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diff --git a/5605-0.txt b/5605-0.txt new file mode 100644 index 0000000..8309a55 --- /dev/null +++ b/5605-0.txt @@ -0,0 +1,19798 @@ +The Project Gutenberg eBook of The Power of Movement in Plants, by Charles Darwin + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +Title: The Power of Movement in Plants + +Author: Charles Darwin + +Release Date: August 14, 2002 [eBook #5605] +[Most recently updated: September 11, 2022] + +Language: English + +Character set encoding: UTF-8 + +Produced by: Sue Asscher + +*** START OF THE PROJECT GUTENBERG EBOOK THE POWER OF MOVEMENT IN PLANTS *** + + + + +THE POWER OF MOVEMENT IN PLANTS + +By Charles Darwin + +Assisted By Francis Darwin + + +CONTENTS + + DETAILED TABLE OF CONTENTS. + THE MOVEMENTS OF PLANTS. + INTRODUCTION. + CHAPTER I. THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS. + CHAPTER II. GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS. + CHAPTER III. SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS. + CHAPTER IV. THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS. + CHAPTER V. MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS. + CHAPTER VI. MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS. + CHAPTER VII. MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES. + CHAPTER VIII. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT. + CHAPTER IX. SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS. + CHAPTER X. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION. + CHAPTER XI. LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS. + CHAPTER XII. CONCLUDING REMARKS. + INDEX + + + + +DETAILED TABLE OF CONTENTS. + + +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, +Æsculus, 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. + + +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. + + +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—Æsculus, 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. + + +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. + + +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. + + +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—A 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. + + +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. + + +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. + + +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. + + +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—Amphicarpæa—Diageotropism—Conclusion. + + +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. + + +CHAPTER XII.—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. + + + + +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 +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.[1] 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[2] 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[3] 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 +increased 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.[4] + + [1] Sachs first showed (‘Lehrbuch,’ etc., 4th edit. p. 452) the + intimate connection between turgescence and growth. For De Vries’ + interesting essay, ‘Wachsthumskrümmungen mehrzelliger Organe,’ see + ‘Bot. Zeitung,’ Dec. 19, 1879, p. 830. + + + [2] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875. + + + [3] ‘Untersuchungen über den Heliotropismus,’ Sitzb. der K. Akad. der + Wissenschaft. (Vienna), Jan. 1880. + + + [4] See Mr. Vines’ excellent discussion (‘Arbeiten des Bot. Instituts + in Würzburg,’ B. II. pp. 142, 143, 1878) on this intricate subject. + Hofmeister’s observations (‘Jahreschrifte des Vereins für Vaterl. + Naturkunde in Würtemberg,’ 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. + + +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 +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 +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,[5]—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. + + [5] The highly useful terms of Heliotropism and Geotropism were first + used by Dr. A. B. Frank: see his remarkable ‘Beiträge zur + Pflanzenphysiologie,’ 1868. + + +The term epinasty is now often used in Germany, and implies that the +upper surface of an organ grows more quickly than the +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.[6] + + [6] These terms are used in the sense given them by De Vries, + ‘Würzburg Arbeiten,’ Heft ii 1872, p. 252. + + +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 +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.[7] +Whenever it could be approximately told how much the movement had been +magnified, this is stated. We have perhaps +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. + + [7] We are much indebted to Mr. Cooper for the care with which he has + reduced and engraved our diagrams. + + +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 +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. + + + + +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, +Æsculus, 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 +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 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 +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. + +Fig. 2. Brassica oleracea: circumnutating and geotropic movement of +radicle, traced on horizontal glass during 46 hours. + +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 +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: 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 +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. 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. + +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 33½ 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. +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 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. + +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. + +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 +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 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 3½ ellipses in 10 3/4 h.; each +being completed on an average in 3 h. 4 m. + +Fig. 6. Brassica oleracea: conjoint circumnutation of the hypocotyl and +cotyledons during 10 hours 45 minutes. Figure here reduced to one-half +original scale. + +On the previous day another seedling had been observed under similar +conditions, excepting that the plant was so +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 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. + +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. + +By 9.25 A.M. on this second day the same cotyledon had +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 +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, +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 +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 +hypocotyl 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, 1½ 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. +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...25° above horizon. Oct. 20 4.20 P.M...22° above +horizon. Oct. 20 5.20 P.M...15° above horizon. Oct. 20 10.40 P.M...8° +above horizon. Oct. 21 8.40 A.M...28° above horizon. Oct. 21 11.15 +A.M...35° above horizon. Oct. 21 9.11 P.M...10° 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. +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 +cotyledons 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 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 +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. + +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. + +Oxalis corniculata (var. cuprea).—The cotyledons rise at night to a +variable degree above the horizon, generally about 45°: 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 22½° to 24½° C. One cotyledon rose 70° in 11 m.; +another, on a distinct seedling, fell 80° 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 180° in under 2 h. We have met with no other +instance of a circumnutating movement of such great amplitude as 180°; +nor of such rapidity of movement as the passage through 80° in 12 m. +The cotyledons of this plant sleep at night by rising +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. + + +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 10½, in figure here +given, and afterwards 36 times, or 18 as here given; seedling +illuminated from above. + +Æsculus 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 70° and 65° 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 +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. Æsculus hippocastanum: outlines of tracks left on inclined +glass-plates by tips of radicles. In A the plate was inclined at 70° +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 65° 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 Æsculus, 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 60°, B inclined at 68° 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 +abruptly, then made a small loop and then a larger zigzag curve. During +the night and till 11 A.M. on the following 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. 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. + +Fig. 21. Vicia faba: tracks left on inclined smoked glass-plates, by +tips of radicles in growing downwards. Plate C was inclined at 63°, +plates A and D at 71°, plate B at 75°, and plate E at a few degrees +beneath the horizon. + + +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 Æsculus and Phaseolus. Some of the plates were +inclined only a few degrees beneath the horizon, but most of them +between 60° and 75°. 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 +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 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. + +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. + +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 +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 movement of which is +here shown, would probably have just risen above the surface of the +ground. + +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. + +Lathyrus nissolia (Leguminosae).—This plant was selected for +observation from being an abnormal form with grass-like leaves. 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, +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. + +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. + +Cassia tora[1] (Leguminosae).—A seedling was placed before a +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. + + [1] Seeds of this plant, which grew near the sea-side, were sent to us + by Fritz Müller 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. + + +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. + +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 45°. They continued to act thus for about an +additional fortnight. Subsequently they remained horizontal at night, +though still green +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 (1½ 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 16° to 17½° 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 +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. 16° to 17° C. or 63° to 65° 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 56° and 57° 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 +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 14½ 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 (Cucurbitaceæ).—Radicle: a seed which had germinated +on damp sand was fixed so that the slightly curved radicle, which was +only .07 inch in length, stood almost vertically +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 45° 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 45° 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 50° 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. + +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. + +Four radicles were allowed to grow downwards over plates of smoked +glass, inclined at 70° to the horizon, under the same conditions as in +the cases of Æsculus, 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 +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. 27. Cucurbita ovifera: tracks left by tips of radicles in growing +downwards over smoked glass-plates, inclined at 70° 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. + +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 4½ 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 +35° 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 +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 +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 14½ 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 60°, 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) (Cucurbitaceæ).—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 53°, and at 10 P.M. they were in +close contact, so that each had risen 26½°. At noon, on the 29th, they +included an angle of 118°, and at 10 P.M. an angle of 54°, so each had +risen 32°. 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, +open more and more on each succeeding day, and rise each night about +30°; consequently during the first two nights of their life they stand +vertically and come into contact. + +Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1½ 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 17½° to 18° C. (63° to 65° 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 (Cucurbitaceæ).—Two seedlings had opened +their cotyledons for the first time during the day,—one to the extent +of 90° 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 90° 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 (Cacteæ).—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 10° 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 +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 (Compositæ).—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 73° 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 +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[2] 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 24½ 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. + + [2] ‘Bot. Zeitung,’ 1874, p. 837. + + +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. 23–24° 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 +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. + +Ipomœa 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.[3] 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 3½ 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. + + [3] ‘Movements and Habits of Climbing Plants,’ p. 33, 1875. + + +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 +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. Ipomœa 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. 20° 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 30° and 35°; but three stood at about 50° and one at even 70° +beneath the horizon. The blades of all these cotyledons had attained +almost their full size, viz. from 1 to 1½ 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 +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 Ipomœa likewise +sink downwards late in the evening. + +Cerinthe major (Boragineae).—The circumnutation of the hypocotyl of a +young seedling with the cotyledons hardly 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. + +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. + + +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. + +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. + +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 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 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. + +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. + +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 27° +above the horizon; at 7 A.M. on the following +morning, before it was light, they had risen to 59° 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 39°; 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 +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 +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 65° and 69° 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 +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 65° +and plate B at 68° 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 +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 45° 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 (Coniferæ).—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, +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. + +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. + +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 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. + +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. + + +Cycas pectinata (Cycadeæ).—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 2½ 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 +circumnutating movements. The extreme distance from side to side which +it passed over amounted to between .6 and .7 of an inch. + +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. + +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 +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,[4] 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,[5] and answers the same end as the +knife-like white crest on the summit of the straight cotyledon of the +Gramineæ. +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. + + [4] This is the expression used by Sachs in his ‘Text-book of Botany.’ + + + [5] 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. + + +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 45° 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 +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 movements 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 +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 +3½ 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. + +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. + +Phalaris Canariensis (Gramineæ).—With the Gramineæ 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.[6] 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. + + [6] We are indebted to the Rev. G. Henslow for an abstract of the + views which have been held on this subject, together with references. + + +The movements of three rather old seedlings, about 1½ 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 +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 +microscope 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 (Gramineæ).—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 +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 45° +and 80° 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 (Gramineæ).—A cotyledon, 1½ 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. + + +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 5½ 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 (?) (Lycopodiaceæ).—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. + + + + +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.[1] 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 +types amongst plants, were continually circumnutating, we may infer +that this kind of movement is common to every seedling species. + + [1] As given in the ‘General System of Botany,’ by Le Maout and + Decaisne, 1873. + + +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. Cucurbitaceæ. XII. PASSIFLORALES. 109. Cacteæ. XIV. +FICOIDALES. 122. Compositæ. 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. Coniferæ. 224. Cycadeæ. + +Class II.—MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae. +XI. LILIALES. 41. Asparageae. DITTO 55. Gramineæ. XV. GLUMALES. + +SUB-KINGDOM II.—Cryptogamic Plants. + +1. Filices. I. FILICALES. 6. Lycopodiaceæ. DITTO + + +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[2] 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. + + [2] ‘Physiologie Végétale,’ 1868, pp. 199, 205. + + +The tip of the radicle, as soon as it protrudes from the seed-coats, +begins to circumnutate, and the whole +growing part continues to do so, probably for as long as growth +continues. This movement of the radicle has been described in Brassica, +Æsculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability +of its occurrence was inferred by Sachs,[3] 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 +xwhen 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. + + [3] ‘Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in + Würzburg,’ 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 (‘Beiträge 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. + + +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 +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.[4] + + [4] See, also, Prof. Hensen’s statements (‘Zeitschrift für 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. + + +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[5] has shown that whilst thus bending, the +growth of the lower surface is greatly retarded, whilst that +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. + + [5] ‘Arbeiten des bot. Inst. Würzburg,’ vol. i. 1873, p. 461. See also + p. 397 for the length of the growing part, and p. 451 on the force of + geotropism. + + +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 +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 +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. 50°–60° 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 +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 +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;[6] its petiole +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. + + [6] 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. + + +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 +instrucive 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 +either had proved decidedly advantageous or disadvantageous, one of +them no doubt would soon have prevailed. + +Asa Gray has described[7] 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. + + [7] ‘Botanical Text-Book,’ 1879, p. 22. + + +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. + + +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,[8] 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 2½ 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 2½ inches, and the radicle by this time had also become +well developed. The plumule, still enclosed within the tube, was now +.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 2½ 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. + + [8] ‘American Journal of Science,’ vol. xiv. 1877, p. 21. + + +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 +plumules 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. + +Ipomœa 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 +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 90° 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 ½ 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 Ipomœa pandurata, +the germination of which, as Asa Gray informs us, resembles that of I. +leptophylla. + +The following case is interesting in connection with +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,[9] 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. + + [9] ‘Transact. St. Louis Acad. Science,’ vol. iv. p. 190. + + +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;[10] so does the +flower-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, +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. + + [10] 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 subterranean + 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. + + +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 +Gramineæ, 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[11] 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 +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. + + [11] ‘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. + + +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 +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 +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 Ipomœa (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 Ipomœa 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 +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.[12] 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. + + [12] ‘Arbeiten des bot. Instit. Würzburg,’ vol. i. 1873, p. 403. + + +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 +likewise 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 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 +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,[13] 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 63° 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. + + [13] An instrument devised by Sachs, consisting essentially of a + slowly revolving horizontal axis, on which the plant under observation + is supported: see ‘Würzburg Arbeiten,’ 1879, p. 209. + + +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. + +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 +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. +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[14] In all these seedlings the hypocotyl was enlarged or +swollen. + +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. + +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. + + [14] 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. + + +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 +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,[15] 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.[16] Again, +certain species of Chaerophyllum and Corydalis produce only a single +cotyledon;[17] in the former the hypocotyl, and in the latter the +radicle is enlarged, according to Irmisch, into a bulb. + + [15] Dr. H. Gressner, ‘Bot. Zeitung,’ 1874, p. 824. + + + [16] Irmisch, ‘Beiträge zur Morphologie der Pflanzen,’ 1854, pp. 11, + 12; ‘Bot. Zeitung,’ 1874, p. 805. + + + [17] 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. + + +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 +hand, with several other Cacteæ, 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 +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 +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[18] 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 +hypocotyl, 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. + + [18] ‘Die undulirende Nutation der Internodien,’ Akad. der Wissench. + (Vienna), Jan. 17th, 1878. Also published separately, see p. 32. + + +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 +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 +Cucurbitaceæ, the seed-coats are ruptured by a curious contrivance, +described by M. Flahault.[19] 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. + + [19] ‘Bull. Soc. Bot. de France,’ tom. xxiv. 1877, p. 201. + + +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 +casting 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. + +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 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 +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. + + +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.[20] + + [20] 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. + + +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 +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 +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, +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.[21] He shows that +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. + + [21] ‘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. + + +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 Ipomœa 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 +Jacobæus 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 Ipomœa 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 +coincided; 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,[22] 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 Gramineæ 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. + + [22] 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 subaërial and subterranean cotyledons. + + +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 +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 60° 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 +cotyledons which at noon were nearly horizontal, stood at night at more +than 20° and less than 60° 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 +20°—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 20° and less than 60° 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, +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,[23] 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.[24] +As long as a leaf provided with a pulvinus is young and continues to +grow, its movement depends on both these causes combined;[25] 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 +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. + + [23] ‘Die Periodische Bewegungen der Blattorgane,’ 1875. + + + [24] Batalin, ‘Flora,’ Oct. 1st, 1873 + + + [25] Pfeffer, ibid. p. 5. + + +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. + +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 +comparing the diagrams given in the last chapter. Thus the movements of +the cotyledons of Brassica oleracea and of Ipomœa 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 Jacobæus 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 +90°. 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, Cucurbitaceæ, 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 45° +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 45° 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 45° 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 +by F. Müller).—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[26] 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 (3½ 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. + + [26] Pfeffer, ‘Die Period. Bewegungen,’ 1875, p. 157. + + +Trifolium is a natural genus, and the leaves of all +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 45°; and in one +lot of seedlings (purchased under the name of O. tropaeoloides, but +certainly belonging to the above variety) they rose only from 5° to 15° +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. + + +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 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 +same pulvinus and in different individuals. In the accompanying +figures, A and B (Fig. 64), we have views of the epidermis[27] 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. + + [27] Longitudinal sections show that the forms of the epidermic cells + may be taken as a fair representation of those constituting the + pulvinus. + + +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. + +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.[28] 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 + + + [28] Each division equalled .003 mm. + + +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 63°) 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 +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 2½ 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 +Jacobæus 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 +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. + + +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 +remained horizontal; it was then brought back to the table, and after +50 m. the cotyledons had risen 68° 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 63° above the horizon; +the pot was then replaced on the table, and after 50 m. the cotyledons +had fallen 33°. 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 90°, so that each had risen +45°. A single cotyledon of another seedling was tapped in a like manner +for 1 m., and it rose 27° in 9 m.; and after eight additional minutes +it had risen 10° 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 +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 31° 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 8°. 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 6°o. Several cotyledons of C. neglecta +(likewise from S. Brazil) rose in from 5 m. to 15 m. to various angles +between 16v and 34°, 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. 50° 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 34°; a second only 6°; a third 13°; and +a fourth 17°. A cotyledon of C. florida similarly treated rose 9°; one +of C. corymbosa rose 7½°, and one of the very distinct C. mimosoides +only 6°. 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 32°; they +remained in this position for 15 m., but when looked at again 40 m. +after the rubbing, each had fallen 14°. 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 30°. 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.[29] + + [29] The sole notice which we have met with on the sensitiveness of + cotyledons, relates to Mimosa; for Aug. P. De Candolle says (‘Phys. + Vég.,’ 1832, tom. ii. p. 865), “les cotyledons du M. pudica tendent à + se raprocher par leurs faces supérieures lorsqu’on les irrite.” + + +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 48°; when looked at again in 35 m. +after being rubbed they had risen 4° 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 56°, a second 45°, a third almost 90°, and a +fourth 90°. 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 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 1½ 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. + + + + +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—Æsculus, 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 +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 +their curvature; and Sachs has shown[1] 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. + + [1] ‘Arbeiten Bot. Inst. Würzburg,’ Heft iii. 1873, p. 398. + + +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 +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 1½ +mm.), or oblong bits of nearly the same size, were found to +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 +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 as about 45° 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 +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. + +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. + +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, 50° 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 20° from the perpendicular and from the card, +and in opposition to Sachs’ curvature: after 23 h. no change. + + +(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected +in the plane of the bean 40° 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 7° or 8° 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 30° 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, 20° 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 40° 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 +40° 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 20° +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 40° and 50° from the vertical and from the card. In the plane +of the bean itself the deflection amounted to 8° or 9° 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 90° from +the vertical and from the card, in a plane intermediate between that of +the bean itself and one at right +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 90° from the perpendicular +and from the card. After 24 h. deflection much decreased, and after an +additional day, reduced to 23° 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 45° 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 45° from the perpendicular and from the cards. + +(23.) Square of card fixed with shellac to young radicle: after +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 40° 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 +20° 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 45° from the perpendicular and from the card. +After 36 additional hours angle of deflection reduced to about 30°. + +(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 +morning deflection reduced to about 40° 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 35° 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 +90° from the perpendicular and from the card. + +(34.) Small oblong of sanded card gummed to apex: after 15 h. deflected +90° 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 90° 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 90°. 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. + + +(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 90° 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 50° 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 +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 69° and 72° 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 +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 70° F. We therefore allowed the radicles of 12 beans +to grow at a temperature of between 55° and 60° 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, 70° 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 53° and 59° F., eleven +germinating beans were tried in the +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. + +A 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 +fragments 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 +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 90° 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[2] 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, +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. + + [2] ‘Arbeiten Bot. Instit., Würzburg,’ Heft iii. p. 456. + + +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 +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 90° 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 19°; 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 26° 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 +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: +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 63° 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 +curvature 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 +14°–16° C. (57°–61° 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 +to bend over towards the injured side.[3] 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 58° 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. + + [3] Ciesielski found this to be the case (‘Untersuchungen über die + Abwartskrümmung 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. + + +_The Power of an Irritant on the apex of the Radicle_ +_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, 1½ 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 40° beneath the horizon. +Thirdly, 5 beans were fastened +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 +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[4] 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 +curvature. The curvature amounted in some cases to about 45° above the +horizon, in others to 90°, 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[5] the interesting fact that each +individual secondary radicle possesses its own peculiar constitution. + + [4] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iv. 1874, p. 605–617. + + + [5] ‘Arbeiten Bot. Instit., Würzburg,’ Heft, iv. 1874, p. 620. + + +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[6] 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,[7] 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 +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. + + [6] Ibid. Heft iii. 1873, p. 437. + + + [7] ‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25. + + +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.[8] We experimented on a variety +(_Yorkshire 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. + + [8] Sachs, ‘Arbeiten Bot. Institut., Würzburg,’ Heft iii. p. 438. + + +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 62° 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 +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 +60°–61° 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 +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 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 58°–60° 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. + +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. + +We now tried the effects of widely different temperatures on the +sensitiveness of these radicles with squares +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 44° to 47° 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 +68° and 72° 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 72° and 85° 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 60° 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 58°–60° F. The +result was somewhat different; for +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 45° +beneath the horizon. These four radicles therefore did not seem 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 +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 45° 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. + +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. + +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 +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 72° to 77° F., and on +the second being almost steadily 78° 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 +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 60° and 72° 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 +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 65°–67° 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 45°—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 34°, which +increased in nine additional hours to 54°. + +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 +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 65° 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 +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 70° from the +perpendicular and from the cut surface; and one was deflected at nearly +90°, 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 78° 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 40° and 65° from the perpendicular; and in +two of them it amounted after 15 h. or 16 h. to about 90°. In one +instance a loop +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 66° to 69° 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 74° to 79° 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 70° to 75° 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 +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 15° and 65° 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 +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 ½ 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 90°, and 3 at about 45° 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 +they grow better and more naturally than in damp air; and their tips +were slightly cauterised on the lower side, brown marks about ½ 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 60°, a second at 40°, +and a third very slightly from the perpendicular and from the +cauterised side. + +Æsculus 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 52° and 58° 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 60° from the perpendicular; +in four days at 90°; on the fifth day it was curved up about 40° above +the horizon; so that it had passed through an angle of 130° in the five +days, and this was the greatest amount of curvature observed. + +(2.) In two days radicle slightly deflected; after seven days +deflected 69° from the perpendicular and from the cauterised side; +after eight days the angle amounted to nearly 90°. + +(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 78°, which in an additional +day increased to 90°. + +(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 56° 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 45° from the perpendicular, and this on the seventh day +increased to about 90°. + +(7.) After two days slightly deflected; on the third day the deflection +amounted to 25° from the perpendicular, and this did not afterwards +increase. + +(8.) After one day deflection distinct; on the third day it amounted to +44°, and on the fourth day to 72° 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 50° from the perpendicular and the cauterised side. + +(11.) After one day decided deflection, which after six days increased +to 62° from the perpendicular and from the cauterised side. + +(12.) After one day slight deflection, which on the second day amounted +to 35°, on the fourth day to 50°, and the sixth day to 63° 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 90° +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 90°; on the following day the +radicle appeared unhealthy. + +(15.) Here we had the anomalous case of a radicle bending +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 +90° 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 +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 40°, and on the +23rd to 53° 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 +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 Æsculus. 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 65°–66° 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 +90°, 72°, 60°, and 49°, but in two other cases to only 18° and 15°. It +should, however, be stated that in the +case in which the deflection was 49°, 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 65°–66° F., and others to between 74° +and 79°; 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 +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 76° to 82° 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 +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 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! + +Fig. 69. Zea mays: radicles excited to bend away from the little +squares of card attached to one side of their tips. + +Secondary Radicles of Zea.—A short time after the first radicle has +appeared, others protrude from the +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 76° and 82° 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.[9] 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 +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 40° +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 50° 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. + + [9] ‘Arbeiten des Bot. Institut., in Würzburg,’ vol. i. 1872, p. 209. + + +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 1½ mm.; +they were kept at a temperature of 15°–16° 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 10°. 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 +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 1½ to 2½ 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 +(1½ mm. in length) removed, and during the next 15½ 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 +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 20° and 12° 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 +them not quite perpendicularly downwards, were coated with the black +grease for a length of from 1 to 1½ mm. The sieves were inclined at 30° +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 10° and 35°; 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 1½ 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 10° and 15°, 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 4.8° 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., 12° to 13° 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 +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 1½ 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 +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[10] 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 +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 1½ 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. + + [10] ‘Arbeiten Bot. Institut., Würzburg,’ Heft iv. 1874, p. 622. + + +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 +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 8° +above the horizon; by Sept. 8th the unpinched one had risen 35°; by +Oct. 4th it had risen 46°, and by Jan. 26th 48°, and it had now become +a little curved inwards. Part of this rise of 48° may be attributed to +ordinary growth, for the pinched shoot rose 12° within the same period. +It thus follows that the unpinched shoot stood, on Jan. 26th, 56° above +the horizon, or 34° 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, Æcidium 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[11], when the mycelium penetrates a bud +beginning to elongate, the shoot developed from it grows vertically +upwards. Such upright shoots +afterwards 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 Æcidium, 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. + + [11] See his valuable article in ‘Bot. Zeitung,’ 1867, p. 257, on + these monstrous growths, which are called in German “Hexenbesen,” or + “witch-brooms.” + + +According to Mr. Meehan,[12] the stems of three species of Euphorbia +and of Portulaca oleracea are “normally prostrate or procumbent;” but +when they are attacked by an Æcidium, 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.[13] + + [12] ‘Proc. Acad. Nat. Sc. Philadelphia,’ June 16th, 1874, and July + 23rd, 1875. + + + [13] See F. Elfving’s interesting paper in ‘Arbeiten Bot. Institut., + in Würzburg,’ 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. + + +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 +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[14] 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 +possible, or even probable, that this tendency to reversion may have +been increased, as it is manifestly of service to the plant. + + [14] 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. + +A 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 Æsculus 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 Æsculus, the tips were not at all sensitive to +small bodies attached to them; but it does not follow from this +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. + + +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 Æsculus. 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 +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. +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_, +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[15] 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. + + [15] ‘Arbeiten Bot. Institut, Würzburg,’ Heft iv. 1874, pp. 605–631. + + +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 +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 +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 Æsculus, 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,[16] 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. + + [16] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iii. p. 456. + +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 +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 +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. + + + + +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 +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 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. + +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. + +(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 +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 Müller +(‘Jenaische Zeitschrift,’ B. v. p. 137) to revolve, or circumnutate. + +(4.) Pelargonium zonale (Geraniaceae, Fam. 47).—A young plant, 7½ +inches in height, was observed in the usual manner; but, in order to +see the bead at the end of the glass filament 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. + +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. + +(5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb); +(Geraniaceae, Fam. 47).—The species of this genus climb by the +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 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. 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. + +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 +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 idæus (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 idæus) 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 40½ 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 +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) (Cacteæ, 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 1½ 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 +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 +they were found to be still circumnutating, but on a yet smaller scale. + +(12.) Gazania ringens (Compositæ, 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 main lines may be observed running +at nearly right angles to two other main lines; but these are +interrupted by small loops. + +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. + +(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 +35° 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 +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 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. + +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. + +(15.) Aloysia citriodora (Verbenaceae, Fam. 173).—The following figure +(Fig. 81) gives the movements of a shoot during +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 1½ inch. A glass filament, +with a bead at the end, was fixed 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 +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.[1] 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 +200° in 6 h., and in one instance through 220° 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 mérithalles supérieurs, qu’il se propage ensuite, en +s’amoindrissant du haut en bas; tandis qu’au contraire le movement de +redressement commence par la partie inférieur pour se terminer a la +partie supérieure qui, quelquefois, peu de temps avant de se relever +tout à fait, forme avec l’axe un angle très aigu.” + + [1] ‘Comptes Rendus,’ April 30th, 1877. Also a second notice published + separately in Bourdeaux, Nov. 12th, 1877. + + +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. + +(18.) Coniferæ.—Dr. Maxwell Masters states (‘Journal Linn. Soc.,’ Dec. +2nd, 1879) that the leading shoots of many Coniferæ 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. + + +(19.) Lilium auratum (Fam. Liliaceae).—The circumnutation of the stem +of a plant 24 inches in height is represented in the above figure (Fig. +83). + +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. + +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 +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 +therefore, 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 11½ 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 +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 acted on +within this time by geotropism;[2] nor had its own weight caused it to +bend downwards. + + [2] 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. + + +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. + +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 +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 45° 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, +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 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. + +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. + +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[3] 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 +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. + + [3] ‘Arbeiten Bot Inst., Würzburg,’ 1872, p. 434. + + +Saxifraga sarmentosa (Saxifrageae).—A plant in a suspended pot had +emitted long branched stolons, which depended like 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 17½ 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 +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. + +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. + +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 (Crassulaceæ).—A plant growing in a pan +of damp moss had emitted 2 stolons, 22 and 20 inches in length. One of +these was supported, so that a length of 4½ 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.; 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 +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 observed 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. + +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. + +Another stolon, which resembled the last in almost every +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. + +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. + +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. + +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. 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 +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. + +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. + +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 19° 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 +_Trifolium repens_ circumnutate in a complicated course during several +days. I may add that the gynophore of _Arachis hypogoea_, which looks +exactly like a peduncle, circumnutates whilst growing vertically +downwards, in order to bury the young pod in the ground. + +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. + +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, 1½ inch in length, was carefully observed +during a whole day, and it made 4½ 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.[4] According to Sachs[5] the flower-stems, whilst +growing, +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 Müller has described[6] the spontaneous +revolving movements of the flower-stems of an Alisma, which he compares +with those of a climbing plant. + + [4] ‘The Movements and Habits of Climbing Plants,’ 2nd edit., 1875, p. + 68. + + + [5] ‘Text-Book of Botany,’ 1875, p. 766. Linnæus 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. + + + [6] ‘Jenaische Zeitsch.,’ B. v. p. 133. + + +We made no observations on the movements of the different parts of +flowers. Morren, however, has observed[7] in the stamens of Sparmannia +and Cereus a “fremissement spontané,” which, it may be suspected, is a +circumnutating movement. The circumnutation of the gynostemium of +Stylidium, as described by Gad,[8] 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. + + [7] ‘N. Mem. de l’Acad. R. de Bruxelles,’ tom. xiv. 1841, p. 3. + + + [8] ‘Sitzungbericht des bot. Vereins der P. Brandenburg,’ xxi. p. 84. + + +We have now seen that the flower-stems of plants belonging to such +widely different families as the Cruciferae, Oxalidæ, 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 +observed, 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. 17°–18° C. Apex of +pitcher 20 inches from glass, so movement greatly magnified. + +(1.) Sarracenia purpurea (Sarraceneae, Fam. 11).—A young leaf, or +pitcher, 8½ inches in height, with the bladder swollen but with the +hood not as yet open, had a filament fixed transversely +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 5½ inches from the glass. + +(3.) Crambe maritima (Cruciferae, Fam. 14).—A leaf 9½ 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 +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 9° 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[9] 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 +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 11° +in the course of 24 h., and the distance travelled by the apex, up and +down, was between .8 and .9 of an inch. + + [9] ‘Flora,’ 1873, p. 437. + + +In order to ascertain the effect of darkness, a filament was fixed to a +leaf 5½ inches in length, borne by a plant which after forming a head +had produced a stem. The leaf was inclined 44° 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,[10] 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. + + [10] ‘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. + + +(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).—The +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 +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 2½ days. As the leaf continued to move all the +time to the left, it is evident that the zigzag line represents many +circumnutations. + +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. +15½°–17½° C. + +(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 40° beneath the horizon. As +it was thick and rigid, and its +petiole very short, much movement could not be expected. Nevertheless, +the apex changed its course completely seven times in the course of 11½ +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. 16°–16½° 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. 15°–16½° C. + +at night; and as they often do not then occupy a vertical position, +especially if they have not been well illuminated during +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 (15°–16° +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 51½ 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. + + +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 19° between 12.20 P.M. +and 10.45 P.M., and to have fallen 23° 30 seconds between the latter +hour and 10.20 A.M. on the following morning. + +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. 17°–18° C. + +The main petiole was now secured to a stick close to the base +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 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. + +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. 16°–18° C. + + +(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. 16½°–17½° 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 12½ h. +the leaf moved eight times up and seven times down; so that it +described 7½ 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. + + +(12.) Echeveria stolonifera (Crassulaceæ, 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. 23°–24½° C. + +(13.) Bryophyllum (vel Calanchæ) calycinum (Crassulaceæ).—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 +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 +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 7½ 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 +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 13½ 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 +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 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. + +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 8½ +inches from the vertical glass. Figure here reduced to two-thirds of +original scale. + +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 +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. 84° to 86° F.), with light admitted +only from above, and with any lateral currents of air +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 +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 2½ 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 14½ inches from the horizontal glass, so figures +considerably magnified. + +(17.) Dahlia (garden var.) (Compositæ, Fam. 122).—A fine young +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 +45° 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 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 +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. + +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. + +(18.) Mutisia clematis (Compositæ).—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[11] 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 +2°, the second 21°, and the third 10° higher than in the middle of the +day; so that instead of sinking they rise a little at night. + + [11] ‘The Movements and Habits of Climbing Plants,’ 1875, p. 118. + + +(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. + + +(20.) Allamanda Schottii (Apocyneae, Fam. 144).—The young leaves of +this shrub are elongated, with the blade bowed so much 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 +Dec. 5th at an angle of 13° 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 37° above the horizon, and had +therefore risen 50°. On the next day similar angular measurements of +the same leaf were made; and at noon the chord stood 36° beneath the +horizon, and 9.30 P.M. 3½° above it, so had risen 39½°. The chief cause +of the rising movement lies in the straightening of the blade, but the +short petiole rises between 4° and 5°. On the third night the chord +stood at 35° above the horizon, and if the leaf occupied the same +position at noon, as on the previous day, it had risen 71°. 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. + +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. + +(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 17½° C. + + +(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. 15°–16½° 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 10°. +This rise was chiefly +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.[12] 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 21° and 12° beneath the +horizon, and at 10 P.M. at 38° beneath it. Two other leaves on a +younger plant were horizontal at 2 P.M., and at 10 P.M. had sunk to 36° +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. + + [12] We were led to observe this plant by Dr. Carl Kraus’ paper, + ‘Beiträge zur Kentniss der Bewegungen Wachsender Laubblätter,’ Flora, + 1879, p. 66. We regret that we cannot fully understand parts of this + paper. + +(25.) Pinus pinaster (Coniferæ, 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, +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 growth, and at the close +of our observations the leaf was not nearly so horizontal as it would +ultimately become. + +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 14½ inches from the vertical glass, so movements +much magnified. + +Pinus austriaca.—Two leaves, 3 inches in length, but not +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 (Cycadeæ, Fam. 224).—A young leaf, 11½ 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. +19°–21° C. + +CIRCUMNUTATION OF LEAVES: MONOCOTYLEDONS. + +(27.) Canna Warscewiczii (Cannaceae, Fam. 2).—The movements of a young +leaf, 8 inches in length and 3½ 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 +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. 15°–16° 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 +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 5½ 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 2½ days. During the first day (22nd) the tip travelled +laterally far to the left, perhaps in consequence of the plant having +been 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. + +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. + +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, +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 20½ h. During the next 24 h. it travelled 2½ 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 45° +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 48°, and the largest +50°; so that it rose only 2° 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.[13] 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 18½° and 23½° C. At 9 A.M. the filament stood at +32° above the horizon; at 3.34 P.M. at 10° and at 11.50 P.M. at 55°; +these two latter angles being the highest and the lowest observed +during the day, showing a difference of 45°. The rising did not become +strongly marked until between +5 and 6 P.M. On the next day the leaf stood at only 10° above the +horizon at 8.25 A.M., and it remained at about 15° till past 3 P.M.; at +5.40 P.M. it was 23°, and at 9.30 P.M. 58°; so that the rise was more +sudden this evening than on the previous one, and the difference in the +angle amounted to 48°. The movement is obviously periodical, and as the +leaf stood on the first night at 55°, and on the second night at 58° +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. + + [13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327. + + +(33.) Pontederia (sp.?) (from the highlands of St. Catharina, Brazil) +(Pontederiaceae, Fam. 46).—A filament was fixed across the apex of a +moderately young leaf, 7½ inches in height, and its movements were +traced during 42½ 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 +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. + +Fig. 118. Pontederia (sp.?): circumnutation of leaf, traced from 4.50 +P.M. July 2nd to 10.15 A.M. 4th. Apex of leaf 16½ inches from the +vertical glass, so tracing greatly magnified. Temp. about 17° C., and +therefore rather too low. + +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 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 15° and 16° 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.[14] + + [14] 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 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.” + + +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. + + +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 +(Lycopodiaceæ, Fam. 6), only .4 inch in height, plainly circumnutated; +we may therefore conclude that older plants, whilst growing, would do +the same. + +(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, +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. + +Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9 +A.M. Oct 25th to 8 A.M. 27th. + +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[15] has given a minute account of the curious, but less +regular though constant, movements of Spirogyra: during 2½ 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.[16] 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. + + [15] ‘Ueber die Bewegungen der Faden der _Spirogyra princeps:_ + Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg,’ + 1874, p. 211. + + + [16] 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.” + +CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES. + +The circumnutating movements of young leaves in 33 genera, belonging to +25 families, widely distributed +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 2° (and probably even less in some instances) +to about 10°; but it amounted to 23° 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 +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 Gramineæ, 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, +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. + + + + +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. + + +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 +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 +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;[1] 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 +45° 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. + + [1] ‘The Movements and Habits of Climbing Plants,’ p. 36. + + +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[2] to express greater +longitudinal growth along the upper than +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. + + [2] ‘Arbeiten des Bot. Inst., in Würzburg,’ 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. + + +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 +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 135°, 154°, and 163°. 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 +young leaf was traced during 4½ 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 Cobœa 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 +well-marked zigzags (one of them forming an angle of 112°), 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[3] 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 +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. + + [3] ‘The Movements and Habits of Climbing Plants,’ 2nd edit. p. 13. + + +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[4] 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. + + [4] ‘Sitzb. der k. Akad. der Wissensch.,’ Vienna, Jan. 1880, p. 16. + + +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 +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 +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. + +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 5½ inches from the vertical glass. Plant illuminated through a +skylight. Temp. 17½°–19° C. Diagram reduced to one-third of original +scale. + +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 9½ inches from the vertical glass. Diagram reduced to +one-fifth of original scale. Plant illuminated through skylight; temp. +17½°–19° C. + +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 +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 +and inclined positions of the parts of plants[5] will see how difficult +a subject this is, and will feel no surprise at our expressing +ourselves doubtfully in this and other such cases. + + [5] ‘Ueber Orthotrope und Plagiotrope Pflanzentheile;’ ‘Arbeiten des + Bot. Inst., in Würzburg,’ Heft ii. 1879, p. 226. + + +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 + +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. + +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. + +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 +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 57° 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. + + + + +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;[1] and since Linnæus +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 +between the sleep of animals and that of plants,[2] 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. + + [1] Pfeffer has given a clear and interesting sketch of the history of + this subject in his ‘Die Periodischen Bewegungen der Blattorgane,’ + 1875, P. 163. + + + [2] Ch. Royer must, however, be excepted; see ‘Annales des Sc. Nat.’ + (5th series), Bot. vol. ix. 1868, p. 378. + + +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 +compound 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 90°; 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 +great advantage from such remarkable powers of movement. + +The nyctitropic movements of leaves and cotyledons are effected in two +ways,[3] 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.[4] But as it has +been shown by De Vries[5] 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 +importance 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;[6] but this statement applies only to periodic and nyctitropic +movements as may be inferred from other cases given by Frank.[7] 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.[8] 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,[9] 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[10] has remarked +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. + + [3] This distinction was first pointed out (according to Pfeffer, ‘Die + Periodischen Bewegungen der Blattorgane,’ 1875, p. 161) by Dassen in + 1837. + + + [4] ‘Flora,’ 1873, p. 433. + + + [5] ‘Bot. Zeitung,’ 1879, Dec. 19th, p. 830. + + + [6] Pfeffer, ‘Die Period. Beweg. der Blattorgane.’ 1875, p. 159. + + + [7] ‘Die Nat. Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 52 + + + [8] 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. + + + [9] ‘Loudon’s Gardener’s Mag.,’ vol. iv. 1828, p. 112. + + + [10] Mr. Rivers in ‘Gardener’s Chron.,’ 1866, p. 732 + + +This view that the sleep of leaves saves them from being chilled at +night by radiation, would no doubt have occurred to Linnæus, 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 +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 +thin pins, so as to stand from ½ 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 +–4° C. (24° to 25° 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 (–3° to +–4° C. on the grass), and the pot was again exposed, but this time for +only 30 m. Eight leaves had been pinned out, +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 –2° C., and not one leaf was injured: the pinned open +leaves, however, all stood from ½ 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 25° to 26° F. (–3.3° to –3.8° C.), but when +looked at after 1 h. had fallen to 22° F. (–5.5° 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 –2° 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., –3° to –4° 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-purpurea) were exposed for 2 h. and 3 h. to a clear sky with the +temp. on grass –2° 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 –2° 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 –3° and –4° 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 –3.3° to –3.8° 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 –2° C., and not a +leaf was injured.[11] It was again exposed on +another night for 1 h., when the temperature of the grass was –4° 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., –4° 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. + + [11] 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 –2° 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 –3° and –4° C., every leaf was killed. + + +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., -3° to –4° C. Ten leaves +had been pinned out, and the result was striking, for on the following +morning all these were found much injured or +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 –1° 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 –2° 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 –3° and –4° 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 –3° to –4° C. Six leaves had +been pinned out so as to stand about ½ 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. + + +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 –1.5° 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 +–2° C., and on a second night for 30 m. with the temperature of the +grass between –3° and –4° 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 –2° 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., –4° 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 –4° 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 +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 29° 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.[12] 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. + + [12] 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 29° 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 –2° C., and they were not in the least + injured. + + +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 +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 +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 ½ 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 –2° C.), it was manifest that more dew had +congealed into hoar-frost on the closely pinned leaflets, than on those +which stood horizontally +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 +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,[13] 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,[14] 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 +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 60° 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 60°, 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 31° at noon to 55° 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 30°, 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 27° above the horizon, and this would lessen +radiation by only 11 per cent.: those of Linum Berendieri rose to 33°, +and this would lessen radiation by 16 per cent. + + [13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327. + + + [14] ‘Adansonia,’ March 10th, 1869. + + +There are, however, some other sources of doubt with +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 60° 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 +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 Oxalidæ 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 60° 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. Oxalidæ (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. — Jacobæus. +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. Cucurbitaceæ (Fam. 106). — aurantia. Lagenaria vulgaris. +Cucurbitaceæ. Cucumis dudaim. Cucurbitaceæ. Apium petroselinum. +Umbelliferae (Fam. 113). — graveolens. Lactuca scariola. Compositæ +(Fam. 122). Helianthus annuus (?). Compositæ. Ipomœa caerulea. +Convolvulaceae (Fam. 151). — purpurea. — bona-nox. — coccinea. +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, +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 75° +above the horizon; at night they moved upwards, each through an angle +of 15° so as to stand quite vertical and in contact with one another. +On the second day they stood at noon at 59° above the horizon, and +again at night were completely closed, each having risen 31°. 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 35° 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. +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 62° 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 +70° 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. 2½ 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. + + +Cucurbitaceæ.—The cotyledons of Cucurbita aurantia and ovifera, and of +Lagenaria vulgaris, stand from the 1st to the 3rd day of their life at +about 60° 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 45° 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_[15] 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. + + [15] 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. + + +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 27° 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, +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 60° 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 61° above the horizon. + +Lactuca scariola (Compositæ).—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 (Compositæ).—This case is rather doubtful; the +cotyledons rise at night, and on one occasion they stood at 73° above +the horizon, so that they might then be said to have been asleep. + +Ipomœa 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 55° and 70° 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. + +Ipomœa 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 77° and the other at 82° beneath the +horizon. Before being placed in the klinostat they stood at 15° and 29° +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. + +Ipomœa 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 63° beneath +the horizon. five days afterwards they were 4½ inches in breadth, and +at night one stood at 64° and the other 48° 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. + +Ipomœa 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 +64° and the other at 47° 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 Ipomœa, 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 27° 30 minutes; but +on the following morning before it was light they stood at 59° 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. + + +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 50° 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 45° 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 90°. Their complicated circumnutating movements during the day +have +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 50° above the horizon, and at +night rose to 77°; 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. + + +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 45° to 55° +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 35° above the horizon; at night they rose to about 75°. 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 10°; so that they had passed through an angle of 100°. 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 45° 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 +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, 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 +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. + +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. + +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 60°. 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. + + +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 60° 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 +90° 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 Ipomœa, 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, Ipomœa 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 +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 Jacobæus, 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 +Raphanus for only a single night. There are, however, some strong +exceptions to this rule, as the cotyledons of Gossypium, Anoda and +Ipomœa 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 Oxalidæ, 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 +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 +cotyledon 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. + + + + +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 30° from +the vertical,—that is, at least 60° 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 30° 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 +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 60° 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,[1] even if the air is very dry +the leaves of Impatiens and Malva are rendered motionless. Carl Kraus +has also lately insisted[2] 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,[3] +referring I suppose to the native plants in France, that they do not +sleep when the temperature is below 5° C. or 41° F. In the case of +several sleeping plants, viz., species of +Tropaeolum, Lupinus, Ipomœa, 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. + + [1] 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. + + + [2] ‘Beiträge zur Kentniss der Bewegungen,’ etc., in ‘Flora,’ 1879, + pp. 42, 43, 67, etc. + + + [3] ‘Annal. des Sc. Nat. Bot.’ (5th Series), ix. 1868, p. 366. + + +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 +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 +Linnæus; 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 Linnæus, 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 +(Linnæus and Pfeffer). ” Hibiscus (Linnæus). ” Anoda. ” Gossypium. ” +Ayenia (Linnæus). Sterculaceae (37). Triumfetta (Linnæus). Tiliaceae +(38). Linum (Batalin). Lineae (39). Oxalis. Oxalidæ (41). Averrhoa. ” +Porlieria. Zygophylleæ (45). Guiacum. ” Impatiens (Linnæus, 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. ” ” +Genus Family. Onobrychis. Leguminosae (75) Tr. VI. Smithia. ” ” +Arachis. ” ” Desmodium. ” ” Urania. ” ” Vicia. ” Tr. VII. Centrosema. ” +Tr. VIII. Amphicarpæa. ” ” 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 (Bouché). Myrtaceae (94). Genus +Family. Aenothera (Linnæus). Omagrarieae (100). Passiflora. +Passifloracea (105). Siegesbeckia. Compositæ (122). Ipomœa. +Convolvulacea (151). Nicotiana. Solaneae (157). Mirabilis. Nyctagineae +(177). Polygonum (Batalin). Polygoneae (179). Amaranthus. Amaranthaceae +(180). Chenopodium. Chenopodieae (181). Pimelia (Bouché). Thymeteae +(188). Euphorbia. Euphorbiaceae (202) Phyllanthus (Pfeffer). ” + +Sub-class II. GYMNOSPERMS. Aies (Chatin). + +CLASS II. MONOCOTYLEDONS. + + +Thalia. Cannaceae (21). Maranta. ” Colocasia. Aroideae (30). Strephium. +Gramineæ (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 55° above the horizon, and +at night at 86°, so each had risen 31°. 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 +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 +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 9° above the horizon, +and at 10 P.M. at 28°, so it had risen only 19°; another leaf (1.4 inch +in length) on a seedling of the same height, stood at the same two +periods at 7° and 32°, and therefore had risen 25°. These leaves, which +moved so little, had a fairly well-developed pulvinus. After an +interval of some weeks, when the same seedlings were 2½ 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. + +Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep) +movements of a leaf on a young plant, 9½ 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. + +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 (15°–16° 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 +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. Linnæus 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 Linnæus 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 45° 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 7½ 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 68° and 90° 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 (Oxalidæ).—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 92° and 150°; in three of the +best folded leaflets of O. fragrans it was 76°, 74°, and 54°. 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 +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 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. + +Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B, +diagram of leaf asleep, also seen from vertically above. + +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 +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[4] the autonomous movements of the leaves +of this plant. + + [4] Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period. + Bewegungen,’ etc., 1875, p. 53. + + +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. + + +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. + +Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep; +traced on vertical glass during 3 h. 40 m. + +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 20° between noon and 10 P.M., whilst the petiole of an older +leaf rose only 13°. 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 +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 +4½ inches from the glass, so movement not much magnified. Plant 9 +inches high, illuminated from above. Temp. 23½°–24½° 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 +more than 90°, so as to assume at night their vertical position. On the +first day the leaflet simply moved up and down; on the +second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after +which hour the great evening fall commenced. + +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. + +Averrhoa bilimbi (Oxalidæ).—It has long been known,[5] 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[6] 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 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 +midrib. This filament acted as an index; and as the leaf rose and fell, +rotating about its basal joint, its angular movement could be recorded +by reading off at short intervals of time the position of the glass +filament on the graduated arc. In order +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.[7] 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 85° with the vertical, or was only 5° below +the horizontal; but in order that the diagram might get into our page, +the leaflet is represented falling from 75° instead of 85°. 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 2° 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. + + [5] Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356. + + + [6] ‘Journal Linn. Soc.,’ vol. xvi. 1877, p. 231. + + + [7] 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 0.2° F. + + +Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced. + +Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its +evening descent, when going to sleep. Temp. 78°–81° F. + +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 +47°, 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 +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 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 31° and + +32° 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 25° C.) to rise during 40 m. before it fell and completed +its oscillation. + +Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a +change from bright illumination to shade; temperature (broken line) +remaining nearly the same. + +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. + +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 7½ inches from the vertical glass. +Temp. 19½°–20½° C. + +Porlieria hygrometrica (Zygophylleæ).—The leaves of this plant (Chilian +form) are from 1 to 1½ 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 Würzburg 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 +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.[8] In the Botanic Garden at Würzburg, 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, +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. + + [8] ‘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. + + +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 +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 +Würzburg, 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 ½ 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 45°, 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 +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; +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 50° 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 23°. 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 38° and the latter of 36°, beneath the horizon; but their +petioles +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 were to be observed +at all seasons, some of the leaves would be found to sleep in a +different manner. + +Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in +daytime; B, leaf asleep, seen laterally at night. + +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 +36° above the horizon, and at night at 51°, thus forming together a +hollow cone with moderately steep sides. The petiole of one leaf rose +14° and of a second 11° at night. With L. luteus a leaflet rose from +47° at noon to 65° above the horizon at night, and another on a +distinct leaf rose from 45° to 69°. The petioles, however, sink at +night to a small extent, viz., in three instances by 2°, 6°, and 9° 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 +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 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 +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. + +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. + +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 42° above the horizon, and during the night at +72°, so had risen 30°. The petiole of another leaf, the leaflets of +which occupied a similar position at night, rose only 6°. On the other +hand, the petiole of a leaf with all its leaflets sloping down at +night, fell at this time 4°. The petioles of two rather older leaves +were subsequently observed; both of which stood during the day at +exactly the same angle, viz., 50° above the horizon, and one of these +rose 7°–8°, and the other fell 3°–4° 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 7°. Other +leaves which likewise stood horizontally during the day, had at night +all their leaflets sloping downwards at 46° 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 45° above the horizon, rose at night to 65° and 69°, 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, +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 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 23° and on another 33°. The three leaflets also bend +upwards, and at the same time +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. + +Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep +at night. + +Trigonella Cretica resembles a Melilotus in its sleep, which will be +immediately described. According to M. Royer,[9] the leaves of Medicago +maculata rise up at night, and “se renversent un peu de manière à +presenter obliquement au ciel leur face inférieure.” 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. + + [9] ‘Annales des Sc. Nat. Bot.’ (5th series), ix. 1868, p. 368. + + +Melilotus (Tribe 3).—The species in this genus sleep in a remarkable +manner. The three leaflets of each leaf twist through an angle of 90°, +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 +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 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 +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. + +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. + +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 59°, whilst three others +rose only 7° and 9°. 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 +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 +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 45° 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 twisting 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 90°, generally of 130° or 140°, and +not rarely—as was often observed with T. subterraneum—of 180°. 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. + +Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at +night. + +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. +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[10] 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.[11] He states that the terminal leaflet of this species, +observed at different times, passed through angles of from 30° to 120° +in the course of from 1½ to 4 h. We observed the movements of T. +subterraneum, resupinatum, and repens. + + [10] ‘Flora,’ 1863, p. 497. + + + [11] ‘Die Period. Bewegungen,’ 1875, pp. 35, 52. + + +Trifolium subterraneum.—A petiole was secured close to the base of the +three leaflets, and the movement of the terminal leaflet was traced +during 26½ 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 48° beneath +the horizon, and by 11.30 A.M. it had risen 50° above the horizon; so +that it passed through 98° 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. + +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. 16°–17° 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, +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. + +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. + +Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, +with leaves asleep at night. SS, stipule-like leaflets. + +In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12 +genera have been observed by ourselves and +others, but only in Robinia with any care. Psoralea acaulis raises its +three leaflets at night; whilst Amorpha fruticosa,[12] Dalea +alopecuroides, and Indigofera tinctoria depress them. Ducharte[13] +states that Tephrosia caribaea is the sole example of “folioles +couchées le long du pétiole et vers la base;” but a similar movement +occurs, as we have already seen, and shall again see in other cases. +Wistaria Sinensis, according to Royer,[14] “abaisse les folioles qui +par une disposition bizarre sont inclinées dans la même feuille, les +supérieures vers +le sommet, les inférieures 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 Linnæus, in Glycyrrhiza. The leaflets of Robinia +pseudo-acacia likewise sink vertically down at night, but the petioles +rise a little, viz., in one case 3°, and in another 4°. 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. + + [12] Ducharte, ‘Eléments de Botanique’, 1867, p. 349. + + + [13] Ibid., p. 347. + + + [14] ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868. + + +Coronilla rosea (Tribe 6).—the leaves bear 9 or 10 pairs of opposite +leaflets, which during the day stand horizontally, with 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 +40° to 50° 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 +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 3° above the horizon at noon, to 9° +at 10 P.M.; from 11° to 33°; and from 5° to 33°—the amount of angular +movement in this latter case amounting to 28°. In several other species +of Coronilla the leaflets showed only feeble movements of a similar +kind. + +Fig. 146. Coronilla rosea: leaf asleep. + +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, 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 +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 39°. 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 17°–17½° 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. 147. Arachis hypogoea: A, leaf during the day, seen from +vertically above; B, leaf asleep, seen laterally, copied from a +photograph. Figures much reduced. + +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 46½°, 36°, +20°, and 19.5° 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 23°, and at night at 69½° above the +horizon. In the evening the rising of the petioles is almost completed +before the leaflets sink perpendicularly downwards. + + +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. 91° F.) it changed its course by as much +as a rectangle six times, describing a figure which apparently +represented two ellipses. 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[15] these leaflets move through an angle of 8° in the course of +from 10 to 30 seconds. + + [15] ‘Die Period. Beweg.,’ p. 35. + + +Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves +asleep. Figures reduced. + +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. +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. + +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. 19°–20° C. + +Had the above plant been subjected to a higher temperature than 67°–70° +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 92°–93° 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 +41° 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 95° F., which +was slowly raised to 103° F., and afterwards allowed to sink to 70° F., +and the sub-petiole of the terminal leaflet then curved downwards. The +water was afterwards + +raised to 120° 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 122° 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[16] 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. + + [16] ‘Die Period. Beweg.,’ p. 39. + + +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 180°; 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 103° 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 100° F., and the +leaflets again began to move quickly. On another occasion a tuft of +fine leaves was immersed in water at 53° F., and the leaflets were of +course motionless. The water was raised to 99°, and the leaflets soon +began to move; it was raised to 105°, 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 71°–72° F., and this agrees with our +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 64° 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 62° 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. 61° 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 +61° to 62° 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 82° 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. 77° 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 +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 45° +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 +subsequent appearance, may be attributed to reversion to more or less +distant progenitors.[17] + + [17] Desmodium vespertilionis is closely allied to D. gyrans, and it + seems only occasionally to bear rudimentary lateral leaflets. + Duchartre, ‘Eléments de Botanique,’ 1867, p. 353. + + +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 +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 16½°. + +Amphicarpoea monoica (Tribe 8).—The leaflets sink down vertically at +night, and the petioles likewise fall considerably. A petiole, which +was carefully observed, stood during the day 25° above the horizon and +at night 32° below it; it therefore fell 57°. A filament was fixed +transversely across the terminal leaflet of a fine young leaf (2 1/4 +inches in length including the +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. + +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 17½°–18½° C. + +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 +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 3½ inches from the vertical glass. Figure reduced to +one-half of original scale. Plant illuminated from above; temp. +17½°–18½° 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. + + +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 16°, +and that of an older leaf 10° 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 20° to about 60° 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 26° beneath the horizon at noon, to 20° 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 +Amphicarpæa. + +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 +Poinciana 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 Linnæus, and since by Duchartre. Our +observations were made chiefly on C. floribunda[18] 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 +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. In one instance we found +that the midrib of a terminal leaflet formed at night an angle of 36°, +with a line dropped +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 12°, and +others as much as 41°. + + [18] 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. + + +Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at +night. Both figures copied from photographs. + +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 3°. Cassia pubescens.—The chief difference in the +nyctitropic movements of this species, compared with those of the +former species, consists in the leaflets not rotating nearly so much; +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. + +Fig. 155. Cassia pubescens: A, upper part of plant during the day; B, +same plant at night. Figures reduced from photographs. + +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 8° 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 104°, diverted at night only 72°; so that each had risen 16° +above its diurnal position. The petiole of a young leaf rose at night +34°, and that of an older leaf 19°. 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 +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 5½ inches from the vertical +glass. Main petiole 3 3/4 inches long. Temp. 16°–17½° 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 Müller, 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 45° above the +horizon, at night stood at 75°; it thus rose 30°; another rose 34°. +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 27° beneath the horizon, being directed towards the +stem. It had thus travelled through 153°. +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 57°–59° 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. + + +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 +was observed every two or three minutes, the plants being kept at a +rather high temperature, viz., on the first occasion at 77°–81° F., and +the filament then described 2½ ellipses in 69 m. On the second +occasion, when the temperature was 81°–86° 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. 84° 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 +80° 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 7°; but not continuously in the same direction, as it moved +four times to one side, and three times to the opposite side, +in one instance to the extent of 16°. This pinna, therefore +circumnutated. Later in the evening the four pinnae approach each +other, and the one which was observed moved inwards 59° 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., 62°–64° 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. 57°–59° 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 +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, 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, +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. + +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. + +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.[19]—The leaves of this plant, one of which is here +figured (Fig. 159) reduced to 2/3 of the natural size, present some +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 +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. + + [19] 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.” + + +Fig. 159. Mimosa albida: leaf seen from vertically above. + +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 +(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 +45°, 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 +27° 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 15°; +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 +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 100° during +the day, and at night of only 38°, so each had moved 31° forwards. The +penultimate pinnae during the day formed together an angle of 180°, +that is, they stood in a straight line opposite one another, and at +night each had moved 65° forwards. The basal pair of pinnae were +directed during the day, each about 21° backwards, and at night 38° +forwards, so each had moved 59° 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 +34° higher than at noon, and by 6.40 A.M. on the following morning it +was still higher by 10°; 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 +represented five ellipses, with their longer axes differently directed. + +Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf +at night. + +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 16° +higher than in the day-time. + +Melaleuca ericaefolia (Myrtaceae).—According to Bouché (‘Bot. Zeit.,’ +1874, p. 359) the leaves sleep at night, in nearly the same manner as +those of certain species of Pimelia. + +Œnothera mollissima (Onagrarieae).—According to Linnæus (‘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 33° above the horizon; and at 10.30 P.M., +when the blade was vertically dependent, at only 15°, so the petiole +had fallen 18°. That of the next older leaf fell only 7°. 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 40° 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 +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. 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. + +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. + +Siegesbeckia orientalis (Compositæ).—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 +considerable angle beneath the horizon at 10 P.M. In the case of four +youngish leaves which were from 2 to 2½ inches in length, these angles +were found to be 50°, 56°, 60°, and 65°. 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 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. + +Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, +and asleep at night. Figures copied from photographs, and reduced. + + +Ipomœa caerulea and purpurea (Convolvulaceae).—The leaves on very young +plants, a foot or two in height, are depressed at night to between 68° +and 80° 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. +17½°–18½° 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 +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 Linnæus, 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 Bouché (‘Bot. Zeitung,’ 1874, p. 359) the leaves +of Pimelia linoides and spectabilis (Thymeleae) sleep at night. + + +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 30° 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 38°. 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,[20] 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 +outwards. They are furnished as might have been expected from this +complex kind of movement, with a pulvinus. + + [20] ‘Die Period. Beweg.,’ p. 159. + +GYMNOSPERMS. + +Pinus Nordmanniana (Coniferæ).—M. Chatin states[21] 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 même +temps, ce mouvement d’érection est accompangé d’un mouvement de torsion +imprimé à la partie basilaire de la feuille, et pouvant souvent +parcourir un arc de 90 degrés.” 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. + + [21] ‘Comptes Rendus,’ Jan. 1876, p. 171. + +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 6½ in breadth, formed at noon an angle with its tall petiole of +121°, and at night stood vertically in a line with it, and so had risen +59°. 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 10½ 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 +the day or between 10° and 20° above the horizon, and at night +vertically upwards. They therefore rise between 70° and 90° 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 70° and 80°. 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., +20°; at 6 P.M. 43°; at 7.20 P.M., 69°; and at 8.30 P.M., 68°; so it had +now begun to rise; at 10.15 P.M. it stood at 65°, and on the following +early morning at 11° 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 +rise so much at night that they may almost be said to sleep. + +Strephium floribundum[22] (Gramineæ).—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.) 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 90°. Thus +the surface of the blade always stands vertically, whatever may be the +position of the midrib or of the leaf as a whole. + + [22] 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. + + +Fig. 164. Strephium floribundum: culms with leaves during the day, and +when asleep at night. Figures reduced. + +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. +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., 24°–26½° 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. 23½°–24½° 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 60° +F., and the movement of one of the leaflets (the petiole having been +secured) was traced +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 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, 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 +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 72° and 83° 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. + +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. + +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. + +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 90° upwards, and of others +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 90° 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 +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 +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 +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 +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 +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 +Amphicarpæa, the petioles of some leaves sank down as much as 57° at +night; with Arachis they sank 39°, 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 46°, whilst the +young ones rose up vertically; that of a young leaf of Cassia +floribunda rose 41°, whilst that of an older leaf rose only 12°. 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 30° and 34°, 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 59°, and others only 7° and 9° 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 +30°, others only 6°, and others sank 4° 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 34°. +These several facts apparently indicate that the movements +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 +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, +Amphicarpæa, 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 +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 +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, +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 Amphicarpæa, and is +probably common to all leaflets provided with a pulvinus. + +With respect to the periodicity of the movements of sleeping leaves, +Pfeffer[23] has so clearly shown that this depends on the daily +alternations of light and darkness, that nothing farther need be said +on this +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. + + [23] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, p. 30, et + passim. + + +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.[24] The strength of such +inheritance differs +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.[25] + + [24] 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 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. + + + [25] Pfeffer, ibid., p. 46. + + +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 +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 +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 +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 3½ 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 +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 +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 Gramineæ, 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.[26] We have also observed the +same phenomenon in Elymus arenareus. + + [26] ‘Annal. des Sc. Nat. (Bot.),’ 1875, tom. i. pp. 326–329. + + +There is another movement, which since the time of Linnæus 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[27] 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. + + [27] ‘Die Schutzmittel des Pollens,’ 1873, pp. 30–39. + + +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 +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 +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, +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. + + + + +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.[1] 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. + + [1] ‘Physiologie Veg.’ (French Translation), 1868, pp. 42, 517, etc. + + +Apheliotropism, or, as usually designated, negative +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 +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 +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 (1½ 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. +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 +rectangularly 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 +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 +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 +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 +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 exposed is shown. +It may be observed that during the first 16 m. the hypocotyl moved +obliquely from the light, and this, +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. 172. Brassica oleracea: ordinary circumnutating movement of the +hypocotyl of a seedling plant. + +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. + +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 +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. + +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. + +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. + + +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 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 +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. 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. + +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, +which, whilst young, is so extremely sensitive to light, worth giving. + +Fig. 177. Cassia tora: heliotropic movement and circumnutation of a +hypocotyl (1½ 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 +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 +20° 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 +contact with the trunks of trees.[2] 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. + + [2] ‘The Movements and Habits of Climbing Plants,’ 1875, p. 97. + + +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.[3] 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 +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 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. + + [3] 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. + + +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. + +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 +movement, 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, +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.[4] +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. + + [4] In his paper, ‘Ueber orthotrope und plagiotrope Pflanzentheile’ + (‘Arbeiten des Bot. Inst. in Würzburg,’ Band ii. Heft ii. 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. + + +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 +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_[5]) _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 +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 +branches which had been fastened in various positions or turned upside +down. + + [5] ‘Die natürliche Wagerechte Richtung von Pflanzentheilen,’ 1870. + See also some interesting articles by the same author, “Zur Frage über + Transversal-Geo-und Heliotropismus,” ‘Bot. Zeitung,’ 1873, p. 17 et + seq. + + +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[6] 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. + + [6] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp. + 223–277. + + +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 +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 90°, 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[7] 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. + + [7] See former note, in reference to Sachs’ remarks on this subject. + + +We may therefore conclude that diaheliotropic movements cannot be fully +explained by the direct action of light, gravitation, weight, etc., any +more +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 2° or 3°, or even of 10° or 20°, can be of +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;[8] +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 +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. + + [8] 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 Néerlandaises,’ 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.) + + +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 30° and 40° 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 Ipomœa, 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 +the Anoda and of some species of Ipomœa, 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[9] 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 Amphicarpæa 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 20° +above the horizon, and no doubt they afterwards sank a little lower. At +noon, after having been exposed for about 2 h. to +a bright sun, they stood at 56° 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 40°, for they now stood at only +16° 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 90° 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 1½ 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 45½° 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. + + [9] Pfeffer gives the names and dates of several ancient writers in + his ‘Die Periodischen Bewegungen,’ 1875, p. 62. + + +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[10] 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. + + [10] ‘Die Näturlichen 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. + + +In the cases above given, the leaflets move either upwards +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 Oxalidæ); 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 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. + +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. + +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 +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. + + + + +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 +Gramineæ, 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 +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.[1] 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. + + [1] 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. + + +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 +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.[2] Three young +plants (about a foot in height) of Ipomœa 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 +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. + + [2] 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. + + +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, +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 Ipomœa +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 +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 +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.[3] + + [3] Strasburger says (‘Wirkung des Lichtes auf Schwärmsporen,’ 1878, + p. 52), that the spores of Haematococcus moved to a light which only + just sufficed to allow middle-sized type to be read. + + +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 +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 +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,[4] 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. + + [4] ‘Sitz. der k. Akad. der Wissensch.’ (Vienna), Jan. 1880, p. 12. + + +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. +After 8 h. 40 m. the chords of their arcs were deflected from the +perpendicular by an average angle of only 16°. 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 +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 +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 19°, and of those +which had previously been illuminated only 7°. 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 16° for those in the pot which had previously +been kept in darkness, and only 5° for those which had previously been +illuminated. + +The curvature of the cotyledons of Phalaris towards a lateral light is +therefore certainly influenced by the +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 180° in 4 +h., having in the morning previously passed through about 90°. 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 +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, 7½, 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 +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 +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 4° and 11°) from the perpendicular. Their +directions were also changed after two additional hours, and again on +the following morning. We may +therefore conclude that the bending back of plants from a light, when +this becomes obscure or is extinguished, is wholly due to +apogeotropism.[5] + + [5] It appears from a reference in Wiesner (‘Die Undulirende Nutation + der Internodien,’ p. 7), that H. Müller 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. + + +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 +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, 35°, 83°, 31°, 43°, and 39°. 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 +cotyledons 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, 44°, 44°, +55°, 51°, and 57°. 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 +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 +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 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. + +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. + +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 5½ h. they were directed towards +the light with their bases bent at right angles; and this abrupt +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 +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 +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, +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. + + +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 +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 11°, 13°, 15°, and 18°, 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 +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 +not directly in the line of the window, but obliquely towards it; one +pointed at an angle of only 18°, but the remaining 12 at angles varying +between 45° and 62° 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 31°, 35°, and 83°. 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. +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 +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 20° and 21° 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 +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 +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 11° and 15° 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 +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.[6] 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 +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. + + [6] Sachs, ‘Physiologie Végétale,’ 1868, p. 44. + +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 Gramineæ, 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 +lately such movements were believed to result simply from increased +growth on the shaded side. At present it is commonly admitted[7] 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.[8] 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, +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. + + [7] 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 Würzburg,’ 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. + + + [8] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, pp. 7, 63, + 123, etc. Frank has also insisted (‘Die Naturliche wägerechte 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. + + +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.[9] So it is with many aërial roots, according +to Wiesner;[10] but there are other opposed cases. It appears, +therefore, that light does not determine the growth of apheliotropic +parts in any uniform manner. + + [9] Francis Darwin, ‘Über das Wachsthum negativ heliotropischer + Wurzeln’: ‘Arbeiten des Bot. Inst. in Würzburg,’ B. ii., Heft iii., + 1880, p. 521. + + + [10] ‘Sitzb. der k. Akad. der Wissensch’ (Vienna), 1880, p. 12. + + +We should bear in mind that the power of bending to the light is highly +beneficial to most plants. There +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.[11] 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. + + [11] 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 Würzburg,’ 1879, B. ii. p. 282. + + +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 +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.[12] +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 Ipomœa, 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 +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. + + [12] Strasburger has shown in his interesting work (‘Wirkung des + Lichtes...auf Schwärmsporen,’ 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 Schwärmsporen’ Verh. d. phys.-med. + Geselsshalft in Würzburg, B. xii. 1878. + + +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[13]—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 +up, and that nearest to the light sinking down, or both twisting +laterally.[14] We may, also, suspect that the extreme sensitiveness to +light of the upper part of the sheath-like cotyledons of the Gramineæ, +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. + + [13] Francis Darwin, ‘On the Hygroscopic Mechanism,’ etc., + ‘Transactions Linn. Soc.,’ series ii. vol. i. p. 149, 1876. + + + [14] 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. + + +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 +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 +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. + + + + +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—Amphicarpæa—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 +circumnutation 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 40° 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 10° 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, +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 60° above +the horizon, and had therefore risen 70°. 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 10° +beneath to 60° 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 +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 23° 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 19° +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 19° 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 +oblique. During the next 3 h. 8 m. it rose in a nearly straight line, +passing through an angle of 109°, 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 31° 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 40° 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 130° +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 +seen how gradually circumnutation becomes changed into apogeotropism, +under circumstances to be specified in each instance. + +Rubus idæus (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 67°. 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 15½ h. was +clearly zigzag, and the rate of the upward movement was not equable. +During the third day, also of 15½ 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 23° 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 idæus (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 +horizontally, and the upper part of the stem rose 58° 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 15½ 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 32° 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 40° beneath the +horizon into a vertical position, passing through an angle of 130° 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 40° 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 45° beneath the horizon +rose 90° in 7 or 8 h. in lines almost absolutely straight. An oldish +cotyledon, from which the first leaf began to +protrude whilst the following observations were being made, was placed +at 10° beneath the horizon, and it rose only 59° in 24h. It behaved +rather differently from any other plant, observed by us, for during the +first 4½ h. it rose in a line not far from straight; during the next 6½ +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 4½ 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 30° and 40° beneath +the horizon. The upper part alone became curved +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 32° 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 +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 20° 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 +Gramineæ, and are effected by means of the thickened bases of their +sheathing leaves; the stem within being in this part thinner than +elsewhere.[1] 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 Gramineæ; 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. + + [1] This structure has been recently described by De Vries in an + interesting article, ‘Ueber die Aufrichtung des gelagerten Getreides,’ + in ‘Landwirthschaftliche Jahrbücher,’ 1880, p. 473. + + +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 +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 50° 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 3½ h.; but it +sometimes moved at a quicker rate, for at one time it crossed 5 +divisions in 1½ 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-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 40°. 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, +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. + + +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 the +middle, so that the lower part answers to the main peduncle, +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 90°, 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. + +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. + +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, +continually 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 109° in 3 h. 8 m., and a cotyledon of Phalaris an +angle of 130° in 4 h. 30 m. On the other hand, the stem of a herbaceous +Verbena rose 90° in about 24 h.; that of Rubus 67°, in 70 h.; that of +Cytisus 70°, in 72 h.; that of a young American Oak only 37°, in 72 h. +The stem of a young Cyperus alternifolius rose only 11° 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 Ipomœa +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 40° 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 +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.[2] 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 45° above the horizon, where it +circumnutated during the remaining 8 h. of observation. + + [2] For details see ‘The Movements and Habits of Climbing Plants,’ + 1875, p. 131. + + +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, +became so in 28 h.; and that of another still older one, 1½ 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 +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-aërial 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 Ipomœa leptophylla) are +guided in their downward course by geotropism; and so are many aërial +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 +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 +Æsculus (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[3] 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 +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 180° in this time. Ten were extended +sub-horizontally, and these had moved through about 90°. Three very +young peduncles had as yet moved only a little downwards, but after an +additional 24 h. were greatly inclined. + + [3] ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. 1841, p. 106. + + +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 ½ 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 +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 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 19° above the +horizon, and it circumnutated during two days. Another +which was already curved 36° 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. 190. Trifolium subterraneum: downward movement of peduncle from +19° 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. + +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. + + +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;[4] 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,[5] 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, +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. + + [4] Vaucher, ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. p. 110. + + + [5] See his interesting article in ‘Nature,’ April 4th, 1878, p. 446. + + +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[6] never to produce pods. + + [6] ‘Gard. Chronicle,’ 1857, p. 566. + + +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 +represented four ellipses. Lastly, a long gynophore, the apex of which +had buried itself to the depth of about half an inch, was pulled up and +extended horizontally: it quickly began to curve downwards in a zigzag +line; but on the following day the +terminal 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. + +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. + +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 25° 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[7] 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. + + [7] ‘Manual of the Botany of the Northern United States,’ 1856, p. + 106. + +DIAGEOTROPISM. + +Besides geotropism and apogeotropism, there is, according to Frank, an +allied form of movement, +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.[8] 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. + + [8] Elfving has lately described (‘Arbeiten des Bot. Instituts in + Würzburg,’ B. ii. 1880, p. 489) an excellent instance of such + movements in the rhizomes of certain plants. + + +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, +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. + + + + +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[1] 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 +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.[2] 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. + + [1] ‘Abwartskrümmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p. + 29. + + + [2] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. iii. 1873, p. + 432. + + +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 +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 65° 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 40° +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 ½ to +1 inch, sixteen were selected which +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 +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 45° 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 45° 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 +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 65°. 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 20° 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 45°. 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 +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 +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., 55°–59° 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 68° F., or 20° 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 +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 3½ 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. +55°–56° F.) these three radicles, 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 +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. + +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. + +Three other radicles, the tips of which had been touched with caustic +curing 2 or 3 seconds, remained (temp. 58°–59° 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. 55°–57° F.) the three control +specimens were plainly geotropic in 8½ 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 8½ h., during which time no geotropic action had occurred. When +the specimens were re-examined after an additional interval of 15½ 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 +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 180°, others through +about 135°, and others through only 90°. 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 +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 +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 45° 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., 59°, 60°, 65°, 57°, and 43°. 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 45° 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 20°–21° C.) plainly geotropic in 8 h. 30 m., for +they then stood at an average angle of 63° beneath the horizon. A +rather greater length of the radicle is bowed downwards by geotropism +than in the case of Vicia faba, +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 10° 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 30° beneath the horizon, whilst the controls stood at +an average of 54°. 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 +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. +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 ½ 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 +75°–76° 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 +45° and 90°, 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 +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 21° 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 +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 +70°–71°) 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 +20° 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 +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. +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 8½ 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 +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 +part which is sensitive to geotropism in the members of such distinct +families as the Leguminosae, Malvaceae, Cucurbitaceæ and Gramineæ, 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 +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. + + + + +CHAPTER XII. +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. + + +In some cases, as with the hypocotyls of Brassica, the leaves of +Dionaea and the joints of the Gramineæ, 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 +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 +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 +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 +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 +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.[1] We ascertained +in one instance that such sensitiveness resided in the tip, and caused +the adjoining parts to bend from the light. The sub-aërial 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. + + [1] 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. + + +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 +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 +completely 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 Gramineæ 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 +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 Gramineæ, and in +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 +Cucurbitaceæ 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_, +_Ipomœa 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 Ipomœa 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 +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. + + +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 +circumnutating 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 +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 +cotyledons 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 Gramineæ. 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 +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. + + +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-aërial 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 +disadvantageous 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 +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 +transmitted 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 +upwards through 109° 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 +1½ 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 +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 +opposite direction might gradually be acquired through natural +selection.[2] + + [2] 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. + + +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 +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.[3] + + [3] For the evidence on this head, see the ‘Movements and Habits of + Climbing Plants,’ 1875, pp. 173, 174. + + +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, Cucurbitaceæ, 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.[4] With plants an +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. + + [4] Sachs remarks to nearly the same effect: “Dass sich die lebende + Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theile + mit specifischen Energien ausgerüstet sind, ähnlich, wie die + verschiedenen Sinnesnerven des Thiere” (‘Arbeiten des Bot. Inst. in + Würzburg,’ Bd. ii. 1879, p. 282). + + +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 +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. + + + + +INDEX. + + + + A. + + Abies communis, effect of killing or injuring the leading shoot, 187 + — pectinata, effect of killing or injuring the leading shoot, 187 + —, affected by Æcidium 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 + + Æcidium elatinum, effect on the lateral branches of the silver fir, 188 + + Æsculus 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 + + 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 Æcidium 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 + + Bouché 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 + —, 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 + + 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 + + Cobœa 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 + + Coniferæ, 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 + —, 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 nyctitropic movement of leaves, 358–360 + —, 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 Gramineæ, 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 + + 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 Ipomœa 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 + + H. + + Haberlandt, Dr., on the protuberance on the hypocotyl of Allium, 59; the + importance of the arch to seedling plants, 87; sub-aërial 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 + + Ipomœa 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 + + Ipomœa 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 + + Linnæus, ‘Somnus Plantarum’, 280; on plants sleeping, 320; on the leaves + of Sida abutilon, 324; on Œnothera 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 + — Jacobæus, movements of cotyledons, 35, 109 + —, pulvini of, 115 + + Lotus Jacobæus, 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 Coniferæ, 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 Æcidium 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 + + 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 + + Müller, 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 + + 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 + + Œnothera 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 + + 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 + + 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 Æcidium 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 + + 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 idæus (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; 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 Æcidium on shoot, 189; on the influence of + light on swarm-spores, 488, n. + + Stapelia sarpedon, circumnutation of hypocotyl, 46, 47 + —, 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 + + 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 Gramineæ, 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 aërial 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. + + + + +*** END OF THE PROJECT GUTENBERG EBOOK THE POWER OF MOVEMENT IN PLANTS *** + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the +United States without permission and without paying copyright +royalties. 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