing to this movement of the
+petioles, the outer and longer leaflets have to bend up a little more than
+the shorter and inner ones, in order that all should stand symmetrically at
+night. We shall presently see that some leaves on the same individual
+plants of L. luteus sleep in a very different manner.
+
+We now come to a remarkable position of the leaves when asleep, which is
+common to several species of Lupines. On the same leaf the shorter
+leaflets, which generally face the centre of the plant, sink at night,
+whilst the longer ones on the opposite side rise; the intermediate and
+lateral ones merely twisting on their own axes. But there is some
+variability with respect to which leaflets rise or fall. As might have been
+expected from such diverse and complicated movements, the
+[page 342]
+base of each leaflet is developed (at least in the case of L. luteus) into
+a pulvinus. The result is that all the leaflets on the same leaf stand at
+night more or less highly inclined, or even quite vertically, forming in
+this latter case a vertical star. This occurs with the leaves of a species
+purchased under the name of
+
+Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day; B,
+same leaf at night; C, another leaf with the leaflet forming a vertical
+star at night. Figures reduced.
+
+L. pubescens; and in the accompanying figures we see at A (Fig. 138) the
+leaves in their diurnal position; and at B the same plant at night with the
+two upper leaves having their leaflets almost vertical. At C another leaf,
+viewed laterally, is shown with the leaflets quite vertical. It is chiefly
+or exclusively the youngest leaves which form at night vertical stars. But
+there
+[page 343]
+is much variability in the position of the leaves at night on the same
+plant; some remaining with their leaflets almost horizontal, others forming
+more or less highly inclined or vertical stars, and some with all their
+leaflets sloping downwards, as in our first class of cases. It is also a
+remarkable fact, that although all the plants produced from the same lot of
+seeds were identical in appearance, yet some individuals at night had the
+leaflets of all their leaves arranged so as to form more or less highly
+inclined stars; others had them all sloping downwards and never forming a
+star; and others, again, retained them either in a horizontal position or
+raised them a little.
+
+We have as yet referred only to the different positions of the leaflets of
+L. pubescens at night; but the petioles likewise differ in their movements.
+That of a young leaf which formed a highly inclined star at night, stood at
+noon at 42o above the horizon, and during the night at 72o, so had risen
+30o. The petiole of another leaf, the leaflets of which occupied a similar
+position at night, rose only 6o. On the other hand, the petiole of a leaf
+with all its leaflets sloping down at night, fell at this time 4o. The
+petioles of two rather older leaves were subsequently observed; both of
+which stood during the day at exactly the same angle, viz., 50o above the
+horizon, and one of these rose 7o - 8o, and the other fell 3o - 4o at night.
+We meet with cases like that of L. pubescens with some other species. On a
+single plant of L. mutabilis some leaves, which stood horizontally during
+the day, formed highly inclined stars at night, and the petiole of one rose
+7o. Other leaves which likewise stood horizontally during the day, had at
+night all their leaflets sloping downwards at 46o beneath the horizon, but
+their petioles had hardly moved. Again, L. luteus offered a still more
+remarkable case, for on two leaves, the leaflets which stood at noon at
+about 45o above the horizon, rose at night to 65o and 69o, so that they
+formed a hollow cone with steep sides. Four leaves on the same plant, which
+had their leaflets horizontal at noon, formed vertical stars at night; and
+three other leaves equally horizontal at noon, had all their leaflets
+sloping downwards at night. So that the leaves on this one plant assumed at
+night three different positions. Though we cannot account for this fact, we
+can see that such a stock might readily give birth to species having widely
+different nyctitropic habits.
+
+Little more need be said about the sleep of the species of Lupinus;
+several, namely, L. polyphyllus, nanus, Menziesii, speciosus,
+[page 344]
+and albifrons, though observed out of doors and in the greenhouse, did not
+change the position of their leaves sufficiently at night to be said to
+sleep. From observations made on two sleeping species, it appears that, as
+with Tropaeolum majus, the leaves must be well illuminated during the day
+in order to sleep at night. For several plants, kept all day in a
+sitting-room with north-east windows, did not sleep at night; but when the
+pots were placed on the following day out of doors, and were brought in at
+night, they slept in the usual manner. the trial was repeated on the
+following day and night with the same result.
+
+Some observations were made on the circumnutation of the leaves of L.
+luteus and arboreus. It will suffice to say that the leaflets of the latter
+exhibited a double oscillation in the course of 24 h.; for they fell from
+the early morning until 10.15 A.M., then rose and zigzagged greatly till 4
+P.M., after which hour the great nocturnal fall commenced. By 8 A.M. on the
+following morning the leaflets had risen to their proper height. We have
+seen in the fourth chapter, that the leaves of Lupinus speciosus, which do
+not sleep, circumnutate to an extraordinary extent, making many ellipses in
+the course of the day.
+
+Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).--Only
+
+Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep at
+night.
+
+a few observations were made on these three genera. The petioles on a young
+plant, about a foot in height, of Cytisus fragrans rose at night, on one
+occasion 23o and on another 33o. The three leaflets also bend upwards, and
+at the same time
+[page 345]
+approach each other, so that the base of the central leaflet overlaps the
+bases of the two lateral leaflets. They bend up so much that they press
+against the stem; and on looking down on one of these young plants from
+vertically above, the lower surfaces of the leaflets are visible; and thus
+their upper surfaces, in accordance with the general rule, are best
+protected from radiation. Whilst the leaves on these young plants were thus
+behaving, those on an old bush in full flower did not sleep at night.
+
+Trigonella Cretica resembles a Melilotus in its sleep, which will be
+immediately described. According to M. Royer,* the leaves of Medicago
+maculata rise up at night, and "se renversent un peu de mani�re � presenter
+obliquement au ciel leur face inf�rieure." A drawing is here given (Fig.
+139) of the leaves of M. marina awake and asleep; and this would almost
+serve for Cytisus fragrans in the same two states.
+
+Melilotus (Tribe 3).--The species in this genus sleep in a remarkable
+manner. The three leaflets of each leaf twist through an angle of 90o, so
+that their blades stand vertically at night with one lateral edge presented
+to the zenith (Fig. 140). We shall best understand the other and more
+complicated movements, if we imagine ourselves always to hold the leaf with
+the tip of the terminal leaflet pointed to the north. The leaflets in
+becoming vertical at night could of course twist so that their upper
+surfaces should face to either side; but the two lateral leaflets always
+twist so that this surface tends to face the north, but as they move at the
+same time towards the terminal leaflet, the upper surface of the one faces
+about N.N.W., and that of the other N.N.E. The terminal leaflet behaves
+differently, for it twists to either side, the upper surface facing
+sometimes east and sometimes west, but rather more commonly west than east.
+The terminal leaflet also moves in another and more remarkable manner, for
+whilst its blade is twisting and becoming vertical, the whole leaflet bends
+to one side, and invariably to the side towards which the upper surface is
+directed; so that if this surface faces the west the whole leaflet bends to
+the west, until it comes into contact with the upper and vertical surface
+of the western lateral leaflet. Thus the upper surface of the terminal and
+of one of the two lateral leaflets is well protected.
+
+The fact of the terminal leaflet twisting indifferently to either
+
+* 'Annales des Sc. Nat. Bot.' (5th series), ix. 1868, p. 368.
+[page 346]
+
+side and afterwards bending to the same side, seemed to us so remarkable,
+that we endeavoured to discover the cause. We imagined that at the
+commencement of the movement it might be determined by one of the two
+halves of the leaflet being a little heavier than the other. Therefore bits
+of wood were gummed on one side of several leaflets, but this produced no
+effect; and they continued to twist in the same direction as
+
+Fig. 140. Melilotus officinalis: A, leaf during the daytime. B, another
+leaf asleep. C, a leaf asleep as viewed from vertically above; but in this
+case the terminal leaflet did not happen to be in such close contact with
+the lateral one, as is usual.
+
+they had previously done. In order to discover whether the same leaflet
+twisted permanently in the same direction, black threads were tied to 20
+leaves, the terminal leaflets of which twisted so that their upper surfaces
+faced west, and 14 white threads to leaflets which twisted to the east.
+These were observed occasionally during 14 days, and they all continued,
+with a single exception, to twist and bend in the same direction; for
+[page 347]
+one leaflet, which had originally faced east, was observed after 9 days to
+face west. The seat of both the twisting and bending movement is in the
+pulvinus of the sub-petioles.
+
+We believe that the leaflets, especially the two lateral ones, in
+performing the above described complicated movements generally bend a
+little downwards; but we are not sure of this, for, as far as the main
+petiole is concerned, its nocturnal movement is largely determined by the
+position which the leaf happens to occupy during the day. Thus one main
+petiole was observed to rise at night 59o, whilst three others rose only 7o
+and 9o. The petioles and sub-petioles are continually circumnutating during
+the whole 24 h., as we shall presently see.
+
+The leaves of the following 15 species, M. officinalis, suaveolens,
+parviflora, alba, infesta, dentata, gracilis, sulcata, elegans, coerulea,
+petitpierreana, macrorrhiza, Italica, secundiflora, and Taurica, sleep in
+nearly the same manner as just described; but the bending to one side of
+the terminal leaflet is apt to fail unless the plants are growing
+vigorously. With M. petitpierreana and secundiflora the terminal leaflet
+was rarely seen to bend to one side. In young plants of M. Italica it bent
+in the usual manner, but with old plants in full flower, growing in the
+same pot and observed at the same hour, viz., 8.30 P.M., none of the
+terminal leaflets on several scores of leaves had bent to one side, though
+they stood vertically; nor had the two lateral leaflets, though standing
+vertically, moved towards the terminal one. At 10.30 P.M., and again one
+hour after midnight, the terminal leaflets had become very slightly bent to
+one side, and the lateral leaflets had moved a very little towards the
+terminal one, so that the position of the leaflets even at this late hour
+was far from the ordinary one. Again, with M. Taurica the terminal leaflets
+were never seen to bend towards either of the two lateral leaflets, though
+these, whilst becoming vertical, had bent towards the terminal one. The
+sub-petiole of the terminal leaflet in this species is of unusual length,
+and if the leaflet had bent to one side, its upper surface could have come
+into contact only with the apex of either lateral leaflet; and this,
+perhaps, is the meaning of the loss of the lateral movement.
+
+The cotyledons do not sleep at night. the first leaf consists of a single
+orbicular leaflet, which twists at night so that the blade stands
+vertically. It is a remarkable fact that with M. Taurica, and in a somewhat
+less degree with M. macrorrhiza and petitpierreana, all the many small and
+young leaves produced during
+[page 348]
+the early spring from shoots on some cut-down plants in the greenhouse,
+slept in a totally different manner from the normal one; for the three
+leaflets, instead of twisting on their own axes so as to present their
+lateral edges to the zenith, turned upwards and stood vertically with their
+apices pointing to the zenith. They thus assumed nearly the same position
+as in the allied genus Trifolium; and on the same principle that
+embryological characters reveal the lines of descent in the animal kingdom,
+so the movements of the small leaves in the above three species of
+Melilotus, perhaps indicate that this genus is descended from a form which
+was closely allied to and slept like a Trifolium. Moreover, there is one
+species, M. messanensis, the leaves of which, on full-grown plants between
+2 and 3 feet in height, sleep like the foregoing small leaves and like
+those of a Trifolium. We were so much surprised at this latter case that,
+until the flowers and fruit were examined, we thought that the seeds of
+some Trifolium had been sown by mistake instead of those of a Melilotus. It
+appears therefore probable that M. messanensis has either retained or
+recovered a primordial habit.
+
+The circumnutation of a leaf of M. officinalis was traced, the stem being
+left free; and the apex of the terminal leaflet described three laterally
+extended ellipses, between 8 A.M. and 4 P.M.; after the latter hour the
+nocturnal twisting movement commenced. It was afterwards ascertained that
+the above movement was compounded of the circumnutation of the stem on a
+small scale, of the main petiole which moved most, and of the sub-petiole
+of the terminal leaflet. The main petiole of a leaf having been secured to
+a stick, close to the base of the sub-petiole of the terminal leaflet, the
+latter described two small ellipses between 10.30 A.M., and 2 P.M. At 7.15
+P.M., after this same leaflet (as well as another) had twisted themselves
+into their vertical nocturnal position, they began to rise slowly, and
+continued to do so until 10.35 P.M., after which hour they were no longer
+observed.
+
+As M. messanensis sleeps in an anomalous manner, unlike that of any other
+species in the genus, the circumnutation of a terminal leaflet, with the
+stem secured, was traced during two days. On each morning the leaflet fell,
+until about noon, and then began to rise very slowly; but on the first day
+the rising movement was interrupted between 1 and 3 P.M. by the formation
+of a laterally extended ellipse, and on the second day, at the same time,
+by two smaller ellipses. The rising movement then
+[page 349]
+recommenced, and became rapid late in the evening, when the leaflet was
+beginning to go to sleep. The awaking or sinking movement had already
+commenced by 6.45 A.M. on both mornings.
+
+Trifolium (Tribe 3).--The nyctitropic movements of 11 species were
+observed, and were found to be closely similar. If we select a leaf of T.
+repens having an upright petiole, and with the three leaflets expanded
+horizontally, the two lateral leaflets will be seen in the evening to twist
+and approach each other, until their upper surfaces come into contact. At
+the same time they bend downwards in a plane at right angles to that of
+their former position, until their midribs form an angle of about 45o with
+the upper part of the petiole. This peculiar change of position requires a
+considerable amount of torsion in the pulvinus. The terminal leaflet merely
+rises up without any twist-
+
+Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at
+night.
+
+ing and bends over until it rests on and forms a roof over the edges of the
+now vertical and united lateral leaflets. Thus the terminal leaflet always
+passes through an angle of at least 90o, generally of 130o or 140o, and not
+rarely--as was often observed with T. subterraneum--of 180o. In this latter
+case the terminal leaflet stands at night horizontally (as in Fig. 141),
+with its lower surface fully exposed to the zenith. Besides the difference
+in the angles, at which the terminal leaflets stand at night in the
+individuals of the same species, the degree to which the lateral leaflets
+approach each other often likewise differs.
+
+We have seen that the cotyledons of some species and not of others rise up
+vertically at night. The first true leaf is generally unifoliate and
+orbicular; it always rises, and either stands vertically at night or more
+commonly bends a little over so as to expose the lower surface obliquely to
+the zenith, in the same manner as does the terminal leaflet of the mature
+leaf. But it does not twist itself like the corresponding first simple leaf
+of Melilotus.
+[page 350]
+With T. Pannonicum the first true leaf was generally unifoliate, but
+sometimes trifoliate, or again partially lobed and in an intermediate
+condition.
+
+Circumnutation.--Sachs described in 1863* the spontaneous up and down
+movements of the leaflets of T. incarnatum, when kept in darkness. Pfeffer
+made many observations on the similar movements in T. pratense.** He states
+that the terminal leaflet of this species, observed at different times,
+passed through angles of from 30o to 120o in the course of from 1 � to 4 h.
+We observed the movements of T. subterraneum, resupinatum, and repens.
+
+Trifolium subterraneum.--A petiole was secured close to the base of the
+three leaflets, and the movement of the terminal leaflet was traced during
+26 � h., as shown in the figure on the next page.
+
+Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times down,
+completing 3 ellipses in 11 h. 15 m. The ascending and descending lines
+stand nearer to one another than is usual with most plants, yet there was
+some lateral motion. At 6 P.M. the great nocturnal rise commenced, and on
+the next morning the sinking of the leaflet was continued until 8.30 A.M.,
+after which hour it circumnutated in the manner just described. In the
+figure the great nocturnal rise and the morning fall are greatly
+abbreviated, from the want of space, and are merely represented by a short
+curved line. The leaflet stood horizontally when at a point a little
+beneath the middle of the diagram; so that during the daytime it oscillated
+almost equally above and beneath a horizontal position. At 8.30 A.M. it
+stood 48o beneath the horizon, and by 11.30 A.M. it had risen 50o above the
+horizon; so that it passed through 98o in 3 h. By the aid of the tracing we
+ascertained that the distance travelled in the 3 h. by the apex of this
+leaflet was 1.03 inch. If we look at the figure, and prolong upwards in our
+mind's eye the short curved broken line, which represents the nocturnal
+course, we see that the latter movement is merely an exaggeration or
+prolongation of one of the diurnal ellipses. The same leaflet had been
+observed on the previous day, and the course then pursued was almost
+identically the same as that here described.
+
+* 'Flora,' 1863, p. 497.
+
+** 'Die Period. Bewegungen,' 1875, pp. 35, 52.
+[page 351]
+
+Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic movement
+of terminal leaflet (.68 inch in length), traced from 6.45 A.M. July 4th to
+9.15 A.M. 5th. Apex of leaf 3 7/8 inches from the vertical glass, and
+movement, as here shown, magnified 5 1/4 times, reduced to one-half of
+original scale. Plant illuminated from above; temp. 16o - 17o C.
+
+Trifolium resupinatum.--A plant left entirely free was placed before a
+north-east window, in such a position that a terminal leaflet projected at
+right angles to the source of the light, the sky being uniformly clouded
+all day. The movements of this leaflet were traced during two days, and on
+both were closely similar. Those executed on the second day are shown in
+Fig. 143. The obliquity of the several lines is due partly to the manner in
+which the leaflet was viewed, and partly to its having moved a little
+towards the light. From 7.50 A.M. to 8.40 A.M. the leaflet fell, that is,
+the awakening movement was continued. It then rose and moved a little
+laterally towards the light. At 12.30 it retrograded, and at 2.30 resumed
+its original course, having thus completed a small ellipse during the
+middle of the day. In the evening it rose rapidly, and by 8 A.M. on the
+following morning had returned to exactly the same spot as on the previous
+morning. The line representing the nocturnal course ought to be extended
+much higher up, and is here abbreviated into a short,
+[page 352]
+curved, broken line. The terminal leaflet, therefore, of this species
+described during the daytime only a single additional ellipse, instead of
+two additional ones, as in the case of T. subterraneum. But we should
+remember that it was shown in the fourth chapter that the stem
+circumnutates, as no doubt does the main petiole and the sub-petioles; so
+that the movement represented in Fig. 143 is a compounded one. We tried to
+observe the movements of a leaf kept during the day in darkness, but it
+began to go to sleep after 2 h. 15 m., and this was well pronounced after 4
+h. 30 m.
+
+Fig 143. Trifolium resupinatum: circumnutation and nyctitropic movements of
+the terminal leaflet during 24 hours.
+
+Trifolium repens.--A stem was secured close to the base of a moderately old
+leaf, and the movement of the terminal leaflet was observed during two
+days. This case is interesting solely from the simplicity of the movements,
+in contrast with those of the two preceding species. On the first day the
+leaflet fell between 8 A.M. and 3 P.M., and on the second between 7 A.M.
+and 1 P.M. On both days the descending course was somewhat zigzag, and this
+evidently represents the circumnutating movement of the two previous
+species during the middle of the day. After 1 P.M., Oct. 1st (Fig. 144),
+the leaflet began to rise, but the movement was slow on both days, both
+before and after this hour, until 4 P.M. The rapid evening and nocturnal
+rise then commenced. Thus in this species the course during 24 h. consists
+of a single great ellipse; in T. resupinatum of two ellipses, one of which
+includes the nocturnal movement and is much elongated; and in T.
+subterraneum of three ellipses, of which the nocturnal one is likewise of
+great length.
+
+Securigera coronilla (Tribe 4).--The leaflets, which stand opposite one
+another and are numerous, rise up at night, come into close contact, and
+bend backwards at a moderate angle towards the base of the petiole.
+[page 353]
+
+Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of a
+nearly full-grown terminal leaflet, traced on a vertical glass from 7 A.M.
+Sept. 30th to 8 A.M. Oct. 1st. Nocturnal course, represented by curved
+broken line, much abbreviated.
+
+Lotus (Tribe 4).--The nyctitropic movements of 10 species in this genus
+were observed, and found to be the same. The main petiole rises a little at
+night, and the three leaflets rise till they become vertical, and at the
+same time approach each other. This was conspicuous with L. Jacoboeus, in
+which the leaflets are almost linear. In most of the species the leaflets
+rise so much as to press against the stem, and not rarely they become
+inclined a little inwards with their lower surfaces exposed obliquely to
+the zenith. This was clearly the case with L. major, as its petioles are
+unusually long, and the leaflets are thus enabled to bend further inwards.
+The young leaves on the summits of the stems close up at night so much, as
+often to resemble large buds. The stipule-like leaflets, which are often of
+large size, rise up like the other leaflets, and press against the stem
+(Fig. 145). All the leaflets of L. Gebelii, and probably of the other
+species, are provided at their bases with distinct pulvini, of a yellowish
+colour, and formed of very small cells. The circumnutation of a terminal
+leaflet of L. peregrinus (with the stem secured) was traced during two
+days, but the movement was so simple that it is not worth while to give the
+diagram. The leaflet fell slowly from the early morning till about 1 P.M.
+It then rose gradually at first, but rapidly late in the evening. It
+occasionally stood still for about 20 m. during the day, and sometimes
+zigzagged a little. The movement of one of the basal, stipule-like leaflets
+was likewise traced in the same manner and at the same time, and its course
+was closely similar to that of the terminal leaflet.
+
+In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12
+genera have been observed by ourselves and
+[page 354]
+others, but only in Robinia with any care. Psoralea acaulis raises its
+three leaflets at night; whilst Amorpha fruticosa,* Dalea alopecuroides,
+and Indigofera tinctoria depress them. Ducharte** states that Tephrosia
+caribaea is the sole example of "folioles couch�es le long du p�tiole et
+vers la base;" but a
+
+Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, with
+leaves asleep at night. SS, stipule-like leaflets.
+
+similar movement occurs, as we have already seen, and shall again see in
+other cases. Wistaria Sinensis, according to Royer,*** "abaisse les
+folioles qui par une disposition bizarre sont inclin�es dans la m�me
+feuille, les sup�rieures vers le
+
+* Ducharte, 'El�ments de Botanique', 1867, p. 349.
+
+** Ibid., p. 347.
+
+*** 'Ann. des Sciences Nats. Bot.' (5th series), ix. 1868.
+[page 355]
+
+sommet, les inf�rieures vers la base du petiole commun;" but the leaflets
+on a young plant observed by us in the greenhouse merely sank vertically
+downwards at night. The leaflets are raised in Sphaerophysa salsola,
+Colutea arborea, and Astragalus uliginosus, but are depressed, according to
+Linnaeus, in Glycyrrhiza. The leaflets of Robinia pseudo-acacia likewise
+sink vertically down at night, but the petioles rise a little, viz., in one
+case 3o, and in another 4o. The circumnutating movements of a terminal
+leaflet on a rather old leaf were traced during two days, and were simple.
+The leaflet fell slowly, in a slightly zigzag line, from 8 A.M. to 5 P.M.,
+and then more rapidly; by 7 A.M. on the following morning it had risen to
+its diurnal position. There was only one peculiarity in the movement,
+namely, that on both days there was a distinct though small oscillation up
+and down between 8.30 and 10 A.M., and this would probably have been more
+strongly pronounced if the leaf had been younger.
+
+Coronilla rosea (Tribe 6).--the leaves bear 9 or 10 pairs of opposite
+leaflets, which during the day stand horizontally, with
+
+Fig. 146. Coronilla rosea: leaf asleep.
+
+their midribs at right angles to the petiole. At night they rise up so that
+the opposite leaflets come nearly into contact, and those on the younger
+leaves into close contact. At the same time they bend back towards the base
+of the petiole, until their midribs form with it angles of from 40o to 50o
+in a vertical plane, as here figured (Fig. 146). The leaflets, however,
+sometimes bend so much back that their midribs become parallel to and lie
+on the petiole. They thus occupy a reversed position to what they do in
+several Leguminosae, for instance, in Mimosa
+[page 356]
+pudica; but, from standing further apart, they do not overlap one another
+nearly so much as in this latter plant. The main petiole is curved slightly
+downwards during the day, but straightens itself at night. In three cases
+it rose from 3o above the horizon at noon, to 9o at 10 P.M.; from 11o to
+33o; and from 5o to 33o--the amount of angular movement in this latter case
+amounting to 28o. In several other species of Coronilla the leaflets showed
+only feeble movements of a similar kind.
+
+Hedysarum coronarium (Tribe 6).--The small lateral leaflets on plants
+growing out of doors rose up vertically at night, but the large terminal
+one became only moderately inclined. The petioles apparently did not rise
+at all.
+
+Smithia Pfundii (Tribe 6).--The leaflets rise up vertically, and the main
+petiole also rises considerably.
+
+Arachis hypogoea (Tribe 6).--The shape of a leaf, with its two pairs of
+leaflets, is shown at A (Fig. 147); and a leaf asleep,
+
+Fig. 147. Arachis hypogoea: A, leaf during the day, seen from vertically
+above; B, leaf asleep, seen laterally, copied from a photograph. Figures
+much reduced.
+
+traced from a photograph (made by the aid of aluminium light), is given at
+B. The two terminal leaflets twist round at night until their blades stand
+vertically, and approach each other until they meet, at the same time
+moving a little upwards and backwards. The two lateral leaflets meet each
+other in this same manner, but move to a greater extent forwards, that is,
+in a contrary direction to the two terminal leaflets, which they partially
+embrace. Thus all four leaflets form together a single packet, with their
+edges directed to the zenith, and with their lower surfaces turned
+outwards. On a plant which was not growing vigorously the closed leaflets
+seemed too heavy for the
+[page 357]
+petioles to support them in a vertical position, so that each night the
+main petiole became twisted, and all the packets were extended
+horizontally, with the lower surfaces of the leaflets on one side directed
+to the zenith in a most anomalous manner. This fact is mentioned solely as
+a caution, as it surprised us greatly, until we discovered that it was an
+anomaly. The petioles are inclined upwards during the day, but sink at
+night, so as to stand at about right angles with the stem. The amount of
+sinking was measured only on one occasion, and found to be 39o. A petiole
+was secured to a stick at the base of the two terminal leaflets, and the
+circumnutating movement of one of these leaflets was traced from 6.40 A.M.
+to 10.40 P.M., the plant being illuminated from above. The temperature was
+17o - 17 1/2o C., and therefore rather too low. During the 16 h. the
+leaflet moved thrice up and thrice down, and as the ascending and
+descending lines did not coincide, three ellipses were formed.
+
+Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half
+natural size. The minute stipules unusually large.
+
+Desmodium gyrans (Tribe 6).--A large and full-grown leaf of this plant, so
+famous for the spontaneous movements of the two little lateral leaflets, is
+here represented (Fig. 148). The large terminal leaflet sleeps by sinking
+vertically down, whilst the petiole rises up. The cotyledons do not sleep,
+but the first-formed leaf sleeps equally well as the older ones. The
+appearance presented by a sleeping branch and one in the day-time, copied
+from two photographs, are shown at A and B (Fig. 149), and we see how at
+night the leaves are crowded together, as if for mutual protection, by the
+rising of the petioles. The petioles of the younger leaves near the summits
+of the shoots rise up at night, so as to stand vertical and parallel to the
+stem; whilst those on the sides were found in four cases to have risen
+respectively 46 1/2o, 36o, 20o, and 19.5o above the inclined positions which
+they had occupied during the day. For instance, in the first of these four
+cases the petiole stood in the day at 23o, and at night at 69 1/2o above
+the horizon. In the evening the rising of the petioles is almost completed
+before the leaflets sink perpendicularly downwards.
+[page 358]
+
+Circumnutation.--The circumnutating movements of four young shoots were
+observed during 5 h. 15 m.; and in this time each completed an oval figure
+of small size. The main petiole also circumnutates rapidly, for in the
+course of 31 m. (temp. 91o F.) it changed its course by as much as a
+rectangle six times, describing a figure which apparently represented two
+ellipses.
+
+Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves
+asleep. Figures reduced.
+
+The movement of the terminal leaflet by means of its sub-petiole or
+pulvinus is quite as rapid, or even more so, than that of the main petiole,
+and has much greater amplitude. Pfeffer has seen* these leaflets move
+through an angle of 8o in the course of from 10 to 30 seconds.
+
+A fine, nearly full-grown leaf on a young plant, 8 inches in height, with
+the stem secured to a stick at the base of the leaf, was observed from 8.30
+A.M. June 22nd to 8 A.M. June 24th.
+
+* 'Die Period. Beweg.,' p. 35.
+[page 359]
+
+In the diagram given on the next page (Fig. 150), the two curved broken
+lines at the base, which represent the nocturnal courses, ought to be
+prolonged far downwards. On the first day the leaflet moved thrice down and
+thrice up, and to a considerable distance laterally; the course was also
+remarkably crooked. The dots were generally made every hour; if they had
+been made every few minutes all the lines would have been zigzag to an
+extraordinary degree, with here and there a loop formed. We may infer that
+this would have been the case, because five dots were made in the course of
+31 m. (between 12.34 and 1.5 P.M.), and we see in the upper part of the
+diagram how crooked the course here is; if only the first and last dots had
+been joined we should have had a straight line. Exactly the same fact may
+be seen in the lines representing the course between 2.24 P.M. and 3 P.M.,
+when six intermediate dots were made; and again at 4.46 and 4.50. But the
+result was widely different after 6 P.M.,--that is, after the great
+nocturnal descent had commenced; for though nine dots were then made in the
+course of 32 m., when these were joined (see Figure) the line thus formed
+was almost straight. The leaflets, therefore, begin to descend in the
+afternoon by zigzag lines, but as soon as the descent becomes rapid their
+whole energy is expended in thus moving, and their course becomes
+rectilinear. After the leaflets are completely asleep they move very little
+or not at all.
+
+Had the above plant been subjected to a higher temperature than 67o - 70o
+F., the movements of the terminal leaflet would probably have been even
+more rapid and wider in extent than those shown in the diagram; for a plant
+was kept for some time in the hot-house at from 92o - 93o F., and in the
+course of 35 m. the apex of a leaflet twice descended and once ascended,
+travelling over a space of 1.2 inch in a vertical direction and of .82 inch
+in a horizontal direction. Whilst thus moving the leaflet also rotated on
+its own axis (and this was a point to which no attention had been before
+paid), for the plane of the blade differed by 41o after an interval of only
+a few minutes. Occasionally the leaflet stood still for a short time. There
+was no jerking movement, which is so characteristic of the little lateral
+leaflets. A sudden and considerable fall of temperature causes the terminal
+leaflet to sink downwards; thus a cut-off leaf was immersed in water at 95o
+F., which was slowly raised to 103o F., and afterwards allowed to sink to
+70o F., and the sub-petiole of the terminal leaflet then curved downwards.
+The water was afterwards
+[page 360]
+
+Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of leaf
+(3 3/4 inches in length, petiole included) during 48 h. Filament affixed to
+midrib of terminal leaflet; its apex 6 inches from the vertical glass.
+Diagram reduced to one-third of original scale. Plant illuminated from
+above. Temp. 19o - 20o C.
+[page 361]
+
+raised to 120o F., and the sub-petiole straightened itself. Similar
+experiments with leaves in water were twice repeated, with nearly the same
+result. It should be added, that water raised to even 122o F. does not soon
+kill a leaf. A plant was placed in darkness at 8.37 A.M., and at 2 P.M.
+(i.e. after 5 h. 23 m.), though the leaflets had sunk considerably, they
+had by no means acquired their nocturnal vertically dependent position.
+Pfeffer, on the other hand, says* that this occurred with him in from 3/4
+h. to 2 h.; perhaps the difference in our results may be due to the plant
+on which we experimented being a very young and vigorous seedling.
+
+The Movements of the little Lateral Leaflets .--These have been so often
+described, that we will endeavour to be as brief as possible in giving a
+few new facts and conclusions. The leaflets sometimes quickly change their
+position by as much as nearly 180o; and their sub-petioles can then be seen
+to become greatly curved. They rotate on their own axes, so that their
+upper surfaces are directed to all points of the compass. The figure
+described by the apex is an irregular oval or ellipse. They sometimes
+remain stationary for a period. In these several respects there is no
+difference, except in rapidity and extent, between their movements and the
+lesser ones performed by the large terminal leaflet whilst making its great
+oscillations. The movements of the little leaflets are much influenced, as
+is well known, by temperature. This was clearly shown by immersing leaves
+with motionless leaflets in cold water, which was slowly raised to 103o F.,
+and the leaflets then moved quickly, describing about a dozen little
+irregular circles in 40 m. By this time the water had become much cooler,
+and the movements became slower or almost ceased; it was then raised to
+100o F., and the leaflets again began to move quickly. On another occasion
+a tuft of fine leaves was immersed in water at 53o F., and the leaflets
+were of course motionless. The water was raised to 99o, and the leaflets
+soon began to move; it was raised to 105o, and the movements became much
+more rapid; each little circle or oval being completed in from 1 m. 30 s.
+to 1 m. 45 s. There was, however, no jerking, and this fact may perhaps be
+attributed to the resistance of the water.
+
+Sachs states that the leaflets do not move until the surrounding air is as
+high as 71o - 72o F., and this agrees with our
+
+* 'Die Period. Beweg.,' p. 39.
+[page 362]
+
+experience on full-grown, or nearly full-grown, plants. But the leaflets of
+young seedlings exhibit a jerking movement at much lower temperatures. A
+seedling was kept (April 16th) in a room for half the day where the
+temperature was steady at 64o F., and the one leaflet which it bore was
+continually jerking, but not so rapidly as in the hot-house. The pot was
+taken in the evening into a bed-room where the temperature remained at 62o
+during nearly the whole night; at 10 and 11 P.M. and at 1 A.M. the leaflet
+was still jerking rapidly; at 3.30 A.M. it was not seen to jerk, but was
+observed during only a short time. It was, however, now inclined at a much
+lower angle than that occupied at 1 A.M. At 6.30 A.M. (temp. 61o F.) its
+inclination was still less than before, and again less at 6.45 A.M.; by
+7.40 A.M. it had risen, and at 8.30 A.M. was again seen to jerk. This
+leaflet, therefore, was moving during the whole night, and the movement was
+by jerks up to 1 A.M. (and possibly later) and again at 8.30 A.M., though
+the temperature was only 61o to 62o F. We must therefore conclude that the
+lateral leaflets produced by young plants differ somewhat in constitution
+from those on older plants.
+
+In the large genus Desmodium by far the greater number of the species are
+trifoliate; but some are unifoliate, and even the same plant may bear uni-
+and trifoliate leaves. In most of the species the lateral leaflets are only
+a little smaller than the terminal one. Therefore the lateral leaflets of
+D. gyrans (see Fig. 148) must be considered as almost rudimentary. They are
+also rudimentary in function, if this expression may be used; for they
+certainly do not sleep like the full-sized terminal leaflets. It is,
+however, possible that the sinking down of the leaflets between 1 A.M. and
+6.45 A.M., as above described, may represent sleep. It is well known that
+the leaflets go on jerking during the early part of the night; but my
+gardener observed (Oct. 13th) a plant in the hot-house between 5 and 5.30
+A.M., the temperature having been kept up to 82o F., and found that all the
+leaflets were inclined, but he saw no jerking movement until 6.55 A.M., by
+which time the terminal leaflet had risen and was awake. Two days
+afterwards (Oct. 15th) the same plant was observed by him at 4.47 A.M.
+(temp. 77o F.), and he found that the large terminal leaflets were awake,
+though not quite horizontal; and the only cause which we could assign for
+this anomalous wakefulness was that the plant had been kept for
+experimental purposes during
+[page 363]
+the previous day at an unusually high temperature; the little lateral
+leaflets were also jerking at this hour, but whether there was any
+connection between this latter fact and the sub-horizontal position of the
+terminal leaflets we do not know. Anyhow, it is certain that the lateral
+leaflets do not sleep like the terminal leaflets; and in so far they may be
+said to be in a functionally rudimentary condition. They are in a similar
+condition in relation to irritability; for if a plant be shaken or
+syringed, the terminal leaflets sink down to about 45o beneath the horizon;
+but we could never detect any effect thus produced on the lateral leaflets;
+yet we are not prepared to assert positively that rubbing or pricking the
+pulvinus produces no effect.
+
+As in the case of most rudimentary organs, the leaflets are variable in
+size; they often depart from their normal position and do not stand
+opposite one another; and one of the two is frequently absent. This absence
+appeared in some, but not in all the cases, to be due to the leaflet having
+become completely confluent with the main petiole, as might be inferred
+from the presence of a slight ridge along its upper margin, and from the
+course of the vessels. In one instance there was a vestige of the leaflet,
+in the shape of a minute point, at the further end of the ridge. The
+frequent, sudden and complete disappearance of one or both of the
+rudimentary leaflets is a rather singular fact; but it is a much more
+surprising one that the leaves which are first developed on seedling plants
+are not provided with them. Thus, on one seedling the seventh leaf above
+the cotyledons was the first which bore any lateral leaflets, and then only
+a single one. On another seedling, the eleventh leaf first bore a leaflet;
+of the nine succeeding leaves five bore a single lateral leaflet, and four
+bore none at all; at last a leaf, the twenty-first above the cotyledons,
+was provided with two rudimentary lateral leaflets. From a widespread
+analogy in the animal kingdom, it might have been expected that these
+rudimentary leaflets would have been better developed and more regularly
+present on very young than on older plants. But bearing in mind, firstly,
+that long-lost characters sometimes reappear late in life, and secondly,
+that the species of Desmodium are generally trifoliate, but that some are
+unifoliate, the suspicion arises that D. gyrans is descended from a
+unifoliate species, and that this was descended from a trifoliate one; for
+in this case both the absence of the little lateral leaflets on very young
+seedlings, and their sub-
+[page 364]
+sequent appearance, may be attributed to reversion to more or less distant
+progenitors.*
+
+No one supposes that the rapid movements of the lateral leaflets of 'D.
+gyrans' are of any use to the plant; and why they should behave in this
+manner is quite unknown. We imagined that their power of movement might
+stand in some relation with their rudimentary condition, and therefore
+observed the almost rudimentary leaflets of Mimosa albida vel sensitiva (of
+which a drawing will hereafter be given, Fig. 159); but they exhibited no
+extraordinary movements, and at night they went to sleep like the
+full-sized leaflets. There is, however, this remarkable difference in the
+two cases; in Desmodium the pulvinus of the rudimentary leaflets has not
+been reduced in length, in correspondence with the reduction of the blade,
+to the same extent as has occurred in the Mimosa; and it is on the length
+and degree of curvature of the pulvinus that the amount of movement of the
+blade depends. Thus the average length of the pulvinus in the large
+terminal leaflets of Desmodium is 3 mm., whilst that of the rudimentary
+leaflets is 2.86 mm.; so that they differ only a little in length. But in
+diameter they differ much, that of the pulvinus of the little leaflets
+being only 0.3 mm. to 0.4 mm.; whilst that of the terminal leaflets is 1.33
+mm. If we now turn to the Mimosa, we find that the average length of the
+pulvinus of the almost rudimentary leaflets is only 0.466 mm., or rather
+more than a quarter of the length of the pulvinus of the full-sized
+leaflets, namely, 1.66 mm. In this small reduction in length of the
+pulvinus of the rudimentary leaflets of Desmodium, we apparently have the
+proximate cause of their great and rapid circumnutating movement, in
+contrast with that of the almost rudimentary leaflets of the Mimosa. The
+small size and weight of the blade, and the little resistance opposed by
+the air to its movement, no doubt also come into play; for we have seen
+that these leaflets if immersed in water, when the resistance would be much
+greater, were prevented from jerking forwards. Why, during the reduction of
+the lateral leaflets of Desmodium, or during their reappearance--if they
+owe their origin to reversion--the pulvinus should have been so much less
+affected than the blade, whilst with the
+
+* Desmodium vespertilionis is closely allied to D. gyrans, and it seems
+only occasionally to bear rudimentary lateral leaflets. Duchartre,
+'El�ments de Botanique,' 1867, p. 353.
+[page 365]
+
+Mimosa the pulvinus has been greatly reduced, we do not know. Nevertheless,
+it deserves notice that the reduction of the leaflets in these two genera
+has apparently been effected by a different process and for a different
+end; for with the Mimosa the reduction of the inner and basal leaflets was
+necessary from the want of space; but no such necessity exists with
+Desmodium, and the reduction of its lateral leaflets seems to have been due
+to the principle of compensation, in consequence of the great size of the
+terminal leaflet.
+Uraria (Tribe 6) and Centrosema (Tribe 8).--The leaflets of Uraria lagopus
+and the leaves of a Centrosema from Brazil both sink vertically down at
+night. In the latter plant the petiole at the same time rose 16 1/2o.
+
+Amphicarpoea monoica (Tribe 8).--The leaflets sink down vertically at
+night, and the petioles likewise fall considerably.
+
+Fig. 151. Amphicarpoea monoica: circumnutation and nyctitropic movement of
+leaf during 48 h.; its apex 9 inches from the vertical glass. Figure
+reduced to one-third of original scale. Plant illuminated from above; temp
+17 1/2o - 18 1/2o C.
+
+A petiole, which was carefully observed, stood during the day 25o above the
+horizon and at night 32o below it; it therefore fell 57o. A filament was
+fixed transversely across the terminal leaflet of a fine young leaf (2 1/4
+inches in length including the
+[page 366]
+petiole), and the movement of the whole leaf was traced on a vertical
+glass. This was a bad plan in some respects, because the rotation of the
+leaflet, independently of its rising or falling, raised and depressed the
+filament; but it was the best plan for our special purpose of observing
+whether the leaf moved much after it had gone to sleep. The plant had
+twined closely round a thin stick, so that the circumnutation of the stem
+was prevented. The movement of the leaf was traced during 48 h., from 9
+A.M. July 10th to 9 A.M. July 12th.  In the figure given (Fig. 151) we see
+how complicated its course was on both days: during the second day it
+changed its course greatly 13 times. The leaflets began to go to sleep a
+little after 6 P.M., and by 7.15 P.M. hung vertically down and were
+completely asleep; but on both nights they continued to move from 7.15 P.M.
+to 10.40 and 10.50 P.M., quite as much as during the day; and this was the
+point which we wished to ascertain. We see in the figure that the great
+sinking movement late in the evening does not differ essentially from the
+circumnutation during the day.
+
+Glycine hispida (Tribe 8).--The three leaflets sink vertically down at
+night.
+
+Erythrina (Tribe 8).--Five species were observed, and the leaflets of all
+sank vertically down at night; with E. caffra and with a second unnamed
+species, the petioles at the same time rose slightly. The movements of the
+terminal leaflet of E. crista-galli (with the main petiole secured to a
+stick) were traced from 6.40 A.M. June 8th, to 8 A.M. on the 10th. In order
+to observe the nyctitropic movements of this plant, it is necessary that it
+should have grown in a warm greenhouse, for out of doors in our climate it
+does not sleep. We see in the tracing (Fig. 152) that the leaflet
+oscillated twice up and down between early morning and noon; it then fell
+greatly, afterwards rising till 3 P.M. At this latter hour the great
+nocturnal fall commenced. On the second day (of which the tracing is not
+given) there was exactly the same double oscillation before noon, but only
+a very small one in the afternoon. On the third morning the leaflet moved
+laterally, which was due to its beginning to assume an oblique position, as
+seems invariably to occur with the leaflets of this species as they grow
+old. On both nights after the leaflets were asleep and hung vertically
+down, they continued to move a little both up and down, and from side to
+side.
+
+Erythrina caffra.--A filament was fixed transversely across
+[page 367]
+
+a terminal leaflet, as we wished to observe its movements when asleep. The
+plant was placed in the morning of June 10th under a skylight, where the
+light was not bright; and we do not know whether it was owing to this cause
+or to the plant having been disturbed, but the leaflet hung vertically down
+all day; nevertheless it circumnutated in this position, describing a
+figure which represented two irregular ellipses. On the next day it
+circumnutated in a greater degree, describing four irregular ellipses, and
+by 3 P.M. had risen into a horizontal position. By 7.15 P.M. it was asleep
+and vertically dependent, but continued to circumnutate as long as
+observed, until 11 P.M.
+
+Fig. 152. Erythrina crista-galli: circumnutation and nyctitropic movement
+of terminal leaflet, 3 3/4 inches in length, traced during 25 h.; apex of
+leaf 3 � inches from the vertical glass. Figure reduced to one-half of
+original scale. Plant illuminated from above; temp. 17 1/2o - 18 1/2o C.
+
+Erythrina corallodendron.--The movements of a terminal leaflet were traced.
+During the second day it oscillated four times up and four times down
+between 8 A.M. and 4 P.M., after which hour the great nocturnal fall
+commenced. On the third day the movement was equally great in amplitude,
+but was remarkably simple, for the leaflet rose in an almost perfectly
+straight line from 6.50 A.M. to 3 P.M., and then sank down in an equally
+straight line until vertically dependent and asleep.
+[page 368]
+
+Apios tuberosa (Tribe 8).--The leaflets sink vertically down at night.
+
+Phaseolus vulgaris (Tribe 8).--The leaflets likewise sink vertically down
+at night. In the greenhouse the petiole of a young leaf rose 16o, and that
+of an older leaf 10o at night. With plants growing out of doors the
+leaflets apparently do not sleep until somewhat late in the season, for on
+the nights of July 11th and 12th none of them were asleep; whereas on the
+night of August 15th the same plants had most of their leaflets vertically
+dependent and asleep. With Ph. caracalla and Hernandesii, the primary
+unifoliate leaves and the leaflets of the secondary trifoliate leaves sink
+vertically down at night. This holds good with the secondary trifoliate
+leaves of Ph. Roxburghii, but it is remarkable that the primary unifoliate
+leaves which are much elongated, rise at night from about 20o to about 60o
+above the horizon. With older seedlings, however, having the secondary
+leaves just developed, the primary leaves stand in the middle of the day
+horizontally, or are deflected a little beneath the horizon. In one such
+case the primary leaves rose from 26o beneath the horizon at noon, to 20o
+above it at 10 P.M.; whilst at this same hour the leaflets of the secondary
+leaves were vertically dependent. Here, then, we have the extraordinary
+case of the primary and secondary leaves on the same plant moving at the
+same time in opposite directions.
+
+We have now seen that the leaflets in the six genera of Phaseoleae observed
+by us (with the exception of the primary leaves of Phaseolus Roxburghii)
+all sleep in the same manner, namely, by sinking vertically down. The
+movements of the petioles were observed in only three of these genera. They
+rose in Centrosema and Phaseolus, and sunk in Amphicarpaea.
+
+Sophora chrysophylla (Tribe 10).--The leaflets rise at night, and are at
+the same time directed towards the apex of the leaf, as in Mimosa pudica.
+
+Caesalpinia, Hoematoxylon, Gleditschia, Poinciana.--The leaflets of two
+species of Caesalpinia (Tribe 13) rose at night. With Haematoxylon
+Campechianum (Tribe 13) the leaflets move forwards at night, so that their
+midribs stand parallel to the petiole, and their now vertical lower
+surfaces are turned outwards (Fig. 153). The petiole sinks a little. In
+Gleditschia, if we understand correctly Duchartre's description, and in
+Poin-
+[page 369]
+ciana Gilliesii (both belonging to Tribe 13), the leaves behave in the same
+manner.
+
+Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B, branch
+with leaves asleep, reduced to two-thirds of natural scale.
+
+Cassia (Tribe 14).--The nyctitropic movements of the leaves in many species
+in this genus are closely alike, and are highly complex. They were first
+briefly described by Linnaeus, and since by Duchartre. Our observations
+were made chiefly on C. floribunda* and corymbosa, but several other
+species were casually observed. The horizontally extended leaflets sink
+down vertically at night; but not simply, as in so many other genera, for
+each leaflet rotates on its own axis, so that its lower surface faces
+outwards. The upper surfaces of the opposite leaflets are thus brought into
+contact with one another beneath the petiole, and are well protected (Fig.
+154). The rotation and other movements are effected by means of a
+well-developed pulvinus at the base of each leaflet, as could be plainly
+seen when a straight narrow black line had been painted along it during the
+day. The two terminal leaflets in the daytime include rather less than a
+right angle; but their divergence increases greatly whilst they
+
+* I am informed by Mr. Dyer that Mr. Bentham believes that C. floribunda (a
+common greenhouse bush) is a hybrid raised in France, and that it comes
+very near to C. laevigata. It is no doubt the same as the form described by
+Lindley ('Bot. Reg.,' Tab. 1422) as C. Herbertiana.
+[page 370]
+
+sink downwards and rotate, so that they stand laterally at night, as may be
+seen in the figure. Moreover, they move somewhat backwards, so as to point
+towards the base of the petiole.
+
+Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at night.
+Both figures copied from photographs.
+
+In one instance we found that the midrib of a terminal leaflet formed at
+night an angle of 36o, with a line dropped
+[page 371]
+perpendicularly from the end of the petiole. The second pair of leaflets
+likewise moves a little backwards, but less than the terminal pair; and the
+third pair moves vertically downwards, or even a little forwards. Thus all
+the leaflets, in those species which bear only 3 or 4 pairs, tend to form a
+single packet, with their upper surfaces in contact, and their lower
+surfaces turned outwards. Lastly, the main petiole rises at night, but with
+leaves of different ages to very different degrees, namely some rose
+through an angle of only 12o, and others as much as 41o.
+
+Cassia calliantha.--The leaves bear a large number of leaflets, which move
+at night in nearly the same manner as just described; but the petioles
+apparently do not rise, and one which was carefully observed certainly fell
+3o.
+Cassia pubescens.--The chief difference in the nyctitropic
+
+Fig. 155. Cassia pubescens: A, upper part of plant during the day; B, same
+plant at night. Figures reduced from photographs.
+
+movements of this species, compared with those of the former species,
+consists in the leaflets not rotating nearly so much;
+[page 372]
+therefore their lower surfaces face but little outwards at night. The
+petioles, which during the day are inclined only a little above the
+horizon, rise at night in a remarkable manner, and stand nearly or quite
+vertically. This, together with the dependent position of the leaflets,
+makes the whole plant wonderfully compact at night. In the two foregoing
+figures, copied from photographs, the same plant is represented awake and
+asleep (Fig. 155), and we see how different is its appearance.
+
+Cassia mimosoides.--At night the numerous leaflets on each leaf rotate on
+their axes, and their tips move towards the apex of the leaf; they thus
+become imbricated with their lower surfaces directed upwards, and with
+their midribs almost parallel to the petiole. Consequently, this species
+differs from all the others seen by us, with the exception of the following
+one, in the leaflets not sinking down at night. A petiole, the movement of
+which was measured, rose 8o at night.
+
+Cassia Barclayana.--The leaflets of this Australian species are numerous,
+very narrow, and almost linear. At night they rise up a little, and also
+move towards the apex of the leaf. For instance, two opposite leaflets
+which diverged from one another during the day at an angle of 104o,
+diverted at night only 72o; so that each had risen 16o above its diurnal
+position. The petiole of a young leaf rose at night 34o, and that of an
+older leaf 19o. Owing to the slight movement of the leaflets and the
+considerable movement of the petiole, the bush presents a different
+appearance at night to what it does by day; yet the leaves can hardly be
+said to sleep.
+
+The circumnutating movements of the leaves of C. floribunda, calliantha,
+and pubescens were observed, each during three or four days; they were
+essentially alike, those of the last-named species being the simplest. The
+petiole of C. floribunda was secured to a stick at the base of the two
+terminal leaflets, and a filament was fixed along the midrib of one of
+them. Its movements were traced from 1 P.M. on August 13th to 8.30 A.M.
+17th; but those during the last 2 h. are alone given in Fig. 156. From 8
+A.M. on each day (by which hour the leaf had assumed its diurnal position)
+to 2 or 3 P.M., it either zigzagged or circumnutated over nearly the same
+small space; at between 2 and 3 P.M. the great evening fall commenced. The
+lines representing this fall and the early morning rise are oblique, owing
+to the peculiar manner in which the leaflets sleep, as already described.
+After the leaflet was asleep at 6 P.M., and whilst the glass filament hung
+[page 373]
+perpendicularly down, the movement of its apex was traced until 10.30 P.M.;
+and during this whole time it swayed from side to side, completing more
+than one ellipse.
+
+Fig 156. Cassia floribunda: circumnutation and nyctitropic movement of a
+terminal leaflet (1 5/6 inch in length) traced from 8.30 A.M. to same hour
+on following morning. Apex of leaflet 5 � inches from the vertical glass.
+Main petiole 3 3/4 inches long. Temp. 16o - 17 1/2o C. Figure reduced to
+one-half of the original scale.
+
+Bauhinia (Tribe 15).--The nyctitropic movements of four species were alike,
+and were highly peculiar. A plant raised from seed sent us from South
+Brazil by Fritz M�ller, was more especially observed. The leaves are large
+and deeply notched at their ends. At night the two halves rise up and close
+completely together, like the opposite leaflets of many Leguminosae. With
+very young plants the petioles rise considerably at the same time; one,
+which was inclined at noon 45o above the horizon, at night stood at 75o; it
+thus rose 30o; another rose 34o. Whilst the two halves of the leaf are
+closing, the midrib at first sinks vertically downwards and afterwards
+bends backwards, so as to pass close along one side of its own upwardly
+inclined petiole; the midrib being thus directed towards the stem or axis
+of the plant. The angle which the midrib formed with the horizon was
+measured in one case at different hours: at noon it stood horizontally;
+late in the evening it depended vertically; then rose to the opposite side,
+and at 10.15 P.M. stood at only 27o beneath the horizon, being directed
+towards the stem. It had thus travelled through 153o.
+[page 374]
+Owing to this movement--to the leaves being folded--and to the petioles
+rising, the whole plant is as much more compact at night than during the
+day, as a fastigiate Lombardy poplar is compared with any other species of
+poplar. It is remarkable that when our plants had grown a little older,
+viz., to a height of 2 or 3 feet, the petioles did not rise at night, and
+the midribs of the folded leaves were no longer bent back along one side of
+the petiole. We have noticed in some other genera that the petioles of very
+young plants rise much more at night than do those of older plants.
+
+Tamarindus Indica (Tribe 16).--The leaflets approach or meet each other at
+night, and are all directed towards the apex of the leaf. They thus become
+imbricated with their midribs parallel to the petiole. The movement is
+closely similar to that of Haematoxylon (see Fig. 153), but more striking
+from the greater number of the leaflets.
+
+Adenanthera, Prosopis, and Neptunia (Tribe 20).--With Adenanthera pavonia
+the leaflets turn edgeways and sink at night. In Prosopis they turn
+upwards. With Neptunia oleracea the leaflets on the opposite sides of the
+same pinna come into contact at night and are directed forwards. The pinnae
+themselves move downwards, and at the same time backwards or towards the
+stem of the plant. The main petiole rises.
+
+Mimosa pudica (Tribe 20).--This plant has been the subject of innumerable
+observations; but there are some points in relation to our subject which
+have not been sufficiently attended to. At night, as is well known, the
+opposite leaflets come into contact and point towards the apex of the leaf;
+they thus become neatly imbricated with their upper surfaces protected. The
+four pinnae also approach each other closely, and the whole leaf is thus
+rendered very compact. The main petiole sinks downwards during the day till
+late in the evening, and rises until very early in the morning. The stem is
+continually circumnutating at a rapid rate, though not to a wide extent.
+Some very young plants, kept in darkness, were observed during two days,
+and although subjected to a rather low temperature of 57o - 59o F., the
+stem of one described four small ellipses in the course of 12 h. We shall
+immediately see that the main petiole is likewise continually
+circumnutating, as is each separate pinna and each separate leaflet.
+Therefore, if the movement of the apex of any one leaflet were to be
+traced, the course described would be compounded of the movements of four
+separate parts.
+[page 375]
+A filament had been fixed on the previous evening, longitudinally to the
+main petiole of a nearly full-grown, highly-sensitive leaf (four inches in
+length), the stem having been secured to a stick at its base; and a tracing
+was made on a vertical glass in the hot-house under a high temperature. In
+the figure given (Fig. 157), the first dot was made at 8.30 A.M. August
+2nd, and the last at 7 P.M. on the 3rd. During 12 h. on the first day the
+petiole moved thrice downwards and twice upwards. Within the same length of
+time on the second day, it moved five times downwards and four times
+upwards. As the ascending and descending lines do not coincide, the petiole
+manifestly circumnutates; the great evening fall and nocturnal rise being
+an exaggeration of one of the circumnutations. It should, however, be
+observed that the petiole fell much lower down in the evenings than could
+be seen on the vertical glass or is represented in the diagram. After 7
+P.M. on the 3rd (when the last dot in Fig. 157 was made) the pot was
+carried into a bed-room, and the petiole was found at 12.50 A.M. (i.e.
+after midnight) standing almost upright, and much more highly inclined than
+it was at 10.40 P.M. When observed again at 4 A.M. it had begun to fall,
+and continued falling till 6.15 A.M., after which hour it zigzagged and
+again circumnutated. Similar observations were made on another petiole,
+with nearly the same result.
+
+Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of main
+petiole, traced during 34 h. 30 m.
+
+On two other occasions the movement of the main petiole
+[page 376]
+was observed every two or three minutes, the plants being kept at a rather
+high temperature, viz., on the first occasion at 77o - 81o F., and the
+filament then described 2 � ellipses in 69 m. On the second occasion, when
+the temperature was 81o - 86o F., it made rather more than 3 ellipses in 67
+m. therefore, Fig. 157, though now sufficiently complex, would have been
+incomparably more so, if dots had been made on the glass every 2 or 3
+minutes, instead of every hour or half-hour. Although the main petiole is
+continually and rapidly describing small ellipses during the day, yet after
+the great nocturnal rising movement has commenced, if dots are made every 2
+or 3 minutes, as was done for an hour between 9.30 and 10.30 P.M. (temp.
+84o F.), and the dots are then joined, an almost absolutely straight line
+is the result.
+
+To show that the movement of the petiole is in all probability due to the
+varying turgescence of the pulvinus, and not to growth (in accordance with
+the conclusions of Pfeffer), a very old leaf, with some of its leaflets
+yellowish and hardly at all sensitive, was selected for observation, and
+the plant was kept at the highly favourable temp. of 80o F. The petiole
+fell from 8 A.M. till 10.15 A.M., it then rose a little in a somewhat
+zigzag line, often remaining stationary, till 5 P.M., when the great
+evening fall commenced, which was continued till at least 10 P.M. By 7 A.M.
+on the following morning it had risen to the same level as on the previous
+morning, and then descended in a zigzag line. But from 10.30 A.M. till 4.15
+P.M. it remained almost motionless, all power of movement being now lost.
+The petiole, therefore, of this very old leaf, which must have long ceased
+growing, moved periodically; but instead of circumnutating several times
+during the day, it moved only twice down and twice up in the course of 24
+h., with the ascending and descending lines not coincident.
+
+It has already been stated that the pinnae move independently of the main
+petiole. The petiole of a leaf was fixed to a cork support, close to the
+point whence the four pinnae diverge, with a short fine filament cemented
+longitudinally to one of the two terminal pinnae, and a graduated
+semicircle was placed close beneath it. By looking vertically down, its
+angular or lateral movements could be measured with accuracy. Between noon
+and 4.15 P.M. the pinna changed its position to one side by only 7o; but
+not continuously in the same direction, as it moved four times to one side,
+and three times to the opposite side,
+[page 377]
+in one instance to the extent of 16o. This pinna, therefore circumnutated.
+Later in the evening the four pinnae approach each other, and the one which
+was observed moved inwards 59o between noon and 6.45 P.M. Ten observations
+were made in the course of 2 h. 20 m. (at average intervals of 14 m.),
+between 4.25 and 6.45 P.M.; and there was now, when the leaf was going to
+sleep, no swaying from side to side, but a steady inward movement. Here
+therefore there is in the evening the same conversion of a circumnutating
+into a steady movement in one direction, as in the case of the main
+petiole.
+
+It has also been stated that each separate leaflet circumnutates. A pinna
+was cemented with shellac on the summit of a little stick driven firmly
+into the ground, immediately beneath a pair of leaflets, to the midribs of
+both of which excessively fine glass filaments were attached. This
+treatment did not injure the leaflets, for they went to sleep in the usual
+manner, and long retained their sensitiveness. the movements of one of them
+were traced during 49 h., as shown in Fig. 158. On the first day the
+leaflet sank down till 11.30 A.M., and then rose till late in the evening
+in a zigzag line, indicating circumnutation. On the second day, when more
+accustomed to its new state, it oscillated twice up and twice down during
+the 24 h. This plant was subjected to a rather low temperature, viz., 62o -
+64o F.; had it been kept warmer, no doubt the movements of the leaflet
+would have been much more rapid and complicated. It may be seen in the
+diagram that the ascending and descending lines do not coincide; but the
+large amount of lateral movement in the evening is the result of the
+leaflets bending towards the apex of the leaf when going to sleep. Another
+leaflet was casually observed, and found to be continually circumnutating
+during the same length of time.
+
+The circumnutation of the leaves is not destroyed by their being subjected
+to moderately long continued darkness; but the proper periodicity of their
+movements is lost. Some very young seedlings were kept during two days in
+the dark (temp. 57o - 59o F.) except when the circumnutation of their stems
+was occasionally observed; and on the evening of the second day the
+leaflets did not fully and properly go to sleep. The pot was then placed
+for three days in a dark cupboard, under nearly the same temperature, and
+at the close of this period the leaflets showed no signs of sleeping, and
+were only slightly sensitive to a touch. On the following day the stem was
+cemented to a
+[page 378]
+stick, and the movements of two leaves were traced on a vertical glass
+during 72 h. The plants were still kept in the dark, excepting that at each
+observation, which lasted 3 or 4 minutes,
+
+Fig 158. Mimosa pudica: circumnutation and nyctitropic movement of a
+leaflet (with pinna secured), traced on a vertical glass, from 8 A.M. Sept.
+14th to 9 A.M. 16th.
+
+they were illuminated by two candles. On the third day the leaflets still
+exhibited a vestige of sensitiveness when forcibly pressed, but in the
+evening they showed no signs of sleep. Nevertheless, their petioles
+continued to circumnutate distinctly,
+[page 379]
+although the proper order of their movements in relation to the day and
+night was wholly lost. Thus, one leaf descended during the first two nights
+(i.e. between 10 P.M. and 7 A.M. next morning) instead of ascending, and on
+the third night it moved chiefly in a lateral direction. The second leaf
+behaved in an equally abnormal manner, moving laterally during the first
+night, descending greatly during the second, and ascending to an unusual
+height during the third night.
+
+With plants kept at a high temperature and exposed to the light, the most
+rapid circumnutating movement of the apex of a leaf which was observed,
+amounted to 1/500 of an inch in one second; and this would have equalled
+1/8 of an inch in a minute, had not the leaf occasionally stood still. The
+actual distance travelled by the apex (as ascertained by a measure placed
+close to the leaf) was on one occasion nearly 3/4 of an inch in a vertical
+direction in 15 m.; and on another occasion 5/8 of an inch in 60 m.; but
+there was also some lateral movement.
+
+Mimosa albida.*--The leaves of this plant, one of which is here figured
+(Fig. 159) reduced to 2/3 of the natural size, present some
+
+Fig. 159. Mimosa albida: leaf seen from vertically above.
+
+interesting peculiarities. It consists of a long petiole bearing only two
+pinnae (here represented as rather more divergent than is usual), each with
+two pairs of leaflets. But the inner
+
+* Mr. Thiselton Dyer informs us that this Peruvian plant (which was sent to
+us from Kew) is considered by Mr. Bentham ('Trans. Linn. Soc.,' vol. xxx.
+p. 390) to be "the species or variety which most commonly represents the M.
+sensitiva of our gardens."
+[page 380]
+
+basal leaflets are greatly reduced in size, owing probably to the want of
+space for their full development, so that they may be considered as almost
+rudimentary. They vary somewhat in size, and both occasionally disappear,
+or only one. Nevertheless, they are not in the least rudimentary in
+function, for they are sensitive, extremely heliotropic, circumnutate at
+nearly the same rate as the fully developed leaflets, and assume when
+asleep exactly the same position. With M. pudica the inner leaflets at the
+base and between the pinnae are likewise much shortened and obliquely
+truncated; this fact was well seen in some seedlings of M. pudica, in which
+the third leaf above the cotyledons bore only two pinnae, each with only 3
+or 4 pairs of leaflets, of which the inner basal one was less than half as
+long as its fellow; so that the whole leaf resembled pretty closely that of
+M. albida. In this latter species the main petiole terminates in a little
+point, and on each side of this there is a pair of minute, flattened,
+lancet-shaped projections, hairy on their margins, which drop off and
+disappear soon after the leaf is fully developed. There can hardly be a
+doubt that these little projections are the last and fugacious
+representatives of an additional pair of leaflets to each pinna; for the
+outer one is twice as broad as the inner one, and a little longer, viz.
+7/100 of an inch, whilst the inner one is only 5/100 - 6/100 long. Now if
+the basal pair of leaflets of the existing leaves were to become
+rudimentary, we should expect that the rudiments would still exhibit some
+trace of their present great inequality of size. The conclusion that the
+pinnae of the parent-form of M. albida possessed at least three pairs of
+leaflets, instead of, as at present, only two, is supported by the
+structure of the first true leaf; for this consists of a simple petiole,
+often bearing three pairs of leaflets. This latter fact, as well as the
+presence of the rudiments, both lead to the conclusion that M. albida is
+descended from a form the leaves of which bore more than two pairs of
+leaflets. The second leaf above the cotyledons resembles in all respects
+the leaves on fully developed plants.
+
+When the leaves go to sleep, each leaflet twists half round, so as to
+present its edge to the zenith, and comes into close contact with its
+fellow. The pinnae also approach each other closely, so that the four
+terminal leaflets come together. The large basal leaflets (with the little
+rudimentary ones in contact with them) move inwards and forwards, so as to
+embrace the outside of the united terminal leaflets, and thus all eight
+leaflets
+[page 381]
+(the rudimentary ones included) form together a single vertical packet. The
+two pinnae at the same time that they approach each other sink downwards,
+and thus instead of extending horizontally in the same line with the main
+petiole, as during the day, they depend at night at about 45o, or even at a
+greater angle, beneath the horizon. The movement of the main petiole seems
+to be variable; we have seen it in the evening 27o lower than during the
+day; but sometimes in nearly the same position. Nevertheless, a sinking
+movement in the evening and a rising one during the night is probably the
+normal course, for this was well-marked in the petiole of the first-formed
+true leaf.
+
+The circumnutation of the main petiole of a young leaf was traced during 2
+3/4 days, and was considerable in extent, but less complex than that of M.
+pudica. The movement was much more lateral than is usual with
+circumnutating leaves, and this was the sole peculiarity which it
+presented. The apex of one of the terminal leaflets was seen under the
+microscope to travel 1/50 of an inch in 3 minutes.
+
+Mimosa marginata.--The opposite leaflets rise up and approach each other at
+night, but do not come into close contact, except in the case of very young
+leaflets on vigorous shoots. Full-grown leaflets circumnutate during the
+day slowly and on a small scale.
+
+Schrankia uncinata (Tribe 20).--A leaf consists of two or three pairs of
+pinnae, each bearing many small leaflets. These, when the plant is asleep,
+are directed forwards and become imbricated. The angle between the two
+terminal pinnae was diminished at night, in one case by 15o; and they sank
+almost vertically downwards. The hinder pairs of pinnae likewise sink
+downwards, but do not converge, that is, move towards the apex of the leaf.
+The main petiole does not become depressed, at least during the evening. In
+this latter respect, as well as in the sinking of the pinnae, there is a
+marked difference between the nyctitropic movements of the present plant
+and of Mimosa pudica. It should, however, be added that our specimen was
+not in a very vigorous condition. The pinnae of Schrankia aculeata also
+sink at night.
+
+Acacia Farnesiana (Tribe 22).--The different appearance presented by a bush
+of this plant when asleep and awake is wonderful. The same leaf in the two
+states is shown in the following figure (Fig. 160). The leaflets move
+towards the apex of the pinna and become imbricated, and the pinnae then
+look like bits of dangling string. The following remarks and measurements
+[page 382]
+do not fully apply to the small leaf here figured. The pinnae move forwards
+and at the same time sink downwards, whilst the main petiole rises
+considerably. With respect to the degree of movement: the two terminal
+pinnae of one specimen formed together an angle of 100o during the day, and
+at night of only 38o, so each had moved 31o forwards. The penultimate
+pinnae during the day formed together an angle of 180o, that is, they stood
+in a straight line opposite one another, and at night each had moved 65o
+forwards. The basal pair of pinnae were directed
+
+Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf at
+night.
+
+during the day, each about 21o backwards, and at night 38o forwards, so
+each had moved 59o forwards. But the pinnae at the same time sink greatly,
+and sometimes hang almost perpendicularly downwards. The main petiole, on
+the other hand, rises much: by 8.30 P.M. one stood 34o higher than at noon,
+and by 6.40 A.M. on the following morning it was still higher by 10o;
+shortly after this hour the diurnal sinking movement commenced. The course
+of a nearly full-grown leaf was traced during 14 h.; it was strongly
+zigzag, and apparently
+[page 383]
+represented five ellipses, with their longer axes differently directed.
+
+Albizzia lophantha (Tribe 23).--The leaflets at night come into contact
+with one another, and are directed towards the apex of the pinna. The
+pinnae approach one another, but remain in the same plane as during the
+day; and in this respect they differ much from those of the above Schrankia
+and Acacia. The main petiole rises but little. The first-formed leaf above
+the cotyledons bore 11 leaflets on each side, and these slept like those on
+the subsequently formed leaves; but the petiole of this first leaf was
+curved downwards during the day and at night straightened itself, so that
+the chord of its arc then stood 16o higher than in the day-time.
+
+Melaleuca ericaefolia (Myrtaceae).--According to Bouch� ('Bot. Zeit.,'
+1874, p. 359) the leaves sleep at night, in nearly the same manner as those
+of certain species of Pimelia.
+
+Oenothera mollissima (Onagrarieae).--According to Linnaeus ('Somnus
+Plantarum'), the leaves rise up vertically at night.
+
+Passiflora gracilis (Passifloracae).--The young leaves sleep by their
+blades hanging vertically downwards, and the whole length of the petiole
+then becomes somewhat curved downwards. Externally no trace of a pulvinus
+can be seen. The petiole of the uppermost leaf on a young shoot stood at
+10.45 A.M. at 33o above the horizon; and at 10.30 P.M., when the blade was
+vertically dependent, at only 15o, so the petiole had fallen 18o. That of
+the next older leaf fell only 7o. From some unknown cause the leaves do not
+always sleep properly. The stem of a plant, which had stood for some time
+before a north-east window, was secured to a stick at the base of a young
+leaf, the blade of which was inclined at 40o below the horizon. From its
+position the leaf had to be viewed obliquely, consequently the vertically
+ascending and descending movements appeared when traced oblique. On the
+first day (Oct. 12th) the leaf descended in a zigzag line until late in the
+evening; and by 8.15 A.M. on the 13th had risen to nearly the same level as
+on the previous morning. A new tracing was now begun (Fig. 161). The leaf
+continued to rise until 8.50 A.M., then moved a little to the right, and
+afterwards descended. Between 11 A.M. and 5 P.M. it circumnutated, and
+after the latter hour the great nocturnal fall commenced. At 7.15 P.M. it
+depended vertically. The dotted line ought to have been prolonged much
+lower down in the figure. By 6.50 A.M. on the following morning (14th) the
+[page 384]
+leaf had risen greatly, and continued to rise till 7.50 A.M., after which
+hour it redescended. It should be observed that the lines traced on this
+second morning would have coincided with and confused those previously
+traced, had not the pot been slided a very little to the left. In the
+evening (14th) a mark was placed behind the filament attached to the apex
+of the leaf, and its movement was carefully traced from 5 P.M. to 10.15
+P.M.
+
+Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement of
+leaf, traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 A.M. 14th.
+Figure reduced to two-thirds of original scale.
+
+Between 5 and 7.15 P.M. the leaf descended in a straight line, and at the
+latter hour it appeared vertically dependent. But between 7.15 and 10.15
+P.M. the line consisted of a succession of steps, the cause of which we
+could not understand; it was, however, manifest that the movement was no
+longer a simple descending one.
+
+Siegesbeckia orientalis (Compositae).--Some seedlings were raised in the
+middle of winter and kept in the hot-house; they flowered, but did not grow
+well, and their leaves never showed any signs of sleep. The leaves on other
+seedlings raised in May were horizontal at noon (June 22nd), and depended
+at a consi-
+[page 385]
+derable angle beneath the horizon at 10 P.M. In the case of four youngish
+leaves which were from 2 to 2 � inches in length, these angles were found
+to be 50o, 56o, 60o, and 65o. At the end of August when the plants had grown
+to a height of 10 to 11 inches, the younger leaves were so much curved
+downwards at night that they might truly be said to be asleep. This is one
+
+Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, and
+asleep at night. Figures copied from photographs, and reduced.
+
+of the species which must be well illuminated during the day in order to
+sleep, for on two occasions when plants were kept all day in a room with
+north-east windows, the leaves did not sleep at night. The same cause
+probably accounts for the leaves on our seedlings raised in the dead of the
+winter not sleeping. Professor Pfeffer informs us that the leaves of
+another species (S. Jorullensis ?) hang vertically down at night.
+[page 386]
+
+Ipomoea caerulea and purpurea (Convolvulaceae).--The leaves on very young
+plants, a foot or two in height, are depressed at night to between 68o and
+80o beneath the horizon; and some hang quite vertically downwards. On the
+following morning they again rise into a horizontal position. The petioles
+become at night downwardly curved, either through their entire length or in
+the upper part alone; and this apparently causes the depression of the
+blade. It seems necessary that the leaves should be well illuminated during
+the day in order to sleep, for those which stood on the back of a plant
+before a north-east window did not sleep.
+
+Nicotiana tabacum (var. Virginian) and glauca (Solaneae).--The young leaves
+of both these species sleep by bending vertically upwards. Figures of two
+shoots of N. glauca, awake and asleep (Fig. 162), are given on p. 385: one
+of the shoots, from which the photographs were taken, was accidentally bent
+to one side.
+
+Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of a
+leaf (5 inches in length), traced on a vertical glass, from 3 P.M. July
+10th to 8.10 A.M. 13th. Apex of leaf 4 inches from glass. Temp. 17 1/2o -
+18 1/2o C. Figure reduced to one-half original scale.
+
+At the base of the petiole of N. tabacum, on the outside, there is a mass
+of cells, which are rather smaller than elsewhere, and
+[page 387]
+have their longer axes differently directed from the cells of the
+parenchyma, and may therefore be considered as forming a sort of pulvinus.
+A young plant of N. tabacum was selected, and the circumnutation of the
+fifth leaf above the cotyledons was observed during three days. On the
+first morning (July 10th) the leaf fell from 9 to 10 A.M., which is its
+normal course, but rose during the remainder of the day; and this no doubt
+was due to its being illuminated exclusively from above; for properly the
+evening rise does not commence until 3 or 4 P.M. In the figure as given on
+p. 386 (Fig. 163) the first dot was made at 3 P.M.; and the tracing was
+continued for the following 65 h. When the leaf pointed to the dot next
+above that marked 3 P.M. it stood horizontally. The tracing is remarkable
+only from its simplicity and the straightness of the lines. The leaf each
+day described a single great ellipse; for it should be observed that the
+ascending and descending lines do not coincide. On the evening of the 11th
+the leaf did not descend quite so low as usual, and it now zigzagged a
+little. The diurnal sinking movement had already commenced each morning by
+7 A.M. The broken lines at the top of the figure, representing the
+nocturnal vertical position of the leaf, ought to be prolonged much higher
+up.
+
+Mirabilis longiflora and jalapa (Nyctagineae).--The first pair of leaves
+above the cotyledons, produced by seedlings of both these species, were
+considerably divergent during the day, and at night stood up vertically in
+close contact with one another. The two upper leaves on an older seedling
+were almost horizontal by day, and at night stood up vertically, but were
+not in close contact, owing to the resistance offered by the central bud.
+
+Polygonum aviculare (Polygoneae).--Professor Batalin informs us that the
+young leaves rise up vertically at night. This is likewise the case,
+according to Linnaeus, with several species of Amaranthus (Amaranthaceae);
+and we observed a sleep movement of this kind in one member of the genus.
+Again, with Chenopodium album (Chenopodieae), the upper young leaves of
+some seedlings, about 4 inches in height, were horizontal or sub-horizontal
+during the day, and at 10 P.M. on March 7th were quite, or almost quite,
+vertical. Other seedlings raised in the greenhouse during the winter (Jan.
+28th) were observed day and night, and no difference could be perceived in
+the position of their leaves. According to Bouch� ('Bot. Zeitung,' 1874, p.
+359) the leaves of Pimelia linoides and spectabilis (Thymeleae) sleep at
+night.
+[page 388]
+
+Euphorbia jacquiniaeflora (Euphorbiaceae).--Mr. Lynch called our attention
+to the fact that the young leaves of this plant sleep by depending
+vertically. The third leaf from the summit (March 11th) was inclined during
+the day 30o beneath the horizon, and at night hung vertically down, as did
+some of the still younger leaves. It rose up to its former level on the
+following morning. The fourth and fifth leaves from the summit stood
+horizontally during the day, and sank down at night only 38o. The sixth
+leaf did not sensibly alter its position. The sinking movement is due to
+the downward curvature of the petiole, no part of which exhibits any
+structure like that of a pulvinus. Early on the morning of June 7th a
+filament was fixed longitudinally to a young leaf (the third from the
+summit, and 2 5/8 inches in length), and its movements were traced on a
+vertical glass during 72 h., the plant being illuminated from above through
+a skylight. Each day the leaf fell in a nearly straight line from 7 A.M. to
+5 P.M., after which hour it was so much inclined downwards that the
+movement could no longer be traced; and during the latter part of each
+night, or early in the morning, the leaf rose. It therefore circumnutated
+in a very simple manner, making a single large ellipse every 24 h., for the
+ascending and descending lines did not coincide. On each successive morning
+it stood at a less height than on the previous one, and this was probably
+due partly to the increasing age of the leaf, and partly to the
+illumination being insufficient; for although the leaves are very slightly
+heliotropic, yet, according to Mr. Lynch's and our own observations, their
+inclination during the day is determined by the intensity of the light. On
+the third day, by which time the extent of the descending movement had much
+decreased, the line traced was plainly much more zigzag than on any
+previous day, and it appeared as if some of its powers of movement were
+thus expended. At 10 P.M. on June 7th, when the leaf depended vertically,
+its movements were observed by a mark being placed behind it, and the end
+of the attached filament was seen to oscillate slowly and slightly from
+side to side, as well as upwards and downwards.
+
+Phyllanthus Niruri (Euphorbiaceae).--The leaflets of this plant sleep, as
+described by Pfeffer,* in a remarkable manner, apparently like those of
+Cassia, for they sink downwards at night and twist round, so that their
+lower surfaces are turned
+
+* 'Die Period. Beweg.,' p. 159.
+[page 389]
+
+outwards. They are furnished as might have been expected from this complex
+kind of movement, with a pulvinus.
+
+GYMNOSPERMS.
+
+Pinus Nordmanniana (Coniferae).--M. Chatin states* that the leaves, which
+are horizontal during the day, rise up at night, so as to assume a position
+almost perpendicular to the branch from which they arise; we presume that
+he here refers to a horizontal branch. He adds: "En m�me temps, ce
+mouvement d'�rection est accompang� d'un mouvement de torsion imprim� � la
+partie basilaire de la feuille, et pouvant souvent parcourir un arc de 90
+degr�s." As the lower surfaces of the leaves are white, whilst the upper
+are dark green, the tree presents a widely different appearance by day and
+night. The leaves on a small tree in a pot did not exhibit with us any
+nyctitropic movements. We have seen in a former chapter that the leaves of
+Pinus pinaster and Austriaca are continually circumnutating.
+
+MONOCOTYLEDONS.
+
+Thalia dealbata (Cannaceae).--the leaves of this plant sleep by turning
+vertically upwards; they are furnished with a well-developed pulvinus. It
+is the only instance known to us of a very large leaf sleeping. The blade
+of a young leaf, which was as yet only 13 1/4 inches in length and 6 � in
+breadth, formed at noon an angle with its tall petiole of 121o, and at
+night stood vertically in a line with it, and so had risen 59o. The actual
+distance travelled by the apex (as measured by an orthogonic tracing) of
+another large leaf, between 7.30 A.M. and 10 P.M., was 10 � inches. The
+circumnutation of two young and dwarfed leaves, arising amongst the taller
+leaves at the base of the plant, was traced on a vertical glass during two
+days. On the first day the apex of one, and on the second day the apex of
+the other leaf, described between 6.40 A.M. and 4 P.M. two ellipses, the
+longer axes of which were extended in very different directions from the
+lines representing the great diurnal sinking and nocturnal rising movement.
+
+Maranta arundinacea (Cannaceae).--The blades of the leaves, which are
+furnished with a pulvinus, stand horizontally during
+
+* 'Comptes Rendus,' Jan. 1876, p. 171.
+[page 390]
+
+the day or between 10o and 20o above the horizon, and at night vertically
+upwards. They therefore rise between 70o and 90o at night. The plant was
+placed at noon in the dark in the hot-house, and on the following day the
+movements of the leaves were traced. Between 8.40 and 10.30 A.M. they rose,
+and then fell greatly till 1.37 P.M. But by 3 P.M. they had again risen a
+little, and continued to rise during the rest of the afternoon and night;
+on the following morning they stood at the same level as on the previous
+day. Darkness, therefore, during a day and a half does not interfere with
+the periodicity of their movements. On a warm but stormy evening, the plant
+whilst being brought into the house, had its leaves violently shaken, and
+at night not one went to sleep. On the next morning the plant was taken
+back to the hot-house, and again at night the leaves did not sleep; but on
+the ensuing night they rose in the usual manner between 70o and 80o. This
+fact is analogous with what we have observed with climbing plants, namely,
+that much agitation checks for a time their power of circumnutation; but
+the effect in this instance was much more strongly marked and prolonged.
+
+Colocasia antiquorum (Caladium esculentum, Hort.) (Aroideae).--The leaves
+of this plant sleep by their blades sinking in the evening, so as to stand
+highly inclined, or even quite vertically with their tips pointing to the
+ground. They are not provided with a pulvinus. The blade of one stood at
+noon 1 degree beneath the horizon; at 4.20 P.M., 20o; at 6 P.M. 43o; at
+7.20 P.M., 69o; and at 8.30 P.M., 68o; so it had now begun to rise; at
+10.15 P.M. it stood at 65o, and on the following early morning at 11o
+beneath the horizon. The circumnutation of another young leaf (with its
+petiole only 3 1/4 inches, and the blade 4 inches in length), was traced on
+a vertical glass during 48 h.; it was dimly illuminated through a skylight,
+and this seemed to disturb the proper periodicity of its movements.
+Nevertheless, the leaf fell greatly during both afternoons, till either
+7.10 P.M. or 9 P.M., when it rose a little and moved laterally. By an early
+hour on both mornings, it had assumed its diurnal position. The well-marked
+lateral movement for a short time in the early part of the night, was the
+only interesting fact which it presented, as this caused the ascending and
+descending lines not to coincide, in accordance with the general rule with
+circumnutating organs. The movements of the leaves of this plant are thus
+of the most simple kind; and the tracing is not worth giving. We have seen
+that in another genus of the Aroideae, namely, Pistia, the leaves
+[page 391]
+rise so much at night that they may almost be said to sleep.
+
+Strephium floribundum* (Gramineae).--The oval leaves are provided with a
+pulvinus, and are extended horizontally or declined a little beneath the
+horizon during the day. Those on the upright culms simply rise up
+vertically at night, so that their tips are directed towards the zenith.
+(Fig. 164.)
+
+Fig. 164. Strephium floribundum: culms with leaves during the day, and when
+asleep at night. Figures reduced.
+
+Horizontally extended leaves arising from much inclined or almost
+horizontal culms, move at night so that their tips point towards the apex
+of the culm, with one lateral margin directed towards the zenith; and in
+order to assume this position the leaves have to twist on their own axes
+through an angle of nearly 90o. Thus the surface of the blade always stands
+vertically, whatever may be the position of the midrib or of the leaf as a
+whole.
+
+The circumnutation of a young leaf (2.3 inches in length) was traced during
+48 h. (Fig. 165). The movement was remarkably simple; the leaf descended
+from before 6.40 A.M. until 2 or 2.50 P.M., and then rose so as to stand
+vertically at about 6 P.M., descending again late in the night or in the
+very early morning.
+
+* A. Brongniart first observed that the leaves of this plant and of
+Marsilea sleep: see 'Bull. de la Soc. Bot. de France,' tom. vii. 1860, p.
+470.
+[page 392]
+
+On the second day the descending line zigzagged slightly. As usual, the
+ascending and descending lines did not coincide. On another occasion, when
+the temperature was a little higher, viz., 24o - 26 1/2o C., a leaf was
+observed 17 times between 8.50 A.M. and 12.16 P.M.; it changed its course
+by as much as a rectangle six times in this interval of 3 h. 26 m., and
+described two irregular triangles and a half. The leaf, therefore, on this
+occasion circumnutated rapidly and in a complex manner.
+
+Fig. 165. Strephium floribundum: circumnutation and nyctitropic movement of
+a leaf, traced from 9 A.M. June 26th to 8.45 A.M. 27th; filament fixed
+along the midrib. Apex of leaf 8 1/4 inches from the vertical glass; plant
+illuminated from above. Temp. 23 1/2o - 24 1/2o C.
+
+ACOTYLEDONS.
+
+Marsilea quadrifoliata (Marsileaceae).--The shape of a leaf, expanded
+horizontally during the day, is shown at A (Fig. 166). Each leaflet is
+provided with a well-developed pulvinus. When the leaves sleep, the two
+terminal leaflets rise up, twist half round and come into contact with one
+another (B), and are afterwards embraced by the two lower leaflets (C); so
+that the four leaflets with their lower surfaces turned outwards form a
+vertical packet. The curvature of the summit of the petiole of the leaf
+figured asleep, is merely accidental. The plant was brought into a room,
+where the temperature was only a little above 60o F., and the movement of
+one of the leaflets (the petiole having been secured) was traced
+[page 393]
+during 24 h. (Fig. 167). The leaf fell from the early morning till 1.50
+P.M., and then rose till 6 P.M., when it was asleep. A
+
+Fig. 166. Marsilea quadrifoliata: A, leaf during the day, seen from
+vertically above; B, leaf beginning to go to sleep, seen laterally; C, the
+same asleep. Figures reduced to one-half of natural scale.
+
+vertically dependent glass filament was now fixed to one of the terminal
+and inner leaflets; and part of the tracing in Fig. 167, after 6 P.M.,
+shows that it continued to sink, making one zigzag, until 10.40 P.M. At
+6.45 A.M. on the following morning, the leaf was awaking, and the filament
+pointed above the vertical glass,
+
+Fig. 167. Marsilea quadrifoliata: circumnutation and nyctitropic movement
+of leaflet traced on vertical glass, during nearly 24 h. Figure reduced to
+two-thirds of original scale. Plant kept at rather too low a temperature.
+
+but by 8.25 A.M. it occupied the position shown in the figure. The diagram
+differs greatly in appearance from most of those previously given; and this
+is due to the leaflet twisting and moving laterally as it approaches and
+comes into contact with
+[page 394]
+its fellow. The movement of another leaflet, when asleep, was traced
+between 6 P.M. and 10.35 P.M., and it clearly circumnutated, for it
+continued for two hours to sink, then rose, and then sank still lower than
+it was at 6 P.M. It may be seen in the preceding figure (167) that the
+leaflet, when the plant was subjected to a rather low temperature in the
+house, descended and ascended during the middle of the day in a somewhat
+zigzag line; but when kept in the hot-house from 9 A.M. to 3 P.M. at a high
+but varying temperature (viz., between 72o and 83o F.) a leaflet (with the
+petiole secured) circumnutated rapidly, for it made three large vertical
+ellipses in the course of the six hours. According to Brongniart, Marsilea
+pubescens sleeps like the present species. These plants are the sole
+cryptogamic ones known to sleep.]
+
+Summary and Concluding Remarks on the Nyctitropic or Sleep-movements of
+Leaves.--That these movements are in some manner of high importance to the
+plants which exhibit them, few will dispute who have observed how complex
+they sometimes are. Thus with Cassia, the leaflets which are horizontal
+during the day not only bend at night vertically downwards with the
+terminal pair directed considerably backwards, but they also rotate on
+their own axes, so that their lower surfaces are turned outwards. The
+terminal leaflet of Melilotus likewise rotates, by which movement one of
+its lateral edges is directed upwards, and at the same time it moves either
+to the left or to the right, until its upper surface comes into contact
+with that of the lateral leaflet on the same side, which has likewise
+rotated on its own axis. With Arachis, all four leaflets form together
+during the night a single vertical packet; and to the effect this the two
+anterior leaflets have to move upwards and the two posterior ones forwards,
+besides all twisting on their own axes. In the genus Sida the leaves of
+some species move at night through an angle of 90o upwards, and of others
+[page 395]
+through the same angle downwards. We have seen a similar difference in the
+nyctitropic movements of the cotyledons in the genus Oxalis. In Lupinus,
+again, the leaflets move either upwards or downwards; and in some species,
+for instance L. luteus, those on one side of the star-shaped leaf move up,
+and those on the opposite side move down; the intermediate ones rotating on
+their axes; and by these varied movements, the whole leaf forms at night a
+vertical star instead of a horizontal one, as during the day. Some leaves
+and leaflets, besides moving either upwards or downwards, become more or
+less folded at night, as in Bauhinia and in some species of Oxalis. The
+positions, indeed, which leaves occupy when asleep are almost infinitely
+diversified; they may point either vertically upwards or downwards, or, in
+the case of leaflets, towards the apex or towards the base of the leaf, or
+in any intermediate position. They often rotate at least as much as 90o on
+their own axes. The leaves which arise from upright and from horizontal or
+much inclined branches on the same plant, move in some few cases in a
+different manner, as with Porlieria and Strephium. The whole appearance of
+many plants is wonderfully changed at night, as may be seen with Oxalis,
+and still more plainly with Mimosa. A bush of Acacia Farnesiana appears at
+night as if covered with little dangling bits of string instead of leaves.
+Excluding a few genera not seen by ourselves, about which we are in doubt,
+and excluding a few others the leaflets of which rotate at night, and do
+not rise or sink much, there are 37 genera in which the leaves or leaflets
+rise, often moving at the same time towards the apex or towards the base of
+the leaf, and 32 genera in which they sink at night.
+
+The nyctitropic movements of leaves, leaflets, and
+[page 396]
+petioles are effected in two different ways; firstly, by alternately
+increased growth on their opposite sides, preceded by increased turgescence
+of the cells; and secondly by means of a pulvinus or aggregate of small
+cells, generally destitute of chlorophyll, which become alternately more
+turgescent on nearly opposite sides; and this turgescence is not followed
+by growth except during the early age of the plant. A pulvinus seems to be
+formed (as formerly shown) by a group of cells ceasing to grow at a very
+early age, and therefore does not differ essentially from the surrounding
+tissues. The cotyledons of some species of Trifolium are provided with a
+pulvinus, and others are destitute of one, and so it is with the leaves in
+the genus Sida. We see also in this same genus gradations in the state of
+the development of the pulvinus; and in Nicotiana we have what may probably
+be considered as the commencing development of one. The nature of the
+movement is closely similar, whether a pulvinus is absent or present, as is
+evident from many of the diagrams given in this chapter. It deserves notice
+that when a pulvinus is present, the ascending and descending lines hardly
+ever coincide, so that ellipses are habitually described by the leaves thus
+provided, whether they are young or so old as to have quite ceased growing.
+This fact of ellipses being described, shows that the alternately increased
+turgescence of the cells does not occur on exactly opposite sides of the
+pulvinus, any more than the increased growth which causes the movements of
+leaves not furnished with pulvini. When a pulvinus is present, the
+nyctitropic movements are continued for a very much longer period than when
+such do not exist. This has been amply proved in the case of cotyledons,
+and Pfeffer has given observations to the same effect with respect
+[page 379[97]]
+to leaves. We have seen that a leaf of Mimosa pudica continued to move in
+the ordinary manner, though somewhat more simply, until it withered and
+died. It may be added that some leaflets of Trifolium pratense were pinned
+open during 10 days, and on the first evening after being released they
+rose up and slept in the usual manner. Besides the long continuance of the
+movements when effected by the aid of a pulvinus (and this appears to be
+the final cause of its development), a twisting movement at night, as
+Pfeffer has remarked, is almost confined to leaves thus provided.
+
+It is a very general rule that the first true leaf, though it may differ
+somewhat in shape from the leaves on the mature plant, yet sleeps like
+them; and this occurs quite independently of the fact whether or not the
+cotyledons themselves sleep, or whether they sleep in the same manner. But
+with Phaseolus Roxburghii the first unifoliate leaves rise at night almost
+sufficiently to be said to sleep, whilst the leaflets of the secondary
+trifoliate leaves sink vertically at night. On young plants of Sida
+rhombaefolia, only a few inches in height, the leaves did not sleep, though
+on rather older plants they rose up vertically at night. On the other hand,
+the leaves on very young plants of Cytisus fragrans slept in a conspicuous
+manner, whilst on old and vigorous bushes kept in the greenhouse, the
+leaves did not exhibit any plain nyctitropic movement. In the genus Lotus
+the basal stipule-like leaflets rise up vertically at night, and are
+provided with pulvini.
+
+As already remarked, when leaves or leaflets change their position greatly
+at night and by complicated movements, it can hardly be doubted that these
+must be in some manner beneficial to the plant. If so, we
+[page 398]
+must extend the same conclusion to a large number of sleeping plants; for
+the most complicated and the simplest nyctitropic movements are connected
+together by the finest gradations. But owing to the causes specified in the
+beginning of this chapter, it is impossible in some few cases to determine
+whether or not certain movements should be called nyctitropic. Generally,
+the position which the leaves occupy at night indicates with sufficient
+clearness, that the benefit thus derived, is the protection of their upper
+surfaces from radiation into the open sky, and in many cases the mutual
+protection of all the parts from cold by their being brought into close
+approximation. It should be remembered that it was proved in the last
+chapter, that leaves compelled to remain extended horizontally at night,
+suffered much more from radiation than those which were allowed to assume
+their normal vertical position.
+
+The fact of the leaves of several plants not sleeping unless they have been
+well illuminated during the day, made us for a time doubt whether the
+protection of their upper surfaces from radiation was in all cases the
+final cause of their well-pronounced nyctitropic movements. But we have no
+reason to suppose that the illumination from the open sky, during even the
+most clouded day, is insufficient for this purpose; and we should bear in
+mind that leaves which are shaded from being seated low down on the plant,
+and which sometimes do not sleep, are likewise protected at night from full
+radiation. Nevertheless, we do not wish to deny that there may exist cases
+in which leaves change their position considerably at night, without their
+deriving any benefit from such movements.
+
+Although with sleeping plants the blades almost
+[page 399]
+always assume at night a vertical, or nearly vertical position, it is a
+point of complete indifference whether the apex, or the base, or one of the
+lateral edges, is directed to the zenith. It is a rule of wide generality,
+that whenever there is any difference in the degree of exposure to
+radiation between the upper and the lower surfaces of leaves and leaflets,
+it is the upper which is the least exposed, as may be seen in Lotus,
+Cytisus, Trifolium, and other genera. In several species of Lupinus the
+leaflets do not, and apparently from their structure cannot, place
+themselves vertically at night, and consequently their upper surfaces,
+though highly inclined, are more exposed than the lower; and here we have
+an exception to our rule. But in other species of this genus the leaflets
+succeed in placing themselves vertically; this, however, is effected by a
+very unusual movement, namely, by the leaflets on the opposite sides of the
+same leaf moving in opposite directions.
+
+It is again a very common rule that when leaflets come into close contact
+with one another, they do so by their upper surfaces, which are thus best
+protected. In some cases this may be the direct result of their rising
+vertically; but it is obviously for the protection of the upper surfaces
+that the leaflets of Cassia rotate in so wonderful a manner whilst sinking
+downwards; and that the terminal leaflet of Melilotus rotates and moves to
+one side until it meets the lateral leaflet on the same side. When opposite
+leaves or leaflets sink vertically down without any twisting, their lower
+surfaces approach each other and sometimes come into contact; but this is
+the direct and inevitable result of their position. With many species of
+Oxalis the lower surfaces of the adjoining leaflets are pressed together,
+and are thus better protected
+[page 400]
+than the upper surfaces; but this depends merely on each leaflet becoming
+folded at night so as to be able to sink vertically downwards. The torsion
+or rotation of leaves and leaflets, which occurs in so many cases,
+apparently always serves to bring their upper surfaces into close
+approximation with one another, or with other parts of the plant, for their
+mutual protection. We see this best in such cases as those of Arachis,
+Mimosa albida, and Marsilea, in which all the leaflets form together at
+night a single vertical packet. If with Mimosa pudica the opposite leaflets
+had merely moved upwards, their upper surfaces would have come into contact
+and been well protected; but as it is, they all successively move towards
+the apex of the leaf; and thus not only their upper surfaces are protected,
+but the successive pairs become imbricated and mutually protect one another
+as well as the petioles. This imbrication of the leaflets of sleeping
+plants is a common phenomenon.
+
+The nyctitropic movement of the blade is generally effected by the
+curvature of the uppermost part of the petiole, which has often been
+modified into a pulvinus; or the whole petiole, when short, may be thus
+modified. But the blade itself sometimes curves or moves, of which fact
+Bauhinia offers a striking instance, as the two halves rise up and come
+into close contact at night. Or the blade and the upper part of the petiole
+may both move. Moreover, the petiole as a whole commonly either rises or
+sinks at night. This movement is sometimes large: thus the petioles of
+Cassia pubescens stand only a little above the horizon during the day, and
+at night rise up almost, or quite, perpendicularly. The petioles of the
+younger leaves of Desmodium gyrans also rise up vertically at night. On the
+other hand, with Amphi-
+[page 401]
+carpaea, the petioles of some leaves sank down as much as 57o at night;
+with Arachis they sank 39o, and then stood at right angles to the stem.
+Generally, when the rising or sinking of several petioles on the same plant
+was measured, the amount differed greatly. This is largely determined by
+the age of the leaf: for instance, the petiole of a moderately old leaf of
+Desmodium gyrans rose only 46o, whilst the young ones rose up vertically;
+that of a young leaf of Cassia floribunda rose 41o, whilst that of an older
+leaf rose only 12o. It is a more singular fact that the age of the plant
+sometimes influences greatly the amount of movement; thus with some young
+seedlings of a Bauhinia the petioles rose at night 30o and 34o, whereas
+those on these same plants, when grown to a height of 2 or 3 feet, hardly
+moved at all. The position of the leaves on the plant as determined by the
+light, seems also to influence the amount of movement of the petiole; for
+no other cause was apparent why the petioles of some leaves of Melilotus
+officinalis rose as much as 59o, and others only 7o and 9o at night.
+
+In the case of many plants, the petioles move at night in one direction and
+the leaflets in a directly opposite one. Thus, in three genera of
+Phaseoleae the leaflets moved vertically downwards at night, and the
+petioles rose in two of them, whilst in the third they sank. Species in the
+same genus often differ widely in the movements of their petioles. Even on
+the same plant of Lupinus pubescens some of the petioles rose 30o, others
+only 6o, and others sank 4o at night. The leaflets of Cassia Barclayana
+moved so little at night that they could not be said to sleep, yet the
+petioles of some young leaves rose as much as 34o. These several facts
+apparently indicate that the movements
+[page 402]
+of the petioles are not performed for any special purpose; though a
+conclusion of this kind is generally rash. When the leaflets sink
+vertically down at night and the petioles rise, as often occurs, it is
+certain that the upward movement of the latter does not aid the leaflets in
+placing themselves in their proper position at night, for they have to move
+through a greater angular space than would otherwise have been necessary.
+
+Notwithstanding what has just been said, it may be strongly suspected that
+in some cases the rising of the petioles, when considerable, does
+beneficially serve the plant by greatly reducing the surface exposed to
+radiation at night. If the reader will compare the two drawings (Fig. 155,
+p. 371) of Cassia pubescens, copied from photographs, he will see that the
+diameter of the plant at night is about one-third of what it is by day, and
+therefore the surface exposed to radiation is nearly nine times less. A
+similar conclusion may be deduced from the drawings (Fig. 149, p. 358) of a
+branch awake and asleep of Desmodium gyrans. So it was in a very striking
+manner with young plants of Bauhinia, and with Oxalis Ortegesii.
+
+We are led to an analogous conclusion with respect to the movements of the
+secondary petioles of certain pinnate leaves. The pinnae of Mimosa pudica
+converge at night; and thus the imbricated and closed leaflets on each
+separate pinna are all brought close together into a single bundle, and
+mutually protect one another, with a somewhat smaller surface exposed to
+radiation. With Albizzia lophantha the pinnae close together in the same
+manner. Although the pinnae of Acacia Farnesiana do not converge much, they
+sink downwards. Those of Neptunia oleracea likewise
+[page 403]
+move downwards, as well as backwards, towards the base of the leaf, whilst
+the main petiole rises. With Schrankia, again, the pinnae are depressed at
+night. Now in these three latter cases, though the pinnae do not mutually
+protect one another at night, yet after having sunk down they expose, as
+does a dependent sleeping leaf, much less surface to the zenith and to
+radiation than if they had remained horizontal.
+
+Any one who had never observed continuously a sleeping plant, would
+naturally suppose that the leaves moved only in the evening when going to
+sleep, and in the morning when awaking; but he would be quite mistaken, for
+we have found no exception to the rule that leaves which sleep continue to
+move during the whole twenty-four hours; they move, however, more quickly
+when going to sleep and when awaking than at other times. That they are not
+stationary during the day is shown by all the diagrams given, and by the
+many more which were traced. It is troublesome to observe the movements of
+leaves in the middle of the night, but this was done in a few cases; and
+tracings were made during the early part of the night of the movements in
+the case of Oxalis, Amphicarpaea, two species of Erythrina, a Cassia,
+Passiflora, Euphorbia and Marsilea; and the leaves after they had gone to
+sleep, were found to be in constant movement. When, however, opposite
+leaflets come into close contact with one another or with the stem at
+night, they are, as we believe, mechanically prevented from moving, but
+this point was not sufficiently investigated.
+
+When the movements of sleeping leaves are traced during twenty-four hours,
+the ascending and descending lines do not coincide, except occasionally and
+by accident for a short space; so that with many plants a
+[page 404]
+single large ellipse is described during each twenty-four hours. Such
+ellipses are generally narrow and vertically directed, for the amount of
+lateral movement is small. That there is some lateral movement is shown by
+the ascending and descending lines not coinciding, and occasionally, as
+with Desmodium gyrans and Thalia dealbata, it was strongly marked. In the
+case of Melilotus the ellipses described by the terminal leaflet during the
+day are laterally extended, instead of vertically, as is usual; and this
+fact evidently stands in relation with the terminal leaflet moving
+laterally when it goes to sleep. With the majority of sleeping plants the
+leaves oscillate more than once up and down in the twenty-four hours; so
+that frequently two ellipses, one of moderate size, and one of very large
+size which includes the nocturnal movement, are described within the
+twenty-four hours. For instance, a leaf which stands vertically up during
+the night will sink in the morning, then rise considerably, again sink in
+the afternoon, and in the evening reascend and assume its vertical
+nocturnal position. It will thus describe, in the course of the twenty-four
+hours, two ellipses of unequal sizes. Other plants describe within the same
+time, three, four, or five ellipses. Occasionally the longer axes of the
+several ellipses extend in different directions, of which Acacia Farnesiana
+offered a good instance. The following cases will give an idea of the rate
+of movement: Oxalis acetosella completed two ellipses at the rate of 1 h.
+25 m. for each; Marsilea quadrifoliata, at the rate of 2 h.; Trifolium
+subterraneum, one in 3 h. 30 m.; and Arachis hypogaea, in 4 h. 50 m. But
+the number of ellipses described within a given time depends largely on the
+state of the plant and on the conditions to which it is exposed. It often
+happens that a single ellipse may be described during one
+[page 405]
+day, and two on the next. Erythrina corallodendron made four ellipses on
+the first day of observation and only a single one on the third, apparently
+owing to having been kept not sufficiently illuminated and perhaps not warm
+enough. But there seems likewise to be an innate tendency in different
+species of the same genus to make a different number of ellipses in the
+twenty-four hours: the leaflets of Trifolium repens made only one; those of
+T. resupinatum two, and those of T. subterraneum three in this time. Again,
+the leaflets of Oxalis Plumierii made a single ellipse; those of O.
+bupleurifolia, two; those of O. Valdiviana, two or three; and those of O.
+acetosella, at least five in the twenty-four hours.
+
+The line followed by the apex of a leaf or leaflet, whilst describing one
+or more ellipses during the day, is often zigzag, either throughout its
+whole course or only during the morning or evening: Robinia offered an
+instance of zigzagging confined to the morning, and a similar movement in
+the evening is shown in the diagram (Fig. 126) given under Sida. The amount
+of the zigzag movement depends largely on the plant being placed under
+highly favourable conditions. But even under such favourable conditions, if
+the dots which mark the position of the apex are made at considerable
+intervals of time, and the dots are then joined, the course pursued will
+still appear comparatively simple, although the number of the ellipses will
+be increased; but if dots are made every two or three minutes and these are
+joined, the result often is that all the lines are strongly zigzag, many
+small loops, triangles, and other figures being also formed. This fact is
+shown in two parts of the diagram (Fig. 150) of the movements of Desmodium
+gyrans. Strephium floribundum, observed under a high temperature,
+[page 406]
+made several little triangles at the rate of 43 m. for each. Mimosa pudica,
+similarly observed, described three little ellipses in 67 m.; and the apex
+of a leaflet crossed 1/500 of an inch in a second, or 0.12 inch in a
+minute. The leaflets of Averrhoa made a countless number of little
+oscillations when the temperature was high and the sun shining. The zigzag
+movement may in all cases be considered as an attempt to form small loops,
+which are drawn out by a prevailing movement in some one direction. The
+rapid gyrations of the little lateral leaflets of Desmodium belong to the
+same class of movements, somewhat exaggerated in rapidity and amplitude.
+The jerking movements, with a small advance and still smaller retreat,
+apparently not exactly in the same line, of the hypocotyl of the cabbage
+and of the leaves of Dionaea, as seen under the microscope, all probably
+come under this same head. We may suspect that we here see the energy which
+is freed during the incessant chemical changes in progress in the tissues,
+converted into motion. Finally, it should be noted that leaflets and
+probably some leaves, whilst describing their ellipses, often rotate
+slightly on their axes; so that the plane of the leaf is directed first to
+one and then to another side. This was plainly seen to be the case with the
+large terminal leaflets of Desmodium, Erythrina and Amphicarpaea, and is
+probably common to all leaflets provided with a pulvinus.
+
+With respect to the periodicity of the movements of sleeping leaves,
+Pfeffer* has so clearly shown that this depends on the daily alternations
+of light and darkness, that nothing farther need be said on this
+
+* 'Die Periodischen Bewegungen der Blattorgane,' 1875, p. 30, et passim.
+[page 407]
+
+head. But we may recall the behaviour of Mimosa in the North, where the sun
+does not set, and the complete inversion of the daily movements by
+artificial light and darkness. It has also been shown by us, that although
+leaves subjected to darkness for a moderately long time continue to
+circumnutate, yet the periodicity of their movements is soon greatly
+disturbed, or quite annulled. The presence of light or its absence cannot
+be supposed to be the direct cause of the movements, for these are
+wonderfully diversified even with the leaflets of the same leaf, although
+all have of course been similarly exposed. The movements depend on innate
+causes, and are of an adaptive nature. The alternations of light and
+darkness merely give notice to the leaves that the period has arrived for
+them to move in a certain manner. We may infer from the fact of several
+plants (Tropaeolum, Lupinus, etc.) not sleeping unless they have been well
+illuminated during the day, that it is not the actual decrease of light in
+the evening, but the contrast between the amount at this hour and during
+the early part of the day, which excites the leaves to modify their
+ordinary mode of circumnutation.
+
+As the leaves of most plants assume their proper diurnal position in the
+morning, although light be excluded, and as the leaves of some plants
+continue to move in the normal manner in darkness during at least a whole
+day, we may conclude that the periodicity of their movements is to a
+certain extent inherited.* The strength of such inheritance differs
+
+* Pfeffer denies such inheritance; he attributes ('Die Period. Bewegungen,'
+pp. 30-56) the periodicity when prolonged for a day or two in darkness, to
+"Nachwirkung," or the after-effects of light and darkness. But we are
+unable to follow his train of reasoning. There does not seem to be any more
+reason for
+[[page 408]]
+attributing such movements to this cause than, for instance, the inherited
+habit of winter and summer wheat to grow best at different seasons; for
+this habit is lost after a few years, like the movements of leaves in
+darkness after a few days. No doubt some effect must be produced on the
+seeds by the long-continued cultivation of the parent-plants under
+different climates, but no one probably would call this the "Nachwirkung"
+of the climates.
+[page 408]
+much in different species, and seems never to be rigid; for plants have
+been introduced from all parts of the world into our gardens and
+greenhouses; and if their movements had been at all strictly fixed in
+relation to the alternations of day and night, they would have slept in
+this country at very different hours, which is not the case. Moreover, it
+has been observed that sleeping plants in their native homes change their
+times of sleep with the changing seasons.*
+
+We may now turn to the systematic list. This contains the names of all the
+sleeping plants known to us, though the list undoubtedly is very imperfect.
+It may be premised that, as a general rule, all the species in the same
+genus sleep in nearly the same manner. But there are some exceptions; in
+several large genera including many sleeping species (for instance,
+Oxalis), some do not sleep. One species of Melilotus sleeps like a
+Trifolium, and therefore very differently from its congeners; so does one
+species of Cassia. In the genus Sida, the leaves either rise or fall at
+night; and with Lupinus they sleep in three different methods. Returning to
+the list, the first point which strikes us, is that there are many more
+genera amongst the Leguminosae (and in almost every one of the Leguminous
+tribes) than in all the other families put together; and we are tempted to
+connect this fact with the great
+
+* Pfeffer, ibid., p. 46.
+[page 409]
+
+mobility of the stems and leaves in this family, as shown by the large
+number of climbing species which it contains. Next to the Leguminosae come
+the Malvaceae, together with some closely allied families. But by far the
+most important point in the list, is that we meet with sleeping plants in
+28 families, in all the great divisions of the Phanerogamic series, and in
+one Cryptogam. Now, although it is probable that with the Leguminosae the
+tendency to sleep may have been inherited from one or a few progenitors,
+and possibly so in the cohorts of the Malvales and Chenopodiales, yet it is
+manifest that the tendency must have been acquired by the several genera in
+the other families, quite independently of one another. Hence the question
+naturally arises, how has this been possible? and the answer, we cannot
+doubt is that leaves owe their nyctitropic movements to their habit of
+circumnutating,--a habit common to all plants, and everywhere ready for any
+beneficial development or modification.
+
+It has been shown in the previous chapters that the leaves and cotyledons
+of all plants are continually moving up and down, generally to a slight but
+sometimes to a considerable extent, and that they describe either one or
+several ellipses in the course of twenty-four hours; they are also so far
+affected by the alternations of day and night that they generally, or at
+least often, move periodically to a small extent; and here we have a basis
+for the development of the greater nyctitropic movements. That the
+movements of leaves and cotyledons which do not sleep come within the class
+of circumnutating movements cannot be doubted, for they are closely similar
+to those of hypocotyls, epicotyls, the stems of mature plants, and of
+various other organs. Now, if we take the simplest
+[page 410]
+case of a sleeping leaf, we see that it makes a single ellipse in the
+twenty-four hours, which resembles one described by a non-sleeping leaf in
+every respect, except that it is much larger. In both cases the course
+pursued is often zigzag. As all non-sleeping leaves are incessantly
+circumnutating, we must conclude that a part at least of the upward and
+downward movement of one that sleeps, is due to ordinary circumnutation;
+and it seems altogether gratuitous to rank the remainder of the movement
+under a wholly different head. With a multitude of climbing plants the
+ellipses which they describe have been greatly increased for another
+purpose, namely, catching hold of a support. With these climbing plants,
+the various circumnutating organs have been so far modified in relation to
+light that, differently from all ordinary plants, they do not bend towards
+it. with sleeping plants the rate and amplitude of the movements of the
+leaves have been so far modified in relation to light, that they move in a
+certain direction with the waning light of the evening and with the
+increasing light of the morning more rapidly, and to a greater extent, than
+at other hours.
+
+But the leaves and cotyledons of many non-sleeping plants move in a much
+more complex manner than in the cases just alluded to, for they describe
+two, three, or more ellipses in the course of a day. Now, if a plant of
+this kind were converted into one that slept, one side of one of the
+several ellipses which each leaf daily describes, would have to be greatly
+increased in length in the evening, until the leaf stood vertically, when
+it would go on circumnutating about the same spot. On the following
+morning, the side of another ellipse would have to be similarly increased
+in length so as to bring the leaf back again into its diurnal position,
+when it would again circumnutate
+[page 411]
+until the evening. If the reader will look, for instance, at the diagram
+(Fig. 142, p. 351), representing the nyctitropic movements of the terminal
+leaflet of Trifolium subterraneum, remembering that the curved broken lines
+at the top ought to be prolonged much higher up, he will see that the great
+rise in the evening and the great fall in the morning together form a large
+ellipse like one of those described during the daytime, differing only in
+size. Or, he may look at the diagram (Fig. 103, p. 236) of the 3 � ellipses
+described in the course of 6 h. 35 m. by a leaf of Lupinus speciosus, which
+is one of the species in this genus that does not sleep; and he will see
+that by merely prolonging upwards the line which was already rising late in
+the evening, and bringing it down again next morning, the diagram would
+represent the movements of a sleeping plant.
+
+With those sleeping plants which describe several ellipses in the daytime,
+and which travel in a strongly zigzag line, often making in their course
+minute loops, triangles, etc., if as soon as one of the ellipses begins in
+the evening to be greatly increased in size, dots are made every 2 or 3
+minutes and these are joined, the line then described is almost strictly
+rectilinear, in strong contrast with the lines made during the daytime.
+This was observed with Desmodium gyrans and Mimosa pudica. With this latter
+plant, moreover, the pinnae converge in the evening by a steady movement,
+whereas during the day they are continually converging and diverging to a
+slight extent. In all such cases it was scarcely possible to observe the
+difference in the movement during the day and evening, without being
+convinced that in the evening the plant saves the expenditure of force by
+not moving laterally, and that its whole energy is now expended
+[page 412]
+in gaining quickly its proper nocturnal position by a direct course. In
+several other cases, for instance, when a leaf after describing during the
+day one or more fairly regular ellipses, zigzags much in the evening, it
+appears as if energy was being expended, so that the great evening rise or
+fall might coincide with the period of the day proper for this movement.
+
+The most complex of all the movements performed by sleeping plants, is that
+when leaves or leaflets, after describing in the daytime several vertically
+directed ellipses, rotate greatly on their axes in the evening, by which
+twisting movement they occupy a wholly different position at night to what
+they do during the day. For instance, the terminal leaflets of Cassia not
+only move vertically downwards in the evening, but twist round, so that
+their lower surfaces face outwards. Such movements are wholly, or almost
+wholly, confined to leaflets provided with a pulvinus. But this torsion is
+not a new kind of movement introduced solely for the purpose of sleep; for
+it has been shown that some leaflets whilst describing their ordinary
+ellipses during the daytime rotate slightly, causing their blades to face
+first to one side and then to another. Although we can see how the slight
+periodical movements of leaves in a vertical plane could be easily
+converted into the greater yet simple nyctitropic movements, we do not at
+present know by what graduated steps the more complex movements, effected
+by the torsion of the pulvini, have been acquired. A probable explanation
+could be given in each case only after a close investigation of the
+movements in all the allied forms.
+
+From the facts and considerations now advanced we may conclude that
+nyctitropism, or the sleep of leaves
+[page 413]
+and cotyledons, is merely a modification of their ordinary circumnutating
+movement, regulated in its period and amplitude by the alternations of
+light and darkness. The object gained is the protection of the upper
+surfaces of the leaves from radiation at night, often combined with the
+mutual protection of the several parts by their close approximation. In
+such cases as those of the leaflets of Cassia--of the terminal leaflets of
+Melilotus--of all the leaflets of Arachis, Marsilea, etc.--we have ordinary
+circumnutation modified to the extreme extent known to us in any of the
+several great classes of modified circumnutation. On this view of the
+origin of nyctitropism we can understand how it is that a few plants,
+widely distributed throughout the Vascular series, have been able to
+acquire the habit of placing the blades of their leaves vertically at
+night, that is, of sleeping,--a fact otherwise inexplicable.
+
+The leaves of some plants move during the day in a manner, which has
+improperly been called diurnal sleep; for when the sun shines brightly on
+them, they direct their edges towards it. To such cases we shall recur in
+the following chapter on Heliotropism. It has been shown that the leaflets
+of one form of Porlieria hygrometrica keep closed during the day, as long
+as the plant is scantily supplied with water, in the same manner as when
+asleep; and this apparently serves to check evaporation. There is only one
+other analogous case known to us, namely, that of certain Gramineae, which
+fold inwards the sides of their narrow leaves, when these are exposed to
+the sun and to a dry atmosphere, as described by Duval-Jouve.* We have also
+observed the same phenomenon in Elymus arenareus.
+
+* 'Annal. des Sc. Nat. (Bot.),' 1875, tom. i. pp. 326-329.
+[page 414]
+
+There is another movement, which since the time of Linnaeus has generally
+been called sleep, namely, that of the petals of the many flowers which
+close at night. These movements have been ably investigated by Pfeffer, who
+has shown (as was first observed by Hofmeister) that they are caused or
+regulated more by temperature than by the alternations of light and
+darkness. Although they cannot fail to protect the organs of reproduction
+from radiation at night, this does not seem to be their chief function, but
+rather the protection of the organs from cold winds, and especially from
+rain, during the day. the latter seems probable, as Kerner* has shown that
+a widely different kind of movement, namely, the bending down of the upper
+part of the peduncle, serves in many cases the same end. The closure of the
+flowers will also exclude nocturnal insects which may be ill-adapted for
+their fertilisation, and the well-adapted kinds at periods when the
+temperature is not favourable for fertilisation. Whether these movements of
+the petals consist, as is probable, of modified circumnutation we do not
+know.
+
+Embryology of Leaves.--A few facts have been incidentally given in this
+chapter on what may be called the embryology of leaves. With most plants
+the first leaf which is developed after the cotyledons, resembles closely
+the leaves produced by the mature plant, but this is not always the case.
+the first leaves produced by some species of Drosera, for instance by D.
+Capensis, differ widely in shape from those borne by the mature plant, and
+resemble closely the leaves of D. rotundifolia, as was shown to us by Prof.
+Williamson of Manchester. The first true leaf of
+
+* 'Die Schutzmittel des Pollens,' 1873, pp. 30-39.
+[page 415]
+
+the gorse, or Ulex, is not narrow and spinose like the older leaves. On the
+other hand, with many Leguminous plants, for instance, Cassia, Acacia
+lophantha, etc., the first leaf has essentially the same character as the
+older leaves, excepting that it bears fewer leaflets. In Trifolium the
+first leaf generally bears only a single leaflet instead of three, and this
+differs somewhat in shape from the corresponding leaflet on the older
+leaves. Now, with Trifolium Pannonicum the first true leaf on some
+seedlings was unifoliate, and on others completely trifoliate; and between
+these two extreme states there were all sorts of gradations, some seedlings
+bearing a single leaflet more or less deeply notched on one or both sides,
+and some bearing a single additional and perfect lateral leaflet. Here,
+then, we have the rare opportunity of seeing a structure proper to a more
+advanced age, in the act of gradually encroaching on and replacing an
+earlier or embryological condition.
+
+The genus Melilotus is closely allied to Trifolium, and the first leaf
+bears only a single leaflet, which at night rotates on its axis so as to
+present one lateral edge to the zenith. Hence it sleeps like the terminal
+leaflet of a mature plant, as was observed in 15 species, and wholly unlike
+the corresponding leaflet of Trifolium, which simply bends upwards. It is
+therefore a curious fact that in one of these 15 species, viz., M. Taurica
+(and in a lesser degree in two others), leaves arising from young shoots,
+produced on plants which had been cut down and kept in pots during the
+winter in the green-house, slept like the leaves of a Trifolium, whilst the
+leaves on the fully-grown branches on these same plants afterwards slept
+normally like those of a Melilotus. If young shoots rising from the ground
+may be considered as new individuals, partaking to a certain extent of the
+nature of seedlings, then the peculiar manner in which their leaves slept
+may be considered
+[page 416]
+as an embryological habit, probably the result of Melilotus being descended
+from some form which slept like a Trifolium. This view is partially
+supported by the leaves on old and young branches of another species, M.
+Messanensis (not included in the above 15 species), always sleeping like
+those of a Trifolium.
+
+The first true leaf of Mimosa albida consists of a simple petiole, often
+bearing three pairs of leaflets, all of which are of nearly equal size and
+of the same shape: the second leaf differs widely from the first, and
+resembles that on a mature plant (see Fig. 159, p. 379), for it consists of
+two pinnae, each of which bears two pairs of leaflets, of which the inner
+basal one is very small. But at the base of each pinna there is a pair of
+minute points, evidently rudiments of leaflets, for they are of unequal
+sizes, like the two succeeding leaflets. These rudiments are in one sense
+embryological, for they exist only during the youth of the leaf, falling
+off and disappearing as soon as it is fully grown.
+
+With Desmodium gyrans the two lateral leaflets are very much smaller than
+the corresponding leaflets in most of the species in this large genus; they
+vary also in position and size; one or both are sometimes absent; and they
+do not sleep like the fully-developed leaflets. They may therefore be
+considered as almost rudimentary; and in accordance with the general
+principles of embryology, they ought to be more constantly and fully
+developed on very young than on old plants. But this is not the case, for
+they were quite absent on some young seedlings, and did not appear until
+from 10 to 20 leaves had been formed. This fact leads to the suspicion that
+D. gyrans is descended through a unifoliate form (of which some exist) from
+a trifoliate species; and that the little lateral leaflets reappear through
+reversion. However this may be,
+[page 417]
+the interesting fact of the pulvini or organs of movement of these little
+leaflets, not having been reduced nearly so much as their blades--taking
+the large terminal leaflet as the standard of comparison--gives us probably
+the proximate cause of their extraordinary power of gyration.
+[page 418]
+
+
+CHAPTER VIII.
+
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
+
+Distinction between heliotropism and the effects of light on the
+periodicity of the movements of leaves--Heliotropic movements of Beta,
+Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in
+Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements
+of tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of the
+pods--Heliotropism and apheliotropism modified forms of circumnutation--
+Steps by which one movement is converted into the other--
+Transversal-heliotropismus or diaheliotropism influenced by epinasty, the
+weight of the part and apogeotropism--Apogeotropism overcome during the
+middle of the day by diaheliotropism--Effects of the weight of the blades
+of cotyledons--So called diurnal sleep--Chlorophyll injured by intense
+light--Movements to avoid intense light
+
+SACHS first clearly pointed out the important difference between the action
+of light in modifying the periodic movements of leaves, and in causing them
+to bend towards its source.* The latter, or heliotropic movements are
+determined by the direction of the light, whilst periodic movements are
+affected by changes in its intensity and not by its direction. The
+periodicity of the circumnutating movement often continues for some time in
+darkness, as we have seen in the last chapter; whilst heliotropic bending
+ceases very quickly when the light fails. Nevertheless, plants which have
+ceased through long-continued darkness to move periodically, if re-exposed
+to the light are still, according to Sachs, heliotropic.
+
+Apheliotropism, or, as usually designated, negative
+
+* 'Physiologie Veg.' (French Translation), 1868, pp. 42, 517, etc.
+[page 419]
+
+heliotropism, implies that a plant, when unequally illuminated on the two
+sides, bends from the light, instead of, as in the last sub-class of cases,
+towards it; but apheliotropism is comparatively rare, at least in a
+well-marked degree. There is a third and large sub-class of cases, namely,
+those of "transversal-Heliotropismus" of Frank, which we will here call
+diaheliotropism. Parts of plants, under this influence, place themselves
+more or less transversely to the direction whence the light proceeds, and
+are thus fully illuminated. There is a fourth sub-class, as far as the
+final cause of the movement is concerned; for the leaves of some plants
+when exposed to an intense and injurious amount of light direct themselves,
+by rising or sinking or twisting, so as to be less intensely illuminated.
+Such movements have sometimes been called diurnal sleep. If thought
+advisable, they might be called paraheliotropic, and this term would
+correspond with our other terms.
+
+It will be shown in the present chapter that all the movements included in
+these four sub-classes, consist of modified circumnutation. We do not
+pretend to say that if a part of a plant, whilst still growing, did not
+circumnutate--though such a supposition is most improbable--it could not
+bend towards the light; but, as a matter of fact, heliotropism seems always
+to consist of modified circumnutation. Any kind of movement in relation to
+light will obviously be much facilitated by each part circumnutating or
+bending successively in all directions, so that an already existing
+movement has only to be increased in some one direction, and to be lessened
+or stopped in the other directions, in order that it should become
+heliotropic, apheliotropic, etc., as the case may be. In the next chapter
+some observations on the sensitiveness of plants to light, their
+[page 420]
+rate of bending towards it, and the accuracy with which they point towards
+its source, etc., will be given. Afterwards it will be shown--and this
+seems to us a point of much interest--that sensitiveness to light is
+sometimes confined to a small part of the plant; and that this part when
+stimulated by light, transmits an influence to distant parts, exciting them
+to bend.
+
+Heliotropism.--When a plant which is strongly heliotropic (and species
+differ much in this respect) is exposed to a bright lateral light, it bends
+quickly towards it, and the course pursued by the stem is quite or nearly
+straight. But if the light is much dimmed, or occasionally interrupted, or
+admitted in only a slightly oblique direction, the course pursued is more
+or less zigzag; and as we have seen and shall again see, such zigzag
+movement results from the elongation or drawing out of the ellipses, loops,
+etc., which the plant would have described, if it had been illuminated from
+above. On several occasions we were much struck with this fact, whilst
+observing the circumnutation of highly sensitive seedlings, which were
+unintentionally illuminated rather obliquely, or only at successive
+intervals of time.
+
+Fig. 168. Beta vulgaris: circumnutation of hypocotyl, deflected by the
+light being slightly lateral, traced on a horizontal glass from 8.30 A.M.
+to 5.30 P.M. Direction of the lighted taper by which it was illuminated
+shown by a line joining the first and penultimate dots. Figure reduced to
+one-third of the original scale.
+
+[For instance two young seedlings of Beta vulgaris were placed in the
+middle of a room with north-east windows, and were kept covered up, except
+during each observation which lasted for only a minute or two; but the
+result was that their hypocotyls bowed themselves to the side, whence some
+light occasionally entered, in lines which were
+[page 421]
+only slightly zigzag. Although not a single ellipse was even approximately
+formed, we inferred from the zigzag lines - and, as it proved, correctly--
+that their hypocotyls were circumnutating, for on the following day these
+same seedlings were placed in a completely darkened room, and were observed
+each time by the aid of a small wax taper held almost directly above them,
+and their movements were traced on a horizontal glass above; and now their
+hypocotyls clearly circumnutated (Fig. 168, and Fig. 39, formerly given, p.
+52); yet they moved a short distance towards the side where the taper was
+held up. If we look at these diagrams, and suppose that the taper had been
+held more on one side, and that the hypocotyls, still circumnutating, had
+bent themselves within the same time much more towards the light, long
+zigzag lines would obviously have been the result.
+
+Fig. 169. Avena sativa: heliotropic movement and circumnutation of
+sheath-like cotyledon (1 � inch in height) traced on horizontal glass from
+8 A.M. to 10.25 P.M. Oct. 16th.
+
+Again, two seedlings of Solanum lycopersicum were illuminated from above,
+but accidentally a little more light entered on one than on any other side,
+and their hypocotyls became slightly bowed towards the brighter side; they
+moved in a zigzag line and described in their course two little triangles,
+as seen in Fig. 37 (p. 50), and in another tracing not given. The
+sheath-like cotyledons of Zea mays behaved, under nearly similar
+circumstances, in a nearly similar manner as described in our first chapter
+(p. 64), for they bowed themselves during the whole day towards one side,
+making, however, in their course some conspicuous flexures. Before we knew
+how greatly ordinary circumnutation was modified by a lateral light, some
+seedling oats, with rather old and therefore not highly sensitive
+cotyledons, were placed in front of a north-east window, towards which they
+bent all day in a strongly zigzag course. On the following day they
+continued to bend in the same direction (Fig. 169), but zigzagged much
+less. The sky, however, became between 12.40 and 2.35 P.M.
+[page 422]
+overcast with extraordinarily dark thunder-clouds, and it was interesting
+to note how plainly the cotyledons circumnutated during this interval.
+
+The foregoing observations are of some value, from having been made when we
+were not attending to heliotropism; and they led us to experiment on
+several kinds of seedlings, by exposing them to a dim lateral light, so as
+to observe the gradations between ordinary circumnutation and heliotropism.
+Seedlings in pots were placed in front of, and about a yard from, a
+north-east window; on each side and over the pots black boards were placed;
+in the rear the pots were open to the diffused light of the room, which had
+a second north-east and a north-west window. By hanging up one or more
+blinds before the window where the seedlings stood, it was easy to dim the
+light, so that very little more entered on this side than on the opposite
+one, which received the diffused light of the room. Late in the evening the
+blinds were successively removed, and as the plants had been subjected
+during the day to a very obscure light, they continued to bend towards the
+window later in the evening than would otherwise have occurred. Most of the
+seedlings were selected because they were known to be highly sensitive to
+light, and some because they were but little sensitive, or had become so
+from having grown old. The movements were traced in the usual manner on a
+horizontal glass cover; a fine glass filament with little triangles of
+paper having been cemented in an upright position to the hypocotyls.
+Whenever the stem or hypocotyl became much bowed towards the light, the
+latter part of its course had to be traced on a vertical glass, parallel to
+the window, and at right angles to the horizontal glass cover.
+Fig. 170. Apios graveolens: heliotropic movement of hypocotyl (.45 of inch
+in height) towards a moderately bright lateral light, traced on a
+horizontal glass from 8.30 A.M. to 11.30 A.M. Sept. 18th. Figure reduced to
+one-third of original scale.
+
+Apios graveolens.--The hypocotyl bends in a few hours rectan-
+[page 423]
+gularly towards a bright lateral light. In order to ascertain how straight
+a course it would pursue when fairly well illuminated on one side,
+seedlings were first placed before a south-west window on a cloudy and
+rainy morning; and the movement of two hypocotyls were traced for 3 h.,
+during which time they became greatly bowed towards the light. One of these
+tracings is given on p. 422 (Fig. 170), and the course may be seen to be
+almost straight. But the amount of light on this occasion was superfluous,
+for two seedlings were placed before a north-east window, protected by an
+ordinary linen and two muslin blinds, yet their hypocotyls moved towards
+this rather dim light in only slightly zigzag lines; but after 4 P.M., as
+the light waned, the lines became distinctly zigzag. One of these
+seedlings, moreover, described in the afternoon an ellipse of considerable
+size, with its longer axis directed towards the window.
+
+We now determined that the light should be made dim enough, so we began by
+exposing several seedlings before a north-east window, protected by one
+linen blind, three muslin blinds, and a towel. But so little light entered
+that a pencil cast no perceptible shadow on a white card, and the
+hypocotyls did not bend at all towards the window. During this time, from
+8.15 to 10.50 A.M., the hypocotyls zigzagged or circumnutated near the same
+spot, as may be seen at A, in Fig. 171. The towel, therefore, was removed
+at 10.50 A.M., and replaced by two muslin blinds, and now the light passed
+through one ordinary linen and four muslin blinds. When a pencil was held
+upright on a card close to the seedlings, it cast a shadow (pointing from
+the window) which could only just be distinguished. Yet this very slight
+excess of light on one side sufficed to cause the hypocotyls of all the
+seedlings immediately to begin bending in zigzag lines towards the window.
+The course of one is shown at A (Fig. 171): after moving towards the window
+from 10.50 A.M. to 12.48 P.M. it bent from the window, and then returned in
+a nearly parallel line; that is, it almost completed between 12.48 and 2
+P.M. a narrow ellipse. Late in the evening, as the light waned, the
+hypocotyl ceased to bend towards the window, and circumnutated on a small
+scale round the same spot; during the night it moved considerably
+backwards, that is, became more upright, through the action of
+apogeotropism. At B, we have a tracing of the movements of another seedling
+from the hour (10.50 A.M.) when the towel was removed; and it is in all
+essential respects
+[page 424]
+similar to the previous one. In these two cases there could be no doubt
+that the ordinary circumnutating movement of the hypocotyl was modified and
+rendered heliotropic.
+
+Fig. 171. Apios graveolens: heliotropic movement and circumnutation of the
+hypocotyls of two seedlings towards a dim lateral light, traced on a
+horizontal glass during the day. The broken lines show their return
+nocturnal courses. Height of hypocotyl of A .5, and of B .55 inch. Figure
+reduced to one-half of original scale.
+
+Brassica oleracea.--The hypocotyl of the cabbage, when not disturbed by a
+lateral light, circumnutates in a complicated
+[page 425]
+manner over nearly the same space, and a figure formerly given is here
+reproduced (Fig. 172). If the hypocotyl is exposed to a moderately strong
+lateral light it moves quickly towards this side, travelling in a straight,
+or nearly straight, line. But when the lateral light is very dim its course
+is extremely tortuous, and evidently consists of modified circumnutation.
+Seedlings were placed before a north-east window, protected by a linen and
+muslin blind and by a towel. The sky was cloudy, and whenever the clouds
+grew a little lighter an additional muslin blind was temporarily suspended.
+The light from the window was
+
+Fig. 172. Brassica oleracea: ordinary circumnutating movement of the
+hypocotyl of a seedling plant.
+
+thus so much obscured that, judging by the unassisted eye, the seedlings
+appeared to receive more light from the interior of the room than from the
+window; but this was not really the case, as was shown by a very faint
+shadow cast by a pencil on a card. Nevertheless, this extremely small
+excess of light on one side caused the hypocotyls, which in the morning had
+stood upright, to bend at right angles towards the window, so that in the
+evening (after 4.23 P.M.) their course had to be traced on a vertical glass
+parallel to the window. It should be stated that at 3.30 P.M., by which
+time the sky had become darker, the towel was removed and replaced by an
+additional muslin blind, which itself was removed at 4 P.M., the other two
+[page 426]
+blinds being left suspended. In Fig. 173 the course pursued, between 8.9
+A.M. and 7.10 P.M., by one of the hypocotyls thus
+
+Fig. 173. Brassica oleracea: heliotropic movement and circumnutation of a
+hypocotyl towards a very dim lateral light, traced during 11 hours, on a
+horizontal glass in the morning, and on a vertical glass in the evening.
+Figure reduced to one-third of the original scale.
+
+exposed is shown. It may be observed that during the first 16 m. the
+hypocotyl moved obliquely from the light, and this,
+[page 427]
+no doubt, was due to its then circumnutating in this direction. Similar
+cases were repeatedly observed, and a dim light rarely or never produced
+any effect until from a quarter to three-quarters of an hour had elapsed.
+After 5.15 P.M., by which time the light had become obscure, the hypocotyl
+began to circumnutate about the same spot. The contrast between the two
+figures (172 and 173) would have been more striking, if they had been
+originally drawn on the same scale, and had been equally reduced. But the
+movements shown in Fig. 172 were at first more magnified, and have been
+reduced to only one-half of the original scale; whereas those in Fig. 173
+were at first less magnified, and have been reduced to a one-third scale. A
+tracing made at the same time with the last of the movements of a second
+hypocotyl, presented a closely analogous appearance; but it did not bend
+quite so much towards the light, and it circumnutated rather more plainly.
+
+Fig. 174. Phalaris Canariensis: heliotropic movement and circumnutation of
+a rather old cotyledon, towards a dull lateral light, traced on a
+horizontal glass from 8.15 A.M. Sept. 16th to 7.45 A.M. 17th. Figure
+reduced to one-third of original scale.
+
+Phalaris Canariensis.--The sheath-like cotyledons of this monocotyledonous
+plant were selected for trial, because they are very sensitive to light and
+circumnutate well, as formerly shown (see Fig. 49, p. 63). Although we felt
+no doubt about the result, some seedlings were first placed before a
+south-west window on a moderately bright morning, and the movements of one
+were traced. As is so common, it moved
+[page 428]
+for the first 45 m. in a zigzag line; it then felt the full influence of
+the light, and travelled towards it for the next 2 h. 30 m. in an almost
+straight line. The tracing has not been given, as it was almost identical
+with that of Apios under similar circumstances (Fig. 170). By noon it had
+bowed itself to its full extent; it then circumnutated about the same spot
+and described two ellipses; by 5 P.M. it had retreated considerably from
+the light, through the action of apogeotropism. After some preliminary
+trials for ascertaining the right degree of obscurity, some seedlings were
+placed (Sept. 16th) before a north-east window, and light was admitted
+through an ordinary linen and three muslin blinds. A pencil held close by
+the pot now cast a very faint shadow on a white card, pointing from the
+window. In the evening, at 4.30 and again at 6 P.M., some of the blinds
+were removed. In Fig. 174 we see the course pursued under these
+circumstances by a rather old and not very sensitive cotyledon, 1.9 inch in
+height, which became much bowed, but was never rectangularly bent towards
+the light. From 11 A.M., when the sky became rather duller, until 6.30
+P.M., the zigzagging was conspicuous, and evidently consisted of drawn-out
+ellipses. After 6.30 P.M. and during the night, it retreated in a crooked
+line from the window. Another and younger seedling moved during the same
+time much more quickly and to a much greater distance, in an only slightly
+zigzag line towards the light; by 11 A.M. it was bent almost rectangularly
+in this direction, and now circumnutated about the same place.
+
+Tropaeolum majus.--Some very young seedlings, bearing only two leaves, and
+therefore not as yet arrived at the climbing stage of growth, were first
+tried before a north-east window without any blind. The epicotyls bowed
+themselves towards the light so rapidly that in little more than 3 h. their
+tips pointed rectangularly towards it. The lines traced were either nearly
+straight or slightly zigzag; and in this latter case we see that a trace of
+circumnutation was retained even under the influence of a moderately bright
+light. Twice whilst these epicotyls were bending towards the window, dots
+were made every 5 or 6 minutes, in order to detect any trace of lateral
+movement, but there was hardly any; and the lines formed by their junction
+were nearly straight, or only very slightly zigzag, as in the other parts
+of the figures. After the epicotyls had bowed themselves to the full extent
+towards the light, ellipses of considerable size were described in the
+usual manner.
+[page 429]
+
+After having seen how the epicotyls moved towards a moderately bright
+light, seedlings were placed at 7.48 A.M. (Sept. 7th) before a north-east
+window, covered by a towel, and shortly afterwards by an ordinary linen
+blind, but the epicotyls still moved towards the window. At 9.13 A.M. two
+additional muslin blinds were suspended, so that the seedlings received
+very little more light from the window than from the interior of the room.
+The sky varied in brightness, and the seedlings occasionally
+
+Fig. 175. Tropaeolum majus: heliotropic movement and circumnutation of the
+epicotyl of a young seedling towards a dull lateral light, traced on a
+horizontal glass from 7.48 A.M. to 10.40 P.M. Figure reduced to one-half of
+the original scale.
+
+received for a short time less light from the window than from the opposite
+side (as ascertained by the shadow cast), and then one of the blinds was
+temporarily removed. In the evening the blinds were taken away, one by one.
+the course pursued by an epicotyl under these circumstances is shown in
+Fig. 175. During the whole day, until 6.45 P.M., it plainly bowed itself
+towards the light; and the tip moved over a considerable space. After 6.45
+P.M. it moved backwards, or from the window, till
+[page 430]
+10.40 P.M., when the last dot was made. Here, then, we have a distinct
+heliotropic movement, effected by means of six elongated figures (which if
+dots had been made every few minutes would have been more or less elliptic)
+directed towards the light, with the apex of each successive ellipse nearer
+to the window than the previous one. Now, if the light had been only a
+little brighter, the epicotyl would have bowed itself more to the light, as
+we may safely conclude from the previous trials; there would also have been
+less lateral movement, and the ellipses or other figures would have been
+drawn out into a strongly marked zigzag line, with probably one or two
+small loops still formed. If the light had been much brighter, we should
+have had a slightly zigzag line, or one quite straight, for there would
+have been more movement in the direction of the light, and much less from
+side to side.
+
+Fig. 176. Tropaeolum majus: heliotropic movement and circumnutation of an
+old internode towards a lateral light, traced on a horizontal glass from 8
+A.M. Nov. 2nd to 10.20 A.M. Nov. 4th. Broken lines show the nocturnal
+course.
+
+Sachs states that the older internodes of this Tropaeolum are
+apheliotropic; we therefore placed a plant, 11 3/4 inches high, in a box,
+blackened within, but open on one side in front of a north-east window
+without any blind. A filament was fixed to the third internode from the
+summit on one plant, and to the fourth internode of another. These
+internodes were either not old enough, or the light was not sufficiently
+bright, to induce apheliotropism, for both plants bent slowly towards,
+instead of from the window during four days. The course, during two days of
+the first-mentioned internode, is given in Fig. 176; and we see that it
+either circumnutated on a small scale, or travelled in a zigzag line
+towards the light. We have thought this case of feeble heliotropism in one
+of the older internodes of a plant,
+[page 431]
+which, whilst young, is so extremely sensitive to light, worth giving.
+
+Fig. 177. Cassia tora: heliotropic movement and circumnutation of a
+hypocotyl (1 � inch in height) traced on a horizontal glass from 8 A.M. to
+10.10 P.M. Oct. 7th. Also its circumnutation in darkness from 7 A.M. Oct.
+8th to 7.45 A.M. Oct. 9th.
+
+Cassia tora.--The cotyledons of this plant are extremely sensitive to
+light, whilst the hypocotyls are much less sensitive than those of most
+other seedlings, as we had often observed with surprise. It seemed
+therefore worth while to trace their movements. They were exposed to a
+lateral light before a north-east window, which was at first covered merely
+by a muslin blind, but as the sky grew brighter about 11 A.M., an
+additional linen blind was suspended. After 4 P.M. one blind and then the
+other was removed. The seedlings were protected on each side and above, but
+were open to the diffused light of the room in the rear. Upright filaments
+were fixed to the hypocotyls of two seedlings, which stood vertically in
+the morning. The accompanying figure (Fig. 177) shows the course pursued by
+one of them during two days; but it should be particularly noticed that
+during the second day the seedlings were kept in darkness, and they then
+circumnutated round nearly the same small space. On the first day (Oct.
+7th) the hypocotyl moved from 8 A.M. to 12.23 P.M., toward the light in a
+zigzag line, then turned abruptly to the left and afterwards described a
+small ellipse. Another irregular
+[page 432]
+ellipse was completed between 3 P.M. and about 5.30 P.M., the hypocotyl
+still bending towards the light. The hypocotyl was straight and upright in
+the morning, but by 6 P.M. its upper half was bowed towards the light, so
+that the chord of the arc thus formed stood at an angle of 20o with the
+perpendicular. After 6 P.M. its course was reversed through the action of
+apogeotropism, and it continued to bend from the window during the night,
+as shown by the broken line. On the next day it was kept in the dark
+(excepting when each observation was made by the aid of a taper), and the
+course followed from 7 A.M. on the 8th to 7.45 A.M. on the 9th is here
+likewise shown. The difference between the two parts of the figure (177),
+namely that described during the daytime on the 7th, when exposed to a
+rather dim lateral light, and that on the 8th in darkness, is striking. The
+difference consists in the lines during the first day having been drawn out
+in the direction of the light. The movements of the other seedling, traced
+under the same circumstances, were closely similar.
+
+Apheliotropism.--We succeeded in observing only two cases of
+apheliotropism, for these are somewhat rare; and the movements are
+generally so slow that they would have been very troublesome to trace.
+
+Fig. 178. Bignonia capreolata: apheliotropic movement of a tendril, traced
+on a horizontal glass from 6.45 A.M. July 19th to 10 A.M. 20th. Movements
+as originally traced, little magnified, here reduced to two-thirds of the
+original scale.
+
+Bignonia capreolata.--No organ of any plant, as far as we have seen, bends
+away so quickly from the light as do the tendrils of this Bignonia. They
+are also remarkable from circumnutating much less regularly than most other
+tendrils, often remaining stationary; they depend on apheliotropism for
+coming into
+[page 433]
+contact with the trunks of trees.* The stem of a young plant was tied to a
+stick at the base of a pair of fine tendrils, which projected almost
+vertically upwards; and it was placed in front of a north-east window,
+being protected on all other sides from the light. The first dot was made
+at 6.45 A.M., and by 7.35 A.M. both tendrils felt the full influence of the
+light, for they moved straight away from it until 9.20 A.M., when they
+circumnutated for a time, still moving, but only a little, from the light
+(see Fig. 178 of the left-hand tendril). After 3 P.M. they again moved
+rapidly away from the light in zigzag lines. By a late hour in the evening
+both had moved so far, that they pointed in a direct line from the light.
+During the night they returned a little in a nearly opposite direction. On
+the following morning they again moved from the light and converged, so
+that by the evening they had become interlocked, still pointing from the
+light. The right-hand tendril, whilst converging, zigzagged much more than
+the one figured. Both tracings showed that the apheliotropic movement was a
+modified form of circumnutation.
+
+Cyclamen Persicum.--Whilst this plant is in flower the peduncles stand
+upright, but their uppermost part is hooked so that the flower itself hangs
+downwards. As soon as the pods begin to swell, the peduncles increase much
+in length and slowly curve downwards, but the short, upper, hooked part
+straightens itself. Ultimately the pods reach the ground, and if this is
+covered with moss or dead leaves, they bury themselves. We have often seen
+saucer-like depressions formed by the pods in damp sand or sawdust; and one
+pod (.3 of inch in diameter) buried itself in sawdust for three-quarters of
+its length.** We shall have occasion hereafter to consider the object
+gained by this burying process. The peduncles can change the direction of
+their curvature, for if a pot, with plants having their peduncles already
+bowed downwards, be placed horizontally, they slowly bend at right angles
+to their former direction towards the centre of the earth. We therefore at
+first attributed the movement to geotropism; but a pot which had lain
+horizontally with the pods
+
+* 'The Movements and Habits of Climbing Plants,' 1875, p. 97.
+
+** The peduncles of several other species of Cyclamen twist themselves into
+a spire, and according to Erasmus Darwin ('Botanic Garden,' Canto., iii. p.
+126), the pods forcibly penetrate the earth. See also Grenier and Godron,
+'Flore de France,' tom. ii. p. 459.
+[page 434]
+
+all pointing to the ground, was reversed, being still kept horizontal, so
+that the pods now pointed directly upwards; it was then placed in a dark
+cupboard, but the pods still pointed upwards after four days and nights.
+The pot, in the same position, was next brought back into the light, and
+after two days there was some bending downwards of the peduncles, and on
+the fourth day two of them pointed to the centre of the earth, as did the
+others after an additional day or two. Another plant, in a pot which had
+always stood upright, was left in the dark cupboard for six days; it bore 3
+peduncles, and only one became within this
+
+Fig. 179. Cyclamen Persicum: downward apheliotropic movement of a
+flower-peduncle, greatly magnified (about 47 times?), traced on a
+horizontal glass from 1 P.M. Feb. 18th to 8 A.M. 21st.
+
+time at all bowed downwards, and that doubtfully. The weight, therefore, of
+the pods is not the cause of the bending down. This pot was then brought
+back into the light, and after three days the peduncles were considerably
+bowed downwards. We are thus led to infer that the downward curvature is
+due to apheliotropism; though more trials ought to have been made.
+
+In order to observe the nature of this movement, a peduncle bearing a large
+pod which had reached and rested on the ground, was lifted a little up and
+secured to a stick. A filament was fixed across the pod with a mark
+beneath, and its move-
+[page 435]
+ment, greatly magnified, was traced on a horizontal glass during 67 h. The
+plant was illuminated during the day from above. A copy of the tracing is
+given on p. 434 (Fig. 179); and there can be no doubt that the descending
+movement is one of modified circumnutation, but on an extremely small
+scale. The observation was repeated on another pod, which had partially
+buried itself in sawdust, and which was lifted up a quarter of an inch
+above the surface; it described three very small circles in 24 h.
+Considering the great length and thinness of the peduncles and the
+lightness of the pods, we may conclude that they would not be able to
+excavate saucer-like depressions in sand or sawdust, or bury themselves in
+moss, etc., unless they were aided by their continued rocking or
+circumnutating movement.]
+
+Relation between Circumnutation and Heliotropism.--Any one who will look at
+the foregoing diagrams, showing the movements of the stems of various
+plants towards a lateral and more or less dimmed light, will be forced to
+admit that ordinary circumnutation and heliotropism graduate into one
+another. When a plant is exposed to a dim lateral light and continues
+during the whole day bending towards it, receding late in the evening, the
+movement unquestionably is one of heliotropism. Now, in the case of
+Tropaeolum (Fig. 175) the stem or epicotyl obviously circumnutated during
+the whole day, and yet it continued at the same time to move
+heliotropically; this latter movement being effected by the apex of each
+successive elongated figure or ellipse standing nearer to the light than
+the previous one. In the case of Cassia (Fig. 177) the comparison of the
+movement of the hypocotyl, when exposed to a dim lateral light and to
+darkness, is very instructive; as is that between the ordinary
+circumnutating movement of a seedling Brassica (Figs. 172, 173), or that of
+Phalaris (Figs. 49, 174), and their heliotropic movement towards a window
+protected by blinds. In both these cases,
+[page 436]
+and in many others, it was interesting to notice how gradually the stems
+began to circumnutate as the light waned in the evening. We have therefore
+many kinds of gradations from a movement towards the light, which must be
+considered as one of circumnutation very slightly modified and still
+consisting of ellipses or circles,--though a movement more or less strongly
+zigzag, with loops or ellipses occasionally formed,--to a nearly straight,
+or even quite straight, heliotropic course.
+
+A plant, when exposed to a lateral light, though this may be bright,
+commonly moves at first in a zigzag line, or even directly from the light;
+and this no doubt is due to its circumnutating at the time in a direction
+either opposite to the source of the light, or more or less transversely to
+it. As soon, however, as the direction of the circumnutating movement
+nearly coincides with that of the entering light, the plant bends in a
+straight course towards the light, if this is bright. The course appears to
+be rendered more and more rapid and rectilinear, in accordance with the
+degree of brightness of the light--firstly, by the longer axes of the
+elliptical figures, which the plant continues to describe as long as the
+light remains very dim, being directed more or less accurately towards its
+source, and by each successive ellipse being described nearer to the light.
+Secondly, if the light is only somewhat dimmed, by the acceleration and
+increase of the movement towards it, and by the retardation or arrestment
+of that from the light, some lateral movement being still retained, for the
+light will interfere less with a movement at right angles to its direction,
+than with one in its own direction.*
+
+* In his paper, 'Ueber orthotrope und plagiotrope Pflanzentheile'
+('Arbeiten des Bot. Inst. in W�rzburg,' Band ii. Heft ii.
+[[page 437]]
+1879), Sachs has discussed the manner in which geotropism and heliotropism
+are affected by differences in the angles at which the organs of plants
+stand with respect to the direction of the incident force.
+[page 437]
+
+The result is that the course is rendered more or less zigzag and unequal
+in rate. Lastly, when the light is very bright all lateral movement is
+lost; and the whole energy of the plant is expended in rendering the
+circumnutating movement rectilinear and rapid in one direction alone,
+namely, towards the light.
+
+The common view seems to be that heliotropism is a quite distinct kind of
+movement from circumnutation; and it may be urged that in the foregoing
+diagrams we see heliotropism merely combined with, or superimposed on,
+circumnutation. But if so, it must be assumed that a bright lateral light
+completely stops circumnutation, for a plant thus exposed moves in a
+straight line towards it, without describing any ellipses or circles. If
+the light be somewhat obscured, though amply sufficient to cause the plant
+to bend towards it, we have more or less plain evidence of still-continued
+circumnutation. It must further be assumed that it is only a lateral light
+which has this extraordinary power of stopping circumnutation, for we know
+that the several plants above experimented on, and all the others which
+were observed by us whilst growing, continue to circumnutate, however
+bright the light may be, if it comes from above. Nor should it be forgotten
+that in the life of each plant, circumnutation precedes heliotropism, for
+hypocotyls, epicotyls, and petioles circumnutate before they have broken
+through the ground and have ever felt the influence of light.
+
+We are therefore fully justified, as it seems to us, in believing that
+whenever light enters laterally, it is the
+[page 438]
+movement of circumnutation which gives rise to, or is converted into,
+heliotropism and apheliotropism. On this view we need not assume against
+all analogy that a lateral light entirely stops circumnutation; it merely
+excites the plant to modify its movement for a time in a beneficial manner.
+The existence of every possible gradation, between a straight course
+towards a lateral light and a course consisting of a series of loops or
+ellipses, becomes perfectly intelligible. Finally, the conversion of
+circumnutation into heliotropism or apheliotropism, is closely analogous to
+what takes place with sleeping plants, which during the daytime describe
+one or more ellipses, often moving in zigzag lines and making little loops;
+for when they begin in the evening to go to sleep, they likewise expend all
+their energy in rendering their course rectilinear and rapid. In the case
+of sleep-movements, the exciting or regulating cause is a difference in the
+intensity of the light, coming from above, at different periods of the
+twenty-four hours; whilst with heliotropic and apheliotropic movements, it
+is a difference in the intensity of the light on the two sides of the
+plant.
+
+Transversal-heliotropismus (of Frank*) or Diaheliotropism.--The cause of
+leaves placing themselves more or less transversely to the light, with
+their upper surfaces directed towards it, has been of late the subject of
+much controversy. We do not here refer to the object of the movement, which
+no doubt is that their upper surfaces may be fully illuminated, but the
+means by which this position is gained. Hardly a better or more simple
+instance can be given
+
+* 'Die nat�rliche Wagerechte Richtung von Pflanzentheilen,' 1870. See also
+some interesting articles by the same author, "Zur Frage �ber
+Transversal-Geo-und Heliotropismus," 'Bot. Zeitung,' 1873, p. 17 et seq.
+[page 439]
+
+of diaheliotropism than that offered by many seedlings, the cotyledons of
+which are extended horizontally. When they first burst from their
+seed-coats they are in contact and stand in various positions, often
+vertically upwards; they soon diverge, and this is effected by epinasty,
+which, as we have seen, is a modified form of circumnutation. After they
+have diverged to their full extent, they retain nearly the same position,
+though brightly illuminated all day long from above, with their lower
+surfaces close to the ground and thus much shaded. There is therefore a
+great contrast in the degree of illumination of their upper and lower
+surfaces, and if they were heliotropic they would bend quickly upwards. It
+must not, however, be supposed that such cotyledons are immovably fixed in
+a horizontal position. When seedlings are exposed before a window, their
+hypocotyls, which are highly heliotropic, bend quickly towards it, and the
+upper surfaces of their cotyledons still remain exposed at right angles to
+the light; but if the hypocotyl is secured so that it cannot bend, the
+cotyledons themselves change their position. If the two are placed in the
+line of the entering light, the one furthest from it rises up and that
+nearest to it often sinks down; if placed transversely to the light, they
+twist a little laterally; so that in every case they endeavour to place
+their upper surfaces at right angles to the light. So it notoriously is
+with the leaves on plants nailed against a wall, or grown in front of a
+window. A moderate amount of light suffices to induce such movements; all
+that is necessary is that the light should steadily strike the plants in an
+oblique direction. With respect to the above twisting movement of
+cotyledons, Frank has given many and much more striking instances in the
+case of the leaves on
+[page 440]
+branches which had been fastened in various positions or turned upside
+down.
+
+In our observations on the cotyledons of seedling plants, we often felt
+surprise at their persistent horizontal position during the day, and were
+convinced before we had read Frank's essay, that some special explanation
+was necessary. De Vries has shown* that the more or less horizontal
+position of leaves is in most cases influenced by epinasty, by their own
+weight, and by apogeotropism. A young cotyledon or leaf after bursting free
+is brought down into its proper position, as already remarked, by epinasty,
+which, according to De Vries, long continues to act on the midribs and
+petioles. Weight can hardly be influential in the case of cotyledons,
+except in a few cases presently to be mentioned, but must be so with large
+and thick leaves. With respect to apogeotropism, De Vries maintains that it
+generally comes into play, and of this fact we shall presently advance some
+indirect evidence. But over these and other constant forces we believe that
+there is in many cases, but we do not say in all, a preponderant tendency
+in leaves and cotyledons to place themselves more or less transversely with
+respect to the light.
+
+In the cases above alluded to of seedlings exposed to a lateral light with
+their hypocotyls secured, it is impossible that epinasty, weight and
+apogeotropism, either in opposition or combined, can be the cause of the
+rising of one cotyledon, and of the sinking of the other, since the forces
+in question act equally on both; and since epinasty, weight and
+apogeotropism all act in a vertical plane, they cannot cause the twisting
+of the petioles, which occurs in seedlings under the
+
+* 'Arbeiten des Bot. Instituts in W�rzburg,' Heft. ii. 1872, pp. 223-277.
+[page 441]
+
+above conditions of illumination. All these movements evidently depend in
+some manner on the obliquity of the light, but cannot be called
+heliotropic, as this implies bending towards the light; whereas the
+cotyledon nearest to the light bends in an opposed direction or downwards,
+and both place themselves as nearly as possible at right angles to the
+light. The movement, therefore, deserves a distinct name. As cotyledons and
+leaves are continually oscillating up and down, and yet retain all day long
+their proper position with their upper surfaces directed transversely to
+the light, and if displaced reassume this position, diaheliotropism must be
+considered as a modified form of circumnutation. This was often evident
+when the movements of cotyledons standing in front of a window were traced.
+We see something analogous in the case of sleeping leaves or cotyledons,
+which after oscillating up and down during the whole day, rise into a
+vertical position late in the evening, and on the following morning sink
+down again into their horizontal or diaheliotropic position, in direct
+opposition to heliotropism. This return into their diurnal position, which
+often requires an angular movement of 90o, is analogous to the movement of
+leaves on displaced branches, which recover their former positions. It
+deserves notice that any force such as apogeotropism, will act with
+different degrees of power* in the different positions of those leaves or
+cotyledons which oscillate largely up and down during the day; and yet they
+recover their horizontal or diaheliotropic position.
+
+We may therefore conclude that diaheliotropic movements cannot be fully
+explained by the direct action of light, gravitation, weight, etc., any
+more
+
+* See former note, in reference to Sachs' remarks on this subject.
+[page 442]
+
+than can the nyctitropic movements of cotyledons and leaves. In the latter
+case they place themselves so that their upper surfaces may radiate at
+night as little as possible into open space, with the upper surfaces of the
+opposite leaflets often in contact. These movements, which are sometimes
+extremely complex, are regulated, though not directly caused, by the
+alternations of light and darkness. In the case of diaheliotropism,
+cotyledons and leaves place themselves so that their upper surfaces may be
+exposed to the light, and this movement is regulated, though not directly
+caused, by the direction whence the light proceeds. In both cases the
+movement consists of circumnutation modified by innate or constitutional
+causes, in the same manner as with climbing plants, the circumnutation of
+which is increased in amplitude and rendered more circular, or again with
+very young cotyledons and leaves which are thus brought down into a
+horizontal position by epinasty.
+
+We have hitherto referred only to those leaves and cotyledons which occupy
+a permanently horizontal position; but many stand more or less obliquely,
+and some few upright. the cause of these differences of position is not
+known; but in accordance with Wiesner's views, hereafter to be given, it is
+probable that some leaves and cotyledons would suffer, if they were fully
+illuminated by standing at right angles to the light.
+
+We have seen in the second and fourth chapters that those cotyledons and
+leaves which do not alter their positions at night sufficiently to be said
+to sleep, commonly rise a little in the evening and fall again on the next
+morning, so that they stand during the night at a rather higher inclination
+than during the middle of the day. It is incredible that a rising movement
+of 2o or 3o, or even of 10o or 20o, can be of
+[page 443]
+any service to the plant, so as to have been specially acquired. It must be
+the result of some periodical change in the conditions to which they are
+subjected, and there can hardly be a doubt that this is the daily
+alternations of light and darkness. De Vries states in the paper before
+referred to, that most petioles and midribs are apogeotropic;* and
+apogeotropism would account for the above rising movement, which is common
+to so many widely distinct species, if we suppose it to be conquered by
+diaheliotropism during the middle of the day, as long as it is of
+importance to the plant that its cotyledons and leaves should be fully
+exposed to the light. The exact hour in the afternoon at which they begin
+to bend slightly upwards, and the extent of the movement, will depend on
+their degree of sensitiveness to gravitation and on their power of
+resisting its action during the middle of the day, as well as on the
+amplitude of their ordinary circumnutating movements; and as these
+qualities differ much in different species, we might expect that the hour
+in the afternoon at which they begin to rise would differ much in different
+species, as is the case. Some other agency, however, besides apogeotropism,
+must come into play, either directly or indirectly, in this upward
+movement. Thus a young bean (Vicia faba), growing in a small pot, was
+placed in front of a window in a klinostat; and at night the leaves rose a
+little, although
+
+* According to Frank ('Die nat. Wagerechte Richtung von Pflanzentheilen,'
+1870, p. 46) the root-leaves of many plants, kept in darkness, rise up and
+even become vertical; and so it is in some cases with shoots. (See
+Rauwenhoff, 'Archives N�erlandaises,' tom. xii. p. 32.) These movements
+indicate apogeotropism; but when organs have been long kept in the dark,
+the amount of water and of mineral matter which they contain is so much
+altered, and their regular growth is so much disturbed, that it is perhaps
+rash to infer from their movements what would occur under normal
+conditions. (See Godlewski, 'Bot. Zeitung,' Feb. 14th, 1879.)
+[page 444]
+
+the action of apogeotropism was quite eliminated. Nevertheless, they did
+not rise nearly so much at night, as when subjected to apogeotropism. Is it
+not possible, or even probable, that leaves and cotyledons, which have
+moved upwards in the evening through the action of apogeotropism during
+countless generations, may inherit a tendency to this movement? We have
+seen that the hypocotyls of several Leguminous plants have from a remote
+period inherited a tendency to arch themselves; and we know that the
+sleep-movements of leaves are to a certain extent inherited, independently
+of the alternations of light and darkness.
+
+In our observations on the circumnutation of those cotyledons and leaves
+which do not sleep at night, we met with hardly any distinct cases of their
+sinking a little in the evening, and rising again in the morning,--that is,
+of movements the reverse of those just discussed. We have no doubt that
+such cases occur, inasmuch as the leaves of many plants sleep by sinking
+vertically downwards. How to account for the few cases which were observed
+must be left doubtful. The young leaves of Cannabis sativa sink at night
+between 30o and 40o beneath the horizon; and Kraus attributes this to
+epinasty in conjunction with the absorption of water. Whenever epinastic
+growth is vigorous, it might conquer diaheliotropism in the evening, at
+which time it would be of no importance to the plant to keep its leaves
+horizontal. The cotyledons of Anoda Wrightii, of one variety of Gossypium,
+and of several species of Ipomoea, remain horizontal in the evening whilst
+they are very young; as they grow a little older they curve a little
+downwards, and when large and heavy sink so much that they come under our
+definition of sleep. In the case of
+[page 445]
+the Anoda and of some species of Ipomoea, it was proved that the downward
+movement did not depend on the weight of the cotyledons; but from the fact
+of the movement being so much more strongly pronounced after the cotyledons
+have grown large and heavy, we may suspect that their weight aboriginally
+played some part in determining that the modification of the circumnutating
+movement should be in a downward direction.
+
+The so-called Diurnal Sleep of Leaves, Or Paraheliotropism.--This is
+another class of movements, dependent on the action of light, which
+supports to some extent the belief that the movements above described are
+only indirectly due to its action. We refer to the movements of leaves and
+cotyledons which when moderately illuminated are diaheliotropic; but which
+change their positions and present their edges to the light, when the sun
+shines brightly on them. These movements have sometimes been called diurnal
+sleep, but they differ wholly with respect to the object gained from those
+properly called nyctitropic; and in some cases the position occupied during
+the day is the reverse of that during the night.
+
+[It has long been known* that when the sun shines brightly on the leaflets
+of Robinia, they rise up and present their edges to the light; whilst their
+position at night is vertically downwards. We have observed the same
+movement, when the sun shone brightly on the leaflets of an Australian
+Acacia. Those of Amphicarpaea monoica turned their edges to the sun; and an
+analogous movement of the little almost rudimentary basal leaflets of
+Mimosa albida was on one occasion so rapid that it could be distinctly seen
+through a lens. the elongated, unifoliate, first leaves of Phaseolus
+Roxburghii stood at 7 A.M. at 20o above the horizon, and no doubt they
+afterwards sank a little lower. At noon, after having been exposed for
+about 2 h. to
+
+* Pfeffer gives the names and dates of several ancient writers in his 'Die
+Periodischen Bewegungen,' 1875, p. 62.
+[page 446]
+
+a bright sun, they stood at 56o above the horizon; they were then protected
+from the rays of the sun, but were left well illuminated from above, and
+after 30 m. they had fallen 40o, for they now stood at only 16o above the
+horizon. Some young plants of Phaseolus Hernandesii had been exposed to the
+same bright sunlight, and their broad, unifoliate, first leaves now stood
+up almost or quite vertically, as did many of the leaflets on the
+trifoliate secondary leaves; but some of the leaflets had twisted round on
+their own axes by as much as 90o without rising, so as to present their
+edges to the sun. The leaflets on the same leaf sometimes behaved in these
+two different manners, but always with the result of being less intensely
+illuminated. These plants were then protected from the sun, and were looked
+at after 1 � h.; and now all the leaves and leaflets had reassumed their
+ordinary sub-horizontal positions. The copper-coloured cotyledons of some
+seedlings of Cassia mimosoides were horizontal in the morning, but after
+the sun had shone on them, each had risen 45 1/2o above the horizon. the
+movement in these several cases must not be confounded with the sudden
+closing of the leaflets of Mimosa pudica, which may sometimes be noticed
+when a plant which has been kept in an obscure place is suddenly exposed to
+the sun; for in this case the light seems to act, as if it were a touch.
+
+From Prof. Wiesner's interesting observations, it is probable that the
+above movements have been acquired for a special purpose. the chlorophyll
+in leaves is often injured by too intense a light, and Prof. Wiesner*
+believes that it is protected by the most diversified means, such as the
+presence of hairs, colouring matter, etc., and amongst other means by the
+leaves presenting their edges to the sun, so that the blades then receive
+much less light. He experimented on the young leaflets of Robinia, by
+fixing them in such a position that they could not escape being intensely
+illuminated, whilst others were allowed to place themselves obliquely; and
+the former began to suffer from the light in the course of two days.
+
+In the cases above given, the leaflets move either upwards
+
+* 'Die N�turlichen Einrichtungen zum Schutze des Chlorophylls,' etc., 1876.
+Pringsheim has recently observed under the microscope the destruction of
+chlorophyll in a few minutes by the action of concentrated light from the
+sun, in the presence of oxygen. See, also, Stahl on the protection of
+chlorophyll from intense light, in 'Bot. Zeitung,' 1880.
+[page 447]
+
+or twist laterally, so as to place their edges in the direction of the
+sun's light; but Cohn long ago observed that the leaflets of Oxalis bend
+downwards when fully exposed to the sun. We witnessed a striking instance
+of this movement in the very large leaflets of O. Ortegesii. A similar
+movement may frequently be observed with the leaflets of Averrhoa bilimbi
+(a member of the Oxalidae); and a leaf is here represented (Fig. 180) on
+which the sun had shone. A diagram (Fig. 134) was given in the last
+chapter, representing the oscillations by which a leaflet rapidly descended
+under these circumstances; and the movement may be seen closely to resemble
+that (Fig. 133) by
+
+Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure to
+sunshine; but the leaflets are sometimes more depressed than is here shown.
+Figure much reduced.
+
+which it assumed its nocturnal position. It is an interesting fact in
+relation to our present subject that, as Prof. Batalin informs us in a
+letter, dated February, 1879, the leaflets of Oxalis acetosella may be
+daily exposed to the sun during many weeks, and they do not suffer if they
+are allowed to depress themselves; but if this be prevented, they lose
+their colour and wither in two or three days. Yet the duration of a leaf is
+about two months, when subjected only to diffused light; and in this case
+the leaflets never sink downwards during the day.]
+
+As the upward movements of the leaflets of Robinia, and the downward
+movements of those of Oxalis, have been proved to be highly beneficial to
+these plants when subjected to bright sunshine, it seems probable that they
+have been acquired for the special purpose of avoiding too intense an
+illumination. As it would have been very troublesome in all the above cases
+to
+[page 448]
+have watched for a fitting opportunity and to have traced the movement of
+the leaves whilst they were fully exposed to the sunshine, we did not
+ascertain whether paraheliotropism always consisted of modified
+circumnutation; but this certainly was the case with the Averrhoa, and
+probably with the other species, as their leaves were continually
+circumnutating.
+[page 449]
+
+
+CHAPTER IX.
+
+SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
+
+Uses of heliotropism--Insectivorous and climbing plants not heliotropic--
+Same organ heliotropic at one age and not at another--Extraordinary
+sensitiveness of some plants to light--The effects of light do not
+correspond with its intensity--Effects of previous illumination--Time
+required for the action of light--After-effects of light--Apogeotropism
+acts as soon as light fails--Accuracy with which plants bend to the light--
+This dependent on the illumination of one whole side of the part--Localised
+sensitiveness to light and its transmitted effects--Cotyledons of Phalaris,
+manner of bending--Results of the exclusion of light from their tips--
+Effects transmitted beneath the surface of the ground--Lateral illumination
+of the tip determines the direction of the curvature of the base--
+Cotyledons of Avena, curvature of basal part due to the illumination of
+upper part--Similar results with the hypocotyls of Brassica and Beta--
+Radicles of Sinapis apheliotropic, due to the sensitiveness of their tips--
+Concluding remarks and summary of chapter--Means by which circumnutation
+has been converted into heliotropism or apheliotropism.
+
+NO one can look at the plants growing on a bank or on the borders of a
+thick wood, and doubt that the young stems and leaves place themselves so
+that the leaves may be well illuminated. They are thus enabled to decompose
+carbonic acid. But the sheath-like cotyledons of some Gramineae, for
+instance, those of Phalaris, are not green and contain very little starch;
+from which fact we may infer that they decompose little or no carbonic
+acid. Nevertheless, they are extremely heliotropic; and this probably
+serves them in another way, namely, as a guide from the buried seeds
+through fissures in the ground or through overlying masses of vegetation,
+into the light and air. This view
+[page 450]
+is strengthened by the fact that with Phalaris and Avena the first true
+leaf, which is bright green and no doubt decomposes carbonic acid, exhibits
+hardly a trace of heliotropism. The heliotropic movements of many other
+seedlings probably aid them in like manner in emerging from the ground; for
+apogeotropism by itself would blindly guide them upwards, against any
+overlying obstacle.
+
+Heliotropism prevails so extensively among the higher plants, that there
+are extremely few, of which some part, either the stem, flower-peduncle,
+petiole, or leaf, does not bend towards a lateral light. Drosera
+rotundifolia is one of the few plants the leaves of which exhibit no trace
+of heliotropism. Nor could we see any in Dionaea, though the plants were
+not so carefully observed. Sir J. Hooker exposed the pitchers of Sarracenia
+for some time to a lateral light, but they did not bend towards it.* We can
+understand the reason why these insectivorous plants should not be
+heliotropic, as they do not live chiefly by decomposing carbonic acid; and
+it is much more important to them that their leaves should occupy the best
+position for capturing insects, than that they should be fully exposed to
+the light.
+
+Tendrils, which consist of leaves or of other organs modified, and the
+stems of twining plants, are, as Mohl long ago remarked, rarely
+heliotropic; and here again we can see the reason why, for if they had
+moved towards a lateral light they would have been drawn away from their
+supports. But some tendrils are apheliotropic, for instance those of
+Bignonia capreolata
+
+* According to F. Kurtz ('Verhandl. des Bot. Vereins der Provinz
+Brandenburg,' Bd. xx. 1878) the leaves or pitchers of Darlingtonia
+Californica are strongly apheliotropic. We failed to detect this movement
+in a plant which we possessed for a short time.
+[page 451]
+
+and of Smilax aspera; and the stems of some plants which climb by rootlets,
+as those of the Ivy and Tecoma radicans, are likewise apheliotropic, and
+they thus find a support. The leaves, on the other hand, of most climbing
+plants are heliotropic; but we could detect no signs of any such movement
+in those of Mutisia clematis.
+
+As heliotropism is so widely prevalent, and as twining plants are
+distributed throughout the whole vascular series, the apparent absence of
+any tendency in their stems to bend towards the light, seemed to us so
+remarkable a fact as to deserve further investigation, for it implies that
+heliotropism can be readily eliminated. When twining plants are exposed to
+a lateral light, their stems go on revolving or circumnutating about the
+same spot, without any evident deflection towards the light; but we thought
+that we might detect some trace of heliotropism by comparing the average
+rate at which the stems moved to and from the light during their successive
+revolutions.* Three young plants (about a foot in height) of Ipomoea
+caerulea and four of I. purpurea, growing in separate pots, were placed on
+a bright day before a north-east window in a room otherwise darkened, with
+the tips of their revolving stems fronting the window. When the tip of each
+plant pointed directly from the window, and when again towards it, the
+times were recorded. This was continued from 6.45 A.M. till a little after
+2 P.M. on June 17th. After a few observations we concluded that we could
+safely estimate the time
+
+* Some erroneous statements are unfortunately given on this subject, in
+'The Movements and Habits of Climbing Plants,' 1875, pp. 28, 32, 40, and
+53. Conclusions were drawn from an insufficient number of observations, for
+we did not then know at how unequal a rate the stems and tendrils of
+climbing plants sometimes travel in different parts of the same revolution.
+[page 452]
+
+taken by each semicircle, within a limit of error of at most 5 minutes.
+Although the rate of movement in different parts of the same revolution
+varied greatly, yet 22 semicircles to the light were completed, each on an
+average in 73.95 minutes; and 22 semicircles from the light each in 73.5
+minutes. It may, therefore, be said that they travelled to and from the
+light at exactly the same average rate; though probably the accuracy of the
+result was in part accidental. In the evening the stems were not in the
+least deflected towards the window. Nevertheless, there appears to exist a
+vestige of heliotropism, for with 6 out of the 7 plants, the first
+semicircle from the light, described in the early morning after they had
+been subjected to darkness during the night and thus probably rendered more
+sensitive, required rather more time, and the first semicircle to the light
+considerably less time, than the average. Thus with all 7 plants, taken
+together, the mean time of the first semicircle in the morning from the
+light, was 76.8 minutes, instead of 73.5 minutes, which is the mean of all
+the semicircles during the day from the light; and the mean of the first
+semicircle to the light was only 63.1, instead of 73.95 minutes, which was
+the mean of all the semicircles during the day to the light.
+
+Similar observations were made on Wistaria Sinensis, and the mean of 9
+semicircles from the light was 117 minutes, and of 7 semicircles to the
+light 122 minutes, and this difference does not exceed the probable limit
+of error. During the three days of exposure, the shoot did not become at
+all bent towards the window before which it stood. In this case the first
+semicircle from the light in the early morning of each day, required rather
+less time for its performance than did the first semicircle to the light;
+and this result,
+[page 453]
+if not accidental, appears to indicate that the shoots retain a trace of an
+original apheliotropic tendency. With Lonicera brachypoda the semicircles
+from and to the light differed considerably in time; for 5 semicircles from
+the light required on a mean 202.4 minutes, and 4 to the light, 229.5
+minutes; but the shoot moved very irregularly, and under these
+circumstances the observations were much too few.
+
+It is remarkable that the same part on the same plant may be affected by
+light in a widely different manner at different ages, and as it appears at
+different seasons. The hypocotyledonous stems of Ipomoea caerulea and
+purpurea are extremely heliotropic, whilst the stems of older plants, only
+about a foot in height, are, as we have just seen, almost wholly insensible
+to light. Sachs states (and we have observed the same fact) that the
+hypocotyls of the Ivy (Hedera helix) are slightly heliotropic; whereas the
+stems of plants grown to a few inches in height become so strongly
+apheliotropic, that they bend at right angles away from the light.
+Nevertheless, some young plants which had behaved in this manner early in
+the summer again became distinctly heliotropic in the beginning of
+September; and the zigzag courses of their stems, as they slowly curved
+towards a north-east window, were traced during 10 days. The stems of very
+young plants of Tropaeolum majus are highly heliotropic, whilst those of
+older plants, according to Sachs, are slightly apheliotropic. In all these
+cases the heliotropism of the very young stems serves to expose the
+cotyledons, or when the cotyledons are hypogean the first true leaves,
+fully to the light; and the loss of this power by the older stems, or their
+becoming apheliotropic, is connected with their habit of climbing.
+
+Most seedling plants are strongly heliotropic, and
+[page 454]
+it is no doubt a great advantage to them in their struggle for life to
+expose their cotyledons to the light as quickly and as fully as possible,
+for the sake of obtaining carbon. It has been shown in the first chapter
+that the greater number of seedlings circumnutate largely and rapidly; and
+as heliotropism consists of modified circumnutation, we are tempted to look
+at the high development of these two powers in seedlings as intimately
+connected. Whether there are any plants which circumnutate slowly and to a
+small extent, and yet are highly heliotropic, we do not know; but there are
+several, and there is nothing surprising in this fact, which circumnutate
+largely and are not at all, or only slightly, heliotropic. Of such cases
+Drosera rotundifolia offers an excellent instance. The stolons of the
+strawberry circumnutate almost like the stems of climbing plants, and they
+are not at all affected by a moderate light; but when exposed late in the
+summer to a somewhat brighter light they were slightly heliotropic; in
+sunlight, according to De Vries, they are apheliotropic. Climbing plants
+circumnutate much more widely than any other plants, yet they are not at
+all heliotropic.
+
+Although the stems of most seedling plants are strongly heliotropic, some
+few are but slightly heliotropic, without our being able to assign any
+reason. This is the case with the hypocotyl of Cassia tora, and we were
+struck with the same fact with some other seedlings, for instance, those of
+Reseda odorata. With respect to the degree of sensitiveness of the more
+sensitive kinds, it was shown in the last chapter that seedlings of several
+species, placed before a north-east window protected by several blinds, and
+exposed in the rear to the diffused light of the room, moved with unerring
+certainty towards the window, although
+[page 455]
+it was impossible to judge, excepting by the shadow cast by an upright
+pencil on a white card, on which side most light entered, so that the
+excess on one side must have been extremely small.
+
+A pot with seedlings of Phalaris Canariensis, which had been raised in
+darkness, was placed in a completely darkened room, at 12 feet from a very
+small lamp. After 3 h. the cotyledons were doubtfully curved towards the
+light, and after 7 h. 40 m. from the first exposure, they were all plainly,
+though slightly, curved towards the lamp. Now, at this distance of 12 feet,
+the light was so obscure that we could not see the seedlings themselves,
+nor read the large Roman figures on the white face of a watch, nor see a
+pencil line on paper, but could just distinguish a line made with Indian
+ink. It is a more surprising fact that no visible shadow was cast by a
+pencil held upright on a white card; the seedlings, therefore, were acted
+on by a difference in the illumination of their two sides, which the human
+eye could not distinguish. On another occasion even a less degree of light
+acted, for some cotyledons of Phalaris became slightly curved towards the
+same lamp at a distance of 20 feet; at this distance we could not see a
+circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on white
+paper, though we could just see a dot 3.56 mm. (.14 inch) in diameter; yet
+a dot of the former size appears large when seen in the light.*
+
+We next tried how small a beam of light would act; as this bears on light
+serving as a guide to seedlings whilst they emerge through fissured or
+encumbered ground. A pot with seedlings of Phalaris was covered
+
+* Strasburger says ('Wirkung des Lichtes auf Schw�rmsporen,' 1878, p. 52),
+that the spores of Haematococcus moved to a light which only just sufficed
+to allow middle-sized type to be read.
+[page 456]
+
+by a tin-vessel, having on one side a circular hole 1.23 mm. in diameter
+(i.e. a little less than the 1/20th of an inch); and the box was placed in
+front of a paraffin lamp and on another occasion in front of a window; and
+both times the seedlings were manifestly bent after a few hours towards the
+little hole.
+
+A more severe trial was now made; little tubes of very thin glass, closed
+at their upper ends and coated with black varnish, were slipped over the
+cotyledons of Phalaris (which had germinated in darkness) and just fitted
+them. Narrow stripes of the varnish had been previously scraped off one
+side, through which alone light could enter; and their dimensions were
+afterwards measured under the microscope. As a control experiment, similar
+unvarnished and transparent tubes were tried, and they did not prevent the
+cotyledons bending towards the light. Two cotyledons were placed before a
+south-west window, one of which was illuminated by a stripe in the varnish,
+only .004 inch (0.1 mm.) in breadth and .016 inch (0.4 mm.) in length; and
+the other by a stripe .008 inch in breadth and .06 inch in length. The
+seedlings were examined after an exposure of 7 h. 40 m., and were found to
+be manifestly bowed towards the light. Some other cotyledons were at the
+same time treated similarly, excepting that the little stripes were
+directed not to the sky, but in such a manner that they received only the
+diffused light from the room; and these cotyledons did not become at all
+bowed. Seven other cotyledons were illuminated through narrow, but
+comparatively long, cleared stripes in the varnish--namely, in breadth
+between .01 and .026 inch, and in length between .15 and .3 inch; and these
+all became bowed to the side, by which light entered through the stripes,
+whether these were directed towards the sky or to one side of
+[page 457]
+the room. That light passing through a hole only .004 inch in breadth by
+.016 in length, should induce curvature, seems to us a surprising fact.
+
+Before we knew how extremely sensitive the cotyledons of Phalaris were to
+light, we endeavoured to trace their circumnutation in darkness by the aid
+of a small wax taper, held for a minute or two at each observation in
+nearly the same position, a little on the left side in front of the
+vertical glass on which the tracing was made. The seedlings were thus
+observed seventeen times in the course of the day, at intervals of from
+half to three-quarters of an hour; and late in the evening we were
+surprised to find that all the 29 cotyledons were greatly curved and
+pointed towards the vertical glass, a little to the left where the taper
+had been held. The tracings showed that they had travelled in zigzag lines.
+Thus, an exposure to a feeble light for a very short time at the above
+specified intervals, sufficed to induce well-marked heliotropism. An
+analogous case was observed with the hypocotyls of Solanum lycopersicum. We
+at first attributed this result to the after-effects of the light on each
+occasion; but since reading Wiesner's observations,* which will be referred
+to in the last chapter, we cannot doubt that an intermittent light is more
+efficacious than a continuous one, as plants are especially sensitive to
+any contrast in its amount.
+
+The cotyledons of Phalaris bend much more slowly towards a very obscure
+light than towards a bright one. Thus, in the experiments with seedlings
+placed in a dark room at 12 feet from a very small lamp, they were just
+perceptibly and doubtfully curved towards it after 3 h., and only slightly,
+yet certainly, after 4 h.
+
+* 'Sitz. der k. Akad. der Wissensch.' (Vienna), Jan. 1880, p. 12.
+[page 458]
+
+After 8 h. 40 m. the chords of their arcs were deflected from the
+perpendicular by an average angle of only 16o. Had the light been bright,
+they would have become much more curved in between 1 and 2 h. Several
+trials were made with seedlings placed at various distances from a small
+lamp in a dark room; but we will give only one trial. Six pots were placed
+at distances of 2, 4, 8, 12, 16, and 20 feet from the lamp, before which
+they were left for 4 h. As light decreases in a geometrical ratio, the
+seedlings in the 2nd pot received 1/4th, those in the 3rd pot 1/16th, those
+in the 4th 1/36th, those in the 5th 1/64th, and those in the 6th 1/100th of
+the light received by the seedlings in the first or nearest pot. Therefore
+it might have been expected that there would have been an immense
+difference in the degree of their heliotropic curvature in the several
+pots; and there was a well-marked difference between those which stood
+nearest and furthest from the lamp, but the difference in each successive
+pair of pots was extremely small. In order to avoid prejudice, we asked
+three persons, who knew nothing about the experiment, to arrange the pots
+in order according to the degree of curvature of the cotyledons. The first
+person arranged them in proper order, but doubted long between the 12 feet
+and 16 feet pots; yet these two received light in the proportion of 36 to
+64. The second person also arranged them properly, but doubted between the
+8 feet and 12 feet pots, which received light in the proportion of 16 to
+36. The third person arranged them in wrong order, and doubted about four
+of the pots. This evidence shows conclusively how little the curvature of
+the seedlings differed in the successive pots, in comparison with the great
+difference in the amount of light which they received; and it should be
+noted that there was no
+[page 459]
+excess of superfluous light, for the cotyledons became but little and
+slowly curved even in the nearest pot. Close to the 6th pot, at the
+distance of 20 feet from the lamp, the light allowed us just to distinguish
+a dot 3.56 mm. (.14 inch) in diameter, made with Indian ink on white paper,
+but not a dot 2.29 mm. (.09 inch) in diameter.
+
+The degree of curvature of the cotyledons of Phalaris within a given time,
+depends not merely on the amount of lateral light which they may then
+receive, but on that which they have previously received from above and on
+all sides. Analogous facts have been given with respect to the nyctitropic
+and periodic movements of plants. Of two pots containing seedlings of
+Phalaris which had germinated in darkness, one was still kept in the dark,
+and the other was exposed (Sept. 26th) to the light in a greenhouse during
+a cloudy day and on the following bright morning. On this morning (27th),
+at 10.30 A.M., both pots were placed in a box, blackened within and open in
+front, before a north-east window, protected by a linen and muslin blind
+and by a towel, so that but little light was admitted, though the sky was
+bright. Whenever the pots were looked at, this was done as quickly as
+possible, and the cotyledons were then held transversely with respect to
+the light, so that their curvature could not have been thus increased or
+diminished. After 50 m. the seedlings which had previously been kept in
+darkness, were perhaps, and after 70 m. were certainly, curved, though very
+slightly, towards the window. After 85 m. some of the seedlings, which had
+previously been illuminated, were perhaps a little affected, and after 100
+m. some of the younger ones were certainly a little curved towards the
+light. At this time (i.e. after 100 m.) there was a plain difference
+[page 460]
+in the curvature of the seedlings in the two pots. After 2 h. 12 m. the
+chords of the arcs of four of the most strongly curved seedlings in each
+pot were measured, and the mean angle from the perpendicular of those which
+had previously been kept in darkness was 19o, and of those which had
+previously been illuminated only 7o. Nor did this difference diminish
+during two additional hours. As a check, the seedlings in both pots were
+then placed in complete darkness for two hours, in order that apogeotropism
+should act on them; and those in the one pot which were little curved
+became in this time almost completely upright, whilst the more curved ones
+in the other pot still remained plainly curved.
+
+Two days afterwards the experiment was repeated, with the sole difference
+that even less light was admitted through the window, as it was protected
+by a linen and muslin blind and by two towels; the sky, moreover, was
+somewhat less bright. The result was the same as before, excepting that
+everything occurred rather slower. The seedlings which had been previously
+kept in darkness were not in the least curved after 54 m., but were so
+after 70 m. Those which had previously been illuminated were not at all
+affected until 130 m. had elapsed, and then only slightly. After 145 m.
+some of the seedlings in this latter pot were certainly curved towards the
+light; and there was now a plain difference between the two pots. After 3
+h. 45 m. the chords of the arcs of 3 seedlings in each pot were measured,
+and the mean angle from the perpendicular was 16o for those in the pot
+which had previously been kept in darkness, and only 5o for those which had
+previously been illuminated.
+
+The curvature of the cotyledons of Phalaris towards a lateral light is
+therefore certainly influenced by the
+[page 461]
+degree to which they have been previously illuminated. We shall presently
+see that the influence of light on their bending continues for a short time
+after the light has been extinguished. These facts, as well as that of the
+curvature not increasing or decreasing in nearly the same ratio with that
+of the amount of light which they receive, as shown in the trials with the
+plants before the lamp, all indicate that light acts on them as a stimulus,
+in somewhat the same manner as on the nervous system of animals, and not in
+a direct manner on the cells or cell-walls which by their contraction or
+expansion cause the curvature.
+
+It has already been incidentally shown how slowly the cotyledons of
+Phalaris bend towards a very dim light; but when they were placed before a
+bright paraffin lamp their tips were all curved rectangularly towards it in
+2 h. 20 m. The hypocotyls of Solanum lycopersicum had bent in the morning
+at right angles towards a north-east window. At 1 P.M. (Oct. 21st) the pot
+was turned round, so that the seedlings now pointed from the light, but by
+5 P.M. they had reversed their curvature and again pointed to the light.
+They had thus passed through 180o in 4 h., having in the morning previously
+passed through about 90o. But the reversal of the first half of the
+curvature will have been aided by apogeotropism. Similar cases were
+observed with other seedlings, for instance, with those of Sinapis alba.
+
+We attempted to ascertain in how short a time light acted on the cotyledons
+of Phalaris, but this was difficult on account of their rapid
+circumnutating movement; moreover, they differ much in sensibility,
+according to age; nevertheless, some of our observations are worth giving.
+Pots with seedlings were
+[page 462]
+placed under a microscope provided with an eye-piece micrometer, of which
+each division equalled 1/500th of an inch (0.051 mm.); and they were at
+first illuminated by light from a paraffin lamp passing through a solution
+of bichromate of potassium, which does not induce heliotropism. Thus the
+direction in which the cotyledons were circumnutating could be observed
+independently of any action from the light; and they could be made, by
+turning round the pots, to circumnutate transversely to the line in which
+the light would strike them, as soon as the solution was removed. The fact
+that the direction of the circumnutating movement might change at any
+moment, and thus the plant might bend either towards or from the lamp
+independently of the action of the light, gave an element of uncertainty to
+the results. After the solution had been removed, five seedlings which were
+circumnutating transversely to the line of light, began to move towards it,
+in 6, 4, 7 1/2, 6, and 9 minutes. In one of these cases, the apex of the
+cotyledon crossed five of the divisions of the micrometer (i.e. 1/100th of
+an inch, or 0.254 mm.) towards the light in 3 m. Of two seedlings which
+were moving directly from the light at the time when the solution was
+removed, one began to move towards it in 13 m., and the other in 15 m. This
+latter seedling was observed for more than an hour and continued to move
+towards the light; it crossed at one time 5 divisions of the micrometer
+(0.254 mm.) in 2 m. 30 s. In all these cases, the movement towards the
+light was extremely unequal in rate, and the cotyledons often remained
+almost stationary for some minutes, and two of them retrograded a little.
+Another seedling which was circumnutating transversely to the line of
+light, moved towards it in 4 m. after the solution was removed; it then
+remained
+[page 463]
+almost stationary for 10 m.; then crossed 5 divisions of the micrometer in
+6 m.; and then 8 divisions in 11m. This unequal rate of movement,
+interrupted by pauses, and at first with occasional retrogressions, accords
+well with our conclusion that heliotropism consists of modified
+circumnutation.
+
+In order to observe how long the after-effects of light lasted, a pot with
+seedlings of Phalaris, which had germinated in darkness, was placed at
+10.40 A.M. before a north-east window, being protected on all other sides
+from the light; and the movement of a cotyledon was traced on a horizontal
+glass. It circumnutated about the same space for the first 24 m., and
+during the next 1 h. 33 m. moved rapidly towards the light. The light was
+now (i.e. after 1 h. 57 m.) completely excluded, but the cotyledon
+continued bending in the same direction as before, certainly for more than
+15 m., probably for about 27 m. The doubt arose from the necessity of not
+looking at the seedlings often, and thus exposing them, though momentarily,
+to the light. This same seedling was now kept in the dark, until 2.18 P.M.,
+by which time it had reacquired through apogeotropism its original upright
+position, when it was again exposed to the light from a clouded sky. By 3
+P.M. it had moved a very short distance towards the light, but during the
+next 45 m. travelled quickly towards it. After this exposure of 1 h. 27 m.
+to a rather dull sky, the light was again completely excluded, but the
+cotyledon continued to bend in the same direction as before for 14 m.
+within a very small limit of error. It was then placed in the dark, and it
+now moved backwards, so that after 1 h. 7 m. it stood close to where it had
+started from at 2.18 P.M. These observations show that the cotyledons of
+Phalaris, after being exposed to a lateral
+[page 464]
+light, continue to bend in the same direction for between a quarter and
+half an hour.
+
+In the two experiments just given, the cotyledons moved backwards or from
+the window shortly after being subjected to darkness; and whilst tracing
+the circumnutation of various kinds of seedlings exposed to a lateral
+light, we repeatedly observed that late in the evening, as the light waned,
+they moved from it. This fact is shown in some of the diagrams given in the
+last chapter. We wished therefore to learn whether this was wholly due to
+apogeotropism, or whether an organ after bending towards the light tended
+from any other cause to bend from it, as soon as the light failed.
+Accordingly, two pots of seedling Phalaris and one pot of seedling Brassica
+were exposed for 8 h. before a paraffin lamp, by which time the cotyledons
+of the former and the hypocotyls of the latter were bent rectangularly
+towards the light. The pots were now quickly laid horizontally, so that the
+upper parts of the cotyledons and of the hypocotyls of 9 seedlings
+projected vertically upwards, as proved by a plumb-line. In this position
+they could not be acted on by apogeotropism, and if they possessed any
+tendency to straighten themselves or to bend in opposition to their former
+heliotropic curvature, this would be exhibited, for it would be opposed at
+first very slightly by apogeotropism. They were kept in the dark for 4 h.,
+during which time they were twice looked at; but no uniform bending in
+opposition to their former heliotropic curvature could be detected. We have
+said uniform bending, because they circumnutated in their new position, and
+after 2 h. were inclined in different directions (between 4o and 11o) from
+the perpendicular. Their directions were also changed after two additional
+hours, and again on the following morning. We may
+[page 465]
+therefore conclude that the bending back of plants from a light, when this
+becomes obscure or is extinguished, is wholly due to apogeotropism.*
+
+In our various experiments we were often struck with the accuracy with
+which seedlings pointed to a light although of small size. To test this,
+many seedlings of Phalaris, which had germinated in darkness in a very
+narrow box several feet in length, were placed in a darkened room near to
+and in front of a lamp having a small cylindrical wick. The cotyledons at
+the two ends and in the central part of the box, would therefore have to
+bend in widely different directions in order to point to the light. After
+they had become rectangularly bent, a long white thread was stretched by
+two persons, close over and parallel, first to one and then to another
+cotyledon; and the thread was found in almost every case actually to
+intersect the small circular wick of the now extinguished lamp. The
+deviation from accuracy never exceeded, as far as we could judge, a degree
+or two. This extreme accuracy seems at first surprising, but is not really
+so, for an upright cylindrical stem, whatever its position may be with
+respect to the light, would have exactly half its circumference illuminated
+and half in shadow; and as the difference in illumination of the two sides
+is the exciting cause of heliotropism, a cylinder would naturally bend with
+much accuracy towards the light. The cotyledons, however, of Phalaris are
+not cylindrical, but oval in section; and the longer axis was to the
+shorter axis (in the one which was measured) as 100 to 70. Nevertheless, no
+difference could be
+
+* It appears from a reference in Wiesner ('Die Undulirende Nutation der
+Internodien,' p. 7), that H. M�ller of Thurgau found that a stem which is
+bending heliotropically is at the same time striving, through
+apogeotropism, to raise itself into a vertical position.
+[page 466]
+
+detected in the accuracy of their bending, whether they stood with their
+broad or narrow sides facing the light, or in any intermediate position;
+and so it was with the cotyledons of Avena sativa, which are likewise oval
+in section. Now, a little reflection will show that in whatever position
+the cotyledons may stand, there will be a line of greatest illumination,
+exactly fronting the light, and on each side of this line an equal amount
+of light will be received; but if the oval stands obliquely with respect to
+the light, this will be diffused over a wider surface on one side of the
+central line than on the other. We may therefore infer that the same amount
+of light, whether diffused over a wider surface or concentrated on a
+smaller surface, produces exactly the same effect; for the cotyledons in
+the long narrow box stood in all sorts of positions with reference to the
+light, yet all pointed truly towards it.
+
+That the bending of the cotyledons to the light depends on the illumination
+of one whole side or on the obscuration of the whole opposite side, and not
+on a narrow longitudinal zone in the line of the light being affected, was
+shown by the effects of painting longitudinally with Indian ink one side of
+five cotyledons of Phalaris. These were then placed on a table near to a
+south-west window, and the painted half was directed either to the right or
+left. The result was that instead of bending in a direct line towards the
+window, they were deflected from the window and towards the unpainted side,
+by the following angles, 35o, 83o, 31o, 43o, and 39o. It should be remarked
+that it was hardly possible to paint one-half accurately, or to place all
+the seedlings which are oval in section in quite the same position
+relatively to the light; and this will account for the differences in the
+angles. Five coty-
+[page 467]
+ledons of Avena were also painted in the same manner, but with greater
+care; and they were laterally deflected from the line of the window,
+towards the unpainted side, by the following angles, 44o, 44o, 55o, 51o, and
+57o. This deflection of the cotyledons from the window is intelligible, for
+the whole unpainted side must have received some light, whereas the
+opposite and painted side received none; but a narrow zone on the unpainted
+side directly in front of the window will have received most light, and all
+the hinder parts (half an oval in section) less and less light in varying
+degrees; and we may conclude that the angle of deflection is the resultant
+of the action of the light over the whole of the unpainted side.
+
+It should have been premised that painting with Indian ink does not injure
+plants, at least within several hours; and it could injure them only by
+stopping respiration. To ascertain whether injury was thus soon caused, the
+upper halves of 8 cotyledons of Avena were thickly coated with transparent
+matter,--4 with gum, and 4 with gelatine; they were placed in the morning
+before a window, and by the evening they were normally bowed towards the
+light, although the coatings now consisted of dry crusts of gum and
+gelatine. Moreover, if the seedlings which were painted longitudinally with
+Indian ink had been injured on the painted side, the opposite side would
+have gone on growing, and they would consequently have become bowed towards
+the painted side; whereas the curvature was always, as we have seen, in the
+opposite direction, or towards the unpainted side which was exposed to the
+light. We witnessed the effects of injuring longitudinally one side of the
+cotyledons of Avena and Phalaris; for before we knew that grease was highly
+injurious to them, several were painted down one side
+[page 468]
+with a mixture of oil and lamp-black, and were then exposed before a
+window; others similarly treated were afterwards tried in darkness. These
+cotyledons soon became plainly bowed towards the blackened side, evidently
+owing to the grease on this side having checked their growth, whilst growth
+continued on the opposite side. But it deserves notice that the curvature
+differed from that caused by light, which ultimately becomes abrupt near
+the ground. These seedlings did not afterwards die, but were much injured
+and grew badly.
+
+LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS.
+
+Phalaris Canariensis.--Whilst observing the accuracy with which the
+cotyledons of this plant became bent towards the light of a small lamp, we
+were impressed with the idea that the uppermost part determined the
+direction of the curvature of the lower part. When the cotyledons are
+exposed to a lateral light, the upper part bends first, and afterwards the
+bending gradually extends down to the base, and, as we shall presently see,
+even a little beneath the ground. This holds good with cotyledons from less
+than .1 inch (one was observed to act in this manner which was only .03 in
+height) to about .5 of an inch in height; but when they have grown to
+nearly an inch in height, the basal part, for a length of .15 to .2 of an
+inch above the ground, ceases to bend. As with young cotyledons the lower
+part goes on bending, after the upper part has become well arched towards a
+lateral light, the apex would ultimately point to the ground instead of to
+the light, did not the upper part reverse its curvature and straighten
+itself, as
+[page 469]
+soon as the upper convex surface of the bowed-down portion received more
+light than the lower concave surface. The position ultimately assumed by
+young and upright cotyledons, exposed to light entering obliquely from
+above through a window, is shown in the accompanying figure (Fig. 181); and
+here it may be seen that the whole upper part has become very nearly
+straight. When the cotyledons were exposed before a bright lamp, standing
+on the same level with them, the upper part, which was at first
+
+Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open on
+one side in front of a south-west window during 8 h. Curvature towards the
+light accurately traced. The short horizontal lines show the level of the
+ground.
+
+greatly arched towards the light, became straight and strictly parallel
+with the surface of the soil in the pots; the basal part being now
+rectangularly bent. All this great amount of curvature, together with the
+subsequent straightening of the upper part, was often effected in a few
+hours.
+
+[After the uppermost part has become bowed a little to the light, its
+overhanging weight must tend to increase the curvature of the lower part;
+but any such effect was shown in several ways to be quite insignificant.
+When little caps of tin-foil (hereafter to be described) were placed on the
+summits of the cotyledons, though this must have added considerably to
+their weight, the rate or amount of bending was not thus increased. But the
+best evidence was afforded by placing pots with seedlings of Phalaris
+before a lamp in such a position, that the cotyledons were horizontally
+extended and projected at right angles to the line of light. In the course
+of 5 � h. they were directed towards the light with their bases bent at
+right angles; and this abrupt
+[page 470]
+curvature could not have been aided in the least by the weight of the upper
+part, which acted at right angles to the plane of curvature.
+
+It will be shown that when the upper halves of the cotyledons of Phalaris
+and Avena were enclosed in little pipes of tin-foil or of blackened glass,
+in which case the upper part was mechanically prevented from bending, the
+lower and unenclosed part did not bend when exposed to a lateral light; and
+it occurred to us that this fact might be due, not to the exclusion of the
+light from the upper part, but to some necessity of the bending gradually
+travelling down the cotyledons, so that unless the upper part first became
+bent, the lower could not bend, however much it might be stimulated. It was
+necessary for our purpose to ascertain whether this notion was true, and it
+was proved false; for the lower halves of several cotyledons became bowed
+to the light, although their upper halves were enclosed in little glass
+tubes (not blackened), which prevented, as far as we could judge, their
+bending. Nevertheless, as the part within the tube might possibly bend a
+very little, fine rigid rods or flat splinters of thin glass were cemented
+with shellac to one side of the upper part of 15 cotyledons; and in six
+cases they were in addition tied on with threads. They were thus forced to
+remain quite straight. The result was that the lower halves of all became
+bowed to the light, but generally not in so great a degree as the
+corresponding part of the free seedlings in the same pots; and this may
+perhaps be accounted for by some slight degree of injury having been caused
+by a considerable surface having been smeared with shellac. It may be
+added, that when the cotyledons of Phalaris and Avena are acted on by
+apogeotropism, it is the upper part which begins first to bend; and when
+this part was rendered rigid in the manner just described, the upward
+curvature of the basal part was not thus prevented.
+
+To test our belief that the upper part of the cotyledons of Phalaris, when
+exposed to a lateral light, regulates the bending of the lower part, many
+experiments were tried; but most of our first attempts proved useless from
+various causes not worth specifying. Seven cotyledons had their tips cut
+off for lengths varying between .1 and .16 of an inch, and these, when left
+exposed all day to a lateral light, remained upright. In another set of 7
+cotyledons, the tips were cut off for a length of only about .05 of an inch
+(1.27 mm.) and these became bowed towards
+[page 471]
+a lateral light, but not nearly so much as the many other seedlings in the
+same pots. This latter case shows that cutting off the tips does not by
+itself injure the plants so seriously as to prevent heliotropism; but we
+thought at the time, that such injury might follow when a greater length
+was cut off, as in the first set of experiments. Therefore, no more trials
+of this kind were made, which we now regret; as we afterwards found that
+when the tips of three cotyledons were cut off for a length of .2 inch, and
+of four others for lengths of .14, .12, .1, and .07 inch, and they were
+extended horizontally, the amputation did not interfere in the least with
+their bending vertically upwards, through the action of apogeotropism, like
+unmutilated specimens. It is therefore extremely improbable that the
+amputation of the tips for lengths of from .1 to .14 inch, could from the
+injury thus caused have prevented the lower part from bending towards the
+light.
+
+We next tried the effects of covering the upper part of the cotyledons of
+Phalaris with little caps which were impermeable to light; the whole lower
+part being left fully exposed before a south-west window or a bright
+paraffin lamp. Some of the caps were made of extremely thin tin-foil
+blackened within; these had the disadvantage of occasionally, though
+rarely, being too heavy, especially when twice folded. The basal edges
+could be pressed into close contact with the cotyledons; though this again
+required care to prevent injuring them. Nevertheless, any injury thus
+caused could be detected by removing the caps, and trying whether the
+cotyledons were then sensitive to light. Other caps were made of tubes of
+the thinnest glass, which when painted black served well, with the one
+great disadvantage that the lower ends could not be closed. But tubes were
+used which fitted the cotyledons almost closely, and black paper was placed
+on the soil round each, to check the upward reflection of light from the
+soil. Such tubes were in one respect far better than caps of tin-foil, as
+it was possible to cover at the same time some cotyledons with transparent
+and others with opaque tubes; and thus our experiments could be controlled.
+It should be kept in mind that young cotyledons were selected for trial,
+and that these when not interfered with become bowed down to the ground
+towards the light.
+
+We will begin with the glass-tubes. The summits of nine cotyledons,
+differing somewhat in height, were enclosed for rather less than half their
+lengths in uncoloured or transparent
+[page 472]
+tubes; and these were then exposed before a south-west window on a bright
+day for 8 h. All of them became strongly curved towards the light, in the
+same degree as the many other free seedlings in the same pots; so that the
+glass-tubes certainly did not prevent the cotyledons from bending towards
+the light. Nineteen other cotyledons were, at the same time, similarly
+enclosed in tubes thickly painted with Indian ink. On five of them, the
+paint, to our surprise, contracted after exposure to the sunlight, and very
+narrow cracks were formed, through which a little light entered; and these
+five cases were rejected. Of the remaining 14 cotyledons, the lower halves
+of which had been fully exposed to the light for the whole time, 7
+continued quite straight and upright; 1 was considerably bowed to the
+light, and 6 were slightly bowed, but with the exposed bases of most of
+them almost or quite straight. It is possible that some light may have been
+reflected upwards from the soil and entered the bases of these 7 tubes, as
+the sun shone brightly, though bits of blackened paper had been placed on
+the soil round them. Nevertheless, the 7 cotyledons which were slightly
+bowed, together with the 7 upright ones, presented a most remarkable
+contrast in appearance with the many other seedlings in the same pots to
+which nothing had been done. The blackened tubes were then removed from 10
+of these seedlings, and they were now exposed before a lamp for 8 h.; 9 of
+them became greatly, and 1 moderately, curved towards the light, proving
+that the previous absence of any curvature in the basal part, or the
+presence of only a slight degree of curvature there, was due to the
+exclusion of light from the upper part.
+
+Similar observations were made on 12 younger cotyledons with their upper
+halves enclosed within glass-tubes coated with black varnish, and with
+their lower halves fully exposed to bright sunshine. In these younger
+seedlings the sensitive zone seems to extend rather lower down, as was
+observed on some other occasions, for two became almost as much curved
+towards the light as the free seedlings; and the remaining ten were
+slightly curved, although the basal part of several of them, which normally
+becomes more curved than any other part, exhibited hardly a trace of
+curvature. These 12 seedlings taken together differed greatly in their
+degree of curvature from all the many other seedlings in the same pots.
+
+Better evidence of the efficiency of the blackened tubes was incidentally
+afforded by some experiments hereafter to be given,
+[page 473]
+in which the upper halves of 14 cotyledons were enclosed in tubes from
+which an extremely narrow stripe of the black varnish had been scraped off.
+These cleared stripes were not directed towards the window, but obliquely
+to one side of the room, so that only a very little light could act on the
+upper halves of the cotyledons. These 14 seedlings remained during eight
+hours of exposure before a south-west window on a hazy day quite upright;
+whereas all the other many free seedlings in the same pots became greatly
+bowed towards the light.
+
+We will now turn to the trials with caps made of very thin tin-foil. These
+were placed at different times on the summits of 24 cotyledons, and they
+extended down for a length of between .15 and .2 of an inch. The seedlings
+were exposed to a lateral light for periods varying between 6 h. 30 m. and
+7 h. 45 m., which sufficed to cause all the other seedlings in the same
+pots to become almost rectangularly bent towards the light. They varied in
+height from only .04 to 1.15 inch, but the greater number were about .75
+inch. Of the 24 cotyledons with their summits thus protected, 3 became much
+bent, but not in the direction of the light, and as they did not straighten
+themselves through apogeotropism during the following night, either the
+caps were too heavy or the plants themselves were in a weak condition; and
+these three cases may be excluded. There are left for consideration 21
+cotyledons; of these 17 remained all the time quite upright; the other 4
+became slightly inclined to the light, but not in a degree comparable with
+that of the many free seedlings in the same pots. As the glass-tubes, when
+unpainted, did not prevent the cotyledons from becoming greatly bowed, it
+cannot be supposed that the caps of very thin tin-foil did so, except
+through the exclusion of the light. To prove that the plants had not been
+injured, the caps were removed from 6 of the upright seedlings, and these
+were exposed before a paraffin lamp for the same length of time as before,
+and they now all became greatly curved towards the light.
+
+As caps between .15 and .2 of an inch in depth were thus proved to be
+highly efficient in preventing the cotyledons from bending towards the
+light, 8 other cotyledons were protected with caps between only .06 and .12
+in depth. Of these, two remained vertical, one was considerably and five
+slightly curved towards the light, but far less so than the free seedlings
+in the same pots.
+[page 474]
+
+Another trial was made in a different manner, namely, by bandaging with
+strips of tin-foil, about .2 in breadth, the upper part, but not the actual
+summit, of eight moderately young seedlings a little over half an inch in
+height. The summits and the basal parts were thus left fully exposed to a
+lateral light during 8 h.; an upper intermediate zone being protected. With
+four of these seedlings the summits were exposed for a length of .05 inch,
+and in two of them this part became curved towards the light, but the whole
+lower part remained quite upright; whereas the entire length of the other
+two seedlings became slightly curved towards the light. The summits of the
+four other seedlings were exposed for a length of .04 inch, and of these
+one remained almost upright, whilst the other three became considerably
+curved towards the light. The many free seedlings in the same pots were all
+greatly curved towards the light.
+
+From these several sets of experiments, including those with the
+glass-tubes, and those when the tips were cut off, we may infer that the
+exclusion of light from the upper part of the cotyledons of Phalaris
+prevents the lower part, though fully exposed to a lateral light, from
+becoming curved. The summit for a length of .04 or .05 of an inch, though
+it is itself sensitive and curves towards the light, has only a slight
+power of causing the lower part to bend. Nor has the exclusion of light
+from the summit for a length of .1 of an inch a strong influence on the
+curvature of the lower part. On the other hand, an exclusion for a length
+of between .15 and .2 of an inch, or of the whole upper half, plainly
+prevents the lower and fully illuminated part from becoming curved in the
+manner (see Fig. 181) which invariably occurs when a free cotyledon is
+exposed to a lateral light. With very young seedlings the sensitive zone
+seems to extend rather lower down relatively to their height than in older
+seedlings. We must therefore conclude that when seedlings are freely
+exposed to a lateral light some influence is transmitted from the upper to
+the lower part, causing the latter to bend.
+
+This conclusion is supported by what may be seen to occur on a small scale,
+especially with young cotyledons, without any artificial exclusion of the
+light; for they bend beneath the earth where no light can enter. Seeds of
+Phalaris were covered with a layer one-fourth of an inch in thickness of
+very fine sand, consisting of extremely minute grains of silex coated with
+[page 475]
+oxide of iron. A layer of this sand, moistened to the same degree as that
+over the seeds, was spread over a glass-plate; and when the layer was .05
+of an inch in thickness (carefully measured) no light from a bright sky
+could be seen to pass through it, unless it was viewed through a long
+blackened tube, and then a trace of light could be detected, but probably
+much too little to affect any plant. A layer .1 of an inch in thickness was
+quite impermeable to light, as judged by the eye aided by the tube. It may
+be worth adding that the layer, when dried, remained equally impermeable to
+light. This sand yielded to very slight pressure whilst kept moist, and in
+this state did not contract or crack in the least. In a first trial,
+cotyledons which had grown to a moderate height were exposed for 8 h.
+before a paraffin lamp, and they became greatly bowed. At their bases on
+the shaded side opposite to the light, well-defined, crescentic, open
+furrows were formed, which (measured under a microscope with a micrometer)
+were from .02 to .03 of an inch in breadth, and these had evidently been
+left by the bending of the buried bases of the cotyledons towards the
+light. On the side of the light the cotyledons were in close contact with
+the sand, which was a very little heaped up. By removing with a sharp knife
+the sand on one side of the cotyledons in the line of the light, the bent
+portion and the open furrows were found to extend down to a depth of about
+.1 of an inch, where no light could enter. The chords of the short buried
+arcs formed in four cases angles of 11o, 13o, 15o, and 18o, with the
+perpendicular. By the following morning these short bowed portions had
+straightened themselves through apogeotropism.
+
+In the next trial much younger cotyledons were similarly treated, but were
+exposed to a rather obscure lateral light. After some hours, a bowed
+cotyledon, .3 inch in height, had an open furrow on the shaded side .04
+inch in breadth; another cotyledon, only .13 inch in height, had left a
+furrow .02 inch in breadth. But the most curious case was that of a
+cotyledon which had just protruded above the ground and was only .03 inch
+in height, and this was found to be bowed in the direction of the light to
+a depth of .2 of an inch beneath the surface. From what we know of the
+impermeability of this sand to light, the upper illuminated part in these
+several cases must have determined the curvature of the lower buried
+portions. But an apparent cause of doubt may be suggested: as the
+cotyledons are continually circumnutating, they tend to form a minute
+[page 476]
+crack or furrow all round their bases, which would admit a little light on
+all sides; but this would not happen when they were illuminated laterally,
+for we know that they quickly bend towards a lateral light, and they then
+press so firmly against the sand on the illuminated side as to furrow it,
+and this would effectually exclude light on this side. Any light admitted
+on the opposite and shaded side, where an open furrow is formed, would tend
+to counteract the curvature towards the lamp or other source of the light.
+It may be added, that the use of fine moist sand, which yields easily to
+pressure, was indispensable in the above experiments; for seedlings raised
+in common soil, not kept especially damp, and exposed for 9 h. 30 m. to a
+strong lateral light, did not form an open furrow at their bases on the
+shaded side, and were not bowed beneath the surface.
+Perhaps the most striking proof of the action of the upper on the lower
+part of the cotyledons of Phalaris, when laterally illuminated, was
+afforded by the blackened glass-tubes (before alluded to) with very narrow
+stripes of the varnish scraped off on one side, through which a little
+light was admitted. The breadth of these stripes or slits varied between
+.01 and .02 inch (.25 and .51 mm.). Cotyledons with their upper halves
+enclosed in such tubes were placed before a south-west window, in such a
+position, that the scraped stripes did not directly face the window, but
+obliquely to one side. The seedlings were left exposed for 8 h., before the
+close of which time the many free seedlings in the same pots had become
+greatly bowed towards the window. Under these circumstances, the whole
+lower halves of the cotyledons, which had their summits enclosed in the
+tubes, were fully exposed to the light of the sky, whilst their upper
+halves received exclusively or chiefly diffused light from the room, and
+this only through a very narrow slit on one side. Now, if the curvature of
+the lower part had been determined by the illumination of this part, all
+the cotyledons assuredly would have become curved towards the window; but
+this was far from being the case. Tubes of the kind just described were
+placed on several occasions over the upper halves of 27 cotyledons; 14 of
+them remained all the time quite vertical; so that sufficient diffused
+light did not enter through the narrow slits to produce any effect
+whatever; and they behaved in the same manner as if their upper halves had
+been enclosed in completely blackened tubes. The lower halves of the 13
+other cotyledons became bowed
+[page 477]
+not directly in the line of the window, but obliquely towards it; one
+pointed at an angle of only 18o, but the remaining 12 at angles varying
+between 45o and 62o from the line of the window. At the commencement of the
+experiment, pins had been laid on the earth in the direction towards which
+the slits in the varnish faced; and in this direction alone a small amount
+of diffused light entered. At the close of the experiment, 7 of the bowed
+cotyledons pointed exactly in the line of the pins, and 6 of them in a line
+between that of the pins and that of the window. This intermediate position
+is intelligible, for any light from the sky which entered obliquely through
+the slits would be much more efficient than the diffused light which
+entered directly through them. After the 8 h. exposure, the contrast in
+appearance between these 13 cotyledons and the many other seedlings in the
+same pots, which were all (excepting the above 14 vertical ones) greatly
+bowed in straight and parallel lines towards the window, was extremely
+remarkable. It is therefore certain that a little weak light striking the
+upper halves of the cotyledons of Phalaris, is far more potent in
+determining the direction of the curvature of the lower halves, than the
+full illumination of the latter during the whole time of exposure.
+
+In confirmation of the above results, the effect of thickly painting with
+Indian ink one side of the upper part of three cotyledons of Phalaris, for
+a length of .2 inch from their tips, may be worth giving. These were placed
+so that the unpainted surface was directed not towards the window, but a
+little to one side; and they all became bent towards the unpainted side,
+and from the line of the window by angles amounting to 31o, 35o, and 83o.
+The curvature in this direction extended down to their bases, although the
+whole lower part was fully exposed to the light from the window.
+
+Finally, although there can be no doubt that the illumination of the upper
+part of the cotyledons of Phalaris greatly affects the power and manner of
+bending of the lower part, yet some observations seemed to render it
+probable that the simultaneous stimulation of the lower part by light
+greatly favours, or is almost necessary, for its well-marked curvature; but
+our experiments were not conclusive, owing to the difficulty of excluding
+light from the lower halves without mechanically preventing their
+curvature.
+
+Avena sativa.--The cotyledons of this plant become quickly bowed towards a
+lateral light, exactly like those of Phalaris.
+[page 478]
+Experiments similar to the foregoing ones were tried, and we will give the
+results as briefly as possible. They are somewhat less conclusive than in
+the case of Phalaris, and this may possibly be accounted for by the
+sensitive zone varying in extension, in a species so long cultivated and
+variable as the common Oat. Cotyledons a little under three-quarters of an
+inch in height were selected for trial: six had their summits protected
+from light by tin-foil caps, .25 inch in depth, and two others by caps .3
+inch in depth. Of these 8 cotyledons, five remained upright during 8 hours
+of exposure, although their lower parts were fully exposed to the light all
+the time; two were very slightly, and one considerably, bowed towards it.
+Caps only .2 or .22 inch in depth were placed over 4 other cotyledons, and
+now only one remained upright, one was slightly, and two considerably bowed
+to the light. In this and the following cases all the free seedlings in the
+same pots became greatly bowed to the light.
+
+Our next trial was made with short lengths of thin and fairly transparent
+quills; for glass-tubes of sufficient diameter to go over the cotyledons
+would have been too heavy. Firstly, the summits of 13 cotyledons were
+enclosed in unpainted quills, and of these 11 became greatly and 2 slightly
+bowed to the light; so that the mere act of enclosure did not prevent the
+lower part from becoming bowed. Secondly, the summits of 11 cotyledons were
+enclosed in quills .3 inch in length, painted so as to be impermeable to
+light; of these, 7 did not become at all inclined towards the light, but 3
+of them were slightly bent more or less transversely with respect to the
+line of light, and these might perhaps have been altogether excluded; one
+alone was slightly bowed towards the light. Painted quills, .25 inch in
+length, were placed over the summits of 4 other cotyledons; of these, one
+alone remained upright, a second was slightly bowed, and the two others as
+much bowed to the light as the free seedlings in the same pots. These two
+latter cases, considering that the caps were .25 in length, are
+inexplicable.
+
+Lastly, the summits of 8 cotyledons were coated with flexible and highly
+transparent gold-beaters' skin, and all became as much bowed to the light
+as the free seedlings. The summits of 9 other cotyledons were similarly
+coated with gold-beaters' skin, which was then painted to a depth of
+between .25 and .3 inch, so as to be impermeable to light; of these 5
+remained upright, and 4 were well bowed to the light, almost or quite as
+well as
+[page 479]
+the free seedlings. These latter four cases, as well as the two in the last
+paragraph, offer a strong exception to the rule that the illumination of
+the upper part determines the curvature of the lower part. Nevertheless, 5
+of these 8 cotyledons remained quite upright, although their lower halves
+were fully illuminated all the time; and it would almost be a prodigy to
+find five free seedlings standing vertically after an exposure for several
+hours to a lateral light.
+
+The cotyledons of Avena, like those of Phalaris, when growing in soft,
+damp, fine sand, leave an open crescentric furrow on the shaded side, after
+bending to a lateral light; and they become bowed beneath the surface at a
+depth to which, as we know, light cannot penetrate. The arcs of the chords
+of the buried bowed portions formed in two cases angles of 20o and 21o with
+the perpendicular. The open furrows on the shaded side were, in four cases,
+.008, .016, .024, and .024 of an inch in breadth.
+Brassica oleracea (Common Red).--It will here be shown that the upper half
+of the hypocotyl of the cabbage, when illuminated by a lateral light,
+determines the curvature of the lower half. It is necessary to
+experimentise on young seedlings about half an inch or rather less in
+height, for when grown to an inch and upwards the basal part ceases to
+bend. We first tried painting the hypocotyls with Indian ink, or cutting
+off their summits for various lengths; but these experiments are not worth
+giving, though they confirm, as far as they can be trusted, the results of
+the following ones. These were made by folding gold-beaters' skin once
+round the upper halves of young hypocotyls, and painting it thickly with
+Indian ink or with black grease. As a control experiment, the same
+transparent skin, left unpainted, was folded round the upper halves of 12
+hypocotyls; and these all became greatly curved to the light, excepting
+one, which was only moderately curved. Twenty other young hypocotyls had
+the skin round their upper halves painted, whilst their lower halves were
+left quite uncovered. These seedlings were then exposed, generally for
+between 7 and 8 h., in a box blackened within and open in front, either
+before a south-west window or a paraffin lamp. This exposure was amply
+sufficient, as was shown by the strongly-marked heliotropism of all the
+free seedlings in the same pots; nevertheless, some were left exposed to
+the light for a much longer time. Of the 20 hypocotyls thus treated, 14
+remained quite upright, and 6 became slightly bowed to the light; but 2 of
+these latter cases were not really
+[page 480]
+exceptions, for on removing the skin the paint was found imperfect and was
+penetrated by many small transparent spaces on the side which faced the
+light. Moreover, in two other cases the painted skin did not extend quite
+halfway down the hypocotyl. Although there was a wonderful contrast in the
+several pots between these 20 hypocotyls and the other many free seedlings,
+which were all greatly bowed down to their bases in the direction of the
+light, some being almost prostrate on the ground.
+
+The most successful trial on any one day (included in the above results) is
+worth describing in detail. Six young seedlings were selected, the
+hypocotyls of which were nearly .45 inch, excepting one, which was .6 inch
+in height, measured from the bases of their petioles to the ground. Their
+upper halves, judged as accurately as could be done by the eye, were folded
+once round with gold-beaters' skin, and this was painted thickly with
+Indian ink. They were exposed in an otherwise darkened room before a bright
+paraffin lamp, which stood on a level with the two pots containing the
+seedlings. They were first looked at after an interval of 5 h. 10 m., and
+five of the protected hypocotyls were found quite erect, the sixth being
+very slightly inclined to the light; whereas all the many free seedlings in
+the same two pots were greatly bowed to the light. They were again examined
+after a continuous exposure to the light of 20 h. 35m.; and now the
+contrast between the two sets was wonderfully great; for the free seedlings
+had their hypocotyls extended almost horizontally in the direction of the
+light, and were curved down to the ground; whilst those with the upper
+halves protected by the painted skin, but with their lower halves fully
+exposed to the light, still remained quite upright, with the exception of
+the one which retained the same slight inclination to the light which it
+had before. This latter seedling was found to have been rather badly
+painted, for on the side facing the light the red colour of the hypocotyl
+could be distinguished through the paint.
+
+We next tried nine older seedlings, the hypocotyls of which varied between
+1 and 1.6 inch in height. the gold-beaters' skin round their upper parts
+was painted with black grease to a depth of only .3 inch, that is, from
+less than a third to a fourth or fifth of their total heights. They were
+exposed to the light for 7 h. 15 m.; and the result showed that the whole
+of the sensitive zone, which determines the curvature of the lower
+[page 481]
+part, was not protected from the action of the light; for all 9 became
+curved towards it, 4 of them very slightly, 3 moderately, and 2 almost as
+much as the unprotected seedlings. Nevertheless, the whole 9 taken together
+differed plainly in their degree of curvature from the many free seedlings,
+and from some which were wrapped in unpainted skin, growing in the same two
+pots.
+
+Seeds were covered with about a quarter of an inch of the fine sand
+described under Phalaris; and when the hypocotyls had grown to a height of
+between .4 and .55 inch, they were exposed during 9 h. before a paraffin
+lamp, their bases being at first closely surrounded by the damp sand. They
+all became bowed down to the ground, so that their upper parts lay near to
+and almost parallel to the surface of the soil. On the side of the light
+their bases were in close contact with the sand, which was here a very
+little heaped up; on the opposite or shaded side there were open,
+crescentic cracks or furrows, rather above .01 of an inch in width; but
+they were not so sharp and regular as those made by Phalaris and Avena, and
+therefore could not be so easily measured under the microscope. The
+hypocotyls were found, when the sand was removed on one side, to be curved
+to a depth beneath the surface in three cases of at least .1 inch, in a
+fourth case of .11, and in a fifth of .15 inch. The chords of the arcs of
+the short, buried, bowed portions formed angles of between 11o and 15o with
+the perpendicular. From what we have seen of the impermeability of this
+sand to light, the curvature of the hypocotyls certainly extended down to a
+depth where no light could enter; and the curvature must have been caused
+by an influence transmitted from the upper illuminated part.
+
+The lower halves of five young hypocotyls were surrounded by unpainted
+gold-beaters' skin, and these, after an exposure of 8 h. before a paraffin
+lamp, all became as much bowed to the light as the free seedlings. The
+lower halves of 10 other young hypocotyls, similarly surrounded with the
+skin, were thickly painted with Indian ink; their upper and unprotected
+halves became well curved to the light, but their lower and protected
+halves remained vertical in all the cases excepting one, and on this the
+layer of paint was imperfect. This result seems to prove that the influence
+transmitted from the upper part is not sufficient to cause the lower part
+to bend, unless it be at the same time illuminated; but there remains the
+doubt, as in
+[page 482]
+the case of Phalaris, whether the skin covered with a rather thick crust of
+dry Indian ink did not mechanically prevent their curvature.
+
+Beta vulgaris.--A few analogous experiments were tried on this plant, which
+is not very well adapted for the purpose, as the basal part of the
+hypocotyl, after it has grown to above half an inch in height, does not
+bend much on exposure to a lateral light. Four hypocotyls were surrounded
+close beneath their petioles with strips of thin tin-foil, .2 inch in
+breadth, and they remained upright all day before a paraffin lamp; two
+others were surrounded with strips .15 inch in breadth, and one of these
+remained upright, the other becoming bowed; the bandages in two other cases
+were only .1 inch in breadth, and both of these hypocotyls became bowed,
+though one only slightly, towards the light. The free seedlings in the same
+pots were all fairly well curved towards the light; and during the
+following night became nearly upright. The pots were now turned round and
+placed before a window, so that the opposite sides of the seedlings were
+exposed to the light, towards which all the unprotected hypocotyls became
+bent in the course of 7 h. Seven out of the 8 seedlings with bandages of
+tin-foil remained upright, but one which had a bandage only .1 inch in
+breadth, became curved to the light. On another occasion, the upper halves
+of 7 hypocotyls were surrounded with painted gold-beaters' skin; of these 4
+remained upright, and 3 became a little curved to the light: at the same
+time 4 other seedlings surrounded with unpainted skin, as well as the free
+ones in the same pots, all became bowed towards the lamp, before which they
+had been exposed during 22 hours.
+
+Radicles of Sinapis alba.--The radicles of some plants are indifferent, as
+far as curvature is concerned, to the action of light; whilst others bend
+towards and others from it.* Whether these movements are of any service to
+the plant is very doubtful, at least in the case of subterranean roots;
+they probably result from the radicles being sensitive to contact,
+moisture, and gravitation, and as a consequence to other irritants which
+are never naturally encountered. The radicles of Sinapis alba, when
+immersed in water and exposed to a lateral light, bend from it, or are
+apheliotropic. They become bent for a length of about 4 mm. from their
+tips. To ascertain whether this movement
+
+* Sachs, 'Physiologie V�g�tale,' 1868, p. 44.
+[page 483]
+
+generally occurred, 41 radicles, which had germinated in damp sawdust, were
+immersed in water and exposed to a lateral light; and they all, with two
+doubtful exceptions, became curved from the light. At the same time the
+tips of 54 other radicles, similarly exposed, were just touched with
+nitrate of silver. They were blackened for a length of from .05 to .07 mm.,
+and probably killed; but it should be observed that this did not check
+materially, if at all, the growth of the upper part; for several, which
+were measured, increased in the course of only 8 -9 h. by 5 to 7 mm. in
+length. Of the 54 cauterised radicles one case was doubtful, 25 curved
+themselves from the light in the normal manner, and 28, or more than half,
+were not in the least apheliotropic. There was a considerable difference,
+which we cannot account for, in the results of the experiments tried
+towards the end of April and in the middle of September. Fifteen radicles
+(part of the above 54) were cauterised at the former period and were
+exposed to sunshine, of which 12 failed to be apheliotropic, 2 were still
+apheliotropic, and 1 was doubtful. In September, 39 cauterised radicles
+were exposed to a northern light, being kept at a proper temperature; and
+now 23 continued to be apheliotropic in the normal manner, and only 16
+failed to bend from the light. Looking at the aggregate results at both
+periods, there can be no doubt that the destruction of the tip for less
+than a millimeter in length destroyed in more than half the cases their
+power of moving from the light. It is probable that if the tips had been
+cauterised for the length of a whole millimeter, all signs of
+apheliotropism would have disappeared. It may be suggested that although
+the application of caustic does not stop growth, yet enough may be absorbed
+to destroy the power of movement in the upper part; but this suggestion
+must be rejected, for we have seen and shall again see, that cauterising
+one side of the tip of various kinds of radicles actually excites movement.
+The conclusion seems inevitable that sensitiveness to light resides in the
+tip of the radicle of Sinapis alba; and that the tip when thus stimulated
+transmits some influence to the upper part, causing it to bend. The case in
+this respect is parallel with that of the radicles of several plants, the
+tips of which are sensitive to contact and to other irritants, and, as will
+be shown in the eleventh chapter, to gravitation.
+[page 484]
+
+CONCLUDING REMARKS AND SUMMARY OF CHAPTER.
+
+We do not know whether it is a general rule with seedling plants that the
+illumination of the upper part determines the curvature of the lower part.
+But as this occurred in the four species examined by us, belonging to such
+distinct families as the Gramineae, Cruciferae, and Chenopodeae, it is
+probably of common occurrence. It can hardly fail to be of service to
+seedlings, by aiding them to find the shortest path from the buried seed to
+the light, on nearly the same principle that the eyes of most of the lower
+crawling animals are seated at the anterior ends of their bodies. It is
+extremely doubtful whether with fully developed plants the illumination of
+one part ever affects the curvature of another part. The summits of 5 young
+plants of Asparagus officinalis (varying in height between 1.1 and 2.7
+inches, and consisting of several short internodes) were covered with caps
+of tin-foil from 0.3 to 0.35 inch in depth; and the lower uncovered parts
+became as much curved towards a lateral light, as were the free seedlings
+in the same pots. Other seedlings of the same plant had their summits
+painted with Indian ink with the same negative result. Pieces of blackened
+paper were gummed to the edges and over the blades of some leaves on young
+plants of Tropaeolum majus and Ranunculus ficaria; these were then placed
+in a box before a window, and the petioles of the protected leaves became
+curved towards the light, as much as those of the unprotected leaves.
+
+The foregoing cases with respect to seedling plants have been fully
+described, not only because the transmission of any effect from light is a
+new physiological fact, but because we think it tends to modify somewhat
+the current views on heliotropic movements. Until
+[page 485]
+lately such movements were believed to result simply from increased growth
+on the shaded side. At present it is commonly admitted* that diminished
+light increases the turgescence of the cells, or the extensibility of the
+cell-walls, or of both together, on the shaded side, and that this is
+followed by increased growth. But Pfeffer has shown that a difference in
+the turgescence on the two sides of a pulvinus,--that is, an aggregate of
+small cells which have ceased to grow at an early age,--is excited by a
+difference in the amount of light received by the two sides; and that
+movement is thus caused without being followed by increased growth on the
+more turgescent side.** All observers apparently believe that light acts
+directly on the part which bends, but we have seen with the above described
+seedlings that this is not the case. Their lower halves were brightly
+illuminated for hours, and yet did not bend in the least towards the light,
+though this is the part which under ordinary circumstances bends the most.
+It is a still more striking fact, that the faint illumination of a narrow
+stripe on one side of the upper part of the cotyledons of Phalaris
+determined the direction of the curvature of the lower part; so that this
+latter part did not bend towards the bright light by which it had been
+fully illuminated,
+
+* Emil Godlewski has given ('Bot. Zeitung,' 1879, Nos. 6-9) an excellent
+account (p. 120) of the present state of the question. See also Vines in
+'Arbeiten des Bot. Inst. in W�rzburg,' 1878, B. ii. pp. 114-147. Hugo de
+Vries has recently published a still more important article on this
+subject: 'Bot Zeitung,' Dec. 19th and 26th, 1879.
+
+** 'Die Periodischen Bewegungen der Blattorgane,' 1875, pp. 7, 63, 123,
+etc. Frank has also insisted ('Die Naturliche w�gerechte Richtung von
+Pflanzentheilen,' 1870, p. 53) on the important part which the pulvini of
+the leaflets of compound leaves play in placing the leaflets in a proper
+position with respect to the light. This holds good, especially with the
+leaves of climbing plants, which are carried into all sorts of positions,
+ill-adapted for the action of the light.
+[page 486]
+
+but obliquely towards one side where only a little light entered. These
+results seem to imply the presence of some matter in the upper part which
+is acted on by light, and which transmits its effects to the lower part. It
+has been shown that this transmission is independent of the bending of the
+upper sensitive part. We have an analogous case of transmission in Drosera,
+for when a gland is irritated, the basal and not the upper or intermediate
+part of the tentacle bends. The flexible and sensitive filament of Dionaea
+likewise transmits a stimulus, without itself bending; as does the stem of
+Mimosa.
+
+Light exerts a powerful influence on most vegetable tissues, and there can
+be no doubt that it generally tends to check their growth. But when the two
+sides of a plant are illuminated in a slightly different degree, it does
+not necessarily follow that the bending towards the illuminated side is
+caused by changes in the tissues of the same nature as those which lead to
+increased growth in darkness. We know at least that a part may bend from
+the light, and yet its growth may not be favoured by light. This is the
+case with the radicles of Sinapis alba, which are plainly apheliotropic;
+nevertheless, they grow quicker in darkness than in light.* So it is with
+many a�rial roots, according to Wiesner;** but there are other opposed
+cases. It appears, therefore, that light does not determine the growth of
+apheliotropic parts in any uniform manner.
+
+We should bear in mind that the power of bending to the light is highly
+beneficial to most plants. There
+
+* Francis Darwin, '�ber das Wachsthum negativ heliotropischer Wurzeln':
+'Arbeiten des Bot. Inst. in W�rzburg,' B. ii., Heft iii., 1880, p. 521.
+
+** 'Sitzb. der k. Akad. der Wissensch' (Vienna), 1880, p. 12.
+[page 487]
+
+is therefore no improbability in this power having been specially acquired.
+In several respects light seems to act on plants in nearly the same manner
+as it does on animals by means of the nervous system.* With seedlings the
+effect, as we have just seen, is transmitted from one part to another. An
+animal may be excited to move by a very small amount of light; and it has
+been shown that a difference in the illumination of the two sides of the
+cotyledons of Phalaris, which could not be distinguished by the human eye,
+sufficed to cause them to bend. It has also been shown that there is no
+close parallelism between the amount of light which acts on a plant and its
+degree of curvature; it was indeed hardly possible to perceive any
+difference in the curvature of some seedlings of Phalaris exposed to a
+light, which, though dim, was very much brighter than that to which others
+had been exposed. The retina, after being stimulated by a bright light,
+feels the effect for some time; and Phalaris continued to bend for nearly
+half an hour towards the side which had been illuminated. The retina cannot
+perceive a dim light after it has been exposed to a bright one; and plants
+which had been kept in the daylight during the previous day and morning,
+did not move so soon towards an obscure lateral light as did others which
+had been kept in complete darkness.
+
+Even if light does act in such a manner on the growing parts of plants as
+always to excite in them a tendency to bend towards the more illuminated
+side--a supposition contradicted by the foregoing experiments on seedlings
+and by all apheliotropic
+* Sachs has made some striking remarks to the same effect with respect to
+the various stimuli which excite movement in plants. See his paper 'Ueber
+orthotrope und plagiotrope Pflanzentheile,' 'Arb. des Bot. Inst. in
+W�rzburg,' 1879, B. ii. p. 282.
+[page 488]
+
+organs--yet the tendency differs greatly in different species, and is
+variable in degree in the individuals of the same species, as may be seen
+in almost any pot of seedlings of a long cultivated plant.* There is
+therefore a basis for the modification of this tendency to almost any
+beneficial extent. That it has been modified, we see in many cases: thus,
+it is of more importance for insectivorous plants to place their leaves in
+the best position for catching insects than to turn their leaves to the
+light, and they have no such power. If the stems of twining plants were to
+bend towards the light, they would often be drawn away from their supports;
+and as we have seen they do not thus bend. As the stems of most other
+plants are heliotropic, we may feel almost sure that twining plants, which
+are distributed throughout the whole vascular series, have lost a power
+that their non-climbing progenitors possessed. Moreover, with Ipomoea, and
+probably all other twiners, the stem of the young plant, before it begins
+to twine, is highly heliotropic, evidently in order to expose the
+cotyledons or the first true leaves fully to the light. With the Ivy the
+stems of seedlings are moderately heliotropic, whilst those of the same
+plants when grown a little older
+
+* Strasburger has shown in his interesting work ('Wirkung des Lichtes...auf
+Schw�rmsporen,' 1878), that the movement of the swarm-spores of various
+lowly organised plants to a lateral light is influenced by their stage of
+development, by the temperature to which they are subjected, by the degree
+of illumination under which they have been raised, and by other unknown
+causes; so that the swarm-spores of the same species may move across the
+field of the microscope either to or from the light. Some individuals,
+moreover, appear to be indifferent to the light; and those of different
+species behave very differently. The brighter the light, the straighter is
+their course. They exhibit also for a short time the after-effects of
+light. In all these respects they resemble the higher plants. See, also,
+Stahl, 'Ueber den einfluss der Lichts auf die Bewegungs-erscheinungen der
+Schw�rmsporen' Verh. d. phys.-med. Geselsshalft in W�rzburg, B. xii. 1878.
+[page 489]
+
+are apheliotropic. Some tendrils which consist of modified leaves--organs
+in all ordinary cases strongly diaheliotropic--have been rendered
+apheliotropic, and their tips crawl into any dark crevice.
+
+Even in the case of ordinary heliotropic movements, it is hardly credible
+that they result directly from the action of the light, without any special
+adaptation. We may illustrate what we mean by the hygroscopic movements of
+plants: if the tissues on one side of an organ permit of rapid evaporation,
+they will dry quickly and contract, causing the part to bend to this side.
+Now the wonderfully complex movements of the pollinia of Orchis
+pyramidalis, by which they clasp the proboscis of a moth and afterwards
+change their position for the sake of depositing the pollen-masses on the
+double stigma--or again the twisting movements, by which certain seeds bury
+themselves in the ground*--follow from the manner of drying of the parts in
+question; yet no one will suppose that these results have been gained
+without special adaptation. Similarly, we are led to believe in adaptation
+when we see the hypocotyl of a seedling, which contains chlorophyll,
+bending to the light; for although it thus receives less light, being now
+shaded by its own cotyledons, it places them--the more important organs--in
+the best position to be fully illuminated. The hypocotyl may therefore be
+said to sacrifice itself for the good of the cotyledons, or rather of the
+whole plant. But if it be prevented from bending, as must sometimes occur
+with seedlings springing up in an entangled mass of vegetation, the
+cotyledons themselves bend so as to face the light; the one farthest off
+rising
+
+* Francis Darwin, 'On the Hygroscopic Mechanism,' etc., 'Transactions Linn.
+Soc.,' series ii. vol. i. p. 149, 1876.
+[page 490]
+
+up, and that nearest to the light sinking down, or both twisting
+laterally.* We may, also, suspect that the extreme sensitiveness to light
+of the upper part of the sheath-like cotyledons of the Gramineae, and their
+power of transmitting its effects to the lower part, are specialised
+arrangements for finding the shortest path to the light. With plants
+growing on a bank, or thrown prostrate by the wind, the manner in which the
+leaves move, even rotating on their own axes, so that their upper surfaces
+may be again directed to the light, is a striking phenomenon. Such facts
+are rendered more striking when we remember that too intense a light
+injures the chlorophyll, and that the leaflets of several Leguminosae when
+thus exposed bend upwards and present their edges to the sun, thus escaping
+injury. On the other hand, the leaflets of Averrhoa and Oxalis, when
+similarly exposed, bend downwards.
+
+It was shown in the last chapter that heliotropism is a modified form of
+circumnutation; and as every growing part of every plant circumnutates more
+or less, we can understand how it is that the power of bending to the light
+has been acquired by such a multitude of plants throughout the vegetable
+kingdom. The manner in which a circumnutating movement--that is, one
+consisting of a succession of irregular ellipses or loops--is gradually
+converted into a rectilinear course towards the light, has been already
+explained. First, we have a succession of ellipses with their longer axes
+directed towards the light, each of which
+
+* Wiesner has made remarks to nearly the same effect with respect to
+leaves: 'Die undulirende Nutation der Internodien,' p. 6, extracted from B.
+lxxvii. (1878). Sitb. der k. Akad. der Wissensch. Wien.
+[page 491]
+
+is described nearer and nearer to its source; then the loops are drawn out
+into a strongly pronounced zigzag line, with here and there a small loop
+still formed. At the same time that the movement towards the light is
+increased in extent and accelerated, that in the opposite direction is
+lessened and retarded, and at last stopped. The zigzag movement to either
+side is likewise gradually lessened, so that finally the course becomes
+rectilinear. Thus under the stimulus of a fairly bright light there is no
+useless expenditure of force.
+
+As with plants every character is more or less variable, there seems to be
+no great difficulty in believing that their circumnutating movements may
+have been increased or modified in any beneficial manner by the
+preservation of varying individuals. The inheritance of habitual movements
+is a necessary contingent for this process of selection, or the survival of
+the fittest; and we have seen good reason to believe that habitual
+movements are inherited by plants. In the case of twining species the
+circumnutating movements have been increased in amplitude and rendered more
+circular; the stimulus being here an internal or innate one. With sleeping
+plants the movements have been increased in amplitude and often changed in
+direction; and here the stimulus is the alternation of light and darkness,
+aided, however, by inheritance. In the case of heliotropism, the stimulus
+is the unequal illumination of the two sides of the plant, and this
+determines, as in the foregoing cases, the modification of the
+circumnutating movement in such a manner that the organ bends to the light.
+A plant which has been rendered heliotropic by the above means, might
+readily lose this tendency, judging from the cases already given, as soon
+as it became useless or
+[page 492]
+injurious. A species which has ceased to be heliotropic might also be
+rendered apheliotropic by the preservation of the individuals which tended
+to circumnutate (though the cause of this and most other variations is
+unknown) in a direction more or less opposed to that whence the light
+proceeded. In like manner a plant might be rendered diaheliotropic.
+[page 493]
+
+
+CHAPTER X.
+
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
+
+Means of observation - Apogeotropism--Cytisus--Verbena--Beta--Gradual
+conversion of the movement of circumnutation into apogeotropism in Rubus,
+Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded by
+heliotropism--Effected by the aid of joints or pulvini--Movements of
+flower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism--
+Movements of radicles--Burying of seed-capsules--Use of process--Trifolium
+subterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion
+
+OUR object in the present chapter is to show that geotropism,
+apogeotropism, and diageotropism are modified forms of circumnutation.
+Extremely fine filaments of glass, bearing two minute triangles of paper,
+were fixed to the summits of young stems, frequently to the hypocotyls of
+seedlings, to flower-peduncles, radicles, etc., and the movements of the
+parts were then traced in the manner already described on vertical and
+horizontal glass-plates. It should be remembered that as the stems or other
+parts become more and more oblique with respect to the glasses, the figures
+traced on them necessarily become more and more magnified. The plants were
+protected from light, excepting whilst each observation was being made, and
+then the light, which was always a dim one, was allowed to enter so as to
+interfere as little as possible with the movement in progress; and we did
+not detect any evidence of such interference.
+
+When observing the gradations between circumnu-
+[page 494]
+tation and heliotropism, we had the great advantage of being able to lessen
+the light; but with geotropism analogous experiments were of course
+impossible. We could, however, observe the movements of stems placed at
+first only a little from the perpendicular, in which case geotropism did
+not act with nearly so much power, as when the stems were horizontal and at
+right angles to the force. Plants, also, were selected which were but
+feebly geotropic or apogeotropic, or had become so from having grown rather
+old. Another plan was to place the stems at first so that they pointed 30
+or 40o beneath the horizon, and then apogeotropism had a great amount of
+work to do before the stem was rendered upright; and in this case ordinary
+circumnutation was often not wholly obliterated. Another plan was to
+observe in the evening plants which during the day had become greatly
+curved heliotropically; for their stems under the gradually waning light
+very slowly became upright through the action of apogeotropism; and in this
+case modified circumnutation was sometimes well displayed.
+
+[Apogeotropism.--Plants were selected for observation almost by chance,
+excepting that they were taken from widely different families. If the stem
+of a plant which is even moderately sensitive to apogeotropism be placed
+horizontally, the upper growing part bends quickly upwards, so as to become
+perpendicular; and the line traced by joining the dots successively made on
+a glass-plate, is generally almost straight. For instance, a young Cytisus
+fragrans, 12 inches in height, was placed so that the stem projected 10o
+beneath the horizon, and its course was traced during 72 h. At first it
+bent a very little downwards (Fig. 182), owing no doubt to the weight of
+the stem, as this occurred with most of the other plants observed, though,
+as they were of course circumnutating, the short downward lines were often
+oblique. After three-quarters of an hour the stem began to curve upwards,
+quickly during the first two hours, but much more slowly during the
+afternoon and night,
+[page 495]
+and on the following day. During the second night it fell a little, and
+circumnutated during the following day; but it also moved a short distance
+to the right, which was caused by a little light having been accidentally
+admitted on this side. The stem was now inclined 60o above the horizon, and
+had therefore risen 70o. With time allowed it would probably have become
+upright, and no doubt would have continued circumnutating. The sole
+remarkable feature in the figure here given is the straightness of the
+course pursued. The stem, however, did not move upwards at an equable rate,
+and it sometimes stood almost or quite still. Such periods probably
+represent attempts to circumnutate in a direction opposite to
+apogeotropism.
+
+Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10o beneath
+to 60o above horizon, traced on vertical glass, from 8.30 A.M. March 12th
+to 10.30 P.M. 13th. The subsequent circumnutating movement is likewise
+shown up to 6.45 A.M. on the 15th. Nocturnal course represented, as usual,
+by a broken line. Movement not greatly magnified, and tracing reduced to
+two-thirds of original scale.
+
+ The herbaceous stem of a Verbena melindres (?) laid horizontally, rose in
+7 h. so much that it could no longer be observed on the vertical glass
+which stood in front of the plant. The long line which was traced was
+almost absolutely straight. After the 7 h. it still continued to rise, but
+now circumnutated slightly. On the following day it stood upright, and
+circumnutated regularly, as shown in Fig. 82, given in the fourth chapter.
+The stems of several other plants which were highly sensitive to
+apogeotropism rose up in almost straight lines, and
+[page 496]
+then suddenly began to circumnutate. A partially etiolated and somewhat old
+hypocotyl of a seedling cabbage (2 3/4 inches in height) was so sensitive
+that when placed at an angle of only 23o from the perpendicular, it became
+vertical in 33 minutes. As it could not have been strongly acted upon by
+apogeotropism in the above slightly inclined position, we expected that it
+would have circumnutated, or at least have moved in a zigzag course.
+Accordingly, dots were made every 3 minutes; but, when these were joined,
+the line was nearly straight. After this hypocotyl had become upright it
+still moved onwards for half an hour in the same general direction, but in
+a zigzag manner. During the succeeding 9 h. it circumnutated regularly, and
+described 3 large ellipses. In this case apogeotropism, although acting at
+a very unfavourable angle, quite overcame the ordinary circumnutating
+movement.
+
+Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19o
+beneath horizon to a vertical position, with subsequent circumnutation,
+traced on a vertical and on a horizontal glass-plate, from 8.28 A.M. Sept.
+28th to 8.40 A.M. 29th. Figure reduced to one-third of original scale.
+
+The hypocotyls of Beta vulgaris are highly sensitive to apogeotropism. One
+was placed so as to project 19o beneath the horizon; it fell at first a
+very little (see Fig. 183), no doubt owing to its weight; but as it was
+circumnutating the line was
+[page 497]
+oblique. During the next 3 h. 8 m. it rose in a nearly straight line,
+passing through an angle of 109o, and then (at 12.3 P.M.) stood upright. It
+continued for 55 m. to move in the same general direction beyond the
+perpendicular, but in a zigzag course. It returned also in a zigzag line,
+and then circumnutated regularly, describing three large ellipses during
+the remainder of the day. It should be observed that the ellipses in this
+figure are exaggerated in size, relatively to the length of the upward
+straight line, owing to the position of the vertical and horizontal
+glass-plates. Another and somewhat old hypocotyl was placed so as to stand
+at only 31o from the perpendicular, in which position apogeotropism acted
+on it with little force, and its course accordingly was slightly zigzag.
+
+The sheath-like cotyledons of Phalaris Canariensis are extremely sensitive
+to apogeotropism. One was placed so as to project 40o beneath the horizon.
+Although it was rather old and 1.3 inch in height, it became vertical in 4
+h. 30 m., having passed through an angle of 130o in a nearly straight line.
+It then suddenly began to circumnutate in the ordinary manner. The
+cotyledons of this plant, after the first leaf has begun to protrude, are
+but slightly apogeotropic, though they still continue to circumnutate. One
+at this stage of development was placed horizontally, and did not become
+upright even after 13 h., and its course was slightly zigzag. So, again, a
+rather old hypocotyl of Cassia tora (1 1/4 inch in height) required 28 h.
+to become upright, and its course was distinctly zigzag; whilst younger
+hypocotyls moved much more quickly and in a nearly straight line.
+
+When a horizontally placed stem or other organ rises in a zigzag line, we
+may infer from the many cases given in our previous chapters, that we have
+a modified form of circumnutation; but when the course is straight, there
+is no evidence of circumnutation, and any one might maintain that this
+latter movement had been replaced by one of a wholly distinct kind. This
+view seems the more probable when (as sometimes occurred with the
+hypocotyls of Brassica and Beta, the stems of Cucurbita, and the cotyledons
+of Phalaris) the part in question, after bending up in a straight course,
+suddenly begins to circumnutate to the full extent and in the usual manner.
+A fairly good instance of a sudden change of this kind--that is, from a
+nearly straight upward movement to one of circumnutation--is shown in Fig.
+183; but more striking instances were occasionally observed with Beta,
+Brassica, and Phalaris.
+
+We will now describe a few cases in which it may be
+[page 498]
+seen how gradually circumnutation becomes changed into apogeotropism, under
+circumstances to be specified in each instance.
+
+Rubus idaeus (hybrid).--A young plant, 11 inches in height, growing in a
+pot, was placed horizontally; and the upward movement was traced during
+nearly 70 h.; but the plant, though growing vigorously, was not highly
+sensitive to apogeotropism, or it was not capable of quick movement, for
+during the above time it rose only 67o. We may see in the diagram (Fig.
+184) that during the first day of 12 h. it rose in a nearly straight line.
+When placed horizontally, it was evidently circumnutating, for it rose at
+first a little, notwithstanding the weight of the stem, and then sank down;
+so that it did not start on its permanently upward course until 1 h. 25 m.
+had elapsed. On the second day, by which time it had risen considerably,
+and when apogeotropism acted on it with somewhat less power, its course
+during 15 � h. was clearly zigzag, and the rate of the upward movement was
+not equable. During the third day, also of 15 � h., when apogeotropism
+acted on it with still less power, the stem plainly circumnutated, for it
+moved during this day 3 times up and 3 times down, 4 times to the left and
+4 to the right. But the course was so complex that it could hardly be
+traced on the glass. We can, however, see that the successively formed
+irregular ellipses rose higher and higher. Apogeotropism continued to act
+on the fourth morning, as the stem was still rising, though it now stood
+only 23o from the perpendicular. In this diagram the several stages may be
+followed by which an almost rectilinear, upward, apogeotropic course first
+becomes zigzag, and then changes into a circumnutating movement, with most
+of the successively formed, irregular ellipses directed upwards.
+
+Fig 184: Rubus idaeus (hybrid): apogeotropic movement of stem, traced on a
+vertical glass during 3 days and 3 nights, from 10.40 A.M. March 18th to 8
+A.M. 21st. Figure reduced to one-half of the original scale.
+
+Lilium auratum.--A plant 23 inches in height was placed
+[page 499]
+horizontally, and the upper part of the stem rose 58o in 46 h., in the
+manner shown in the accompanying diagram (Fig. 185). We here see that
+during the whole of the second day of 15 � h., the stem plainly
+circumnutated whilst bending upwards through apogeotropism. It had still to
+rise considerably, for when the last dot in the figure was made, it stood
+32o from an upright position.
+
+Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on a
+vertical glass during 2 days and 2 nights, from 10.40 A.M. March 18th to 8
+A.M. 20th. Figure reduced to one-half of the original scale.
+
+Phalaris Canariensis.--A cotyledon of this plant (1.3 inch in height) has
+already been described as rising in 4 h. 30 m. from 40o beneath the horizon
+into a vertical position, passing through an angle of 130o in a nearly
+straight line, and then abruptly beginning to circumnutate. Another
+somewhat old cotyledon of the same height (but from which a true leaf had
+not yet protruded), was similarly placed at 40o beneath the horizon. For
+the first 4 h. it rose in a nearly straight course (Fig. 186), so that by
+1.10 P.M. it was highly inclined, and now apogeotropism acted on it with
+much less power than before, and it began to zigzag. At 4.15 P.M. (i.e. in
+7 h. from the commencement) it stood vertically, and afterwards continued
+to circumnutate in the usual manner about the same spot. Here then we have
+a graduated change from a straight upward apogeotropic course into
+circumnutation, instead of an abrupt change, as in the former case.
+
+Avena sativa.--The sheath-like cotyledons, whilst young, are strongly
+apogeotropic; and some which were placed at 45o beneath the horizon rose
+90o in 7 or 8 h. in lines almost absolutely straight. An oldish cotyledon,
+from which the first leaf began to
+[page 500]
+protrude whilst the following observations were being made, was placed at
+10o beneath the horizon, and it rose only 59o in 24h. It behaved rather
+differently from any other plant, observed by us, for during the first 4 �
+h. it rose in a line not far from straight; during the next 6 � h. it
+circumnutated, that is, it descended and again ascended in a strongly
+marked zigzag course; it then resumed its upward movement in a moderately
+straight line, and, with time allowed, no doubt would have become upright.
+In this case, after the first 4 � h., ordinary circumnutation almost
+completely conquered for a time apogeotropism.
+
+Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon, traced
+on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to 9 A.M.
+20th. Figure here reduced to one-fifth of original scale.
+
+Brassica oleracea.--The hypocotyls of several young seedlings placed
+horizontally, rose up vertically in the course of 6 or 7 h. in nearly
+straight lines. A seedling which had grown in darkness to a height of 2 1/4
+inches, and was therefore rather old and not highly sensitive, was placed
+so that the hypocotyl projected at between 30o and 40o beneath the horizon.
+The upper part alone became curved
+[page 501]
+upwards, and rose during the first 3 h. 10 m. in a nearly straight line
+(Fig. 187); but it was not possible to trace the upward movement on the
+vertical glass for the first 1 h. 10 m., so that the nearly straight line
+in the diagram ought to have been much longer. During the next 11 h. the
+hypocotyl circumnutated, describing irregular figures, each of which rose a
+little above the one previously formed. During the night and following
+early morning it continued to rise in a zigzag course, so that
+apogeotropism was still acting. At the close of our observations, after 23
+h. (represented by the highest dot in the diagram) the hypocotyl was still
+32o from the perpendicular. There can be little doubt that it would
+ultimately have become upright by describing an additional number of
+irregular ellipses, one above the other.
+
+Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced on
+vertical glass, from 9.20 A.M., Sept. 12th to 8.30 A.M. 13th. The upper
+part of the figure is more magnified than the lower part. If the whole
+course had been traced, the straight upright line would have been much
+longer. Figure here reduced to one-third of the original scale.
+
+Apogeotropism retarded by Heliotropism.--When the stem of any plant bends
+during the day towards a lateral light, the movement is opposed by
+apogeotropism; but as the light gradually wanes in the evening the latter
+power slowly gains the upper hand, and draws the stem back into a vertical
+position. Here then we have a good opportunity for observing how
+apogeotropism acts when very nearly balanced by an opposing force. For
+instance, the plumule of Tropaeolum majus (see former Fig. 175) moved
+towards the dim evening light in a slightly zigzag line until 6.45 P.M., it
+then returned on its course until
+[page 502]
+10.40 P.M., during which time it zigzagged and described an ellipse of
+considerable size. The hypocotyl of Brassica oleracea (see former Fig. 173)
+moved in a straight line to the light until 5.15 P.M., and then from the
+light, making in its backward course a great rectangular bend, and then
+returned for a short distance towards the former source of the light; no
+observations were made after 7.10 P.M., but during the night it recovered
+its vertical position. A hypocotyl of Cassia tora moved in the evening in a
+somewhat zigzag line towards the failing light until 6 P.M., and was now
+bowed 20o from the perpendicular; it then returned on its course, making
+before 10.30 P.M. four great, nearly rectangular bends and almost
+completing an ellipse. Several other analogous cases were casually
+observed, and in all of them the apogeotropic movement could be seen to
+consist of modified circumnutation.
+
+Apogeotropic Movements effected by the aid of joints or pulvini.--Movements
+of this kind are well known to occur in the Gramineae, and are effected by
+means of the thickened bases of their sheathing leaves; the stem within
+being in this part thinner than elsewhere.* According to the analogy of all
+other pulvini, such joints ought to continue circumnutating for a long
+period, after the adjoining parts have ceased to grow. We therefore wished
+to ascertain whether this was the case with the Gramineae; for if so, the
+upward curvature of their stems, when extended horizontally or laid
+prostrate, would be explained in accordance with our view--namely, that
+apogeotropism results from modified circumnutation. After these joints have
+curved upwards, they are fixed in their new position by increased growth
+along their lower sides.
+
+Lolium perenne.--A young stem, 7 inches in height, consisting of 3
+internodes, with the flower-head not yet protruded, was selected for
+observation. A long and very thin glass filament was cemented horizontally
+to the stem close above the second joint, 3 inches above the ground. This
+joint was subsequently proved to be in an active condition, as its lower
+side swelled much through the action of apogeotropism (in the manner
+described by De Vries) after the haulm had been fastened down for 24 h. in
+a horizontal position. The pot was
+
+* This structure has been recently described by De Vries in an interesting
+article, 'Ueber die Aufrichtung des gelagerten Getreides,' in
+'Landwirthschaftliche Jahrb�cher,' 1880, p. 473.
+[page 503]
+
+so placed that the end of the filament stood beneath the 2-inch object
+glass of a microscope with an eye-piece micrometer, each division of which
+equalled 1/500 of an inch. The end of the filament was repeatedly observed
+during 6 h., and was seen to be in constant movement; and it crossed 5
+divisions of the micrometer (1/100 inch) in 2 h. Occasionally it moved
+forwards by jerks, some of which were 1/1000 inch in length, and then
+slowly retreated a little, afterwards again jerking forwards. These
+oscillations were exactly like those described under Brassica and Dionaea,
+but they occurred only occasionally. We may therefore conclude that this
+moderately old joint was continually circumnutating on a small scale.
+
+Alopecurus pratensis.--A young plant, 11 inches in height, with the
+flower-head protruded, but with the florets not yet expanded, had a glass
+filament fixed close above the second joint, at a height of only 2 inches
+above the ground. The basal internode, 2 inches in length, was cemented to
+a stick to prevent any possibility of its circumnutating. The extremity of
+the filament, which projected about 50o above the horizon, was often
+observed during 24 h. in the same manner as in the last case. Whenever
+looked at, it was always in movement, and it crossed 30 divisions of the
+micrometer (3/50 inch) in 3 � h.; but it sometimes moved at a quicker rate,
+for at one time it crossed 5 divisions in 1 � h. The pot had to be moved
+occasionally, as the end of the filament travelled beyond the field of
+vision; but as far as we could judge it followed during the daytime a
+semicircular course; and it certainly travelled in two different directions
+at right angles to one another. It sometimes oscillated in the same manner
+as in the last species, some of the jerks forwards being as much as 1/1000
+of an inch. We may therefore conclude that the joints in this and the last
+species of grass long continue to circumnutate; so that this movement would
+be ready to be converted into an apogeotropic movement, whenever the stem
+was placed in an inclined or horizontal position.
+
+Movements of the Flower-peduncles of Oxalis carnosa, due to apogeotropism
+and other forces.--The movements of the main peduncle, and of the three or
+four sub-peduncles which each main peduncle of this plant bears, are
+extremely complex, and are determined by several distinct causes. Whilst
+the flowers are expanded, both kinds of peduncles circumnutate about the
+same spot, as we have seen (Fig. 91) in the fourth chapter. But soon after
+the flowers have begun to wither the sub-
+[page 504]
+peduncles bend downwards, and this is due to epinasty; for on two occasions
+when pots were laid horizontally, the sub-peduncles assumed the same
+position relatively to the main peduncle, as would have been the case if
+they had remained upright; that is, each of them formed with it an angle of
+about 40o. If they had been acted on by geotropism or apheliotropism (for
+the plant was illuminated from above), they would have directed themselves
+to the centre of the earth. A main peduncle was secured to a stick in an
+upright position, and one of the upright sub-peduncles which had been
+observed circumnutating whilst the flower was expanded, continued to do so
+for at least 24 h. after it had withered. It then began to bend downwards,
+and after 36 h. pointed a little beneath the horizon. A new figure was now
+begun (A, Fig. 188), and the sub-peduncle was traced descending in a zigzag
+line from 7.20 P.M. on the 19th to 9 A.M. on the 22nd. It now pointed
+almost perpendicularly downwards, and the glass filament had to be removed
+and fastened transversely across the base of the young capsule. We expected
+that the sub-peduncle would have been motionless in its new position; but
+it continued slowly to swing, like a pendulum, from side to side, that is,
+in a plane at right angles to that in which it had descended. This
+circumnutating movement was observed from 9 A.M. on 22nd to 9 A.M. 24th, as
+shown at B in the diagram. We were not able to observe this particular
+sub-peduncle any longer; but it would certainly have gone on circumnutating
+until the capsule was nearly ripe (which requires only a short time), and
+it would then have moved upwards.
+
+The upward movement (C, Fig. 188) is effected in part by the whole
+sub-peduncle rising in the same manner as it had previously descended
+through epinasty--namely, at the joint where united to the main peduncle.
+As this upward movement occurred with plants kept in the dark and in
+whatever position the main peduncle was fastened, it could not have been
+caused by heliotropism or apogeotropism, but by hyponasty. Besides this
+movement at the joint, there is another of a very different kind, for the
+sub-peduncle becomes upwardly bent in the middle part. If the sub-peduncle
+happens at the time to be inclined much downwards, the upward curvature is
+so great that the whole forms a hook. The upper end bearing the capsule,
+thus always places itself upright, and as this occurs in darkness, and in
+whatever position the main peduncle may have been secured,
+[page 505]
+the upward curvature cannot be due to heliotropism or hyponasty, but to
+apogeotropism.
+
+Fig. 188. Oxalis carnosa: movements of flower-peduncle, traced on a
+vertical glass: A, epinastic downward movement; B, circumnutation whilst
+depending vertically; C, subsequent upward movement, due to apogeotropism
+and hyponasty combined.
+[page 506]
+
+In order to trace this upward movement, a filament was fixed to a
+sub-peduncle bearing a capsule nearly ripe, which was beginning to bend
+upwards by the two means just described. Its course was traced (see C, Fig
+188) during 53 h., by which time it had become nearly upright. The course
+is seen to be strongly zigzag, together with some little loops. We may
+therefore conclude that the movement consists of modified circumnutation.
+
+The several species of Oxalis probably profit in the following manner by
+their sub-peduncles first bending downwards and then upwards. They are
+known to scatter their seeds by the bursting of the capsule; the walls of
+which are so extremely thin, like silver paper, that they would easily be
+permeated by rain. But as soon as the petals wither, the sepals rise up and
+enclose the young capsule, forming a perfect roof over it as soon as the
+sub-peduncle has bent itself downwards. By its subsequent upward movement,
+the capsule stands when ripe at a greater height above the ground by twice
+the length of the sub-peduncle, than it did when dependent, and is thus
+able to scatter its seeds to a greater distance. The sepals, which enclose
+the ovarium whilst it is young, present an additional adaptation by
+expanding widely when the seeds are ripe, so as not to interfere with their
+dispersal. In the case of Oxalis acetosella, the capsules are said
+sometimes to bury themselves under loose leaves or moss on the ground, but
+this cannot occur with those of O. carnosa, as the woody stem is too high.
+
+Oxalis acetosella.--The peduncles are furnished with a joint in
+
+Fig. 189. Oxalis acetosella: course pursued by the upper part of a
+peduncle, whilst rising, traced from 11 A.M. June 1st to 9 A.M. 3rd. Figure
+here reduced to one-half of the original scale.
+
+the middle, so that the lower part answers to the main peduncle,
+[page 507]
+and the upper part to one of the sub-peduncles of O. carnosa. The upper
+part bends downwards, after the flower has begun to wither, and the whole
+peduncle then forms a hook; that this bending is due to epinasty we may
+infer from the case of O. carnosa. When the pod is nearly ripe, the upper
+part straightens itself and becomes erect; and this is due to hyponasty or
+apogeotropism, or both combined, and not to heliotropism, for it occurred
+in darkness. The short, hooked part of the peduncle of a cleistogamic
+flower, bearing a pod nearly ripe, was observed in the dark during three
+days. The apex of the pod at first pointed perpendicularly down, but in the
+course of three days rose 90o, so that it now projected horizontally. The
+course during the two latter days is shown in Fig. 189; and it may be seen
+how greatly the peduncle, whilst rising, circumnutated. The lines of chief
+movement were at right angles to the plane of the originally hooked part.
+The tracing was not continued any longer; but after two additional days,
+the peduncle with its capsule had become straight and stood upright.]
+
+Concluding Remarks on Apogeotropism.--When apogeotropism is rendered by any
+means feeble, it acts, as shown in the several foregoing cases, by
+increasing the always present circumnutating movement in a direction
+opposed to gravity, and by diminishing that in the direction of gravity, as
+well as that to either side. The upward movement thus becomes unequal in
+rate, and is sometimes interrupted by stationary periods. Whenever
+irregular ellipses or loops are still formed, their longer axes are almost
+always directed in the line of gravity, in an analogous manner as occurred
+with heliotropic movements in reference to the light. As apogeotropism acts
+more and more energetically, ellipses or loops cease to be formed, and the
+course becomes at first strongly, and then less and less zigzag, and
+finally rectilinear. From this gradation in the nature of the movement, and
+more especially from all growing parts, which alone (except when pulvini
+are present) are acted on by apogeotropism, con-
+[page 508]
+tinually circumnutating, we may conclude that even a rectilinear course is
+merely an extremely modified form of circumnutation. It is remarkable that
+a stem or other organ which is highly sensitive to apogeotropism, and which
+has bowed itself rapidly upwards in a straight line, is often carried
+beyond the vertical, as if by momentum. It then bends a little backwards to
+a point round which it finally circumnutates. Two instances of this were
+observed with the hypocotyls of Beta vulgaris, one of which is shown in
+Fig. 183, and two other instances with the hypocotyls of Brassica. This
+momentum-like movement probably results from the accumulated effects of
+apogeotropism. For the sake of observing how long such after-effects
+lasted, a pot with seedlings of Beta was laid on its side in the dark, and
+the hypocotyls in 3 h. 15 m. became highly inclined. The pot, still in the
+dark, was then placed upright, and the movements of the two hypocotyls were
+traced; one continued to bend in its former direction, now in opposition to
+apogeotropism, for about 37 m., perhaps for 48 m.; but after 61 m. it moved
+in an opposite direction. The other hypocotyl continued to move in its
+former course, after being placed upright, for at least 37 m.
+
+Different species and different parts of the same species are acted on by
+apogeotropism in very different degrees. Young seedlings, most of which
+circumnutate quickly and largely, bend upwards and become vertical in much
+less time than do any older plants observed by us; but whether this is due
+to their greater sensitiveness to apogeotropism, or merely to their greater
+flexibility we do not know. A hypocotyl of Beta traversed an angle of 109o
+in 3 h. 8 m., and a cotyledon of Phalaris an angle of 130o in 4 h. 30 m. On
+the other hand, the stem of a herbaceous
+[page 509]
+Verbena rose 90o in about 24 h.; that of Rubus 67o, in 70 h.; that of
+Cytisus 70o, in 72 h.; that of a young American Oak only 37o, in 72 h. The
+stem of a young Cyperus alternifolius rose only 11o in 96 h.; the bending
+being confined to near its base. Though the sheath-like cotyledons of
+Phalaris are so extremely sensitive to apogeotropism, the first true leaves
+which protrude from them exhibited only a trace of this action. Two fronds
+of a fern, Nephrodium molle, both of them young and one with the tip still
+inwardly curled, were kept in a horizontal position for 46 h., and during
+this time they rose so little that it was doubtful whether there was any
+true apogeotropic movement.
+
+The most curious case known to us of a difference in sensitiveness to
+gravitation, and consequently of movement, in different parts of the same
+organ, is that offered by the petioles of the cotyledons of Ipomoea
+leptophylla. The basal part for a short length where united to the
+undeveloped hypocotyl and radicle is strongly geotropic, whilst the whole
+upper part is strongly apogeotropic. But a portion near the blades of the
+cotyledons is after a time acted on by epinasty and curves downwards, for
+the sake of emerging in the form of an arch from the ground; it
+subsequently straightens itself, and is then again acted on by
+apogeotropism.
+
+A branch of Cucurbita ovifera, placed horizontally, moved upwards during 7
+h. in a straight line, until it stood at 40o above the horizon; it then
+began to circumnutate, as if owing to its trailing nature it had no
+tendency to rise any higher. Another upright branch was secured to a stick,
+close to the base of a tendril, and the pot was then laid horizontally in
+the dark. In this position the tendril circumnutated and made
+[page 510]
+several large ellipses during 14 h., as it likewise did on the following
+day; but during this whole time it was not in the least affected by
+apogeotropism. On the other hand, when branches of another Cucurbitaceous
+plant, Echinocytis lobata, were fixed in the dark so that the tendrils
+depended beneath the horizon, these began immediately to bend upwards, and
+whilst thus moving they ceased to circumnutate in any plain manner; but as
+soon as they had become horizontal they recommenced to revolve
+conspicuously.* The tendrils of Passiflora gracilis are likewise
+apogeotropic. Two branches were tied down so that their tendrils pointed
+many degrees beneath the horizon. One was observed for 8 h., during which
+time it rose, describing two circles, one above the other. The other
+tendril rose in a moderately straight line during the first 4 h., making
+however one small loop in its course; it then stood at about 45o above the
+horizon, where it circumnutated during the remaining 8 h. of observation.
+
+A part or organ which whilst young is extremely sensitive to apogeotropism
+ceases to be so as it grows old; and it is remarkable, as showing the
+independence of this sensitiveness and of the circumnutating movement, that
+the latter sometimes continues for a time after all power of bending from
+the centre of the earth has been lost. Thus a seedling Orange bearing only
+3 young leaves, with a rather stiff stem, did not curve in the least
+upwards during 24 h. whilst extended horizontally; yet it circumnutated all
+the time over a small space. The hypocotyl of a young seedling of Cassia
+tora, similarly placed, became vertical in 12 h.; that of an older
+seedling, 1 1/4 inch in height,
+
+* For details see 'The Movements and Habits of Climbing Plants,' 1875, p.
+131.
+[page 511]
+
+became so in 28 h.; and that of another still older one, 1 � inch in
+height, remained horizontal during two days, but distinctly circumnutated
+during this whole time.
+
+When the cotyledons of Phalaris or Avena are laid horizontally, the
+uppermost part first bends upwards, and then the lower part; consequently,
+after the lower part has become much curved upwards, the upper part is
+compelled to curve backwards in an opposite direction, in order to
+straighten itself and to stand vertically; and this subsequent
+straightening process is likewise due to apogeotropism. The upper part of 8
+young cotyledons of Phalaris were made rigid by being cemented to thin
+glass rods, so that this part could not bend in the least; nevertheless,
+the basal part was not prevented from curving upward. A stem or other organ
+which bends upwards through apogeotropism exerts considerable force; its
+own weight, which has of course to be lifted, was sufficient in almost
+every instance to cause the part at first to bend a little downwards; but
+the downward course was often rendered oblique by the simultaneous
+circumnutating movement. The cotyledons of Avena placed horizontally,
+besides lifting their own weight, were able to furrow the soft sand above
+them, so as to leave little crescentic open spaces on the lower sides of
+their bases; and this is a remarkable proof of the force exerted.
+
+As the tips of the cotyledons of Phalaris and Avena bend upwards through
+the action of apogeotropism before the basal part, and as these same tips
+when excited by a lateral light transmit some influence to the lower part,
+causing it to bend, we thought that the same rule might hold good with
+apogeotropism. Consequently, the tips of 7 cotyledons of Phalaris were
+[page 512]
+cut off for a length in three cases of .2 inch and in the four other cases
+of .14, .12, .1, and .07 inch. But these cotyledons, after being extended
+horizontally, bowed themselves upwards as effectually as the unmutilated
+specimens in the same pots, showing that sensitiveness to gravitation is
+not confined to their tips.
+
+GEOTROPISM.
+
+This movement is directly the reverse of apogeotropism. Many organs bend
+downwards through epinasty or apheliotropism or from their own weight; but
+we have met with very few cases of a downward movement in sub-a�rial organs
+due to geotropism. We shall however, give one good instance in the
+following section, in the case of Trifolium subterraneum, and probably in
+that of Arachis hypogaea.
+
+On the other hand, all roots which penetrate the ground (including the
+modified root-like petioles of Megarrhiza and Ipomoea leptophylla) are
+guided in their downward course by geotropism; and so are many a�rial
+roots, whilst others, as those of the Ivy, appear to be indifferent to its
+action. In our first chapter the movements of the radicles of several
+seedlings were described. We may there see (Fig. 1) how a radicle of the
+cabbage, when pointing vertically upwards so as to be very little acted on
+by geotropism, circumnutated; and how another (Fig. 2) which was at first
+placed in an inclined position bowed itself downwards in a zigzag line,
+sometimes remaining stationary for a time. Two other radicles of the
+cabbage travelled downwards in almost rectilinear courses. A radicle of the
+bean placed upright (Fig. 20) made a great sweep and zigzagged; but as it
+sank downwards and was more strongly acted on by geotropism, it moved in an
+[page 513]
+almost straight course. A radicle of Cucurbita, directed upwards (Fig. 26),
+also zigzagged at first, and described small loops; it then moved in a
+straight line. Nearly the same result was observed with the radicles of Zea
+mays. But the best evidence of the intimate connection between
+circumnutation and geotropism was afforded by the radicles of Phaseolus,
+Vicia, and Quercus, and in a less degree by those of Zea and Aesculus (see
+Figs. 18, 19, 21, 41, and 52); for when these were compelled to grow and
+slide down highly inclined surfaces of smoked glass, they left distinctly
+serpentine tracks.
+
+[The Burying of Seed-capsules: Trifolium subterraneum.--The flower-heads of
+this plant are remarkable from producing only 3 or 4 perfect flowers, which
+are situated exteriorly. All the other many flowers abort, and are modified
+into rigid points, with a bundle of vessels running up their centres. After
+a time 5 long, elastic, claw-like projections, which represent the
+divisions of the calyx, are developed on their summits. As soon as the
+perfect flowers wither they bend downwards, supposing the peduncle to stand
+upright, and they then closely surround its upper part. This movement is
+due to epinasty, as is likewise the case with the flowers of T. repens. The
+imperfect central flowers ultimately follow, one after the other, the same
+course. Whilst the perfect flowers are thus bending down, the whole
+peduncle curves downwards and increases much in length, until the
+flower-head reaches the ground. Vaucher* says that when the plant is so
+placed that the heads cannot soon reach the ground, the peduncles grow to
+the extraordinary length of from 6 to 9 inches. In whatever position the
+branches may be placed, the upper part of the peduncle at first bends
+vertically upwards through heliotropism; but as soon as the flowers begin
+to wither the downward curvature of the whole peduncle commences. As this
+latter movement occurred in complete darkness, and with peduncles arising
+from upright and from dependent branches, it cannot be due to
+apheliotropism or to epinasty, but must be attributed to geotropism.
+Nineteen
+
+* 'Hist. Phys. des Plantes d'Europe,' tom. ii. 1841, p. 106.
+[page 514]
+
+upright flower-heads, arising from branches in all sorts of positions, on
+plants growing in a warm greenhouse, were marked with thread, and after 24
+h. six of them were vertically dependent; these therefore had travelled
+through 180o in this time. Ten were extended sub-horizontally, and these
+had moved through about 90o. Three very young peduncles had as yet moved
+only a little downwards, but after an additional 24 h. were greatly
+inclined.
+
+At the time when the flower-heads reach the ground, the younger imperfect
+flowers in the centre are still pressed closely together, and form a
+conical projection; whereas the perfect and imperfect flowers on the
+outside are upturned and closely surround the peduncle. They are thus
+adapted to offer as little resistance, as the case admits of, in
+penetrating the ground, though the diameter of the flower-head is still
+considerable. The means by which this penetration is effected will
+presently be described. The flower-heads are able to bury themselves in
+common garden mould, and easily in sand or in fine sifted cinders packed
+rather closely. The depth to which they penetrated, measured from the
+surface to the base of the head, was between 1/4 and � inch, but in one
+case rather above 0.6 inch. With a plant kept in the house, a head partly
+buried itself in sand in 6 h.: after 3 days only the tips of the reflexed
+calyces were visible, and after 6 days the whole had disappeared. But with
+plants growing out of doors we believe, from casual observations, that they
+bury themselves in a much shorter time.
+
+After the heads have buried themselves, the central aborted flowers
+increase considerably in length and rigidity, and become bleached. They
+gradually curve, one after the other, upwards or towards the peduncle, in
+the same manner as did the perfect flowers at first. In thus moving, the
+long claws on their summits carry with them some earth. Hence a flower-head
+which has been buried for a sufficient time, forms a rather large ball,
+consisting of the aborted flowers, separated from one another by earth, and
+surrounding the little pods (the product of the perfect flowers) which lie
+close round the upper part of the peduncle. The calyces of the perfect and
+imperfect flowers are clothed with simple and multicellular hairs, which
+have the power of absorption; for when placed in a weak solution of
+carbonate of ammonia (2 gr. to 1 oz. of water) their protoplasmic contents
+immediately became aggregated and afterwards displayed the usual slow
+movements. This clover generally
+[page 515]
+grows in dry soil, but whether the power of absorption by the hairs on the
+buried flower-heads is of any importance to them we do not know. Only a few
+of the flower-heads, which from their position are not able to reach the
+ground and bury themselves, yield seeds; whereas the buried ones never
+failed, as far as we observed, to produce as many seeds as there had been
+perfect flowers.
+
+We will now consider the movements of the peduncle whilst
+
+Fig. 190. Trifolium subterraneum: downward movement of peduncle from 19o
+beneath the horizon to a nearly vertically dependent position, traced from
+11 A.M. July 22nd to the morning of 25th. Glass filament fixed transversely
+across peduncle, at base of flower-head.
+
+curving down to the ground. We have seen in Chap. IV., Fig. 92, p. 225,
+that an upright young flower-head circumnutated conspicuously; and that
+this movement continued after the peduncle had begun to bend downwards. The
+same peduncle was observed when inclined at an angle of 19o above the
+horizon, and it circumnutated during two days. Another
+[page 516]
+which was already curved 36o beneath the horizon, was observed from 11 A.M.
+July 22nd to the 27th, by which latter date it had become vertically
+dependent. Its course during the first 12 h. is shown in Fig. 190, and its
+position on the three succeeding mornings until the 25th, when it was
+nearly vertical. During the first day the peduncle clearly circumnutated,
+for it moved 4 times down and 3 times up; and on each succeeding day, as it
+sank downwards, the same movement continued, but was only occasionally
+observed and was less strongly marked. It should be stated that these
+peduncles were observed under a double skylight in the house, and that they
+generally moved downwards very much more slowly than those on plants
+growing out of doors or in the greenhouse.
+
+Fig. 191. Trifolium subterraneum: circumnutating movement of peduncle,
+whilst the flower-head was burying itself in sand, with the reflexed tips
+of the calyx still visible; traced from 8 A.M. July 26th to 9 A.M. on 27th.
+Glass filament fixed transversely across peduncle, near flower-head.
+
+Fig. 192. Trifolium subterraneum: movement of same peduncle, with
+flower-head completely buried beneath the sand; traced from 8 A.M. to 7.15
+P.M. on July 29th.
+
+The movement of another vertically dependent peduncle with the flower-head
+standing half an inch above the ground, was traced, and again when it first
+touched the ground; in both cases irregular ellipses were described every 4
+or 5 h. A peduncle on a plant which had been brought into the house, moved
+from an upright into a vertically dependent position in a single day; and
+here the course during the first 12 h. was nearly straight, but with a few
+well-marked zigzags which betrayed the essential nature of the movement.
+Lastly the circumnutation of a peduncle was traced during 51 h. whilst in
+the act of burying itself obliquely in a little heap of sand. After it had
+buried itself to such a depth that the tips of the sepals were alone
+visible, the above figure (Fig 191) was traced during 25 h. When the
+flower-head had completely disappeared beneath the sand, another tracing
+was made during 11 h. 45 m. (Fig. 192); and here again we see that the
+peduncle was circumnutating.
+[page 517]
+
+Any one who will observe a flower-head burying itself, will be convinced
+that the rocking movement, due to the continued circumnutation of the
+peduncle, plays an important part in the act. Considering that the
+flower-heads are very light, that the peduncles are long, thin, and
+flexible, and that they arise from flexible branches, it is incredible that
+an object as blunt as one of these flower-heads could penetrate the ground
+by means of the growing force of the peduncle, unless it were aided by the
+rocking movement. After a flower-head has penetrated the ground to a small
+depth, another and efficient agency comes into play; the central rigid
+aborted flowers, each terminating in five long claws, curve up towards the
+peduncle; and in doing so can hardly fail to drag the head down to a
+greater depth, aided as this action is by the circumnutating movement,
+which continues after the flower-head has completely buried itself. The
+aborted flowers thus act something like the hands of the mole, which force
+the earth backwards and the body forwards.
+
+It is well known that the seed-capsules of various widely distinct plants
+either bury themselves in the ground, or are produced from imperfect
+flowers developed beneath the surface. Besides the present case, two other
+well-marked instances will be immediately given. It is probable that one
+chief good thus gained is the protection of the seeds from animals which
+prey on them. In the case of T. subterraneum, the seeds are not only
+concealed by being buried, but are likewise protected by being closely
+surrounded by the rigid, aborted flowers. We may the more confidently infer
+that protection is here aimed at, because the seeds of several species in
+this same genus are protected in other ways;* namely, by the swelling and
+closure of the calyx, or by the persistence and bending down of the
+standard-petal, etc. But the most curious instance is that of T. globosum,
+in which the upper flowers are sterile, as in T. subterraneum, but are here
+developed into large brushes of hairs which envelop and protect the
+seed-bearing flowers. Nevertheless, in all these cases the capsules, with
+their seeds, may profit, as Mr. T. Thiselton Dyer has remarked,** by their
+being kept somewhat damp; and the advantage of such dampness perhaps throws
+light on the presence of the absorbent hairs on the buried flower-heads of
+T. subterraneum. According to Mr. Bentham, as quoted by Mr. Dyer,
+
+* Vaucher, 'Hist. Phys. des Plantes d'Europe,' tom. ii. p. 110.
+
+** See his interesting article in 'Nature,' April 4th, 1878, p. 446.
+[page 518]
+
+the prostrate habit of Helianthemum prostratum "brings the capsules in
+contact with the surface of the ground, postpones their maturity, and so
+favours the seeds attaining a larger size." The capsules of Cyclamen and of
+Oxalis acetosella are only occasionally buried, and this only beneath dead
+leaves or moss. If it be an advantage to a plant that its capsules should
+be kept damp and cool by being laid on the ground, we have in these latter
+cases the first step, from which the power of penetrating the ground, with
+the aid of the always present movement of circumnutation, might afterwards
+have been gained.
+
+Arachis hypogoea.--The flowers which bury themselves, rise from stiff
+branches a few inches above the ground, and stand upright. After they have
+fallen off, the gynophore, that is the part which supports the ovarium,
+grows to a great length, even to 3 or 4 inches, and bends perpendicularly
+downwards. It resembles closely a peduncle, but has a smooth and pointed
+apex, which contains the ovules, and is at first not in the least enlarged.
+The apex after reaching the ground penetrates it, in one case observed by
+us to a depth of 1 inch, and in another to 0.7 inch. It there becomes
+developed into a large pod. Flowers which are seated too high on the plant
+for the gynophore to reach the ground are said* never to produce pods.
+
+The movement of a young gynophore, rather under an inch in length and
+vertically dependent, was traced during 46 H. by means of a glass filament
+(with sights) fixed transversely a little above the apex. It plainly
+circumnutated (Fig. 193) whilst increasing in length and growing downwards.
+It was then raised up, so as to be extended almost horizontally, and the
+terminal part curved itself downwards, following a nearly straight course
+during 12 h., but with one attempt to circumnutate, as shown in Fig. 194.
+After 24 h. it had become nearly vertical. Whether the exciting cause of
+the downward movement is geotropism or apheliotropism was not ascertained;
+but probably it is not apheliotropism, as all the gynophores grew straight
+down towards the ground, whilst the light in the hot-house entered from one
+side as well as from above. Another and older gynophore, the apex of which
+had nearly reached the ground, was observed during 3 days in the same
+manner as the first-mentioned short one; and it was found to be always
+circumnutating. During the first 34 h. it described a figure which
+
+* 'Gard. Chronicle,' 1857, p. 566.
+[page 519]
+
+represented four ellipses. Lastly, a long gynophore, the apex of which had
+buried itself to the depth of about half an inch, was
+
+Fig. 193 Arachis hypogoea: circumnutation of vertically dependent young
+gynophore, traced on a vertical glass from 10 A.M. July 31st to 8 A.M. Aug.
+2nd.
+
+Fig. 194. Arachis hypogoea: downward movement of same young gynophore,
+after being extended horizontally; traced on a vertical glass from 8.30
+A.M. to 8.30 P.M. Aug. 2nd.
+
+pulled up and extended horizontally: it quickly began to curve downwards in
+a zigzag line; but on the following day the ter-
+[page 520]
+minal bleached portion was a little shrivelled. As the gynophores are rigid
+and arise from stiff branches, and as they terminate in sharp smooth
+points, it is probable that they could penetrate the ground by the mere
+force of growth. But this action must be aided by the circumnutating
+movement, for fine sand, kept moist, was pressed close round the apex of a
+gynophore which had reached the ground, and after a few hours it was
+surrounded by a narrow open crack. After three weeks this gynophore was
+uncovered, and the apex was found at a depth of rather above half an inch
+developed into a small, white, oval pod.
+
+Amphicarpoea monoica.--This plant produces long thin shoots, which twine
+round a support and of course circumnutate. Early in the summer shorter
+shoots are produced from the lower parts of the plant, which grow
+perpendicularly downwards and penetrate the ground. One of these,
+terminating in a minute bud, was observed to bury itself in sand to a depth
+of 0.2 inch in 24 h. It was lifted up and fixed in an inclined position
+about 25o beneath the horizon, being feebly illuminated from above. In this
+position it described two vertical ellipses in 24 h.; but on the following
+day, when brought into the house, it circumnutated only a very little round
+the same spot. Other branches were seen to penetrate the ground, and were
+afterwards found running like roots beneath the surface for a length of
+nearly two inches, and they had grown thick. One of these, after thus
+running, had emerged into the air. How far circumnutation aids these
+delicate branches in entering the ground we do not know; but the reflexed
+hairs with which they are clothed will assist in the work. This plant
+produces pods in the air, and others beneath the ground; which differ
+greatly in appearance. Asa Gray says* that it is the imperfect flowers on
+the creeping branches near the base of the plant which produce the
+subterranean pods; these flowers, therefore, must bury themselves like
+those of Arachis. But it may be suspected that the branches which were seen
+by us to penetrate the ground also produce subterranean flowers and pods.]
+
+DIAGEOTROPISM.
+
+Besides geotropism and apogeotropism, there is, according to Frank, an
+allied form of movement,
+
+* 'Manual of the Botany of the Northern United States,' 1856, p. 106.
+[page 521]
+
+namely, "transverse-geotropism," or diageotropism, as we may call it for
+the sake of matching our other terms. Under the influence of gravitation
+certain parts are excited to place themselves more or less transversely to
+the line of its action.* We made no observations on this subject, and will
+here only remark that the position of the secondary radicles of various
+plants, which extend horizontally or are a little inclined downwards, would
+probably be considered by Frank as due to transverse-geotropism. As it has
+been shown in Chap. I. that the secondary radicles of Cucurbita made
+serpentine tracks on a smoked glass-plate, they clearly circumnutated, and
+there can hardly be a doubt that this holds good with other secondary
+radicles. It seems therefore highly probable that they place themselves in
+their diageotropic position by means of modified circumnutation.
+
+Finally, we may conclude that the three kinds of movement which have now
+been described and which are excited by gravitation, consist of modified
+circumnutation. Different parts or organs on the same plant, and the same
+part in different species, are thus excited to act in a widely different
+manner. We can see no reason why the attraction of gravity should directly
+modify the state of turgescence and subsequent growth of one part on the
+upper side and of another part on the lower side. We are therefore led to
+infer that both geotropic, apogeotropic, and diageotropic movements, the
+purpose of which we can generally understand,
+
+* Elfving has lately described ('Arbeiten des Bot. Instituts in W�rzburg,'
+B. ii. 1880, p. 489) an excellent instance of such movements in the
+rhizomes of certain plants.
+[page 522]
+
+have been acquired for the advantage of the plant by the modification of
+the ever-present movement of circumnutation. This, however, implies that
+gravitation produces some effect on the young tissues sufficient to serve
+as a guide to the plant.
+[page 523]
+
+CHAPTER XI.
+
+LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
+
+General considerations--Vicia faba, effects of amputating the tips of the
+radicles--Regeneration of the tips--Effects of a short exposure of the tips
+to geotropic action and their subsequent amputation--Effects of amputating
+the tips obliquely--Effects of cauterising the tips--Effects of grease on
+the tips--Pisum sativum, tips of radicles cauterised transversely, and on
+their upper and lower sides--Phaseolus, cauterisation and grease on the
+tips--Gossypium--Cucurbita, tips cauterised transversely, and on their
+upper and lower sides--Zea, tips cauterised--Concluding remarks and summary
+of chapter--Advantages of the sensibility to geotropism being localised in
+the tips of the radicles.
+
+CIESIELSKI states* that when the roots of Pisum, Lens and Vicia were
+extended horizontally with their tips cut off, they were not acted on by
+geotropism; but some days afterwards, when a new root-cap and vegetative
+point had been formed, they bent themselves perpendicularly downwards. He
+further states that if the tips are cut off, after the roots have been left
+extended horizontally for some little time, but before they have begun to
+bend downwards, they may be placed in any position, and yet will bend as if
+still acted on by geotropism; and this shows that some influence had been
+already transmitted to the bending part from the tip before it was
+amputated. Sachs repeated these experiments; he cut off a length of between
+.05 and 1 mm. (measured from the apex of the
+
+* 'Abwartskr�mmung der Wurzel,' Inaug. Dissert. Breslau, 1871, p. 29.
+[page 524]
+
+vegetative point) of the tips of the radicles of the bean (Vicia faba), and
+placed them horizontally or vertically in damp air, earth, and water, with
+the result that they became bowed in all sorts of directions.* He therefore
+disbelieved in Ciesielski's conclusions. But as we have seen with several
+plants that the tip of the radicle is sensitive to contact and to other
+irritants, and that it transmits some influence to the upper growing part
+causing it to bend, there seemed to us to be no a priori improbability in
+Ciesielski's statements. We therefore determined to repeat his experiments,
+and to try others on several species by different methods.
+
+Vicia faba.--Radicles of this plant were extended horizontally either over
+water or with their lower surfaces just touching it. Their tips had
+previously been cut off, in a direction as accurately transverse as could
+be done, to different lengths, measured from the apex of the root-cap, and
+which will be specified in each case. Light was always excluded. We had
+previously tried hundreds of unmutilated radicles under similar
+circumstances, and found that every one that was healthy became plainly
+geotropic in under 12 h. In the case of four radicles which had their tips
+cut off for a length of 1.5 mm., new root caps and new vegetative points
+were re-formed after an interval of 3 days 20 h.; and these when placed
+horizontally were acted on by geotropism. On some other occasions this
+regeneration of the tips and reacquired sensitiveness occurred within a
+somewhat shorter time. Therefore, radicles having their tips amputated
+should be observed in from 12 to 48 h. after the operation.
+
+Four radicles were extended horizontally with their lower surfaces touching
+the water, and with their tips cut off for a length of only 0.5 mm.: after
+23 h. three of them were still horizontal; after 47 h. one of the three
+became fairly geotropic; and after 70 h. the other two showed a trace of
+this action. The fourth radicle was vertically geotropic after 23 h.; but
+by an
+
+* 'Arbeiten des Bot. Instituts in W�rzburg,' Heft. iii. 1873, p. 432.
+[page 525]
+
+accident the root-cap alone and not the vegetative point was found to have
+been amputated; so that this case formed no real exception and might have
+been excluded.
+
+Five radicles were extended horizontally like the last, and had their tips
+cut off for a length of 1 mm.; after 22-23 h., four of them were still
+horizontal, and one was slightly geotropic; after 48 h. the latter had
+become vertical; a second was also somewhat geotropic; two remained
+approximately horizontal; and the last or fifth had grown in a disordered
+manner, for it was inclined upwards at an angle of 65o above the horizon.
+
+Fourteen radicles were extended horizontally at a little height over the
+water with their tips cut off for a length of 1.5 mm.; after 12 h. all were
+horizontal, whilst five control or standard specimens in the same jar were
+all bent greatly downwards. After 24 h. several of the amputated radicles
+remained horizontal, but some showed a trace of geotropism, and one was
+plainly geotropic, for it was inclined at 40o beneath the horizon.
+
+Seven horizontally extended radicles from which the tips had been cut off
+for the unusual length of 2 mm. unfortunately were not looked at until 35
+h. had elapsed; three were still horizontal, but to our surprise, four were
+more or less plainly geotropic.
+
+The radicles in the foregoing cases were measured before their tips were
+amputated, and in the course of 24 h. they had all increased greatly in
+length; but the measurements are not worth giving. It is of more importance
+that Sachs found that the rate of growth of the different parts of radicles
+with amputated tips was the same as with unmutilated ones. Altogether
+twenty-nine radicles were operated on in the manner above described, and of
+these only a few showed any geotropic curvature within 24 h.; whereas
+radicles with unmutilated tips always became, as already stated, much bent
+down in less than half of this time. The part of the radicle which bends
+most lies at the distance of from 3 to 6 mm. from the tip, and as the
+bending part continues to grow after the operation, there does not seem any
+reason why it should not have been acted on by geotropism, unless its
+curvature depended on some influence transmitted from the tip. And we have
+clear evidence of such transmission in Ciesielski's experiments, which we
+repeated and extended in the following manner.
+
+Beans were embedded in friable peat with the hilum downwards, and after
+their radicles had grown perpendicularly down for a length of from � to 1
+inch, sixteen were selected which
+[page 526]
+were perfectly straight, and these were placed horizontally on the peat,
+being covered by a thin layer of it. They were thus left for an average
+period of 1 h. 37 m. The tips were then cut off transversely for a length
+of 1.5 mm., and immediately afterwards they were embedded vertically in the
+peat. In this position geotropism would not tend to induce any curvature,
+but if some influence had already been transmitted from the tip to the part
+which bends most, we might expect that this part would become curved in the
+direction in which geotropism had previously acted; for it should be noted
+that these radicles being now destitute of their sensitive tips, would not
+be prevented by geotropism from curving in any direction. The result was
+that of the sixteen vertically embedded radicles, four continued for
+several days to grow straight downwards, whilst twelve became more or less
+bowed laterally. In two of the twelve, a trace of curvature was perceptible
+in 3 h. 30 m., counting from the time when they had first been laid
+horizontally; and all twelve were plainly bowed in 6 h., and still more
+plainly in 9 h. In every one of them the curvature was directed towards the
+side which had been downwards whilst the radicles remained horizontal. The
+curvature extended for a length of from 5 to, in one instance, 8 mm.,
+measured from the cut-off end. Of the twelve bowed radicles five became
+permanently bent into a right angle; the other seven were at first much
+less bent, and their curvature generally decreased after 24 h., but did not
+wholly disappear. This decrease of curvature would naturally follow, if an
+exposure of only 1 h. 37 m. to geotropism, served to modify the turgescence
+of the cells, but not their subsequent growth to the full extent. The five
+radicles which were rectangularly bent became fixed in this position, and
+they continued to grow out horizontally in the peat for a length of about 1
+inch during from 4 to 6 days. By this time new tips had been formed; and it
+should be remarked that this regeneration occurred slower in the peat than
+in water, owing perhaps to the radicles being often looked at and thus
+disturbed. After the tips had been regenerated, geotropism was able to act
+on them, so that they now became bowed vertically downwards. An accurate
+drawing (Fig. 195) is given on the opposite page of one of these five
+radicles, reduced to half the natural size.
+
+We next tried whether a shorter exposure to geotropism would suffice to
+produce an after-effect. Seven radicles were extended horizontally for an
+hour, instead of 1 h. 37 m. as in the
+[page 527]
+former trial; and after their tips (1.5 mm. in length) had been amputated,
+they were placed vertically in damp peat. Of these, three were not in the
+least affected and continued for days to grow straight downwards. Four
+showed after 8 h. 30 m. a mere trace of curvature in the direction in which
+they had been acted on by geotropism; and in this respect they differed
+much from those which had been exposed for 1 h. 37 m., for many of the
+latter were plainly curved in 6 h. The curvature of one of these four
+radicles almost disappeared after 24 h. In the second, the curvature
+increased during two days and then decreased. the third radicle became
+permanently bent, so that its terminal part made an angle of about 45o with
+its original vertical direction. The fourth radicle became horizontal.
+These two, latter radicles continued during two more days to grow in the
+peat in the same directions, that is, at an angle of 45o beneath the
+horizon and horizontally. By the fourth morning new tips had been
+re-formed, and now geotropism was able to act on them again, and they
+became bent perpendicularly downwards, exactly as in the case of the five
+radicles described in the last paragraph and as is shown in (Fig. 195) here
+given.
+
+Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the
+amputation of the tip, due to the previous influence of geotropism. L, side
+of bean which lay on the peat, whilst geotropism acted on the radicle. A,
+point of chief curvature of the radicle, whilst standing vertically
+downwards. B, point of chief curvature after the regeneration of the tip,
+when geotropism again acted. C, regenerated tip.
+
+Lastly, five other radicles were similarly treated, but were exposed to
+geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully
+affected; after 24 h. two were just perceptibly curved towards the side
+which had been acted on by geotropism; after 48 h. the one first mentioned
+had a radius of curvature of 60 mm. That this curvature was due to the
+action of geotropism during the horizontal position of the radicle, was
+shown after 4 days, when a new tip had been re-formed, for it then grew
+perpendicularly downwards. We learn from this
+[page 528]
+case that when the tips are amputated after an exposure to geotropism of
+only 45 m., though a slight influence is sometimes transmitted to the
+adjoining part of the radicle, yet this seldom suffices, and then only
+slowly, to induce even moderately well-pronounced curvature.
+
+In the previously given experiments on 29 horizontally extended radicles
+with their tips amputated, only one grew irregularly in any marked manner,
+and this became bowed upwards at an angle of 65o. In Ciesielski's
+experiments the radicles could not have grown very irregularly, for if they
+had done so, he could not have spoken confidently of the obliteration of
+all geotropic action. It is therefore remarkable that Sachs, who
+experimented on many radicles with their tips amputated, found extremely
+disordered growth to be the usual result. As horizontally extended radicles
+with amputated tips are sometimes acted on slightly by geotropism within a
+short time, and are often acted on plainly after one or two days, we
+thought that this influence might possibly prevent disordered growth,
+though it was not able to induce immediate curvature. Therefore 13
+radicles, of which 6 had their tips amputated transversely for a length of
+1.5 mm., and the other 7 for a length of only 0.5 mm., were suspended
+vertically in damp air, in which position they would not be affected by
+geotropism; but they exhibited no great irregularity of growth, whilst
+observed during 4 to 6 days. We next thought that if care were not taken in
+cutting off the tips transversely, one side of the stump might be irritated
+more than the other, either at first or subsequently during the
+regeneration of the tip, and that this might cause the radicle to bend to
+one side. It has also been shown in Chapter III. that if a thin slice be
+cut off one side of the tip of the radicle, this causes the radicle to bend
+from the sliced side. Accordingly, 30 radicles, with tips amputated for a
+length of 1.5 mm., were allowed to grow perpendicularly downwards into
+water. Twenty of them were amputated at an angle of 20o with a line
+transverse to their longitudinal axes; and such stumps appeared only
+moderately oblique. The remaining ten radicles were amputated at an angle
+of about 45o. Under these circumstances no less than 19 out of the 30
+became much distorted in the course of 2 or 3 days. Eleven other radicles
+were similarly treated, excepting that only 1 mm. (including in this and
+all other cases the root-cap) was amputated; and of these only one grew
+much, and two others slightly
+[page 529]
+distorted; so that this amount of oblique amputation was not sufficient.
+Out of the above 30 radicles, only one or two showed in the first 24 h. any
+distortion, but this became plain in the 19 cases on the second day, and
+still more conspicuous at the close of the third day, by which time new
+tips had been partially or completely regenerated. When therefore a new tip
+is reformed on an oblique stump, it probably is developed sooner on one
+side than on the other: and this in some manner excites the adjoining part
+to bend to one side. Hence it seems probable that Sachs unintentionally
+amputated the radicles on which he experimented, not strictly in a
+transverse direction.
+
+This explanation of the occasional irregular growth of radicles with
+amputated tips, is supported by the results of cauterising their tips; for
+often a greater length on one side than on the other was unavoidably
+injured or killed. It should be remarked that in the following trials the
+tips were first dried with blotting-paper, and then slightly rubbed with a
+dry stick of nitrate of silver or lunar caustic. A few touches with the
+caustic suffice to kill the root-cap and some of the upper layers of cells
+of the vegetative point. Twenty-seven radicles, some young and very short,
+others of moderate length, were suspended vertically over water, after
+being thus cauterised. Of these some entered the water immediately, and
+others on the second day. The same number of uncauterised radicles of the
+same age were observed as controls. After an interval of three or four days
+the contrast in appearance between the cauterised and control specimens was
+wonderfully great. The controls had grown straight downwards, with the
+exception of the normal curvature, which we have called Sachs' curvature.
+Of the 27 cauterised radicles, 15 had become extremely distorted; 6 of them
+grew upwards and formed hoops, so that their tips sometimes came into
+contact with the bean above; 5 grew out rectangularly to one side; only a
+few of the remaining 12 were quite straight, and some of these towards the
+close of our observations became hooked at their extreme lower ends.
+Radicles, extended horizontally instead of vertically, with their tips
+cauterised, also sometimes grew distorted, but not so commonly, as far as
+we could judge, as those suspended vertically; for this occurred with only
+5 out of 19 radicles thus treated.
+
+Instead of cutting off the tips, as in the first set of experiments, we
+next tried the effects of touching horizontally extended radicles with
+caustic in the manner just described. But
+[page 530]
+some preliminary remarks must first be made. It may be objected that the
+caustic would injure the radicles and prevent them from bending; but ample
+evidence was given in Chapter III. that touching the tips of vertically
+suspended radicles with caustic on one side, does not stop their bending;
+on the contrary, it causes them to bend from the touched side. We also
+tried touching both the upper and the lower sides of the tips of some
+radicles of the bean, extended horizontally in damp friable earth. The tips
+of three were touched with caustic on their upper sides, and this would aid
+their geotropic bending; the tips of three were touched on their lower
+sides, which would tend to counteract the bending downwards; and three were
+left as controls. After 24 h. an independent observer was asked to pick out
+of the nine radicles, the two which were most and the two which were least
+bent; he selected as the latter, two of those which had been touched on
+their lower sides, and as the most bent, two of those which had been
+touched on the upper side. Hereafter analogous and more striking
+experiments with Pisum sativum and Cucurbita ovifera will be given. We may
+therefore safely conclude that the mere application of caustic to the tip
+does not prevent the radicles from bending.
+
+In the following experiments, the tips of young horizontally extended
+radicles were just touched with a stick of dry caustic; and this was held
+transversely, so that the tip might be cauterised all round as
+symmetrically as possible. The radicles were then suspended in a closed
+vessel over water, kept rather cool, viz., 55o - 59o F. This was done
+because we had found that the tips were more sensitive to contact under a
+low than under a high temperature; and we thought that the same rule might
+apply to geotropism. In one exceptional trial, nine radicles (which were
+rather too old, for they had grown to a length of from 3 to 5 cm.), were
+extended horizontally in damp friable earth, after their tips had been
+cauterised and were kept at too high a temperature, viz., of 68o F., or 20o
+C. The result in consequence was not so striking as in the subsequent cases
+for although when after 9 h. 40 m. six of them were examined, these did not
+exhibit any geotropic bending, yet after 24 h., when all nine were
+examined, only two remained horizontal, two exhibited a trace of
+geotropism, and five were slightly or moderately geotropic, yet not
+comparable in degree with the control specimens. Marks had been made on
+seven of these cauterised radicles at 10 mm. from the tips, which includes
+[page 531]
+the whole growing portion; and after the 24 h. this part had a mean length
+of 37 mm., so that it had increased to more than 3 � times its original
+length; but it should be remembered that these beans had been exposed to a
+rather high temperature.
+
+Nineteen young radicles with cauterised tips were extended at different
+times horizontally over water. In every trial an equal number of control
+specimens were observed. In the first trial, the tips of three radicles
+were lightly touched with the caustic for 6 or 7 seconds, which was a
+longer application than usual. After 23 h. 30 m. (temp. 55o - 56o F.) these
+three radicles,
+
+Fig. 196. Vicia faba: state of radicles which had been extended
+horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E, F,
+tips uncauterised. Lengths of radicles reduced to one-half scale, but by an
+accident the beans themselves not reduced in the same degree.
+
+A, B, C (Fig. 196), were still horizontal, whilst the three control
+specimens had become within 8 h. slightly geotropic, and strongly so (D, E,
+F) in 23 h. 30 m. A dot had been made on all six radicles at 10 mm. from
+their tips, when first placed horizontally. After the 23 h. 30 m. this
+terminal part, originally 10 mm. in length, had increased in the cauterised
+specimens to a mean length of 17.3 mm., and to 15.7 mm. in the control
+radicles, as shown in the figures by the unbroken transverse line; the
+dotted line being at 10 mm. from the apex. The control or uncauterised
+radicles, therefore, had actually grown less
+[page 532]
+than the cauterised; but this no doubt was accidental, for radicles of
+different ages grow at different rates, and the growth of different
+individuals is likewise affected by unknown causes. The state of the tips
+of these three radicles, which had been cauterised for a rather longer time
+than usual, was as follows: the blackened apex, or the part which had been
+actually touched by the caustic, was succeeded by a yellowish zone, due
+probably to the absorption of some of the caustic; in A, both zones
+together were 1.1 mm. in length, and 1.4 mm. in diameter at the base of the
+yellowish zone; in B, the length of both was only 0.7 mm., and the diameter
+0.7 mm.; in C, the length was 0.8 mm., and the diameter 1.2 mm.
+
+Three other radicles, the tips of which had been touched with caustic
+curing 2 or 3 seconds, remained (temp. 58o - 59o F.) horizontal for 23 h.;
+the control radicles having, of course, become geotropic within this time.
+The terminal growing part, 10 mm. in length, of the cauterised radicles had
+increased in this interval to a mean length of 24.5 mm., and of the
+controls to a mean of 26 mm. A section of one of the cauterised tips showed
+that the blackened part was 0.5 mm. in length, of which 0.2 mm. extended
+into the vegetative point; and a faint discoloration could be detected even
+to 1.6 mm. from the apex of the root-cap.
+
+In another lot of six radicles (temp. 55o - 57o F.) the three control
+specimens were plainly geotropic in 8 � h.; and after 24 h. the mean length
+of their terminal part had increased from 10 mm. to 21 mm. When the caustic
+was applied to the three cauterised specimens, it was held quite motionless
+during 5 seconds, and the result was that the black marks were extremely
+minute. Therefore, caustic was again applied, after 8 � h., during which
+time no geotropic action had occurred. When the specimens were re-examined
+after an additional interval of 15 � h., one was horizontal and the other
+two showed, to our surprise, a trace of geotropism which in one of them
+soon afterwards became strongly marked; but in this latter specimen the
+discoloured tip was only 2/3 mm. in length. The growing part of these three
+radicles increased in 24 h. from 10 mm. to an average of 16.5 mm.
+
+It would be superfluous to describe in detail the behaviour of the 10
+remaining cauterised radicles. The corresponding control specimens all
+became geotropic in 8 h. Of the cauterised, 6 were first looked at after 8
+h., and one alone showed a trace
+[page 533]
+of geotropism; 4 were first looked at after 14 h., and one alone of these
+was slightly geotropic. After 23 - 24h., 5 of the 10 were still horizontal,
+4 slightly, and 1 decidedly, geotropic. After 48 h. some of them became
+strongly geotropic. The cauterised radicles increased greatly in length,
+but the measurements are not worth giving.
+
+As five of the last-mentioned cauterised radicles had become in 24 h.
+somewhat geotropic, these (together with three which were still horizontal)
+had their positions reversed, so that their tips were now a little
+upturned, and they were again touched with caustic. After 24 h. they showed
+no trace of geotropism; whereas the eight corresponding control specimens,
+which had likewise been reversed, in which position the tips of several
+pointed to the zenith, all became geotropic; some having passed in the 24
+h. through an angle of 180o, others through about 135o, and others through
+only 90o. The eight radicles, which had been twice cauterised, were
+observed for an additional day (i.e. for 48 h. after being reversed), and
+they still showed no signs of geotropism. Nevertheless, they continued to
+grow rapidly; four were measured 24 h. after being reversed, and they had
+in this time increased in length between 8 and 11 mm.; the other four were
+measured 48 h. after being reversed, and these had increased by 20, 18, 23,
+and 28 mm.
+
+In coming to a conclusion with respect to the effects of cauterising the
+tips of these radicles, we should bear in mind, firstly, that horizontally
+extended control radicles were always acted on by geotropism, and became
+somewhat bowed downwards in 8 or 9 h.; secondly, that the chief seat of the
+curvature lies at a distance of from 3 to 6 mm. from the tip; thirdly, that
+the tip was discoloured by the caustic rarely for more than 1 mm. in
+length; fourthly, that the greater number of the cauterised radicles,
+although subjected to the full influence of geotropism during the whole
+time, remained horizontal for 24 h., and some for twice as long; and that
+those which did become bowed were so only in a slight degree; fifthly, that
+the cauterised radicles continued to grow almost, and sometimes quite, as
+well as the uninjured ones along the part which bends most. And lastly,
+that a touch on the tip with caustic, if on one side, far from preventing
+curvature, actually induces it. Bearing all these facts in mind, we must
+infer that under normal conditions the geotropic curvature of the root is
+due to an influence transmitted from the apex to the adjoining part where
+the bending
+[page 534]
+takes place; and that when the tip of the root is cauterised it is unable
+to originate the stimulus necessary to produce geotropic curvature.
+
+As we had observed that grease was highly injurious to some plants, we
+determined to try its effects on radicles. When the cotyledons of Phalaris
+and Avena were covered with grease along one side, the growth of this side
+was quite stopped or greatly checked, and as the opposite side continued to
+grow, the cotyledons thus treated became bowed towards the greased side.
+This same matter quickly killed the delicate hypocotyls and young leaves of
+certain plants. The grease which we employed was made by mixing lamp-black
+and olive oil to such a consistence that it could be laid on in a thick
+layer. The tips of five radicles of the bean were coated with it for a
+length of 3 mm., and to our surprise this part increased in length in 23 h.
+to 7.1 mm.; the thick layer of grease being curiously drawn out. It thus
+could not have checked much, if at all, the growth of the terminal part of
+the radicle. With respect to geotropism, the tips of seven horizontally
+extended radicles were coated for a length of 2 mm., and after 24 h. no
+clear difference could be perceived between their downward curvature and
+that of an equal number of control specimens. The tips of 33 other radicles
+were coated on different occasions for a length of 3 mm.; and they were
+compared with the controls after 8 h., 24 h., and 48 h. On one occasion,
+after 24 h., there was very little difference in curvature between the
+greased and control specimens; but generally the difference was
+unmistakable, those with greased tips being considerably less curved
+downwards. The whole growing part (the greased tips included) of six of
+these radicles was measured and was found to have increased in 23 h. from
+10 mm. to a mean length of 17.7 mm.; whilst the corresponding part of the
+controls had increased to 20.8 mm. It appears therefore, that although the
+tip itself, when greased, continues to grow, yet the growth of the whole
+radicle is somewhat checked, and that the geotropic curvature of the upper
+part, which was free from grease, was in most cases considerably lessened.
+
+Pisum sativum.--Five radicles, extended horizontally over water, had their
+tips lightly touched two or three times with dry caustic. These tips were
+measured in two cases, and found to be blackened for a length of only half
+a millimeter. Five other radicles were left as controls. The part which is
+most bowed through geotropism lies at a distance of several millimeters
+from
+[page 535]
+the apex. After 24 h., and again after 32 h. from the commencement, four of
+the cauterised radicles were still horizontal, but one was plainly
+geotropic, being inclined at 45o beneath the horizon. The five controls
+were somewhat geotropic after 7 h. 20 m., and after 24 h. were all strongly
+geotropic; being inclined at the following angles beneath the horizon,
+viz., 59o, 60o, 65o, 57o, and 43o. The length of the radicles was not
+measured in either set, but it was manifest that the cauterised radicles
+had grown greatly.
+
+The following case proves that the action of the caustic by itself does not
+prevent the curvature of the radicle. Ten radicles were extended
+horizontally on and beneath a layer of damp friable peat-earth; and before
+being extended their tips were touched with dry caustic on the upper side.
+Ten other radicles similarly placed were touched on the lower side; and
+this would tend to make them bend from the cauterised side; and therefore,
+as now placed, upwards, or in opposition to geotropism. Lastly, ten
+uncauterised radicles were extended horizontally as controls. After 24 h.
+all the latter were geotropic; and the ten with their tips cauterised on
+the upper side were equally geotropic; and we believe that they became
+curved downwards before the controls. The ten which had been cauterised on
+the lower side presented a widely different appearance: No. 1, however, was
+perpendicularly geotropic, but this was no real exception, for on
+examination under the microscope, there was no vestige of a coloured mark
+on the tip, and it was evident that by a mistake it had not been touched
+with the caustic. No. 2 was plainly geotropic, being inclined at about 45o
+beneath the horizon; No. 3 was slightly, and No. 4 only just perceptibly
+geotropic; Nos. 5 and 6 were strictly horizontal; and the four remaining
+ones were bowed upwards, in opposition to geotropism. In these four cases
+the radius of the upward curvatures (according to Sachs' cyclometer) was 5
+mm., 10 mm., 30 mm., and 70 mm. This curvature was distinct long before the
+24 h. had elapsed, namely, after 8 h. 45 m. from the time when the lower
+sides of the tips were touched with the caustic.
+
+Phaseolus multiflorus.--Eight radicles, serving as controls, were extended
+horizontally, some in damp friable peat and some in damp air. They all
+became (temp 20o - 21o C.) plainly geotropic in 8 h. 30 m., for they then
+stood at an average angle of 63o beneath the horizon. A rather greater
+length of the radicle is bowed downwards by geotropism than in the case of
+Vicia faba,
+[page 536]
+that is to say, rather more than 6 mm. as measured from the apex of the
+root-cap. Nine other radicles were similarly extended, three in damp peat
+and six in damp air, and dry caustic was held transversely to their tips
+during 4 or 5 seconds. Three of their tips were afterwards examined: in (1)
+a length of 0.68 mm. was discoloured, of which the basal 0.136 mm. was
+yellow, the apical part being black; in (2) the discoloration was 0.65 mm.
+in length, of which the basal 0.04 mm. was yellow; in (3) the discoloration
+was 0.6 mm. in length, of which the basal 0.13 mm. was yellow. Therefore
+less than 1 mm. was affected by the caustic, but this sufficed almost
+wholly to prevent geotropic action; for after 24 h. one alone of the nine
+cauterised radicles became slightly geotropic, being now inclined at 10o
+beneath the horizon; the eight others remained horizontal, though one was
+curved a little laterally.
+
+The terminal part (10 mm. in length) of the six cauterised radicles in the
+damp air, had more than doubled in length in the 24 h., for this part was
+now on an average 20.7 mm. long. The increase in length within the same
+time was greater in the control specimens, for the terminal part had grown
+on an average from 10 mm. to 26.6 mm. But as the cauterised radicles had
+more than doubled their length in the 24 h., it is manifest that they had
+not been seriously injured by the caustic. We may here add that when
+experimenting on the effects of touching one side of the tip with caustic,
+too much was applied at first, and the whole tip (but we believe not more
+than 1 mm. in length) of six horizontally extended radicles was killed, and
+these continued for two or three days to grow out horizontally.
+
+Many trials were made, by coating the tips of horizontally extended
+radicles with the before described thick grease. The geotropic curvature of
+12 radicles, which were thus coated for a length of 2 mm., was delayed
+during the first 8 or 9 h., but after 24 h. was nearly as great as that of
+the control specimens. The tips of nine radicles were coated for a length
+of 3 mm., and after 7 h. 10 m. these stood at an average angle of 30o
+beneath the horizon, whilst the controls stood at an average of 54o. After
+24 h. the two lots differed but little in their degree of curvature. In
+some other trials, however, there was a fairly well-marked difference after
+24 h. between those with greased tips and the controls. The terminal part
+of eight control specimens increased in 24 h. from 10 mm. to a mean length
+of
+[page 537]
+24.3 mm., whilst the mean increase of those with greased tips was 20.7 mm.
+The grease, therefore, slightly checked the growth of the terminal part,
+but this part was not much injured; for several radicles which had been
+greased for a length of 2 mm. continued to grow during seven days, and were
+then only a little shorter than the controls. The appearance presented by
+these radicles after the seven days was very curious, for the black grease
+had been drawn out into the finest longitudinal striae, with dots and
+reticulations, which covered their surfaces for a length of from 26 to 44
+mm., or of 1 to 1.7 inch. We may therefore conclude that grease on the tips
+of the radicles of this Phaseolus somewhat delays and lessens the geotropic
+curvature of the part which ought to bend most.
+
+Gossypium herbaceum.--The radicles of this plant bend, through the action
+of geotropism, for a length of about 6 mm. Five radicles, placed
+horizontally in damp air, had their tips touched with caustic, and the
+discoloration extended for a length of from 2/3 to 1 mm. They showed, after
+7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the
+terminal portion, 9 mm. in length, had increased on an average to 15.9 mm.
+Six control radicles, after 7 h. 45 m., were all plainly geotropic, two of
+them being vertically dependent, and after 23 h. all were vertical, or
+nearly so.
+
+Cucurbita ovifera.--A large number of trials proved almost useless, from
+the three following causes: Firstly, the tips of radicles which have grown
+somewhat old are only feebly geotropic if kept in damp air; nor did we
+succeed well in our experiments, until the germinating seeds were placed in
+peat and kept at a rather high temperature. Secondly, the hypocotyls of the
+seeds which were pinned to the lids of the jars gradually became arched;
+and, as the cotyledons were fixed, the movement of the hypocotyl affected
+the position of the radicle, and caused confusion. Thirdly, the point of
+the radicle is so fine that it is difficult not to cauterise it either too
+much or too little. But we managed generally to overcome this latter
+difficulty, as the following experiments show, which are given to prove
+that a touch with caustic on one side of the tip does not prevent the upper
+part of the radicle from bending. Ten radicles were laid horizontally
+beneath and on damp friable peat, and their tips were touched with caustic
+on the upper side. After 8 h. all were plainly geotropic, three of them
+rectangularly; after 19 h.
+[page 538]
+all were strongly geotropic, most of them pointing perpendicularly
+downwards. Ten other radicles, similarly placed, had their tips touched
+with caustic on the lower side; after 8 h. three were slightly geotropic,
+but not nearly so much so as the least geotropic of the foregoing
+specimens; four remained horizontal; and three were curved upwards in
+opposition to geotropism. After 19 h. the three which were slightly
+geotropic had become strongly so. Of the four horizontal radicles, one
+alone showed a trace of geotropism; of the three up-curved radicles, one
+retained this curvature, and the other two had become horizontal.
+
+The radicles of this plant, as already remarked, do not succeed well in
+damp air, but the result of one trial may be briefly given. Nine young
+radicles between .3 and .5 inch in length, with their tips cauterised and
+blackened for a length never exceeding � mm., together with eight control
+specimens, were extended horizontally in damp air. After an interval of
+only 4 h. 10 m. all the controls were slightly geotropic, whilst not one of
+the cauterised specimens exhibited a trace of this action. After 8 h. 35
+m., there was the same difference between the two sets, but rather more
+strongly marked. By this time both sets had increased greatly in length.
+The controls, however, never became much more curved downwards; and after
+24 h. there was no great difference between the two sets in their degree of
+curvature.
+
+Eight young radicles of nearly equal length (average .36 inch) were placed
+beneath and on peat-earth, and were exposed to a temp. of 75o - 76o F.
+Their tips had been touched transversely with caustic, and five of them
+were blackened for a length of about 0.5 mm., whilst the other three were
+only just visibly discoloured. In the same box there were 15 control
+radicles, mostly about .36 inch in length, but some rather longer and
+older, and therefore less sensitive. After 5 h., the 15 control radicles
+were all more or less geotropic: after 9 h., eight of them were bent down
+beneath the horizon at various angles between 45o and 90o, the remaining
+seven being only slightly geotropic: after 25 h. all were rectangularly
+geotropic. The state of the eight cauterised radicles after the same
+intervals of time was as follows: after 5 h. one alone was slightly
+geotropic, and this was one with the tip only a very little discoloured:
+after 9 h. the one just mentioned was rectangularly geotropic, and two
+others were slightly so, and these were the three which had been scarcely
+[page 539]
+affected by the caustic; the other five were still strictly horizontal.
+After 24 h. 40 m. the three with only slightly discoloured tips were bent
+down rectangularly; the other five were not in the least affected, but
+several of them had grown rather tortuously, though still in a horizontal
+plane. The eight cauterised radicles which had at first a mean length of
+.36 inch, after 9 h. had increased to a mean length of .79 inch; and after
+24 h. 40 m. to the extraordinary mean length of 2 inches. There was no
+plain difference in length between the five well cauterised radicles which
+remained horizontal, and the three with slightly cauterised tips which had
+become abruptly bent down. A few of the control radicles were measured
+after 25 h., and they were on an average only a little longer than the
+cauterised, viz., 2.19 inches. We thus see that killing the extreme tip of
+the radicle of this plant for a length of about 0.5 mm., though it stops
+the geotropic bending of the upper part, hardly interferes with the growth
+of the whole radicle.
+
+In the same box with the 15 control specimens, the rapid geotropic bending
+and growth of which have just been described, there were six radicles,
+about .6 inch in length, extended horizontally, from which the tips had
+been cut off in a transverse direction for a length of barely 1 mm. These
+radicles were examined after 9 h. and again after 24 h. 40 m., and they all
+remained horizontal. They had not become nearly so tortuous as those above
+described which had been cauterised. The radicles with their tips cut off
+had grown in the 24 h. 40 m. as much, judging by the eye, as the cauterised
+specimens.
+
+Zea mays.--The tips of several radicles, extended horizontally in damp air,
+were dried with blotting-paper and then touched in the first trial during 2
+or 3 seconds with dry caustic; but this was too long a contact, for the
+tips were blackened for a length of rather above 1 mm. They showed no signs
+of geotropism after an interval of 9 h., and were then thrown away. In a
+second trial the tips of three radicles were touched for a shorter time,
+and were blackened for a length of from 0.5 to 0.75 mm.: they all remained
+horizontal for 4 h., but after 8 h. 30 m. one of them, in which the
+blackened tip was only 0.5 mm. in length, was inclined at 21o beneath the
+horizon. Six control radicles all became slightly geotropic in 4 h., and
+strongly so after 8 h. 30 m., with the chief seat of curvature generally
+between 6 or 7 mm. from the apex. In the cauterised specimens, the terminal
+growing part, 10 mm. in length, increased during
+[page 540]
+the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3 mm.
+
+In a third trial the tips of five radicles (exposed to a temp. of 70o -
+71o) were touched with the caustic only once and very slightly; they were
+afterwards examined under the microscope, and the part which was in any way
+discoloured was on an average .76 mm. in length. After 4 h. 10 m. none were
+bent; after 5 h. 45 m., and again after 23 h. 30 m., they still remained
+horizontal, excepting one which was now inclined 20o beneath the horizon.
+The terminal part, 10 mm. in length, had increased greatly in length during
+the 23 h. 30 m., viz., to an average of 26 mm. Four control radicles became
+slightly geotropic after the 4 h. 10 m., and plainly so after the 5 h. 45
+m. Their mean length after the 23 h. 30 m. had increased from 10 mm. to 31
+mm. Therefore a slight cauterisation of the tip checks slightly the growth
+of the whole radicle, and manifestly stops the bending of that part which
+ought to bend most under the influence of geotropism, and which still
+continues to increase greatly in length.]
+
+Concluding Remarks.--Abundant evidence has now been given, showing that
+with various plants the tip of the radicle is alone sensitive to
+geotropism; and that when thus excited, it causes the adjoining parts to
+bend. The exact length of the sensitive part seems to be somewhat variable,
+depending in part on the age of the radicle; but the destruction of a
+length of from less than 1 to 1.5 mm. (about 1/20th of an inch), in the
+several species observed, generally sufficed to prevent any part of the
+radicle from bending within 24 h., or even for a longer period. The fact of
+the tip alone being sensitive is so remarkable a fact, that we will here
+give a brief summary of the foregoing experiments. The tips were cut off 29
+horizontally extended radicles of Vicia faba, and with a few exceptions
+they did not become geotropic in 22 or 23 h., whilst unmutilated radicles
+were always bowed downwards in 8 or 9 h. It should be borne in mind that
+the mere act of cutting
+[page 541]
+off the tip of a horizontally extended radicle does not prevent the
+adjoining parts from bending, if the tip has been previously exposed for an
+hour or two to the influence of geotropism. The tip after amputation is
+sometimes completely regenerated in three days; and it is possible that it
+may be able to transmit an impulse to the adjoining parts before its
+complete regeneration. The tips of six radicles of Cucurbita ovifera were
+amputated like those of Vicia faba; and these radicles showed no signs of
+geotropism in 24 h.; whereas the control specimens were slightly affected
+in 5 h., and strongly in 9 h.
+
+With plants belonging to six genera, the tips of the radicles were touched
+transversely with dry caustic; and the injury thus caused rarely extended
+for a greater length than 1 mm., and sometimes to a less distance, as
+judged by even the faintest discoloration. We thought that this would be a
+better method of destroying the vegetative point than cutting it off; for
+we knew, from many previous experiments and from some given in the present
+chapter, that a touch with caustic on one side of the apex, far from
+preventing the adjoining part from bending, caused it to bend. In all the
+following cases, radicles with uncauterised tips were observed at the same
+time and under similar circumstances, and they became, in almost every
+instance, plainly bowed downwards in one-half or one-third of the time
+during which the cauterised specimens were observed. With Vicia faba 19
+radicles were cauterised; 12 remained horizontal during 23-24 h.; 6 became
+slightly and 1 strongly geotropic. Eight of these radicles were afterwards
+reversed, and again touched with caustic, and none of them became geotropic
+in 24 h., whilst the reversed control specimens became strongly bowed
+downwards within this time.
+[page 542]
+With Pisum sativum, five radicles had their tips touched with caustic, and
+after 32 h. four were still horizontal. The control specimens were slightly
+geotropic in 7 h. 20 m., and strongly so in 24 h. The tips of 9 other
+radicles of this plant were touched only on the lower side, and 6 of them
+remained horizontal for 24 h., or were upturned in opposition to
+geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus
+multiflorus, 15 radicles were cauterised, and 8 remained horizontal for 24
+h.; whereas all the controls were plainly geotropic in 8 h. 30 m. Of 5
+cauterised radicles of Gossypium herbaceum, 4 remained horizontal for 23 h.
+and 1 became slightly geotropic; 6 control radicles were distinctly
+geotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera remained
+horizontal in peat-earth during 25 h., and 9 remained so in damp air during
+8 � h.; whilst the controls became slightly geotropic in 4 h. 10 m. The
+tips of 10 radicals of this plant were touched on their lower sides, and 6
+of them remained horizontal or were upturned after 19 h., 1 being slightly
+and 3 strongly geotropic.
+
+Lastly, the tips of several radicles of Vicia faba and Phaseolus
+multiflorus were thickly coated with grease for a length of 3 mm. This
+matter, which is highly injurious to most plants, did not kill or stop the
+growth of the tips, and only slightly lessened the rate of growth of the
+whole radicle; but it generally delayed a little the geotropic bending of
+the upper part.
+
+The several foregoing cases would tell us nothing, if the tip itself was
+the part which became most bent; but we know that it is a part distant from
+the tip by some millimeters which grows quickest, and which, under the
+influence of geotropism, bends most. We have no reason to suppose that this
+part is injured by the death or injury of the tip; and it is certain
+[page 543]
+that after the tip has been destroyed this part goes on growing at such a
+rate, that its length was often doubled in a day. We have also seen that
+the destruction of the tip does not prevent the adjoining part from
+bending, if this part has already received some influence from the tip. As
+with horizontally extended radicles, of which the tip has been cut off or
+destroyed, the part which ought to bend most remains motionless for many
+hours or days, although exposed at right angles to the full influence of
+geotropism, we must conclude that the tip alone is sensitive to this power,
+and transmits some influence or stimulus to the adjoining parts, causing
+them to bend. We have direct evidence of such transmission; for when a
+radicle was left extended horizontally for an hour or an hour and a half,
+by which time the supposed influence will have travelled a little distance
+from the tip, and the tip was then cut off, the radicle afterwards became
+bent, although placed perpendicularly. The terminal portions of several
+radicles thus treated continued for some time to grow in the direction of
+their newly-acquired curvature; for as they were destitute of tips, they
+were no longer acted on by geotropism. But after three or four days when
+new vegetative points were formed, the radicles were again acted on by
+geotropism, and now they curved themselves perpendicularly downwards. To
+see anything of the above kind in the animal kingdom, we should have to
+suppose than an animal whilst lying down determined to rise up in some
+particular direction; and that after its head had been cut off, an impulse
+continued to travel very slowly along the nerves to the proper muscles; so
+that after several hours the headless animal rose up in the predetermined
+direction.
+
+As the tip of the radicle has been found to be the
+[page 544]
+part which is sensitive to geotropism in the members of such distinct
+families as the Leguminosae, Malvaceae, Cucurbitaceae and Gramineae, we may
+infer that this character is common to the roots of most seedling plants.
+Whilst a root is penetrating the ground, the tip must travel first; and we
+can see the advantage of its being sensitive to geotropism, as it has to
+determine the course of the whole root. Whenever the tip is deflected by
+any subterranean obstacle, it will also be an advantage that a considerable
+length of the root should be able to bend, more especially as the tip
+itself grows slowly and bends but little, so that the proper downward
+course may be soon recovered. But it appears at first sight immaterial
+whether this were effected by the whole growing part being sensitive to
+geotropism, or by an influence transmitted exclusively from the tip. We
+should, however, remember that it is the tip which is sensitive to the
+contact of hard objects, causing the radicle to bend away from them, thus
+guiding it along the lines of least resistance in the soil. It is again the
+tip which is alone sensitive, at least in some cases, to moisture, causing
+the radicle to bend towards its source. These two kinds of sensitiveness
+conquer for a time the sensitiveness to geotropism, which, however,
+ultimately prevails. Therefore, the three kinds of sensitiveness must often
+come into antagonism; first one prevailing, and then another; and it would
+be an advantage, perhaps a necessity, for the interweighing and reconciling
+of these three kinds of sensitiveness, that they should be all localised in
+the same group of cells which have to transmit the command to the adjoining
+parts of the radicle, causing it to bend to or from the source of
+irritation.
+
+Finally, the fact of the tip alone being sensitive to
+[page 545]
+the attraction of gravity has an important bearing on the theory of
+geotropism. Authors seem generally to look at the bending of a radicle
+towards the centre of the earth, as the direct result of gravitation, which
+is believed to modify the growth of the upper or lower surfaces, in such a
+manner as to induce curvature in the proper direction. But we now know that
+it is the tip alone which is acted on, and that this part transmits some
+influence to the adjoining parts, causing them to curve downwards. Gravity
+does not appear to act in a more direct manner on a radicle, than it does
+on any lowly organised animal, which moves away when it feels some weight
+or pressure.
+[page 546]
+
+
+CHAPTER XII.
+
+SUMMARY AND CONCLUDING REMARKS.
+
+Nature of the circumnutating movement--History of a germinating seed--The
+radicle first protrudes and circumnutates--Its tip highly sensitive--
+Emergence of the hypocotyl or of the epicotyl from the ground under the
+form of an arch - Its circumnutation and that of the cotyledons--The
+seedling throws up a leaf-bearing stem--The circumnutation of all the parts
+or organs--Modified circumnutation--Epinasty and hyponasty--Movements of
+climbing plants--Nyctitropic movements--Movements excited by light and
+gravitation--Localised sensitiveness--Resemblance between the movements of
+plants and animals--The tip of the radicle acts like a brain.
+
+IT may be useful to the reader if we briefly sum up the chief conclusions,
+which, as far as we can judge, have been fairly well established by the
+observations given in this volume. All the parts or organs in every plant
+whilst they continue to grow, and some parts which are provided with
+pulvini after they have ceased to grow, are continually circumnutating.
+This movement commences even before the young seedling has broken through
+the ground. The nature of the movement and its causes, as far as
+ascertained, have been briefly described in the Introduction. Why every
+part of a plant whilst it is growing, and in some cases after growth has
+ceased, should have its cells rendered more turgescent and its cell-walls
+more extensile first on one side and then on another, thus inducing
+circumnutation is not known. It would appear as if the changes in the cells
+required periods of rest.
+[page 547]
+
+In some cases, as with the hypocotyls of Brassica, the leaves of Dionaea
+and the joints of the Gramineae, the circumnutating movement when viewed
+under the microscope is seen to consist of innumerable small oscillations.
+The part under observation suddenly jerks forwards for a length of .002 to
+.001 of an inch, and then slowly retreats for a part of this distance;
+after a few seconds it again jerks forwards, but with many intermissions.
+The retreating movement apparently is due to the elasticity of the
+resisting tissues. How far this oscillatory movement is general we do not
+know, as not many circumnutating plants were observed by us under the
+microscope; but no such movement could be detected in the case of Drosera
+with a 2-inch object-glass which we used. The phenomenon is a remarkable
+one. The whole hypocotyl of a cabbage or the whole leaf of a Dionaea could
+not jerk forwards unless a very large number of cells on one side were
+simultaneously affected. Are we to suppose that these cells steadily become
+more and more turgescent on one side, until the part suddenly yields and
+bends, inducing what may be called a microscopically minute earthquake in
+the plant; or do the cells on one side suddenly become turgescent in an
+intermittent manner; each forward movement thus caused being opposed by the
+elasticity of the tissues?
+
+Circumnutation is of paramount importance in the life of every plant; for
+it is through its modification that many highly beneficial or necessary
+movements have been acquired. When light strikes one side of a plant, or
+light changes into darkness, or when gravitation acts on a displaced part,
+the plant is enabled in some unknown manner to increase the always varying
+turgescence of the cells on one side; so that the ordinary circumnutating
+movement is
+[page 548]
+modified, and the part bends either to or from the exciting cause; or it
+may occupy a new position, as in the so-called sleep of leaves. The
+influence which modifies circumnutation may be transmitted from one part to
+another. Innate or constitutional changes, independently of any external
+agency, often modify the circumnutating movements at particular periods of
+the life of the plant. As circumnutation is universally present, we can
+understand how it is that movements of the same kind have been developed in
+the most distinct members of the vegetable series. But it must not be
+supposed that all the movements of plants arise from modified
+circumnutation; for, as we shall presently see, there is reason to believe
+that this is not the case.
+
+Having made these few preliminary remarks, we will in imagination take a
+germinating seed, and consider the part which the various movements play in
+the life-history of the plant. The first change is the protrusion of the
+radicle, which begins at once to circumnutate. This movement is immediately
+modified by the attraction of gravity and rendered geotropic. The radicle,
+therefore, supposing the seed to be lying on the surface, quickly bends
+downwards, following a more or less spiral course, as was seen on the
+smoked glass-plates. Sensitiveness to gravitation resides in the tip; and
+it is the tip which transmits some influence to the adjoining parts,
+causing them to bend. As soon as the tip, protected by the root-cap,
+reaches the ground, it penetrates the surface, if this be soft or friable;
+and the act of penetration is apparently aided by the rocking or
+circumnutating movement of the whole end of the radicle. If the surface is
+compact, and cannot easily be penetrated, then
+[page 549]
+the seed itself, unless it be a heavy one, is displaced or lifted up by the
+continued growth and elongation of the radicle. But in a state of nature
+seeds often get covered with earth or other matter, or fall into crevices,
+etc., and thus a point of resistance is afforded, and the tip can more
+easily penetrate the ground. But even with seeds lying loose on the surface
+there is another aid: a multitude of excessively fine hairs are emitted
+from the upper part of the radicle, and these attach themselves firmly to
+stones or other objects lying on the surface, and can do so even to glass;
+and thus the upper part is held down whilst the tip presses against and
+penetrates the ground. The attachment of the root-hairs is effected by the
+liquefaction of the outer surface of the cellulose walls, and by the
+subsequent setting hard of the liquefied matter. This curious process
+probably takes place, not for the sake of the attachment of the radicles to
+superficial objects, but in order that the hairs may be brought into the
+closest contact with the particles in the soil, by which means they can
+absorb the layer of water surrounding them, together with any dissolved
+matter.
+
+After the tip has penetrated the ground to a little depth, the increasing
+thickness of the radicle, together with the root-hairs, hold it securely in
+its place; and now the force exerted by the longitudinal growth of the
+radicle drives the tip deeper into the ground. This force, combined with
+that due to transverse growth, gives to the radicle the power of a wedge.
+Even a growing root of moderate size, such as that of a seedling bean, can
+displace a weight of some pounds. It is not probable that the tip when
+buried in compact earth can actually circumnutate and thus aid its downward
+passage, but the circumnutating movement will facilitate the tip entering
+any lateral
+[page 550]
+or oblique fissure in the earth, or a burrow made by an earth-worm or
+larva; and it is certain that roots often run down the old burrows of
+worms. The tip, however, in endeavouring to circumnutate, will continually
+press against the earth on all sides, and this can hardly fail to be of the
+highest importance to the plant; for we have seen that when little bits of
+card-like paper and of very thin paper were cemented on opposite sides of
+the tip, the whole growing part of the radicle was excited to bend away
+from the side bearing the card or more resisting substance, towards the
+side bearing the thin paper. We may therefore feel almost sure that when
+the tip encounters a stone or other obstacle in the ground, or even earth
+more compact on one side than the other, the root will bend away as much as
+it can from the obstacle or the more resisting earth, and will thus follow
+with unerring skill a line of least resistance.
+
+The tip is more sensitive to prolonged contact with an object than to
+gravitation when this acts obliquely on the radicle, and sometimes even
+when it acts in the most favourable direction at right angles to the
+radicle. The tip was excited by an attached bead of shellac weighing less
+than 1/200th of a grain (0.33 mg.); it is therefore more sensitive than the
+most delicate tendril, namely, that of Passiflora gracilis, which was
+barely acted on by a bit of wire weighing 1/50th of a grain. But this
+degree of sensitiveness is as nothing compared with that of the glands of
+Drosera, for these are excited by particles weighing only 1/78740 of a
+grain. The sensitiveness of the tip cannot be accounted for by its being
+covered by a thinner layer of tissue than the other parts, for it is
+protected by the relatively thick root-cap. It is remarkable that although
+the radicle bends away, when one side of the tip is slightly touched
+[page 551]
+with caustic, yet if the side be much cauterised the injury is too great,
+and the power of transmitting some influence to the adjoining parts causing
+them to bend, is lost. Other analogous cases are known to occur.
+
+After a radicle has been deflected by some obstacle, geotropism directs the
+tip again to grow perpendicularly downwards; but geotropism is a feeble
+power, and here, as Sachs has shown, another interesting adaptive movement
+comes into play; for radicles at a distance of a few millimeters from the
+tip are sensitive to prolonged contact in such a manner that they bend
+towards the touching object, instead of from it as occurs when an object
+touches one side of the tip. Moreover, the curvature thus caused is abrupt;
+the pressed part alone bending. Even slight pressure suffices, such as a
+bit of card cemented to one side. therefore a radicle, as it passes over
+the edge of any obstacle in the ground, will through the action of
+geotropism press against it; and this pressure will cause the radicle to
+endeavour to bend abruptly over the edge. It will thus recover as quickly
+as possible its normal downward course.
+
+Radicles are also sensitive to air which contains more moisture on one side
+than the other, and they bend towards its source. It is therefore probable
+that they are in like manner sensitive to dampness in the soil. It was
+ascertained in several cases that this sensitiveness resides in the tip,
+which transmits an influence causing the adjoining upper part to bend in
+opposition to geotropism towards the moist object. We may therefore infer
+that roots will be deflected from their downward course towards any source
+of moisture in the soil.
+
+Again, most or all radicles are slightly sensitive to light, and according
+to Wiesner, generally bend a little
+[page 552]
+from it. Whether this can be of any service to them is very doubtful, but
+with seeds germinating on the surface it will slightly aid geotropism in
+directing the radicles to the ground.* We ascertained in one instance that
+such sensitiveness resided in the tip, and caused the adjoining parts to
+bend from the light. The sub-a�rial roots observed by Wiesner were all
+apheliotropic, and this, no doubt, is of use in bringing them into contact
+with trunks of trees or surfaces of rock, as is their habit.
+
+We thus see that with seedling plants the tip of the radicle is endowed
+with diverse kinds of sensitiveness; and that the tip directs the adjoining
+growing parts to bend to or from the exciting cause, according to the needs
+of the plant. The sides of the radicle are also sensitive to contact, but
+in a widely different manner. Gravitation, though a less powerful cause of
+movement than the other above specified stimuli, is ever present; so that
+it ultimately prevails and determines the downward growth of the root.
+
+The primary radicle emits secondary ones which project sub-horizontally;
+and these were observed in one case to circumnutate. Their tips are also
+sensitive to contact, and they are thus excited to bend away from any
+touching object; so that they resemble in these respects, as far as they
+were observed, the primary radicles. If displaced they resume, as Sachs has
+shown, their original sub-horizontal position; and this apparently is due
+to diageotropism. The secondary radicles emit tertiary ones, but these, in
+the case of the bean, are not affected by gravitation; consequently they
+protrude in all directions. Thus the general
+
+* Dr. Karl Richter, who has especially attended to this subject ('K. Akad.
+der Wissenschaften in Wien,' 1879, p. 149), states that apheliotropism does
+not aid radicles in penetrating the ground.
+[page 553]
+
+arrangement of the three orders of roots is excellently adapted for
+searching the whole soil for nutriment.
+
+Sachs has shown that if the tip of the primary radicle is cut off (and the
+tip will occasionally be gnawed off with seedlings in a state of nature)
+one of the secondary radicles grows perpendicularly downwards, in a manner
+which is analogous to the upward growth of a lateral shoot after the
+amputation of the leading shoot. We have seen with radicles of the bean
+that if the primary radicle is merely compressed instead of being cut off,
+so that an excess of sap is directed into the secondary radicles, their
+natural condition is disturbed and they grow downwards. Other analogous
+facts have been given. As anything which disturbs the constitution is apt
+to lead to reversion, that is, to the resumption of a former character, it
+appears probable that when secondary radicles grow downwards or lateral
+shoots upwards, they revert to the primary manner of growth proper to
+radicles and shoots.
+
+With dicotyledonous seeds, after the protrusion of the radicle, the
+hypocotyl breaks through the seed-coats; but if the cotyledons are
+hypogean, it is the epicotyl which breaks forth. These organs are at first
+invariably arched, with the upper part bent back parallel to the lower; and
+they retain this form until they have risen above the ground. In some
+cases, however, it is the petioles of the cotyledons or of the first true
+leaves which break through the seed-coats as well as the ground, before any
+part of the stem protrudes; and then the petioles are almost invariably
+arched. We have met with only one exception, and that only a partial one,
+namely, with the petioles of the two first leaves of Acanthus candelabrum.
+With Delphinium nudicaule the petioles of the two cotyledons are com-
+[page 554]
+pletely confluent, and they break through the ground as an arch; afterwards
+the petioles of the successively formed early leaves are arched, and they
+are thus enabled to break through the base of the confluent petioles of the
+cotyledons. In the case of Megarrhiza, it is the plumule which breaks as an
+arch through the tube formed by the confluence of the cotyledon-petioles.
+With mature plants, the flower-stems and the leaves of some few species,
+and the rachis of several ferns, as they emerge separately from the ground,
+are likewise arched.
+The fact of so many different organs in plants of many kinds breaking
+through the ground under the form of an arch, shows that this must be in
+some manner highly important to them. According to Haberlandt, the tender
+growing apex is thus saved from abrasion, and this is probably the true
+explanation. But as both legs of the arch grow, their power of breaking
+through the ground will be much increased as long as the tip remains within
+the seed-coats and has a point of support. In the case of monocotyledons
+the plumule or cotyledon is rarely arched, as far as we have seen; but this
+is the case with the leaf-like cotyledon of the onion; and the crown of the
+arch is here strengthened by a special protuberance. In the Gramineae the
+summit of the straight, sheath-like cotyledon is developed into a hard
+sharp crest, which evidently serves for breaking through the earth. With
+dicotyledons the arching of the epicotyl or hypocotyl often appears as if
+it merely resulted from the manner in which the parts are packed within the
+seed; but it is doubtful whether this is the whole of the truth in any
+case, and it certainly was not so in several cases, in which the arching
+was seen to commence after the parts had wholly
+[page 555]
+escaped from the seed-coats. As the arching occurred in whatever position
+the seeds were placed, it is no doubt due to temporarily increased growth
+of the nature of epinasty or hyponasty along one side of the part.
+
+As this habit of the hypocotyl to arch itself appears to be universal, it
+is probably of very ancient origin. It is therefore not surprising that it
+should be inherited, at least to some extent, by plants having hypogean
+cotyledons, in which the hypocotyl is only slightly developed and never
+protrudes above the ground, and in which the arching is of course now quite
+useless. This tendency explains, as we have seen, the curvature of the
+hypocotyl (and the consequent movement of the radicle) which was first
+observed by Sachs, and which we have often had to refer to as Sachs'
+curvature.
+
+The several foregoing arched organs are continually circumnutating, or
+endeavouring to circumnutate, even before they break through the ground. As
+soon as any part of the arch protrudes from the seed-coats it is acted upon
+by apogeotropism, and both the legs bend upwards as quickly as the
+surrounding earth will permit, until the arch stands vertically. By
+continued growth it then forcibly breaks through the ground; but as it is
+continually striving to circumnutate this will aid its emergence in some
+slight degree, for we know that a circumnutating hypocotyl can push away
+damp sand on all sides. As soon as the faintest ray of light reaches a
+seedling, heliotropism will guide it through any crack in the soil, or
+through an entangled mass of overlying vegetation; for apogeotropism by
+itself can direct the seedling only blindly upwards. Hence probably it is
+that sensitiveness to light resides in the tip of the cotyledons of the
+Gramineae, and in
+[page 556]
+the upper part of the hypocotyls of at least some plants.
+
+As the arch grows upwards the cotyledons are dragged out of the ground. The
+seed-coats are either left behind buried, or are retained for a time still
+enclosing the cotyledons. These are afterwards cast off merely by the
+swelling of the cotyledons. But with most of the Cucurbitaceae there is a
+curious special contrivance for bursting the seed-coats whilst beneath the
+ground, namely, a peg at the base of the hypocotyl, projecting at right
+angles, which holds down the lower half of the seed-coats, whilst the
+growth of the arched part of the hypocotyl lifts up the upper half, and
+thus splits them in twain. A somewhat analogous structure occurs in Mimosa
+pudica and some other plants. Before the cotyledons are fully expanded and
+have diverged, the hypocotyl generally straightens itself by increased
+growth along the concave side, thus reversing the process which caused the
+arching. Ultimately not a trace of the former curvature is left, except in
+the case of the leaf-like cotyledons of the onion.
+
+The cotyledons can now assume the function of leaves, and decompose
+carbonic acid; they also yield up to other parts of the plant the nutriment
+which they often contain. When they contain a large stock of nutriment they
+generally remain buried beneath the ground, owing to the small development
+of the hypocotyl; and thus they have a better chance of escaping
+destruction by animals. From unknown causes, nutriment is sometimes stored
+in the hypocotyl or in the radicle, and then one of the cotyledons or both
+become rudimentary, of which several instances have been given. It is
+probable that the extraordinary manner of germination of Megarrhiza
+Californica,
+[page 557]
+Ipomoea leptophylla and pandurata, and of Quercus virens, is connected with
+the burying of the tuber-like roots, which at an early age are stocked with
+nutriment; for in these plants it is the petioles of the cotyledons which
+first protrude from the seeds, and they are then merely tipped with a
+minute radicle and hypocotyl. These petioles bend down geotropically like a
+root and penetrate the ground, so that the true root, which afterwards
+becomes greatly enlarged, is buried at some little depth beneath the
+surface. Gradations of structure are always interesting, and Asa Gray
+informs us that with Ipomoea Jalappa, which likewise forms huge tubers, the
+hypocotyl is still of considerable length, and the petioles of the
+cotyledons are only moderately elongated. But in addition to the advantage
+gained by the concealment of the nutritious matter stored within the
+tubers, the plumule, at least in the case of Megarrhiza, is protected from
+the frosts of winter by being buried.
+
+With many dicotyledonous seedlings, as has lately been described by De
+Vries, the contraction of the parenchyma of the upper part of the radicle
+drags the hypocotyl downwards into the earth; sometimes (it is said) until
+even the cotyledons are buried. The hypocotyl itself of some species
+contracts in a like manner. It is believed that this burying process serves
+to protect the seedlings against the frosts of winter.
+
+Our imaginary seedling is now mature as a seedling, for its hypocotyl is
+straight and its cotyledons are fully expanded. In this state the upper
+part of the hypocotyl and the cotyledons continue for some time to
+circumnutate, generally to a wide extent relatively to the size of the
+parts, and at a rapid rate. But seedlings profit by this power of movement
+only when it is modified, especially by the action of light and
+[page 558]
+gravitation; for they are thus enabled to move more rapidly and to a
+greater extent than can most mature plants. Seedlings are subjected to a
+severe struggle for life, and it appears to be highly important to them
+that they should adapt themselves as quickly and as perfectly as possible
+to their conditions. Hence also it is that they are so extremely sensitive
+to light and gravitation. The cotyledons of some few species are sensitive
+to a touch; but it is probable that this is only an indirect result of the
+foregoing kinds of sensitiveness, for there is no reason to believe that
+they profit by moving when touched.
+
+Our seedling now throws up a stem bearing leaves, and often branches, all
+of which whilst young are continually circumnutating. If we look, for
+instance, at a great acacia tree, we may feel assured that every one of the
+innumerable growing shoots is constantly describing small ellipses; as is
+each petiole, sub-petiole, and leaflet. The latter, as well as ordinary
+leaves, generally move up and down in nearly the same vertical plane, so
+that they describe very narrow ellipses. The flower-peduncles are likewise
+continually circumnutating. If we could look beneath the ground, and our
+eyes had the power of a microscope, we should see the tip of each rootlet
+endeavouring to sweep small ellipses or circles, as far as the pressure of
+the surrounding earth permitted. All this astonishing amount of movement
+has been going on year after year since the time when, as a seedling, the
+tree first emerged from the ground.
+
+Stems are sometimes developed into long runners or stolons. These
+circumnutate in a conspicuous manner, and are thus aided in passing between
+and over surrounding obstacles. But whether the circumnutating movement has
+been increased for this special purpose is doubtful.
+[page 559]
+
+We have now to consider circumnutation in a modified form, as the source of
+several great classes of movement. The modification may be determined by
+innate causes, or by external agencies. Under the first head we see leaves
+which, when first unfolded, stand in a vertical position, and gradually
+bend downwards as they grow older. We see flower-peduncles bending down
+after the flower has withered, and others rising up; or again, stems with
+their tips at first bowed downwards, so as to be hooked, afterwards
+straightening themselves; and many other such cases. These changes of
+position, which are due to epinasty or hyponasty, occur at certain periods
+of the life of the plant, and are independent of any external agency. They
+are effected not by a continuous upward or downward movement, but by a
+succession of small ellipses, or by zigzag lines,--that is, by a
+circumnutating movement which is preponderant in some one direction.
+
+Again, climbing plants whilst young circumnutate in the ordinary manner,
+but as soon as the stem has grown to a certain height, which is different
+for different species, it elongates rapidly, and now the amplitude of the
+circumnutating movement is immensely increased, evidently to favour the
+stem catching hold of a support. The stem also circumnutates rather more
+equally to all sides than in the case of non-climbing plants. This is
+conspicuously the case with those tendrils which consist of modified
+leaves, as these sweep wide circles; whilst ordinary leaves usually
+circumnutate nearly in the same vertical plane. Flower-peduncles when
+converted into tendrils have their circumnutating movement in like manner
+greatly increased.
+
+We now come to our second group of circumnu-
+[page 560]
+tating movements--those modified through external agencies. The so-called
+sleep or nyctitropic movements of leaves are determined by the daily
+alternations of light and darkness. It is not the darkness which excites
+them to move, but the difference in the amount of light which they receive
+during the day and night; for with several species, if the leaves have not
+been brightly illuminated during the day, they do not sleep at night. They
+inherit, however, some tendency to move at the proper periods,
+independently of any change in the amount of light. The movements are in
+some cases extraordinarily complex, but as a full summary has been given in
+the chapter devoted to this subject, we will here say but little on this
+head. Leaves and cotyledons assume their nocturnal position by two means,
+by the aid of pulvini and without such aid. In the former case the movement
+continues as long as the leaf or cotyledon remains in full health; whilst
+in the latter case it continues only whilst the part is growing. Cotyledons
+appear to sleep in a larger proportional number of species than do leaves.
+In some species, the leaves sleep and not the cotyledons; in others, the
+cotyledons and not the leaves; or both may sleep, and yet assume widely
+different positions at night.
+
+Although the nyctitropic movements of leaves and cotyledons are wonderfully
+diversified, and sometimes differ much in the species of the same genus,
+yet the blade is always placed in such a position at night, that its upper
+surface is exposed as little as possible to full radiation. We cannot doubt
+that this is the object gained by these movements; and it has been proved
+that leaves exposed to a clear sky, with their blades compelled to remain
+horizontal, suffered much more from the cold than others which were allowed
+to assume
+[page 561]
+their proper vertical position. Some curious facts have been given under
+this head, showing that horizontally extended leaves suffered more at
+night, when the air, which is not cooled by radiation, was prevented from
+freely circulating beneath their lower surfaces; and so it was, when the
+leaves were allowed to go to sleep on branches which had been rendered
+motionless. In some species the petioles rise up greatly at night, and the
+pinnae close together. The whole plant is thus rendered more compact, and a
+much smaller surface is exposed to radiation.
+
+That the various nyctitropic movements of leaves result from modified
+circumnutation has, we think, been clearly shown. In the simplest cases a
+leaf describes a single large ellipse during the 24 h.; and the movement is
+so arranged that the blade stands vertically during the night, and
+reassumes its former position on the following morning. The course pursued
+differs from ordinary circumnutation only in its greater amplitude, and in
+its greater rapidity late in the evening and early on the following
+morning. Unless this movement is admitted to be one of circumnutation, such
+leaves do not circumnutate at all, and this would be a monstrous anomaly.
+In other cases, leaves and cotyledons describe several vertical ellipses
+during the 24 h.; and in the evening one of them is increased greatly in
+amplitude until the blade stands vertically either upwards or downwards. In
+this position it continues to circumnutate until the following morning,
+when it reassumes its former position. These movements, when a pulvinus is
+present, are often complicated by the rotation of the leaf or leaflet; and
+such rotation on a small scale occurs during ordinary circumnutation. The
+many diagrams showing the movements of sleeping and non-sleeping leaves and
+coty-
+[page 562]
+ledons should be compared, and it will be seen that they are essentially
+alike. Ordinary circumnutation is converted into a nyctitropic movement,
+firstly by an increase in its amplitude, but not to so great a degree as in
+the case of climbing plants, and secondly by its being rendered periodic in
+relation to the alternations of day and night. But there is frequently a
+distinct trace of periodicity in the circumnutating movements of
+non-sleeping leaves and cotyledons. The fact that nyctitropic movements
+occur in species distributed in many families throughout the whole vascular
+series, is intelligible, if they result from the modification of the
+universally present movement of circumnutation; otherwise the fact is
+inexplicable.
+
+In the seventh chapter we have given the case of a Porlieria, the leaflets
+of which remained closed all day, as if asleep, when the plant was kept
+dry, apparently for the sake of checking evaporation. Something of the same
+kind occurs with certain Gramineae. At the close of this same chapter, a
+few observations were appended on what may be called the embryology of
+leaves. The leaves produced by young shoots on cut-down plants of Melilotus
+Taurica slept like those of a Trifolium, whilst the leaves on the older
+branches on the same plants slept in a very different manner, proper to the
+genus; and from the reasons assigned we are tempted to look at this case as
+one of reversion to a former nyctitropic habit. So again with Desmodium
+gyrans, the absence of small lateral leaflets on very young plants, makes
+us suspect that the immediate progenitor of this species did not possess
+lateral leaflets, and that their appearance in an almost rudimentary
+condition at a somewhat more advanced age is the result of reversion to a
+trifoliate predecessor. However this may be, the rapid circumnutating or
+[page 563]
+gyrating movements of the little lateral leaflets, seem to be due
+proximately to the pulvinus, or organ of movement, not having been reduced
+nearly so much as the blade, during the successive modifications through
+which the species has passed.
+
+We now come to the highly important class of movements due to the action of
+a lateral light. When stems, leaves, or other organs are placed, so that
+one side is illuminated more brightly than the other, they bend towards the
+light. This heliotropic movement manifestly results from the modification
+of ordinary circumnutation; and every gradation between the two movements
+could be followed. When the light was dim, and only a very little brighter
+on one side than on the other, the movement consisted of a succession of
+ellipses, directed towards the light, each of which approached nearer to
+its source than the previous one. When the difference in the light on the
+two sides was somewhat greater, the ellipses were drawn out into a
+strongly-marked zigzag line, and when much greater the course became
+rectilinear. We have reason to believe that changes in the turgescence of
+the cells is the proximate cause of the movement of circumnutation; and it
+appears that when a plant is unequally illuminated on the two sides, the
+always changing turgescence is augmented along one side, and is weakened or
+quite arrested along the other sides. Increased turgescence is commonly
+followed by increased growth, so that a plant which has bent itself towards
+the light during the day would be fixed in this position were it not for
+apogeotropism acting during the night. But parts provided with pulvini
+bend, as Pfeffer has shown, towards the light; and here growth does not
+come into play any more than in the ordinary circumnutating movements of
+pulvini.
+[page 564]
+
+Heliotropism prevails widely throughout the vegetable kingdom, but
+whenever, from the changed habits of life of any plant, such movements
+become injurious or useless, the tendency is easily eliminated, as we see
+with climbing and insectivorous plants.
+
+Apheliotropic movements are comparatively rare in a well-marked degree,
+excepting with sub-a�rial roots. In the two cases investigated by us, the
+movement certainly consisted of modified circumnutation.
+
+The position which leaves and cotyledons occupy during the day, namely,
+more or less transversely to the direction of the light, is due, according
+to Frank, to what we call diaheliotropism. As all leaves and cotyledons are
+continually circumnutating, there can hardly be a doubt that
+diaheliotropism results from modified circumnutation. From the fact of
+leaves and cotyledons frequently rising a little in the evening, it appears
+as if diaheliotropism had to conquer during the middle of the day a widely
+prevalent tendency to apogeotropism.
+
+Lastly, the leaflets and cotyledons of some plants are known to be injured
+by too much light; and when the sun shines brightly on them, they move
+upwards or downwards, or twist laterally, so that they direct their edges
+towards the light, and thus they escape being injured. These
+paraheliotropic movements certainly consisted in one case of modified
+circumnutation; and so it probably is in all cases, for the leaves of all
+the species described circumnutate in a conspicuous manner. This movement
+has hitherto been observed only with leaflets provided with pulvini, in
+which the increased turgescence on opposite sides is not followed by
+growth; and we can understand why this should be so, as the movement is
+required only for a temporary purpose. It would manifestly be dis-
+[page 565]
+advantageous for the leaf to be fixed by growth in its inclined position.
+For it has to assume its former horizontal position, as soon as possible
+after the sun has ceased shining too brightly on it.
+
+The extreme sensitiveness of certain seedlings to light, as shown in our
+ninth chapter, is highly remarkable. The cotyledons of Phalaris became
+curved towards a distant lamp, which emitted so little light, that a pencil
+held vertically close to the plants, did not cast any shadow which the eye
+could perceive on a white card. These cotyledons, therefore, were affected
+by a difference in the amount of light on their two sides, which the eye
+could not distinguish. The degree of their curvature within a given time
+towards a lateral light did not correspond at all strictly with the amount
+of light which they received; the light not being at any time in excess.
+They continued for nearly half an hour to bend towards a lateral light,
+after it had been extinguished. They bend with remarkable precision towards
+it, and this depends on the illumination of one whole side, or on the
+obscuration of the whole opposite side. The difference in the amount of
+light which plants at any time receive in comparison with what they have
+shortly before received, seems in all cases to be the chief exciting cause
+of those movements which are influenced by light. Thus seedlings brought
+out of darkness bend towards a dim lateral light, sooner than others which
+had previously been exposed to daylight. We have seen several analogous
+cases with the nyctitropic movements of leaves. A striking instance was
+observed in the case of the periodic movements of the cotyledons of a
+Cassia; in the morning a pot was placed in an obscure part of a room, and
+all the cotyledons rose up closed; another pot had stood in the sunlight,
+and
+[page 566]
+the cotyledons of course remained expanded; both pots were now placed close
+together in the middle of the room, and the cotyledons which had been
+exposed to the sun, immediately began to close, while the others opened; so
+that the cotyledons in the two pots moved in exactly opposite directions
+whilst exposed to the same degree of light.
+
+We found that if seedlings, kept in a dark place, were laterally
+illuminated by a small wax taper for only two or three minutes at intervals
+of about three-quarters of an hour, they all became bowed to the point
+where the taper had been held. We felt much surprised at this fact, and
+until we had read Wiesner's observations, we attributed it to the
+after-effects of the light; but he has shown that the same degree of
+curvature in a plant may be induced in the course of an hour by several
+interrupted illuminations lasting altogether for 20 m., as by a continuous
+illumination of 60 m. We believe that this case, as well as our own, may be
+explained by the excitement from light being due not so much to its actual
+amount, as to the difference in amount from that previously received; and
+in our case there were repeated alternations from complete darkness to
+light. In this, and in several of the above specified respects, light seems
+to act on the tissues of plants, almost in the same manner as it does on
+the nervous system of animals.
+There is a much more striking analogy of the same kind, in the
+sensitiveness to light being localised in the tips of the cotyledons of
+Phalaris and Avena, and in the upper part of the hypocotyls of Brassica and
+Beta; and in the transmission of some influence from these upper to the
+lower parts, causing the latter to bend towards the light. This influence
+is also trans-
+[page 567]
+mitted beneath the soil to a depth where no light enters. It follows from
+this localisation, that the lower parts of the cotyledons of Phalaris,
+etc., which normally become more bent towards a lateral light than the
+upper parts, may be brightly illuminated during many hours, and will not
+bend in the least, if all light be excluded from the tip. It is an
+interesting experiment to place caps over the tips of the cotyledons of
+Phalaris, and to allow a very little light to enter through minute orifices
+on one side of the caps, for the lower part of the cotyledons will then
+bend to this side, and not to the side which has been brightly illuminated
+during the whole time. In the case of the radicles of Sinapis alba,
+sensitiveness to light also resides in the tip, which, when laterally
+illuminated, causes the adjoining part of the root to bend
+apheliotropically.
+
+Gravitation excites plants to bend away from the centre of the earth, or
+towards it, or to place themselves in a transverse position with respect to
+it. Although it is impossible to modify in any direct manner the attraction
+of gravity, yet its influence could be moderated indirectly, in the several
+ways described in the tenth chapter; and under such circumstances the same
+kind of evidence as that given in the chapter on Heliotropism, showed in
+the plainest manner that apogeotropic and geotropic, and probably
+diageotropic movements, are all modified forms of circumnutation.
+
+Different parts of the same plant and different species are affected by
+gravitation in widely different degrees and manners. Some plants and organs
+exhibit hardly a trace of its action. Young seedlings which, as we know,
+circumnutate rapidly, are eminently sensitive; and we have seen the
+hypocotyl of Beta bending
+[page 568]
+upwards through 109o in 3 h. 8 m. The after-effects of apogeotropism last
+for above half an hour; and horizontally-laid hypocotyls are sometimes thus
+carried temporarily beyond an upright position. The benefits derived from
+geotropism, apogeotropism, and diageotropism, are generally so manifest
+that they need not be specified. With the flower-peduncles of Oxalis,
+epinasty causes them to bend down, so that the ripening pods may be
+protected by the calyx from the rain. Afterwards they are carried upwards
+by apogeotropism in combination with hyponasty, and are thus enabled to
+scatter their seeds over a wider space. The capsules and flower-heads of
+some plants are bowed downwards through geotropism, and they then bury
+themselves in the earth for the protection and slow maturation of the
+seeds. This burying process is much facilitated by the rocking movement due
+to circumnutation.
+
+In the case of the radicles of several, probably of all seedling plants,
+sensitiveness to gravitation is confined to the tip, which transmits an
+influence to the adjoining upper part, causing it to bend towards the
+centre of the earth. That there is transmission of this kind was proved in
+an interesting manner when horizontally extended radicles of the bean were
+exposed to the attraction of gravity for 1 or 1 � h., and their tips were
+then amputated. Within this time no trace of curvature was exhibited, and
+the radicles were now placed pointing vertically downwards; but an
+influence had already been transmitted from the tip to the adjoining part,
+for it soon became bent to one side, in the same manner as would have
+occurred had the radicle remained horizontal and been still acted on by
+geotropism. Radicles thus treated continued to grow out horizontally for
+two or three days, until a new tip was
+[page 569]
+re-formed; and this was then acted on by geotropism, and the radicle became
+curved perpendicularly downwards.
+
+It has now been shown that the following important classes of movement all
+arise from modified circumnutation, which is omnipresent whilst growth
+lasts, and after growth has ceased, whenever pulvini are present. These
+classes of movement consist of those due to epinasty and hyponasty,--those
+proper to climbing plants, commonly called revolving nutation,--the
+nyctitropic or sleep movements of leaves and cotyledons,--and the two
+immense classes of movement excited by light and gravitation. When we speak
+of modified circumnutation we mean that light, or the alternations of light
+and darkness, gravitation, slight pressure or other irritants, and certain
+innate or constitutional states of the plant, do not directly cause the
+movement; they merely lead to a temporary increase or diminution of those
+spontaneous changes in the turgescence of the cells which are already in
+progress. In what manner, light, gravitation, etc., act on the cells is not
+known; and we will here only remark that, if any stimulus affected the
+cells in such a manner as to cause some slight tendency in the affected
+part to bend in a beneficial manner, this tendency might easily be
+increased through the preservation of the more sensitive individuals. But
+if such bending were injurious, the tendency would be eliminated unless it
+was overpoweringly strong; for we know how commonly all characters in all
+organisms vary. Nor can we see any reason to doubt, that after the complete
+elimination of a tendency to bend in some one direction under a certain
+stimulus, the power to bend in a directly
+[page 570]
+opposite direction might gradually be acquired through natural selection.*
+
+Although so many movements have arisen through modified circumnutation,
+there are others which appear to have had a quite independent origin; but
+they do not form such large and important classes. When a leaf of a Mimosa
+is touched it suddenly assumes the same position as when asleep, but Brucke
+has shown that this movement results from a different state of turgescence
+in the cells from that which occurs during sleep; and as sleep-movements
+are certainly due to modified circumnutation, those from a touch can hardly
+be thus due. The back of a leaf of Drosera rotundifolia was cemented to the
+summit of a stick driven into the ground, so that it could not move in the
+least, and a tentacle was observed during many hours under the microscope;
+but it exhibited no circumnutating movement, yet after being momentarily
+touched with a bit of raw meat, its basal part began to curve in 23
+seconds. This curving movement therefore could not have resulted from
+modified circumnutation. But when a small object, such as a fragment of a
+bristle, was placed on one side of the tip of a radicle, which we know is
+continually circumnutating, the induced curvature was so similar to the
+movement caused by geotropism, that we can hardly doubt that it is due to
+modified circumnutation. A flower of a Mahonia was cemented to a stick, and
+the stamens exhibited no signs of circumnutation under the microscope, yet
+when they were lightly touched they suddenly moved towards the pistil.
+Lastly, the curling of the extremity of a tendril when
+
+* See the remarks in Frank's 'Die wagerechte Richtung von Pflanzentheilen'
+(1870, pp. 90, 91, etc.), on natural selection in connection with
+geotropism, heliotropism, etc.
+[page 571]
+
+touched seems to be independent of its revolving or circumnutating
+movement. This is best shown by the part which is the most sensitive to
+contact, circumnutating much less than the lower parts, or apparently not
+at all.*
+
+Although in these cases we have no reason to believe that the movement
+depends on modified circumnutation, as with the several classes of movement
+described in this volume, yet the difference between the two sets of cases
+may not be so great as it at first appears. In the one set, an irritant
+causes an increase or diminution in the turgescence of the cells, which are
+already in a state of change; whilst in the other set, the irritant first
+starts a similar change in their state of turgescence. Why a touch, slight
+pressure or any other irritant, such as electricity, heat, or the
+absorption of animal matter, should modify the turgescence of the affected
+cells in such a manner as to cause movement, we do not know. But a touch
+acts in this manner so often, and on such widely distinct plants, that the
+tendency seems to be a very general one; and if beneficial, it might be
+increased to any extent. In other cases, a touch produces a very different
+effect, as with Nitella, in which the protoplasm may be seen to recede from
+the walls of the cell; in Lactuca, in which a milky fluid exudes; and in
+the tendrils of certain Vitaceae, Cucurbitaceae, and Bignoniaceae, in which
+slight pressure causes a cellular outgrowth.
+
+Finally it is impossible not to be struck with the resemblance between the
+foregoing movements of plants and many of the actions performed
+unconsciously by the lower animals.** With plants an
+
+* For the evidence on this head, see the 'Movements and Habits of Climbing
+Plants,' 1875, pp. 173, 174.
+
+** Sachs remarks to nearly the same effect: "Dass sich die le-
+[[page 572]]
+bende Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theile
+mit specifischen Energien ausger�stet sind, �hnlich, wie die verschiedenen
+Sinnesnerven des Thiere" ('Arbeiten des Bot. Inst. in W�rzburg,' Bd. ii.
+1879, p. 282).
+[page 572]
+
+astonishingly small stimulus suffices; and even with allied plants one may
+be highly sensitive to the slightest continued pressure, and another highly
+sensitive to a slight momentary touch. The habit of moving at certain
+periods is inherited both by plants and animals; and several other points
+of similitude have been specified. But the most striking resemblance is the
+localisation of their sensitiveness, and the transmission of an influence
+from the excited part to another which consequently moves. Yet plants do
+not of course possess nerves or a central nervous system; and we may infer
+that with animals such structures serve only for the more perfect
+transmission of impressions, and for the more complete intercommunication
+of the several parts.
+
+We believe that there is no structure in plants more wonderful, as far as
+its functions are concerned, than the tip of the radicle. If the tip be
+lightly pressed or burnt or cut, it transmits an influence to the upper
+adjoining part, causing it to bend away from the affected side; and, what
+is more surprising, the tip can distinguish between a slightly harder and
+softer object, by which it is simultaneously pressed on opposite sides. If,
+however, the radicle is pressed by a similar object a little above the tip,
+the pressed part does not transmit any influence to the more distant parts,
+but bends abruptly towards the object. If the tip perceives the air to be
+moister on one side than on the other, it likewise transmits an influence
+to the upper adjoining part, which bends towards the source of moisture.
+When the tip is excited by light (though
+[page 573]
+in the case of radicles this was ascertained in only a single instance) the
+adjoining part bends from the light; but when excited by gravitation the
+same part bends towards the centre of gravity. In almost every case we can
+clearly perceive the final purpose or advantage of the several movements.
+Two, or perhaps more, of the exciting causes often act simultaneously on
+the tip, and one conquers the other, no doubt in accordance with its
+importance for the life of the plant. The course pursued by the radicle in
+penetrating the ground must be determined by the tip; hence it has acquired
+such diverse kinds of sensitiveness. It is hardly an exaggeration to say
+that the tip of the radicle thus endowed, and having the power of directing
+the movements of the adjoining parts, acts like the brain of one of the
+lower animals; the brain being seated within the anterior end of the body,
+receiving impressions from the sense-organs, and directing the several
+movements.
+
+[page 574]
+
+
+INDEX.
+
+ABIES--AMPHICARPOEA.
+
+A.
+
+Abies communis, effect of killing or injuring the leading shoot, 187
+-- pectinata, effect of killing or injuring the leading shoot, 187
+--, affected by Aecidium elatinum, 188
+
+
+Abronia umbellata, its single, developed cotyledon, 78
+--, rudimentary cotyledon, 95
+--, rupture of the seed coats, 105
+
+Abutilon Darwinii, sleep of leaves and not of cotyledons, 314
+--, nocturnal movement of leaves, 323
+
+Acacia Farnesiana, state of plant when awake and asleep, 381, 382
+--, appearance at night, 395
+--, nyctitropic movements of pinnae, 402
+--, the axes of the ellipses, 404
+-- lophantha, character of first leaf, 415
+-- retinoides, circumnutation of young phyllode, 236
+
+Acanthosicyos horrida, nocturnal movement of cotyledon 304
+
+Acanthus candelabrum, inequality in the two first leaves, 79
+--, petioles not arched, 553
+-- latifolius, variability in first leaves 79
+-- mollis, seedling, manner of breaking through the ground, 78, 79
+--, circumnutation of young leaf, 249, 269
+-- spinosus, 79
+--, movement of leaves, 249
+
+Adenanthera pavonia, nyctitropic movements of leaflets, 374
+
+Aecidium elatinum, effect on the lateral branches of the silver fir, 188
+
+Aesculus hippocastanum, movements of radicle, 28, 29
+--, sensitiveness of apex of radicle, 172-174
+
+Albizzia lophantha, nyctitropic movements of leaflets, 383
+--, of pinnae, 402
+
+Allium cepa, conical protuberance on arched cotyledon, 59
+--, circumnutation of basal half of arched cotyledon, 60
+--, mode of breaking through ground, 87
+--, straightening process, 101
+-- porrum, movements of flower-stems, 226
+
+Alopecurus pratensis, joints affected by apogeotropism, 503
+
+Aloysia citriodora, circumnutation of stem, 210
+
+Amaranthus, sleep of leaves, 387
+-- caudatus, nocturnal movement of cotyledons, 307
+
+Amorpha fruticosa, sleep of leaflets, 354
+
+Ampelopsis tricuspidata, hyponastic movement of hooked tips, 272-275
+
+Amphicarpoea monoica, circumnutation and nyctitropic movements of leaves,
+365
+--, effect of sunshine on leaflets, 445
+--, geotropic movements of, 520
+[page 575]
+
+ANODA--BRASSICA
+
+Anoda Wrightii, sleep of cotyledons, 302, 312
+--, of leaves, 324
+--, downward movement of cotyledons, 444
+
+Apheliotropism, or negative heliotropism, 5, 419, 432
+
+Apios graveolens, heliotropic movements of hypocotyl, 422-424
+-- tuberosa, vertical sinking of leaflets at night, 368
+
+Apium graveolens, sleep of cotyledons, 305
+--, petroselinum, sleep of cotyledons, 304
+
+Apogeotropic movements effected by joints or pulvini, 502
+
+Apogeotropism, 5, 494; retarded by heliotropism, 501; concluding remarks
+on, 507
+
+Arachis hypogoea, circumnutation of gynophore, 225
+--, effects of radiation on leaves, 289, 296
+--, movements of leaves, 357
+-- rate of movement, 404
+--, circumnutation of vertically dependent young gynophores, 519
+--, downward movement of the same, 519
+
+Arching of various organs, importance of, to seedling plants, 87, 88;
+emergence of hypocotyls or epicotyls in the form of an, 553
+
+Asparagus officinalis, circumnutation of plumules, 60-62.
+--, effect of lateral light, 484
+
+Asplenium trichomanes, movement in the fruiting fronds, 257, n.
+
+Astragalus uliginosus, movement of leaflets, 355
+
+Avena sativa, movement of cotyledons, 65, 66.
+--, sensitiveness of tip of radicle to moist air, 183
+--, heliotropic movement and circumnutation of cotyledon, 421, 422
+--, sensitiveness of cotyledon to a lateral light, 477
+--, young sheath-like cotyledons strongly apogeotropic, 499
+
+Avena sativa, movements of oldish cotyledons, 499, 500
+
+Averrhoa bilimbi, leaf asleep, 330
+--, angular movements when going to sleep, 331-335
+--, leaflets exposed to bright sunshine, 447
+
+Azalea Indica, circumnutation of stem, 208
+
+B.
+
+Bary, de, on the effect of the Aecidium on the silver fir, 188
+
+Batalin, Prof., on the nyctitropic movements of leaves, 283; on the sleep
+of leaves of Sida napoea, 322; on Polygonum aviculare, 387; on the effect
+of sunshine on leaflets of Oxalis acetosella, 447
+
+Bauhinia, nyctitropic movements, 373
+--, movements of petioles of young seedlings, 401
+--, appearance of young plants at night, 402
+
+Beta vulgaris, circumnutation of hypocotyl of seedlings, 52
+--, movements of cotyledons, 52, 53
+--, effect of light, 124
+--, nocturnal movement of cotyledons, 307
+--, heliotropic movements of, 420
+--, transmitted effect of light on hypocotyl, 482
+--, apogeotropic movement of hypocotyl, 496
+
+Bignonia capreolata, apheliotropic movement of tendrils, 432, 450
+
+Bouch� on Melaleuca ericaefolia, 383
+
+Brassica napus, circumnutation of flower-stems, 226
+
+Brassica oleracea, circumnutation of seedling, 10
+--, of radicle, 11
+--, geotropic movement of radicle, 11
+[page 576]
+
+Brassica oleracea, movement of buried and arched hypocotyl, 13, 14, 15
+--, conjoint circumnutation of hypocotyl and cotyledons, 16, 17, 18
+--, of hypocotyl in darkness, 19
+--, of a cotyledon with hypocotyl secured to a stick, 19, 20
+--, rate of movement, 20
+--, ellipses described by hypocotyls when erect, 105
+--, movements of cotyledons, 115
+--, -- of stem, 202
+--, -- of leaves at night, 229, 230
+--, sleep of cotyledons, 301
+--, circumnutation of hypocotyl of seedling plant, 425
+--, heliotropic movement and circumnutation of hypocotyls, 426
+--, effect of lateral light on hypocotyls, 479-482
+--, apogeotropic movement of hypocotyls, 500, 501
+
+Brassica rapa, movements of leaves, 230
+
+Brongniart, A., on the sleep of Strephium floribundum, 391
+
+Bruce, Dr., on the sleep of leaves in Averrhoa, 330
+
+Bryophyllum (vel Calanchoe) calycinum, movement of leaves, 237
+
+C.
+
+Camellia Japonica, circumnutation of leaf, 231, 232
+
+Candolle, A. de, on Trapa natans, 95; on sensitiveness of cotyledons, 127
+
+Canna Warscewiczii, circumnutation of plumules, 58, 59
+--, of leaf, 252
+
+Cannabis sativa, movements of leaves, 250
+--, nocturnal movements of cotyledons, 307
+Cannabis sativa, sinking of the young leaves at night, 444
+
+Cassia, nyctitropic movement of leaves, 369
+
+Cassia Barclayana, nocturnal movement of leaves, 372
+--, slight movement of leaflets, 401
+-- calliantha, uninjured by exposure at night, 289, n.
+--, nyctitropic movement of leaves, 371
+-- circumnutating movement of leaves, 372
+-- corymbosa, cotyledons sensitive to contact, 126
+--, nyctitropic movement of leaves, 369
+-- floribunda, use of sleep movements, 289
+--, effect of radiation on the leaves at night, 294
+--, circumnutating and nyctitropic movement of a terminal leaflet, 372, 373
+--, movements of young and older leaves, 400
+-- florida, cotyledons sensitive to contact, 126
+--, sleep of cotyledons, 308
+-- glauca, cotyledons sensitive to contact, 126
+--, sleep of cotyledons, 308
+-- laevigata, effect of radiation on leaves, 289, n.
+-- mimosoides, movement of cotyledons. 116
+--, sensitiveness of, 126
+--, sleep of, 308
+--, nyctitropic movement of leaves, 372
+--, effect of bright sunshine on cotyledons, 446
+-- neglecta, movements of, 117
+--, effect of light, 124
+--, sensitiveness of cotyledons, 126
+-- nodosa, non-sensitive cotyledons, 126
+--, do not rise at night, 308
+-- pubescens, non-sensitive cotyledons, 126
+[page 577]
+
+CASSIA--CRINUM
+
+Cassia pubescens, uninjured by exposure at night, 293
+--, sleep of cotyledons, 308
+--, nyctitropic movement of leaves, 371
+--, circumnutating movement of leaves, 372
+--, nyctitropic movement of petioles, 400
+--, diameter of plant at night, 402
+-- sp. (?) movement of cotyledons, 116
+-- tora, circumnutation of cotyledons and hypocotyls, 34, 35, 109, 308
+--, effect of light, 124, 125
+--, sensitiveness to contact, 125
+--, heliotropic movement and circumnutation of hypocotyl, 431
+--, hypocotyl of seedling slightly heliotropic, 454
+--, apogeotropic movement of old hypocotyl, 497
+--, movement of hypocotyl of young seedling, 510
+
+Caustic (nitrate of silver), effect of, on radicle of bean, 150, 156; on
+the common pea, 160.
+
+Cells, table of the measurement of, in the pulvini of Oxalis corniculata,
+120; changes in, 547
+
+Centrosema, 365
+
+Ceratophyllum demersum, movements of stem, 211
+
+Cereus Landbeckii, its rudimentary cotyledons, 97
+-- speciossimus, circumnutation of stem, 206, 207
+
+Cerinthe major, circumnutation of hypocotyl, 49
+--, of cotyledons, 49
+--, ellipses described by hypocotyls when erect, 107
+-- effect of darkness, 124
+
+Chatin, M., on Pinus Nordmanniana, 389
+
+Chenopodium album, sleep of leaves but not of cotyledons, 314, 319
+
+Chenopodium album, movement of leaves, 387
+
+Chlorophyll injured by bright light, 446
+
+Ciesielski, on the sensitiveness of the tip of the radicles, 4, 523
+
+Circumnutation, meaning explained, 1; modified, 263-279; and heliotropism,
+relation between, 435; of paramount importance to every plant, 547
+
+Cissus discolor, circumnutation of leaf, 233
+
+Citrus aurantium, circumnutation of epicotyl, 28
+--, unequal cotyledons, 95
+
+Clianthus Dampieri, nocturnal movement of leaves, 297
+
+Coboea scandens, circumnutation of, 270
+
+Cohn, on the water secreted by Lathraea squamaria, 86, n.; on the movement
+of leaflets of Oxalis, 447
+
+Colutea arborea, nocturnal movement of leaflets, 355
+
+Coniferae, circumnutation of, 211
+Coronilla rosea, leaflets asleep, 355
+
+Corylus avellana, circumnutation of young shoot, emitted from the epicotyl,
+55, 56
+--, arched epicotyl, 77
+
+Cotyledon umbilicus, circumnutation of stolons, 219, 220
+
+Cotyledons, rudimentary, 94-98; circumnutation of, 109-112; nocturnal
+movements, 111, 112; pulvini or joints of, 112-122; disturbed periodic
+movements by light, 123; sensitiveness of, to contact, 125; nyctitropic
+movements of, 283, 297; list of cotyledons which rise or sink at night,
+300; concluding remarks on their movements, 311
+
+Crambe maritima, circumnutation of leaves, 228, 229
+
+Crinum Capense, shape of leaves, 253
+[page 578]
+
+CRINUM--DESMODIUM
+
+Crinum Capense, circumnutation of, 254
+
+Crotolaria (sp.?), sleep of leaves, 340
+
+Cryptogams, circumnutation of, 257-259
+
+Cucumis dudaim, movement of cotyledons, 43, 44
+--, sleep of cotyledons, 304
+
+Cucurbita aurantia, movement of hypocotyl, 42
+--, cotyledons vertical at night, 304
+--, ovifera, geotropic movement of radicle, 38, 39
+--, circumnutation of arched hypocotyl, 39
+--, of straight and vertical hypocotyl, 40
+--, movements of cotyledons, 41, 42, 115, 124
+--, position of radicle, 89
+--, rupture of the seed-coats, 102
+--, circumnutation of hypocotyl when erect, 107, 108
+--, sensitiveness of apex of radicle, 169-171
+--, cotyledons vertical at night, 304
+--, not affected by apogeotropism, 509
+--, tips cauterised transversely, 537
+
+Curvature of the radicle, 193
+
+Cycas pectinata, circumnutation of young leaf, whilst emerging from the
+ground, 58
+--, first leaf arched, 78
+--, circumnutation of terminal leaflets, 252
+
+Cyclamen Persicum, movement of cotyledon, 46
+--, undeveloped cotyledons, 78, 96
+--, circumnutation of peduncle, 225
+--, --, of leaf, 246, 247
+--, downward apheliotropic movement of a flower-peduncle, 433-435
+
+Cyclamen Persicum, burying of the pods, 433
+
+Cyperus alternifolius, circumnutation of stem, 212
+--, movement of stem, 509
+
+Cytisus fragrans, circumnutation of hypocotyl, 37
+--, sleep of leaves, 344, 397
+--, apogeotropic movement of stem, 494-496
+
+D.
+
+Dahlia, circumnutation of young leaves, 244-246
+
+Dalea alopecuroides, leaflets depressed at night, 354
+
+Darkness, effect of, on the movement of leaves, 407
+
+Darlingtonia Californica, its leaves or pitchers apheliotropic, 450, n.
+
+Darwin, Charles, on Maurandia semperflorens, 225; on the Swedish turnip,
+230, n.; movements of climbing plants, 266, 271; the heliotropic movement
+of the tendrils of Bignonia capreolata, 433; revolution of climbing plants,
+451; on the curling of a tendril, 570
+--, Erasmus, on the peduncles of Cyclamens, 433
+--, Francis, on the radicle of Sinapis alba, 486; on Hygroscopic seeds,
+489, n.
+
+Datura stramonium, nocturnal movement of cotyledons, 298
+
+Delpino, on cotyledons of Chaerophyllum and Corydalis, 96, n.
+
+Delphinium nudicaule, mode of breaking through the ground, 80
+--, confluent petioles of two cotyledons, 553
+
+Desmodium gyrans, movement of leaflets, 257, n.
+--, position of leaves at night, 285
+--, sleep of leaves, not of cotyledons, 314
+--, circumnutation and nycti-
+[page 579]
+
+DESMODIUM--EUCALYPTUS
+
+tropic movement of leaves, 358-360
+Desmodium gyrans, movement of lateral leaflets, 361
+--, jerking of leaflets, 362
+-- nyctitropic movement of petioles, 400, 401
+--, diameter of plant at night, 402
+--, lateral movement of leaves, 404
+--, zigzag movement of apex of leaf, 405
+--, shape of lateral leaflet, 416
+--, vespertilionis, 364, n.
+
+Deutzia gracilis, circumnutation of stem, 205
+
+Diageotropism, 5; or transverse-geotropism, 520
+
+Diaheliotropism, 5; or Transversal-Heliotropismus of Frank, 419; influenced
+by epinasty, 439; by weight and apogeotropism, 440
+
+Dianthus caryophyllus, 230
+--, circumnutation of young leaf, 231, 269
+
+Dicotyledons, circumnutation widely spread among, 68
+
+Dionoea, oscillatory movements of leaves, 261, 271
+
+Dionoea muscipula, circumnutation of young expanding leaf, 239, 240
+--, closure of the lobes and circumnutation of a full-grown leaf, 241
+--, oscillations of, 242-244
+
+Diurnal sleep, 419
+
+Drosera Capensis, structure of first-formed leaves, 414
+-- rotundifolia, movement of young leaf, 237, 238
+--, of the tentacles, 239
+--, sensitiveness of tentacles, 261
+--, shape of leaves, 414
+--, leaves not heliotropic, 450
+--, leaves circumnutate largely, 454
+--, sensitiveness of 570
+
+Duchartre on Trephrosia cariboea, 354; on the nyctitropic movement of the
+Cassia, 369
+
+Duval-Jouve, on the movements of Bryophyllum calycinum, 237; of the narrow
+leaves of the Gramineae, 413
+
+Dyer, Mr. Thiselton, on the leaves of Crotolaria, 340; on Cassia
+floribunda, 369, n., on the absorbent hairs on the buried flower-heads of
+Trifolium subterraneum, 517
+
+E.
+
+Echeveria stolonifera, circumnutation of leaf, 237
+
+Echinocactus viridescens, its rudimentary cotyledons, 97
+
+Echinocystis lobata, movements of tendrils, 266
+--, apogeotropism of tendrils, 510
+
+Elfving, F., on the rhizomes of Sparganium ramosum, 189; on the
+diageotropic movement in the rhizomes of some plants, 521
+
+Elymus arenareus, leaves closed during the day, 413
+
+Embryology of leaves, 414
+
+Engelmann, Dr., on the Quercus virens, 85
+
+Epinasty, 5, 267
+
+Epicotyl, or plumule, 5; manner of breaking through the ground, 77; emerges
+from the ground under the form of an arch, 553
+
+Erythrina caffra, sleep of leaves, 367
+-- corallodendron, movement of terminal leaflet, 367
+-- crista-galli, effect of temperature on sleep of leaves, 318
+--, circumnutation and nyctitropic movement of terminal leaflets, 367
+
+Eucalyptus resinifera, circumnutation of leaves, 244
+[page 580]
+
+EUPHORBIA--GYMNOSPERMS
+
+Euphorbia jacquineaeflora, nyctitropic movement of leaves, 388
+
+F.
+
+Flahault, M., on the rupture of seed-coats, 102-104, 106
+
+Flower-stems, circumnutation of, 223-226
+
+Fragaria Rosacea, circumnutation of stolon, 214-218
+
+Frank, Dr. A. B., the terms Heliotropism and Geotropism, first used by him,
+5, n.; radicles acted on by geotropism, 70, n.; on the stolons of Fragaria,
+215; periodic and nyctitropic movements of leaves, 284; on the root-leaves
+of plants kept in darkness, 443; on pulvini, 485; on natural selection in
+connection with geotropism, heliotropism, etc., 570
+--, on Transversal-Heliotropismus, 419
+
+Fuchsia, circumnutation of stem, 205, 206
+
+G.
+
+Gazania ringens, circumnutation of stem, 208 Genera containing sleeping
+plants, 320, 321
+
+Geotropism, 5; effect of, on the primary radicle, 196; the reverse of
+apogeotropism, 512: effect on the tips of radicles, 543
+
+Geranium cinereum, 304
+-- Endressii, 304
+-- Ibericum, nocturnal movement of cotyledons, 298
+-- Richardsoni, 304
+-- rotundifolium, nocturnal movement of cotyledon, 304, 312
+-- subcaulescens, 304
+
+Germinating seed, history of a, 548
+
+Githago segetum, circumnutation of hypocotyl, 21, 108
+--, burying of hypocotyl, 109
+--, seedlings feebly illuminated, 124, 128
+--, sleep of cotyledon, 302
+--, -- leaves 321
+
+Glaucium luteum, circumnutation of young leaves, 228
+
+Gleditschia, sleep of leaves, 368
+
+Glycine hispida, vertical sinking of leaflets, 366
+
+Glycyrrhiza, leaflets depressed at night, 355
+
+Godlewski, Emil, on the turgescence of the cells, 485
+
+Gooseberry, effect of radiation, 284
+
+Gossypium (var. Nankin cotton), circumnutation of hypocotyl, 22
+--, movement of cotyledon, 22, 23
+--, sleep of leaves, 324
+--, arboreum (?), sleep of cotyledons, 303
+--, Braziliense, nocturnal movement of leaves, 324
+--, sleep of cotyledons, 303
+-- herbaceum, sensitiveness of apex of radicle, 168
+--, radicles cauterised transversely, 537
+-- maritimum, nocturnal movement of leaves, 324
+
+Gravitation, movements excited by, 567
+
+Gray, Asa, on Delphinium nudicaule, 80; on Megarrhiza Californica, 81; on
+the movements in the fruiting fronds of Aesplenium trichomanes, 257; on the
+Amphicarpoea monoica, 520; on the Ipomoea Jalappa, 557
+
+Grease, effect of, on radicles and their tips, 182, 185
+
+Gressner, Dr. H., on the cotyledons of Cyclamen Persicum, 46, 77; on
+hypocotyl of the same, 96
+
+Gymnosperms, 389
+[page 581]
+
+HABERLANDT--IPOMOEA
+
+H.
+
+Haberlandt, Dr., on the protuberance on the hypocotyl of Allium, 59; the
+importance of the arch to seedling plants, 87; sub-a�rial and subterranean
+cotyledons, 110, n.; the arched hypocotyl, 554
+
+Haematoxylon Campechianum, nocturnal movement of leaves, 368, 369
+
+Hedera helix, circumnutation of stem, 207
+
+Hedysarum coronarium, nocturnal movements of leaves, 356
+
+Helianthemum prostratum, geotropic movement of flower-heads, 518
+
+Helianthus annuus, circumnutation of hypocotyl, 45
+--, arching of hypocotyl, 90
+--, nocturnal movement of cotyledons, 305
+
+Heliotropism, 5; uses of, 449; a modified form of circumnutation, 490
+
+Helleborus niger, mode of breaking through the ground, 86
+
+Hensen, Prof., on roots in worm-burrows, 72
+
+Henslow, Rev. G., on the cotyledons of Phalaris Canariensis, 62
+
+Hofmeister, on the curious movement of Spirogyra, 3, 259, n.; of the leaves
+of Pistia stratiotes, 255; of cotyledons at night, 297; of petals, 414
+-- and Batalin on the movements of the cabbage, 229
+
+Hooker, Sir J., on the effect of light on the pitchers of Sarracenia, 450
+
+Hypocotyl, 5; manner of breaking through the ground, 77; emerges under the
+form of an arch, 553
+
+Hypocotyls and Epicotyls, circumnutation and other movements when arched,
+98; power of straightening themselves, 100; rupture of the seed-coats,
+102-106; illustration of, 106; circumnutation when erect, 107; when in
+dark, 108
+
+Hyponasty, 6, 267
+
+I.
+Iberis umbellata, movement of stem, 202.
+
+Illumination, effect of, on the sleep of leaves, 398
+
+Imatophyllum vel Clivia (sp.?), movement of leaves, 255
+
+Indigofera tinctoria, leaflets depressed at night, 354
+
+Inheritance in plants, 407, 491
+
+Insectivorous and climbing plants not heliotropic, 450; influence of light
+on, 488
+
+Ipomoea bona nox, arching of hypocotyl, 90
+--, nocturnal position of cotyledons, 306, 312
+-- coerulea vel Pharbitis nil, circumnutation of seedlings, 47
+--, movement of cotyledons, 47-49, 109
+--, nocturnal movements of cotyledons, 305
+--, sleep of leaves, 386
+--, sensitiveness to light, 451
+--, the hypocotyledonous stems heliotropic, 453
+-- coccinea, position of cotyledons at night, 306, 312
+-- leptophylla, mode of breaking through the ground, 83, 84
+--, arching of the petioles of the cotyledons, 90
+--, difference in sensitiveness to gravitation in different parts, 509
+--, extraordinary manner of germination, 557
+[page 582]
+
+IPOMOEA--LOTUS
+
+Ipomoea pandurata, manner of germination, 84, 557
+-- purpurea (vel Pharbitis hispida), nocturnal movement of cotyledons, 305,
+312
+--, sleep of leaves, 386
+--, sensitiveness to light, 451
+--, the hypocotyledonous stems heliotropic, 453
+
+Iris pseudo-acorus, circumnutation of leaves, 253
+
+Irmisch, on cotyledons of Ranunculus Ficaria, 96
+
+Ivy, its stems heliotropic, 451
+
+K.
+
+Kerner on the bending down of peduncles, 414
+
+Klinostat, the, an instrument devised by Sachs to eliminate geotropism, 93
+
+Kraus, Dr. Carl, on the underground shoots of Triticum repens, 189; on
+Cannabis sativa, 250, 307, 312; on the movements of leaves, 318
+L.
+
+Lactuca scariola, sleep of cotyledons, 305
+
+Lagenaria vulgaris, circumnutation of seedlings, 42
+--, of cotyledons, 43
+--, cotyledons vertical at night, 304
+
+Lathraea squamaria, mode of breaking through the ground, 85
+--, quantity of water secreted, 85, 86, n.
+
+Lathyrus nissolia, circumnutation of stem of young seedling, 33
+--, ellipses described by, 107, 108
+
+Leaves, circumnutation of, 226-262; dicotyledons, 226-252; monocotyledons,
+252-257; nyctitropism of, 280; their temperature affected by their position
+at night, 294; nyctitropic or sleep movements, 315, 394; periodicity of
+their movements inherited, 407; embryology of, 414; so-called diurnal
+sleep, 445
+
+Leguminosae, sleep of cotyledons, 308; sleeping species, 340
+
+Le Maout and Decaisne, 67
+
+Lepidium sativum, sleep of cotyledons, 302
+
+Light, movements excited by 418, 563; influence on most vegetable tissues,
+486; acts on plant as on the nervous system of animals, 487
+
+Lilium auratum, circumnutation of stem, 212
+--, apogeotropic movement of stem, 498, 499
+
+Linnaeus, 'Somnus Plantarum', 280; on plants sleeping, 320; on the leaves
+of Sida abutilon, 324; on Oenothera mollissima, 383
+
+Linum Berendieri, nocturnal movement of cotyledons, 298
+-- usitatissimum, circumnutation of stem, 203
+
+Lolium perenne, joints affected by apogeotropism, 502
+
+Lonicera brachypoda, hooking of the tip, 272
+--, sensitiveness to light, 453
+
+Loomis, Mr., on the movements in the fruiting fronds of Asplenium
+trichomanes, 257
+
+Lotus aristata, effect of radiation on leaves, 292
+-- Creticus, leaves awake and asleep, 354
+-- Gebelii, nocturnal movement of cotyledons, 308
+--, leaflets provided with pulvini, 353
+-- Jacobaeus, movements of cotyledons, 35, 109
+--, pulvini of, 115
+[page 583]
+
+LOTUS--MELILOTUS
+
+Lotus Jacobaeus, movements at night, 116, 121, 124
+--, development of pulvini, 122
+--, sleep of cotyledons, 308, 313
+--, nyctitropic movement of leaves, 353
+-- major, sleep of leaves, 353
+-- perigrinus, movement of leaflets, 353
+
+Lunularia vulgaris, circumnutation of fronds, 258
+
+Lupinus, 340
+-- albifrons, sleep of leaves, 344
+-- Hartwegii, sleep of leaves, 341
+-- luteus, circumnutation of cotyledons, 38, 110
+--, effect of darkness, 124
+
+Lupinus, position of leaves when asleep, 341
+--, different positions of leaves at night, 343
+--, varied movements of leaves and leaflets, 395
+-- Menziesii, sleep of leaves, 343
+-- mutabilis, sleep of leaves, 343
+-- nanus, sleep of leaves, 343
+-- pilosus, sleep of leaves, 340, 341
+-- polyphyllus, sleep of leaves, 343
+-- pubescens, sleep of leaves by day and night, 342
+--, position of petioles at night, 343
+--, movements of petioles, 401
+-- speciosus, circumnutation of leaves, 236
+
+Lynch, Mr. R., on Pachira aquatica, 95, n.; sleep movements of Averrhoa,
+330
+
+M.
+
+Maranta arundinacea, nyctitropic movement of leaves, 389-391
+--, after much agitation do not sleep, 319
+
+Marsilia quadrifoliata, effect of radiation at night, 292
+--, circumnutation and nyctitropic movement of leaflets, 392-394
+--, rate of movement, 404
+
+Martins, on radiation at night, 284, n.
+
+Masters, Dr. Maxwell, on the leading shoots of the Coniferae, 211
+
+Maurandia semperflorens, circumnutation of peduncle, 225
+Medicago maculata, nocturnal position of leaves, 345
+-- marina, leaves awake and asleep, 344
+
+Meehan, Mr., on the effect of an Aecidium on Portulaca oleracea, 189
+
+Megarrhiza Californica, mode of breaking through the ground, 81
+--, germination described by Asa Gray, 82
+--, singular manner of germination, 83, 556
+
+Melaleuca ericaefolia, sleep of leaves, 383
+
+Melilotus, sleep of leaves, 345
+-- alba, sleep of leaves, 347
+-- coerulea, sleep of leaves, 347
+-- dentata, effect of radiation at night, 295
+-- elegans, sleep of leaves, 347
+-- gracilis, sleep of leaves, 347
+-- infesta, sleep of leaves, 347
+-- Italica, leaves exposed at night, 291
+--, sleep of leaves, 347
+-- macrorrhiza, leaves exposed at night, 292
+--, sleep of leaves, 347
+-- messanensis, sleep of leaves on full-grown and young plants, 348, 416
+-- officinalis, effect of exposure of leaves at night, 290, 296
+--, nocturnal movement of leaves, 346, 347
+--, circumnutation of leaves, 348
+--, movement of petioles, 401
+[page 584]
+
+MELILOTUS--NEPTUNIA
+
+Melilotus parviflora, sleep of leaves, 347
+-- Petitpierreana, leaves exposed at night, 291, 296
+--, sleep of leaves, 347
+-- secundiflora, sleep of leaves, 347
+-- suaveolens, leaves exposed at night, 291
+--, sleep of leaves, 347
+-- sulcata, sleep of leaves, 347
+-- Taurica, leaves exposed at night, 291
+--, sleep of leaves, 347, 415
+
+Methods of observation, 6
+
+Mimosa albida, cotyledons vertical at night, 116
+--, not sensitive to contact, 127
+--, sleep of cotyledons, 308
+--, rudimentary leaflets, 364
+--, nyctitropic movements of leaves, 379, 380
+--, circumnutation of the main petiole of young leaf, 381
+--, torsion, or rotation of leaves and leaflets, 400
+--, first true leaf, 416
+--, effect of bright sunshine on basal leaflets, 445
+-- marginata, nyctitropic movements of leaflets, 381
+-- pudica, movement of cotyledons, 105
+--, rupture of the seed-coats, 105
+--, circumnutation of cotyledons, 109
+--, pulvini of, 113, 115
+--, cotyledons vertical at night, 116
+--, hardly sensitive to contact, 127
+--, effect of exposure at night, 293
+--, nocturnal movement of leaves, 297
+--, sleep of cotyledons, 308
+--, circumnutation and nyctitropic movement of main petiole, 374-378
+--, of leaflets, 378
+
+Mimosa albida, circumnutation and nyctitropic movement of pinnae, 402
+--, number of ellipses described in given time, 406
+--, effect of bright sunshine on leaflets, 446
+
+Mirabilis jalapa and longiflora, nocturnal movements of cotyledons, 307
+--, nyctitropic movement of leaves, 387
+
+Mohl, on heliotropism in tendrils, stems, and twining plants, 451
+
+Momentum-like movement, the accumulated effects of apogeotropism, 508
+
+Monocotyledons, sleep of leaves, 389
+
+Monotropa hypopitys, mode of breaking through the ground, 86
+
+Morren, on the movements of stamens of Sparmannia and Cereus, 226
+
+M�ller, Fritz, on Cassia tora, 34; on the circumnutation of Linum
+usitatissimum, 203; movements of the flower-stems of an Alisma, 226
+
+Mutisia clematis, movement of leaves, 246
+--, leaves not heliotropic, 451
+
+N.
+
+Natural selection in connection with geotropism, heliotropism, etc., 570
+
+Nephrodium molle, circumnutation of very young frond, 66
+--, of older frond, 257
+--, slight movement of fronds, 509
+
+Neptunia oleracea, sensitiveness to contact, 128
+--, nyctitropic movement of leaflets, 374
+--, of pinnae, 402
+[page 585]
+
+NICOTIANA--OXALIS
+
+Nicotiana glauca, sleep of leaves, 385, 386
+--, circumnutation of leaves, 386
+
+Nobbe, on the rupture of the seed-coats in a seedling of Martynia, 105
+
+Nolana prostrata, movement of seedlings in the dark, 50
+--, circumnutation of seedling, 108
+
+Nyctitropic movement of leaves, 560
+
+Nyctitropism, or sleep of leaves, 281; in connection with radiation, 286;
+object gained by it, 413
+
+O.
+
+Observation, methods of, 6
+
+Oenothera mollissima, sleep of leaves, 383
+
+Opuntia basilaris, conjoint circumnutation of hypocotyl and cotyledon, 44
+--, thickening of the hypocotyl, 96
+--, circumnutation of hypocotyl when erect, 107
+--, burying of, 109
+
+Orange, seedling, circumnutation of, 510
+
+Orchis pyramidalis, complex movement of pollinia, 489
+
+Oxalis acetosella, circumnutation of flower-stem, 224
+--, effects of exposure to radiation at night, 287, 288, 296
+--, circumnutation and nyctitropic movement in full-grown leaf, 326
+--, circumnutation of leaflet when asleep, 327
+--, rate of circumnutation of leaflets, 404
+--, effect of sunshine on leaflets, 447
+--, circumnutation of peduncle, 506
+Oxalis acetosella, seed-capsules, only occasionally buried, 518
+-- articulata, nocturnal movements of cotyledons, 307
+-- (Biophytum) sensitiva, rapidity of movement of cotyledons during the
+day, 26
+--, pulvinus of, 113
+--, cotyledons vertical at night, 116, 118
+-- bupleurifolia, circumnutation of foliaceous petiole, 328
+--, nyctitropic movement of terminal leaflet, 329
+-- carnosa, circumnutation of flower-stem, 223
+--, epinastic movements of flower-stem, 504
+--, effect of exposure at night, 288, 296
+--, movements of the flower-peduncles due to apogeotropism and other
+forces, 503-506
+-- corniculata (var. cuprea), movements of cotyledons, 26
+--, rising of cotyledons, 116
+--, rudimentary pulvini of cotyledons, 119
+--, development of pulvinus, 122
+--, effect of dull light, 124
+--, experiments on leaves at night, 288
+-- floribunda, pulvinus of cotyledons, 114
+--, nocturnal movement, 118, 307, 313
+-- fragrans, sleep of leaves, 324
+-- Ortegesii, circumnutation of flower-stems, 224
+--, sleep of large leaves, 327
+--, diameter of plant at night, 402
+--, large leaflets affected by bright sunshine, 447
+ -- Plumierii, sleep of leaves, 327
+-- purpurea, exposure of leaflets at night, 293
+-- rosea, circumnutation of cotyledons, 23, 24
+[page 586]
+
+OXALIS--PHASEOLUS
+
+Oxalis rosea, pulvinus of, 113
+--, movement of cotyledons at night, 117, 118, 307
+--, effect of dull light, 124
+--, non-sensitive cotyledons, 127
+-- sensitiva, movement of cotyledons, 109, 127, 128
+--, circumnutation of flower-stem, 224
+--, nocturnal movement of cotyledons, 307, 312
+--, sleep of leaves, 327
+-- tropoeoloides, movement of cotyledons at night, 118, 120
+-- Valdiviana, conjoint circumnutation of cotyledons and hypocotyl, 25
+--, cotyledons rising vertically at night, 114, 115, 117, 118
+--, non-sensitive cotyledons, 127
+--, nocturnal movement of cotyledon, 307, 312
+--, sleep of leaves and not of cotyledons, 315
+--, movements of leaves, 327
+
+P.
+
+Pachira aquatica, unequal cotyledons, 95, n.
+
+Pancratium littorale, movement of leaves, 255
+
+Paraheliotropism, or diurnal sleep of leaves, 445
+
+Passiflora gracilis, circumnutation and nyctitropic movement of leaves,
+383, 384
+--, apogeotropic movement of tendrils, 510
+--, sensitiveness of tendrils, 550
+Pelargonium zonale, circumnutation of stem, 203
+--, and downward movement of young leaf, 232, 233, 269
+
+Petioles, the rising of beneficial to plant at night, 402
+
+Petunia violacea, downward movement and circumnutation of very young leaf,
+248, 249, 269.
+
+Pfeffer, Prof., on the turgescence of the cells, 2; on pulvini of leaves,
+113, 117; sleep movements of leaves, 280, 283, 284; nocturnal rising of
+leaves of Malva, 324; movements of leaflets in Desmodium gyrans, 358; on
+Phyllanthus Niruri, 388; influence of a pulvinus on leaves, 396; periodic
+movements of sleeping leaves, 407, 408; movements of petals, 414; effect of
+bright sunshine on leaflets of Robinia, 445; effect of light on parts
+provided with pulvini, 363
+
+Phalaris Canariensis, movements of old seedlings, 62
+--, circumnutation of cotyledons, 63, 64, 108
+--, heliotropic movement and circumnutation of cotyledon towards a dim
+lateral light, 427
+--, sensitiveness of cotyledon to light, 455
+--, effect of exclusion of light from tips of cotyledons, 456
+--, manner of bending towards light, 457
+--, effects of painting with Indian ink, 467
+--, transmitted effects of light, 469
+--, lateral illumination of tip, 470
+--, apogeotropic movement of the sheath-like cotyledons, 497
+--, change from a straight upward apogeotropic course to circumnutation,
+499
+--, apogeotropic movement of cotyledons, 500
+
+Phaseolus Hernandesii, nocturnal movement of leaves and leaflets, 368
+--, caracalla, 93
+--, nocturnal movement of leaves, 368
+--, effect of bright sunshine on leaflets, 446
+[page 587]
+
+PHASEOLUS--QUERCUS
+
+Phaseolus multiflorus, movement of radicles, 29
+--, of young radicle, 72
+--, of hypocotyl, 91, 93
+--, sensitiveness of apex of radicle, 163-167
+--, to moist air, 181
+--, cauterisation and grease on the tips, 535
+--, nocturnal movement of leaves, 368
+--, nyctitropic movement of the first unifoliate leaves, 397
+-- Roxburghii, effect of bright sunshine on first leaves, 445
+--, vulgaris, 93
+--, sleep of leaves, 318
+--, vertical sinking of leaflets at night, 368
+
+Phyllanthus Niruri, sleep of leaflets, 388
+-- linoides, sleep of leaves, 387
+
+Pilocereus Houlletii, rudimentary cotyledons, 97
+
+Pimelia spectabilis, sleep of leaves, 387
+
+Pincers, wooden, through which the radicle of a bean was allowed to grow,
+75
+
+Pinus austriaca, circumnutation of leaves, 251, 252
+-- Nordmanniana, nyctitropic movement of leaves, 389
+-- pinaster, circumnutation of hypocotyl, 56
+--, movement of two opposite cotyledons, 57
+--, circumnutation of young leaf, 250, 251
+--, epinastic downward movement of young leaf, 270
+
+Pistia stratiotes, movement of leaves, 255
+
+Pisum sativum, sensitiveness of apex of radicle, 158
+--, tips of radicles cauterised transversely, 534
+
+Plants, sensitiveness to light, 449; hygroscopic movements of, 489
+
+Plants, climbing, circumnutation of, 264; movements of, 559
+--, mature, circumnutation of, 201-214
+
+Pliny on the sleep-movements of plants, 280
+
+Plumbago Capensis, circumnutation of stem, 208, 209
+
+Poinciana Gilliesii, sleep of leaves, 368
+
+Polygonum aviculare, leaves vertical at night, 387
+-- convolvulus, sinking of the leaves at night, 318
+
+Pontederia (sp.?), circumnutation of leaves, 256
+
+Porlieria hygrometrica, circumnutation and nyctitropic movements of petiole
+of leaf, 335, 336
+--, effect of watering, 336-338
+--, leaflets closed during the day, 413
+
+Portulaca oleracea, effect of Aecidium on, 189
+
+Primula Sinensis, conjoint circumnutation of hypocotyl and cotyledon, 45,
+46
+
+Pringsheim on the injury to chlorophyll, 446
+
+Prosopis, nyctitropic movements of leaflets, 374
+Psoralea acaulis, nocturnal movements of leaflets, 354
+
+Pteris aquilina, rachis of, 86
+
+Pulvini, or joints; of cotyledons, 112-122; influence of, on the movements
+of cotyledons, 313; effect on nyctitropic movements, 396
+
+Q.
+
+Quercus (American sp.), circumnutation of young stem, 53, 54
+-- robur, movement of radicles, 54, 55
+--, sensitiveness of apex of radicle, 174-176
+[page 588]
+
+QUERCUS--SACHS
+
+Quercus virens, manner of germination, 85, 557
+
+R.
+
+Radiation at night, effect of, on leaves, 284-286
+
+Radicles, manner in which they penetrate the ground, 69-77; circumnutation
+of 69; experiments with split sticks, 74; with wooden pincers, 75;
+sensitiveness of apex to contact and other irritants, 129; of Vicia faba,
+132-158; various experiments, 135-140; summary of results, 143-151; power
+of an irritant on, compared with geotropism, 151-154; sensitiveness of tip
+to moist air, 180; with greased tips, 185; effect of killing or injuring
+the primary radicle, 187-191; curvature of, 193; affected by moisture, 198;
+tip alone sensitive to geotropism, 540; protrusion and circumnutation in a
+germinating seed, 548; tip highly sensitive, 550; the tip acts like the
+brain of one of the lower animals, 573
+--, secondary, sensitiveness of the tips in the bean, 154; become
+vertically geotropic, 186-191
+
+Ramey on the movements of the cotyledons of Mimosa pudica, and Clianthus
+Dampieri at night, 297
+
+Ranunculus Ficaria, mode of breaking through the ground, 86, 90
+--, single cotyledon, 96
+--, effect of lateral light, 484
+
+Raphanus sativa, sensitiveness of apex of radicle, 171
+--, sleep of cotyledons, 301
+
+Rattan, Mr., on the germination of the seeds of Megarrhiza Californica, 82
+
+Relation between circumnutation and heliotropism, 435
+
+Reseda odorata, hypocotyl of seedling slightly heliotropic, 454
+
+Reversion, due to mutilation, 190
+Rhipsalis cassytha, rudimentary cotyledons, 97
+
+Ricinus Borboniensis, circumnutation of arched hypocotyl, 53
+
+Robinia, effect of bright sunshine on its leaves, 445
+-- pseudo-acacia, leaflets vertical at night, 355
+
+Rodier, M., on the movements of Ceratophyllum demersum, 211
+
+Royer, Ch., on the sleep-movements of plants, 281, n.; on the sleep of
+leaves, 318; the leaves of Medicago maculata, 345; on Wistaria Sinensis,
+354
+
+Rubus idaeus (hybrid) circumnutation of stem, 205
+--, apogeotropic movement of stem, 498
+
+Ruiz and Pavon, on Porlieria hygrometrica, 336
+
+S.
+
+SACHS on "revolving nutation," 1; intimate connection between turgescence
+and growth, 2, n.; cotyledon of the onion, 59; adaptation of root-hairs,
+69; the movement of the radicle, 70, 72, 73; movement in the hypocotyls of
+the bean, etc., 91; sensitiveness of radicles, 131, 145, 198; sensitiveness
+of the primary radicle in the bean, 155; in the common pea, 156; effect of
+moist air, 180; of killing or injuring the primary radicle, 186, 187;
+circumnutation of flower-stems, 225; epinasty, 268; movements of leaflets
+of Trifolium incarnatum, 350; action of light in modifying the periodic
+movements of leaves, 418; on geotropism and heliotropism, 436, n.; on
+Tropaeolum majus, 453;
+[page 589]
+
+SARRACENIA--STAPELIA
+
+on the hypocotyls slightly heliotropic, and stems strongly apheliotropic of
+the ivy, 453; heliotropism of radicles, 482; experiments on tips of
+radicles of bean, 523, 524; curvature of the hypocotyl, 555; resemblance
+between plants and animals, 571
+
+Sarracenia purpurea, circumnutation of young pitcher, 227
+
+Saxifraga sarmentosa, circumn utation of an inclined stolon, 218
+
+Schrankia aculeata, nyctitropic movement of the pinnae, 381, 403
+-- uncinata, nyctitropic movements of leaflets, 381
+
+Securigera coronilla, nocturnal movements of leaflets, 352
+
+Seed-capsules, burying of, 513
+
+Seed-coats, rupture of, 102-106
+
+Seedling plants, circumnutating movements of, 10
+Selaginella, circumnutation of 258
+-- Kraussii (?), circumnutation of young plant, 66
+
+Sida napoea, depression of leaves at night, 322
+--, no pulvinus, 322
+-- retusa, vertical rising of leaves, 322
+-- rhombifolia, sleep of cotyledons, 308
+--, sleep of leaves, 314
+--, vertical rising of leaves, 322
+--, no pulvinus, 322
+--, circumnutation and nyctitropic movements of leaf of young plant, 322
+--, nyctitropic movement of leaves, 397
+
+Siegesbeckia orientalis, sleep of leaves, 319, 384
+
+Sinapis alba, hypocotyl bending towards the light, 461
+--, transmitted effect of light on radicles, 482, 483, 567
+--, growth of radicles in darkness, 486
+
+Sinapis nigra, sleep of cotyledons, 301
+
+Smilax aspera, tendrils apheliotropic, 451
+
+Smithia Pfundii, non-sensitive cotyledons, 127
+--, hyponastic movement of the curved summit of the stem, 274-276
+--, cotyledons not sleeping at night, 308
+--, vertical movement of leaves, 356
+-- sensitiva, sensitiveness of cotyledons to contact, 126
+--, sleep of cotyledons, 308
+
+Sophora chrysophylla, leaflets rise at night, 368
+
+Solanum dulcamara, circumnutating stems, 266
+-- lycopersicum, movement of hypocotyl, 50
+--, of cotyledons, 50
+--, effect of darkness, 124
+--, rising of cotyledons at night, 306
+--, heliotropic movements of hypocotyl, 421
+--, effect of an intermittent light, 457
+--, rapid heliotropism, 461
+-- palinacanthum, circumnutation of arched hypocotyl, 51, 100
+--, of cotyledon, 51
+--, ellipses described by hypocotyl when erect, 107
+--, nocturnal movement of cotyledons, 306
+
+Sparganium ramosum, rhizomes of, 189
+
+Sphaerophysa salsola, rising of leaflets, 355
+
+Spirogyra princeps, movements of, 259, n.
+
+Stahl, Dr., on the effect of Aecidium on shoot, 189; on the influence of
+light on swarm-spores, 488, n.
+
+Stapelia sarpedon, circumnutation of hypocotyl, 46, 47
+[page 590]
+
+STAPELIA--TRITICUM
+
+Stapelia sarpedon, minute cotyledons, 97
+
+Stellaria media, nocturnal movement of leaves, 297
+
+Stems, circumnutation of, 201-214
+
+Stolons, or Runners, circumnutation of, 214-222, 558
+
+Strasburger, on the effect of light on spores of Haematococcus, 455, n.;
+the influence of light on the swarm-spores, 488.
+
+Strawberry, stolons of the, circumnutate, but not affected by moderate
+light, 454
+
+Strephium floribundum, circumnutation and nyctitropic movement of leaves,
+391, 392
+
+T.
+
+Tamarindus Indica, nyctitropic movement of leaflets, 374
+
+Transversal - heliotropismus (of Frank) or diaheliotropism, 438
+
+Trapa natans, unequal cotyledons, 95, n.
+
+Tecoma radicans, stems apheliotropic, 451
+
+Tephrosia caribaea, 354
+
+Terminology, 5
+
+Thalia dealbata, sleep of leaves, 389
+--, lateral movement of leaves, 404
+
+Trichosanthes anguina, action of the peg on the radicle, 104
+--, nocturnal movement of cotyledons, 304
+
+Trifolium, position of terminal leaflets at night, 282
+-- globosum, with hairs protecting the seed-bearing flowers, 517
+-- glomeratum, movement of cotyledons, 309
+-- incarnatum, movement of cotyledons, 309
+-- Pannonicum, shape of first true leaf, 350, 415
+Trifolium pratense, leaves exposed at night, 293
+-- repens, circumnutation of flower-stem, 225
+--, circumnutating and epinastic movements of flower-stem, 276-279
+--, nyctitropic movement of leaves, 349
+--, circumnutation and nyctitropic movements of terminal leaflets, 352, 353
+--, sleep movements, 349
+-- resupinatum, no pulvini to cotyledons, 118
+--, circumnutation of stem, 204
+--, effect of exposure at night, 295
+--, cotyledons not rising at night, 118, 309
+--, circumnutation and nyctitropic movements of terminal leaflets, 351, 352
+-- strictum, movements of cotyledons at night, 116, 118
+--, nocturnal and diurnal movements of cotyledons, 309-311, 313
+--, movement of the left-hand cotyledon, 316
+-- subterraneum, movement of flower-heads, 71
+--, of cotyledons at night, 116, 118, 309
+--, circumnutation of flower-stem, 224, 225
+--, circumnutation and nyctitropic movements of leaves, 350
+--, number of ellipses in 24 hours, 405
+--, burying its flower-heads, 513, 514
+--, downward movement of peduncle, 515
+--, circumnutating movement of peduncle, 516
+
+Trigonella Cretica, sleep of leaves, 345
+
+Triticum repens, underground shoots of, become apogeotropic, 189
+[page 591]
+
+TRITICUM--WILSON
+
+Triticum vulgare, sensitiveness of tips of radicle to moist air, 184
+
+Tropaeolum majus (?), sensitiveness of apex of radicle to contact, 167
+--, circumnutation of stem, 204
+--, influence of illumination on nyctitropic movements, 338-340, 344
+--, heliotropic movement and circumnutation of epicotyl of a young
+seedling, 428, 429
+--, of an old internode towards a lateral light, 430
+--, stems of very young plants highly heliotropic, of old plants slightly
+apheliotropic, 453
+--, effect of lateral light, 484
+-- minus (?), circumnutation of buried and arched epicotyl, 27
+
+U.
+
+Ulex, or gorse, first-formed leaf of, 415
+
+Uraria lagopus, vertical sinking of leaflets at night, 365
+
+V.
+Vaucher, on the burying of the flower-heads of Trifolium subterraneum, 513;
+on the protection of seeds, 517
+
+Verbena melindres (?), circumnutation of stem, 210
+--, apogeotropic movement of stem, 495
+
+Vicia faba, circumnutation of radicle, 29, 30
+--, of epicotyl, 31-33
+--, curvature of hypocotyl, 92
+--, sensitiveness of apex of radicle, 132-134
+--, of the tips of secondary radicles, 154
+--, of the primary radicle above the apex, 155-158
+--, various experiments, 135-143
+--, summary of results, 143-151
+--, power of an irritant on, compared with that of geotropism, 151-154
+Vicia faba, circumnutation of leaves, 233-235
+--, circumnutation of terminal leaflet, 235
+--, effect of apogeotropism, 444
+--, effect of amputating the tips of radicles, 523
+--, regeneration of tips, 526
+--, short exposure to geotropic action, 527
+--, effects of amputating the tips obliquely, 528
+--, of cauterising the tips, 529
+--, of grease on the tips, 534
+
+Vines, Mr., on cell growth, 3
+
+Vries, De, on turgescence, 2; on epinasty and hyponasty, 6, 267, 268; the
+protection of hypocotyls during winter, 557; stolons apheliotropic, 108;
+the nyctitropic movement of leaves, 283; the position of leaves influenced
+by epinasty, their own weight and apogeotropism, 440; apogeotropism in
+petioles and midribs, 443; the stolons of strawberries, 454; the joints or
+pulvini of the Gramineae, 502
+
+W.
+
+Watering, effect of, on Porlieria hygrometrica, 336-338
+
+Wells, 'Essay on Dew,' 284, n.
+
+Wiesner, Prof., on the circumnutation of the hypocotyl, 99, 100; on the
+hooked tip of climbing stems, 272; observations on the effect of bright
+sunshine on chlorophyll in leaves, 446; the effects of an intermittent
+light, 457; on a�rial roots, 486; on special adaptations, 490
+
+Wigandia, movement of leaves, 248
+
+Williamson, Prof., on leaves of Drosera Capensis, 414
+
+Wilson, Mr. A. S., on the movements of Swedish turnip leaves, 230, 298
+
+Winkler on the protection of seedlings, 108
+
+Wistaria Sinensis, leaflets depressed at night, 354
+--, circumnutation with lateral light, 452
+
+Z.
+
+Zea mays, circumnutation of cotyledon, 64
+Zea mays, geotropic movement of radicles, 65
+--, sensitiveness of apex of radicle to contact, 177-179
+--, secondary radicles, 179
+--, heliotropic movements of seedling, 64, 421
+--, tips of radicles cauterised, 539
+
+Zukal, on the movements of Spirulina, 259, n.
+
+
+
+
+
+
+THE END.
+
+
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