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diff --git a/5605-h/5605-h.htm b/5605-h/5605-h.htm new file mode 100644 index 0000000..e0e8652 --- /dev/null +++ b/5605-h/5605-h.htm @@ -0,0 +1,21699 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>The Project Gutenberg eBook of The Power of Movement in Plants, by Charles Darwin</title> + +<style type="text/css" xml:space="preserve"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 130%; margin-top: 1em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +.no-break {page-break-before: avoid;} /* for epubs */ + +div.chapter {page-break-before: always; margin-top: 4em;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +pre { font-style: italic; font-size: 90%; margin-left: 10%;} + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + </head> + <body> + +<div style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Power of Movement in Plants, by Charles Darwin</div> +<div style='display:block; margin:1em 0'> +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 <a href="https://www.gutenberg.org">www.gutenberg.org</a>. 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. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Power of Movement in Plants</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block; margin:1em 0'>Release Date: August 14, 2002 [eBook #5605]<br /> +[Most recently updated: September 11, 2022]</div> +<div style='display:block; margin:1em 0'>Language: English</div> +<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> +<div style='display:block; margin-left:2em; text-indent:-2em'>Produced by: Sue Asscher</div> +<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE POWER OF MOVEMENT IN PLANTS ***</div> + +<h1>THE POWER OF MOVEMENT IN PLANTS</h1> + +<h2 class="no-break">By Charles Darwin</h2> + +<h3>Assisted By Francis Darwin</h3> + +<hr /> + +<h2>CONTENTS</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#link2H_4_0001">DETAILED TABLE OF CONTENTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2H_4_0002">THE MOVEMENTS OF PLANTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2H_INTR">INTRODUCTION.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0001">CHAPTER I. THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0002">CHAPTER II. GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0003">CHAPTER III. SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0004">CHAPTER IV. THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0005">CHAPTER V. MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0006">CHAPTER VI. MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0007">CHAPTER VII. MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0008">CHAPTER VIII. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0009">CHAPTER IX. SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0010">CHAPTER X. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0011">CHAPTER XI. LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0012">CHAPTER XII. CONCLUDING REMARKS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0013">INDEX</a></td> +</tr> + +</table> + +<hr /> + +<div class="chapter"> + +<h2><a name="link2H_4_0001"></a> +DETAILED TABLE OF CONTENTS.</h2> + +<p class="noindent"> +<a href="#link2HCH0001">CHAPTER I.—THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0002">CHAPTER II.—GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0003">CHAPTER III.—SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0004">CHAPTER IV.—THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0005">CHAPTER V.—MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0006">CHAPTER VI.—MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0007">CHAPTER VII.—MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0008">CHAPTER VIII.—MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0009">CHAPTER IX.—SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0010">CHAPTER X.—MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0011">CHAPTER XI.—LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.</a><br/> +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.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0012">CHAPTER XII.—CONCLUDING REMARKS.</a><br/> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_4_0002"></a> +<a name="page01"></a> +THE MOVEMENTS OF PLANTS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_INTR"></a> +INTRODUCTION.</h2> + +<p> +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 +<a name="page02"></a> +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. +</p> + +<p> +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.<a href="#fn0.1" +name="fnref0.1"><sup>[1]</sup></a> 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<a href="#fn0.2" +name="fnref0.2"><sup>[2]</sup></a> 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<a href="#fn0.3" +name="fnref0.3"><sup>[3]</sup></a> 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 +<a name="page03"></a> +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.<a href="#fn0.4" name="fnref0.4"><sup>[4]</sup></a> +</p> + +<p class="footnote"> +<a name="fn0.1"></a> <a href="#fnref0.1">[1]</a> +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. +</p> + +<p class="footnote"> +<a name="fn0.2"></a> <a href="#fnref0.2">[2]</a> +‘Die Periodischen Bewegungen der Blattorgane,’ 1875. +</p> + +<p class="footnote"> +<a name="fn0.3"></a> <a href="#fnref0.3">[3]</a> +‘Untersuchungen über den Heliotropismus,’ Sitzb. der K. Akad. der +Wissenschaft. (Vienna), Jan. 1880. +</p> + +<p class="footnote"> +<a name="fn0.4"></a> <a href="#fnref0.4">[4]</a> +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. +</p> + +<p> +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 +<a name="page04"></a> +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. +</p> + +<p> +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 +<a name="page05"></a> +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. +</p> + +<p class="p2"> +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,<a href="#fn0.5" +name="fnref0.5"><sup>[5]</sup></a>—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. +</p> + +<p class="footnote"> +<a name="fn0.5"></a> <a href="#fnref0.5">[5]</a> +The highly useful terms of Heliotropism and Geotropism were first used by Dr. +A. B. Frank: see his remarkable ‘Beiträge zur Pflanzenphysiologie,’ 1868. +</p> + +<p> +The term epinasty is now often used in Germany, and implies that the upper +surface of an organ grows more quickly than the +<a name="page06"></a> +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.<a href="#fn0.6" name="fnref0.6"><sup>[6]</sup></a> +</p> + +<p class="footnote"> +<a name="fn0.6"></a> <a href="#fnref0.6">[6]</a> +These terms are used in the sense given them by De Vries, ‘Würzburg +Arbeiten,’ Heft ii 1872, p. 252. +</p> + +<p> +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 +<a name="page07"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn0.7" name="fnref0.7"><sup>[7]</sup></a> Whenever it +could be approximately told how much the movement had been magnified, this is +stated. We have perhaps +<a name="page08"></a> +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. +</p> + +<p class="footnote"> +<a name="fn0.7"></a> <a href="#fnref0.7">[7]</a> +We are much indebted to Mr. Cooper for the care with which he has reduced and +engraved our diagrams. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +Finally, we must have the pleasure of returning our +<a name="page09"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0001"></a> +<a name="page10"></a> +CHAPTER I.<br /> +THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +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 +<a name="page11"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page12"></a> +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. +</p> + +<p> +Fig. 2. Brassica oleracea: circumnutating and geotropic movement of radicle, +traced on horizontal glass during 46 hours. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page13"></a> +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. +</p> + +<p> +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 +<a name="page14"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page15"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page16"></a> +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. +</p> + +<p> +Fig. 6. Brassica oleracea: conjoint circumnutation of the hypocotyl and +cotyledons during 10 hours 45 minutes. Figure here reduced to one-half original +scale. +</p> + +<p> +On the previous day another seedling had been observed under similar +conditions, excepting that the plant was so +<a name="page17"></a> +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. +</p> + +<p> +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. +</p> + +<p> +By 9.25 A.M. on this second day the same cotyledon had +<a name="page18"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page19"></a> +vertically up and down all day long, and as there was some slight lateral +movement, they circumnutated. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page20"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page21"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Githago segetum (Caryophylleae).—A young seedling was dimly illuminated +from above, and the circumnutation of the +<a name="page22"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page23"></a> +The angles above the horizon at which the cotyledons of another seedling stood +at different hours is recorded in the following short table:— +</p> + +<p> +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. +</p> + +<p> +The position of the two cotyledons was roughly sketched at various hours with +the same general result. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page24"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Oxalis Valdiviana.—This species is interesting, as the +<a name="page25"></a> +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 +<a name="page26"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page27"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page28"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page29"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page30"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page31"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page32"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page33"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page34"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Cassia tora<a href="#fn1.1" name="fnref1.1"><sup>[1]</sup></a> +(Leguminosae).—A seedling was placed before a +<a name="page35"></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. +</p> + +<p class="footnote"> +<a name="fn1.1"></a> <a href="#fnref1.1">[1]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page36"></a> +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. +</p> + +<p> +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 +<a name="page37"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page38"></a> +move both up and down and laterally; but they do not rise up at night in a +conspicuous manner. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page39"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page40"></a> +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. +</p> + +<p> +Fig. 27. Cucurbita ovifera: tracks left by tips of radicles in growing +downwards over smoked glass-plates, inclined at 70° to the horizon. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page41"></a> +the other leg adjoining the radicle likewise circumnutated at an equally early +age. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page42"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page43"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Cucumis dudaim (Cucurbitaceæ).—Two seedlings had opened +<a name="page44"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page45"></a> +filament was, as far as could be ascertained, about .14 of an inch. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page46"></a> +on the following day with nearly the same result; and there can be no doubt +about the circumnutation of the hypocotyl. +</p> + +<p> +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. +</p> + +<p> +Cyclamen Persicum (Primulaceae).—This plant is generally supposed to +produce only a single cotyledon, but Dr. H. Gressner<a href="#fn1.2" +name="fnref1.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn1.2"></a> <a href="#fnref1.2">[2]</a> +‘Bot. Zeitung,’ 1874, p. 837. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page47"></a> +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. +</p> + +<p> +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.<a +href="#fn1.3" name="fnref1.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn1.3"></a> <a href="#fnref1.3">[3]</a> +‘Movements and Habits of Climbing Plants,’ p. 33, 1875. +</p> + +<p> +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 +<a name="page48"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page49"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page50"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page51"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page52"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page53"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page54"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page55"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page56"></a> +this was in parts straight and in parts decidedly zigzag, indicating +circumnutation. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page57"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page58"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page59"></a> +by us of the part that first breaks through the ground not being arched. +</p> + +<p> +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. +</p> + +<p> +Allium cepa (Liliaceae).—The narrow green leaf, which protrudes from the +seed of the common onion as a cotyledon,<a href="#fn1.4" +name="fnref1.4"><sup>[4]</sup></a> 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,<a href="#fn1.5" name="fnref1.5"><sup>[5]</sup></a> and answers the same +end as the knife-like white crest on the summit of the straight cotyledon of +the Gramineæ. +<a name="page60"></a> +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. +</p> + +<p class="footnote"> +<a name="fn1.4"></a> <a href="#fnref1.4">[4]</a> +This is the expression used by Sachs in his ‘Text-book of Botany.’ +</p> + +<p class="footnote"> +<a name="fn1.5"></a> <a href="#fnref1.5">[5]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page61"></a> +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. +</p> + +<p> +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 +<a name="page62"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn1.6" +name="fnref1.6"><sup>[6]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn1.6"></a> <a href="#fnref1.6">[6]</a> +We are indebted to the Rev. G. Henslow for an abstract of the views which have +been held on this subject, together with references. +</p> + +<p> +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 +<a name="page63"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page64"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page65"></a> +concluded that they were circumnutating; but we thought it advisable to make +the tracing above given. +</p> + +<p> +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. +</p> + +<p> +Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip of radicle +in growing downwards. +</p> + +<p> +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. +</p> + +<p> +<a name="page66"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Fig. 54. Selaginella Kraussii (?): circumnutation of young plant, kept in +darkness, traced from 8.45 A.M. to 10 P.M. Oct. 31st. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0002"></a> +<a name="page67"></a> +CHAPTER II.<br /> +GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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.<a href="#fn2.1" name="fnref2.1"><sup>[1]</sup></a> 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 +<a name="page68"></a> +types amongst plants, were continually circumnutating, we may infer that this +kind of movement is common to every seedling species. +</p> + +<p class="footnote"> +<a name="fn2.1"></a> <a href="#fnref2.1">[1]</a> +As given in the ‘General System of Botany,’ by Le Maout and Decaisne, 1873. +</p> + +<p> +SUB-KINGDOM I.—Phaenogamous Plants. +</p> + +<p> +Class I.—DICOTYLEDONS. +</p> + +<p> +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 +</p> + +<p> +Sub-class II.—Gymnosperms. 223. Coniferæ. 224. Cycadeæ. +</p> + +<p> +Class II.—MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae. XI. +LILIALES. 41. Asparageae. DITTO 55. Gramineæ. XV. GLUMALES. +</p> + +<p> +SUB-KINGDOM II.—Cryptogamic Plants. +</p> + +<p> +1. Filices. I. FILICALES. 6. Lycopodiaceæ. DITTO +</p> + +<p> +<a name="page69"></a> +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<a href="#fn2.2" +name="fnref2.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.2"></a> <a href="#fnref2.2">[2]</a> +‘Physiologie Végétale,’ 1868, pp. 199, 205. +</p> + +<p> +The tip of the radicle, as soon as it protrudes from the seed-coats, begins to +circumnutate, and the whole +<a name="page70"></a> +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,<a href="#fn2.3" name="fnref2.3"><sup>[3]</sup></a> 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 +<a name="page71"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.3"></a> <a href="#fnref2.3">[3]</a> +‘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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page72"></a> +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 <i>Phaseolus multiflorus</i> 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.<a href="#fn2.4" name="fnref2.4"><sup>[4]</sup></a> +</p> + +<p class="footnote"> +<a name="fn2.4"></a> <a href="#fnref2.4">[4]</a> +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. +</p> + +<p> +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<a href="#fn2.5" name="fnref2.5"><sup>[5]</sup></a> has shown that whilst +thus bending, the growth of the lower surface is greatly retarded, whilst that +<a name="page73"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.5"></a> <a href="#fnref2.5">[5]</a> +‘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. +</p> + +<p> +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 +<a name="page74"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page75"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page76"></a> +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. +</p> + +<p> +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 +<a name="page77"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Cyclamen does not produce any distinct stem, and only a single cotyledon +appears at first;<a href="#fn2.6" name="fnref2.6"><sup>[6]</sup></a> its +petiole +<a name="page78"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.6"></a> <a href="#fnref2.6">[6]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page79"></a> +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 +<a name="page80"></a> +either had proved decidedly advantageous or disadvantageous, one of them no +doubt would soon have prevailed. +</p> + +<p> +Asa Gray has described<a href="#fn2.7" name="fnref2.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.7"></a> <a href="#fnref2.7">[7]</a> +‘Botanical Text-Book,’ 1879, p. 22. +</p> + +<p> +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. +</p> + +<p> +<a name="page81"></a> +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,<a href="#fn2.8" name="fnref2.8"><sup>[8]</sup></a> 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 +<a name="page82"></a> +.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. +</p> + +<p class="footnote"> +<a name="fn2.8"></a> <a href="#fnref2.8">[8]</a> +‘American Journal of Science,’ vol. xiv. 1877, p. 21. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page83"></a> +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. +</p> + +<p> +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 +<a name="page84"></a> +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. +</p> + +<p> +The following case is interesting in connection with +<a name="page85"></a> +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. +</p> + +<p> +In Quercus virens, according to Dr. Engelmann,<a href="#fn2.9" +name="fnref2.9"><sup>[9]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.9"></a> <a href="#fnref2.9">[9]</a> +‘Transact. St. Louis Acad. Science,’ vol. iv. p. 190. +</p> + +<p> +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;<a href="#fn2.10" name="fnref2.10"><sup>[10]</sup></a> so does the +<a name="page86"></a> +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, +<a name="page87"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.10"></a> <a href="#fnref2.10">[10]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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<a +href="#fn2.11" name="fnref2.11"><sup>[11]</sup></a> 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 +<a name="page88"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.11"></a> <a href="#fnref2.11">[11]</a> +‘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. +</p> + +<p> +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 +<a name="page89"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page90"></a> +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. +</p> + +<p> +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 +<a name="page91"></a> +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. +</p> + +<p> +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.<a href="#fn2.12" +name="fnref2.12"><sup>[12]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.12"></a> <a href="#fnref2.12">[12]</a> +‘Arbeiten des bot. Instit. Würzburg,’ vol. i. 1873, p. 403. +</p> + +<p> +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 +<a name="page92"></a> +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 +<a name="page93"></a> +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,<a href="#fn2.13" name="fnref2.13"><sup>[13]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.13"></a> <a href="#fnref2.13">[13]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page94"></a> +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. +</p> + +<p> +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. +<a name="page95"></a> +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<a href="#fn2.14" +name="fnref2.14"><sup>[14]</sup></a> In all these seedlings the hypocotyl was +enlarged or swollen. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p class="footnote"> +<a name="fn2.14"></a> <a href="#fnref2.14">[14]</a> +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. +</p> + +<p> +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 +<a name="page96"></a> +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. +</p> + +<p> +In <i>Cyclamen Persicum</i> the hypocotyl, even whilst still within the seed, +is enlarged into a regular corm,<a href="#fn2.15" +name="fnref2.15"><sup>[15]</sup></a> and only a single cotyledon is at first +developed (see former Fig. 57). With <i>Ranunculus ficaria</i> two cotyledons +are never produced, and here one of the secondary radicles is developed at an +early age into a so-called bulb.<a href="#fn2.16" +name="fnref2.16"><sup>[16]</sup></a> Again, certain species of Chaerophyllum +and Corydalis produce only a single cotyledon;<a href="#fn2.17" +name="fnref2.17"><sup>[17]</sup></a> in the former the hypocotyl, and in the +latter the radicle is enlarged, according to Irmisch, into a bulb. +</p> + +<p class="footnote"> +<a name="fn2.15"></a> <a href="#fnref2.15">[15]</a> +Dr. H. Gressner, ‘Bot. Zeitung,’ 1874, p. 824. +</p> + +<p class="footnote"> +<a name="fn2.16"></a> <a href="#fnref2.16">[16]</a> +Irmisch, ‘Beiträge zur Morphologie der Pflanzen,’ 1854, pp. 11, 12; ‘Bot. +Zeitung,’ 1874, p. 805. +</p> + +<p class="footnote"> +<a name="fn2.17"></a> <a href="#fnref2.17">[17]</a> +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. +</p> + +<p> +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 +<a name="page97"></a> +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. +</p> + +<p> +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 +<a name="page98"></a> +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.” +</p> + +<p> +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 +<a name="page99"></a> +height upwards the basal part ceases to circumnutate, whilst the upper part +continues to do so. +</p> + +<p> +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<a href="#fn2.18" +name="fnref2.18"><sup>[18]</sup></a> 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 +<a name="page100"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.18"></a> <a href="#fnref2.18">[18]</a> +‘Die undulirende Nutation der Internodien,’ Akad. der Wissench. (Vienna), +Jan. 17th, 1878. Also published separately, see p. 32. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page101"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Nevertheless, in some few cases, especially with the +<a name="page102"></a> +Cucurbitaceæ, the seed-coats are ruptured by a curious contrivance, described +by M. Flahault.<a href="#fn2.19" name="fnref2.19"><sup>[19]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.19"></a> <a href="#fnref2.19">[19]</a> +‘Bull. Soc. Bot. de France,’ tom. xxiv. 1877, p. 201. +</p> + +<p> +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 +<a name="page103"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page104"></a> +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. +</p> + +<p> +<a name="page105"></a> +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.<a href="#fn2.20" +name="fnref2.20"><sup>[20]</sup></a> +</p> + +<p class="footnote"> +<a name="fn2.20"></a> <a href="#fnref2.20">[20]</a> +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. +</p> + +<p> +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 +<a name="page106"></a> +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. +</p> + +<p class="p2"> +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 +<a name="page107"></a> +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. +</p> + +<p> +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. +</p> + +<p> +As most seedling plants before the development of true leaves are of low, +sometimes very low stature, +<a name="page108"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn2.21" name="fnref2.21"><sup>[21]</sup></a> He shows that +<a name="page109"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.21"></a> <a href="#fnref2.21">[21]</a> +‘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. +</p> + +<p> +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. +</p> + +<p> +Although the movements of cotyledons were generally in nearly the same vertical +plane, yet their upward and downward courses never exactly +<a name="page110"></a> +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,<a href="#fn2.22" name="fnref2.22"><sup>[22]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.22"></a> <a href="#fnref2.22">[22]</a> +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. +</p> + +<p> +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 +<a name="page111"></a> +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. +</p> + +<p> +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 +<a name="page112"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page113"></a> +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,<a href="#fn2.23" name="fnref2.23"><sup>[23]</sup></a> 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.<a href="#fn2.24" name="fnref2.24"><sup>[24]</sup></a> As long as a leaf +provided with a pulvinus is young and continues to grow, its movement depends +on both these causes combined;<a href="#fn2.25" name="fnref2.25"><sup>[25]</sup></a> 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 +<a name="page114"></a> +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. +</p> + +<p class="footnote"> +<a name="fn2.23"></a> <a href="#fnref2.23">[23]</a> +‘Die Periodische Bewegungen der Blattorgane,’ 1875. +</p> + +<p class="footnote"> +<a name="fn2.24"></a> <a href="#fnref2.24">[24]</a> +Batalin, ‘Flora,’ Oct. 1st, 1873 +</p> + +<p class="footnote"> +<a name="fn2.25"></a> <a href="#fnref2.25">[25]</a> +Pfeffer, ibid. p. 5. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page"></a> +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. +</p> + +<p class="p2"> +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° +<a name="page116"></a> +above it. After 24 days from the first observation (begun after a true leaf had +been developed) the cotyledons ceased to rise at night. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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! +</p> + +<p> +Trifolium strictum.—After 17 days the cotyledons still rose at night, but +were not afterwards observed. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Cassia sp? (a large S. Brazilian tree raised from seeds sent us +<a name="page117"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +It is known<a href="#fn2.26" name="fnref2.26"><sup>[26]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.26"></a> <a href="#fnref2.26">[26]</a> +Pfeffer, ‘Die Period. Bewegungen,’ 1875, p. 157. +</p> + +<p> +Trifolium is a natural genus, and the leaves of all +<a name="page118"></a> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page119"></a> +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 +<a name="page120"></a> +same pulvinus and in different individuals. In the accompanying figures, A and +B (Fig. 64), we have views of the epidermis<a href="#fn2.27" name="fnref2.27"><sup>[27]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn2.27"></a> <a href="#fnref2.27">[27]</a> +Longitudinal sections show that the forms of the epidermic cells may be taken +as a fair representation of those constituting the pulvinus. +</p> + +<p> +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. +</p> + +<p> +In the following Table some measurements of the cells in fairly well-developed +pulvini of O. corniculata are given:— +</p> + +<p> +Seedling 1 day old, with cotyledon 2.3 mm. in length. Divisions of +Micrometer.<a href="#fn2.28" name="fnref2.28"><sup>[28]</sup></a> +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 +</p> + +<p> +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 +</p> + +<p> +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 +</p> + +<p> +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 +</p> + +<hr /> + +<p class="footnote"> +<a name="fn2.28"></a> <a href="#fnref2.28">[28]</a> +Each division equalled .003 mm. +</p> + +<p> +<a name="page121"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page122"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page123"></a> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page124"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page125"></a> +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. +</p> + +<p> +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 +<a name="page126"></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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page127"></a> +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. +</p> + +<p> +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.<a href="#fn2.29" name="fnref2.29"><sup>[29]</sup></a> +</p> + +<p class="footnote"> +<a name="fn2.29"></a> <a href="#fnref2.29">[29]</a> +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.” +</p> + +<p> +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. +</p> + +<p> +Finally, there seems to exist some relation between +<a name="page128"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0003"></a> +<a name="page129"></a> +CHAPTER III.<br /> +SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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 +<a name="page130"></a> +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. +</p> + +<p> +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 +<a name="page131"></a> +their curvature; and Sachs has shown<a href="#fn3.1" +name="fnref3.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn3.1"></a> <a href="#fnref3.1">[1]</a> +‘Arbeiten Bot. Inst. Würzburg,’ Heft iii. 1873, p. 398. +</p> + +<p> +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 +<a name="page132"></a> +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. +</p> + +<p class="p2"> +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 +<a name="page133"></a> +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. +</p> + +<p> +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 +<a name="page134"></a> +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 +<a name="page135"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +<a name="page136"></a> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(9.) Card fixed laterally: after 11 h. 30 m. no effect, the radicle being still +almost vertical. +</p> + +<p> +(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 +<a name="page137"></a> +angles to it. Radicle consequently partially deflected from Sachs’ curvature. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(14.) Case like the last in all respects, except that a length of only .25 of +an inch of the radicle was thus deflected. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(20.) Square of card affixed with shellac: after 24 h. no effect. +</p> + +<p> +(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. +</p> + +<p> +(23.) Square of card fixed with shellac to young radicle: after +<a name="page138"></a> +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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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°. +</p> + +<p> +(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 +<a name="page139"></a> +morning deflection reduced to about 40° from the perpendicular. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(33.) Square of card gummed to apex: after 15 h. deflected at nearly 90° from +the perpendicular and from the card. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +<a name="page140"></a> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p class="p2"> +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 +<a name="page141"></a> +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 +<a name="page142"></a> +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. +</p> + +<p> +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 +<a name="page143"></a> +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. +</p> + +<p> +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 +<a name="page144"></a> +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 +<a name="page145"></a> +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. +</p> + +<p> +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<a href="#fn3.2" +name="fnref3.2"><sup>[2]</sup></a> 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, +<a name="page146"></a> +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. +</p> + +<p class="footnote"> +<a name="fn3.2"></a> <a href="#fnref3.2">[2]</a> +‘Arbeiten Bot. Instit., Würzburg,’ Heft iii. p. 456. +</p> + +<p> +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 +<a name="page147"></a> +other in 8 h., within which time squares of card usually act; but after 24 h. +there was slight deflection. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page148"></a> +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. +</p> + +<p> +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: +<a name="page149"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page150"></a> +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. +</p> + +<p> +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 +<a name="page151"></a> +to bend over towards the injured side.<a href="#fn3.3" +name="fnref3.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn3.3"></a> <a href="#fnref3.3">[3]</a> +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. +</p> + +<p> +<i>The Power of an Irritant on the apex of the Radicle</i> +<a name="page152"></a> +<i>of the Bean, compared with that of Geotropism</i>.—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 +<a name="page153"></a> +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 +<a name="page154"></a> +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. +</p> + +<p> +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<a href="#fn3.4" name="fnref3.4"><sup>[4]</sup></a> +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 +<a name="page155"></a> +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<a href="#fn3.5" +name="fnref3.5"><sup>[5]</sup></a> the interesting fact that each individual +secondary radicle possesses its own peculiar constitution. +</p> + +<p class="footnote"> +<a name="fn3.4"></a> <a href="#fnref3.4">[4]</a> +‘Arbeiten Bot. Inst., Würzburg,’ Heft iv. 1874, p. 605–617. +</p> + +<p class="footnote"> +<a name="fn3.5"></a> <a href="#fnref3.5">[5]</a> +‘Arbeiten Bot. Instit., Würzburg,’ Heft, iv. 1874, p. 620. +</p> + +<p> +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<a href="#fn3.6" +name="fnref3.6"><sup>[6]</sup></a> 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,<a href="#fn3.7" +name="fnref3.7"><sup>[7]</sup></a> 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 +<a name="page156"></a> +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. +</p> + +<p class="footnote"> +<a name="fn3.6"></a> <a href="#fnref3.6">[6]</a> +Ibid. Heft iii. 1873, p. 437. +</p> + +<p class="footnote"> +<a name="fn3.7"></a> <a href="#fnref3.7">[7]</a> +‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn3.8" name="fnref3.8"><sup>[8]</sup></a> We +experimented on a variety +<a name="page157"></a> +(<i>Yorkshire Hero</i>) 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. +</p> + +<p class="footnote"> +<a name="fn3.8"></a> <a href="#fnref3.8">[8]</a> +Sachs, ‘Arbeiten Bot. Institut., Würzburg,’ Heft iii. p. 438. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page158"></a> +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. +</p> + +<p> +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 +<a name="page159"></a> +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. +</p> + +<p> +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. +</p> + +<p> +We now tried the effects of widely different temperatures on the sensitiveness +of these radicles with squares +<a name="page160"></a> +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. + +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page161"></a> +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 +<a name="page162"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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). +</p> + +<p> +An analogous trial was now made, but instead of attaching squares of card to +the lower sides of the +<a name="page163"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Phaseolus multiflorus: Sensitiveness of the apex of the +Radicle.—Fifty-nine radicles were tried with squares +<a name="page164"></a> +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. +</p> + +<p> +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 +<a name="page165"></a> +curvature extending for some little distance above the apex. +</p> + +<p> +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°. +</p> + +<p> +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 +<a name="page166"></a> +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. +</p> + +<p> +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 +<a name="page167"></a> +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. +</p> + +<p> +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 +<a name="page168"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page169"></a> +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. +</p> + +<p class="p2"> +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 +<a name="page170"></a> +of card, and 13 were not acted on. Rather large squares, though difficult to +affix, seemed more efficient than very small ones. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page171"></a> +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°, +<a name="page172"></a> +and a third very slightly from the perpendicular and from the cauterised side. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +(2.) In two days radicle slightly deflected; after seven days +<a name="page173"></a> +deflected 69° from the perpendicular and from the cauterised side; after eight +days the angle amounted to nearly 90°. +</p> + +<p> +(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°. +</p> + +<p> +(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. +</p> + +<p> +(5.) After two days very slight deflection; but this on the fourth day amounted +to 56° from the perpendicular and from the cauterised side. +</p> + +<p> +(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°. +</p> + +<p> +(7.) After two days slightly deflected; on the third day the deflection +amounted to 25° from the perpendicular, and this did not afterwards increase. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(10.) After one day slight deflection, which after six days increased to 50° +from the perpendicular and the cauterised side. +</p> + +<p> +(11.) After one day decided deflection, which after six days increased to 62° +from the perpendicular and from the cauterised side. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(15.) Here we had the anomalous case of a radicle bending +<a name="page174"></a> +slightly <i>towards</i> 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. +</p> + +<p class="p2"> +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. +</p> + +<p> +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 +<a name="page175"></a> +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. +</p> + +<p class="p2"> +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). +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +No. 5. Square attached on the 21st; on the 22nd decided +<a name="page176"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page177"></a> +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. +</p> + +<p> +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 +<a name="page178"></a> +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. +</p> + +<p> +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 +<a name="page179"></a> +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! +</p> + +<p> +Fig. 69. Zea mays: radicles excited to bend away from the little squares of +card attached to one side of their tips. +</p> + +<p> +Secondary Radicles of Zea.—A short time after the first radicle has +appeared, others protrude from the +<a name="page180"></a> +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. +</p> + +<h3>SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR.</h3> + +<p> +Sachs made the interesting discovery, a few years ago, that the radicles of +many seedling plants bend towards an adjoining damp surface.<a href="#fn3.9" name="fnref3.9"><sup>[9]</sup></a> 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 +<a name="page181"></a> +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. +</p> + +<p class="footnote"> +<a name="fn3.9"></a> <a href="#fnref3.9">[9]</a> +‘Arbeiten des Bot. Institut., in Würzburg,’ vol. i. 1872, p. 209. +</p> + +<p> +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 +<a name="page182"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page183"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Avena sativa.—The tips of 13 radicles, which projected between 2 and 4 +mm. from the bottom of the sieve, many of +<a name="page184"></a> +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. +</p> + +<p> +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 +<a name="page185"></a> +of the air may have caused the difference in the results at the two periods. +</p> + +<p class="p2"> +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 +<a name="page186"></a> +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. +</p> + +<p class="center"> +SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY THE DESTRUCTION OR INJURY +OF THE TERMINAL PART OF THE PRIMARY RADICLE. +</p> + +<p> +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<a href="#fn3.10" name="fnref3.10"><sup>[10]</sup></a> 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 +<a name="page187"></a> +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. +</p> + +<p class="footnote"> +<a name="fn3.10"></a> <a href="#fnref3.10">[10]</a> +‘Arbeiten Bot. Institut., Würzburg,’ Heft iv. 1874, p. 622. +</p> + +<p> +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 +<a name="page188"></a> +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. +</p> + +<p> +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<a +href="#fn3.11" name="fnref3.11"><sup>[11]</sup></a>, when the mycelium +penetrates a bud beginning to elongate, the shoot developed from it grows +vertically upwards. Such upright shoots +<a name="page189"></a> +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. +</p> + +<p class="footnote"> +<a name="fn3.11"></a> <a href="#fnref3.11">[11]</a> +See his valuable article in ‘Bot. Zeitung,’ 1867, p. 257, on these monstrous +growths, which are called in German “Hexenbesen,” or “witch-brooms.” +</p> + +<p> +According to Mr. Meehan,<a href="#fn3.12" name="fnref3.12"><sup>[12]</sup></a> +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.<a href="#fn3.13" +name="fnref3.13"><sup>[13]</sup></a> +</p> + +<p class="footnote"> +<a name="fn3.12"></a> <a href="#fnref3.12">[12]</a> +‘Proc. Acad. Nat. Sc. Philadelphia,’ June 16th, 1874, and July 23rd, 1875. +</p> + +<p class="footnote"> +<a name="fn3.13"></a> <a href="#fnref3.13">[13]</a> +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. +</p> + +<p> +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 +<a name="page190"></a> +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<a href="#fn3.14" name="fnref3.14"><sup>[14]</sup></a> 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 +<a name="page191"></a> +possible, or even probable, that this tendency to reversion may have been +increased, as it is manifestly of service to the plant. +</p> + +<p class="footnote"> +<a name="fn3.14"></a> <a href="#fnref3.14">[14]</a> +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. +</p> + +<h3>A SUMMARY OF CHAPTER.</h3> + +<p> +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 +<a name="page192"></a> +fact that they would not have been sensitive to somewhat greater continued +pressure, if this could have been applied. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page193"></a> +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. +</p> + +<p> +When the tip of a radicle is lightly touched on one side with dry nitrate of +silver, the injury caused is +<a name="page194"></a> +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. +</p> + +<p> +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. +<a name="page195"></a> +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. +</p> + +<p> +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 <i>Cucurbita ovifera</i>, +<a name="page196"></a> +when their tips were slightly cauterised on the lower side. +</p> + +<p> +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<a href="#fn3.15" +name="fnref3.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn3.15"></a> <a href="#fnref3.15">[15]</a> +‘Arbeiten Bot. Institut, Würzburg,’ Heft iv. 1874, pp. 605–631. +</p> + +<p> +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 +<a name="page197"></a> +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 +<a name="page198"></a> +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. +</p> + +<p> +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,<a href="#fn3.16" name="fnref3.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn3.16"></a> <a href="#fnref3.16">[16]</a> +‘Arbeiten Bot. Inst., Würzburg,’ Heft iii. p. 456. +</p> +<p> +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 +<a name="page199"></a> +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. +</p> + +<p> +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 +<a name="page200"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0004"></a> +<a name="page201"></a> +CHAPTER IV.<br /> +THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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 +<a name="page202"></a> +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. +</p> + +<p class="p2"> +(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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page203"></a> +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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb); (Geraniaceae, Fam. +47).—The species of this genus climb by the +<a name="page204"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page205"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page206"></a> +accompanying figure (Fig. 76) gives the necessary particulars, and shows that +the stem circumnutated, though rather slowly. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page207"></a> +extreme amount of movement during the time was very small, probably rather less +than the 1/20th of an inch. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page208"></a> +they were found to be still circumnutating, but on a yet smaller scale. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(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). +</p> + +<p> +(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 +<a name="page209"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +(15.) Aloysia citriodora (Verbenaceae, Fam. 173).—The following figure +(Fig. 81) gives the movements of a shoot during +<a name="page210"></a> +31 h. 40 m., and shows that it circumnutated. The bush was 15 inches in height. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page211"></a> +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.<a href="#fn4.1" +name="fnref4.1"><sup>[1]</sup></a> 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.” +</p> + +<p class="footnote"> +<a name="fn4.1"></a> <a href="#fnref4.1">[1]</a> +‘Comptes Rendus,’ April 30th, 1877. Also a second notice published separately +in Bourdeaux, Nov. 12th, 1877. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +<a name="page212"></a> +(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). +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +(20.) Cyperus alternifolius (Fam. Cyperaceae.)—A glass +<a name="page213"></a> +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. +</p> + +<p class="p2"> +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 +<a name="page214"></a> +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. +</p> + +<h3>CIRCUMNUTATION OF STOLONS OR RUNNERS.</h3> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page215"></a> +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;<a href="#fn4.2" name="fnref4.2"><sup>[2]</sup></a> nor had its +own weight caused it to bend downwards. +</p> + +<p class="footnote"> +<a name="fn4.2"></a> <a href="#fnref4.2">[2]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page216"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page217"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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<a href="#fn4.3" name="fnref4.3"><sup>[3]</sup></a> 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 +<a name="page218"></a> +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. +</p> + +<p class="footnote"> +<a name="fn4.3"></a> <a href="#fnref4.3">[3]</a> +‘Arbeiten Bot Inst., Würzburg,’ 1872, p. 434. +</p> + +<p> +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 +<a name="page219"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Cotyledon umbilicus (Crassulaceæ).—A plant growing in a pan +<a name="page220"></a> +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 +<a name="page221"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Another stolon, which resembled the last in almost every +<a name="page222"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<h3><a name="page223"></a>CIRCUMNUTATION OF FLOWER-STEMS.</h3> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page224"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page225"></a> +<i>Trifolium repens</i> circumnutate in a complicated course during several +days. I may add that the gynophore of <i>Arachis hypogoea</i>, which looks +exactly like a peduncle, circumnutates whilst growing vertically downwards, in +order to bury the young pod in the ground. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn4.4" name="fnref4.4"><sup>[4]</sup></a> +According to Sachs<a href="#fn4.5" name="fnref4.5"><sup>[5]</sup></a> the +flower-stems, whilst growing, +<a name="page226"></a> +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<a href="#fn4.6" name="fnref4.6"><sup>[6]</sup></a> the +spontaneous revolving movements of the flower-stems of an Alisma, which he +compares with those of a climbing plant. +</p> + +<p class="footnote"> +<a name="fn4.4"></a> <a href="#fnref4.4">[4]</a> +‘The Movements and Habits of Climbing Plants,’ 2nd edit., 1875, p. 68. +</p> + +<p class="footnote"> +<a name="fn4.5"></a> <a href="#fnref4.5">[5]</a> +‘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. +</p> + +<p class="footnote"> +<a name="fn4.6"></a> <a href="#fnref4.6">[6]</a> +‘Jenaische Zeitsch.,’ B. v. p. 133. +</p> + +<p> +We made no observations on the movements of the different parts of flowers. +Morren, however, has observed<a href="#fn4.7" +name="fnref4.7"><sup>[7]</sup></a> 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,<a href="#fn4.8" name="fnref4.8"><sup>[8]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn4.7"></a> <a href="#fnref4.7">[7]</a> +‘N. Mem. de l’Acad. R. de Bruxelles,’ tom. xiv. 1841, p. 3. +</p> + +<p class="footnote"> +<a name="fn4.8"></a> <a href="#fnref4.8">[8]</a> +‘Sitzungbericht des bot. Vereins der P. Brandenburg,’ xxi. p. 84. +</p> + +<p> +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. +</p> + +<h3>CIRCUMNUTATION OF LEAVES: DICOTYLEDONS.</h3> + +<p> +Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries, Batalin, +Millardet, etc., have +<a name="page227"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page228"></a> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page229"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(4.) Brassica oleracea (Cruciferae).—Hofmeister and Batalin<a +href="#fn4.9" name="fnref4.9"><sup>[9]</sup></a> 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 +<a name="page230"></a> +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. +</p> + +<p class="footnote"> +<a name="fn4.9"></a> <a href="#fnref4.9">[9]</a> +‘Flora,’ 1873, p. 437. +</p> + +<p> +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. +</p> + +<p> +We may add that, according to Mr. A. Stephen Wilson,<a href="#fn4.10" +name="fnref4.10"><sup>[10]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn4.10"></a> <a href="#fnref4.10">[10]</a> +‘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. +</p> + +<p> +(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).—The +<a name="page231"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page232"></a> +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. +</p> + +<p> +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. +</p> + +<p> +In the chapter on the Sleep of Plants we shall see that the leaves in several +Malvaceous genera sink +</p> + +<p> +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. +</p> + +<p> +at night; and as they often do not then occupy a vertical position, especially +if they have not been well illuminated during +<a name="page233"></a> +the day, it is doubtful whether some of these cases ought not to have been +included in the present chapter. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +<a name="page234"></a> +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. +</p> + +<p> +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. +</p> + +<p> +The main petiole was now secured to a stick close to the base +<a name="page235"></a> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page236"></a> +(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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +<a name="page237"></a> +(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. +</p> + +<p> +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. +</p> + +<p> +(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:— +</p> + +<p> +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 ” +</p> + +<p class="noindent"> +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. +</p> + +<p> +(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 +<a name="page238"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page239"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page240"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page241"></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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +The previous observations relate to the movements of the whole leaf, but the +lobes move independently of the petiole, and +<a name="page242"></a> +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. +</p> + +<p> +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 +<a name="page243"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page244"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(17.) Dahlia (garden var.) (Compositæ, Fam. 122).—A fine young +<a name="page245"></a> +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 +<a name="page246"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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<a href="#fn4.11" +name="fnref4.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn4.11"></a> <a href="#fnref4.11">[11]</a> +‘The Movements and Habits of Climbing Plants,’ 1875, p. 118. +</p> + +<p> +(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. +</p> + +<p> +<a name="page247"></a> +(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 +<a name="page248"></a> +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. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +<a name="page249"></a> +(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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page250"></a> +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. +</p> + +<p> +(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.<a href="#fn4.12" name="fnref4.12"><sup>[12]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn4.12"></a> <a href="#fnref4.12">[12]</a> +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. +</p> +<p> +(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, +<a name="page251"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Pinus austriaca.—Two leaves, 3 inches in length, but not +<a name="page252"></a> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<h3>CIRCUMNUTATION OF LEAVES: MONOCOTYLEDONS.</h3> + +<p> +(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 +<a name="page253"></a> +part of the night. On the evening of the 11th it circumnutated on a small scale +for some time about the same spot. +</p> + +<p> +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. +</p> + +<p> +(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. +</p> + +<p> +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. +</p> + +<p> +(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 +<a name="page254"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page255"></a> +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. +</p> + +<p> +(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. +</p> + +<p> +(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. +</p> + +<p> +(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.<a href="#fn4.13" name="fnref4.13"><sup>[13]</sup></a> 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 +<a name="page256"></a> +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. +</p> + +<p class="footnote"> +<a name="fn4.13"></a> <a href="#fnref4.13">[13]</a> +‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327. +</p> + +<p> +(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 +<a name="page257"></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. +</p> + +<p> +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. +</p> + +<h3>CRYPTOGAMS.</h3> + +<p> +(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.<a href="#fn4.14" name="fnref4.14"><sup>[14]</sup></a> +</p> + +<p class="footnote"> +<a name="fn4.14"></a> <a href="#fnref4.14">[14]</a> +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.” +</p> + +<p> +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. +</p> + +<p> +<a name="page258"></a> +In the chapter on the Sleep of Plants the conspicuous circumnutation of +Marsilea quadrifoliata (Marsileaceae, Fam. 4) will be described. +</p> + +<p> +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. +</p> + +<p> +(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, +<a name="page259"></a> +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. +</p> + +<p> +Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9 A.M. Oct +25th to 8 A.M. 27th. +</p> + +<p> +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<a href="#fn4.15" +name="fnref4.15"><sup>[15]</sup></a> 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.<a href="#fn4.16" name="fnref4.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn4.15"></a> <a href="#fnref4.15">[15]</a> +‘Ueber die Bewegungen der Faden der <i>Spirogyra princeps:</i> Jahreshefte des +Vereins für vaterländische Naturkunde in Württemberg,’ 1874, p. 211. +</p> + +<p class="footnote"> +<a name="fn4.16"></a> <a href="#fnref4.16">[16]</a> +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.” +</p> + +<h3>CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES.</h3> + +<p> +The circumnutating movements of young leaves in 33 genera, belonging to 25 +families, widely distributed +<a name="page260"></a> +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 +<a name="page261"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page262"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0005"></a> +<a name="page263"></a> +CHAPTER V.<br /> +MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page264"></a> +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. +</p> + +<h3>THE CIRCUMNUTATION OF CLIMBING PLANTS.</h3> + +<p> +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 +<a name="page265"></a> +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. +</p> + +<p> +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. +</p> + +<p> +We have said that the circumnutation of climbing plants differs from that of +ordinary plants chiefly by its greater amplitude. But most leaves circumnutate +<a name="page266"></a> +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;<a href="#fn5.1" name="fnref5.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn5.1"></a> <a href="#fnref5.1">[1]</a> +‘The Movements and Habits of Climbing Plants,’ p. 36. +</p> + +<p> +<a name="page267"></a> +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. +</p> + +<p class="p2"> +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. +</p> + +<h3>EPINASTY—HYPONASTY.</h3> + +<p> +The term epinasty is used by De Vries<a href="#fn5.2" +name="fnref5.2"><sup>[2]</sup></a> to express greater longitudinal growth along +the upper than +<a name="page268"></a> +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. +</p> + +<p class="footnote"> +<a name="fn5.2"></a> <a href="#fnref5.2">[2]</a> +‘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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page269"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page270"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page271"></a> +well-marked zigzags (one of them forming an angle of 112°), and this indicates +circumnutation. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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<a href="#fn5.3" name="fnref5.3"><sup>[3]</sup></a> 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 +<a name="page272"></a> +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. +</p> + +<p class="footnote"> +<a name="fn5.3"></a> <a href="#fnref5.3">[3]</a> +‘The Movements and Habits of Climbing Plants,’ 2nd edit. p. 13. +</p> + +<p> +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<a href="#fn5.4" +name="fnref5.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn5.4"></a> <a href="#fnref5.4">[4]</a> +‘Sitzb. der k. Akad. der Wissensch.,’ Vienna, Jan. 1880, p. 16. +</p> + +<p> +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 +<a name="page273"></a> +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 +<a name="page274"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page275"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page276"></a> +and inclined positions of the parts of plants<a href="#fn5.5" +name="fnref5.5"><sup>[5]</sup></a> will see how difficult a subject this is, +and will feel no surprise at our expressing ourselves doubtfully in this and +other such cases. +</p> + +<p class="footnote"> +<a name="fn5.5"></a> <a href="#fnref5.5">[5]</a> +‘Ueber Orthotrope und Plagiotrope Pflanzentheile;’ ‘Arbeiten des Bot. Inst., +in Würzburg,’ Heft ii. 1879, p. 226. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page277"></a> +<a name="page278"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page279"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0006"></a> +<a name="page280"></a> +CHAPTER VI.<br /> +MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF +COTYLEDONS.</h2> + +<p class="letter"> +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. +</p> + +<p> +The so-called sleep of leaves is so conspicuous a phenomenon that it was +observed as early as the time of Pliny;<a href="#fn6.1" +name="fnref6.1"><sup>[1]</sup></a> 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 +<a name="page281"></a> +between the sleep of animals and that of plants,<a href="#fn6.2" +name="fnref6.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn6.1"></a> <a href="#fnref6.1">[1]</a> +Pfeffer has given a clear and interesting sketch of the history of this +subject in his ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, P. 163. +</p> + +<p class="footnote"> +<a name="fn6.2"></a> <a href="#fnref6.2">[2]</a> +Ch. Royer must, however, be excepted; see ‘Annales des Sc. Nat.’ (5th +series), Bot. vol. ix. 1868, p. 378. +</p> + +<p> +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. +</p> + +<p> +Leaves, when they go to sleep, move either upwards or downwards, or in the case +of the leaflets of +<a name="page282"></a> +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. +</p> + +<p> +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 +<a name="page283"></a> +great advantage from such remarkable powers of movement. +</p> + +<p> +The nyctitropic movements of leaves and cotyledons are effected in two ways,<a +href="#fn6.3" name="fnref6.3"><sup>[3]</sup></a> 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.<a +href="#fn6.4" name="fnref6.4"><sup>[4]</sup></a> But as it has been shown by De +Vries<a href="#fn6.5" name="fnref6.5"><sup>[5]</sup></a> 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 +<a name="page284"></a> +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;<a href="#fn6.6" +name="fnref6.6"><sup>[6]</sup></a> but this statement applies only to periodic +and nyctitropic movements as may be inferred from other cases given by Frank.<a +href="#fn6.7" name="fnref6.7"><sup>[7]</sup></a> 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.<a href="#fn6.8" name="fnref6.8"><sup>[8]</sup></a> 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,<a href="#fn6.9" +name="fnref6.9"><sup>[9]</sup></a> 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<a href="#fn6.10" +name="fnref6.10"><sup>[10]</sup></a> has remarked +<a name="page285"></a> +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. +</p> + +<p class="footnote"> +<a name="fn6.3"></a> <a href="#fnref6.3">[3]</a> +This distinction was first pointed out (according to Pfeffer, ‘Die +Periodischen Bewegungen der Blattorgane,’ 1875, p. 161) by Dassen in 1837. +</p> + +<p class="footnote"> +<a name="fn6.4"></a> <a href="#fnref6.4">[4]</a> +‘Flora,’ 1873, p. 433. +</p> + +<p class="footnote"> +<a name="fn6.5"></a> <a href="#fnref6.5">[5]</a> +‘Bot. Zeitung,’ 1879, Dec. 19th, p. 830. +</p> + +<p class="footnote"> +<a name="fn6.6"></a> <a href="#fnref6.6">[6]</a> +Pfeffer, ‘Die Period. Beweg. der Blattorgane.’ 1875, p. 159. +</p> + +<p class="footnote"> +<a name="fn6.7"></a> <a href="#fnref6.7">[7]</a> +‘Die Nat. Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 52 +</p> + +<p class="footnote"> +<a name="fn6.8"></a> <a href="#fnref6.8">[8]</a> +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. +</p> + +<p class="footnote"> +<a name="fn6.9"></a> <a href="#fnref6.9">[9]</a> +‘Loudon’s Gardener’s Mag.,’ vol. iv. 1828, p. 112. +</p> + +<p class="footnote"> +<a name="fn6.10"></a> <a href="#fnref6.10">[10]</a> +Mr. Rivers in ‘Gardener’s Chron.,’ 1866, p. 732 +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page286"></a> +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. +</p> + +<p> +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 +<a name="page287"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +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, +<a name="page288"></a> +and in the morning two of them were dead, whilst not a single other leaf on the +many plants was even injured. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page289"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn6.11" name="fnref6.11"><sup>[11]</sup></a> It was again +exposed on +<a name="page290"></a> +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. +</p> + +<p class="footnote"> +<a name="fn6.11"></a> <a href="#fnref6.11">[11]</a> +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. +</p> + +<p> +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 +<a name="page291"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page292"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page293"></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. +</p> + +<p> +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.<a href="#fn6.12" +name="fnref6.12"><sup>[12]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn6.12"></a> <a href="#fnref6.12">[12]</a> +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. +</p> + +<p class="p2"> +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 +<a name="page294"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page295"></a> +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. +</p> + +<p> +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 name="page296"></a> +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. +</p> + +<p> +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 +<a name="page297"></a> +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. +</p> + +<h3>NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.</h3> + +<p> +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,<a href="#fn6.13" +name="fnref6.13"><sup>[13]</sup></a> 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,<a href="#fn6.14" +name="fnref6.14"><sup>[14]</sup></a> 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 +<a name="page298"></a> +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. +</p> + +<p class="footnote"> +<a name="fn6.13"></a> <a href="#fnref6.13">[13]</a> +‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327. +</p> + +<p class="footnote"> +<a name="fn6.14"></a> <a href="#fnref6.14">[14]</a> +‘Adansonia,’ March 10th, 1869. +</p> + +<p> +There are, however, some other sources of doubt with +<a name="page299"></a> +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. +</p> + +<p> +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 +<a name="page300"></a> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +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. +<a name="page301"></a> +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). +</p> + +<p class="p2"> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page302"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page303"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page304"></a> +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 <i>Acanthosicyos horrida</i><a href="#fn6.15" +name="fnref6.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn6.15"></a> <a href="#fnref6.15">[15]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page305"></a> +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. +</p> + +<p> +Apium graveolens.—The cotyledons at noon were horizontal, and at 10 P.M. +stood at an angle of 61° above the horizon. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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° +<a name="page306"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page307"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page308"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page309"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page310"></a> +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 +<a name="page311"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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. +</p> + +<p> +<a name="page312"></a> +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 +<a name="page313"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page314"></a> +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. +</p> + +<p> +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 +<a name="page315"></a> +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. +</p> + +<p> +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 +<a name="page316"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0007"></a> +<a name="page317"></a> +CHAPTER VII.<br /> +MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.</h2> + +<p class="letter"> +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. +</p> + +<p> +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 +<a name="page318"></a> +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. +</p> + +<p> +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,<a href="#fn7.1" name="fnref7.1"><sup>[1]</sup></a> even if the air is +very dry the leaves of Impatiens and Malva are rendered motionless. Carl Kraus +has also lately insisted<a href="#fn7.2" name="fnref7.2"><sup>[2]</sup></a> 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,<a href="#fn7.3" name="fnref7.3"><sup>[3]</sup></a> +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 +<a name="page319"></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.1"></a> <a href="#fnref7.1">[1]</a> +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. +</p> + +<p class="footnote"> +<a name="fn7.2"></a> <a href="#fnref7.2">[2]</a> +‘Beiträge zur Kentniss der Bewegungen,’ etc., in ‘Flora,’ 1879, pp. 42, 43, 67, +etc. +</p> + +<p class="footnote"> +<a name="fn7.3"></a> <a href="#fnref7.3">[3]</a> +‘Annal. des Sc. Nat. Bot.’ (5th Series), ix. 1868, p. 366. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page320"></a> +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. +</p> + +<p class="center"> +[List of Genera, including species the leaves of which sleep. +</p> + +<h4>CLASS I. DICOTYLEDONS.</h4> + +<p> +Sub-class I. ANGIOSPERMS. +</p> + +<p> +Genus Family. +</p> + +<p> +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. ” ” +<a name="page321"></a> +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). ” +</p> + +<p> +Sub-class II. GYMNOSPERMS. Aies (Chatin). +</p> + +<h4>CLASS II. MONOCOTYLEDONS.</h4> + +<p> +Thalia. Cannaceae (21). Maranta. ” Colocasia. Aroideae (30). Strephium. +Gramineæ (55). +</p> + +<h4>CLASS III. ACOTYLEDONS.</h4> + +<p> +Marsilea. Marsileaceae (4). +</p> + +<p> +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. +</p> + +<p> +Sida (Malvaceae).—the nyctitropic movements of the leaves in this genus +are remarkable in some respects. Batalin informs +<a name="page322"></a> +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 +<a name="page323"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page324"></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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page325"></a> +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. +</p> + +<p> +Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B, diagram of +leaf asleep, also seen from vertically above. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page326"></a> +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<a href="#fn7.4" +name="fnref7.4"><sup>[4]</sup></a> the autonomous movements of the leaves of +this plant. +</p> + +<p class="footnote"> +<a name="fn7.4"></a> <a href="#fnref7.4">[4]</a> +Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period. Bewegungen,’ etc., +1875, p. 53. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page327"></a> +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. +</p> + +<p> +Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep; traced on +vertical glass during 3 h. 40 m. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page328"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page329"></a> +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 +<a name="page330"></a> +second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after which +hour the great evening fall commenced. +</p> + +<p> +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. +</p> + +<p> +Averrhoa bilimbi (Oxalidæ).—It has long been known,<a href="#fn7.5" +name="fnref7.5"><sup>[5]</sup></a> 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<a href="#fn7.6" name="fnref7.6"><sup>[6]</sup></a> 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 +<a name="page331"></a> +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 +<a name="page332"></a> +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.<a href="#fn7.7" name="fnref7.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn7.5"></a> <a href="#fnref7.5">[5]</a> +Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356. +</p> + +<p class="footnote"> +<a name="fn7.6"></a> <a href="#fnref7.6">[6]</a> +‘Journal Linn. Soc.,’ vol. xvi. 1877, p. 231. +</p> + +<p class="footnote"> +<a name="fn7.7"></a> <a href="#fnref7.7">[7]</a> +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. +</p> + +<p> +Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced. +</p> + +<p> +Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its evening +descent, when going to sleep. Temp. 78°–81° F. +</p> + +<p> +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. +</p> + +<p> +The effect of an increase of temperature in diffused light is +<a name="page333"></a> +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 +<a name="page334"></a> +<a name="page335"></a> +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. +</p> + +<p> +Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a change from +bright illumination to shade; temperature (broken line) remaining nearly the +same. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page336"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn7.8" name="fnref7.8"><sup>[8]</sup></a> 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, +<a name="page337"></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.8"></a> <a href="#fnref7.8">[8]</a> +‘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. +</p> + +<p> +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 +<a name="page338"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page339"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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; +<a name="page340"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page341"></a> +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. +</p> + +<p> +Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in daytime; B, +leaf asleep, seen laterally at night. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page342"></a> +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 +<a name="page343"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Little more need be said about the sleep of the species of Lupinus; several, +namely, L. polyphyllus, nanus, Menziesii, speciosus, +<a name="page344"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page345"></a> +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. +</p> + +<p> +Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep at night. +</p> + +<p> +Trigonella Cretica resembles a Melilotus in its sleep, which will be +immediately described. According to M. Royer,<a href="#fn7.9" +name="fnref7.9"><sup>[9]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn7.9"></a> <a href="#fnref7.9">[9]</a> +‘Annales des Sc. Nat. Bot.’ (5th series), ix. 1868, p. 368. +</p> + +<p> +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. +</p> + +<p> +The fact of the terminal leaflet twisting indifferently to either +<a name="page346"></a> +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 +<a name="page347"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page348"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page349"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at night. +</p> + +<p> +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. +<a name="page350"></a> +With T. Pannonicum the first true leaf was generally unifoliate, but sometimes +trifoliate, or again partially lobed and in an intermediate condition. +</p> + +<p> +Circumnutation.—Sachs described in 1863<a href="#fn7.10" +name="fnref7.10"><sup>[10]</sup></a> 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.<a href="#fn7.11" +name="fnref7.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn7.10"></a> <a href="#fnref7.10">[10]</a> +‘Flora,’ 1863, p. 497. +</p> + +<p class="footnote"> +<a name="fn7.11"></a> <a href="#fnref7.11">[11]</a> +‘Die Period. Bewegungen,’ 1875, pp. 35, 52. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page351"></a> +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, +<a name="page352"></a> +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. +</p> + +<p> +Fig 143. Trifolium resupinatum: circumnutation and nyctitropic movements of the +terminal leaflet during 24 hours. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page353"></a> +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. +</p> + +<p> +Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B, with +leaves asleep at night. SS, stipule-like leaflets. +</p> + +<p> +In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12 genera +have been observed by ourselves and +<a name="page354"></a> +others, but only in Robinia with any care. Psoralea acaulis raises its three +leaflets at night; whilst Amorpha fruticosa,<a href="#fn7.12" +name="fnref7.12"><sup>[12]</sup></a> Dalea alopecuroides, and Indigofera +tinctoria depress them. Ducharte<a href="#fn7.13" +name="fnref7.13"><sup>[13]</sup></a> 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,<a href="#fn7.14" +name="fnref7.14"><sup>[14]</sup></a> “abaisse les folioles qui par une +disposition bizarre sont inclinées dans la même feuille, les supérieures vers +<a name="page355"></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.12"></a> <a href="#fnref7.12">[12]</a> +Ducharte, ‘Eléments de Botanique’, 1867, p. 349. +</p> + +<p class="footnote"> +<a name="fn7.13"></a> <a href="#fnref7.13">[13]</a> +Ibid., p. 347. +</p> + +<p class="footnote"> +<a name="fn7.14"></a> <a href="#fnref7.14">[14]</a> +‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868. +</p> + +<p> +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 +<a name="page356"></a> +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. +</p> + +<p> +Fig. 146. Coronilla rosea: leaf asleep. +</p> + +<p> +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. +</p> + +<p> +Smithia Pfundii (Tribe 6).—The leaflets rise up vertically, and the main +petiole also rises considerably. +</p> + +<p> +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 +<a name="page357"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half natural +size. The minute stipules unusually large. +</p> + +<p> +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. +</p> + +<p> +<a name="page358"></a> +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<a href="#fn7.15" name="fnref7.15"><sup>[15]</sup></a> these +leaflets move through an angle of 8° in the course of from 10 to 30 seconds. +</p> + +<p class="footnote"> +<a name="fn7.15"></a> <a href="#fnref7.15">[15]</a> +‘Die Period. Beweg.,’ p. 35. +</p> + +<p> +Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves asleep. +Figures reduced. +</p> + +<p> +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. +<a name="page359"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page360"></a> +<a name="page361"></a> +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<a href="#fn7.16" name="fnref7.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn7.16"></a> <a href="#fnref7.16">[16]</a> +‘Die Period. Beweg.,’ p. 39. +</p> + +<p> +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. +</p> + +<p> +Sachs states that the leaflets do not move until the surrounding air is as high +as 71°–72° F., and this agrees with our +<a name="page362"></a> +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. +</p> + +<p> +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 +<a name="page363"></a> +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. +</p> + +<p> +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 +<a name="page364"></a> +subsequent appearance, may be attributed to reversion to more or less distant +progenitors.<a href="#fn7.17" name="fnref7.17"><sup>[17]</sup></a> +</p> + +<p class="footnote"> +<a name="fn7.17"></a> <a href="#fnref7.17">[17]</a> +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. +</p> + +<p> +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 +<a name="page365"></a> +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°. +</p> + +<p> +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 +<a name="page366"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Glycine hispida (Tribe 8).—The three leaflets sink vertically down at +night. +</p> + +<p> +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. +</p> + +<p> +Erythrina caffra.—A filament was fixed transversely across +<a name="page367"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page368"></a> +Apios tuberosa (Tribe 8).—The leaflets sink vertically down at night. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page369"></a> +Poinciana Gilliesii (both belonging to Tribe 13), the leaves behave in the same +manner. +</p> + +<p> +Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B, branch with +leaves asleep, reduced to two-thirds of natural scale. +</p> + +<p> +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<a href="#fn7.18" +name="fnref7.18"><sup>[18]</sup></a> 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 +<a name="page370"></a> +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 +<a name="page371"></a> +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°. +</p> + +<p class="footnote"> +<a name="fn7.18"></a> <a href="#fnref7.18">[18]</a> +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. +</p> + +<p> +Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at night. Both +figures copied from photographs. +</p> + +<p> +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; +<a name="page372"></a> +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. +</p> + +<p> +Fig. 155. Cassia pubescens: A, upper part of plant during the day; B, same +plant at night. Figures reduced from photographs. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page373"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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°. +<a name="page374"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page375"></a> +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. +</p> + +<p> +Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of main +petiole, traced during 34 h. 30 m. +</p> + +<p> +On two other occasions the movement of the main petiole +<a name="page376"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page377"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page378"></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, +<a name="page379"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Mimosa albida.<a href="#fn7.19" name="fnref7.19"><sup>[19]</sup></a>—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 +<a name="page380"></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.19"></a> <a href="#fnref7.19">[19]</a> +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.” +</p> + +<p> +Fig. 159. Mimosa albida: leaf seen from vertically above. +</p> + +<p> +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 +<a name="page381"></a> +(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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page382"></a> +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 +<a name="page383"></a> +represented five ellipses, with their longer axes differently directed. +</p> + +<p> +Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf at night. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Œnothera mollissima (Onagrarieae).—According to Linnæus (‘Somnus +Plantarum’), the leaves rise up vertically at night. +</p> + +<p> +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 +<a name="page384"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page385"></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. +</p> + +<p> +Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, and +asleep at night. Figures copied from photographs, and reduced. +</p> + +<p> +<a name="page386"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page387"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page388"></a> +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. +</p> + +<p> +Phyllanthus Niruri (Euphorbiaceae).—The leaflets of this plant sleep, as +described by Pfeffer,<a href="#fn7.20" name="fnref7.20"><sup>[20]</sup></a> 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 +<a name="page389"></a> +outwards. They are furnished as might have been expected from this complex kind +of movement, with a pulvinus. +</p> + +<p class="footnote"> +<a name="fn7.20"></a> <a href="#fnref7.20">[20]</a> +‘Die Period. Beweg.,’ p. 159. +</p> + +<h3>GYMNOSPERMS.</h3> + +<p> +Pinus Nordmanniana (Coniferæ).—M. Chatin states<a href="#fn7.21" +name="fnref7.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn7.21"></a> <a href="#fnref7.21">[21]</a> +‘Comptes Rendus,’ Jan. 1876, p. 171. +</p> + +<h3>MONOCOTYLEDONS.</h3> + +<p> +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. +</p> + +<p> +Maranta arundinacea (Cannaceae).—The blades of the leaves, which are +furnished with a pulvinus, stand horizontally during +<a name="page390"></a> +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. +</p> + +<p> +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 +<a name="page391"></a> +rise so much at night that they may almost be said to sleep. +</p> + +<p> +Strephium floribundum<a href="#fn7.22" name="fnref7.22"><sup>[22]</sup></a> +(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. +</p> + +<p class="footnote"> +<a name="fn7.22"></a> <a href="#fnref7.22">[22]</a> +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. +</p> + +<p> +Fig. 164. Strephium floribundum: culms with leaves during the day, and when +asleep at night. Figures reduced. +</p> + +<p> +The circumnutation of a young leaf (2.3 inches in length) was traced during 48 +h. (Fig. 165). The movement was remarkably simple; the leaf descended from +before 6.40 A.M. until 2 or 2.50 P.M., and then rose so as to stand vertically +at about 6 P.M., descending again late in the night or in the very early +morning. +<a name="page392"></a> +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. +</p> + +<p> +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. +</p> + +<h3>ACOTYLEDONS.</h3> + +<p> +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 +<a name="page393"></a> +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 +<a name="page394"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page395"></a> +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. +</p> + +<p> +The nyctitropic movements of leaves, leaflets, and +<a name="page396"></a> +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 +<a name="page397"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page398"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Although with sleeping plants the blades almost +<a name="page399"></a> +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. +</p> + +<p> +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 +<a name="page400"></a> +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. +</p> + +<p> +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 +<a name="page401"></a> +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. +</p> + +<p> +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 +<a name="page402"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page403"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page404"></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 +<a name="page405"></a> +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. +</p> + +<p> +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, +<a name="page406"></a> +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. +</p> + +<p> +With respect to the periodicity of the movements of sleeping leaves, Pfeffer<a +href="#fn7.23" name="fnref7.23"><sup>[23]</sup></a> has so clearly shown that +this depends on the daily alternations of light and darkness, that nothing +farther need be said on this +<a name="page407"></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.23"></a> <a href="#fnref7.23">[23]</a> +‘Die Periodischen Bewegungen der Blattorgane,’ 1875, p. 30, et passim. +</p> + +<p> +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.<a href="#fn7.24" name="fnref7.24"><sup>[24]</sup></a> The strength +of such inheritance differs +<a name="page408"></a> +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.<a href="#fn7.25" name="fnref7.25"><sup>[25]</sup></a> +</p> + +<p class="footnote"> +<a name="fn7.24"></a> <a href="#fnref7.24">[24]</a> +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. +</p> + +<p class="footnote"> +<a name="fn7.25"></a> <a href="#fnref7.25">[25]</a> +Pfeffer, ibid., p. 46. +</p> + +<p class="p2"> +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 +<a name="page409"></a> +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. +</p> + +<p> +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 +<a name="page410"></a> +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. +</p> + +<p> +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 +<a name="page411"></a> +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. +</p> + +<p> +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 +<a name="page412"></a> +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. +</p> + +<p> +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. +</p> + +<p> +From the facts and considerations now advanced we may conclude that +nyctitropism, or the sleep of leaves +<a name="page413"></a> +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. +</p> + +<p class="p2"> +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.<a href="#fn7.26" +name="fnref7.26"><sup>[26]</sup></a> We have also observed the same phenomenon +in Elymus arenareus. +</p> + +<p class="footnote"> +<a name="fn7.26"></a> <a href="#fnref7.26">[26]</a> +‘Annal. des Sc. Nat. (Bot.),’ 1875, tom. i. pp. 326–329. +</p> + +<p> +<a name="page414"></a> +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<a href="#fn7.27" name="fnref7.27"><sup>[27]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn7.27"></a> <a href="#fnref7.27">[27]</a> +‘Die Schutzmittel des Pollens,’ 1873, pp. 30–39. +</p> + +<p class="p2"> +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 +<a name="page415"></a> +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. +</p> + +<p> +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 +<a name="page416"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page417"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0008"></a> +<a name="page418"></a> +CHAPTER VIII.<br /> +MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.</h2> + +<p class="letter"> +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 +</p> + +<p> +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.<a href="#fn8.1" name="fnref8.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn8.1"></a> <a href="#fnref8.1">[1]</a> +‘Physiologie Veg.’ (French Translation), 1868, pp. 42, 517, etc. +</p> + +<p> +Apheliotropism, or, as usually designated, negative +<a name="page419"></a> +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. +</p> + +<p> +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 +<a name="page420"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page421"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page422"></a> +overcast with extraordinarily dark thunder-clouds, and it was interesting to +note how plainly the cotyledons circumnutated during this interval. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Apios graveolens.—The hypocotyl bends in a few hours +<a name="page423"></a> +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. +</p> + +<p> +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 +<a name="page424"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Brassica oleracea.—The hypocotyl of the cabbage, when not disturbed by a +lateral light, circumnutates in a complicated +<a name="page425"></a> +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 +<a name="page426"></a> +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, +<a name="page427"></a> +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. +</p> + +<p> +Fig. 172. Brassica oleracea: ordinary circumnutating movement of the hypocotyl +of a seedling plant. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page428"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page429"></a> +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 +<a name="page430"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page431"></a> +which, whilst young, is so extremely sensitive to light, worth giving. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page432"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page433"></a> +contact with the trunks of trees.<a href="#fn8.2" +name="fnref8.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn8.2"></a> <a href="#fnref8.2">[2]</a> +‘The Movements and Habits of Climbing Plants,’ 1875, p. 97. +</p> + +<p> +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.<a href="#fn8.3" name="fnref8.3"><sup>[3]</sup></a> 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 +<a name="page434"></a> +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. +</p> + +<p class="footnote"> +<a name="fn8.3"></a> <a href="#fnref8.3">[3]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page435"></a> +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. +</p> + +<p class="p2"> +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, +<a name="page436"></a> +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. +</p> + +<p> +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.<a href="#fn8.4" name="fnref8.4"><sup>[4]</sup></a> +<a name="page437"></a> +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. +</p> + +<p class="footnote"> +<a name="fn8.4"></a> <a href="#fnref8.4">[4]</a> +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. +</p> + +<p> +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. +</p> + +<p> +We are therefore fully justified, as it seems to us, in believing that whenever +light enters laterally, it is the +<a name="page438"></a> +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. +</p> + +<p> +<i>Transversal-heliotropismus</i> (<i>of Frank</i><a href="#fn8.5" +name="fnref8.5"><sup>[5]</sup></a>) <i>or Diaheliotropism</i>.—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 +<a name="page439"></a> +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 +<a name="page440"></a> +branches which had been fastened in various positions or turned upside down. +</p> + +<p class="footnote"> +<a name="fn8.5"></a> <a href="#fnref8.5">[5]</a> +‘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. +</p> + +<p> +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<a href="#fn8.6" +name="fnref8.6"><sup>[6]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn8.6"></a> <a href="#fnref8.6">[6]</a> +‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp. 223–277. +</p> + +<p> +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 +<a name="page441"></a> +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<a href="#fn8.7" name="fnref8.7"><sup>[7]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn8.7"></a> <a href="#fnref8.7">[7]</a> +See former note, in reference to Sachs’ remarks on this subject. +</p> + +<p> +We may therefore conclude that diaheliotropic movements cannot be fully +explained by the direct action of light, gravitation, weight, etc., any more +<a name="page442"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page443"></a> +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;<a href="#fn8.8" +name="fnref8.8"><sup>[8]</sup></a> 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 +<a name="page444"></a> +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. +</p> + +<p class="footnote"> +<a name="fn8.8"></a> <a href="#fnref8.8">[8]</a> +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.) +</p> + +<p> +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 +<a name="page445"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +It has long been known<a href="#fn8.9" name="fnref8.9"><sup>[9]</sup></a> 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 name="page446"></a> +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. +</p> + +<p class="footnote"> +<a name="fn8.9"></a> <a href="#fnref8.9">[9]</a> +Pfeffer gives the names and dates of several ancient writers in his ‘Die +Periodischen Bewegungen,’ 1875, p. 62. +</p> + +<p> +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<a href="#fn8.10" +name="fnref8.10"><sup>[10]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn8.10"></a> <a href="#fnref8.10">[10]</a> +‘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. +</p> + +<p> +In the cases above given, the leaflets move either upwards +<a name="page447"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page448"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0009"></a> +<a name="page449"></a> +CHAPTER IX.<br /> +SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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 +<a name="page450"></a> +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. +</p> + +<p> +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.<a href="#fn9.1" +name="fnref9.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn9.1"></a> <a href="#fnref9.1">[1]</a> +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. +</p> + +<p> +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 +<a name="page451"></a> +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. +</p> + +<p> +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.<a href="#fn9.2" +name="fnref9.2"><sup>[2]</sup></a> 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 +<a name="page452"></a> +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. +</p> + +<p class="footnote"> +<a name="fn9.2"></a> <a href="#fnref9.2">[2]</a> +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. +</p> + +<p> +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, +<a name="page453"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Most seedling plants are strongly heliotropic, and +<a name="page454"></a> +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. +</p> + +<p> +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 +<a name="page455"></a> +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. +</p> + +<p> +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.<a href="#fn9.3" name="fnref9.3"><sup>[3]</sup></a> +</p> + +<p class="footnote"> +<a name="fn9.3"></a> <a href="#fnref9.3">[3]</a> +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. +</p> + +<p> +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 +<a name="page456"></a> +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. +</p> + +<p> +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 +<a name="page457"></a> +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. +</p> + +<p> +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,<a href="#fn9.4" +name="fnref9.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn9.4"></a> <a href="#fnref9.4">[4]</a> +‘Sitz. der k. Akad. der Wissensch.’ (Vienna), Jan. 1880, p. 12. +</p> + +<p> +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. +<a name="page458"></a> +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 +<a name="page459"></a> +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. +</p> + +<p> +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 +<a name="page460"></a> +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. +</p> + +<p> +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. +</p> + +<p> +The curvature of the cotyledons of Phalaris towards a lateral light is +therefore certainly influenced by the +<a name="page461"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page462"></a> +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 +<a name="page463"></a> +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. +</p> + +<p> +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 +<a name="page464"></a> +light, continue to bend in the same direction for between a quarter and half an +hour. +</p> + +<p> +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 +<a name="page465"></a> +therefore conclude that the bending back of plants from a light, when this +becomes obscure or is extinguished, is wholly due to +apogeotropism.<a href="#fn9.5" name="fnref9.5"><sup>[5]</sup></a> +</p> + +<p class="footnote"> +<a name="fn9.5"></a> <a href="#fnref9.5">[5]</a> +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. +</p> + +<p> +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 +<a name="page466"></a> +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. +</p> + +<p> +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 +<a name="page467"></a> +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. +</p> + +<p> +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 +<a name="page468"></a> +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. +</p> + +<h3>LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS.</h3> + +<p> +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 +<a name="page469"></a> +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. +</p> + +<p> +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. +</p> + +<p class="p2"> +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 +<a name="page470"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 name="page471"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page472"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Better evidence of the efficiency of the blackened tubes was incidentally +afforded by some experiments hereafter to be given, +<a name="page473"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page474"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page475"></a> +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. +</p> + +<p> +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 +<a name="page476"></a> +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. +</p> + +<p> +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 +<a name="page477"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Avena sativa.—The cotyledons of this plant become quickly bowed towards a +lateral light, exactly like those of Phalaris. +<a name="page478"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page479"></a> +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. +</p> + +<p> +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 +<a name="page480"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page481"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page482"></a> +the case of Phalaris, whether the skin covered with a rather thick crust of dry +Indian ink did not mechanically prevent their curvature. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn9.6" name="fnref9.6"><sup>[6]</sup></a> +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 +<a name="page483"></a> +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. +</p> + +<p class="footnote"> +<a name="fn9.6"></a> <a href="#fnref9.6">[6]</a> +Sachs, ‘Physiologie Végétale,’ 1868, p. 44. +</p> + +<h3><a name="page484"></a>CONCLUDING REMARKS AND SUMMARY OF CHAPTER.</h3> + +<p> +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. +</p> + +<p> +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 +<a name="page485"></a> +lately such movements were believed to result simply from increased growth on +the shaded side. At present it is commonly admitted<a href="#fn9.7" +name="fnref9.7"><sup>[7]</sup></a> 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.<a href="#fn9.8" +name="fnref9.8"><sup>[8]</sup></a> 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, +<a name="page486"></a> +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. +</p> + +<p class="footnote"> +<a name="fn9.7"></a> <a href="#fnref9.7">[7]</a> +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. +</p> + +<p class="footnote"> +<a name="fn9.8"></a> <a href="#fnref9.8">[8]</a> +‘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. +</p> + +<p> +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.<a href="#fn9.9" +name="fnref9.9"><sup>[9]</sup></a> So it is with many aërial roots, according +to Wiesner;<a href="#fn9.10" name="fnref9.10"><sup>[10]</sup></a> but there are +other opposed cases. It appears, therefore, that light does not determine the +growth of apheliotropic parts in any uniform manner. +</p> + +<p class="footnote"> +<a name="fn9.9"></a> <a href="#fnref9.9">[9]</a> +Francis Darwin, ‘Über das Wachsthum negativ heliotropischer Wurzeln’: +‘Arbeiten des Bot. Inst. in Würzburg,’ B. ii., Heft iii., 1880, p. 521. +</p> + +<p class="footnote"> +<a name="fn9.10"></a> <a href="#fnref9.10">[10]</a> +‘Sitzb. der k. Akad. der Wissensch’ (Vienna), 1880, p. 12. +</p> + +<p> +We should bear in mind that the power of bending to the light is highly +beneficial to most plants. There +<a name="page487"></a> +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.<a href="#fn9.11" +name="fnref9.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn9.11"></a> <a href="#fnref9.11">[11]</a> +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. +</p> + +<p> +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 +<a name="page488"></a> +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.<a href="#fn9.12" +name="fnref9.12"><sup>[12]</sup></a> 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 +<a name="page489"></a> +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. +</p> + +<p class="footnote"> +<a name="fn9.12"></a> <a href="#fnref9.12">[12]</a> +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. +</p> + +<p> +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<a href="#fn9.13" name="fnref9.13"><sup>[13]</sup></a>—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 +<a name="page490"></a> +up, and that nearest to the light sinking down, or both twisting laterally.<a +href="#fn9.14" name="fnref9.14"><sup>[14]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn9.13"></a> <a href="#fnref9.13">[13]</a> +Francis Darwin, ‘On the Hygroscopic Mechanism,’ etc., ‘Transactions Linn. +Soc.,’ series ii. vol. i. p. 149, 1876. +</p> + +<p class="footnote"> +<a name="fn9.14"></a> <a href="#fnref9.14">[14]</a> +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. +</p> + +<p> +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 +<a name="page491"></a> +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. +</p> + +<p> +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 +<a name="page492"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0010"></a> +<a name="page493"></a> +CHAPTER X.<br /> +MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.</h2> + +<p class="letter"> +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 +</p> + +<p> +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. +</p> + +<p> +When observing the gradations between +<a name="page494"></a> +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. +</p> + +<p class="p2"> +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, +<a name="page495"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page496"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page497"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +We will now describe a few cases in which it may be +<a name="page498"></a> +seen how gradually circumnutation becomes changed into apogeotropism, under +circumstances to be specified in each instance. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Lilium auratum.—A plant 23 inches in height was placed +<a name="page499"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page500"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page501"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page502"></a> +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. +</p> + +<p> +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.<a href="#fn10.1" +name="fnref10.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn10.1"></a> <a href="#fnref10.1">[1]</a> +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. +</p> + +<p> +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 +<a name="page503"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page504"></a> +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. +</p> + +<p> +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, +<a name="page505"></a> +the upward curvature cannot be due to heliotropism or hyponasty, but to +apogeotropism. +</p> + +<p> +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. +</p> + +<p> +<a name="page506"></a> +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. +</p> + +<p> +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. +</p> + +<p> +Oxalis acetosella.—The peduncles are furnished with a joint in the +middle, so that the lower part answers to the main peduncle, +<a name="page507"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page508"></a> +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. +</p> + +<p> +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 +<a name="page509"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page510"></a> +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.<a href="#fn10.2" +name="fnref10.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn10.2"></a> <a href="#fnref10.2">[2]</a> +For details see ‘The Movements and Habits of Climbing Plants,’ 1875, p. 131. +</p> + +<p> +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, +<a name="page511"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page512"></a> +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. +</p> + +<h3>GEOTROPISM.</h3> + +<p> +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. +</p> + +<p> +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 +<a name="page513"></a> +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. +</p> + +<p class="p2"> +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<a +href="#fn10.3" name="fnref10.3"><sup>[3]</sup></a> 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 +<a name="page514"></a> +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. +</p> + +<p class="footnote"> +<a name="fn10.3"></a> <a href="#fnref10.3">[3]</a> +‘Hist. Phys. des Plantes d’Europe,’ tom. ii. 1841, p. 106. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page515"></a> +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. +</p> + +<p> +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 +<a name="page516"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page517"></a> +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. +</p> + +<p> +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;<a href="#fn10.4" name="fnref10.4"><sup>[4]</sup></a> 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,<a href="#fn10.5" +name="fnref10.5"><sup>[5]</sup></a> 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, +<a name="page518"></a> +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. +</p> + +<p class="footnote"> +<a name="fn10.4"></a> <a href="#fnref10.4">[4]</a> +Vaucher, ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. p. 110. +</p> + +<p class="footnote"> +<a name="fn10.5"></a> <a href="#fnref10.5">[5]</a> +See his interesting article in ‘Nature,’ April 4th, 1878, p. 446. +</p> + +<p> +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<a href="#fn10.6" name="fnref10.6"><sup>[6]</sup></a> never to +produce pods. +</p> + +<p class="footnote"> +<a name="fn10.6"></a> <a href="#fnref10.6">[6]</a> +‘Gard. Chronicle,’ 1857, p. 566. +</p> + +<p> +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 +<a name="page519"></a> +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 +<a name="page520"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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<a href="#fn10.7" +name="fnref10.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn10.7"></a> <a href="#fnref10.7">[7]</a> +‘Manual of the Botany of the Northern United States,’ 1856, p. 106. +</p> + +<h3>DIAGEOTROPISM.</h3> + +<p> +Besides geotropism and apogeotropism, there is, according to Frank, an allied +form of movement, +<a name="page521"></a> +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.<a href="#fn10.8" name="fnref10.8"><sup>[8]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn10.8"></a> <a href="#fnref10.8">[8]</a> +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. +</p> + +<p class="p2"> +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, +<a name="page522"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0011"></a> +<a name="page523"></a> +CHAPTER XI.<br /> +LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.</h2> + +<p class="letter"> +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. +</p> + +<p> +Ciesielski states<a href="#fn11.1" name="fnref11.1"><sup>[1]</sup></a> 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 +<a name="page524"></a> +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.<a href="#fn11.2" +name="fnref11.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn11.1"></a> <a href="#fnref11.1">[1]</a> +‘Abwartskrümmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p. 29. +</p> + +<p class="footnote"> +<a name="fn11.2"></a> <a href="#fnref11.2">[2]</a> +‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. iii. 1873, p. 432. +</p> + +<p class="p2"> +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. +</p> + +<p> +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 +<a name="page525"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page526"></a> +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. +</p> + +<p> +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 +<a name="page527"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page528"></a> +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. +</p> + +<p> +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 +<a name="page529"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page530"></a> +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. +</p> + +<p> +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 +<a name="page531"></a> +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. +</p> + +<p> +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 +<a name="page532"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page533"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page534"></a> +takes place; and that when the tip of the root is cauterised it is unable to +originate the stimulus necessary to produce geotropic curvature. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page535"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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, +<a name="page536"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page537"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +<a name="page538"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page539"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page540"></a> +the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3 mm. +</p> + +<p> +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.] +</p> + +<p class="p2"> +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 +<a name="page541"></a> +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. +</p> + +<p> +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. +<a name="page542"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page543"></a> +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. +</p> + +<p> +As the tip of the radicle has been found to be the +<a name="page544"></a> +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. +</p> + +<p> +Finally, the fact of the tip alone being sensitive to +<a name="page545"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0012"></a> +<a name="page546"></a> +CHAPTER XII.<br /> +CONCLUDING REMARKS.</h2> + +<p class="letter"> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page547"></a> +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? +</p> + +<p> +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 +<a name="page548"></a> +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. +</p> + +<p class="p2"> +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 +<a name="page549"></a> +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. +</p> + +<p> +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 +<a name="page550"></a> +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. +</p> + +<p> +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 +<a name="page551"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +Again, most or all radicles are slightly sensitive to light, and according to +Wiesner, generally bend a little +<a name="page552"></a> +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.<a href="#fn12.1" +name="fnref12.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn12.1"></a> <a href="#fnref12.1">[1]</a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page553"></a> +arrangement of the three orders of roots is excellently adapted for searching +the whole soil for nutriment. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page554"></a> +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. +</p> + +<p> +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 +<a name="page555"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page556"></a> +the upper part of the hypocotyls of at least some plants. +</p> + +<p> +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. +</p> + +<p> +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 <i>Megarrhiza Californica</i>, +<a name="page557"></a> +<i>Ipomœa leptophylla</i> and <i>pandurata</i>, and of <i>Quercus virens</i>, +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page558"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page559"></a> +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. +</p> + +<p> +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. +</p> + +<p> +We now come to our second group of +<a name="page560"></a> +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. +</p> + +<p> +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 +<a name="page561"></a> +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. +</p> + +<p> +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 +<a name="page562"></a> +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. +</p> + +<p> +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 +<a name="page563"></a> +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. +</p> + +<p> +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. +</p> + +<p> +<a name="page564"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page565"></a> +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. +</p> + +<p> +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 +<a name="page566"></a> +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. +</p> + +<p> +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 +<a name="page567"></a> +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. +</p> + +<p> +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. +</p> + +<p> +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 +<a name="page568"></a> +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. +</p> + +<p> +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 +<a name="page569"></a> +re-formed; and this was then acted on by geotropism, and the radicle became +curved perpendicularly downwards. +</p> + +<p class="p2"> +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 +<a name="page570"></a> +opposite direction might gradually be acquired through natural +selection.<a href="#fn12.2" name="fnref12.2"><sup>[2]</sup></a> +</p> + +<p class="footnote"> +<a name="fn12.2"></a> <a href="#fnref12.2">[2]</a> +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. +</p> + +<p> +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 +<a name="page571"></a> +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.<a href="#fn12.3" name="fnref12.3"><sup>[3]</sup></a> +</p> + +<p class="footnote"> +<a name="fn12.3"></a> <a href="#fnref12.3">[3]</a> +For the evidence on this head, see the ‘Movements and Habits of Climbing +Plants,’ 1875, pp. 173, 174. +</p> + +<p> +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. +</p> + +<p> +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.<a href="#fn12.4" name="fnref12.4"><sup>[4]</sup></a> With +plants an +<a name="page572"></a> +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. +</p> + +<p class="footnote"> +<a name="fn12.4"></a> <a href="#fnref12.4">[4]</a> +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). +</p> + +<p> +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 +<a name="page573"></a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page574"></a><a name="link2HCH0013"></a>INDEX.</h2> + +<pre xml:space="preserve"> + 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. +</pre> + +</div><!--end chapter--> + +<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE POWER OF MOVEMENT IN PLANTS ***</div> +<div style='text-align:left'> + +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +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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