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+The Project Gutenberg eBook of The Power of Movement in Plants, by Charles Darwin
+
+This eBook is for the use of anyone anywhere in the United States and
+most other parts of the world at no cost and with almost no restrictions
+whatsoever. You may copy it, give it away or re-use it under the terms
+of the Project Gutenberg License included with this eBook or online at
+www.gutenberg.org. If you are not located in the United States, you
+will have to check the laws of the country where you are located before
+using this eBook.
+
+Title: The Power of Movement in Plants
+
+Author: Charles Darwin
+
+Release Date: August 14, 2002 [eBook #5605]
+[Most recently updated: September 11, 2022]
+
+Language: English
+
+Character set encoding: UTF-8
+
+Produced by: Sue Asscher
+
+*** START OF THE PROJECT GUTENBERG EBOOK THE POWER OF MOVEMENT IN PLANTS ***
+
+
+
+
+THE POWER OF MOVEMENT IN PLANTS
+
+By Charles Darwin
+
+Assisted By Francis Darwin
+
+
+CONTENTS
+
+ DETAILED TABLE OF CONTENTS.
+ THE MOVEMENTS OF PLANTS.
+ INTRODUCTION.
+ CHAPTER I. THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
+ CHAPTER II. GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
+ CHAPTER III. SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.
+ CHAPTER IV. THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.
+ CHAPTER V. MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.
+ CHAPTER VI. MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS.
+ CHAPTER VII. MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
+ CHAPTER VIII. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
+ CHAPTER IX. SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
+ CHAPTER X. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
+ CHAPTER XI. LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
+ CHAPTER XII. CONCLUDING REMARKS.
+ INDEX
+
+
+
+
+DETAILED TABLE OF CONTENTS.
+
+
+CHAPTER I.—THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
+Brassica oleracea, circumnutation of the radicle, of the arched
+hypocotyl whilst still buried beneath the ground, whilst rising above
+the ground and straightening itself, and when erect—Circumnutation of
+the cotyledons—Rate of movement—Analogous observations on various
+organs in species of Githago, Gossypium, Oxalis, Tropaeolum, Citrus,
+Æsculus, of several Leguminous and Cucurbitaceous genera, Opuntia,
+Helianthus, Primula, Cyclamen, Stapelia, Cerinthe, Nolana, Solanum,
+Beta, Ricinus, Quercus, Corylus, Pinus, Cycas, Canna, Allium,
+Asparagus, Phalaris, Zea, Avena, Nephrodium, and Selaginella.
+
+
+CHAPTER II.—GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF
+SEEDLING PLANTS.
+Generality of the circumnutating movement—Radicles, their
+circumnutation of service—Manner in which they penetrate the
+ground—Manner in which hypocotyls and other organs break through the
+ground by being arched—Singular manner of germination in Megarrhiza,
+etc.—Abortion of cotyledons—Circumnutation of hypocotyls and epicotyls
+whilst still buried and arched—Their power of straightening
+themselves—Bursting of the seed-coats—Inherited effect of the arching
+process in hypogean hypocotyls—Circumnutation of hypocotyls and
+epicotyls when erect—Circumnutation of cotyledons—Pulvini or joints of
+cotyledons, duration of their activity, rudimentary in Oxalis
+corniculata, their development—Sensitiveness of cotyledons to light and
+consequent disturbance of their periodic movements—Sensitiveness of
+cotyledons to contact.
+
+
+CHAPTER III.—SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO
+OTHER IRRITANTS.
+Manner in which radicles bend when they encounter an obstacle in the
+soil—Vicia faba, tips of radicles highly sensitive to contact and other
+irritants—Effects of too high a temperature—Power of discriminating
+between objects attached on opposite sides—Tips of secondary radicles
+sensitive—Pisum, tips of radicles sensitive—Effects of such
+sensitiveness in overcoming geotropism—Secondary radicles—Phaseolus,
+tips of radicles hardly sensitive to contact, but highly sensitive to
+caustic and to the removal of a
+slice—Tropaeolum—Gossypium—Cucurbita—Raphanus—Æsculus, tip not
+sensitive to slight contact, highly sensitive to caustic—Quercus, tip
+highly sensitive to contact—Power of discrimination—Zea, tip highly
+sensitive, secondary radicles—Sensitiveness of radicles to moist
+air—Summary of chapter.
+
+
+CHAPTER IV.—THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE
+PLANTS.
+Circumnutation of stems: concluding remarks on—Circumnutation of
+stolons: aid thus afforded in winding amongst the stems of surrounding
+plants—Circumnutation of flower-stems—Circumnutation of Dicotyledonous
+leaves—Singular oscillatory movement of leaves of Dionaea—Leaves of
+Cannabis sink at night—Leaves of Gymnosperms—Of
+Monocotyledons—Cryptogams—Concluding remarks on the circumnutation of
+leaves; generally rise in the evening and sink in the morning.
+
+
+CHAPTER V.—MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND
+HYPONASTIC MOVEMENTS.
+Circumnutation modified through innate causes or through the action of
+external conditions—Innate causes—Climbing plants; similarity of their
+movements with those of ordinary plants; increased amplitude;
+occasional points of difference—Epinastic growth of young
+leaves—Hyponastic growth of the hypocotyls and epicotyls of
+seedlings—Hooked tips of climbing and other plants due to modified
+circumnutation—Ampelopsis tricuspidata—Smithia Pfundii—Straightening of
+the tip due to hyponasty—Epinastic growth and circumnutation of the
+flower-peduncles of Trifolium repens and Oxalis carnosa.
+
+
+CHAPTER VI.—MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS,
+THEIR USE: SLEEP OF COTYLEDONS.
+Preliminary sketch of the sleep or nyctitropic movements of
+leaves—Presence of pulvini—The lessening of radiation the final cause
+of nyctitropic movements—Manner of trying experiments on leaves of
+Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the
+cotyledons of Mimosa—Concluding remarks on radiation from leaves—Small
+differences in the conditions make a great difference in the
+result—Description of the nyctitropic position and movements of the
+cotyledons of various plants—A List of species—Concluding
+remarks—Independence of the nyctitropic movements of the leaves and
+cotyledons of the same species—Reasons for believing that the movements
+have been acquired for a special purpose.
+
+
+CHAPTER VII.—MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF
+LEAVES.
+Conditions necessary for these movements—List of Genera and Families,
+which include sleeping plants—Description of the movements in the
+several Genera—Oxalis: leaflets folded at night—Averrhoa: rapid
+movements of the leaflets—Porlieria: leaflets close when plant kept
+very dry—Tropaeolum: leaves do not sleep unless well illuminated during
+day—Lupinus: various modes of sleeping—Melilotus: singular movements of
+terminal leaflet—Trifolium—Desmodium: rudimentary lateral leaflets,
+movements of, not developed on young plants, state of their
+pulvini—Cassia: complex movements of the leaflets—Bauhinia: leaves
+folded at night—Mimosa pudica: compounded movements of leaves, effect
+of darkness—Mimosa albida, reduced leaflets of—Schrankia: downward
+movement of the pinnae—Marsilea: the only cryptogam known to
+sleep—Concluding remarks and summary—Nyctitropism consists of modified
+circumnutation, regulated by the alternations of light and
+darkness—Shape of first true leaves.
+
+
+CHAPTER VIII.—MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
+Distinction between heliotropism and the effects of light on the
+periodicity of the movements of leaves—Heliotropic movements of Beta,
+Solanum, Zea, and Avena—Heliotropic movements towards an obscure light
+in Apios, Brassica, Phalaris, Tropaeolum, and Cassia—Apheliotropic
+movements of tendrils of Bignonia—Of flower-peduncles of
+Cyclamen—Burying of the pods—Heliotropism and apheliotropism modified
+forms of circumnutation—Steps by which one movement is converted into
+the other—Transversal-heliotropismus or diaheliotropism influenced by
+epinasty, the weight of the part and apogeotropism—Apogeotropism
+overcome during the middle of the day by diaheliotropism—Effects of the
+weight of the blades of cotyledons—So called diurnal sleep—Chlorophyll
+injured by intense light—Movements to avoid intense light.
+
+
+CHAPTER IX.—SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
+Uses of heliotropism—Insectivorous and climbing plants not
+heliotropic—Same organ heliotropic at one age and not at
+another—Extraordinary sensitiveness of some plants to light—The effects
+of light do not correspond with its intensity—Effects of previous
+illumination—Time required for the action of light—After-effects of
+light—Apogeotropism acts as soon as light fails—Accuracy with which
+plants bend to the light—This dependent on the illumination of one
+whole side of the part—Localised sensitiveness to light and its
+transmitted effects—Cotyledons of Phalaris, manner of bending—Results
+of the exclusion of light from their tips—Effects transmitted beneath
+the surface of the ground—Lateral illumination of the tip determines
+the direction of the curvature of the base—Cotyledons of Avena,
+curvature of basal part due to the illumination of upper part—Similar
+results with the hypocotyls of Brassica and Beta—Radicles of Sinapis
+apheliotropic, due to the sensitiveness of their tips—Concluding
+remarks and summary of chapter—Means by which circumnutation has been
+converted into heliotropism or apheliotropism.
+
+
+CHAPTER X.—MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
+Means of observation—Apogeotropism—Cytisus—Verbena—Beta—Gradual
+conversion of the movement of circumnutation into apogeotropism in
+Rubus, Lilium, Phalaris, Avena, and Brassica—Apogeotropism retarded by
+heliotropism—Effected by the aid of joints or pulvini—Movements of
+flower-peduncles of Oxalis—General remarks on
+apogeotropism—Geotropism—Movements of radicles—Burying of
+seed-capsules—Use of process—Trifolium
+subterraneum—Arachis—Amphicarpæa—Diageotropism—Conclusion.
+
+
+CHAPTER XI.—LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED
+EFFECTS.
+General considerations—Vicia faba, effects of amputating the tips of
+the radicles—Regeneration of the tips—Effects of a short exposure of
+the tips to geotropic action and their subsequent amputation—Effects of
+amputating the tips obliquely—Effects of cauterising the tips—Effects
+of grease on the tips—Pisum sativum, tips of radicles cauterised
+transversely, and on their upper and lower sides—Phaseolus,
+cauterisation and grease on the tips—Gossypium—Cucurbita, tips
+cauterised transversely, and on their upper and lower sides—Zea, tips
+cauterised—Concluding remarks and summary of chapter—Advantages of the
+sensibility to geotropism being localised in the tips of the radicles.
+
+
+CHAPTER XII.—CONCLUDING REMARKS.
+Nature of the circumnutating movement—History of a germinating seed—The
+radicle first protrudes and circumnutates—Its tip highly
+sensitive—Emergence of the hypocotyl or of the epicotyl from the ground
+under the form of an arch—Its circumnutation and that of the
+cotyledons—The seedling throws up a leaf-bearing stem—The
+circumnutation of all the parts or organs—Modified
+circumnutation—Epinasty and hyponasty—Movements of climbing
+plants—Nyctitropic movements—Movements excited by light and
+gravitation—Localised sensitiveness—Resemblance between the movements
+of plants and animals—The tip of the radicle acts like a brain.
+
+
+
+
+THE MOVEMENTS OF PLANTS.
+
+
+
+
+INTRODUCTION.
+
+
+The chief object of the present work is to describe and connect
+together several large classes of movement, common to almost all
+plants. The most widely prevalent movement is essentially of the same
+nature as that of the stem of a climbing plant, which bends
+successively to all points of the compass, so that the tip revolves.
+This movement has been called by Sachs “revolving nutation;” but we
+have found it much more convenient to use the terms circumnutation and
+circumnutate. As we shall have to say much about this movement, it will
+be useful here briefly to describe its nature. If we observe a
+circumnutating stem, which happens at the time to be bent, we will say
+towards the north, it will be found gradually to bend more and more
+easterly, until it faces the east; and so onwards to the south, then to
+the west, and back again to the north. If the movement had been quite
+regular, the apex would have described a circle, or rather, as the stem
+is always growing upwards, a circular spiral. But it generally
+describes irregular elliptical or oval figures; for the apex, after
+pointing in any one direction, commonly moves back to the opposite
+side, not, however, returning along the same line. Afterwards other
+irregular ellipses or ovals are successively described, with their
+longer
+axes directed to different points of the compass. Whilst describing
+such figures, the apex often travels in a zigzag line, or makes small
+subordinate loops or triangles. In the case of leaves the ellipses are
+generally narrow.
+
+Until recently the cause of all such bending movements was believed to
+be due to the increased growth of the side which becomes for a time
+convex; that this side does temporarily grow more quickly than the
+concave side has been well established; but De Vries has lately shown
+that such increased growth follows a previously increased state of
+turgescence on the convex side.[1] In the case of parts provided with a
+so-called joint, cushion or pulvinus, which consists of an aggregate of
+small cells that have ceased to increase in size from a very early age,
+we meet with similar movements; and here, as Pfeffer has shown[2] and
+as we shall see in the course of this work, the increased turgescence
+of the cells on opposite sides is not followed by increased growth.
+Wiesner denies in certain cases the accuracy of De Vries’ conclusion
+about turgescence, and maintains[3] that the increased extensibility of
+the cell-walls is the more important element. That such extensibility
+must accompany increased turgescence in order that the part may bend is
+manifest, and this has been insisted on by several botanists; but in
+the case of unicellular plants it can hardly fail to be the more
+important element. On the whole we may at present conclude that
+increased growth, first on one side and then on another, is a secondary
+effect, and that the increased turgescence of the cells, together with
+the extensibility of their walls, is the primary cause of the movement
+of circumnutation.[4]
+
+ [1] Sachs first showed (‘Lehrbuch,’ etc., 4th edit. p. 452) the
+ intimate connection between turgescence and growth. For De Vries’
+ interesting essay, ‘Wachsthumskrümmungen mehrzelliger Organe,’ see
+ ‘Bot. Zeitung,’ Dec. 19, 1879, p. 830.
+
+
+ [2] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875.
+
+
+ [3] ‘Untersuchungen über den Heliotropismus,’ Sitzb. der K. Akad. der
+ Wissenschaft. (Vienna), Jan. 1880.
+
+
+ [4] See Mr. Vines’ excellent discussion (‘Arbeiten des Bot. Instituts
+ in Würzburg,’ B. II. pp. 142, 143, 1878) on this intricate subject.
+ Hofmeister’s observations (‘Jahreschrifte des Vereins für Vaterl.
+ Naturkunde in Würtemberg,’ 1874, p. 211) on the curious movements of
+ Spirogyra, a plant consisting of a single row of cells, are valuable
+ in relation to this subject.
+
+
+In the course of the present volume it will be shown that apparently
+every growing part of every plant is continually circumnutating, though
+often on a small scale. Even the stems of seedlings before they have
+broken through the ground, as well as their buried radicles,
+circumnutate, as far as the pressure of the surrounding earth permits.
+In this universally present movement we have the basis or groundwork
+for the acquirement, according to the requirements of the plant, of the
+most diversified movements. Thus, the great sweeps made by the stems of
+twining plants, and by the tendrils of other climbers, result from a
+mere increase in the amplitude of the ordinary movement of
+circumnutation. The position which young leaves and other organs
+ultimately assume is acquired by the circumnutating movement being
+increased in some one direction. the leaves of various plants are said
+to sleep at night, and it will be seen that their blades then assume a
+vertical position through modified circumnutation, in order to protect
+their upper surfaces from being chilled through radiation. The
+movements of various organs to the light, which are so general
+throughout the vegetable kingdom, and occasionally from the light, or
+transversely with respect to it, are all modified
+forms of circumnutation; as again are the equally prevalent movements
+of stems, etc., towards the zenith, and of roots towards the centre of
+the earth. In accordance with these conclusions, a considerable
+difficulty in the way of evolution is in part removed, for it might
+have been asked, how did all these diversified movements for the most
+different purposes first arise? As the case stands, we know that there
+is always movement in progress, and its amplitude, or direction, or
+both, have only to be modified for the good of the plant in relation
+with internal or external stimuli.
+
+Besides describing the several modified forms of circumnutation, some
+other subjects will be discussed. The two which have interested us most
+are, firstly, the fact that with some seedling plants the uppermost
+part alone is sensitive to light, and transmits an influence to the
+lower part, causing it to bend. If therefore the upper part be wholly
+protected from light, the lower part may be exposed for hours to it,
+and yet does not become in the least bent, although this would have
+occurred quickly if the upper part had been excited by light. Secondly,
+with the radicles of seedlings, the tip is sensitive to various
+stimuli, especially to very slight pressure, and when thus excited,
+transmits an influence to the upper part, causing it to bend from the
+pressed side. On the other hand, if the tip is subjected to the vapour
+of water proceeding from one side, the upper part of the radicle bends
+towards this side. Again it is the tip, as stated by Ciesielski, though
+denied by others, which is sensitive to the attraction of gravity, and
+by transmission causes the adjoining parts of the radicle to bend
+towards the centre of the earth. These several cases of the effects of
+contact, other irritants, vapour, light, and the
+attraction of gravity being transmitted from the excited part for some
+little distance along the organ in question, have an important bearing
+on the theory of all such movements.
+
+Terminology.—A brief explanation of some terms which will be used, must
+here be given. With seedlings, the stem which supports the cotyledons
+(i.e. the organs which represent the first leaves) has been called by
+many botanists the hypocotyledonous stem, but for brevity sake we will
+speak of it merely as the hypocotyl: the stem immediately above the
+cotyledons will be called the epicotyl or plumule. The radicle can be
+distinguished from the hypocotyl only by the presence of root-hairs and
+the nature of its covering. The meaning of the word circumnutation has
+already been explained. Authors speak of positive and negative
+heliotropism,[5]—that is, the bending of an organ to or from the light;
+but it is much more convenient to confine the word heliotropism to
+bending towards the light, and to designate as apheliotropism bending
+from the light. There is another reason for this change, for writers,
+as we have observed, occasionally drop the adjectives positive and
+negative, and thus introduce confusion into their discussions.
+Diaheliotropism may express a position more or less transverse to the
+light and induced by it. In like manner positive geotropism, or bending
+towards the centre of the earth, will be called by us geotropism;
+apogeotropism will mean bending in opposition to gravity or from the
+centre of the earth; and diageotropism, a position more or less
+transverse to the radius of the earth. The words heliotropism and
+geotropism properly mean the act of moving in relation to the light or
+the earth; but in the same manner as gravitation, though defined as
+“the act of tending to the centre,” is often used to express the cause
+of a body falling, so it will be found convenient occasionally to
+employ heliotropism and geotropism, etc., as the cause of the movements
+in question.
+
+ [5] The highly useful terms of Heliotropism and Geotropism were first
+ used by Dr. A. B. Frank: see his remarkable ‘Beiträge zur
+ Pflanzenphysiologie,’ 1868.
+
+
+The term epinasty is now often used in Germany, and implies that the
+upper surface of an organ grows more quickly than the
+lower surface, and thus causes it to bend downwards. Hyponasty is the
+reverse, and implies increased growth along the lower surface, causing
+the part to bend upwards.[6]
+
+ [6] These terms are used in the sense given them by De Vries,
+ ‘Würzburg Arbeiten,’ Heft ii 1872, p. 252.
+
+
+Methods of Observation.—The movements, sometimes very small and
+sometimes considerable in extent, of the various organs observed by us,
+were traced in the manner which after many trials we found to be best,
+and which must be described. Plants growing in pots were protected
+wholly from the light, or had light admitted from above, or on one side
+as the case might require, and were covered above by a large horizontal
+sheet of glass, and with another vertical sheet on one side. A glass
+filament, not thicker than a horsehair, and from a quarter to
+three-quarters of an inch in length, was affixed to the part to be
+observed by means of shellac dissolved in alcohol. The solution was
+allowed to evaporate, until it became so thick that it set hard in two
+or three seconds, and it never injured the tissues, even the tips of
+tender radicles, to which it was applied. To the end of the glass
+filament an excessively minute bead of black sealing-wax was cemented,
+below or behind which a bit of card with a black dot was fixed to a
+stick driven into the ground. The weight of the filament was so slight
+that even small leaves were not perceptibly pressed down. another
+method of observation, when much magnification of the movement was not
+required, will presently be described. The bead and the dot on the card
+were viewed through the horizontal or vertical glass-plate (according
+to the position of the object), and when one exactly covered the other,
+a dot was made on the glass-plate with a sharply pointed stick dipped
+in thick Indian-ink. Other dots were made at short intervals of time
+and these were afterwards joined by straight lines. The figures thus
+traced were therefore angular; but if dots had been made every 1 or 2
+minutes, the lines would have been more curvilinear, as occurred when
+radicles were allowed to trace their own courses on smoked
+glass-plates. To make the dots accurately was the sole difficulty, and
+required some practice. Nor could this be done quite accurately, when
+the movement was much magnified, such as 30 times and upwards; yet even
+in this case the general course may be trusted. To test the accuracy of
+the above method of observation, a filament was fixed to an
+inanimate object which was made to slide along a straight edge and dots
+were repeatedly made on a glass-plate; when these were joined, the
+result ought to have been a perfectly straight line, and the line was
+very nearly straight. It may be added that when the dot on the card was
+placed half-an-inch below or behind the bead of sealing-wax, and when
+the glass-plate (supposing it to have been properly curved) stood at a
+distance of 7 inches in front (a common distance), then the tracing
+represented the movement of the bead magnified 15 times.
+
+Whenever a great increase of the movement was not required, another,
+and in some respects better, method of observation was followed. This
+consisted in fixing two minute triangles of thin paper, about 1/20 inch
+in height, to the two ends of the attached glass filament; and when
+their tips were brought into a line so that they covered one another,
+dots were made as before on the glass-plate. If we suppose the
+glass-plate to stand at a distance of seven inches from the end of the
+shoot bearing the filament, the dots when joined, will give nearly the
+same figure as if a filament seven inches long, dipped in ink, had been
+fixed to the moving shoot, and had inscribed its own course on the
+plate. The movement is thus considerably magnified; for instance, if a
+shoot one inch in length were bending, and the glass-plate stood at the
+distance of seven inches, the movement would be magnified eight times.
+It would, however, have been very difficult to have ascertained in each
+case how great a length of the shoot was bending; and this is
+indispensable for ascertaining the degree to which the movement is
+magnified.
+
+After dots had been made on the glass-plates by either of the above
+methods, they were copied on tracing paper and joined by ruled lines,
+with arrows showing the direction of the movement. The nocturnal
+courses are represented by straight broken lines. the first dot is
+always made larger than the others, so as to catch the eye, as may be
+seen in the diagrams. The figures on the glass-plates were often drawn
+on too large a scale to be reproduced on the pages of this volume, and
+the proportion in which they have been reduced is always given.[7]
+Whenever it could be approximately told how much the movement had been
+magnified, this is stated. We have perhaps
+introduced a superfluous number of diagrams; but they take up less
+space than a full description of the movements. Almost all the sketches
+of plants asleep, etc., were carefully drawn for us by Mr. George
+Darwin.
+
+ [7] We are much indebted to Mr. Cooper for the care with which he has
+ reduced and engraved our diagrams.
+
+
+As shoots, leaves, etc., in circumnutating bend more and more, first in
+one direction and then in another, they were necessarily viewed at
+different times more or less obliquely; and as the dots were made on a
+flat surface, the apparent amount of movement is exaggerated according
+to the degree of obliquity of the point of view. It would, therefore,
+have been a much better plan to have used hemispherical glasses, if we
+had possessed them of all sizes, and if the bending part of the shoot
+had been distinctly hinged and could have been placed so as to have
+formed one of the radii of the sphere. But even in this case it would
+have been necessary afterwards to have projected the figures on paper;
+so that complete accuracy could not have been attained. From the
+distortion of our figures, owing to the above causes, they are of no
+use to any one who wishes to know the exact amount of movement, or the
+exact course pursued; but they serve excellently for ascertaining
+whether or not the part moved at all, as well as the general character
+of the movement.
+
+In the following chapters, the movements of a considerable number of
+plants are described; and the species have been arranged according to
+the system adopted by Hooker in Le Maout and Decaisne’s ‘Descriptive
+Botany.’ No one who is not investigating the present subject need read
+all the details, which, however, we have thought it advisable to give.
+To save the reader trouble, the conclusions and most of the more
+important parts have been printed in larger type than the other parts.
+He may, if he thinks fit, read the last chapter first, as it includes a
+summary of the whole volume; and he will thus see what points interest
+him, and on which he requires the full evidence.
+
+Finally, we must have the pleasure of returning our
+sincere thanks to Sir Joseph Hooker and to Mr. W. Thiselton Dyer for
+their great kindness, in not only sending us plants from Kew, but in
+procuring others from several sources when they were required for our
+observations; also, for naming many species, and giving us information
+on various points.
+
+
+
+
+CHAPTER I.
+THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
+
+
+Brassica oleracea, circumnutation of the radicle, of the arched
+hypocotyl whilst still buried beneath the ground, whilst rising above
+the ground and straightening itself, and when erect—Circumnutation of
+the cotyledons—Rate of movement—Analogous observations on various
+organs in species of Githago, Gossypium, Oxalis, Tropaeolum, Citrus,
+Æsculus, of several Leguminous and Cucurbitaceous genera, Opuntia,
+Helianthus, Primula, Cyclamen, Stapelia, Cerinthe, Nolana, Solanum,
+Beta, Ricinus, Quercus, Corylus, Pinus, Cycas, Canna, Allium,
+Asparagus, Phalaris, Zea, Avena, Nephrodium, and Selaginella.
+
+
+The following chapter is devoted to the circumnutating movements of the
+radicles, hypocotyls, and cotyledons of seedling plants; and, when the
+cotyledons do not rise above the ground, to the movements of the
+epicotyl. But in a future chapter we shall have to recur to the
+movements of certain cotyledons which sleep at night.
+
+Brassica oleracea (Cruciferae)’.—Fuller details will be given with
+respect to the movements in this case than in any other, as space and
+time will thus ultimately be saved.
+
+Radicle.—A seed with the radicle projecting .05 inch was fastened with
+shellac to a little plate of zinc, so that the radicle stood up
+vertically; and a fine glass filament was then fixed near its base,
+that is, close to the seed-coats. The seed was surrounded by little
+bits of wet sponge, and the movement of the bead at the end of the
+filament was traced (Fig. 1) during sixty hours. In this time the
+radicle increased in length from .05 to .11 inch. Had the filament been
+attached at first close to the apex of the radicle, and if it could
+have remained there all the time, the movement exhibited would have
+been much greater, for at the close of our observations the tip,
+instead of standing vertically upwards, had become bowed downwards
+through geotropism, so as almost to touch the zinc plate. As far as we
+could roughly ascertain by measurements made with compasses on other
+seeds, the tip alone, for a length of only 2/100 to 3/100 of an inch,
+is acted on by geotropism. But the tracing shows that the basal part of
+the radicle continued to circumnutate irregularly during the whole
+time. The actual extreme amount of movement of the bead at the end of
+the filament was nearly .05 inch, but to what extent the movement of
+the radicle was magnified by the filament, which was nearly 3/4 inch in
+length, it was impossible to estimate.
+
+Fig. 1. Brassica oleracea: circumnutation of radicle, traced on
+horizontal glass, from 9 A.M. Jan. 31st to 9 P.M. Feb. 2nd. Movement of
+bead at end of filament magnified about 40 times.
+
+Another seed was treated and observed in the same manner, but the
+radicle in this case protruded .1 inch, and was not fastened so as to
+project quite vertically upwards. The filament was affixed close to its
+base. The tracing (Fig. 2, reduced by half) shows the movement from 9
+A.M. Jan. 31st to 7 A.M. Feb. 2nd; but it continued to move during the
+whole of the
+2nd in the same general direction, and in a similar zigzag manner. From
+the radicle not being quite perpendicular when the filament was affixed
+geotropism came into play at once; but the irregular zigzag course
+shows that there was growth (probably preceded by turgescence),
+sometimes on one and sometimes on another side. Occasionally the bead
+remained stationary for about an hour, and then probably growth
+occurred on the side opposite to that which caused the geotropic
+curvature. In the case previously described the basal part of the very
+short radicle from being turned vertically upwards, was at first very
+little affected by geotropism. Filaments were affixed in two other
+instances to rather longer radicles protruding obliquely from seeds
+which had been turned upside down; and in these cases the lines traced
+on the horizontal glasses were only slightly zigzag, and the movement
+was always in the same general direction, through the action of
+geotropism. All these observations are liable to several causes of
+error, but we believe, from what will hereafter be shown with respect
+to the movements of the radicles of other plants, that they may be
+largely trusted.
+
+Fig. 2. Brassica oleracea: circumnutating and geotropic movement of
+radicle, traced on horizontal glass during 46 hours.
+
+Hypocotyl.—The hypocotyl protrudes through the seed-coats as a
+rectangular projection, which grows rapidly into an arch like the
+letter U turned upside down; the cotyledons being still enclosed within
+the seed. In whatever position the seed may be embedded in the earth or
+otherwise fixed, both legs of the arch bend upwards through
+apogeotropism, and thus rise vertically above the ground. As soon as
+this has taken place, or even earlier, the inner or concave surface of
+the arch grows more quickly than the upper or convex surface; and this
+tends to separate the two legs and aids in drawing the cotyledons out
+of the buried seed-coats. By the growth of the whole arch the
+cotyledons are ultimately dragged from beneath the ground, even from a
+considerable depth; and now the hypocotyl quickly straightens itself by
+the increased growth of the concave side.
+
+Even whilst the arched or doubled hypocotyl is still beneath the
+ground, it circumnutates as much as the pressure of the surrounding
+soil will permit; but this was difficult to observe, because as soon as
+the arch is freed from lateral pressure the two legs begin to separate,
+even at a very early age, before the arch would naturally have reached
+the surface. Seeds were allowed to germinate on the surface of damp
+earth, and after they had fixed themselves by their radicles, and after
+the, as yet, only
+slightly arched hypocotyl had become nearly vertical, a glass filament
+was affixed on two occasions near to the base of the basal leg (i.e.
+the one in connection with the radicle), and its movements were traced
+in darkness on a horizontal glass. The result was that long lines were
+formed running in nearly the plane of the vertical arch, due to the
+early separation of the two legs now freed from pressure; but as the
+lines were zigzag, showing lateral movement, the arch must have been
+circumnutating, whilst it was straightening itself by growth along its
+inner or concave surface.
+
+A somewhat different method of observation was next followed: as soon
+as the earth with seeds in a pot began to crack, the surface was
+removed in parts to the depth of .2 inch; and a filament was fixed to
+the basal leg of a buried and arched hypocotyl, just above the summit
+of the radicle. The cotyledons were still almost completely enclosed
+within the much-cracked seed-coats; and these were again covered up
+with damp adhesive soil pressed pretty firmly down. The movement of the
+filament was traced (Fig. 3) from 11 A.M. Feb. 5th till 8 A.M. Feb.
+7th. By this latter period the cotyledons had been dragged from beneath
+the pressed-down earth, but the upper part of the hypocotyl still
+formed nearly a right angle with the lower part. The tracing shows that
+the arched hypocotyl tends at this early
+age to circumnutate irregularly. On the first day the greater movement
+(from right to left in the figure) was not in the plane of the vertical
+and arched hypocotyl, but at right angles to it, or in the plane of the
+two cotyledons, which were still in close contact. The basal leg of the
+arch at the time when the filament was affixed to it, was already bowed
+considerably backwards, or from the cotyledons; had the filament been
+affixed before this bowing occurred, the chief movement would have been
+at right angles to that shown in the figure. A filament was attached to
+another buried hypocotyl of the same age, and it moved in a similar
+general manner, but the line traced was not so complex. This hypocotyl
+became almost straight, and the cotyledons were dragged from beneath
+the ground on the evening of the second day.
+
+Fig. 3. Brassica oleracea: circumnutating movement of buried and arched
+hypocotyl (dimly illuminated from above), traced on horizontal glass
+during 45 hours. Movement of bead of filament magnified about 25 times,
+and here reduced to one-half of original scale.
+
+Fig. 4. Brassica oleracea: circumnutating movement of buried and arched
+hypocotyl, with the two legs of the arch tied together, traced on
+horizontal glass during 33½ hours. Movement of the bead of filament
+magnified about 26 times, and here reduced to one-half original scale.
+
+Before the above observations were made, some arched hypocotyls buried
+at the depth of a quarter of an inch were uncovered; and in order to
+prevent the two legs of the arch from beginning to separate at once,
+they were tied together with fine silk. This was done partly because we
+wished to ascertain how long the hypocotyl, in its arched condition,
+would continue to move, and whether the movement when not masked and
+disturbed by the straightening process, indicated circumnutation.
+Firstly a filament was fixed to the basal leg of an arched hypocotyl
+close above the summit of the radicle. The cotyledons were still
+partially enclosed within the seed-coats. The movement was traced (Fig.
+4) from 9.20 A.M. on Dec.
+23rd to 6.45 A.M. on Dec. 25th. No doubt the natural movement was much
+disturbed by the two legs having been tied together; but we see that it
+was distinctly zigzag, first in one direction and then in an almost
+opposite one. After 3 P.M. on the 24th the arched hypocotyl sometimes
+remained stationary for a considerable time, and when moving, moved far
+slower than before. Therefore, on the morning of the 25th, the glass
+filament was removed from the base of the basal leg, and was fixed
+horizontally on the summit of the arch, which, from the legs having
+been tied, had grown broad and almost flat. The movement was now traced
+during 23 hours (Fig. 5), and we see that the course was still zigzag,
+which indicates a tendency to circumnutation. The base of the basal leg
+by this time had almost completely ceased to move.
+
+Fig. 5. Brassica oleracea: circumnutating movement of the crown of a
+buried and arched hypocotyl, with the two legs tied together, traced on
+a horizontal glass during 23 hours. Movement of the bead of the
+filament magnified about 58 times, and here reduced to one-half
+original scale.
+
+As soon as the cotyledons have been naturally dragged from beneath the
+ground, and the hypocotyl has straightened itself by growth along the
+inner or concave surface, there is nothing to interfere with the free
+movements of the parts; and the circumnutation now becomes much more
+regular and clearly displayed, as shown in the following cases:—A
+seedling was placed in front and near a north-east window with a line
+joining the
+two cotyledons parallel to the window. It was thus left the whole day
+so as to accommodate itself to the light. On the following morning a
+filament was fixed to the midrib of the larger and taller cotyledon
+(which enfolds the other and smaller one, whilst still within the
+seed), and a mark being placed close behind, the movement of the whole
+plant, that is, of the hypocotyl and cotyledon, was traced greatly
+magnified on a vertical glass. At first the plant bent so much towards
+the light that it was useless to attempt to trace the movement; but at
+10 A.M. heliotropism almost wholly ceased and the first dot was made on
+the glass. The last was made at 8.45 P.M.; seventeen dots being
+altogether made in this interval of 10 h. 45 m. (see Fig. 6). It should
+be noticed that when I looked shortly after 4 P.M. the bead was
+pointing off the glass, but it came on again at 5.30 P.M., and the
+course during this interval of 1 h. 30 m. has been filled up by
+imagination, but cannot be far from correct. The bead moved seven times
+from side to side, and thus described 3½ ellipses in 10 3/4 h.; each
+being completed on an average in 3 h. 4 m.
+
+Fig. 6. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons during 10 hours 45 minutes. Figure here reduced to one-half
+original scale.
+
+On the previous day another seedling had been observed under similar
+conditions, excepting that the plant was so
+placed that a line joining the two cotyledons pointed towards the
+window; and the filament was attached to the smaller cotyledon on the
+side furthest from the window. Moreover the plant was now for the first
+time placed in this position. The cotyledons bowed themselves greatly
+towards the light from 8 to 10.50 A.M., when the first dot was made
+(Fig. 7). During the next 12 hours the bead swept obliquely up and down
+8 times and described 4 figures representing ellipses; so that it
+travelled at nearly the same rate as in the previous case. during the
+night it moved upwards, owing to the sleep-movement of the cotyledons,
+and continued to move in the same direction till 9 A.M. on the
+following morning; but this latter movement would not have occurred
+with seedlings under their natural conditions fully exposed to the
+light.
+
+Fig. 7. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons, from 10.50 A.M. to 8 A.M. on the following morning. Tracing
+made on a vertical glass.
+
+By 9.25 A.M. on this second day the same cotyledon had
+begun to fall, and a dot was made on a fresh glass. The movement was
+traced until 5.30 P.M. as shown in (Fig. 8), which is given, because
+the course followed was much more irregular than on the two previous
+occasions. During these 8 hours the bead changed its course greatly 10
+times. The upward movement of the cotyledon during the afternoon and
+early part of the night is here plainly shown.
+
+Fig. 8. Brassica oleracea: conjoint circumnutation of the hypocotyl and
+cotyledons during 8 hours. Figure here reduced to one-third of the
+original scale, as traced on a vertical glass.
+
+As the filaments were fixed in the three last cases to one of the
+cotyledons, and as the hypocotyl was left free, the tracings show the
+movement of both organs conjoined; and we now wished to ascertain
+whether both circumnutated. Filaments were therefore fixed horizontally
+to two hypocotyls close beneath the petioles of their cotyledons. These
+seedlings had stood for two days in the same position before a
+north-east window. In the morning, up to about 11 A.M., they moved in
+zigzag lines towards the light; and at night they again became almost
+upright through apogeotropism. After about 11 A.M. they moved a little
+back from the light, often crossing and recrossing their former path in
+zigzag lines. the sky on this day varied much in brightness, and these
+observations merely proved that the hypocotyls were continually moving
+in a manner resembling circumnutation. On a previous day which was
+uniformly cloudy, a hypocotyl was firmly secured to a little stick, and
+a filament was fixed to the larger of the two cotyledons, and its
+movement was traced on a vertical glass. It fell greatly from 8.52
+A.M., when the first dot was made, till 10.55 A.M.; it then rose
+greatly until 12.17 P.M. Afterwards it fell a little and made a loop,
+but by 2.22 P.M. it had risen a little and continued rising till 9.23
+P.M., when it made another loop, and at 10.30 P.M. was again rising.
+These observations show that the cotyledons move
+vertically up and down all day long, and as there was some slight
+lateral movement, they circumnutated.
+
+Fig. 9. Brassica oleracea: circumnutation of hypocotyl, in darkness,
+traced on a horizontal glass, by means of a filament with a bead fixed
+across its summit, between 9.15 A.M. and 8.30 A.M. on the following
+morning. Figure here reduced to one-half of original scale.
+
+The cabbage was one of the first plants, the seedlings of which were
+observed by us, and we did not then know how far the circumnutation of
+the different parts was affected by light. Young seedlings were
+therefore kept in complete darkness except for a minute or two during
+each observation, when they were illuminated by a small wax taper held
+almost vertically above them. During the first day the hypocotyl of one
+changed its course 13 times (see Fig. 9); and it deserves notice that
+the longer axes of the figures described often cross one another at
+right or nearly right angles. Another seedling was observed in the same
+manner, but it was much older, for it had formed a true leaf a quarter
+of an inch in length, and the hypocotyl was 1 3/8 inch in height. The
+figure traced was a very complex one, though the movement was not so
+great in extent as in the last case.
+
+The hypocotyl of another seedling of the same age was secured to a
+little stick, and a filament having been fixed to the midrib of one of
+the cotyledons, the movement of the bead was traced during 14 h. 15 m.
+(see Fig. 10) in darkness. It should be noted that the chief movement
+of the cotyledons, namely, up and down, would be shown on a horizontal
+glass-plate only by the lines in the direction of the midrib (that is,
+up and down, as Fig. 10 here stands) being a little lengthened or
+shortened; whereas any lateral movement would be well exhibited. The
+present tracing shows that the cotyledon did thus move laterally (that
+is, from side to side in the tracing) 12 times in the 14 h. 15 m. of
+observation. Therefore the cotyledons certainly circumnutated, though
+the chief movement was up and down in a vertical plane.
+
+Fig. 10. Brassica oleracea: circumnutation of a cotyledon, the
+hypocotyl having been secured to a stick, traced on a horizontal glass,
+in darkness, from 8.15 A.M. to 10.30 P.M. Movement of the bead of the
+filament magnified 13 times.
+
+Rate of Movement.—The movements of the hypocotyls and cotyledons of
+seedling cabbages of different ages have now been sufficiently
+illustrated. With respect to the rate, seedlings were placed under the
+microscope with the stage removed, and with a micrometer eye-piece so
+adjusted that each division equalled 1/500 inch; the plants were
+illuminated by light passing through a solution of bichromate of
+potassium so as to eliminate heliotropism. Under these circumstances it
+was interesting to observe how rapidly the circumnutating apex of a
+cotyledon passed across the divisions of the micrometer. Whilst
+travelling in any direction the apex generally oscillated backwards and
+forwards to the extent of 1/500 and sometimes of nearly 1/250 of an
+inch. These oscillations were quite different from the trembling caused
+by any disturbance in the same room or by the shutting of a distant
+door. The first seedling observed was nearly two inches in height and
+had been etiolated by having been grown in darkness. The tip of the
+cotyledon passed across 10 divisions of the micrometer, that is, 1/50
+of an inch, in 6 m. 40 s. Short glass filaments were then fixed
+vertically to the hypocotyls of several seedlings so as to project a
+little above the cotyledons, thus exaggerating the rate of movement;
+but only a few of the observations thus made are worth giving. The most
+remarkable fact was the oscillatory movement above described, and the
+difference of rate at which the point crossed the divisions of the
+micrometer, after short intervals of time. For instance, a tall
+not-etiolated seedling had been kept for 14 h. in darkness; it was
+exposed before a north-east window for only
+two or three minutes whilst a glass filament was fixed vertically to
+the hypocotyl; it was then again placed in darkness for half an hour
+and afterwards observed by light passing through bichromate of
+potassium. The point, oscillating as usual, crossed five divisions of
+the micrometer (i.e. 1/100 inch) in 1 m. 30 s. The seedling was then
+left in darkness for an hour, and now it required 3 m. 6 s. to cross
+one division, that is, 15 m. 30 s. to have crossed five divisions.
+Another seedling, after being occasionally observed in the back part of
+a northern room with a very dull light, and left in complete darkness
+for intervals of half an hour, crossed five divisions in 5 m. in the
+direction of the window, so that we concluded that the movement was
+heliotropic. But this was probably not the case, for it was placed
+close to a north-east window and left there for 25 m., after which
+time, instead of moving still more quickly towards the light, as might
+have been expected, it travelled only at the rate of 12 m. 30 s. for
+five divisions. It was then again left in complete darkness for 1 h.,
+and the point now travelled in the same direction as before, but at the
+rate of 3 m. 18 s. for five divisions.
+
+We shall have to recur to the cotyledons of the cabbage in a future
+chapter, when we treat of their sleep-movements. The circumnutation,
+also, of the leaves of fully-developed plants will hereafter be
+described.
+
+Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on a
+horizontal glass, by means of a filament fixed transversely across its
+summit, from 8.15 A.M. to 12.15 P.M. on the following day. Movement of
+bead of filament magnified about 13 times, here reduced to one-half the
+original scale.
+
+Githago segetum (Caryophylleae).—A young seedling was dimly illuminated
+from above, and the circumnutation of the
+hypocotyl was observed during 28 h., as shown in Fig. 11. It moved in
+all directions; the lines from right and to left in the figure being
+parallel to the blades of the cotyledons. The actual distance travelled
+from side to side by the summit of the hypocotyl was about .2 of an
+inch; but it was impossible to be accurate on this head, as the more
+obliquely the plant was viewed, after it had moved for some time, the
+more the distances were exaggerated.
+
+We endeavoured to observe the circumnutation of the cotyledons, but as
+they close together unless kept exposed to a moderately bright light,
+and as the hypocotyl is extremely heliotropic, the necessary
+arrangements were too troublesome. We shall recur to the nocturnal or
+sleep-movements of the cotyledons in a future chapter.
+
+Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontal
+glass, from 10.30 A.M. to 9.30 A.M. on following morning, by means of a
+filament fixed across its summit. Movement of bead of filament
+magnified about twice; seedling illuminated from above.
+
+Gossypium (var. Nankin cotton) (Malvaceae).—The circumnutation of a
+hypocotyl was observed in the hot-house, but the movement was so much
+exaggerated that the bead twice passed for a time out of view. It was,
+however, manifest that two somewhat irregular ellipses were nearly
+completed in 9 h. Another seedling, 1½ in. in height, was then observed
+during 23 h.; but the observations were not made at sufficiently short
+intervals, as shown by the few dots in Fig. 12, and the tracing was not
+now sufficiently enlarged. Nevertheless there could be no doubt about
+the circumnutation of the hypocotyl, which described in 12 h. a figure
+representing three irregular ellipses of unequal sizes.
+
+The cotyledons are in constant movement up and down during the whole
+day, and as they offer the unusual case of moving downwards late in the
+evening and in the early part of the night, many observations were made
+on them. A filament was fixed along the middle of one, and its movement
+traced on a vertical glass; but the tracing is not given, as the
+hypocotyl was not secured, so that it was impossible to distinguish
+clearly between its movement and that of the cotyledon. The cotyledons
+rose from 10.30 A.M. to about 3 P.M.; they then sank till 10 P.M.,
+rising, however, greatly in the latter part of the night.
+The angles above the horizon at which the cotyledons of another
+seedling stood at different hours is recorded in the following short
+table:—
+
+Oct. 20 2.50 P.M...25° above horizon. Oct. 20 4.20 P.M...22° above
+horizon. Oct. 20 5.20 P.M...15° above horizon. Oct. 20 10.40 P.M...8°
+above horizon. Oct. 21 8.40 A.M...28° above horizon. Oct. 21 11.15
+A.M...35° above horizon. Oct. 21 9.11 P.M...10° below horizon.
+
+The position of the two cotyledons was roughly sketched at various
+hours with the same general result.
+
+In the following summer, the hypocotyl of a fourth seedling was secured
+to a little stick, and a glass filament with triangles of paper having
+been fixed to one of the cotyledons, its movements were traced on a
+vertical glass under a double skylight in the house. The first dot was
+made at 4.20 P.M. June 20th; and the cotyledon fell till 10.15 P.M. in
+a nearly straight line. Just past midnight it was found a little lower
+and somewhat to one side. By the early morning, at 3.45 A.M., it had
+risen greatly, but by 6.20 A.M. had fallen a little. During the whole
+of this day (21st) it fell in a slightly zigzag line, but its normal
+course was disturbed by the want of sufficient illumination, for during
+the night it rose only a little, and travelled irregularly during the
+whole of the following day and night of June 22nd. The ascending and
+descending lines traced during the three days did not coincide, so that
+the movement was one of circumnutation. This seedling was then taken
+back to the hot-house, and after five days was inspected at 10 P.M.,
+when the cotyledons were found hanging so nearly vertically down, that
+they might justly be said to have been asleep. On the following morning
+they had resumed their usual horizontal position.
+
+Oxalis rosea (Oxalideae).—The hypocotyl was secured to a little stick,
+and an extremely thin glass filament, with two triangles of paper, was
+attached to one of the cotyledons, which was .15 inch in length. In
+this and the following species the end of the petiole, where united to
+the blade, is developed into a pulvinus. The apex of the cotyledon
+stood only 5 inches from the vertical glass, so that its movement was
+not greatly exaggerated as long as it remained nearly horizontal; but
+in the course of the day it both rose considerably above and fell
+beneath a horizontal position, and then of course the movement was much
+exaggerated.
+In Fig. 13 its course is shown from 6.45 A.M. on June 17th, to 7.40
+A.M. on the following morning; and we see that during the daytime, in
+the course of 11 h. 15 m., it travelled thrice down and twice up. After
+5.45 P.M. it moved rapidly downwards, and in an hour or two depended
+vertically; it thus remained all night asleep. This position could not
+be represented on the vertical glass nor in the figure here given. By
+6.40 A.M. on the following morning (18th) both cotyledons had risen
+greatly, and they continued to rise until 8 A.M., when they stood
+almost horizontally. Their movement was traced during the whole of this
+day and until the next morning; but a tracing is not given, as it was
+closely similar to Fig. 13, excepting that the lines were more zigzag.
+The cotyledons moved 7 times, either upwards or downwards; and at about
+4 P.M. the great nocturnal sinking movement commenced.
+
+Fig. 13. Oxalis rosea: circumnutation of cotyledons, the hypocotyl
+being secured to a stick; illuminated from above. Figure here given
+one-half of original scale.
+
+Another seedling was observed in a similar manner during nearly 24 h.,
+but with the difference that the hypocotyl was left free. The movement
+also was less magnified. Between 8.12 A.M. and 5 P.M. on the 18th, the
+apex of the cotyledon moved 7 times upwards or downwards (Fig. 14). The
+nocturnal sinking movement, which is merely a great increase of one of
+the diurnal oscillations, commenced about 4 P.M.
+
+Oxalis Valdiviana.—This species is interesting, as the
+cotyledons rise perpendicularly upwards at night so as to come into
+close contact, instead of sinking vertically downwards, as in the case
+of O. rosea. A glass filament was fixed to a cotyledon, .17 of an inch
+in length, and the hypocotyl was left free. On the first day the
+seedling was placed too far from the vertical glass; so that the
+tracing was enormously exaggerated and the movement could not be traced
+when the cotyledon either rose or sank much; but it was clearly seen
+that the cotyledons rose thrice and fell twice between 8.15 A.M. and
+4.15 P.M. Early on the following morning (June 19th) the apex of a
+cotyledon was
+placed only 1 7/8 inch from the vertical glass. At 6.40 A.M. it stood
+horizontally; it then fell till 8.35, and then rose. Altogether in the
+course of 12 h. it rose thrice and fell thrice, as may be seen in Fig.
+15. The great nocturnal rise of the cotyledons usually commences about
+4 or 5 P.M., and on the following morning they are expanded or stand
+horizontally at about 6.30 A.M. In the present instance, however, the
+great nocturnal rise did not commence till 7 P.M.; but this was due to
+the hypocotyl having from some unknown cause temporarily bent to the
+left side, as is shown in the tracing. To ascertain positively that the
+hypocotyl circumnutated, a mark was placed at 8.15 P.M. behind the two
+now closed and vertical cotyledons; and the movement of a glass
+filament fixed upright to the top of the hypocotyl was traced until
+10.40 P.M. During this time it moved from side to side, as well as
+backwards and forwards, plainly showing circumnutation; but the
+movement was small in extent. Therefore Fig. 15 represents fairly well
+the movements of the cotyledons alone, with the exception of the one
+great afternoon curvature to the left.
+
+Fig. 14. Oxalis rosea: conjoint circumnutation of the cotyledons and
+hypocotyl, traced from 8.12 A.M. on June 18th to 7.30 A.M. 19th. The
+apex of the cotyledon stood only 3 3/4 inches from the vertical glass.
+Figure here given one-half of original scale.
+
+Fig. 15. Oxalis Valdiviana: conjoint circumnutation of a cotyledon and
+the hypocotyl, traced on vertical glass, during 24 hours. Figure here
+given one-half of original scale; seedling illuminated from above.
+
+Oxalis corniculata (var. cuprea).—The cotyledons rise at night to a
+variable degree above the horizon, generally about 45°: those on some
+seedlings between 2 and 5 days old were found to be in continued
+movement all day long; but the movements were more simple than in the
+last two species. This may have partly resulted from their not being
+sufficiently illuminated whilst being observed, as was shown by their
+not beginning to rise until very late in the evening.
+
+Oxalis (Biophytum) sensitiva.—The cotyledons are highly remarkable from
+the amplitude and rapidity of their movements during the day. The
+angles at which they stood above or beneath the horizon were measured
+at short intervals of time; and we regret that their course was not
+traced during the whole day. We will give only a few of the
+measurements, which were made whilst the seedlings were exposed to a
+temperature of 22½° to 24½° C. One cotyledon rose 70° in 11 m.;
+another, on a distinct seedling, fell 80° in 12 m. Immediately before
+this latter fall the same cotyledon had risen from a vertically
+downward to a vertically upward position in 1 h. 48 m., and had
+therefore passed through 180° in under 2 h. We have met with no other
+instance of a circumnutating movement of such great amplitude as 180°;
+nor of such rapidity of movement as the passage through 80° in 12 m.
+The cotyledons of this plant sleep at night by rising
+vertically and coming into close contact. This upward movement differs
+from one of the great diurnal oscillations above described only by the
+position being permanent during the night and by its periodicity, as it
+always commences late in the evening.
+
+Tropaeolum minus (?) (var. Tom Thumb) (Tropaeoleae).—The cotyledons are
+hypogean, or never rise above the ground. By removing the soil a buried
+epicotyl or plumule was found, with its summit arched abruptly
+downwards, like the arched hypocotyl of the cabbage previously
+described. A glass filament with a bead at its end was affixed to the
+basal half or leg, just above the hypogean cotyledons, which were again
+almost surrounded by loose earth. The tracing (Fig. 16) shows the
+course of the bead during 11 h. After the last dot given in the figure,
+the bead moved to a great distance, and finally off the glass, in the
+direction indicated by the broken line. This great movement, due to
+increased growth along the concave surface of the arch, was caused by
+the basal leg bending backwards from the upper part, that is in a
+direction opposite to the dependent tip, in the same manner as occurred
+with the hypocotyl of the cabbage. Another buried and arched epicotyl
+was observed in the same manner, excepting that the two legs of the
+arch were tied together with fine silk for the sake of preventing the
+great movement just mentioned. It moved, however, in the evening in the
+same direction as before, but the line followed was not so straight.
+During the morning the tied arch moved in an irregularly circular,
+strongly zigzag course, and to a greater distance than in the previous
+case, as was shown in a tracing, magnified 18 times. The movements of a
+young plant bearing a few leaves and of a mature plant, will hereafter
+be described.
+
+Fig. 16. Tropaeolum minus (?): circumnutation of buried and arched
+epicotyl, traced on a horizontal glass, from 9.20 A.M. to 8.15 P.M.
+Movement of bead of filament magnified 27 times.
+
+
+Citrus aurantium (Orange) (Aurantiaceae).—The cotyledons are hypogean.
+The circumnutation of an epicotyl, which at the close of our
+observations was .59 of an inch (15 mm.) in height above the ground, is
+shown in the annexed figure (Fig. 17), as observed during a period of
+44 h. 40 m.
+
+Fig. 17. Citrus aurantium: circumnutation of epicotyl with a filament
+fixed transversely near its apex, traced on a horizontal glass, from
+12.13 P.M. on Feb. 20th to 8.55 A.M. on 22nd. The movement of the bead
+of the filament was at first magnified 21 times, or 10½, in figure here
+given, and afterwards 36 times, or 18 as here given; seedling
+illuminated from above.
+
+Æsculus hippocastanum (Hippocastaneae).—Germinating seeds were placed
+in a tin box, kept moist internally, with a sloping bank of damp
+argillaceous sand, on which four smoked glass-plates rested, inclined
+at angles of 70° and 65° with the horizon. The tips of the radicles
+were placed so as just to touch the upper end of the glass-plates, and,
+as they grew downwards they pressed lightly, owing to geotropism, on
+the smoked surfaces, and left tracks of their course. In the middle
+part of each track the glass was swept clean, but the margins were much
+blurred and irregular. Copies of two of these tracks (all four being
+nearly alike) were made on tracing paper placed over the glass-plates
+after they had been varnished; and they are as exact as possible
+considering the nature of the margins (Fig. 18). They suffice to show
+that there was some lateral, almost serpentine movement, and that the
+tips in their downward course pressed with unequal force on the plates,
+as
+the tracks varied in breadth. The more perfectly serpentine tracks made
+by the radicles of Phaseolus multiflorus and Vicia faba (presently to
+be described), render it almost certain that the radicles of the
+present plant circumnutated.
+
+Fig. 18. Æsculus hippocastanum: outlines of tracks left on inclined
+glass-plates by tips of radicles. In A the plate was inclined at 70°
+with the horizon, and the radicle was 1.9 inch in length, and .23 inch
+in diameter at base. In B the plate was inclined 65° with the horizon,
+and the radicle was a trifle larger.
+
+Phaseolus multiflorus (Leguminosae).—Four smoked glass-plates were
+arranged in the same manner as described under Æsculus, and the tracks
+left by the tips of four radicles of the present plant, whilst growing
+downwards, were photographed as transparent objects. Three of them are
+here exactly copied (Fig. 19). Their serpentine courses show that the
+tips moved regularly from side to side; they also pressed alternately
+with greater or less force on the plates, sometimes rising up and
+leaving them altogether for a very short distance; but this was better
+seen on the original plates than in the copies. These radicles
+therefore were continually moving in all directions—that is, they
+circumnutated. The distance between the extreme right and left
+positions of the radicle A, in its lateral movement, was 2 mm., as
+ascertained by measurement with an eye-piece micrometer.
+
+Fig. 19. Phaseolus multiflorus: tracks left on inclined smoked
+glass-plates by tips of radicles in growing downwards. A and C, plates
+inclined at 60°, B inclined at 68° with the horizon.
+
+Vicia faba (Common Bean) (Leguminosae).—Radicle.—Some beans were
+allowed to germinate on bare sand, and after one had protruded its
+radicle to a length of .2 of an inch, it was turned upside down, so
+that the radicle, which was kept in damp air, now stood upright. A
+filament, nearly an inch in length, was affixed obliquely near its tip;
+and the movement of the terminal bead was traced from 8.30 A.M. to
+10.30 P.M., as shown in Fig. 18. The radicle at first changed its
+course twice
+abruptly, then made a small loop and then a larger zigzag curve. During
+the night and till 11 A.M. on the following morning, the bead moved to
+a great distance in a nearly straight line, in the direction indicated
+by the broken line in the figure. This resulted from the tip bending
+quickly downwards, as it had now become much declined, and had thus
+gained a position highly favourable for the action of geotropism.
+
+Fig. 20. Vicia faba: circumnutation of a radicle, at first pointing
+vertically upwards, kept in darkness, traced on a horizontal glass,
+during 14 hours. Movement of bead of filament magnified 23 times, here
+reduced to one-half of original scale.
+
+Fig. 21. Vicia faba: tracks left on inclined smoked glass-plates, by
+tips of radicles in growing downwards. Plate C was inclined at 63°,
+plates A and D at 71°, plate B at 75°, and plate E at a few degrees
+beneath the horizon.
+
+
+We next experimented on nearly a score of radicles by allowing them to
+grow downwards over inclined plates of smoked glass, in exactly the
+same manner as with Æsculus and Phaseolus. Some of the plates were
+inclined only a few degrees beneath the horizon, but most of them
+between 60° and 75°. In the latter cases the radicles in growing
+downwards were deflected only a little from the direction which they
+had followed whilst germinating in sawdust, and they pressed lightly on
+the glass-plates (Fig. 21). Five of the most distinct tracks are here
+copied, and they are all slightly sinuous, showing circumnutation.
+Moreover, a close examination of almost every one of the tracks clearly
+showed that the tips in their downward course had alternately pressed
+with greater or less force on the plates, and had sometimes risen up so
+as nearly to leave them for short intervals. The distance between the
+extreme right and left positions of the radicle A was 0.7 mm.,
+ascertained in the same manner as in the case of Phaseolus.
+
+Epicotyl.—At the point where the radicle had protruded from a bean laid
+on its side, a flattened solid lump projected .1 of an inch, in the
+same horizontal plane with the bean. This protuberance consisted of the
+convex summit of the arched epicotyl; and as it became developed the
+two legs of the arch curved themselves laterally upwards, owing to
+apogeotropism, at such a rate that the arch stood highly inclined after
+14 h., and vertically in 48 h. A filament was fixed to the crown of the
+protuberance before any arch was visible, but the basal half grew so
+quickly that on the second morning the end of the filament was bowed
+greatly downwards. It was therefore removed and fixed lower down. The
+line traced during these two days extended in the same general
+direction, and was in parts nearly straight, and in others plainly
+zigzag, thus giving some evidence of circumnutation.
+
+As the arched epicotyl, in whatever position it may be placed, bends
+quickly upwards through apogeotropism, and as the two legs tend at a
+very early age to separate from one another, as soon as they are
+relieved from the pressure of the surrounding earth, it was difficult
+to ascertain positively whether the epicotyl, whilst remaining arched,
+circumnutated. Therefore some rather deeply buried beans were
+uncovered, and the two legs of the arches were tied together, as had
+been done with the epicotyl of Tropaeolum and the hypocotyl of the
+Cabbage. The movements of the tied arches were traced in the usual
+manner on
+two occasions during three days. But the tracings made under such
+unnatural conditions are not worth giving; and it need only be said
+that the lines were decidedly zigzag, and that small loops were
+occasionally formed. We may therefore conclude that the epicotyl
+circumnutates whilst still arched and before it has grown tall enough
+to break through the surface of the ground.
+
+In order to observe the movements of the epicotyl at a somewhat more
+advanced age, a filament was fixed near the base of one which was no
+longer arched, for its upper half now formed a right angle with the
+lower half. This bean had germinated on bare damp sand, and the
+epicotyl began to straighten itself much sooner than would have
+occurred if it had been properly planted. The course pursued during 50
+h. (from 9 A.M. Dec. 26th, to 11 A.M. 28th) is here shown (Fig. 22);
+and we see that the epicotyl circumnutated during the whole time. Its
+basal part grew so much during the 50 h. that the filament at the end
+of our observations was attached at the height of .4 inch above the
+upper surface of the bean, instead of close to it. If the bean had been
+properly planted, this part of the epicotyl would still have been
+beneath the soil.
+
+Fig. 22. Vicia faba: circumnutation of young epicotyl, traced in
+darkness during 50 hours on a horizontal glass. Movement of bead of
+filament magnified 20 times, here reduced to one-half of original
+scale.
+
+Late in the evening of the 28th, some hours after the above
+observations were completed, the epicotyl had grown much straighter,
+for the upper part now formed a widely open angle with the lower part.
+A filament was fixed to the upright basal part, higher up than before,
+close beneath the lowest scale-like process or homologue of a leaf; and
+its movement was traced
+during 38 h. (Fig. 23). We here again have plain evidence of continued
+circumnutation. Had the bean been properly planted, the part of the
+epicotyl to which the filament was attached, the movement of which is
+here shown, would probably have just risen above the surface of the
+ground.
+
+Fig. 23. Vicia faba: circumnutation of the same epicotyl as in Fig. 22,
+a little more advanced in age, traced under similar conditions as
+before, from 8.40 A.M. Dec. 28th, to 10.50 A.M. 30th. Movement of bead
+here magnified 20 times.
+
+Lathyrus nissolia (Leguminosae).—This plant was selected for
+observation from being an abnormal form with grass-like leaves. The
+cotyledons are hypogean, and the epicotyl breaks through the ground in
+an arched form. The movements of a stem, 1.2 inch in height, consisting
+of three internodes, the lower one almost wholly subterranean, and the
+upper one bearing a short,
+narrow leaf, is shown during 24 h., in Fig. 24. No glass filament was
+employed, but a mark was placed beneath the apex of the leaf. The
+actual length of the longer of the two ellipses described by the stem
+was about .14 of an inch. On the previous day the chief line of
+movement was nearly at right angles to that shown in the present
+figure, and it was more simple.
+
+Fig. 24. Lathyrus nissolia: circumnutation of stem of young seedling,
+traced in darkness on a horizontal glass, from 6.45 A.M. Nov. 22nd, to
+7 A.M. 23rd. Movement of end of leaf magnified about 12 times, here
+reduced to one-half of original scale.
+
+Cassia tora[1] (Leguminosae).—A seedling was placed before a
+north-east window; it bent very little towards it, as the hypocotyl
+which was left free was rather old, and therefore not highly
+heliotropic. A filament had been fixed to the midrib of one of the
+cotyledons, and the movement of the whole seedling was traced during
+two days. The circumnutation of the hypocotyl is quite insignificant
+compared with that of the cotyledons. These rise up vertically at night
+and come into close contact; so that they may be said to sleep. This
+seedling was so old that a very small true leaf had been developed,
+which at night was completely hidden by the closed cotyledons. On Sept.
+24th, between 8 A.M. and 5 P.M., the cotyledons moved five times up and
+five times down; they therefore described five irregular ellipses in
+the course of the 9 h. The great nocturnal rise commenced about 4.30
+P.M.
+
+ [1] Seeds of this plant, which grew near the sea-side, were sent to us
+ by Fritz Müller from S. Brazil. The seedlings did not flourish or
+ flower well with us; they were sent to Kew, and were pronounced not to
+ be distinguishable from C. tora.
+
+
+Fig. 25. Cassia tora: conjoint circumnutation of cotyledons and
+hypocotyl, traced on vertical glass, from 7.10 A.M. Sept. 25th to 7.30
+A.M. 26th. Figure here given reduced to one-half of original scale.
+
+On the following morning (Sept. 25th) the movement of the same
+cotyledon was again traced in the same manner during 24 h.; and a copy
+of the tracing is here given (Fig. 25). The morning was cold, and the
+window had been accidentally left open for a short time, which must
+have chilled the plant; and this probably prevented it from moving
+quite as freely as on the previous day; for it rose only four and sank
+only four times during the day, one of the oscillations being very
+small. At 7.10 A.M., when the first dot was made, the cotyledons were
+not fully open or awake; they continued to open till about 9 A.M., by
+which time they had sunk a little beneath the horizon: by 9.30 A.M.
+they had risen, and then they oscillated up and down; but the upward
+and downward lines never quite coincided. At about 4.30 P.M. the great
+nocturnal rise commenced. At 7 A.M. on the following morning (Sept.
+26th) they occupied nearly the same level as on the previous morning,
+as shown in the diagram: they then began to open or sink in the usual
+manner. The diagram leads to the belief that the great periodical daily
+rise and fall does not differ essentially, excepting in amplitude, from
+the oscillations during the middle of the day.
+
+Lotus Jacoboeus (Leguminosae).—The cotyledons of this plant, after the
+few first days of their life, rise so as to stand almost, though rarely
+quite, vertically at night. They continue to act in this manner for a
+long time even after the development of some of the true leaves. With
+seedlings, 3 inches in height, and bearing five or six leaves, they
+rose at night about 45°. They continued to act thus for about an
+additional fortnight. Subsequently they remained horizontal at night,
+though still green
+and at last dropped off. Their rising at night so as to stand almost
+vertically appears to depend largely on temperature; for when the
+seedlings were kept in a cool house, though they still continued to
+grow, the cotyledons did not become vertical at night. It is remarkable
+that the cotyledons do not generally rise at night to any conspicuous
+extent during the first four or five days after germination; but the
+period was extremely variable with seedlings kept under the same
+conditions; and many were observed. Glass filaments with minute
+triangles of paper were fixed to the cotyledons (1½ mm. in breadth) of
+two seedlings, only 24 h. old, and the hypocotyl was secured to a
+stick; their movements greatly magnified were traced, and they
+certainly circumnutated the whole time on a small scale, but they did
+not exhibit any distinct nocturnal and diurnal movement. The
+hypocotyls, when left free, circumnutated over a large space.
+
+Another and much older seedling, bearing a half-developed leaf, had its
+movements traced in a similar manner during the three first days and
+nights of June; but seedlings at this age appear to be very sensitive
+to a deficiency of light; they were observed under a rather dim
+skylight, at a temperature of between 16° to 17½° C.’ and apparently,
+in consequence of these conditions, the great daily movement of the
+cotyledons ceased on the third day. During the first two days they
+began rising in the early afternoon in a nearly straight line, until
+between 6 and 7 P.M., when they stood vertically. During the latter
+part of the night, or more probably in the early morning, they began to
+fall or open, so that by 6.45 A.M. they stood fully expanded and
+horizontal. They continued to fall slowly for some time, and during the
+second day described a single small ellipse, between 9 A.M. and 2 P.M.,
+in addition to the great diurnal movement. The course pursued during
+the whole 24 h. was far less complex than in the foregoing case of
+Cassia. On the third morning they fell very much, and then
+circumnutated on a small scale round the same spot; by 8.20 P.M. they
+showed no tendency to rise at night. Nor did the cotyledons of any of
+the many other seedlings in the same pot rise; and so it was on the
+following night of June 5th. The pot was then taken back into the
+hot-house, where it was exposed to the sun, and on the succeeding night
+all the cotyledons rose again to a high angle, but did not stand quite
+vertically. On each of the above days the line representing the great
+nocturnal
+rise did not coincide with that of the great diurnal fall, so that
+narrow ellipses were described, as is the usual rule with
+circumnutating organs. The cotyledons are provided with a pulvinus, and
+its development will hereafter be described.
+
+Mimosa pudica (Leguminosae).—The cotyledons rise up vertically at
+night, so as to close together. Two seedlings were observed in the
+greenhouse (temp. 16° to 17° C. or 63° to 65° F.). Their hypocotyls
+were secured to sticks, and glass filaments bearing little triangles of
+paper were affixed to the cotyledons of both. Their movements were
+traced on a vertical glass during 24 h. on November 13th. The pot had
+stood for some time in the same position, and they were chiefly
+illuminated through the glass-roof. The cotyledons of one of these
+seedlings moved downward in the morning till 11.30 A.M., and then rose,
+moving rapidly in the evening until they stood vertically, so that in
+this case there was simply a single great daily fall and rise. The
+other seedling behaved rather differently, for it fell in the morning
+until 11.30 A.M., and then rose, but after 12.10 P.M. again fell; and
+the great evening rise did not begin until 1.22 P.M. On the following
+morning this cotyledon had fallen greatly from its vertical position by
+8.15 A.M. Two other seedlings (one seven and the other eight days old)
+had been previously observed under unfavourable circumstances, for they
+had been brought into a room and placed before a north-east window,
+where the temperature was between only 56° and 57° F. They had,
+moreover, to be protected from lateral light, and perhaps were not
+sufficiently illuminated. Under these circumstances the cotyledons
+moved simply downwards from 7 A.M. till 2 P.M., after which hour and
+during a large part of the night they continued to rise. Between 7 and
+8 A.M. on the following morning they fell again; but on this second and
+likewise on the third day the movements became irregular, and between 3
+and 10.30 P.M. they circumnutated to a small extent about the same
+spot; but they did not rise at night. Nevertheless, on the following
+night they rose as usual.
+
+Cytisus fragrans (Leguminosae).—Only a few observations were made on
+this plant. The hypocotyl circumnutated to a considerable extent, but
+in a simple manner—namely, for two hours in one direction, and then
+much more slowly back again in a zigzag course, almost parallel to the
+first line, and beyond the starting-point. It moved in the same
+direction all night, but next morning began to return. The cotyledons
+continually
+move both up and down and laterally; but they do not rise up at night
+in a conspicuous manner.
+
+Lupinus luteus (Leguminosae).—Seedlings of this plant were observed
+because the cotyledons are so thick (about .08 of an inch) that it
+seemed unlikely that they would move. Our observations were not very
+successful, as the seedlings are strongly heliotropic, and their
+circumnutation could not be accurately observed near a north-east
+window, although they had been kept during the previous day in the same
+position. A seedling was then placed in darkness with the hypocotyl
+secured to a stick; both cotyledons rose a little at first, and then
+fell during the rest of the day; in the evening between 5 and 6 P.M.
+they moved very slowly; during the night one continued to fall and the
+other rose, though only a little. The tracing was not much magnified,
+and as the lines were plainly zigzag, the cotyledons must have moved a
+little laterally, that is, they must have circumnutated.
+
+The hypocotyl is rather thick, about .12 of inch; nevertheless it
+circumnutated in a complex course, though to a small extent. The
+movement of an old seedling with two true leaves partially developed,
+was observed in the dark. As the movement was magnified about 100 times
+it is not trustworthy and is not given; but there could be no doubt
+that the hypocotyl moved in all directions during the day, changing its
+course 19 times. The extreme actual distance from side to side through
+which the upper part of the hypocotyl passed in the course of 14½ hours
+was only 1/60 of an inch; it sometimes travelled at the rate of 1/50 of
+an inch in an hour.
+
+Cucurbita ovifera (Cucurbitaceæ).—Radicle: a seed which had germinated
+on damp sand was fixed so that the slightly curved radicle, which was
+only .07 inch in length, stood almost vertically
+upwards, in which position geotropism would act at first with little
+power. A filament was attached near to its base, and projected at about
+an angle of 45° above the horizon. The general course followed during
+the 11 hours of observation and during the following night is shown in
+the accompanying diagram (Fig. 26), and was plainly due to geotropism;
+but it was also clear that the radicle circumnutated. By the next
+morning the tip had curved so much downwards that the filament, instead
+of projecting at 45° above the horizon, was nearly horizontal. Another
+germinating seed was turned upside down and covered with damp sand; and
+a filament was fastened to the radicle so as to project at an angle of
+about 50° above the horizon; this radicle was .35 of an inch in length
+and a little curved. The course pursued was mainly governed, as in the
+last case, by geotropism, but the line traced during 12 hours and
+magnified as before was more strongly zigzag, again showing
+circumnutation.
+
+Fig. 26. Cucurbita ovifera: course followed by a radicle in bending
+geotropically downwards, traced on a horizontal glass, between 11.25
+A.M. and 10.25 P.M.; the direction during the night is indicated by the
+broken line. Movement of bead magnified 14 times.
+
+Four radicles were allowed to grow downwards over plates of smoked
+glass, inclined at 70° to the horizon, under the same conditions as in
+the cases of Æsculus, Phaseolus, and Vicia. Facsimiles are here given
+(Fig. 27) of two of these tracks; and a third short one was almost as
+plainly serpentine as that at A. It was also manifest by a greater or
+less amount of soot having been swept off the glasses, that the tips
+had
+pressed alternately with greater and less force on them. There must,
+therefore, have been movement in at least two planes at right angles to
+one another. These radicles were so delicate that they rarely had the
+power to sweep the glasses quite clean. One of them had developed some
+lateral or secondary rootlets, which projected a few degrees beneath
+the horizon; and it is an important fact that three of them left
+distinctly serpentine tracks on the smoked surface, showing beyond
+doubt that they had circumnutated like the main or primary radicle. But
+the tracks were so slight that they could not be traced and copied
+after the smoked surface had been varnished.
+
+Fig. 27. Cucurbita ovifera: tracks left by tips of radicles in growing
+downwards over smoked glass-plates, inclined at 70° to the horizon.
+
+Fig. 28. Cucurbita ovifera: circumnutation of arched hypocotyl at a
+very early age, traced in darkness on a horizontal glass, from 8 A.M.
+to 10.20 A.M. on the following day. The movement of the bead magnified
+20 times, here reduced to one-half of original scale.
+
+Fig. 29. Cucurbita ovifera: circumnutation of straight and vertical
+hypocotyl, with filament fastened transversely across its upper end,
+traced in darkness on a horizontal glass, from 8.30 A.M. to 8.30 P.M.
+The movement of the terminal bead originally magnified about 18 times,
+here only 4½ times.
+
+Hypocotyl.—A seed lying on damp sand was firmly fixed by two crossed
+wires and by its own growing radicle. The cotyledons were still
+enclosed within the seed-coats; and the short hypocotyl, between the
+summit of the radicle and the cotyledons, was as yet only slightly
+arched. A filament (.85 of inch in length) was attached at an angle of
+35° above the horizon to the side of the arch adjoining the cotyledons.
+This part would ultimately form the upper end of the hypocotyl, after
+it had grown straight and vertical. Had the seed been properly planted,
+the hypocotyl at this stage of growth would have been deeply buried
+beneath the surface. The course followed by the bead of the filament is
+shown in Fig. 28. The chief lines of movement from left to right in the
+figure were parallel to the plane of the two united cotyledons and of
+the flattened seed; and this movement would aid in dragging them out of
+the seed-coats, which are held down by a special structure hereafter to
+be described. The movement at right angles to the above lines was due
+to the arched hypocotyl becoming more arched as it increased in height.
+The foregoing observations apply to the leg of the arch next to the
+cotyledons, but
+the other leg adjoining the radicle likewise circumnutated at an
+equally early age.
+
+The movement of the same hypocotyl after it had become straight and
+vertical, but with the cotyledons only partially expanded, is shown in
+Fig. 29. The course pursued during 12 h. apparently represents four and
+a half ellipses or ovals, with the longer axis of the first at nearly
+right angles to that of the others. The longer axes of all were oblique
+to a line joining the opposite cotyledons. The actual extreme distance
+from side to side over which the summit of the tall hypocotyl passed in
+the course of 12 h. was .28 of an inch. The original figure was traced
+on a large scale, and from the obliquity of the line of view the outer
+parts of the diagram are much exaggerated.
+
+Cotyledons.—On two occasions the movements of the cotyledons were
+traced on a vertical glass, and as the ascending and descending lines
+did not quite coincide, very narrow ellipses were formed; they
+therefore circumnutated. Whilst young they rise vertically up at night,
+but their tips always remain reflexed; on the following morning they
+sink down again. With a seedling kept in complete darkness they moved
+in the same manner, for they sank from 8.45 A.M. to 4.30 P.M.; they
+then began to rise and remained close together until 10 P.M., when they
+were last observed. At 7 A.M. on the following morning they were as
+much expanded as at any hour on the previous day. The cotyledons of
+another young seedling, exposed to the light, were fully open for the
+first time on a certain day, but were found completely closed at 7 A.M.
+on the following morning. They soon began to expand again, and
+continued doing so till about 5 P.M.; they then began to rise, and by
+10.30 P.M. stood vertically and were almost closed. At 7 A.M. on the
+third morning they were nearly vertical, and again expanded during the
+day; on the fourth morning they were not closed, yet they opened a
+little in the course of the day and rose a little on the following
+night. By this time a minute true leaf had become developed. Another
+seedling, still older, bearing a well-developed leaf, had a sharp rigid
+filament affixed to one of its cotyledons (85 mm. in length), which
+recorded its own movements on a revolving drum with smoked paper. The
+observations were made in the hot-house, where the plant had lived, so
+that there was no change in temperature or light. The record commenced
+at 11 A.M. on February 18th; and from this hour till 3 P.M. the
+cotyledon fell; it then rose rapidly till 9 P.M., then very gradually
+till 3 A.M. February 19th, after which hour it sank gradually till 4.30
+P.M.; but the downward movement was interrupted by one slight rise or
+oscillation about 1.30 P.M. After 4.30 P.M. (19th) the cotyledon rose
+till 1 A.M. (in the night of February 20th) and then sank very
+gradually till 9.30 A.M., when our observations ceased. The amount of
+movement was greater on the 18th than on the 19th or on the morning of
+the 20th.
+
+Cucurbita aurantia.—An arched hypocotyl was found buried a little
+beneath the surface of the soil; and in order to prevent it
+straightening itself quickly, when relieved from the surrounding
+pressure of the soil, the two legs of the arch were tied together. The
+seed was then lightly covered with loose damp earth. A filament with a
+bead at the end was affixed to the basal leg, the movements of which
+were observed during two days in the usual manner. On the first day the
+arch moved in a zigzag line towards the side of the basal leg. On the
+next day, by which time the dependent cotyledons had been dragged above
+the surface of the soil, the tied arch changed its course greatly nine
+times in the course of 14½ h. It swept a large, extremely irregular,
+circular figure, returning at night to nearly the same spot whence it
+had started early in the morning. The line was so strongly zigzag that
+it apparently represented five ellipses, with their longer axes
+pointing in various directions. With respect to the periodical
+movements of the cotyledons, those of several young seedlings formed
+together at 4 P.M. an angle of about 60°, and at 10 P.M. their lower
+parts stood vertically and were in contact; their tips, however, as is
+usual in the genus, were permanently reflexed. These cotyledons, at 7
+A.M. on the following morning, were again well expanded.
+
+Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbitaceæ).—A
+seedling opened its cotyledons, the movements of which were alone
+observed, slightly on June 27th and closed them at night: next day, at
+noon (28th), they included an angle of 53°, and at 10 P.M. they were in
+close contact, so that each had risen 26½°. At noon, on the 29th, they
+included an angle of 118°, and at 10 P.M. an angle of 54°, so each had
+risen 32°. On the following day they were still more open, and the
+nocturnal rise was greater, but the angles were not measured. Two other
+seedlings were observed, and behaved during three days in a closely
+similar manner. The cotyledons, therefore,
+open more and more on each succeeding day, and rise each night about
+30°; consequently during the first two nights of their life they stand
+vertically and come into contact.
+
+Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1½ inch in
+length, apex only 4 3/4 inches from the vertical glass, on which its
+movements were traced from 7.35 A.M. July 11th to 9.5 A.M. on the 14th.
+Figure here given reduced to one-third of original scale.
+
+In order to ascertain more accurately the nature of these movements,
+the hypocotyl of a seedling, with its cotyledons well expanded, was
+secured to a little stick, and a filament with triangles of paper was
+affixed to one of the cotyledons. The observations were made under a
+rather dim skylight, and the temperature during the whole time was
+between 17½° to 18° C. (63° to 65° F.). Had the temperature been higher
+and the light brighter, the movements would probably have been greater.
+On July 11th (see Fig. 30), the cotyledon fell from 7.35 A.M. till 10
+A.M.; it then rose (rapidly after 4 P.M.) till it stood quite
+vertically at 8.40 P.M. During the early morning of the next day (12th)
+it fell, and continued to fall till 8 A.M., after which hour it rose,
+then fell, and again rose, so that by 10.35 P.M. it stood much higher
+than it did in the morning, but was not vertical as on the preceding
+night. During the following early morning and whole day (13th) it fell
+and circumnutated, but had not risen when observed late in the evening;
+and this was probably due to the deficiency of heat or light, or of
+both. We thus see that the cotyledons became more widely open at noon
+on each succeeding day; and that they rose considerably each night,
+though not acquiring a vertical position, except during the first two
+nights.
+
+Cucumis dudaim (Cucurbitaceæ).—Two seedlings had opened
+their cotyledons for the first time during the day,—one to the extent
+of 90° and the other rather more; they remained in nearly the same
+position until 10.40 P.M.; but by 7 A.M. on the following morning the
+one which had been previously open to the extent of 90° had its
+cotyledons vertical and completely shut; the other seedling had them
+nearly shut. Later in the morning they opened in the ordinary manner.
+It appears therefore that the cotyledons of this plant close and open
+at somewhat different periods from those of the foregoing species of
+the allied genera of Cucurbita and Lagenaria.
+
+Fig. 31. Opuntia basilaris: conjoint circumnutation of hypocotyl and
+cotyledon; filament fixed longitudinally to cotyledon, and movement
+traced during 66 h. on horizontal glass. Movement of the terminal bead
+magnified about 30 times, here reduced to one-third scale. Seedling
+kept in hot-house, feebly illuminated from above.
+
+Opuntia basilaris (Cacteæ).—A seedling was carefully observed, because,
+considering its appearance and the nature of the mature plant, it
+seemed very unlikely that either the hypocotyl or cotyledons would
+circumnutate to an appreciable extent. The cotyledons were well
+developed, being .9 of an inch in length, .22 in breadth, and .15 in
+thickness. The almost cylindrical hypocotyl, now bearing a minute
+spinous bud on its summit, was only .45 of an inch in height, and .19
+in diameter. The tracing (Fig. 31) shows the combined movement of the
+hypocotyl and of one of the cotyledons, from 4.45 P.M. on May 28th to
+11 A.M. on the 31st. On the 29th a nearly perfect ellipse was
+completed. On the 30th the hypocotyl moved, from some unknown cause, in
+the same general direction in a zigzag line; but between 4.30 and 10
+P.M. almost completed a second small ellipse. The cotyledons move only
+a little up and down: thus at 10.15 P.M. they stood only 10° higher
+than at noon. The chief seat of movement therefore, at least when the
+cotyledons are rather old as in the present case, lies in the
+hypocotyl. The ellipse described on the 29th had its longer axis
+directed at nearly right angles to a line joining the two cotyledons.
+The actual amount of movement of the bead at the end of the
+filament was, as far as could be ascertained, about .14 of an inch.
+
+Fig. 32. Helianthus annuus: circumnutation of hypocotyl, with filament
+fixed across its summit, traced on a horizontal glass in darkness, from
+8.45 A.M. to 10.45 P.M., and for an hour on following morning. Movement
+of bead magnified 21 times, here reduced to one-half of original scale.
+
+Helianthus annuus (Compositæ).—The upper part of the hypocotyl moved
+during the day-time in the course shown in the annexed figure (Fig.
+32). As the line runs in various directions, crossing itself several
+times, the movement may be considered as one of circumnutation. The
+extreme actual distance travelled was at least .1 of an inch. The
+movements of the cotyledons of two seedlings were observed; one facing
+a north-east window, and the other so feebly illuminated from above us
+as to be almost in darkness. They continued to sink till about noon,
+when they began to rise; but between 5 and 7 or 8 P.M. they either sank
+a little, or moved laterally, and then again began to rise. At 7 A.M.
+on the following morning those on the plant before the north-east
+window had opened so little that they stood at an angle of 73° above
+the horizon, and were not observed any longer. Those on the seedling
+which had been kept in almost complete darkness, sank during the whole
+day, without rising about mid-day, but rose during the night. On the
+third and fourth days they continued sinking without any alternate
+ascending movement; and this, no doubt, was due to the absence of
+light.
+
+Primula Sinensis (Primulaceae).—A seedling was placed with the two
+cotyledons parallel to a north-east window on a day when the light was
+nearly uniform, and a filament was affixed to one of them. From
+observations subsequently made on another seedling with the stem
+secured to a stick, the greater part of the movement shown in the
+annexed figure (Fig. 33), must have been that of the hypocotyl, though
+the cotyledons certainly move up and down to a certain extent both
+during the day and night. The movements of the same seedling were
+traced
+on the following day with nearly the same result; and there can be no
+doubt about the circumnutation of the hypocotyl.
+
+Fig. 33. Primula Sinensis: conjoint circumnutation of hypocotyl and
+cotyledon, traced on vertical glass, from 8.40 A.M. to 10.45 P.M.
+Movements of bead magnified about 26 times.
+
+Cyclamen Persicum (Primulaceae).—This plant is generally supposed to
+produce only a single cotyledon, but Dr. H. Gressner[2] has shown that
+a second one is developed after a long interval of time. The hypocotyl
+is converted into a globular corm, even before the first cotyledon has
+broken through the ground with its blade closely enfolded and with its
+petiole in the form of an arch, like the arched hypocotyl or epicotyl
+of any ordinary dicotyledonous plant. A glass filament was affixed to a
+cotyledon, .55 of an inch in height, the petiole of which had
+straightened itself and stood nearly vertical, but with the blade not
+as yet fully expanded. Its movements were traced during 24½ h. on a
+horizontal glass, magnified 50 times; and in this interval it described
+two irregular small circles; it therefore circumnutates, though on an
+extremely small scale.
+
+ [2] ‘Bot. Zeitung,’ 1874, p. 837.
+
+
+Fig. 34. Stapelia sarpedon: circumnutation of hypocotyl, illuminated
+from above, traced on horizontal glass, from 6.45 A.M. June 26th to
+8.45 A.M. 28th. Temp. 23–24° C. Movement of bead magnified 21 times.
+
+Stapelia sarpedon (Asclepiadeae).—This plant, when mature, resembles a
+cactus. The flattened hypocotyl is fleshy, enlarged in the upper part,
+and bears two rudimentary cotyledons. It breaks through the ground in
+an arched form, with the rudimentary cotyledons closed or in contact. A
+filament was affixed almost
+vertically to the hypocotyl of a seedling half an inch high; and its
+movements were traced during 50 h. on a horizontal glass (Fig. 34).
+From some unknown cause it bowed itself to one side, and as this was
+effected by a zigzag course, it probably circumnutated; but with hardly
+any other seedling observed by us was this movement so obscurely shown.
+
+Ipomœa caerulea vel Pharbitis nil (Convolvulaceae).—Seedlings of this
+plant were observed because it is a twiner, the upper internodes of
+which circumnutate conspicuously; but like other twining plants, the
+first few internodes which rise above the ground are stiff enough to
+support themselves, and therefore do not circumnutate in any plainly
+recognisable manner.[3] In this particular instance the fifth internode
+(including the hypocotyl) was the first which plainly circumnutated and
+twined round a stick. We therefore wished to learn whether
+circumnutation could be observed in the hypocotyl if carefully observed
+in our usual manner. Two seedlings were kept in the dark with filaments
+fixed to the upper part of their hypocotyls; but from circumstances not
+worth explaining their movements were traced for only a short time. One
+moved thrice forwards and twice backwards in nearly opposite
+directions, in the course of 3 h. 15 m.; and the other twice forwards
+and twice backwards in 2 h. 22 m. The hypocotyl therefore circumnutated
+at a remarkably rapid rate. It may here be added that a filament was
+affixed transversely to the summit of the second internode above the
+cotyledons of a little plant 3½ inches in height; and its movements
+were traced on a horizontal glass. It circumnutated, and the actual
+distance travelled from side to side was a quarter of an inch, which
+was too small an amount to be perceived without the aid of marks.
+
+ [3] ‘Movements and Habits of Climbing Plants,’ p. 33, 1875.
+
+
+The movements of the cotyledons are interesting from their complexity
+and rapidity, and in some other respects. The hypocotyl (2 inches high)
+of a vigorous seedling was secured to a stick, and a filament with
+triangles of paper was affixed to one of the cotyledons. The plant was
+kept all day in the hot-house, and at 4.20 P.M. (June 20th) was placed
+under a skylight in the house, and observed occasionally during the
+evening and night. It fell in a slightly zigzag line to a moderate
+extent from 4.20 P.M. till 10.15 P.M. When looked at shortly after
+midnight (12.30 P.M.) it had risen a very little, and considerably by
+3.45 A.M. When again looked at, at 6.10 A.M. (21st), it had fallen
+largely. A new tracing was now begun (see Fig. 35), and soon
+afterwards, at 6.42 A.M., the cotyledon had risen a little. During the
+forenoon it was observed about every hour; but between 12.30 and 6 P.M.
+every half-hour. If the observations had been made at these short
+intervals during the whole day, the figure would have been too
+intricate to have been copied. As it was, the cotyledon moved up and
+down in the course of 16 h. 20 m. (i.e. between 6.10 A.M. and 10.30
+P.M.) thirteen times.
+
+Fig. 35. Ipomœa caerulea: circumnutation of cotyledon, traced on
+vertical glass, from 6.10 A.M. June 21st to 6.45 A.M. 22nd. Cotyledon
+with petiole 1.6 inch in length, apex of blade 4.1 inch from the
+vertical glass; so movement not greatly magnified; temp. 20° C.
+
+The cotyledons of this seedling sank downwards during both evenings and
+the early part of the night, but rose during the latter part. As this
+is an unusual movement, the cotyledons of twelve other seedlings were
+observed; they stood almost or quite horizontally at mid-day, and at 10
+P.M. were all declined at various angles. The most usual angle was
+between 30° and 35°; but three stood at about 50° and one at even 70°
+beneath the horizon. The blades of all these cotyledons had attained
+almost their full size, viz. from 1 to 1½ inches in length, measured
+along their midribs. It is a remarkable fact that whilst young—that is,
+when less than half an inch in length, measured in the same manner—they
+do not sink
+downwards in the evening. Therefore their weight, which is considerable
+when almost fully developed, probably came into play in originally
+determining the downward movement. The periodicity of this movement is
+much influenced by the degree of light to which the seedlings have been
+exposed during the day; for three kept in an obscure place began to
+sink about noon, instead of late in the evening; and those of another
+seedling were almost paralysed by having been similarly kept during two
+whole days. The cotyledons of several other species of Ipomœa likewise
+sink downwards late in the evening.
+
+Cerinthe major (Boragineae).—The circumnutation of the hypocotyl of a
+young seedling with the cotyledons hardly expanded, is shown in the
+annexed figure (Fig. 36), which apparently represents four or five
+irregular ellipses, described in the course of a little over 12 hours.
+Two older seedlings were similarly observed, excepting that one of them
+was kept in the dark; their hypocotyls also circumnutated, but in a
+more simple manner. The cotyledons on a seedling exposed to the light
+fell from the early morning until a little after noon, and then
+continued to rise until 10.30 P.M. or later. The cotyledons of this
+same seedling acted in the same general manner during the two following
+days. It had previously been tried in the dark, and after being thus
+kept for only 1 h. 40 m. the cotyledons began at 4.30 P.M. to sink,
+instead of continuing to rise till late at night.
+
+Fig. 36. Cerinthe major: circumnutation of hypocotyl, with filament
+fixed across its summit, illuminated from above, traced on horizontal
+glass, from 9.26 A.M. to 9.53 P.M. on Oct. 25th. Movement of the bead
+magnified 30 times, here reduced to one-third of original scale.
+
+
+Nolana prostrata (Nolaneae).—The movements were not traced, but a pot
+with seedlings, which had been kept in the dark for an hour, was placed
+under the microscope, with the micrometer eye-piece so adjusted that
+each division equalled 1/500th of an inch. The apex of one of the
+cotyledons crossed rather obliquely four divisions in 13 minutes; it
+was also sinking, as shown by getting out of focus. The seedlings were
+again placed in darkness for another hour, and the apex now crossed two
+divisions in 6 m. 18 s.; that is, at very nearly the same rate as
+before. After another interval of an hour in darkness, it crossed two
+divisions in 4 m. 15 s., therefore at a quicker rate. In the afternoon,
+after a longer interval in the dark, the apex was motionless, but after
+a time it recommenced moving, though slowly; perhaps the room was too
+cold. Judging from previous cases, there can hardly be a doubt that
+this seedling was circumnutating.
+
+Solanum lycopersicum (Solaneae).—The movements of the hypocotyls of two
+seedling tomatoes were observed during seven hours, and there could be
+no doubt that both circumnutated. They were illuminated from above, but
+by an accident a little light entered on one side, and in the
+accompanying figure (Fig. 37) it may be seen that the hypocotyl moved
+to this side (the upper one in the figure), making small loops and
+zigzagging in its course. The movements of the cotyledons were also
+traced both on vertical and horizontal glasses; their angles with the
+horizon were likewise measured at various hours. They fell from 8.30
+A.M. (October 17th) to about noon; then moved laterally in a zigzag
+line, and at about 4 P.M. began to rise; they continued to do so until
+10.30 P.M., by which hour they stood vertically and were asleep. At
+what hour of the night or early morning they began to fall was not
+ascertained. Owing to the lateral movement shortly after mid-day, the
+descending and ascending lines did not coincide, and irregular ellipses
+were described during each 24 h. The regular periodicity of these
+movements is destroyed, as we shall hereafter see, if the seedlings are
+kept in the dark.
+
+Fig. 37. Solanum lycopersicum: circumnutation of hypocotyl, with
+filament fixed across its summit, traced on horizontal glass, from 10
+A.M. to 5 P.M. Oct. 24th. Illuminated obliquely from above. Movement of
+bead magnified about 35 times, here reduced to one-third of original
+scale.
+
+
+Solanum palinacanthum.—Several arched hypocotyls rising nearly .2 of an
+inch above the ground, but with the cotyledons still buried beneath the
+surface, were observed, and the tracings showed that they
+circumnutated. Moreover, in several cases little open circular spaces
+or cracks in the argillaceous sand which surrounded the arched
+hypocotyls were visible, and these appeared to have been made by the
+hypocotyls having bent first to one and then to another side whilst
+growing upwards. In two instances the vertical arches were observed to
+move to a considerable distance backwards from the point where the
+cotyledons lay buried; this movement, which has been noticed in some
+other cases, and which seems to aid in extracting the cotyledons from
+the buried seed-coats, is due to the commencement of the straightening
+of the hypocotyl. In order to prevent this latter movement, the two
+legs of an arch, the summit of which was on a level with the surface of
+the soil, were tied together; the earth having been previously removed
+to a little depth all round. The movement of the arch during 47 hours
+under these unnatural circumstances is exhibited in the annexed figure.
+
+Fig. 38. Solanum palinacanthum: circumnutation of an arched hypocotyl,
+just emerging from the ground, with the two legs tied together, traced
+in darkness on a horizontal glass, from 9.20 A.M. Dec. 17th to 8.30
+A.M. 19th. Movement of bead magnified 13 times; but the filament, which
+was affixed obliquely to the crown of the arch, was of unusual length.
+
+The cotyledons of some seedlings in the hot-house were horizontal about
+noon on December 13th; and at 10 P.M. had risen to an angle of 27°
+above the horizon; at 7 A.M. on the following
+morning, before it was light, they had risen to 59° above the horizon;
+in the afternoon of the same day they were found again horizontal.
+
+Beta vulgaris (Chenopodeae).—The seedlings are excessively sensitive to
+light, so that although on the first day they were uncovered only
+during two or three minutes at each observation, they all moved
+steadily towards the side of the room whence the light proceeded, and
+the tracings consisted only of slightly zigzag lines directed towards
+the light. On the next day the plants were placed in a completely
+darkened room, and at each observation were illuminated as much as
+possible from vertically above by a small wax taper. The annexed figure
+(Fig. 39) shows the movement of the hypocotyl during 9 h. under these
+circumstances. A second seedling was similarly observed at the same
+time, and the tracing had the same peculiar character, due to the
+hypocotyl often moving and returning in nearly parallel lines. The
+movement of a third hypocotyl differed greatly.
+
+Fig. 39. Beta vulgaris: circumnutation of hypocotyl, with filament
+fixed obliquely across its summit, traced in darkness on horizontal
+glass, from 8.25 A.M. to 5.30 P.M. Nov. 4th. Movement of bead magnified
+23 times, here reduced to one-third of original scale.
+
+We endeavoured to trace the movements of the cotyledons, and for this
+purpose some seedlings were kept in the dark, but they moved in an
+abnormal manner; they continued rising from 8.45 A.M. to 2 P.M., then
+moved laterally, and from 3 to 6 P.M. descended; whereas cotyledons
+which have been exposed all the day to the light rise in the evening so
+as to stand vertically at night; but this statement applies only to
+young seedlings. For instance, six seedlings in the greenhouse had
+their cotyledons partially open for the first time on the morning of
+November 15th, and at 8.45 P.M. all were completely closed, so that
+they might properly be said to be asleep. Again, on the morning of
+November 27th, the cotyledons of four other seedlings, which were
+surrounded by a collar of brown paper so that they received light only
+from above, were open to the extent of 39°; at 10 P.M. they were
+completely closed; next morning (November 28th) at 6.45 A.M. whilst it
+was still dark, two of them
+were partially open and all opened in the course of the morning; but at
+10.20 P.M. all four (not to mention nine others which had been open in
+the morning and six others on another occasion) were again completely
+closed. On the morning of the 29th they were open, but at night only
+one of the four was closed, and this only partially; the three others
+had their cotyledons much more raised than during the day. On the night
+of the 30th the cotyledons of the four were only slightly raised.
+
+Ricinus Borboniensis (Euphorbiaceae).—Seeds were purchased under the
+above name—probably a variety of the common castor-oil plant. As soon
+as an arched hypocotyl had risen clear above the ground, a filament was
+attached to the upper leg bearing the cotyledons which were still
+buried beneath the surface, and the movement of the bead was traced on
+a horizontal glass during a period of 34 h. The lines traced were
+strongly zigzag, and as the bead twice returned nearly parallel to its
+former course in two different directions, there could be no doubt that
+the arched hypocotyl circumnutated. At the close of the 34 h. the upper
+part began to rise and straighten itself, dragging the cotyledons out
+of the ground, so that the movements of the bead could no longer be
+traced on the glass.
+
+Quercus (American sp.) (Cupuliferae).—Acorns of an American oak which
+had germinated at Kew were planted in a pot in the greenhouse. This
+transplantation checked their growth; but after a time one grew to a
+height of five inches, measured to the tips of the small partially
+unfolded leaves on the summit, and now looked vigorous. It consisted of
+six very thin internodes of unequal lengths. Considering these
+circumstances and the nature of the plant, we hardly expected that it
+would circumnutate; but the annexed figure (Fig. 40) shows that it did
+so in a conspicuous manner, changing its course many times and
+travelling in all directions during the 48 h. of observation. The
+figure seems to represent 5 or 6 irregular ovals or ellipses. The
+actual amount of movement from side to side (excluding one great bend
+to the left) was about .2 of an inch; but this was difficult to
+estimate, as owing to the rapid growth of the stem, the attached
+filament was much further from the mark beneath at the close than at
+the commencement of the observations. It deserves notice that the pot
+was placed in a north-east room within a deep box, the top of which was
+not at first covered up, so that the inside facing
+the windows was a little more illuminated than the opposite side; and
+during the first morning the stem travelled to a greater distance in
+this direction (to the left in the figure) than it did afterwards when
+the box was completely protected from light.
+
+Fig. 40. Quercus (American sp.): circumnutation of young stem, traced
+on horizontal glass, from 12.50 P.M. Feb. 22nd to 12.50 P.M. 24th.
+Movement of bead greatly magnified at first, but slightly towards the
+close of the observations—about 10 times on an average.
+
+Quercus robur.—Observations were made only on the movements of the
+radicles from germinating acorns, which were allowed to grow downwards
+in the manner previously described, over plates of smoked glass,
+inclined at angles between 65° and 69° to the horizon. In four cases
+the tracks left were almost straight, but the tips had pressed
+sometimes with more and sometimes with less force on the glass, as
+shown by the varying thickness of the tracks and by little bridges of
+soot left across them. In the fifth case the track was slightly
+serpentine, that is, the tip had moved a little from side to side. In
+the sixth case (Fig. 41, A) it was plainly serpentine, and the tip had
+pressed almost equably on the glass in its whole course. In the seventh
+case (B) the tip had moved both laterally and had pressed
+alternately with unequal force on the glass; so that it had moved a
+little in two planes at right angles to one another. In the eighth and
+last case (C) it had moved very little laterally, but had alternately
+left the glass and come into contact with it again. There can be no
+doubt that in the last four cases the radicle of the oak circumnutated
+whilst growing downwards.
+
+Fig. 41. Quercus robur: tracks left on inclined smoked glass-plates by
+tips of radicles in growing downwards. Plates A and C inclined at 65°
+and plate B at 68° to the horizon.
+
+Corylus avellana (Corylaceae).—The epicotyl breaks through the ground
+in an arched form; but in the specimen which was first examined, the
+apex had become decayed, and the epicotyl grew to some distance through
+the soil, in a tortuous, almost horizontal direction, like a root. In
+consequence of this injury it had emitted near the hypogean cotyledons
+two secondary shoots, and it was remarkable that both of these were
+arched, like the normal epicotyl in ordinary cases. The soil was
+removed from around one of these arched secondary shoots, and a glass
+filament was affixed to the basal leg. The whole was kept damp beneath
+a metal-box with a glass lid, and was thus illuminated only from above.
+Owing apparently to the lateral pressure of the earth being removed,
+the terminal and bowed-down part of the shoot began at once to move
+upwards, so that after 24 h. it formed a right angle with the lower
+part. This lower part, to which the filament was attached, also
+straightened itself, and moved a little backwards from the upper part.
+Consequently a long line was traced on the horizontal glass; and
+this was in parts straight and in parts decidedly zigzag, indicating
+circumnutation.
+
+On the following day the other secondary shoot was observed; it was a
+little more advanced in age, for the upper part, instead of depending
+vertically downwards, stood at an angle of 45° above the horizon. The
+tip of the shoot projected obliquely .4 of an inch above the ground,
+but by the close of our observations, which lasted 47 h., it had grown,
+chiefly towards its base, to a height of .85 of an inch. The filament
+was fixed transversely to the basal and almost upright half of the
+shoot, close beneath the lowest scale-like appendage. The
+circumnutating course pursued is shown in the accompanying figure (Fig.
+42). The actual distance traversed from side to side was about .04 of
+an inch.
+
+Fig. 42. Corylus avellana: circumnutation of a young shoot emitted from
+the epicotyl, the apex of which had been injured, traced on a
+horizontal glass, from 9 A.M. Feb. 2nd to 8 A.M. 4th. Movement of bead
+magnified about 27 times.
+
+Pinus pinaster (Coniferæ).—A young hypocotyl, with the tips of the
+cotyledons still enclosed within the seed-coats, was at first only .35
+of an inch in height; but the upper part grew so rapidly that at the
+end of our observations it was .6 in height,
+and by this time the filament was attached some way down the little
+stem. From some unknown cause, the hypocotyl moved far towards the
+left, but there could be no doubt (Fig. 43) that it circumnutated.
+Another hypocotyl was similarly observed, and it likewise moved in a
+strongly zigzag line to the same side. This lateral movement was not
+caused by the attachment of the glass filaments, nor by the action of
+light; for no light was allowed to enter when each observation was
+made, except from vertically above.
+
+Fig. 43. Pinus pinaster: circumnutation of hypocotyl, with filament
+fixed across its summit, traced on horizontal glass, from 10 A.M. March
+21st to 9 A.M. 23rd. Seedling kept in darkness. Movement of bead
+magnified about 35 times.
+
+The hypocotyl of a seedling was secured to a little stick; it bore nine
+in appearance distinct cotyledons, arranged in a circle. The movements
+of two nearly opposite ones were observed. The tip of one was painted
+white, with a mark placed below, and the figure described (Fig. 44, A)
+shows that it made an irregular circle in the course of about 8 h.
+during the night it travelled to a considerable distance in the
+direction indicated by the broken line. A glass filament was attached
+longitudinally to the other cotyledon, and this nearly completed (Fig,
+44, B) an irregular circular figure in about 12 hours. During the night
+it also moved to a considerable distance, in the direction indicated by
+the broken line. The cotyledons therefore circumnutate independently of
+the movement of the hypocotyl. Although they moved much during the
+night, they did not approach each other so as to stand more vertically
+than during the day.
+
+Fig. 44. Pinus pinaster: circumnutation of two opposite cotyledons,
+traced on horizontal glass in darkness, from 8.45 A.M. to 8.35 P.M.
+Nov. 25th. Movement of tip in A magnified about 22 times, here reduced
+to one-half of original scale.
+
+
+Cycas pectinata (Cycadeæ).—The large seeds of this plant in germinating
+first protrude a single leaf, which breaks through the ground with the
+petiole bowed into an arch and with the leaflets involuted. A leaf in
+this condition, which at the close of our observations was 2½ inches in
+height, had its movements traced in a warm greenhouse by means of a
+glass filament bearing paper triangles attached across its tip. The
+tracing (Fig. 45) shows how large, complex, and rapid were the
+circumnutating movements. The extreme distance from side to side which
+it passed over amounted to between .6 and .7 of an inch.
+
+Fig. 45. Cycas pectinata: circumnutation of young leaf whilst emerging
+from the ground, feebly illuminated from above, traced on vertical
+glass, from 5 P.M. May 28th to 11 A.M. 31st. Movement magnified 7
+times, here reduced to two-thirds of original scale.
+
+Canna Warscewiczii (Cannaceae).—A seedling with the plumule projecting
+one inch above the ground was observed, but not under fair conditions,
+as it was brought out of the hot-house and kept in a room not
+sufficiently warm. Nevertheless the tracing (Fig. 46) shows that it
+made two or three incomplete irregular circles or ellipses in the
+course of 48 hours. The plumule is straight; and this was the first
+instance observed
+by us of the part that first breaks through the ground not being
+arched.
+
+Fig. 46. Canna Warscewiczii: circumnutation of plumule with filament
+affixed obliquely to outer sheath-like leaf, traced in darkness on
+horizontal glass from 8.45 A.M. Nov. 9th to 8.10 A.M. 11th. Movement of
+bead magnified 6 times.
+
+Allium cepa (Liliaceae).—The narrow green leaf, which protrudes from
+the seed of the common onion as a cotyledon,[4] breaks through the
+ground in the form of an arch, in the same manner as the hypocotyl or
+epicotyl of a dicotyledonous plant. Long after the arch has risen above
+the surface the apex remains within the seed-coats, evidently absorbing
+the still abundant contents. The summit or crown of the arch, when it
+first protrudes from the seed and is still buried beneath the ground,
+is simply rounded; but before it reaches the surface it is developed
+into a conical protuberance of a white colour (owing to the absence of
+chlorophyll), whilst the adjoining parts are green, with the epidermis
+apparently rather thicker and tougher than elsewhere. We may therefore
+conclude that this conical protuberance is a special adaptation for
+breaking through the ground,[5] and answers the same end as the
+knife-like white crest on the summit of the straight cotyledon of the
+Gramineæ.
+After a time the apex is drawn out of the empty seed-coats, and rises
+up, forming a right angle, or more commonly a still larger angle with
+the lower part, and occasionally the whole becomes nearly straight. The
+conical protuberance, which originally formed the crown of the arch, is
+now seated on one side, and appears like a joint or knee, which from
+acquiring chlorophyll becomes green, and increases in size. In rarely
+or never becoming perfectly straight, these cotyledons differ
+remarkably from the ultimate condition of the arched hypocotyls or
+epicotyls of dicotyledons. It is, also, a singular circumstance that
+the attenuated extremity of the upper bent portion invariably withers
+and dies.
+
+ [4] This is the expression used by Sachs in his ‘Text-book of Botany.’
+
+
+ [5] Haberlandt has briefly described (‘Die
+ Schutzeinrichtungen...Keimpflanze,’ 1877, p. 77) this curious
+ structure and the purpose which it subserves. He states that good
+ figures of the cotyledon of the onion have been given by Tittmann and
+ by Sachs in his ‘Experimental Physiologie,’ p. 93.
+
+
+A filament, 1.7 inch in length, was affixed nearly upright beneath the
+knee to the basal and vertical portion of a cotyledon; and its
+movements were traced during 14 h. in the usual manner. The tracing
+here given (Fig. 47) indicates circumnutation. The movement of the
+upper part above the knee of the same cotyledon, which projected at
+about an angle of 45° above the horizon, was observed at the same time.
+A filament was not affixed to it, but a mark was placed beneath the
+apex, which was almost white from beginning to wither, and its
+movements were thus traced. The figure described resembled pretty
+closely that above given; and this shows that the chief seat of
+movement is in the lower or basal part of the cotyledon.
+
+Fig. 47. Allium cepa: circumnutation of basal half of arched cotyledon,
+traced in darkness on horizontal glass, from 8.15 A.M. to 10 P.M. Oct.
+31st. Movement of bead magnified about 17 times.
+
+Asparagus officinalis (Asparageae).—The tip of a straight plumule or
+cotyledon (for we do not know which it should be called) was found at a
+depth of .1 inch beneath the surface, and the earth was then removed
+all round to the dept of .3 inch. a glass filament was affixed
+obliquely to it, and the movement of the bead, magnified 17 times, was
+traced in darkness. During the first 1 h. 15 m. the plumule moved to
+the right, and during the next two hours it returned in a roughly
+parallel but strongly zigzag course. From some unknown cause it had
+grown up through the soil in an inclined direction, and now through
+apogeotropism it moved during nearly 24 h. in
+the same general direction, but in a slightly zigzag manner, until it
+became upright. On the following morning it changed its course
+completely. There can therefore hardly be a doubt that the plumule
+circumnutates, whilst buried beneath the ground, as much as the
+pressure of the surrounding earth will permit. The surface of the soil
+in the pot was now covered with a thin layer of very fine argillaceous
+sand, which was kept damp; and after the tapering seedlings had grown a
+few tenths of an inch in height, each was found surrounded by a little
+open space or circular crack; and this could be accounted for only by
+their having circumnutated and thus pushed away the sand on all sides;
+for there was no vestige of a crack in any other part.
+
+In order to prove that there was circumnutation, the movements of five
+seedlings, varying in height from .3 inch to 2 inches, were traced.
+They were placed within a box and illuminated from above; but in all
+five cases the longer axes of the figures described were directed to
+nearly the same point; so that more light seemed to have come through
+the glass roof of the greenhouse on one side than on any other. All
+five tracings resembled each other to a certain extent, and it will
+suffice to give two of them. In A (Fig. 48) the seedling was only .45
+of an
+inch in height, and consisted of a single internode bearing a bud on
+its summit. The apex described between 8.30 A.M. and 10.20 P.M. (i.e.
+during nearly 14 hours) a figure which would probably have consisted of
+3½ ellipses, had not the stem been drawn to one side until 1 P.M.,
+after which hour it moved backwards. On the following morning it was
+not far distant from the point whence it had first started. The actual
+amount of movement of the apex from side to side was very small, viz.
+about 1/18th of an inch. The seedling of which the movements are shown
+in Fig. 48, B, was 1 3/4 inch in height, and consisted of three
+internodes besides the bud on the summit. The figure, which was
+described during 10 h., apparently represents two irregular and unequal
+ellipses or circles. The actual amount of movement of the apex, in the
+line not influenced by the light, was .11 of an inch, and in that thus
+influenced .37 of an inch. With a seedling 2 inches in height it was
+obvious, even without the aid of any tracing, that the uppermost part
+of the stem bent successively to all points of the compass, like the
+stem of a twining plant. A little increase in the power of
+circumnutating and in the flexibility of the stem, would convert the
+common asparagus into a twining plant, as has occurred with one species
+in this genus, namely, A. scandens.
+
+Fig. 48. Asparagus officinalis: circumnutation of plumules with tips
+whitened and marks placed beneath, traced on a horizontal glass. A,
+young plumule; movement traced from 8.30 A.M. Nov. 30th to 7.15 A.M.
+next morning; magnified about 35 times. B, older plumule; movement
+traced from 10.15 A.M. to 8.10 P.M. Nov. 29th; magnified 9 times, but
+here reduced to one-half of original scale.
+
+Phalaris Canariensis (Gramineæ).—With the Gramineæ the part which first
+rises above the ground has been called by some authors the pileole; and
+various views have been expressed on its homological nature. It is
+considered by some great authorities to be a cotyledon, which term we
+will use without venturing to express any opinion on the subject.[6] It
+consists in the present case of a slightly flattened reddish sheath,
+terminating upwards in a sharp white edge; it encloses a true green
+leaf, which protrudes from the sheath through a slit-like orifice,
+close beneath and at right angles to the sharp edge on the summit. The
+sheath is not arched when it breaks through the ground.
+
+ [6] We are indebted to the Rev. G. Henslow for an abstract of the
+ views which have been held on this subject, together with references.
+
+
+The movements of three rather old seedlings, about 1½ inch in height,
+shortly before the protrusion of the leaves, were first traced. They
+were illuminated exclusively from above; for, as will hereafter be
+shown, they are excessively sensitive to the
+action of light; and if any enters even temporarily on one side, they
+merely bend to this side in slightly zigzag lines. Of the three
+tracings one alone (Fig. 49) is here given. Had the observations been
+more frequent during the 12 h. two oval figures would have been
+described with their longer axes at right angles to one another. The
+actual amount of movement of the apex from side to side was about .3 of
+an inch. The figures described by the other two seedlings resembled to
+a certain extent the one here given.
+
+Fig. 49. Phalaris Canariensis: circumnutation of a cotyledon, with a
+mark placed below the apex, traced on a horizontal glass, from 8.35
+A.M. Nov. 26th to 8.45 A.M. 27th. Movement of apex magnified 7 times,
+here reduced to one-half scale.
+
+A seedling which had just broken through the ground and projected only
+1/20th of an inch above the surface, was next observed in the same
+manner as before. It was necessary to clear away the earth all round
+the seedling to a little depth in order to place a mark beneath the
+apex. The figure (Fig. 50) shows that the apex moved to one side, but
+changed its course ten times in the course of the ten hours of
+observation; so that there can be no doubt about its circumnutation.
+The cause of the general movement in one direction could hardly be
+attributed to the entrance of lateral light, as this was carefully
+guarded against; and we suppose it was in some manner connected with
+the removal of the earth round the little seedling.
+
+Fig. 50. Phalaris Canariensis: circumnutation of a very young
+cotyledon, with a mark placed below the apex, traced on a horizontal
+glass, from 11.37 A.M. to 9.30 P.M. Dec. 13th. Movement of apex greatly
+magnified, here reduced to one-fourth of original scale.
+
+Lastly, the soil in the same pot was searched with the aid of a lens,
+and the white knife-like apex of a seedling was found on an exact level
+with that of the surrounding surface. The soil was removed all round
+the apex to the depth of a quarter of an inch, the seed itself
+remaining covered. The pot, protected from lateral light, was placed
+under the
+microscope with a micrometer eye-piece, so arranged that each division
+equalled 1/500th of an inch. After an interval of 30 m. the apex was
+observed, and it was seen to cross a little obliquely two divisions of
+the micrometer in 9 m. 15 s.; and after a few minutes it crossed the
+same space in 8 m. 50s. The seedling was again observed after an
+interval of three-quarters of an hour, and now the apex crossed rather
+obliquely two divisions in 10 m. We may therefore conclude that it was
+travelling at about the rate of 1/50th of an inch in 45 minutes. We may
+also conclude from these and the previous observations, that the
+seedlings of Phalaris in breaking through the surface of the soil
+circumnutate as much as the surrounding pressure will permit. This fact
+accounts (as in the case before given of the asparagus) for a circular,
+narrow, open space or crack being distinctly visible round several
+seedlings which had risen through very fine argillaceous sand, kept
+uniformly damp.
+
+Fig. 51. Zea mays: circumnutation of cotyledon, traced on horizontal
+glass, from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead
+magnified on an average about 25 times.
+
+Zea mays (Gramineæ).—A glass filament was fixed obliquely to the summit
+of a cotyledon, rising .2 of an inch above the ground; but by the third
+morning it had grown to exactly thrice this height, so that the
+distance of the bead from the mark below was greatly increased,
+consequently the tracing (Fig. 51) was much more magnified on the first
+than on the second day. The upper part of the cotyledon changed its
+course by at least as much as a rectangle six times on each of the two
+days. The plant was illuminated by an obscure light from vertically
+above. This was a necessary precaution, as on the previous day we had
+traced the movements of cotyledons placed in a deep box, the inner side
+of which was feebly illuminated on one side from a distant north-east
+window, and at each observation by a wax taper held for a minute or two
+on the same side; and the result was that the cotyledons travelled all
+day long to this side, though making in their course some conspicuous
+flexures, from which fact alone we might have
+concluded that they were circumnutating; but we thought it advisable to
+make the tracing above given.
+
+Radicles.—Glass filaments were fixed to two short radicles, placed so
+as to stand almost upright, and whilst bending downwards through
+geotropism their courses were strongly zigzag; from this latter
+circumstance circumnutation might have been inferred, had not their
+tips become slightly withered after the first 24 h., though they were
+watered and the air kept very damp. Nine radicles were next arranged in
+the manner formerly described, so that in growing downwards they left
+tracks on smoked glass-plates, inclined at various angles between 45°
+and 80° beneath the horizon. Almost every one of these tracks offered
+evidence in their greater or less breadth in different parts, or in
+little bridges of soot being left, that the apex had come alternately
+into more and less close contact with the glass. In the accompanying
+figure (Fig. 52) we have an accurate copy of one such track. In two
+instances alone (and in these the plates were highly inclined) there
+was some evidence of slight lateral movement. We presume therefore that
+the friction of the apex on the smoked surface, little as this could
+have been, sufficed to check the movement from side to side of these
+delicate radicles.
+
+Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip of
+radicle in growing downwards.
+
+Avena sativa (Gramineæ).—A cotyledon, 1½ inch in height, was placed in
+front of a north-east window, and the movement of the apex was traced
+on a horizontal glass during two days. It moved towards the light in a
+slightly zigzag line from 9 to 11.30 A.M. on October 15th; it then
+moved a little backwards and zigzagged much until 5 P.M., after which
+hour, and curing the night, it continued to move towards the window. On
+the following morning the same movement was continued in a nearly
+straight line until 12.40 P.M., when the sky remained until 2.35
+extraordinarily dark from thunder-clouds. During this interval of 1 h.
+55 m., whilst the light was obscure, it was interesting to observe how
+circumnutation overcame heliotropism, for the apex, instead of
+continuing to move towards the window in a slightly zigzag line,
+reversed its course four times, making two small narrow ellipses. A
+diagram of this case will be given in the chapter on Heliotropism.
+
+
+A filament was next fixed to a cotyledon only 1/4 of an inch in height,
+which was illuminated exclusively from above, and as it was kept in a
+warm greenhouse, it grew rapidly; and now there could be no doubt about
+its circumnutation, for it described a figure of 8 as well as two small
+ellipses in 5½ hours.
+
+Nephrodium molle (Filices).—A seedling fern of this species came up by
+chance in a flowerpot near its parent. The frond, as yet only slightly
+lobed, was only .16 of an inch in length and .2 in breadth, and was
+supported on a rachis as fine as a hair and .23 of an inch in height. A
+very thin glass filament, which projected for a length of .36 of an
+inch, was fixed to the end of the frond. The movement was so highly
+magnified that the figure (Fig. 53) cannot be fully trusted; but the
+frond was constantly moving in a complex manner, and the bead greatly
+changed its course eighteen times in the 12 hours of observation.
+Within half an hour it often returned in a line almost parallel to its
+former course. The greatest amount of movement occurred between 4 and 6
+P.M. The circumnutation of this plant is interesting, because the
+species in the genus Lygodium are well known to circumnutate
+conspicuously and to twine round any neighbouring object.
+
+Fig. 53. Nephrodium molle: circumnutation of very young frond, traced
+in darkness on horizontal glass, from 9 A.M. to 9 P.M. Oct. 30th.
+Movement of bead magnified 48 times.
+
+Selaginella Kraussii (?) (Lycopodiaceæ).—A very young plant, only .4 of
+an inch in height, had sprung up in a pot in the hot-house. An
+extremely fine glass filament was fixed to the end of the frond-like
+stem, and the movement of the bead traced on a horizontal glass. It
+changed its course several times, as shown in Fig. 54, whilst observed
+during 13 h. 15 m., and returned at night to a point not far distant
+from that whence it had started in the morning. There can be no doubt
+that this little plant circumnutated.
+
+Fig. 54. Selaginella Kraussii (?): circumnutation of young plant, kept
+in darkness, traced from 8.45 A.M. to 10 P.M. Oct. 31st.
+
+
+
+
+CHAPTER II.
+GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
+
+
+Generality of the circumnutating movement—Radicles, their
+circumnutation of service—Manner in which they penetrate the
+ground—Manner in which hypocotyls and other organs break through the
+ground by being arched—Singular manner of germination in Megarrhiza,
+etc.—Abortion of cotyledons—Circumnutation of hypocotyls and epicotyls
+whilst still buried and arched—Their power of straightening
+themselves—Bursting of the seed-coats—Inherited effect of the arching
+process in hypogean hypocotyls—Circumnutation of hypocotyls and
+epicotyls when erect—Circumnutation of cotyledons—Pulvini or joints of
+cotyledons, duration of their activity, rudimentary in Oxalis
+corniculata, their development—Sensitiveness of cotyledons to light and
+consequent disturbance of their periodic movements—Sensitiveness of
+cotyledons to contact.
+
+
+The circumnutating movements of the several parts or organs of a
+considerable number of seedling plants have been described in the last
+chapter. A list is here appended of the Families, Cohorts, Sub-classes,
+etc., to which they belong, arranged and numbered according to the
+classification adopted by Hooker.[1] Any one who will consider this
+list will see that the young plants selected for observation, fairly
+represent the whole vegetable series excepting the lowest cryptogams,
+and the movements of some of the latter when mature will hereafter be
+described. As all the seedlings which were observed, including
+Conifers, Cycads and Ferns, which belong to the most ancient
+types amongst plants, were continually circumnutating, we may infer
+that this kind of movement is common to every seedling species.
+
+ [1] As given in the ‘General System of Botany,’ by Le Maout and
+ Decaisne, 1873.
+
+
+SUB-KINGDOM I.—Phaenogamous Plants.
+
+Class I.—DICOTYLEDONS.
+
+Sub-class I.—Angiosperms. Family. Cohort. 14. Cruciferae. II.
+PARIETALES. 26. Caryophylleae. IV. CARYOPHYLLALES. 36. Malvaceae. VI
+MALVALES. 41. Oxalideae. VII. GERANIALES. 49. Tropaeoleae. DITTO 52.
+Aurantiaceae. DITTO 70. Hippocastaneae. X. SAPINDALES. 75. Leguminosae.
+XI. ROSALES. 106. Cucurbitaceæ. XII. PASSIFLORALES. 109. Cacteæ. XIV.
+FICOIDALES. 122. Compositæ. XVII. ASTRALES. 135. Primulaceae. XX.
+PRIMULALES. 145. Asclepiadeae. XXII. GENTIANALES. 151. Convolvulaceae.
+XXIII. POLEMONIALES. 154. Boragineae. DITTO 156. Nolaneae. DITTO 157.
+Solaneae. XXIV. SOLANALES. 181. Chenopodieae. XXVII. CHENOPODIALES.
+202. Euphorbiaceae. XXXII. EUPHORBIALES. 211. Cupuliferae. XXXVI.
+QUERNALES. 212. Corylaceae. DITTO
+
+Sub-class II.—Gymnosperms. 223. Coniferæ. 224. Cycadeæ.
+
+Class II.—MONOCOTYLEDONS. 2. Cannaceae. II. AMOMALES. 34. Liliaceae.
+XI. LILIALES. 41. Asparageae. DITTO 55. Gramineæ. XV. GLUMALES.
+
+SUB-KINGDOM II.—Cryptogamic Plants.
+
+1. Filices. I. FILICALES. 6. Lycopodiaceæ. DITTO
+
+
+Radicles.—In all the germinating seeds observed by us, the first change
+is the protrusion of the radicle, which immediately bends downwards and
+endeavours to penetrate the ground. In order to effect this, it is
+almost necessary that the seed should be pressed down so as to offer
+some resistance, unless indeed the soil is extremely loose; for
+otherwise the seed is lifted up, instead of the radicle penetrating the
+surface. But seeds often get covered by earth thrown up by burrowing
+quadrupeds or scratching birds, by the castings of earth-worms, by
+heaps of excrement, the decaying branches of trees, etc., and will thus
+be pressed down; and they must often fall into cracks when the ground
+is dry, or into holes. Even with seeds lying on the bare surface, the
+first developed root-hairs, by becoming attached to stones or other
+objects on the surface, are able to hold down the upper part of the
+radicle, whilst the tip penetrates the ground. Sachs has shown[2] how
+well and closely root-hairs adapt themselves by growth to the most
+irregular particles in the soil, and become firmly attached to them.
+This attachment seems to be effected by the softening or liquefaction
+of the outer surface of the wall of the hair and its subsequent
+consolidation, as will be on some future occasion more fully described.
+This intimate union plays an important part, according to Sachs, in the
+absorption of water and of the inorganic matter dissolved in it. The
+mechanical aid afforded by the root-hairs in penetrating the ground is
+probably only a secondary service.
+
+ [2] ‘Physiologie Végétale,’ 1868, pp. 199, 205.
+
+
+The tip of the radicle, as soon as it protrudes from the seed-coats,
+begins to circumnutate, and the whole
+growing part continues to do so, probably for as long as growth
+continues. This movement of the radicle has been described in Brassica,
+Æsculus, Phaseolus, Vicia, Cucurbita, Quercus and Zea. The probability
+of its occurrence was inferred by Sachs,[3] from radicles placed
+vertically upwards being acted on by geotropism (which we likewise
+found to be the case), for if they had remained absolutely
+perpendicular, the attraction of gravity could not have caused them to
+bend to any one side. Circumnutation was observed in the above
+specified cases, either by means of extremely fine filaments of glass
+affixed to the radicles in the manner previously described, or by their
+being allowed to grow downwards over inclined smoked glass-plates, on
+which they left their tracks. In the latter cases the serpentine course
+(see Figs. 19, 21, 27, 41) showed unequivocally that the apex had
+continually moved from side to side. This lateral movement was small in
+extent, being in the case of Phaseolus at most about 1 mm. from a
+medial line to both sides. But there was also movement in a vertical
+plane at right angles to the inclined glass-plates. This was shown by
+the tracks often being alternately a little broader and narrower, due
+to the radicles having alternately pressed with greater and less force
+on the plates. Occasionally little bridges of soot were left across the
+tracks, showing that the apex had at these spots been lifted up. This
+latter fact was especially apt to occur
+xwhen the radicle instead of travelling straight down the glass made a
+semicircular bend; but Fig. 52 shows that this may occur when the track
+is rectilinear. The apex by thus rising, was in one instance able to
+surmount a bristle cemented across an inclined glass-plate; but slips
+of wood only 1/40 of an inch in thickness always caused the radicles to
+bend rectangularly to one side, so that the apex did not rise to this
+small height in opposition to geotropism.
+
+ [3] ‘Ueber das Wachsthum der Wurzeln: Arbeiten des bot. Instituts in
+ Würzburg,’ Heft iii. 1873, p. 460. This memoir, besides its intrinsic
+ and great interest, deserves to be studied as a model of careful
+ investigation, and we shall have occasion to refer to it repeatedly.
+ Dr. Frank had previously remarked (‘Beiträge zur Pflanzenphysiologie,
+ 1868, p. 81) on the fact of radicles placed vertically upwards being
+ acted on by geotropism, and he explained it by the supposition that
+ their growth was not equal on all sides.
+
+
+In those cases in which radicles with attached filaments were placed so
+as to stand up almost vertically, they curved downwards through the
+action of geotropism, circumnutating at the same time, and their
+courses were consequently zigzag. Sometimes, however, they made great
+circular sweeps, the lines being likewise zigzag.
+
+Radicles closely surrounded by earth, even when this is thoroughly
+soaked and softened, may perhaps be quite prevented from
+circumnutating. Yet we should remember that the circumnutating
+sheath-like cotyledons of Phalaris, the hypocotyls of Solanum, and the
+epicotyls of Asparagus formed round themselves little circular cracks
+or furrows in a superficial layer of damp argillaceous sand. They were
+also able, as well as the hypocotyls of Brassica, to form straight
+furrows in damp sand, whilst circumnutating and bending towards a
+lateral light. In a future chapter it will be shown that the rocking or
+circumnutating movement of the flower-heads of Trifolium subterraneum
+aids them in burying themselves. It is therefore probable that the
+circumnutation of the tip of the radicle aids it slightly in
+penetrating the ground; and it may be observed in several of the
+previously given diagrams, that the movement is more strongly
+pronounced in radicles when they first
+protrude from the seed than at a rather later period; but whether this
+is an accidental or an adaptive coincidence we do not pretend to
+decide. Nevertheless, when young radicles of _Phaseolus multiflorus_
+were fixed vertically close over damp sand, in the expectation that as
+soon as they reached it they would form circular furrows, this did not
+occur,—a fact which may be accounted for, as we believe, by the furrow
+being filled up as soon as formed by the rapid increase of thickness in
+the apex of the radicle. Whether or not a radicle, when surrounded by
+softened earth, is aided in forming a passage for itself by
+circumnutating, this movement can hardly fail to be of high importance,
+by guiding the radicle along a line of least resistance, as will be
+seen in the next chapter when we treat of the sensibility of the tip to
+contact. If, however, a radicle in its downward growth breaks obliquely
+into any crevice, or a hole left by a decayed root, or one made by the
+larva of an insect, and more especially by worms, the circumnutating
+movement of the tip will materially aid it in following such open
+passage; and we have observed that roots commonly run down the old
+burrows of worms.[4]
+
+ [4] See, also, Prof. Hensen’s statements (‘Zeitschrift für Wissen,
+ Zool.,’ B. xxviii. p. 354, 1877) to the same effect. He goes so far as
+ to believe that roots are able to penetrate the ground to a great
+ depth only by means of the burrows made by worms.
+
+
+When a radicle is placed in a horizontal or inclined position, the
+terminal growing part, as is well known, bends down towards the centre
+of the earth; and Sachs[5] has shown that whilst thus bending, the
+growth of the lower surface is greatly retarded, whilst that
+of the upper surface continues at the normal rate, or may be even
+somewhat increased. He has further shown by attaching a thread, running
+over a pulley, to a horizontal radicle of large size, namely that of
+the common bean, that it was able to pull up a weight of only one
+gramme, or 15.4 grains. We may therefore conclude that geotropism does
+not give a radicle force sufficient to penetrate the ground, but merely
+tells it (if such an expression may be used) which course to pursue.
+Before we knew of Sachs’ more precise observations we covered a flat
+surface of damp sand with the thinnest tin-foil which we could procure
+(.02 to .03 mm., or .00012 to .00079 of an inch in thickness), and
+placed a radicle close above, in such a position that it grew almost
+perpendicularly downwards. When the apex came into contact with the
+polished level surface it turned at right angles and glided over it
+without leaving any impression; yet the tin-foil was so flexible, that
+a little stick of soft wood, pointed to the same degree as the end of
+the radicle and gently loaded with a weight of only a quarter of an
+ounce (120 grains) plainly indented the tin-foil.
+
+ [5] ‘Arbeiten des bot. Inst. Würzburg,’ vol. i. 1873, p. 461. See also
+ p. 397 for the length of the growing part, and p. 451 on the force of
+ geotropism.
+
+
+Radicles are able to penetrate the ground by the force due to their
+longitudinal and transverse growth; the seeds themselves being held
+down by the weight of the superincumbent soil. In the case of the bean
+the apex, protected by the root-cap, is sharp, and the growing part,
+from 8 to 10 mm. in length, is much more rigid, as Sachs has proved,
+than the part immediately above, which has ceased to increase in
+length. We endeavoured to ascertain the downward pressure of the
+growing part, by placing germinating beans between two small metal
+plates, the upper one of which was loaded with a known weight; and the
+radicle was then allowed to grow into a narrow hole in wood, 2 or 3
+tenths of an inch in depth, and closed at the bottom. The wood was so
+cut that the short space of radicle between the mouth of the hole and
+the bean could not bend laterally on three sides; but it was impossible
+to protect the fourth side, close to the bean. Consequently, as long as
+the radicle continued to increase in length and remained straight, the
+weighted bean would be lifted up after the tip had reached the bottom
+of the shallow hole. Beans thus arranged, surrounded by damp sand,
+lifted up a quarter of a pound in 24 h. after the tip of the radicle
+had entered the hole. With a greater weight the radicles themselves
+always became bent on the one unguarded side; but this probably would
+not have occurred if they had been closely surrounded on all sides by
+compact earth. There was, however, a possible, but not probable, source
+of error in these trials, for it was not ascertained whether the beans
+themselves go on swelling for several days after they have germinated,
+and after having been treated in the manner in which ours had been;
+namely, being first left for 24 h. in water, then allowed to germinate
+in very damp air, afterwards placed over the hole and almost surrounded
+by damp sand in a closed box.
+
+Fig. 55. Outline of piece of stick (reduced to one-half natural size)
+with a hole through which the radicle of a bean grew. Thickness of
+stick at narrow end .08 inch, at broad end .16; depth of hole .1 inch.
+
+We succeeded better in ascertaining the force exerted transversely by
+these radicles. Two were so placed as to penetrate small holes made in
+little sticks, one of which was cut into the shape here exactly copied
+(Fig. 55). The short end of the stick beyond the hole was purposely
+split, but not the opposite
+end. As the wood was highly elastic, the split or fissure closed
+immediately after being made. After six days the stick and bean were
+dug out of the damp sand, and the radicle was found to be much enlarged
+above and beneath the hole. The fissure which was at first quite
+closed, was now open to a width of 4 mm.; as soon as the radicle was
+extracted, it immediately closed to a width of 2 mm. The stick was then
+suspended horizontally by a fine wire passing through the hole lately
+filled by the radicle, and a little saucer was suspended beneath to
+receive the weights; and it required 8 lbs. 8 ozs. to open the fissure
+to the width of 4 mm.—that is, the width before the root was extracted.
+But the part of the radicle (only .1 of an inch in length) which was
+embedded in the hole, probably exerted a greater transverse strain even
+than 8 lbs. 8 ozs., for it had split the solid wood for a length of
+rather more than a quarter of an inch (exactly .275 inch), and this
+fissure is shown in Fig. 55. A second stick was tried in the same
+manner with almost exactly the same result.
+
+Fig. 56. Wooden pincers, kept closed by a spiral brass spring, with a
+hole (.14 inch in diameter and .6 inch in depth) bored through the
+narrow closed part, through which a radicle of a bean was allowed to
+grow. Temp. 50°–60° F.
+
+We then followed a better plan. Holes were bored near the narrow end of
+two wooden clips or pincers (Fig. 56), kept closed by brass spiral
+springs. Two radicles in damp sand were allowed to grow through these
+holes. The
+pincers rested on glass-plates to lessen the friction from the sand.
+The holes were a little larger (viz..14 inch) and considerably deeper
+(viz..6 inch) than in the trials with the sticks; so that a greater
+length of a rather thicker radicle exerted a transverse strain. After
+13 days they were taken up. The distance of two dots (see the figure)
+on the longer ends of the pincers was now carefully measured; the
+radicles were then extracted from the holes, and the pincers of course
+closed. They were then suspended horizontally in the same manner as
+were the bits of sticks, and a weight of 1500 grams (or 3 pounds 4
+ounces) was necessary with one of the pincers to open them to the same
+extent as had been effected by the transverse growth of the radicle. As
+soon as this radicle had slightly opened the pincers, it had grown into
+a flattened form and had escaped a little beyond the hole; its diameter
+in one direction being 4.2 mm., and at rightangles 3.5 mm. If this
+escape and flattening could have been prevented, the radicle would
+probably have exerted a greater strain than the 3 pounds 4 ounces. With
+the other pincers the radicle escaped still further out of the hole;
+and the weight required to open them to the same extent as had been
+effected by the radicle, was only 600 grams.
+
+With these facts before us, there seems little difficulty in
+understanding how a radicle penetrates the ground. The apex is pointed
+and is protected by the root-cap; the terminal growing part is rigid,
+and increases in length with a force equal, as far as our observations
+can be trusted, to the pressure of at least a quarter of a pound,
+probably with a much greater force when prevented from bending to any
+side by the surrounding earth. Whilst thus increasing in length it
+increases in thickness, pushing away the damp
+earth on all sides, with a force of above 8 pounds in one case, of 3
+pounds in another case. It was impossible to decide whether the actual
+apex exerts, relatively to its diameter, the same transverse strain as
+the parts a little higher up; but there seems no reason to doubt that
+this would be the case. The growing part therefore does not act like a
+nail when hammered into a board, but more like a wedge of wood, which
+whilst slowly driven into a crevice continually expands at the same
+time by the absorption of water; and a wedge thus acting will split
+even a mass of rock.
+
+Manner in which Hypocotyls, Epicotyls, etc., rise up and break through
+the ground.—After the radicle has penetrated the ground and fixed the
+seed, the hypocotyls of all the dicotyledonous seedlings observed by
+us, which lift their cotyledons above the surface, break through the
+ground in the form of an arch. When the cotyledons are hypogean, that
+is, remain buried in the soil, the hypocotyl is hardly developed, and
+the epicotyl or plumule rises in like manner as an arch through the
+ground. In all, or at least in most of such cases, the downwardly bent
+apex remains for a time enclosed within the seed-coats. With Corylus
+avellena the cotyledons are hypogean, and the epicotyl is arched; but
+in the particular case described in the last chapter its apex had been
+injured, and it grew laterally through the soil like a root; and in
+consequence of this it had emitted two secondary shoots, which likewise
+broke through the ground as arches.
+
+Cyclamen does not produce any distinct stem, and only a single
+cotyledon appears at first;[6] its petiole
+breaks through the ground as an arch (Fig. 57). Abronia has only a
+single fully developed cotyledon, but in this case it is the hypocotyl
+which first emerges and is arched. Abronia umbellata, however, presents
+this peculiarity, that the enfolded blade of the one developed
+cotyledon (with the enclosed endosperm) whilst still beneath the
+surface has its apex upturned and parallel to the descending leg of the
+arched hypocotyl; but it is dragged out of the ground by the continued
+growth of the hypocotyl, with the apex pointing downward. With Cycas
+pectinata the cotyledons are hypogean, and a true leaf first breaks
+through the ground with its petiole forming an arch.
+
+ [6] This is the conclusion arrived at by Dr. H. Gressner (‘Bot.
+ Zeitung,’ 1874, p. 837), who maintains that what has been considered
+ by other botanists as the first true leaf is really the second
+ cotyledon, which is greatly delayed in its development.
+
+
+Fig. 57. Cyclamen Persicum: seedling, figure enlarged: c, blade of
+cotyledon, not yet expanded, with arched petiole beginning to
+straighten itself; h, hypocotyl developed into a corm; r, secondary
+radicles.
+
+Fig. 58. Acanthus mollis: seedling with the hypogean cotyledon on the
+near side removed and the radicles cut off; a, blade of first leaf
+beginning to expand, with petiole still partially arched; b, second and
+opposite leaf, as yet very imperfectly developed; c, hypogean cotyledon
+on the opposite side.
+
+In the genus Acanthus the cotyledons are likewise hypogean. In A.
+mollis, a single leaf first breaks through the ground with its petiole
+arched, and with the opposite leaf much less developed, short,
+straight, of a yellowish colour, and with the petiole at first not half
+as thick as that of the other. The undeveloped leaf is protected by
+standing beneath its arched fellow; and it is an
+instrucive fact that it is not arched, as it has not to force for
+itself a passage through the ground. In the accompanying sketch (Fig.
+58) the petiole of the first leaf has already partially straightened
+itself, and the blade is beginning to unfold. The small second leaf
+ultimately grows to an equal size with the first, but this process is
+effected at very different rates in different individuals: in one
+instance the second leaf did not appear fully above the ground until
+six weeks after the first leaf. As the leaves in the whole family of
+the Acanthaceae stand either opposite one another or in whorls, and as
+these are of equal size, the great inequality between the first two
+leaves is a singular fact. We can see how this inequality of
+development and the arching of the petiole could have been gradually
+acquired, if they were beneficial to the seedlings by favouring their
+emergence; for with A. candelabrum, spinosus, and latifolius there was
+a great variability in the inequality between the two first leaves and
+in the arching of their petioles. In one seedling of A. candelabrum the
+first leaf was arched and nine times as long as the second, which
+latter consisted of a mere little, yellowish-white, straight, hairy
+style. In other seedlings the difference in length between the two
+leaves was as 3 to 2, or as 4 to 3, or as only .76 to .62 inch. In
+these latter cases the first and taller leaf was not properly arched.
+Lastly, in another seedling there was not the least difference in size
+between the two first leaves, and both of them had their petioles
+straight; their laminae were enfolded and pressed against each other,
+forming a lance or wedge, by which means they had broken through the
+ground. Therefore in different individuals of this same species of
+Acanthus the first pair of leaves breaks through the ground by two
+widely different methods; and if
+either had proved decidedly advantageous or disadvantageous, one of
+them no doubt would soon have prevailed.
+
+Asa Gray has described[7] the peculiar manner of germination of three
+widely different plants, in which the hypocotyl is hardly at all
+developed. These were therefore observed by us in relation to our
+present subject.
+
+ [7] ‘Botanical Text-Book,’ 1879, p. 22.
+
+
+Delphinium nudicaule.—The elongated petioles of the two cotyledons are
+confluent (as are sometimes their blades at the base), and they break
+through the ground as an arch. They thus resemble in a most deceptive
+manner a hypocotyl. At first they are solid, but after a time become
+tubular; and the basal part beneath the ground is enlarged into a
+hollow chamber, within which the young leaves are developed without any
+prominent plumule. Externally root-hairs are formed on the confluent
+petioles, either a little above, or on a level with, the plumule. The
+first leaf at an early period of its growth and whilst within the
+chamber is quite straight, but the petiole soon becomes arched; and the
+swelling of this part (and probably of the blade) splits open one side
+of the chamber, and the leaf then emerges. The slit was found in one
+case to be 3.2 mm. in length, and it is seated on the line of
+confluence of the two petioles. The leaf when it first escapes from the
+chamber is buried beneath the ground, and now an upper part of the
+petiole near the blade becomes arched in the usual manner. The second
+leaf comes out of the slit either straight or somewhat arched, but
+afterwards the upper part of the petiole,—certainly in some, and we
+believe in all cases,—arches itself whilst forcing a passage through
+the soil.
+
+
+Megarrhiza Californica.—The cotyledons of this Gourd never free
+themselves from the seed-coats and are hypogean. Their petioles are
+completely confluent, forming a tube which terminates downwards in a
+little solid point, consisting of a minute radicle and hypocotyl, with
+the likewise minute plumule enclosed within the base of the tube. This
+structure was well exhibited in an abnormal specimen, in which one of
+the two cotyledons failed to produce a petiole, whilst the other
+produced one consisting of an open semicylinder ending in a sharp
+point, formed of the parts just described. As soon as the confluent
+petioles protrude from the seed they bend down, as they are strongly
+geotropic, and penetrate the ground. The seed itself retains its
+original position, either on the surface or buried at some depth, as
+the case may be. If, however, the point of the confluent petioles meets
+with some obstacle in the soil, as appears to have occurred with the
+seedlings described and figured by Asa Gray,[8] the cotyledons are
+lifted up above the ground. The petioles are clothed with root-hairs
+like those on a true radicle, and they likewise resemble radicles in
+becoming brown when immersed in a solution of permanganate of
+potassium. Our seeds were subjected to a high temperature, and in the
+course of three or four days the petioles penetrated the soil
+perpendicularly to a depth of from 2 to 2½ inches; and not until then
+did the true radicle begin to grow. In one specimen which was closely
+observed, the petioles in 7 days after their first protrusion attained
+a length of 2½ inches, and the radicle by this time had also become
+well developed. The plumule, still enclosed within the tube, was now
+.3 inch in length, and was quite straight; but from having increased in
+thickness it had just begun to split open the lower part of the
+petioles on one side, along the line of their confluence. By the
+following morning the upper part of the plumule had arched itself into
+a right angle, and the convex side or elbow had thus been forced out
+through the slit. Here then the arching of the plumule plays the same
+part as in the case of the petioles of the Delphinium. As the plumule
+continued to grow, the tip became more arched, and in the course of six
+days it emerged through the 2½ inches of superincumbent soil, still
+retaining its arched form. After reaching the surface it straightened
+itself in the usual manner. In the accompanying figure (Fig. 58, A) we
+have a sketch of a seedling in this advanced state of development; the
+surface of the ground being represented by the line G...........G.
+
+ [8] ‘American Journal of Science,’ vol. xiv. 1877, p. 21.
+
+
+Fig. 58, A. Megarrhiza Californica: sketch of seedling, copied from Asa
+Gray, reduced to one-half scale: c, cotyledons within seed-coats; p,
+the two confluent petioles; h and r, hypocotyl and radicle; p1,
+plumule; G..........G, surface of soil.
+
+The germination of the seeds in their native Californian home proceeds
+in a rather different manner, as we infer from an interesting letter
+from Mr. Rattan, sent to us by Prof. Asa Gray. The petioles protrude
+from the seeds soon after the autumnal rains, and penetrate the ground,
+generally in a vertical direction, to a depth of from 4 to even 6
+inches. they were found in this state by Mr. Rattan during the
+Christmas vacation, with the
+plumules still enclosed within the tubes; and he remarks that if the
+plumules had been at once developed and had reached the surface (as
+occurred with our seeds which were exposed to a high temperature), they
+would surely have been killed by the frost. As it is, they lie dormant
+at some depth beneath the surface, and are thus protected from the
+cold; and the root-hairs on the petioles would supply them with
+sufficient moisture. We shall hereafter see that many seedlings are
+protected from frost, but by a widely different process, namely, by
+being drawn beneath the surface by the contraction of their radicles.
+We may, however, believe that the extraordinary manner of germination
+of Megarrhiza has another and secondary advantage. The radicle begins
+in a few weeks to enlarge into a little tuber, which then abounds with
+starch and is only slightly bitter. It would therefore be very liable
+to be devoured by animals, were it not protected by being buried whilst
+young and tender, at a depth of some inches beneath the surface.
+Ultimately it grows to a huge size.
+
+Ipomœa leptophylla.—In most of the species of this genus the hypocotyl
+is well developed, and breaks through the ground as an arch. But the
+seeds of the present species in germinating behave like those of
+Megarrhiza, excepting that the elongated petioles of the cotyledons are
+not confluent. After they have protruded from the seed, they are united
+at their lower ends with the undeveloped hypocotyl and undeveloped
+radicle, which together form a point only about .1 inch in length. They
+are at first highly geotropic, and penetrate the ground to a depth of
+rather above half an inch. The radicle then begins to grow. On four
+occasions after the petioles had grown for a short distance vertically
+downwards, they
+were placed in a horizontal position in damp air in the dark, and in
+the course of 4 hours they again became curved vertically downwards,
+having passed through 90° in this time. But their sensitiveness to
+geotropism lasts for only 2 or 3 days; and the terminal part alone, for
+a length of between .2 and .4 inch, is thus sensitive. Although the
+petioles of our specimens did not penetrate the ground to a greater
+depth than about ½ inch, yet they continued for some time to grow
+rapidly, and finally attained the great length of about 3 inches. The
+upper part is apogeotropic, and therefore grows vertically upwards,
+excepting a short portion close to the blades, which at an early period
+bends downwards and becomes arched, and thus breaks through the ground.
+Afterwards this portion straightens itself, and the cotyledons then
+free themselves from the seed-coats. Thus we here have in different
+parts of the same organ widely different kinds of movement and of
+sensitiveness; for the basal part is geotropic, the upper part
+apogeotropic, and a portion near the blades temporarily and
+spontaneously arches itself. The plumule is not developed for some
+little time; and as it rises between the bases of the parallel and
+closely approximate petioles of the cotyledons, which in breaking
+through the ground have formed an almost open passage, it does not
+require to be arched and is consequently always straight. Whether the
+plumule remains buried and dormant for a time in its native country,
+and is thus protected from the cold of winter, we do not know. The
+radicle, like that of the Megarrhiza, grows into a tuber-like mass,
+which ultimately attains a great size. So it is with Ipomœa pandurata,
+the germination of which, as Asa Gray informs us, resembles that of I.
+leptophylla.
+
+The following case is interesting in connection with
+the root-like nature of the petioles. The radicle of a seedling was cut
+off, as it was completely decayed, and the two now separated cotyledons
+were planted. They emitted roots from their bases, and continued green
+and healthy for two months. The blades of both then withered, and on
+removing the earth the bases of the petioles (instead of the radicle)
+were found enlarged into little tubers. Whether these would have had
+the power of producing two independent plants in the following summer,
+we do not know.
+
+In Quercus virens, according to Dr. Engelmann,[9] both the cotyledons
+and their petioles are confluent. The latter grow to a length “of an
+inch or even more;” and, if we understand rightly, penetrate the
+ground, so that they must be geotropic. The nutriment within the
+cotyledons is then quickly transferred to the hypocotyl or radicle,
+which thus becomes developed into a fusiform tuber. The fact of tubers
+being formed by the foregoing three widely distinct plants, makes us
+believe that their protection from animals at an early age and whilst
+tender, is one at least of the advantages gained by the remarkable
+elongation of the petioles of the cotyledons, together with their power
+of penetrating the ground like roots under the guidance of geotropism.
+
+ [9] ‘Transact. St. Louis Acad. Science,’ vol. iv. p. 190.
+
+
+The following cases may be here given, as they bear on our present
+subject, though not relating to seedlings. The flower-stem of the
+parasitic Lathraea squamaria, which is destitute of true leaves, breaks
+through the ground as an arch;[10] so does the
+flower-stem of the parasitic and leafless Monotropa hypopitys. With
+Helleborus niger, the flower-stems, which rise up independently of the
+leaves, likewise break through the ground as arches. This is also the
+case with the greatly elongated flower-stems, as well as with the
+petioles of Epimedium pinnatum. So it is with the petioles of
+Ranunculus ficaria, when they have to break through the ground, but
+when they arise from the summit of the bulb above ground, they are from
+the first quite straight; and this is a fact which deserves notice. The
+rachis of the bracken fern (Pteris aquilina), and of some, probably
+many, other ferns, likewise rises above ground under the form of an
+arch. No doubt other analogous instances could be found by careful
+search. In all ordinary cases of bulbs, rhizomes,
+root-stocks, etc., buried beneath the ground, the surface is broken by
+a cone formed by the young imbricated leaves, the combined growth of
+which gives them force sufficient for the purpose.
+
+ [10] The passage of the flower-stem of the Lathraea through the ground
+ cannot fail to be greatly facilitated by the extraordinary quantity of
+ water secreted at this period of the year by the subterranean
+ scale-like leaves; not that there is any reason to suppose that the
+ secretion is a special adaptation for this purpose: it probably
+ follows from the great quantity of sap absorbed in the early spring by
+ the parasitic roots. After a long period without any rain, the earth
+ had become light-coloured and very dry, but it was dark-coloured and
+ damp, even in parts quite wet, for a distance of at least six inches
+ all round each flower-stem. The water is secreted by glands (described
+ by Cohn, ‘Bericht. Bot. Sect. der Schlesischen Gesell.,’ 1876, p. 113)
+ which line the longitudinal channels running through each scale-like
+ leaf. A large plant was dug up, washed so as to remove the earth, left
+ for some time to drain, and then placed in the evening on a dry
+ glass-plate, covered with a bell-glass, and by next morning it had
+ secreted a large pool of water. The plate was wiped dry, and in the
+ course of the succeeding 7 or 8 hours another little pool was
+ secreted, and after 16 additional hours several large drops. A smaller
+ plant was washed and placed in a large jar, which was left inclined
+ for an hour, by which time no more water drained off. The jar was then
+ placed upright and closed: after 23 hours two drachms of water were
+ collected from the bottom, and a little more after 25 additional
+ hours. The flower-stems were now cut off, for they do not secrete, and
+ the subterranean part of the plant was found to weigh 106.8 grams
+ (1611 grains), and the water secreted during the 48 hours weighed 11.9
+ grams (183 grains),—that is, one-ninth of the whole weight of the
+ plant, excluding the flower-stems. We should remember that plants in a
+ state of nature would probably secrete in 48 hours much more than the
+ above large amount, for their roots would continue all the time
+ absorbing sap from the plant on which they were parasitic.
+
+
+With germinating monocotyledonous seeds, of which, however, we did not
+observe a large number, the plumules, for instance, those of Asparagus
+and Canna, are straight whilst breaking through the ground. With the
+Gramineæ, the sheath-like cotyledons are likewise straight; they,
+however, terminate in a sharp crest, which is white and somewhat
+indurated; and this structure obviously facilitates their emergence
+from the soil: the first true leaves escape from the sheath through a
+slit beneath the chisel-like apex and at right angles to it. In the
+case of the onion (Allium cepa) we again meet with an arch; the
+leaf-like cotyledon being abruptly bowed, when it breaks through the
+ground, with the apex still enclosed within the seed-coats. The crown
+of the arch, as previously described, is developed into a white conical
+protuberance, which we may safely believe to be a special adaptation
+for this office.
+
+The fact of so many organs of different kinds—hypocotyls and epicotyls,
+the petioles of some cotyledons and of some first leaves, the
+cotyledons of the onion, the rachis of some ferns, and some
+flower-stems—being all arched whilst they break through the ground,
+shows how just are Dr. Haberlandt’s[11] remarks on the importance of
+the arch to seedling plants. He attributes its chief importance to the
+upper, young, and more tender parts of the hypocotyl
+or epicotyl, being thus saved from abrasion and pressure whilst
+breaking through the ground. But we think that some importance may be
+attributed to the increased force gained by the hypocotyl, epicotyl, or
+other organ by being at first arched; for both legs of the arch
+increase in length, and both have points of resistance as long as the
+tip remains enclosed within the seed-coats; and thus the crown of the
+arch is pushed up through the earth with twice as much force as that
+which a straight hypocotyl, etc., could exert. As soon, however, as the
+upper end has freed itself, all the work has to be done by the basal
+leg. In the case of the epicotyl of the common bean, the basal leg (the
+apex having freed itself from the seed-coats) grew upwards with a force
+sufficient to lift a thin plate of zinc, loaded with 12 ounces. Two
+more ounces were added, and the 14 ounces were lifted up to a very
+little height, and then the epicotyl yielded and bent to one side.
+
+ [11] ‘Die Schutzeinrichtungen in der Entwickelung der Keimpflanze,’
+ 1877. We have learned much from this interesting essay, though our
+ observations lead us to differ on some points from the author.
+
+
+With respect to the primary cause of the arching process, we long
+thought in the case of many seedlings that this might be attributed to
+the manner in which the hypocotyl or epicotyl was packed and curved
+within the seed-coats; and that the arched shape thus acquired was
+merely retained until the parts in question reached the surface of the
+ground. But it is doubtful whether this is the whole of the truth in
+any case. For instance, with the common bean, the epicotyl or plumule
+is bowed into an arch whilst breaking through the seed-coats, as shown
+in Fig. 59 (p. 92). The plumule first protrudes as a solid knob (e in
+A), which after twenty-four hours’ growth is seen (e in B) to be the
+crown of an arch. Nevertheless, with several beans which germinated in
+damp air, and had otherwise been treated in an unnatural manner, little
+plumules were developed in the axils of the petioles of both
+cotyledons, and these were as perfectly arched as the normal plumule;
+yet they had not been subjected to any confinement or pressure, for the
+seed-coats were completely ruptured, and they grew in the open air.
+This proves that the plumule has an innate or spontaneous tendency to
+arch itself.
+
+In some other cases the hypocotyl or epicotyl protrudes from the seed
+at first only slightly bowed; but the bowing afterwards increases
+independently of any constraint. The arch is thus made narrow, with the
+two legs, which are sometimes much elongated, parallel and close
+together, and thus it becomes well fitted for breaking through the
+ground.
+
+With many kinds of plants, the radicle, whilst still enclosed within
+the seed and likewise after its first protrusion, lies in a straight
+line with the future hypocotyl and with the longitudinal axis of the
+cotyledons. This is the case with Cucurbita ovifera: nevertheless, in
+whatever position the seeds were buried, the hypocotyl always came up
+arched in one particular direction. Seeds were planted in friable peat
+at a depth of about an inch in a vertical position, with the end from
+which the radicle protrudes downwards. Therefore all the parts occupied
+the same relative positions which they would ultimately hold after the
+seedlings had risen clear above the surface. Notwithstanding this fact,
+the hypocotyl arched itself; and as the arch grew upwards through the
+peat, the buried seeds were turned either upside down, or were laid
+horizontally, being afterwards dragged above the ground. Ultimately the
+hypocotyl straightened itself in the usual manner; and now after all
+these movements the several parts occupied the same position relatively
+to one another and to the centre of the earth, which they
+had done when the seeds were first buried. But it may be argued in this
+and other such cases that, as the hypocotyl grows up through the soil,
+the seed will almost certainly be tilted to one side; and then from the
+resistance which it must offer during its further elevation, the upper
+part of the hypocotyl will be doubled down and thus become arched. This
+view seems the more probable, because with Ranunculus ficaria only the
+petioles of the leaves which forced a passage through the earth were
+arched; and not those which arose from the summits of the bulbs above
+the ground. Nevertheless, this explanation does not apply to the
+Cucurbita, for when germinating seeds were suspended in damp air in
+various positions by pins passing through the cotyledons, fixed to the
+inside of the lids of jars, in which case the hypocotyls were not
+subjected to any friction or constraint, yet the upper part became
+spontaneously arched. This fact, moreover, proves that it is not the
+weight of the cotyledons which causes the arching. Seeds of Helianthus
+annuus and of two species of Ipomœa (those of ‘I. bona nox’ being for
+the genus large and heavy) were pinned in the same manner, and the
+hypocotyls became spontaneously arched; the radicles, which had been
+vertically dependent, assumed in consequence a horizontal position. In
+the case of Ipomœa leptophylla it is the petioles of the cotyledons
+which become arched whilst rising through the ground; and this occurred
+spontaneously when the seeds were fixed to the lids of jars.
+
+It may, however, be suggested with some degree of probability that the
+arching was aboriginally caused by mechanical compulsion, owing to the
+confinement of the parts in question within the seed-coats, or to
+friction whilst they were being dragged upwards. But
+if this is so, we must admit from the cases just given, that a tendency
+in the upper part of the several specified organs to bend downwards and
+thus to become arched, has now become with many plants firmly
+inherited. The arching, to whatever cause it may be due, is the result
+of modified circumnutation, through increased growth along the convex
+side of the part; such growth being only temporary, for the part always
+straightens itself subsequently by increased growth along the concave
+side, as will hereafter be described.
+
+It is a curious fact that the hypocotyls of some plants, which are but
+little developed and which never raise their cotyledons above the
+ground, nevertheless inherit a slight tendency to arch themselves,
+although this movement is not of the least use to them. We refer to a
+movement observed by Sachs in the hypocotyls of the bean and some other
+Leguminosae, and which is shown in the accompanying figure (Fig. 59),
+copied from his Essay.[12] The hypocotyl and radicle at first grow
+perpendicularly downwards, as at A, and then bend, often in the course
+of 24 hours, into the position shown at B. As we shall hereafter often
+have to recur to this movement, we will, for brevity sake, call it
+“Sachs’ curvature.” At first sight it might be thought that the altered
+position of the radicle in B was wholly due to the outgrowth of the
+epicotyl (e), the petiole (p) serving as a hinge; and it is probable
+that this is partly the cause; but the hypocotyl and upper part of the
+radicle themselves become slightly curved.
+
+ [12] ‘Arbeiten des bot. Instit. Würzburg,’ vol. i. 1873, p. 403.
+
+
+The above movement in the bean was repeatedly seen by us; but our
+observations were made chiefly on Phaseolus multiflorus, the cotyledons
+of which are
+likewise hypogean. Some seedlings with well-developed radicles were
+first immersed in a solution of permanganate of potassium; and, judging
+from the changes of colour (though these were not very clearly
+defined), the hypocotyl is about .3 inch in length. Straight, thin,
+black lines of this length were now drawn from the bases of the short
+petioles along the hypocotyls of 23 germinating seeds, which were
+pinned to the lids of jars, generally with the hilum downwards, and
+with their radicles pointing to the centre of the earth. After an
+interval of from 24 to 48 hours the black lines on the hypocotyls of 16
+out of the 23 seedlings became distinctly curved, but in very various
+degrees (namely, with radii between 20 and
+80 mm. on Sachs’ cyclometer) in the same relative direction as shown at
+B in Fig. 59. As geotropism will obviously tend to check this
+curvature, seven seeds were allowed to germinate with proper
+precautions for their growth in a klinostat,[13] by which means
+geotropism was eliminated. The position of the hypocotyls was observed
+during four successive days, and they continued to bend towards the
+hilum and lower surface of the seed. On the fourth day they were
+deflected by an average angle of 63° from a line perpendicular to the
+lower surface, and were therefore considerably more curved than the
+hypocotyl and radicle in the bean at B (Fig. 59), though in the same
+relative direction.
+
+ [13] An instrument devised by Sachs, consisting essentially of a
+ slowly revolving horizontal axis, on which the plant under observation
+ is supported: see ‘Würzburg Arbeiten,’ 1879, p. 209.
+
+
+Fig. 59. Vicia faba: germinating seeds, suspended in damp air: A, with
+radicle growing perpendicularly downwards; B, the same bean after 24
+hours and after the radicle has curved itself; r. radicle; h, short
+hypocotyl; e, epicotyl appearing as a knob in A and as an arch in B; p,
+petiole of the cotyledon, the latter enclosed within the seed-coats.
+
+It will, we presume, be admitted that all leguminous plants with
+hypogean cotyledons are descended from forms which once raised their
+cotyledons above the ground in the ordinary manner; and in doing so, it
+is certain that their hypocotyls would have been abruptly arched, as in
+the case of every other dicotyledonous plant. This is especially clear
+in the case of Phaseolus, for out of five species, the seedlings of
+which we observed, namely, P. multiflorus, caracalla, vulgaris,
+Hernandesii and Roxburghii (inhabitants of the Old and New Worlds), the
+three last-named species have well-developed hypocotyls which break
+through the ground as arches. Now, if we imagine a seedling of the
+common bean or of P. multiflorus, to behave as its progenitors once
+did, the hypocotyl (h, Fig. 59), in whatever position the seed may have
+been buried, would become so much arched that the upper part would be
+doubled down parallel to the lower part; and
+this is exactly the kind of curvature which actually occurs in these
+two plants, though to a much less degree. Therefore we can hardly doubt
+that their short hypocotyls have retained by inheritance a tendency to
+curve themselves in the same manner as they did at a former period,
+when this movement was highly important to them for breaking through
+the ground, though now rendered useless by the cotyledons being
+hypogean. Rudimentary structures are in most cases highly variable, and
+we might expect that rudimentary or obsolete actions would be equally
+so; and Sachs’ curvature varies extremely in amount, and sometimes
+altogether fails. This is the sole instance known to us of the
+inheritance, though in a feeble degree, of movements which have become
+superfluous from changes which the species has undergone.
+
+Rudimentary Cotyledons.—A few remarks on this subject may be here
+interpolated. It is well known that some dicotyledonous plants produce
+only a single cotyledon; for instance, certain species of Ranunculus,
+Corydalis, Chaerophyllum; and we will here endeavour to show that the
+loss of one or both cotyledons is apparently due to a store of
+nutriment being laid up in some other part, as in the hypocotyl or one
+of the two cotyledons, or one of the secondary radicles.
+With the orange (Citrus aurantium) the cotyledons are hypogean, and one
+is larger than the other, as may be seen in A (Fig. 60). In B the
+inequality is rather greater, and the stem has grown between the points
+of insertion of the two petioles, so that they do not stand opposite to
+one another; in another case the separation amounted to one-fifth of an
+inch. The smaller cotyledon of one seedling was extremely thin, and not
+half the length of the larger one, so that it was clearly becoming
+rudimentary[14] In all these seedlings the hypocotyl was enlarged or
+swollen.
+
+Fig. 60. Citrus aurantium: two young seedlings: c, larger cotyledon;
+c’, smaller cotyledon; h, thickened hypocotyl; r, radicle. In A the
+epicotyl is still arched, in B it has become erect.
+
+Fig. 61. Abronia umbellata: seedling twice natural size: c cotyledon;
+c’, rudimentary cotyledon; h, enlarged hypocotyl, with a heel or
+projection (h’) at the lower end; r, radicle.
+
+ [14] In Pachira aquatica, as described by Mr. R. I. Lynch (‘Journal
+ Linn. Soc. Bot.’ vol. xvii. 1878, p. 147), one of the hypogean
+ cotyledons is of immense size; the other is small and soon falls off;
+ the pair do not always stand opposite. In another and very different
+ water-plant, ‘Trapa natans’, one of the cotyledons, filled with
+ farinaceous matter, is much larger than the other, which is scarcely
+ visible, as is stated by Aug. de Candolle, ‘Physiologie Veg.’ tom. ii.
+ p. 834, 1832.
+
+
+With Abronia umbellata one of the cotyledons is quite rudimentary, as
+may be seen (c’) in Fig. 61. In this specimen it consisted of a little
+green flap, 1/84th inch in length, destitute of a petiole and covered
+with glands like those on the fully developed cotyledon (c). At first
+it stood opposite to the larger cotyledon; but as the petiole of the
+latter increased in length and grew in the same line with the hypocotyl
+(h), the rudiment appeared in older seedlings as if seated some way
+down the hypocotyl. With Abronia arenaria there is a similar rudiment,
+which in one
+specimen was only 1/100th and in another 1/60th inch in length; it
+ultimately appeared as if seated halfway down the hypocotyl. In both
+these species the hypocotyl is so much enlarged, especially at a very
+early age, that it might almost be called a corm. The lower end forms a
+heel or projection, the use of which will hereafter be described.
+
+In _Cyclamen Persicum_ the hypocotyl, even whilst still within the
+seed, is enlarged into a regular corm,[15] and only a single cotyledon
+is at first developed (see former Fig. 57). With _Ranunculus ficaria_
+two cotyledons are never produced, and here one of the secondary
+radicles is developed at an early age into a so-called bulb.[16] Again,
+certain species of Chaerophyllum and Corydalis produce only a single
+cotyledon;[17] in the former the hypocotyl, and in the latter the
+radicle is enlarged, according to Irmisch, into a bulb.
+
+ [15] Dr. H. Gressner, ‘Bot. Zeitung,’ 1874, p. 824.
+
+
+ [16] Irmisch, ‘Beiträge zur Morphologie der Pflanzen,’ 1854, pp. 11,
+ 12; ‘Bot. Zeitung,’ 1874, p. 805.
+
+
+ [17] Delpino, ‘Rivista Botanica,’ 1877, p. 21. It is evident from
+ Vaucher’s account (‘Hist. Phys. des Plantes d’Europe,’ tom. i. 1841,
+ p. 149) of the germination of the seeds of several species of
+ Corydalis, that the bulb or tubercule begins to be formed at an
+ extremely early age.
+
+
+In the several foregoing cases one of the cotyledons is delayed in its
+development, or reduced in size, or rendered rudimentary, or quite
+aborted; but in other cases both cotyledons are represented by mere
+rudiments. With Opuntia basilaris this is not the case, for both
+cotyledons are thick and large, and the hypocotyl shows at first no
+signs of enlargement; but afterwards, when the cotyledons have withered
+and disarticulated themselves, it becomes thickened, and from its
+tapering form, together with its smooth, tough, brown skin, appears,
+when ultimately drawn down to some depth into the soil, like a root. On
+the other
+hand, with several other Cacteæ, the hypocotyl is from the first much
+enlarged, and both cotyledons are almost or quite rudimentary. Thus
+with Cereus Landbeckii two little triangular projections, representing
+the cotyledons, are narrower than the hypocotyl, which is pear-shaped,
+with the point downwards. In Rhipsalis cassytha the cotyledons are
+represented by mere points on the enlarged hypocotyl. In Echinocactus
+viridescens the hypocotyl is globular, with two little prominences on
+its summit. In Pilocereus Houlletii the hypocotyl, much swollen in the
+upper part, is merely notched on the summit; and each side of the notch
+evidently represents a cotyledon. Stapelia sarpedon, a member of the
+very distinct family of the Asclepiadeae, is fleshy like a cactus; and
+here again the upper part of the flattened hypocotyl is much thickened
+and bears two minute cotyledons, which, measured internally, were only
+.15 inch in length, and in breadth not equal to one-fourth of the
+diameter of the hypocotyl in its narrow axis; yet these minute
+cotyledons are probably not quite useless, for when the hypocotyl
+breaks through the ground in the form of an arch, they are closed or
+pressed against one another, and thus protect the plumule. They
+afterwards open.
+
+From the several cases now given, which refer to widely distinct
+plants, we may infer that there is some close connection between the
+reduced size of one or both cotyledons and the formation, by the
+enlargement of the hypocotyl or of the radicle, of a so-called bulb.
+But it may be asked, did the cotyledons first tend to abort, or did a
+bulb first begin to be formed? As all dicotyledons naturally produce
+two well-developed cotyledons, whilst the thickness of the hypocotyl
+and of the radicle differs much in different plants, it seems probable
+that these latter organs first became from
+some cause thickened—in several instances apparently in correlation
+with the fleshy nature of the mature plant—so as to contain a store of
+nutriment sufficient for the seedling, and then that one or both
+cotyledons, from being superfluous, decreased in size. It is not
+surprising that one cotyledon alone should sometimes have been thus
+affected, for with certain plants, for instance the cabbage, the
+cotyledons are at first of unequal size, owing apparently to the manner
+in which they are packed within the seed. It does not, however, follow
+from the above connection, that whenever a bulb is formed at an early
+age, one or both cotyledons will necessarily become superfluous, and
+consequently more or less rudimentary. Finally, these cases offer a
+good illustration of the principle of compensation or balancement of
+growth, or, as Goethe expresses it, “in order to spend on one side,
+Nature is forced to economise on the other side.”
+
+Circumnutation and other movements of Hypocotyls and Epicotyls, whilst
+still arched and buried beneath the ground, and whilst breaking through
+it.—According to the position in which a seed may chance to have been
+buried, the arched hypocotyl or epicotyl will begin to protrude in a
+horizontal, a more or less inclined, or in a vertical plane. Except
+when already standing vertically upwards, both legs of the arch are
+acted on from the earliest period by apogeotropism. Consequently they
+both bend upwards until the arch becomes vertical. During the whole of
+this process, even before the arch has broken through the ground, it is
+continually trying to circumnutate to a slight extent; as it likewise
+does if it happens at first to stand vertically up,—all which cases
+have been observed and described, more or less fully, in the last
+chapter. After the arch has grown to some
+height upwards the basal part ceases to circumnutate, whilst the upper
+part continues to do so.
+
+That an arched hypocotyl or epicotyl, with the two legs fixed in the
+ground, should be able to circumnutate, seemed to us, until we had read
+Prof. Wiesner’s observations, an inexplicable fact. He has shown[18] in
+the case of certain seedlings, whose tips are bent downwards (or which
+nutate), that whilst the posterior side of the upper or dependent
+portion grows quickest, the anterior and opposite side of the basal
+portion of the same internode grows quickest; these two portions being
+separated by an indifferent zone, where the growth is equal on all
+sides. There may be even more than one indifferent zone in the same
+internode; and the opposite sides of the parts above and below each
+such zone grow quickest. This peculiar manner of growth is called by
+Wiesner “undulatory nutation.” Circumnutation depends on one side of an
+organ growing quickest (probably preceded by increased turgescence),
+and then another side, generally almost the opposite one, growing
+quickest. Now if we look at an arch like this [upside down U] and
+suppose the whole of one side—we will say the whole convex side of both
+legs—to increase in length, this would not cause the arch to bend to
+either side. But if the outer side or surface of the left leg were to
+increase in length the arch would be pushed over to the right, and this
+would be aided by the inner side of the right leg increasing in length.
+If afterwards the process were reversed, the arch would be pushed over
+to the opposite or left side, and so on alternately,—that is, it would
+circumnutate. As an arched
+hypocotyl, with the two legs fixed in the ground, certainly
+circumnutates, and as it consists of a single internode, we may
+conclude that it grows in the manner described by Wiesner. It may be
+added, that the crown of the arch does not grow, or grows very slowly,
+for it does not increase much in breadth, whilst the arch itself
+increases greatly in height.
+
+ [18] ‘Die undulirende Nutation der Internodien,’ Akad. der Wissench.
+ (Vienna), Jan. 17th, 1878. Also published separately, see p. 32.
+
+
+The circumnutating movements of arched hypocotyls and epicotyls can
+hardly fail to aid them in breaking through the ground, if this be damp
+and soft; though no doubt their emergence depends mainly on the force
+exerted by their longitudinal growth. Although the arch circumnutates
+only to a slight extent and probably with little force, yet it is able
+to move the soil near the surface, though it may not be able to do so
+at a moderate depth. A pot with seeds of Solanum palinacanthum, the
+tall arched hypocotyls of which had emerged and were growing rather
+slowly, was covered with fine argillaceous sand kept damp, and this at
+first closely surrounded the bases of the arches; but soon a narrow
+open crack was formed round each of them, which could be accounted for
+only by their having pushed away the sand on all sides; for no such
+cracks surrounded some little sticks and pins which had been driven
+into the sand. It has already been stated that the cotyledons of
+Phalaris and Avena, the plumules of Asparagus and the hypocotyls of
+Brassica, were likewise able to displace the same kind of sand, either
+whilst simply circumnutating or whilst bending towards a lateral light.
+
+As long as an arched hypocotyl or epicotyl remains buried beneath the
+ground, the two legs cannot separate from one another, except to a
+slight extent from the yielding of the soil; but as soon as the arch
+rises above the ground, or at an earlier period if
+the pressure of the surrounding earth be artificially removed, the arch
+immediately begins to straighten itself. This no doubt is due to growth
+along the whole inner surface of both legs of the arch; such growth
+being checked or prevented, as long as the two legs of the arch are
+firmly pressed together. When the earth is removed all round an arch
+and the two legs are tied together at their bases, the growth on the
+under side of the crown causes it after a time to become much flatter
+and broader than naturally occurs. The straightening process consists
+of a modified form of circumnutation, for the lines described during
+this process (as with the hypocotyl of Brassica, and the epicotyls of
+Vicia and Corylus) were often plainly zigzag and sometimes looped.
+After hypocotyls or epicotyls have emerged from the ground, they
+quickly become perfectly straight. No trace is left of their former
+abrupt curvature, excepting in the case of Allium cepa, in which the
+cotyledon rarely becomes quite straight, owing to the protuberance
+developed on the crown of the arch.
+
+The increased growth along the inner surface of the arch which renders
+it straight, apparently begins in the basal leg or that which is united
+to the radicle; for this leg, as we often observed, is first bowed
+backwards from the other leg. This movement facilitates the withdrawal
+of the tip of the epicotyl or of the cotyledons, as the case may be,
+from within the seed-coats and from the ground. But the cotyledons
+often emerge from the ground still tightly enclosed within the
+seed-coats, which apparently serve to protect them. The seed-coats are
+afterwards ruptured and cast off by the swelling of the closely
+conjoined cotyledons, and not by any movement or their separation from
+one another.
+
+Nevertheless, in some few cases, especially with the
+Cucurbitaceæ, the seed-coats are ruptured by a curious contrivance,
+described by M. Flahault.[19] A heel or peg is developed on one side of
+the summit of the radicle or base of the hypocotyl; and this holds down
+the lower half of the seed-coats (the radicle being fixed into the
+ground) whilst the continued growth of the arched hypocotyl forced
+upwards the upper half, and tears asunder the seed-coats at one end,
+and the cotyledons are then easily withdrawn.
+
+ [19] ‘Bull. Soc. Bot. de France,’ tom. xxiv. 1877, p. 201.
+
+
+The accompanying figure (Fig. 62) will render this description
+intelligible. Forty-one seeds of Cucurbita ovifera were laid on friable
+peat and were covered by a layer about an inch in thickness, not much
+pressed down, so that the cotyledons in being dragged up were subjected
+to very little friction, yet forty of them came up naked, the
+seed-coats being left buried in the peat. This was certainly due to the
+action of the peg, for when it was prevented from acting, the
+cotyledons, as we shall presently see, were lifted up still enclosed in
+their seed-coats. They were, however, cast off in the course of two or
+three days by the swelling of the cotyledons. Until this occurs light
+is excluded, and the cotyledons cannot decompose carbonic acid; but no
+one probably would have thought that the advantage thus gained by a
+little earlier
+casting off of the seed-coats would be sufficient to account for the
+development of the peg. Yet according to M. Flahault, seedlings which
+have been prevented from casting their seed-coats whilst beneath the
+ground, are inferior to those which have emerged with their cotyledons
+naked and ready to act.
+
+Fig. 62. Cucurbita ovifera: germinating seed, showing the heel or peg
+projecting on one side from summit of radicle and holding down lower
+tip of seed-coats, which have been partially ruptured by the growth of
+the arched hypocotyl.
+
+The peg is developed with extraordinary rapidity; for it could only
+just be distinguished in two seedlings, having radicles .35 inch in
+length, but after an interval of only 24 hours was well developed in
+both. It is formed, according to Flahault, by the enlargement of the
+layers of the cortical parenchyma at the base of the hypocotyl. If,
+however, we judge by the effects of a solution of permanganate of
+potassium, it is developed on the exact line of junction between the
+hypocotyl and radicle; for the flat lower surface, as well as the
+edges, were coloured brown like the radicle; whilst the upper slightly
+inclined surface was left uncoloured like the hypocotyl, excepting
+indeed in one out of 33 immersed seedlings in which a large part of the
+upper surface was coloured brown. Secondary roots sometimes spring from
+the lower surface of the peg, which thus seems in all respects to
+partake of the nature of the radicle. The peg is always developed on
+the side which becomes concave by the arching of the hypocotyl; and it
+would be of no service if it were formed on any other side. It is also
+always developed with the flat lower side, which, as just stated, forms
+a part of the radicle, at right angles to it, and in a horizontal
+plane. This fact was clearly shown by burying some of the thin flat
+seeds in the same position as in Fig. 62, excepting that they were not
+laid on their flat broad sides, but with one edge downwards. Nine seeds
+were thus planted, and the peg was developed in the
+same position, relatively to the radicle, as in the figure;
+consequently it did not rest on the flat tip of the lower half of the
+seed-coats, but was inserted like a wedge between the two tips. As the
+arched hypocotyl grew upwards it tended to draw up the whole seed, and
+the peg necessarily rubbed against both tips, but did not hold either
+down. The result was, that the cotyledons of five out of the nine seeds
+thus placed were raised above the ground still enclosed within their
+seed-coats. Four seeds were buried with the end from which the radicle
+protrudes pointing vertically downwards, and owing to the peg being
+always developed in the same position, its apex alone came into contact
+with, and rubbed against the tip on one side; the result was, that the
+cotyledons of all four emerged still within their seed-coats. These
+cases show us how the peg acts in co-ordination with the position which
+the flat, thin, broad seeds would almost always occupy when naturally
+sown. When the tip of the lower half of the seed-coats was cut off,
+Flahault found (as we did likewise) that the peg could not act, since
+it had nothing to press on, and the cotyledons were raised above the
+ground with their seed-coats not cast off. Lastly, nature shows us the
+use of the peg; for in the one Cucurbitaceous genus known to us, in
+which the cotyledons are hypogean and do not cast their seed-coats,
+namely, Megarrhiza, there is no vestige of a peg. This structure seems
+to be present in most of the other genera in the family, judging from
+Flahault’s statements’ we found it well-developed and properly acting
+in Trichosanthes anguina, in which we hardly expected to find it, as
+the cotyledons are somewhat thick and fleshy. Few cases can be advanced
+of a structure better adapted for a special purpose than the present
+one.
+
+
+With Mimosa pudica the radicle protrudes from a small hole in the sharp
+edge of the seed; and on its summit, where united with the hypocotyl, a
+transverse ridge is developed at an early age, which clearly aids in
+splitting the tough seed-coats; but it does not aid in casting them
+off, as this is subsequently effected by the swelling of the cotyledons
+after they have been raised above the ground. The ridge or heel
+therefore acts rather differently from that of Cucurbita. Its lower
+surface and the edges were coloured brown by the permanganate of
+potassium, but not the upper surface. It is a singular fact that after
+the ridge has done its work and has escaped from the seed-coats, it is
+developed into a frill all round the summit of the radicle.[20]
+
+ [20] Our attention was called to this case by a brief statement by
+ Nobbe in his ‘Handbuch der Samenkunde,’ 1876, p. 215, where a figure
+ is also given of a seedling of Martynia with a heel or ridge at the
+ junction of the radicle and hypocotyl. This seed possesses a very hard
+ and tough coat, and would be likely to require aid in bursting and
+ freeing the cotyledons.
+
+
+At the base of the enlarged hypocotyl of Abronia umbellata, where it
+blends into the radicle, there is a projection or heel which varies in
+shape, but its outline is too angular in our former figure (Fig. 61).
+The radicle first protrudes from a small hole at one end of the tough,
+leathery, winged fruit. At this period the upper part of the radicle is
+packed within the fruit parallel to the hypocotyl, and the single
+cotyledon is doubled back parallel to the latter. The swelling of these
+three parts, and especially the rapid development of the thick heel
+between the hypocotyl and radicle at the point where they are doubled,
+ruptures the tough fruit at the upper end and allows the arched
+hypocotyl to emerge; and this seems to be the function of the heel. A
+seed was cut out of the fruit and
+allowed to germinate in damp air, and now a thin flat disc was
+developed all round the base of the hypocotyl and grew to an
+extraordinary breadth, like the frill described under Mimosa, but
+somewhat broader. Flahault says that with Mirabilis, a member of the
+same family with Abronia, a heel or collar is developed all round the
+base of the hypocotyl, but more on one side than on the other; and that
+it frees the cotyledons from their seed-coats. We observed only old
+seeds, and these were ruptured by the absorption of moisture,
+independently of any aid from the heel and before the protrusion of the
+radicle; but it does not follow from our experience that fresh and
+tough fruits would behave in a like manner.
+
+In concluding this section of the present chapter it may be convenient
+to summarise, under the form of an illustration, the usual movements of
+the hypocotyls and epicotyls of seedlings, whilst breaking through the
+ground and immediately afterwards. We may suppose a man to be thrown
+down on his hands and knees, and at the same time to one side, by a
+load of hay falling on him. He would first endeavour to get his arched
+back upright, wriggling at the same time in all directions to free
+himself a little from the surrounding pressure; and this may represent
+the combined effects of apogeotropism and circumnutation, when a seed
+is so buried that the arched hypocotyl or epicotyl protrudes at first
+in a horizontal or inclined plane. The man, still wriggling, would then
+raise his arched back as high as he could; and this may represent the
+growth and continued circumnutation of an arched hypocotyl or epicotyl,
+before it has reached the surface of the ground. As soon as the man
+felt himself at all free, he would raise the upper part of his body,
+whilst still on
+his knees and still wriggling; and this may represent the bowing
+backwards of the basal leg of the arch, which in most cases aids in the
+withdrawal of the cotyledons from the buried and ruptured seed-coats,
+and the subsequent straightening of the whole hypocotyl or
+epicotyl—circumnutation still continuing.
+
+Circumnutation of Hypocotyls and Epicotyls, when erect.—The hypocotyls,
+epicotyls, and first shoots of the many seedlings observed by us, after
+they had become straight and erect, circumnutated continuously. The
+diversified figures described by them, often during two successive
+days, have been shown in the woodcuts in the last chapter. It should be
+recollected that the dots were joined by straight lines, so that the
+figures are angular; but if the observations had been made every few
+minutes the lines would have been more or less curvilinear, and
+irregular ellipses or ovals, or perhaps occasionally circles, would
+have been formed. The direction of the longer axes of the ellipses made
+during the same day or on successive days generally changed completely,
+so as to stand at right angles to one another. The number of irregular
+ellipses or circles made within a given time differs much with
+different species. Thus with Brassica oleracea, Cerinthe major, and
+Cucurbita ovifera about four such figures were completed in 12 h.;
+whereas with Solanum palinacanthum and Opuntia basilaris, scarcely more
+than one. The figures likewise differ greatly in size; thus they were
+very small and in some degree doubtful in Stapelia, and large in
+Brassica, etc. The ellipses described by Lathyrus nissolia and Brassica
+were narrow, whilst those made by the Oak were broad. The figures are
+often complicated by small loops and zigzag lines.
+
+As most seedling plants before the development of true leaves are of
+low, sometimes very low stature,
+the extreme amount of movement from side to side of their
+circumnutating stems was small; that of the hypocotyl of Githago
+segetum was about .2 of an inch, and that of Cucurbita ovifera about
+.28. A very young shoot of Lathyrus nissolia moved about .14, that of
+an American oak .2, that of the common nut only .04, and a rather tall
+shoot of the Asparagus .11 of an inch. The extreme amount of movement
+of the sheath-like cotyledon of Phalaris Canariensis was .3 of an inch;
+but it did not move very quickly, the tip crossing on one occasion five
+divisions of the micrometer, that is, 1/100th of an inch, in 22 m. 5 s.
+A seedling Nolana prostrata travelled the same distance in 10 m. 38 s.
+Seedling cabbages circumnutate much more quickly, for the tip of a
+cotyledon crossed 1/100th of an inch on the micrometer in 3 m. 20 s.;
+and this rapid movement, accompanied by incessant oscillations, was a
+wonderful spectacle when beheld under the microscope.
+
+The absence of light, for at least a day, does not interfere in the
+least with the circumnutation of the hypocotyls, epicotyls, or young
+shoots of the various dicotyledonous seedlings observed by us; nor with
+that of the young shoots of some monocotyledons. The circumnutation was
+indeed much plainer in darkness than in light, for if the light was at
+all lateral the stem bent towards it in a more or less zigzag course.
+
+Finally, the hypocotyls of many seedlings are drawn during the winter
+into the ground, or even beneath it so that they disappear. This
+remarkable process, which apparently serves for their protection, has
+been fully described by De Vries.[21] He shows that
+it is effected by the contraction of the parenchyma-cells of the root.
+But the hypocotyl itself in some cases contracts greatly, and although
+at first smooth becomes covered with zigzag ridges, as we observed with
+Githago segetum. How much of the drawing down and burying of the
+hypocotyl of Opuntia basilaris was due to the contraction of this part
+and how much to that of the radicle, we did not observe.
+
+ [21] ‘Bot. Zeitung,’ 1879, p. 649. See also Winkler in ‘Verhandl. des
+ Bot. Vereins der P. Brandenburg,’ Jahrg. xvi. p. 16, as quoted by
+ Haberlandt, ‘Schutzeinrichungen der Keimpflanze,’ 1877, p. 52.
+
+
+Circumnutation of Cotyledons.—With all the dicotyledonous seedlings
+described in the last chapter, the cotyledons were in constant
+movement, chiefly in a vertical plane, and commonly once up and once
+down in the course of the 24 hours. But there were many exceptions to
+such simplicity of movement; thus the cotyledons of Ipomœa caerulea
+moved 13 times either upwards or downwards in the course of 16 h.. 18
+m. Those of Oxalis rosea moved in the same manner 7 times in the course
+of 24 h.; and those of Cassia tora described 5 irregular ellipses in 9
+h. The cotyledons of some individuals of Mimosa pudica and of Lotus
+Jacobæus moved only once up and down in 24 h., whilst those of others
+performed within the same period an additional small oscillation. Thus
+with different species, and with different individuals of the same
+species, there were many gradations from a single diurnal movement to
+oscillations as complex as those of the Ipomœa and Cassia. The opposite
+cotyledons on the same seedling move to a certain extent independently
+of one another. This was conspicuous with those of Oxalis sensitiva, in
+which one cotyledon might be seen during the daytime rising up until it
+stood vertically, whilst the opposite one was sinking down.
+
+Although the movements of cotyledons were generally in nearly the same
+vertical plane, yet their upward and downward courses never exactly
+coincided; so that ellipses, more or less narrow, were described, and
+the cotyledons may safely be said to have circumnutated. Nor could this
+fact be accounted for by the mere increase in length of the cotyledons
+through growth, for this by itself would not induce any lateral
+movement. That there was lateral movement in some instances, as with
+the cotyledons of the cabbage, was evident; for these, besides moving
+up and down, changed their course from right to left 12 times in 14 h.
+15 m. With Solanum lycopersicum the cotyledons, after falling in the
+forenoon, zigzagged from side to side between 12 and 4 P.M., and then
+commenced rising. The cotyledons of Lupinus luteus are so thick (about
+.08 of an inch) and fleshy,[22] that they seemed little likely to move,
+and were therefore observed with especial interest; they certainly
+moved largely up and down, and as the line traced was zigzag there was
+some lateral movement. The nine cotyledons of a seedling Pinus pinaster
+plainly circumnutated; and the figures described approached more nearly
+to irregular circles than to irregular ovals or ellipses. The
+sheath-like cotyledons of the Gramineæ circumnutate, that is, move to
+all sides, as plainly as do the hypocotyls or epicotyls of any
+dicotyledonous plants. Lastly, the very young fronds of a Fern and of a
+Selaginella circumnutated.
+
+ [22] The cotyledons, though bright green, resemble to a certain extent
+ hypogean ones; see the interesting discussion by Haberlandt (‘Die
+ Schutzeinrichtungen,’ etc., 1877, p. 95), on the gradations in the
+ Leguminosae between subaërial and subterranean cotyledons.
+
+
+In a large majority of the cases which were carefully observed, the
+cotyledons sink a little downwards in the forenoon, and rise a little
+in the afternoon or evening. They thus stand rather more highly
+inclined during the night than during the mid-day, at which
+time they are expanded almost horizontally. The circumnutating movement
+is thus at least partially periodic, no doubt in connection, as we
+shall hereafter see, with the daily alternations of light and darkness.
+The cotyledons of several plants move up so much at night as to stand
+nearly or quite vertically; and in this latter case they come into
+close contact with one another. On the other hand, the cotyledons of a
+few plants sink almost or quite vertically down at night; and in this
+latter case they clasp the upper part of the hypocotyl. In the same
+genus Oxalis the cotyledons of certain species stand vertically up, and
+those of other species vertically down, at night. In all such cases the
+cotyledons may be said to sleep, for they act in the same manner as do
+the leaves of many sleeping plants. This is a movement for a special
+purpose, and will therefore be considered in a future chapter devoted
+to this subject.
+
+In order to gain some rude notion of the proportional number of cases
+in which the cotyledons of dicotyledonous plants (hypogean ones being
+of course excluded) changed their position in a conspicuous manner at
+night, one or more species in several genera were cursorily observed,
+besides those described in the last chapter. Altogether 153 genera,
+included in as many families as could be procured, were thus observed
+by us. The cotyledons were looked at in the middle of the day and again
+at night; and those were noted as sleeping which stood either
+vertically or at an angle of at least 60° above or beneath the horizon.
+Of such genera there were 26; and in 21 of them the cotyledons of some
+of the species rose, and in only 6 sank at night; and some of these
+latter cases are rather doubtful from causes to be explained in the
+chapter on the sleep of cotyledons. When
+cotyledons which at noon were nearly horizontal, stood at night at more
+than 20° and less than 60° above the horizon, they were recorded as
+“plainly raised;” and of such genera there were 38. We did not meet
+with any distinct instances of cotyledons periodically sinking only a
+few degrees at night, although no doubt such occur. We have now
+accounted for 64 genera out of the 153, and there remain 89 in which
+the cotyledons did not change their position at night by as much as
+20°—that is, in a conspicuous manner which could easily be detected by
+the unaided eye and by memory; but it must not be inferred from this
+statement that these cotyledons did not move at all, for in several
+cases a rise of a few degrees was recorded, when they were carefully
+observed. The number 89 might have been a little increased, for the
+cotyledons remained almost horizontal at night in some species in a few
+genera, for instance, Trifolium and Geranium, which are included
+amongst the sleepers, such genera might therefore have been added to
+the 89. Again, one species of Oxalis generally raised its cotyledons at
+night more than 20° and less than 60° above the horizon; so that this
+genus might have been included under two heads. But as several species
+in the same genus were not often observed, such double entries have
+been avoided.
+
+In a future chapter it will be shown that the leaves of many plants
+which do not sleep, rise a few degrees in the evening and during the
+early part of the night; and it will be convenient to defer until then
+the consideration of the periodicity of the movements of cotyledons.
+
+On the Pulvini or Joints of Cotyledons.—With several of the seedlings
+described in this and the last chapter, the summit of the petiole is
+developed into a pulvinus,
+cushion, or joint (as this organ has been variously called), like that
+with which many leaves are provided. It consists of a mass of small
+cells usually of a pale colour from the absence of chlorophyll, and
+with its outline more or less convex, as shown in the annexed figure.
+In the case of Oxalis sensitiva two-thirds of the petiole, and in that
+of Mimosa pudica, apparently the whole of the short sub-petioles of the
+leaflets have been converted into pulvini. With pulvinated leaves (i.e.
+those provided with a pulvinus) their periodical movements depend,
+according to Pfeffer,[23] on the cells of the pulvinus alternately
+expanding more quickly on one side than on the other; whereas the
+similar movements of leaves not provided with pulvini, depend on their
+growth being alternately more rapid on one side than on the other.[24]
+As long as a leaf provided with a pulvinus is young and continues to
+grow, its movement depends on both these causes combined;[25] and if
+the view now held by many botanists be sound, namely, that growth is
+always preceded by the expansion of the growing cells, then the
+difference between the movements induced by the aid of pulvini and
+without such aid, is reduced to the expansion of the cells not being
+followed by growth in the first case, and being so followed in the
+second case.
+
+ [23] ‘Die Periodische Bewegungen der Blattorgane,’ 1875.
+
+
+ [24] Batalin, ‘Flora,’ Oct. 1st, 1873
+
+
+ [25] Pfeffer, ibid. p. 5.
+
+
+Fig. 63. Oxalis rosea: longitudinal section of a pulvinus on the summit
+of the petiole of a cotyledon, drawn with the camera lucida, magnified
+75 times: p, p, petiole; f, fibro-vascular bundle: b, b, commencement
+of blade of cotyledon.
+
+Dots were made with Indian ink along the midrib of both pulvinated
+cotyledons of a rather old seedling of Oxalis Valdiviana; their
+distances were repeatedly measured with an eye-piece micrometer during
+8 3/4 days, and they did not exhibit the least trace of increase. It is
+therefore almost certain that the pulvinus itself was not then growing.
+Nevertheless, during this whole time and for ten days afterwards, these
+cotyledons rose vertically every night. In the case of some seedlings
+raised from seeds purchased under the name of Oxalis floribunda, the
+cotyledons continued for a long time to move vertically down at night,
+and the movement apparently depended exclusively on the pulvini, for
+their petioles were of nearly the same length in young, and in old
+seedlings which had produced true leaves. With some species of Cassia,
+on the other hand, it was obvious without any measurement that the
+pulvinated cotyledons continued to increase greatly in length during
+some weeks; so that here the expansion of the cells of the pulvini and
+the growth of the petiole were probably combined in causing their
+prolonged periodic movements. It was equally evident that the
+cotyledons of many plants, not provided with pulvini, increased rapidly
+in length; and their periodic movements no doubt were exclusively due
+to growth.
+
+In accordance with the view that the periodic movements of all
+cotyledons depend primarily on the expansion of the cells, whether or
+not followed by growth, we can understand the fact that there is but
+little difference in the kind or form of movement in the two sets of
+cases. This may be seen by
+comparing the diagrams given in the last chapter. Thus the movements of
+the cotyledons of Brassica oleracea and of Ipomœa caerulea, which are
+not provided with pulvini, are as complex as those of Oxalis and Cassia
+which are thus provided. The pulvinated cotyledons of some individuals
+of Mimosa pudica and Lotus Jacobæus made only a single oscillation,
+whilst those of other individuals moved twice up and down in the course
+of 24 hours; so it was occasionally with the cotyledons of Cucurbita
+ovifera, which are destitute of a pulvinus. The movements of pulvinated
+cotyledons are generally larger in extent than those without a
+pulvinus; nevertheless some of the latter moved through an angle of
+90°. There is, however, one important difference in the two sets of
+cases; the nocturnal movements of cotyledons without pulvini, for
+instance, those in the Cruciferae, Cucurbitaceæ, Githago, and Beta,
+never last even for a week, to any conspicuous degree. Pulvinated
+cotyledons, on the other hand, continue to rise at night for a much
+longer period, even for more than a month, as we shall now show. But
+the period no doubt depends largely on the temperature to which the
+seedlings are exposed and their consequent rate of development.
+
+Oxalis Valdiviana.—Some cotyledons which had lately opened and were
+horizontal on March 6th at noon, stood at night vertically up; on the
+13th the first true leaf was formed, and was embraced at night by the
+cotyledons; on April 9th, after an interval of 35 days, six leaves were
+developed, and yet the cotyledons rose almost vertically at night. The
+cotyledons of another seedling, which when first observed had already
+produced a leaf, stood vertically at night and continued to do so for
+11 additional days. After 16 days from the first observation two leaves
+were developed, and the cotyledons were still greatly raised at night.
+After 21 days the cotyledons during the day were deflected beneath the
+horizon, but at night were raised 45°
+above it. After 24 days from the first observation (begun after a true
+leaf had been developed) the cotyledons ceased to rise at night.
+
+Oxalis (Biophytum) sensitiva.—The cotyledons of several seedlings, 45
+days after their first expansion, stood nearly vertical at night, and
+closely embraced either one or two true leaves which by this time had
+been formed. These seedlings had been kept in a very warm house, and
+their development had been rapid.
+
+Oxalis corniculata.—The cotyledons do not stand vertical at night, but
+generally rise to an angle of about 45° above the horizon. They
+continued thus to act for 23 days after their first expansion, by which
+time two leaves had been formed; even after 29 days they still rose
+moderately above their horizontal or downwardly deflected diurnal
+position.
+
+Mimosa pudica.—The cotyledons were expanded for the first time on Nov.
+2nd, and stood vertical at night. On the 15th the first leaf was
+formed, and at night the cotyledons were vertical. On the 28th they
+behaved in the same manner. On Dec. 15th, that is after 44 days, the
+cotyledons were still considerably raised at night; but those of
+another seedling, only one day older, were raised very little.
+
+Mimosa albida.—A seedling was observed during only 12 days, by which
+time a leaf had been formed, and the cotyledons were then quite
+vertical at night.
+
+Trifolium subterraneum.—A seedling, 8 days old, had its cotyledons
+horizontal at 10.30 A.M. and vertical at 9.15 P.M. After an interval of
+two months, by which time the first and second true leaves had been
+developed, the cotyledons still performed the same movement. They had
+now increased greatly in size, and had become oval; and their petioles
+were actually .8 of an inch in length!
+
+Trifolium strictum.—After 17 days the cotyledons still rose at night,
+but were not afterwards observed.
+
+Lotus Jacoboeus.—The cotyledons of some seedlings having well-developed
+leaves rose to an angle of about 45° at night; and even after 3 or 4
+whorls of leaves had been formed, the cotyledons rose at night
+considerably above their diurnal horizontal position.
+
+Cassia mimosoides.—The cotyledons of this Indian species, 14 days after
+their first expansion, and when a leaf had been formed, stood during
+the day horizontal, and at night vertical.
+
+Cassia sp? (a large S. Brazilian tree raised from seeds sent us
+by F. Müller).—The cotyledons, after 16 days from their first
+expansion, had increased greatly in size with two leaves just formed.
+They stood horizontally during the day and vertically at night, but
+were not afterwards observed.
+
+Cassia neglecta (likewise a S. Brazilian species).—A seedling, 34 days
+after the first expansion of its cotyledons, was between 3 and 4 inches
+in height, with 3 well-developed leaves; and the cotyledons, which
+during the day were nearly horizontal, at night stood vertical, closely
+embracing the young stem. The cotyledons of another seedling of the
+same age, 5 inches in height, with 4 well-developed leaves, behaved at
+night in exactly the same manner.
+
+It is known[26] that there is no difference in structure between the
+upper and lower halves of the pulvini of leaves, sufficient to account
+for their upward or downward movements. In this respect cotyledons
+offer an unusually good opportunity for comparing the structure of the
+two halves; for the cotyledons of Oxalis Valdiviana rise vertically at
+night, whilst those of O. rosea sink vertically; yet when sections of
+their pulvini were made, no clear difference could be detected between
+the corresponding halves of this organ in the two species which move so
+differently. With O. rosea, however, there were rather more cells in
+the lower than in the upper half, but this was likewise the case in one
+specimen of O. Valdiviana. the cotyledons of both species (3½ mm. in
+length) were examined in the morning whilst extended horizontally, and
+the upper surface of the pulvinus of O. rosea was then wrinkled
+transversely, showing that it was in a state of compression, and this
+might have been expected, as the cotyledons sink at night; with O.
+Valdiviana it was the lower surface which was wrinkled, and its
+cotyledons rise at night.
+
+ [26] Pfeffer, ‘Die Period. Bewegungen,’ 1875, p. 157.
+
+
+Trifolium is a natural genus, and the leaves of all
+the species seen by us are pulvinated; so it is with the cotyledons of
+T. subterraneum and strictum, which stand vertically at night; whereas
+those of T. resupinatum exhibit not a trace of a pulvinus, nor of any
+nocturnal movement. This was ascertained by measuring the distance
+between the tips of the cotyledons of four seedlings at mid-day and at
+night. In this species, however, as in the others, the first-formed
+leaf, which is simple or not trifoliate, rises up and sleeps like the
+terminal leaflet on a mature plant.
+
+In another natural genus, Oxalis, the cotyledons of O. Valdiviana,
+rosea, floribunda, articulata, and sensitiva are pulvinated, and all
+move at night into an upward or downward vertical position. In these
+several species the pulvinus is seated close to the blade of the
+cotyledon, as is the usual rule with most plants. Oxalis corniculata
+(var. Atro-purpurea) differs in several respects; the cotyledons rise
+at night to a very variable amount, rarely more than 45°; and in one
+lot of seedlings (purchased under the name of O. tropaeoloides, but
+certainly belonging to the above variety) they rose only from 5° to 15°
+above the horizon. The pulvinus is developed imperfectly and to an
+extremely variable degree, so that apparently it is tending towards
+abortion. No such case has hitherto, we believe, been described. It is
+coloured green from its cells containing chlorophyll; and it is seated
+nearly in the middle of the petiole, instead of at the upper end as in
+all the other species. The nocturnal movement is effected partly by its
+aid, and partly by the growth of the upper part of the petiole as in
+the case of plants destitute of a pulvinus. From these several reasons
+and from our having partially traced the development of the pulvinus
+from an early age, the case seems worth describing in some detail.
+
+
+When the cotyledons of O. corniculata were dissected out of a seed from
+which they would soon have naturally emerged, no trace of a pulvinus
+could be detected; and all the cells forming the short petiole, 7 in
+number in a longitudinal row, were of nearly equal size. In seedlings
+one or two days old, the pulvinus was so indistinct that we thought at
+first that it did not exist; but in the middle of the petiole an
+ill-defined transverse zone of cells could be seen, which were much
+shorter than those both above and below, although of the same breadth
+with them. They presented the appearance of having been just formed by
+the transverse division of longer cells; and there can be little doubt
+that this had occurred, for the cells in the petiole which had been
+dissected out of the seed averaged in length 7 divisions of the
+micrometer (each division equalling .003 mm.), and were a little longer
+than those forming a well-developed pulvinus, which varied between 4
+and 6 of these same divisions. After a few additional days the
+ill-defined zone of cells becomes distinct, and although it does not
+extend across the whole width of the petiole, and although the cells
+are of a green colour from containing chlorophyll, yet they certainly
+constitute a pulvinus, which as we shall presently see, acts as one.
+These small cells were arranged in longitudinal rows, and varied from 4
+to 7 in number; and the cells themselves varied in length in different
+parts of the
+same pulvinus and in different individuals. In the accompanying
+figures, A and B (Fig. 64), we have views of the epidermis[27] in the
+middle part of the petioles of two seedlings, in which the pulvinus was
+for this species well developed. They offer a striking contrast with
+the pulvinus of O. rosea (see former Fig. 63), or of O. Valdiviana.
+With the seedlings, falsely called O. tropaeoloides, the cotyledons of
+which rise very little at night, the small cells were still fewer in
+number and in parts formed a single transverse row, and in other parts
+short longitudinal rows of only two or three. Nevertheless they
+sufficed to attract the eye, when the whole petiole was viewed as a
+transparent object beneath the microscope. In these seedlings there
+could hardly be a doubt that the pulvinus was becoming rudimentary and
+tending to disappear; and this accounts for its great variability in
+structure and function.
+
+ [27] Longitudinal sections show that the forms of the epidermic cells
+ may be taken as a fair representation of those constituting the
+ pulvinus.
+
+
+Fig. 64. Oxalis corniculata: A and B the almost rudimentary pulvini of
+the cotyledons of two rather old seedlings, viewed as transparent
+objects. Magnified 50 times.
+
+In the following Table some measurements of the cells in fairly
+well-developed pulvini of O. corniculata are given:—
+
+Seedling 1 day old, with cotyledon 2.3 mm. in length. Divisions of
+Micrometer.[28] Average length of cells of
+pulvinus..................................................6 to 7 Length
+of longest cell below the pulvinus.....................................
+13 Length of longest cell above the
+pulvinus...................................... 20
+
+Seedling 5 days old, cotyledon 3.1 mm. in length, with the pulvinus
+quite distinct. Average length of cells of
+pulvinus.................................................. 6 Length of
+longest cell below the pulvinus..................................... 22
+Length of longest cell above the
+pulvinus...................................... 40
+
+Seedling 8 days old, cotyledon 5 mm. in length, with a true leaf formed
+but not yet expanded. Average length of cells of
+pulvinus.................................................. 9 Length of
+longest cell below the pulvinus..................................... 44
+Length of longest cell above the
+pulvinus...................................... 70
+
+Seedling 13 days old, cotyledon 4.5 mm. in length, with a small true
+leaf fully developed. Average length of cells of
+pulvinus.................................................. 7 Length of
+longest cell below the pulvinus..................................... 30
+Length of longest cell above the
+pulvinus...................................... 60
+
+
+ [28] Each division equalled .003 mm.
+
+
+We here see that the cells of the pulvinus increase but little in
+length with advancing age, in comparison with those of the petiole both
+above and below it; but they continue to grow in width, and keep equal
+in this respect with the other cells of the petiole. The rate of
+growth, however, varies in all parts of the cotyledons, as may be
+observed in the measurements of the 8-days’ old seedling.
+
+The cotyledons of seedlings only a day old rise at night considerably,
+sometimes as much as afterwards; but there was much variation in this
+respect. As the pulvinus is so indistinct at first, the movement
+probably does not then depend on the expansion of its cells, but on
+periodically unequal growth in the petiole. By the comparison of
+seedlings of different known ages, it was evident that the chief seat
+of growth of the petiole was in the upper part between the pulvinus and
+the blade; and this agrees with the fact (shown in the measurements
+above given) that the cells grow to a greater length in the upper than
+in the lower part. With a seedling 11 days old, the nocturnal rise was
+found to depend largely on the action of the pulvinus, for the petiole
+at night was curved upwards at this point; and during the day, whilst
+the petiole was horizontal, the lower surface of the pulvinus was
+wrinkled with the upper surface tense. Although the cotyledons at an
+advanced age do not rise at night to a higher inclination than whilst
+young, yet they have to pass through a larger angle (in one instance
+amounting to 63°) to gain their nocturnal position, as they are
+generally deflected beneath the horizon during the day. Even with the
+11-days’ old seedling the movement did not depend exclusively on the
+pulvinus, for the blade where joined to the petiole was curved upwards,
+and this must be attributed to unequal growth. Therefore the periodic
+movements of the cotyledons of ‘O. corniculata’ depend on two distinct
+but conjoint actions, namely, the expansion of the cells of the
+pulvinus and on the growth of the upper part of the petiole, including
+the base of the blade.
+
+Lotus Jacoboeus.—The seedlings of this plant present a case parallel to
+that of Oxalis corniculata in some respects, and in others unique, as
+far as we have seen. The cotyledons during the first 4 or 5 days of
+their life do not exhibit any plain nocturnal movement; but afterwards
+they stand vertically or almost vertically up at night. There is,
+however, some degree of variability in this respect, apparently
+dependent on the season and on the degree to which they have been
+illuminated during
+the day. With older seedlings, having cotyledons 4 mm. in length, which
+rise considerably at night, there is a well-developed pulvinus close to
+the blade, colourless, and rather narrower than the rest of the
+petiole, from which it is abruptly separated. It is formed of a mass of
+small cells of an average length of .021 mm.; whereas the cells in the
+lower part of the petiole are about .06 mm., and those in the blade
+from .034 to .04 mm. in length. The epidermic cells in the lower part
+of the petiole project conically, and thus differ in shape from those
+over the pulvinus.
+
+Turning now to very young seedlings, the cotyledons of which do not
+rise at night and are only from 2 to 2½ mm. in length, their petioles
+do not exhibit any defined zone of small cells, destitute of
+chlorophyll and differing in shape exteriorly from the lower ones.
+Nevertheless, the cells at the place where a pulvinus will afterwards
+be developed are smaller (being on an average .015 mm. in length) than
+those in the lower parts of the same petiole, which gradually become
+larger in proceeding downwards, the largest being .030 mm. in length.
+At this early age the cells of the blade are about .027 mm. in length.
+We thus see that the pulvinus is formed by the cells in the uppermost
+part of the petiole, continuing for only a short time to increase in
+length, then being arrested in their growth, accompanied by the loss of
+their chlorophyll grains; whilst the cells in the lower part of the
+petiole continue for a long time to increase in length, those of the
+epidermis becoming more conical. The singular fact of the cotyledons of
+this plant not sleeping at first is therefore due to the pulvinus not
+being developed at an early age.
+
+We learn from these two cases of Lotus and Oxalis, that the development
+of a pulvinus follows from the growth of the cells over a small defined
+space of the petiole being almost arrested at an early age. With Lotus
+Jacobæus the cells at first increase a little in length; in Oxalis
+corniculata they decrease a little, owing to self-division. A mass of
+such small cells forming a pulvinus, might therefore be either acquired
+or lost without any special difficulty, by different species in the
+same natural genus: and we know that
+with seedlings of Trifolium, Lotus, and Oxalis some of the species have
+a well-developed pulvinus, and others have none, or one in a
+rudimentary condition. As the movements caused by the alternate
+turgescence of the cells in the two halves of a pulvinus, must be
+largely determined by the extensibility and subsequent contraction of
+their walls, we can perhaps understand why a large number of small
+cells will be more efficient than a small number of large cells
+occupying the same space. As a pulvinus is formed by the arrestment of
+the growth of its cells, movements dependent on their action may be
+long-continued without any increase in length of the part thus
+provided; and such long-continued movements seem to be one chief end
+gained by the development of a pulvinus. Long-continued movement would
+be impossible in any part, without an inordinate increase in its
+length, if the turgescence of the cells was always followed by growth.
+
+Disturbance of the Periodic Movements of Cotyledons by Light.—The
+hypocotyls and cotyledons of most seedling plants are, as is well
+known, extremely heliotropic; but cotyledons, besides being
+heliotropic, are affected paratonically (to use Sachs’ expression) by
+light; that is, their daily periodic movements are greatly and quickly
+disturbed by changes in its intensity or by its absence. It is not that
+they cease to circumnutate in darkness, for in all the many cases
+observed by us they continued to do so; but the normal order of their
+movements in relation to the alternations of day and night is much
+disturbed or quite annulled. This holds good with species the
+cotyledons of which rise or sink so much at night that they may be said
+to sleep, as well as with others which rise only a little. But
+different species are affected in very different degrees by changes in
+the light.
+
+
+For instance, the cotyledons of Beta vulgaris, Solanum lycopersicum,
+Cerinthe major, and Lupinus luteus, when placed in darkness, moved down
+during the afternoon and early night, instead of rising as they would
+have done if they had been exposed to the light. All the individuals of
+the Solanum did not behave in the same manner, for the cotyledons of
+one circumnutated about the same spot between 2.30 and 10 P.M. The
+cotyledons of a seedling of Oxalis corniculata, which was feebly
+illuminated from above, moved downwards during the first morning in the
+normal manner, but on the second morning it moved upwards. The
+cotyledons of Lotus Jacoboeus were not affected by 4 h. of complete
+darkness, but when placed under a double skylight and thus feebly
+illuminated, they quite lost their periodical movements on the third
+morning. On the other hand, the cotyledons of Cucurbita ovifera moved
+in the normal manner during a whole day in darkness.
+
+Seedlings of Githago segetum were feebly illuminated from above in the
+morning before their cotyledons had expanded, and they remained closed
+for the next 40 h. Other seedlings were placed in the dark after their
+cotyledons had opened in the morning and these did not begin to close
+until about 4 h. had elapsed. The cotyledons of Oxalis rosea sank
+vertically downwards after being left for 1 h. 20 m. in darkness; but
+those of some other species of Oxalis were not affected by several
+hours of darkness. The cotyledons of several species of Cassia are
+eminently susceptible to changes in the degree of light to which they
+are exposed: thus seedlings of an unnamed S. Brazilian species (a large
+and beautiful tree) were brought out of the hot-house and placed on a
+table in the middle of a room with two north-east and one north-west
+window, so that they were fairly well illuminated, though of course
+less so than in the hot-house, the day being moderately bright; and
+after 36 m. the cotyledons which had been horizontal rose up vertically
+and closed together as when asleep; after thus remaining on the table
+for 1 h. 13 m. they began to open. The cotyledons of young seedlings of
+another Brazilian species and of C. neglecta, treated in the same
+manner, behaved similarly, excepting that they did not rise up quite so
+much: they again became horizontal after about an hour.
+
+Here is a more interesting case: seedlings of Cassia tora in two pots,
+which had stood for some time on the table in the room just described,
+had their cotyledons horizontal. One pot was now exposed for 2 h. to
+dull sunshine, and the cotyledons
+remained horizontal; it was then brought back to the table, and after
+50 m. the cotyledons had risen 68° above the horizon. The other pot was
+placed during the same 2 h. behind a screen in the room, where the
+light was very obscure, and the cotyledons rose 63° above the horizon;
+the pot was then replaced on the table, and after 50 m. the cotyledons
+had fallen 33°. These two pots with seedlings of the same age stood
+close together, and were exposed to exactly the same amount of light,
+yet the cotyledons in the one pot were rising, whilst those in the
+other pot were at the same time sinking. This fact illustrates in a
+striking manner that their movements are not governed by the actual
+amount, but by a change in the intensity or degree of the light. A
+similar experiment was tried with two sets of seedlings, both exposed
+to a dull light, but different in degree, and the result was the same.
+The movements of the cotyledons of this Cassia are, however, determined
+(as in many other cases) largely by habit or inheritance, independently
+of light; for seedlings which had been moderately illuminated during
+the day, were kept all night and on the following morning in complete
+darkness; yet the cotyledons were partially open in the morning and
+remained open in the dark for about 6 h. The cotyledons in another pot,
+similarly treated on another occasion, were open at 7 A.M. and remained
+open in the dark for 4 h. 30 m., after which time they began to close.
+Yet these same seedlings, when brought in the middle of the day from a
+moderately bright into only a moderately dull light raised, as we have
+seen, their cotyledons high above the horizon.
+
+Sensitiveness of Cotyledons to contact.—This subject does not possess
+much interest, as it is not known that sensitiveness of this kind is of
+any service to seedling plants. We have observed cases in only four
+genera, though we have vainly observed the cotyledons of many others.
+The genus cassia seems to be pre-eminent in this respect: thus, the
+cotyledons of C. tora, when extended horizontally, were both lightly
+tapped with a very thin twig for 3 m. and in the course of a few
+minutes they formed together an angle of 90°, so that each had risen
+45°. A single cotyledon of another seedling was tapped in a like manner
+for 1 m., and it rose 27° in 9 m.; and after eight additional minutes
+it had risen 10° more; the opposite cotyledon, which was not tapped,
+hardly moved at all. The cotyledons in all these cases became
+horizontal again in less than half an hour. The pulvinus is the most
+sensitive part, for on slightly pricking three cotyledons with a
+pin in this part, they rose up vertically; but the blade was found also
+to be sensitive, care having been taken that the pulvinus was not
+touched. Drops of water placed quietly on these cotyledons produced no
+effect, but an extremely fine stream of water, ejected from a syringe,
+caused them to move upwards. When a pot of seedlings was rapidly hit
+with a stick and thus jarred, the cotyledons rose slightly. When a
+minute drop of nitric acid was placed on both pulvini of a seedling,
+the cotyledons rose so quickly that they could easily be seen to move,
+and almost immediately afterwards they began to fall; but the pulvini
+had been killed and became brown.
+
+The cotyledons of an unnamed species of Cassia (a large tree from S.
+Brazil) rose 31° in the course of 26 m. after the pulvini and the
+blades had both been rubbed during 1 m. with a twig; but when the blade
+alone was similarly rubbed the cotyledons rose only 8°. The remarkably
+long and narrow cotyledons, of a third unnamed species from S. Brazil,
+did not move when their blades were rubbed on six occasions with a
+pointed stick for 30 s. or for 1 m.; but when the pulvinus was rubbed
+and slightly pricked with a pin, the cotyledons rose in the course of a
+few minutes through an angle of 6°o. Several cotyledons of C. neglecta
+(likewise from S. Brazil) rose in from 5 m. to 15 m. to various angles
+between 16v and 34°, after being rubbed during 1 m. with a twig. Their
+sensitiveness is retained to a somewhat advanced age, for the
+cotyledons of a little plant of C. neglecta, 34 days old and bearing
+three true leaves, rose when lightly pinched between the finger and
+thumb. Some seedlings were exposed for 30 m. to a wind (temp. 50° F.)
+sufficiently strong to keep the cotyledons vibrating, but this to our
+surprise did not cause any movement. The cotyledons of four seedlings
+of the Indian C. glauca were either rubbed with a thin twig for 2 m. or
+were lightly pinched: one rose 34°; a second only 6°; a third 13°; and
+a fourth 17°. A cotyledon of C. florida similarly treated rose 9°; one
+of C. corymbosa rose 7½°, and one of the very distinct C. mimosoides
+only 6°. Those of C. pubescens did not appear to be in the least
+sensitive; nor were those of C. nodosa, but these latter are rather
+thick and fleshy, and do not rise at night or go to sleep.
+
+Smithia sensitiva.—This plant belongs to a distinct sub-order of the
+Leguminosae from Cassia. Both cotyledons of an oldish seedling, with
+the first true leaf partially unfolded, were rubbed for 1 m. with a
+fine twig, and in 5 m. each rose 32°; they
+remained in this position for 15 m., but when looked at again 40 m.
+after the rubbing, each had fallen 14°. Both cotyledons of another and
+younger seedling were lightly rubbed in the same manner for 1 m., and
+after an interval of 32 m. each had risen 30°. They were hardly at all
+sensitive to a fine jet of water. The cotyledons of S. Pfundii, an
+African water plant, are thick and fleshy; they are not sensitive and
+do not go to sleep.
+
+Mimosa pudica and albida.—The blades of several cotyledons of both
+these plants were rubbed or slightly scratched with a needle during 1
+m. or 2 m.; but they did not move in the least. When, however, the
+pulvini of six cotyledons of M. pudica were thus scratched, two of them
+were slightly raised. In these two cases perhaps the pulvinus was
+accidentally pricked, for on pricking the pulvinus of another cotyledon
+it rose a little. It thus appears that the cotyledons of Mimosa are
+less sensitive than those of the previously mentioned plants.[29]
+
+ [29] The sole notice which we have met with on the sensitiveness of
+ cotyledons, relates to Mimosa; for Aug. P. De Candolle says (‘Phys.
+ Vég.,’ 1832, tom. ii. p. 865), “les cotyledons du M. pudica tendent à
+ se raprocher par leurs faces supérieures lorsqu’on les irrite.”
+
+
+Oxalis sensitiva.—The blades and pulvini of two cotyledons, standing
+horizontally, were rubbed or rather tickled for 30 s. with a fine split
+bristle, and in 10 m. each had risen 48°; when looked at again in 35 m.
+after being rubbed they had risen 4° more; after 30 additional minutes
+they were again horizontal. On hitting a pot rapidly with a stick for 1
+m., the cotyledons of two seedlings were considerably raised in the
+course of 11 m. A pot was carried a little distance on a tray and thus
+jolted; and the cotyledons of four seedlings were all raised in 10 m.;
+after 17 m. one had risen 56°, a second 45°, a third almost 90°, and a
+fourth 90°. After an additional interval of 40 m. three of them had
+re-expanded to a considerable extent. These observations were made
+before we were aware at what an extraordinarily rapid rate the
+cotyledons circumnutate, and are therefore liable to error.
+Nevertheless it is extremely improbable that the cotyledons in the
+eight cases given, should all have been rising at the time when they
+were irritated. The cotyledons of Oxalis Valdiviana and rosea were
+rubbed and did not exhibit any sensitiveness.
+
+Finally, there seems to exist some relation between
+the habit of cotyledons rising vertically at night or going to sleep,
+and their sensitiveness, especially that of their pulvini, to a touch;
+for all the above-named plants sleep at night. On the other hand, there
+are many plants the cotyledons of which sleep, and are not in the least
+sensitive. As the cotyledons of several species of Cassia are easily
+affected both by slightly diminished light and by contact, we thought
+that these two kinds of sensitiveness might be connected; but this is
+not necessarily the case, for the cotyledons of Oxalis sensitiva did
+not rise when kept on one occasion for 1½ h., and on a second occasion
+for nearly 4 h., in a dark closet. Some other cotyledons, as those of
+Githago segetum, are much affected by a feeble light, but do not move
+when scratched by a needle. That with the same plant there is some
+relation between the sensitiveness of its cotyledons and leaves seems
+highly probable, for the above described Smithia and Oxalis have been
+called sensitiva, owing to their leaves being sensitive; and though the
+leaves of the several species of Cassia are not sensitive to a touch,
+yet if a branch be shaken or syringed with water, they partially assume
+their nocturnal dependent position. But the relation between the
+sensitiveness to contact of the cotyledons and of the leaves of the
+same plant is not very close, as may be inferred from the cotyledons of
+Mimosa pudica being only slightly sensitive, whilst the leaves are well
+known to be so in the highest degree. Again, the leaves of Neptunia
+oleracea are very sensitive to a touch, whilst the cotyledons do not
+appear to be so in any degree.
+
+
+
+
+CHAPTER III.
+SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER
+IRRITANTS.
+
+
+Manner in which radicles bend when they encounter an obstacle in the
+soil—Vicia faba, tips of radicles highly sensitive to contact and other
+irritants—Effects of too high a temperature—Power of discriminating
+between objects attached on opposite sides—Tips of secondary radicles
+sensitive—Pisum, tips of radicles sensitive—Effects of such
+sensitiveness in overcoming geotropism—Secondary radicles—Phaseolus,
+tips of radicles hardly sensitive to contact, but highly sensitive to
+caustic and to the removal of a
+slice—Tropaeolum—Gossypium—Cucurbita—Raphanus—Æsculus, tip not
+sensitive to slight contact, highly sensitive to caustic—Quercus, tip
+highly sensitive to contact—Power of discrimination—Zea, tip highly
+sensitive, secondary radicles—Sensitiveness of radicles to moist
+air—Summary of chapter.
+
+
+In order to see how the radicles of seedlings would pass over stones,
+roots, and other obstacles, which they must incessantly encounter in
+the soil, germinating beans (Vicia faba) were so placed that the tips
+of the radicles came into contact, almost rectangularly or at a high
+angle, with underlying plates of glass. In other cases the beans were
+turned about whilst their radicles were growing, so that they descended
+nearly vertically on their own smooth, almost flat, broad upper
+surfaces. The delicate root-cap, when it first touched any directly
+opposing surface, was a little flattened transversely; the flattening
+soon became oblique, and in a few hours quite disappeared, the apex now
+pointing at right angles, or at nearly right angles, to its former
+course. The radicle then seemed to glide in its new direction over the
+surface which had opposed
+it, pressing on it with very little force. How far such abrupt changes
+in its former course are aided by the circumnutation of the tip must be
+left doubtful. Thin slips of wood were cemented on more or less steeply
+inclined glass-plates, at right angles to the radicles which were
+gliding down them. Straight lines had been painted along the growing
+terminal part of some of these radicles, before they met the opposing
+slip of wood; and the lines became sensibly curved in 2 h. after the
+apex had come into contact with the slips. In one case of a radicle,
+which was growing rather slowly, the root-cap, after encountering a
+rough slip of wood at right angles, was at first slightly flattened
+transversely: after an interval of 2 h. 30 m. the flattening became
+oblique; and after an additional 3 hours the flattening had wholly
+disappeared, and the apex now pointed at right angles to its former
+course. It then continued to grow in its new direction alongside the
+slip of wood, until it came to the end of it, round which it bent
+rectangularly. Soon afterwards when coming to the edge of the plate of
+glass, it was again bent at a large angle, and descended
+perpendicularly into the damp sand.
+
+When, as in the above cases, radicles encountered an obstacle at right
+angles to their course, the terminal growing part became curved for a
+length of between .3 and .4 of an inch (8–10 mm.), measured from the
+apex. This was well shown by the black lines which had been previously
+painted on them. The first and most obvious explanation of the
+curvature is, that it results merely from the mechanical resistance to
+the growth of the radicle in its original direction. Nevertheless, this
+explanation did not seem to us satisfactory. The radicles did not
+present the appearance of having been subjected to a sufficient
+pressure to account for
+their curvature; and Sachs has shown[1] that the growing part is more
+rigid than the part immediately above which has ceased to grow, so that
+the latter might have been expected to yield and become curved as soon
+as the apex encountered an unyielding object; whereas it was the stiff
+growing part which became curved. Moreover, an object which yields with
+the greatest ease will deflect a radicle: thus, as we have seen, when
+the apex of the radicle of the bean encountered the polished surface of
+extremely thin tin-foil laid on soft sand, no impression was left on
+it, yet the radicle became deflected at right angles. A second
+explanation occurred to us, namely, that even the gentlest pressure
+might check the growth of the apex, and in this case growth could
+continue only on one side, and thus the radicle would assume a
+rectangular form; but this view leaves wholly unexplained the curvature
+of the upper part, extending for a length of 8–10 mm.
+
+ [1] ‘Arbeiten Bot. Inst. Würzburg,’ Heft iii. 1873, p. 398.
+
+
+We were therefore led to suspect that the apex was sensitive to
+contact, and that an effect was transmitted from it to the upper part
+of the radicle, which was thus excited to bend away from the touching
+object. As a little loop of fine thread hung on a tendril or on the
+petiole of a leaf-climbing plant, causes it to bend, we thought that
+any small hard object affixed to the tip of a radicle, freely suspended
+and growing in damp air, might cause it to bend, if it were sensitive,
+and yet would not offer any mechanical resistance to its growth. Full
+details will be given of the experiments which were tried, as the
+result proved remarkable. The fact of the apex of a radicle being
+sensitive to contact has never been observed, though, as we shall
+hereafter see, Sachs discovered that the radicle a little above the
+apex is sensitive, and bends like a tendril towards the touching
+object. But when one side of the apex is pressed by any object, the
+growing part bends away from the object; and this seems a beautiful
+adaptation for avoiding obstacles in the soil, and, as we shall see,
+for following the lines of least resistance. Many organs, when touched,
+bend in one fixed direction, such as the stamens of Berberis, the lobes
+of Dionaea, etc.; and many organs, such as tendrils, whether modified
+leaves or flower-peduncles, and some few stems, bend towards a touching
+object; but no case, we believe, is known of an organ bending away from
+a touching object.
+
+Sensitiveness of the Apex of the Radicle of Vicia faba.—Common beans,
+after being soaked in water for 24 h., were pinned with the hilum
+downwards (in the manner followed by Sachs), inside the cork lids of
+glass-vessels, which were half filled with water; the sides and the
+cork were well moistened, and light was excluded. As soon as the beans
+had protruded radicles, some to a length of less than a tenth of an
+inch, and others to a length of several tenths, little squares or
+oblongs of card were affixed to the short sloping sides of their
+conical tips. The squares therefore adhered obliquely with reference to
+the longitudinal axis of the radicle; and this is a very necessary
+precaution, for if the bits of card accidentally became displaced, or
+were drawn by the viscid matter employed so as to adhere parallel to
+the side of the radicle, although only a little way above the conical
+apex, the radicle did not bend in the peculiar manner which we are here
+considering. Squares of about the 1/20th of an inch (i.e. about 1½
+mm.), or oblong bits of nearly the same size, were found to
+be the most convenient and effective. We employed at first ordinary
+thin card, such as visiting cards, or bits of very thin glass, and
+various other objects; but afterwards sand-paper was chiefly employed,
+for it was almost as stiff as thin card, and the roughened surface
+favoured its adhesion. At first we generally used very thick gum-water;
+and this of course, under the circumstances, never dried in the least;
+on the contrary, it sometimes seemed to absorb vapour, so that the bits
+of card became separated by a layer of fluid from the tip. When there
+was no such absorption and the card was not displaced, it acted well
+and caused the radicle to bend to the opposite side. I should state
+that thick gum-water by itself induces no action. In most cases the
+bits of card were touched with an extremely small quantity of a
+solution of shellac in spirits of wine, which had been left to
+evaporate until it was thick; it then set hard in a few seconds, and
+fixed the bits of card well. When small drops of the shellac were
+placed on the tips without any card, they set into hard little beads,
+and these acted like any other hard object, causing the radicles to
+bend to the opposite side. Even extremely minute beads of the shellac
+occasionally acted in a slight degree, as will hereafter be described.
+But that it was the cards which chiefly acted in our many trials, was
+proved by coating one side of the tip with a little bit of goldbeaters’
+skin (which by itself hardly acts), and then fixing a bit of card to
+the skin with shellac which never came into contact with the radicle:
+nevertheless the radicle bent away from the attached card in the
+ordinary manner.
+
+Some preliminary trials were made, presently to be described, by which
+the proper temperature was determined, and then the following
+experiments were made. It should be premised that the beans were
+always fixed to the cork-lids, for the convenience of manipulation,
+with the edge from which the radicle and plumule protrudes, outwards;
+and it must be remembered that owing to what we have called Sachs’
+curvature, the radicles, instead of growing perpendicularly downwards,
+often bend somewhat, even as much as about 45° inwards, or under the
+suspended bean. Therefore when a square of card was fixed to the apex
+in front, the bowing induced by it coincided with Sachs’ curvature, and
+could be distinguished from it only by being more strongly pronounced
+or by occurring more quickly. To avoid this source of doubt, the
+squares
+were fixed either behind, causing a curvature in direct opposition to
+that of Sachs’, or more commonly to the right or left sides. For the
+sake of brevity, we will speak of the bits of card, etc., as fixed in
+front, or behind, or laterally. As the chief curvature of the radicle
+is at a little distance from the apex, and as the extreme terminal and
+basal portions are nearly straight, it is possible to estimate in a
+rough manner the amount of curvature by an angle; and when it is said
+that the radicle became deflected at any angle from the perpendicular,
+this implies that the apex was turned upwards by so many degrees from
+the downward direction which it would naturally have followed, and to
+the side opposite to that to which the card was affixed. That the
+reader may have a clear idea of the kind of movement excited by the
+bits of attached card, we append here accurate sketches of three
+germinating beans thus treated, and selected out of several specimens
+to show the gradations in the degrees of curvature. We will now give in
+detail a series of experiments, and afterwards a summary of the
+results.
+
+Fig. 65. Vicia faba: A, radicle beginning to bend from the attached
+little square of card; B, bent at a rectangle; C, bent into a circle or
+loop, with the tip beginning to bend downwards through the action of
+geotropism.
+
+In the first 12 trials, little squares or oblongs of sanded card, 1.8
+mm. in length, and 1.5 or only 0.9 mm. in breadth (i.e. .071 of an inch
+in length and .059 or .035 of an inch in breadth) were fixed with
+shellac to the tips of the radicles. In the subsequent trials the
+little squares were only occasionally measured, but were of about the
+same size.
+
+(1.) A young radicle, 4 mm. in length, had a card fixed behind: after 9
+h. deflected in the plane in which the bean is flattened, 50° from the
+perpendicular and from the card, and in opposition to Sachs’ curvature:
+no change next morning, 23 h. from the time of attachment.
+
+(2.) Radicle 5.5 mm. in length, card fixed behind: after 9 h. deflected
+in the plane of the bean 20° from the perpendicular and from the card,
+and in opposition to Sachs’ curvature: after 23 h. no change.
+
+
+(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected
+in the plane of the bean 40° from the perpendicular and from the card,
+and in opposition to Sachs’ curvature. The tip of the radicle more
+curved than the upper part, but in the same plane. After 23 h. the
+extreme tip was slightly bent towards the card; the general curvature
+of the radicle remaining the same.
+
+(4.) Radicle 9 mm. long, card fixed behind and a little laterally:
+after 9 h. deflected in the plane of the bean only about 7° or 8° from
+the perpendicular and from the card, in opposition to Sachs’ curvature.
+There was in addition a slight lateral curvature directed partly from
+the card. After 23 h. no change.
+
+(5.) Radicle 8 mm. long, card affixed almost laterally: after 9 h.
+deflected 30° from the perpendicular, in the plane of the bean and in
+opposition to Sachs’ curvature; also deflected in a plane at right
+angles to the above one, 20° from the perpendicular: after 23 h. no
+change.
+
+(6.) Radicle 9 mm. long, card affixed in front: after 9 h. deflected in
+the plane of the bean about 40° from the vertical, away from the card
+and in the direction of Sachs’ curvature. Here therefore we have no
+evidence of the card being the cause of the deflection, except that a
+radicle never moves spontaneously, as far as we have seen, as much as
+40° in the course of 9 h. After 23 h. no change.
+
+(7.) Radicle 7 mm. long, card affixed to the back: after 9 h. the
+terminal part of the radicle deflected in the plane of the bean 20°
+from the vertical, away from the card and in opposition to Sachs’
+curvature. After 22 h. 30 m. this part of the radicle had become
+straight.
+
+(8.) Radicle 12 mm. long, card affixed almost laterally: after 9 h.
+deflected laterally in a plane at right angles to that of the bean
+between 40° and 50° from the vertical and from the card. In the plane
+of the bean itself the deflection amounted to 8° or 9° from the
+vertical and from the card, in opposition to Sachs’ curvature. After 22
+h. 30 m. the extreme tip had become slightly curved towards the card.
+
+(9.) Card fixed laterally: after 11 h. 30 m. no effect, the radicle
+being still almost vertical.
+
+(10.) Card fixed almost laterally: after 11 h. 30 m. deflected 90° from
+the vertical and from the card, in a plane intermediate between that of
+the bean itself and one at right
+angles to it. Radicle consequently partially deflected from Sachs’
+curvature.
+
+(11.) Tip of radicle protected with goldbeaters’ skin, with a square of
+card of the usual dimensions affixed with shellac: after 11 h. greatly
+deflected in the plane of the bean, in the direction of Sachs’
+curvature, but to a much greater degree and in less time than ever
+occurs spontaneously.
+
+(12.) Tip of radicle protected as in last case: after 11 h. no effect,
+but after 24 h. 40 m. radicle clearly deflected from the card. This
+slow action was probably due to a portion of the goldbeaters’ skin
+having curled round and lightly touched the opposite side of the tip
+and thus irritated it.
+
+(13.) A radicle of considerable length had a small square of card fixed
+with shellac to its apex laterally: after only 7 h. 15 m. a length of
+.4 of an inch from the apex, measured along the middle, was
+considerably curved from the side bearing the card.
+
+(14.) Case like the last in all respects, except that a length of only
+.25 of an inch of the radicle was thus deflected.
+
+(15.) A small square of card fixed with shellac to the apex of a young
+radicle; after 9 h. 15 m. deflected through 90° from the perpendicular
+and from the card. After 24 h. deflection much decreased, and after an
+additional day, reduced to 23° from the perpendicular.
+
+(16.) Square of card fixed with shellac behind the apex of a radicle,
+which from its position having been changed during growth had become
+very crooked; but the terminal portion was straight, and this became
+deflected to about 45° from the perpendicular and from the card, in
+opposition to Sachs’ curvature.
+
+(17.) Square of card affixed with shellac: after 8 h. radicle curved at
+right angles from the perpendicular and from the card. After 15
+additional hours curvature much decreased.
+
+(18.) Square of card affixed with shellac: after 8 h. no effect; after
+23 h. 3 m. from time of affixing, radicle much curved from the square.
+(19.) Square of card affixed with shellac: after 24 h. no effect, but
+the radicle had not grown well and seemed sickly.
+
+(20.) Square of card affixed with shellac: after 24 h. no effect.
+
+(21, 22.) Squares of card affixed with shellac: after 24 h. radicles of
+both curved at about 45° from the perpendicular and from the cards.
+
+(23.) Square of card fixed with shellac to young radicle: after
+9 h. very slightly curved from the card; after 24 h. tip curved towards
+card. Refixed new square laterally, after 9 h. distinctly curved from
+the card, and after 24 h. curved at right angles from the perpendicular
+and from the card.
+
+(24.) A rather large oblong piece of card fixed with shellac to apex:
+after 24 h. no effect, but the card was found not to be touching the
+apex. A small square was now refixed with shellac; after 16 h. slight
+deflection from the perpendicular and from the card. After an
+additional day the radicle became almost straight.
+
+(25.) Square of card fixed laterally to apex of young radicle; after 9
+h. deflection from the perpendicular considerable; after 24 h.
+deflection reduced. Refixed a fresh square with shellac: after 24 h.
+deflection about 40° from the perpendicular and from the card.
+
+(26.) A very small square of card fixed with shellac to apex of young
+radicle: after 9 h. the deflection from the perpendicular and from the
+card amounted to nearly a right angle; after 24 h. deflection much
+reduced; after an additional 24 h. radicle almost straight.
+
+(27.) Square of card fixed with shellac to apex of young radicle: after
+9 h. deflection from the card and from the perpendicular a right angle;
+next morning quite straight. Refixed a square laterally with shellac;
+after 9 h. a little deflection, which after 24 h. increased to nearly
+20° from the perpendicular and from the card.
+
+(28.) Square of card fixed with shellac; after 9 h. some deflection;
+next morning the card dropped off; refixed it with shellac; it again
+became loose and was refixed; and now on the third trial the radicle
+was deflected after 14 h. at right angles from the card.
+
+(29.) A small square of card was first fixed with thick gum-water to
+the apex. It produced a slight effect but soon fell off. A similar
+square was now affixed laterally with shellac: after 9 h. the radicle
+was deflected nearly 45° from the perpendicular and from the card.
+After 36 additional hours angle of deflection reduced to about 30°.
+
+(30.) A very small piece, less than 1/20th of an inch square, of thin
+tin-foil fixed with shellac to the apex of a young radicle; after 24 h.
+no effect. Tin-foil removed, and a small square of sanded card fixed
+with shellac; after 9 h. deflection at nearly right angles from the
+perpendicular and from the card. Next
+morning deflection reduced to about 40° from the perpendicular.
+
+(31.) A splinter of thin glass gummed to apex, after 9 h. no effect,
+but it was then found not to be touching the apex of the radicle. Next
+morning a square of card was fixed with shellac to it, and after 9 h.
+radicle greatly deflected from the card. After two additional days the
+deflection had decreased and was only 35° from the perpendicular.
+
+(32.) Small square of sanded card, attached with thick gum-water
+laterally to the apex of a long straight radicle: after 9 h. greatly
+deflected from the perpendicular and from the card. Curvature extended
+for a length of .22 of an inch from the apex. After 3 additional hours
+terminal portion deflected at right angles from the perpendicular. Next
+morning the curved portion was .36 in length.
+
+(33.) Square of card gummed to apex: after 15 h. deflected at nearly
+90° from the perpendicular and from the card.
+
+(34.) Small oblong of sanded card gummed to apex: after 15 h. deflected
+90° from the perpendicular and from the card: in the course of the
+three following days the terminal portion became much contorted and
+ultimately coiled into a helix.
+
+(35.) Square of card gummed to apex: after 9 h. deflected from card:
+after 24 h. from time of attachment greatly deflected obliquely and
+partly in opposition to Sachs’ curvature.
+
+(36.) Small piece of card, rather less than 1/20th of an inch square,
+gummed to apex: in 9 h. considerably deflected from card and in
+opposition to Sachs’ curvature; after 24 h. greatly deflected in the
+same direction. After an additional day the extreme tip was curved
+towards the card.
+
+(37.) Square of card, gummed to apex in front, caused after 8 h. 30 m.
+hardly any effect; refixed fresh square laterally, after 15 h.
+deflected almost 90° from the perpendicular and from the card. After 2
+additional days deflection much reduced.
+
+(38.) Square of card gummed to apex: after 9 h. much deflection, which
+after 24 h. from time of fixing increased to nearly 90°. After an
+additional day terminal portion was curled into a loop, and on the
+following day into a helix.
+
+(39.) Small oblong piece of card gummed to apex, nearly in front, but a
+little to one side; in 9 h. slightly deflected in the direction of
+Sachs’ curvature, but rather obliquely, and to side opposite to card.
+Next day more curved in the same direction, and after 2 additional days
+coiled into a ring.
+
+
+(40.) Square of card gummed to apex: after 9 h. slightly curved from
+card; next morning radicle straight, and apex had grown beyond the
+card. Refixed another square laterally with shellac; in 9 h. deflected
+laterally, but also in the direction of Sachs’ curvature. After 2
+additional days’ curvature considerably increased in the same
+direction.
+
+(41.) Little square of tin-foil fixed with gum to one side of apex of a
+young and short radicle: after 15 h. no effect, but tin-foil had become
+displaced. A little square of card was now gummed to one side of apex,
+which after 8 h. 40 m. was slightly deflected; in 24 h. from the time
+of attachment deflected at 90° from the perpendicular and from the
+card; after 9 additional hours became hooked, with the apex pointing to
+the zenith. In 3 days from the time of attachment the terminal portion
+of the radicle formed a ring or circle.
+
+(42.) A little square of thick letter-paper gummed to the apex of a
+radicle, which after 9 h. was deflected from it. In 24 h. from time
+when the paper was affixed the deflection much increased, and after 2
+additional days it amounted to 50° from the perpendicular and from the
+paper.
+
+(43.) A narrow chip of a quill was fixed with shellac to the apex of a
+radicle. After 9 h. no effect; after 24 h. moderate deflection, but now
+the quill had ceased to touch the apex. Removed quill and gummed a
+little square of card to apex, which after 8 h. caused slight
+deflection. On the fourth day from the first attachment of any object,
+the extreme tip was curved towards the card.
+
+(44.) A rather long and narrow splinter of extremely thin glass, fixed
+with shellac to apex, it caused in 9 h. slight deflection, which
+disappeared in 24 h.; the splinter was then found not touching the
+apex. It was twice refixed, with nearly similar results, that is, it
+caused slight deflection, which soon disappeared. On the fourth day
+from the time of first attachment the tip was bent towards the
+splinter.
+
+From these experiments it is clear that the apex of the radicle of the
+bean is sensitive to contact, and that it causes the upper part to bend
+away from the touching object. But before giving a summary of the
+results, it will be convenient briefly to give a few other
+observations. Bits of very thin glass and little squares
+of common card were affixed with thick gum-water to the tips of the
+radicles of seven beans, as a preliminary trial. Six of these were
+plainly acted on, and in two cases the radicles became coiled up into
+complete loops. One radicle was curved into a semi-circle in so short a
+period as 6 h. 10 m. The seventh radicle which was not affected was
+apparently sickly, as it became brown on the following day; so that it
+formed no real exception. Some of these trials were made in the early
+spring during cold weather in a sitting-room, and others in a
+greenhouse, but the temperature was not recorded. These six striking
+cases almost convinced us that the apex was sensitive, but of course we
+determined to make many more trials. As we had noticed that the
+radicles grew much more quickly when subjected to considerable heat,
+and as we imagined that heat would increase their sensitiveness,
+vessels with germinating beans suspended in damp air were placed on a
+chimney-piece, where they were subjected during the greater part of the
+day to a temperature of between 69° and 72° F.; some, however, were
+placed in the hot-house where the temperature was rather higher. Above
+two dozen beans were thus tried; and when a square of glass or card did
+not act, it was removed, and a fresh one affixed, this being often done
+thrice to the same radicle. Therefore between five and six dozen trials
+were altogether made. But there was moderately distinct deflection from
+the perpendicular and from the attached object in only one radicle out
+of this large number of cases. In five other cases there was very
+slight and doubtful deflection. We were astonished at this result, and
+concluded that we had made some inexplicable mistake in the first six
+experiments. But before finally relinquishing the subject, we resolved
+to make one
+other trial for it occurred to us that sensitiveness is easily affected
+by external conditions, and that radicles growing naturally in the
+earth in the early spring would not be subjected to a temperature
+nearly so high as 70° F. We therefore allowed the radicles of 12 beans
+to grow at a temperature of between 55° and 60° F. The result was that
+in every one of these cases (included in the above-described
+experiments) the radicle was deflected in the course of a few hours
+from the attached object. All the above recorded successful trials, and
+some others presently to be given, were made in a sitting-room at the
+temperatures just specified. It therefore appears that a temperature of
+about, or rather above, 70° F. destroys the sensitiveness of the
+radicles, either directly, or indirectly through abnormally accelerated
+growth; and this curious fact probably explains why Sachs, who
+expressly states that his beans were kept at a high temperature, failed
+to detect the sensitiveness of the apex of the radicle.
+
+But other causes interfere with this sensibility. Eighteen radicles
+were tried with little squares of sanded card, some affixed with
+shellac and some with gum-water, during the few last days of 1878, and
+few first days of the next year. They were kept in a room at the proper
+temperature during the day, but were probably too cold at night, as
+there was a hard frost at the time. The radicles looked healthy but
+grew very slowly. The result was that only 6 out of the 18 were
+deflected from the attached cards, and this only to a slight degree and
+at a very slow rate. These radicles therefore presented a striking
+contrast with the 44 above described. On March 6th and 7th, when the
+temperature of the room varied between 53° and 59° F., eleven
+germinating beans were tried in the
+same manner, and now every one of the radicles became curved away from
+the cards, though one was only slightly deflected. Some horticulturists
+believe that certain kinds of seeds will not germinate properly in the
+middle of the winter, although kept at a right temperature. If there
+really is any proper period for the germination of the bean, the feeble
+degree of sensibility of the above radicles may have resulted from the
+trial having been made in the middle of the winter, and not simply from
+the nights being too cold. Lastly, the radicles of four beans, which
+from some innate cause germinated later than all the others of the same
+lot, and which grew slowly though appearing healthy, were similarly
+tried, and even after 24 h. they were hardly at all deflected from the
+attached cards. We may therefore infer that any cause which renders the
+growth of the radicles either slower or more rapid than the normal
+rate, lessens or annuls the sensibility of their tips to contact. It
+deserves particular attention that when the attached objects failed to
+act, there was no bending of any kind, excepting Sachs’ curvature. The
+force of our evidence would have been greatly weakened if occasionally,
+though rarely, the radicles had become curved in any direction
+independently of the attached objects. In the foregoing numbered
+paragraphs, however, it may be observed that the extreme tip sometimes
+becomes, after a considerable interval of time, abruptly curved towards
+the bit of card; but this is a totally distinct phenomenon, as will
+presently be explained.
+
+A Summary of the Results of the foregoing Experiments on the Radicles
+of Vicia faba.—Altogether little squares (about 1/20th of an inch),
+generally of sanded paper as stiff as thin card (between .15 and .20
+mm. in thickness), sometimes of ordinary card, or little
+fragments of very thin glass etc., were affixed at different times to
+one side of the conical tips of 55 radicles. The 11 last-mentioned
+cases, but not the preliminary ones, are here included. The squares,
+etc., were most commonly affixed with shellac, but in 19 cases with
+thick gum-water. When the latter was used, the squares were sometimes
+found, as previously stated, to be separated from the apex by a layer
+of thick fluid, so that there was no contact, and consequently no
+bending of the radicle; and such few cases were not recorded. But in
+every instance in which shellac was employed, unless the square fell
+off very soon, the result was recorded. In several instances when the
+squares became displaced, so as to stand parallel to the radicle, or
+were separated by fluid from the apex, or soon fell off, fresh squares
+were attached, and these cases (described under the numbered
+paragraphs) are here included. Out of 55 radicles experimented on under
+the proper temperature, 52 became bent, generally to a considerable
+extent from the perpendicular, and away from the side to which the
+object was attached. Of the three failures, one can be accounted for,
+as the radicle became sickly on the following day; and a second was
+observed only during 11 h. 30 m. As in several cases the terminal
+growing part of the radicle continued for some time to bend from the
+attached object, it formed itself into a hook, with the apex pointing
+to the zenith, or even into a ring, and occasionally into a spire or
+helix. It is remarkable that these latter cases occurred more
+frequently when objects were attached with thick gum-water, which never
+became dry, than when shellac was employed. The curvature was often
+well-marked in from 7 h. to 11 h.; and in one instance a semicircle was
+formed in 6 h. 10 m, from the time
+of attachment. But in order to see the phenomenon as well displayed as
+in the above described cases, it is indispensable that the bits of
+card, etc., should be made to adhere closely to one side of the conical
+apex; that healthy radicles should be selected and kept at not too high
+or too low a temperature, and apparently that the trials should not be
+made in the middle of the winter.
+
+In ten instances, radicles which had curved away from a square of card
+or other object attached to their tips, straightened themselves to a
+certain extent, or even completely, in the course of from one to two
+days from the time of attachment. This was more especially apt to occur
+when the curvature was slight. But in one instance (No. 27) a radicle
+which in 9 h. had been deflected about 90° from the perpendicular,
+became quite straight in 24 h. from the period of attachment. With No.
+26, the radicle was almost straight in 48 h. We at first attributed the
+straightening process to the radicles becoming accustomed to a slight
+stimulus, in the same manner as a tendril or sensitive petiole becomes
+accustomed to a very light loop of thread, and unbends itself though
+the loop remains still suspended; but Sachs states[2] that radicles of
+the bean placed horizontally in damp air after curving downwards
+through geotropism, straighten themselves a little by growth along
+their lower or concave sides. Why this should occur is not clear: but
+perhaps it likewise occurred in the above ten cases. There is another
+occasional movement which must not be passed over: the tip of the
+radicle, for a length of from 2 to 3 mm., was found in six instances,
+after an interval of about 24 or more hours, bent towards the bit of
+still attached card,—that is, in a direction exactly opposite to the
+previously induced curvature of the whole growing part for a length of
+from 7 to 8 mm. This occurred chiefly when the first curvature was
+small, and when an object had been affixed more than once to the apex
+of the same radicle. The attachment of a bit of card by shellac to one
+side of the tender apex may sometimes mechanically prevent its growth;
+or the application of thick gum-water more than once to the same side
+may injure it; and then checked growth on this side with continued
+growth on the opposite and unaffected side would account for the
+reversed curvature of the apex.
+
+ [2] ‘Arbeiten Bot. Instit., Würzburg,’ Heft iii. p. 456.
+
+
+Various trials were made for ascertaining, as far as we could, the
+nature and degree of irritation to which the apex must be subjected, in
+order that the terminal growing part should bend away, as if to avoid
+the cause of irritation. We have seen in the numbered experiments, that
+a little square of rather thick letter-paper gummed to the apex
+induced, though slowly, considerable deflection. Judging from several
+cases in which various objects had been affixed with gum, and had soon
+become separated from the apex by a layer of fluid, as well as from
+some trials in which drops of thick gum-water alone had been applied,
+this fluid never causes bending. We have also seen in the numbered
+experiments that narrow splinters of quill and of very thin glass,
+affixed with shellac, caused only a slight degree of deflection, and
+this may perhaps have been due to the shellac itself. Little squares of
+goldbeaters’ skin, which is excessively thin, were damped, and thus
+made to adhere to one side of the tips of two radicles; one of these,
+after 24 h., produced no effect; nor did the
+other in 8 h., within which time squares of card usually act; but after
+24 h. there was slight deflection.
+
+An oval bead, or rather cake, of dried shellac, 1.01 mm. in length and
+0.63 in breadth, caused a radicle to become deflected at nearly right
+angles in the course of only 6 h.; but after 23 h. it had nearly
+straightened itself. A very small quantity of dissolved shellac was
+spread over a bit of card, and the tips of 9 radicles were touched
+laterally with it; only two of them became slightly deflected to the
+side opposite to that bearing the speck of dried shellac, and they
+afterwards straightened themselves. These specks were removed, and both
+together weighed less than 1/100th of a grain; so that a weight of
+rather less than 1/200th of a grain (0.32 mg.) sufficed to excite
+movement in two out of the nine radicles. Here then we have apparently
+reached nearly the minimum weight which will act.
+
+A moderately thick bristle (which on measurement was found rather
+flattened, being 0.33 mm. in one diameter, and 0.20 mm. in the other)
+was cut into lengths of about 1/20th of an inch. These after being
+touched with thick gum-water, were placed on the tips of eleven
+radicles. Three of them were affected; one being deflected in 8 h. 15
+m. to an angle of about 90° from the perpendicular; a second to the
+same amount when looked at after 9 h.; but after 24 h. from the time of
+first attachment the deflection had decreased to only 19°; the third
+was only slightly deflected after 9 h., and the bit of bristle was then
+found not touching the apex; it was replaced, and after 15 additional
+hours the deflection amounted to 26° from the perpendicular. The
+remaining eight radicles were not at all acted on by the bits of
+bristle, so that we here appear to have nearly reached the minimum
+of size of an object which will act on the radicle of the bean. But it
+is remarkable that when the bits of bristle did act, that they should
+have acted so quickly and efficiently.
+
+As the apex of a radicle in penetrating the ground must be pressed on
+all sides, we wished to learn whether it could distinguish between
+harder or more resisting, and softer substances. A square of the sanded
+paper, almost as stiff as card, and a square of extremely thin paper
+(too thin for writing on), of exactly the same size (about 1/20th of an
+inch), were fixed with shellac on opposite sides of the apices of 12
+suspended radicles. The sanded card was between 0.15 and 0.20 mm. (or
+between 0.0059 and 0.0079 of an inch), and the thin paper only 0.045
+mm. (or 0.00176 of an inch) in thickness. In 8 out of the 12 cases
+there could be no doubt that the radicle was deflected from the side to
+which the card-like paper was attached, and towards the opposite side,
+bearing the very thin paper. This occurred in some instances in 9 h.,
+but in others not until 24 h. had elapsed. Moreover, some of the four
+failures can hardly be considered as really failures: thus, in one of
+them, in which the radicle remained quite straight, the square of thin
+paper was found, when both were removed from the apex, to have been so
+thickly coated with shellac that it was almost as stiff as the card: in
+the second case, the radicle was bent upwards into a semicircle, but
+the deflection was not directly from the side bearing the card, and
+this was explained by the two squares having become cemented laterally
+together, forming a sort of stiff gable, from which the radicle was
+deflected: in the third case, the square of card had been fixed by
+mistake in front, and though there was deflection from it, this might
+have been due to Sachs’ curvature:
+in the fourth case alone no reason could be assigned why the radicle
+had not been at all deflected. These experiments suffice to prove that
+the apex of the radicle possesses the extraordinary power of
+discriminating between thin card and very thin paper, and is deflected
+from the side pressed by the more resisting or harder substance.
+
+Some trials were next made by irritating the tips without any object
+being left in contact with them. Nine radicles, suspended over water,
+had their tips rubbed, each six times with a needle, with sufficient
+force to shake the whole bean; the temperature was favourable, viz.
+about 63° F. In 7 out of these cases no effect whatever was produced;
+in the eighth case the radicle became slightly deflected from, and in
+the ninth case slightly deflected towards, the rubbed side; but these
+two latter opposed curvatures were probably accidental, as radicles do
+not always grow perfectly straight downwards. The tips of two other
+radicles were rubbed in the same manner for 15 seconds with a little
+round twig, two others for 30 seconds, and two others for 1 minute, but
+without any effect being produced. We may therefore conclude from these
+15 trials that the radicles are not sensitive to temporary contact, but
+are acted on only by prolonged, though very slight, pressure.
+
+We then tried the effects of cutting off a very thin slice parallel to
+one of the sloping sides of the apex, as we thought that the wound
+would cause prolonged irritation, which might induce bending towards
+the opposite side, as in the case of an attached object. Two
+preliminary trials were made: firstly, slices were cut from the
+radicles of 6 beans suspended in damp air, with a pair of scissors,
+which, though sharp, probably caused considerable crushing, and no
+curvature followed. Secondly, thin slices were cut with a razor
+obliquely off the tips of three radicles similarly suspended; and after
+44 h. two were found plainly bent from the sliced surface; and the
+third, the whole apex of which had been cut off obliquely by accident,
+was curled upwards over the bean, but it was not clearly ascertained
+whether the curvature had been at first directed from the cut surface.
+These results led us to pursue the experiment, and 18 radicles, which
+had grown vertically downwards in damp air, had one side of their
+conical tips sliced off with a razor. The tips were allowed just to
+enter the water in the jars, and they were exposed to a temperature
+14°–16° C. (57°–61° F.). The observations were made at different times.
+Three were examined 12 h. after being sliced, and were all slightly
+curved from the cut surface; and the curvature increased considerably
+after an additional 12 h. Eight were examined after 19 h.; four after
+22 h. 30 m.; and three after 25 h. The final result was that out of the
+18 radicles thus tried, 13 were plainly bent from the cut surface after
+the above intervals of time; and one other became so after an
+additional interval of 13 h. 30 m. So that only 4 out of the 18
+radicles were not acted on. To these 18 cases the 3 previously
+mentioned ones should be added. It may, therefore, be concluded that a
+thin slice removed by a razor from one side of the conical apex of the
+radicle causes irritation, like that from an attached object, and
+induces curvature from the injured surface.
+
+Lastly, dry caustic (nitrate of silver) was employed to irritate one
+side of the apex. If one side of the apex or of the whole terminal
+growing part of a radicle, is by any means killed or badly injured, the
+other side continues to grow; and this causes the part
+to bend over towards the injured side.[3] But in the following
+experiments we endeavoured, generally with success, to irritate the
+tips on one side, without badly injuring them. This was effected by
+first drying the tip as far as possible with blotting-paper, though it
+still remained somewhat damp, and then touching it once with quite dry
+caustic. Seventeen radicles were thus treated, and were suspended in
+moist air over water at a temperature of 58° F. They were examined
+after an interval of 21 h. or 24h. The tips of two were found blackened
+equally all round, so that they could tell nothing and were rejected,
+15 being left. Of these, 10 were curved from the side which had been
+touched, where there was a minute brown or blackish mark. Five of these
+radicles, three of which were already slightly deflected, were allowed
+to enter the water in the jar, and were re-examined after an additional
+interval of 27 h. (i.e. in 48 h. after the application of the caustic),
+and now four of them had become hooked, being bent from the discoloured
+side, with their points directed to the zenith; the fifth remained
+unaffected and straight. Thus 11 radicles out of the 15 were acted on.
+But the curvature of the four just described was so plain, that they
+alone would have sufficed to show that the radicles of the bean bend
+away from that side of the apex which has been slightly irritated by
+caustic.
+
+ [3] Ciesielski found this to be the case (‘Untersuchungen über die
+ Abwartskrümmung der Wurzel,’ 1871, p. 28) after burning with heated
+ platinum one side of a radicle. So did we when we painted
+ longitudinally half of the whole length of 7 radicles, suspended over
+ water, with a thick layer of grease, which is very injurious or even
+ fatal to growing parts; for after 48 hours five of these radicles were
+ curved towards the greased side, two remaining straight.
+
+
+_The Power of an Irritant on the apex of the Radicle_
+_of the Bean, compared with that of Geotropism_.—We know that when a
+little square of card or other object is fixed to one side of the tip
+of a vertically dependent radicle, the growing part bends from it often
+into a semicircle, in opposition to geotropism, which force is
+conquered by the effect of the irritation from the attached object.
+Radicles were therefore extended horizontally in damp air, kept at the
+proper low temperature for full sensitiveness, and squares of card were
+affixed with shellac on the lower sides of their tips, so that if the
+squares acted, the terminal growing part would curve upwards. Firstly,
+eight beans were so placed that their short, young, horizontally
+extended radicles would be simultaneously acted on both by geotropism
+and by Sachs’ curvature, if the latter came into play; and they all
+eight became bowed downwards to the centre of the earth in 20 h.,
+excepting one which was only slightly acted on. Two of them were a
+little bowed downwards in only 5 h.! Therefore the cards, affixed to
+the lower sides of their tips, seemed to produce no effect; and
+geotropism easily conquered the effects of the irritation thus caused.
+Secondly, 5 oldish radicles, 1½ inch in length, and therefore less
+sensitive than the above-mentioned young ones, were similarly placed
+and similarly treated. From what has been seen on many other occasions,
+it may be safely inferred that if they had been suspended vertically
+they would have bent away from the cards; and if they had been extended
+horizontally, without cards attached to them, they would have quickly
+bent vertically downwards through geotropism; but the result was that
+two of these radicles were still horizontal after 23 h.; two were
+curved only slightly, and the fifth as much as 40° beneath the horizon.
+Thirdly, 5 beans were fastened
+with their flat surfaces parallel to the cork-lid, so that Sachs’
+curvature would not tend to make the horizontally extended radicles
+turn either upwards or downwards, and little squares of card were
+affixed as before, to the lower sides of their tips. The result was
+that all five radicles were bent down, or towards the centre of the
+earth, after only 8 h. 20 m. At the same time and within the same jars,
+3 radicles of the same age, with squares affixed to one side, were
+suspended vertically; and after 8 h. 20 m. they were considerably
+deflected from the cards, and therefore curved upwards in opposition to
+geotropism. In these latter cases the irritation from the squares had
+over-powered geotropism; whilst in the former cases, in which the
+radicles were extended horizontally, geotropism had overpowered the
+irritation. Thus within the same jars, some of the radicles were
+curving upwards and others downwards at the same time—these opposite
+movements depending on whether the radicles, when the squares were
+first attached to them, projected vertically down, or were extended
+horizontally. This difference in their behaviour seems at first
+inexplicable, but can, we believe, be simply explained by the
+difference between the initial power of the two forces under the above
+circumstances, combined with the well-known principle of the
+after-effects of a stimulus. When a young and sensitive radicle is
+extended horizontally, with a square attached to the lower side of the
+tip, geotropism acts on it at right angles, and, as we have seen, is
+then evidently more efficient than the irritation from the square; and
+the power of geotropism will be strengthened at each successive period
+by its previous action—that is, by its after-effects. On the other
+hand, when a square is affixed to a vertically dependent radicle, and
+the apex begins to
+curve upwards, this movement will be opposed by geotropism acting only
+at a very oblique angle, and the irritation from the card will be
+strengthened by its previous action. We may therefore conclude that the
+initial power of an irritant on the apex of the radicle of the bean, is
+less than that of geotropism when acting at right angles, but greater
+than that of geotropism when acting obliquely on it.
+
+Sensitiveness of the tips of the Secondary Radicles of the Bean to
+contact.—All the previous observations relate to the main or primary
+radicle. Some beans suspended to cork-lids, with their radicles dipping
+into water, had developed secondary or lateral radicles, which were
+afterwards kept in very damp air, at the proper low temperature for
+full sensitiveness. They projected, as usual, almost horizontally, with
+only a slight downward curvature, and retained this position during
+several days. Sachs has shown[4] that these secondary roots are acted
+on in a peculiar manner by geotropism, so that if displaced they
+reassume their former sub-horizontal position, and do not bend
+vertically downwards like the primary radicle. Minute squares of the
+stiff sanded paper were affixed by means of shellac (but in some
+instances with thick gum-water) to the tips of 39 secondary radicles of
+different ages, generally the uppermost ones. Most of the squares were
+fixed to the lower sides of the apex, so that if they acted the radicle
+would bend upwards; but some were fixed laterally, and a few on the
+upper side. Owing to the extreme tenuity of these radicles, it was very
+difficult to attach the square to the actual apex. Whether owing to
+this or some other circumstance, only nine of the squares induced any
+curvature. The curvature amounted in some cases to about 45° above the
+horizon, in others to 90°, and then the tip pointed to the zenith. In
+one instance a distinct upward curvature was observed in 8 h. 15 m.,
+but usually not until 24 h. had elapsed. Although only 9 out of 39
+radicles were affected, yet the curvature was so distinct in several of
+them, that there could be no doubt that the tip is sensitive to slight
+contact, and that the growing part bends away from the touching object.
+It is possible that some secondary radicles are more sensitive than
+others; for Sachs has proved[5] the interesting fact that each
+individual secondary radicle possesses its own peculiar constitution.
+
+ [4] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iv. 1874, p. 605–617.
+
+
+ [5] ‘Arbeiten Bot. Instit., Würzburg,’ Heft, iv. 1874, p. 620.
+
+
+Sensitiveness to contact of the Primary Radicle, a little above the
+apex, in the Bean (Vicia faba) and Pea (Pisum sativum).—The
+sensitiveness of the apex of the radicle in the previously described
+cases, and the consequent curvature of the upper part from the touching
+object or other source of irritation, is the more remarkable, because
+Sachs[6] has shown that pressure at the distance of a few millimeters
+above the apex causes the radicle to bend, like a tendril, towards the
+touching object. By fixing pins so that they pressed against the
+radicles of beans suspended vertically in damp air, we saw this kind of
+curvature; but rubbing the part with a twig or needle for a few minutes
+produced no effect. Haberlandt remarks,[7] that these radicles in
+breaking through the seed-coats often rub and press against the
+ruptured edges, and consequently bend round them. As little squares of
+the card-like paper affixed with shellac to the tips were highly
+efficient in causing the radicles to bend away from them, similar
+pieces (of about 1/20th
+inch square, or rather less) were attached in the same manner to one
+side of the radicle at a distance of 3 or 4 mm. above the apex. In our
+first trial on 15 radicles no effect was produced. In a second trial on
+the same number, three became abruptly curved (but only one strongly)
+towards the card within 24 h. From these cases we may infer that the
+pressure from a bit of card affixed with shellac to one side above the
+apex, is hardly a sufficient irritant; but that it occasionally causes
+the radicle to bend like a tendril towards this side.
+
+ [6] Ibid. Heft iii. 1873, p. 437.
+
+
+ [7] ‘Die Schutzeinrichtungen der Keimpflanze,’ 1877, p. 25.
+
+
+We next tried the effect of rubbing several radicles at a distance of 4
+mm. from the apex for a few seconds with lunar caustic (nitrate of
+silver); and although the radicles had been wiped dry and the stick of
+caustic was dry, yet the part rubbed was much injured and a slight
+permanent depression was left. In such cases the opposite side
+continues to grow, and the radicle necessarily becomes bent towards the
+injured side. But when a point 4 mm. from the apex was momentarily
+touched with dry caustic, it was only faintly discoloured, and no
+permanent injury was caused. This was shown by several radicles thus
+treated straightening themselves after one or two days; yet at first
+they became curved towards the touched side, as if they had been there
+subjected to slight continued pressure. These cases deserve notice,
+because when one side of the apex was just touched with caustic, the
+radicle, as we have seen, curved itself in an opposite direction, that
+is, away from the touched side.
+
+The radicle of the common pea at a point a little above the apex is
+rather more sensitive to continued pressure than that of the bean, and
+bends towards the pressed side.[8] We experimented on a variety
+(_Yorkshire Hero_) which has a much wrinkled tough skin, too large for
+the included cotyledons; so that out of 30 peas which had been soaked
+for 24 h. and allowed to germinate on damp sand, the radicles of three
+were unable to escape, and were crumpled up in a strange manner within
+the skin; four other radicles were abruptly bent round the edges of the
+ruptured skin against which they had pressed. Such abnormalities would
+probably never, or very rarely, occur with forms developed in a state
+of nature and subjected to natural selection. One of the four radicles
+just mentioned in doubling backwards came into contact with the pin by
+which the pea was fixed to the cork-lid; and now it bent at right
+angles round the pin, in a direction quite different from that of the
+first curvature due to contact with the ruptured skin; and it thus
+afforded a good illustration of the tendril-like sensitiveness of the
+radicle a little above the apex.
+
+ [8] Sachs, ‘Arbeiten Bot. Institut., Würzburg,’ Heft iii. p. 438.
+
+
+Little squares of the card-like paper were next affixed to radicles of
+the pea at 4 mm. above the apex, in the same manner as with the bean.
+Twenty-eight radicles suspended vertically over water were thus treated
+on different occasions, and 13 of them became curved towards the cards.
+The greatest degree of curvature amounted to 62° from the
+perpendicular; but so large an angle was only once formed. On one
+occasion a slight curvature was perceptible after 5 h. 45 m., and it
+was generally well-marked after 14 h. There can therefore be no doubt
+that with the pea, irritation from a bit of card attached to one side
+of the radicle above the apex suffices to induce curvature.
+
+Squares of card were attached to one side of the tips of 11 radicles
+within the same jars in which the above trials were made, and five of
+them became plainly, and one slightly, curved away from this side.
+Other
+analogous cases will be immediately described. The fact is here
+mentioned because it was a striking spectacle, showing the difference
+in the sensitiveness of the radicle in different parts, to behold in
+the same jar one set of radicles curved away from the squares on their
+tips, and another set curved towards the squares attached a little
+higher up. Moreover, the kind of curvature in the two cases is
+different. The squares attached above the apex cause the radicle to
+bend abruptly, the part above and beneath remaining nearly straight; so
+that here there is little or no transmitted effect. On the other hand,
+the squares attached to the apex affect the radicle for a length of
+from about 4 to even 8 mm., inducing in most cases a symmetrical
+curvature; so that here some influence is transmitted from the apex for
+this distance along the radicle.
+
+Pisum sativum (var. Yorkshire Hero): Sensitiveness of the apex of the
+Radicle.—Little squares of the same card-like paper were affixed (April
+24th) with shellac to one side of the apex of 10 vertically suspended
+radicles: the temperature of the water in the bottom of the jars was
+60°–61° F. Most of these radicles were acted on in 8 h. 30 m.; and
+eight of them became in the course of 24 h. conspicuously, and the
+remaining two slightly, deflected from the perpendicular and from the
+side bearing the attached squares. Thus all were acted on; but it will
+suffice to describe two conspicuous cases. In one the terminal portion
+of the radicle was bent at right angles (A, Fig. 66) after 24h.; and in
+the other (B) it had by this time become hooked, with the apex pointing
+to the zenith. The two bits of card here used were .07 inch in length
+and .04 inch in breadth. Two other radicles, which after 8 h. 30 m.
+were moderately deflected, became straight again after 24h. Another
+trial was made in the same manner with 15 radicles; but from
+circumstances, not worth explaining, they were only once and briefly
+examined after the short interval of 5 h. 30 m.; and we merely record
+in our notes “almost all bent slightly from the perpendicular, and away
+from the squares; the deflection amounting in one or two instances to
+nearly a rectangle.” These two sets of cases, especially the first one,
+prove that the apex of the radicle is sensitive to slight contact and
+that the upper part bends from the touching object. Nevertheless, on
+June 1st and 4th, 8 other radicles were tried in the same manner at a
+temperature of 58°–60° F., and after 24 h. only 1 was decidedly bent
+from the card, 4 slightly, 2 doubtfully, and 1 not in the least. The
+amount of curvature was unaccountably small; but all the radicles which
+were at all bent, were bent away from the cards.
+
+Fig. 66. Pisum sativum: deflection produced within 24 hours in the
+growth of vertically dependent radicles, by little squares of card
+affixed with shellac to one side of apex: A, bent at right angles; B,
+hooked.
+
+We now tried the effects of widely different temperatures on the
+sensitiveness of these radicles with squares
+of card attached to their tips. Firstly, 13 peas, most of them having
+very short and young radicles, were placed in an ice-box, in which the
+temperature rose during three days from 44° to 47° F. They grew slowly,
+but 10 out of the 13 became in the course of the three days very
+slightly curved from the squares; the other 3 were not affected; so
+that this temperature was too low for any high degree of sensitiveness
+or for much movement. Jars with 13 other radicles were next placed on a
+chimney-piece, where they were subjected to a temperature of between
+68° and 72° F., and after 24 h., 4 were conspicuously curved from the
+cards, 2 slightly, and 7 not at all; so that this temperature was
+rather too high. Lastly 12 radicles were subjected to a temperature
+varying between 72° and 85° F., and none of them were in the least
+affected by the squares. The above several trials, especially the first
+recorded one, indicate that the most favourable temperature for the
+sensitiveness of the radicle of the pea is about 60° F.
+
+
+The tips of 6 vertically dependent radicles were touched once with dry
+caustic, in the manner described under Vicia faba. After 24 h. four of
+them were bent from the side bearing a minute black mark; and the
+curvature increased in one case after 38 h., and in another case after
+48 h., until the terminal part projected almost horizontally. The two
+remaining radicles were not affected.
+
+With radicles of the bean, when extended horizontally in damp air,
+geotropism always conquered the effects of the irritation caused by
+squares of card attached to the lower sides of their tips. A similar
+experiment was tried on 13 radicles of the pea; the squares being
+attached with shellac, and the temperature between 58°–60° F. The
+result was somewhat different; for
+these radicles are either less strongly acted on by geotropism, or,
+what is more probable, are more sensitive to contact. After a time
+geotropism always prevailed, but its action was often delayed; and in
+three instances there was a most curious struggle between geotropism
+and the irritation caused by the cards. Four of the 13 radicles were a
+little curved downwards within 6 or 8 h., always reckoning from the
+time when the squares were first attached, and after 23 h. three of
+them pointed vertically downwards, and the fourth at an angle of 45°
+beneath the horizon. These four radicles therefore did not seem to have
+been at all affected by the attached squares. Four others were not
+acted on by geotropism within the first 6 or 8 h., but after 23 h. were
+much bowed down. Two others remained almost horizontal for 23 h., but
+afterwards were acted on. So that in these latter six cases the action
+of geotropism was much delayed. The eleventh radicle was slightly
+curved down after 8 h., but when looked at again after 23 h. the
+terminal portion was curved upwards; if it had
+been longer observed, the tip no doubt would have been found again
+curved down, and it would have formed a loop as in the following case.
+The twelfth radicle after 6 h. was slightly curved downwards; but when
+looked at again after 21 h., this curvature had disappeared and the
+apex pointed upwards; after 30 h. the radicle formed a hook, as shown
+at A (Fig. 67); which hook after 45 h. was converted into a loop (B).
+The thirteenth radicle after 6 h. was slightly curved downwards, but
+within 21 h. had curved considerably up, and then down again at an
+angle of 45° beneath the horizon, afterwards becoming perpendicular. In
+these three last cases geotropism and the irritation caused by the
+attached squares alternately prevailed in a highly remarkable manner;
+geotropism being ultimately victorious.
+
+Fig. 67. Pisum sativum: a radicle extended horizontally in damp air
+with a little square of card affixed to the lower side of its tip,
+causing it to bend upwards in opposition to geotropism. The deflection
+of the radicle after 21 hours is shown at A, and of the same radicle
+after 45 hours at B, now forming a loop.
+
+Similar experiments were not always quite so successful as in the above
+cases. Thus 6 radicles, horizontally extended with attached squares,
+were tried on June 8th at a proper temperature, and after 7 h. 30 m.
+none were in the least curved upwards and none were distinctly
+geotropic; whereas of 6 radicles without any attached squares, which
+served as standards of comparison or controls, 3 became slightly and 3
+almost rectangularly geotropic within the 7 h. 30 m.; but after 23 h.
+the two lots were equally geotropic. On July 10th another trial was
+made with 6 horizontally extended radicles, with squares attached in
+the same manner beneath their tips; and after 7 h. 30 m., 4 were
+slightly geotropic, 1 remained horizontal, and 1 was curved upwards in
+opposition to gravity or geotropism. This latter radicle after 48 h.
+formed a loop, like that at B (Fig. 67).
+
+An analogous trial was now made, but instead of attaching squares of
+card to the lower sides of the
+tips, these were touched with dry caustic. The details of the
+experiment will be given in the chapter on Geotropism, and it will
+suffice here to say that 10 peas, with radicles extended horizontally
+and not cauterised, were laid on and under damp friable peat; these,
+which served as standards or controls, as well as 10 others which had
+been touched on the upper side with the caustic, all became strongly
+geotropic in 24 h. Nine radicles, similarly placed, had their tips
+touched on the lower side with the caustic; and after 24 h., 3 were
+slightly geotropic, 2 remained horizontal, and 4 were bowed upwards in
+opposition to gravity and to geotropism. This upward curvature was
+distinctly visible in 8 h. 45m. after the lower sides of the tips had
+been cauterised.
+
+Little squares of card were affixed with shellac on two occasions to
+the tips of 22 young and short secondary radicles, which had been
+emitted from the primary radicle whilst growing in water, but were now
+suspended in damp air. Besides the difficulty of attaching the squares
+to such finely pointed objects as were these radicles, the temperature
+was too high,—varying on the first occasion from 72° to 77° F., and on
+the second being almost steadily 78° F.; and this probably lessened the
+sensitiveness of the tips. The result was that after an interval of 8
+h. 30 m., 6 of the 22 radicles were bowed upwards (one of them greatly)
+in opposition to gravity, and 2 laterally; the remaining 14 were not
+affected. Considering the unfavourable circumstances, and bearing in
+mind the case of the bean, the evidence appears sufficient to show that
+the tips of the secondary radicles of the pea are sensitive to slight
+contact.
+
+Phaseolus multiflorus: Sensitiveness of the apex of the
+Radicle.—Fifty-nine radicles were tried with squares
+of various sizes of the same card-like paper, also with bits of thin
+glass and rough cinders, affixed with shellac to one side of the apex.
+Rather large drops of the dissolved shellac were also placed on them
+and allowed to set into hard beads. The specimens were subjected to
+various temperatures between 60° and 72° F., more commonly at about the
+latter. But out of this considerable number of trials only 5 radicles
+were plainly bent, and 8 others slightly or even doubtfully, from the
+attached objects; the remaining 46 not being at all affected. It is
+therefore clear that the tips of the radicles of this Phaseolus are
+much less sensitive to contact than are those of the bean or pea. We
+thought that they might be sensitive to harder pressure, but after
+several trials we could not devise any method for pressing harder on
+one side of the apex than on the other, without at the same time
+offering mechanical resistance to its growth. We therefore tried other
+irritants.
+
+The tips of 13 radicles, dried with blotting-paper, were thrice touched
+or just rubbed on one side with dry nitrate of silver. They were rubbed
+thrice, because we supposed from the foregoing trials, that the tips
+were not highly sensitive. After 24 h. the tips were found greatly
+blackened; 6 were blackened equally all round, so that no curvature to
+any one side could be expected; 6 were much blackened on one side for a
+length of about 1/10th of an inch, and this length became curved at
+right angles towards the blackened surface, the curvature afterwards
+increasing in several instances until little hooks were formed. It was
+manifest that the blackened side was so much injured that it could not
+grow, whilst the opposite side continued to grow. One alone out of
+these 13 radicles became curved from the blackened side, the
+curvature extending for some little distance above the apex.
+
+After the experience thus gained, the tips of six almost dry radicles
+were once touched with the dry caustic on one side; and after an
+interval of 10 m. were allowed to enter water, which was kept at a
+temperature of 65°–67° F. The result was that after an interval of 8 h.
+a minute blackish speck could just be distinguished on one side of the
+apex of five of these radicles, all of which became curved towards the
+opposite side—in two cases at about an angle of 45°—in two other cases
+at nearly a rectangle—and in the fifth case at above a rectangle, so
+that the apex was a little hooked; in this latter case the black mark
+was rather larger than in the others. After 24 h. from the application
+of the caustic, the curvature of three of these radicles (including the
+hooked one) had diminished; in the fourth it remained the same, and in
+the fifth it had increased, the tip being now hooked. It has been said
+that after 8 h. black specks could be seen on one side of the apex of
+five of the six radicles; on the sixth the speck, which was extremely
+minute, was on the actual apex and therefore central; and this radicle
+alone did not become curved. It was therefore again touched on one side
+with caustic, and after 15 h. 30 m. was found curved from the
+perpendicular and from the blackened side at an angle of 34°, which
+increased in nine additional hours to 54°.
+
+It is therefore certain that the apex of the radicle of this Phaseolus
+is extremely sensitive to caustic, more so than that of the bean,
+though the latter is far more sensitive to pressure. In the experiments
+just given, the curvature from the slightly cauterised side of the tip,
+extended along the radicle for a length of nearly 10 mm.; whereas in
+the first set
+of experiments, when the tips of several were greatly blackened and
+injured on one side, so that their growth was arrested, a length of
+less than 3 mm. became curved towards the much blackened side, owing to
+the continued growth of the opposite side. This difference in the
+results is interesting, for it shows that too strong an irritant does
+not induce any transmitted effect, and does not cause the adjoining,
+upper and growing part of the radicle to bend. We have analogous cases
+with Drosera, for a strong solution of carbonate of ammonia when
+absorbed by the glands, or too great heat suddenly applied to them, or
+crushing them, does not cause the basal part of the tentacles to bend,
+whilst a weak solution of the carbonate, or a moderate heat, or slight
+pressure always induced such bending. Similar results were observed
+with Dionaea and Pinguicula.
+
+The effect of cutting off with a razor a thin slice from one side of
+the conical apex of 14 young and short radicles was next tried. Six of
+them after being operated on were suspended in damp air; the tips of
+the other eight, similarly suspended, were allowed to enter water at a
+temperature of about 65° F. It was recorded in each case which side of
+the apex had been sliced off, and when they were afterwards examined
+the direction of the curvature was noted, before the record was
+consulted. Of the six radicles in damp air, three had their tips curved
+after an interval of 10 h. 15 m. directly away from the sliced surface,
+whilst the other three were not affected and remained straight;
+nevertheless, one of them after 13 additional hours became slightly
+curved from the sliced surface. Of the eight radicles with their tips
+immersed in water, seven were plainly curved away from the sliced
+surfaces after 10 h. 15 m.; and with
+respect to the eighth which remained quite straight, too thick a slice
+had been accidentally removed, so that it hardly formed a real
+exception to the general result. When the seven radicles were looked at
+again, after an interval of 23 h. from the time of slicing, two had
+become distorted; four were deflected at an angle of about 70° from the
+perpendicular and from the cut surface; and one was deflected at nearly
+90°, so that it projected almost horizontally, but with the extreme tip
+now beginning to bend downwards through the action of geotropism. It is
+therefore manifest that a thin slice cut off one side of the conical
+apex, causes the upper growing part of the radicle of this Phaseolus to
+bend, through the transmitted effects of the irritation, away from the
+sliced surface.
+
+Tropaeolum majus: Sensitiveness of the apex of the Radicle to
+contact.—Little squares of card were attached with shellac to one side
+of the tips of 19 radicles, some of which were subjected to 78° F., and
+others to a much lower temperature. Only 3 became plainly curved from
+the squares, 5 slightly, 4 doubtfully, and 7 not at all. These seeds
+were, as we believed, old, so we procured a fresh lot, and now the
+results were widely different. Twenty-three were tried in the same
+manner; five of the squares produced no effect, but three of these
+cases were no real exceptions, for in two of them the squares had
+slipped and were parallel to the apex, and in the third the shellac was
+in excess and had spread equally all round the apex. One radicle was
+deflected only slightly from the perpendicular and from the card;
+whilst seventeen were plainly deflected. The angles in several of these
+latter cases varied between 40° and 65° from the perpendicular; and in
+two of them it amounted after 15 h. or 16 h. to about 90°. In one
+instance a loop
+was nearly completed in 16 h. There can, therefore, be no doubt that
+the apex is highly sensitive to slight contact, and that the upper part
+of the radicle bends away from the touching object.
+
+Gossypium herbaceum: Sensitiveness of the apex of the Radicle.—Radicles
+were experimented on in the same manner as before, but they proved
+ill-fitted for our purpose, as they soon became unhealthy when
+suspended in damp air. Of 38 radicles thus suspended, at temperatures
+varying from 66° to 69° F., with squares of card attached to their
+tips, 9 were plainly and 7 slightly or even doubtfully deflected from
+the squares and from the perpendicular; 22 not being affected. We
+thought that perhaps the above temperature was not high enough, so 19
+radicles with attached squares, likewise suspended in damp air, were
+subjected to a temperature of from 74° to 79° F., but not one of them
+was acted on, and they soon became unhealthy. Lastly, 19 radicles were
+suspended in water at a temperature from 70° to 75° F., with bits of
+glass or squares of the card attached to their tips by means of
+Canada-balsam or asphalte, which adhered rather better than shellac
+beneath the water. The radicles did not keep healthy for long. The
+result was that 6 were plainly and 2 doubtfully deflected from the
+attached objects and the perpendicular; 11 not being affected. The
+evidence consequently is hardly conclusive, though from the two sets of
+cases tried under a moderate temperature, it is probable that the
+radicles are sensitive to contact; and would be more so under
+favourable conditions.
+
+Fifteen radicles which had germinated in friable peat were suspended
+vertically over water. Seven of them served as controls, and they
+remained quite straight during 24 h. The tips of the other eight
+radicles
+were just touched with dry caustic on one side. After only 5 h. 10 m.
+five of them were slightly curved from the perpendicular and from the
+side bearing the little blackish marks. After 8 h. 40 m., 4 out of
+these 5 were deflected at angles between 15° and 65° from the
+perpendicular. On the other hand, one which had been slightly curved
+after 5 h. 10 m., now became straight. After 24 h. the curvature in two
+cases had considerably increased; also in four other cases, but these
+latter radicles had now become so contorted, some being turned upwards,
+that it could no longer be ascertained whether they were still curved
+from the cauterised side. The control specimens exhibited no such
+irregular growth, and the two sets presented a striking contrast. Out
+of the 8 radicles which had been touched with caustic, two alone were
+not affected, and the marks left on their tips by the caustic were
+extremely minute. These marks in all cases were oval or elongated; they
+were measured in three instances, and found to be of nearly the same
+size, viz. 2/3 of a mm. in length. Bearing this fact in mind, it should
+be observed that the length of the curved part of the radicle, which
+had become deflected from the cauterised side in the course of 8 h. 40
+m. was found to be in three cases 6, 7, and 9 mm.
+
+Cucurbita ovifera: Sensitiveness of the apex of the Radicle.—The tips
+proved ill-fitted for the attachment of cards, as they are extremely
+fine and flexible. Moreover, owing to the hypocotyls being soon
+developed and becoming arched, the whole radicle is quickly displaced
+and confusion is thus caused. A large number of trials were made, but
+without any definite result, excepting on two occasions, when out of 23
+radicles 10 were deflected from the attached squares
+of card, and 13 were not acted on. Rather large squares, though
+difficult to affix, seemed more efficient than very small ones.
+
+We were much more successful with caustic; but in our first trial, 15
+radicles were too much cauterised, and only two became curved from the
+blackened side; the others being either killed on one side, or
+blackened equally all round. In our next trial the dried tips of 11
+radicles were touched momentarily with dry caustic, and after a few
+minutes were immersed in water. The elongated marks thus caused were
+never black, only brown, and about ½ mm. in length, or even less. In 4
+h. 30 m. after the cauterisation, 6 of them were plainly curved from
+the side with the brown mark, 4 slightly, and 1 not at all. The latter
+proved unhealthy, and never grew; and the marks on 2 of the 4 slightly
+curved radicles were excessively minute, one being distinguishable only
+with the aid of a lens. Of 10 control specimens tried in the same jars
+at the same time, not one was in the least curved. In 8 h. 40 m. after
+the cauterisation, 5 of the radicles out of the 10 (the one unhealthy
+one being omitted) were deflected at about 90°, and 3 at about 45° from
+the perpendicular and from the side bearing the brown mark. After 24 h.
+all 10 radicles had increased immensely in length; in 5 of them the
+curvature was nearly the same, in 2 it had increased, and in 3 it had
+decreased. The contrast presented by the 10 controls, after both the 8
+h. 40 m. and the 24 h. intervals, was very great; for they had
+continued to grow vertically downwards, excepting two which, from some
+unknown cause, had become somewhat tortuous.
+
+In the chapter on Geotropism we shall see that 10 radicles of this
+plant were extended horizontally on and beneath damp friable peat,
+under which conditions
+they grow better and more naturally than in damp air; and their tips
+were slightly cauterised on the lower side, brown marks about ½ mm. in
+length being thus caused. Uncauterised specimens similarly placed
+became much bent downwards through geotropism in the course of 5 or 6
+hours. After 8 h. only 3 of the cauterised ones were bowed downwards,
+and this in a slight degree; 4 remained horizontal; and 3 were curved
+upwards in opposition to geotropism and from the side bearing the brown
+mark. Ten other specimens had their tips cauterised at the same time
+and in the same degree, on the upper side; and this, if it produced any
+effect, would tend to increase the power of geotropism; and all these
+radicles were strongly bowed downwards after 8 h. From the several
+foregoing facts, there can be no doubt that the cauterisation of the
+tip of the radicle of this Cucurbita on one side, if done lightly
+enough, causes the whole growing part to bend to the opposite side.
+Raphanus sativus: Sensitiveness of the apex of the Radicle.—We here
+encountered many difficulties in our trials, both with squares of card
+and with caustic; for when seeds were pinned to a cork-lid, many of the
+radicles, to which nothing had been done, grew irregularly, often
+curving upwards, as if attracted by the damp surface above; and when
+they were immersed in water they likewise often grew irregularly. We
+did not therefore dare to trust our experiments with attached squares
+of card; nevertheless some of them seemed to indicate that the tips
+were sensitive to contact. Our trials with caustic generally failed
+from the difficulty of not injuring too greatly the extremely fine
+tips. Out of 7 radicles thus tried, one became bowed after 22 h. at an
+angle of 60°, a second at 40°,
+and a third very slightly from the perpendicular and from the
+cauterised side.
+
+Æsculus hippocastanum: Sensitiveness of the apex of the Radicle.—Bits
+of glass and squares of card were affixed with shellac or gum-water to
+the tips of 12 radicles of the horse-chestnut; and when these objects
+fell off, they were refixed; but not in a single instance was any
+curvature thus caused. These massive radicles, one of which was above 2
+inches in length and .3 inch in diameter at its base, seemed insensible
+to so slight a stimulus as any small attached object. Nevertheless,
+when the apex encountered an obstacle in its downward course, the
+growing part became so uniformly and symmetrically curved, that its
+appearance indicated not mere mechanical bending, but increased growth
+along the whole convex side, due to the irritation of the apex.
+
+That this is the correct view may be inferred from the effects of the
+more powerful stimulus of caustic. The bending from the cauterised side
+occurred much slower than in the previously described species, and it
+will perhaps be worth while to give our trials in detail.
+
+The seeds germinated in sawdust, and one side of the tips of the
+radicles were slightly rubbed once with dry nitrate of silver; and
+after a few minutes were allowed to dip into water. They were subjected
+to a rather varying temperature, generally between 52° and 58° F. A few
+cases have not been thought worth recording, in which the whole tip was
+blackened, or in which the seedling soon became unhealthy.
+
+(1.) The radicle was slightly deflected from the cauterised side in one
+day (i.e. 24 h.); in three days it stood at 60° from the perpendicular;
+in four days at 90°; on the fifth day it was curved up about 40° above
+the horizon; so that it had passed through an angle of 130° in the five
+days, and this was the greatest amount of curvature observed.
+
+(2.) In two days radicle slightly deflected; after seven days
+deflected 69° from the perpendicular and from the cauterised side;
+after eight days the angle amounted to nearly 90°.
+
+(3.) After one day slight deflection, but the cauterised mark was so
+faint that the same side was again touched with caustic. In four days
+from the first touch deflection amounted to 78°, which in an additional
+day increased to 90°.
+
+(4.) After two days slight deflection, which during the next three days
+certainly increased but never became great; the radicle did not grow
+well and died on the eighth day.
+
+(5.) After two days very slight deflection; but this on the fourth day
+amounted to 56° from the perpendicular and from the cauterised side.
+
+(6.) After three days doubtfully, but after four days certainly
+deflected from the cauterised side. On the fifth day deflection
+amounted to 45° from the perpendicular, and this on the seventh day
+increased to about 90°.
+
+(7.) After two days slightly deflected; on the third day the deflection
+amounted to 25° from the perpendicular, and this did not afterwards
+increase.
+
+(8.) After one day deflection distinct; on the third day it amounted to
+44°, and on the fourth day to 72° from the perpendicular and the
+cauterised side.
+
+(9.) After two days deflection slight, yet distinct; on the third day
+the tip was again touched on the same side with caustic and thus
+killed.
+
+(10.) After one day slight deflection, which after six days increased
+to 50° from the perpendicular and the cauterised side.
+
+(11.) After one day decided deflection, which after six days increased
+to 62° from the perpendicular and from the cauterised side.
+
+(12.) After one day slight deflection, which on the second day amounted
+to 35°, on the fourth day to 50°, and the sixth day to 63° from the
+perpendicular and the cauterised side.
+
+(13.) Whole tip blackened, but more on one side than the other; on the
+fourth day slightly, and on the sixth day greatly deflected from the
+more blackened side; the deflection on the ninth day amounted to 90°
+from the perpendicular.
+
+(14.) Whole tip blackened in the same manner as in the last case: on
+the second day decided deflection from the more blackened side, which
+increased on the seventh day to nearly 90°; on the following day the
+radicle appeared unhealthy.
+
+(15.) Here we had the anomalous case of a radicle bending
+slightly _towards_ the cauterised side on the first day, and continuing
+to do so for the next three days, when the deflection amounted to about
+90° from the perpendicular. The cause appeared to lie in the
+tendril-like sensitiveness of the upper part of the radicle, against
+which the point of a large triangular flap of the seed-coats pressed
+with considerable force; and this irritation apparently conquered that
+from the cauterised apex.
+
+These several cases show beyond doubt that the irritation of one side
+of the apex, excites the upper part of the radicle to bend slowly
+towards the opposite side. This fact was well exhibited in one lot of
+five seeds pinned to the cork-lid of a jar; for when after 6 days the
+lid was turned upside down and viewed from directly above, the little
+black marks made by the caustic were now all distinctly visible on the
+upper sides of the tips of the laterally bowed radicles. A thin slice
+was shaved off with a razor from one side of the tips of 22 radicles,
+in the manner described under the common bean; but this kind of
+irritation did not prove very effective. Only 7 out of the 22 radicles
+became moderately deflected in from 3 to 5 days from the sliced
+surface, and several of the others grew irregularly. The evidence,
+therefore, is far from conclusive.
+
+Quercus robur: Sensitiveness of the apex of the Radicle.—The tips of
+the radicles of the common oak are fully as sensitive to slight contact
+as are those of any plant examined by us. They remained healthy in damp
+air for 10 days, but grew slowly. Squares of the card-like paper were
+fixed with shellac to the tips of 15 radicles, and ten of these became
+conspicuously bowed from the perpendicular and from the squares; two
+slightly, and three not at all. But two of the latter were not real
+exceptions, as they were at first very short, and hardly grew
+afterwards. Some of the more
+remarkable cases are worth describing. The radicles were examined on
+each successive morning, at nearly the same hour, that is, after
+intervals of 24 h.
+
+No. 1. This radicle suffered from a series of accidents, and acted in
+an anomalous manner, for the apex appeared at first insensible and
+afterwards sensitive to contact. The first square was attached on Oct
+19th; on the 21st the radicle was not at all curved, and the square was
+accidentally knocked off; it was refixed on the 22nd, and the radicle
+became slightly curved from the square, but the curvature disappeared
+on the 23rd, when the square was removed and refixed. No curvature
+ensued, and the square was again accidentally knocked off, and refixed.
+On the morning of the 27th it was washed off by having reached the
+water in the bottom of the jar. The square was refixed, and on the
+29th, that is, ten days after the first square had been attached, and
+two days after the attachment of the last square, the radicle had grown
+to the great length of 3.2 inches, and now the terminal growing part
+had become bent away from the square into a hook (see Fig. 68).
+
+Fig. 68. Quercus robur: radicle with square of card attached to one
+side of apex, causing it to become hooked. Drawing one-half natural
+scale.
+
+No. 2. Square attached on the 19th; on the 20th radicle slightly
+deflected from it and from the perpendicular; on the 21st deflected at
+nearly right angles; it remained during the next two days in this
+position, but on the 25th the upward curvature was lessened through the
+action of geotropism, and still more so on the 26th.
+
+No. 3. Square attached on the 19th; on the 21st a trace of curvature
+from the square, which amounted on the 22nd to about 40°, and on the
+23rd to 53° from the perpendicular.
+
+No. 4. Square attached on the 21st; on the 22nd trace of curvature from
+the square; on the 23rd completely hooked with the point turned up to
+the zenith. Three days afterwards (i.e. 26th) the curvature had wholly
+disappeared and the apex pointed perpendicularly downwards.
+
+No. 5. Square attached on the 21st; on the 22nd decided
+though slight curvature from the square; on the 23rd the tip had curved
+up above the horizon, and on the 24th was hooked with the apex pointing
+almost to the zenith, as in Fig. 68.
+
+No. 6. Square attached on the 21st; on the 22nd slightly curved from
+the square; 23rd more curved; 25th considerably curved; 27th all
+curvature lost, and the radicle was now directed perpendicularly
+downwards.
+
+No. 7. Square attached on the 21st; on the 22nd a trace of curvature
+from the square, which increased next day, and on the 24th amounted to
+a right angle.
+
+It is, therefore, manifest that the apex of the radicle of the oak is
+highly sensitive to contact, and retains its sensitiveness during
+several days. The movement thus induced was, however, slower than in
+any of the previous cases, with the exception of that of Æsculus. As
+with the bean, the terminal growing part, after bending, sometimes
+straightened itself through the action of geotropism, although the
+object still remained attached to the tip.
+
+The same remarkable experiment was next tried, as in the case of the
+bean; namely, little squares of exactly the same size of the card-like
+sanded paper and of very thin paper (the thicknesses of which have been
+given under Vicia faba) were attached with shellac on opposite sides
+(as accurately as could be done) of the tips of 13 radicles, suspended
+in damp air, at a temperature of 65°–66° F. The result was striking,
+for 9 out of these 13 radicles became plainly, and 1 very slightly,
+curved from the thick paper towards the side bearing the thin paper. In
+two of these cases the apex became completely hooked after two days; in
+four cases the deflection from the perpendicular and from the side
+bearing the thick paper, amounted in from two to four days to angles of
+90°, 72°, 60°, and 49°, but in two other cases to only 18° and 15°. It
+should, however, be stated that in the
+case in which the deflection was 49°, the two squares had accidentally
+come into contact on one side of the apex, and thus formed a lateral
+gable; and the deflection was directed in part from this gable and in
+part from the thick paper. In three cases alone the radicles were not
+affected by the difference in thickness of the squares of paper
+attached to their tips, and consequently did not bend away from the
+side bearing the stiffer paper.
+
+Zea mays: Sensitiveness of the apex of the Radicle to contact.—A large
+number of trials were made on this plant, as it was the only
+monocotyledon on which we experimented. An abstract of the results will
+suffice. In the first place, 22 germinating seeds were pinned to
+cork-lids without any object being attached to their radicles, some
+being exposed to a temperature of 65°–66° F., and others to between 74°
+and 79°; and none of them became curved, though some were a little
+inclined to one side. A few were selected, which from having germinated
+on sand were crooked, but when suspended in damp air the terminal part
+grew straight downwards. This fact having been ascertained, little
+squares of the card-like paper were affixed with shellac, on several
+occasions, to the tips of 68 radicles. Of these the terminal growing
+part of 39 became within 24 h. conspicuously curved away from the
+attached squares and from the perpendicular; 13 out of the 39 forming
+hooks with their points directed towards the zenith, and 8 forming
+loops. Moreover, 7 other radicles out of the 68, were slightly and two
+doubtfully deflected from the cards. There remain 20 which were not
+affected; but 10 of these ought not to be counted; for one was
+diseased, two had their tips quite surrounded by shellac, and the
+squares on 7 had slipped so as to stand parallel to the apex, instead
+of obliquely
+on it. There were therefore only 10 out of the 68 which certainly were
+not acted on. Some of the radicles which were experimented on were
+young and short, most of them of moderate length, and two or three
+exceeded three inches in length. The curvature in the above cases
+occurred within 24 h., but it was often conspicuous within a much
+shorter period. For instance, the terminal growing part of one radicle
+was bent upwards into a rectangle in 8 h. 15 m., and of another in 9 h.
+On one occasion a hook was formed in 9 h. Six of the radicles in a jar
+containing nine seeds, which stood on a sand-bath, raised to a
+temperature varying from 76° to 82° F., became hooked, and a seventh
+formed a complete loop, when first looked at after 15 hours.
+
+The accompanying figures of four germinating seeds (Fig. 69) show,
+firstly, a radicle (A) the apex of which has become so much bent away
+from the attached square as to form a hook. Secondly (B), a hook
+converted through the continued irritation of the card, aided perhaps
+by geotropism, into an almost complete circle or loop. The tip in the
+act of forming a loop generally rubs against the upper part of the
+radicle, and pushes off the attached square; the loop then contracts or
+closes, but never disappears; and the apex afterwards grows vertically
+downwards, being no longer irritated by any attached object. This
+frequently occurred, and is represented at C. The jar above mentioned
+with the six hooked radicles and another jar were kept for two
+additional days, for the sake of observing how the hooks would be
+modified. Most of them became converted into simple loops, like that
+figured at C; but in one case the apex did not rub against the upper
+part of the radicle and thus remove the card; and it consequently made,
+owing
+to the continued irritation from the card, two complete loops, that is,
+a helix of two spires; which afterwards became pressed closely
+together. Then geotropism prevailed and caused the apex to grow
+perpendicularly downwards. In another case, shown at (D), the apex in
+making a second turn or spire, passed through the first loop, which was
+at first widely open, and in doing so knocked off the card; it then
+grew perpendicularly downwards, and thus tied itself into a knot, which
+soon became tight!
+
+Fig. 69. Zea mays: radicles excited to bend away from the little
+squares of card attached to one side of their tips.
+
+Secondary Radicles of Zea.—A short time after the first radicle has
+appeared, others protrude from the
+seed, but not laterally from the primary one. Ten of these secondary
+radicles, which were directed obliquely downwards, were experimented on
+with very small squares of card attached with shellac to the lower
+sides of their tips. If therefore the squares acted, the radicles would
+bend upwards in opposition to gravity. The jar stood (protected from
+light) on a sand-bath, which varied between 76° and 82° F. After only 5
+h. one appeared to be a little deflected from the square, and after 20
+h. formed a loop. Four others were considerably curved from the squares
+after 20 h., and three of them became hooked, with their tips pointing
+to the zenith,—one after 29 h. and the two others after 44 h. By this
+latter time a sixth radicle had become bent at a right angle from the
+side bearing the square. Thus altogether six out of the ten secondary
+radicles were acted on, four not being affected. There can, therefore,
+be no doubt that the tips of these secondary radicles are sensitive to
+slight contact, and that when thus excited they cause the upper part to
+bend from the touching object; but generally, as it appears, not in so
+short a time as in the case of the first-formed radicle.
+
+SENSITIVENESS OF THE TIP OF THE RADICLE TO MOIST AIR.
+
+Sachs made the interesting discovery, a few years ago, that the
+radicles of many seedling plants bend towards an adjoining damp
+surface.[9] We shall here endeavour to show that this peculiar form of
+sensitiveness resides in their tips. The movement is directly the
+reverse of that excited by the irritants hitherto considered, which
+cause the growing part of the
+radicle to bend away from the source of irritation. In our experiments
+we followed Sachs’ plan, and sieves with seeds germinating in damp
+sawdust were suspended so that the bottom was generally inclined at 40°
+with the horizon. If the radicles had been acted on solely by
+geotropism, they would have grown out of the bottom of the sieve
+perpendicularly downwards; but as they were attracted by the adjoining
+damp surface they bent towards it and were deflected 50° from the
+perpendicular. For the sake of ascertaining whether the tip or the
+whole growing part of the radicle was sensitive to the moist air, a
+length of from 1 to 2 mm. was coated in a certain number of cases with
+a mixture of olive-oil and lamp-black. This mixture was made in order
+to give consistence to the oil, so that a thick layer could be applied,
+which would exclude, at least to a large extent, the moist air, and
+would be easily visible. A greater number of experiments than those
+which were actually tried would have been necessary, had not it been
+clearly established that the tip of the radicle is the part which is
+sensitive to various other irritants.
+
+ [9] ‘Arbeiten des Bot. Institut., in Würzburg,’ vol. i. 1872, p. 209.
+
+
+Phaseolus multiflorus.—Twenty-nine radicles, to which nothing had been
+done, growing out of a sieve, were observed at the same time with those
+which had their tips greased, and for an equal length of time. Of the
+29, 24 curved themselves so as to come into close contact with the
+bottom of the sieve. The place of chief curvature was generally at a
+distance of 5 or 6 mm. from the apex. Eight radicles had their tips
+greased for a length of 2 mm., and two others for a length of 1½ mm.;
+they were kept at a temperature of 15°–16° C. After intervals of from
+19 h. to 24 h. all were still vertically or almost vertically
+dependent, for some of them had moved towards the adjoining damp
+surface by about 10°. They had therefore not been acted on, or only
+slightly acted on, by the damper air on one side, although the whole
+upper part was freely exposed. After 48 h. three of these radicles
+became
+considerably curved towards the sieve; and the absence of curvature in
+some of the others might perhaps be accounted for by their not having
+grown very well. But it should be observed that during the first 19 h.
+to 24 h. all grew well; two of them having increased 2 and 3 mm. in
+length in 11 h.; five others increased 5 to 8 mm. in 19 h.; and two,
+which had been at first 4 and 6 mm. in length, increased in 24 h. to 15
+and 20 mm.
+
+The tips of 10 radicles, which likewise grew well, were coated with the
+grease for a length of only 1 mm., and now the result was somewhat
+different; for of these 4 curved themselves to the sieve in from 21 h.
+to 24h., whilst 6 did not do so. Five of the latter were observed for
+an additional day, and now all excepting one became curved to the
+sieve.
+
+The tips of 5 radicles were cauterised with nitrate of silver, and
+about 1 mm. in length was thus destroyed. They were observed for
+periods varying between 11 h. and 24h., and were found to have grown
+well. One of them had curved until it came into contact with the sieve;
+another was curving towards it; whilst the remaining three were still
+vertically dependent. Of 7 not cauterised radicles observed at the same
+time, all had come into contact with the sieve.
+
+The tips of 11 radicles were protected by moistened gold-beaters’ skin,
+which adheres closely, for a length varying from 1½ to 2½ mm. After 22
+h. to 24 h., 6 of these radicles were clearly bent towards or had come
+into contact with the sieve; 2 were slightly curved in this direction,
+and 3 not at all. All had grown well. Of 14 control specimens observed
+at the same time, all excepting one had closely approached the sieve.
+It appears from these cases that a cap of goldbeaters’ skin checks,
+though only to a slight degree, the bending of the radicles to an
+adjoining damp surface. Whether an extremely thin sheet of this
+substance when moistened allows moisture from the air to pass through
+it, we do not know. One case indicated that the caps were sometimes
+more efficient than appears from the above results; for a radicle,
+which after 23 h. had only slightly approached the sieve, had its cap
+(1½ mm. in length) removed, and during the next 15½ h. it curved itself
+abruptly towards the source of moisture, the chief seat of curvature
+being at a distance of 2 to 3 mm. from the apex.
+
+Vicia faba.—The tips of 13 radicles were coated with the grease for a
+length of 2 mm.; and it should be remembered that with these radicles
+the seat of chief curvature is about
+4 or 5 mm. from the apex. Four of them were examined after 22h., three
+after 26 h., and six after 36 h., and none had been attracted towards
+the damp lower surface of the sieve. In another trial 7 radicles were
+similarly treated, and 5 of them still pointed perpendicularly
+downwards after 11 h., whilst 2 were a little curved towards the sieve;
+by an accident they were not subsequently observed. In both these
+trials the radicles grew well; 7 of them, which were at first from 4 to
+11 mm. in length, were after 11 h. between 7 and 16 mm.; 3 which were
+at first from 6 to 8 mm. after 26 h. were 11.5 to 18 mm. in length; and
+lastly, 4 radicles which were at first 5 to 8 mm. after 46 h. were 18
+to 23 mm. in length. The control or ungreased radicles were not
+invariably attracted towards the bottom of the sieve. But on one
+occasion 12 out of 13, which were observed for periods between 22 h.
+and 36 h., were thus attracted. On two other occasions taken together,
+38 out of 40 were similarly attracted. On another occasion only 7 out
+of 14 behaved in this manner, but after two more days the proportion of
+the curved increased to 17 out of 23. On a last occasion only 11 out of
+20 were thus attracted. If we add up these numbers, we find that 78 out
+of 96 of the control specimens curved themselves towards the bottom of
+the sieve. Of the specimens with greased tips, 2 alone out of the 20
+(but 7 of these were not observed for a sufficiently long time) thus
+curved themselves. We can, therefore, hardly doubt that the tip for a
+length of 2 mm. is the part which is sensitive to a moist atmosphere,
+and causes the upper part to bend towards its source.
+
+The tips of 15 radicles were cauterised with nitrate of silver, and
+they grew as well as those above described with greased tips. After an
+interval of 24 h., 9 of them were not at all curved towards the bottom
+of the sieve; 2 were curved towards it at angles of 20° and 12° from
+their former vertical position, and 4 had come into close contact with
+it. Thus the destruction of the tip for a length of about 1 mm.
+prevented the curvature of the greater number of these radicles to the
+adjoining damp surface. Of 24 control specimens, 23 were bent to the
+sieve, and on a second occasion 15 out of 16 were similarly curved in a
+greater or less degree. These control trials are included in those
+given in the foregoing paragraph.
+
+Avena sativa.—The tips of 13 radicles, which projected between 2 and 4
+mm. from the bottom of the sieve, many of
+them not quite perpendicularly downwards, were coated with the black
+grease for a length of from 1 to 1½ mm. The sieves were inclined at 30°
+with the horizon. The greater number of these radicles were examined
+after 22 h., and a few after 25 h., and within these intervals they had
+grown so quickly as to have nearly doubled their lengths. With the
+ungreased radicles the chief seat of curvature is at a distance of not
+less than between 3.5 and 5.5 mm., and not more than between 7 and 10
+mm. from the apex. Out of the 13 radicles with greased tips, 4 had not
+moved at all towards the sieve; 6 were deflected towards it and from
+the perpendicular by angles varying between 10° and 35°; and 3 had come
+into close contact with it. It appears, therefore, at first sight that
+greasing the tips of these radicles had checked but little their
+bending to the adjoining damp surface. But the inspection of the sieves
+on two occasions produced a widely different impression on the mind;
+for it was impossible to behold the radicles with the black greased
+tips projecting from the bottom, and all those with ungreased tips, at
+least 40 to 50 in number, clinging closely to it, and feel any doubt
+that the greasing had produced a great effect. On close examination
+only a single ungreased radicle could be found which had not become
+curved towards the sieve. It is probable that if the tips had been
+protected by grease for a length of 2 mm. instead of from 1 to 1½ mm.,
+they would not have been affected by the moist air and none would have
+become curved.
+
+Triticum vulgare.—Analogous trials were made on 8 radicles of the
+common wheat; and greasing their tips produced much less effect than in
+the case of the oats. After 22 h., 5 of them had come into contact with
+the bottom of the sieve; 2 had moved towards it 10° and 15°, and one
+alone remained perpendicular. Not one of the very numerous ungreased
+radicles failed to come into close contact with the sieve. These trials
+were made on Nov. 28th, when the temperature was only 4.8° C. at 10
+A.M. We should hardly have thought this case worth notice, had it not
+been for the following circumstance. In the beginning of October, when
+the temperature was considerably higher, viz., 12° to 13° C., we found
+that only a few of the ungreased radicles became bent towards the
+sieve; and this indicates that sensitiveness to moisture in the air is
+increased by a low temperature, as we have seen with the radicles of
+Vicia faba relatively to objects attached to their tips. But in the
+present instance it is possible that a difference in the dryness
+of the air may have caused the difference in the results at the two
+periods.
+
+Finally, the facts just given with respect to Phaseolus multiflorus,
+Vicia faba, and Avena sativa show, as it seems to us, that a layer of
+grease spread for a length of 1½ to 2 mm. over the tip of the radicle,
+or the destruction of the tip by caustic, greatly lessens or quite
+annuls in the upper and exposed part the power of bending towards a
+neighbouring source of moisture. We should bear in mind that the part
+which bends most, lies at some little distance above the greased or
+cauterised tip; and that the rapid growth of this part, proves that it
+has not been injured by the tips having been thus treated. In those
+cases in which the radicles with greased tips became curved, it is
+possible that the layer of grease was not sufficiently thick wholly to
+exclude moisture, or that a sufficient length was not thus protected,
+or, in the case of the caustic, not destroyed. When radicles with
+greased tips are left to grow for several days in damp air, the grease
+is drawn out into the finest reticulated threads and dots, with narrow
+portions of the surface left clean. Such portions would, it is
+probable, be able to absorb moisture, and thus we can account for
+several of the radicles with greased tips having become curved towards
+the sieve after an interval of one or two days. On the whole, we may
+infer that sensitiveness to a difference in the amount of moisture in
+the air on the two sides of a radicle resides in the tip, which
+transmits some influence to the upper part, causing it to bend towards
+the source of moisture. Consequently, the movement is the reverse of
+that caused by objects attached to one side of the tip, or by a thin
+slice being cut off, or by being slightly cauterised. In a future
+chapter it will be shown that sensitiveness to the attraction of
+gravity likewise resides in the tip; so that it is the tip which
+excites the adjoining parts of a horizontally extended radicle to bend
+towards the centre of the earth.
+
+SECONDARY RADICLES BECOMING VERTICALLY GEOTROPIC BY THE DESTRUCTION OR
+INJURY OF THE TERMINAL PART OF THE PRIMARY RADICLE.
+
+
+Sachs has shown that the lateral or secondary radicles of the bean, and
+probably of other plants, are acted on by geotropism in so peculiar a
+manner, that they grow out horizontally or a little inclined downwards;
+and he has further shown[10] the interesting fact, that if the end of
+the primary radicle be cut off, one of the nearest secondary radicles
+changes its nature and grows perpendicularly downwards, thus replacing
+the primary radicle. We repeated this experiment, and planted beans
+with amputated radicles in friable peat, and saw the result described
+by Sachs; but generally two or three of the secondary radicles grew
+perpendicularly downwards. We also modified the experiment, by pinching
+young radicles a little way above their tips, between the arms of a
+U-shaped piece of thick leaden wire. The part pinched was thus
+flattened, and was afterwards prevented from growing thicker. Five
+radicles had their ends cut off, and served as controls or standards.
+Eight were pinched; of these 2 were pinched too severely and their ends
+died and dropped off; 2 were not pinched enough and were not sensibly
+affected; the remaining 4 were pinched sufficiently to check the growth
+of the terminal part, but did not appear otherwise injured. When the
+U-shaped wires were removed, after an
+interval of 15 days, the part beneath the wire was found to be very
+thin and easily broken, whilst the part above was thickened. Now in
+these four cases, one or more of the secondary radicles, arising from
+the thickened part just above the wire, had grown perpendicularly
+downwards. In the best case the primary radicle (the part below the
+wire being 1½ inch in length) was somewhat distorted, and was not half
+as long as three adjoining secondary radicles, which had grown
+vertically, or almost vertically, downwards. Some of these secondary
+radicles adhered together or had become confluent. We learn from these
+four cases that it is not necessary, in order that a secondary radicle
+should assume the nature of a primary one, that the latter should be
+actually amputated; it is sufficient that the flow of sap into it
+should be checked, and consequently should be directed into the
+adjoining secondary radicles; for this seems to be the most obvious
+result of the primary radicle being pinched between the arms of a
+U-shaped wire.
+
+ [10] ‘Arbeiten Bot. Institut., Würzburg,’ Heft iv. 1874, p. 622.
+
+
+This change in the nature of secondary radicles is clearly analogous,
+as Sachs has remarked, to that which occurs with the shoots of trees,
+when the leading one is destroyed and is afterwards replaced by one or
+more of the lateral shoots; for these now grow upright instead of
+sub-horizontally. But in this latter case the lateral shoots are
+rendered apogeotropic, whereas with radicles the lateral ones are
+rendered geotropic. We are naturally led to suspect that the same cause
+acts with shoots as with roots, namely, an increased flow of sap into
+the lateral ones. We made some trials with Abies communis and
+pectinata, by pinching with wire the leading and all the lateral shoots
+excepting one. But we believe that they were too old when experimented
+on; and some were pinched too severely, and
+some not enough. Only one case succeeded, namely, with the spruce-fir.
+The leading shoot was not killed, but its growth was checked; at its
+base there were three lateral shoots in a whorl, two of which were
+pinched, one being thus killed; the third was left untouched. These
+lateral shoots, when operated on (July 14th) stood at an angle of 8°
+above the horizon; by Sept. 8th the unpinched one had risen 35°; by
+Oct. 4th it had risen 46°, and by Jan. 26th 48°, and it had now become
+a little curved inwards. Part of this rise of 48° may be attributed to
+ordinary growth, for the pinched shoot rose 12° within the same period.
+It thus follows that the unpinched shoot stood, on Jan. 26th, 56° above
+the horizon, or 34° from the vertical; and it was thus obviously almost
+ready to replace the slowly growing, pinched, leading shoot.
+Nevertheless, we feel some doubt about this experiment, for we have
+since observed with spruce-firs growing rather unhealthily, that the
+lateral shoots near the summit sometimes become highly inclined, whilst
+the leading shoot remains apparently sound.
+
+A widely different agency not rarely causes shoots which naturally
+would have brown out horizontally to grow up vertically. The lateral
+branches of the Silver Fir (A. pectinata) are often affected by a
+fungus, Æcidium elatinum, which causes the branch to enlarge into an
+oval knob formed of hard wood, in one of which we counted 24 rings of
+growth. According to De Bary[11], when the mycelium penetrates a bud
+beginning to elongate, the shoot developed from it grows vertically
+upwards. Such upright shoots
+afterwards produce lateral and horizontal branches; and they then
+present a curious appearance, as if a young fir-tree had grown out of a
+ball of clay surrounding the branch. These upright shoots have
+manifestly changed their nature and become apogeotropic; for if they
+had not been affected by the Æcidium, they would have grown out
+horizontally like all the other twigs on the same branches. This change
+can hardly be due to an increased flow of sap into the part; but the
+presence of the mycelium will have greatly disturbed its natural
+constitution.
+
+ [11] See his valuable article in ‘Bot. Zeitung,’ 1867, p. 257, on
+ these monstrous growths, which are called in German “Hexenbesen,” or
+ “witch-brooms.”
+
+
+According to Mr. Meehan,[12] the stems of three species of Euphorbia
+and of Portulaca oleracea are “normally prostrate or procumbent;” but
+when they are attacked by an Æcidium, they “assume an erect habit.” Dr.
+Stahl informs us that he knows of several analogous cases; and these
+seem to be closely related to that of the Abies. The rhizomes of
+Sparganium ramosum grow out horizontally in the soil to a considerable
+length, or are diageotropic; but F. Elfving found that when they were
+cultivated in water their tips turned upwards, and they became
+apogeotropic. The same result followed when the stem of the plant was
+bent until it cracked or was merely much bowed.[13]
+
+ [12] ‘Proc. Acad. Nat. Sc. Philadelphia,’ June 16th, 1874, and July
+ 23rd, 1875.
+
+
+ [13] See F. Elfving’s interesting paper in ‘Arbeiten Bot. Institut.,
+ in Würzburg,’ vol. ii. 1880, p. 489. Carl Kraus (Triesdorf) had
+ previously observed (‘Flora,’ 1878, p. 324) that the underground
+ shoots of Triticum repens bend vertically up when the parts above
+ ground are removed, and when the rhizomes are kept partly immersed in
+ water.
+
+
+No explanation has hitherto been attempted of such cases as the
+foregoing,—namely, of secondary radicles growing vertically downwards,
+and of lateral shoots growing vertically upwards, after the amputation
+of
+the primary radicle or of the leading shoot. The following
+considerations give us, as we believe, the clue. Firstly, any cause
+which disturbs the constitution[14] is apt to induce reversion; such as
+the crossing of two distinct races, or a change of conditions, as when
+domestic animals become feral. But the case which most concerns us, is
+the frequent appearance of peloric flowers on the summit of a stem, or
+in the centre of the inflorescence,—parts which, it is believed,
+receive the most sap; for when an irregular flower becomes perfectly
+regular or peloric, this may be attributed, at least partly, to
+reversion to a primitive and normal type. Even the position of a seed
+at the end of the capsule sometimes gives to the seedling developed
+from it a tendency to revert. Secondly, reversions often occur by means
+of buds, independently of reproduction by seed; so that a bud may
+revert to the character of a former state many bud-generations ago. In
+the case of animals, reversions may occur in the individual with
+advancing age. Thirdly and lastly, radicles when they first protrude
+from the seed are always geotropic, and plumules or shoots almost
+always apogeotropic. If then any cause, such as an increased flow of
+sap or the presence of mycelium, disturbs the constitution of a lateral
+shoot or of a secondary radicle, it is apt to revert to its primordial
+state; and it becomes either apogeotropic or geotropic, as the case may
+be, and consequently grows either vertically upwards or downwards. It
+is indeed
+possible, or even probable, that this tendency to reversion may have
+been increased, as it is manifestly of service to the plant.
+
+ [14] The facts on which the following conclusions are founded are
+ given in ‘The Variation of Animals and Plants under Domestication,’
+ 2nd edit. 1875. On the causes leading to reversion see chap. xii. vol.
+ ii. and p. 59, chap. xiv. On peloric flowers, chap. xiii. p. 32; and
+ see p. 337 on their position on the plant. With respect to seeds, p.
+ 340. On reversion by means of buds, p. 438, chap. xi. vol. i.
+
+A SUMMARY OF CHAPTER.
+
+A part or organ may be called sensitive, when its irritation excites
+movement in an adjoining part. Now it has been shown in this chapter,
+that the tip of the radicle of the bean is in this sense sensitive to
+the contact of any small object attached to one side by shellac or
+gum-water; also to a slight touch with dry caustic, and to a thin slice
+cut off one side. The radicles of the pea were tried with attached
+objects and caustic, both of which acted. With Phaseolus multiflorus
+the tip was hardly sensitive to small squares of attached card, but was
+sensitive to caustic and to slicing. The radicles of Tropaeolum were
+highly sensitive to contact; and so, as far as we could judge, were
+those of Gossypium herbaceum, and they were certainly sensitive to
+caustic. The tips of the radicles of Cucurbita ovifera were likewise
+highly sensitive to caustic, though only moderately so to contact.
+Raphanus sativus offered a somewhat doubtful case. With Æsculus the
+tips were quite indifferent to bodies attached to them, though
+sensitive to caustic. Those of Quercus robur and Zea mays were highly
+sensitive to contact, as were the radicles of the latter to caustic. In
+several of these cases the difference in sensitiveness of the tip to
+contact and to caustic was, as we believe, merely apparent; for with
+Gossypium, Raphanus, and Cucurbita, the tip was so fine and flexible
+that it was very difficult to attach any object to one of its sides.
+With the radicles of Æsculus, the tips were not at all sensitive to
+small bodies attached to them; but it does not follow from this
+fact that they would not have been sensitive to somewhat greater
+continued pressure, if this could have been applied.
+
+The peculiar form of sensitiveness which we are here considering, is
+confined to the tip of the radicle for a length of from 1 mm. to 1.5
+mm. When this part is irritated by contact with any object, by caustic,
+or by a thin slice being cut off, the upper adjoining part of the
+radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend
+away from the side which has been irritated. Some influence must
+therefore be transmitted from the tip along the radicle for this
+length. The curvature thus caused is generally symmetrical. The part
+which bends most apparently coincides with that of the most rapid
+growth. The tip and the basal part grow very slowly and they bend very
+little.
+
+Considering the widely separated position in the vegetable series of
+the several above-named genera, we may conclude that the tips of the
+radicles of all, or almost all, plants are similarly sensitive, and
+transmit an influence causing the upper part to bend. With respect to
+the tips of the secondary radicles, those of Vicia faba, Pisum sativum,
+and Zea mays were alone observed, and they were found similarly
+sensitive.
+
+In order that these movements should be properly displayed, it appears
+necessary that the radicles should grow at their normal rate. If
+subjected to a high temperature and made to grow rapidly, the tips seem
+either to lose their sensitiveness, or the upper part to lose the power
+of bending. So it appears to be if they grow very slowly from not being
+vigorous, or from being kept at too low a temperature; also when they
+are forced to germinate in the middle of the winter.
+
+
+The curvature of the radicle sometimes occurs within from 6 to 8 hours
+after the tip has been irritated, and almost always within 24 h.,
+excepting in the case of the massive radicles of Æsculus. The curvature
+often amounts to a rectangle,—that is, the terminal part bends upwards
+until the tip, which is but little curved, projects almost
+horizontally. Occasionally the tip, from the continued irritation of
+the attached object, continues to bend up until it forms a hook with
+the point directed towards the zenith, or a loop, or even a spire.
+After a time the radicle apparently becomes accustomed to the
+irritation, as occurs in the case of tendrils, for it again grows
+downwards, although the bit of card or other object may remain attached
+to the tip. It is evident that a small object attached to the free
+point of a vertically suspended radicle can offer no mechanical
+resistance to its growth as a whole, for the object is carried
+downwards as the radicle elongates, or upwards as the radicle curves
+upwards. Nor can the growth of the tip itself be mechanically checked
+by an object attached to it by gum-water, which remains all the time
+perfectly soft. The weight of the object, though quite insignificant,
+is opposed to the upward curvature. We may therefore conclude that it
+is the irritation due to contact which excites the movement. The
+contact, however, must be prolonged, for the tips of 15 radicles were
+rubbed for a short time, and this did not cause them to bend. Here then
+we have a case of specialised sensibility, like that of the glands of
+Drosera; for these are exquisitely sensitive to the slightest pressure
+if prolonged, but not to two or three rough touches.
+
+When the tip of a radicle is lightly touched on one side with dry
+nitrate of silver, the injury caused is
+very slight, and the adjoining upper part bends away from the
+cauterised point, with more certainty in most cases than from an object
+attached on one side. Here it obviously is not the mere touch, but the
+effect produced by the caustic, which induces the tip to transmit some
+influence to the adjoining part, causing it to bend away. If one side
+of the tip is badly injured or killed by the caustic, it ceases to
+grow, whilst the opposite side continues growing; and the result is
+that the tip itself bends towards the injured side and often becomes
+completely hooked; and it is remarkable that in this case the adjoining
+upper part does not bend. The stimulus is too powerful or the shock too
+great for the proper influence to be transmitted from the tip. We have
+strictly analogous cases with Drosera, Dionaea and Pinguicula, with
+which plants a too powerful stimulus does not excite the tentacles to
+become incurved, or the lobes to close, or the margin to be folded
+inwards.
+
+With respect to the degree of sensitiveness of the apex to contact
+under favourable conditions, we have seen that with Vicia faba a little
+square of writing-paper affixed with shellac sufficed to cause
+movement; as did on one occasion a square of merely damped goldbeaters’
+skin, but it acted very slowly. Short bits of moderately thick bristle
+(of which measurements have been given) affixed with gum-water acted in
+only three out of eleven trials, and beads of dried shellac under
+1/200th of a grain in weight acted only twice in nine cases; so that
+here we have nearly reached the minimum of necessary irritation. The
+apex, therefore, is much less sensitive to pressure than the glands of
+Drosera, for these are affected by far thinner objects than bits of
+bristle, and by a very much less weight than 1/200th of a grain.
+But the most interesting evidence of the delicate sensitiveness of the
+tip of the radicle, was afforded by its power of discriminating between
+equal-sized squares of card-like and very thin paper, when these were
+attached on opposite sides, as was observed with the radicles of the
+bean and oak.
+
+When radicles of the bean are extended horizontally with squares of
+card attached to the lower sides of their tips, the irritation thus
+caused was always conquered by geotropism, which then acts under the
+most favourable conditions at right angles to the radicle. But when
+objects were attached to the radicles of any of the above-named genera,
+suspended vertically, the irritation conquered geotropism, which latter
+power at first acted obliquely on the radicle; so that the immediate
+irritation from the attached object, aided by its after-effects,
+prevailed and caused the radicle to bend upwards, until sometimes the
+point was directed to the zenith. We must, however, assume that the
+after-effects of the irritation of the tip by an attached object come
+into play, only after movement has been excited. The tips of the
+radicles of the pea seem to be more sensitive to contact than those of
+the bean, for when they were extended horizontally with squares of card
+adhering to their lower sides, a most curious struggle occasionally
+arose, sometimes one and sometimes the other force prevailing, but
+ultimately geotropism was always victorious; nevertheless, in two
+instances the terminal part became so much curved upwards that loops
+were subsequently formed. With the pea, therefore, the irritation from
+an attached object, and from geotropism when acting at right angles to
+the radicle, are nearly balanced forces. Closely similar results were
+observed with the horizontally extended radicles of _Cucurbita
+ovifera_,
+when their tips were slightly cauterised on the lower side.
+
+Finally, the several co-ordinated movements by which radicles are
+enabled to perform their proper functions are admirably perfect. In
+whatever direction the primary radicle first protrudes from the seed,
+geotropism guides it perpendicularly downwards; and the capacity to be
+acted on by the attraction of gravity resides in the tip. But Sachs has
+proved[15] that the secondary radicles, or those emitted by the primary
+one, are acted on by geotropism in such a manner that they tend to bend
+only obliquely downwards. If they had been acted on like the primary
+radicle, all the radicles would have penetrated the ground in a close
+bundle. We have seen that if the end of the primary radicle is cut off
+or injured, the adjoining secondary radicles become geotropic and grow
+vertically downwards. This power must often be of great service to the
+plant, when the primary radicle has been destroyed by the larvae of
+insects, burrowing animals, or any other accident. The tertiary
+radicles, or those emitted by the secondary ones, are not influenced,
+at least in the case of the bean, by geotropism; so they grow out
+freely in all directions. From this manner of growth of the various
+kinds of radicles, they are distributed, together with their absorbent
+hairs, throughout the surrounding soil, as Sachs has remarked, in the
+most advantageous manner; for the whole soil is thus closely searched.
+
+ [15] ‘Arbeiten Bot. Institut, Würzburg,’ Heft iv. 1874, pp. 605–631.
+
+
+Geotropism, as was shown in the last chapter, excites the primary
+radicle to bend downwards with very little force, quite insufficient to
+penetrate the ground. Such penetration is effected by the pointed
+apex (protected by the root-cap) being pressed down by the longitudinal
+expansion or growth of the terminal rigid portion, aided by its
+transverse expansion, both of which forces act powerfully. It is,
+however, indispensable that the seeds should be at first held down in
+some manner. When they lie on the bare surface they are held down by
+the attachment of the root-hairs to any adjoining objects; and this
+apparently is effected by the conversion of their outer surfaces into a
+cement. But many seeds get covered up by various accidents, or they
+fall into crevices or holes. With some seeds their own weight suffices.
+The circumnutating movement of the terminal growing part both of the
+primary and secondary radicles is so feeble that it can aid them very
+little in penetrating the ground, excepting when the superficial layer
+is very soft and damp. But it must aid them materially when they happen
+to break obliquely into cracks, or into burrows made by earth-worms or
+larvae. This movement, moreover, combined with the sensitiveness of the
+tip to contact, can hardly fail to be of the highest importance; for as
+the tip is always endeavouring to bend to all sides it will press on
+all sides, and will thus be able to discriminate between the harder and
+softer adjoining surfaces, in the same manner as it discriminated
+between the attached squares of card-like and thin paper. Consequently
+it will tend to bend from the harder soil, and will thus follow the
+lines of least resistance. So it will be if it meets with a stone or
+the root of another plant in the soil, as must incessantly occur. If
+the tip were not sensitive, and if it did not excite the upper part of
+the root to bend away, whenever it encountered at right angles some
+obstacle in the ground, it would be liable
+to be doubled up into a contorted mass. But we have seen with radicles
+growing down inclined plates of glass, that as soon as the tip merely
+touched a slip of wood cemented across the plate, the whole terminal
+growing part curved away, so that the tip soon stood at right angles to
+its former direction; and thus it would be with an obstacle encountered
+in the ground, as far as the pressure of the surrounding soil would
+permit. We can also understand why thick and strong radicles, like
+those of Æsculus, should be endowed with less sensitiveness than more
+delicate ones; for the former would be able by the force of their
+growth to overcome any slight obstacle.
+
+After a radicle, which has been deflected by some stone or root from
+its natural downward course, reaches the edge of the obstacle,
+geotropism will direct it to grow again straight downward; but we know
+that geotropism acts with very little force, and here another excellent
+adaptation, as Sachs has remarked,[16] comes into play. For the upper
+part of the radicle, a little above the apex, is, as we have seen,
+likewise sensitive; and this sensitiveness causes the radicle to bend
+like a tendril towards the touching object, so that as it rubs over the
+edge of an obstacle, it will bend downwards; and the curvature thus
+induced is abrupt, in which respect it differs from that caused by the
+irritation of one side of the tip. This downward bending coincides with
+that due to geotropism, and both will cause the root to resume its
+original course.
+
+ [16] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iii. p. 456.
+
+As radicles perceive an excess of moisture in the air on one side and
+bend towards this side, we may infer that they will act in the same
+manner with respect to moisture in the earth. The sensitiveness to
+moisture
+resides in the tip, which determines the bending of the upper part.
+This capacity perhaps partly accounts for the extent to which
+drain-pipes often become choked with roots.
+
+Considering the several facts given in this chapter, we see that the
+course followed by a root through the soil is governed by
+extraordinarily complex and diversified agencies,—by geotropism acting
+in a different manner on the primary, secondary, and tertiary
+radicles,—by sensitiveness to contact, different in kind in the apex
+and in the part immediately above the apex, and apparently by
+sensitiveness to the varying dampness of different parts of the soil.
+These several stimuli to movement are all more powerful than
+geotropism, when this acts obliquely on a radicle, which has been
+deflected from its perpendicular downward course. The roots, moreover,
+of most plants are excited by light to bend either to or from it; but
+as roots are not naturally exposed to the light it is doubtful whether
+this sensitiveness, which is perhaps only the indirect result of the
+radicles being highly sensitive to other stimuli, is of any service to
+the plant. The direction which the apex takes at each successive period
+of the growth of a root, ultimately determines its whole course; it is
+therefore highly important that the apex should pursue from the first
+the most advantageous direction; and we can thus understand why
+sensitiveness to geotropism, to contact and to moisture, all reside in
+the tip, and why the tip determines the upper growing part to bend
+either from or to the exciting cause. A radicle may be compared with a
+burrowing animal such as a mole, which wishes to penetrate
+perpendicularly down into the ground. By continually moving his head
+from side to side, or circumnutating, he will feel any stone
+or other obstacle, as well as any difference in the hardness of the
+soil, and he will turn from that side; if the earth is damper on one
+than on the other side he will turn thitherward as a better
+hunting-ground. Nevertheless, after each interruption, guided by the
+sense of gravity, he will be able to recover his downward course and to
+burrow to a greater depth.
+
+
+
+
+CHAPTER IV.
+THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.
+
+
+Circumnutation of stems: concluding remarks on—Circumnutation of
+stolons: aid thus afforded in winding amongst the stems of surrounding
+plants—Circumnutation of flower-stems—Circumnutation of Dicotyledonous
+leaves—Singular oscillatory movement of leaves of Dionaea—Leaves of
+Cannabis sink at night—Leaves of Gymnosperms—Of
+Monocotyledons—Cryptogams—Concluding remarks on the circumnutation of
+leaves; generally rise in the evening and sink in the morning.
+
+
+We have seen in the first chapter that the stems of all seedlings,
+whether hypocotyls or epicotyls, as well as the cotyledons and the
+radicles, are continually circumnutating—that is they grow first on one
+side and then on another, such growth being probably preceded by
+increased turgescence of the cells. As it was unlikely that plants
+should change their manner of growth with advancing age, it seemed
+probable that the various organs of all plants at all ages, as long as
+they continued to grow, would be found to circumnutate, though perhaps
+to an extremely small extent. As it was important for us to discover
+whether this was the case, we determined to observe carefully a certain
+number of plants which were growing vigorously, and which were not
+known to move in any manner. We commenced with stems. Observations of
+this kind are tedious, and it appeared to us that it would be
+sufficient to observe the stems in about a score of genera, belonging
+to widely distinct families and inhabitants of various countries.
+Several plants
+were selected which, from being woody, or for other reasons, seemed the
+least likely to circumnutate. The observations and the diagrams were
+made in the manner described in the Introduction. Plants in pots were
+subjected to a proper temperature, and whilst being observed, were kept
+either in darkness or were feebly illuminated from above. They are
+arranged in the order adopted by Hooker in Le Maout and Decaisne’s
+‘System of Botany.’ The number of the family to which each genus
+belongs is appended, as this serves to show the place of each in the
+series.
+
+(1.) Iberis umbellata (Cruciferae, Fam. 14).—The movement of the stem
+of a young plant, 4 inches in height, consisting of four internodes
+(the hypocotyl included) besides a large bud on the summit, was traced,
+as here shown, during 24 h. (Fig. 70). As far as we could judge the
+uppermost inch alone of the stem circumnutated, and this in a simple
+manner. The movement was slow, and the rate very unequal at different
+times. In part of its course an irregular ellipse, or rather triangle,
+was completed in 6 h. 30 m.
+
+Fig. 70. Iberis umbellata: circumnutation of stem of young plant,
+traced from 8.30 A.M. Sept. 13th to same hour on following morning.
+Distance of summit of stem beneath the horizontal glass 7.6 inches.
+Diagram reduced to half of original size. Movement as here shown
+magnified between 4 and 5 times.
+
+(2.) Brassica oleracea (Cruciferae).—A very young plant, bearing three
+leaves, of which the longest was only three-quarters of an inch in
+length, was placed under a microscope, furnished with an eye-piece
+micrometer, and the tip of the largest leaf was
+found to be in constant movement. It crossed five divisions of the
+micrometer, that is, 1/100th of an inch, in 6 m. 20 s. There could
+hardly be a doubt that it was the stem which chiefly moved, for the tip
+did not get quickly out of focus; and this would have occurred had the
+movement been confined to the leaf, which moves up or down in nearly
+the same vertical plane.
+
+(3.) Linum usitatissimum (Lineae, Fam. 39).—The stems of this plant,
+shortly before the flowering period, are stated by Fritz Müller
+(‘Jenaische Zeitschrift,’ B. v. p. 137) to revolve, or circumnutate.
+
+(4.) Pelargonium zonale (Geraniaceae, Fam. 47).—A young plant, 7½
+inches in height, was observed in the usual manner; but, in order to
+see the bead at the end of the glass filament and at the same time the
+mark beneath, it was necessary to cut off three leaves on one side. We
+do not know whether it was owing to this cause, or to the plant having
+previously become bent to one side through heliotropism, but from the
+morning of the 7th of March to 10.30 P.M. on the 8th, the stem moved a
+considerable distance in a zigzag line in the same general direction.
+During the night of the 8th it moved to some distance at right angles
+to its former course, and next morning (9th) stood for a time almost
+still. At noon on the 9th a new tracing was begun (see Fig. 71), which
+was continued till 8 A.M. on the 11th. Between noon on the 9th and 5
+P.M. on the 10th (i.e. in the course of 29 h.), the stem described a
+circle. This plant therefore circumnutates, but at a very slow rate,
+and to a small extent.
+
+Fig. 71. Pelargonium zonale: circumnutation of stem of young plant,
+feebly illuminated from above. Movement of bead magnified about 11
+times; traced on a horizontal glass from noon on March 9th to 8 A.M. on
+the 11th.
+
+(5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb);
+(Geraniaceae, Fam. 47).—The species of this genus climb by the
+aid of their sensitive petioles, but some of them also twine round
+supports; but even these latter species do not begin to circumnutate in
+a conspicuous manner whilst young. The variety here treated of has a
+rather thick stem, and is so dwarf that apparently it does not climb in
+any manner. We therefore wished to ascertain whether the stem of a
+young plant, consisting of two internodes, together 3.2 inches in
+height, circumnutated. It was observed during 25 h., and we see in Fig.
+72 that the stem moved in a zigzag course, indicating circumnutation.
+
+Fig. 72. Tropaeolum majus (?): circumnutation of stem of young plant,
+traced on a horizontal glass from 9 A.M. Dec. 26th to 10 A.M. on 27th.
+Movement of bead magnified about 5 times, and here reduced to half of
+original scale.
+
+Fig. 73. Trifolium resupinatum: circumnutation of stem, traced on
+vertical glass from 9.30 A.M. to 4.30 P.M. Nov. 3rd. Tracing not
+greatly magnified, reduced to half of original size. Plant feebly
+illuminated from above.
+
+(6.) Trifolium resupinatum (Leguminosae, Fam. 75).—When we treat of the
+sleep of plants, we shall see that the stems in several Leguminous
+genera, for instance, those of Hedysarum, Mimosa, Melilotus, etc.,
+which are not climbers, circumnutate in a conspicuous manner. We will
+here give only a single instance (Fig. 73), showing the circumnutation
+of the stem of a large plant of a clover, Trifolium resupinatum. In the
+course of 7 h. the stem changed
+its course greatly eight times and completed three irregular circles or
+ellipses. It therefore circumnutated rapidly. Some of the lines run at
+right angles to one another.
+
+Fig. 74. Rubus (hybrid): circumnutation of stem, traced on horizontal
+glass, from 4 P.M. March 14th to 8.30 A.M. 16th. Tracing much
+magnified, reduced to half of original size. Plant illuminated feebly
+from above.
+
+(7.) Rubus idæus (hybrid) (Rosaceae, Fam. 76).—As we happened to have a
+young plant, 11 inches in height and growing vigorously, which had been
+raised from a cross between the raspberry (Rubus idæus) and a North
+American Rubus, it was observed in the usual manner. During the morning
+of March 14th the stem almost completed a circle, and then moved far to
+the right. At 4 P.M. it reversed its course, and now a fresh tracing
+was begun, which was continued during 40½ h., and is given in Fig. 74.
+We here have well-marked circumnutation.
+
+(8.) Deutzia gracilis (Saxifrageae, Fam. 77).—A shoot on a bush about
+18 inches in height was observed. The bead changed its course greatly
+eleven times in the course of 10 h. 30 m. (Fig. 75), and there could be
+no doubt about the circumnutation of the stem.
+
+Fig. 75. Deutzia gracilis: circumnutation of stem, kept in darkness,
+traced on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th.
+Movement of bead originally magnified about 20 times, here reduced to
+half scale.
+
+(9.) Fuchsia (greenhouse var., with large flowers, probably a hybrid)
+(Onagrarieae, Fam. 100).—A young plant, 15 inches in height, was
+observed during nearly 48 h. The
+accompanying figure (Fig. 76) gives the necessary particulars, and
+shows that the stem circumnutated, though rather slowly.
+
+Fig. 76. Fuchsia (garden var.): circumnutation of stem, kept in
+darkness, traced on horizontal glass, from 8.30 A.M. to 7 P.M. March
+20th. Movement of bead originally magnified about 40 times, here
+reduced to half scale.
+
+(10.) Cereus speciocissimus (garden var., sometimes called Phyllocactus
+multiflorus) (Cacteæ, Fam. 109).—This plant, which was growing
+vigorously from having been removed a few days before from the
+greenhouse to the hot-house, was observed with especial interest, as it
+seemed so little probable that the stem would circumnutate. The
+branches are flat, or flabelliform; but some of them are triangular in
+section, with the three sides hollowed out. A branch of this latter
+shape, 9 inches in length and 1½ in diameter, was chosen for
+observation, as less likely to circumnutate than a flabelliform branch.
+The movement of the bead at the end of the glass filament, affixed to
+the summit of the branch, was traced (A, Fig. 77) from 9.23 A.M. to
+4.30 P.M. on Nov. 23rd, during which time it changed its course greatly
+six times. On the 24th another tracing was made (see B), and the bead
+on this day changed its course oftener, making in 8 h. what may be
+considered as four ellipses, with their longer axes differently
+directed. The position of the stem and its commencing course on the
+following morning are likewise shown. There can be no doubt that this
+branch, though appearing quite rigid, circumnutated; but the
+extreme amount of movement during the time was very small, probably
+rather less than the 1/20th of an inch.
+
+Fig 77. Cereus speciocissimus: circumnutation of stem, illuminated from
+above, traced on a horizontal glass, in A from 9 A.M. to 4.30 P.M. on
+Nov. 23rd; and in B from 8.30 A.M. on the 24th to 8 A.M. on the 25th.
+Movement of the bead in B magnified about 38 times.
+
+(11.) Hedera helix (Araliaceae, Fam. 114).—The stem is known to be
+apheliotropic, and several seedlings growing in a pot in the greenhouse
+became bent in the middle of the summer at right angles from the light.
+On Sept. 2nd some of these stems were tied up so as to stand
+vertically, and were placed before a north-east window; but to our
+surprise they were now decidedly heliotropic, for during 4 days they
+curved themselves towards the light, and their course being traced on a
+horizontal glass, was strongly zigzag. During the 6 succeeding days
+they circumnutated over the same small space at a slow rate, but there
+could be no doubt about their circumnutation. The plants were kept
+exactly in the same place before the window, and after an interval of
+15 days the stems were again observed during 2 days and their movements
+traced, and
+they were found to be still circumnutating, but on a yet smaller scale.
+
+(12.) Gazania ringens (Compositæ, Fam. 122).—The circumnutation of the
+stem of a young plant, 7 inches in height, as measured to the tip of
+the highest leaf, was traced during 33 h., and is shown in the
+accompanying figure (Fig. 78). Two main lines may be observed running
+at nearly right angles to two other main lines; but these are
+interrupted by small loops.
+
+Fig. 78. Gazania ringens: circumnutation of stem traced from 9 A.M.
+March 21st to 6 P.M. on 22nd; plant kept in darkness. Movement of bead
+at the close of the observations magnified 34 times, here reduced to
+half the original scale.
+
+(13.) Azalea Indica (Ericineae, Fam. 128).—A bush 21 inches in height
+was selected for observation, and the circumnutation of its leading
+shoot was traced during 26 h. 40 m., as shown in the following figure
+(Fig. 79).
+
+(14.) Plumbago Capensis (Plumbagineae, Fam. 134).—A small lateral
+branch which projected from a tall freely growing bush, at an angle of
+35° above the horizon, was selected for observation. For the first 11
+h. it moved to a considerable distance in a nearly straight line to one
+side, owing probably to its having been previously deflected by the
+light whilst standing in the greenhouse. At 7.20 P.M. on March 7th a
+fresh tracing was begun and continued for the next 43 h. 40 m. (see
+Fig. 80). During the first 2 h. it followed nearly the same direction
+as before, and then changed it a little; during the night it moved at
+nearly right angles to its previous course. Next
+day (8th) it zigzagged greatly, and on the 9th moved irregularly round
+and round a small circular space. By 3 P.M. on the 9th the figure had
+become so complicated that no more dots could be made; but the shoot
+continued during the evening of the 9th, the whole of the 10th, and the
+morning of the 11th to circumnutate over the same small space, which
+was only about the 1/26th of an inch (.97 mm.) in diameter. Although
+this branch circumnutated to a very small extent, yet it changed its
+course frequently. The movements ought to have been more magnified.
+
+Fig. 79. Azalea Indica: circumnutation of stem, illuminated from above,
+traced on horizontal glass, from 9.30 A.M. March 9th to 12.10 P.M. on
+the 10th. But on the morning of the 10th only four dots were made
+between 8.30 A.M. and 12.10 P.M., both hours included, so that the
+circumnutation is not fairly represented in this part of the diagram.
+Movement of the bead here magnified about 30 times.
+
+Fig. 80. Plumbago Capensis: circumnutation of tip of a lateral branch,
+traced on horizontal glass, from 7.20 P.M. on March 7th to 3 P.M. on
+the 9th. Movement of bead magnified 13 times. Plant feebly illuminated
+from above.
+
+(15.) Aloysia citriodora (Verbenaceae, Fam. 173).—The following figure
+(Fig. 81) gives the movements of a shoot during
+31 h. 40 m., and shows that it circumnutated. The bush was 15 inches in
+height.
+
+Fig. 81. Aloysia citriodora: circumnutation of stem, traced from 8.20
+A.M. on March 22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement
+magnified about 40 times.
+
+(16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.)
+(Verbenaceae).—A shoot 8 inches in height had been laid horizontally,
+for the sake of observing its apogeotropism, and the terminal portion
+had grown vertically upwards for a length of 1½ inch. A glass filament,
+with a bead at the end, was fixed upright to the tip, and its movements
+were traced during 41 h. 30 m. on a vertical glass (Fig. 82). Under
+these circumstances the lateral movements were chiefly shown; but as
+the lines from side to side are not on the same level, the shoot
+must have moved in a plane at right angles to that of the lateral
+movement, that is, it must have circumnutated. On the next day (6th)
+the shoot moved in the course of 16 h. four times to the right, and
+four times to the left; and this apparently represents the formation of
+four ellipses, so that each was completed in 4 h. (17.) Ceratophyllum
+demersum (Ceratophylleae, Fam. 220).—An interesting account of the
+movements of the stem of this water-plant has been published by M. E.
+Rodier.[1] The movements are confined to the young internodes, becoming
+less and less lower down the stem; and they are extraordinary from
+their amplitude. The stems sometimes moved through an angle of above
+200° in 6 h., and in one instance through 220° in 3 h. They generally
+bent from right to left in the morning, and in an opposite direction in
+the afternoon; but the movement was sometimes temporarily reversed or
+quite arrested. It was not affected by light. It does not appear that
+M. Rodier made any diagram on a horizontal plane representing the
+actual course pursued by the apex, but he speaks of the “branches
+executing round their axes of growth a movement of torsion.” From the
+particulars above given, and remembering in the case of twining plants
+and of tendrils, how difficult it is not to mistake their bending to
+all points of the compass for true torsion, we are led to believe that
+the stems of this Ceratophyllum circumnutate, probably in the shape of
+narrow ellipses, each completed in about 26 h. The following statement,
+however, seems to indicate something different from ordinary
+circumnutation, but we cannot fully understand it. M. Rodier says: “Il
+est alors facile de voir que le mouvement de flexion se produit d’abord
+dans les mérithalles supérieurs, qu’il se propage ensuite, en
+s’amoindrissant du haut en bas; tandis qu’au contraire le movement de
+redressement commence par la partie inférieur pour se terminer a la
+partie supérieure qui, quelquefois, peu de temps avant de se relever
+tout à fait, forme avec l’axe un angle très aigu.”
+
+ [1] ‘Comptes Rendus,’ April 30th, 1877. Also a second notice published
+ separately in Bourdeaux, Nov. 12th, 1877.
+
+
+Fig. 82. Verbena melindres: circumnutation of stem in darkness, traced
+on vertical glass, from 5.30 P.M. on June 5th to 11 A.M. June 7th.
+Movement of bead magnified 9 times.
+
+(18.) Coniferæ.—Dr. Maxwell Masters states (‘Journal Linn. Soc.,’ Dec.
+2nd, 1879) that the leading shoots of many Coniferæ during the season
+of their active growth exhibit very remarkable movements of revolving
+nutation, that is, they circumnutate. We may feel sure that the lateral
+shoots whilst growing would exhibit the same movement if carefully
+observed.
+
+
+(19.) Lilium auratum (Fam. Liliaceae).—The circumnutation of the stem
+of a plant 24 inches in height is represented in the above figure (Fig.
+83).
+
+Fig. 83. Lilium auratum: circumnutation of a stem in darkness, traced
+on a horizontal glass, from 8 A.M. on March 14th to 8.35 A.M. on 16th.
+But it should be noted that our observations were interrupted between 6
+P.M. on the 14th and 12.15 P.M. on the 15th, and the movements during
+this interval of 18 h. 15 m. are represented by a long broken line.
+Diagram reduced to half original scale.
+
+Fig. 84. Cyperus alternifolius: circumnutation of stem, illuminated
+from above, traced on horizontal glass, from 9.45 A.M. March 9th to 9
+P.M. on 10th. The stem grew so rapidly whilst being observed, that it
+was not possible to estimate how much its movements were magnified in
+the tracing.
+
+(20.) Cyperus alternifolius (Fam. Cyperaceae.)—A glass
+filament, with a bead at the end, was fixed across the summit of a
+young stem 10 inches in height, close beneath the crown of elongated
+leaves. On March 8th, between 12.20 and 7.20 P.M. the stem described an
+ellipse, open at one end. On the following day a new tracing was begun
+(Fig. 84), which plainly shows that the stem completed three irregular
+figures in the course of 35 h. 15 m.
+
+Concluding Remarks on the Circumnutation of Stems.—Any one who will
+inspect the diagrams now given, and will bear in mind the widely
+separated position of the plants described in the series,—remembering
+that we have good grounds for the belief that the hypocotyls and
+epicotyls of all seedlings circumnutate,—not forgetting the number of
+plants distributed in the most distinct families which climb by a
+similar movement,—will probably admit that the growing stems of all
+plants, if carefully observed, would be found to circumnutate to a
+greater or less extent. When we treat of the sleep and other movements
+of plants, many other cases of circumnutating stems will be
+incidentally given. In looking at the diagrams, we should remember that
+the stems were always growing, so that in each case the circumnutating
+apex as it rose will have described a spire of some kind. The dots were
+made on the glasses generally at intervals of an hour, or hour and a
+half, and were then joined by straight lines. If they had been made at
+intervals of 2 or 3 minutes, the lines would have been more
+curvilinear, as in the case of the tracks left on the smoked
+glass-plates by the tips of the circumnutating radicles of seedling
+plants. The diagrams generally approach in form to a succession of more
+or less irregular ellipses or ovals, with their longer axes directed to
+different points of the compass during the same day or on succeeding
+days. The stems
+therefore, sooner or later, bend to all sides; but after a stem has
+bent in any one direction, it commonly bends back at first in nearly,
+though not quite, the opposite direction; and this gives the tendency
+to the formation of ellipses, which are generally narrow, but not so
+narrow as those described by stolons and leaves. On the other hand, the
+figures sometimes approach in shape to circles. Whatever the figure may
+be, the course pursued is often interrupted by zigzags, small
+triangles, loops, or ellipses. A stem may describe a single large
+ellipse one day, and two on the next. With different plants the
+complexity, rate, and amount of movement differ much. The stems, for
+instance, of Iberis and Azalea described only a single large ellipse in
+24 h.; whereas those of the Deutzia made four or five deep zigzags or
+narrow ellipses in 11½ h., and those of the Trifolium three triangular
+or quadrilateral figures in 7 h.
+
+CIRCUMNUTATION OF STOLONS OR RUNNERS.
+
+Stolons consist of much elongated, flexible branches, which run along
+the surface of the ground and form roots at a distance from the
+parent-plant. They are therefore of the same homological nature as
+stems; and the three following cases may be added to the twenty
+previously given cases.
+
+Fragaria (cultivated garden var.): Rosaceae.—A plant growing in a pot
+had emitted a long stolon; this was supported by a stick, so that it
+projected for the length of several inches horizontally. A glass
+filament bearing two minute triangles of paper was affixed to the
+terminal bud, which was a little upturned; and its movements were
+traced during 21 h., as shown in Fig. 85. In the course of the first 12
+h. it moved twice up and twice down in somewhat zigzag lines, and no
+doubt travelled in the same manner during the night. On the following
+morning after an interval of 20 h. the apex stood a little higher than
+it did at first, and this shows that the stolon had not been acted on
+within this time by geotropism;[2] nor had its own weight caused it to
+bend downwards.
+
+ [2] Dr. A. B. Frank states (‘Die Naturliche wagerechte Richtung von
+ Pflanzentheilen,’ 1870, p. 20) that the stolons of this plant are
+ acted on by geotropism, but only after a considerable interval of
+ time.
+
+
+Fig. 85. Fragaria: circumnutation of stolon, kept in darkness, traced
+on vertical glass, from 10.45 A.M. May 18th to 7.45 A.M. on 19th.
+
+On the following morning (19th) the glass filament was detached and
+refixed close behind the bud, as it appeared possible that the
+circumnutation of the terminal bud and of the adjoining part of the
+stolon might be different. The movement was now traced during two
+consecutive days (Fig. 86). During the first day the filament travelled
+in the course of 14 h. 30 m. five times up and four times down, besides
+some lateral movement. On the 20th the course was even more
+complicated, and can hardly be followed in the figure; but the filament
+moved in 16 h. at least five times up and five times down, with very
+little
+lateral deflection. The first and last dots made on this second day,
+viz., at 7 A.M. and 11 P.M., were close together, showing that the
+stolon had not fallen or risen. Nevertheless, by comparing its position
+on the morning of the 19th and 21st, it is obvious that the stolon had
+sunk; and this may be attributed to slow bending down either from its
+own weight or from geotropism.
+
+Fig. 86. Fragaria: circumnutation of the same stolon as in the last
+figure, observed in the same manner, and traced from 8 A.M. May 19th to
+8 A.M. 21st.
+
+During a part of the 20th an orthogonal tracing was made by applying a
+cube of wood to the vertical glass and bringing the apex of the stolon
+at successive periods into a line with one edge; a dot being made each
+time on the glass. This tracing therefore represented very nearly the
+actual amount of movement of the apex; and in the course of 9 h. the
+distance of the extreme dots from one another was .45 inch. By the same
+method it was ascertained that the apex moved between 7 A.M. on the
+20th and 8 A.M. on the 21st a distance of .82 inch.
+
+A younger and shorter stolon was supported so that it projected at
+about 45° above the horizon, and its movement was traced by the same
+orthogonal method. On the first day the apex soon rose above the field
+of vision. By the next morning it had sunk, and the course pursued was
+now traced during 14 h. 30 m. (Fig. 87). The amount of movement was
+almost the same,
+from side to side as up and down; and differed in this respect
+remarkably from the movement in the previous cases. During the latter
+part of the day, viz., between 3 and 10.30 P.M., the actual distance
+travelled by the apex amounted to 1.15 inch; and in the course of the
+whole day to at least 2.67 inches. This is an amount of movement almost
+comparable with that of some climbing plants. The same stolon was
+observed on the following day, and now it moved in a somewhat less
+complex manner, in a plane not far from vertical. The extreme amount of
+actual movement was 1.55 inch in one direction, and .6 inch in another
+direction at right angles. During neither of these days did the stolon
+bend downwards through geotropism or its own weight.
+
+Fig. 87. Fragaria: circumnutation of another and younger stolon, traced
+from 8 A.M. to 10.30 P.M. Figure reduced to one-half of original scale.
+
+Four stolons still attached to the plant were laid on damp sand in the
+back of a room, with their tips facing the north-east windows. They
+were thus placed because De Vries says[3] that they are apheliotropic
+when exposed to the light of the sun; but we could not perceive any
+effect from the above feeble degree of illumination. We may add that on
+another occasion, late in the summer, some stolons, placed upright
+before a south-west window
+on a cloudy day, became distinctly curved towards the light, and were
+therefore heliotropic. Close in front of the tips of the prostrate
+stolons, a crowd of very thin sticks and the dried haulms of grasses
+were driven into the sand, to represent the crowded stems of
+surrounding plants in a state of nature. This was done for the sake of
+observing how the growing stolons would pass through them. They did so
+easily in the course of 6 days, and their circumnutation apparently
+facilitated their passage. When the tips encountered sticks so close
+together that they could not pass between them, they rose up and passed
+over them. The sticks and haulms were removed after the passage of the
+four stolons, two of which were found to have assumed a permanently
+sinuous shape, and two were still straight. But to this subject we
+shall recur under Saxifraga.
+
+ [3] ‘Arbeiten Bot Inst., Würzburg,’ 1872, p. 434.
+
+
+Saxifraga sarmentosa (Saxifrageae).—A plant in a suspended pot had
+emitted long branched stolons, which depended like threads on all
+sides. Two were tied up so as to stand vertically, and their upper ends
+became gradually bent downwards, but so slowly in the course of several
+days, that the bending was probably due to their weight and not to
+geotropism. A glass filament with little triangles of paper was fixed
+to the end of one of these stolons, which was 17½ inches in length, and
+had already become much bent down, but still projected at a
+considerable angle above the horizon. It moved only slightly three
+times from side to side and then upwards; on the following day
+the movement was even less. As this stolon was so long we thought that
+its growth was nearly completed, so we tried another which was thicker
+and shorter, viz., 10 1/4 inches in length. It moved greatly, chiefly
+upwards, and changed its course five times in the course of the day.
+During the night it curved so much upwards in opposition to gravity,
+that the movement could no longer be traced on the vertical glass, and
+a horizontal one had to be used. The movement was followed during the
+next 25 h., as shown in Fig. 88. Three irregular ellipses, with their
+longer axes somewhat differently directed, were almost completed in the
+first 15 h. The extreme actual amount of movement of the tip during the
+25 h. was .75 inch. Several stolons were laid on a flat surface of damp
+sand, in the same manner as with those of the strawberry. The friction
+of the sand did not interfere with their circumnutation; nor could we
+detect any evidence of their being sensitive to contact. In order to
+see how in a state of nature they would act, when encountering a stone
+or other obstacle on the ground, short pieces of smoked glass, an inch
+in height, were stuck upright into the sand in front of two thin
+lateral branches. Their tips scratched the smoked surface in various
+directions; one made three upward and two downward lines, besides a
+nearly horizontal one; the other curled quite away from the glass; but
+ultimately both surmounted the glass and pursued their original course.
+The apex of a third thick stolon swept up the glass in a curved line,
+recoiled and again came into contact with it; it then moved to the
+right, and after ascending, descended vertically; ultimately it passed
+round one end of the glass instead of over it.
+
+Fig. 88. Saxifraga sarmentosa: circumnutation of an inclined stolon,
+traced in darkness on a horizontal glass, from 7.45 A.M. April 18th to
+9 A.M. on 19th. Movement of end of stolon magnified 2.2 times.
+
+Many long pins were next driven rather close together into the sand, so
+as to form a crowd in front of the same two thin lateral branches; but
+these easily wound their way through the crowd. A thick stolon was much
+delayed in its passage; at one place it was forced to turn at right
+angles to its former course; at another place it could not pass through
+the pins, and the hinder part became bowed; it then curved upwards and
+passed through an opening between the upper part of some pins which
+happened to diverge; it then descended and finally emerged through the
+crowd. This stolon was rendered permanently sinuous to a slight degree,
+and was thicker where sinuous than elsewhere, apparently from its
+longitudinal growth having been checked.
+
+Cotyledon umbilicus (Crassulaceæ).—A plant growing in a pan
+of damp moss had emitted 2 stolons, 22 and 20 inches in length. One of
+these was supported, so that a length of 4½ inches projected in a
+straight and horizontal line, and the movement of the apex was traced.
+The first dot was made at 9.10 A.M.; the terminal portion soon began to
+bend downwards and continued to do so until noon. Therefore a straight
+line, very nearly as long as the whole figure here given (Fig. 89), was
+first traced on the glass; but the upper part of this line has not been
+copied in the diagram. The curvature occurred in the middle
+of the penultimate internode; and its chief seat was at the distance of
+1 1/4 inch from the apex; it appeared due to the weight of the terminal
+portion, acting on the more flexible part of the internode, and not to
+geotropism. The apex after thus sinking down from 9.10 A.M. to noon,
+moved a little to the left; it then rose up and circumnutated in a
+nearly vertical plane until 10.35 P.M. On the following day (26th) it
+was observed from 6.40 A.M. to 5.20 P.M., and within this time it moved
+twice up and twice down. On the morning of the 27th the apex stood as
+high as it did at 11.30 A.M. on the 25th. Nor did it sink down during
+the 28th, but continued to circumnutate about the same place.
+
+Fig. 89. Cotyledon umbilicus: circumnutation of stolon, traced from
+11.15 A.M. Aug. 25th to 11 A.M. 27th. Plant illuminated from above. The
+terminal internode was .25 inch in length, the penultimate 2.25 and the
+third 3.0 inches in length. Apex of stolon stood at a distance of 5.75
+inches from the vertical glass; but it was not possible to ascertain
+how much the tracing was magnified, as it was not known how great a
+length of the internode circumnutated.
+
+Another stolon, which resembled the last in almost every
+respect, was observed during the same two days, but only two inches of
+the terminal portion was allowed to project freely and horizontally. On
+the 25th it continued from 9.10 A.M. to 1.30 P.M. to bend straight
+downwards, apparently owing to its weight (Fig. 90); but after this
+hour until 10.35 P.M. it zigzagged. This fact deserves notice, for we
+here probably see the combined effects of the bending down from weight
+and of circumnutation. The stolon, however, did not circumnutate when
+it first began to bend down, as may be observed in the present diagram,
+and as was still more evident in the last case, when a longer portion
+of the stolon was left unsupported. On the following day (26th) the
+stolon moved twice up and twice down, but still continued to fall; in
+the evening and during the night it travelled from some unknown cause
+in an oblique direction.
+
+Fig. 90. Cotyledon umbilicus: circumnutation and downward movement of
+another stolon, traced on vertical glass, from 9.11 A.M. Aug. 25th to
+11 A.M. 27th. Apex close to glass, so that figure but little magnified,
+and here reduced to two-thirds of original size.
+
+We see from these three cases that stolons or runners circumnutate in a
+very complex manner. The lines generally extend in a vertical plane,
+and this may probably be attributed to the effect of the weight of the
+unsupported end of the stolon; but there is always some, and
+occasionally a considerable, amount of lateral movement. The
+circumnutation is so great in amplitude that it may almost be compared
+with that of climbing plants. That the stolons are thus aided in
+passing over obstacles and in winding between the stems of the
+surrounding plants, the observations above given render almost certain.
+If they had not circumnutated, their tips would have been liable to
+have been doubled up, as often as they met with obstacles in their
+path; but as it is, they easily avoid them. This must be a considerable
+advantage to the plant in spreading from its parent-stock; but we are
+far from supposing that the power has been gained by the stolons for
+this purpose, for circumnutation seems to be of universal occurrence
+with all growing parts; but it is not improbable that the amplitude of
+the movement may have been specially increased for this purpose.
+
+CIRCUMNUTATION OF FLOWER-STEMS.
+
+We did not think it necessary to make any special observations on the
+circumnutation of flower-stems, these being axial in their nature, like
+stems or stolons; but some were incidentally made whilst attending to
+other subjects, and these we will here briefly give. A few observations
+have also been made by other botanists. These taken together suffice to
+render it probable that all peduncles and sub-peduncles circumnutate
+whilst growing.
+
+Oxalis carnosa.—The peduncle which springs from the thick and woody
+stem of this plant bears three or four sub-peduncles. A filament with
+little triangles of paper was fixed within the calyx of a flower which
+stood upright. Its movements were observed for 48 h.; during the first
+half of this time the flower was fully expanded, and during the second
+half withered. The figure here given (Fig. 91) represents 8 or 9
+ellipses. Although the main peduncle circumnutated, and described one
+large and
+two smaller ellipses in the course of 24 h., yet the chief seat of
+movement lies in the sub-peduncles, which ultimately bend vertically
+downwards, as will be described in a future chapter. The peduncles of
+Oxalis acetosella likewise bend downwards, and afterwards, when the
+pods are nearly mature, upwards; and this is effected by a
+circumnutating movement.
+
+Fig. 91. Oxalis carnosa: flower-stem, feebly illuminated from above,
+its circumnutation traced from 9 A.M. April 13th to 9 A.M. 15th. Summit
+of flower 8 inches beneath the horizontal glass. Movement probably
+magnified about 6 times.
+
+It may be seen in the above figure that the flower-stem of O. carnosa
+circumnutated during two days about the same spot. On the other hand,
+the flower-stem of O. sensitiva undergoes a strongly marked, daily,
+periodical change of position, when kept at a proper temperature. In
+the middle of the day it stands vertically up, or at a high angle; in
+the afternoon it sinks, and in the evening projects horizontally, or
+almost horizontally, rising again during the night. This movement
+continues from the period when the flowers are in bud to when, as we
+believe, the pods are mature: and it ought perhaps to have been
+included amongst the so-called sleep-movements of plants. A tracing was
+not made, but the angles were measured at successive periods during one
+whole day; and these showed that the movement was not continuous, but
+that the peduncle oscillated up and down. We may therefore conclude
+that it circumnutated. At the base of the peduncle there is a mass of
+small cells, forming a well-developed pulvinus, which is exteriorly
+coloured purple and hairy. In no other genus, as far as we know, is the
+peduncle furnished with a pulvinus. The peduncle of O. Ortegesii
+behaved differently from that of O. sensitiva, for it stood at a less
+angle above the horizon in the middle of the day, then in the morning
+or evening. By 10.20 P.M. it had risen greatly. During the middle of
+the day it oscillated much up and down.
+
+Trifolium subterraneum.—A filament was fixed vertically to the
+uppermost part of the peduncle of a young and upright flower-head (the
+stem of the plant having been secured to a stick); and its movements
+were traced during 36 h. Within this time it described (see Fig. 92) a
+figure which represents four ellipses; but during the latter part of
+the time the peduncle began to bend downwards, and after 10.30 P.M. on
+the 24th it curved so rapidly down, that by 6.45 A.M. on the 25th it
+stood only 19° above the horizon. It went on circumnutating in nearly
+the same position for two days. Even after the flower-heads have buried
+themselves in the ground they continue, as will hereafter be shown, to
+circumnutate. It will also be seen in the next chapter that the
+sub-peduncles of the separate flowers of
+_Trifolium repens_ circumnutate in a complicated course during several
+days. I may add that the gynophore of _Arachis hypogoea_, which looks
+exactly like a peduncle, circumnutates whilst growing vertically
+downwards, in order to bury the young pod in the ground.
+
+Fig. 92. Trifolium subterraneum: main flower-peduncle, illuminated from
+above, circumnutation traced on horizontal glass, from 8.40 A.M. July
+23rd to 10.30 P.M. 24th.
+
+The movements of the flowers of Cyclamen Persicum were not observed;
+but the peduncle, whilst the pod is forming, increases much in length,
+and bows itself down by a circumnutating movement. A young peduncle of
+Maurandia semperflorens, 1½ inch in length, was carefully observed
+during a whole day, and it made 4½ narrow, vertical, irregular and
+short ellipses, each at an average rate of about 2 h. 25 m. An
+adjoining peduncle described during the same time similar, though
+fewer, ellipses.[4] According to Sachs[5] the flower-stems, whilst
+growing,
+of many plants, for instance, those of Brassica napus, revolve or
+circumnutate; those of Allium porrum bend from side to side, and, if
+this movement had been traced on a horizontal glass, no doubt ellipses
+would have been formed. Fritz Müller has described[6] the spontaneous
+revolving movements of the flower-stems of an Alisma, which he compares
+with those of a climbing plant.
+
+ [4] ‘The Movements and Habits of Climbing Plants,’ 2nd edit., 1875, p.
+ 68.
+
+
+ [5] ‘Text-Book of Botany,’ 1875, p. 766. Linnæus and Treviranus
+ (according to Pfeffer, ‘Die Periodischen Bewegungen,’ etc., p. 162)
+ state that the flower-stalks of many plants occupy different positions
+ by night and day, and we shall see in the chapter on the Sleep of
+ Plants that this implies circumnutation.
+
+
+ [6] ‘Jenaische Zeitsch.,’ B. v. p. 133.
+
+
+We made no observations on the movements of the different parts of
+flowers. Morren, however, has observed[7] in the stamens of Sparmannia
+and Cereus a “fremissement spontané,” which, it may be suspected, is a
+circumnutating movement. The circumnutation of the gynostemium of
+Stylidium, as described by Gad,[8] is highly remarkable, and apparently
+aids in the fertilisation of the flowers. The gynostemium, whilst
+spontaneously moving, comes into contact with the viscid labellum, to
+which it adheres, until freed by the increasing tension of the parts or
+by being touched.
+
+ [7] ‘N. Mem. de l’Acad. R. de Bruxelles,’ tom. xiv. 1841, p. 3.
+
+
+ [8] ‘Sitzungbericht des bot. Vereins der P. Brandenburg,’ xxi. p. 84.
+
+
+We have now seen that the flower-stems of plants belonging to such
+widely different families as the Cruciferae, Oxalidæ, Leguminosae,
+Primulaceae, Scrophularineae, Alismaceae, and Liliaceae, circumnutate;
+and that there are indications of this movement in many other families.
+With these facts before us, bearing also in mind that the tendrils of
+not a few plants consist of modified peduncles, we may admit without
+much doubt that all growing flower-stems circumnutate.
+
+CIRCUMNUTATION OF LEAVES: DICOTYLEDONS.
+
+Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries,
+Batalin, Millardet, etc., have
+observed, and some of them with the greatest care, the periodical
+movements of leaves; but their attention has been chiefly, though not
+exclusively, directed to those which move largely and are commonly said
+to sleep at night. From considerations hereafter to be given, plants of
+this nature are here excluded, and will be treated of separately. As we
+wished to ascertain whether all young and growing leaves circumnutated,
+we thought that it would be sufficient if we observed between 30 and 40
+genera, widely distributed throughout the vegetable series, selecting
+some unusual forms and others on woody plants. All the plants were
+healthy and grew in pots. They were illuminated from above, but the
+light perhaps was not always sufficiently bright, as many of them were
+observed under a skylight of ground-glass. Except in a few specified
+cases, a fine glass filament with two minute triangles of paper was
+fixed to the leaves, and their movements were traced on a vertical
+glass (when not stated to the contrary) in the manner already
+described. I may repeat that the broken lines represent the nocturnal
+course. The stem was always secured to a stick, close to the base of
+the leaf under observation. The arrangement of the species, with the
+number of the Family appended, is the same as in the case of stems.
+
+Fig. 93. Sarracenia purpurea: circumnutation of young pitcher, traced
+from 8 A.M. July 3rd to 10.15 A.M. 4th. Temp. 17°–18° C. Apex of
+pitcher 20 inches from glass, so movement greatly magnified.
+
+(1.) Sarracenia purpurea (Sarraceneae, Fam. 11).—A young leaf, or
+pitcher, 8½ inches in height, with the bladder swollen but with the
+hood not as yet open, had a filament fixed transversely
+across its apex; it was observed for 48 h., and during the whole of
+this time it circumnutated in a nearly similar manner, but to a very
+small extent. The tracing given (Fig. 93) relates only to the movement
+during the first 26 h.
+
+(2) Glaucium luteum (Papaveraceae, Fam. 12).—A young plant, bearing
+only 8 leaves, had a filament attached to the youngest leaf but one,
+which was 3 inches in length, including the petiole. The circumnutating
+movement was traced during 47 h. On both days the leaf descended from
+before 7 A.M. until about 11 A.M., and then ascended slightly during
+the rest of the day and the early part of the night. During the latter
+part of the night it fell greatly. It did not ascend so much during the
+second as during the first day, and it descended considerably lower on
+the second night than on the first. This difference was probably due to
+the illumination from above having been insufficient during the two
+days of observation. Its course during the two days is shown in Fig.
+94.
+
+Fig. 94. Glaucium luteum: circumnutation of young leaf, traced from
+9.30 A.M. June 14th to 8.30 A.M. 16th. Tracing not much magnified, as
+apex of leaf stood only 5½ inches from the glass.
+
+(3.) Crambe maritima (Cruciferae, Fam. 14).—A leaf 9½ inches in length
+on a plant not growing vigorously was first observed. Its apex was in
+constant movement, but this could hardly be traced, from being so small
+in extent. The apex, however, certainly changed its course at least 6
+times in the course of 14 h. A more vigorous young plant, bearing only
+4 leaves, was then selected, and a filament was affixed to the midrib
+of the third leaf from the base, which, with the petiole, was 5 inches
+in length. The leaf stood up almost vertically, but the tip
+was deflected, so that the filament projected almost horizontally, and
+its movements were traced during 48 h. on a vertical glass as shown in
+the accompanying figure (Fig. 95). We here plainly see that the leaf
+was continually circumnutating; but the proper periodicity of its
+movements was disturbed by its being only dimly illuminated from above
+through a double skylight. We infer that this was the case, because two
+leaves on plants growing out of doors, had their angles above the
+horizon measured in the middle of the day and at 9 to about 10 P.M. on
+successive nights, and they were found at this latter hour to have
+risen by an average angle of 9° above their mid-day position: on the
+following morning they fell to their former position. Now it may be
+observed in the diagram that the leaf rose during the second night, so
+that it stood at 6.40 A.M. higher than at 10.20 P.M. on the preceding
+night; and this may be attributed to the leaf adjusting itself to the
+dim light, coming exclusively from above.
+
+Fig. 95. Crambe maritima: circumnutation of leaf, disturbed by being
+insufficiently illuminated from above, traced from 7.50 A.M. June 23rd
+to 8 A.M. 25th. Apex of leaf 15 1/4 inches from the vertical glass, so
+that the tracing was much magnified, but is here reduced to one-fourth
+of original scale.
+
+(4.) Brassica oleracea (Cruciferae).—Hofmeister and Batalin[9] state
+that the leaves of the cabbage rise at night, and fall by day. We
+covered a young plant, bearing 8 leaves, under a large bell-glass,
+placing it in the same position with respect to the
+light in which it had long remained, and a filament was fixed at the
+distance of .4 of an inch from the apex of a young leaf nearly 4 inches
+in length. Its movements were then traced during three days, but the
+tracing is not worth giving. The leaf fell during the whole morning,
+and rose in the evening and during the early part of the night. The
+ascending and descending lines did not coincide, so that an irregular
+ellipse was formed each 24 h. The basal part of the midrib did not
+move, as was ascertained by measuring at successive periods the angle
+which it formed with the horizon, so that the movement was confined to
+the terminal portion of the leaf, which moved through an angle of 11°
+in the course of 24 h., and the distance travelled by the apex, up and
+down, was between .8 and .9 of an inch.
+
+ [9] ‘Flora,’ 1873, p. 437.
+
+
+In order to ascertain the effect of darkness, a filament was fixed to a
+leaf 5½ inches in length, borne by a plant which after forming a head
+had produced a stem. The leaf was inclined 44° above the horizon, and
+its movements were traced on a vertical glass every hour by the aid of
+a taper. During the first day the leaf rose from 8 A.M. to 10.40 P.M.
+in a slightly zigzag course, the actual distance travelled by the apex
+being .67 of an inch. During the night the leaf fell, whereas it ought
+to have risen; and by 7 A.M. on the following morning it had fallen .23
+of an inch, and it continued falling until 9.40 A.M. It then rose until
+10.50 P.M., but the rise was interrupted by one considerable
+oscillation, that is, by a fall and re-ascent. During the second night
+it again fell, but only to a very short distance, and on the following
+morning re-ascended to a very short distance. Thus the normal course of
+the leaf was greatly disturbed, or rather completely inverted, by the
+absence of light; and the movements were likewise greatly diminished in
+amplitude.
+
+We may add that, according to Mr. A. Stephen Wilson,[10] the young
+leaves of the Swedish turnip, which is a hybrid between B. oleracea and
+rapa, draw together in the evening so much “that the horizontal breadth
+diminishes about 30 per cent. of the daylight breadth.” Therefore the
+leaves must rise considerably at night.
+
+ [10] ‘Trans. Bot. Soc. Edinburgh,’ vol. xiii. p. 32. With respect to
+ the origin of the Swedish turnip, see Darwin, ‘Animals and Plants
+ under Domestication,’ 2nd edit. vol. i. p. 344.
+
+
+(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).—The
+terminal shoot of a young plant, growing very vigorously, was selected
+for observation. The young leaves at first stand up vertically and
+close together, but they soon bend outwards and downwards, so as to
+become horizontal, and often at the same time a little to one side. A
+filament was fixed to the tip of a young leaf whilst still highly
+inclined, and the first dot was made on the vertical glass at 8.30 A.M.
+June 13th, but it curved downwards so quickly that by 6.40 A.M. on the
+following morning it stood only a little above the horizon. In Fig. 96
+the long, slightly zigzag line representing this rapid downward course,
+which was somewhat inclined to the left, is not given; but the figure
+shows the highly tortuous and zigzag course, together with some loops,
+pursued during the next 2½ days. As the leaf continued to move all the
+time to the left, it is evident that the zigzag line represents many
+circumnutations.
+
+Fig. 96. Dianthus caryophyllus: circumnutation of young leaf, traced
+from 10.15 P.M. June 13th to 10.35 P.M. 16th. Apex of leaf stood, at
+the close of our observations, 8 3/4 inches from the vertical glass, so
+tracing not greatly magnified. The leaf was 5 1/4 inches long. Temp.
+15½°–17½° C.
+
+(6.) Camellia Japonica (Camelliaceae, Fam. 32).—A youngish leaf, which
+together with its petiole was 2 3/4 inches in length and which arose
+from a side branch on a tall bush, had a filament attached to its apex.
+This leaf sloped downwards at an angle of 40° beneath the horizon. As
+it was thick and rigid, and its
+petiole very short, much movement could not be expected. Nevertheless,
+the apex changed its course completely seven times in the course of 11½
+h., but moved to only a very small distance. On the next day the
+movement of the apex was traced during 26 h. 20 m. (as shown in Fig.
+97), and was nearly of the same nature, but rather less complex. The
+movement seems to be periodical, for on both days the leaf
+circumnutated in the forenoon, fell in the afternoon (on the first day
+until between 3 and 4 P.M., and on the second day until 6 P.M.), and
+then rose, falling again during the night or early morning.
+
+Fig. 97. Camellia Japonica: circumnutation of leaf, traced from 6.40
+A.M. June 14th to 6.50 A.M. 15th. Apex of leaf 12 inches from the
+vertical glass, so figure considerably magnified. Temp. 16°–16½° C.
+
+In the chapter on the Sleep of Plants we shall see that the leaves in
+several Malvaceous genera sink
+
+Fig. 98. Pelargonium zonale: circumnutation and downward movement of
+young leaf, traced from 9.30 A.M. June 14th to 6.30 P.M. 16th. Apex of
+leaf 9 1.4 inches from the vertical glass, so figure moderately
+magnified. Temp. 15°–16½° C.
+
+at night; and as they often do not then occupy a vertical position,
+especially if they have not been well illuminated during
+the day, it is doubtful whether some of these cases ought not to have
+been included in the present chapter.
+
+(7.) Pelargonium zonale (Geraniaceae, Fam. 47).—A young leaf, 1 1/4
+inch in breadth, with its petiole 1 inch long, borne on a young plant,
+was observed in the usual manner during 61 h.; and its course is shown
+in the preceding figure (Fig. 98). During the first day and night the
+leaf moved downwards, but circumnutated between 10 A.M. and 4.30 P.M.
+On the second day it sank and rose again, but between 10 A.M. and 6
+P.M. it circumnutated on an extremely small scale. On the third day the
+circumnutation was more plainly marked.
+
+(8.) Cissus discolor (Ampelideae, Fam. 67).—A leaf, not nearly
+full-grown, the third from the apex of a shoot on a cut-down plant, was
+observed during 31 h. 30 m. (see Fig. 99). The day was cold (15°–16°
+C.), and if the plant had been observed in the hot-house, the
+circumnutation, though plain enough as it was, would probably have been
+far more conspicuous.
+
+Fig. 99. Cissus discolor: circumnutation of leaf, traced from 10.35
+A.M. May 28th to 6 P.M. 29th. Apex of leaf 8 3/4 inches from the
+vertical glass.
+
+(9.) Vicia faba (Leguminosae, Fam. 75).—A young leaf, 3.1 inches in
+length, measured from base of petiole to end of leaflets, had a
+filament affixed to the midrib of one of the two terminal leaflets, and
+its movements were traced during 51½ h. The filament fell all morning
+(July 2nd) till 3 P.M., and then rose greatly till 10.35 P.M.; but the
+rise this day was so great, compared with that which subsequently
+occurred, that it was probably due in part to the plant being
+illuminated from above. The latter part of the course on July 2nd is
+alone given in the following figure (Fig. 100). On the next day (July
+3rd) the leaf again fell in the morning, then circumnutated in a
+conspicuous manner, and rose till late at night; but the movement was
+not traced after 7.15 P.M., as by that time the filament pointed
+towards the upper edge of the glass. During the latter part of the
+night or early morning it again fell in the same manner as before.
+
+
+As the evening rise and the early morning fall were unusually large,
+the angle of the petiole above the horizon was measured at the two
+periods, and the leaf was found to have risen 19° between 12.20 P.M.
+and 10.45 P.M., and to have fallen 23° 30 seconds between the latter
+hour and 10.20 A.M. on the following morning.
+
+Fig. 100. Vicia faba: circumnutation of leaf, traced from 7.15 P.M.
+July 2nd to 10.15 A.M. 4th. Apex of the two terminal leaflets 7 1/4
+inches from the vertical glass. Figure here reduced to two-thirds of
+original scale. Temp. 17°–18° C.
+
+The main petiole was now secured to a stick close to the base
+of the two terminal leaflets, which were 1.4 inch in length; and the
+movements of one of them were traced during 48 h. (see Fig. 101). The
+course pursued is closely analogous to that of the whole leaf. The
+zigzag line between 8.30 A.M. and 3.30 P.M. on the second day
+represents 5 very small ellipses, with their longer axes differently
+directed. From these observations it follows that both the whole leaf
+and the terminal leaflets undergo a well-marked daily periodical
+movement, rising in the evening and falling during the latter part of
+the night or early morning; whilst in the middle of the day they
+generally circumnutate round the same small space.
+
+Fig 101. Vicia faba: circumnutation of one of the two terminal
+leaflets, the main petiole having been secured, traced from 10.40 A.M.
+July 4th to 10.30 A.M. 6th. Apex of leaflet 6 5/8 inches from the
+vertical glass. Tracing here reduced to one-half of original scale.
+Temp. 16°–18° C.
+
+
+(10.) Acacia retinoides (Leguminosae).—The movement of a young
+phyllode, 2 3/8 inches in length, and inclined at a considerable angle
+above the horizon, was traced during 45 h. 30 m.; but in the figure
+here given (Fig. 102), its circumnutation is shown during only 21 h. 30
+m. During part of this time (viz., 14 h. 30 m.) the phyllode described
+a figure representing 5 or 6 small ellipses. The actual amount of
+movement in a vertical direction was .3 inch. The phyllode rose
+considerably between 1.30 P.M. and 4 P.M., but there was no evidence on
+either day of a regular periodic movement.
+
+Fig. 102. Acacia retinoides: circumnutation of a young phyllode, traced
+from 10.45 A.M. July 18th to 8.15 A.M. 19th. Apex of phyllode 9 inches
+from the vertical glass; temp. 16½°–17½° C.
+
+(11.) Lupinus speciosus (Leguminosae).—Plants were raised from seed
+purchased under this name. This is one of the species in this large
+genus, the leaves of which do not sleep at night. The petioles rise
+direct from the ground, and are from 5 to 7 inches in length. A
+filament was fixed to the midrib of one of the longer leaflets, and the
+movement of the whole leaf was traced, as shown in Fig. 103. In the
+course of 6 h. 30 m. the filament went four times up and three times
+down. A new tracing was then begun (not here given), and during 12½ h.
+the leaf moved eight times up and seven times down; so that it
+described 7½ ellipses in this time, and this is an extraordinary rate
+of movement. The summit of the petiole was then secured to a stick, and
+the separate leaflets were found to be continually circumnutating.
+
+Fig. 103. Lupinus speciosus: circumnutation of leaf, traced on vertical
+glass, from 10.15 A.M. to 5.45 P.M.; i.e., during 6 h. 30 m.
+
+
+(12.) Echeveria stolonifera (Crassulaceæ, Fam. 84).—The older leaves of
+this plant are so thick and fleshy, and the young ones so short and
+broad, that it seemed very improbable that any circumnutation could be
+detected. A filament was fixed to a young upwardly inclined leaf, .75
+inch in length and .28 in breadth, which stood on the outside of a
+terminal rosette of leaves, produced by a plant growing very
+vigorously. Its movement was traced during 3 days, as here shown (Fig.
+104). The course was chiefly in an upward direction, and this may be
+attributed to the elongation of the leaf through growth; but we see
+that the lines are strongly zigzag, and that occasionally there was
+distinct circumnutation, though on a very small scale.
+
+Fig. 104. Echeveria stolonifera: circumnutation of leaf, traced from
+8.20 A.M. June 25th to 8.45 A.M. 28th. Apex of leaf 12 1/4 inches from
+the glass, so that the movement was much magnified; temp. 23°–24½° C.
+
+(13.) Bryophyllum (vel Calanchæ) calycinum (Crassulaceæ).—Duval-Jouve
+(‘Bull. Soc. Bot. de France,’ Feb. 14th, 1868) measured the distance
+between the tips of the upper pair of leaves on this plant, with the
+result shown in the following Table. It should be noted that the
+measurements on Dec. 2nd were made on a different pair of leaves:—
+
+8 A.M.      2 P.M.      7 P.M. Nov. 16. . . . . . . . . . . . . . . . .
+. .15 mm.. . . . . .25 mm. . . .. . . .(?) ”     19. . . . . . . . . .
+. . . . . . . . .48  ” . . . . . . .  60 ”. . . . . . .  48 mm. Dec.  
+2. . . . . . . . . . . . . . . . . . .22  ”. . . . . . . . 43 ”. . . .
+. . . .28  ”
+
+We see from this Table that the leaves stood considerably further apart
+at 2 P.M. than at either 8 A.M. or 7 P.M.; and this shows that they
+rise a little in the evening and fall or open in the forenoon.
+
+(14.) Drosera rotundifolia (Droseraceae, Fam. 85).—The movements of a
+young leaf, having a long petiole but with its tentacles (or
+gland-bearing hairs) as yet unfolded, were traced during 47 h. 15 m.
+The figure (Fig. 105) shows that it circumnutated largely, chiefly in a
+vertical direction, making two ellipses each
+day. On both days the leaf began to descend after 12 or 1 o’clock, and
+continued to do so all night, though to a very unequal distance on the
+two occasions. We therefore thought that the movement was periodic; but
+on observing three other leaves during several successive days and
+nights, we found this to be an error; and the case is given merely as a
+caution. On the third morning the above leaf occupied almost exactly
+the same position as on the first morning; and the tentacles by this
+time had unfolded sufficiently to project at right angles to the blade
+or disc.
+
+Fig. 105. Drosera rotundifolia: circumnutation of young leaf, with
+filament fixed to back of blade, traced from 9.15 A.M. June 7th to 8.30
+A.M. June 9th. Figure here reduced to one-half original scale.
+
+The leaves as they grow older generally sink more and more downwards.
+The movement of an oldish leaf, the glands of which were still
+secreting freely, was traced for 24 h., during which time it continued
+to sink a little in a slightly zigzag line. On the following morning,
+at 7 A.M., a drop of a solution of carbonate of ammonia (2 gr. to 1 oz.
+of water) was placed on the disc, and this blackened the glands and
+induced inflection of many of the tentacles. The weight of the drop
+caused the leaf at first to sink a little; but immediately afterwards
+it began to rise in a somewhat zigzag course, and continued to do so
+till 3 P.M. It then circumnutated about the same spot on a very small
+scale for 21 h.; and during the next 21 h. it sank in a zigzag line to
+nearly the same level which it had held when the ammonia was first
+administered. By this time the tentacles had re-expanded, and the
+glands had recovered their proper colour. We thus learn that an old
+leaf
+circumnutates on a small scale, at least whilst absorbing carbonate of
+ammonia; for it is probable that this absorption may stimulate growth
+and thus re-excite circumnutation. Whether the rising of the glass
+filament which was attached to the back of the leaf, resulted from its
+margin becoming slightly inflected (as generally occurs), or from the
+rising of the petiole, was not ascertained.
+
+In order to learn whether the tentacles or gland-bearing hairs
+circumnutate, the back of a young leaf, with the innermost tentacles as
+yet incurved, was firmly cemented with shellac to a flat stick driven
+into compact damp argillaceous sand. The plant was placed under a
+microscope with the stage removed and with an eye-piece micrometer, of
+which each division equalled 1/500 of an inch. It should be stated that
+as the leaves grow older the tentacles of the exterior rows bend
+outwards and downwards, so as ultimately to become deflected
+considerably beneath the horizon. A tentacle in the second row from the
+margin was selected for observation, and was found to be moving
+outwards at a rate of 1/500 of an inch in 20 m., or 1/100 of inch in 1
+h. 40 m.; but as it likewise moved from side to side to an extent of
+above 1/500 of inch, the movement was probably one of modified
+circumnutation. A tentacle on an old leaf was next observed in the same
+manner. In 15 m. after being placed under the microscope it had moved
+about 1/1000 of an inch. During the next 7½ h. it was looked at
+repeatedly, and during this whole time it moved only another 1/1000 of
+an inch; and this small movement may have been due to the settling of
+the damp sand (on which the plant rested), though the sand had been
+firmly pressed down. We may therefore conclude that the tentacles when
+old do not circumnutate; yet this tentacle was so sensitive, that in 23
+seconds after its gland had been merely touched with a bit of raw meat,
+it began to curl inwards. This fact is of some importance, as it
+apparently shows that the inflection of the tentacles from the stimulus
+of absorbed animal matter (and no doubt from that of contact with any
+object) is not due to modified circumnutation.
+
+(15.) Dionoea muscipula (Droseraceae).—It should be premised that the
+leaves at an early stage of their development have the two lobes
+pressed closely together. These are at first directed back towards the
+centre of the plant; but they gradually rise up and soon stand at right
+angles to the petiole, and ultimately in nearly a straight line with
+it. A young leaf, which with the
+petiole was only 1.2 inch in length, had a filament fixed externally
+along the midrib of the still closed lobes, which projected at right
+angles to the petiole. In the evening this leaf completed an ellipse in
+the course of 2 h. On the following day (Sept. 25th) its movements were
+traced during 22 h.; and we see in Fig. 106 that it moved in the same
+general direction, due to the straightening of the leaf, but in an
+extremely zigzag line. This line represents several drawn-out or
+modified ellipses. There can therefore be no doubt that this young leaf
+circumnutated.
+
+Fig. 106. Dionaea muscipula: circumnutation of a young and expanding
+leaf, traced on a horizontal glass in darkness, from noon Sept. 24th to
+10 A.M. 25th. Apex of leaf 13½ inches from the glass, so tracing
+considerably magnified.
+
+A rather old, horizontally extended leaf, with a filament attached
+along the under side of the midrib, was next observed during 7 h. It
+hardly moved, but when one of its sensitive hairs was touched, the
+blades closed, though not very quickly. A new dot was now made on the
+glass, but in the course of 14 h. 20 m. there was hardly any change in
+the position of the filament. We may therefore infer that an old and
+only moderately sensitive leaf does not circumnutate plainly; but we
+shall soon see that it by no means follows that such a leaf is
+absolutely motionless. We may further infer that the stimulus from a
+touch does not re-excite plain circumnutation.
+
+Another full-grown leaf had a filament attached externally along one
+side of the midrib and parallel to it, so that the filament would move
+if the lobes closed. It should be first stated that, although a touch
+on one of the sensitive hairs of a vigorous leaf causes it to close
+quickly, often almost instantly, yet when a bit of damp meat or some
+solution of carbonate of ammonia is placed on the lobes, they close so
+slowly that generally 24 h. is required for the completion of the act.
+The above leaf was first observed for 2 h. 30 m., and did not
+circumnutate, but it ought to have been observed for a
+longer period; although, as we have seen, a young leaf completed a
+fairly large ellipse in 2 h. A drop of an infusion of raw meat was then
+placed on the leaf, and within 2 h. the glass filament rose a little;
+and this implies that the lobes had begun to close, and perhaps the
+petiole to rise. It continued to rise with extreme slowness for the
+next 8 h. 30 m. The position of the pot was then (7.15 P.M., Sept.
+24th) slightly changed and an additional drop of the infusion given,
+and a new tracing was begun (Fig. 107). By 10.50 P.M. the filament had
+risen only a little more, and it fell during the night. On the
+following morning the lobes were closing more quickly, and by 5 P.M. it
+was evident to the eye that they had closed considerably; by 8.48. P.M.
+this was still plainer, and by 10.45 P.M. the marginal spikes were
+interlocked. The leaf fell a little during the night, and next morning
+(25th) at 7 A.M. the lobes were completely shut. The course pursued, as
+may be seen in the figure, was strongly zigzag, and this indicates that
+the closing of the lobes was combined with the circumnutation of the
+whole leaf; and there cannot be much doubt, considering how motionless
+the leaf was during 2 h. 30 m. before it received the infusion, that
+the absorption of the animal matter had excited it to circumnutate. The
+leaf was occasionally observed for the next four days, but was kept in
+rather too cool a place; nevertheless, it continued to circumnutate to
+a small extent, and the lobes remained closed.
+
+Fig. 107. Dionoea muscipula: closure of the lobes and circumnutation of
+a full-grown leaf, whilst absorbing an infusion of raw meat, traced in
+darkness, from 7.15 P.M. Sept. 24th to 9 A.M. 26th. Apex of leaf 8½
+inches from the vertical glass. Figure here reduced to two-thirds of
+original scale.
+
+It is sometimes stated in botanical works that the lobes close or sleep
+at night; but this is an error. To test the statement, very long glass
+filaments were fixed inside the two lobes of three leaves, and the
+distances between their tips were measured in the middle of the day and
+at night; but no difference could be detected.
+
+The previous observations relate to the movements of the whole leaf,
+but the lobes move independently of the petiole, and
+seem to be continually opening and shutting to a very small extent. A
+nearly full-grown leaf (afterwards proved to be highly sensitive to
+contact) stood almost horizontally, so that by driving a long thin pin
+through the foliaceous petiole close to the blade, it was rendered
+motionless. The plant, with a little triangle of paper attached to one
+of the marginal spikes, was placed under a microscope with an eye-piece
+micrometer, each division of which equalled 1/500 of an inch. The apex
+of the paper-triangle was now seen to be in constant slight movement;
+for in 4 h. it crossed nine divisions, or 9/500 of an inch, and after
+ten additional hours it moved back and had crossed 5/500 in an opposite
+direction. The plant was kept in rather too cool a place, and on the
+following day it moved rather less, namely, 1/500 in 3 h., and 2/500 in
+an opposite direction during the next 6 h. The two lobes, therefore,
+seem to be constantly closing or opening, though to a very small
+distance; for we must remember that the little triangle of paper
+affixed to the marginal spike increased its length, and thus
+exaggerated somewhat the movement. Similar observations, with the
+important difference that the petiole was left free and the plant kept
+under a high temperature, were made on a leaf, which was healthy, but
+so old that it did not close when its sensitive hairs were repeatedly
+touched, though judging from other cases it would have slowly closed if
+it had been stimulated by animal matter. The apex of the triangle was
+in almost, though not quite, constant movement, sometimes in one
+direction and sometimes in an opposite one; and it thrice crossed five
+divisions of the micrometer (i.e. 1/100 of an inch) in 30 m. This
+movement on so small a scale is hardly comparable with ordinary
+circumnutation; but it may perhaps be compared with the zigzag lines
+and little loops, by which the larger ellipses made by other plants are
+often interrupted.
+
+In the first chapter of this volume, the remarkable oscillatory
+movements of the circumnutating hypocotyl of the cabbage have been
+described. The leaves of Dionaea present the same phenomenon, which is
+a wonderful one, as viewed under a low power (2-inch object-glass),
+with an eye-piece micrometer of which each division (1/500 of an inch)
+appeared as a rather wide space. The young unexpanded leaf, of which
+the circumnutating movements were traced (Fig. 106), had a glass
+filament fixed perpendicularly to it; and the movement of the apex was
+observed in the hot-house (temp. 84° to 86° F.), with light admitted
+only from above, and with any lateral currents of air
+excluded. The apex sometimes crossed one or two divisions of the
+micrometer at an imperceptibly slow rate, but generally it moved
+onwards by rapid starts or jerks of 2/1000 or 3/1000, and in one
+instance of 4/1000 of an inch. After each jerk forwards, the apex drew
+itself backwards with comparative slowness for part of the distance
+which had just been gained; and then after a very short time made
+another jerk forwards. Four conspicuous jerks forwards, with slower
+retreats, were seen on one occasion to occur in exactly one minute,
+besides some minor oscillations. As far as we could judge, the
+advancing and retreating lines did not coincide, and if so, extremely
+minute ellipses were each time described. Sometimes the apex remained
+quite motionless for a short period. Its general course during the
+several hours of observation was in two opposite directions, so that
+the leaf was probably circumnutating.
+
+An older leaf with the lobes fully expanded, and which was afterwards
+proved to be highly sensitive to contact, was next observed in a
+similar manner, except that the plant was exposed to a lower
+temperature in a room. The apex oscillated forwards and backwards in
+the same manner as before; but the jerks forward were less in extent,
+viz. about 1/1000 inch; and there were longer motionless periods. As it
+appeared possible that the movements might be due to currents of air, a
+wax taper was held close to the leaf during one of the motionless
+periods, but no oscillations were thus caused. After 10 m., however,
+vigorous oscillations commenced, perhaps owing to the plant having been
+warmed and thus stimulated. The candle was then removed and before long
+the oscillations ceased; nevertheless, when looked at again after an
+interval of 1 h. 30 m., it was again oscillating. The plant was taken
+back into the hot-house, and on the following morning was seen to be
+oscillating, though not very vigorously. Another old but healthy leaf,
+which was not in the least sensitive to a touch, was likewise observed
+during two days in the hot-house, and the attached filament made many
+little jerks forwards of about 2/1000 or only 1/1000 of an inch.
+
+Finally, to ascertain whether the lobes independently of the petiole
+oscillated, the petiole of an old leaf was cemented close to the blade
+with shellac to the top of a little stick driven into the soil. But
+before this was done the leaf was observed, and found to be vigorously
+oscillating or jerking; and after it had been cemented to the stick,
+the oscillations of about 2/1000 of an inch still continued. On the
+following day a little infusion
+of raw meat was placed on the leaf, which caused the lobes to close
+together very slowly in the course of two days; and the oscillations
+continued during this whole time and for the next two days. After nine
+additional days the leaf began to open and the margins were a little
+everted, and now the apex of the glass filament remained for long
+periods motionless, and then moved backwards and forwards for a
+distance of about 1/1000 of an inch slowly, without any jerks.
+Nevertheless, after warming the leaf with a taper held close to it, the
+jerking movement recommenced.
+
+This same leaf had been observed 2½ months previously, and was then
+found to be oscillating or jerking. We may therefore infer that this
+kind of movement goes on night and day for a very long period; and it
+is common to young unexpanded leaves and to leaves so old as to have
+lost their sensitiveness to a touch, but which were still capable of
+absorbing nitrogenous matter. The phenomenon when well displayed, as in
+the young leaf just described, is a very interesting one. It often
+brought before our minds the idea of effort, or of a small animal
+struggling to escape from some constraint.
+
+(16.) Eucalyptus resinifera (Myrtaceae, Fam. 94).—A young leaf, two
+inches in length together with the petiole, produced by a lateral shoot
+from a cut-down tree, was observed in the usual manner. The blade had
+not as yet assumed its vertical position. On June 7th only a few
+observations were made, and the tracing merely showed that the leaf had
+moved three times upwards and three downwards. On the following day it
+was observed more frequently; and two tracings were made (see A and B,
+Fig. 108), as a single one would have been too complicated. The apex
+changed its course 13 times in the course of 16 h., chiefly up and
+down, but with some lateral movement. The actual amount of movement in
+any one direction was small.
+
+Fig. 108. Eucalyptus resinifera: circumnutation of a leaf, traced, A,
+from 6.40 A.M. to 1 P.M. June 8th; B, from 1 P.M. 8th to 8.30 A.M. 9th.
+Apex of leaf 14½ inches from the horizontal glass, so figures
+considerably magnified.
+
+(17.) Dahlia (garden var.) (Compositæ, Fam. 122).—A fine young
+leaf 5 3/4 inches in length, produced by a young plant 2 feet high,
+growing vigorously in a large pot, was directed at an angle of about
+45° beneath the horizon. On June 18th the leaf descended from 10 A.M.
+till 11.35 A.M. (see Fig. 109); it then ascended greatly till 6 P.M.,
+this ascent being probably due to the light coming only from above. It
+zigzagged between 6 P.M. and 10.35 P.M., and ascended a little during
+the night. It should be remarked that the vertical distances in the
+lower part of the diagram are much exaggerated, as the leaf was at
+first deflected beneath the horizon, and after it had sunk downwards,
+the filament pointed in a very oblique line towards the glass. Next
+day the leaf descended from 8.20 A.M. till 7.15 P.M., then zigzagged
+and ascended greatly during the night. On the morning of the 20th the
+leaf was probably beginning to descend, though the short line in the
+diagram is horizontal. The actual distances travelled by the apex of
+the leaf were considerable, but could not be calculated with safety.
+From the course pursued on the second day, when the plant had
+accommodated itself to the light from above, there cannot be much doubt
+that the leaves undergo a daily periodic movement, sinking during the
+day and rising at night.
+
+Fig. 109. Dahlia: circumnutation of leaf, traced from 10 A.M. June 18th
+to 8.10 A.M. 20th, but with a break of 1 h. 40 m. on the morning of the
+19th, as, owing to the glass filament pointing too much to one side,
+the pot had to be slightly moved; therefore the relative position of
+the two tracings is somewhat arbitrary. The figure here given is
+reduced to one-fifth of the original scale. Apex of leaf 9 inches from
+the glass in the line of its inclination, and 4 3/4 in a horizontal
+line.
+
+(18.) Mutisia clematis (Compositæ).—The leaves terminate in tendrils
+and circumnutate like those of other tendril-bearers; but this plant is
+here mentioned, on account of an erroneous statement[11] which has been
+published, namely, that the leaves sink at night and rise during the
+day. The leaves which behaved in this manner had been kept for some
+days in a northern room and had not been sufficiently illuminated. A
+plant therefore was left undisturbed in the hot-house, and three leaves
+had their angles measured at noon and at 10 P.M. All three were
+inclined a little beneath the horizon at noon, but one stood at night
+2°, the second 21°, and the third 10° higher than in the middle of the
+day; so that instead of sinking they rise a little at night.
+
+ [11] ‘The Movements and Habits of Climbing Plants,’ 1875, p. 118.
+
+
+(19.) Cyclamen Persicum (Primulaceae, Fam. 135).—A young leaf, 1.8 of
+an inch in length, petiole included, produced by an old root-stock, was
+observed during three days in the usual manner (Fig. 110). On the first
+day the leaf fell more than afterwards, apparently from adjusting
+itself to the light from above. On all three days it fell from the
+early morning to about 7 P.M., and from that hour rose during the
+night, the course being slightly zigzag. The movement therefore is
+strictly periodic. It should be noted that the leaf would have sunk
+each evening a little lower down than it did, had not the glass
+filament rested between 5 and 6 P.M. on the rim of the pot. The amount
+of movement was considerable; for if we assume that the whole leaf to
+the base of the petiole became bent, the tracing would be magnified
+rather less than five times, and this would give to the apex a rise and
+fall of half an inch, with some lateral movement. This amount, however,
+would not attract attention without the aid of a tracing or measurement
+of some kind.
+
+
+(20.) Allamanda Schottii (Apocyneae, Fam. 144).—The young leaves of
+this shrub are elongated, with the blade bowed so much downwards as
+almost to form a semicircle. The chord—that is, a line drawn from the
+apex of the blade to the base of the petiole—of a young leaf, 4 3/4
+inches in length, stood at 2.50 P.M. on
+Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 P.M. the
+blade had straightened itself so much, which implies the raising of the
+apex, that the chord now stood at 37° above the horizon, and had
+therefore risen 50°. On the next day similar angular measurements of
+the same leaf were made; and at noon the chord stood 36° beneath the
+horizon, and 9.30 P.M. 3½° above it, so had risen 39½°. The chief cause
+of the rising movement lies in the straightening of the blade, but the
+short petiole rises between 4° and 5°. On the third night the chord
+stood at 35° above the horizon, and if the leaf occupied the same
+position at noon, as on the previous day, it had risen 71°. With older
+leaves no such change of curvature could be detected. The plant was
+then brought into the house and kept in a north-east room, but at night
+there was no change in the curvature of the young leaves; so that
+previous exposure to a strong light is apparently requisite for the
+periodical change of curvature in the blade, and for the slight rising
+of the petiole.
+
+Fig. 110. Cyclamen Persicum: circumnutation of leaf, traced from 6.45
+A.M. June 2nd to 6.40 A.M. 5th. Apex of leaf 7 inches from the vertical
+glass.
+
+(21.) Wigandia (Hydroleaceae, Fam. 149).—Professor Pfeffer informs us
+that the leaves of this plant rise in the evening; but as we do not
+know whether or not the rising is great, this species ought perhaps to
+be classed amongst sleeping plants.
+
+Fig. 111. Petunia violacea: downward movement and circumnutation of a
+very young leaf, traced from 10 A.M. June 2nd to 9.20 A.M. June 6th.
+N.B.—At 6.40 A.M. on the 5th it was necessary to move the pot a little,
+and a new tracing was begun at the point where two dots are not joined
+in the diagram. Apex of leaf 7 inches from the vertical glass. Temp.
+generally 17½° C.
+
+
+(22.) Petunia violacea (Solaneae, Fam. 157).—A very young leaf, only
+3/4 inch in length, highly inclined upwards, was observed for four
+days. During the whole of this time it bent outwards and downwards, so
+as to become more and more nearly horizontal. The strongly marked
+zigzag line in the figure on p. 248 (Fig. 111), shows that this was
+effected by modified circumnutation; and during the latter part of the
+time there was much ordinary circumnutation on a small scale. The
+movement in the diagram is magnified between 10 and 11 times. It
+exhibits a clear trace of periodicity, as the leaf rose a little each
+evening; but this upward tendency appeared to be almost conquered by
+the leaf striving to become more and more horizontal as it grew older.
+The angles which two older leaves formed together, were measured in the
+evening and about noon on 3 successive days, and each night the angle
+decreased a little, though irregularly.
+
+Fig. 112. Acanthus mollis: circumnutation of young leaf, traced from
+9.20 A.M. June 14th to 8.30 A.M. 16th. Apex of leaf 11 inches from the
+vertical glass, so movement considerably magnified. Figure here reduced
+to one-half of original scale. Temp. 15°–16½° C.
+
+(23.) Acanthus mollis (Acanthaceae, Fam. 168).—The younger of two
+leaves, 2 1/4 inches in length, petiole included, produced by a
+seedling plant, was observed during 47 h. Early on each of the three
+mornings, the apex of the leaf fell; and it continued to fall till 3
+P.M., on the two afternoons when observed. After 3 P.M. it rose
+considerably, and continued to rise on the second night until the early
+morning. But on the first night it fell instead of rising, and we have
+little doubt that this was owing to the leaf being very young and
+becoming through epinastic growth more and more horizontal; for it may
+be seen in the diagram (Fig. 112), that the leaf stood on a higher
+level on the first than on the second day. The leaves of an allied
+species (‘A. spinosus’) certainly rose every night; and the rise
+between noon and 10.15 P.M., when measured on one occasion, was 10°.
+This rise was chiefly
+or exclusively due to the straightening of the blade, and not to the
+movement of the petiole. We may therefore conclude that the leaves of
+Acanthus circumnutate periodically, falling in the morning and rising
+in the afternoon and night.
+
+(24.) Cannabis sativa (Cannabineae, Fam. 195).—We have here the rare
+case of leaves moving downwards in the evening, but not to a sufficient
+degree to be called sleep.[12] In the early morning, or in the latter
+part of the night, they move upwards. For instance, all the young
+leaves near the summits of several stems stood almost horizontally at 8
+A.M. May 29th and at 10.30 P.M. were considerably declined. On a
+subsequent day two leaves stood at 2 P.M. at 21° and 12° beneath the
+horizon, and at 10 P.M. at 38° beneath it. Two other leaves on a
+younger plant were horizontal at 2 P.M., and at 10 P.M. had sunk to 36°
+beneath the horizon. With respect to this downward movement of the
+leaves, Kraus believes that it is due to their epinastic growth. He
+adds, that the leaves are relaxed during the day, and tense at night,
+both in sunny and rainy weather.
+
+ [12] We were led to observe this plant by Dr. Carl Kraus’ paper,
+ ‘Beiträge zur Kentniss der Bewegungen Wachsender Laubblätter,’ Flora,
+ 1879, p. 66. We regret that we cannot fully understand parts of this
+ paper.
+
+(25.) Pinus pinaster (Coniferæ, Fam. 223).—The leaves on the summits of
+the terminal shoots stand at first in a bundle almost upright, but they
+soon diverge and ultimately become almost horizontal. The movements of
+a young leaf, nearly one inch in length, on the summit of a seedling
+plant only 3 inches high, were traced from the early morning of June
+2nd to the evening of the 7th. During these five days the leaf
+diverged, and its apex descended at first in an almost straight line;
+but during the two latter days it zigzagged so much that it was
+evidently circumnutating. The same little plant, when grown to a height
+of 5 inches, was again observed during four days. A filament was fixed
+transversely to the apex of a leaf, one inch in length, and which had
+already diverged considerably from its originally upright position. It
+continued to diverge (see A, Fig. 113), and to descend from 11.45 A.M.
+July 31st to 6.40 A.M. Aug. 1st. On August 1st it circumnutated about
+the same small space, and again descended at night. Next morning the
+pot was moved nearly one inch to the right, and a new tracing was begun
+(B). From this time, viz., 7 A.M. August 2nd to 8.20 A.M. on the 4th,
+the leaf manifestly circumnutated. It does not appear from the diagram
+that the leaves move periodically, for the descending course during the
+first two nights, was clearly due to epinastic growth, and at the close
+of our observations the leaf was not nearly so horizontal as it would
+ultimately become.
+
+Fig. 113. Pinus pinaster: circumnutation of young leaf, traced from
+11.45 A.M. July 31st to 8.20 A.M. Aug. 4th. At 7 A.M. Aug. 2nd the pot
+was moved an inch to one side, so that the tracing consists of two
+figures. Apex of leaf 14½ inches from the vertical glass, so movements
+much magnified.
+
+Pinus austriaca.—Two leaves, 3 inches in length, but not
+quite fully grown, produced by a lateral shoot, on a young tree 3 feet
+in height, were observed during 29 h. (July 31st), in the same manner
+as the leaves of the previous species. Both these leaves certainly
+circumnutated, making within the above period two, or two and a half,
+small, irregular ellipses.
+
+(26.) Cycas pectinata (Cycadeæ, Fam. 224).—A young leaf, 11½ inches in
+length, of which the leaflets had only recently become uncurled, was
+observed during 47 h. 30 m. The main petiole was secured to a stick at
+the base of the two terminal leaflets. To one of the latter, 3 3/4
+inches in length, a filament was fixed; the leaflet was much bowed
+downward, but as the terminal part was upturned, the filament projected
+almost horizontally. The leaflet moved (see Fig. 114) largely and
+periodically, for it fell until about 7 P.M. and rose during the night,
+falling again next morning after 6.40 A.M. The descending lines are in
+a marked manner zigzag, and so probably would have been the ascending
+lines, if they had been traced throughout the night.
+
+Fig. 114. Cycas pectinata: circumnutation of one of the terminal
+leaflets, traced from 8.30 A.M. June 22nd to 8 A.M. June 24th. Apex of
+leaflet 7 3/4 inches from the vertical glass, so tracing not greatly
+magnified, and here reduced to one-third of original scale; temp.
+19°–21° C.
+
+CIRCUMNUTATION OF LEAVES: MONOCOTYLEDONS.
+
+(27.) Canna Warscewiczii (Cannaceae, Fam. 2).—The movements of a young
+leaf, 8 inches in length and 3½ in breadth, produced by a vigorous
+young plant, were observed during 45 h. 50 m., as shown in Fig. 115.
+The pot was slided about an inch to the right on the morning of the
+11th, as a single figure would have been too complicated; but the two
+figures are continuous in time. The movement is periodical, as the leaf
+descended from the early morning until about 5 P.M., and ascended
+during the rest of the evening and
+part of the night. On the evening of the 11th it circumnutated on a
+small scale for some time about the same spot.
+
+Fig. 115. Canna Warscewiczii: circumnutation of leaf, traced (A) from
+11.30 A.M. June 10th to 6.40 A.M. 11th; and (B) from 6.40 A.M. 11th to
+8.40 A.M. 12th. Apex of leaf 9 inches from the vertical glass.
+
+(28.) Iris pseudo-acorus (Irideae, Fam. 10).—The movements of a young
+leaf, rising 13 inches above the water in which the plant grew, were
+traced as shown in the figure (Fig. 116), during 27 h. 30 m. It
+manifestly circumnutated, though only to a small extent. On the second
+morning, between 6.40 A.M. and 2 P.M. (at which latter hour the figure
+here given ends), the apex changed its course five times. During the
+next 8 h. 40 m. it zigzagged much, and descended as far as the lowest
+dot in the figure, making in its course two very small ellipses; but if
+these lines had been added to the diagram it would have been too
+complex.
+
+Fig. 116. Iris pseudo-acorus: circumnutation of leaf, traced from 10.30
+A.M. May 28th to 2 P.M. 29th. Tracing continued to 11 P.M., but not
+here copied. Apex of leaf 12 inches beneath the horizontal glass, so
+figure considerably magnified. Temp. 15°–16° C.
+
+(29.) Crinum Capense (Amaryllideae, Fam. 11).—The leaves of this plant
+are remarkable for their great length and narrowness: one was measured
+and found to be 53 inches long and only 1.4 broad at the base. Whilst
+quite young they stand up almost vertically to the height of about a
+foot; afterwards
+their tips begin to bend over, and subsequently hang vertically down,
+and thus continue to grow. A rather young leaf was selected, of which
+the dependent tapering point was as yet only 5½ inches in length, the
+upright basal part being 20 inches high, though this part would
+ultimately become shorter by being more bent over. A large bell-glass
+was placed over the plant, with a black dot on one side; and by
+bringing the dependent apex of the leaf into a line with this dot, the
+accompanying figure (Fig. 117) was traced on the other side of the
+bell, during 2½ days. During the first day (22nd) the tip travelled
+laterally far to the left, perhaps in consequence of the plant having
+been disturbed; and the last dot made at 10.30 P.M. on this day is
+alone here given. As we see in the figure, there can be no doubt that
+the apex of this leaf circumnutated.
+
+Fig. 117. Crinum Capense: circumnutation of dependent tip of young
+leaf, traced on a bell-glass, from 10.30 P.M. May 22nd to 10.15 A.M.
+25th. Figure not greatly magnified.
+
+A glass filament with little triangles of paper was at the same time
+fixed obliquely across the tip of a still younger leaf, which stood
+vertically up and was as yet straight. Its movements were traced from 3
+P.M. May 22nd to 10.15 A.M. 25th. The leaf was growing rapidly, so that
+the apex ascended greatly during this period; as it zigzagged much it
+was clearly circumnutating, and it apparently tended to form one
+ellipse each day. The lines traced during the night were much more
+vertical than those traced during the day; and this indicates that the
+tracing would have exhibited a nocturnal rise and a diurnal fall, if
+the leaf had not grown so quickly. The movement of this same leaf after
+an interval of six days (May 31st), by which time the tip had curved
+outwards into a horizontal position,
+and had thus made the first step towards becoming dependent, was traced
+orthogonically by the aid of a cube of wood (in the manner before
+explained); and it was thus ascertained that the actual distance
+travelled by the apex, and due to circumnutation, was 3 1/8 inches in
+the course of 20½ h. During the next 24 h. it travelled 2½ inches. The
+circumnutating movement, therefore, of this young leaf was strongly
+marked.
+
+(30.) Pancratium littorale (Amaryllideae).—The movements, much
+magnified, of a leaf, 9 inches in length and inclined at about 45°
+above the horizon, were traced during two days. On the first day it
+changed its course completely, upwards and downwards and laterally, 9
+times in 12 h.; and the figure traced apparently represented five
+ellipses. On the second day it was observed seldomer, and was therefore
+not seen to change its course so often, viz., only 6 times, but in the
+same complex manner as before. The movements were small in extent, but
+there could be no doubt about the circumnutation of the leaf.
+
+(31.) Imatophyllum vel Clivia (sp.?) (Amaryllideae).—A long glass
+filament was fixed to a leaf, and the angle formed by it with the
+horizon was measured occasionally during three successive days. It fell
+each morning until between 3 and 4 P.M., and rose at night. The
+smallest angle at any time above the horizon was 48°, and the largest
+50°; so that it rose only 2° at night; but as this was observed each
+day, and as similar observations were nightly made on another leaf on a
+distinct plant, there can be no doubt that the leaves move
+periodically, though to a very small extent. The position of the apex
+when it stood highest was .8 of an inch above its lowest point.
+
+(32.) Pistia stratiotes (Aroideae, Fam. 30).—Hofmeister remarks that
+the leaves of this floating water-plant are more highly inclined at
+night than by day.[13] We therefore fastened a fine glass filament to
+the midrib of a moderately young leaf, and on Sept. 19th measured the
+angle which it formed with the horizon 14 times between 9 A.M. and
+11.50 P.M. The temperature of the hot-house varied during the two days
+of observation between 18½° and 23½° C. At 9 A.M. the filament stood at
+32° above the horizon; at 3.34 P.M. at 10° and at 11.50 P.M. at 55°;
+these two latter angles being the highest and the lowest observed
+during the day, showing a difference of 45°. The rising did not become
+strongly marked until between
+5 and 6 P.M. On the next day the leaf stood at only 10° above the
+horizon at 8.25 A.M., and it remained at about 15° till past 3 P.M.; at
+5.40 P.M. it was 23°, and at 9.30 P.M. 58°; so that the rise was more
+sudden this evening than on the previous one, and the difference in the
+angle amounted to 48°. The movement is obviously periodical, and as the
+leaf stood on the first night at 55°, and on the second night at 58°
+above the horizon, it appeared very steeply inclined. This case, as we
+shall see in a future chapter, ought perhaps to have been included
+under the head of sleeping plants.
+
+ [13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327.
+
+
+(33.) Pontederia (sp.?) (from the highlands of St. Catharina, Brazil)
+(Pontederiaceae, Fam. 46).—A filament was fixed across the apex of a
+moderately young leaf, 7½ inches in height, and its movements were
+traced during 42½ h. (see Fig. 118). On the first evening, when the
+tracing was begun, and during the night, the leaf descended
+considerably. On the next morning it ascended in a strongly marked
+zigzag line, and descended again in the evening and during the night.
+The movement, therefore, seems to be periodic, but some doubt is thrown
+on this conclusion, because another leaf, 8 inches in height, appearing
+older and standing more highly inclined, behaved differently. During
+the first 12 h. it circumnutated over a
+small space, but during the night and the whole following day it
+ascended in the same general direction; the ascent being effected by
+repeated up and down well-pronounced oscillations.
+
+Fig. 118. Pontederia (sp.?): circumnutation of leaf, traced from 4.50
+P.M. July 2nd to 10.15 A.M. 4th. Apex of leaf 16½ inches from the
+vertical glass, so tracing greatly magnified. Temp. about 17° C., and
+therefore rather too low.
+
+CRYPTOGAMS.
+
+(34.) Nephrodium molle (Filices, Fam. 1).—A filament was fixed near the
+apex of a young frond of this Fern, 17 inches in height, which was not
+as yet fully uncurled; and its movements were traced during 24 h. We
+see in Fig. 119 that it plainly circumnutated. The movement was not
+greatly magnified as the frond was placed near to the vertical glass,
+and would probably have been greater and more rapid had the day been
+warmer. For the plant was brought out of a warm greenhouse and observed
+under a skylight, where the temperature was between 15° and 16° C. We
+have seen in Chap. I. that a frond of this Fern, as yet only slightly
+lobed and with a rachis only .23 inch in height, plainly
+circumnutated.[14]
+
+ [14] Mr. Loomis and Prof. Asa Gray have described (‘Botanical
+ Gazette,’ 1880, pp. 27, 43), an extremely curious case of movement in
+ the fronds, but only in the fruiting fronds, of Asplenium trichomanes.
+ They move almost as rapidly as the little leaflets of Desmodium
+ gyrans, alternately backwards and forwards through from 20 to 40
+ degrees, in a plane at right angles to that of the frond. The apex of
+ the frond describes “a long and very narrow ellipse,” so that it
+ circumnutates. But the movement differs from ordinary circumnutation
+ as it occurs only when the plant is exposed to the light; even
+ artificial light “is sufficient to excite motion for a few minutes.”
+
+
+Fig. 119. Nephrodium molle: circumnutation of rachis, traced from 9.15
+A.M. May 28th to 9 A.M. 29th. Figure here given two-thirds of original
+scale.
+
+
+In the chapter on the Sleep of Plants the conspicuous circumnutation of
+Marsilea quadrifoliata (Marsileaceae, Fam. 4) will be described.
+
+It has also been shown in Chap. I. that a very young Selaginella
+(Lycopodiaceæ, Fam. 6), only .4 inch in height, plainly circumnutated;
+we may therefore conclude that older plants, whilst growing, would do
+the same.
+
+(35.) Lunularia vulgaris (Hepaticae, Fam. 11, Muscales).—The earth in
+an old flower-pot was coated with this plant, bearing gemmae. A highly
+inclined frond, which projected .3 inch above the soil and was .4 inch
+in breadth, was selected for observation. A glass hair of extreme
+tenuity, .75 inch in length, with its end whitened, was cemented with
+shellac to the frond at right angles to its breadth; and a white stick
+with a minute black spot was driven into the soil close behind the end
+of the hair. The white end could be accurately brought into a line with
+the black spot, and dots could thus be successively made on the
+vertical glass-plate in front. Any movement of the frond would of
+course be exhibited and increased by the long glass hair; and the black
+spot was placed so close behind the end of the hair, relatively to the
+distance of the glass-plate in front, that the movement of the end was
+magnified about 40 times. Nevertheless, we are convinced that our
+tracing gives a fairly faithful representation of the movements of the
+frond. In the intervals between each observation, the plant was covered
+by a small bell-glass. The frond, as already stated,
+was highly inclined, and the pot stood in front of a north-east window.
+During the five first days the frond moved downwards or became less
+inclined; and the long line which was traced was strongly zigzag, with
+loops occasionally formed or nearly formed; and this indicated
+circumnutation. Whether the sinking was due to epinastic growth, or
+apheliotropism, we do not know. As the sinking was slight on the fifth
+day, a new tracing was begun on the sixth day (Oct. 25th), and was
+continued for 47 h.; it is here given (Fig. 120). Another tracing was
+made on the next day (27th) and the frond was found to be still
+circumnutating, for during 14 h. 30 m. it changed its course completely
+(besides minor changes) 10 times. It was casually observed for two more
+days, and was seen to be continually moving.
+
+Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9
+A.M. Oct 25th to 8 A.M. 27th.
+
+The lowest members of the vegetable series, the Thallogens, apparently
+circumnutate. If an Oscillaria be watched under the microscope, it may
+be seen to describe circles about every 40 seconds. After it has bent
+to one side, the tip first begins to bend back to the opposite side and
+then the whole filament curves over in the same direction.
+Hofmeister[15] has given a minute account of the curious, but less
+regular though constant, movements of Spirogyra: during 2½ h. the
+filament moved 4 times to the left and 3 times to the right, and he
+refers to a movement at right angles to the above. The tip moved at the
+rate of about 0.1 mm. in five minutes. He compares the movement with
+the nutation of the higher plants.[16] We shall hereafter see that
+heliotropic movements result from modified circumnutation, and as
+unicellular Moulds bend to the light we may infer that they also
+circumnutate.
+
+ [15] ‘Ueber die Bewegungen der Faden der _Spirogyra princeps:_
+ Jahreshefte des Vereins für vaterländische Naturkunde in Württemberg,’
+ 1874, p. 211.
+
+
+ [16] Zukal also remarks (as quoted in ‘Journal R. Microscop. Soc.,’
+ 1880, vol. iii. p. 320) that the movements of Spirulina, a member of
+ the Oscillatorieae, are closely analogous “to the well-known rotation
+ of growing shoots and tendrils.”
+
+CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES.
+
+The circumnutating movements of young leaves in 33 genera, belonging to
+25 families, widely distributed
+amongst ordinary and gymnospermous Dicotyledons and amongst
+Monocotyledons, together with several Cryptogams, have now been
+described. It would, therefore, not be rash to assume that the growing
+leaves of all plants circumnutate, as we have seen reason to conclude
+is the case with cotyledons. The seat of movement generally lies in the
+petiole, but sometimes both in the petiole and blade, or in the blade
+alone. The extent of the movement differed much in different plants;
+but the distance passed over was never great, except with Pistia, which
+ought perhaps to have been included amongst sleeping plants. The
+angular movement of the leaves was only occasionally measured; it
+commonly varied from only 2° (and probably even less in some instances)
+to about 10°; but it amounted to 23° in the common bean. The movement
+is chiefly in a vertical plane, but as the ascending and descending
+lines never coincided, there was always some lateral movement, and thus
+irregular ellipses were formed. The movement, therefore, deserves to be
+called one of circumnutation; for all circumnutating organs tend to
+describe ellipses,—that is, growth on one side is succeeded by growth
+on nearly but not quite the opposite side. The ellipses, or the zigzag
+lines representing drawn-out ellipses, are generally very narrow; yet
+with the Camellia, their minor axes were half as long, and with the
+Eucalyptus more than half as long as their major axes. In the case of
+Cissus, parts of the figure more nearly represented circles than
+ellipses. The amount of lateral movement is therefore sometimes
+considerable. Moreover, the longer axes of the successively formed
+ellipses (as with the Bean, Cissus, and Sea-kale), and in several
+instances the zigzag lines representing ellipses, were extended in very
+different directions during the same day or on
+the next day. The course followed was curvilinear or straight, or
+slightly or strongly zigzag, and little loops or triangles were often
+formed. A single large irregular ellipse may be described on one day,
+and two smaller ones by the same plant on the next day. With Drosera
+two, and with Lupinus, Eucalyptus and Pancratium, several were formed
+each day.
+
+The oscillatory and jerking movements of the leaves of Dionaea, which
+resemble those of the hypocotyl of the cabbage, are highly remarkable,
+as seen under the microscope. They continue night and day for some
+months, and are displayed by young unexpanded leaves, and by old ones
+which have lost their sensibility to a touch, but which, after
+absorbing animal matter, close their lobes. We shall hereafter meet
+with the same kind of movement in the joints of certain Gramineæ, and
+it is probably common to many plants while circumnutating. It is,
+therefore, a strange fact that no such movement could be detected in
+the tentacles of Drosera rotundifolia, though a member of the same
+family with Dionaea; yet the tentacle which was observed was so
+sensitive, that it began to curl inwards in 23 seconds after being
+touched by a bit of raw meat.
+
+One of the most interesting facts with respect to the circumnutation of
+leaves is the periodicity of their movements; for they often, or even
+generally, rise a little in the evening and early part of the night,
+and sink again on the following morning. Exactly the same phenomenon
+was observed in the case of cotyledons. The leaves in 16 genera out of
+the 33 which were observed behaved in this manner, as did probably 2
+others. Nor must it be supposed that in the remaining 15 genera there
+was no periodicity in their movements; for 6 of them were observed
+during too short a period for any judgment to be formed on this head,
+and 3 were so young that their epinastic growth, which serves to bring
+them down into a horizontal position, overpowered every other kind of
+movement. In only one genus, Cannabis, did the leaves sink in the
+evening, and Kraus attributes this movement to the prepotency of their
+epinastic growth. That the periodicity is determined by the daily
+alternations of light and darkness there can hardly be a doubt, as will
+hereafter be shown. Insectivorous plants are very little affected, as
+far as their movements are concerned, by light; and hence probably it
+is that their leaves, at least in the cases of Sarracenia, Drosera, and
+Dionaea, do not move periodically. The upward movement in the evening
+is at first slow, and with different plants begins at very different
+hours;—with Glaucium as early as 11 A.M., commonly between 3 and 5
+P.M., but sometimes as late as 7 P.M. It should be observed that none
+of the leaves described in this chapter (except, as we believe, those
+of Lupinus speciosus) possess a pulvinus; for the periodical movements
+of leaves thus provided have generally been amplified into so-called
+sleep-movements, with which we are not here concerned. The fact of
+leaves and cotyledons frequently, or even generally, rising a little in
+the evening and sinking in the morning, is of interest as giving the
+foundation from which the specialised sleep-movements of many leaves
+and cotyledons, not provided with a pulvinus, have been developed. the
+above periodicity should be kept in mind, by any one considering the
+problem of the horizontal position of leaves and cotyledons during the
+day, whilst illuminated from above.
+
+
+
+
+CHAPTER V.
+MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC
+MOVEMENTS.
+
+
+Circumnutation modified through innate causes or through the action of
+external conditions—Innate causes—Climbing plants; similarity of their
+movements with those of ordinary plants; increased amplitude;
+occasional points of difference—Epinastic growth of young
+leaves—Hyponastic growth of the hypocotyls and epicotyls of
+seedlings—Hooked tips of climbing and other plants due to modified
+circumnutation—Ampelopsis tricuspidata—Smithia Pfundii—Straightening of
+the tip due to hyponasty—Epinastic growth and circumnutation of the
+flower-peduncles of Trifolium repens and Oxalis carnosa.
+
+
+The radicles, hypocotyls and epicotyls of seedling plants, even before
+they emerge from the ground, and afterwards the cotyledons, are all
+continually circumnutating. So it is with the stems, stolons,
+flower-peduncles, and leaves of older plants. We may, therefore, infer
+with a considerable degree of safety that all the growing parts of all
+plants circumnutate. Although this movement, in its ordinary or
+unmodified state, appears in some cases to be of service to plants,
+either directly or indirectly—for instance, the circumnutation of the
+radicle in penetrating the ground, or that of the arched hypocotyl and
+epicotyl in breaking through the surface—yet circumnutation is so
+general, or rather so universal a phenomenon, that we cannot suppose it
+to have been gained for any special purpose. We must believe that it
+follows in some unknown way from the manner in which vegetable tissues
+grow.
+
+
+We shall now consider the many cases in which circumnutation has been
+modified for various special purposes; that is, a movement already in
+progress is temporarily increased in some one direction, and
+temporarily diminished or quite arrested in other directions. These
+cases may be divided in two sub-classes; in one of which the
+modification depends on innate or constitutional causes, and is
+independent of external conditions, excepting in so far that the proper
+ones for growth must be present. In the second sub-class the
+modification depends to a large extent on external agencies, such as
+the daily alternations of light and darkness, or light alone,
+temperature, or the attraction of gravity. The first small sub-class
+will be considered in the present chapter, and the second sub-class in
+the remainder of this volume.
+
+THE CIRCUMNUTATION OF CLIMBING PLANTS.
+
+The simplest case of modified circumnutation is that offered by
+climbing plants, with the exception of those which climb by the aid of
+motionless hooks or of rootlets: for the modification consists chiefly
+in the greatly increased amplitude of the movement. This would follow
+either from greatly increased growth over a small length, or more
+probably from moderately increased growth spread over a considerable
+length of the moving organ, preceded by turgescence, and acting
+successively on all sides. The circumnutation of climbers is more
+regular than that of ordinary plants; but in almost every other respect
+there is a close similarity between their movements, namely, in their
+tendency to describe ellipses directed successively to all points of
+the compass—in their courses being often interrupted by zigzag lines,
+triangles, loops, or small
+ellipses—in the rate of movement, and in different species revolving
+once or several times within the same length of time. In the same
+internode, the movements cease first in the lower part and then slowly
+upwards. In both sets of cases the movement may be modified in a
+closely analogous manner by geotropism and by heliotropism; though few
+climbing plants are heliotropic. Other points of similarity might be
+pointed out.
+
+That the movements of climbing plants consist of ordinary
+circumnutation, modified by being increased in amplitude, is well
+exhibited whilst the plants are very young; for at this early age they
+move like other seedlings, but as they grow older their movements
+gradually increase without undergoing any other change. That this power
+is innate, and is not excited by any external agencies, beyond those
+necessary for growth and vigour, is obvious. No one doubts that this
+power has been gained for the sake of enabling climbing plants to
+ascend to a height, and thus to reach the light. This is effected by
+two very different methods; first, by twining spirally round a support,
+but to do so their stems must be long and flexible; and, secondly, in
+the case of leaf-climbers and tendril-bearers, by bringing these organs
+into contact with a support, which is then seized by the aid of their
+sensitiveness. It may be here remarked that these latter movements have
+no relation, as far as we can judge, with circumnutation. In other
+cases the tips of tendrils, after having been brought into contact with
+a support, become developed into little discs which adhere firmly to
+it.
+
+We have said that the circumnutation of climbing plants differs from
+that of ordinary plants chiefly by its greater amplitude. But most
+leaves circumnutate
+in an almost vertical plane, and therefore describe very narrow
+ellipses, whereas the many kinds of tendrils which consist of
+metamorphosed leaves, make much broader ellipses or nearly circular
+figures; and thus they have a far better chance of catching hold of a
+support on any side. The movements of climbing plants have also been
+modified in some few other special ways. Thus the circumnutating stems
+of Solnanum dulcamara can twine round a support only when this is as
+thin and flexible as a string or thread. The twining stems of several
+British plants cannot twine round a support when it is more than a few
+inches in thickness; whilst in tropical forests some can embrace thick
+trunks;[1] and this great difference in power depends on some unknown
+difference in their manner of circumnutation. The most remarkable
+special modification of this movement which we have observed is in the
+tendrils of Echinocystis lobata; these are usually inclined at about
+45° above the horizon, but they stiffen and straighten themselves so as
+to stand upright in a part of their circular course, namely, when they
+approach and have to pass over the summit or the shoot from which they
+arise. If they had not possessed and exercised this curious power, they
+would infallibly have struck against the summit of the shoot and been
+arrested in their course. As soon as one of these tendrils with its
+three branches begins to stiffen itself and rise up vertically, the
+revolving motion becomes more rapid; and as soon as it has passed over
+the point of difficulty, its motion coinciding with that from its own
+weight, causes it to fall into its previously inclined position so
+quickly, that the apex can be seen travelling like the hand of a
+gigantic clock.
+
+ [1] ‘The Movements and Habits of Climbing Plants,’ p. 36.
+
+
+A large number of ordinary leaves and leaflets and a few
+flower-peduncles are provided with pulvini; but this is not the case
+with a single tendril at present known. The cause of this difference
+probably lies in the fact, that the chief service of a pulvinus is to
+prolong the movement of the part thus provided after growth has ceased;
+and as tendrils or other climbing-organs are of use only whilst the
+plant is increasing in height or growing, a pulvinus which served to
+prolong their movements would be useless.
+
+It was shown in the last chapter that the stolons or runners of certain
+plants circumnutate largely, and that this movement apparently aids
+them in finding a passage between the crowded stems of adjoining
+plants. If it could be proved that their movements had been modified
+and increased for this special purpose, they ought to have been
+included in the present chapter; but as the amplitude of their
+revolutions is not so conspicuously different from that of ordinary
+plants, as in the case of climbers, we have no evidence on this head.
+We encounter the same doubt in the case of some plants which bury their
+pods in the ground. This burying process is certainly favoured by the
+circumnutation of the flower-peduncle; but we do not know whether it
+has been increased for this special purpose.
+
+EPINASTY—HYPONASTY.
+
+The term epinasty is used by De Vries[2] to express greater
+longitudinal growth along the upper than
+along the lower side of a part, which is thus caused to bend downwards;
+and hyponasty is used for the reversed process, by which the part is
+made to bend upwards. These actions come into play so frequently that
+the use of the above two terms is highly convenient. The movements thus
+induced result from a modified form of circumnutation; for, as we shall
+immediately see, an organ under the influence of epinasty does not
+generally move in a straight line downwards, or under that of hyponasty
+upwards, but oscillates up and down with some lateral movement: it
+moves, however, in a preponderant manner in one direction. This shows
+that there is some growth on all sides of the part, but more on the
+upper side in the case of epinasty, and more on the lower side in that
+of hyponasty, than on the other sides. At the same time there may be in
+addition, as De Vries insists, increased growth on one side due to
+geotropism, and on another side due to heliotropism; and thus the
+effects of epinasty or of hyponasty may be either increased or
+lessened.
+
+ [2] ‘Arbeiten des Bot. Inst., in Würzburg,’ Heft ii. 1872, p. 223. De
+ Vries has slightly modified (p. 252) the meaning of the above two
+ terms as first used by Schimper, and they have been adopted in this
+ sense by Sachs.
+
+
+He who likes, may speak of ordinary circumnutation as being combined
+with epinasty, hyponasty, the effects of gravitation, light, etc.; but
+it seems to us, from reasons hereafter to be given, to be more correct
+to say that circumnutation is modified by these several agencies. We
+will therefore speak of circumnutation, which is always in progress, as
+modified by epinasty, hyponasty, geotropism, or other agencies, whether
+internal or external.
+
+One of the commonest and simplest cases of epinasty is that offered by
+leaves, which at an early age are crowded together round the buds, and
+diverge as they grow older. Sachs first remarked that this was due to
+increased growth along the upper side of the petiole and blade; and De
+Vries has now shown in more detail that the movement is thus caused,
+aided slightly by
+the weight of the leaf, and resisted as he believes by apogeotropism,
+at least after the leaf has somewhat diverged. In our observations on
+the circumnutation of leaves, some were selected which were rather too
+young, so that they continued to diverge or sink downwards whilst their
+movements were being traced. This may be seen in the diagrams (Figs. 98
+and 112, pp. 232 and 249) representing the circumnutation of the young
+leaves of Acanthus mollis and Pelargonium zonale. Similar cases were
+observed with Drosera. The movements of a young leaf, only 3/4 inch in
+length, of Petunia violacea were traced during four days, and offers a
+better instance (Fig. 111, p. 248) as it diverged during the whole of
+this time in a curiously zigzag line with some of the angles sharply
+acute, and during the latter days plainly circumnutated. Some young
+leaves of about the same age on a plant of this Petunia, which had been
+laid horizontally, and on another plant which was left upright, both
+being kept in complete darkness, diverged in the same manner for 48 h.,
+and apparently were not affected by apogeotropism; though their stems
+were in a state of high tension, for when freed from the sticks to
+which they had been tied, they instantly curled upwards.
+
+The leaves, whilst very young, on the leading shoots of the Carnation
+(Dianthus caryophyllus) are highly inclined or vertical; and if the
+plant is growing vigorously they diverge so quickly that they become
+almost horizontal in a day. But they move downwards in a rather oblique
+line and continue for some time afterwards to move in the same
+direction, in connection, we presume, with their spiral arrangement on
+the stem. The course pursued by a young leaf whilst thus obliquely
+descending was traced, and the line was distinctly yet not strongly
+zigzag; the larger angles formed by the successive lines amounting only
+to 135°, 154°, and 163°. The subsequent lateral movement (shown in Fig.
+96, p. 231) was strongly zigzag with occasional circumnutations. The
+divergence and sinking of the young leaves of this plant seem to be
+very little affected by geotropism or heliotropism; for a plant, the
+leaves of which were growing rather slowly (as ascertained by
+measurement) was laid horizontally, and the opposite young leaves
+diverged from one another symmetrically in the usual manner, without
+any upturning in the direction of gravitation or towards the light.
+
+The needle-like leaves of Pinus pinaster form a bundle whilst young;
+afterwards they slowly diverge, so that those on the upright shoots
+become horizontal. The movements of one such
+young leaf was traced during 4½ days, and the tracing here given (Fig.
+121) shows that it descended at first in a nearly straight line, but
+afterwards zigzagged, making one or two little loops. The diverging and
+descending movements of a rather older leaf were also traced (see
+former Fig. 113, p. 251): it descended during the first day and night
+in a somewhat zigzag line; it then circumnutated round a small space
+and again descended. By this time the leaf had nearly assumed its final
+position, and now plainly circumnutated. As in the case of the
+Carnation, the leaves, whilst very young, do not seem to be much
+affected by geotropism or heliotropism, for those on a young plant laid
+horizontally, and those on another plant left upright, both kept in the
+dark, continued to diverge in the usual manner without bending to
+either side.
+
+Fig. 121. Pinus pinaster: epinastic downward movement of a young leaf,
+produced by a young plant in a pot, traced on a vertical glass under a
+skylight, from 6.45 A.M. June 2nd to 10.40 P.M. 6th.
+
+With Cobœa scandens, the young leaves, as they successively diverge
+from the leading shoot which is bent to one side, rise up so as to
+project vertically, and they retain this position for some time whilst
+the tendril is revolving. The diverging and ascending movements of the
+petiole of one such a leaf, were traced on a vertical glass under a
+skylight; and the course pursued was in most parts nearly straight, but
+there were two
+well-marked zigzags (one of them forming an angle of 112°), and this
+indicates circumnutation.
+
+The still closed lobes of a young leaf of Dionaea projected at right
+angles to the petiole, and were in the act of slowly rising. A glass
+filament was attached to the under side of the midrib, and its
+movements were traced on a vertical glass. It circumnutated once in the
+evening, and on the next day rose, as already described (see Fig. 106,
+p. 240), by a number of acutely zigzag lines, closely approaching in
+character to ellipses. This movement no doubt was due to epinasty,
+aided by apogeotropism, for the closed lobes of a very young leaf on a
+plant which had been placed horizontally, moved into nearly the same
+line with the petiole, as if the plant had stood upright; but at the
+same time the lobes curved laterally upwards, and thus occupied an
+unnatural position, obliquely to the plane of the foliaceous petiole.
+
+As the hypocotyls and epicotyls of some plants protrude from the
+seed-coats in an arched form, it is doubtful whether the arching of
+these parts, which is invariably present when they break through the
+ground, ought always to be attributed to epinasty; but when they are at
+first straight and afterwards become arched, as often happens, the
+arching is certainly due to epinasty. As long as the arch is surrounded
+by compact earth it must retain its form; but as soon as it rises above
+the surface, or even before this period if artificially freed from the
+surrounding pressure, it begins to straighten itself, and this no doubt
+is mainly due to hyponasty. The movement of the upper and lower half of
+the arch, and of the crown, was occasionally traced; and the course was
+more or less zigzag, showing modified circumnutation.
+
+With not a few plants, especially climbers, the summit of the shoot is
+hooked, so that the apex points vertically downwards. In seven genera
+of twining plants[3] the hooking, or as it has been called by Sachs,
+the nutation of the tip, is mainly due to an exaggerated form of
+circumnutation. That is, the growth is so great along one side that it
+bends the shoot completely over to the opposite side, thus forming a
+hook; the longitudinal line or zone of growth then travels a little
+laterally round the shoot, and the hook points in a slightly different
+direction, and so onwards until the hook is completely reversed.
+Ultimately it
+comes back to the point whence it started. This was ascertained by
+painting narrow lines with Indian ink along the convex surface of
+several hooks, and the line was found slowly to become at first
+lateral, then to appear along the concave surface, and ultimately back
+again on the convex surface. In the case of Lonicera brachypoda the
+hooked terminal part of the revolving shoot straightens itself
+periodically, but is never reversed; that is, the periodically
+increased growth of the concave side of the hook is sufficient only to
+straighten it, and not to bend it over to the opposite side. The
+hooking of the tip is of service to twining plants by aiding them to
+catch hold of a support, and afterwards by enabling this part to
+embrace the support much more closely than it could otherwise have done
+at first, thus preventing it, as we often observed, from being blown
+away by a strong wind. Whether the advantage thus gained by twining
+plants accounts for their summits being so frequently hooked, we do not
+know, as this structure is not very rare with plants which do not
+climb, and with some climbers (for instance, Vitis, Ampelopsis, Cissus,
+etc.) to whom it does not afford any assistance in climbing.
+
+ [3] ‘The Movements and Habits of Climbing Plants,’ 2nd edit. p. 13.
+
+
+With respect to those cases in which the tip remains always bent or
+hooked towards the same side, as in the genera just named, the most
+obvious explanation is that the bending is due to continued growth in
+excess along the convex side. Wiesner, however, maintains[4] that in
+all cases the hooking of the tip is the result of its plasticity and
+weight,—a conclusion which from what we have already seen with several
+climbing plants is certainly erroneous. Nevertheless, we fully admit
+that the weight of the part, as well as geotropism, etc., sometimes
+come into play.
+
+ [4] ‘Sitzb. der k. Akad. der Wissensch.,’ Vienna, Jan. 1880, p. 16.
+
+
+Ampelopsis tricuspidata.—This plant climbs by the aid of adhesive
+tendrils, and the hooked tips of the shoots do not appear to be of any
+service to it. The hooking depends chiefly, as far as we could
+ascertain, on the tip being affected by epinasty and geotropism; the
+lower and older parts continually straightening themselves through
+hyponasty and apogeotropism. We believe that the weight of the apex is
+an unimportant element, because on horizontal or inclined shoots the
+hook is often extended horizontally or even faces upwards. Moreover
+shoots frequently form loops instead of hooks; and in this case the
+extreme part, instead of hanging vertically down as would follow if
+weight was the efficient cause, extends horizontally or even points
+upwards. A shoot, which terminated in a rather open hook, was fastened
+in a highly inclined downward position, so that the concave side faced
+upwards, and the result was that the apex at first curved upwards. This
+apparently was due to epinasty and not to apogeotropism, for the apex,
+soon after passing the perpendicular, curved so rapidly downwards that
+we could not doubt that the movement was at least aided by geotropism.
+In the course of a few hours the hook was thus converted into a loop
+with the apex of the shoot pointing straight downwards. The longer axis
+of the loop was at first horizontal, but afterwards became vertical.
+During this same time the basal part of the hook (and subsequently of
+the loop) curved itself slowly upwards; and this must have been wholly
+due to apogeotropism in opposition to hyponasty. The loop was then
+fastened upside down, so that its basal half would be simultaneously
+acted on by hyponasty (if present) and by apogeotropism; and now it
+curved itself so greatly upwards in the course of only 4 h. that there
+could hardly be a doubt that both forces were acting
+together. At the same time the loop became open and was thus
+reconverted into a hook, and this apparently was effected by the
+geotropic movement of the apex in opposition to epinasty. In the case
+of Ampelopsis hederacea, weight plays, as far as we could judge, a more
+important part in the hooking of the tip.
+
+Fig. 122. Ampelopsis tricuspidata: hyponastic movement of hooked tip of
+leading shoot, traced from 8.10 A.M. July 13th to 8 A.M. 15th. Apex of
+shoot 5½ inches from the vertical glass. Plant illuminated through a
+skylight. Temp. 17½°–19° C. Diagram reduced to one-third of original
+scale.
+
+Fig. 123. Smithia Pfundii: hyponastic movement of the curved summit of
+a stem, whilst straightening itself, traced from 9 A.M. July 10th to 3
+P.M. 13th. Apex 9½ inches from the vertical glass. Diagram reduced to
+one-fifth of original scale. Plant illuminated through skylight; temp.
+17½°–19° C.
+
+In order to ascertain whether the shoots of A. tricuspidata in
+straightening themselves under the combined action of hyponasty and
+apogeotropism moved in a simple straight course, or whether they
+circumnutated, glass filaments were fixed to the crowns of four hooked
+tips standing in their natural position; and the movements of the
+filaments were traced on a vertical glass. All four tracings resembled
+each other in a general manner; but we will give only one (see Fig.
+122, p. 273). The filament rose at first, which shows that the hook was
+straightening itself; it then zigzagged, moving a little to the left
+between 9.25 A.M. and 9 P.M. From this latter hour on the 13th to 10.50
+A.M. on the following morning (14th) the hook continued to straighten
+itself, and then zigzagged a short distance to the right. But from 1
+P.M. to 10.40 P.M. on the 14th the movement
+was reversed and the shoot became more hooked. During the night, after
+10.40 P.M. to 8.15 A.M. on the 15th, the hook again opened or
+straightened itself. By this time the glass filament had become so
+highly inclined that its movements could no longer be traced with
+accuracy; and by 1.30 P.M. on this same day, the crown of the former
+arch or hook had become perfectly straight and vertical. There can
+therefore be no doubt that the straightening of the hooked shoot of
+this plant is effected by the circumnutation of the arched portion—that
+is, by growth alternating between the upper and lower surface, but
+preponderant on the lower surface, with some little lateral movement.
+
+We were enabled to trace the movement of another straightening shoot
+for a longer period (owing to its slower growth and to its having been
+placed further from the vertical glass), namely, from the early morning
+on July 13th to late in the evening of the 16th. During the whole
+daytime of the 14th, the hook straightened itself very little, but
+zigzagged and plainly circumnutated about nearly the same spot. By the
+16th it had become nearly straight, and the tracing was no longer
+accurate, yet it was manifest that there was still a considerable
+amount of movement both up and down and laterally; for the crown whilst
+continuing to straighten itself occasionally became for a short time
+more curved, causing the filament to descend twice during the day.
+
+Smithia Pfundii.—The stiff terminal shoots of this Leguminous
+water-plant from Africa project so as to make a rectangle with the stem
+below; but this occurs only when the plants are growing vigorously, for
+when kept in a cool place, the summits of the stems become straight, as
+they likewise did at the close of the growing season. The direction of
+the rectangularly bent part is independent of the chief source of
+light. But from observing the effects of placing plants in the dark, in
+which case several shoots became in two or three days upright or nearly
+upright, and when brought back into the light again became
+rectangularly curved, we believe that the bending is in part due to
+apheliotropism, apparently somewhat opposed by apogeotropism. On the
+other hand, from observing the effects of tying a shoot downwards, so
+that the rectangle faced upwards, we are led to believe that the
+curvature is partly due to epinasty. As the rectangularly bent portion
+of an upright stem grows older, the lower part straightens itself; and
+this is effected through hyponasty. He who has read Sachs’ recent Essay
+on the vertical
+and inclined positions of the parts of plants[5] will see how difficult
+a subject this is, and will feel no surprise at our expressing
+ourselves doubtfully in this and other such cases.
+
+ [5] ‘Ueber Orthotrope und Plagiotrope Pflanzentheile;’ ‘Arbeiten des
+ Bot. Inst., in Würzburg,’ Heft ii. 1879, p. 226.
+
+
+A plant, 20 inches in height, was secured to a stick close beneath the
+curved summit, which formed rather less than a rectangle with the stem
+below. The shoot pointed away from the observer; and a glass filament
+pointing towards the vertical glass on which the tracing was made, was
+fixed to the convex surface of the curved portion. Therefore the
+descending lines in the figure represent the straightening of the
+curved portion as it grew older. The tracing (Fig. 123, p. 274) was
+begun at 9 A.M. on July 10th; the filament at first moved but little in
+a zigzag line, but at 2 P.M. it began rising and continued to do so
+till 9 P.M.; and this proves that the terminal portion was being more
+bent downwards. After 9 P.M. on the 10th an opposite movement
+commenced, and the curved portion began to straighten itself, and this
+continued till 11.10 A.M. on the 12th, but was interrupted by some
+small oscillations and zigzags, showing movement in different
+directions. After 11.10 A.M. on the 12th this part of the stem, still
+considerably curved, circumnutated in a conspicuous manner until nearly
+3 P.M. on the 13th; but during all this time a downward movement of the
+filament prevailed, caused by the continued straightening of the stem.
+By the afternoon of the 13th, the summit, which had originally been
+deflected more than a right angle from the perpendicular, had grown so
+nearly straight that the tracing could no longer be continued on the
+vertical glass. There can therefore be no doubt that the straightening
+of the abruptly curved portion of the growing stem of this plant, which
+appears to be wholly due to hyponasty, is the result of modified
+circumnutation. We will only add that a filament was fixed in a
+different manner across the curved summit of another plant, and the
+same general kind of movement was observed.
+
+Trifolium repens.—In many, but not in all the species of Trifolium, as
+the separate little flowers wither, the sub-peduncles bend downwards,
+so as to depend parallel to the upper part of the main peduncle. In Tr.
+subterraneum the main peduncle curves downwards for the sake of burying
+its capsules, and in this species the sub-peduncles of the separate
+flowers bend
+
+upwards, so as to occupy the same position relatively to the upper part
+of the main peduncle as in Tr. repens. This fact alone would render it
+probable that the movements of the sub-peduncles in Tr. repens were
+independent of geotropism. Nevertheless, to make sure, some
+flower-heads were tied to little sticks upside down and others in a
+horizontal position; their sub-peduncles, however, all quickly curved
+upwards through the action of heliotropism. We therefore protected some
+flower-heads, similarly secured to sticks, from the light, and although
+some of them rotted, many of their sub-peduncles turned very slowly
+from their reversed or from their horizontal positions, so as to stand
+in the normal manner parallel to the upper part of the main peduncle.
+These facts show that the movement is independent of geotropism or
+apheliotropism; it must there[fore] be attributed to epinasty, which
+however is checked, at least as long as the flowers are young, by
+heliotropism. Most of the above flowers were never fertilised owing to
+the exclusion of bees; they consequently withered very slowly, and the
+movements of the sub-peduncles were in like manner much retarded.
+
+Fig. 124. Trifolium repens: circumnutating and epinastic movements of
+the sub-peduncle of a single flower, traced on a vertical glass under a
+skylight, in A from 11.30 A.M. Aug. 27th to 7 A.M. 30th; in B from 7
+A.M. Aug. 30th to a little after 6 P.M. Sept. 8th.
+
+To ascertain the nature of the movement of the sub-peduncle, whilst
+bending downwards, a filament was fixed across the summit of the calyx
+of a not fully expanded and almost upright flower, nearly in the centre
+of the head. The main peduncle was secured to a stick close beneath the
+head. In order to see the marks on the glass filament, a few flowers
+had to be cut away on the lower side of the head. The flower under
+observation at first diverged a little from its upright position, so as
+to occupy the open space caused by the removal of the adjoining
+flowers. This required two days, after which time a new tracing was
+begun (Fig. 124). In A we see the complex circumnutating course pursued
+from 11.30 A.M. Aug. 26th to 7 A.M. on the 30th. The pot was then moved
+a very little to the right, and the tracing (B) was continued without
+interruption from 7 A.M. Aug. 30th to after 6 P.M. Sept. 8th. It should
+be observed that on most of these days, only a single dot was made each
+morning at the same hour. Whenever the flower was observed carefully,
+as on Aug. 30th and Sept. 5th and 6th, it was found to be
+circumnutating over a small space. At last, on Sept. 7th, it began to
+bend downwards, and continued to do so until after 6 P.M. on the 8th,
+and indeed until the morning of the 9th, when its movements could no
+longer be traced on the vertical glass. It was carefully observed
+during the whole of the 8th, and by
+10.30 P.M. it had descended to a point lower down by two-thirds of the
+length of the figure as here given; but from want of space the tracing
+has been copied in B, only to a little after 6 P.M. On the morning of
+the 9th the flower was withered, and the sub-peduncle now stood at an
+angle of 57° beneath the horizon. If the flower had been fertilised it
+would have withered much sooner, and have moved much more quickly. We
+thus see that the sub-peduncle oscillated up and down, or
+circumnutated, during its whole downward epinastic course.
+
+The sub-peduncles of the fertilised and withered flowers of Oxalis
+carnosa likewise bend downwards through epinasty, as will be shown in a
+future chapter; and their downward course is strongly zigzag,
+indicating circumnutation.
+
+The number of instances in which various organs move through epinasty
+or hyponasty, often in combination with other forces, for the most
+diversified purposes, seems to be inexhaustibly great; and from the
+several cases which have been here given, we may safely infer that such
+movements are due to modified circumnutation.
+
+
+
+
+CHAPTER VI.
+MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE:
+SLEEP OF COTYLEDONS.
+
+
+Preliminary sketch of the sleep or nyctitropic movements of
+leaves—Presence of pulvini—The lessening of radiation the final cause
+of nyctitropic movements—Manner of trying experiments on leaves of
+Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the
+cotyledons of Mimosa—Concluding remarks on radiation from leaves—Small
+differences in the conditions make a great difference in the
+result—Description of the nyctitropic position and movements of the
+cotyledons of various plants—List of species—Concluding
+remarks—Independence of the nyctitropic movements of the leaves and
+cotyledons of the same species—Reasons for believing that the movements
+have been acquired for a special purpose.
+
+
+The so-called sleep of leaves is so conspicuous a phenomenon that it
+was observed as early as the time of Pliny;[1] and since Linnæus
+published his famous Essay, ‘Somnus Plantarum,’ it has been the subject
+of several memoirs. Many flowers close at night, and these are likewise
+said to sleep; but we are not here concerned with their movements, for
+although effected by the same mechanism as in the case of young leaves,
+namely, unequal growth on the opposite sides (as first proved by
+Pfeffer), yet they differ essentially in being excited chiefly by
+changes of temperature instead of light; and in being effected, as far
+as we can judge, for a different purpose. Hardly any one supposes that
+there is any real analogy
+between the sleep of animals and that of plants,[2] whether of leaves
+or flowers. It seems therefore, advisable to give a distinct name to
+the so-called sleep-movements of plants. These have also generally been
+confounded, under the term “periodic,” with the slight daily rise and
+fall of leaves, as described in the fourth chapter; and this makes it
+all the more desirable to give some distinct name to sleep-movements.
+Nyctitropism and nyctitropic, i.e. night-turning, may be applied both
+to leaves and flowers, and will be occasionally used by us; but it
+would be best to confine the term to leaves. The leaves of some few
+plants move either upwards or downwards when the sun shines intensely
+on them, and this movement has sometimes been called diurnal sleep; but
+we believe it to be of an essentially different nature from the
+nocturnal movement, and it will be briefly considered in a future
+chapter.
+
+ [1] Pfeffer has given a clear and interesting sketch of the history of
+ this subject in his ‘Die Periodischen Bewegungen der Blattorgane,’
+ 1875, P. 163.
+
+
+ [2] Ch. Royer must, however, be excepted; see ‘Annales des Sc. Nat.’
+ (5th series), Bot. vol. ix. 1868, p. 378.
+
+
+The sleep or nyctitropism of leaves is a large subject, and we think
+that the most convenient plan will be first to give a brief account of
+the position which leaves assume at night, and of the advantages
+apparently thus gained. Afterwards the more remarkable cases will be
+described in detail, with respect to cotyledons in the present chapter,
+and to leaves in the next chapter. Finally, it will be shown that these
+movements result from circumnutation, much modified and regulated by
+the alternations of day and night, or light and darkness; but that they
+are also to a certain extent inherited.
+
+Leaves, when they go to sleep, move either upwards or downwards, or in
+the case of the leaflets of
+compound leaves, forwards, that is, towards the apex of the leaf, or
+backwards, that is, towards its base; or, again, they may rotate on
+their own axes without moving either upwards or downwards. But in
+almost every case the plane of the blade is so placed as to stand
+nearly or quite vertically at night. Therefore the apex, or the base,
+or either lateral edge, may be directed towards the zenith. Moreover,
+the upper surface of each leaf, and more especially of each leaflet, is
+often brought into close contact with that of the opposite one; and
+this is sometimes effected by singularly complicated movements. This
+fact suggests that the upper surface requires more protection than the
+lower one. For instance, the terminal leaflet in Trifolium, after
+turning up at night so as to stand vertically, often continues to bend
+over until the upper surface is directed downwards whilst the lower
+surface is fully exposed to the sky; and an arched roof is thus formed
+over the two lateral leaflets, which have their upper surfaces pressed
+closely together. Here we have the unusual case of one of the leaflets
+not standing vertically, or almost vertically, at night.
+
+Considering that leaves in assuming their nyctitropic positions often
+move through an angle of 90°; that the movement is rapid in the
+evening; that in some cases, as we shall see in the next chapter, it is
+extraordinarily complicated; that with certain seedlings, old enough to
+bear true leaves, the cotyledons move vertically upwards at night,
+whilst at the same time the leaflets move vertically downwards; and
+that in the same genus the leaves or cotyledons of some species move
+upwards, whilst those of other species move downwards;—from these and
+other such facts, it is hardly possible to doubt that plants must
+derive some
+great advantage from such remarkable powers of movement.
+
+The nyctitropic movements of leaves and cotyledons are effected in two
+ways,[3] firstly, by means of pulvini which become, as Pfeffer has
+shown, alternately more turgescent on opposite sides; and secondly, by
+increased growth along one side of the petiole or midrib, and then on
+the opposite side, as was first proved by Batalin.[4] But as it has
+been shown by De Vries[5] that in these latter cases increased growth
+is preceded by the increased turgescence of the cells, the difference
+between the above two means of movement is much diminished, and
+consists chiefly in the turgescence of the cells of a fully developed
+pulvinus, not being followed by growth. When the movements of leaves or
+cotyledons, furnished with a pulvinus and destitute of one, are
+compared, they are seen to be closely similar, and are apparently
+effected for the same purpose. Therefore, with our object in view, it
+does not appear advisable to separate the above two sets of cases into
+two distinct classes. There is, however, one important distinction
+between them, namely, that movements effected by growth on the
+alternate sides, are confined to young growing leaves, whilst those
+effected by means of a pulvinus last for a long time. We have already
+seen well-marked instances of this latter fact with cotyledons, and so
+it is with leaves, as has been observed by Pfeffer and by ourselves.
+The long endurance of the nyctitropic movements when effected by the
+aid of pulvini indicates, in addition to the evidence already advanced,
+the functional
+importance of such movements to the plant. There is another difference
+between the two sets of cases, namely, that there is never, or very
+rarely, any torsion of the leaves, excepting when a pulvinus is
+present;[6] but this statement applies only to periodic and nyctitropic
+movements as may be inferred from other cases given by Frank.[7] The
+fact that the leaves of many plants place themselves at night in widely
+different positions from what they hold during the day, but with the
+one point in common, that their upper surfaces avoid facing the zenith,
+often with the additional fact that they come into close contact with
+opposite leaves or leaflets, clearly indicates, as it seems to us, that
+the object gained is the protection of the upper surfaces from being
+chilled at night by radiation. There is nothing improbable in the upper
+surface needing protection more than the lower, as the two differ in
+function and structure. All gardeners know that plants suffer from
+radiation. It is this and not cold winds which the peasants of Southern
+Europe fear for their olives.[8] Seedlings are often protected from
+radiation by a very thin covering of straw; and fruit-trees on walls by
+a few fir-branches, or even by a fishing-net, suspended over them.
+There is a variety of the gooseberry,[9] the flowers of which from
+being produced before the leaves, are not protected by them from
+radiation, and consequently often fail to yield fruit. An excellent
+observer[10] has remarked
+that one variety of the cherry has the petals of its flowers much
+curled backwards, and after a severe frost all the stigmas were killed;
+whilst at the same time, in another variety with incurved petals, the
+stigmas were not in the least injured.
+
+ [3] This distinction was first pointed out (according to Pfeffer, ‘Die
+ Periodischen Bewegungen der Blattorgane,’ 1875, p. 161) by Dassen in
+ 1837.
+
+
+ [4] ‘Flora,’ 1873, p. 433.
+
+
+ [5] ‘Bot. Zeitung,’ 1879, Dec. 19th, p. 830.
+
+
+ [6] Pfeffer, ‘Die Period. Beweg. der Blattorgane.’ 1875, p. 159.
+
+
+ [7] ‘Die Nat. Wagerechte Richtung von Pflanzentheilen,’ 1870, p. 52
+
+
+ [8] Martins in ‘Bull. Soc. Bot. de France,’ tom. xix. 1872. Wells, in
+ his famous ‘Essay on Dew,’ remarks that an exposed thermometer rises
+ as soon as even a fleecy cloud, high in the sky, passes over the
+ zenith.
+
+
+ [9] ‘Loudon’s Gardener’s Mag.,’ vol. iv. 1828, p. 112.
+
+
+ [10] Mr. Rivers in ‘Gardener’s Chron.,’ 1866, p. 732
+
+
+This view that the sleep of leaves saves them from being chilled at
+night by radiation, would no doubt have occurred to Linnæus, had the
+principle of radiation been then discovered; for he suggests in many
+parts of his ‘Somnus Plantarum’ that the position of the leaves at
+night protects the young stems and buds, and often the young
+inflorescence, against cold winds. We are far from doubting that an
+additional advantage may be thus gained; and we have observed with
+several plants, for instance, Desmodium gyrans, that whilst the blade
+of the leaf sinks vertically down at night, the petiole rises, so that
+the blade has to move through a greater angle in order to assume its
+vertical position than would otherwise have been necessary; but with
+the result that all the leaves on the same plant are crowded together
+as if for mutual protection.
+
+We doubted at first whether radiation would affect in any important
+manner objects so thin as are many cotyledons and leaves, and more
+especially affect differently their upper and lower surfaces; for
+although the temperature of their upper surfaces would undoubtedly fall
+when freely exposed to a clear sky, yet we thought that they would so
+quickly acquire by conduction the temperature of the surrounding air,
+that it could hardly make any sensible difference to them, whether they
+stood horizontally and radiated into the open sky, or vertically and
+radiated chiefly in a lateral direction towards neighbouring plants and
+other objects. We endeavoured, therefore, to ascertain something on
+this head by preventing the leaves
+of several plants from going to sleep, and by exposing to a clear sky
+when the temperature was beneath the freezing-point, these, as well as
+the other leaves on the same plants which had already assumed their
+nocturnal vertical position. Our experiments show that leaves thus
+compelled to remain horizontal at night, suffered much more injury from
+frost than those which were allowed to assume their normal vertical
+position. It may, however, be said that conclusions drawn from such
+observations are not applicable to sleeping plants, the inhabitants of
+countries where frosts do not occur. But in every country, and at all
+seasons, leaves must be exposed to nocturnal chills through radiation,
+which might be in some degree injurious to them, and which they would
+escape by assuming a vertical position.
+
+In our experiments, leaves were prevented from assuming their
+nyctitropic position, generally by being fastened with the finest
+entomological pins (which did not sensibly injure them) to thin sheets
+of cork supported on sticks. But in some instances they were fastened
+down by narrow strips of card, and in others by their petioles being
+passed through slits in the cork. The leaves were at first fastened
+close to the cork, for as this is a bad conductor, and as the leaves
+were not exposed for long periods, we thought that the cork, which had
+been kept in the house, would very slightly warm them; so that if they
+were injured by the frost in a greater degree than the free vertical
+leaves, the evidence would be so much the stronger that the horizontal
+position was injurious. But we found that when there was any slight
+difference in the result, which could be detected only occasionally,
+the leaves which had been fastened closely down suffered rather more
+than those fastened with very long and
+thin pins, so as to stand from ½ to 3/4 inch above the cork. This
+difference in the result, which is in itself curious as showing what a
+very slight difference in the conditions influences the amount of
+injury inflicted, may be attributed, as we believe, to the surrounding
+warmer air not circulating freely beneath the closely pinned leaves and
+thus slightly warming them. This conclusion is supported by some
+analogous facts hereafter to be given.
+
+We will now describe in detail the experiments which were tried. These
+were troublesome from our not being able to predict how much cold the
+leaves of the several species could endure. Many plants had every leaf
+killed, both those which were secured in a horizontal position and
+those which were allowed to sleep—that is, to rise up or sink down
+vertically. Others again had not a single leaf in the least injured,
+and these had to be re-exposed either for a longer time or to a lower
+temperature.
+
+Oxalis acetosella.—A very large pot, thickly covered with between 300
+and 400 leaves, had been kept all winter in the greenhouse. Seven
+leaves were pinned horizontally open, and were exposed on March 16th
+for 2 h. to a clear sky, the temperature on the surrounding grass being
+–4° C. (24° to 25° F.). Next morning all seven leaves were found quite
+killed, so were many of the free ones which had previously gone to
+sleep, and about 100 of them, either dead or browned and injured were
+picked off. Some leaves showed that they had been slightly injured by
+not expanding during the whole of the next day, though they afterwards
+recovered. As all the leaves which were pinned open were killed, and
+only about a third or fourth of the others were either killed or
+injured, we had some little evidence that those which were prevented
+from assuming their vertically dependent position suffered most.
+
+The following night (17th) was clear and almost equally cold (–3° to
+–4° C. on the grass), and the pot was again exposed, but this time for
+only 30 m. Eight leaves had been pinned out,
+and in the morning two of them were dead, whilst not a single other
+leaf on the many plants was even injured.
+
+On the 23rd the pot was exposed for 1 h. 30 m., the temperature on the
+grass being only –2° C., and not one leaf was injured: the pinned open
+leaves, however, all stood from ½ to 3/4 of an inch above the cork.
+
+On the 24th the pot was again placed on the ground and exposed to a
+clear sky for between 35 m. and 40 m. By a mistake the thermometer was
+left on an adjoining sun-dial 3 feet high, instead of being placed on
+the grass; it recorded 25° to 26° F. (–3.3° to –3.8° C.), but when
+looked at after 1 h. had fallen to 22° F. (–5.5° C.); so that the pot
+was perhaps exposed to rather a lower temperature than on the two first
+occasions. Eight leaves had been pinned out, some close to the cork and
+some above it, and on the following morning five of them (i.e. 63 per
+cent.) were found killed. By counting a portion of the leaves we
+estimated that about 250 had been allowed to go to sleep, and of these
+about 20 were killed (i.e. only 8 per cent.), and about 30 injured.
+
+Considering these cases, there can be no doubt that the leaves of this
+Oxalis, when allowed to assume their normal vertically dependent
+position at night, suffer much less from frost than those (23 in
+number) which had their upper surfaces exposed to the zenith.
+
+Oxalis carnosa.—A plant of this Chilian species was exposed for 30 m.
+to a clear sky, the thermometer on the grass standing at –2° C., with
+some of its leaves pinned open, and not one leaf on the whole bushy
+plant was in the least injured. On the 16th of March another plant was
+similarly exposed for 30 m., when the temperature on the grass was only
+a little lower, viz., –3° to –4° C. Six of the leaves had been pinned
+open, and next morning five of them were found much browned. The plant
+was a large one, and none of the free leaves, which were asleep and
+depended vertically, were browned, excepting four very young ones. But
+three other leaves, though not browned, were in a rather flaccid
+condition, and retained their nocturnal position during the whole of
+the following day. In this case it was obvious that the leaves which
+were exposed horizontally to the zenith suffered most. This same pot
+was afterwards exposed for 35–40 m. on a slightly colder night, and
+every leaf, both the pinned open and the free ones, was killed. It may
+be added that two pots of O. corniculata (var.
+Atro-purpurea) were exposed for 2 h. and 3 h. to a clear sky with the
+temp. on grass –2° C., and none of the leaves, whether free or pinned
+open, were at all injured.
+
+Arachis hypogoea.—Some plants in a pot were exposed at night for 30 m.
+to a clear sky, the temperature on the surrounding grass being –2° C.,
+and on two nights afterwards they were again exposed to the same
+temperature, but this time during 1 h. 30 m. On neither occasion was a
+single leaf, whether pinned open or free, injured; and this surprised
+us much, considering its native tropical African home. Two plants were
+next exposed (March 16th) for 30 m. to a clear sky, the temperature of
+the surrounding grass being now lower, viz., between –3° and –4° C.,
+and all four pinned-open leaves were killed and blackened. These two
+plants bore 22 other and free leaves (excluding some very young
+bud-like ones) and only two of these were killed and three somewhat
+injured; that is, 23 per cent. were either killed or injured, whereas
+all four pinned-open leaves were utterly killed.
+
+On another night two pots with several plants were exposed for between
+35 m. and 40 m. to a clear sky, and perhaps to a rather lower
+temperature, for a thermometer on a dial, 3 feet high, close by stood
+at –3.3° to –3.8° C. In one pot three leaves were pinned open, and all
+were badly injured; of the 44 free leaves, 26 were injured, that is, 59
+per cent. In the other pot 3 leaves were pinned open and all were
+killed; four other leaves were prevented from sleeping by narrow strips
+of stiff paper gummed across them, and all were killed; of 24 free
+leaves, 10 were killed, 2 much injured, and 12 unhurt; that is, 50 per
+cent. of the free leaves were either killed or much injured. Taking the
+two pots together, we may say that rather more than half of the free
+leaves, which were asleep, were either killed or injured, whilst all
+the ten horizontally extended leaves, which had been prevented from
+going to sleep, were either killed or much injured.
+
+Cassia floribunda.—A bush was exposed at night for 40 m. to a clear
+sky, the temperature on the surrounding grass being –2° C., and not a
+leaf was injured.[11] It was again exposed on
+another night for 1 h., when the temperature of the grass was –4° C.;
+and now all the leaves on a large bush, whether pinned flat open or
+free, were killed, blackened, and shrivelled, with the exception of
+those on one small branch, low down, which was very slightly protected
+by the leaves on the branches above. Another tall bush, with four of
+its large compound leaves pinned out horizontally, was afterwards
+exposed (temp. of surrounding grass exactly the same, viz., –4° C.),
+but only for 30 m. On the following morning every single leaflet on
+these four leaves was dead, with both their upper and lower surfaces
+completely blackened. Of the many free leaves on the bush, only seven
+were blackened, and of these only a single one (which was a younger and
+more tender leaf than any of the pinned ones) had both surfaces of the
+leaflets blackened. The contrast in this latter respect was well shown
+by a free leaf, which stood between two pinned-open ones; for these
+latter had the lower surfaces of their leaflets as black as ink, whilst
+the intermediate free leaf, though badly injured, still retained a
+plain tinge of green on the lower surface of the leaflets. This bush
+exhibited in a striking manner the evil effects of the leaves not being
+allowed to assume at night their normal dependent position; for had
+they all been prevented from doing so, assuredly every single leaf on
+the bush would have been utterly killed by this exposure of only 30 m.
+The leaves whilst sinking downwards in the evening twist round, so that
+the upper surface is turned inwards, and is thus better protected than
+the outwardly turned lower surface. Nevertheless, it was always the
+upper surface which was more blackened than the lower, whenever any
+difference could be perceived between them; but whether this was due to
+the cells near the upper surface being more tender, or merely to their
+containing more chlorophyll, we do not know.
+
+ [11] Cassia laevigata was exposed to a clear sky for 35 m., and C.
+ calliantha (a Guiana species) for 60 m., the temperature on the
+ surrounding grass being –2° C., and neither was in the least injured.
+ But when C. laevigata was exposed for 1 h., the temp. on the
+ surrounding grass being between –3° and –4° C., every leaf was killed.
+
+
+Melilotus officinalis.—A large pot with many plants, which had been
+kept during the winter in the greenhouse, was exposed during 5 h. at
+night to a slight frost and clear sky. Four leaves had been pinned out,
+and these died after a few days; but so did many of the free leaves.
+Therefore nothing certain could be inferred from this trial, though it
+indicated that the horizontally extended leaves suffered most. Another
+large pot with many plants was next exposed for 1 h., the temperature
+on the surrounding grass being lower, viz., -3° to –4° C. Ten leaves
+had been pinned out, and the result was striking, for on the following
+morning all these were found much injured or
+killed, and none of the many free leaves on the several plants were at
+all injured, with the doubtful exception of two or three very young
+ones.
+
+Melilotus Italica.—Six leaves were pinned out horizontally, three with
+their upper and three with their lower surfaces turned to the zenith.
+The plants were exposed for 5 h. to a clear sky, the temperature on
+ground being about –1° C. Next morning the six pinned-open leaves
+seemed more injured even than the younger and more tender free ones on
+the same branches. The exposure, however, had been too long, for after
+an interval of some days many of the free leaves seemed in almost as
+bad a condition as the pinned-out ones. It was not possible to decide
+whether the leaves with their upper or those with their lower surfaces
+turned to the zenith had suffered most.
+
+Melilotus suaveolens.—Some plants with 8 leaves pinned out were exposed
+to a clear sky during 2 h., the temperature on the surrounding grass
+being –2° C. Next morning 6 out of these 8 leaves were in a flaccid
+condition. There were about 150 free leaves on the plant, and none of
+these were injured, except 2 or 3 very young ones. But after two days,
+the plants having been brought back into the greenhouse, the 6
+pinned-out leaves all recovered.
+
+Melilotus Taurica.—Several plants were exposed for 5 h. during two
+nights to a clear sky and slight frost, accompanied by some wind; and 5
+leaves which had been pinned out suffered more than those both above
+and below on the same branches which had gone to sleep. Another pot,
+which had likewise been kept in the greenhouse, was exposed for 35–40
+m. to a clear sky, the temperature of the surrounding grass being
+between –3° and –4° C. Nine leaves had been pinned out, and all of
+these were killed. On the same plants there were 210 free leaves, which
+had been allowed to go to sleep, and of these about 80 were killed,
+i.e. only 38 per cent.
+
+Melilotus Petitpierreana.—The plants were exposed to a clear sky for
+35–40 m.: temperature on surrounding grass –3° to –4° C. Six leaves had
+been pinned out so as to stand about ½ inch above the cork, and four
+had been pinned close to it. These 10 leaves were all killed, but the
+closely pinned ones suffered most, as 4 of the 6 which stood above the
+cork still retained small patches of a green colour. A considerable
+number, but not nearly all, of the free leaves, were killed or much
+injured, whereas all the pinned out ones were killed.
+
+
+Melilotus macrorrhiza.—The plants were exposed in the same manner as in
+the last case. Six leaves had been pinned out horizontally, and five of
+them were killed, that is, 83 percent. We estimated that there were 200
+free leaves on the plants, and of these about 50 were killed and 20
+badly injured, so that about 35 per cent of the free leaves were killed
+or injured.
+
+Lotus aristata.—Six plants were exposed for nearly 5 h. to a clear sky;
+temperature on surrounding grass –1.5° C. Four leaves had been pinned
+out horizontally, and 2 of these suffered more than those above or
+below on the same branches, which had been allowed to go to sleep. It
+is rather a remarkable fact that some plants of Lotus Jacoboeus, an
+inhabitant of so hot a country as the Cape Verde Islands, were exposed
+one night to a clear sky, with the temperature of the surrounding grass
+–2° C., and on a second night for 30 m. with the temperature of the
+grass between –3° and –4° C., and not a single leaf, either the
+pinned-out or free ones, was in the least injured.
+
+Marsilea quadrifoliata.—A large plant of this species—the only
+Cryptogamic plant known to sleep—with some leaves pinned open, was
+exposed for 1 h. 35 m. to a clear sky, the temperature on the
+surrounding ground being –2° C., and not a single leaf was injured.
+After an interval of some days the plant was again exposed for 1 h. to
+a clear sky, with the temperature on the surrounding ground lower,
+viz., –4° C. Six leaves had been pinned out horizontally, and all of
+them were utterly killed. The plant had emitted long trailing stems,
+and these had been wrapped round with a blanket, so as to protect them
+from the frozen ground and from radiation; but a very large number of
+leaves were left freely exposed, which had gone to sleep, and of these
+only 12 were killed. After another interval, the plant, with 9 leaves
+pinned out, was again exposed for 1 h., the temperature on the ground
+being again –4° C. Six of the leaves were killed, and one which did not
+at first appear injured afterwards became streaked with brown. The
+trailing branches, which rested on the frozen ground, had one-half or
+three-quarters of their leaves killed, but of the many other leaves on
+the plant, which alone could be fairly compared with the pinned-out
+ones, none appeared at first sight to have been killed, but on careful
+search 12 were found in this state. After another interval, the plant
+with 9 leaves pinned out, was exposed for 35–40 m. to a clear sky and
+to nearly the same, or perhaps a rather lower, temperature (for the
+thermometer by an accident had been left on a
+sun-dial close by), and 8 of these leaves were killed. Of the free
+leaves (those on the trailing branches not being considered), a good
+many were killed, but their number, compared with the uninjured ones,
+was small. Finally, taking the three trials together, 24 leaves,
+extended horizontally, were exposed to the zenith and to unobstructed
+radiation, and of these 20 were killed and 1 injured; whilst a
+relatively very small proportion of the leaves, which had been allowed
+to go to sleep with their leaflets vertically dependent, were killed or
+injured.
+
+The cotyledons of several plants were prepared for trial, but the
+weather was mild and we succeeded only in a single instance in having
+seedlings of the proper age on nights which were clear and cold. The
+cotyledons of 6 seedlings of Mimosa pudica were fastened open on cork
+and were thus exposed for 1 h. 45 m. to a clear sky, with the
+temperature on the surrounding ground at 29° F.; of these, 3 were
+killed. Two other seedlings, after their cotyledons had risen up and
+had closed together, were bent over and fastened so that they stood
+horizontally, with the lower surface of one cotyledon fully exposed to
+the zenith, and both were killed. Therefore of the 8 seedlings thus
+tried 5, or more than half, were killed. Seven other seedlings with
+their cotyledons in their normal nocturnal position, viz., vertical and
+closed, were exposed at the same time, and of these only 2 were
+killed.[12] Hence it appears, as far as these few trials tell anything,
+that the vertical position at night of the cotyledons of Mimosa pudica
+protects them to a certain degree from the evil effects of radiation
+and cold.
+
+ [12] We were surprised that young seedlings of so tropical a plant as
+ Mimosa pudica were able to resist, as well as they did, exposure for 1
+ hr. 45 m. to a clear sky, the temperature on the surrounding ground
+ being 29° F. It may be added that seedlings of the Indian ‘Cassia
+ pubescens’ were exposed for 1 h. 30 m. to a clear sky, with the temp.
+ on the surrounding ground at –2° C., and they were not in the least
+ injured.
+
+
+Concluding Remarks on the Radiation from Leaves at Night.—We exposed on
+two occasions during the summer to a clear sky several pinned-open
+leaflets of Trifolium pratense, which naturally rise at night, and of
+Oxalis purpurea, which naturally sink at night (the plants growing out
+of doors), and looked at
+them early on several successive mornings, after they had assumed their
+diurnal positions. The difference in the amount of dew on the
+pinned-open leaflets and on those which had gone to sleep was generally
+conspicuous; the latter being sometimes absolutely dry, whilst the
+leaflets which had been horizontal were coated with large beads of dew.
+This shows how much cooler the leaflets fully exposed to the zenith
+must have become, than those which stood almost vertically, either
+upwards or downwards, during the night.
+
+From the several cases above given, there can be no doubt that the
+position of the leaves at night affects their temperature through
+radiation to such a degree, that when exposed to a clear sky during a
+frost, it is a question of life and death. We may therefore admit as
+highly probable, seeing that their nocturnal position is so well
+adapted to lessen radiation, that the object gained by their often
+complicated sleep movements, is to lessen the degree to which they are
+chilled at night. It should be kept in mind that it is especially the
+upper surface which is thus protected, as it is never directed towards
+the zenith, and is often brought into close contact with the upper
+surface of an opposite leaf or leaflet.
+
+We failed to obtain sufficient evidence, whether the better protection
+of the upper surface has been gained from its being more easily injured
+than the lower surface, or from its injury being a greater evil to the
+plant. That there is some difference in constitution between the two
+surfaces is shown by the following cases. Cassia floribunda was exposed
+to a clear sky on a sharp frosty night, and several leaflets which had
+assumed their nocturnal dependent position with their lower surfaces
+turned outwards so as to be
+exposed obliquely to the zenith, nevertheless had these lower surfaces
+less blackened than the upper surfaces which were turned inwards and
+were in close contact with those of the opposite leaflets. Again, a pot
+full of plants of Trifolium resupinatum, which had been kept in a warm
+room for three days, was turned out of doors (Sept. 21st) on a clear
+and almost frosty night. Next morning ten of the terminal leaflets were
+examined as opaque objects under the microscope. These leaflets, in
+going to sleep, either turn vertically upwards, or more commonly bend a
+little over the lateral leaflets, so that their lower surfaces are more
+exposed to the zenith than their upper surfaces. Nevertheless, six of
+these ten leaflets were distinctly yellower on the upper than on the
+lower and more exposed surface. In the remaining four, the result was
+not so plain, but certainly whatever difference there was leaned to the
+side of the upper surface having suffered most.
+
+It has been stated that some of the leaflets experimented on were
+fastened close to the cork, and others at a height of from ½ to 3/4 of
+an inch above it; and that whenever, after exposure to a frost, any
+difference could be detected in their states, the closely pinned ones
+had suffered most. We attributed this difference to the air, not cooled
+by radiation, having been prevented from circulating freely beneath the
+closely pinned leaflets. That there was really a difference in the
+temperature of leaves treated in these two different methods, was
+plainly shown on one occasion; for after the exposure of a pot with
+plants of Melilotus dentata for 2 h. to a clear sky (the temperature on
+the surrounding grass being –2° C.), it was manifest that more dew had
+congealed into hoar-frost on the closely pinned leaflets, than on those
+which stood horizontally
+a little above the cork. Again, the tips of some few leaflets, which
+had been pinned close to the cork, projected a little beyond the edge,
+so that the air could circulate freely round them. This occurred with
+six leaflets of Oxalis acetosella, and their tips certainly suffered
+rather less then the rest of the same leaflets; for on the following
+morning they were still slightly green. The same result followed, even
+still more clearly, in two cases with leaflets of Melilotus officinalis
+which projected a little beyond the cork; and in two other cases some
+leaflets which were pinned close to the cork were injured, whilst other
+free leaflets on the same leaves, which had not space to rotate and
+assume their proper vertical position, were not at all injured.
+
+Another analogous fact deserves notice: we observed on several
+occasions that a greater number of free leaves were injured on the
+branches which had been kept motionless by some of their leaves having
+been pinned to the corks, than on the other branches. This was
+conspicuously the case with those of Melilotus Petitpierreana, but the
+injured leaves in this instance were not actually counted. With Arachis
+hypogaea, a young plant with 7 stems bore 22 free leaves, and of these
+5 were injured by the frost, all of which were on two stems, bearing
+four leaves pinned to the cork-supports. With Oxalis carnosa, 7 free
+leaves were injured, and every one of them belonged to a cluster of
+leaves, some of which had been pinned to the cork. We could account for
+these cases only by supposing that the branches which were quite free
+had been slightly waved about by the wind, and that their leaves had
+thus been a little warmed by the surrounding warmer air. If we hold our
+hands motionless before a hot fire, and then wave them about, we
+immediately feel relief; and this is evidently an analogous, though
+reversed, case. These several facts—in relation to leaves pinned close
+to or a little above the cork-supports—to their tips projecting beyond
+it—and to the leaves on branches kept motionless—seem to us curious, as
+showing how a difference, apparently trifling, may determine the
+greater or less injury of the leaves. We may even infer as probable
+that the less or greater destruction during a frost of the leaves on a
+plant which does not sleep, may often depend on the greater or less
+degree of flexibility of their petioles and of the branches which bear
+them.
+
+NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.
+
+We now come to the descriptive part of our work, and will begin with
+cotyledons, passing on to leaves in the next chapter. We have met with
+only two brief notices of cotyledons sleeping. Hofmeister,[13] after
+stating that the cotyledons of all the observed seedlings of the
+Caryophylleae (Alsineae and Sileneae) bend upwards at night (but to
+what angle he does not state), remarks that those of Stellaria media
+rise up so as to touch one another; they may therefore safely be said
+to sleep. Secondly, according to Ramey,[14] the cotyledons of Mimosa
+pudica and of Clianthus Dampieri rise up almost vertically at night and
+approach each other closely. It has been shown in a previous chapter
+that the cotyledons of a large number of plants bend a little upwards
+at night, and we here have to meet the difficult question at what
+inclination may they be said to sleep? According to the view which we
+maintain, no movement deserves to be called
+nyctitropic, unless it has been acquired for the sake of lessening
+radiation; but this could be discovered only by a long series of
+experiments, showing that the leaves of each species suffered from this
+cause, if prevented from sleeping. We must therefore take an arbitrary
+limit. If a cotyledon or leaf is inclined at 60° above or beneath the
+horizon, it exposes to the zenith about one-half of its area;
+consequently the intensity of its radiation will be lessened by about
+half, compared with what it would have been if the cotyledon or leaf
+had remained horizontal. This degree of diminution certainly would make
+a great difference to a plant having a tender constitution. We will
+therefore speak of a cotyledon and hereafter of a leaf as sleeping,
+only when it rises at night to an angle of about 60°, or to a still
+higher angle, above the horizon, or sinks beneath it to the same
+amount. Not but that a lesser diminution of radiation may be
+advantageous to a plant, as in the case of Datura stramonium, the
+cotyledons of which rose from 31° at noon to 55° at night above the
+horizon. The Swedish turnip may profit by the area of its leaves being
+reduced at night by about 30 per cent., as estimated by Mr. A. S.
+Wilson; though in this case the angle through which the leaves rose was
+not observed. On the other hand, when the angular rise of cotyledons or
+of leaves is small, such as less than 30°, the diminution of radiation
+is so slight that it probably is of no significance to the plant in
+relation to radiation. For instance, the cotyledons of Geranium
+Ibericum rose at night to 27° above the horizon, and this would lessen
+radiation by only 11 per cent.: those of Linum Berendieri rose to 33°,
+and this would lessen radiation by 16 per cent.
+
+ [13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327.
+
+
+ [14] ‘Adansonia,’ March 10th, 1869.
+
+
+There are, however, some other sources of doubt with
+respect to the sleep of cotyledons. In certain cases, the cotyledons
+whilst young diverge during the day to only a very moderate extent, so
+that a small rise at night, which we know occurs with the cotyledons of
+many plants, would necessarily cause them to assume a vertical or
+nearly vertical position at night; and in this case it would be rash to
+infer that the movement was effected for any special purpose. On this
+account we hesitated long whether we should introduce several
+Cucurbitaceous plants into the following list; but from reasons,
+presently to be given, we thought that they had better be at least
+temporarily included. This same source of doubt applies in some few
+other cases; for at the commencement of our observations we did not
+always attend sufficiently to whether the cotyledons stood nearly
+horizontally in the middle of the day. With several seedlings, the
+cotyledons assume a highly inclined position at night during so short a
+period of their life, that a doubt naturally arises whether this can be
+of any service to the plant. Nevertheless, in most of the cases given
+in the following list, the cotyledons may be as certainly said to sleep
+as may the leaves of any plant. In two cases, namely with the cabbage
+and radish, the cotyledons of which rise almost vertically during the
+few first nights of their life, it was ascertained by placing young
+seedlings in the klinostat, that the upward movement was not due to
+apogeotropism.
+
+The names of the plants, the cotyledons of which stand at night at an
+angle of at least 60° with the horizon, are arranged in the appended
+list on the same system as previously followed. The numbers of the
+Families, and with the Leguminosae the numbers of the Tribes, have been
+added to show how widely the plants in question are distributed
+throughout the
+dicotyledonous series. A few remarks will have to be made about many of
+the plants in the list. In doing so, it will be convenient not to
+follow strictly any systematic order, but to treat of the Oxalidæ and
+the Leguminosae at the close; for in these two Families the cotyledons
+are generally provided with a pulvinus, and their movements endure for
+a much longer time than those of the other plants in the list.
+
+List of Seedling Plants, the cotyledons of which rise or sink at night
+to an angle of at least 60° above or beneath the horizon.
+
+Brassica oleracea. Cruciferae (Fam. 14). — napus (as we are informed by
+Prof. Pfeffer). Raphanus sativus. Cruciferae. Githago segetum.
+Caryophylleae (Fam. 26). Stellaria media (according to Hofmeister, as
+quoted). Caryophylleae. Anoda Wrightii. Malvaceae (Fam. 36). Gossypium
+(var. Nankin cotton). Malvaceae. Oxalis rosea. Oxalidæ (Fam. 41). —
+floribunda. — articulata. — Valdiviana. — sensitiva. Geranium
+rotundifolium. Geraniaceae (Fam. 47). Trifolium subterraneum.
+Leguminosae (Fam. 75, Tribe 3). — strictum. — leucanthemum. Lotus
+ornithopopoides. Leguminosae (Tribe 4). — peregrinus. — Jacobæus.
+Clianthus Dampieri. Leguminosae (Tribe 5)—according to M. Ramey.
+Smithia sensitiva. Leguminosae (Tribe 6). Haematoxylon Campechianum.
+Leguminosae (Tribe 13)—according to Mr. R. I. Lynch. Cassia mimosoides.
+Leguminosae (Tribe 14). — glauca. — florida. — corymbosa. — pubescens.
+— tora. — neglecta. — 3 other Brazilian unnamed species. Bauhinia
+(sp.?. Leguminosae (Tribe 15). Neptunia oleracea. Leguminosae (Tribe
+20). Mimosa pudica. Leguminosae (Tribe 21). — albida. Cucurbita
+ovifera. Cucurbitaceæ (Fam. 106). — aurantia. Lagenaria vulgaris.
+Cucurbitaceæ. Cucumis dudaim. Cucurbitaceæ. Apium petroselinum.
+Umbelliferae (Fam. 113). — graveolens. Lactuca scariola. Compositæ
+(Fam. 122). Helianthus annuus (?). Compositæ. Ipomœa caerulea.
+Convolvulaceae (Fam. 151). — purpurea. — bona-nox. — coccinea.
+Solanum lycopersicum. Solaneae (Fam. 157.) Mimulus, (sp. ?)
+Scrophularineae (Fam. 159)—from information given us by Prof. Pfeffer.
+Mirabilis jalapa. Nyctagineae (Fam. 177). Mirabilis longiflora. Beta
+vulgaris. Polygoneae (Fam. 179). Amaranthus caudatus. Amaranthaceae
+(Fam. 180). Cannabis sativa (?). Cannabineae (Fam. 195).
+
+Brassica oleracea (Cruciferae).—It was shown in the first chapter that
+the cotyledons of the common cabbage rise in the evening and stand
+vertically up at night with their petioles in contact. But as the two
+cotyledons are of unequal height, they frequently interfere a little
+with each other’s movements, the shorter one often not standing quite
+vertically. They awake early in the morning; thus at 6.45 A.M. on Nov.
+27th, whilst if was still dark, the cotyledons, which had been vertical
+and in contact on the previous evening, were reflexed, and thus
+presented a very different appearance. It should be borne in mind that
+seedlings in germinating at the proper season, would not be subjected
+to darkness at this hour in the morning. The above amount of movement
+of the cotyledons is only temporary, lasting with plants kept in a warm
+greenhouse from four to six days; how long it would last with seedlings
+growing out of doors we do not know.
+
+Raphanus sativus.—In the middle of the day the blades of the cotyledons
+of 10 seedlings stood at right angles to their hypocotyls, with their
+petioles a little divergent; at night the blades stood vertically, with
+their bases in contact and with their petioles parallel. Next morning,
+at 6.45 A.M., whilst it was still dark, the blades were horizontal. On
+the following night they were much raised, but hardly stood
+sufficiently vertical to be said to be asleep, and so it was in a still
+less degree on the third night. Therefore the cotyledons of this plant
+(kept in the greenhouse) go to sleep for even a shorter time than those
+of the cabbage. Similar observations were made, but only during a
+single day and night, on 13 other seedlings likewise raised in the
+greenhouse, with the same result.
+
+The petioles of the cotyledons of 11 young seedlings of Sinapis nigra
+were slightly divergent at noon, and the blades stood at right angles
+to the hypocotyls; at night the petioles were in close contact, and the
+blades considerably raised, with their bases in contact, but only a few
+stood sufficiently upright to be called asleep. On the following
+morning,
+the petioles diverged before it was light. The hypocotyl is slightly
+sensitive, so that if rubbed with a needle it bends towards the rubbed
+side. In the case of Lepidium sativum, the petioles of the cotyledons
+of young seedlings diverge during the day and converge so as to touch
+each other during the night, by which means the bases of the tripartite
+blades are brought into contact; but the blades are so little raised
+that they cannot be said to sleep. The cotyledons of several other
+cruciferous plants were observed, but they did not rise sufficiently
+during the night to be said to sleep.
+
+Githago segetum (Caryophylleae).—On the first day after the cotyledons
+had burst through the seed-coats, they stood at noon at an angle of 75°
+above the horizon; at night they moved upwards, each through an angle
+of 15° so as to stand quite vertical and in contact with one another.
+On the second day they stood at noon at 59° above the horizon, and
+again at night were completely closed, each having risen 31°. On the
+fourth day the cotyledons did not quite close at night. The first and
+succeeding pairs of young true leaves behaved in exactly the same
+manner. We think that the movement in this case may be called
+nyctitropic, though the angle passed through was small. The cotyledons
+are very sensitive to light and will not expand if exposed to an
+extremely dim one.
+
+Anoda Wrightii (Malvaceae).—The cotyledons whilst moderately young, and
+only from .2 to .3 inch in diameter, sink in the evening from their
+mid-day horizontal position to about 35° beneath the horizon. But when
+the same seedlings were older and had produced small true leaves, the
+almost orbicular cotyledons, now .55 inch in diameter, moved vertically
+downwards at night. This fact made us suspect that their sinking might
+be due merely to their weight; but they were not in the least flaccid,
+and when lifted up sprang back through elasticity into their former
+dependent position. A pot with some old seedlings was turned upside
+down in the afternoon, before the nocturnal fall had commenced, and at
+night they assumed in opposition to their own weight (and to any
+geotropic action) an upwardly directed vertical position. When pots
+were thus reversed, after the evening fall had already commenced, the
+sinking movement appeared to be somewhat disturbed; but all their
+movements were occasionally variable without any apparent cause. This
+latter fact, as well as that of the young cotyledons not sinking nearly
+so much as the older ones, deserves notice.
+Although the movement of the cotyledons endured for a long time, no
+pulvinus was exteriorly visible; but their growth continued for a long
+time. The cotyledons appear to be only slightly heliotropic, though the
+hypocotyl is strongly so.
+
+Gossypium arboreum (?) (var. Nankin cotton) (Malvaceae).—The cotyledons
+behave in nearly the same manner as those of the Anoda. On June 15th
+the cotyledons of two seedlings were .65 inch in length (measured along
+the midrib) and stood horizontally at noon; at 10 P.M. they occupied
+the same position and had not fallen at all. On June 23rd, the
+cotyledons of one of these seedlings were 1.1 inch in length, and by 10
+P.M. they had fallen from a horizontal position to 62° beneath the
+horizon. The cotyledons of the other seedling were 1.3 inch in length,
+and a minute true leaf had been formed; they had fallen at 10 P.M. to
+70° beneath the horizon. On June 25th, the true leaf of this latter
+seedling was .9 inch in length, and the cotyledons occupied nearly the
+same position at night. By July 9th the cotyledons appeared very old
+and showed signs of withering; but they stood at noon almost
+horizontally, and at 10 P.M. hung down vertically.
+
+Gossypium herbaceum.—It is remarkable that the cotyledons of this
+species behave differently from those of the last. They were observed
+during 6 weeks from their first development until they had grown to a
+very large size (still appearing fresh and green), viz. 2½ inches in
+breadth. At this age a true leaf had been formed, which with its
+petiole was 2 inches long. During the whole of these 6 weeks the
+cotyledons did not sink at night; yet when old their weight was
+considerable and they were borne by much elongated petioles. Seedlings
+raised from some seed sent us from Naples, behaved in the same manner;
+as did those of a kind cultivated in Alabama and of the Sea-island
+cotton. To what species these three latter forms belong we do not know.
+We could not make out in the case of the Naples cotton, that the
+position of the cotyledons at night was influenced by the soil being
+more or less dry; care being taken that they were not rendered flaccid
+by being too dry. The weight of the large cotyledons of the Alabama and
+Sea-island kinds caused them to hang somewhat downwards, when the pots
+in which they grew were left for a time upside down. It should,
+however, be observed that these three kinds were raised in the middle
+of the winter, which sometimes greatly interferes with the proper
+nyctitropic movements of leaves and cotyledons.
+
+
+Cucurbitaceæ.—The cotyledons of Cucurbita aurantia and ovifera, and of
+Lagenaria vulgaris, stand from the 1st to the 3rd day of their life at
+about 60° above the horizon, and at night rise up so as to become
+vertical and in close contact with one another. With Cucumis dudaim
+they stood at noon at 45° above the horizon, and closed at night. The
+tips of the cotyledons of all these species are, however, reflexed, so
+that this part is fully exposed to the zenith at night; and this fact
+is opposed to the belief that the movement is of the same nature as
+that of sleeping plants. After the first two or three days the
+cotyledons diverge more during the day and cease to close at night.
+Those of Trichosanthes anguina are somewhat thick and fleshy, and did
+not rise at night; and they could perhaps hardly be expected to do so.
+On the other hand, those of _Acanthosicyos horrida_[15] present nothing
+in their appearance opposed to their moving at night in the same manner
+as the preceding species; yet they did not rise up in any plain manner.
+This fact leads to the belief that the nocturnal movements of the
+above-named species has been acquired for some special purpose, which
+may be to protect the young plumule from radiation, by the close
+contact of the whole basal portion of the two cotyledons.
+
+ [15] This plant, from Dammara Land in S. Africa, is remarkable from
+ being the one known member of the Family which is not a climber; it
+ has been described in ‘Transact. Linn. Soc.,’ xxvii. p. 30.
+
+
+Geranium rotundifolium (Geraniaceae).—A single seedling came up
+accidentally in a pot, and its cotyledons were observed to bend
+perpendicularly downwards during several successive nights, having been
+horizontal at noon. It grew into a fine plant but died before
+flowering: it was sent to Kew and pronounced to be certainly a
+Geranium, and in all probability the above-named species. This case is
+remarkable because the cotyledons of G. cinereum, Endressii, Ibericum,
+Richardsoni, and subcaulescens were observed during some weeks in the
+winter, and they did not sink, whilst those of G. Ibericum rose 27° at
+night.
+
+Apium petroselinum (Umbelliferae).—A seedling had its cotyledons (Nov.
+22nd) almost fully expanded during the day; by 8.30 P.M. they had risen
+considerably, and at 10.30 P.M. were almost closed, their tips being
+only 8/100 of an inch apart. On the following morning (23rd) the tips
+were 58/100 of an inch apart,
+or more than seven times as much. On the next night the cotyledons
+occupied nearly the same position as before. On the morning of the 24th
+they stood horizontally, and at night were 60° above the horizon; and
+so it was on the night of the 25th. But four days afterwards (on the
+29th), when the seedlings were a week old, the cotyledons had ceased to
+rise at night to any plain degree.
+
+Apium graveolens.—The cotyledons at noon were horizontal, and at 10
+P.M. stood at an angle of 61° above the horizon.
+
+Lactuca scariola (Compositæ).—The cotyledons whilst young stood
+sub-horizontally during the day, and at night rose so as to be almost
+vertical, and some were quite vertical and closed; but this movement
+ceased when they had grown old and large, after an interval of 11 days.
+
+Helianthus annuus (Compositæ).—This case is rather doubtful; the
+cotyledons rise at night, and on one occasion they stood at 73° above
+the horizon, so that they might then be said to have been asleep.
+
+Ipomœa caerulea vel Pharbitis nil (Convolvulaceae).—The cotyledons
+behave in nearly the same manner as those of the Anoda and Nankin
+cotton, and like them grow to a large size. Whilst young and small, so
+that their blades were from .5 to .6 of an inch in length, measured
+along the middle to the base of the central notch, they remained
+horizontal both during the middle of the day and at night. As they
+increased in size they began to sink more and more in the evening and
+early night; and when they had grown to a length (measured in the above
+manner) of from 1 to 1.25 inch, they sank between 55° and 70° beneath
+the horizon. They acted, however, in this manner only when they had
+been well illuminated during the day. Nevertheless, the cotyledons have
+little or no power of bending towards a lateral light, although the
+hypocotyl is strongly heliotropic. They are not provided with a
+pulvinus, but continue to grow for a long time.
+
+Ipomœa purpurea (vel Pharbitis hispida).—The cotyledons behave in all
+respects like those of I. caerulea. A seedling with cotyledons .75 inch
+in length (measured as before) and 1.65 inch in breadth, having a small
+true leaf developed, was placed at 5.30 P.M. on a klinostat in a
+darkened box, so that neither weight nor geotropism could act on them.
+At 10 P.M. one cotyledon stood at 77° and the other at 82° beneath the
+horizon. Before being placed in the klinostat they stood at 15° and 29°
+beneath the horizon. The nocturnal position depends chiefly on the
+curvature of the petiole close to the blade, but the whole petiole
+becomes slightly curved downwards. It deserves notice that seedlings of
+this and the last-named species were raised at the end of February and
+another lot in the middle of March, and the cotyledons in neither case
+exhibited any nyctitropic movement.
+
+Ipomœa bona-nox.—The cotyledons after a few days grow to an enormous
+size, those on a young seedling being 3 1/4 inches in breadth. They
+were extended horizontally at noon, and at 10 P.M. stood at 63° beneath
+the horizon. five days afterwards they were 4½ inches in breadth, and
+at night one stood at 64° and the other 48° beneath the horizon. Though
+the blades are thin, yet from their great size and from the petioles
+being long, we imagined that their depression at night might be
+determined by their weight; but when the pot was laid horizontally,
+they became curved towards the hypocotyl, which movement could not have
+been in the least aided by their weight, at the same time they were
+somewhat twisted upwards through apogeotropism. Nevertheless, the
+weight of the cotyledons is so far influential, that when on another
+night the pot was turned upside down, they were unable to rise and thus
+to assume their proper nocturnal position.
+
+Ipomœa coccinea.—The cotyledons whilst young do not sink at night, but
+when grown a little older, but still only .4 inch in length (measured
+as before) and .82 in breadth, they became greatly depressed. In one
+case they were horizontal at noon, and at 10 P.M. one of them stood at
+64° and the other at 47° beneath the horizon. The blades are thin, and
+the petioles, which become much curved down at night, are short, so
+that here weight can hardly have produced any effect. With all the
+above species of Ipomœa, when the two cotyledons on the same seedling
+were unequally depressed at night, this seemed to depend on the
+position which they had held during the day with reference to the
+light.
+
+Solanum lycopersicum (Solaneae).—The cotyledons rise so much at night
+as to come nearly in contact. Those of ‘S. palinacanthum’ were
+horizontal at noon, and by 10 P.M. had risen only 27° 30 minutes; but
+on the following morning before it was light they stood at 59° above
+the horizon, and in the afternoon of the same day were again
+horizontal. The behaviour of the cotyledons of this latter species
+seems, therefore, to be anomalous.
+
+
+Mirabilis jalapa and longiflora (Nyctagineae).—The cotyledons, which
+are of unequal size, stand horizontally during the middle of the day,
+and at night rise up vertically and come into close contact with one
+another. But this movement with M. longiflora lasted for only the three
+first nights.
+
+Beta vulgaris (Polygoneae).—A large number of seedlings were observed
+on three occasions. During the day the cotyledons sometimes stood
+sub-horizontally, but more commonly at an angle of about 50° above the
+horizon, and for the first two or three nights they rose up vertically
+so as to be completely closed. During the succeeding one or two nights
+they rose only a little, and afterwards hardly at all.
+
+Amaranthus caudatus (Amaranthaceae).—At noon the cotyledons of many
+seedlings, which had just germinated, stood at about 45° above the
+horizon, and at 10.15 P.M. some were nearly and the others quite
+closed. On the following morning they were again well expanded or open.
+
+Cannabis sativa (Cannabineae).—We are very doubtful whether this plant
+ought to be here included. The cotyledons of a large number of
+seedlings, after being well illuminated during the day, were curved
+downwards at night, so that the tips of some pointed directly to the
+ground, but the basal part did not appear to be at all depressed. On
+the following morning they were again flat and horizontal. the
+cotyledons of many other seedlings were at the same time not in any way
+affected. Therefore this case seems very different from that of
+ordinary sleep, and probably comes under the head of epinasty, as is
+the case with the leaves of this plant according to Kraus. The
+cotyledons are heliotropic, and so is the hypocotyl in a still stronger
+degree.
+
+Oxalis.—We now come to cotyledons provided with a pulvinus, all of
+which are remarkable from the continuance of the nocturnal movements
+during several days or even weeks, and apparently after growth has
+ceased. The cotyledons of O. rosea, floribunda and articulata sink
+vertically down at night and clasp the upper part of the hypocotyl.
+Those of O. Valdiviana and sensitiva, on the contrary, rise vertically
+up, so that their upper surfaces come into close contact; and after the
+young leaves are developed these are clasped by the cotyledons. As in
+the daytime they stand horizontally, or are even a little deflected
+beneath the horizon, they move in the evening through an angle of at
+least 90°. Their complicated circumnutating movements during the day
+have
+been described in the first chapter. The experiment was a superfluous
+one, but pots with seedlings of O. rosea and floribunda were turned
+upside down, as soon as the cotyledons began to show any signs of
+sleep, and this made no difference in their movements.
+
+Leguminosae.—It may be seen in our list that the cotyledons of several
+species in nine genera, widely distributed throughout the Family, sleep
+at night; and this probably is the case with many others. The
+cotyledons of all these species are provided with a pulvinus; and the
+movement in all is continued during many days or weeks. In Cassia the
+cotyledons of the ten species in the list rise up vertically at night
+and come into close contact with one another. We observed that those of
+C. florida opened in the morning rather later than those of C. glauca
+and pubescens. The movement is exactly the same in C. mimosoides as in
+the other species, though its subsequently developed leaves sleep in a
+different manner. The cotyledons of an eleventh species, namely, C.
+nodosa, are thick and fleshy, and do not rise up at night. The
+circumnutation of the cotyledons during the day of C. tora has been
+described in the first chapter. Although the cotyledons of Smithia
+sensitiva rose from a horizontal position in the middle of the day to a
+vertical one at night, those of S. Pfundii, which are thick and fleshy,
+did not sleep. When Mimosa pudica and albida have been kept at a
+sufficiently high temperature during the day, the cotyledons come into
+close contact at night; otherwise they merely rise up almost
+vertically. The circumnutation of those of M. pudica has been
+described. The cotyledons of a Bauhinia from St. Catharina in Brazil
+stood during the day at an angle of about 50° above the horizon, and at
+night rose to 77°; but it is probable that they would have closed
+completely, if the seedlings had been kept in a warmer place.
+
+Lotus.—In three species of Lotus the cotyledons were observed to sleep.
+Those of L. Jacoboeus present the singular case of not rising at night
+in any conspicuous manner for the first 5 or 6 days of their life, and
+the pulvinus is not well developed at this period. Afterwards the
+sleeping movement is well displayed, though to a variable degree, and
+is long continued. We shall hereafter meet with a nearly parallel case
+with the leaves of Sida rhombifolia. The cotyledons of L. Gebelii are
+only slightly raised at night, and differ much in this respect from the
+three species in our list.
+
+
+Trifolium.—The germination of 21 species was observed. In most of them
+the cotyledons rise hardly at all, or only slightly, at night; but
+those of T. glomeratum, striatum and incarnactum rose from 45° to 55°
+above the horizon. With T. subterraneum, leucanthemum and strictum,
+they stood up vertically; and with T. strictum the rising movement is
+accompanied, as we shall see, by another movement, which makes us
+believe that the rising is truly nyctitropic. We did not carefully
+examine the cotyledons of all the species for a pulvinus, but this
+organ was distinctly present in those of T. subterraneum and strictum;
+whilst there was no trace of a pulvinus in some species, for instance,
+in T. resupinatum, the cotyledons of which do not rise at night.
+
+Trifolium subterraneum.—The blades of the cotyledons on the first day
+after germination (Nov. 21st) were not fully expanded, being inclined
+at about 35° above the horizon; at night they rose to about 75°. Two
+days afterwards the blades at noon were horizontal, with the petioles
+highly inclined upwards; and it is remarkable that the nocturnal
+movement is almost wholly confined to the blades, being effected by the
+pulvinus at their bases; whilst the petioles retain day and night
+nearly the same inclination. On this night (Nov. 23rd), and for some
+few succeeding nights, the blades rose from a horizontal into a
+vertical position, and then became bowed inwards at about an average
+angle of 10°; so that they had passed through an angle of 100°. Their
+tips now almost touched one another, their bases being slightly
+divergent. The two blades thus formed a highly inclined roof over the
+axis of the seedling. This movement is the same as that of the terminal
+leaflet of the tripartite leaves of many species of Trifolium. After an
+interval of 8 days (Nov. 29th) the blades were horizontal during the
+day, and vertical at night, and now they were no longer bowed inwards.
+They continued to move in the same manner for the following two months,
+by which time they had increased greatly in size, their petioles being
+no less than .8 of an inch in length, and two true leaves had by this
+time been developed.
+
+Trifolium strictum.—On the first day after germination the cotyledons,
+which are provided with a pulvinus, stood at noon horizontally, and at
+night rose to only about 45° above the horizon. Four days afterwards
+the seedlings were again observed at night, and now the blades stood
+vertically and were in contact, excepting the tips, which were much
+deflexed, so that they faced the zenith. At this age the petioles are
+curved
+upwards, and at night, when the bases of the blades are in contact, the
+two petioles together form a vertical ring surrounding the plumule. The
+cotyledons continued to act in nearly the same manner for 8 or 10 days
+from the period of germination; but the petioles had by this time
+become straight and had increased much in length. After from 12 to 14
+days the first simple true leaf was formed, and during the ensuing
+fortnight a remarkable movement was repeatedly observed. At I. (Fig.
+125) we have a sketch, made in the middle of the day, of a seedling
+about a fortnight old. The two cotyledons, of which Rc is the right and
+Lc the left one, stand directly opposite one another, and the first
+true leaf (F) projects at right angles to them. At night (see II. and
+III.) the right cotyledon (Rc) is greatly raised, but is not otherwise
+changed in position. The left cotyledon (Lc) is likewise raised, but it
+is also twisted so that its blade, instead of exactly facing the
+opposite one, now stands at nearly right angles to it. This nocturnal
+twisting movement is effected not by means of the pulvinus, but by the
+twisting of the whole length of the petiole, as could be seen by the
+curved line of its upper concave surface. At the same time the true
+leaf (F) rises up, so as to stand vertically, or it even passes the
+vertical and is inclined a little inwards. It also twists a little, by
+which means the upper surface of its blade fronts, and almost comes
+into contact with, the upper surface of the twisted
+left cotyledon. This seems to be the object gained by these singular
+movements. Altogether 20 seedlings were examined on successive nights,
+and in 19 of them it was the left cotyledon alone which became twisted,
+with the true leaf always so twisted that its upper surface approached
+closely and fronted that of the left cotyledon. In only one instance
+was the right cotyledon twisted, with the true leaf twisted towards it;
+but this seedling was in an abnormal condition, as the left cotyledon
+did not rise up properly at night. This whole case is remarkable, as
+with the cotyledons of no other plant have we seen any nocturnal
+movement except vertically upwards or downwards. It is the more
+remarkable, because we shall meet with an analogous case in the leaves
+of the allied genus Melilotus, in which the terminal leaflet rotates at
+night so as to present one edge to the zenith and at the same time
+bends to one side, so that its upper surface comes into contact with
+that of one of the two now vertical lateral leaflets.
+
+Fig. 125. Trifolium strictum: diurnal and nocturnal positions of the
+two cotyledons and of the first leaf. I. Seedling viewed obliquely from
+above, during the day: Rc, right cotyledon; Lc, left cotyledon; F,
+first true leaf. II. A rather younger seedling, viewed at night: Rc,
+right cotyledon raised, but its position not otherwise changed; Lc,
+left cotyledon raised and laterally twisted; F, first leaf raised and
+twisted so as to face the left twisted cotyledon. III. Same seedling
+viewed at night from the opposite side. The back of the first leaf, F,
+is here shown instead of the front, as in II.
+
+Concluding Remarks on the Nyctitropic Movements of Cotyledons.—The
+sleep of cotyledons (though this is a subject which has been little
+attended to), seems to be a more common phenomenon than that of leaves.
+We observed the position of the cotyledons during the day and night in
+153 genera, widely distributed throughout the dicotyledonous series,
+but otherwise selected almost by hazard; and one or more species in 26
+of these genera placed their cotyledons at night so as to stand
+vertically or almost vertically, having generally moved through an
+angle of at least 60°. If we lay on one side the Leguminosae, the
+cotyledons of which are particularly liable to sleep, 140 genera
+remain; and out of these, the cotyledons of at least one species in 19
+genera slept. Now if we were to select by hazard 140 genera, excluding
+the Leguminosae, and observed their leaves at night, assuredly not
+nearly so many as 19 would be found to include sleeping species. We
+here refer exclusively to the plants observed by ourselves.
+
+
+In our entire list of seedlings, there are 30 genera, belonging to 16
+Families, the cotyledons of which in some of the species rise or sink
+in the evening or early night, so as to stand at least 60° above or
+beneath the horizon. In a large majority of the genera, namely, 24, the
+movement is a rising one; so that the same direction prevails in these
+nyctitropic movements as in the lesser periodic ones described in the
+second chapter. The cotyledons move downwards during the early part of
+the night in only 6 of the genera; and in one of them, Cannabis, the
+curving down of the tip is probably due to epinasty, as Kraus believes
+to be the case with the leaves. The downward movement to the amount of
+90° is very decided in Oxalis Valdiviana and sensitiva, and in Geranium
+rotundifolium. It is a remarkable fact that with Anoda Wrightii, one
+species of Gossypium and at least 3 species of Ipomœa, the cotyledons
+whilst young and light sink at night very little or not at all;
+although this movement becomes well pronounced as soon as they have
+grown large and heavy. Although the downward movement cannot be
+attributed to the weight of the cotyledons in the several cases which
+were investigated, namely, in those of the Anoda, Ipomœa purpurea and
+bona-nox, nor in that of I. coccinea, yet bearing in mind that
+cotyledons are continually circumnutating, a slight cause might at
+first have determined whether the great nocturnal movement should be
+upwards or downwards. We may therefore suspect that in some aboriginal
+member of the groups in question, the weight of the cotyledons first
+determined the downward direction. The fact of the cotyledons of these
+species not sinking down much whilst they are young and tender, seems
+opposed to the belief that the greater movement when they are
+grown older, has been acquired for the sake of protecting them from
+radiation at night; but then we should remember that there are many
+plants, the leaves of which sleep, whilst the cotyledons do not; and if
+in some cases the leaves are protected from cold at night whilst the
+cotyledons are not protected, so in other cases it may be of more
+importance to the species that the nearly full-grown cotyledons should
+be better protected than the young ones.
+
+In all the species of Oxalis observed by us, the cotyledons are
+provided with pulvini; but this organ has become more or less
+rudimentary in O. corniculata, and the amount of upward movement of its
+cotyledons at night is very variable, but is never enough to be called
+sleep. We omitted to ascertain whether the cotyledons of Geranium
+rotundifolium possess pulvini. In the Leguminosae all the cotyledons
+which sleep, as far as we have seen, are provided with pulvini. But
+with Lotus Jacobæus, these are not fully developed during the first few
+days of the life of the seedling, and the cotyledons do not then rise
+much at night. With Trifolium strictum the blades of the cotyledons
+rise at night by the aid of their pulvini; whilst the petiole of one
+cotyledon twists half-round at the same time, independently of its
+pulvinus.
+
+As a general rule, cotyledons which are provided with pulvini continue
+to rise or sink at night during a much longer period than those
+destitute of this organ. In this latter case the movement no doubt
+depends on alternately greater growth on the upper and lower side of
+the petiole, or of the blade, or of both, preceded probably by the
+increased turgescence of the growing cells. Such movements generally
+last for a very short period—for instance, with Brassica and Githago
+for 4 or 5 nights, with Beta for 2 or 3, and with
+Raphanus for only a single night. There are, however, some strong
+exceptions to this rule, as the cotyledons of Gossypium, Anoda and
+Ipomœa do not possess pulvini, yet continue to move and to grow for a
+long time. We thought at first that when the movement lasted for only 2
+or 3 nights, it could hardly be of any service to the plant, and hardly
+deserved to be called sleep; but as many quickly-growing leaves sleep
+for only a few nights, and as cotyledons are rapidly developed and soon
+complete their growth, this doubt now seems to us not well-founded,
+more especially as these movements are in many instances so strongly
+pronounced. We may here mention another point of similarity between
+sleeping leaves and cotyledons, namely, that some of the latter (for
+instance, those of Cassia and Githago) are easily affected by the
+absence of light; and they then either close, or if closed do not open;
+whereas others (as with the cotyledons of Oxalis) are very little
+affected by light. In the next chapter it will be shown that the
+nyctitropic movements both of cotyledons and leaves consist of a
+modified form of circumnutation.
+
+As in the Leguminosae and Oxalidæ, the leaves and the cotyledons of the
+same species generally sleep, the idea at first naturally occurred to
+us, that the sleep of the cotyledons was merely an early development of
+a habit proper to a more advanced stage of life. But no such
+explanation can be admitted, although there seems to be some
+connection, as might have been expected, between the two sets of cases.
+For the leaves of many plants sleep, whilst their cotyledons do not do
+so—of which fact Desmodium gyrans offers a good instance, as likewise
+do three species of Nicotiana observed by us; also Sida rhombifolia,
+Abutilon Darwinii, and Chenopodium album. On the other
+hand, the cotyledons of some plants sleep and not the leaves, as with
+the species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis,
+named in our list. Still more striking is the fact that, in the same
+genus, the leaves of several or of all the species may sleep, but the
+cotyledons of only some of them, as occurs with Trifolium, Lotus,
+Gossypium, and partially with Oxalis. Again, when both the cotyledons
+and the leaves of the same plant sleep, their movements may be of a
+widely dissimilar nature: thus with Cassia the cotyledons rise
+vertically up at night, whilst their leaves sink down and twist round
+so as to turn their lower surfaces outwards. With seedlings of Oxalis
+Valdiviana, having 2 or 3 well-developed leaves, it was a curious
+spectacle to behold at night each leaflet folded inwards and hanging
+perpendicularly downwards, whilst at the same time and on the same
+plant the cotyledons stood vertically upwards.
+
+These several facts, showing the independence of the nocturnal
+movements of the leaves and cotyledons on the same plant, and on plants
+belonging to the same genus, lead to the belief that the cotyledons
+have acquired their power of movement for some special purpose. Other
+facts lead to the same conclusion, such as the presence of pulvini, by
+the aid of which the nocturnal movement is continued during some weeks.
+In Oxalis the cotyledons of some species move vertically upwards, and
+of others vertically downwards at night; but this great difference
+within the same natural genus is not so surprising as it may at first
+appear, seeing that the cotyledons of all the species are continually
+oscillating up and down during the day, so that a small cause might
+determine whether they should rise or sink at night. Again, the
+peculiar nocturnal movement of the left-hand
+cotyledon of Trifolium strictum, in combination with that of the first
+true leaf. Lastly, the wide distribution in the dicotyledonous series
+of plants with cotyledons which sleep. Reflecting on these several
+facts, our conclusion seems justified, that the nyctitropic movements
+of cotyledons, by which the blade is made to stand either vertically or
+almost vertically upwards or downwards at night, has been acquired, at
+least in most cases, for some special purpose; nor can we doubt that
+this purpose is the protection of the upper surface of the blade, and
+perhaps of the central bud or plumule, from radiation at night.
+
+
+
+
+CHAPTER VII.
+MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.
+
+
+Conditions necessary for these movements—List of Genera and Families,
+which include sleeping plants—Description of the movements in the
+several Genera—Oxalis: leaflets folded at night—Averrhoa: rapid
+movements of the leaflets—Porlieria: leaflets close when plant kept
+very dry—Tropaeolum: leaves do not sleep unless well illuminated during
+day—Lupinus: various modes of sleeping—Melilotus: singular movements of
+terminal leaflet—Trifolium—Desmodium: rudimentary lateral leaflets,
+movements of, not developed on young plants, state of their
+pulvini—Cassia: complex movements of the leaflets—Bauhinia: leaves
+folded at night—Mimosa pudica: compounded movements of leaves, effect
+of darkness—Mimosa albida, reduced leaflets of—Schrankia: downward
+movement of the pinnae—Marsilea: the only cryptogam known to
+sleep—Concluding remarks and summary—Nyctitropism consists of modified
+circumnutation, regulated by the alternations of light and
+darkness—Shape of first true leaves.
+
+
+We now come to the nyctitropic or sleep movements of leaves. It should
+be remembered that we confine this term to leaves which place their
+blades at night either in a vertical position or not more than 30° from
+the vertical,—that is, at least 60° above or beneath the horizon. In
+some few cases this is effected by the rotation of the blade, the
+petiole not being either raised or lowered to any considerable extent.
+The limit of 30° from the vertical is obviously an arbitrary one, and
+has been selected for reasons previously assigned, namely, that when
+the blade approaches the perpendicular as nearly as this, only half as
+much of the surface is exposed at night to the
+zenith and to free radiation as when the blade is horizontal.
+Nevertheless, in a few instances, leaves which seem to be prevented by
+their structure from moving to so great an extent as 60° above or
+beneath the horizon, have been included amongst sleeping plants.
+
+It should be premised that the nyctitropic movements of leaves are
+easily affected by the conditions to which the plants have been
+subjected. If the ground is kept too dry, the movements are much
+delayed or fail: according to Dassen,[1] even if the air is very dry
+the leaves of Impatiens and Malva are rendered motionless. Carl Kraus
+has also lately insisted[2] on the great influence which the quantity
+of water absorbed has on the periodic movements of leaves; and he
+believes that this cause chiefly determines the variable amount of
+sinking of the leaves of Polygonum convolvulus at night; and if so,
+their movements are not in our sense strictly nyctitropic. Plants in
+order to sleep must have been exposed to a proper temperature:
+Erythrina crista-galli, out of doors and nailed against a wall, seemed
+in fairly good health, but the leaflets did not sleep, whilst those on
+another plant kept in a warm greenhouse were all vertically dependent
+at night. In a kitchen-garden the leaflets of Phaseolus vulgaris did
+not sleep during the early part of the summer. Ch. Royer says,[3]
+referring I suppose to the native plants in France, that they do not
+sleep when the temperature is below 5° C. or 41° F. In the case of
+several sleeping plants, viz., species of
+Tropaeolum, Lupinus, Ipomœa, Abutilon, Siegesbeckia, and probably other
+genera, it is indispensable that the leaves should be well illuminated
+during the day in order that they may assume at night a vertical
+position; and it was probably owing to this cause that seedlings of
+Chenopodium album and Siegesbeckia orientalis, raised by us during the
+middle of the winter, though kept at a proper temperature, did not
+sleep. Lastly, violent agitation by a strong wind, during a few
+minutes, of the leaves of Maranta arundinacea (which previously had not
+been disturbed in the hot-house), prevented their sleeping during the
+two next nights.
+
+ [1] Dassen,’Tijdschrift vor. Naturlijke Gesch. en Physiologie,’ 1837,
+ vol. iv. p. 106. See also Ch. Royer on the importance of a proper
+ state of turgescence of the cells, in ‘Annal. des Sc. Nat. Bot.’ (5th
+ series), ix. 1868, p. 345.
+
+
+ [2] ‘Beiträge zur Kentniss der Bewegungen,’ etc., in ‘Flora,’ 1879,
+ pp. 42, 43, 67, etc.
+
+
+ [3] ‘Annal. des Sc. Nat. Bot.’ (5th Series), ix. 1868, p. 366.
+
+
+We will now give our observations on sleeping plants, made in the
+manner described in the Introduction. The stem of the plant was always
+secured (when not stated to the contrary) close to the base of the
+leaf, the movements of which were being observed, so as to prevent the
+stem from circumnutating. As the tracings were made on a vertical glass
+in front of the plant, it was obviously impossible to trace its course
+as soon as the leaf became in the evening greatly inclined either
+upwards or downwards; it must therefore be understood that the broken
+lines in the diagrams, which represent the evening and nocturnal
+courses, ought always to be prolonged to a much greater distance,
+either upwards or downwards, than appears in them. The conclusions
+which may be deduced from our observations will be given near the end
+of this chapter.
+
+In the following list all the genera which include sleeping plants are
+given, as far as known to us. The same arrangement is followed as in
+former cases, and the number of the Family is appended. This list
+possesses some interest, as it shows that the habit of
+sleeping is common to some few plants throughout the whole vascular
+series. The greater number of the genera in the list have been observed
+by ourselves with more or less care; but several are given on the
+authority of others (whose names are appended in the list), and about
+these we have nothing more to say. No doubt the list is very imperfect,
+and several genera might have been added from the ‘Somnus Plantarum’ by
+Linnæus; but we could not judge in some of his cases, whether the
+blades occupied at night a nearly vertical position. He refers to some
+plants as sleeping, for instance, Lathyrus odoratus and Vicia faba, in
+which we could observe no movement deserving to be called sleep, and as
+no one can doubt the accuracy of Linnæus, we are left in doubt.
+
+[List of Genera, including species the leaves of which sleep.
+
+CLASS I. DICOTYLEDONS.
+
+
+Sub-class I. ANGIOSPERMS.
+
+Genus Family.
+
+Githago Caryophylleae (26). Stellaria (Batalin). ” Portulaca
+(Ch.Royer). Portulaceae (27). Sida Malvaceae (36). Abutilon. ” Malva
+(Linnæus and Pfeffer). ” Hibiscus (Linnæus). ” Anoda. ” Gossypium. ”
+Ayenia (Linnæus). Sterculaceae (37). Triumfetta (Linnæus). Tiliaceae
+(38). Linum (Batalin). Lineae (39). Oxalis. Oxalidæ (41). Averrhoa. ”
+Porlieria. Zygophylleæ (45). Guiacum. ” Impatiens (Linnæus, Pfeffer,
+Batalin). Balsamineae (48). Tropaeolum. Tropaeoleae (49). Crotolaria
+(Thiselton Dyer). Leguminosae (75) Tribe II. Lupinus. ” ” Cytisus. ” ”
+Trigonella. ” Tr. III. Medicago. ” Melilotus. ” ” Trifolium. ” ”
+Securigera. ” Tr. IV. Lotus. ” ” Psoralea. ” Tr. V. Amorpha
+(Cuchartre). ” ” Daelea. ” ” Indigofera. ” ” Tephrosia. ” ” Wistaria. ”
+” Robinia. ” ” Sphaerophysa. ” ” Colutea. ” ” Astragalus. ” ”
+Glycyrrhiza. ” ” Coronilla. ” Tr. VI. Hedysarum. ” ”
+Genus Family. Onobrychis. Leguminosae (75) Tr. VI. Smithia. ” ”
+Arachis. ” ” Desmodium. ” ” Urania. ” ” Vicia. ” Tr. VII. Centrosema. ”
+Tr. VIII. Amphicarpæa. ” ” Glycine. ” ” Erythrina. ” ” Apios. ” ”
+Phaseolus. ” ” Sophora. ” Tr. X. Caesalpinia. ” Tr. XIII. Haematoxylon.
+” ” Gleditschia (Duchartre). ” ” Poinciana. ” ” Cassia. ” Tr. XIV.
+Bauhinia. ” Tr. XV. Tamarindus. ” Tr. XVI. Adenanthera. ” Tr. XX.
+Prosopis. ” ” Neptunia. ” ” Mimosa. ” ” Schrankia. ” ” Acacia. ” Tr.
+XXII. Albizzia. ” Tr. XXIII. Melaleuca (Bouché). Myrtaceae (94). Genus
+Family. Aenothera (Linnæus). Omagrarieae (100). Passiflora.
+Passifloracea (105). Siegesbeckia. Compositæ (122). Ipomœa.
+Convolvulacea (151). Nicotiana. Solaneae (157). Mirabilis. Nyctagineae
+(177). Polygonum (Batalin). Polygoneae (179). Amaranthus. Amaranthaceae
+(180). Chenopodium. Chenopodieae (181). Pimelia (Bouché). Thymeteae
+(188). Euphorbia. Euphorbiaceae (202) Phyllanthus (Pfeffer). ”
+
+Sub-class II. GYMNOSPERMS. Aies (Chatin).
+
+CLASS II. MONOCOTYLEDONS.
+
+
+Thalia. Cannaceae (21). Maranta. ” Colocasia. Aroideae (30). Strephium.
+Gramineæ (55).
+
+CLASS III. ACOTYLEDONS.
+
+
+Marsilea. Marsileaceae (4).
+
+Githago segetum (Caryophylleae).—The first leaves produced by young
+seedlings, rise up and close together at night. On a rather older
+seedling, two young leaves stood at noon at 55° above the horizon, and
+at night at 86°, so each had risen 31°. The angle, however, was less in
+some cases. Similar observations were occasionally made on young leaves
+(for the older ones moved very little) produced by nearly full-grown
+plants. Batalin says (‘Flora,’ Oct. 1st, 1873, p. 437) that the young
+leaves of Stellaria close up so completely at night that they form
+together great buds.
+
+Sida (Malvaceae).—the nyctitropic movements of the leaves in this genus
+are remarkable in some respects. Batalin informs
+us (see also ‘Flora,’ Oct. 1st, 1873, p. 437) that those of S. napaea
+fall at night, but to what angle he cannot remember. The leaves of S.
+rhombifolia and retusa, on the other hand, rise up vertically, and are
+pressed against the stem. We have therefore here within the same genus,
+directly opposite movements. Again, the leaves of S. rhombifolia are
+furnished with a pulvinus, formed of a mass of small cells destitute of
+chlorophyll, and with their longer axes perpendicular to the axis of
+the petiole. As measured along this latter line, these cells are only
+1/5th of the length of those of the petiole; but instead of being
+abruptly separated from them (as is usual with the pulvinus in most
+plants), they graduate into the larger cells of the petiole. On the
+other hand, S. napaea, according to Batalin, does not possess a
+pulvinus; and he informs us that a gradation may be traced in the
+several species of the genus between these two states of the petiole.
+Sida rhombifolia presents another peculiarity, of which we have seen no
+other instance with leaves that sleep: for those on very young plants,
+though they rise somewhat in the evening, do not go to sleep, as we
+observed
+on several occasions; whilst those on rather older plants sleep in a
+conspicuous manner. For instance a leaf (.85 of an inch in length) on a
+very young seedling 2 inches high, stood at noon 9° above the horizon,
+and at 10 P.M. at 28°, so it had risen only 19°; another leaf (1.4 inch
+in length) on a seedling of the same height, stood at the same two
+periods at 7° and 32°, and therefore had risen 25°. These leaves, which
+moved so little, had a fairly well-developed pulvinus. After an
+interval of some weeks, when the same seedlings were 2½ and 3 inches in
+height, some of the young leaves stood up at night quite vertically,
+and others were highly inclined; and so it was with bushes which were
+fully grown and were flowering.
+
+Fig. 126. Sida rhombifolia: circumnutation and nyctitropic (or sleep)
+movements of a leaf on a young plant, 9½ inches high; filament fixed to
+midrib of nearly full-grown leaf, 2 3/8 inches in length; movement
+traced under a sky-light. Apex of leaf 5 5/8 inches from the vertical
+glass, so diagram not greatly enlarged.
+
+The movement of a leaf was traced from 9.15 A.M. on May 28th to 8.30
+A.M. on the 30th. The temperature was too low (15°–16° C.), and the
+illumination hardly sufficient; consequently the leaves did not become
+quite so highly inclined at night, as they had done previously and as
+they did subsequently in the hot-house: but the movements did not
+appear otherwise disturbed. On the first day the leaf sank till 5.15
+P.M.; it then rose rapidly and greatly till 10.5 P.M., and only a
+little higher during the rest of the night (Fig. 126). Early on the
+next day (29th) it fell in a slightly zigzag line rapidly until 9 A.M.,
+by which time it had reached nearly the same place as on the previous
+morning. During the remainder of the day it fell slowly, and zigzagged
+laterally. The evening rise began after 4 P.M. in the same manner as
+before, and on the second morning it again fell rapidly. The ascending
+and descending lines do not coincide, as may be seen in the diagram. On
+the 30th a new tracing was made (not here given) on a rather enlarged
+scale, as the apex of the leaf now stood 9 inches from the vertical
+glass. In order to observe more carefully the course pursued at the
+time when the diurnal fall changes into the nocturnal rise, dots were
+made every half-hour between 4 P.M. and 10.30 P.M. This rendered the
+lateral zigzagging movement during the evening more conspicuous than in
+the diagram given, but it was of the same nature as there shown. The
+impression forced on our minds was that the leaf was expending
+superfluous movement, so that the great nocturnal rise might not occur
+at too early an hour.
+
+Abutilon Darwinii (Malvaceae).—The leaves on some very young plants
+stood almost horizontally during the day, and hung down vertically at
+night. Very fine plants kept in a
+large hall, lighted only from the roof, did not sleep at night for in
+order to do so the leaves must be well illuminated during the day. The
+cotyledons do not sleep. Linnæus says that the leaves of his Sida
+abutilon sink perpendicularly down at night, though the petioles rise.
+Prof. Pfeffer informs us that the leaves of a Malva, allied to M.
+sylvestris, rise greatly at night; and this genus, as well as that of
+Hibiscus, are included by Linnæus in his list of sleeping plants.
+
+Anoda Wrightii (Malvaceae).—The leaves, produced by very young plants,
+when grown to a moderate size, sink at night either almost vertically
+down or to an angle of about 45° beneath the horizon; for there is a
+considerable degree of variability in the amount of sinking at night,
+which depends in part on the degree to which they have been illuminated
+during the day. But the leaves, whilst quite young, do not sink down at
+night, and this is a very unusual circumstance. The summit of the
+petiole, where it joins the blade, is developed into a pulvinus, and
+this is present in very young leaves which do not sleep; though it is
+not so well defined as in older leaves.
+
+Gossypium (var. Nankin cotton, Malvaceae).—Some young leaves, between 1
+and 2 inches in length, borne by two seedlings 6 and 7½ inches in
+height, stood horizontally, or were raised a little above the horizon
+at noon on July 8th and 9th; but by 10 P.M. they had sunk down to
+between 68° and 90° beneath the horizon. When the same plants had grown
+to double the above height, their leaves stood at night almost or quite
+vertically dependent. The leaves on some large plants of G. maritimum
+and Brazilense, which were kept in a very badly lighted hot-house, only
+occasionally sank much downwards at night, and hardly enough to be
+called sleep.
+
+Oxalis (Oxalidæ).—In most of the species in this large genus the three
+leaflets sink vertically down at night; but as their sub-petioles are
+short the blades could not assume this position from the want of space,
+unless they were in some manner rendered narrower; and this is effected
+by their becoming more or less folded (Fig. 127). The angle formed by
+the two halves of the same leaflet was found to vary in different
+individuals of several species between 92° and 150°; in three of the
+best folded leaflets of O. fragrans it was 76°, 74°, and 54°. The angle
+is often different in the three leaflets of the same leaf. As the
+leaflets sink down at night and become folded, their lower surfaces are
+brought near together (see B), or even into
+close contact; and from this circumstance it might be thought that the
+object of the folding was the protection of their lower surfaces. If
+this had been the case, it would have formed a strongly marked
+exception to the rule, that when there is any difference in the degree
+of protection from radiation of the two surfaces of the leaves, it is
+always the upper surface which is the best protected. But that the
+folding of the leaflets, and consequent mutual approximation of their
+lower surfaces, serves merely to allow them to sink down vertically,
+may be inferred from the fact that when the leaflets do not radiate
+from the summit of a common petiole, or, again, when there is plenty of
+room from the sub-petioles not being very short, the leaflets sink down
+without becoming folded. This occurs with the leaflets of O. sensitiva,
+Plumierii, and bupleurifolia.
+
+Fig. 127. Oxalis acetosella: A, leaf seen from vertically above; B,
+diagram of leaf asleep, also seen from vertically above.
+
+There is no use in giving a long list of the many species which sleep
+in the above described manner. This holds good with species having
+rather fleshy leaves, like those of O. carnosa, or large leaves like
+those of O. Ortegesii, or four leaflets like those of O. variabilis.
+There are, however, some species which show no signs of sleep, viz., O.
+pentaphylla, enneaphylla, hirta, and rubella. We will now describe the
+nature of the movements in some of the species.
+
+Oxalis acetosella.—The movement of a leaflet, together with that of the
+main petiole, are shown in the following diagram (Fig. 128), traced
+between 11 A.M. on October 4th and 7.45 A.M. on the 5th. After 5.30
+P.M. on the 4th the leaflet sank rapidly, and at 7 P.M. depended
+vertically. for some time before it assumed this latter position, its
+movements could, of course, no longer be traced on the vertical glass,
+and the broken line in the diagram ought to be extended much further
+down in this and all other cases. By 6.45 A.M. on the following morning
+it had risen considerably, and continued to rise for the next hour;
+but, judging from other observations, it would soon have begun to fall
+again. Between 11 A.M. and 5.30 P.M. the leaflet moved at least four
+times up and four times down before the great nocturnal fall commenced;
+it reached its highest point at noon. Similar observations were made on
+two other leaflets, with nearly the same results. Sachs and Pfeffer
+have also described briefly[4] the autonomous movements of the leaves
+of this plant.
+
+ [4] Sachs in ‘Flora,’ 1863, p. 470, etc; Pfeffer, ‘Die Period.
+ Bewegungen,’ etc., 1875, p. 53.
+
+
+Fig 128. Oxalis acetosella: circumnutation and nyctitropic movements of
+a nearly full-grown leaf, with filament attached to the midrib of one
+of the leaflets; traced on vertical glass during 20 h. 45m.
+
+On another occasion the petiole of a leaf was secured to a little stick
+close beneath the leaflets, and a filament tipped with a bead of
+sealing-wax was affixed to the mid-rib of one of them, and a mark was
+placed close behind. At 7 P.M., when the leaflets were asleep, the
+filament depended vertically down, and the movements of the bead were
+then traced till 10.40 P.M., as shown in the following diagram (Fig.
+129). We here see that the leaflet moved a little from side to side, as
+well as a little up and down, whilst asleep.
+
+
+Oxalis Valdiviana.—The leaves resemble those of the last species, and
+the movements of two leaflets (the main petioles of both having been
+secured) were traced during two days; but the tracings are not given,
+as they resembled that of O. acetosella, with the exception that the up
+and down oscillations were not so frequent during the day, and there
+was more lateral movement, so that broader ellipses were described. The
+leaves awoke early in the morning, for by 6.45 A.M. on June 12th and
+13th they had not only risen to their full height, but had already
+begun to fall, that is, they were circumnutating. We have seen in the
+last chapter that the cotyledons, instead of sinking, rise up
+vertically at night.
+
+Fig 129. Oxalis acetosella: circumnutation of leaflet when asleep;
+traced on vertical glass during 3 h. 40 m.
+
+Oxalis Ortegesii.—The large leaves of this plant sleep like those of
+the previous species. The main petioles are long, and that of a young
+leaf rose 20° between noon and 10 P.M., whilst the petiole of an older
+leaf rose only 13°. Owing to this rising of the petioles, and the
+vertical sinking of the large leaflets, the leaves become crowded
+together at night, and the whole plant then exposes a much smaller
+surface to radiation than during the day.
+
+Oxalis Plumierii.—In this species the three leaflets do not surround
+the summit of the petiole, but the terminal leaflet projects in the
+line of the petiole, with a lateral leaflet on each side. They all
+sleep by bending vertically downwards, but do not become at all folded.
+The petiole is rather long, and, one having been secured to a stick,
+the movement of the terminal leaflet was traced during 45 h. on a
+vertical glass. It moved in a very simple manner, sinking rapidly after
+5 P.M., and rising rapidly early next morning. During the middle of the
+day it moved slowly and a little laterally. Consequently the ascending
+and descending lines did not coincide, and a single great ellipse was
+formed each day. There was no other evidence of circumnutation, and
+this fact is of interest, as we shall hereafter see.
+
+Oxalis sensitiva.—The leaflets, as in the last species, bend vertically
+down at night, without becoming folded. The much elongated main petiole
+rises considerably in the evening, but in
+some very young plants the rise did not commence until late at night.
+We have seen that the cotyledons, instead of sinking like the leaflets,
+rise up vertically at night.
+
+Oxalis bupleurifolia.—This species is rendered remarkable by the
+petioles being foliaceous, like the phyllodes of many Acacias. The
+leaflets are small, of a paler green and more tender consistence than
+the foliaceous petioles. The leaflet which was observed was .55 inch in
+length, and was borne by a petiole 2 inches long and .3 inch broad. It
+may be suspected that the leaflets are on the road to abortion or
+obliteration, as has actually occurred with those of another Brazilian
+species, O. rusciformis. Nevertheless, in the present species the
+nyctitropic movements are perfectly performed. The foliaceous petiole
+was first observed during 48 h., and found to be in continued
+circumnutation, as shown in the accompanying figure (Fig. 130). It rose
+during the day and early part of the night, and fell during the
+remainder of the night and early morning; but the movement was not
+sufficient to be called sleep. The ascending and descending lines did
+not coincide, so that an ellipse was formed each day. There was but
+little zigzagging; if the filament had been fixed longitudinally, we
+should probably have seen that there was more lateral movement than
+appears in the diagram.
+
+Fig. 130. Oxalis bupleurifolia: circumnutation of foliaceous petiole,
+filament fixed obliquely across end of petiole; movements traced on
+vertical glass from 9 A.M. June 26th to 8.50 A.M. 28th. Apex of leaflet
+4½ inches from the glass, so movement not much magnified. Plant 9
+inches high, illuminated from above. Temp. 23½°–24½° C.
+
+A terminal leaflet on another leaf was next observed (the petiole being
+secured), and its movements are shown in Fig. 131. During the day the
+leaflets are extended horizontally, and at night depend vertically; and
+as the petiole rises during the day the leaflets have to bend down in
+the evening
+more than 90°, so as to assume at night their vertical position. On the
+first day the leaflet simply moved up and down; on the
+second day it plainly circumnutated between 8 A.M. and 4.30 P.M., after
+which hour the great evening fall commenced.
+
+Fig. 131. Oxalis bupleurifolia: circumnutation and nyctitropic movement
+of terminal leaflet, with filament affixed along the midrib; traced on
+a vertical glass from 9 A.M. on June 26th to 8.45 A.M. 28th. Conditions
+the same as in the last case.
+
+Averrhoa bilimbi (Oxalidæ).—It has long been known,[5] firstly, that
+the leaflets in this genus sleep; secondly, that they move
+spontaneously during the day; and thirdly, that they are sensitive to a
+touch; but in none of these respects do they differ essentially from
+the species of Oxalis. They differ, however, as Mr. R. I. Lynch[6] has
+lately shown, in their spontaneous movements being strongly marked. In
+the case of A. bilimbi, it is a wonderful spectacle to behold on a warm
+sunny day the leaflets one after the other sinking rapidly downwards,
+and again ascending slowly. Their movements rival those of Desmodium
+gyrans. At night the leaflets hang vertically down; and now they are
+motionless, but this may be due to the opposite ones being pressed
+together (Fig. 132). The main petiole is in constant movement during
+the day, but no careful observations were made on it. The following
+diagrams are graphic representations of the variations in the angle,
+which a given leaflet makes with the vertical. The observations were
+made as follows. The plant growing in a pot was kept in a high
+temperature, the petiole of the leaf to be observed pointing straight
+at the observer, being separated from him by a vertical pane of glass.
+The petiole was secured so that the basal joint, or pulvinus, of one of
+the lateral leaflets was at the centre of a graduated arc placed close
+behind the leaflet. A fine glass filament was fixed to the leaf, so as
+to project like a continuation of the
+midrib. This filament acted as an index; and as the leaf rose and fell,
+rotating about its basal joint, its angular movement could be recorded
+by reading off at short intervals of time the position of the glass
+filament on the graduated arc. In order
+to avoid errors of parallax, all readings were made by looking through
+a small ring painted on the vertical glass, in a line with the joint of
+the leaflet and the centre of the graduated arc. In the following
+diagrams the ordinates represent the angles which the leaflet made with
+the vertical at successive instants.[7] It follows that a fall in the
+curve represents an actual dropping of the leaf, and that the zero line
+represents a vertically dependent position. Fig. 133 represents the
+nature of the movements which occur in the evening, as soon as the
+leaflets begin to assume their nocturnal position. At 4.55 P.M. the
+leaflet formed an angle of 85° with the vertical, or was only 5° below
+the horizontal; but in order that the diagram might get into our page,
+the leaflet is represented falling from 75° instead of 85°. Shortly
+after 6 P.M. it hung vertically down, and had attained its nocturnal
+position. Between 6.10 and 6.35 P.M. it performed a number of minute
+oscillations of about 2° each, occupying periods of 4 or 5 m. The
+complete state of rest of the leaflet which ultimately followed is not
+shown in the diagram. It is manifest that each oscillation consists of
+a gradual rise, followed by a sudden fall. Each time the leaflet fell,
+it approached nearer to the nocturnal position than it did on the
+previous fall. The amplitude of the oscillations diminished, while the
+periods of oscillation became shorter.
+
+ [5] Dr. Bruce, ‘Philosophical Trans.,’ 1785, p. 356.
+
+
+ [6] ‘Journal Linn. Soc.,’ vol. xvi. 1877, p. 231.
+
+
+ [7] In all the diagrams 1 mm. in the horizontal direction represents
+ one minute of time. Each mm. in the vertical direction represents one
+ degree of angular movement. In Figs. 133 and 134 the temperature is
+ represented (along the ordinates) in the scale of 1 mm. to each 0.1
+ degree C. In Fig. 135 each mm. equals 0.2° F.
+
+
+Fig. 132. Averrhoa bilimbi: leaf asleep; drawing reduced.
+
+Fig. 133. Averrhoa bilimbi: angular movements of a leaflet during its
+evening descent, when going to sleep. Temp. 78°–81° F.
+
+In bright sunshine the leaflets assume a highly inclined dependent
+position. A leaflet in diffused light was observed rising for 25 m. A
+blind was then pulled up so that the plant was brightly illuminated (BR
+in Fig. 134), and within a minute it began to fall, and ultimately fell
+47°, as shown in the diagram. This descent was performed by six
+descending steps, precisely similar to those by which the nocturnal
+fall is effected. The plant was then again shaded (SH), and a long slow
+rise occurred until another series of falls commenced at BR’, when the
+sun was again admitted. In this experiment cool air was allowed to
+enter by the windows being opened at the same time that the blinds were
+pulled up, so that in spite of the sun shining on the plant the
+temperature was not raised.
+
+The effect of an increase of temperature in diffused light is
+shown in Fig. 135. The temperature began to rise at 11.35 A.M. (in
+consequence of the fire being lighted), but by 12.42 a marked fall had
+occurred. It may be seen in the diagram that when the temperature was
+highest there were rapid oscillations of small amplitude, the mean
+position of the leaflet being at the time nearer the vertical. When the
+temperature began to fall, the oscillations became slower and larger,
+and the mean position of the leaf again approached the horizontal. The
+rate of oscillation was sometimes quicker than is represented in the
+above diagram. Thus, when the temperature was between 31° and
+
+32° C., 14 oscillations of a few degrees occurred in 19 m. On the other
+hand, an oscillation may be much slower; thus a leaflet was observed
+(temperature 25° C.) to rise during 40 m. before it fell and completed
+its oscillation.
+
+Fig. 134. Averrhoa bilimbi: angular movements of leaflet during a
+change from bright illumination to shade; temperature (broken line)
+remaining nearly the same.
+
+Fig. 135. Averrhoa bilimbi: angular movement of leaflet during a change
+of temperature; light remaining the same. The broken line shows the
+change of temperature.
+
+Fig. 136. Porlieria hygrometrica: circumnutation and nyctitropic
+movements of petiole of leaf, traced from 9.35 A.M. July 7th to about
+midnight on the 8th. Apex of leaf 7½ inches from the vertical glass.
+Temp. 19½°–20½° C.
+
+Porlieria hygrometrica (Zygophylleæ).—The leaves of this plant (Chilian
+form) are from 1 to 1½ inch in length, and bear as many as 16 or 17
+small leaflets on each side, which do not stand opposite one another.
+They are articulated to the petiole, and the petiole to the branch by a
+pulvinus. We must premise that apparently two forms are confounded
+under the same name: the leaves on a bush from Chili, which was sent to
+us from Kew, bore many leaflets, whilst those on plants in the Botanic
+Garden at Würzburg bore only 8 or 9 pairs; and the whole character of
+the bushes appeared somewhat different. We shall also see that they
+differ in a remarkable physiological peculiarity. On the Chilian plant
+the petioles of the younger leaves on upright branches, stood
+horizontally during the day, and at night sank down vertically so as to
+depend parallel and close to the branch beneath. The petioles of rather
+older leaves did not become at night vertically depressed, but only
+highly inclined. In one instance we found a branch which had grown
+perpendicularly downwards, and the petioles on it moved in the same
+direction relatively to the branch as just stated, and therefore moved
+upwards. On horizontal branches the younger petioles likewise move at
+night in the same direction as before, that is, towards the branch, and
+are consequently then extended horizontally; but it is remarkable that
+the older petioles on the
+same branch, though moving a little in the same direction, also bend
+downwards; they thus occupy a somewhat different position, relatively
+to the centre of the earth and to the branch, from that of the petioles
+on the upright branches. With respect to the leaflets, they move at
+night towards the apex of the petiole until their midribs stand nearly
+parallel to it; and they then lie neatly imbricated one over the other.
+Thus half of the upper surface of each leaflet is in close contact with
+half of the lower surface of the one next in advance; and all the
+leaflets, excepting the basal ones, have the whole of their upper
+surfaces and half of their lower surfaces well protected. Those on the
+opposite sides of the same petiole do not come into close contact at
+night, as occurs with the leaflets of so many Leguminosae but are
+separated by an open furrow; nor could they exactly coincide, as they
+stand alternately with respect to one another.
+
+The circumnutation of the petiole of a leaf 3/4 of an inch in length,
+on an upright branch, was observed during 36h., and is shown in the
+preceding diagram (Fig. 136). On the first morning, the leaf fell a
+little and then rose until 1 P.M., and this was probably due to its
+being now illuminated through a skylight from above; it then
+circumnutated on a very small scale round the same spot until about 4
+P.M., when the great evening fall commenced. During the latter part of
+the night or very early on the next morning the leaf rose again. On the
+second day it fell during the morning till 1 P.M., and this no doubt is
+its normal habit. From 1 to 4 P.M. it rose in a zigzag line, and soon
+afterwards the great evening fall commenced. It thus completed a double
+oscillation during the 24 h.
+
+The specific name given to this plant by Ruiz and Pavon, indicates that
+in its native arid home it is affected in some manner by the dryness or
+dampness of the atmosphere.[8] In the Botanic Garden at Würzburg, there
+was a plant in a pot out of doors which was daily watered, and another
+in the open ground which was never watered. After some hot and dry
+weather there was a great difference in the state of the leaflets on
+these two plants; those on the unwatered plant in the open ground
+remaining half,
+or even quite, closed during the day. But twigs cut from this bush,
+with their ends standing in water, or wholly immersed in it, or kept in
+damp air under a bell-glass, opened their leaves though exposed to a
+blazing sun; whilst those on the plant in the ground remained closed.
+The leaves on this same plant, after some heavy rain, remained open for
+two days; they then became half closed during two days, and after an
+additional day were quite closed. This plant was now copiously watered,
+and on the following morning the leaflets were fully expanded. The
+other plant growing in a pot, after having been exposed to heavy rain,
+was placed before a window in the Laboratory, with its leaflets open,
+and they remained so during the daytime for 48 h.; but after an
+additional day were half closed. The plant was then watered, and the
+leaflets on the two following days remained open. On the third day they
+were again half closed, but on being again watered remained open during
+the two next days. From these several facts we may conclude that the
+plant soon feels the want of water; and that as soon as this occurs, it
+partially or quite closes its leaflets, which in their then imbricated
+condition expose a small surface to evaporation. It is therefore
+probable that this sleep-like movement, which occurs only when the
+ground is dry, is an adaptation against the loss of moisture.
+
+ [8] ‘Systema Veg. Florae Peruvianae et Chilensis,’ tom. i. p. 95,
+ 1798. We cannot understand the account given by the authors of the
+ behaviour of this plant in its native home. There is much about its
+ power of foretelling changes in the weather; and it appears as if the
+ brightness of the sky largely determined the opening and closing of
+ the leaflets.
+
+
+A bush about 4 feet in height, a native of Chili, which was thickly
+covered with leaves, behaved very differently, for during the day it
+never closed its leaflets. On July 6th the earth in the small pot in
+which it grew appeared extremely dry, and it was given a very little
+water. After 21 and 22 days (on the 27th and 28th), during the whole of
+which time the plant did not receive a drop of water, the leaves began
+to droop, but they showed no signs of closing during the day. It
+appeared almost incredible that any plant, except a fleshy one, could
+have kept alive in soil so dry, which resembled the dust on a road. On
+the 29th, when the bush was shaken, some leaves fell off, and the
+remaining ones were unable to sleep at night. It was therefore
+moderately watered, as well as syringed, late in the evening. On the
+next morning (30th) the bush looked as fresh as ever, and at night the
+leaves went to sleep. It may be added that a small branch while growing
+on the bush was enclosed, by means of a curtain of bladder, during 13
+days in a large bottle half full of quicklime, so that the air within
+must have been intensely dry; yet the leaves on this branch did not
+suffer in the
+least, and did not close at all during the hottest days. Another trial
+was made with the same bush on August 2nd and 6th (the soil appearing
+at this latter date extremely dry), for it was exposed out of doors
+during the whole day to the wind, but the leaflets showed no signs of
+closing. The Chilian form therefore differs widely from the one at
+Würzburg, in not closing its leaflets when suffering from the want of
+water; and it can live for a surprisingly long time without water.
+
+Tropaeolum majus (?) (cultivated var.) (Tropaeoleae).—Several plants in
+pots stood in the greenhouse, and the blades of the leaves which faced
+the front-lights were during the day highly inclined and at night
+vertical; whilst the leaves on the back of the pots, though of course
+illuminated through the roof, did not become vertical at night. We
+thought, at first, that this difference in their positions was in some
+manner due to heliotropism, for the leaves are highly heliotropic. The
+true explanation, however, is that unless they are well illuminated
+during at least a part of the day they do not sleep at night; and a
+little difference in the degree of illumination determines whether or
+not they shall become vertical at night. We have observed no other so
+well-marked a case as this, of the influence of previous illumination
+on nyctitropic movements. The leaves present also another peculiarity
+in their habit of rising or awaking in the morning, being more strongly
+fixed or inherited than that of sinking or sleeping at night. The
+movements are caused by the bending of an upper part of the petiole,
+between ½ and 1 inch in length; but the part close to the blade, for
+about 1/4 of an inch in length, does not bend and always remains at
+right angles to the blade. The bending portion does not present any
+external or internal difference in structure from the rest of the
+petiole. We will now give the experiments on which the above
+conclusions are founded.
+
+A large pot with several plants was brought on the morning of Sept. 3rd
+out of the greenhouse and placed before a north-east window, in the
+same position as before with respect to the light, as far as that was
+possible. On the front of the plants, 24 leaves were marked with
+thread, some of which had their blades horizontal, but the greater
+number were inclined at about 45°, beneath the horizon; at night all
+these, without exception, became vertical. Early on the following
+morning (4th) they reassumed their former positions, and at night again
+became vertical. On the 5th the shutters were opened at 6.15 A.M., and
+by 8.18 A.M., after the leaves had been illuminated for 2 h. 3 m. and
+had acquired their diurnal position, they were placed in a dark
+cupboard. They were looked at twice during the day and thrice in the
+evening, the last time at 10.30 P.M., and not one had become vertical.
+At 8 A.M. on the following morning (6th) they still retained the same
+diurnal position, and were now replaced before the north-east window.
+At night all the leaves which had faced the light had their petioles
+curved and their blades vertical; whereas none of the leaves on the
+back of the plants, although they had been moderately illuminated by
+the diffused light of the room, were vertical. They were now at night
+placed in the same dark cupboard; at 9 A.M. on the next morning (7th)
+all those which had been asleep had reassumed their diurnal position.
+The pot was then placed for 3 h. in the sunshine, so as to stimulate
+the plants; at noon they were placed before the same north-east window,
+and at night the leaves slept in the usual manner and awoke on the
+following morning. At noon on this day (8th) the plants, after having
+been left before the north-east window for 5 h. 45 m. and thus
+illuminated (though not brightly, as the sky was cloudy during the
+whole time), were replaced in the dark cupboard, and at 3 P.M. the
+position of the leaves was very little, if at all, altered, so that
+they are not quickly affected by darkness; but by 10.15 P.M. all the
+leaves which had faced the north-east sky during the 5 h. 45 m. of
+illumination stood vertical, whereas those on the back of the plant
+retained their diurnal position. On the following morning (9th) the
+leaves awoke as on the two former occasions in the dark, and they were
+kept in the dark during the whole day; at night a very few of them
+became vertical, and this was the one instance in which we observed any
+inherited tendency or habit in this plant to sleep at the proper time.
+That it was real sleep was shown by these same leaves reassuming their
+diurnal position on the following morning (10th) whilst still kept in
+the dark.
+
+The pot was then (9.45 A.M. 10th) replaced, after having been kept for
+36 h. in darkness, before the north-east window; and at night the
+blades of all the leaves (excepting a few on the back of the plants)
+became conspicuously vertical.
+
+At 6.45 A.M. (11th) after the plants had been illuminated on the same
+side as before during only 25 m., the pot was turned round, so that the
+leaves which had faced the light now faced the interior of the room,
+and not one of these went to sleep at night;
+whilst some, but not many, of those which had formerly stood facing the
+back of the room and which had never before been well illuminated or
+gone to sleep, now assumed a vertical position at night. On the next
+day (12th) the plant was turned round into its original position, so
+that the same leaves faced the light as formerly, and these now went to
+sleep in the usual manner. We will only add that with some young
+seedlings kept in the greenhouse, the blades of the first pair of true
+leaves (the cotyledons being hypogean) stood during the day almost
+horizontally and at night almost vertically.
+
+A few observations were subsequently made on the circumnutation of
+three leaves, whilst facing a north-east window; but the tracings are
+not given, as the leaves moved somewhat towards the light. It was,
+however, manifest that they rose and fell more than once during the
+daytime, the ascending and descending lines being in parts extremely
+zigzag. The nocturnal fall commenced about 7 P.M., and the leaves had
+risen considerably by 6.45 A.M. on the following morning.
+
+Leguminosae.—This Family includes many more genera with sleeping
+species than all the other families put together. The number of the
+tribes to which each genus belongs, according to Bentham and Hooker’s
+arrangement, has been added.
+
+Crotolaria (sp.?) (Tribe 2).—This plant is monophyllous, and we are
+informed by Mr. T. Thiselton Dyer that the leaves rise up vertically at
+night and press against the stem.
+
+Lupinus (Tribe 2).—The palmate or digitate leaves of the species in
+this large genus sleep in three different manners. One of the simplest,
+is that all the leaflets become steeply inclined downwards at night,
+having been during the day extended horizontally. This is shown in the
+accompanying figures (Fig. 137), of a leaf of L. pilosus, as seen
+during the day from vertically above, and of another leaf asleep with
+the leaflets inclined downwards. As in this position they are crowded
+together, and as they do not become folded like those in the genus
+Oxalis, they cannot occupy a vertically dependent position; but they
+are often inclined at an angle of 50° beneath the horizon. In this
+species, whilst the leaflets are sinking, the petioles rise up, in two
+instances when the angles were measured to the extent of 23°. The
+leaflets of L. sub-carnosus and arboreus, which were horizontal during
+the day, sank down at night in nearly the same manner; the former to an
+angle of 38° and the latter of 36°, beneath the horizon; but their
+petioles
+did not move in any plainly perceptible degree. It is, however, quite
+possible, as we shall presently see, that if a large number of plants
+of the three foregoing and of the following species were to be observed
+at all seasons, some of the leaves would be found to sleep in a
+different manner.
+
+Fig. 137. Lupinus pilosus: A, leaf seen from vertically above in
+daytime; B, leaf asleep, seen laterally at night.
+
+In the two following species the leaflets, instead of moving downwards,
+rise at night. With L. Hartwegii some stood at noon at a mean angle of
+36° above the horizon, and at night at 51°, thus forming together a
+hollow cone with moderately steep sides. The petiole of one leaf rose
+14° and of a second 11° at night. With L. luteus a leaflet rose from
+47° at noon to 65° above the horizon at night, and another on a
+distinct leaf rose from 45° to 69°. The petioles, however, sink at
+night to a small extent, viz., in three instances by 2°, 6°, and 9° 30
+seconds. Owing to this movement of the petioles, the outer and longer
+leaflets have to bend up a little more than the shorter and inner ones,
+in order that all should stand symmetrically at night. We shall
+presently see that some leaves on the same individual plants of L.
+luteus sleep in a very different manner.
+
+We now come to a remarkable position of the leaves when asleep, which
+is common to several species of Lupines. On the same leaf the shorter
+leaflets, which generally face the centre of the plant, sink at night,
+whilst the longer ones on the opposite side rise; the intermediate and
+lateral ones merely twisting on their own axes. But there is some
+variability with respect to which leaflets rise or fall. As might have
+been expected from such diverse and complicated movements, the
+base of each leaflet is developed (at least in the case of L. luteus)
+into a pulvinus. The result is that all the leaflets on the same leaf
+stand at night more or less highly inclined, or even quite vertically,
+forming in this latter case a vertical star. This occurs with the
+leaves of a species purchased under the name of L. pubescens; and in
+the accompanying figures we see at A (Fig. 138) the leaves in their
+diurnal position; and at B the same plant at night with the two upper
+leaves having their leaflets almost vertical. At C another leaf, viewed
+laterally, is shown with the leaflets quite vertical. It is chiefly or
+exclusively the youngest leaves which form at night vertical stars. But
+there
+is much variability in the position of the leaves at night on the same
+plant; some remaining with their leaflets almost horizontal, others
+forming more or less highly inclined or vertical stars, and some with
+all their leaflets sloping downwards, as in our first class of cases.
+It is also a remarkable fact, that although all the plants produced
+from the same lot of seeds were identical in appearance, yet some
+individuals at night had the leaflets of all their leaves arranged so
+as to form more or less highly inclined stars; others had them all
+sloping downwards and never forming a star; and others, again, retained
+them either in a horizontal position or raised them a little.
+
+Fig. 138. Lupinus pubescens: A, leaf viewed laterally during the day;
+B, same leaf at night; C, another leaf with the leaflet forming a
+vertical star at night. Figures reduced.
+
+We have as yet referred only to the different positions of the leaflets
+of L. pubescens at night; but the petioles likewise differ in their
+movements. That of a young leaf which formed a highly inclined star at
+night, stood at noon at 42° above the horizon, and during the night at
+72°, so had risen 30°. The petiole of another leaf, the leaflets of
+which occupied a similar position at night, rose only 6°. On the other
+hand, the petiole of a leaf with all its leaflets sloping down at
+night, fell at this time 4°. The petioles of two rather older leaves
+were subsequently observed; both of which stood during the day at
+exactly the same angle, viz., 50° above the horizon, and one of these
+rose 7°–8°, and the other fell 3°–4° at night. We meet with cases like
+that of L. pubescens with some other species. On a single plant of L.
+mutabilis some leaves, which stood horizontally during the day, formed
+highly inclined stars at night, and the petiole of one rose 7°. Other
+leaves which likewise stood horizontally during the day, had at night
+all their leaflets sloping downwards at 46° beneath the horizon, but
+their petioles had hardly moved. Again, L. luteus offered a still more
+remarkable case, for on two leaves, the leaflets which stood at noon at
+about 45° above the horizon, rose at night to 65° and 69°, so that they
+formed a hollow cone with steep sides. Four leaves on the same plant,
+which had their leaflets horizontal at noon, formed vertical stars at
+night; and three other leaves equally horizontal at noon, had all their
+leaflets sloping downwards at night. So that the leaves on this one
+plant assumed at night three different positions. Though we cannot
+account for this fact, we can see that such a stock might readily give
+birth to species having widely different nyctitropic habits.
+
+Little more need be said about the sleep of the species of Lupinus;
+several, namely, L. polyphyllus, nanus, Menziesii, speciosus,
+and albifrons, though observed out of doors and in the greenhouse, did
+not change the position of their leaves sufficiently at night to be
+said to sleep. From observations made on two sleeping species, it
+appears that, as with Tropaeolum majus, the leaves must be well
+illuminated during the day in order to sleep at night. For several
+plants, kept all day in a sitting-room with north-east windows, did not
+sleep at night; but when the pots were placed on the following day out
+of doors, and were brought in at night, they slept in the usual manner.
+the trial was repeated on the following day and night with the same
+result.
+
+Some observations were made on the circumnutation of the leaves of L.
+luteus and arboreus. It will suffice to say that the leaflets of the
+latter exhibited a double oscillation in the course of 24 h.; for they
+fell from the early morning until 10.15 A.M., then rose and zigzagged
+greatly till 4 P.M., after which hour the great nocturnal fall
+commenced. By 8 A.M. on the following morning the leaflets had risen to
+their proper height. We have seen in the fourth chapter, that the
+leaves of Lupinus speciosus, which do not sleep, circumnutate to an
+extraordinary extent, making many ellipses in the course of the day.
+
+Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).—Only a few
+observations were made on these three genera. The petioles on a young
+plant, about a foot in height, of Cytisus fragrans rose at night, on
+one occasion 23° and on another 33°. The three leaflets also bend
+upwards, and at the same time
+approach each other, so that the base of the central leaflet overlaps
+the bases of the two lateral leaflets. They bend up so much that they
+press against the stem; and on looking down on one of these young
+plants from vertically above, the lower surfaces of the leaflets are
+visible; and thus their upper surfaces, in accordance with the general
+rule, are best protected from radiation. Whilst the leaves on these
+young plants were thus behaving, those on an old bush in full flower
+did not sleep at night.
+
+Fig. 139. Medicago marina: A, leaves during the day; B, leaves asleep
+at night.
+
+Trigonella Cretica resembles a Melilotus in its sleep, which will be
+immediately described. According to M. Royer,[9] the leaves of Medicago
+maculata rise up at night, and “se renversent un peu de manière à
+presenter obliquement au ciel leur face inférieure.” A drawing is here
+given (Fig. 139) of the leaves of M. marina awake and asleep; and this
+would almost serve for Cytisus fragrans in the same two states.
+
+ [9] ‘Annales des Sc. Nat. Bot.’ (5th series), ix. 1868, p. 368.
+
+
+Melilotus (Tribe 3).—The species in this genus sleep in a remarkable
+manner. The three leaflets of each leaf twist through an angle of 90°,
+so that their blades stand vertically at night with one lateral edge
+presented to the zenith (Fig. 140). We shall best understand the other
+and more complicated movements, if we imagine ourselves always to hold
+the leaf with the tip of the terminal leaflet pointed to the north. The
+leaflets in becoming vertical at night could of course twist so that
+their upper surfaces should face to either side; but the two lateral
+leaflets always twist so that this surface tends to face the north, but
+as they move at the same time towards the terminal leaflet, the upper
+surface of the one faces about N.N.W., and that of the other N.N.E. The
+terminal leaflet behaves differently, for it twists to either side, the
+upper surface facing sometimes east and sometimes west, but rather more
+commonly west than east. The terminal leaflet also moves in another and
+more remarkable manner, for whilst its blade is twisting and becoming
+vertical, the whole leaflet bends to one side, and invariably to the
+side towards which the upper surface is directed; so that if this
+surface faces the west the whole leaflet bends to the west, until it
+comes into contact with the upper and vertical surface of the western
+lateral leaflet. Thus the upper surface of the terminal and of one of
+the two lateral leaflets is well protected.
+
+The fact of the terminal leaflet twisting indifferently to either
+side and afterwards bending to the same side, seemed to us so
+remarkable, that we endeavoured to discover the cause. We imagined that
+at the commencement of the movement it might be determined by one of
+the two halves of the leaflet being a little heavier than the other.
+Therefore bits of wood were gummed on one side of several leaflets, but
+this produced no effect; and they continued to twist in the same
+direction as they had previously done. In order to discover whether the
+same leaflet twisted permanently in the same direction, black threads
+were tied to 20 leaves, the terminal leaflets of which twisted so that
+their upper surfaces faced west, and 14 white threads to leaflets which
+twisted to the east. These were observed occasionally during 14 days,
+and they all continued, with a single exception, to twist and bend in
+the same direction; for
+one leaflet, which had originally faced east, was observed after 9 days
+to face west. The seat of both the twisting and bending movement is in
+the pulvinus of the sub-petioles.
+
+Fig. 140. Melilotus officinalis: A, leaf during the daytime. B, another
+leaf asleep. C, a leaf asleep as viewed from vertically above; but in
+this case the terminal leaflet did not happen to be in such close
+contact with the lateral one, as is usual.
+
+We believe that the leaflets, especially the two lateral ones, in
+performing the above described complicated movements generally bend a
+little downwards; but we are not sure of this, for, as far as the main
+petiole is concerned, its nocturnal movement is largely determined by
+the position which the leaf happens to occupy during the day. Thus one
+main petiole was observed to rise at night 59°, whilst three others
+rose only 7° and 9°. The petioles and sub-petioles are continually
+circumnutating during the whole 24 h., as we shall presently see.
+
+The leaves of the following 15 species, M. officinalis, suaveolens,
+parviflora, alba, infesta, dentata, gracilis, sulcata, elegans,
+coerulea, petitpierreana, macrorrhiza, Italica, secundiflora, and
+Taurica, sleep in nearly the same manner as just described; but the
+bending to one side of the terminal leaflet is apt to fail unless the
+plants are growing vigorously. With M. petitpierreana and secundiflora
+the terminal leaflet was rarely seen to bend to one side. In young
+plants of M. Italica it bent in the usual manner, but with old plants
+in full flower, growing in the same pot and observed at the same hour,
+viz., 8.30 P.M., none of the terminal leaflets on several scores of
+leaves had bent to one side, though they stood vertically; nor had the
+two lateral leaflets, though standing vertically, moved towards the
+terminal one. At 10.30 P.M., and again one hour after midnight, the
+terminal leaflets had become very slightly bent to one side, and the
+lateral leaflets had moved a very little towards the terminal one, so
+that the position of the leaflets even at this late hour was far from
+the ordinary one. Again, with M. Taurica the terminal leaflets were
+never seen to bend towards either of the two lateral leaflets, though
+these, whilst becoming vertical, had bent towards the terminal one. The
+sub-petiole of the terminal leaflet in this species is of unusual
+length, and if the leaflet had bent to one side, its upper surface
+could have come into contact only with the apex of either lateral
+leaflet; and this, perhaps, is the meaning of the loss of the lateral
+movement.
+
+The cotyledons do not sleep at night. the first leaf consists of a
+single orbicular leaflet, which twists at night so that the blade
+stands vertically. It is a remarkable fact that with M. Taurica, and in
+a somewhat less degree with M. macrorrhiza and petitpierreana, all the
+many small and young leaves produced during
+the early spring from shoots on some cut-down plants in the greenhouse,
+slept in a totally different manner from the normal one; for the three
+leaflets, instead of twisting on their own axes so as to present their
+lateral edges to the zenith, turned upwards and stood vertically with
+their apices pointing to the zenith. They thus assumed nearly the same
+position as in the allied genus Trifolium; and on the same principle
+that embryological characters reveal the lines of descent in the animal
+kingdom, so the movements of the small leaves in the above three
+species of Melilotus, perhaps indicate that this genus is descended
+from a form which was closely allied to and slept like a Trifolium.
+Moreover, there is one species, M. messanensis, the leaves of which, on
+full-grown plants between 2 and 3 feet in height, sleep like the
+foregoing small leaves and like those of a Trifolium. We were so much
+surprised at this latter case that, until the flowers and fruit were
+examined, we thought that the seeds of some Trifolium had been sown by
+mistake instead of those of a Melilotus. It appears therefore probable
+that M. messanensis has either retained or recovered a primordial
+habit.
+
+The circumnutation of a leaf of M. officinalis was traced, the stem
+being left free; and the apex of the terminal leaflet described three
+laterally extended ellipses, between 8 A.M. and 4 P.M.; after the
+latter hour the nocturnal twisting movement commenced. It was
+afterwards ascertained that the above movement was compounded of the
+circumnutation of the stem on a small scale, of the main petiole which
+moved most, and of the sub-petiole of the terminal leaflet. The main
+petiole of a leaf having been secured to a stick, close to the base of
+the sub-petiole of the terminal leaflet, the latter described two small
+ellipses between 10.30 A.M., and 2 P.M. At 7.15 P.M., after this same
+leaflet (as well as another) had twisted themselves into their vertical
+nocturnal position, they began to rise slowly, and continued to do so
+until 10.35 P.M., after which hour they were no longer observed.
+
+As M. messanensis sleeps in an anomalous manner, unlike that of any
+other species in the genus, the circumnutation of a terminal leaflet,
+with the stem secured, was traced during two days. On each morning the
+leaflet fell, until about noon, and then began to rise very slowly; but
+on the first day the rising movement was interrupted between 1 and 3
+P.M. by the formation of a laterally extended ellipse, and on the
+second day, at the same time, by two smaller ellipses. The rising
+movement then
+recommenced, and became rapid late in the evening, when the leaflet was
+beginning to go to sleep. The awaking or sinking movement had already
+commenced by 6.45 A.M. on both mornings.
+
+Trifolium (Tribe 3).—The nyctitropic movements of 11 species were
+observed, and were found to be closely similar. If we select a leaf of
+T. repens having an upright petiole, and with the three leaflets
+expanded horizontally, the two lateral leaflets will be seen in the
+evening to twist and approach each other, until their upper surfaces
+come into contact. At the same time they bend downwards in a plane at
+right angles to that of their former position, until their midribs form
+an angle of about 45° with the upper part of the petiole. This peculiar
+change of position requires a considerable amount of torsion in the
+pulvinus. The terminal leaflet merely rises up without any twisting and
+bends over until it rests on and forms a roof over the edges of the now
+vertical and united lateral leaflets. Thus the terminal leaflet always
+passes through an angle of at least 90°, generally of 130° or 140°, and
+not rarely—as was often observed with T. subterraneum—of 180°. In this
+latter case the terminal leaflet stands at night horizontally (as in
+Fig. 141), with its lower surface fully exposed to the zenith. Besides
+the difference in the angles, at which the terminal leaflets stand at
+night in the individuals of the same species, the degree to which the
+lateral leaflets approach each other often likewise differs.
+
+Fig. 141. Trifolium repens: A, leaf during the day; B, leaf asleep at
+night.
+
+We have seen that the cotyledons of some species and not of others rise
+up vertically at night. The first true leaf is generally unifoliate and
+orbicular; it always rises, and either stands vertically at night or
+more commonly bends a little over so as to expose the lower surface
+obliquely to the zenith, in the same manner as does the terminal
+leaflet of the mature leaf. But it does not twist itself like the
+corresponding first simple leaf of Melilotus.
+With T. Pannonicum the first true leaf was generally unifoliate, but
+sometimes trifoliate, or again partially lobed and in an intermediate
+condition.
+
+Circumnutation.—Sachs described in 1863[10] the spontaneous up and down
+movements of the leaflets of T. incarnatum, when kept in darkness.
+Pfeffer made many observations on the similar movements in T.
+pratense.[11] He states that the terminal leaflet of this species,
+observed at different times, passed through angles of from 30° to 120°
+in the course of from 1½ to 4 h. We observed the movements of T.
+subterraneum, resupinatum, and repens.
+
+ [10] ‘Flora,’ 1863, p. 497.
+
+
+ [11] ‘Die Period. Bewegungen,’ 1875, pp. 35, 52.
+
+
+Trifolium subterraneum.—A petiole was secured close to the base of the
+three leaflets, and the movement of the terminal leaflet was traced
+during 26½ h., as shown in the figure on the next page.
+
+Between 6.45 A.M. and 6 P.M. the apex moved 3 times up and 3 times
+down, completing 3 ellipses in 11 h. 15 m. The ascending and descending
+lines stand nearer to one another than is usual with most plants, yet
+there was some lateral motion. At 6 P.M. the great nocturnal rise
+commenced, and on the next morning the sinking of the leaflet was
+continued until 8.30 A.M., after which hour it circumnutated in the
+manner just described. In the figure the great nocturnal rise and the
+morning fall are greatly abbreviated, from the want of space, and are
+merely represented by a short curved line. The leaflet stood
+horizontally when at a point a little beneath the middle of the
+diagram; so that during the daytime it oscillated almost equally above
+and beneath a horizontal position. At 8.30 A.M. it stood 48° beneath
+the horizon, and by 11.30 A.M. it had risen 50° above the horizon; so
+that it passed through 98° in 3 h. By the aid of the tracing we
+ascertained that the distance travelled in the 3 h. by the apex of this
+leaflet was 1.03 inch. If we look at the figure, and prolong upwards in
+our mind’s eye the short curved broken line, which represents the
+nocturnal course, we see that the latter movement is merely an
+exaggeration or prolongation of one of the diurnal ellipses. The same
+leaflet had been observed on the previous day, and the course then
+pursued was almost identically the same as that here described.
+
+Fig. 142. Trifolium subterraneum: circumnutation and nyctitropic
+movement of terminal leaflet (.68 inch in length), traced from 6.45
+A.M. July 4th to 9.15 A.M. 5th. Apex of leaf 3 7/8 inches from the
+vertical glass, and movement, as here shown, magnified 5 1/4 times,
+reduced to one-half of original scale. Plant illuminated from above;
+temp. 16°–17° C.
+
+
+Trifolium resupinatum.—A plant left entirely free was placed before a
+north-east window, in such a position that a terminal leaflet projected
+at right angles to the source of the light, the sky being uniformly
+clouded all day. The movements of this leaflet were traced during two
+days, and on both were closely similar. Those executed on the second
+day are shown in Fig. 143. The obliquity of the several lines is due
+partly to the manner in which the leaflet was viewed, and partly to its
+having moved a little towards the light. From 7.50 A.M. to 8.40 A.M.
+the leaflet fell, that is, the awakening movement was continued. It
+then rose and moved a little laterally towards the light. At 12.30 it
+retrograded, and at 2.30 resumed its original course, having thus
+completed a small ellipse during the middle of the day. In the evening
+it rose rapidly, and by 8 A.M. on the following morning had returned to
+exactly the same spot as on the previous morning. The line representing
+the nocturnal course ought to be extended much higher up, and is here
+abbreviated into a short,
+curved, broken line. The terminal leaflet, therefore, of this species
+described during the daytime only a single additional ellipse, instead
+of two additional ones, as in the case of T. subterraneum. But we
+should remember that it was shown in the fourth chapter that the stem
+circumnutates, as no doubt does the main petiole and the sub-petioles;
+so that the movement represented in Fig. 143 is a compounded one. We
+tried to observe the movements of a leaf kept during the day in
+darkness, but it began to go to sleep after 2 h. 15 m., and this was
+well pronounced after 4 h. 30 m.
+
+Fig 143. Trifolium resupinatum: circumnutation and nyctitropic
+movements of the terminal leaflet during 24 hours.
+
+Trifolium repens.—A stem was secured close to the base of a moderately
+old leaf, and the movement of the terminal leaflet was observed during
+two days. This case is interesting solely from the simplicity of the
+movements, in contrast with those of the two preceding species. On the
+first day the leaflet fell between 8 A.M. and 3 P.M., and on the second
+between 7 A.M. and 1 P.M. On both days the descending course was
+somewhat zigzag, and this evidently represents the circumnutating
+movement of the two previous species during the middle of the day.
+After 1 P.M., Oct. 1st (Fig. 144), the leaflet began to rise, but the
+movement was slow on both days, both before and after this hour, until
+4 P.M. The rapid evening and nocturnal rise then commenced. Thus in
+this species the course during 24 h. consists of a single great
+ellipse; in T. resupinatum of two ellipses, one of which includes the
+nocturnal movement and is much elongated; and in T. subterraneum of
+three ellipses, of which the nocturnal one is likewise of great length.
+
+Securigera coronilla (Tribe 4).—The leaflets, which stand opposite one
+another and are numerous, rise up at night, come into close contact,
+and bend backwards at a moderate angle towards the base of the petiole.
+
+Fig. 144. Trifolium repens: circumnutation and nyctitropic movements of
+a nearly full-grown terminal leaflet, traced on a vertical glass from 7
+A.M. Sept. 30th to 8 A.M. Oct. 1st. Nocturnal course, represented by
+curved broken line, much abbreviated.
+
+
+Lotus (Tribe 4).—The nyctitropic movements of 10 species in this genus
+were observed, and found to be the same. The main petiole rises a
+little at night, and the three leaflets rise till they become vertical,
+and at the same time approach each other. This was conspicuous with L.
+Jacoboeus, in which the leaflets are almost linear. In most of the
+species the leaflets rise so much as to press against the stem, and not
+rarely they become inclined a little inwards with their lower surfaces
+exposed obliquely to the zenith. This was clearly the case with L.
+major, as its petioles are unusually long, and the leaflets are thus
+enabled to bend further inwards. The young leaves on the summits of the
+stems close up at night so much, as often to resemble large buds. The
+stipule-like leaflets, which are often of large size, rise up like the
+other leaflets, and press against the stem (Fig. 145). All the leaflets
+of L. Gebelii, and probably of the other species, are provided at their
+bases with distinct pulvini, of a yellowish colour, and formed of very
+small cells. The circumnutation of a terminal leaflet of L. peregrinus
+(with the stem secured) was traced during two days, but the movement
+was so simple that it is not worth while to give the diagram. The
+leaflet fell slowly from the early morning till about 1 P.M. It then
+rose gradually at first, but rapidly late in the evening. It
+occasionally stood still for about 20 m. during the day, and sometimes
+zigzagged a little. The movement of one of the basal, stipule-like
+leaflets was likewise traced in the same manner and at the same time,
+and its course was closely similar to that of the terminal leaflet.
+
+Fig. 145. Lotus Creticus: A, stem with leaves awake during the day; B,
+with leaves asleep at night. SS, stipule-like leaflets.
+
+In Tribe 5 of Bentham and Hooker, the sleep-movements of species in 12
+genera have been observed by ourselves and
+others, but only in Robinia with any care. Psoralea acaulis raises its
+three leaflets at night; whilst Amorpha fruticosa,[12] Dalea
+alopecuroides, and Indigofera tinctoria depress them. Ducharte[13]
+states that Tephrosia caribaea is the sole example of “folioles
+couchées le long du pétiole et vers la base;” but a similar movement
+occurs, as we have already seen, and shall again see in other cases.
+Wistaria Sinensis, according to Royer,[14] “abaisse les folioles qui
+par une disposition bizarre sont inclinées dans la même feuille, les
+supérieures vers
+le sommet, les inférieures vers la base du petiole commun;” but the
+leaflets on a young plant observed by us in the greenhouse merely sank
+vertically downwards at night. The leaflets are raised in Sphaerophysa
+salsola, Colutea arborea, and Astragalus uliginosus, but are depressed,
+according to Linnæus, in Glycyrrhiza. The leaflets of Robinia
+pseudo-acacia likewise sink vertically down at night, but the petioles
+rise a little, viz., in one case 3°, and in another 4°. The
+circumnutating movements of a terminal leaflet on a rather old leaf
+were traced during two days, and were simple. The leaflet fell slowly,
+in a slightly zigzag line, from 8 A.M. to 5 P.M., and then more
+rapidly; by 7 A.M. on the following morning it had risen to its diurnal
+position. There was only one peculiarity in the movement, namely, that
+on both days there was a distinct though small oscillation up and down
+between 8.30 and 10 A.M., and this would probably have been more
+strongly pronounced if the leaf had been younger.
+
+ [12] Ducharte, ‘Eléments de Botanique’, 1867, p. 349.
+
+
+ [13] Ibid., p. 347.
+
+
+ [14] ‘Ann. des Sciences Nats. Bot.’ (5th series), ix. 1868.
+
+
+Coronilla rosea (Tribe 6).—the leaves bear 9 or 10 pairs of opposite
+leaflets, which during the day stand horizontally, with their midribs
+at right angles to the petiole. At night they rise up so that the
+opposite leaflets come nearly into contact, and those on the younger
+leaves into close contact. At the same time they bend back towards the
+base of the petiole, until their midribs form with it angles of from
+40° to 50° in a vertical plane, as here figured (Fig. 146). The
+leaflets, however, sometimes bend so much back that their midribs
+become parallel to and lie on the petiole. They thus occupy a reversed
+position to what they do in several Leguminosae, for instance, in
+Mimosa
+pudica; but, from standing further apart, they do not overlap one
+another nearly so much as in this latter plant. The main petiole is
+curved slightly downwards during the day, but straightens itself at
+night. In three cases it rose from 3° above the horizon at noon, to 9°
+at 10 P.M.; from 11° to 33°; and from 5° to 33°—the amount of angular
+movement in this latter case amounting to 28°. In several other species
+of Coronilla the leaflets showed only feeble movements of a similar
+kind.
+
+Fig. 146. Coronilla rosea: leaf asleep.
+
+Hedysarum coronarium (Tribe 6).—The small lateral leaflets on plants
+growing out of doors rose up vertically at night, but the large
+terminal one became only moderately inclined. The petioles apparently
+did not rise at all.
+
+Smithia Pfundii (Tribe 6).—The leaflets rise up vertically, and the
+main petiole also rises considerably.
+
+Arachis hypogoea (Tribe 6).—The shape of a leaf, with its two pairs of
+leaflets, is shown at A (Fig. 147); and a leaf asleep, traced from a
+photograph (made by the aid of aluminium light), is given at B. The two
+terminal leaflets twist round at night until their blades stand
+vertically, and approach each other until they meet, at the same time
+moving a little upwards and backwards. The two lateral leaflets meet
+each other in this same manner, but move to a greater extent forwards,
+that is, in a contrary direction to the two terminal leaflets, which
+they partially embrace. Thus all four leaflets form together a single
+packet, with their edges directed to the zenith, and with their lower
+surfaces turned outwards. On a plant which was not growing vigorously
+the closed leaflets seemed too heavy for the
+petioles to support them in a vertical position, so that each night the
+main petiole became twisted, and all the packets were extended
+horizontally, with the lower surfaces of the leaflets on one side
+directed to the zenith in a most anomalous manner. This fact is
+mentioned solely as a caution, as it surprised us greatly, until we
+discovered that it was an anomaly. The petioles are inclined upwards
+during the day, but sink at night, so as to stand at about right angles
+with the stem. The amount of sinking was measured only on one occasion,
+and found to be 39°. A petiole was secured to a stick at the base of
+the two terminal leaflets, and the circumnutating movement of one of
+these leaflets was traced from 6.40 A.M. to 10.40 P.M., the plant being
+illuminated from above. The temperature was 17°–17½° C., and therefore
+rather too low. During the 16 h. the leaflet moved thrice up and thrice
+down, and as the ascending and descending lines did not coincide, three
+ellipses were formed.
+
+Fig. 147. Arachis hypogoea: A, leaf during the day, seen from
+vertically above; B, leaf asleep, seen laterally, copied from a
+photograph. Figures much reduced.
+
+Fig. 148. Desmodium gyrans: leaf seen from above, reduced to one-half
+natural size. The minute stipules unusually large.
+
+Desmodium gyrans (Tribe 6).—A large and full-grown leaf of this plant,
+so famous for the spontaneous movements of the two little lateral
+leaflets, is here represented (Fig. 148). The large terminal leaflet
+sleeps by sinking vertically down, whilst the petiole rises up. The
+cotyledons do not sleep, but the first-formed leaf sleeps equally well
+as the older ones. The appearance presented by a sleeping branch and
+one in the day-time, copied from two photographs, are shown at A and B
+(Fig. 149), and we see how at night the leaves are crowded together, as
+if for mutual protection, by the rising of the petioles. The petioles
+of the younger leaves near the summits of the shoots rise up at night,
+so as to stand vertical and parallel to the stem; whilst those on the
+sides were found in four cases to have risen respectively 46½°, 36°,
+20°, and 19.5° above the inclined positions which they had occupied
+during the day. For instance, in the first of these four cases the
+petiole stood in the day at 23°, and at night at 69½° above the
+horizon. In the evening the rising of the petioles is almost completed
+before the leaflets sink perpendicularly downwards.
+
+
+Circumnutation.—The circumnutating movements of four young shoots were
+observed during 5 h. 15 m.; and in this time each completed an oval
+figure of small size. The main petiole also circumnutates rapidly, for
+in the course of 31 m. (temp. 91° F.) it changed its course by as much
+as a rectangle six times, describing a figure which apparently
+represented two ellipses. The movement of the terminal leaflet by means
+of its sub-petiole or pulvinus is quite as rapid, or even more so, than
+that of the main petiole, and has much greater amplitude. Pfeffer has
+seen[15] these leaflets move through an angle of 8° in the course of
+from 10 to 30 seconds.
+
+ [15] ‘Die Period. Beweg.,’ p. 35.
+
+
+Fig. 149. Desmodium gyrans: A, stem during the day; B, stem with leaves
+asleep. Figures reduced.
+
+A fine, nearly full-grown leaf on a young plant, 8 inches in height,
+with the stem secured to a stick at the base of the leaf, was observed
+from 8.30 A.M. June 22nd to 8 A.M. June 24th.
+In the diagram given on the next page (Fig. 150), the two curved broken
+lines at the base, which represent the nocturnal courses, ought to be
+prolonged far downwards. On the first day the leaflet moved thrice down
+and thrice up, and to a considerable distance laterally; the course was
+also remarkably crooked. The dots were generally made every hour; if
+they had been made every few minutes all the lines would have been
+zigzag to an extraordinary degree, with here and there a loop formed.
+We may infer that this would have been the case, because five dots were
+made in the course of 31 m. (between 12.34 and 1.5 P.M.), and we see in
+the upper part of the diagram how crooked the course here is; if only
+the first and last dots had been joined we should have had a straight
+line. Exactly the same fact may be seen in the lines representing the
+course between 2.24 P.M. and 3 P.M., when six intermediate dots were
+made; and again at 4.46 and 4.50. But the result was widely different
+after 6 P.M.,—that is, after the great nocturnal descent had commenced;
+for though nine dots were then made in the course of 32 m., when these
+were joined (see Figure) the line thus formed was almost straight. The
+leaflets, therefore, begin to descend in the afternoon by zigzag lines,
+but as soon as the descent becomes rapid their whole energy is expended
+in thus moving, and their course becomes rectilinear. After the
+leaflets are completely asleep they move very little or not at all.
+
+Fig. 150. Desmodium gyrans: circumnutation and nyctitropic movement of
+leaf (3 3/4 inches in length, petiole included) during 48 h. Filament
+affixed to midrib of terminal leaflet; its apex 6 inches from the
+vertical glass. Diagram reduced to one-third of original scale. Plant
+illuminated from above. Temp. 19°–20° C.
+
+Had the above plant been subjected to a higher temperature than 67°–70°
+F., the movements of the terminal leaflet would probably have been even
+more rapid and wider in extent than those shown in the diagram; for a
+plant was kept for some time in the hot-house at from 92°–93° F., and
+in the course of 35 m. the apex of a leaflet twice descended and once
+ascended, travelling over a space of 1.2 inch in a vertical direction
+and of .82 inch in a horizontal direction. Whilst thus moving the
+leaflet also rotated on its own axis (and this was a point to which no
+attention had been before paid), for the plane of the blade differed by
+41° after an interval of only a few minutes. Occasionally the leaflet
+stood still for a short time. There was no jerking movement, which is
+so characteristic of the little lateral leaflets. A sudden and
+considerable fall of temperature causes the terminal leaflet to sink
+downwards; thus a cut-off leaf was immersed in water at 95° F., which
+was slowly raised to 103° F., and afterwards allowed to sink to 70° F.,
+and the sub-petiole of the terminal leaflet then curved downwards. The
+water was afterwards
+
+raised to 120° F., and the sub-petiole straightened itself. Similar
+experiments with leaves in water were twice repeated, with nearly the
+same result. It should be added, that water raised to even 122° F. does
+not soon kill a leaf. A plant was placed in darkness at 8.37 A.M., and
+at 2 P.M. (i.e. after 5 h. 23 m.), though the leaflets had sunk
+considerably, they had by no means acquired their nocturnal vertically
+dependent position. Pfeffer, on the other hand, says[16] that this
+occurred with him in from 3/4 h. to 2 h.; perhaps the difference in our
+results may be due to the plant on which we experimented being a very
+young and vigorous seedling.
+
+ [16] ‘Die Period. Beweg.,’ p. 39.
+
+
+The Movements of the little Lateral Leaflets .—These have been so often
+described, that we will endeavour to be as brief as possible in giving
+a few new facts and conclusions. The leaflets sometimes quickly change
+their position by as much as nearly 180°; and their sub-petioles can
+then be seen to become greatly curved. They rotate on their own axes,
+so that their upper surfaces are directed to all points of the compass.
+The figure described by the apex is an irregular oval or ellipse. They
+sometimes remain stationary for a period. In these several respects
+there is no difference, except in rapidity and extent, between their
+movements and the lesser ones performed by the large terminal leaflet
+whilst making its great oscillations. The movements of the little
+leaflets are much influenced, as is well known, by temperature. This
+was clearly shown by immersing leaves with motionless leaflets in cold
+water, which was slowly raised to 103° F., and the leaflets then moved
+quickly, describing about a dozen little irregular circles in 40 m. By
+this time the water had become much cooler, and the movements became
+slower or almost ceased; it was then raised to 100° F., and the
+leaflets again began to move quickly. On another occasion a tuft of
+fine leaves was immersed in water at 53° F., and the leaflets were of
+course motionless. The water was raised to 99°, and the leaflets soon
+began to move; it was raised to 105°, and the movements became much
+more rapid; each little circle or oval being completed in from 1 m. 30
+s. to 1 m. 45 s. There was, however, no jerking, and this fact may
+perhaps be attributed to the resistance of the water.
+
+Sachs states that the leaflets do not move until the surrounding air is
+as high as 71°–72° F., and this agrees with our
+experience on full-grown, or nearly full-grown, plants. But the
+leaflets of young seedlings exhibit a jerking movement at much lower
+temperatures. A seedling was kept (April 16th) in a room for half the
+day where the temperature was steady at 64° F., and the one leaflet
+which it bore was continually jerking, but not so rapidly as in the
+hot-house. The pot was taken in the evening into a bed-room where the
+temperature remained at 62° during nearly the whole night; at 10 and 11
+P.M. and at 1 A.M. the leaflet was still jerking rapidly; at 3.30 A.M.
+it was not seen to jerk, but was observed during only a short time. It
+was, however, now inclined at a much lower angle than that occupied at
+1 A.M. At 6.30 A.M. (temp. 61° F.) its inclination was still less than
+before, and again less at 6.45 A.M.; by 7.40 A.M. it had risen, and at
+8.30 A.M. was again seen to jerk. This leaflet, therefore, was moving
+during the whole night, and the movement was by jerks up to 1 A.M. (and
+possibly later) and again at 8.30 A.M., though the temperature was only
+61° to 62° F. We must therefore conclude that the lateral leaflets
+produced by young plants differ somewhat in constitution from those on
+older plants.
+
+In the large genus Desmodium by far the greater number of the species
+are trifoliate; but some are unifoliate, and even the same plant may
+bear uni- and trifoliate leaves. In most of the species the lateral
+leaflets are only a little smaller than the terminal one. Therefore the
+lateral leaflets of D. gyrans (see Fig. 148) must be considered as
+almost rudimentary. They are also rudimentary in function, if this
+expression may be used; for they certainly do not sleep like the
+full-sized terminal leaflets. It is, however, possible that the sinking
+down of the leaflets between 1 A.M. and 6.45 A.M., as above described,
+may represent sleep. It is well known that the leaflets go on jerking
+during the early part of the night; but my gardener observed (Oct.
+13th) a plant in the hot-house between 5 and 5.30 A.M., the temperature
+having been kept up to 82° F., and found that all the leaflets were
+inclined, but he saw no jerking movement until 6.55 A.M., by which time
+the terminal leaflet had risen and was awake. Two days afterwards (Oct.
+15th) the same plant was observed by him at 4.47 A.M. (temp. 77° F.),
+and he found that the large terminal leaflets were awake, though not
+quite horizontal; and the only cause which we could assign for this
+anomalous wakefulness was that the plant had been kept for experimental
+purposes during
+the previous day at an unusually high temperature; the little lateral
+leaflets were also jerking at this hour, but whether there was any
+connection between this latter fact and the sub-horizontal position of
+the terminal leaflets we do not know. Anyhow, it is certain that the
+lateral leaflets do not sleep like the terminal leaflets; and in so far
+they may be said to be in a functionally rudimentary condition. They
+are in a similar condition in relation to irritability; for if a plant
+be shaken or syringed, the terminal leaflets sink down to about 45°
+beneath the horizon; but we could never detect any effect thus produced
+on the lateral leaflets; yet we are not prepared to assert positively
+that rubbing or pricking the pulvinus produces no effect.
+
+As in the case of most rudimentary organs, the leaflets are variable in
+size; they often depart from their normal position and do not stand
+opposite one another; and one of the two is frequently absent. This
+absence appeared in some, but not in all the cases, to be due to the
+leaflet having become completely confluent with the main petiole, as
+might be inferred from the presence of a slight ridge along its upper
+margin, and from the course of the vessels. In one instance there was a
+vestige of the leaflet, in the shape of a minute point, at the further
+end of the ridge. The frequent, sudden and complete disappearance of
+one or both of the rudimentary leaflets is a rather singular fact; but
+it is a much more surprising one that the leaves which are first
+developed on seedling plants are not provided with them. Thus, on one
+seedling the seventh leaf above the cotyledons was the first which bore
+any lateral leaflets, and then only a single one. On another seedling,
+the eleventh leaf first bore a leaflet; of the nine succeeding leaves
+five bore a single lateral leaflet, and four bore none at all; at last
+a leaf, the twenty-first above the cotyledons, was provided with two
+rudimentary lateral leaflets. From a widespread analogy in the animal
+kingdom, it might have been expected that these rudimentary leaflets
+would have been better developed and more regularly present on very
+young than on older plants. But bearing in mind, firstly, that
+long-lost characters sometimes reappear late in life, and secondly,
+that the species of Desmodium are generally trifoliate, but that some
+are unifoliate, the suspicion arises that D. gyrans is descended from a
+unifoliate species, and that this was descended from a trifoliate one;
+for in this case both the absence of the little lateral leaflets on
+very young seedlings, and their
+subsequent appearance, may be attributed to reversion to more or less
+distant progenitors.[17]
+
+ [17] Desmodium vespertilionis is closely allied to D. gyrans, and it
+ seems only occasionally to bear rudimentary lateral leaflets.
+ Duchartre, ‘Eléments de Botanique,’ 1867, p. 353.
+
+
+No one supposes that the rapid movements of the lateral leaflets of ‘D.
+gyrans’ are of any use to the plant; and why they should behave in this
+manner is quite unknown. We imagined that their power of movement might
+stand in some relation with their rudimentary condition, and therefore
+observed the almost rudimentary leaflets of Mimosa albida vel sensitiva
+(of which a drawing will hereafter be given, Fig. 159); but they
+exhibited no extraordinary movements, and at night they went to sleep
+like the full-sized leaflets. There is, however, this remarkable
+difference in the two cases; in Desmodium the pulvinus of the
+rudimentary leaflets has not been reduced in length, in correspondence
+with the reduction of the blade, to the same extent as has occurred in
+the Mimosa; and it is on the length and degree of curvature of the
+pulvinus that the amount of movement of the blade depends. Thus the
+average length of the pulvinus in the large terminal leaflets of
+Desmodium is 3 mm., whilst that of the rudimentary leaflets is 2.86
+mm.; so that they differ only a little in length. But in diameter they
+differ much, that of the pulvinus of the little leaflets being only 0.3
+mm. to 0.4 mm.; whilst that of the terminal leaflets is 1.33 mm. If we
+now turn to the Mimosa, we find that the average length of the pulvinus
+of the almost rudimentary leaflets is only 0.466 mm., or rather more
+than a quarter of the length of the pulvinus of the full-sized
+leaflets, namely, 1.66 mm. In this small reduction in length of the
+pulvinus of the rudimentary leaflets of Desmodium, we apparently have
+the proximate cause of their great and rapid circumnutating movement,
+in contrast with that of the almost rudimentary leaflets of the Mimosa.
+The small size and weight of the blade, and the little resistance
+opposed by the air to its movement, no doubt also come into play; for
+we have seen that these leaflets if immersed in water, when the
+resistance would be much greater, were prevented from jerking forwards.
+Why, during the reduction of the lateral leaflets of Desmodium, or
+during their reappearance—if they owe their origin to reversion—the
+pulvinus should have been so much less affected than the blade, whilst
+with the
+Mimosa the pulvinus has been greatly reduced, we do not know.
+Nevertheless, it deserves notice that the reduction of the leaflets in
+these two genera has apparently been effected by a different process
+and for a different end; for with the Mimosa the reduction of the inner
+and basal leaflets was necessary from the want of space; but no such
+necessity exists with Desmodium, and the reduction of its lateral
+leaflets seems to have been due to the principle of compensation, in
+consequence of the great size of the terminal leaflet. Uraria (Tribe 6)
+and Centrosema (Tribe 8).—The leaflets of Uraria lagopus and the leaves
+of a Centrosema from Brazil both sink vertically down at night. In the
+latter plant the petiole at the same time rose 16½°.
+
+Amphicarpoea monoica (Tribe 8).—The leaflets sink down vertically at
+night, and the petioles likewise fall considerably. A petiole, which
+was carefully observed, stood during the day 25° above the horizon and
+at night 32° below it; it therefore fell 57°. A filament was fixed
+transversely across the terminal leaflet of a fine young leaf (2 1/4
+inches in length including the
+petiole), and the movement of the whole leaf was traced on a vertical
+glass. This was a bad plan in some respects, because the rotation of
+the leaflet, independently of its rising or falling, raised and
+depressed the filament; but it was the best plan for our special
+purpose of observing whether the leaf moved much after it had gone to
+sleep. The plant had twined closely round a thin stick, so that the
+circumnutation of the stem was prevented. The movement of the leaf was
+traced during 48 h., from 9 A.M. July 10th to 9 A.M. July 12th. In the
+figure given (Fig. 151) we see how complicated its course was on both
+days: during the second day it changed its course greatly 13 times. The
+leaflets began to go to sleep a little after 6 P.M., and by 7.15 P.M.
+hung vertically down and were completely asleep; but on both nights
+they continued to move from 7.15 P.M. to 10.40 and 10.50 P.M., quite as
+much as during the day; and this was the point which we wished to
+ascertain. We see in the figure that the great sinking movement late in
+the evening does not differ essentially from the circumnutation during
+the day.
+
+Fig. 151. Amphicarpoea monoica: circumnutation and nyctitropic movement
+of leaf during 48 h.; its apex 9 inches from the vertical glass. Figure
+reduced to one-third of original scale. Plant illuminated from above;
+temp 17½°–18½° C.
+
+Glycine hispida (Tribe 8).—The three leaflets sink vertically down at
+night.
+
+Erythrina (Tribe 8).—Five species were observed, and the leaflets of
+all sank vertically down at night; with E. caffra and with a second
+unnamed species, the petioles at the same time rose slightly. The
+movements of the terminal leaflet of E. crista-galli (with the main
+petiole secured to a stick) were traced from 6.40 A.M. June 8th, to 8
+A.M. on the 10th. In order to observe the nyctitropic movements of this
+plant, it is necessary that it should have grown in a warm greenhouse,
+for out of doors in our climate it does not sleep. We see in the
+tracing (Fig. 152) that the leaflet oscillated twice up and down
+between early morning and noon; it then fell greatly, afterwards rising
+till 3 P.M. At this latter hour the great nocturnal fall commenced. On
+the second day (of which the tracing is not given) there was exactly
+the same double oscillation before noon, but only a very small one in
+the afternoon. On the third morning the leaflet moved laterally, which
+was due to its beginning to assume an oblique position, as seems
+invariably to occur with the leaflets of this species as they grow old.
+On both nights after the leaflets were asleep and hung vertically down,
+they continued to move a little both up and down, and from side to
+side.
+
+Erythrina caffra.—A filament was fixed transversely across
+a terminal leaflet, as we wished to observe its movements when asleep.
+The plant was placed in the morning of June 10th under a skylight,
+where the light was not bright; and we do not know whether it was owing
+to this cause or to the plant having been disturbed, but the leaflet
+hung vertically down all day; nevertheless it circumnutated in this
+position, describing a figure which represented two irregular ellipses.
+On the next day it circumnutated in a greater degree, describing four
+irregular ellipses, and by 3 P.M. had risen into a horizontal position.
+By 7.15 P.M. it was asleep and vertically dependent, but continued to
+circumnutate as long as observed, until 11 P.M.
+
+Fig. 152. Erythrina crista-galli: circumnutation and nyctitropic
+movement of terminal leaflet, 3 3/4 inches in length, traced during 25
+h.; apex of leaf 3½ inches from the vertical glass. Figure reduced to
+one-half of original scale. Plant illuminated from above; temp.
+17½°–18½° C.
+
+Erythrina corallodendron.—The movements of a terminal leaflet were
+traced. During the second day it oscillated four times up and four
+times down between 8 A.M. and 4 P.M., after which hour the great
+nocturnal fall commenced. On the third day the movement was equally
+great in amplitude, but was remarkably simple, for the leaflet rose in
+an almost perfectly straight line from 6.50 A.M. to 3 P.M., and then
+sank down in an equally straight line until vertically dependent and
+asleep.
+
+
+Apios tuberosa (Tribe 8).—The leaflets sink vertically down at night.
+
+Phaseolus vulgaris (Tribe 8).—The leaflets likewise sink vertically
+down at night. In the greenhouse the petiole of a young leaf rose 16°,
+and that of an older leaf 10° at night. With plants growing out of
+doors the leaflets apparently do not sleep until somewhat late in the
+season, for on the nights of July 11th and 12th none of them were
+asleep; whereas on the night of August 15th the same plants had most of
+their leaflets vertically dependent and asleep. With Ph. caracalla and
+Hernandesii, the primary unifoliate leaves and the leaflets of the
+secondary trifoliate leaves sink vertically down at night. This holds
+good with the secondary trifoliate leaves of Ph. Roxburghii, but it is
+remarkable that the primary unifoliate leaves which are much elongated,
+rise at night from about 20° to about 60° above the horizon. With older
+seedlings, however, having the secondary leaves just developed, the
+primary leaves stand in the middle of the day horizontally, or are
+deflected a little beneath the horizon. In one such case the primary
+leaves rose from 26° beneath the horizon at noon, to 20° above it at 10
+P.M.; whilst at this same hour the leaflets of the secondary leaves
+were vertically dependent. Here, then, we have the extraordinary case
+of the primary and secondary leaves on the same plant moving at the
+same time in opposite directions.
+
+We have now seen that the leaflets in the six genera of Phaseoleae
+observed by us (with the exception of the primary leaves of Phaseolus
+Roxburghii) all sleep in the same manner, namely, by sinking vertically
+down. The movements of the petioles were observed in only three of
+these genera. They rose in Centrosema and Phaseolus, and sunk in
+Amphicarpæa.
+
+Sophora chrysophylla (Tribe 10).—The leaflets rise at night, and are at
+the same time directed towards the apex of the leaf, as in Mimosa
+pudica.
+
+Caesalpinia, Hoematoxylon, Gleditschia, Poinciana.—The leaflets of two
+species of Caesalpinia (Tribe 13) rose at night. With Haematoxylon
+Campechianum (Tribe 13) the leaflets move forwards at night, so that
+their midribs stand parallel to the petiole, and their now vertical
+lower surfaces are turned outwards (Fig. 153). The petiole sinks a
+little. In Gleditschia, if we understand correctly Duchartre’s
+description, and in
+Poinciana Gilliesii (both belonging to Tribe 13), the leaves behave in
+the same manner.
+
+Fig. 153. Haematoxylon Campechianum: A, branch during daytime; B,
+branch with leaves asleep, reduced to two-thirds of natural scale.
+
+Cassia (Tribe 14).—The nyctitropic movements of the leaves in many
+species in this genus are closely alike, and are highly complex. They
+were first briefly described by Linnæus, and since by Duchartre. Our
+observations were made chiefly on C. floribunda[18] and corymbosa, but
+several other species were casually observed. The horizontally extended
+leaflets sink down vertically at night; but not simply, as in so many
+other genera, for each leaflet rotates on its own axis, so that its
+lower surface faces outwards. The upper surfaces of the opposite
+leaflets are thus brought into contact with one another beneath the
+petiole, and are well protected (Fig. 154). The rotation and other
+movements are effected by means of a well-developed pulvinus at the
+base of each leaflet, as could be plainly seen when a straight narrow
+black line had been painted along it during the day. The two terminal
+leaflets in the daytime include rather less than a right angle; but
+their divergence increases greatly whilst they
+sink downwards and rotate, so that they stand laterally at night, as
+may be seen in the figure. Moreover, they move somewhat backwards, so
+as to point towards the base of the petiole. In one instance we found
+that the midrib of a terminal leaflet formed at night an angle of 36°,
+with a line dropped
+perpendicularly from the end of the petiole. The second pair of
+leaflets likewise moves a little backwards, but less than the terminal
+pair; and the third pair moves vertically downwards, or even a little
+forwards. Thus all the leaflets, in those species which bear only 3 or
+4 pairs, tend to form a single packet, with their upper surfaces in
+contact, and their lower surfaces turned outwards. Lastly, the main
+petiole rises at night, but with leaves of different ages to very
+different degrees, namely some rose through an angle of only 12°, and
+others as much as 41°.
+
+ [18] I am informed by Mr. Dyer that Mr. Bentham believes that C.
+ floribunda (a common greenhouse bush) is a hybrid raised in France,
+ and that it comes very near to C. laevigata. It is no doubt the same
+ as the form described by Lindley (‘Bot. Reg.,’ Tab. 1422) as C.
+ Herbertiana.
+
+
+Fig. 154. Cassia corymbosa: A, plant during day; B, same plant at
+night. Both figures copied from photographs.
+
+Cassia calliantha.—The leaves bear a large number of leaflets, which
+move at night in nearly the same manner as just described; but the
+petioles apparently do not rise, and one which was carefully observed
+certainly fell 3°. Cassia pubescens.—The chief difference in the
+nyctitropic movements of this species, compared with those of the
+former species, consists in the leaflets not rotating nearly so much;
+therefore their lower surfaces face but little outwards at night. The
+petioles, which during the day are inclined only a little above the
+horizon, rise at night in a remarkable manner, and stand nearly or
+quite vertically. This, together with the dependent position of the
+leaflets, makes the whole plant wonderfully compact at night. In the
+two foregoing figures, copied from photographs, the same plant is
+represented awake and asleep (Fig. 155), and we see how different is
+its appearance.
+
+Fig. 155. Cassia pubescens: A, upper part of plant during the day; B,
+same plant at night. Figures reduced from photographs.
+
+Cassia mimosoides.—At night the numerous leaflets on each leaf rotate
+on their axes, and their tips move towards the apex of the leaf; they
+thus become imbricated with their lower surfaces directed upwards, and
+with their midribs almost parallel to the petiole. Consequently, this
+species differs from all the others seen by us, with the exception of
+the following one, in the leaflets not sinking down at night. A
+petiole, the movement of which was measured, rose 8° at night.
+
+Cassia Barclayana.—The leaflets of this Australian species are
+numerous, very narrow, and almost linear. At night they rise up a
+little, and also move towards the apex of the leaf. For instance, two
+opposite leaflets which diverged from one another during the day at an
+angle of 104°, diverted at night only 72°; so that each had risen 16°
+above its diurnal position. The petiole of a young leaf rose at night
+34°, and that of an older leaf 19°. Owing to the slight movement of the
+leaflets and the considerable movement of the petiole, the bush
+presents a different appearance at night to what it does by day; yet
+the leaves can hardly be said to sleep.
+
+The circumnutating movements of the leaves of C. floribunda,
+calliantha, and pubescens were observed, each during three or four
+days; they were essentially alike, those of the last-named species
+being the simplest. The petiole of C. floribunda was secured to a stick
+at the base of the two terminal leaflets, and a filament was fixed
+along the midrib of one of them. Its movements were traced from 1 P.M.
+on August 13th to 8.30 A.M. 17th; but those during the last 2 h. are
+alone given in Fig. 156. From 8 A.M. on each day (by which hour the
+leaf had assumed its diurnal position) to 2 or 3 P.M., it either
+zigzagged or circumnutated over nearly the same small space; at between
+2 and 3 P.M. the great evening fall commenced. The lines representing
+this fall and the early morning rise are oblique, owing to the peculiar
+manner in which the leaflets sleep, as already described. After the
+leaflet was asleep at 6 P.M., and whilst the glass filament hung
+perpendicularly down, the movement of its apex was traced until 10.30
+P.M.; and during this whole time it swayed from side to side,
+completing more than one ellipse.
+
+Fig 156. Cassia floribunda: circumnutation and nyctitropic movement of
+a terminal leaflet (1 5/6 inch in length) traced from 8.30 A.M. to same
+hour on following morning. Apex of leaflet 5½ inches from the vertical
+glass. Main petiole 3 3/4 inches long. Temp. 16°–17½° C. Figure reduced
+to one-half of the original scale.
+
+Bauhinia (Tribe 15).—The nyctitropic movements of four species were
+alike, and were highly peculiar. A plant raised from seed sent us from
+South Brazil by Fritz Müller, was more especially observed. The leaves
+are large and deeply notched at their ends. At night the two halves
+rise up and close completely together, like the opposite leaflets of
+many Leguminosae. With very young plants the petioles rise considerably
+at the same time; one, which was inclined at noon 45° above the
+horizon, at night stood at 75°; it thus rose 30°; another rose 34°.
+Whilst the two halves of the leaf are closing, the midrib at first
+sinks vertically downwards and afterwards bends backwards, so as to
+pass close along one side of its own upwardly inclined petiole; the
+midrib being thus directed towards the stem or axis of the plant. The
+angle which the midrib formed with the horizon was measured in one case
+at different hours: at noon it stood horizontally; late in the evening
+it depended vertically; then rose to the opposite side, and at 10.15
+P.M. stood at only 27° beneath the horizon, being directed towards the
+stem. It had thus travelled through 153°.
+Owing to this movement—to the leaves being folded—and to the petioles
+rising, the whole plant is as much more compact at night than during
+the day, as a fastigiate Lombardy poplar is compared with any other
+species of poplar. It is remarkable that when our plants had grown a
+little older, viz., to a height of 2 or 3 feet, the petioles did not
+rise at night, and the midribs of the folded leaves were no longer bent
+back along one side of the petiole. We have noticed in some other
+genera that the petioles of very young plants rise much more at night
+than do those of older plants.
+
+Tamarindus Indica (Tribe 16).—The leaflets approach or meet each other
+at night, and are all directed towards the apex of the leaf. They thus
+become imbricated with their midribs parallel to the petiole. The
+movement is closely similar to that of Haematoxylon (see Fig. 153), but
+more striking from the greater number of the leaflets.
+
+Adenanthera, Prosopis, and Neptunia (Tribe 20).—With Adenanthera
+pavonia the leaflets turn edgeways and sink at night. In Prosopis they
+turn upwards. With Neptunia oleracea the leaflets on the opposite sides
+of the same pinna come into contact at night and are directed forwards.
+The pinnae themselves move downwards, and at the same time backwards or
+towards the stem of the plant. The main petiole rises.
+
+Mimosa pudica (Tribe 20).—This plant has been the subject of
+innumerable observations; but there are some points in relation to our
+subject which have not been sufficiently attended to. At night, as is
+well known, the opposite leaflets come into contact and point towards
+the apex of the leaf; they thus become neatly imbricated with their
+upper surfaces protected. The four pinnae also approach each other
+closely, and the whole leaf is thus rendered very compact. The main
+petiole sinks downwards during the day till late in the evening, and
+rises until very early in the morning. The stem is continually
+circumnutating at a rapid rate, though not to a wide extent. Some very
+young plants, kept in darkness, were observed during two days, and
+although subjected to a rather low temperature of 57°–59° F., the stem
+of one described four small ellipses in the course of 12 h. We shall
+immediately see that the main petiole is likewise continually
+circumnutating, as is each separate pinna and each separate leaflet.
+Therefore, if the movement of the apex of any one leaflet were to be
+traced, the course described would be compounded of the movements of
+four separate parts.
+
+
+A filament had been fixed on the previous evening, longitudinally to
+the main petiole of a nearly full-grown, highly-sensitive leaf (four
+inches in length), the stem having been secured to a stick at its base;
+and a tracing was made on a vertical glass in the hot-house under a
+high temperature. In the figure given (Fig. 157), the first dot was
+made at 8.30 A.M. August 2nd, and the last at 7 P.M. on the 3rd. During
+12 h. on the first day the petiole moved thrice downwards and twice
+upwards. Within the same length of time on the second day, it moved
+five times downwards and four times upwards. As the ascending and
+descending lines do not coincide, the petiole manifestly circumnutates;
+the great evening fall and nocturnal rise being an exaggeration of one
+of the circumnutations. It should, however, be observed that the
+petiole fell much lower down in the evenings than could be seen on the
+vertical glass or is represented in the diagram. After 7 P.M. on the
+3rd (when the last dot in Fig. 157 was made) the pot was carried into a
+bed-room, and the petiole was found at 12.50 A.M. (i.e. after midnight)
+standing almost upright, and much more highly inclined than it was at
+10.40 P.M. When observed again at 4 A.M. it had begun to fall, and
+continued falling till 6.15 A.M., after which hour it zigzagged and
+again circumnutated. Similar observations were made on another petiole,
+with nearly the same result.
+
+Fig. 157 Mimosa pudica: circumnutation and nyctitropic movement of main
+petiole, traced during 34 h. 30 m.
+
+On two other occasions the movement of the main petiole
+was observed every two or three minutes, the plants being kept at a
+rather high temperature, viz., on the first occasion at 77°–81° F., and
+the filament then described 2½ ellipses in 69 m. On the second
+occasion, when the temperature was 81°–86° F., it made rather more than
+3 ellipses in 67 m. therefore, Fig. 157, though now sufficiently
+complex, would have been incomparably more so, if dots had been made on
+the glass every 2 or 3 minutes, instead of every hour or half-hour.
+Although the main petiole is continually and rapidly describing small
+ellipses during the day, yet after the great nocturnal rising movement
+has commenced, if dots are made every 2 or 3 minutes, as was done for
+an hour between 9.30 and 10.30 P.M. (temp. 84° F.), and the dots are
+then joined, an almost absolutely straight line is the result.
+
+To show that the movement of the petiole is in all probability due to
+the varying turgescence of the pulvinus, and not to growth (in
+accordance with the conclusions of Pfeffer), a very old leaf, with some
+of its leaflets yellowish and hardly at all sensitive, was selected for
+observation, and the plant was kept at the highly favourable temp. of
+80° F. The petiole fell from 8 A.M. till 10.15 A.M., it then rose a
+little in a somewhat zigzag line, often remaining stationary, till 5
+P.M., when the great evening fall commenced, which was continued till
+at least 10 P.M. By 7 A.M. on the following morning it had risen to the
+same level as on the previous morning, and then descended in a zigzag
+line. But from 10.30 A.M. till 4.15 P.M. it remained almost motionless,
+all power of movement being now lost. The petiole, therefore, of this
+very old leaf, which must have long ceased growing, moved periodically;
+but instead of circumnutating several times during the day, it moved
+only twice down and twice up in the course of 24 h., with the ascending
+and descending lines not coincident.
+
+It has already been stated that the pinnae move independently of the
+main petiole. The petiole of a leaf was fixed to a cork support, close
+to the point whence the four pinnae diverge, with a short fine filament
+cemented longitudinally to one of the two terminal pinnae, and a
+graduated semicircle was placed close beneath it. By looking vertically
+down, its angular or lateral movements could be measured with accuracy.
+Between noon and 4.15 P.M. the pinna changed its position to one side
+by only 7°; but not continuously in the same direction, as it moved
+four times to one side, and three times to the opposite side,
+in one instance to the extent of 16°. This pinna, therefore
+circumnutated. Later in the evening the four pinnae approach each
+other, and the one which was observed moved inwards 59° between noon
+and 6.45 P.M. Ten observations were made in the course of 2 h. 20 m.
+(at average intervals of 14 m.), between 4.25 and 6.45 P.M.; and there
+was now, when the leaf was going to sleep, no swaying from side to
+side, but a steady inward movement. Here therefore there is in the
+evening the same conversion of a circumnutating into a steady movement
+in one direction, as in the case of the main petiole.
+
+It has also been stated that each separate leaflet circumnutates. A
+pinna was cemented with shellac on the summit of a little stick driven
+firmly into the ground, immediately beneath a pair of leaflets, to the
+midribs of both of which excessively fine glass filaments were
+attached. This treatment did not injure the leaflets, for they went to
+sleep in the usual manner, and long retained their sensitiveness. the
+movements of one of them were traced during 49 h., as shown in Fig.
+158. On the first day the leaflet sank down till 11.30 A.M., and then
+rose till late in the evening in a zigzag line, indicating
+circumnutation. On the second day, when more accustomed to its new
+state, it oscillated twice up and twice down during the 24 h. This
+plant was subjected to a rather low temperature, viz., 62°–64° F.; had
+it been kept warmer, no doubt the movements of the leaflet would have
+been much more rapid and complicated. It may be seen in the diagram
+that the ascending and descending lines do not coincide; but the large
+amount of lateral movement in the evening is the result of the leaflets
+bending towards the apex of the leaf when going to sleep. Another
+leaflet was casually observed, and found to be continually
+circumnutating during the same length of time.
+
+The circumnutation of the leaves is not destroyed by their being
+subjected to moderately long continued darkness; but the proper
+periodicity of their movements is lost. Some very young seedlings were
+kept during two days in the dark (temp. 57°–59° F.) except when the
+circumnutation of their stems was occasionally observed; and on the
+evening of the second day the leaflets did not fully and properly go to
+sleep. The pot was then placed for three days in a dark cupboard, under
+nearly the same temperature, and at the close of this period the
+leaflets showed no signs of sleeping, and were only slightly sensitive
+to a touch. On the following day the stem was cemented to a
+stick, and the movements of two leaves were traced on a vertical glass
+during 72 h. The plants were still kept in the dark, excepting that at
+each observation, which lasted 3 or 4 minutes, they were illuminated by
+two candles. On the third day the leaflets still exhibited a vestige of
+sensitiveness when forcibly pressed, but in the evening they showed no
+signs of sleep. Nevertheless, their petioles continued to circumnutate
+distinctly,
+although the proper order of their movements in relation to the day and
+night was wholly lost. Thus, one leaf descended during the first two
+nights (i.e. between 10 P.M. and 7 A.M. next morning) instead of
+ascending, and on the third night it moved chiefly in a lateral
+direction. The second leaf behaved in an equally abnormal manner,
+moving laterally during the first night, descending greatly during the
+second, and ascending to an unusual height during the third night.
+
+Fig 158. Mimosa pudica: circumnutation and nyctitropic movement of a
+leaflet (with pinna secured), traced on a vertical glass, from 8 A.M.
+Sept. 14th to 9 A.M. 16th.
+
+With plants kept at a high temperature and exposed to the light, the
+most rapid circumnutating movement of the apex of a leaf which was
+observed, amounted to 1/500 of an inch in one second; and this would
+have equalled 1/8 of an inch in a minute, had not the leaf occasionally
+stood still. The actual distance travelled by the apex (as ascertained
+by a measure placed close to the leaf) was on one occasion nearly 3/4
+of an inch in a vertical direction in 15 m.; and on another occasion
+5/8 of an inch in 60 m.; but there was also some lateral movement.
+
+Mimosa albida.[19]—The leaves of this plant, one of which is here
+figured (Fig. 159) reduced to 2/3 of the natural size, present some
+interesting peculiarities. It consists of a long petiole bearing only
+two pinnae (here represented as rather more divergent than is usual),
+each with two pairs of leaflets. But the inner
+basal leaflets are greatly reduced in size, owing probably to the want
+of space for their full development, so that they may be considered as
+almost rudimentary. They vary somewhat in size, and both occasionally
+disappear, or only one. Nevertheless, they are not in the least
+rudimentary in function, for they are sensitive, extremely heliotropic,
+circumnutate at nearly the same rate as the fully developed leaflets,
+and assume when asleep exactly the same position. With M. pudica the
+inner leaflets at the base and between the pinnae are likewise much
+shortened and obliquely truncated; this fact was well seen in some
+seedlings of M. pudica, in which the third leaf above the cotyledons
+bore only two pinnae, each with only 3 or 4 pairs of leaflets, of which
+the inner basal one was less than half as long as its fellow; so that
+the whole leaf resembled pretty closely that of M. albida. In this
+latter species the main petiole terminates in a little point, and on
+each side of this there is a pair of minute, flattened, lancet-shaped
+projections, hairy on their margins, which drop off and disappear soon
+after the leaf is fully developed. There can hardly be a doubt that
+these little projections are the last and fugacious representatives of
+an additional pair of leaflets to each pinna; for the outer one is
+twice as broad as the inner one, and a little longer, viz. 7/100 of an
+inch, whilst the inner one is only 5/100–6/100 long. Now if the basal
+pair of leaflets of the existing leaves were to become rudimentary, we
+should expect that the rudiments would still exhibit some trace of
+their present great inequality of size. The conclusion that the pinnae
+of the parent-form of M. albida possessed at least three pairs of
+leaflets, instead of, as at present, only two, is supported by the
+structure of the first true leaf; for this consists of a simple
+petiole, often bearing three pairs of leaflets. This latter fact, as
+well as the presence of the rudiments, both lead to the conclusion that
+M. albida is descended from a form the leaves of which bore more than
+two pairs of leaflets. The second leaf above the cotyledons resembles
+in all respects the leaves on fully developed plants.
+
+ [19] Mr. Thiselton Dyer informs us that this Peruvian plant (which was
+ sent to us from Kew) is considered by Mr. Bentham (‘Trans. Linn.
+ Soc.,’ vol. xxx. p. 390) to be “the species or variety which most
+ commonly represents the M. sensitiva of our gardens.”
+
+
+Fig. 159. Mimosa albida: leaf seen from vertically above.
+
+When the leaves go to sleep, each leaflet twists half round, so as to
+present its edge to the zenith, and comes into close contact with its
+fellow. The pinnae also approach each other closely, so that the four
+terminal leaflets come together. The large basal leaflets (with the
+little rudimentary ones in contact with them) move inwards and
+forwards, so as to embrace the outside of the united terminal leaflets,
+and thus all eight leaflets
+(the rudimentary ones included) form together a single vertical packet.
+The two pinnae at the same time that they approach each other sink
+downwards, and thus instead of extending horizontally in the same line
+with the main petiole, as during the day, they depend at night at about
+45°, or even at a greater angle, beneath the horizon. The movement of
+the main petiole seems to be variable; we have seen it in the evening
+27° lower than during the day; but sometimes in nearly the same
+position. Nevertheless, a sinking movement in the evening and a rising
+one during the night is probably the normal course, for this was
+well-marked in the petiole of the first-formed true leaf.
+
+The circumnutation of the main petiole of a young leaf was traced
+during 2 3/4 days, and was considerable in extent, but less complex
+than that of M. pudica. The movement was much more lateral than is
+usual with circumnutating leaves, and this was the sole peculiarity
+which it presented. The apex of one of the terminal leaflets was seen
+under the microscope to travel 1/50 of an inch in 3 minutes.
+
+Mimosa marginata.—The opposite leaflets rise up and approach each other
+at night, but do not come into close contact, except in the case of
+very young leaflets on vigorous shoots. Full-grown leaflets
+circumnutate during the day slowly and on a small scale.
+
+Schrankia uncinata (Tribe 20).—A leaf consists of two or three pairs of
+pinnae, each bearing many small leaflets. These, when the plant is
+asleep, are directed forwards and become imbricated. The angle between
+the two terminal pinnae was diminished at night, in one case by 15°;
+and they sank almost vertically downwards. The hinder pairs of pinnae
+likewise sink downwards, but do not converge, that is, move towards the
+apex of the leaf. The main petiole does not become depressed, at least
+during the evening. In this latter respect, as well as in the sinking
+of the pinnae, there is a marked difference between the nyctitropic
+movements of the present plant and of Mimosa pudica. It should,
+however, be added that our specimen was not in a very vigorous
+condition. The pinnae of Schrankia aculeata also sink at night.
+
+Acacia Farnesiana (Tribe 22).—The different appearance presented by a
+bush of this plant when asleep and awake is wonderful. The same leaf in
+the two states is shown in the following figure (Fig. 160). The
+leaflets move towards the apex of the pinna and become imbricated, and
+the pinnae then look like bits of dangling string. The following
+remarks and measurements
+do not fully apply to the small leaf here figured. The pinnae move
+forwards and at the same time sink downwards, whilst the main petiole
+rises considerably. With respect to the degree of movement: the two
+terminal pinnae of one specimen formed together an angle of 100° during
+the day, and at night of only 38°, so each had moved 31° forwards. The
+penultimate pinnae during the day formed together an angle of 180°,
+that is, they stood in a straight line opposite one another, and at
+night each had moved 65° forwards. The basal pair of pinnae were
+directed during the day, each about 21° backwards, and at night 38°
+forwards, so each had moved 59° forwards. But the pinnae at the same
+time sink greatly, and sometimes hang almost perpendicularly downwards.
+The main petiole, on the other hand, rises much: by 8.30 P.M. one stood
+34° higher than at noon, and by 6.40 A.M. on the following morning it
+was still higher by 10°; shortly after this hour the diurnal sinking
+movement commenced. The course of a nearly full-grown leaf was traced
+during 14 h.; it was strongly zigzag, and apparently
+represented five ellipses, with their longer axes differently directed.
+
+Fig. 160. Acacia Farnesiana: A, leaf during the day; B, the same leaf
+at night.
+
+Albizzia lophantha (Tribe 23).—The leaflets at night come into contact
+with one another, and are directed towards the apex of the pinna. The
+pinnae approach one another, but remain in the same plane as during the
+day; and in this respect they differ much from those of the above
+Schrankia and Acacia. The main petiole rises but little. The
+first-formed leaf above the cotyledons bore 11 leaflets on each side,
+and these slept like those on the subsequently formed leaves; but the
+petiole of this first leaf was curved downwards during the day and at
+night straightened itself, so that the chord of its arc then stood 16°
+higher than in the day-time.
+
+Melaleuca ericaefolia (Myrtaceae).—According to Bouché (‘Bot. Zeit.,’
+1874, p. 359) the leaves sleep at night, in nearly the same manner as
+those of certain species of Pimelia.
+
+Œnothera mollissima (Onagrarieae).—According to Linnæus (‘Somnus
+Plantarum’), the leaves rise up vertically at night.
+
+Passiflora gracilis (Passifloracae).—The young leaves sleep by their
+blades hanging vertically downwards, and the whole length of the
+petiole then becomes somewhat curved downwards. Externally no trace of
+a pulvinus can be seen. The petiole of the uppermost leaf on a young
+shoot stood at 10.45 A.M. at 33° above the horizon; and at 10.30 P.M.,
+when the blade was vertically dependent, at only 15°, so the petiole
+had fallen 18°. That of the next older leaf fell only 7°. From some
+unknown cause the leaves do not always sleep properly. The stem of a
+plant, which had stood for some time before a north-east window, was
+secured to a stick at the base of a young leaf, the blade of which was
+inclined at 40° below the horizon. From its position the leaf had to be
+viewed obliquely, consequently the vertically ascending and descending
+movements appeared when traced oblique. On the first day (Oct. 12th)
+the leaf descended in a zigzag line until late in the evening; and by
+8.15 A.M. on the 13th had risen to nearly the same level as on the
+previous morning. A new tracing was now begun (Fig. 161). The leaf
+continued to rise until 8.50 A.M., then moved a little to the right,
+and afterwards descended. Between 11 A.M. and 5 P.M. it circumnutated,
+and after the latter hour the great nocturnal fall commenced. At 7.15
+P.M. it depended vertically. The dotted line ought to have been
+prolonged much lower down in the figure. By 6.50 A.M. on the following
+morning (14th) the
+leaf had risen greatly, and continued to rise till 7.50 A.M., after
+which hour it redescended. It should be observed that the lines traced
+on this second morning would have coincided with and confused those
+previously traced, had not the pot been slided a very little to the
+left. In the evening (14th) a mark was placed behind the filament
+attached to the apex of the leaf, and its movement was carefully traced
+from 5 P.M. to 10.15 P.M. Between 5 and 7.15 P.M. the leaf descended in
+a straight line, and at the latter hour it appeared vertically
+dependent. But between 7.15 and 10.15 P.M. the line consisted of a
+succession of steps, the cause of which we could not understand; it
+was, however, manifest that the movement was no longer a simple
+descending one.
+
+Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement
+of leaf, traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 A.M.
+14th. Figure reduced to two-thirds of original scale.
+
+Siegesbeckia orientalis (Compositæ).—Some seedlings were raised in the
+middle of winter and kept in the hot-house; they flowered, but did not
+grow well, and their leaves never showed any signs of sleep. The leaves
+on other seedlings raised in May were horizontal at noon (June 22nd),
+and depended at a
+considerable angle beneath the horizon at 10 P.M. In the case of four
+youngish leaves which were from 2 to 2½ inches in length, these angles
+were found to be 50°, 56°, 60°, and 65°. At the end of August when the
+plants had grown to a height of 10 to 11 inches, the younger leaves
+were so much curved downwards at night that they might truly be said to
+be asleep. This is one of the species which must be well illuminated
+during the day in order to sleep, for on two occasions when plants were
+kept all day in a room with north-east windows, the leaves did not
+sleep at night. The same cause probably accounts for the leaves on our
+seedlings raised in the dead of the winter not sleeping. Professor
+Pfeffer informs us that the leaves of another species (S. Jorullensis
+?) hang vertically down at night.
+
+Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day,
+and asleep at night. Figures copied from photographs, and reduced.
+
+
+Ipomœa caerulea and purpurea (Convolvulaceae).—The leaves on very young
+plants, a foot or two in height, are depressed at night to between 68°
+and 80° beneath the horizon; and some hang quite vertically downwards.
+On the following morning they again rise into a horizontal position.
+The petioles become at night downwardly curved, either through their
+entire length or in the upper part alone; and this apparently causes
+the depression of the blade. It seems necessary that the leaves should
+be well illuminated during the day in order to sleep, for those which
+stood on the back of a plant before a north-east window did not sleep.
+
+Nicotiana tabacum (var. Virginian) and glauca (Solaneae).—The young
+leaves of both these species sleep by bending vertically upwards.
+Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are
+given on p. 385: one of the shoots, from which the photographs were
+taken, was accidentally bent to one side.
+
+Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of
+a leaf (5 inches in length), traced on a vertical glass, from 3 P.M.
+July 10th to 8.10 A.M. 13th. Apex of leaf 4 inches from glass. Temp.
+17½°–18½° C. Figure reduced to one-half original scale.
+
+At the base of the petiole of N. tabacum, on the outside, there is a
+mass of cells, which are rather smaller than elsewhere, and
+have their longer axes differently directed from the cells of the
+parenchyma, and may therefore be considered as forming a sort of
+pulvinus. A young plant of N. tabacum was selected, and the
+circumnutation of the fifth leaf above the cotyledons was observed
+during three days. On the first morning (July 10th) the leaf fell from
+9 to 10 A.M., which is its normal course, but rose during the remainder
+of the day; and this no doubt was due to its being illuminated
+exclusively from above; for properly the evening rise does not commence
+until 3 or 4 P.M. In the figure as given on p. 386 (Fig. 163) the first
+dot was made at 3 P.M.; and the tracing was continued for the following
+65 h. When the leaf pointed to the dot next above that marked 3 P.M. it
+stood horizontally. The tracing is remarkable only from its simplicity
+and the straightness of the lines. The leaf each day described a single
+great ellipse; for it should be observed that the ascending and
+descending lines do not coincide. On the evening of the 11th the leaf
+did not descend quite so low as usual, and it now zigzagged a little.
+The diurnal sinking movement had already commenced each morning by 7
+A.M. The broken lines at the top of the figure, representing the
+nocturnal vertical position of the leaf, ought to be prolonged much
+higher up.
+
+Mirabilis longiflora and jalapa (Nyctagineae).—The first pair of leaves
+above the cotyledons, produced by seedlings of both these species, were
+considerably divergent during the day, and at night stood up vertically
+in close contact with one another. The two upper leaves on an older
+seedling were almost horizontal by day, and at night stood up
+vertically, but were not in close contact, owing to the resistance
+offered by the central bud.
+
+Polygonum aviculare (Polygoneae).—Professor Batalin informs us that the
+young leaves rise up vertically at night. This is likewise the case,
+according to Linnæus, with several species of Amaranthus
+(Amaranthaceae); and we observed a sleep movement of this kind in one
+member of the genus. Again, with Chenopodium album (Chenopodieae), the
+upper young leaves of some seedlings, about 4 inches in height, were
+horizontal or sub-horizontal during the day, and at 10 P.M. on March
+7th were quite, or almost quite, vertical. Other seedlings raised in
+the greenhouse during the winter (Jan. 28th) were observed day and
+night, and no difference could be perceived in the position of their
+leaves. According to Bouché (‘Bot. Zeitung,’ 1874, p. 359) the leaves
+of Pimelia linoides and spectabilis (Thymeleae) sleep at night.
+
+
+Euphorbia jacquiniaeflora (Euphorbiaceae).—Mr. Lynch called our
+attention to the fact that the young leaves of this plant sleep by
+depending vertically. The third leaf from the summit (March 11th) was
+inclined during the day 30° beneath the horizon, and at night hung
+vertically down, as did some of the still younger leaves. It rose up to
+its former level on the following morning. The fourth and fifth leaves
+from the summit stood horizontally during the day, and sank down at
+night only 38°. The sixth leaf did not sensibly alter its position. The
+sinking movement is due to the downward curvature of the petiole, no
+part of which exhibits any structure like that of a pulvinus. Early on
+the morning of June 7th a filament was fixed longitudinally to a young
+leaf (the third from the summit, and 2 5/8 inches in length), and its
+movements were traced on a vertical glass during 72 h., the plant being
+illuminated from above through a skylight. Each day the leaf fell in a
+nearly straight line from 7 A.M. to 5 P.M., after which hour it was so
+much inclined downwards that the movement could no longer be traced;
+and during the latter part of each night, or early in the morning, the
+leaf rose. It therefore circumnutated in a very simple manner, making a
+single large ellipse every 24 h., for the ascending and descending
+lines did not coincide. On each successive morning it stood at a less
+height than on the previous one, and this was probably due partly to
+the increasing age of the leaf, and partly to the illumination being
+insufficient; for although the leaves are very slightly heliotropic,
+yet, according to Mr. Lynch’s and our own observations, their
+inclination during the day is determined by the intensity of the light.
+On the third day, by which time the extent of the descending movement
+had much decreased, the line traced was plainly much more zigzag than
+on any previous day, and it appeared as if some of its powers of
+movement were thus expended. At 10 P.M. on June 7th, when the leaf
+depended vertically, its movements were observed by a mark being placed
+behind it, and the end of the attached filament was seen to oscillate
+slowly and slightly from side to side, as well as upwards and
+downwards.
+
+Phyllanthus Niruri (Euphorbiaceae).—The leaflets of this plant sleep,
+as described by Pfeffer,[20] in a remarkable manner, apparently like
+those of Cassia, for they sink downwards at night and twist round, so
+that their lower surfaces are turned
+outwards. They are furnished as might have been expected from this
+complex kind of movement, with a pulvinus.
+
+ [20] ‘Die Period. Beweg.,’ p. 159.
+
+GYMNOSPERMS.
+
+Pinus Nordmanniana (Coniferæ).—M. Chatin states[21] that the leaves,
+which are horizontal during the day, rise up at night, so as to assume
+a position almost perpendicular to the branch from which they arise; we
+presume that he here refers to a horizontal branch. He adds: “En même
+temps, ce mouvement d’érection est accompangé d’un mouvement de torsion
+imprimé à la partie basilaire de la feuille, et pouvant souvent
+parcourir un arc de 90 degrés.” As the lower surfaces of the leaves are
+white, whilst the upper are dark green, the tree presents a widely
+different appearance by day and night. The leaves on a small tree in a
+pot did not exhibit with us any nyctitropic movements. We have seen in
+a former chapter that the leaves of Pinus pinaster and Austriaca are
+continually circumnutating.
+
+ [21] ‘Comptes Rendus,’ Jan. 1876, p. 171.
+
+MONOCOTYLEDONS.
+
+Thalia dealbata (Cannaceae).—the leaves of this plant sleep by turning
+vertically upwards; they are furnished with a well-developed pulvinus.
+It is the only instance known to us of a very large leaf sleeping. The
+blade of a young leaf, which was as yet only 13 1/4 inches in length
+and 6½ in breadth, formed at noon an angle with its tall petiole of
+121°, and at night stood vertically in a line with it, and so had risen
+59°. The actual distance travelled by the apex (as measured by an
+orthogonic tracing) of another large leaf, between 7.30 A.M. and 10
+P.M., was 10½ inches. The circumnutation of two young and dwarfed
+leaves, arising amongst the taller leaves at the base of the plant, was
+traced on a vertical glass during two days. On the first day the apex
+of one, and on the second day the apex of the other leaf, described
+between 6.40 A.M. and 4 P.M. two ellipses, the longer axes of which
+were extended in very different directions from the lines representing
+the great diurnal sinking and nocturnal rising movement.
+
+Maranta arundinacea (Cannaceae).—The blades of the leaves, which are
+furnished with a pulvinus, stand horizontally during
+the day or between 10° and 20° above the horizon, and at night
+vertically upwards. They therefore rise between 70° and 90° at night.
+The plant was placed at noon in the dark in the hot-house, and on the
+following day the movements of the leaves were traced. Between 8.40 and
+10.30 A.M. they rose, and then fell greatly till 1.37 P.M. But by 3
+P.M. they had again risen a little, and continued to rise during the
+rest of the afternoon and night; on the following morning they stood at
+the same level as on the previous day. Darkness, therefore, during a
+day and a half does not interfere with the periodicity of their
+movements. On a warm but stormy evening, the plant whilst being brought
+into the house, had its leaves violently shaken, and at night not one
+went to sleep. On the next morning the plant was taken back to the
+hot-house, and again at night the leaves did not sleep; but on the
+ensuing night they rose in the usual manner between 70° and 80°. This
+fact is analogous with what we have observed with climbing plants,
+namely, that much agitation checks for a time their power of
+circumnutation; but the effect in this instance was much more strongly
+marked and prolonged.
+
+Colocasia antiquorum (Caladium esculentum, Hort.) (Aroideae).—The
+leaves of this plant sleep by their blades sinking in the evening, so
+as to stand highly inclined, or even quite vertically with their tips
+pointing to the ground. They are not provided with a pulvinus. The
+blade of one stood at noon 1 degree beneath the horizon; at 4.20 P.M.,
+20°; at 6 P.M. 43°; at 7.20 P.M., 69°; and at 8.30 P.M., 68°; so it had
+now begun to rise; at 10.15 P.M. it stood at 65°, and on the following
+early morning at 11° beneath the horizon. The circumnutation of another
+young leaf (with its petiole only 3 1/4 inches, and the blade 4 inches
+in length), was traced on a vertical glass during 48 h.; it was dimly
+illuminated through a skylight, and this seemed to disturb the proper
+periodicity of its movements. Nevertheless, the leaf fell greatly
+during both afternoons, till either 7.10 P.M. or 9 P.M., when it rose a
+little and moved laterally. By an early hour on both mornings, it had
+assumed its diurnal position. The well-marked lateral movement for a
+short time in the early part of the night, was the only interesting
+fact which it presented, as this caused the ascending and descending
+lines not to coincide, in accordance with the general rule with
+circumnutating organs. The movements of the leaves of this plant are
+thus of the most simple kind; and the tracing is not worth giving. We
+have seen that in another genus of the Aroideae, namely, Pistia, the
+leaves
+rise so much at night that they may almost be said to sleep.
+
+Strephium floribundum[22] (Gramineæ).—The oval leaves are provided with
+a pulvinus, and are extended horizontally or declined a little beneath
+the horizon during the day. Those on the upright culms simply rise up
+vertically at night, so that their tips are directed towards the
+zenith. (Fig. 164.) Horizontally extended leaves arising from much
+inclined or almost horizontal culms, move at night so that their tips
+point towards the apex of the culm, with one lateral margin directed
+towards the zenith; and in order to assume this position the leaves
+have to twist on their own axes through an angle of nearly 90°. Thus
+the surface of the blade always stands vertically, whatever may be the
+position of the midrib or of the leaf as a whole.
+
+ [22] A. Brongniart first observed that the leaves of this plant and of
+ Marsilea sleep: see ‘Bull. de la Soc. Bot. de France,’ tom. vii. 1860,
+ p. 470.
+
+
+Fig. 164. Strephium floribundum: culms with leaves during the day, and
+when asleep at night. Figures reduced.
+
+The circumnutation of a young leaf (2.3 inches in length) was traced
+during 48 h. (Fig. 165). The movement was remarkably simple; the leaf
+descended from before 6.40 A.M. until 2 or 2.50 P.M., and then rose so
+as to stand vertically at about 6 P.M., descending again late in the
+night or in the very early morning.
+On the second day the descending line zigzagged slightly. As usual, the
+ascending and descending lines did not coincide. On another occasion,
+when the temperature was a little higher, viz., 24°–26½° C., a leaf was
+observed 17 times between 8.50 A.M. and 12.16 P.M.; it changed its
+course by as much as a rectangle six times in this interval of 3 h. 26
+m., and described two irregular triangles and a half. The leaf,
+therefore, on this occasion circumnutated rapidly and in a complex
+manner.
+
+Fig. 165. Strephium floribundum: circumnutation and nyctitropic
+movement of a leaf, traced from 9 A.M. June 26th to 8.45 A.M. 27th;
+filament fixed along the midrib. Apex of leaf 8 1/4 inches from the
+vertical glass; plant illuminated from above. Temp. 23½°–24½° C.
+
+ACOTYLEDONS.
+
+Marsilea quadrifoliata (Marsileaceae).—The shape of a leaf, expanded
+horizontally during the day, is shown at A (Fig. 166). Each leaflet is
+provided with a well-developed pulvinus. When the leaves sleep, the two
+terminal leaflets rise up, twist half round and come into contact with
+one another (B), and are afterwards embraced by the two lower leaflets
+(C); so that the four leaflets with their lower surfaces turned
+outwards form a vertical packet. The curvature of the summit of the
+petiole of the leaf figured asleep, is merely accidental. The plant was
+brought into a room, where the temperature was only a little above 60°
+F., and the movement of one of the leaflets (the petiole having been
+secured) was traced
+during 24 h. (Fig. 167). The leaf fell from the early morning till 1.50
+P.M., and then rose till 6 P.M., when it was asleep. A vertically
+dependent glass filament was now fixed to one of the terminal and inner
+leaflets; and part of the tracing in Fig. 167, after 6 P.M., shows that
+it continued to sink, making one zigzag, until 10.40 P.M. At 6.45 A.M.
+on the following morning, the leaf was awaking, and the filament
+pointed above the vertical glass, but by 8.25 A.M. it occupied the
+position shown in the figure. The diagram differs greatly in appearance
+from most of those previously given; and this is due to the leaflet
+twisting and moving laterally as it approaches and comes into contact
+with
+its fellow. The movement of another leaflet, when asleep, was traced
+between 6 P.M. and 10.35 P.M., and it clearly circumnutated, for it
+continued for two hours to sink, then rose, and then sank still lower
+than it was at 6 P.M. It may be seen in the preceding figure (167) that
+the leaflet, when the plant was subjected to a rather low temperature
+in the house, descended and ascended during the middle of the day in a
+somewhat zigzag line; but when kept in the hot-house from 9 A.M. to 3
+P.M. at a high but varying temperature (viz., between 72° and 83° F.) a
+leaflet (with the petiole secured) circumnutated rapidly, for it made
+three large vertical ellipses in the course of the six hours. According
+to Brongniart, Marsilea pubescens sleeps like the present species.
+These plants are the sole cryptogamic ones known to sleep.
+
+Fig. 166. Marsilea quadrifoliata: A, leaf during the day, seen from
+vertically above; B, leaf beginning to go to sleep, seen laterally; C,
+the same asleep. Figures reduced to one-half of natural scale.
+
+Fig. 167. Marsilea quadrifoliata: circumnutation and nyctitropic
+movement of leaflet traced on vertical glass, during nearly 24 h.
+Figure reduced to two-thirds of original scale. Plant kept at rather
+too low a temperature.
+
+Summary and Concluding Remarks on the Nyctitropic or Sleep-movements of
+Leaves.—That these movements are in some manner of high importance to
+the plants which exhibit them, few will dispute who have observed how
+complex they sometimes are. Thus with Cassia, the leaflets which are
+horizontal during the day not only bend at night vertically downwards
+with the terminal pair directed considerably backwards, but they also
+rotate on their own axes, so that their lower surfaces are turned
+outwards. The terminal leaflet of Melilotus likewise rotates, by which
+movement one of its lateral edges is directed upwards, and at the same
+time it moves either to the left or to the right, until its upper
+surface comes into contact with that of the lateral leaflet on the same
+side, which has likewise rotated on its own axis. With Arachis, all
+four leaflets form together during the night a single vertical packet;
+and to the effect this the two anterior leaflets have to move upwards
+and the two posterior ones forwards, besides all twisting on their own
+axes. In the genus Sida the leaves of some species move at night
+through an angle of 90° upwards, and of others
+through the same angle downwards. We have seen a similar difference in
+the nyctitropic movements of the cotyledons in the genus Oxalis. In
+Lupinus, again, the leaflets move either upwards or downwards; and in
+some species, for instance L. luteus, those on one side of the
+star-shaped leaf move up, and those on the opposite side move down; the
+intermediate ones rotating on their axes; and by these varied
+movements, the whole leaf forms at night a vertical star instead of a
+horizontal one, as during the day. Some leaves and leaflets, besides
+moving either upwards or downwards, become more or less folded at
+night, as in Bauhinia and in some species of Oxalis. The positions,
+indeed, which leaves occupy when asleep are almost infinitely
+diversified; they may point either vertically upwards or downwards, or,
+in the case of leaflets, towards the apex or towards the base of the
+leaf, or in any intermediate position. They often rotate at least as
+much as 90° on their own axes. The leaves which arise from upright and
+from horizontal or much inclined branches on the same plant, move in
+some few cases in a different manner, as with Porlieria and Strephium.
+The whole appearance of many plants is wonderfully changed at night, as
+may be seen with Oxalis, and still more plainly with Mimosa. A bush of
+Acacia Farnesiana appears at night as if covered with little dangling
+bits of string instead of leaves. Excluding a few genera not seen by
+ourselves, about which we are in doubt, and excluding a few others the
+leaflets of which rotate at night, and do not rise or sink much, there
+are 37 genera in which the leaves or leaflets rise, often moving at the
+same time towards the apex or towards the base of the leaf, and 32
+genera in which they sink at night.
+
+The nyctitropic movements of leaves, leaflets, and
+petioles are effected in two different ways; firstly, by alternately
+increased growth on their opposite sides, preceded by increased
+turgescence of the cells; and secondly by means of a pulvinus or
+aggregate of small cells, generally destitute of chlorophyll, which
+become alternately more turgescent on nearly opposite sides; and this
+turgescence is not followed by growth except during the early age of
+the plant. A pulvinus seems to be formed (as formerly shown) by a group
+of cells ceasing to grow at a very early age, and therefore does not
+differ essentially from the surrounding tissues. The cotyledons of some
+species of Trifolium are provided with a pulvinus, and others are
+destitute of one, and so it is with the leaves in the genus Sida. We
+see also in this same genus gradations in the state of the development
+of the pulvinus; and in Nicotiana we have what may probably be
+considered as the commencing development of one. The nature of the
+movement is closely similar, whether a pulvinus is absent or present,
+as is evident from many of the diagrams given in this chapter. It
+deserves notice that when a pulvinus is present, the ascending and
+descending lines hardly ever coincide, so that ellipses are habitually
+described by the leaves thus provided, whether they are young or so old
+as to have quite ceased growing. This fact of ellipses being described,
+shows that the alternately increased turgescence of the cells does not
+occur on exactly opposite sides of the pulvinus, any more than the
+increased growth which causes the movements of leaves not furnished
+with pulvini. When a pulvinus is present, the nyctitropic movements are
+continued for a very much longer period than when such do not exist.
+This has been amply proved in the case of cotyledons, and Pfeffer has
+given observations to the same effect with respect
+to leaves. We have seen that a leaf of Mimosa pudica continued to move
+in the ordinary manner, though somewhat more simply, until it withered
+and died. It may be added that some leaflets of Trifolium pratense were
+pinned open during 10 days, and on the first evening after being
+released they rose up and slept in the usual manner. Besides the long
+continuance of the movements when effected by the aid of a pulvinus
+(and this appears to be the final cause of its development), a twisting
+movement at night, as Pfeffer has remarked, is almost confined to
+leaves thus provided.
+
+It is a very general rule that the first true leaf, though it may
+differ somewhat in shape from the leaves on the mature plant, yet
+sleeps like them; and this occurs quite independently of the fact
+whether or not the cotyledons themselves sleep, or whether they sleep
+in the same manner. But with Phaseolus Roxburghii the first unifoliate
+leaves rise at night almost sufficiently to be said to sleep, whilst
+the leaflets of the secondary trifoliate leaves sink vertically at
+night. On young plants of Sida rhombaefolia, only a few inches in
+height, the leaves did not sleep, though on rather older plants they
+rose up vertically at night. On the other hand, the leaves on very
+young plants of Cytisus fragrans slept in a conspicuous manner, whilst
+on old and vigorous bushes kept in the greenhouse, the leaves did not
+exhibit any plain nyctitropic movement. In the genus Lotus the basal
+stipule-like leaflets rise up vertically at night, and are provided
+with pulvini.
+
+As already remarked, when leaves or leaflets change their position
+greatly at night and by complicated movements, it can hardly be doubted
+that these must be in some manner beneficial to the plant. If so, we
+must extend the same conclusion to a large number of sleeping plants;
+for the most complicated and the simplest nyctitropic movements are
+connected together by the finest gradations. But owing to the causes
+specified in the beginning of this chapter, it is impossible in some
+few cases to determine whether or not certain movements should be
+called nyctitropic. Generally, the position which the leaves occupy at
+night indicates with sufficient clearness, that the benefit thus
+derived, is the protection of their upper surfaces from radiation into
+the open sky, and in many cases the mutual protection of all the parts
+from cold by their being brought into close approximation. It should be
+remembered that it was proved in the last chapter, that leaves
+compelled to remain extended horizontally at night, suffered much more
+from radiation than those which were allowed to assume their normal
+vertical position.
+
+The fact of the leaves of several plants not sleeping unless they have
+been well illuminated during the day, made us for a time doubt whether
+the protection of their upper surfaces from radiation was in all cases
+the final cause of their well-pronounced nyctitropic movements. But we
+have no reason to suppose that the illumination from the open sky,
+during even the most clouded day, is insufficient for this purpose; and
+we should bear in mind that leaves which are shaded from being seated
+low down on the plant, and which sometimes do not sleep, are likewise
+protected at night from full radiation. Nevertheless, we do not wish to
+deny that there may exist cases in which leaves change their position
+considerably at night, without their deriving any benefit from such
+movements.
+
+Although with sleeping plants the blades almost
+always assume at night a vertical, or nearly vertical position, it is a
+point of complete indifference whether the apex, or the base, or one of
+the lateral edges, is directed to the zenith. It is a rule of wide
+generality, that whenever there is any difference in the degree of
+exposure to radiation between the upper and the lower surfaces of
+leaves and leaflets, it is the upper which is the least exposed, as may
+be seen in Lotus, Cytisus, Trifolium, and other genera. In several
+species of Lupinus the leaflets do not, and apparently from their
+structure cannot, place themselves vertically at night, and
+consequently their upper surfaces, though highly inclined, are more
+exposed than the lower; and here we have an exception to our rule. But
+in other species of this genus the leaflets succeed in placing
+themselves vertically; this, however, is effected by a very unusual
+movement, namely, by the leaflets on the opposite sides of the same
+leaf moving in opposite directions.
+
+It is again a very common rule that when leaflets come into close
+contact with one another, they do so by their upper surfaces, which are
+thus best protected. In some cases this may be the direct result of
+their rising vertically; but it is obviously for the protection of the
+upper surfaces that the leaflets of Cassia rotate in so wonderful a
+manner whilst sinking downwards; and that the terminal leaflet of
+Melilotus rotates and moves to one side until it meets the lateral
+leaflet on the same side. When opposite leaves or leaflets sink
+vertically down without any twisting, their lower surfaces approach
+each other and sometimes come into contact; but this is the direct and
+inevitable result of their position. With many species of Oxalis the
+lower surfaces of the adjoining leaflets are pressed together, and are
+thus better protected
+than the upper surfaces; but this depends merely on each leaflet
+becoming folded at night so as to be able to sink vertically downwards.
+The torsion or rotation of leaves and leaflets, which occurs in so many
+cases, apparently always serves to bring their upper surfaces into
+close approximation with one another, or with other parts of the plant,
+for their mutual protection. We see this best in such cases as those of
+Arachis, Mimosa albida, and Marsilea, in which all the leaflets form
+together at night a single vertical packet. If with Mimosa pudica the
+opposite leaflets had merely moved upwards, their upper surfaces would
+have come into contact and been well protected; but as it is, they all
+successively move towards the apex of the leaf; and thus not only their
+upper surfaces are protected, but the successive pairs become
+imbricated and mutually protect one another as well as the petioles.
+This imbrication of the leaflets of sleeping plants is a common
+phenomenon.
+
+The nyctitropic movement of the blade is generally effected by the
+curvature of the uppermost part of the petiole, which has often been
+modified into a pulvinus; or the whole petiole, when short, may be thus
+modified. But the blade itself sometimes curves or moves, of which fact
+Bauhinia offers a striking instance, as the two halves rise up and come
+into close contact at night. Or the blade and the upper part of the
+petiole may both move. Moreover, the petiole as a whole commonly either
+rises or sinks at night. This movement is sometimes large: thus the
+petioles of Cassia pubescens stand only a little above the horizon
+during the day, and at night rise up almost, or quite, perpendicularly.
+The petioles of the younger leaves of Desmodium gyrans also rise up
+vertically at night. On the other hand, with
+Amphicarpæa, the petioles of some leaves sank down as much as 57° at
+night; with Arachis they sank 39°, and then stood at right angles to
+the stem. Generally, when the rising or sinking of several petioles on
+the same plant was measured, the amount differed greatly. This is
+largely determined by the age of the leaf: for instance, the petiole of
+a moderately old leaf of Desmodium gyrans rose only 46°, whilst the
+young ones rose up vertically; that of a young leaf of Cassia
+floribunda rose 41°, whilst that of an older leaf rose only 12°. It is
+a more singular fact that the age of the plant sometimes influences
+greatly the amount of movement; thus with some young seedlings of a
+Bauhinia the petioles rose at night 30° and 34°, whereas those on these
+same plants, when grown to a height of 2 or 3 feet, hardly moved at
+all. The position of the leaves on the plant as determined by the
+light, seems also to influence the amount of movement of the petiole;
+for no other cause was apparent why the petioles of some leaves of
+Melilotus officinalis rose as much as 59°, and others only 7° and 9° at
+night.
+
+In the case of many plants, the petioles move at night in one direction
+and the leaflets in a directly opposite one. Thus, in three genera of
+Phaseoleae the leaflets moved vertically downwards at night, and the
+petioles rose in two of them, whilst in the third they sank. Species in
+the same genus often differ widely in the movements of their petioles.
+Even on the same plant of Lupinus pubescens some of the petioles rose
+30°, others only 6°, and others sank 4° at night. The leaflets of
+Cassia Barclayana moved so little at night that they could not be said
+to sleep, yet the petioles of some young leaves rose as much as 34°.
+These several facts apparently indicate that the movements
+of the petioles are not performed for any special purpose; though a
+conclusion of this kind is generally rash. When the leaflets sink
+vertically down at night and the petioles rise, as often occurs, it is
+certain that the upward movement of the latter does not aid the
+leaflets in placing themselves in their proper position at night, for
+they have to move through a greater angular space than would otherwise
+have been necessary.
+
+Notwithstanding what has just been said, it may be strongly suspected
+that in some cases the rising of the petioles, when considerable, does
+beneficially serve the plant by greatly reducing the surface exposed to
+radiation at night. If the reader will compare the two drawings (Fig.
+155, p. 371) of Cassia pubescens, copied from photographs, he will see
+that the diameter of the plant at night is about one-third of what it
+is by day, and therefore the surface exposed to radiation is nearly
+nine times less. A similar conclusion may be deduced from the drawings
+(Fig. 149, p. 358) of a branch awake and asleep of Desmodium gyrans. So
+it was in a very striking manner with young plants of Bauhinia, and
+with Oxalis Ortegesii.
+
+We are led to an analogous conclusion with respect to the movements of
+the secondary petioles of certain pinnate leaves. The pinnae of Mimosa
+pudica converge at night; and thus the imbricated and closed leaflets
+on each separate pinna are all brought close together into a single
+bundle, and mutually protect one another, with a somewhat smaller
+surface exposed to radiation. With Albizzia lophantha the pinnae close
+together in the same manner. Although the pinnae of Acacia Farnesiana
+do not converge much, they sink downwards. Those of Neptunia oleracea
+likewise
+move downwards, as well as backwards, towards the base of the leaf,
+whilst the main petiole rises. With Schrankia, again, the pinnae are
+depressed at night. Now in these three latter cases, though the pinnae
+do not mutually protect one another at night, yet after having sunk
+down they expose, as does a dependent sleeping leaf, much less surface
+to the zenith and to radiation than if they had remained horizontal.
+
+Any one who had never observed continuously a sleeping plant, would
+naturally suppose that the leaves moved only in the evening when going
+to sleep, and in the morning when awaking; but he would be quite
+mistaken, for we have found no exception to the rule that leaves which
+sleep continue to move during the whole twenty-four hours; they move,
+however, more quickly when going to sleep and when awaking than at
+other times. That they are not stationary during the day is shown by
+all the diagrams given, and by the many more which were traced. It is
+troublesome to observe the movements of leaves in the middle of the
+night, but this was done in a few cases; and tracings were made during
+the early part of the night of the movements in the case of Oxalis,
+Amphicarpæa, two species of Erythrina, a Cassia, Passiflora, Euphorbia
+and Marsilea; and the leaves after they had gone to sleep, were found
+to be in constant movement. When, however, opposite leaflets come into
+close contact with one another or with the stem at night, they are, as
+we believe, mechanically prevented from moving, but this point was not
+sufficiently investigated.
+
+When the movements of sleeping leaves are traced during twenty-four
+hours, the ascending and descending lines do not coincide, except
+occasionally and by accident for a short space; so that with many
+plants a
+single large ellipse is described during each twenty-four hours. Such
+ellipses are generally narrow and vertically directed, for the amount
+of lateral movement is small. That there is some lateral movement is
+shown by the ascending and descending lines not coinciding, and
+occasionally, as with Desmodium gyrans and Thalia dealbata, it was
+strongly marked. In the case of Melilotus the ellipses described by the
+terminal leaflet during the day are laterally extended, instead of
+vertically, as is usual; and this fact evidently stands in relation
+with the terminal leaflet moving laterally when it goes to sleep. With
+the majority of sleeping plants the leaves oscillate more than once up
+and down in the twenty-four hours; so that frequently two ellipses, one
+of moderate size, and one of very large size which includes the
+nocturnal movement, are described within the twenty-four hours. For
+instance, a leaf which stands vertically up during the night will sink
+in the morning, then rise considerably, again sink in the afternoon,
+and in the evening reascend and assume its vertical nocturnal position.
+It will thus describe, in the course of the twenty-four hours, two
+ellipses of unequal sizes. Other plants describe within the same time,
+three, four, or five ellipses. Occasionally the longer axes of the
+several ellipses extend in different directions, of which Acacia
+Farnesiana offered a good instance. The following cases will give an
+idea of the rate of movement: Oxalis acetosella completed two ellipses
+at the rate of 1 h. 25 m. for each; Marsilea quadrifoliata, at the rate
+of 2 h.; Trifolium subterraneum, one in 3 h. 30 m.; and Arachis
+hypogaea, in 4 h. 50 m. But the number of ellipses described within a
+given time depends largely on the state of the plant and on the
+conditions to which it is exposed. It often happens that a single
+ellipse may be described during one
+day, and two on the next. Erythrina corallodendron made four ellipses
+on the first day of observation and only a single one on the third,
+apparently owing to having been kept not sufficiently illuminated and
+perhaps not warm enough. But there seems likewise to be an innate
+tendency in different species of the same genus to make a different
+number of ellipses in the twenty-four hours: the leaflets of Trifolium
+repens made only one; those of T. resupinatum two, and those of T.
+subterraneum three in this time. Again, the leaflets of Oxalis
+Plumierii made a single ellipse; those of O. bupleurifolia, two; those
+of O. Valdiviana, two or three; and those of O. acetosella, at least
+five in the twenty-four hours.
+
+The line followed by the apex of a leaf or leaflet, whilst describing
+one or more ellipses during the day, is often zigzag, either throughout
+its whole course or only during the morning or evening: Robinia offered
+an instance of zigzagging confined to the morning, and a similar
+movement in the evening is shown in the diagram (Fig. 126) given under
+Sida. The amount of the zigzag movement depends largely on the plant
+being placed under highly favourable conditions. But even under such
+favourable conditions, if the dots which mark the position of the apex
+are made at considerable intervals of time, and the dots are then
+joined, the course pursued will still appear comparatively simple,
+although the number of the ellipses will be increased; but if dots are
+made every two or three minutes and these are joined, the result often
+is that all the lines are strongly zigzag, many small loops, triangles,
+and other figures being also formed. This fact is shown in two parts of
+the diagram (Fig. 150) of the movements of Desmodium gyrans. Strephium
+floribundum, observed under a high temperature,
+made several little triangles at the rate of 43 m. for each. Mimosa
+pudica, similarly observed, described three little ellipses in 67 m.;
+and the apex of a leaflet crossed 1/500 of an inch in a second, or 0.12
+inch in a minute. The leaflets of Averrhoa made a countless number of
+little oscillations when the temperature was high and the sun shining.
+The zigzag movement may in all cases be considered as an attempt to
+form small loops, which are drawn out by a prevailing movement in some
+one direction. The rapid gyrations of the little lateral leaflets of
+Desmodium belong to the same class of movements, somewhat exaggerated
+in rapidity and amplitude. The jerking movements, with a small advance
+and still smaller retreat, apparently not exactly in the same line, of
+the hypocotyl of the cabbage and of the leaves of Dionaea, as seen
+under the microscope, all probably come under this same head. We may
+suspect that we here see the energy which is freed during the incessant
+chemical changes in progress in the tissues, converted into motion.
+Finally, it should be noted that leaflets and probably some leaves,
+whilst describing their ellipses, often rotate slightly on their axes;
+so that the plane of the leaf is directed first to one and then to
+another side. This was plainly seen to be the case with the large
+terminal leaflets of Desmodium, Erythrina and Amphicarpæa, and is
+probably common to all leaflets provided with a pulvinus.
+
+With respect to the periodicity of the movements of sleeping leaves,
+Pfeffer[23] has so clearly shown that this depends on the daily
+alternations of light and darkness, that nothing farther need be said
+on this
+head. But we may recall the behaviour of Mimosa in the North, where the
+sun does not set, and the complete inversion of the daily movements by
+artificial light and darkness. It has also been shown by us, that
+although leaves subjected to darkness for a moderately long time
+continue to circumnutate, yet the periodicity of their movements is
+soon greatly disturbed, or quite annulled. The presence of light or its
+absence cannot be supposed to be the direct cause of the movements, for
+these are wonderfully diversified even with the leaflets of the same
+leaf, although all have of course been similarly exposed. The movements
+depend on innate causes, and are of an adaptive nature. The
+alternations of light and darkness merely give notice to the leaves
+that the period has arrived for them to move in a certain manner. We
+may infer from the fact of several plants (Tropaeolum, Lupinus, etc.)
+not sleeping unless they have been well illuminated during the day,
+that it is not the actual decrease of light in the evening, but the
+contrast between the amount at this hour and during the early part of
+the day, which excites the leaves to modify their ordinary mode of
+circumnutation.
+
+ [23] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, p. 30, et
+ passim.
+
+
+As the leaves of most plants assume their proper diurnal position in
+the morning, although light be excluded, and as the leaves of some
+plants continue to move in the normal manner in darkness during at
+least a whole day, we may conclude that the periodicity of their
+movements is to a certain extent inherited.[24] The strength of such
+inheritance differs
+much in different species, and seems never to be rigid; for plants have
+been introduced from all parts of the world into our gardens and
+greenhouses; and if their movements had been at all strictly fixed in
+relation to the alternations of day and night, they would have slept in
+this country at very different hours, which is not the case. Moreover,
+it has been observed that sleeping plants in their native homes change
+their times of sleep with the changing seasons.[25]
+
+ [24] Pfeffer denies such inheritance; he attributes (‘Die Period.
+ Bewegungen,’ pp. 30–56) the periodicity when prolonged for a day or
+ two in darkness, to “Nachwirkung,” or the after-effects of light and
+ darkness. But we are unable to follow his train of reasoning. There
+ does not seem to be any more reason for attributing such movements to
+ this cause than, for instance, the inherited habit of winter and
+ summer wheat to grow best at different seasons; for this habit is lost
+ after a few years, like the movements of leaves in darkness after a
+ few days. No doubt some effect must be produced on the seeds by the
+ long-continued cultivation of the parent-plants under different
+ climates, but no one probably would call this the “Nachwirkung” of the
+ climates.
+
+
+ [25] Pfeffer, ibid., p. 46.
+
+
+We may now turn to the systematic list. This contains the names of all
+the sleeping plants known to us, though the list undoubtedly is very
+imperfect. It may be premised that, as a general rule, all the species
+in the same genus sleep in nearly the same manner. But there are some
+exceptions; in several large genera including many sleeping species
+(for instance, Oxalis), some do not sleep. One species of Melilotus
+sleeps like a Trifolium, and therefore very differently from its
+congeners; so does one species of Cassia. In the genus Sida, the leaves
+either rise or fall at night; and with Lupinus they sleep in three
+different methods. Returning to the list, the first point which strikes
+us, is that there are many more genera amongst the Leguminosae (and in
+almost every one of the Leguminous tribes) than in all the other
+families put together; and we are tempted to connect this fact with the
+great
+mobility of the stems and leaves in this family, as shown by the large
+number of climbing species which it contains. Next to the Leguminosae
+come the Malvaceae, together with some closely allied families. But by
+far the most important point in the list, is that we meet with sleeping
+plants in 28 families, in all the great divisions of the Phanerogamic
+series, and in one Cryptogam. Now, although it is probable that with
+the Leguminosae the tendency to sleep may have been inherited from one
+or a few progenitors, and possibly so in the cohorts of the Malvales
+and Chenopodiales, yet it is manifest that the tendency must have been
+acquired by the several genera in the other families, quite
+independently of one another. Hence the question naturally arises, how
+has this been possible? and the answer, we cannot doubt is that leaves
+owe their nyctitropic movements to their habit of circumnutating,—a
+habit common to all plants, and everywhere ready for any beneficial
+development or modification.
+
+It has been shown in the previous chapters that the leaves and
+cotyledons of all plants are continually moving up and down, generally
+to a slight but sometimes to a considerable extent, and that they
+describe either one or several ellipses in the course of twenty-four
+hours; they are also so far affected by the alternations of day and
+night that they generally, or at least often, move periodically to a
+small extent; and here we have a basis for the development of the
+greater nyctitropic movements. That the movements of leaves and
+cotyledons which do not sleep come within the class of circumnutating
+movements cannot be doubted, for they are closely similar to those of
+hypocotyls, epicotyls, the stems of mature plants, and of various other
+organs. Now, if we take the simplest
+case of a sleeping leaf, we see that it makes a single ellipse in the
+twenty-four hours, which resembles one described by a non-sleeping leaf
+in every respect, except that it is much larger. In both cases the
+course pursued is often zigzag. As all non-sleeping leaves are
+incessantly circumnutating, we must conclude that a part at least of
+the upward and downward movement of one that sleeps, is due to ordinary
+circumnutation; and it seems altogether gratuitous to rank the
+remainder of the movement under a wholly different head. With a
+multitude of climbing plants the ellipses which they describe have been
+greatly increased for another purpose, namely, catching hold of a
+support. With these climbing plants, the various circumnutating organs
+have been so far modified in relation to light that, differently from
+all ordinary plants, they do not bend towards it. with sleeping plants
+the rate and amplitude of the movements of the leaves have been so far
+modified in relation to light, that they move in a certain direction
+with the waning light of the evening and with the increasing light of
+the morning more rapidly, and to a greater extent, than at other hours.
+
+But the leaves and cotyledons of many non-sleeping plants move in a
+much more complex manner than in the cases just alluded to, for they
+describe two, three, or more ellipses in the course of a day. Now, if a
+plant of this kind were converted into one that slept, one side of one
+of the several ellipses which each leaf daily describes, would have to
+be greatly increased in length in the evening, until the leaf stood
+vertically, when it would go on circumnutating about the same spot. On
+the following morning, the side of another ellipse would have to be
+similarly increased in length so as to bring the leaf back again into
+its diurnal position, when it would again circumnutate
+until the evening. If the reader will look, for instance, at the
+diagram (Fig. 142, p. 351), representing the nyctitropic movements of
+the terminal leaflet of Trifolium subterraneum, remembering that the
+curved broken lines at the top ought to be prolonged much higher up, he
+will see that the great rise in the evening and the great fall in the
+morning together form a large ellipse like one of those described
+during the daytime, differing only in size. Or, he may look at the
+diagram (Fig. 103, p. 236) of the 3½ ellipses described in the course
+of 6 h. 35 m. by a leaf of Lupinus speciosus, which is one of the
+species in this genus that does not sleep; and he will see that by
+merely prolonging upwards the line which was already rising late in the
+evening, and bringing it down again next morning, the diagram would
+represent the movements of a sleeping plant.
+
+With those sleeping plants which describe several ellipses in the
+daytime, and which travel in a strongly zigzag line, often making in
+their course minute loops, triangles, etc., if as soon as one of the
+ellipses begins in the evening to be greatly increased in size, dots
+are made every 2 or 3 minutes and these are joined, the line then
+described is almost strictly rectilinear, in strong contrast with the
+lines made during the daytime. This was observed with Desmodium gyrans
+and Mimosa pudica. With this latter plant, moreover, the pinnae
+converge in the evening by a steady movement, whereas during the day
+they are continually converging and diverging to a slight extent. In
+all such cases it was scarcely possible to observe the difference in
+the movement during the day and evening, without being convinced that
+in the evening the plant saves the expenditure of force by not moving
+laterally, and that its whole energy is now expended
+in gaining quickly its proper nocturnal position by a direct course. In
+several other cases, for instance, when a leaf after describing during
+the day one or more fairly regular ellipses, zigzags much in the
+evening, it appears as if energy was being expended, so that the great
+evening rise or fall might coincide with the period of the day proper
+for this movement.
+
+The most complex of all the movements performed by sleeping plants, is
+that when leaves or leaflets, after describing in the daytime several
+vertically directed ellipses, rotate greatly on their axes in the
+evening, by which twisting movement they occupy a wholly different
+position at night to what they do during the day. For instance, the
+terminal leaflets of Cassia not only move vertically downwards in the
+evening, but twist round, so that their lower surfaces face outwards.
+Such movements are wholly, or almost wholly, confined to leaflets
+provided with a pulvinus. But this torsion is not a new kind of
+movement introduced solely for the purpose of sleep; for it has been
+shown that some leaflets whilst describing their ordinary ellipses
+during the daytime rotate slightly, causing their blades to face first
+to one side and then to another. Although we can see how the slight
+periodical movements of leaves in a vertical plane could be easily
+converted into the greater yet simple nyctitropic movements, we do not
+at present know by what graduated steps the more complex movements,
+effected by the torsion of the pulvini, have been acquired. A probable
+explanation could be given in each case only after a close
+investigation of the movements in all the allied forms.
+
+From the facts and considerations now advanced we may conclude that
+nyctitropism, or the sleep of leaves
+and cotyledons, is merely a modification of their ordinary
+circumnutating movement, regulated in its period and amplitude by the
+alternations of light and darkness. The object gained is the protection
+of the upper surfaces of the leaves from radiation at night, often
+combined with the mutual protection of the several parts by their close
+approximation. In such cases as those of the leaflets of Cassia—of the
+terminal leaflets of Melilotus—of all the leaflets of Arachis,
+Marsilea, etc.—we have ordinary circumnutation modified to the extreme
+extent known to us in any of the several great classes of modified
+circumnutation. On this view of the origin of nyctitropism we can
+understand how it is that a few plants, widely distributed throughout
+the Vascular series, have been able to acquire the habit of placing the
+blades of their leaves vertically at night, that is, of sleeping,—a
+fact otherwise inexplicable.
+
+The leaves of some plants move during the day in a manner, which has
+improperly been called diurnal sleep; for when the sun shines brightly
+on them, they direct their edges towards it. To such cases we shall
+recur in the following chapter on Heliotropism. It has been shown that
+the leaflets of one form of Porlieria hygrometrica keep closed during
+the day, as long as the plant is scantily supplied with water, in the
+same manner as when asleep; and this apparently serves to check
+evaporation. There is only one other analogous case known to us,
+namely, that of certain Gramineæ, which fold inwards the sides of their
+narrow leaves, when these are exposed to the sun and to a dry
+atmosphere, as described by Duval-Jouve.[26] We have also observed the
+same phenomenon in Elymus arenareus.
+
+ [26] ‘Annal. des Sc. Nat. (Bot.),’ 1875, tom. i. pp. 326–329.
+
+
+There is another movement, which since the time of Linnæus has
+generally been called sleep, namely, that of the petals of the many
+flowers which close at night. These movements have been ably
+investigated by Pfeffer, who has shown (as was first observed by
+Hofmeister) that they are caused or regulated more by temperature than
+by the alternations of light and darkness. Although they cannot fail to
+protect the organs of reproduction from radiation at night, this does
+not seem to be their chief function, but rather the protection of the
+organs from cold winds, and especially from rain, during the day. the
+latter seems probable, as Kerner[27] has shown that a widely different
+kind of movement, namely, the bending down of the upper part of the
+peduncle, serves in many cases the same end. The closure of the flowers
+will also exclude nocturnal insects which may be ill-adapted for their
+fertilisation, and the well-adapted kinds at periods when the
+temperature is not favourable for fertilisation. Whether these
+movements of the petals consist, as is probable, of modified
+circumnutation we do not know.
+
+ [27] ‘Die Schutzmittel des Pollens,’ 1873, pp. 30–39.
+
+
+Embryology of Leaves.—A few facts have been incidentally given in this
+chapter on what may be called the embryology of leaves. With most
+plants the first leaf which is developed after the cotyledons,
+resembles closely the leaves produced by the mature plant, but this is
+not always the case. the first leaves produced by some species of
+Drosera, for instance by D. Capensis, differ widely in shape from those
+borne by the mature plant, and resemble closely the leaves of D.
+rotundifolia, as was shown to us by Prof. Williamson of Manchester. The
+first true leaf of
+the gorse, or Ulex, is not narrow and spinose like the older leaves. On
+the other hand, with many Leguminous plants, for instance, Cassia,
+Acacia lophantha, etc., the first leaf has essentially the same
+character as the older leaves, excepting that it bears fewer leaflets.
+In Trifolium the first leaf generally bears only a single leaflet
+instead of three, and this differs somewhat in shape from the
+corresponding leaflet on the older leaves. Now, with Trifolium
+Pannonicum the first true leaf on some seedlings was unifoliate, and on
+others completely trifoliate; and between these two extreme states
+there were all sorts of gradations, some seedlings bearing a single
+leaflet more or less deeply notched on one or both sides, and some
+bearing a single additional and perfect lateral leaflet. Here, then, we
+have the rare opportunity of seeing a structure proper to a more
+advanced age, in the act of gradually encroaching on and replacing an
+earlier or embryological condition.
+
+The genus Melilotus is closely allied to Trifolium, and the first leaf
+bears only a single leaflet, which at night rotates on its axis so as
+to present one lateral edge to the zenith. Hence it sleeps like the
+terminal leaflet of a mature plant, as was observed in 15 species, and
+wholly unlike the corresponding leaflet of Trifolium, which simply
+bends upwards. It is therefore a curious fact that in one of these 15
+species, viz., M. Taurica (and in a lesser degree in two others),
+leaves arising from young shoots, produced on plants which had been cut
+down and kept in pots during the winter in the green-house, slept like
+the leaves of a Trifolium, whilst the leaves on the fully-grown
+branches on these same plants afterwards slept normally like those of a
+Melilotus. If young shoots rising from the ground may be considered as
+new individuals, partaking to a certain extent of the nature of
+seedlings, then the peculiar manner in which their leaves slept may be
+considered
+as an embryological habit, probably the result of Melilotus being
+descended from some form which slept like a Trifolium. This view is
+partially supported by the leaves on old and young branches of another
+species, M. Messanensis (not included in the above 15 species), always
+sleeping like those of a Trifolium.
+
+The first true leaf of Mimosa albida consists of a simple petiole,
+often bearing three pairs of leaflets, all of which are of nearly equal
+size and of the same shape: the second leaf differs widely from the
+first, and resembles that on a mature plant (see Fig. 159, p. 379), for
+it consists of two pinnae, each of which bears two pairs of leaflets,
+of which the inner basal one is very small. But at the base of each
+pinna there is a pair of minute points, evidently rudiments of
+leaflets, for they are of unequal sizes, like the two succeeding
+leaflets. These rudiments are in one sense embryological, for they
+exist only during the youth of the leaf, falling off and disappearing
+as soon as it is fully grown.
+
+With Desmodium gyrans the two lateral leaflets are very much smaller
+than the corresponding leaflets in most of the species in this large
+genus; they vary also in position and size; one or both are sometimes
+absent; and they do not sleep like the fully-developed leaflets. They
+may therefore be considered as almost rudimentary; and in accordance
+with the general principles of embryology, they ought to be more
+constantly and fully developed on very young than on old plants. But
+this is not the case, for they were quite absent on some young
+seedlings, and did not appear until from 10 to 20 leaves had been
+formed. This fact leads to the suspicion that D. gyrans is descended
+through a unifoliate form (of which some exist) from a trifoliate
+species; and that the little lateral leaflets reappear through
+reversion. However this may be,
+the interesting fact of the pulvini or organs of movement of these
+little leaflets, not having been reduced nearly so much as their
+blades—taking the large terminal leaflet as the standard of
+comparison—gives us probably the proximate cause of their extraordinary
+power of gyration.
+
+
+
+
+CHAPTER VIII.
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
+
+
+Distinction between heliotropism and the effects of light on the
+periodicity of the movements of leaves—Heliotropic movements of Beta,
+Solanum, Zea, and Avena—Heliotropic movements towards an obscure light
+in Apios, Brassica, Phalaris, Tropaeolum, and Cassia—Apheliotropic
+movements of tendrils of Bignonia—Of flower-peduncles of
+Cyclamen—Burying of the pods—Heliotropism and apheliotropism modified
+forms of circumnutation—Steps by which one movement is converted into
+the other—Transversal-heliotropismus or diaheliotropism influenced by
+epinasty, the weight of the part and apogeotropism—Apogeotropism
+overcome during the middle of the day by diaheliotropism—Effects of the
+weight of the blades of cotyledons—So called diurnal sleep—Chlorophyll
+injured by intense light—Movements to avoid intense light
+
+
+Sachs first clearly pointed out the important difference between the
+action of light in modifying the periodic movements of leaves, and in
+causing them to bend towards its source.[1] The latter, or heliotropic
+movements are determined by the direction of the light, whilst periodic
+movements are affected by changes in its intensity and not by its
+direction. The periodicity of the circumnutating movement often
+continues for some time in darkness, as we have seen in the last
+chapter; whilst heliotropic bending ceases very quickly when the light
+fails. Nevertheless, plants which have ceased through long-continued
+darkness to move periodically, if re-exposed to the light are still,
+according to Sachs, heliotropic.
+
+ [1] ‘Physiologie Veg.’ (French Translation), 1868, pp. 42, 517, etc.
+
+
+Apheliotropism, or, as usually designated, negative
+heliotropism, implies that a plant, when unequally illuminated on the
+two sides, bends from the light, instead of, as in the last sub-class
+of cases, towards it; but apheliotropism is comparatively rare, at
+least in a well-marked degree. There is a third and large sub-class of
+cases, namely, those of “transversal-Heliotropismus” of Frank, which we
+will here call diaheliotropism. Parts of plants, under this influence,
+place themselves more or less transversely to the direction whence the
+light proceeds, and are thus fully illuminated. There is a fourth
+sub-class, as far as the final cause of the movement is concerned; for
+the leaves of some plants when exposed to an intense and injurious
+amount of light direct themselves, by rising or sinking or twisting, so
+as to be less intensely illuminated. Such movements have sometimes been
+called diurnal sleep. If thought advisable, they might be called
+paraheliotropic, and this term would correspond with our other terms.
+
+It will be shown in the present chapter that all the movements included
+in these four sub-classes, consist of modified circumnutation. We do
+not pretend to say that if a part of a plant, whilst still growing, did
+not circumnutate—though such a supposition is most improbable—it could
+not bend towards the light; but, as a matter of fact, heliotropism
+seems always to consist of modified circumnutation. Any kind of
+movement in relation to light will obviously be much facilitated by
+each part circumnutating or bending successively in all directions, so
+that an already existing movement has only to be increased in some one
+direction, and to be lessened or stopped in the other directions, in
+order that it should become heliotropic, apheliotropic, etc., as the
+case may be. In the next chapter some observations on the sensitiveness
+of plants to light, their
+rate of bending towards it, and the accuracy with which they point
+towards its source, etc., will be given. Afterwards it will be
+shown—and this seems to us a point of much interest—that sensitiveness
+to light is sometimes confined to a small part of the plant; and that
+this part when stimulated by light, transmits an influence to distant
+parts, exciting them to bend.
+
+Heliotropism.—When a plant which is strongly heliotropic (and species
+differ much in this respect) is exposed to a bright lateral light, it
+bends quickly towards it, and the course pursued by the stem is quite
+or nearly straight. But if the light is much dimmed, or occasionally
+interrupted, or admitted in only a slightly oblique direction, the
+course pursued is more or less zigzag; and as we have seen and shall
+again see, such zigzag movement results from the elongation or drawing
+out of the ellipses, loops, etc., which the plant would have described,
+if it had been illuminated from above. On several occasions we were
+much struck with this fact, whilst observing the circumnutation of
+highly sensitive seedlings, which were unintentionally illuminated
+rather obliquely, or only at successive intervals of time.
+
+Fig. 168. Beta vulgaris: circumnutation of hypocotyl, deflected by the
+light being slightly lateral, traced on a horizontal glass from 8.30
+A.M. to 5.30 P.M. Direction of the lighted taper by which it was
+illuminated shown by a line joining the first and penultimate dots.
+Figure reduced to one-third of the original scale.
+
+For instance two young seedlings of Beta vulgaris were placed in the
+middle of a room with north-east windows, and were kept covered up,
+except during each observation which lasted for only a minute or two;
+but the result was that their hypocotyls bowed themselves to the side,
+whence some light occasionally entered, in lines which were
+only slightly zigzag. Although not a single ellipse was even
+approximately formed, we inferred from the zigzag lines—and, as it
+proved, correctly—that their hypocotyls were circumnutating, for on the
+following day these same seedlings were placed in a completely darkened
+room, and were observed each time by the aid of a small wax taper held
+almost directly above them, and their movements were traced on a
+horizontal glass above; and now their hypocotyls clearly circumnutated
+(Fig. 168, and Fig. 39, formerly given, p. 52); yet they moved a short
+distance towards the side where the taper was held up. If we look at
+these diagrams, and suppose that the taper had been held more on one
+side, and that the hypocotyls, still circumnutating, had bent
+themselves within the same time much more towards the light, long
+zigzag lines would obviously have been the result.
+
+Fig. 169. Avena sativa: heliotropic movement and circumnutation of
+sheath-like cotyledon (1½ inch in height) traced on horizontal glass
+from 8 A.M. to 10.25 P.M. Oct. 16th.
+
+Again, two seedlings of Solanum lycopersicum were illuminated from
+above, but accidentally a little more light entered on one than on any
+other side, and their hypocotyls became slightly bowed towards the
+brighter side; they moved in a zigzag line and described in their
+course two little triangles, as seen in Fig. 37 (p. 50), and in another
+tracing not given. The sheath-like cotyledons of Zea mays behaved,
+under nearly similar circumstances, in a nearly similar manner as
+described in our first chapter (p. 64), for they bowed themselves
+during the whole day towards one side, making, however, in their course
+some conspicuous flexures. Before we knew how greatly ordinary
+circumnutation was modified by a lateral light, some seedling oats,
+with rather old and therefore not highly sensitive cotyledons, were
+placed in front of a north-east window, towards which they bent all day
+in a strongly zigzag course. On the following day they continued to
+bend in the same direction (Fig. 169), but zigzagged much less. The
+sky, however, became between 12.40 and 2.35 P.M.
+overcast with extraordinarily dark thunder-clouds, and it was
+interesting to note how plainly the cotyledons circumnutated during
+this interval.
+
+The foregoing observations are of some value, from having been made
+when we were not attending to heliotropism; and they led us to
+experiment on several kinds of seedlings, by exposing them to a dim
+lateral light, so as to observe the gradations between ordinary
+circumnutation and heliotropism. Seedlings in pots were placed in front
+of, and about a yard from, a north-east window; on each side and over
+the pots black boards were placed; in the rear the pots were open to
+the diffused light of the room, which had a second north-east and a
+north-west window. By hanging up one or more blinds before the window
+where the seedlings stood, it was easy to dim the light, so that very
+little more entered on this side than on the opposite one, which
+received the diffused light of the room. Late in the evening the blinds
+were successively removed, and as the plants had been subjected during
+the day to a very obscure light, they continued to bend towards the
+window later in the evening than would otherwise have occurred. Most of
+the seedlings were selected because they were known to be highly
+sensitive to light, and some because they were but little sensitive, or
+had become so from having grown old. The movements were traced in the
+usual manner on a horizontal glass cover; a fine glass filament with
+little triangles of paper having been cemented in an upright position
+to the hypocotyls. Whenever the stem or hypocotyl became much bowed
+towards the light, the latter part of its course had to be traced on a
+vertical glass, parallel to the window, and at right angles to the
+horizontal glass cover.
+
+Fig. 170. Apios graveolens: heliotropic movement of hypocotyl (.45 of
+inch in height) towards a moderately bright lateral light, traced on a
+horizontal glass from 8.30 A.M. to 11.30 A.M. Sept. 18th. Figure
+reduced to one-third of original scale.
+
+Apios graveolens.—The hypocotyl bends in a few hours
+rectangularly towards a bright lateral light. In order to ascertain how
+straight a course it would pursue when fairly well illuminated on one
+side, seedlings were first placed before a south-west window on a
+cloudy and rainy morning; and the movement of two hypocotyls were
+traced for 3 h., during which time they became greatly bowed towards
+the light. One of these tracings is given on p. 422 (Fig. 170), and the
+course may be seen to be almost straight. But the amount of light on
+this occasion was superfluous, for two seedlings were placed before a
+north-east window, protected by an ordinary linen and two muslin
+blinds, yet their hypocotyls moved towards this rather dim light in
+only slightly zigzag lines; but after 4 P.M., as the light waned, the
+lines became distinctly zigzag. One of these seedlings, moreover,
+described in the afternoon an ellipse of considerable size, with its
+longer axis directed towards the window.
+
+We now determined that the light should be made dim enough, so we began
+by exposing several seedlings before a north-east window, protected by
+one linen blind, three muslin blinds, and a towel. But so little light
+entered that a pencil cast no perceptible shadow on a white card, and
+the hypocotyls did not bend at all towards the window. During this
+time, from 8.15 to 10.50 A.M., the hypocotyls zigzagged or
+circumnutated near the same spot, as may be seen at A, in Fig. 171. The
+towel, therefore, was removed at 10.50 A.M., and replaced by two muslin
+blinds, and now the light passed through one ordinary linen and four
+muslin blinds. When a pencil was held upright on a card close to the
+seedlings, it cast a shadow (pointing from the window) which could only
+just be distinguished. Yet this very slight excess of light on one side
+sufficed to cause the hypocotyls of all the seedlings immediately to
+begin bending in zigzag lines towards the window. The course of one is
+shown at A (Fig. 171): after moving towards the window from 10.50 A.M.
+to 12.48 P.M. it bent from the window, and then returned in a nearly
+parallel line; that is, it almost completed between 12.48 and 2 P.M. a
+narrow ellipse. Late in the evening, as the light waned, the hypocotyl
+ceased to bend towards the window, and circumnutated on a small scale
+round the same spot; during the night it moved considerably backwards,
+that is, became more upright, through the action of apogeotropism. At
+B, we have a tracing of the movements of another seedling from the hour
+(10.50 A.M.) when the towel was removed; and it is in all essential
+respects
+similar to the previous one. In these two cases there could be no doubt
+that the ordinary circumnutating movement of the hypocotyl was modified
+and rendered heliotropic.
+
+Fig. 171. Apios graveolens: heliotropic movement and circumnutation of
+the hypocotyls of two seedlings towards a dim lateral light, traced on
+a horizontal glass during the day. The broken lines show their return
+nocturnal courses. Height of hypocotyl of A .5, and of B .55 inch.
+Figure reduced to one-half of original scale.
+
+Brassica oleracea.—The hypocotyl of the cabbage, when not disturbed by
+a lateral light, circumnutates in a complicated
+manner over nearly the same space, and a figure formerly given is here
+reproduced (Fig. 172). If the hypocotyl is exposed to a moderately
+strong lateral light it moves quickly towards this side, travelling in
+a straight, or nearly straight, line. But when the lateral light is
+very dim its course is extremely tortuous, and evidently consists of
+modified circumnutation. Seedlings were placed before a north-east
+window, protected by a linen and muslin blind and by a towel. The sky
+was cloudy, and whenever the clouds grew a little lighter an additional
+muslin blind was temporarily suspended. The light from the window was
+thus so much obscured that, judging by the unassisted eye, the
+seedlings appeared to receive more light from the interior of the room
+than from the window; but this was not really the case, as was shown by
+a very faint shadow cast by a pencil on a card. Nevertheless, this
+extremely small excess of light on one side caused the hypocotyls,
+which in the morning had stood upright, to bend at right angles towards
+the window, so that in the evening (after 4.23 P.M.) their course had
+to be traced on a vertical glass parallel to the window. It should be
+stated that at 3.30 P.M., by which time the sky had become darker, the
+towel was removed and replaced by an additional muslin blind, which
+itself was removed at 4 P.M., the other two
+blinds being left suspended. In Fig. 173 the course pursued, between
+8.9 A.M. and 7.10 P.M., by one of the hypocotyls thus exposed is shown.
+It may be observed that during the first 16 m. the hypocotyl moved
+obliquely from the light, and this,
+no doubt, was due to its then circumnutating in this direction. Similar
+cases were repeatedly observed, and a dim light rarely or never
+produced any effect until from a quarter to three-quarters of an hour
+had elapsed. After 5.15 P.M., by which time the light had become
+obscure, the hypocotyl began to circumnutate about the same spot. The
+contrast between the two figures (172 and 173) would have been more
+striking, if they had been originally drawn on the same scale, and had
+been equally reduced. But the movements shown in Fig. 172 were at first
+more magnified, and have been reduced to only one-half of the original
+scale; whereas those in Fig. 173 were at first less magnified, and have
+been reduced to a one-third scale. A tracing made at the same time with
+the last of the movements of a second hypocotyl, presented a closely
+analogous appearance; but it did not bend quite so much towards the
+light, and it circumnutated rather more plainly.
+
+Fig. 172. Brassica oleracea: ordinary circumnutating movement of the
+hypocotyl of a seedling plant.
+
+Fig. 173. Brassica oleracea: heliotropic movement and circumnutation of
+a hypocotyl towards a very dim lateral light, traced during 11 hours,
+on a horizontal glass in the morning, and on a vertical glass in the
+evening. Figure reduced to one-third of the original scale.
+
+Phalaris Canariensis.—The sheath-like cotyledons of this
+monocotyledonous plant were selected for trial, because they are very
+sensitive to light and circumnutate well, as formerly shown (see Fig.
+49, p. 63). Although we felt no doubt about the result, some seedlings
+were first placed before a south-west window on a moderately bright
+morning, and the movements of one were traced. As is so common, it
+moved
+for the first 45 m. in a zigzag line; it then felt the full influence
+of the light, and travelled towards it for the next 2 h. 30 m. in an
+almost straight line. The tracing has not been given, as it was almost
+identical with that of Apios under similar circumstances (Fig. 170). By
+noon it had bowed itself to its full extent; it then circumnutated
+about the same spot and described two ellipses; by 5 P.M. it had
+retreated considerably from the light, through the action of
+apogeotropism. After some preliminary trials for ascertaining the right
+degree of obscurity, some seedlings were placed (Sept. 16th) before a
+north-east window, and light was admitted through an ordinary linen and
+three muslin blinds. A pencil held close by the pot now cast a very
+faint shadow on a white card, pointing from the window. In the evening,
+at 4.30 and again at 6 P.M., some of the blinds were removed. In Fig.
+174 we see the course pursued under these circumstances by a rather old
+and not very sensitive cotyledon, 1.9 inch in height, which became much
+bowed, but was never rectangularly bent towards the light. From 11
+A.M., when the sky became rather duller, until 6.30 P.M., the
+zigzagging was conspicuous, and evidently consisted of drawn-out
+ellipses. After 6.30 P.M. and during the night, it retreated in a
+crooked line from the window. Another and younger seedling moved during
+the same time much more quickly and to a much greater distance, in an
+only slightly zigzag line towards the light; by 11 A.M. it was bent
+almost rectangularly in this direction, and now circumnutated about the
+same place.
+
+Fig. 174. Phalaris Canariensis: heliotropic movement and circumnutation
+of a rather old cotyledon, towards a dull lateral light, traced on a
+horizontal glass from 8.15 A.M. Sept. 16th to 7.45 A.M. 17th. Figure
+reduced to one-third of original scale.
+
+Tropaeolum majus.—Some very young seedlings, bearing only two leaves,
+and therefore not as yet arrived at the climbing stage of growth, were
+first tried before a north-east window without any blind. The epicotyls
+bowed themselves towards the light so rapidly that in little more than
+3 h. their tips pointed rectangularly towards it. The lines traced were
+either nearly straight or slightly zigzag; and in this latter case we
+see that a trace of circumnutation was retained even under the
+influence of a moderately bright light. Twice whilst these epicotyls
+were bending towards the window, dots were made every 5 or 6 minutes,
+in order to detect any trace of lateral movement, but there was hardly
+any; and the lines formed by their junction were nearly straight, or
+only very slightly zigzag, as in the other parts of the figures. After
+the epicotyls had bowed themselves to the full extent towards the
+light, ellipses of considerable size were described in the usual
+manner.
+
+
+After having seen how the epicotyls moved towards a moderately bright
+light, seedlings were placed at 7.48 A.M. (Sept. 7th) before a
+north-east window, covered by a towel, and shortly afterwards by an
+ordinary linen blind, but the epicotyls still moved towards the window.
+At 9.13 A.M. two additional muslin blinds were suspended, so that the
+seedlings received very little more light from the window than from the
+interior of the room. The sky varied in brightness, and the seedlings
+occasionally received for a short time less light from the window than
+from the opposite side (as ascertained by the shadow cast), and then
+one of the blinds was temporarily removed. In the evening the blinds
+were taken away, one by one. the course pursued by an epicotyl under
+these circumstances is shown in Fig. 175. During the whole day, until
+6.45 P.M., it plainly bowed itself towards the light; and the tip moved
+over a considerable space. After 6.45 P.M. it moved backwards, or from
+the window, till
+10.40 P.M., when the last dot was made. Here, then, we have a distinct
+heliotropic movement, effected by means of six elongated figures (which
+if dots had been made every few minutes would have been more or less
+elliptic) directed towards the light, with the apex of each successive
+ellipse nearer to the window than the previous one. Now, if the light
+had been only a little brighter, the epicotyl would have bowed itself
+more to the light, as we may safely conclude from the previous trials;
+there would also have been less lateral movement, and the ellipses or
+other figures would have been drawn out into a strongly marked zigzag
+line, with probably one or two small loops still formed. If the light
+had been much brighter, we should have had a slightly zigzag line, or
+one quite straight, for there would have been more movement in the
+direction of the light, and much less from side to side.
+
+Fig. 175. Tropaeolum majus: heliotropic movement and circumnutation of
+the epicotyl of a young seedling towards a dull lateral light, traced
+on a horizontal glass from 7.48 A.M. to 10.40 P.M. Figure reduced to
+one-half of the original scale.
+
+Fig. 176. Tropaeolum majus: heliotropic movement and circumnutation of
+an old internode towards a lateral light, traced on a horizontal glass
+from 8 A.M. Nov. 2nd to 10.20 A.M. Nov. 4th. Broken lines show the
+nocturnal course.
+
+Sachs states that the older internodes of this Tropaeolum are
+apheliotropic; we therefore placed a plant, 11 3/4 inches high, in a
+box, blackened within, but open on one side in front of a north-east
+window without any blind. A filament was fixed to the third internode
+from the summit on one plant, and to the fourth internode of another.
+These internodes were either not old enough, or the light was not
+sufficiently bright, to induce apheliotropism, for both plants bent
+slowly towards, instead of from the window during four days. The
+course, during two days of the first-mentioned internode, is given in
+Fig. 176; and we see that it either circumnutated on a small scale, or
+travelled in a zigzag line towards the light. We have thought this case
+of feeble heliotropism in one of the older internodes of a plant,
+which, whilst young, is so extremely sensitive to light, worth giving.
+
+Fig. 177. Cassia tora: heliotropic movement and circumnutation of a
+hypocotyl (1½ inch in height) traced on a horizontal glass from 8 A.M.
+to 10.10 P.M. Oct. 7th. Also its circumnutation in darkness from 7 A.M.
+Oct. 8th to 7.45 A.M. Oct. 9th.
+
+Cassia tora.—The cotyledons of this plant are extremely sensitive to
+light, whilst the hypocotyls are much less sensitive than those of most
+other seedlings, as we had often observed with surprise. It seemed
+therefore worth while to trace their movements. They were exposed to a
+lateral light before a north-east window, which was at first covered
+merely by a muslin blind, but as the sky grew brighter about 11 A.M.,
+an additional linen blind was suspended. After 4 P.M. one blind and
+then the other was removed. The seedlings were protected on each side
+and above, but were open to the diffused light of the room in the rear.
+Upright filaments were fixed to the hypocotyls of two seedlings, which
+stood vertically in the morning. The accompanying figure (Fig. 177)
+shows the course pursued by one of them during two days; but it should
+be particularly noticed that during the second day the seedlings were
+kept in darkness, and they then circumnutated round nearly the same
+small space. On the first day (Oct. 7th) the hypocotyl moved from 8
+A.M. to 12.23 P.M., toward the light in a zigzag line, then turned
+abruptly to the left and afterwards described a small ellipse. Another
+irregular
+ellipse was completed between 3 P.M. and about 5.30 P.M., the hypocotyl
+still bending towards the light. The hypocotyl was straight and upright
+in the morning, but by 6 P.M. its upper half was bowed towards the
+light, so that the chord of the arc thus formed stood at an angle of
+20° with the perpendicular. After 6 P.M. its course was reversed
+through the action of apogeotropism, and it continued to bend from the
+window during the night, as shown by the broken line. On the next day
+it was kept in the dark (excepting when each observation was made by
+the aid of a taper), and the course followed from 7 A.M. on the 8th to
+7.45 A.M. on the 9th is here likewise shown. The difference between the
+two parts of the figure (177), namely that described during the daytime
+on the 7th, when exposed to a rather dim lateral light, and that on the
+8th in darkness, is striking. The difference consists in the lines
+during the first day having been drawn out in the direction of the
+light. The movements of the other seedling, traced under the same
+circumstances, were closely similar.
+
+Apheliotropism.—We succeeded in observing only two cases of
+apheliotropism, for these are somewhat rare; and the movements are
+generally so slow that they would have been very troublesome to trace.
+
+Fig. 178. Bignonia capreolata: apheliotropic movement of a tendril,
+traced on a horizontal glass from 6.45 A.M. July 19th to 10 A.M. 20th.
+Movements as originally traced, little magnified, here reduced to
+two-thirds of the original scale.
+
+Bignonia capreolata.—No organ of any plant, as far as we have seen,
+bends away so quickly from the light as do the tendrils of this
+Bignonia. They are also remarkable from circumnutating much less
+regularly than most other tendrils, often remaining stationary; they
+depend on apheliotropism for coming into
+contact with the trunks of trees.[2] The stem of a young plant was tied
+to a stick at the base of a pair of fine tendrils, which projected
+almost vertically upwards; and it was placed in front of a north-east
+window, being protected on all other sides from the light. The first
+dot was made at 6.45 A.M., and by 7.35 A.M. both tendrils felt the full
+influence of the light, for they moved straight away from it until 9.20
+A.M., when they circumnutated for a time, still moving, but only a
+little, from the light (see Fig. 178 of the left-hand tendril). After 3
+P.M. they again moved rapidly away from the light in zigzag lines. By a
+late hour in the evening both had moved so far, that they pointed in a
+direct line from the light. During the night they returned a little in
+a nearly opposite direction. On the following morning they again moved
+from the light and converged, so that by the evening they had become
+interlocked, still pointing from the light. The right-hand tendril,
+whilst converging, zigzagged much more than the one figured. Both
+tracings showed that the apheliotropic movement was a modified form of
+circumnutation.
+
+ [2] ‘The Movements and Habits of Climbing Plants,’ 1875, p. 97.
+
+
+Cyclamen Persicum.—Whilst this plant is in flower the peduncles stand
+upright, but their uppermost part is hooked so that the flower itself
+hangs downwards. As soon as the pods begin to swell, the peduncles
+increase much in length and slowly curve downwards, but the short,
+upper, hooked part straightens itself. Ultimately the pods reach the
+ground, and if this is covered with moss or dead leaves, they bury
+themselves. We have often seen saucer-like depressions formed by the
+pods in damp sand or sawdust; and one pod (.3 of inch in diameter)
+buried itself in sawdust for three-quarters of its length.[3] We shall
+have occasion hereafter to consider the object gained by this burying
+process. The peduncles can change the direction of their curvature, for
+if a pot, with plants having their peduncles already bowed downwards,
+be placed horizontally, they slowly bend at right angles to their
+former direction towards the centre of the earth. We therefore at first
+attributed the movement to geotropism; but a pot which had lain
+horizontally with the pods
+all pointing to the ground, was reversed, being still kept horizontal,
+so that the pods now pointed directly upwards; it was then placed in a
+dark cupboard, but the pods still pointed upwards after four days and
+nights. The pot, in the same position, was next brought back into the
+light, and after two days there was some bending downwards of the
+peduncles, and on the fourth day two of them pointed to the centre of
+the earth, as did the others after an additional day or two. Another
+plant, in a pot which had always stood upright, was left in the dark
+cupboard for six days; it bore 3 peduncles, and only one became within
+this time at all bowed downwards, and that doubtfully. The weight,
+therefore, of the pods is not the cause of the bending down. This pot
+was then brought back into the light, and after three days the
+peduncles were considerably bowed downwards. We are thus led to infer
+that the downward curvature is due to apheliotropism; though more
+trials ought to have been made.
+
+ [3] The peduncles of several other species of Cyclamen twist
+ themselves into a spire, and according to Erasmus Darwin (‘Botanic
+ Garden,’ Canto., iii. p. 126), the pods forcibly penetrate the earth.
+ See also Grenier and Godron, ‘Flore de France,’ tom. ii. p. 459.
+
+
+Fig. 179. Cyclamen Persicum: downward apheliotropic movement of a
+flower-peduncle, greatly magnified (about 47 times?), traced on a
+horizontal glass from 1 P.M. Feb. 18th to 8 A.M. 21st.
+
+In order to observe the nature of this movement, a peduncle bearing a
+large pod which had reached and rested on the ground, was lifted a
+little up and secured to a stick. A filament was fixed across the pod
+with a mark beneath, and its
+movement, greatly magnified, was traced on a horizontal glass during 67
+h. The plant was illuminated during the day from above. A copy of the
+tracing is given on p. 434 (Fig. 179); and there can be no doubt that
+the descending movement is one of modified circumnutation, but on an
+extremely small scale. The observation was repeated on another pod,
+which had partially buried itself in sawdust, and which was lifted up a
+quarter of an inch above the surface; it described three very small
+circles in 24 h. Considering the great length and thinness of the
+peduncles and the lightness of the pods, we may conclude that they
+would not be able to excavate saucer-like depressions in sand or
+sawdust, or bury themselves in moss, etc., unless they were aided by
+their continued rocking or circumnutating movement.
+
+Relation between Circumnutation and Heliotropism.—Any one who will look
+at the foregoing diagrams, showing the movements of the stems of
+various plants towards a lateral and more or less dimmed light, will be
+forced to admit that ordinary circumnutation and heliotropism graduate
+into one another. When a plant is exposed to a dim lateral light and
+continues during the whole day bending towards it, receding late in the
+evening, the movement unquestionably is one of heliotropism. Now, in
+the case of Tropaeolum (Fig. 175) the stem or epicotyl obviously
+circumnutated during the whole day, and yet it continued at the same
+time to move heliotropically; this latter movement being effected by
+the apex of each successive elongated figure or ellipse standing nearer
+to the light than the previous one. In the case of Cassia (Fig. 177)
+the comparison of the movement of the hypocotyl, when exposed to a dim
+lateral light and to darkness, is very instructive; as is that between
+the ordinary circumnutating movement of a seedling Brassica (Figs. 172,
+173), or that of Phalaris (Figs. 49, 174), and their heliotropic
+movement towards a window protected by blinds. In both these cases,
+and in many others, it was interesting to notice how gradually the
+stems began to circumnutate as the light waned in the evening. We have
+therefore many kinds of gradations from a movement towards the light,
+which must be considered as one of circumnutation very slightly
+modified and still consisting of ellipses or circles,—though a movement
+more or less strongly zigzag, with loops or ellipses occasionally
+formed,—to a nearly straight, or even quite straight, heliotropic
+course.
+
+A plant, when exposed to a lateral light, though this may be bright,
+commonly moves at first in a zigzag line, or even directly from the
+light; and this no doubt is due to its circumnutating at the time in a
+direction either opposite to the source of the light, or more or less
+transversely to it. As soon, however, as the direction of the
+circumnutating movement nearly coincides with that of the entering
+light, the plant bends in a straight course towards the light, if this
+is bright. The course appears to be rendered more and more rapid and
+rectilinear, in accordance with the degree of brightness of the
+light—firstly, by the longer axes of the elliptical figures, which the
+plant continues to describe as long as the light remains very dim,
+being directed more or less accurately towards its source, and by each
+successive ellipse being described nearer to the light. Secondly, if
+the light is only somewhat dimmed, by the acceleration and increase of
+the movement towards it, and by the retardation or arrestment of that
+from the light, some lateral movement being still retained, for the
+light will interfere less with a movement at right angles to its
+direction, than with one in its own direction.[4]
+The result is that the course is rendered more or less zigzag and
+unequal in rate. Lastly, when the light is very bright all lateral
+movement is lost; and the whole energy of the plant is expended in
+rendering the circumnutating movement rectilinear and rapid in one
+direction alone, namely, towards the light.
+
+ [4] In his paper, ‘Ueber orthotrope und plagiotrope Pflanzentheile’
+ (‘Arbeiten des Bot. Inst. in Würzburg,’ Band ii. Heft ii. 1879), Sachs
+ has discussed the manner in which geotropism and heliotropism are
+ affected by differences in the angles at which the organs of plants
+ stand with respect to the direction of the incident force.
+
+
+The common view seems to be that heliotropism is a quite distinct kind
+of movement from circumnutation; and it may be urged that in the
+foregoing diagrams we see heliotropism merely combined with, or
+superimposed on, circumnutation. But if so, it must be assumed that a
+bright lateral light completely stops circumnutation, for a plant thus
+exposed moves in a straight line towards it, without describing any
+ellipses or circles. If the light be somewhat obscured, though amply
+sufficient to cause the plant to bend towards it, we have more or less
+plain evidence of still-continued circumnutation. It must further be
+assumed that it is only a lateral light which has this extraordinary
+power of stopping circumnutation, for we know that the several plants
+above experimented on, and all the others which were observed by us
+whilst growing, continue to circumnutate, however bright the light may
+be, if it comes from above. Nor should it be forgotten that in the life
+of each plant, circumnutation precedes heliotropism, for hypocotyls,
+epicotyls, and petioles circumnutate before they have broken through
+the ground and have ever felt the influence of light.
+
+We are therefore fully justified, as it seems to us, in believing that
+whenever light enters laterally, it is the
+movement of circumnutation which gives rise to, or is converted into,
+heliotropism and apheliotropism. On this view we need not assume
+against all analogy that a lateral light entirely stops circumnutation;
+it merely excites the plant to modify its movement for a time in a
+beneficial manner. The existence of every possible gradation, between a
+straight course towards a lateral light and a course consisting of a
+series of loops or ellipses, becomes perfectly intelligible. Finally,
+the conversion of circumnutation into heliotropism or apheliotropism,
+is closely analogous to what takes place with sleeping plants, which
+during the daytime describe one or more ellipses, often moving in
+zigzag lines and making little loops; for when they begin in the
+evening to go to sleep, they likewise expend all their energy in
+rendering their course rectilinear and rapid. In the case of
+sleep-movements, the exciting or regulating cause is a difference in
+the intensity of the light, coming from above, at different periods of
+the twenty-four hours; whilst with heliotropic and apheliotropic
+movements, it is a difference in the intensity of the light on the two
+sides of the plant.
+
+_Transversal-heliotropismus_ (_of Frank_[5]) _or Diaheliotropism_.—The
+cause of leaves placing themselves more or less transversely to the
+light, with their upper surfaces directed towards it, has been of late
+the subject of much controversy. We do not here refer to the object of
+the movement, which no doubt is that their upper surfaces may be fully
+illuminated, but the means by which this position is gained. Hardly a
+better or more simple instance can be given
+of diaheliotropism than that offered by many seedlings, the cotyledons
+of which are extended horizontally. When they first burst from their
+seed-coats they are in contact and stand in various positions, often
+vertically upwards; they soon diverge, and this is effected by
+epinasty, which, as we have seen, is a modified form of circumnutation.
+After they have diverged to their full extent, they retain nearly the
+same position, though brightly illuminated all day long from above,
+with their lower surfaces close to the ground and thus much shaded.
+There is therefore a great contrast in the degree of illumination of
+their upper and lower surfaces, and if they were heliotropic they would
+bend quickly upwards. It must not, however, be supposed that such
+cotyledons are immovably fixed in a horizontal position. When seedlings
+are exposed before a window, their hypocotyls, which are highly
+heliotropic, bend quickly towards it, and the upper surfaces of their
+cotyledons still remain exposed at right angles to the light; but if
+the hypocotyl is secured so that it cannot bend, the cotyledons
+themselves change their position. If the two are placed in the line of
+the entering light, the one furthest from it rises up and that nearest
+to it often sinks down; if placed transversely to the light, they twist
+a little laterally; so that in every case they endeavour to place their
+upper surfaces at right angles to the light. So it notoriously is with
+the leaves on plants nailed against a wall, or grown in front of a
+window. A moderate amount of light suffices to induce such movements;
+all that is necessary is that the light should steadily strike the
+plants in an oblique direction. With respect to the above twisting
+movement of cotyledons, Frank has given many and much more striking
+instances in the case of the leaves on
+branches which had been fastened in various positions or turned upside
+down.
+
+ [5] ‘Die natürliche Wagerechte Richtung von Pflanzentheilen,’ 1870.
+ See also some interesting articles by the same author, “Zur Frage über
+ Transversal-Geo-und Heliotropismus,” ‘Bot. Zeitung,’ 1873, p. 17 et
+ seq.
+
+
+In our observations on the cotyledons of seedling plants, we often felt
+surprise at their persistent horizontal position during the day, and
+were convinced before we had read Frank’s essay, that some special
+explanation was necessary. De Vries has shown[6] that the more or less
+horizontal position of leaves is in most cases influenced by epinasty,
+by their own weight, and by apogeotropism. A young cotyledon or leaf
+after bursting free is brought down into its proper position, as
+already remarked, by epinasty, which, according to De Vries, long
+continues to act on the midribs and petioles. Weight can hardly be
+influential in the case of cotyledons, except in a few cases presently
+to be mentioned, but must be so with large and thick leaves. With
+respect to apogeotropism, De Vries maintains that it generally comes
+into play, and of this fact we shall presently advance some indirect
+evidence. But over these and other constant forces we believe that
+there is in many cases, but we do not say in all, a preponderant
+tendency in leaves and cotyledons to place themselves more or less
+transversely with respect to the light.
+
+ [6] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp.
+ 223–277.
+
+
+In the cases above alluded to of seedlings exposed to a lateral light
+with their hypocotyls secured, it is impossible that epinasty, weight
+and apogeotropism, either in opposition or combined, can be the cause
+of the rising of one cotyledon, and of the sinking of the other, since
+the forces in question act equally on both; and since epinasty, weight
+and apogeotropism all act in a vertical plane, they cannot cause the
+twisting of the petioles, which occurs in seedlings under the
+above conditions of illumination. All these movements evidently depend
+in some manner on the obliquity of the light, but cannot be called
+heliotropic, as this implies bending towards the light; whereas the
+cotyledon nearest to the light bends in an opposed direction or
+downwards, and both place themselves as nearly as possible at right
+angles to the light. The movement, therefore, deserves a distinct name.
+As cotyledons and leaves are continually oscillating up and down, and
+yet retain all day long their proper position with their upper surfaces
+directed transversely to the light, and if displaced reassume this
+position, diaheliotropism must be considered as a modified form of
+circumnutation. This was often evident when the movements of cotyledons
+standing in front of a window were traced. We see something analogous
+in the case of sleeping leaves or cotyledons, which after oscillating
+up and down during the whole day, rise into a vertical position late in
+the evening, and on the following morning sink down again into their
+horizontal or diaheliotropic position, in direct opposition to
+heliotropism. This return into their diurnal position, which often
+requires an angular movement of 90°, is analogous to the movement of
+leaves on displaced branches, which recover their former positions. It
+deserves notice that any force such as apogeotropism, will act with
+different degrees of power[7] in the different positions of those
+leaves or cotyledons which oscillate largely up and down during the
+day; and yet they recover their horizontal or diaheliotropic position.
+
+ [7] See former note, in reference to Sachs’ remarks on this subject.
+
+
+We may therefore conclude that diaheliotropic movements cannot be fully
+explained by the direct action of light, gravitation, weight, etc., any
+more
+than can the nyctitropic movements of cotyledons and leaves. In the
+latter case they place themselves so that their upper surfaces may
+radiate at night as little as possible into open space, with the upper
+surfaces of the opposite leaflets often in contact. These movements,
+which are sometimes extremely complex, are regulated, though not
+directly caused, by the alternations of light and darkness. In the case
+of diaheliotropism, cotyledons and leaves place themselves so that
+their upper surfaces may be exposed to the light, and this movement is
+regulated, though not directly caused, by the direction whence the
+light proceeds. In both cases the movement consists of circumnutation
+modified by innate or constitutional causes, in the same manner as with
+climbing plants, the circumnutation of which is increased in amplitude
+and rendered more circular, or again with very young cotyledons and
+leaves which are thus brought down into a horizontal position by
+epinasty.
+
+We have hitherto referred only to those leaves and cotyledons which
+occupy a permanently horizontal position; but many stand more or less
+obliquely, and some few upright. the cause of these differences of
+position is not known; but in accordance with Wiesner’s views,
+hereafter to be given, it is probable that some leaves and cotyledons
+would suffer, if they were fully illuminated by standing at right
+angles to the light.
+
+We have seen in the second and fourth chapters that those cotyledons
+and leaves which do not alter their positions at night sufficiently to
+be said to sleep, commonly rise a little in the evening and fall again
+on the next morning, so that they stand during the night at a rather
+higher inclination than during the middle of the day. It is incredible
+that a rising movement of 2° or 3°, or even of 10° or 20°, can be of
+any service to the plant, so as to have been specially acquired. It
+must be the result of some periodical change in the conditions to which
+they are subjected, and there can hardly be a doubt that this is the
+daily alternations of light and darkness. De Vries states in the paper
+before referred to, that most petioles and midribs are apogeotropic;[8]
+and apogeotropism would account for the above rising movement, which is
+common to so many widely distinct species, if we suppose it to be
+conquered by diaheliotropism during the middle of the day, as long as
+it is of importance to the plant that its cotyledons and leaves should
+be fully exposed to the light. The exact hour in the afternoon at which
+they begin to bend slightly upwards, and the extent of the movement,
+will depend on their degree of sensitiveness to gravitation and on
+their power of resisting its action during the middle of the day, as
+well as on the amplitude of their ordinary circumnutating movements;
+and as these qualities differ much in different species, we might
+expect that the hour in the afternoon at which they begin to rise would
+differ much in different species, as is the case. Some other agency,
+however, besides apogeotropism, must come into play, either directly or
+indirectly, in this upward movement. Thus a young bean (Vicia faba),
+growing in a small pot, was placed in front of a window in a klinostat;
+and at night the leaves rose a little, although
+the action of apogeotropism was quite eliminated. Nevertheless, they
+did not rise nearly so much at night, as when subjected to
+apogeotropism. Is it not possible, or even probable, that leaves and
+cotyledons, which have moved upwards in the evening through the action
+of apogeotropism during countless generations, may inherit a tendency
+to this movement? We have seen that the hypocotyls of several
+Leguminous plants have from a remote period inherited a tendency to
+arch themselves; and we know that the sleep-movements of leaves are to
+a certain extent inherited, independently of the alternations of light
+and darkness.
+
+ [8] According to Frank (‘Die nat. Wagerechte Richtung von
+ Pflanzentheilen,’ 1870, p. 46) the root-leaves of many plants, kept in
+ darkness, rise up and even become vertical; and so it is in some cases
+ with shoots. (See Rauwenhoff, ‘Archives Néerlandaises,’ tom. xii. p.
+ 32.) These movements indicate apogeotropism; but when organs have been
+ long kept in the dark, the amount of water and of mineral matter which
+ they contain is so much altered, and their regular growth is so much
+ disturbed, that it is perhaps rash to infer from their movements what
+ would occur under normal conditions. (See Godlewski, ‘Bot. Zeitung,’
+ Feb. 14th, 1879.)
+
+
+In our observations on the circumnutation of those cotyledons and
+leaves which do not sleep at night, we met with hardly any distinct
+cases of their sinking a little in the evening, and rising again in the
+morning,—that is, of movements the reverse of those just discussed. We
+have no doubt that such cases occur, inasmuch as the leaves of many
+plants sleep by sinking vertically downwards. How to account for the
+few cases which were observed must be left doubtful. The young leaves
+of Cannabis sativa sink at night between 30° and 40° beneath the
+horizon; and Kraus attributes this to epinasty in conjunction with the
+absorption of water. Whenever epinastic growth is vigorous, it might
+conquer diaheliotropism in the evening, at which time it would be of no
+importance to the plant to keep its leaves horizontal. The cotyledons
+of Anoda Wrightii, of one variety of Gossypium, and of several species
+of Ipomœa, remain horizontal in the evening whilst they are very young;
+as they grow a little older they curve a little downwards, and when
+large and heavy sink so much that they come under our definition of
+sleep. In the case of
+the Anoda and of some species of Ipomœa, it was proved that the
+downward movement did not depend on the weight of the cotyledons; but
+from the fact of the movement being so much more strongly pronounced
+after the cotyledons have grown large and heavy, we may suspect that
+their weight aboriginally played some part in determining that the
+modification of the circumnutating movement should be in a downward
+direction.
+
+The so-called Diurnal Sleep of Leaves, Or Paraheliotropism.—This is
+another class of movements, dependent on the action of light, which
+supports to some extent the belief that the movements above described
+are only indirectly due to its action. We refer to the movements of
+leaves and cotyledons which when moderately illuminated are
+diaheliotropic; but which change their positions and present their
+edges to the light, when the sun shines brightly on them. These
+movements have sometimes been called diurnal sleep, but they differ
+wholly with respect to the object gained from those properly called
+nyctitropic; and in some cases the position occupied during the day is
+the reverse of that during the night.
+
+It has long been known[9] that when the sun shines brightly on the
+leaflets of Robinia, they rise up and present their edges to the light;
+whilst their position at night is vertically downwards. We have
+observed the same movement, when the sun shone brightly on the leaflets
+of an Australian Acacia. Those of Amphicarpæa monoica turned their
+edges to the sun; and an analogous movement of the little almost
+rudimentary basal leaflets of Mimosa albida was on one occasion so
+rapid that it could be distinctly seen through a lens. the elongated,
+unifoliate, first leaves of Phaseolus Roxburghii stood at 7 A.M. at 20°
+above the horizon, and no doubt they afterwards sank a little lower. At
+noon, after having been exposed for about 2 h. to
+a bright sun, they stood at 56° above the horizon; they were then
+protected from the rays of the sun, but were left well illuminated from
+above, and after 30 m. they had fallen 40°, for they now stood at only
+16° above the horizon. Some young plants of Phaseolus Hernandesii had
+been exposed to the same bright sunlight, and their broad, unifoliate,
+first leaves now stood up almost or quite vertically, as did many of
+the leaflets on the trifoliate secondary leaves; but some of the
+leaflets had twisted round on their own axes by as much as 90° without
+rising, so as to present their edges to the sun. The leaflets on the
+same leaf sometimes behaved in these two different manners, but always
+with the result of being less intensely illuminated. These plants were
+then protected from the sun, and were looked at after 1½ h.; and now
+all the leaves and leaflets had reassumed their ordinary sub-horizontal
+positions. The copper-coloured cotyledons of some seedlings of Cassia
+mimosoides were horizontal in the morning, but after the sun had shone
+on them, each had risen 45½° above the horizon. the movement in these
+several cases must not be confounded with the sudden closing of the
+leaflets of Mimosa pudica, which may sometimes be noticed when a plant
+which has been kept in an obscure place is suddenly exposed to the sun;
+for in this case the light seems to act, as if it were a touch.
+
+ [9] Pfeffer gives the names and dates of several ancient writers in
+ his ‘Die Periodischen Bewegungen,’ 1875, p. 62.
+
+
+From Prof. Wiesner’s interesting observations, it is probable that the
+above movements have been acquired for a special purpose. the
+chlorophyll in leaves is often injured by too intense a light, and
+Prof. Wiesner[10] believes that it is protected by the most diversified
+means, such as the presence of hairs, colouring matter, etc., and
+amongst other means by the leaves presenting their edges to the sun, so
+that the blades then receive much less light. He experimented on the
+young leaflets of Robinia, by fixing them in such a position that they
+could not escape being intensely illuminated, whilst others were
+allowed to place themselves obliquely; and the former began to suffer
+from the light in the course of two days.
+
+ [10] ‘Die Näturlichen Einrichtungen zum Schutze des Chlorophylls,’
+ etc., 1876. Pringsheim has recently observed under the microscope the
+ destruction of chlorophyll in a few minutes by the action of
+ concentrated light from the sun, in the presence of oxygen. See, also,
+ Stahl on the protection of chlorophyll from intense light, in ‘Bot.
+ Zeitung,’ 1880.
+
+
+In the cases above given, the leaflets move either upwards
+or twist laterally, so as to place their edges in the direction of the
+sun’s light; but Cohn long ago observed that the leaflets of Oxalis
+bend downwards when fully exposed to the sun. We witnessed a striking
+instance of this movement in the very large leaflets of O. Ortegesii. A
+similar movement may frequently be observed with the leaflets of
+Averrhoa bilimbi (a member of the Oxalidæ); and a leaf is here
+represented (Fig. 180) on which the sun had shone. A diagram (Fig. 134)
+was given in the last chapter, representing the oscillations by which a
+leaflet rapidly descended under these circumstances; and the movement
+may be seen closely to resemble that (Fig. 133) by which it assumed its
+nocturnal position. It is an interesting fact in relation to our
+present subject that, as Prof. Batalin informs us in a letter, dated
+February, 1879, the leaflets of Oxalis acetosella may be daily exposed
+to the sun during many weeks, and they do not suffer if they are
+allowed to depress themselves; but if this be prevented, they lose
+their colour and wither in two or three days. Yet the duration of a
+leaf is about two months, when subjected only to diffused light; and in
+this case the leaflets never sink downwards during the day.
+
+Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure
+to sunshine; but the leaflets are sometimes more depressed than is here
+shown. Figure much reduced.
+
+As the upward movements of the leaflets of Robinia, and the downward
+movements of those of Oxalis, have been proved to be highly beneficial
+to these plants when subjected to bright sunshine, it seems probable
+that they have been acquired for the special purpose of avoiding too
+intense an illumination. As it would have been very troublesome in all
+the above cases to
+have watched for a fitting opportunity and to have traced the movement
+of the leaves whilst they were fully exposed to the sunshine, we did
+not ascertain whether paraheliotropism always consisted of modified
+circumnutation; but this certainly was the case with the Averrhoa, and
+probably with the other species, as their leaves were continually
+circumnutating.
+
+
+
+
+CHAPTER IX.
+SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.
+
+
+Uses of heliotropism—Insectivorous and climbing plants not
+heliotropic—Same organ heliotropic at one age and not at
+another—Extraordinary sensitiveness of some plants to light—The effects
+of light do not correspond with its intensity—Effects of previous
+illumination—Time required for the action of light—After-effects of
+light—Apogeotropism acts as soon as light fails—Accuracy with which
+plants bend to the light—This dependent on the illumination of one
+whole side of the part—Localised sensitiveness to light and its
+transmitted effects—Cotyledons of Phalaris, manner of bending—Results
+of the exclusion of light from their tips—Effects transmitted beneath
+the surface of the ground—Lateral illumination of the tip determines
+the direction of the curvature of the base—Cotyledons of Avena,
+curvature of basal part due to the illumination of upper part—Similar
+results with the hypocotyls of Brassica and Beta—Radicles of Sinapis
+apheliotropic, due to the sensitiveness of their tips—Concluding
+remarks and summary of chapter—Means by which circumnutation has been
+converted into heliotropism or apheliotropism.
+
+
+No one can look at the plants growing on a bank or on the borders of a
+thick wood, and doubt that the young stems and leaves place themselves
+so that the leaves may be well illuminated. They are thus enabled to
+decompose carbonic acid. But the sheath-like cotyledons of some
+Gramineæ, for instance, those of Phalaris, are not green and contain
+very little starch; from which fact we may infer that they decompose
+little or no carbonic acid. Nevertheless, they are extremely
+heliotropic; and this probably serves them in another way, namely, as a
+guide from the buried seeds through fissures in the ground or through
+overlying masses of vegetation, into the light and air. This view
+is strengthened by the fact that with Phalaris and Avena the first true
+leaf, which is bright green and no doubt decomposes carbonic acid,
+exhibits hardly a trace of heliotropism. The heliotropic movements of
+many other seedlings probably aid them in like manner in emerging from
+the ground; for apogeotropism by itself would blindly guide them
+upwards, against any overlying obstacle.
+
+Heliotropism prevails so extensively among the higher plants, that
+there are extremely few, of which some part, either the stem,
+flower-peduncle, petiole, or leaf, does not bend towards a lateral
+light. Drosera rotundifolia is one of the few plants the leaves of
+which exhibit no trace of heliotropism. Nor could we see any in
+Dionaea, though the plants were not so carefully observed. Sir J.
+Hooker exposed the pitchers of Sarracenia for some time to a lateral
+light, but they did not bend towards it.[1] We can understand the
+reason why these insectivorous plants should not be heliotropic, as
+they do not live chiefly by decomposing carbonic acid; and it is much
+more important to them that their leaves should occupy the best
+position for capturing insects, than that they should be fully exposed
+to the light.
+
+ [1] According to F. Kurtz (‘Verhandl. des Bot. Vereins der Provinz
+ Brandenburg,’ Bd. xx. 1878) the leaves or pitchers of Darlingtonia
+ Californica are strongly apheliotropic. We failed to detect this
+ movement in a plant which we possessed for a short time.
+
+
+Tendrils, which consist of leaves or of other organs modified, and the
+stems of twining plants, are, as Mohl long ago remarked, rarely
+heliotropic; and here again we can see the reason why, for if they had
+moved towards a lateral light they would have been drawn away from
+their supports. But some tendrils are apheliotropic, for instance those
+of Bignonia capreolata
+and of Smilax aspera; and the stems of some plants which climb by
+rootlets, as those of the Ivy and Tecoma radicans, are likewise
+apheliotropic, and they thus find a support. The leaves, on the other
+hand, of most climbing plants are heliotropic; but we could detect no
+signs of any such movement in those of Mutisia clematis.
+
+As heliotropism is so widely prevalent, and as twining plants are
+distributed throughout the whole vascular series, the apparent absence
+of any tendency in their stems to bend towards the light, seemed to us
+so remarkable a fact as to deserve further investigation, for it
+implies that heliotropism can be readily eliminated. When twining
+plants are exposed to a lateral light, their stems go on revolving or
+circumnutating about the same spot, without any evident deflection
+towards the light; but we thought that we might detect some trace of
+heliotropism by comparing the average rate at which the stems moved to
+and from the light during their successive revolutions.[2] Three young
+plants (about a foot in height) of Ipomœa caerulea and four of I.
+purpurea, growing in separate pots, were placed on a bright day before
+a north-east window in a room otherwise darkened, with the tips of
+their revolving stems fronting the window. When the tip of each plant
+pointed directly from the window, and when again towards it, the times
+were recorded. This was continued from 6.45 A.M. till a little after 2
+P.M. on June 17th. After a few observations we concluded that we could
+safely estimate the time
+taken by each semicircle, within a limit of error of at most 5 minutes.
+Although the rate of movement in different parts of the same revolution
+varied greatly, yet 22 semicircles to the light were completed, each on
+an average in 73.95 minutes; and 22 semicircles from the light each in
+73.5 minutes. It may, therefore, be said that they travelled to and
+from the light at exactly the same average rate; though probably the
+accuracy of the result was in part accidental. In the evening the stems
+were not in the least deflected towards the window. Nevertheless, there
+appears to exist a vestige of heliotropism, for with 6 out of the 7
+plants, the first semicircle from the light, described in the early
+morning after they had been subjected to darkness during the night and
+thus probably rendered more sensitive, required rather more time, and
+the first semicircle to the light considerably less time, than the
+average. Thus with all 7 plants, taken together, the mean time of the
+first semicircle in the morning from the light, was 76.8 minutes,
+instead of 73.5 minutes, which is the mean of all the semicircles
+during the day from the light; and the mean of the first semicircle to
+the light was only 63.1, instead of 73.95 minutes, which was the mean
+of all the semicircles during the day to the light.
+
+ [2] Some erroneous statements are unfortunately given on this subject,
+ in ‘The Movements and Habits of Climbing Plants,’ 1875, pp. 28, 32,
+ 40, and 53. Conclusions were drawn from an insufficient number of
+ observations, for we did not then know at how unequal a rate the stems
+ and tendrils of climbing plants sometimes travel in different parts of
+ the same revolution.
+
+
+Similar observations were made on Wistaria Sinensis, and the mean of 9
+semicircles from the light was 117 minutes, and of 7 semicircles to the
+light 122 minutes, and this difference does not exceed the probable
+limit of error. During the three days of exposure, the shoot did not
+become at all bent towards the window before which it stood. In this
+case the first semicircle from the light in the early morning of each
+day, required rather less time for its performance than did the first
+semicircle to the light; and this result,
+if not accidental, appears to indicate that the shoots retain a trace
+of an original apheliotropic tendency. With Lonicera brachypoda the
+semicircles from and to the light differed considerably in time; for 5
+semicircles from the light required on a mean 202.4 minutes, and 4 to
+the light, 229.5 minutes; but the shoot moved very irregularly, and
+under these circumstances the observations were much too few.
+
+It is remarkable that the same part on the same plant may be affected
+by light in a widely different manner at different ages, and as it
+appears at different seasons. The hypocotyledonous stems of Ipomœa
+caerulea and purpurea are extremely heliotropic, whilst the stems of
+older plants, only about a foot in height, are, as we have just seen,
+almost wholly insensible to light. Sachs states (and we have observed
+the same fact) that the hypocotyls of the Ivy (Hedera helix) are
+slightly heliotropic; whereas the stems of plants grown to a few inches
+in height become so strongly apheliotropic, that they bend at right
+angles away from the light. Nevertheless, some young plants which had
+behaved in this manner early in the summer again became distinctly
+heliotropic in the beginning of September; and the zigzag courses of
+their stems, as they slowly curved towards a north-east window, were
+traced during 10 days. The stems of very young plants of Tropaeolum
+majus are highly heliotropic, whilst those of older plants, according
+to Sachs, are slightly apheliotropic. In all these cases the
+heliotropism of the very young stems serves to expose the cotyledons,
+or when the cotyledons are hypogean the first true leaves, fully to the
+light; and the loss of this power by the older stems, or their becoming
+apheliotropic, is connected with their habit of climbing.
+
+Most seedling plants are strongly heliotropic, and
+it is no doubt a great advantage to them in their struggle for life to
+expose their cotyledons to the light as quickly and as fully as
+possible, for the sake of obtaining carbon. It has been shown in the
+first chapter that the greater number of seedlings circumnutate largely
+and rapidly; and as heliotropism consists of modified circumnutation,
+we are tempted to look at the high development of these two powers in
+seedlings as intimately connected. Whether there are any plants which
+circumnutate slowly and to a small extent, and yet are highly
+heliotropic, we do not know; but there are several, and there is
+nothing surprising in this fact, which circumnutate largely and are not
+at all, or only slightly, heliotropic. Of such cases Drosera
+rotundifolia offers an excellent instance. The stolons of the
+strawberry circumnutate almost like the stems of climbing plants, and
+they are not at all affected by a moderate light; but when exposed late
+in the summer to a somewhat brighter light they were slightly
+heliotropic; in sunlight, according to De Vries, they are
+apheliotropic. Climbing plants circumnutate much more widely than any
+other plants, yet they are not at all heliotropic.
+
+Although the stems of most seedling plants are strongly heliotropic,
+some few are but slightly heliotropic, without our being able to assign
+any reason. This is the case with the hypocotyl of Cassia tora, and we
+were struck with the same fact with some other seedlings, for instance,
+those of Reseda odorata. With respect to the degree of sensitiveness of
+the more sensitive kinds, it was shown in the last chapter that
+seedlings of several species, placed before a north-east window
+protected by several blinds, and exposed in the rear to the diffused
+light of the room, moved with unerring certainty towards the window,
+although
+it was impossible to judge, excepting by the shadow cast by an upright
+pencil on a white card, on which side most light entered, so that the
+excess on one side must have been extremely small.
+
+A pot with seedlings of Phalaris Canariensis, which had been raised in
+darkness, was placed in a completely darkened room, at 12 feet from a
+very small lamp. After 3 h. the cotyledons were doubtfully curved
+towards the light, and after 7 h. 40 m. from the first exposure, they
+were all plainly, though slightly, curved towards the lamp. Now, at
+this distance of 12 feet, the light was so obscure that we could not
+see the seedlings themselves, nor read the large Roman figures on the
+white face of a watch, nor see a pencil line on paper, but could just
+distinguish a line made with Indian ink. It is a more surprising fact
+that no visible shadow was cast by a pencil held upright on a white
+card; the seedlings, therefore, were acted on by a difference in the
+illumination of their two sides, which the human eye could not
+distinguish. On another occasion even a less degree of light acted, for
+some cotyledons of Phalaris became slightly curved towards the same
+lamp at a distance of 20 feet; at this distance we could not see a
+circular dot 2.29 mm. (.09 inch) in diameter made with Indian ink on
+white paper, though we could just see a dot 3.56 mm. (.14 inch) in
+diameter; yet a dot of the former size appears large when seen in the
+light.[3]
+
+ [3] Strasburger says (‘Wirkung des Lichtes auf Schwärmsporen,’ 1878,
+ p. 52), that the spores of Haematococcus moved to a light which only
+ just sufficed to allow middle-sized type to be read.
+
+
+We next tried how small a beam of light would act; as this bears on
+light serving as a guide to seedlings whilst they emerge through
+fissured or encumbered ground. A pot with seedlings of Phalaris was
+covered
+by a tin-vessel, having on one side a circular hole 1.23 mm. in
+diameter (i.e. a little less than the 1/20th of an inch); and the box
+was placed in front of a paraffin lamp and on another occasion in front
+of a window; and both times the seedlings were manifestly bent after a
+few hours towards the little hole.
+
+A more severe trial was now made; little tubes of very thin glass,
+closed at their upper ends and coated with black varnish, were slipped
+over the cotyledons of Phalaris (which had germinated in darkness) and
+just fitted them. Narrow stripes of the varnish had been previously
+scraped off one side, through which alone light could enter; and their
+dimensions were afterwards measured under the microscope. As a control
+experiment, similar unvarnished and transparent tubes were tried, and
+they did not prevent the cotyledons bending towards the light. Two
+cotyledons were placed before a south-west window, one of which was
+illuminated by a stripe in the varnish, only .004 inch (0.1 mm.) in
+breadth and .016 inch (0.4 mm.) in length; and the other by a stripe
+.008 inch in breadth and .06 inch in length. The seedlings were
+examined after an exposure of 7 h. 40 m., and were found to be
+manifestly bowed towards the light. Some other cotyledons were at the
+same time treated similarly, excepting that the little stripes were
+directed not to the sky, but in such a manner that they received only
+the diffused light from the room; and these cotyledons did not become
+at all bowed. Seven other cotyledons were illuminated through narrow,
+but comparatively long, cleared stripes in the varnish—namely, in
+breadth between .01 and .026 inch, and in length between .15 and .3
+inch; and these all became bowed to the side, by which light entered
+through the stripes, whether these were directed towards the sky or to
+one side of
+the room. That light passing through a hole only .004 inch in breadth
+by .016 in length, should induce curvature, seems to us a surprising
+fact.
+
+Before we knew how extremely sensitive the cotyledons of Phalaris were
+to light, we endeavoured to trace their circumnutation in darkness by
+the aid of a small wax taper, held for a minute or two at each
+observation in nearly the same position, a little on the left side in
+front of the vertical glass on which the tracing was made. The
+seedlings were thus observed seventeen times in the course of the day,
+at intervals of from half to three-quarters of an hour; and late in the
+evening we were surprised to find that all the 29 cotyledons were
+greatly curved and pointed towards the vertical glass, a little to the
+left where the taper had been held. The tracings showed that they had
+travelled in zigzag lines. Thus, an exposure to a feeble light for a
+very short time at the above specified intervals, sufficed to induce
+well-marked heliotropism. An analogous case was observed with the
+hypocotyls of Solanum lycopersicum. We at first attributed this result
+to the after-effects of the light on each occasion; but since reading
+Wiesner’s observations,[4] which will be referred to in the last
+chapter, we cannot doubt that an intermittent light is more efficacious
+than a continuous one, as plants are especially sensitive to any
+contrast in its amount.
+
+ [4] ‘Sitz. der k. Akad. der Wissensch.’ (Vienna), Jan. 1880, p. 12.
+
+
+The cotyledons of Phalaris bend much more slowly towards a very obscure
+light than towards a bright one. Thus, in the experiments with
+seedlings placed in a dark room at 12 feet from a very small lamp, they
+were just perceptibly and doubtfully curved towards it after 3 h., and
+only slightly, yet certainly, after 4 h.
+After 8 h. 40 m. the chords of their arcs were deflected from the
+perpendicular by an average angle of only 16°. Had the light been
+bright, they would have become much more curved in between 1 and 2 h.
+Several trials were made with seedlings placed at various distances
+from a small lamp in a dark room; but we will give only one trial. Six
+pots were placed at distances of 2, 4, 8, 12, 16, and 20 feet from the
+lamp, before which they were left for 4 h. As light decreases in a
+geometrical ratio, the seedlings in the 2nd pot received 1/4th, those
+in the 3rd pot 1/16th, those in the 4th 1/36th, those in the 5th
+1/64th, and those in the 6th 1/100th of the light received by the
+seedlings in the first or nearest pot. Therefore it might have been
+expected that there would have been an immense difference in the degree
+of their heliotropic curvature in the several pots; and there was a
+well-marked difference between those which stood nearest and furthest
+from the lamp, but the difference in each successive pair of pots was
+extremely small. In order to avoid prejudice, we asked three persons,
+who knew nothing about the experiment, to arrange the pots in order
+according to the degree of curvature of the cotyledons. The first
+person arranged them in proper order, but doubted long between the 12
+feet and 16 feet pots; yet these two received light in the proportion
+of 36 to 64. The second person also arranged them properly, but doubted
+between the 8 feet and 12 feet pots, which received light in the
+proportion of 16 to 36. The third person arranged them in wrong order,
+and doubted about four of the pots. This evidence shows conclusively
+how little the curvature of the seedlings differed in the successive
+pots, in comparison with the great difference in the amount of light
+which they received; and it should be noted that there was no
+excess of superfluous light, for the cotyledons became but little and
+slowly curved even in the nearest pot. Close to the 6th pot, at the
+distance of 20 feet from the lamp, the light allowed us just to
+distinguish a dot 3.56 mm. (.14 inch) in diameter, made with Indian ink
+on white paper, but not a dot 2.29 mm. (.09 inch) in diameter.
+
+The degree of curvature of the cotyledons of Phalaris within a given
+time, depends not merely on the amount of lateral light which they may
+then receive, but on that which they have previously received from
+above and on all sides. Analogous facts have been given with respect to
+the nyctitropic and periodic movements of plants. Of two pots
+containing seedlings of Phalaris which had germinated in darkness, one
+was still kept in the dark, and the other was exposed (Sept. 26th) to
+the light in a greenhouse during a cloudy day and on the following
+bright morning. On this morning (27th), at 10.30 A.M., both pots were
+placed in a box, blackened within and open in front, before a
+north-east window, protected by a linen and muslin blind and by a
+towel, so that but little light was admitted, though the sky was
+bright. Whenever the pots were looked at, this was done as quickly as
+possible, and the cotyledons were then held transversely with respect
+to the light, so that their curvature could not have been thus
+increased or diminished. After 50 m. the seedlings which had previously
+been kept in darkness, were perhaps, and after 70 m. were certainly,
+curved, though very slightly, towards the window. After 85 m. some of
+the seedlings, which had previously been illuminated, were perhaps a
+little affected, and after 100 m. some of the younger ones were
+certainly a little curved towards the light. At this time (i.e. after
+100 m.) there was a plain difference
+in the curvature of the seedlings in the two pots. After 2 h. 12 m. the
+chords of the arcs of four of the most strongly curved seedlings in
+each pot were measured, and the mean angle from the perpendicular of
+those which had previously been kept in darkness was 19°, and of those
+which had previously been illuminated only 7°. Nor did this difference
+diminish during two additional hours. As a check, the seedlings in both
+pots were then placed in complete darkness for two hours, in order that
+apogeotropism should act on them; and those in the one pot which were
+little curved became in this time almost completely upright, whilst the
+more curved ones in the other pot still remained plainly curved.
+
+Two days afterwards the experiment was repeated, with the sole
+difference that even less light was admitted through the window, as it
+was protected by a linen and muslin blind and by two towels; the sky,
+moreover, was somewhat less bright. The result was the same as before,
+excepting that everything occurred rather slower. The seedlings which
+had been previously kept in darkness were not in the least curved after
+54 m., but were so after 70 m. Those which had previously been
+illuminated were not at all affected until 130 m. had elapsed, and then
+only slightly. After 145 m. some of the seedlings in this latter pot
+were certainly curved towards the light; and there was now a plain
+difference between the two pots. After 3 h. 45 m. the chords of the
+arcs of 3 seedlings in each pot were measured, and the mean angle from
+the perpendicular was 16° for those in the pot which had previously
+been kept in darkness, and only 5° for those which had previously been
+illuminated.
+
+The curvature of the cotyledons of Phalaris towards a lateral light is
+therefore certainly influenced by the
+degree to which they have been previously illuminated. We shall
+presently see that the influence of light on their bending continues
+for a short time after the light has been extinguished. These facts, as
+well as that of the curvature not increasing or decreasing in nearly
+the same ratio with that of the amount of light which they receive, as
+shown in the trials with the plants before the lamp, all indicate that
+light acts on them as a stimulus, in somewhat the same manner as on the
+nervous system of animals, and not in a direct manner on the cells or
+cell-walls which by their contraction or expansion cause the curvature.
+
+It has already been incidentally shown how slowly the cotyledons of
+Phalaris bend towards a very dim light; but when they were placed
+before a bright paraffin lamp their tips were all curved rectangularly
+towards it in 2 h. 20 m. The hypocotyls of Solanum lycopersicum had
+bent in the morning at right angles towards a north-east window. At 1
+P.M. (Oct. 21st) the pot was turned round, so that the seedlings now
+pointed from the light, but by 5 P.M. they had reversed their curvature
+and again pointed to the light. They had thus passed through 180° in 4
+h., having in the morning previously passed through about 90°. But the
+reversal of the first half of the curvature will have been aided by
+apogeotropism. Similar cases were observed with other seedlings, for
+instance, with those of Sinapis alba.
+
+We attempted to ascertain in how short a time light acted on the
+cotyledons of Phalaris, but this was difficult on account of their
+rapid circumnutating movement; moreover, they differ much in
+sensibility, according to age; nevertheless, some of our observations
+are worth giving. Pots with seedlings were
+placed under a microscope provided with an eye-piece micrometer, of
+which each division equalled 1/500th of an inch (0.051 mm.); and they
+were at first illuminated by light from a paraffin lamp passing through
+a solution of bichromate of potassium, which does not induce
+heliotropism. Thus the direction in which the cotyledons were
+circumnutating could be observed independently of any action from the
+light; and they could be made, by turning round the pots, to
+circumnutate transversely to the line in which the light would strike
+them, as soon as the solution was removed. The fact that the direction
+of the circumnutating movement might change at any moment, and thus the
+plant might bend either towards or from the lamp independently of the
+action of the light, gave an element of uncertainty to the results.
+After the solution had been removed, five seedlings which were
+circumnutating transversely to the line of light, began to move towards
+it, in 6, 4, 7½, 6, and 9 minutes. In one of these cases, the apex of
+the cotyledon crossed five of the divisions of the micrometer (i.e.
+1/100th of an inch, or 0.254 mm.) towards the light in 3 m. Of two
+seedlings which were moving directly from the light at the time when
+the solution was removed, one began to move towards it in 13 m., and
+the other in 15 m. This latter seedling was observed for more than an
+hour and continued to move towards the light; it crossed at one time 5
+divisions of the micrometer (0.254 mm.) in 2 m. 30 s. In all these
+cases, the movement towards the light was extremely unequal in rate,
+and the cotyledons often remained almost stationary for some minutes,
+and two of them retrograded a little. Another seedling which was
+circumnutating transversely to the line of light, moved towards it in 4
+m. after the solution was removed; it then remained
+almost stationary for 10 m.; then crossed 5 divisions of the micrometer
+in 6 m.; and then 8 divisions in 11m. This unequal rate of movement,
+interrupted by pauses, and at first with occasional retrogressions,
+accords well with our conclusion that heliotropism consists of modified
+circumnutation.
+
+In order to observe how long the after-effects of light lasted, a pot
+with seedlings of Phalaris, which had germinated in darkness, was
+placed at 10.40 A.M. before a north-east window, being protected on all
+other sides from the light; and the movement of a cotyledon was traced
+on a horizontal glass. It circumnutated about the same space for the
+first 24 m., and during the next 1 h. 33 m. moved rapidly towards the
+light. The light was now (i.e. after 1 h. 57 m.) completely excluded,
+but the cotyledon continued bending in the same direction as before,
+certainly for more than 15 m., probably for about 27 m. The doubt arose
+from the necessity of not looking at the seedlings often, and thus
+exposing them, though momentarily, to the light. This same seedling was
+now kept in the dark, until 2.18 P.M., by which time it had reacquired
+through apogeotropism its original upright position, when it was again
+exposed to the light from a clouded sky. By 3 P.M. it had moved a very
+short distance towards the light, but during the next 45 m. travelled
+quickly towards it. After this exposure of 1 h. 27 m. to a rather dull
+sky, the light was again completely excluded, but the cotyledon
+continued to bend in the same direction as before for 14 m. within a
+very small limit of error. It was then placed in the dark, and it now
+moved backwards, so that after 1 h. 7 m. it stood close to where it had
+started from at 2.18 P.M. These observations show that the cotyledons
+of Phalaris, after being exposed to a lateral
+light, continue to bend in the same direction for between a quarter and
+half an hour.
+
+In the two experiments just given, the cotyledons moved backwards or
+from the window shortly after being subjected to darkness; and whilst
+tracing the circumnutation of various kinds of seedlings exposed to a
+lateral light, we repeatedly observed that late in the evening, as the
+light waned, they moved from it. This fact is shown in some of the
+diagrams given in the last chapter. We wished therefore to learn
+whether this was wholly due to apogeotropism, or whether an organ after
+bending towards the light tended from any other cause to bend from it,
+as soon as the light failed. Accordingly, two pots of seedling Phalaris
+and one pot of seedling Brassica were exposed for 8 h. before a
+paraffin lamp, by which time the cotyledons of the former and the
+hypocotyls of the latter were bent rectangularly towards the light. The
+pots were now quickly laid horizontally, so that the upper parts of the
+cotyledons and of the hypocotyls of 9 seedlings projected vertically
+upwards, as proved by a plumb-line. In this position they could not be
+acted on by apogeotropism, and if they possessed any tendency to
+straighten themselves or to bend in opposition to their former
+heliotropic curvature, this would be exhibited, for it would be opposed
+at first very slightly by apogeotropism. They were kept in the dark for
+4 h., during which time they were twice looked at; but no uniform
+bending in opposition to their former heliotropic curvature could be
+detected. We have said uniform bending, because they circumnutated in
+their new position, and after 2 h. were inclined in different
+directions (between 4° and 11°) from the perpendicular. Their
+directions were also changed after two additional hours, and again on
+the following morning. We may
+therefore conclude that the bending back of plants from a light, when
+this becomes obscure or is extinguished, is wholly due to
+apogeotropism.[5]
+
+ [5] It appears from a reference in Wiesner (‘Die Undulirende Nutation
+ der Internodien,’ p. 7), that H. Müller of Thurgau found that a stem
+ which is bending heliotropically is at the same time striving, through
+ apogeotropism, to raise itself into a vertical position.
+
+
+In our various experiments we were often struck with the accuracy with
+which seedlings pointed to a light although of small size. To test
+this, many seedlings of Phalaris, which had germinated in darkness in a
+very narrow box several feet in length, were placed in a darkened room
+near to and in front of a lamp having a small cylindrical wick. The
+cotyledons at the two ends and in the central part of the box, would
+therefore have to bend in widely different directions in order to point
+to the light. After they had become rectangularly bent, a long white
+thread was stretched by two persons, close over and parallel, first to
+one and then to another cotyledon; and the thread was found in almost
+every case actually to intersect the small circular wick of the now
+extinguished lamp. The deviation from accuracy never exceeded, as far
+as we could judge, a degree or two. This extreme accuracy seems at
+first surprising, but is not really so, for an upright cylindrical
+stem, whatever its position may be with respect to the light, would
+have exactly half its circumference illuminated and half in shadow; and
+as the difference in illumination of the two sides is the exciting
+cause of heliotropism, a cylinder would naturally bend with much
+accuracy towards the light. The cotyledons, however, of Phalaris are
+not cylindrical, but oval in section; and the longer axis was to the
+shorter axis (in the one which was measured) as 100 to 70.
+Nevertheless, no difference could be
+detected in the accuracy of their bending, whether they stood with
+their broad or narrow sides facing the light, or in any intermediate
+position; and so it was with the cotyledons of Avena sativa, which are
+likewise oval in section. Now, a little reflection will show that in
+whatever position the cotyledons may stand, there will be a line of
+greatest illumination, exactly fronting the light, and on each side of
+this line an equal amount of light will be received; but if the oval
+stands obliquely with respect to the light, this will be diffused over
+a wider surface on one side of the central line than on the other. We
+may therefore infer that the same amount of light, whether diffused
+over a wider surface or concentrated on a smaller surface, produces
+exactly the same effect; for the cotyledons in the long narrow box
+stood in all sorts of positions with reference to the light, yet all
+pointed truly towards it.
+
+That the bending of the cotyledons to the light depends on the
+illumination of one whole side or on the obscuration of the whole
+opposite side, and not on a narrow longitudinal zone in the line of the
+light being affected, was shown by the effects of painting
+longitudinally with Indian ink one side of five cotyledons of Phalaris.
+These were then placed on a table near to a south-west window, and the
+painted half was directed either to the right or left. The result was
+that instead of bending in a direct line towards the window, they were
+deflected from the window and towards the unpainted side, by the
+following angles, 35°, 83°, 31°, 43°, and 39°. It should be remarked
+that it was hardly possible to paint one-half accurately, or to place
+all the seedlings which are oval in section in quite the same position
+relatively to the light; and this will account for the differences in
+the angles. Five
+cotyledons of Avena were also painted in the same manner, but with
+greater care; and they were laterally deflected from the line of the
+window, towards the unpainted side, by the following angles, 44°, 44°,
+55°, 51°, and 57°. This deflection of the cotyledons from the window is
+intelligible, for the whole unpainted side must have received some
+light, whereas the opposite and painted side received none; but a
+narrow zone on the unpainted side directly in front of the window will
+have received most light, and all the hinder parts (half an oval in
+section) less and less light in varying degrees; and we may conclude
+that the angle of deflection is the resultant of the action of the
+light over the whole of the unpainted side.
+
+It should have been premised that painting with Indian ink does not
+injure plants, at least within several hours; and it could injure them
+only by stopping respiration. To ascertain whether injury was thus soon
+caused, the upper halves of 8 cotyledons of Avena were thickly coated
+with transparent matter,—4 with gum, and 4 with gelatine; they were
+placed in the morning before a window, and by the evening they were
+normally bowed towards the light, although the coatings now consisted
+of dry crusts of gum and gelatine. Moreover, if the seedlings which
+were painted longitudinally with Indian ink had been injured on the
+painted side, the opposite side would have gone on growing, and they
+would consequently have become bowed towards the painted side; whereas
+the curvature was always, as we have seen, in the opposite direction,
+or towards the unpainted side which was exposed to the light. We
+witnessed the effects of injuring longitudinally one side of the
+cotyledons of Avena and Phalaris; for before we knew that grease was
+highly injurious to them, several were painted down one side
+with a mixture of oil and lamp-black, and were then exposed before a
+window; others similarly treated were afterwards tried in darkness.
+These cotyledons soon became plainly bowed towards the blackened side,
+evidently owing to the grease on this side having checked their growth,
+whilst growth continued on the opposite side. But it deserves notice
+that the curvature differed from that caused by light, which ultimately
+becomes abrupt near the ground. These seedlings did not afterwards die,
+but were much injured and grew badly.
+
+LOCALISED SENSITIVENESS TO LIGHT, AND ITS TRANSMITTED EFFECTS.
+
+Phalaris Canariensis.—Whilst observing the accuracy with which the
+cotyledons of this plant became bent towards the light of a small lamp,
+we were impressed with the idea that the uppermost part determined the
+direction of the curvature of the lower part. When the cotyledons are
+exposed to a lateral light, the upper part bends first, and afterwards
+the bending gradually extends down to the base, and, as we shall
+presently see, even a little beneath the ground. This holds good with
+cotyledons from less than .1 inch (one was observed to act in this
+manner which was only .03 in height) to about .5 of an inch in height;
+but when they have grown to nearly an inch in height, the basal part,
+for a length of .15 to .2 of an inch above the ground, ceases to bend.
+As with young cotyledons the lower part goes on bending, after the
+upper part has become well arched towards a lateral light, the apex
+would ultimately point to the ground instead of to the light, did not
+the upper part reverse its curvature and straighten itself, as
+soon as the upper convex surface of the bowed-down portion received
+more light than the lower concave surface. The position ultimately
+assumed by young and upright cotyledons, exposed to light entering
+obliquely from above through a window, is shown in the accompanying
+figure (Fig. 181); and here it may be seen that the whole upper part
+has become very nearly straight. When the cotyledons were exposed
+before a bright lamp, standing on the same level with them, the upper
+part, which was at first greatly arched towards the light, became
+straight and strictly parallel with the surface of the soil in the
+pots; the basal part being now rectangularly bent. All this great
+amount of curvature, together with the subsequent straightening of the
+upper part, was often effected in a few hours.
+
+Fig. 181. Phalaris Canariensis: cotyledons after exposure in a box open
+on one side in front of a south-west window during 8 h. Curvature
+towards the light accurately traced. The short horizontal lines show
+the level of the ground.
+
+After the uppermost part has become bowed a little to the light, its
+overhanging weight must tend to increase the curvature of the lower
+part; but any such effect was shown in several ways to be quite
+insignificant. When little caps of tin-foil (hereafter to be described)
+were placed on the summits of the cotyledons, though this must have
+added considerably to their weight, the rate or amount of bending was
+not thus increased. But the best evidence was afforded by placing pots
+with seedlings of Phalaris before a lamp in such a position, that the
+cotyledons were horizontally extended and projected at right angles to
+the line of light. In the course of 5½ h. they were directed towards
+the light with their bases bent at right angles; and this abrupt
+curvature could not have been aided in the least by the weight of the
+upper part, which acted at right angles to the plane of curvature.
+
+It will be shown that when the upper halves of the cotyledons of
+Phalaris and Avena were enclosed in little pipes of tin-foil or of
+blackened glass, in which case the upper part was mechanically
+prevented from bending, the lower and unenclosed part did not bend when
+exposed to a lateral light; and it occurred to us that this fact might
+be due, not to the exclusion of the light from the upper part, but to
+some necessity of the bending gradually travelling down the cotyledons,
+so that unless the upper part first became bent, the lower could not
+bend, however much it might be stimulated. It was necessary for our
+purpose to ascertain whether this notion was true, and it was proved
+false; for the lower halves of several cotyledons became bowed to the
+light, although their upper halves were enclosed in little glass tubes
+(not blackened), which prevented, as far as we could judge, their
+bending. Nevertheless, as the part within the tube might possibly bend
+a very little, fine rigid rods or flat splinters of thin glass were
+cemented with shellac to one side of the upper part of 15 cotyledons;
+and in six cases they were in addition tied on with threads. They were
+thus forced to remain quite straight. The result was that the lower
+halves of all became bowed to the light, but generally not in so great
+a degree as the corresponding part of the free seedlings in the same
+pots; and this may perhaps be accounted for by some slight degree of
+injury having been caused by a considerable surface having been smeared
+with shellac. It may be added, that when the cotyledons of Phalaris and
+Avena are acted on by apogeotropism, it is the upper part which begins
+first to bend; and when this part was rendered rigid in the manner just
+described, the upward curvature of the basal part was not thus
+prevented.
+
+To test our belief that the upper part of the cotyledons of Phalaris,
+when exposed to a lateral light, regulates the bending of the lower
+part, many experiments were tried; but most of our first attempts
+proved useless from various causes not worth specifying. Seven
+cotyledons had their tips cut off for lengths varying between .1 and
+.16 of an inch, and these, when left exposed all day to a lateral
+light, remained upright. In another set of 7 cotyledons, the tips were
+cut off for a length of only about .05 of an inch (1.27 mm.) and these
+became bowed towards
+a lateral light, but not nearly so much as the many other seedlings in
+the same pots. This latter case shows that cutting off the tips does
+not by itself injure the plants so seriously as to prevent
+heliotropism; but we thought at the time, that such injury might follow
+when a greater length was cut off, as in the first set of experiments.
+Therefore, no more trials of this kind were made, which we now regret;
+as we afterwards found that when the tips of three cotyledons were cut
+off for a length of .2 inch, and of four others for lengths of .14,
+.12, .1, and .07 inch, and they were extended horizontally, the
+amputation did not interfere in the least with their bending vertically
+upwards, through the action of apogeotropism, like unmutilated
+specimens. It is therefore extremely improbable that the amputation of
+the tips for lengths of from .1 to .14 inch, could from the injury thus
+caused have prevented the lower part from bending towards the light.
+
+We next tried the effects of covering the upper part of the cotyledons
+of Phalaris with little caps which were impermeable to light; the whole
+lower part being left fully exposed before a south-west window or a
+bright paraffin lamp. Some of the caps were made of extremely thin
+tin-foil blackened within; these had the disadvantage of occasionally,
+though rarely, being too heavy, especially when twice folded. The basal
+edges could be pressed into close contact with the cotyledons; though
+this again required care to prevent injuring them. Nevertheless, any
+injury thus caused could be detected by removing the caps, and trying
+whether the cotyledons were then sensitive to light. Other caps were
+made of tubes of the thinnest glass, which when painted black served
+well, with the one great disadvantage that the lower ends could not be
+closed. But tubes were used which fitted the cotyledons almost closely,
+and black paper was placed on the soil round each, to check the upward
+reflection of light from the soil. Such tubes were in one respect far
+better than caps of tin-foil, as it was possible to cover at the same
+time some cotyledons with transparent and others with opaque tubes; and
+thus our experiments could be controlled. It should be kept in mind
+that young cotyledons were selected for trial, and that these when not
+interfered with become bowed down to the ground towards the light.
+
+We will begin with the glass-tubes. The summits of nine cotyledons,
+differing somewhat in height, were enclosed for rather less than half
+their lengths in uncoloured or transparent
+tubes; and these were then exposed before a south-west window on a
+bright day for 8 h. All of them became strongly curved towards the
+light, in the same degree as the many other free seedlings in the same
+pots; so that the glass-tubes certainly did not prevent the cotyledons
+from bending towards the light. Nineteen other cotyledons were, at the
+same time, similarly enclosed in tubes thickly painted with Indian ink.
+On five of them, the paint, to our surprise, contracted after exposure
+to the sunlight, and very narrow cracks were formed, through which a
+little light entered; and these five cases were rejected. Of the
+remaining 14 cotyledons, the lower halves of which had been fully
+exposed to the light for the whole time, 7 continued quite straight and
+upright; 1 was considerably bowed to the light, and 6 were slightly
+bowed, but with the exposed bases of most of them almost or quite
+straight. It is possible that some light may have been reflected
+upwards from the soil and entered the bases of these 7 tubes, as the
+sun shone brightly, though bits of blackened paper had been placed on
+the soil round them. Nevertheless, the 7 cotyledons which were slightly
+bowed, together with the 7 upright ones, presented a most remarkable
+contrast in appearance with the many other seedlings in the same pots
+to which nothing had been done. The blackened tubes were then removed
+from 10 of these seedlings, and they were now exposed before a lamp for
+8 h.; 9 of them became greatly, and 1 moderately, curved towards the
+light, proving that the previous absence of any curvature in the basal
+part, or the presence of only a slight degree of curvature there, was
+due to the exclusion of light from the upper part.
+
+Similar observations were made on 12 younger cotyledons with their
+upper halves enclosed within glass-tubes coated with black varnish, and
+with their lower halves fully exposed to bright sunshine. In these
+younger seedlings the sensitive zone seems to extend rather lower down,
+as was observed on some other occasions, for two became almost as much
+curved towards the light as the free seedlings; and the remaining ten
+were slightly curved, although the basal part of several of them, which
+normally becomes more curved than any other part, exhibited hardly a
+trace of curvature. These 12 seedlings taken together differed greatly
+in their degree of curvature from all the many other seedlings in the
+same pots.
+
+Better evidence of the efficiency of the blackened tubes was
+incidentally afforded by some experiments hereafter to be given,
+in which the upper halves of 14 cotyledons were enclosed in tubes from
+which an extremely narrow stripe of the black varnish had been scraped
+off. These cleared stripes were not directed towards the window, but
+obliquely to one side of the room, so that only a very little light
+could act on the upper halves of the cotyledons. These 14 seedlings
+remained during eight hours of exposure before a south-west window on a
+hazy day quite upright; whereas all the other many free seedlings in
+the same pots became greatly bowed towards the light.
+
+We will now turn to the trials with caps made of very thin tin-foil.
+These were placed at different times on the summits of 24 cotyledons,
+and they extended down for a length of between .15 and .2 of an inch.
+The seedlings were exposed to a lateral light for periods varying
+between 6 h. 30 m. and 7 h. 45 m., which sufficed to cause all the
+other seedlings in the same pots to become almost rectangularly bent
+towards the light. They varied in height from only .04 to 1.15 inch,
+but the greater number were about .75 inch. Of the 24 cotyledons with
+their summits thus protected, 3 became much bent, but not in the
+direction of the light, and as they did not straighten themselves
+through apogeotropism during the following night, either the caps were
+too heavy or the plants themselves were in a weak condition; and these
+three cases may be excluded. There are left for consideration 21
+cotyledons; of these 17 remained all the time quite upright; the other
+4 became slightly inclined to the light, but not in a degree comparable
+with that of the many free seedlings in the same pots. As the
+glass-tubes, when unpainted, did not prevent the cotyledons from
+becoming greatly bowed, it cannot be supposed that the caps of very
+thin tin-foil did so, except through the exclusion of the light. To
+prove that the plants had not been injured, the caps were removed from
+6 of the upright seedlings, and these were exposed before a paraffin
+lamp for the same length of time as before, and they now all became
+greatly curved towards the light.
+
+As caps between .15 and .2 of an inch in depth were thus proved to be
+highly efficient in preventing the cotyledons from bending towards the
+light, 8 other cotyledons were protected with caps between only .06 and
+.12 in depth. Of these, two remained vertical, one was considerably and
+five slightly curved towards the light, but far less so than the free
+seedlings in the same pots.
+
+
+Another trial was made in a different manner, namely, by bandaging with
+strips of tin-foil, about .2 in breadth, the upper part, but not the
+actual summit, of eight moderately young seedlings a little over half
+an inch in height. The summits and the basal parts were thus left fully
+exposed to a lateral light during 8 h.; an upper intermediate zone
+being protected. With four of these seedlings the summits were exposed
+for a length of .05 inch, and in two of them this part became curved
+towards the light, but the whole lower part remained quite upright;
+whereas the entire length of the other two seedlings became slightly
+curved towards the light. The summits of the four other seedlings were
+exposed for a length of .04 inch, and of these one remained almost
+upright, whilst the other three became considerably curved towards the
+light. The many free seedlings in the same pots were all greatly curved
+towards the light.
+
+From these several sets of experiments, including those with the
+glass-tubes, and those when the tips were cut off, we may infer that
+the exclusion of light from the upper part of the cotyledons of
+Phalaris prevents the lower part, though fully exposed to a lateral
+light, from becoming curved. The summit for a length of .04 or .05 of
+an inch, though it is itself sensitive and curves towards the light,
+has only a slight power of causing the lower part to bend. Nor has the
+exclusion of light from the summit for a length of .1 of an inch a
+strong influence on the curvature of the lower part. On the other hand,
+an exclusion for a length of between .15 and .2 of an inch, or of the
+whole upper half, plainly prevents the lower and fully illuminated part
+from becoming curved in the manner (see Fig. 181) which invariably
+occurs when a free cotyledon is exposed to a lateral light. With very
+young seedlings the sensitive zone seems to extend rather lower down
+relatively to their height than in older seedlings. We must therefore
+conclude that when seedlings are freely exposed to a lateral light some
+influence is transmitted from the upper to the lower part, causing the
+latter to bend.
+
+This conclusion is supported by what may be seen to occur on a small
+scale, especially with young cotyledons, without any artificial
+exclusion of the light; for they bend beneath the earth where no light
+can enter. Seeds of Phalaris were covered with a layer one-fourth of an
+inch in thickness of very fine sand, consisting of extremely minute
+grains of silex coated with
+oxide of iron. A layer of this sand, moistened to the same degree as
+that over the seeds, was spread over a glass-plate; and when the layer
+was .05 of an inch in thickness (carefully measured) no light from a
+bright sky could be seen to pass through it, unless it was viewed
+through a long blackened tube, and then a trace of light could be
+detected, but probably much too little to affect any plant. A layer .1
+of an inch in thickness was quite impermeable to light, as judged by
+the eye aided by the tube. It may be worth adding that the layer, when
+dried, remained equally impermeable to light. This sand yielded to very
+slight pressure whilst kept moist, and in this state did not contract
+or crack in the least. In a first trial, cotyledons which had grown to
+a moderate height were exposed for 8 h. before a paraffin lamp, and
+they became greatly bowed. At their bases on the shaded side opposite
+to the light, well-defined, crescentic, open furrows were formed, which
+(measured under a microscope with a micrometer) were from .02 to .03 of
+an inch in breadth, and these had evidently been left by the bending of
+the buried bases of the cotyledons towards the light. On the side of
+the light the cotyledons were in close contact with the sand, which was
+a very little heaped up. By removing with a sharp knife the sand on one
+side of the cotyledons in the line of the light, the bent portion and
+the open furrows were found to extend down to a depth of about .1 of an
+inch, where no light could enter. The chords of the short buried arcs
+formed in four cases angles of 11°, 13°, 15°, and 18°, with the
+perpendicular. By the following morning these short bowed portions had
+straightened themselves through apogeotropism.
+
+In the next trial much younger cotyledons were similarly treated, but
+were exposed to a rather obscure lateral light. After some hours, a
+bowed cotyledon, .3 inch in height, had an open furrow on the shaded
+side .04 inch in breadth; another cotyledon, only .13 inch in height,
+had left a furrow .02 inch in breadth. But the most curious case was
+that of a cotyledon which had just protruded above the ground and was
+only .03 inch in height, and this was found to be bowed in the
+direction of the light to a depth of .2 of an inch beneath the surface.
+From what we know of the impermeability of this sand to light, the
+upper illuminated part in these several cases must have determined the
+curvature of the lower buried portions. But an apparent cause of doubt
+may be suggested: as the cotyledons are continually circumnutating,
+they tend to form a minute
+crack or furrow all round their bases, which would admit a little light
+on all sides; but this would not happen when they were illuminated
+laterally, for we know that they quickly bend towards a lateral light,
+and they then press so firmly against the sand on the illuminated side
+as to furrow it, and this would effectually exclude light on this side.
+Any light admitted on the opposite and shaded side, where an open
+furrow is formed, would tend to counteract the curvature towards the
+lamp or other source of the light. It may be added, that the use of
+fine moist sand, which yields easily to pressure, was indispensable in
+the above experiments; for seedlings raised in common soil, not kept
+especially damp, and exposed for 9 h. 30 m. to a strong lateral light,
+did not form an open furrow at their bases on the shaded side, and were
+not bowed beneath the surface.
+
+Perhaps the most striking proof of the action of the upper on the lower
+part of the cotyledons of Phalaris, when laterally illuminated, was
+afforded by the blackened glass-tubes (before alluded to) with very
+narrow stripes of the varnish scraped off on one side, through which a
+little light was admitted. The breadth of these stripes or slits varied
+between .01 and .02 inch (.25 and .51 mm.). Cotyledons with their upper
+halves enclosed in such tubes were placed before a south-west window,
+in such a position, that the scraped stripes did not directly face the
+window, but obliquely to one side. The seedlings were left exposed for
+8 h., before the close of which time the many free seedlings in the
+same pots had become greatly bowed towards the window. Under these
+circumstances, the whole lower halves of the cotyledons, which had
+their summits enclosed in the tubes, were fully exposed to the light of
+the sky, whilst their upper halves received exclusively or chiefly
+diffused light from the room, and this only through a very narrow slit
+on one side. Now, if the curvature of the lower part had been
+determined by the illumination of this part, all the cotyledons
+assuredly would have become curved towards the window; but this was far
+from being the case. Tubes of the kind just described were placed on
+several occasions over the upper halves of 27 cotyledons; 14 of them
+remained all the time quite vertical; so that sufficient diffused light
+did not enter through the narrow slits to produce any effect whatever;
+and they behaved in the same manner as if their upper halves had been
+enclosed in completely blackened tubes. The lower halves of the 13
+other cotyledons became bowed
+not directly in the line of the window, but obliquely towards it; one
+pointed at an angle of only 18°, but the remaining 12 at angles varying
+between 45° and 62° from the line of the window. At the commencement of
+the experiment, pins had been laid on the earth in the direction
+towards which the slits in the varnish faced; and in this direction
+alone a small amount of diffused light entered. At the close of the
+experiment, 7 of the bowed cotyledons pointed exactly in the line of
+the pins, and 6 of them in a line between that of the pins and that of
+the window. This intermediate position is intelligible, for any light
+from the sky which entered obliquely through the slits would be much
+more efficient than the diffused light which entered directly through
+them. After the 8 h. exposure, the contrast in appearance between these
+13 cotyledons and the many other seedlings in the same pots, which were
+all (excepting the above 14 vertical ones) greatly bowed in straight
+and parallel lines towards the window, was extremely remarkable. It is
+therefore certain that a little weak light striking the upper halves of
+the cotyledons of Phalaris, is far more potent in determining the
+direction of the curvature of the lower halves, than the full
+illumination of the latter during the whole time of exposure.
+
+In confirmation of the above results, the effect of thickly painting
+with Indian ink one side of the upper part of three cotyledons of
+Phalaris, for a length of .2 inch from their tips, may be worth giving.
+These were placed so that the unpainted surface was directed not
+towards the window, but a little to one side; and they all became bent
+towards the unpainted side, and from the line of the window by angles
+amounting to 31°, 35°, and 83°. The curvature in this direction
+extended down to their bases, although the whole lower part was fully
+exposed to the light from the window.
+
+Finally, although there can be no doubt that the illumination of the
+upper part of the cotyledons of Phalaris greatly affects the power and
+manner of bending of the lower part, yet some observations seemed to
+render it probable that the simultaneous stimulation of the lower part
+by light greatly favours, or is almost necessary, for its well-marked
+curvature; but our experiments were not conclusive, owing to the
+difficulty of excluding light from the lower halves without
+mechanically preventing their curvature.
+
+Avena sativa.—The cotyledons of this plant become quickly bowed towards
+a lateral light, exactly like those of Phalaris.
+Experiments similar to the foregoing ones were tried, and we will give
+the results as briefly as possible. They are somewhat less conclusive
+than in the case of Phalaris, and this may possibly be accounted for by
+the sensitive zone varying in extension, in a species so long
+cultivated and variable as the common Oat. Cotyledons a little under
+three-quarters of an inch in height were selected for trial: six had
+their summits protected from light by tin-foil caps, .25 inch in depth,
+and two others by caps .3 inch in depth. Of these 8 cotyledons, five
+remained upright during 8 hours of exposure, although their lower parts
+were fully exposed to the light all the time; two were very slightly,
+and one considerably, bowed towards it. Caps only .2 or .22 inch in
+depth were placed over 4 other cotyledons, and now only one remained
+upright, one was slightly, and two considerably bowed to the light. In
+this and the following cases all the free seedlings in the same pots
+became greatly bowed to the light.
+
+Our next trial was made with short lengths of thin and fairly
+transparent quills; for glass-tubes of sufficient diameter to go over
+the cotyledons would have been too heavy. Firstly, the summits of 13
+cotyledons were enclosed in unpainted quills, and of these 11 became
+greatly and 2 slightly bowed to the light; so that the mere act of
+enclosure did not prevent the lower part from becoming bowed. Secondly,
+the summits of 11 cotyledons were enclosed in quills .3 inch in length,
+painted so as to be impermeable to light; of these, 7 did not become at
+all inclined towards the light, but 3 of them were slightly bent more
+or less transversely with respect to the line of light, and these might
+perhaps have been altogether excluded; one alone was slightly bowed
+towards the light. Painted quills, .25 inch in length, were placed over
+the summits of 4 other cotyledons; of these, one alone remained
+upright, a second was slightly bowed, and the two others as much bowed
+to the light as the free seedlings in the same pots. These two latter
+cases, considering that the caps were .25 in length, are inexplicable.
+
+Lastly, the summits of 8 cotyledons were coated with flexible and
+highly transparent gold-beaters’ skin, and all became as much bowed to
+the light as the free seedlings. The summits of 9 other cotyledons were
+similarly coated with gold-beaters’ skin, which was then painted to a
+depth of between .25 and .3 inch, so as to be impermeable to light; of
+these 5 remained upright, and 4 were well bowed to the light, almost or
+quite as well as
+the free seedlings. These latter four cases, as well as the two in the
+last paragraph, offer a strong exception to the rule that the
+illumination of the upper part determines the curvature of the lower
+part. Nevertheless, 5 of these 8 cotyledons remained quite upright,
+although their lower halves were fully illuminated all the time; and it
+would almost be a prodigy to find five free seedlings standing
+vertically after an exposure for several hours to a lateral light.
+
+The cotyledons of Avena, like those of Phalaris, when growing in soft,
+damp, fine sand, leave an open crescentric furrow on the shaded side,
+after bending to a lateral light; and they become bowed beneath the
+surface at a depth to which, as we know, light cannot penetrate. The
+arcs of the chords of the buried bowed portions formed in two cases
+angles of 20° and 21° with the perpendicular. The open furrows on the
+shaded side were, in four cases, .008, .016, .024, and .024 of an inch
+in breadth. Brassica oleracea (Common Red).—It will here be shown that
+the upper half of the hypocotyl of the cabbage, when illuminated by a
+lateral light, determines the curvature of the lower half. It is
+necessary to experimentise on young seedlings about half an inch or
+rather less in height, for when grown to an inch and upwards the basal
+part ceases to bend. We first tried painting the hypocotyls with Indian
+ink, or cutting off their summits for various lengths; but these
+experiments are not worth giving, though they confirm, as far as they
+can be trusted, the results of the following ones. These were made by
+folding gold-beaters’ skin once round the upper halves of young
+hypocotyls, and painting it thickly with Indian ink or with black
+grease. As a control experiment, the same transparent skin, left
+unpainted, was folded round the upper halves of 12 hypocotyls; and
+these all became greatly curved to the light, excepting one, which was
+only moderately curved. Twenty other young hypocotyls had the skin
+round their upper halves painted, whilst their lower halves were left
+quite uncovered. These seedlings were then exposed, generally for
+between 7 and 8 h., in a box blackened within and open in front, either
+before a south-west window or a paraffin lamp. This exposure was amply
+sufficient, as was shown by the strongly-marked heliotropism of all the
+free seedlings in the same pots; nevertheless, some were left exposed
+to the light for a much longer time. Of the 20 hypocotyls thus treated,
+14 remained quite upright, and 6 became slightly bowed to the light;
+but 2 of these latter cases were not really
+exceptions, for on removing the skin the paint was found imperfect and
+was penetrated by many small transparent spaces on the side which faced
+the light. Moreover, in two other cases the painted skin did not extend
+quite halfway down the hypocotyl. Although there was a wonderful
+contrast in the several pots between these 20 hypocotyls and the other
+many free seedlings, which were all greatly bowed down to their bases
+in the direction of the light, some being almost prostrate on the
+ground.
+
+The most successful trial on any one day (included in the above
+results) is worth describing in detail. Six young seedlings were
+selected, the hypocotyls of which were nearly .45 inch, excepting one,
+which was .6 inch in height, measured from the bases of their petioles
+to the ground. Their upper halves, judged as accurately as could be
+done by the eye, were folded once round with gold-beaters’ skin, and
+this was painted thickly with Indian ink. They were exposed in an
+otherwise darkened room before a bright paraffin lamp, which stood on a
+level with the two pots containing the seedlings. They were first
+looked at after an interval of 5 h. 10 m., and five of the protected
+hypocotyls were found quite erect, the sixth being very slightly
+inclined to the light; whereas all the many free seedlings in the same
+two pots were greatly bowed to the light. They were again examined
+after a continuous exposure to the light of 20 h. 35m.; and now the
+contrast between the two sets was wonderfully great; for the free
+seedlings had their hypocotyls extended almost horizontally in the
+direction of the light, and were curved down to the ground; whilst
+those with the upper halves protected by the painted skin, but with
+their lower halves fully exposed to the light, still remained quite
+upright, with the exception of the one which retained the same slight
+inclination to the light which it had before. This latter seedling was
+found to have been rather badly painted, for on the side facing the
+light the red colour of the hypocotyl could be distinguished through
+the paint.
+
+We next tried nine older seedlings, the hypocotyls of which varied
+between 1 and 1.6 inch in height. the gold-beaters’ skin round their
+upper parts was painted with black grease to a depth of only .3 inch,
+that is, from less than a third to a fourth or fifth of their total
+heights. They were exposed to the light for 7 h. 15 m.; and the result
+showed that the whole of the sensitive zone, which determines the
+curvature of the lower
+part, was not protected from the action of the light; for all 9 became
+curved towards it, 4 of them very slightly, 3 moderately, and 2 almost
+as much as the unprotected seedlings. Nevertheless, the whole 9 taken
+together differed plainly in their degree of curvature from the many
+free seedlings, and from some which were wrapped in unpainted skin,
+growing in the same two pots.
+
+Seeds were covered with about a quarter of an inch of the fine sand
+described under Phalaris; and when the hypocotyls had grown to a height
+of between .4 and .55 inch, they were exposed during 9 h. before a
+paraffin lamp, their bases being at first closely surrounded by the
+damp sand. They all became bowed down to the ground, so that their
+upper parts lay near to and almost parallel to the surface of the soil.
+On the side of the light their bases were in close contact with the
+sand, which was here a very little heaped up; on the opposite or shaded
+side there were open, crescentic cracks or furrows, rather above .01 of
+an inch in width; but they were not so sharp and regular as those made
+by Phalaris and Avena, and therefore could not be so easily measured
+under the microscope. The hypocotyls were found, when the sand was
+removed on one side, to be curved to a depth beneath the surface in
+three cases of at least .1 inch, in a fourth case of .11, and in a
+fifth of .15 inch. The chords of the arcs of the short, buried, bowed
+portions formed angles of between 11° and 15° with the perpendicular.
+From what we have seen of the impermeability of this sand to light, the
+curvature of the hypocotyls certainly extended down to a depth where no
+light could enter; and the curvature must have been caused by an
+influence transmitted from the upper illuminated part.
+
+The lower halves of five young hypocotyls were surrounded by unpainted
+gold-beaters’ skin, and these, after an exposure of 8 h. before a
+paraffin lamp, all became as much bowed to the light as the free
+seedlings. The lower halves of 10 other young hypocotyls, similarly
+surrounded with the skin, were thickly painted with Indian ink; their
+upper and unprotected halves became well curved to the light, but their
+lower and protected halves remained vertical in all the cases excepting
+one, and on this the layer of paint was imperfect. This result seems to
+prove that the influence transmitted from the upper part is not
+sufficient to cause the lower part to bend, unless it be at the same
+time illuminated; but there remains the doubt, as in
+the case of Phalaris, whether the skin covered with a rather thick
+crust of dry Indian ink did not mechanically prevent their curvature.
+
+Beta vulgaris.—A few analogous experiments were tried on this plant,
+which is not very well adapted for the purpose, as the basal part of
+the hypocotyl, after it has grown to above half an inch in height, does
+not bend much on exposure to a lateral light. Four hypocotyls were
+surrounded close beneath their petioles with strips of thin tin-foil,
+.2 inch in breadth, and they remained upright all day before a paraffin
+lamp; two others were surrounded with strips .15 inch in breadth, and
+one of these remained upright, the other becoming bowed; the bandages
+in two other cases were only .1 inch in breadth, and both of these
+hypocotyls became bowed, though one only slightly, towards the light.
+The free seedlings in the same pots were all fairly well curved towards
+the light; and during the following night became nearly upright. The
+pots were now turned round and placed before a window, so that the
+opposite sides of the seedlings were exposed to the light, towards
+which all the unprotected hypocotyls became bent in the course of 7 h.
+Seven out of the 8 seedlings with bandages of tin-foil remained
+upright, but one which had a bandage only .1 inch in breadth, became
+curved to the light. On another occasion, the upper halves of 7
+hypocotyls were surrounded with painted gold-beaters’ skin; of these 4
+remained upright, and 3 became a little curved to the light: at the
+same time 4 other seedlings surrounded with unpainted skin, as well as
+the free ones in the same pots, all became bowed towards the lamp,
+before which they had been exposed during 22 hours.
+
+Radicles of Sinapis alba.—The radicles of some plants are indifferent,
+as far as curvature is concerned, to the action of light; whilst others
+bend towards and others from it.[6] Whether these movements are of any
+service to the plant is very doubtful, at least in the case of
+subterranean roots; they probably result from the radicles being
+sensitive to contact, moisture, and gravitation, and as a consequence
+to other irritants which are never naturally encountered. The radicles
+of Sinapis alba, when immersed in water and exposed to a lateral light,
+bend from it, or are apheliotropic. They become bent for a length of
+about 4 mm. from their tips. To ascertain whether this movement
+generally occurred, 41 radicles, which had germinated in damp sawdust,
+were immersed in water and exposed to a lateral light; and they all,
+with two doubtful exceptions, became curved from the light. At the same
+time the tips of 54 other radicles, similarly exposed, were just
+touched with nitrate of silver. They were blackened for a length of
+from .05 to .07 mm., and probably killed; but it should be observed
+that this did not check materially, if at all, the growth of the upper
+part; for several, which were measured, increased in the course of only
+8–9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles one case
+was doubtful, 25 curved themselves from the light in the normal manner,
+and 28, or more than half, were not in the least apheliotropic. There
+was a considerable difference, which we cannot account for, in the
+results of the experiments tried towards the end of April and in the
+middle of September. Fifteen radicles (part of the above 54) were
+cauterised at the former period and were exposed to sunshine, of which
+12 failed to be apheliotropic, 2 were still apheliotropic, and 1 was
+doubtful. In September, 39 cauterised radicles were exposed to a
+northern light, being kept at a proper temperature; and now 23
+continued to be apheliotropic in the normal manner, and only 16 failed
+to bend from the light. Looking at the aggregate results at both
+periods, there can be no doubt that the destruction of the tip for less
+than a millimeter in length destroyed in more than half the cases their
+power of moving from the light. It is probable that if the tips had
+been cauterised for the length of a whole millimeter, all signs of
+apheliotropism would have disappeared. It may be suggested that
+although the application of caustic does not stop growth, yet enough
+may be absorbed to destroy the power of movement in the upper part; but
+this suggestion must be rejected, for we have seen and shall again see,
+that cauterising one side of the tip of various kinds of radicles
+actually excites movement. The conclusion seems inevitable that
+sensitiveness to light resides in the tip of the radicle of Sinapis
+alba; and that the tip when thus stimulated transmits some influence to
+the upper part, causing it to bend. The case in this respect is
+parallel with that of the radicles of several plants, the tips of which
+are sensitive to contact and to other irritants, and, as will be shown
+in the eleventh chapter, to gravitation.
+
+ [6] Sachs, ‘Physiologie Végétale,’ 1868, p. 44.
+
+CONCLUDING REMARKS AND SUMMARY OF CHAPTER.
+
+We do not know whether it is a general rule with seedling plants that
+the illumination of the upper part determines the curvature of the
+lower part. But as this occurred in the four species examined by us,
+belonging to such distinct families as the Gramineæ, Cruciferae, and
+Chenopodeae, it is probably of common occurrence. It can hardly fail to
+be of service to seedlings, by aiding them to find the shortest path
+from the buried seed to the light, on nearly the same principle that
+the eyes of most of the lower crawling animals are seated at the
+anterior ends of their bodies. It is extremely doubtful whether with
+fully developed plants the illumination of one part ever affects the
+curvature of another part. The summits of 5 young plants of Asparagus
+officinalis (varying in height between 1.1 and 2.7 inches, and
+consisting of several short internodes) were covered with caps of
+tin-foil from 0.3 to 0.35 inch in depth; and the lower uncovered parts
+became as much curved towards a lateral light, as were the free
+seedlings in the same pots. Other seedlings of the same plant had their
+summits painted with Indian ink with the same negative result. Pieces
+of blackened paper were gummed to the edges and over the blades of some
+leaves on young plants of Tropaeolum majus and Ranunculus ficaria;
+these were then placed in a box before a window, and the petioles of
+the protected leaves became curved towards the light, as much as those
+of the unprotected leaves.
+
+The foregoing cases with respect to seedling plants have been fully
+described, not only because the transmission of any effect from light
+is a new physiological fact, but because we think it tends to modify
+somewhat the current views on heliotropic movements. Until
+lately such movements were believed to result simply from increased
+growth on the shaded side. At present it is commonly admitted[7] that
+diminished light increases the turgescence of the cells, or the
+extensibility of the cell-walls, or of both together, on the shaded
+side, and that this is followed by increased growth. But Pfeffer has
+shown that a difference in the turgescence on the two sides of a
+pulvinus,—that is, an aggregate of small cells which have ceased to
+grow at an early age,—is excited by a difference in the amount of light
+received by the two sides; and that movement is thus caused without
+being followed by increased growth on the more turgescent side.[8] All
+observers apparently believe that light acts directly on the part which
+bends, but we have seen with the above described seedlings that this is
+not the case. Their lower halves were brightly illuminated for hours,
+and yet did not bend in the least towards the light, though this is the
+part which under ordinary circumstances bends the most. It is a still
+more striking fact, that the faint illumination of a narrow stripe on
+one side of the upper part of the cotyledons of Phalaris determined the
+direction of the curvature of the lower part; so that this latter part
+did not bend towards the bright light by which it had been fully
+illuminated,
+but obliquely towards one side where only a little light entered. These
+results seem to imply the presence of some matter in the upper part
+which is acted on by light, and which transmits its effects to the
+lower part. It has been shown that this transmission is independent of
+the bending of the upper sensitive part. We have an analogous case of
+transmission in Drosera, for when a gland is irritated, the basal and
+not the upper or intermediate part of the tentacle bends. The flexible
+and sensitive filament of Dionaea likewise transmits a stimulus,
+without itself bending; as does the stem of Mimosa.
+
+ [7] Emil Godlewski has given (‘Bot. Zeitung,’ 1879, Nos. 6–9) an
+ excellent account (p. 120) of the present state of the question. See
+ also Vines in ‘Arbeiten des Bot. Inst. in Würzburg,’ 1878, B. ii. pp.
+ 114–147. Hugo de Vries has recently published a still more important
+ article on this subject: ‘Bot Zeitung,’ Dec. 19th and 26th, 1879.
+
+
+ [8] ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, pp. 7, 63,
+ 123, etc. Frank has also insisted (‘Die Naturliche wägerechte Richtung
+ von Pflanzentheilen,’ 1870, p. 53) on the important part which the
+ pulvini of the leaflets of compound leaves play in placing the
+ leaflets in a proper position with respect to the light. This holds
+ good, especially with the leaves of climbing plants, which are carried
+ into all sorts of positions, ill-adapted for the action of the light.
+
+
+Light exerts a powerful influence on most vegetable tissues, and there
+can be no doubt that it generally tends to check their growth. But when
+the two sides of a plant are illuminated in a slightly different
+degree, it does not necessarily follow that the bending towards the
+illuminated side is caused by changes in the tissues of the same nature
+as those which lead to increased growth in darkness. We know at least
+that a part may bend from the light, and yet its growth may not be
+favoured by light. This is the case with the radicles of Sinapis alba,
+which are plainly apheliotropic; nevertheless, they grow quicker in
+darkness than in light.[9] So it is with many aërial roots, according
+to Wiesner;[10] but there are other opposed cases. It appears,
+therefore, that light does not determine the growth of apheliotropic
+parts in any uniform manner.
+
+ [9] Francis Darwin, ‘Über das Wachsthum negativ heliotropischer
+ Wurzeln’: ‘Arbeiten des Bot. Inst. in Würzburg,’ B. ii., Heft iii.,
+ 1880, p. 521.
+
+
+ [10] ‘Sitzb. der k. Akad. der Wissensch’ (Vienna), 1880, p. 12.
+
+
+We should bear in mind that the power of bending to the light is highly
+beneficial to most plants. There
+is therefore no improbability in this power having been specially
+acquired. In several respects light seems to act on plants in nearly
+the same manner as it does on animals by means of the nervous
+system.[11] With seedlings the effect, as we have just seen, is
+transmitted from one part to another. An animal may be excited to move
+by a very small amount of light; and it has been shown that a
+difference in the illumination of the two sides of the cotyledons of
+Phalaris, which could not be distinguished by the human eye, sufficed
+to cause them to bend. It has also been shown that there is no close
+parallelism between the amount of light which acts on a plant and its
+degree of curvature; it was indeed hardly possible to perceive any
+difference in the curvature of some seedlings of Phalaris exposed to a
+light, which, though dim, was very much brighter than that to which
+others had been exposed. The retina, after being stimulated by a bright
+light, feels the effect for some time; and Phalaris continued to bend
+for nearly half an hour towards the side which had been illuminated.
+The retina cannot perceive a dim light after it has been exposed to a
+bright one; and plants which had been kept in the daylight during the
+previous day and morning, did not move so soon towards an obscure
+lateral light as did others which had been kept in complete darkness.
+
+ [11] Sachs has made some striking remarks to the same effect with
+ respect to the various stimuli which excite movement in plants. See
+ his paper ‘Ueber orthotrope und plagiotrope Pflanzentheile,’ ‘Arb. des
+ Bot. Inst. in Würzburg,’ 1879, B. ii. p. 282.
+
+
+Even if light does act in such a manner on the growing parts of plants
+as always to excite in them a tendency to bend towards the more
+illuminated side—a supposition contradicted by the foregoing
+experiments on seedlings and by all apheliotropic
+organs—yet the tendency differs greatly in different species, and is
+variable in degree in the individuals of the same species, as may be
+seen in almost any pot of seedlings of a long cultivated plant.[12]
+There is therefore a basis for the modification of this tendency to
+almost any beneficial extent. That it has been modified, we see in many
+cases: thus, it is of more importance for insectivorous plants to place
+their leaves in the best position for catching insects than to turn
+their leaves to the light, and they have no such power. If the stems of
+twining plants were to bend towards the light, they would often be
+drawn away from their supports; and as we have seen they do not thus
+bend. As the stems of most other plants are heliotropic, we may feel
+almost sure that twining plants, which are distributed throughout the
+whole vascular series, have lost a power that their non-climbing
+progenitors possessed. Moreover, with Ipomœa, and probably all other
+twiners, the stem of the young plant, before it begins to twine, is
+highly heliotropic, evidently in order to expose the cotyledons or the
+first true leaves fully to the light. With the Ivy the stems of
+seedlings are moderately heliotropic, whilst those of the same plants
+when grown a little older
+are apheliotropic. Some tendrils which consist of modified
+leaves—organs in all ordinary cases strongly diaheliotropic—have been
+rendered apheliotropic, and their tips crawl into any dark crevice.
+
+ [12] Strasburger has shown in his interesting work (‘Wirkung des
+ Lichtes...auf Schwärmsporen,’ 1878), that the movement of the
+ swarm-spores of various lowly organised plants to a lateral light is
+ influenced by their stage of development, by the temperature to which
+ they are subjected, by the degree of illumination under which they
+ have been raised, and by other unknown causes; so that the
+ swarm-spores of the same species may move across the field of the
+ microscope either to or from the light. Some individuals, moreover,
+ appear to be indifferent to the light; and those of different species
+ behave very differently. The brighter the light, the straighter is
+ their course. They exhibit also for a short time the after-effects of
+ light. In all these respects they resemble the higher plants. See,
+ also, Stahl, ‘Ueber den einfluss der Lichts auf die
+ Bewegungs-erscheinungen der Schwärmsporen’ Verh. d. phys.-med.
+ Geselsshalft in Würzburg, B. xii. 1878.
+
+
+Even in the case of ordinary heliotropic movements, it is hardly
+credible that they result directly from the action of the light,
+without any special adaptation. We may illustrate what we mean by the
+hygroscopic movements of plants: if the tissues on one side of an organ
+permit of rapid evaporation, they will dry quickly and contract,
+causing the part to bend to this side. Now the wonderfully complex
+movements of the pollinia of Orchis pyramidalis, by which they clasp
+the proboscis of a moth and afterwards change their position for the
+sake of depositing the pollen-masses on the double stigma—or again the
+twisting movements, by which certain seeds bury themselves in the
+ground[13]—follow from the manner of drying of the parts in question;
+yet no one will suppose that these results have been gained without
+special adaptation. Similarly, we are led to believe in adaptation when
+we see the hypocotyl of a seedling, which contains chlorophyll, bending
+to the light; for although it thus receives less light, being now
+shaded by its own cotyledons, it places them—the more important
+organs—in the best position to be fully illuminated. The hypocotyl may
+therefore be said to sacrifice itself for the good of the cotyledons,
+or rather of the whole plant. But if it be prevented from bending, as
+must sometimes occur with seedlings springing up in an entangled mass
+of vegetation, the cotyledons themselves bend so as to face the light;
+the one farthest off rising
+up, and that nearest to the light sinking down, or both twisting
+laterally.[14] We may, also, suspect that the extreme sensitiveness to
+light of the upper part of the sheath-like cotyledons of the Gramineæ,
+and their power of transmitting its effects to the lower part, are
+specialised arrangements for finding the shortest path to the light.
+With plants growing on a bank, or thrown prostrate by the wind, the
+manner in which the leaves move, even rotating on their own axes, so
+that their upper surfaces may be again directed to the light, is a
+striking phenomenon. Such facts are rendered more striking when we
+remember that too intense a light injures the chlorophyll, and that the
+leaflets of several Leguminosae when thus exposed bend upwards and
+present their edges to the sun, thus escaping injury. On the other
+hand, the leaflets of Averrhoa and Oxalis, when similarly exposed, bend
+downwards.
+
+ [13] Francis Darwin, ‘On the Hygroscopic Mechanism,’ etc.,
+ ‘Transactions Linn. Soc.,’ series ii. vol. i. p. 149, 1876.
+
+
+ [14] Wiesner has made remarks to nearly the same effect with respect
+ to leaves: ‘Die undulirende Nutation der Internodien,’ p. 6, extracted
+ from B. lxxvii. (1878). Sitb. der k. Akad. der Wissensch. Wien.
+
+
+It was shown in the last chapter that heliotropism is a modified form
+of circumnutation; and as every growing part of every plant
+circumnutates more or less, we can understand how it is that the power
+of bending to the light has been acquired by such a multitude of plants
+throughout the vegetable kingdom. The manner in which a circumnutating
+movement—that is, one consisting of a succession of irregular ellipses
+or loops—is gradually converted into a rectilinear course towards the
+light, has been already explained. First, we have a succession of
+ellipses with their longer axes directed towards the light, each of
+which
+is described nearer and nearer to its source; then the loops are drawn
+out into a strongly pronounced zigzag line, with here and there a small
+loop still formed. At the same time that the movement towards the light
+is increased in extent and accelerated, that in the opposite direction
+is lessened and retarded, and at last stopped. The zigzag movement to
+either side is likewise gradually lessened, so that finally the course
+becomes rectilinear. Thus under the stimulus of a fairly bright light
+there is no useless expenditure of force.
+
+As with plants every character is more or less variable, there seems to
+be no great difficulty in believing that their circumnutating movements
+may have been increased or modified in any beneficial manner by the
+preservation of varying individuals. The inheritance of habitual
+movements is a necessary contingent for this process of selection, or
+the survival of the fittest; and we have seen good reason to believe
+that habitual movements are inherited by plants. In the case of twining
+species the circumnutating movements have been increased in amplitude
+and rendered more circular; the stimulus being here an internal or
+innate one. With sleeping plants the movements have been increased in
+amplitude and often changed in direction; and here the stimulus is the
+alternation of light and darkness, aided, however, by inheritance. In
+the case of heliotropism, the stimulus is the unequal illumination of
+the two sides of the plant, and this determines, as in the foregoing
+cases, the modification of the circumnutating movement in such a manner
+that the organ bends to the light. A plant which has been rendered
+heliotropic by the above means, might readily lose this tendency,
+judging from the cases already given, as soon as it became useless or
+injurious. A species which has ceased to be heliotropic might also be
+rendered apheliotropic by the preservation of the individuals which
+tended to circumnutate (though the cause of this and most other
+variations is unknown) in a direction more or less opposed to that
+whence the light proceeded. In like manner a plant might be rendered
+diaheliotropic.
+
+
+
+
+CHAPTER X.
+MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
+
+
+Means of observation—Apogeotropism—Cytisus—Verbena—Beta—Gradual
+conversion of the movement of circumnutation into apogeotropism in
+Rubus, Lilium, Phalaris, Avena, and Brassica—Apogeotropism retarded by
+heliotropism—Effected by the aid of joints or pulvini—Movements of
+flower-peduncles of Oxalis—General remarks on
+apogeotropism—Geotropism—Movements of radicles—Burying of
+seed-capsules—Use of process—Trifolium
+subterraneum—Arachis—Amphicarpæa—Diageotropism—Conclusion
+
+
+Our object in the present chapter is to show that geotropism,
+apogeotropism, and diageotropism are modified forms of circumnutation.
+Extremely fine filaments of glass, bearing two minute triangles of
+paper, were fixed to the summits of young stems, frequently to the
+hypocotyls of seedlings, to flower-peduncles, radicles, etc., and the
+movements of the parts were then traced in the manner already described
+on vertical and horizontal glass-plates. It should be remembered that
+as the stems or other parts become more and more oblique with respect
+to the glasses, the figures traced on them necessarily become more and
+more magnified. The plants were protected from light, excepting whilst
+each observation was being made, and then the light, which was always a
+dim one, was allowed to enter so as to interfere as little as possible
+with the movement in progress; and we did not detect any evidence of
+such interference.
+
+When observing the gradations between
+circumnutation and heliotropism, we had the great advantage of being
+able to lessen the light; but with geotropism analogous experiments
+were of course impossible. We could, however, observe the movements of
+stems placed at first only a little from the perpendicular, in which
+case geotropism did not act with nearly so much power, as when the
+stems were horizontal and at right angles to the force. Plants, also,
+were selected which were but feebly geotropic or apogeotropic, or had
+become so from having grown rather old. Another plan was to place the
+stems at first so that they pointed 30 or 40° beneath the horizon, and
+then apogeotropism had a great amount of work to do before the stem was
+rendered upright; and in this case ordinary circumnutation was often
+not wholly obliterated. Another plan was to observe in the evening
+plants which during the day had become greatly curved heliotropically;
+for their stems under the gradually waning light very slowly became
+upright through the action of apogeotropism; and in this case modified
+circumnutation was sometimes well displayed.
+
+Apogeotropism.—Plants were selected for observation almost by chance,
+excepting that they were taken from widely different families. If the
+stem of a plant which is even moderately sensitive to apogeotropism be
+placed horizontally, the upper growing part bends quickly upwards, so
+as to become perpendicular; and the line traced by joining the dots
+successively made on a glass-plate, is generally almost straight. For
+instance, a young Cytisus fragrans, 12 inches in height, was placed so
+that the stem projected 10° beneath the horizon, and its course was
+traced during 72 h. At first it bent a very little downwards (Fig.
+182), owing no doubt to the weight of the stem, as this occurred with
+most of the other plants observed, though, as they were of course
+circumnutating, the short downward lines were often oblique. After
+three-quarters of an hour the stem began to curve upwards, quickly
+during the first two hours, but much more slowly during the afternoon
+and night,
+and on the following day. During the second night it fell a little, and
+circumnutated during the following day; but it also moved a short
+distance to the right, which was caused by a little light having been
+accidentally admitted on this side. The stem was now inclined 60° above
+the horizon, and had therefore risen 70°. With time allowed it would
+probably have become upright, and no doubt would have continued
+circumnutating. The sole remarkable feature in the figure here given is
+the straightness of the course pursued. The stem, however, did not move
+upwards at an equable rate, and it sometimes stood almost or quite
+still. Such periods probably represent attempts to circumnutate in a
+direction opposite to apogeotropism.
+
+Fig. 182. Cytisus fragrans: apogeotropic movement of stem from 10°
+beneath to 60° above horizon, traced on vertical glass, from 8.30 A.M.
+March 12th to 10.30 P.M. 13th. The subsequent circumnutating movement
+is likewise shown up to 6.45 A.M. on the 15th. Nocturnal course
+represented, as usual, by a broken line. Movement not greatly
+magnified, and tracing reduced to two-thirds of original scale.
+
+The herbaceous stem of a Verbena melindres (?) laid horizontally, rose
+in 7 h. so much that it could no longer be observed on the vertical
+glass which stood in front of the plant. The long line which was traced
+was almost absolutely straight. After the 7 h. it still continued to
+rise, but now circumnutated slightly. On the following day it stood
+upright, and circumnutated regularly, as shown in Fig. 82, given in the
+fourth chapter. The stems of several other plants which were highly
+sensitive to apogeotropism rose up in almost straight lines, and
+then suddenly began to circumnutate. A partially etiolated and somewhat
+old hypocotyl of a seedling cabbage (2 3/4 inches in height) was so
+sensitive that when placed at an angle of only 23° from the
+perpendicular, it became vertical in 33 minutes. As it could not have
+been strongly acted upon by apogeotropism in the above slightly
+inclined position, we expected that it would have circumnutated, or at
+least have moved in a zigzag course. Accordingly, dots were made every
+3 minutes; but, when these were joined, the line was nearly straight.
+After this hypocotyl had become upright it still moved onwards for half
+an hour in the same general direction, but in a zigzag manner. During
+the succeeding 9 h. it circumnutated regularly, and described 3 large
+ellipses. In this case apogeotropism, although acting at a very
+unfavourable angle, quite overcame the ordinary circumnutating
+movement.
+
+Fig. 183. Beta vulgaris: apogeotropic movement of hypocotyl from 19°
+beneath horizon to a vertical position, with subsequent circumnutation,
+traced on a vertical and on a horizontal glass-plate, from 8.28 A.M.
+Sept. 28th to 8.40 A.M. 29th. Figure reduced to one-third of original
+scale.
+
+The hypocotyls of Beta vulgaris are highly sensitive to apogeotropism.
+One was placed so as to project 19° beneath the horizon; it fell at
+first a very little (see Fig. 183), no doubt owing to its weight; but
+as it was circumnutating the line was
+oblique. During the next 3 h. 8 m. it rose in a nearly straight line,
+passing through an angle of 109°, and then (at 12.3 P.M.) stood
+upright. It continued for 55 m. to move in the same general direction
+beyond the perpendicular, but in a zigzag course. It returned also in a
+zigzag line, and then circumnutated regularly, describing three large
+ellipses during the remainder of the day. It should be observed that
+the ellipses in this figure are exaggerated in size, relatively to the
+length of the upward straight line, owing to the position of the
+vertical and horizontal glass-plates. Another and somewhat old
+hypocotyl was placed so as to stand at only 31° from the perpendicular,
+in which position apogeotropism acted on it with little force, and its
+course accordingly was slightly zigzag.
+
+The sheath-like cotyledons of Phalaris Canariensis are extremely
+sensitive to apogeotropism. One was placed so as to project 40° beneath
+the horizon. Although it was rather old and 1.3 inch in height, it
+became vertical in 4 h. 30 m., having passed through an angle of 130°
+in a nearly straight line. It then suddenly began to circumnutate in
+the ordinary manner. The cotyledons of this plant, after the first leaf
+has begun to protrude, are but slightly apogeotropic, though they still
+continue to circumnutate. One at this stage of development was placed
+horizontally, and did not become upright even after 13 h., and its
+course was slightly zigzag. So, again, a rather old hypocotyl of Cassia
+tora (1 1/4 inch in height) required 28 h. to become upright, and its
+course was distinctly zigzag; whilst younger hypocotyls moved much more
+quickly and in a nearly straight line.
+
+When a horizontally placed stem or other organ rises in a zigzag line,
+we may infer from the many cases given in our previous chapters, that
+we have a modified form of circumnutation; but when the course is
+straight, there is no evidence of circumnutation, and any one might
+maintain that this latter movement had been replaced by one of a wholly
+distinct kind. This view seems the more probable when (as sometimes
+occurred with the hypocotyls of Brassica and Beta, the stems of
+Cucurbita, and the cotyledons of Phalaris) the part in question, after
+bending up in a straight course, suddenly begins to circumnutate to the
+full extent and in the usual manner. A fairly good instance of a sudden
+change of this kind—that is, from a nearly straight upward movement to
+one of circumnutation—is shown in Fig. 183; but more striking instances
+were occasionally observed with Beta, Brassica, and Phalaris.
+
+We will now describe a few cases in which it may be
+seen how gradually circumnutation becomes changed into apogeotropism,
+under circumstances to be specified in each instance.
+
+Rubus idæus (hybrid).—A young plant, 11 inches in height, growing in a
+pot, was placed horizontally; and the upward movement was traced during
+nearly 70 h.; but the plant, though growing vigorously, was not highly
+sensitive to apogeotropism, or it was not capable of quick movement,
+for during the above time it rose only 67°. We may see in the diagram
+(Fig. 184) that during the first day of 12 h. it rose in a nearly
+straight line. When placed horizontally, it was evidently
+circumnutating, for it rose at first a little, notwithstanding the
+weight of the stem, and then sank down; so that it did not start on its
+permanently upward course until 1 h. 25 m. had elapsed. On the second
+day, by which time it had risen considerably, and when apogeotropism
+acted on it with somewhat less power, its course during 15½ h. was
+clearly zigzag, and the rate of the upward movement was not equable.
+During the third day, also of 15½ h., when apogeotropism acted on it
+with still less power, the stem plainly circumnutated, for it moved
+during this day 3 times up and 3 times down, 4 times to the left and 4
+to the right. But the course was so complex that it could hardly be
+traced on the glass. We can, however, see that the successively formed
+irregular ellipses rose higher and higher. Apogeotropism continued to
+act on the fourth morning, as the stem was still rising, though it now
+stood only 23° from the perpendicular. In this diagram the several
+stages may be followed by which an almost rectilinear, upward,
+apogeotropic course first becomes zigzag, and then changes into a
+circumnutating movement, with most of the successively formed,
+irregular ellipses directed upwards.
+
+Fig 184: Rubus idæus (hybrid): apogeotropic movement of stem, traced on
+a vertical glass during 3 days and 3 nights, from 10.40 A.M. March 18th
+to 8 A.M. 21st. Figure reduced to one-half of the original scale.
+
+Lilium auratum.—A plant 23 inches in height was placed
+horizontally, and the upper part of the stem rose 58° in 46 h., in the
+manner shown in the accompanying diagram (Fig. 185). We here see that
+during the whole of the second day of 15½ h., the stem plainly
+circumnutated whilst bending upwards through apogeotropism. It had
+still to rise considerably, for when the last dot in the figure was
+made, it stood 32° from an upright position.
+
+Fig. 185. Lilium auratum: apogeotropic movement of stem, traced on a
+vertical glass during 2 days and 2 nights, from 10.40 A.M. March 18th
+to 8 A.M. 20th. Figure reduced to one-half of the original scale.
+
+Phalaris Canariensis.—A cotyledon of this plant (1.3 inch in height)
+has already been described as rising in 4 h. 30 m. from 40° beneath the
+horizon into a vertical position, passing through an angle of 130° in a
+nearly straight line, and then abruptly beginning to circumnutate.
+Another somewhat old cotyledon of the same height (but from which a
+true leaf had not yet protruded), was similarly placed at 40° beneath
+the horizon. For the first 4 h. it rose in a nearly straight course
+(Fig. 186), so that by 1.10 P.M. it was highly inclined, and now
+apogeotropism acted on it with much less power than before, and it
+began to zigzag. At 4.15 P.M. (i.e. in 7 h. from the commencement) it
+stood vertically, and afterwards continued to circumnutate in the usual
+manner about the same spot. Here then we have a graduated change from a
+straight upward apogeotropic course into circumnutation, instead of an
+abrupt change, as in the former case.
+
+Avena sativa.—The sheath-like cotyledons, whilst young, are strongly
+apogeotropic; and some which were placed at 45° beneath the horizon
+rose 90° in 7 or 8 h. in lines almost absolutely straight. An oldish
+cotyledon, from which the first leaf began to
+protrude whilst the following observations were being made, was placed
+at 10° beneath the horizon, and it rose only 59° in 24h. It behaved
+rather differently from any other plant, observed by us, for during the
+first 4½ h. it rose in a line not far from straight; during the next 6½
+h. it circumnutated, that is, it descended and again ascended in a
+strongly marked zigzag course; it then resumed its upward movement in a
+moderately straight line, and, with time allowed, no doubt would have
+become upright. In this case, after the first 4½ h., ordinary
+circumnutation almost completely conquered for a time apogeotropism.
+
+Fig 186. Phalaris Canariensis: apogeotropic movement of cotyledon,
+traced on a vertical and horizontal glass, from 9.10 A.M. Sept. 19th to
+9 A.M. 20th. Figure here reduced to one-fifth of original scale.
+
+Brassica oleracea.—The hypocotyls of several young seedlings placed
+horizontally, rose up vertically in the course of 6 or 7 h. in nearly
+straight lines. A seedling which had grown in darkness to a height of 2
+1/4 inches, and was therefore rather old and not highly sensitive, was
+placed so that the hypocotyl projected at between 30° and 40° beneath
+the horizon. The upper part alone became curved
+upwards, and rose during the first 3 h. 10 m. in a nearly straight line
+(Fig. 187); but it was not possible to trace the upward movement on the
+vertical glass for the first 1 h. 10 m., so that the nearly straight
+line in the diagram ought to have been much longer. During the next 11
+h. the hypocotyl circumnutated, describing irregular figures, each of
+which rose a little above the one previously formed. During the night
+and following early morning it continued to rise in a zigzag course, so
+that apogeotropism was still acting. At the close of our observations,
+after 23 h. (represented by the highest dot in the diagram) the
+hypocotyl was still 32° from the perpendicular. There can be little
+doubt that it would ultimately have become upright by describing an
+additional number of irregular ellipses, one above the other.
+
+Fig 187. Brassica oleracea: apogeotropic movement of hypocotyl, traced
+on vertical glass, from 9.20 A.M., Sept. 12th to 8.30 A.M. 13th. The
+upper part of the figure is more magnified than the lower part. If the
+whole course had been traced, the straight upright line would have been
+much longer. Figure here reduced to one-third of the original scale.
+
+Apogeotropism retarded by Heliotropism.—When the stem of any plant
+bends during the day towards a lateral light, the movement is opposed
+by apogeotropism; but as the light gradually wanes in the evening the
+latter power slowly gains the upper hand, and draws the stem back into
+a vertical position. Here then we have a good opportunity for observing
+how apogeotropism acts when very nearly balanced by an opposing force.
+For instance, the plumule of Tropaeolum majus (see former Fig. 175)
+moved towards the dim evening light in a slightly zigzag line until
+6.45 P.M., it then returned on its course until
+10.40 P.M., during which time it zigzagged and described an ellipse of
+considerable size. The hypocotyl of Brassica oleracea (see former Fig.
+173) moved in a straight line to the light until 5.15 P.M., and then
+from the light, making in its backward course a great rectangular bend,
+and then returned for a short distance towards the former source of the
+light; no observations were made after 7.10 P.M., but during the night
+it recovered its vertical position. A hypocotyl of Cassia tora moved in
+the evening in a somewhat zigzag line towards the failing light until 6
+P.M., and was now bowed 20° from the perpendicular; it then returned on
+its course, making before 10.30 P.M. four great, nearly rectangular
+bends and almost completing an ellipse. Several other analogous cases
+were casually observed, and in all of them the apogeotropic movement
+could be seen to consist of modified circumnutation.
+
+Apogeotropic Movements effected by the aid of joints or
+pulvini.—Movements of this kind are well known to occur in the
+Gramineæ, and are effected by means of the thickened bases of their
+sheathing leaves; the stem within being in this part thinner than
+elsewhere.[1] According to the analogy of all other pulvini, such
+joints ought to continue circumnutating for a long period, after the
+adjoining parts have ceased to grow. We therefore wished to ascertain
+whether this was the case with the Gramineæ; for if so, the upward
+curvature of their stems, when extended horizontally or laid prostrate,
+would be explained in accordance with our view—namely, that
+apogeotropism results from modified circumnutation. After these joints
+have curved upwards, they are fixed in their new position by increased
+growth along their lower sides.
+
+ [1] This structure has been recently described by De Vries in an
+ interesting article, ‘Ueber die Aufrichtung des gelagerten Getreides,’
+ in ‘Landwirthschaftliche Jahrbücher,’ 1880, p. 473.
+
+
+Lolium perenne.—A young stem, 7 inches in height, consisting of 3
+internodes, with the flower-head not yet protruded, was selected for
+observation. A long and very thin glass filament was cemented
+horizontally to the stem close above the second joint, 3 inches above
+the ground. This joint was subsequently proved to be in an active
+condition, as its lower side swelled much through the action of
+apogeotropism (in the manner described by De Vries) after the haulm had
+been fastened down for 24 h. in a horizontal position. The pot was
+so placed that the end of the filament stood beneath the 2-inch object
+glass of a microscope with an eye-piece micrometer, each division of
+which equalled 1/500 of an inch. The end of the filament was repeatedly
+observed during 6 h., and was seen to be in constant movement; and it
+crossed 5 divisions of the micrometer (1/100 inch) in 2 h. Occasionally
+it moved forwards by jerks, some of which were 1/1000 inch in length,
+and then slowly retreated a little, afterwards again jerking forwards.
+These oscillations were exactly like those described under Brassica and
+Dionaea, but they occurred only occasionally. We may therefore conclude
+that this moderately old joint was continually circumnutating on a
+small scale.
+
+Alopecurus pratensis.—A young plant, 11 inches in height, with the
+flower-head protruded, but with the florets not yet expanded, had a
+glass filament fixed close above the second joint, at a height of only
+2 inches above the ground. The basal internode, 2 inches in length, was
+cemented to a stick to prevent any possibility of its circumnutating.
+The extremity of the filament, which projected about 50° above the
+horizon, was often observed during 24 h. in the same manner as in the
+last case. Whenever looked at, it was always in movement, and it
+crossed 30 divisions of the micrometer (3/50 inch) in 3½ h.; but it
+sometimes moved at a quicker rate, for at one time it crossed 5
+divisions in 1½ h. The pot had to be moved occasionally, as the end of
+the filament travelled beyond the field of vision; but as far as we
+could judge it followed during the daytime a semicircular course; and
+it certainly travelled in two different directions at right angles to
+one another. It sometimes oscillated in the same manner as in the last
+species, some of the jerks forwards being as much as 1/1000 of an inch.
+We may therefore conclude that the joints in this and the last species
+of grass long continue to circumnutate; so that this movement would be
+ready to be converted into an apogeotropic movement, whenever the stem
+was placed in an inclined or horizontal position.
+
+Movements of the Flower-peduncles of Oxalis carnosa, due to
+apogeotropism and other forces.—The movements of the main peduncle, and
+of the three or four sub-peduncles which each main peduncle of this
+plant bears, are extremely complex, and are determined by several
+distinct causes. Whilst the flowers are expanded, both kinds of
+peduncles circumnutate about the same spot, as we have seen (Fig. 91)
+in the fourth chapter. But soon after the flowers have begun to wither
+the
+sub-peduncles bend downwards, and this is due to epinasty; for on two
+occasions when pots were laid horizontally, the sub-peduncles assumed
+the same position relatively to the main peduncle, as would have been
+the case if they had remained upright; that is, each of them formed
+with it an angle of about 40°. If they had been acted on by geotropism
+or apheliotropism (for the plant was illuminated from above), they
+would have directed themselves to the centre of the earth. A main
+peduncle was secured to a stick in an upright position, and one of the
+upright sub-peduncles which had been observed circumnutating whilst the
+flower was expanded, continued to do so for at least 24 h. after it had
+withered. It then began to bend downwards, and after 36 h. pointed a
+little beneath the horizon. A new figure was now begun (A, Fig. 188),
+and the sub-peduncle was traced descending in a zigzag line from 7.20
+P.M. on the 19th to 9 A.M. on the 22nd. It now pointed almost
+perpendicularly downwards, and the glass filament had to be removed and
+fastened transversely across the base of the young capsule. We expected
+that the sub-peduncle would have been motionless in its new position;
+but it continued slowly to swing, like a pendulum, from side to side,
+that is, in a plane at right angles to that in which it had descended.
+This circumnutating movement was observed from 9 A.M. on 22nd to 9 A.M.
+24th, as shown at B in the diagram. We were not able to observe this
+particular sub-peduncle any longer; but it would certainly have gone on
+circumnutating until the capsule was nearly ripe (which requires only a
+short time), and it would then have moved upwards.
+
+The upward movement (C, Fig. 188) is effected in part by the whole
+sub-peduncle rising in the same manner as it had previously descended
+through epinasty—namely, at the joint where united to the main
+peduncle. As this upward movement occurred with plants kept in the dark
+and in whatever position the main peduncle was fastened, it could not
+have been caused by heliotropism or apogeotropism, but by hyponasty.
+Besides this movement at the joint, there is another of a very
+different kind, for the sub-peduncle becomes upwardly bent in the
+middle part. If the sub-peduncle happens at the time to be inclined
+much downwards, the upward curvature is so great that the whole forms a
+hook. The upper end bearing the capsule, thus always places itself
+upright, and as this occurs in darkness, and in whatever position the
+main peduncle may have been secured,
+the upward curvature cannot be due to heliotropism or hyponasty, but to
+apogeotropism.
+
+Fig. 188. Oxalis carnosa: movements of flower-peduncle, traced on a
+vertical glass: A, epinastic downward movement; B, circumnutation
+whilst depending vertically; C, subsequent upward movement, due to
+apogeotropism and hyponasty combined.
+
+
+In order to trace this upward movement, a filament was fixed to a
+sub-peduncle bearing a capsule nearly ripe, which was beginning to bend
+upwards by the two means just described. Its course was traced (see C,
+Fig 188) during 53 h., by which time it had become nearly upright. The
+course is seen to be strongly zigzag, together with some little loops.
+We may therefore conclude that the movement consists of modified
+circumnutation.
+
+The several species of Oxalis probably profit in the following manner
+by their sub-peduncles first bending downwards and then upwards. They
+are known to scatter their seeds by the bursting of the capsule; the
+walls of which are so extremely thin, like silver paper, that they
+would easily be permeated by rain. But as soon as the petals wither,
+the sepals rise up and enclose the young capsule, forming a perfect
+roof over it as soon as the sub-peduncle has bent itself downwards. By
+its subsequent upward movement, the capsule stands when ripe at a
+greater height above the ground by twice the length of the
+sub-peduncle, than it did when dependent, and is thus able to scatter
+its seeds to a greater distance. The sepals, which enclose the ovarium
+whilst it is young, present an additional adaptation by expanding
+widely when the seeds are ripe, so as not to interfere with their
+dispersal. In the case of Oxalis acetosella, the capsules are said
+sometimes to bury themselves under loose leaves or moss on the ground,
+but this cannot occur with those of O. carnosa, as the woody stem is
+too high.
+
+Oxalis acetosella.—The peduncles are furnished with a joint in the
+middle, so that the lower part answers to the main peduncle,
+and the upper part to one of the sub-peduncles of O. carnosa. The upper
+part bends downwards, after the flower has begun to wither, and the
+whole peduncle then forms a hook; that this bending is due to epinasty
+we may infer from the case of O. carnosa. When the pod is nearly ripe,
+the upper part straightens itself and becomes erect; and this is due to
+hyponasty or apogeotropism, or both combined, and not to heliotropism,
+for it occurred in darkness. The short, hooked part of the peduncle of
+a cleistogamic flower, bearing a pod nearly ripe, was observed in the
+dark during three days. The apex of the pod at first pointed
+perpendicularly down, but in the course of three days rose 90°, so that
+it now projected horizontally. The course during the two latter days is
+shown in Fig. 189; and it may be seen how greatly the peduncle, whilst
+rising, circumnutated. The lines of chief movement were at right angles
+to the plane of the originally hooked part. The tracing was not
+continued any longer; but after two additional days, the peduncle with
+its capsule had become straight and stood upright.
+
+Fig. 189. Oxalis acetosella: course pursued by the upper part of a
+peduncle, whilst rising, traced from 11 A.M. June 1st to 9 A.M. 3rd.
+Figure here reduced to one-half of the original scale.
+
+Concluding Remarks on Apogeotropism.—When apogeotropism is rendered by
+any means feeble, it acts, as shown in the several foregoing cases, by
+increasing the always present circumnutating movement in a direction
+opposed to gravity, and by diminishing that in the direction of
+gravity, as well as that to either side. The upward movement thus
+becomes unequal in rate, and is sometimes interrupted by stationary
+periods. Whenever irregular ellipses or loops are still formed, their
+longer axes are almost always directed in the line of gravity, in an
+analogous manner as occurred with heliotropic movements in reference to
+the light. As apogeotropism acts more and more energetically, ellipses
+or loops cease to be formed, and the course becomes at first strongly,
+and then less and less zigzag, and finally rectilinear. From this
+gradation in the nature of the movement, and more especially from all
+growing parts, which alone (except when pulvini are present) are acted
+on by apogeotropism,
+continually circumnutating, we may conclude that even a rectilinear
+course is merely an extremely modified form of circumnutation. It is
+remarkable that a stem or other organ which is highly sensitive to
+apogeotropism, and which has bowed itself rapidly upwards in a straight
+line, is often carried beyond the vertical, as if by momentum. It then
+bends a little backwards to a point round which it finally
+circumnutates. Two instances of this were observed with the hypocotyls
+of Beta vulgaris, one of which is shown in Fig. 183, and two other
+instances with the hypocotyls of Brassica. This momentum-like movement
+probably results from the accumulated effects of apogeotropism. For the
+sake of observing how long such after-effects lasted, a pot with
+seedlings of Beta was laid on its side in the dark, and the hypocotyls
+in 3 h. 15 m. became highly inclined. The pot, still in the dark, was
+then placed upright, and the movements of the two hypocotyls were
+traced; one continued to bend in its former direction, now in
+opposition to apogeotropism, for about 37 m., perhaps for 48 m.; but
+after 61 m. it moved in an opposite direction. The other hypocotyl
+continued to move in its former course, after being placed upright, for
+at least 37 m.
+
+Different species and different parts of the same species are acted on
+by apogeotropism in very different degrees. Young seedlings, most of
+which circumnutate quickly and largely, bend upwards and become
+vertical in much less time than do any older plants observed by us; but
+whether this is due to their greater sensitiveness to apogeotropism, or
+merely to their greater flexibility we do not know. A hypocotyl of Beta
+traversed an angle of 109° in 3 h. 8 m., and a cotyledon of Phalaris an
+angle of 130° in 4 h. 30 m. On the other hand, the stem of a herbaceous
+Verbena rose 90° in about 24 h.; that of Rubus 67°, in 70 h.; that of
+Cytisus 70°, in 72 h.; that of a young American Oak only 37°, in 72 h.
+The stem of a young Cyperus alternifolius rose only 11° in 96 h.; the
+bending being confined to near its base. Though the sheath-like
+cotyledons of Phalaris are so extremely sensitive to apogeotropism, the
+first true leaves which protrude from them exhibited only a trace of
+this action. Two fronds of a fern, Nephrodium molle, both of them young
+and one with the tip still inwardly curled, were kept in a horizontal
+position for 46 h., and during this time they rose so little that it
+was doubtful whether there was any true apogeotropic movement.
+
+The most curious case known to us of a difference in sensitiveness to
+gravitation, and consequently of movement, in different parts of the
+same organ, is that offered by the petioles of the cotyledons of Ipomœa
+leptophylla. The basal part for a short length where united to the
+undeveloped hypocotyl and radicle is strongly geotropic, whilst the
+whole upper part is strongly apogeotropic. But a portion near the
+blades of the cotyledons is after a time acted on by epinasty and
+curves downwards, for the sake of emerging in the form of an arch from
+the ground; it subsequently straightens itself, and is then again acted
+on by apogeotropism.
+
+A branch of Cucurbita ovifera, placed horizontally, moved upwards
+during 7 h. in a straight line, until it stood at 40° above the
+horizon; it then began to circumnutate, as if owing to its trailing
+nature it had no tendency to rise any higher. Another upright branch
+was secured to a stick, close to the base of a tendril, and the pot was
+then laid horizontally in the dark. In this position the tendril
+circumnutated and made
+several large ellipses during 14 h., as it likewise did on the
+following day; but during this whole time it was not in the least
+affected by apogeotropism. On the other hand, when branches of another
+Cucurbitaceous plant, Echinocytis lobata, were fixed in the dark so
+that the tendrils depended beneath the horizon, these began immediately
+to bend upwards, and whilst thus moving they ceased to circumnutate in
+any plain manner; but as soon as they had become horizontal they
+recommenced to revolve conspicuously.[2] The tendrils of Passiflora
+gracilis are likewise apogeotropic. Two branches were tied down so that
+their tendrils pointed many degrees beneath the horizon. One was
+observed for 8 h., during which time it rose, describing two circles,
+one above the other. The other tendril rose in a moderately straight
+line during the first 4 h., making however one small loop in its
+course; it then stood at about 45° above the horizon, where it
+circumnutated during the remaining 8 h. of observation.
+
+ [2] For details see ‘The Movements and Habits of Climbing Plants,’
+ 1875, p. 131.
+
+
+A part or organ which whilst young is extremely sensitive to
+apogeotropism ceases to be so as it grows old; and it is remarkable, as
+showing the independence of this sensitiveness and of the
+circumnutating movement, that the latter sometimes continues for a time
+after all power of bending from the centre of the earth has been lost.
+Thus a seedling Orange bearing only 3 young leaves, with a rather stiff
+stem, did not curve in the least upwards during 24 h. whilst extended
+horizontally; yet it circumnutated all the time over a small space. The
+hypocotyl of a young seedling of Cassia tora, similarly placed, became
+vertical in 12 h.; that of an older seedling, 1 1/4 inch in height,
+became so in 28 h.; and that of another still older one, 1½ inch in
+height, remained horizontal during two days, but distinctly
+circumnutated during this whole time.
+
+When the cotyledons of Phalaris or Avena are laid horizontally, the
+uppermost part first bends upwards, and then the lower part;
+consequently, after the lower part has become much curved upwards, the
+upper part is compelled to curve backwards in an opposite direction, in
+order to straighten itself and to stand vertically; and this subsequent
+straightening process is likewise due to apogeotropism. The upper part
+of 8 young cotyledons of Phalaris were made rigid by being cemented to
+thin glass rods, so that this part could not bend in the least;
+nevertheless, the basal part was not prevented from curving upward. A
+stem or other organ which bends upwards through apogeotropism exerts
+considerable force; its own weight, which has of course to be lifted,
+was sufficient in almost every instance to cause the part at first to
+bend a little downwards; but the downward course was often rendered
+oblique by the simultaneous circumnutating movement. The cotyledons of
+Avena placed horizontally, besides lifting their own weight, were able
+to furrow the soft sand above them, so as to leave little crescentic
+open spaces on the lower sides of their bases; and this is a remarkable
+proof of the force exerted.
+
+As the tips of the cotyledons of Phalaris and Avena bend upwards
+through the action of apogeotropism before the basal part, and as these
+same tips when excited by a lateral light transmit some influence to
+the lower part, causing it to bend, we thought that the same rule might
+hold good with apogeotropism. Consequently, the tips of 7 cotyledons of
+Phalaris were
+cut off for a length in three cases of .2 inch and in the four other
+cases of .14, .12, .1, and .07 inch. But these cotyledons, after being
+extended horizontally, bowed themselves upwards as effectually as the
+unmutilated specimens in the same pots, showing that sensitiveness to
+gravitation is not confined to their tips.
+
+GEOTROPISM.
+
+This movement is directly the reverse of apogeotropism. Many organs
+bend downwards through epinasty or apheliotropism or from their own
+weight; but we have met with very few cases of a downward movement in
+sub-aërial organs due to geotropism. We shall however, give one good
+instance in the following section, in the case of Trifolium
+subterraneum, and probably in that of Arachis hypogaea.
+
+On the other hand, all roots which penetrate the ground (including the
+modified root-like petioles of Megarrhiza and Ipomœa leptophylla) are
+guided in their downward course by geotropism; and so are many aërial
+roots, whilst others, as those of the Ivy, appear to be indifferent to
+its action. In our first chapter the movements of the radicles of
+several seedlings were described. We may there see (Fig. 1) how a
+radicle of the cabbage, when pointing vertically upwards so as to be
+very little acted on by geotropism, circumnutated; and how another
+(Fig. 2) which was at first placed in an inclined position bowed itself
+downwards in a zigzag line, sometimes remaining stationary for a time.
+Two other radicles of the cabbage travelled downwards in almost
+rectilinear courses. A radicle of the bean placed upright (Fig. 20)
+made a great sweep and zigzagged; but as it sank downwards and was more
+strongly acted on by geotropism, it moved in an
+almost straight course. A radicle of Cucurbita, directed upwards (Fig.
+26), also zigzagged at first, and described small loops; it then moved
+in a straight line. Nearly the same result was observed with the
+radicles of Zea mays. But the best evidence of the intimate connection
+between circumnutation and geotropism was afforded by the radicles of
+Phaseolus, Vicia, and Quercus, and in a less degree by those of Zea and
+Æsculus (see Figs. 18, 19, 21, 41, and 52); for when these were
+compelled to grow and slide down highly inclined surfaces of smoked
+glass, they left distinctly serpentine tracks.
+
+The Burying of Seed-capsules: Trifolium subterraneum.—The flower-heads
+of this plant are remarkable from producing only 3 or 4 perfect
+flowers, which are situated exteriorly. All the other many flowers
+abort, and are modified into rigid points, with a bundle of vessels
+running up their centres. After a time 5 long, elastic, claw-like
+projections, which represent the divisions of the calyx, are developed
+on their summits. As soon as the perfect flowers wither they bend
+downwards, supposing the peduncle to stand upright, and they then
+closely surround its upper part. This movement is due to epinasty, as
+is likewise the case with the flowers of T. repens. The imperfect
+central flowers ultimately follow, one after the other, the same
+course. Whilst the perfect flowers are thus bending down, the whole
+peduncle curves downwards and increases much in length, until the
+flower-head reaches the ground. Vaucher[3] says that when the plant is
+so placed that the heads cannot soon reach the ground, the peduncles
+grow to the extraordinary length of from 6 to 9 inches. In whatever
+position the branches may be placed, the upper part of the peduncle at
+first bends vertically upwards through heliotropism; but as soon as the
+flowers begin to wither the downward curvature of the whole peduncle
+commences. As this latter movement occurred in complete darkness, and
+with peduncles arising from upright and from dependent branches, it
+cannot be due to apheliotropism or to epinasty, but must be attributed
+to geotropism. Nineteen
+upright flower-heads, arising from branches in all sorts of positions,
+on plants growing in a warm greenhouse, were marked with thread, and
+after 24 h. six of them were vertically dependent; these therefore had
+travelled through 180° in this time. Ten were extended
+sub-horizontally, and these had moved through about 90°. Three very
+young peduncles had as yet moved only a little downwards, but after an
+additional 24 h. were greatly inclined.
+
+ [3] ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. 1841, p. 106.
+
+
+At the time when the flower-heads reach the ground, the younger
+imperfect flowers in the centre are still pressed closely together, and
+form a conical projection; whereas the perfect and imperfect flowers on
+the outside are upturned and closely surround the peduncle. They are
+thus adapted to offer as little resistance, as the case admits of, in
+penetrating the ground, though the diameter of the flower-head is still
+considerable. The means by which this penetration is effected will
+presently be described. The flower-heads are able to bury themselves in
+common garden mould, and easily in sand or in fine sifted cinders
+packed rather closely. The depth to which they penetrated, measured
+from the surface to the base of the head, was between 1/4 and ½ inch,
+but in one case rather above 0.6 inch. With a plant kept in the house,
+a head partly buried itself in sand in 6 h.: after 3 days only the tips
+of the reflexed calyces were visible, and after 6 days the whole had
+disappeared. But with plants growing out of doors we believe, from
+casual observations, that they bury themselves in a much shorter time.
+
+After the heads have buried themselves, the central aborted flowers
+increase considerably in length and rigidity, and become bleached. They
+gradually curve, one after the other, upwards or towards the peduncle,
+in the same manner as did the perfect flowers at first. In thus moving,
+the long claws on their summits carry with them some earth. Hence a
+flower-head which has been buried for a sufficient time, forms a rather
+large ball, consisting of the aborted flowers, separated from one
+another by earth, and surrounding the little pods (the product of the
+perfect flowers) which lie close round the upper part of the peduncle.
+The calyces of the perfect and imperfect flowers are clothed with
+simple and multicellular hairs, which have the power of absorption; for
+when placed in a weak solution of carbonate of ammonia (2 gr. to 1 oz.
+of water) their protoplasmic contents immediately became aggregated and
+afterwards displayed the usual slow movements. This clover generally
+grows in dry soil, but whether the power of absorption by the hairs on
+the buried flower-heads is of any importance to them we do not know.
+Only a few of the flower-heads, which from their position are not able
+to reach the ground and bury themselves, yield seeds; whereas the
+buried ones never failed, as far as we observed, to produce as many
+seeds as there had been perfect flowers.
+
+We will now consider the movements of the peduncle whilst curving down
+to the ground. We have seen in Chap. IV., Fig. 92, p. 225, that an
+upright young flower-head circumnutated conspicuously; and that this
+movement continued after the peduncle had begun to bend downwards. The
+same peduncle was observed when inclined at an angle of 19° above the
+horizon, and it circumnutated during two days. Another
+which was already curved 36° beneath the horizon, was observed from 11
+A.M. July 22nd to the 27th, by which latter date it had become
+vertically dependent. Its course during the first 12 h. is shown in
+Fig. 190, and its position on the three succeeding mornings until the
+25th, when it was nearly vertical. During the first day the peduncle
+clearly circumnutated, for it moved 4 times down and 3 times up; and on
+each succeeding day, as it sank downwards, the same movement continued,
+but was only occasionally observed and was less strongly marked. It
+should be stated that these peduncles were observed under a double
+skylight in the house, and that they generally moved downwards very
+much more slowly than those on plants growing out of doors or in the
+greenhouse.
+
+Fig. 190. Trifolium subterraneum: downward movement of peduncle from
+19° beneath the horizon to a nearly vertically dependent position,
+traced from 11 A.M. July 22nd to the morning of 25th. Glass filament
+fixed transversely across peduncle, at base of flower-head.
+
+Fig. 191. Trifolium subterraneum: circumnutating movement of peduncle,
+whilst the flower-head was burying itself in sand, with the reflexed
+tips of the calyx still visible; traced from 8 A.M. July 26th to 9 A.M.
+on 27th. Glass filament fixed transversely across peduncle, near
+flower-head.
+
+Fig. 192. Trifolium subterraneum: movement of same peduncle, with
+flower-head completely buried beneath the sand; traced from 8 A.M. to
+7.15 P.M. on July 29th.
+
+The movement of another vertically dependent peduncle with the
+flower-head standing half an inch above the ground, was traced, and
+again when it first touched the ground; in both cases irregular
+ellipses were described every 4 or 5 h. A peduncle on a plant which had
+been brought into the house, moved from an upright into a vertically
+dependent position in a single day; and here the course during the
+first 12 h. was nearly straight, but with a few well-marked zigzags
+which betrayed the essential nature of the movement. Lastly the
+circumnutation of a peduncle was traced during 51 h. whilst in the act
+of burying itself obliquely in a little heap of sand. After it had
+buried itself to such a depth that the tips of the sepals were alone
+visible, the above figure (Fig 191) was traced during 25 h. When the
+flower-head had completely disappeared beneath the sand, another
+tracing was made during 11 h. 45 m. (Fig. 192); and here again we see
+that the peduncle was circumnutating.
+
+
+Any one who will observe a flower-head burying itself, will be
+convinced that the rocking movement, due to the continued
+circumnutation of the peduncle, plays an important part in the act.
+Considering that the flower-heads are very light, that the peduncles
+are long, thin, and flexible, and that they arise from flexible
+branches, it is incredible that an object as blunt as one of these
+flower-heads could penetrate the ground by means of the growing force
+of the peduncle, unless it were aided by the rocking movement. After a
+flower-head has penetrated the ground to a small depth, another and
+efficient agency comes into play; the central rigid aborted flowers,
+each terminating in five long claws, curve up towards the peduncle; and
+in doing so can hardly fail to drag the head down to a greater depth,
+aided as this action is by the circumnutating movement, which continues
+after the flower-head has completely buried itself. The aborted flowers
+thus act something like the hands of the mole, which force the earth
+backwards and the body forwards.
+
+It is well known that the seed-capsules of various widely distinct
+plants either bury themselves in the ground, or are produced from
+imperfect flowers developed beneath the surface. Besides the present
+case, two other well-marked instances will be immediately given. It is
+probable that one chief good thus gained is the protection of the seeds
+from animals which prey on them. In the case of T. subterraneum, the
+seeds are not only concealed by being buried, but are likewise
+protected by being closely surrounded by the rigid, aborted flowers. We
+may the more confidently infer that protection is here aimed at,
+because the seeds of several species in this same genus are protected
+in other ways;[4] namely, by the swelling and closure of the calyx, or
+by the persistence and bending down of the standard-petal, etc. But the
+most curious instance is that of T. globosum, in which the upper
+flowers are sterile, as in T. subterraneum, but are here developed into
+large brushes of hairs which envelop and protect the seed-bearing
+flowers. Nevertheless, in all these cases the capsules, with their
+seeds, may profit, as Mr. T. Thiselton Dyer has remarked,[5] by their
+being kept somewhat damp; and the advantage of such dampness perhaps
+throws light on the presence of the absorbent hairs on the buried
+flower-heads of T. subterraneum. According to Mr. Bentham, as quoted by
+Mr. Dyer,
+the prostrate habit of Helianthemum prostratum “brings the capsules in
+contact with the surface of the ground, postpones their maturity, and
+so favours the seeds attaining a larger size.” The capsules of Cyclamen
+and of Oxalis acetosella are only occasionally buried, and this only
+beneath dead leaves or moss. If it be an advantage to a plant that its
+capsules should be kept damp and cool by being laid on the ground, we
+have in these latter cases the first step, from which the power of
+penetrating the ground, with the aid of the always present movement of
+circumnutation, might afterwards have been gained.
+
+ [4] Vaucher, ‘Hist. Phys. des Plantes d’Europe,’ tom. ii. p. 110.
+
+
+ [5] See his interesting article in ‘Nature,’ April 4th, 1878, p. 446.
+
+
+Arachis hypogoea.—The flowers which bury themselves, rise from stiff
+branches a few inches above the ground, and stand upright. After they
+have fallen off, the gynophore, that is the part which supports the
+ovarium, grows to a great length, even to 3 or 4 inches, and bends
+perpendicularly downwards. It resembles closely a peduncle, but has a
+smooth and pointed apex, which contains the ovules, and is at first not
+in the least enlarged. The apex after reaching the ground penetrates
+it, in one case observed by us to a depth of 1 inch, and in another to
+0.7 inch. It there becomes developed into a large pod. Flowers which
+are seated too high on the plant for the gynophore to reach the ground
+are said[6] never to produce pods.
+
+ [6] ‘Gard. Chronicle,’ 1857, p. 566.
+
+
+The movement of a young gynophore, rather under an inch in length and
+vertically dependent, was traced during 46 H. by means of a glass
+filament (with sights) fixed transversely a little above the apex. It
+plainly circumnutated (Fig. 193) whilst increasing in length and
+growing downwards. It was then raised up, so as to be extended almost
+horizontally, and the terminal part curved itself downwards, following
+a nearly straight course during 12 h., but with one attempt to
+circumnutate, as shown in Fig. 194. After 24 h. it had become nearly
+vertical. Whether the exciting cause of the downward movement is
+geotropism or apheliotropism was not ascertained; but probably it is
+not apheliotropism, as all the gynophores grew straight down towards
+the ground, whilst the light in the hot-house entered from one side as
+well as from above. Another and older gynophore, the apex of which had
+nearly reached the ground, was observed during 3 days in the same
+manner as the first-mentioned short one; and it was found to be always
+circumnutating. During the first 34 h. it described a figure which
+represented four ellipses. Lastly, a long gynophore, the apex of which
+had buried itself to the depth of about half an inch, was pulled up and
+extended horizontally: it quickly began to curve downwards in a zigzag
+line; but on the following day the
+terminal bleached portion was a little shrivelled. As the gynophores
+are rigid and arise from stiff branches, and as they terminate in sharp
+smooth points, it is probable that they could penetrate the ground by
+the mere force of growth. But this action must be aided by the
+circumnutating movement, for fine sand, kept moist, was pressed close
+round the apex of a gynophore which had reached the ground, and after a
+few hours it was surrounded by a narrow open crack. After three weeks
+this gynophore was uncovered, and the apex was found at a depth of
+rather above half an inch developed into a small, white, oval pod.
+
+Fig. 193 Arachis hypogoea: circumnutation of vertically dependent young
+gynophore, traced on a vertical glass from 10 A.M. July 31st to 8 A.M.
+Aug. 2nd.
+
+Fig. 194. Arachis hypogoea: downward movement of same young gynophore,
+after being extended horizontally; traced on a vertical glass from 8.30
+A.M. to 8.30 P.M. Aug. 2nd.
+
+Amphicarpoea monoica.—This plant produces long thin shoots, which twine
+round a support and of course circumnutate. Early in the summer shorter
+shoots are produced from the lower parts of the plant, which grow
+perpendicularly downwards and penetrate the ground. One of these,
+terminating in a minute bud, was observed to bury itself in sand to a
+depth of 0.2 inch in 24 h. It was lifted up and fixed in an inclined
+position about 25° beneath the horizon, being feebly illuminated from
+above. In this position it described two vertical ellipses in 24 h.;
+but on the following day, when brought into the house, it circumnutated
+only a very little round the same spot. Other branches were seen to
+penetrate the ground, and were afterwards found running like roots
+beneath the surface for a length of nearly two inches, and they had
+grown thick. One of these, after thus running, had emerged into the
+air. How far circumnutation aids these delicate branches in entering
+the ground we do not know; but the reflexed hairs with which they are
+clothed will assist in the work. This plant produces pods in the air,
+and others beneath the ground; which differ greatly in appearance. Asa
+Gray says[7] that it is the imperfect flowers on the creeping branches
+near the base of the plant which produce the subterranean pods; these
+flowers, therefore, must bury themselves like those of Arachis. But it
+may be suspected that the branches which were seen by us to penetrate
+the ground also produce subterranean flowers and pods.
+
+ [7] ‘Manual of the Botany of the Northern United States,’ 1856, p.
+ 106.
+
+DIAGEOTROPISM.
+
+Besides geotropism and apogeotropism, there is, according to Frank, an
+allied form of movement,
+namely, “transverse-geotropism,” or diageotropism, as we may call it
+for the sake of matching our other terms. Under the influence of
+gravitation certain parts are excited to place themselves more or less
+transversely to the line of its action.[8] We made no observations on
+this subject, and will here only remark that the position of the
+secondary radicles of various plants, which extend horizontally or are
+a little inclined downwards, would probably be considered by Frank as
+due to transverse-geotropism. As it has been shown in Chap. I. that the
+secondary radicles of Cucurbita made serpentine tracks on a smoked
+glass-plate, they clearly circumnutated, and there can hardly be a
+doubt that this holds good with other secondary radicles. It seems
+therefore highly probable that they place themselves in their
+diageotropic position by means of modified circumnutation.
+
+ [8] Elfving has lately described (‘Arbeiten des Bot. Instituts in
+ Würzburg,’ B. ii. 1880, p. 489) an excellent instance of such
+ movements in the rhizomes of certain plants.
+
+
+Finally, we may conclude that the three kinds of movement which have
+now been described and which are excited by gravitation, consist of
+modified circumnutation. Different parts or organs on the same plant,
+and the same part in different species, are thus excited to act in a
+widely different manner. We can see no reason why the attraction of
+gravity should directly modify the state of turgescence and subsequent
+growth of one part on the upper side and of another part on the lower
+side. We are therefore led to infer that both geotropic, apogeotropic,
+and diageotropic movements, the purpose of which we can generally
+understand,
+have been acquired for the advantage of the plant by the modification
+of the ever-present movement of circumnutation. This, however, implies
+that gravitation produces some effect on the young tissues sufficient
+to serve as a guide to the plant.
+
+
+
+
+CHAPTER XI.
+LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.
+
+
+General considerations—Vicia faba, effects of amputating the tips of
+the radicles—Regeneration of the tips—Effects of a short exposure of
+the tips to geotropic action and their subsequent amputation—Effects of
+amputating the tips obliquely—Effects of cauterising the tips—Effects
+of grease on the tips—Pisum sativum, tips of radicles cauterised
+transversely, and on their upper and lower sides—Phaseolus,
+cauterisation and grease on the tips—Gossypium—Cucurbita, tips
+cauterised transversely, and on their upper and lower sides—Zea, tips
+cauterised—Concluding remarks and summary of chapter—Advantages of the
+sensibility to geotropism being localised in the tips of the radicles.
+
+
+Ciesielski states[1] that when the roots of Pisum, Lens and Vicia were
+extended horizontally with their tips cut off, they were not acted on
+by geotropism; but some days afterwards, when a new root-cap and
+vegetative point had been formed, they bent themselves perpendicularly
+downwards. He further states that if the tips are cut off, after the
+roots have been left extended horizontally for some little time, but
+before they have begun to bend downwards, they may be placed in any
+position, and yet will bend as if still acted on by geotropism; and
+this shows that some influence had been already transmitted to the
+bending part from the tip before it was amputated. Sachs repeated these
+experiments; he cut off a length of between .05 and 1 mm. (measured
+from the apex of the
+vegetative point) of the tips of the radicles of the bean (Vicia faba),
+and placed them horizontally or vertically in damp air, earth, and
+water, with the result that they became bowed in all sorts of
+directions.[2] He therefore disbelieved in Ciesielski’s conclusions.
+But as we have seen with several plants that the tip of the radicle is
+sensitive to contact and to other irritants, and that it transmits some
+influence to the upper growing part causing it to bend, there seemed to
+us to be no a priori improbability in Ciesielski’s statements. We
+therefore determined to repeat his experiments, and to try others on
+several species by different methods.
+
+ [1] ‘Abwartskrümmung der Wurzel,’ Inaug. Dissert. Breslau, 1871, p.
+ 29.
+
+
+ [2] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. iii. 1873, p.
+ 432.
+
+
+Vicia faba.—Radicles of this plant were extended horizontally either
+over water or with their lower surfaces just touching it. Their tips
+had previously been cut off, in a direction as accurately transverse as
+could be done, to different lengths, measured from the apex of the
+root-cap, and which will be specified in each case. Light was always
+excluded. We had previously tried hundreds of unmutilated radicles
+under similar circumstances, and found that every one that was healthy
+became plainly geotropic in under 12 h. In the case of four radicles
+which had their tips cut off for a length of 1.5 mm., new root caps and
+new vegetative points were re-formed after an interval of 3 days 20 h.;
+and these when placed horizontally were acted on by geotropism. On some
+other occasions this regeneration of the tips and reacquired
+sensitiveness occurred within a somewhat shorter time. Therefore,
+radicles having their tips amputated should be observed in from 12 to
+48 h. after the operation.
+
+Four radicles were extended horizontally with their lower surfaces
+touching the water, and with their tips cut off for a length of only
+0.5 mm.: after 23 h. three of them were still horizontal; after 47 h.
+one of the three became fairly geotropic; and after 70 h. the other two
+showed a trace of this action. The fourth radicle was vertically
+geotropic after 23 h.; but by an
+accident the root-cap alone and not the vegetative point was found to
+have been amputated; so that this case formed no real exception and
+might have been excluded.
+
+Five radicles were extended horizontally like the last, and had their
+tips cut off for a length of 1 mm.; after 22–23 h., four of them were
+still horizontal, and one was slightly geotropic; after 48 h. the
+latter had become vertical; a second was also somewhat geotropic; two
+remained approximately horizontal; and the last or fifth had grown in a
+disordered manner, for it was inclined upwards at an angle of 65° above
+the horizon.
+
+Fourteen radicles were extended horizontally at a little height over
+the water with their tips cut off for a length of 1.5 mm.; after 12 h.
+all were horizontal, whilst five control or standard specimens in the
+same jar were all bent greatly downwards. After 24 h. several of the
+amputated radicles remained horizontal, but some showed a trace of
+geotropism, and one was plainly geotropic, for it was inclined at 40°
+beneath the horizon.
+
+Seven horizontally extended radicles from which the tips had been cut
+off for the unusual length of 2 mm. unfortunately were not looked at
+until 35 h. had elapsed; three were still horizontal, but to our
+surprise, four were more or less plainly geotropic.
+
+The radicles in the foregoing cases were measured before their tips
+were amputated, and in the course of 24 h. they had all increased
+greatly in length; but the measurements are not worth giving. It is of
+more importance that Sachs found that the rate of growth of the
+different parts of radicles with amputated tips was the same as with
+unmutilated ones. Altogether twenty-nine radicles were operated on in
+the manner above described, and of these only a few showed any
+geotropic curvature within 24 h.; whereas radicles with unmutilated
+tips always became, as already stated, much bent down in less than half
+of this time. The part of the radicle which bends most lies at the
+distance of from 3 to 6 mm. from the tip, and as the bending part
+continues to grow after the operation, there does not seem any reason
+why it should not have been acted on by geotropism, unless its
+curvature depended on some influence transmitted from the tip. And we
+have clear evidence of such transmission in Ciesielski’s experiments,
+which we repeated and extended in the following manner.
+
+Beans were embedded in friable peat with the hilum downwards, and after
+their radicles had grown perpendicularly down for a length of from ½ to
+1 inch, sixteen were selected which
+were perfectly straight, and these were placed horizontally on the
+peat, being covered by a thin layer of it. They were thus left for an
+average period of 1 h. 37 m. The tips were then cut off transversely
+for a length of 1.5 mm., and immediately afterwards they were embedded
+vertically in the peat. In this position geotropism would not tend to
+induce any curvature, but if some influence had already been
+transmitted from the tip to the part which bends most, we might expect
+that this part would become curved in the direction in which geotropism
+had previously acted; for it should be noted that these radicles being
+now destitute of their sensitive tips, would not be prevented by
+geotropism from curving in any direction. The result was that of the
+sixteen vertically embedded radicles, four continued for several days
+to grow straight downwards, whilst twelve became more or less bowed
+laterally. In two of the twelve, a trace of curvature was perceptible
+in 3 h. 30 m., counting from the time when they had first been laid
+horizontally; and all twelve were plainly bowed in 6 h., and still more
+plainly in 9 h. In every one of them the curvature was directed towards
+the side which had been downwards whilst the radicles remained
+horizontal. The curvature extended for a length of from 5 to, in one
+instance, 8 mm., measured from the cut-off end. Of the twelve bowed
+radicles five became permanently bent into a right angle; the other
+seven were at first much less bent, and their curvature generally
+decreased after 24 h., but did not wholly disappear. This decrease of
+curvature would naturally follow, if an exposure of only 1 h. 37 m. to
+geotropism, served to modify the turgescence of the cells, but not
+their subsequent growth to the full extent. The five radicles which
+were rectangularly bent became fixed in this position, and they
+continued to grow out horizontally in the peat for a length of about 1
+inch during from 4 to 6 days. By this time new tips had been formed;
+and it should be remarked that this regeneration occurred slower in the
+peat than in water, owing perhaps to the radicles being often looked at
+and thus disturbed. After the tips had been regenerated, geotropism was
+able to act on them, so that they now became bowed vertically
+downwards. An accurate drawing (Fig. 195) is given on the opposite page
+of one of these five radicles, reduced to half the natural size.
+
+We next tried whether a shorter exposure to geotropism would suffice to
+produce an after-effect. Seven radicles were extended horizontally for
+an hour, instead of 1 h. 37 m. as in the
+former trial; and after their tips (1.5 mm. in length) had been
+amputated, they were placed vertically in damp peat. Of these, three
+were not in the least affected and continued for days to grow straight
+downwards. Four showed after 8 h. 30 m. a mere trace of curvature in
+the direction in which they had been acted on by geotropism; and in
+this respect they differed much from those which had been exposed for 1
+h. 37 m., for many of the latter were plainly curved in 6 h. The
+curvature of one of these four radicles almost disappeared after 24 h.
+In the second, the curvature increased during two days and then
+decreased. the third radicle became permanently bent, so that its
+terminal part made an angle of about 45° with its original vertical
+direction. The fourth radicle became horizontal. These two, latter
+radicles continued during two more days to grow in the peat in the same
+directions, that is, at an angle of 45° beneath the horizon and
+horizontally. By the fourth morning new tips had been re-formed, and
+now geotropism was able to act on them again, and they became bent
+perpendicularly downwards, exactly as in the case of the five radicles
+described in the last paragraph and as is shown in (Fig. 195) here
+given.
+
+Fig. 195. Vicia faba: radicle, rectangularly bent at A, after the
+amputation of the tip, due to the previous influence of geotropism. L,
+side of bean which lay on the peat, whilst geotropism acted on the
+radicle. A, point of chief curvature of the radicle, whilst standing
+vertically downwards. B, point of chief curvature after the
+regeneration of the tip, when geotropism again acted. C, regenerated
+tip.
+
+Lastly, five other radicles were similarly treated, but were exposed to
+geotropism during only 45 m. After 8 h. 30 m. only one was doubtfully
+affected; after 24 h. two were just perceptibly curved towards the side
+which had been acted on by geotropism; after 48 h. the one first
+mentioned had a radius of curvature of 60 mm. That this curvature was
+due to the action of geotropism during the horizontal position of the
+radicle, was shown after 4 days, when a new tip had been re-formed, for
+it then grew perpendicularly downwards. We learn from this
+case that when the tips are amputated after an exposure to geotropism
+of only 45 m., though a slight influence is sometimes transmitted to
+the adjoining part of the radicle, yet this seldom suffices, and then
+only slowly, to induce even moderately well-pronounced curvature.
+
+In the previously given experiments on 29 horizontally extended
+radicles with their tips amputated, only one grew irregularly in any
+marked manner, and this became bowed upwards at an angle of 65°. In
+Ciesielski’s experiments the radicles could not have grown very
+irregularly, for if they had done so, he could not have spoken
+confidently of the obliteration of all geotropic action. It is
+therefore remarkable that Sachs, who experimented on many radicles with
+their tips amputated, found extremely disordered growth to be the usual
+result. As horizontally extended radicles with amputated tips are
+sometimes acted on slightly by geotropism within a short time, and are
+often acted on plainly after one or two days, we thought that this
+influence might possibly prevent disordered growth, though it was not
+able to induce immediate curvature. Therefore 13 radicles, of which 6
+had their tips amputated transversely for a length of 1.5 mm., and the
+other 7 for a length of only 0.5 mm., were suspended vertically in damp
+air, in which position they would not be affected by geotropism; but
+they exhibited no great irregularity of growth, whilst observed during
+4 to 6 days. We next thought that if care were not taken in cutting off
+the tips transversely, one side of the stump might be irritated more
+than the other, either at first or subsequently during the regeneration
+of the tip, and that this might cause the radicle to bend to one side.
+It has also been shown in Chapter III. that if a thin slice be cut off
+one side of the tip of the radicle, this causes the radicle to bend
+from the sliced side. Accordingly, 30 radicles, with tips amputated for
+a length of 1.5 mm., were allowed to grow perpendicularly downwards
+into water. Twenty of them were amputated at an angle of 20° with a
+line transverse to their longitudinal axes; and such stumps appeared
+only moderately oblique. The remaining ten radicles were amputated at
+an angle of about 45°. Under these circumstances no less than 19 out of
+the 30 became much distorted in the course of 2 or 3 days. Eleven other
+radicles were similarly treated, excepting that only 1 mm. (including
+in this and all other cases the root-cap) was amputated; and of these
+only one grew much, and two others slightly
+distorted; so that this amount of oblique amputation was not
+sufficient. Out of the above 30 radicles, only one or two showed in the
+first 24 h. any distortion, but this became plain in the 19 cases on
+the second day, and still more conspicuous at the close of the third
+day, by which time new tips had been partially or completely
+regenerated. When therefore a new tip is reformed on an oblique stump,
+it probably is developed sooner on one side than on the other: and this
+in some manner excites the adjoining part to bend to one side. Hence it
+seems probable that Sachs unintentionally amputated the radicles on
+which he experimented, not strictly in a transverse direction.
+
+This explanation of the occasional irregular growth of radicles with
+amputated tips, is supported by the results of cauterising their tips;
+for often a greater length on one side than on the other was
+unavoidably injured or killed. It should be remarked that in the
+following trials the tips were first dried with blotting-paper, and
+then slightly rubbed with a dry stick of nitrate of silver or lunar
+caustic. A few touches with the caustic suffice to kill the root-cap
+and some of the upper layers of cells of the vegetative point.
+Twenty-seven radicles, some young and very short, others of moderate
+length, were suspended vertically over water, after being thus
+cauterised. Of these some entered the water immediately, and others on
+the second day. The same number of uncauterised radicles of the same
+age were observed as controls. After an interval of three or four days
+the contrast in appearance between the cauterised and control specimens
+was wonderfully great. The controls had grown straight downwards, with
+the exception of the normal curvature, which we have called Sachs’
+curvature. Of the 27 cauterised radicles, 15 had become extremely
+distorted; 6 of them grew upwards and formed hoops, so that their tips
+sometimes came into contact with the bean above; 5 grew out
+rectangularly to one side; only a few of the remaining 12 were quite
+straight, and some of these towards the close of our observations
+became hooked at their extreme lower ends. Radicles, extended
+horizontally instead of vertically, with their tips cauterised, also
+sometimes grew distorted, but not so commonly, as far as we could
+judge, as those suspended vertically; for this occurred with only 5 out
+of 19 radicles thus treated.
+
+Instead of cutting off the tips, as in the first set of experiments, we
+next tried the effects of touching horizontally extended radicles with
+caustic in the manner just described. But
+some preliminary remarks must first be made. It may be objected that
+the caustic would injure the radicles and prevent them from bending;
+but ample evidence was given in Chapter III. that touching the tips of
+vertically suspended radicles with caustic on one side, does not stop
+their bending; on the contrary, it causes them to bend from the touched
+side. We also tried touching both the upper and the lower sides of the
+tips of some radicles of the bean, extended horizontally in damp
+friable earth. The tips of three were touched with caustic on their
+upper sides, and this would aid their geotropic bending; the tips of
+three were touched on their lower sides, which would tend to counteract
+the bending downwards; and three were left as controls. After 24 h. an
+independent observer was asked to pick out of the nine radicles, the
+two which were most and the two which were least bent; he selected as
+the latter, two of those which had been touched on their lower sides,
+and as the most bent, two of those which had been touched on the upper
+side. Hereafter analogous and more striking experiments with Pisum
+sativum and Cucurbita ovifera will be given. We may therefore safely
+conclude that the mere application of caustic to the tip does not
+prevent the radicles from bending.
+
+In the following experiments, the tips of young horizontally extended
+radicles were just touched with a stick of dry caustic; and this was
+held transversely, so that the tip might be cauterised all round as
+symmetrically as possible. The radicles were then suspended in a closed
+vessel over water, kept rather cool, viz., 55°–59° F. This was done
+because we had found that the tips were more sensitive to contact under
+a low than under a high temperature; and we thought that the same rule
+might apply to geotropism. In one exceptional trial, nine radicles
+(which were rather too old, for they had grown to a length of from 3 to
+5 cm.), were extended horizontally in damp friable earth, after their
+tips had been cauterised and were kept at too high a temperature, viz.,
+of 68° F., or 20° C. The result in consequence was not so striking as
+in the subsequent cases for although when after 9 h. 40 m. six of them
+were examined, these did not exhibit any geotropic bending, yet after
+24 h., when all nine were examined, only two remained horizontal, two
+exhibited a trace of geotropism, and five were slightly or moderately
+geotropic, yet not comparable in degree with the control specimens.
+Marks had been made on seven of these cauterised radicles at 10 mm.
+from the tips, which includes
+the whole growing portion; and after the 24 h. this part had a mean
+length of 37 mm., so that it had increased to more than 3½ times its
+original length; but it should be remembered that these beans had been
+exposed to a rather high temperature.
+
+Nineteen young radicles with cauterised tips were extended at different
+times horizontally over water. In every trial an equal number of
+control specimens were observed. In the first trial, the tips of three
+radicles were lightly touched with the caustic for 6 or 7 seconds,
+which was a longer application than usual. After 23 h. 30 m. (temp.
+55°–56° F.) these three radicles, A, B, C (Fig. 196), were still
+horizontal, whilst the three control specimens had become within 8 h.
+slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had
+been made on all six radicles at 10 mm. from their tips, when first
+placed horizontally. After the 23 h. 30 m. this terminal part,
+originally 10 mm. in length, had increased in the cauterised specimens
+to a mean length of 17.3 mm., and to 15.7 mm. in the control radicles,
+as shown in the figures by the unbroken transverse line; the dotted
+line being at 10 mm. from the apex. The control or uncauterised
+radicles, therefore, had actually grown less
+than the cauterised; but this no doubt was accidental, for radicles of
+different ages grow at different rates, and the growth of different
+individuals is likewise affected by unknown causes. The state of the
+tips of these three radicles, which had been cauterised for a rather
+longer time than usual, was as follows: the blackened apex, or the part
+which had been actually touched by the caustic, was succeeded by a
+yellowish zone, due probably to the absorption of some of the caustic;
+in A, both zones together were 1.1 mm. in length, and 1.4 mm. in
+diameter at the base of the yellowish zone; in B, the length of both
+was only 0.7 mm., and the diameter 0.7 mm.; in C, the length was 0.8
+mm., and the diameter 1.2 mm.
+
+Fig. 196. Vicia faba: state of radicles which had been extended
+horizontally for 23 h. 30 m.; A, B, C, tips touched with caustic; D, E,
+F, tips uncauterised. Lengths of radicles reduced to one-half scale,
+but by an accident the beans themselves not reduced in the same degree.
+
+Three other radicles, the tips of which had been touched with caustic
+curing 2 or 3 seconds, remained (temp. 58°–59° F.) horizontal for 23
+h.; the control radicles having, of course, become geotropic within
+this time. The terminal growing part, 10 mm. in length, of the
+cauterised radicles had increased in this interval to a mean length of
+24.5 mm., and of the controls to a mean of 26 mm. A section of one of
+the cauterised tips showed that the blackened part was 0.5 mm. in
+length, of which 0.2 mm. extended into the vegetative point; and a
+faint discoloration could be detected even to 1.6 mm. from the apex of
+the root-cap.
+
+In another lot of six radicles (temp. 55°–57° F.) the three control
+specimens were plainly geotropic in 8½ h.; and after 24 h. the mean
+length of their terminal part had increased from 10 mm. to 21 mm. When
+the caustic was applied to the three cauterised specimens, it was held
+quite motionless during 5 seconds, and the result was that the black
+marks were extremely minute. Therefore, caustic was again applied,
+after 8½ h., during which time no geotropic action had occurred. When
+the specimens were re-examined after an additional interval of 15½ h.,
+one was horizontal and the other two showed, to our surprise, a trace
+of geotropism which in one of them soon afterwards became strongly
+marked; but in this latter specimen the discoloured tip was only 2/3
+mm. in length. The growing part of these three radicles increased in 24
+h. from 10 mm. to an average of 16.5 mm.
+
+It would be superfluous to describe in detail the behaviour of the 10
+remaining cauterised radicles. The corresponding control specimens all
+became geotropic in 8 h. Of the cauterised, 6 were first looked at
+after 8 h., and one alone showed a trace
+of geotropism; 4 were first looked at after 14 h., and one alone of
+these was slightly geotropic. After 23–24h., 5 of the 10 were still
+horizontal, 4 slightly, and 1 decidedly, geotropic. After 48 h. some of
+them became strongly geotropic. The cauterised radicles increased
+greatly in length, but the measurements are not worth giving.
+
+As five of the last-mentioned cauterised radicles had become in 24 h.
+somewhat geotropic, these (together with three which were still
+horizontal) had their positions reversed, so that their tips were now a
+little upturned, and they were again touched with caustic. After 24 h.
+they showed no trace of geotropism; whereas the eight corresponding
+control specimens, which had likewise been reversed, in which position
+the tips of several pointed to the zenith, all became geotropic; some
+having passed in the 24 h. through an angle of 180°, others through
+about 135°, and others through only 90°. The eight radicles, which had
+been twice cauterised, were observed for an additional day (i.e. for 48
+h. after being reversed), and they still showed no signs of geotropism.
+Nevertheless, they continued to grow rapidly; four were measured 24 h.
+after being reversed, and they had in this time increased in length
+between 8 and 11 mm.; the other four were measured 48 h. after being
+reversed, and these had increased by 20, 18, 23, and 28 mm.
+
+In coming to a conclusion with respect to the effects of cauterising
+the tips of these radicles, we should bear in mind, firstly, that
+horizontally extended control radicles were always acted on by
+geotropism, and became somewhat bowed downwards in 8 or 9 h.; secondly,
+that the chief seat of the curvature lies at a distance of from 3 to 6
+mm. from the tip; thirdly, that the tip was discoloured by the caustic
+rarely for more than 1 mm. in length; fourthly, that the greater number
+of the cauterised radicles, although subjected to the full influence of
+geotropism during the whole time, remained horizontal for 24 h., and
+some for twice as long; and that those which did become bowed were so
+only in a slight degree; fifthly, that the cauterised radicles
+continued to grow almost, and sometimes quite, as well as the uninjured
+ones along the part which bends most. And lastly, that a touch on the
+tip with caustic, if on one side, far from preventing curvature,
+actually induces it. Bearing all these facts in mind, we must infer
+that under normal conditions the geotropic curvature of the root is due
+to an influence transmitted from the apex to the adjoining part where
+the bending
+takes place; and that when the tip of the root is cauterised it is
+unable to originate the stimulus necessary to produce geotropic
+curvature.
+
+As we had observed that grease was highly injurious to some plants, we
+determined to try its effects on radicles. When the cotyledons of
+Phalaris and Avena were covered with grease along one side, the growth
+of this side was quite stopped or greatly checked, and as the opposite
+side continued to grow, the cotyledons thus treated became bowed
+towards the greased side. This same matter quickly killed the delicate
+hypocotyls and young leaves of certain plants. The grease which we
+employed was made by mixing lamp-black and olive oil to such a
+consistence that it could be laid on in a thick layer. The tips of five
+radicles of the bean were coated with it for a length of 3 mm., and to
+our surprise this part increased in length in 23 h. to 7.1 mm.; the
+thick layer of grease being curiously drawn out. It thus could not have
+checked much, if at all, the growth of the terminal part of the
+radicle. With respect to geotropism, the tips of seven horizontally
+extended radicles were coated for a length of 2 mm., and after 24 h. no
+clear difference could be perceived between their downward curvature
+and that of an equal number of control specimens. The tips of 33 other
+radicles were coated on different occasions for a length of 3 mm.; and
+they were compared with the controls after 8 h., 24 h., and 48 h. On
+one occasion, after 24 h., there was very little difference in
+curvature between the greased and control specimens; but generally the
+difference was unmistakable, those with greased tips being considerably
+less curved downwards. The whole growing part (the greased tips
+included) of six of these radicles was measured and was found to have
+increased in 23 h. from 10 mm. to a mean length of 17.7 mm.; whilst the
+corresponding part of the controls had increased to 20.8 mm. It appears
+therefore, that although the tip itself, when greased, continues to
+grow, yet the growth of the whole radicle is somewhat checked, and that
+the geotropic curvature of the upper part, which was free from grease,
+was in most cases considerably lessened.
+
+Pisum sativum.—Five radicles, extended horizontally over water, had
+their tips lightly touched two or three times with dry caustic. These
+tips were measured in two cases, and found to be blackened for a length
+of only half a millimeter. Five other radicles were left as controls.
+The part which is most bowed through geotropism lies at a distance of
+several millimeters from
+the apex. After 24 h., and again after 32 h. from the commencement,
+four of the cauterised radicles were still horizontal, but one was
+plainly geotropic, being inclined at 45° beneath the horizon. The five
+controls were somewhat geotropic after 7 h. 20 m., and after 24 h. were
+all strongly geotropic; being inclined at the following angles beneath
+the horizon, viz., 59°, 60°, 65°, 57°, and 43°. The length of the
+radicles was not measured in either set, but it was manifest that the
+cauterised radicles had grown greatly.
+
+The following case proves that the action of the caustic by itself does
+not prevent the curvature of the radicle. Ten radicles were extended
+horizontally on and beneath a layer of damp friable peat-earth; and
+before being extended their tips were touched with dry caustic on the
+upper side. Ten other radicles similarly placed were touched on the
+lower side; and this would tend to make them bend from the cauterised
+side; and therefore, as now placed, upwards, or in opposition to
+geotropism. Lastly, ten uncauterised radicles were extended
+horizontally as controls. After 24 h. all the latter were geotropic;
+and the ten with their tips cauterised on the upper side were equally
+geotropic; and we believe that they became curved downwards before the
+controls. The ten which had been cauterised on the lower side presented
+a widely different appearance: No. 1, however, was perpendicularly
+geotropic, but this was no real exception, for on examination under the
+microscope, there was no vestige of a coloured mark on the tip, and it
+was evident that by a mistake it had not been touched with the caustic.
+No. 2 was plainly geotropic, being inclined at about 45° beneath the
+horizon; No. 3 was slightly, and No. 4 only just perceptibly geotropic;
+Nos. 5 and 6 were strictly horizontal; and the four remaining ones were
+bowed upwards, in opposition to geotropism. In these four cases the
+radius of the upward curvatures (according to Sachs’ cyclometer) was 5
+mm., 10 mm., 30 mm., and 70 mm. This curvature was distinct long before
+the 24 h. had elapsed, namely, after 8 h. 45 m. from the time when the
+lower sides of the tips were touched with the caustic.
+
+Phaseolus multiflorus.—Eight radicles, serving as controls, were
+extended horizontally, some in damp friable peat and some in damp air.
+They all became (temp 20°–21° C.) plainly geotropic in 8 h. 30 m., for
+they then stood at an average angle of 63° beneath the horizon. A
+rather greater length of the radicle is bowed downwards by geotropism
+than in the case of Vicia faba,
+that is to say, rather more than 6 mm. as measured from the apex of the
+root-cap. Nine other radicles were similarly extended, three in damp
+peat and six in damp air, and dry caustic was held transversely to
+their tips during 4 or 5 seconds. Three of their tips were afterwards
+examined: in (1) a length of 0.68 mm. was discoloured, of which the
+basal 0.136 mm. was yellow, the apical part being black; in (2) the
+discoloration was 0.65 mm. in length, of which the basal 0.04 mm. was
+yellow; in (3) the discoloration was 0.6 mm. in length, of which the
+basal 0.13 mm. was yellow. Therefore less than 1 mm. was affected by
+the caustic, but this sufficed almost wholly to prevent geotropic
+action; for after 24 h. one alone of the nine cauterised radicles
+became slightly geotropic, being now inclined at 10° beneath the
+horizon; the eight others remained horizontal, though one was curved a
+little laterally.
+
+The terminal part (10 mm. in length) of the six cauterised radicles in
+the damp air, had more than doubled in length in the 24 h., for this
+part was now on an average 20.7 mm. long. The increase in length within
+the same time was greater in the control specimens, for the terminal
+part had grown on an average from 10 mm. to 26.6 mm. But as the
+cauterised radicles had more than doubled their length in the 24 h., it
+is manifest that they had not been seriously injured by the caustic. We
+may here add that when experimenting on the effects of touching one
+side of the tip with caustic, too much was applied at first, and the
+whole tip (but we believe not more than 1 mm. in length) of six
+horizontally extended radicles was killed, and these continued for two
+or three days to grow out horizontally.
+
+Many trials were made, by coating the tips of horizontally extended
+radicles with the before described thick grease. The geotropic
+curvature of 12 radicles, which were thus coated for a length of 2 mm.,
+was delayed during the first 8 or 9 h., but after 24 h. was nearly as
+great as that of the control specimens. The tips of nine radicles were
+coated for a length of 3 mm., and after 7 h. 10 m. these stood at an
+average angle of 30° beneath the horizon, whilst the controls stood at
+an average of 54°. After 24 h. the two lots differed but little in
+their degree of curvature. In some other trials, however, there was a
+fairly well-marked difference after 24 h. between those with greased
+tips and the controls. The terminal part of eight control specimens
+increased in 24 h. from 10 mm. to a mean length of
+24.3 mm., whilst the mean increase of those with greased tips was 20.7
+mm. The grease, therefore, slightly checked the growth of the terminal
+part, but this part was not much injured; for several radicles which
+had been greased for a length of 2 mm. continued to grow during seven
+days, and were then only a little shorter than the controls. The
+appearance presented by these radicles after the seven days was very
+curious, for the black grease had been drawn out into the finest
+longitudinal striae, with dots and reticulations, which covered their
+surfaces for a length of from 26 to 44 mm., or of 1 to 1.7 inch. We may
+therefore conclude that grease on the tips of the radicles of this
+Phaseolus somewhat delays and lessens the geotropic curvature of the
+part which ought to bend most.
+
+Gossypium herbaceum.—The radicles of this plant bend, through the
+action of geotropism, for a length of about 6 mm. Five radicles, placed
+horizontally in damp air, had their tips touched with caustic, and the
+discoloration extended for a length of from 2/3 to 1 mm. They showed,
+after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet
+the terminal portion, 9 mm. in length, had increased on an average to
+15.9 mm. Six control radicles, after 7 h. 45 m., were all plainly
+geotropic, two of them being vertically dependent, and after 23 h. all
+were vertical, or nearly so.
+
+Cucurbita ovifera.—A large number of trials proved almost useless, from
+the three following causes: Firstly, the tips of radicles which have
+grown somewhat old are only feebly geotropic if kept in damp air; nor
+did we succeed well in our experiments, until the germinating seeds
+were placed in peat and kept at a rather high temperature. Secondly,
+the hypocotyls of the seeds which were pinned to the lids of the jars
+gradually became arched; and, as the cotyledons were fixed, the
+movement of the hypocotyl affected the position of the radicle, and
+caused confusion. Thirdly, the point of the radicle is so fine that it
+is difficult not to cauterise it either too much or too little. But we
+managed generally to overcome this latter difficulty, as the following
+experiments show, which are given to prove that a touch with caustic on
+one side of the tip does not prevent the upper part of the radicle from
+bending. Ten radicles were laid horizontally beneath and on damp
+friable peat, and their tips were touched with caustic on the upper
+side. After 8 h. all were plainly geotropic, three of them
+rectangularly; after 19 h.
+all were strongly geotropic, most of them pointing perpendicularly
+downwards. Ten other radicles, similarly placed, had their tips touched
+with caustic on the lower side; after 8 h. three were slightly
+geotropic, but not nearly so much so as the least geotropic of the
+foregoing specimens; four remained horizontal; and three were curved
+upwards in opposition to geotropism. After 19 h. the three which were
+slightly geotropic had become strongly so. Of the four horizontal
+radicles, one alone showed a trace of geotropism; of the three
+up-curved radicles, one retained this curvature, and the other two had
+become horizontal.
+
+The radicles of this plant, as already remarked, do not succeed well in
+damp air, but the result of one trial may be briefly given. Nine young
+radicles between .3 and .5 inch in length, with their tips cauterised
+and blackened for a length never exceeding ½ mm., together with eight
+control specimens, were extended horizontally in damp air. After an
+interval of only 4 h. 10 m. all the controls were slightly geotropic,
+whilst not one of the cauterised specimens exhibited a trace of this
+action. After 8 h. 35 m., there was the same difference between the two
+sets, but rather more strongly marked. By this time both sets had
+increased greatly in length. The controls, however, never became much
+more curved downwards; and after 24 h. there was no great difference
+between the two sets in their degree of curvature.
+
+Eight young radicles of nearly equal length (average .36 inch) were
+placed beneath and on peat-earth, and were exposed to a temp. of
+75°–76° F. Their tips had been touched transversely with caustic, and
+five of them were blackened for a length of about 0.5 mm., whilst the
+other three were only just visibly discoloured. In the same box there
+were 15 control radicles, mostly about .36 inch in length, but some
+rather longer and older, and therefore less sensitive. After 5 h., the
+15 control radicles were all more or less geotropic: after 9 h., eight
+of them were bent down beneath the horizon at various angles between
+45° and 90°, the remaining seven being only slightly geotropic: after
+25 h. all were rectangularly geotropic. The state of the eight
+cauterised radicles after the same intervals of time was as follows:
+after 5 h. one alone was slightly geotropic, and this was one with the
+tip only a very little discoloured: after 9 h. the one just mentioned
+was rectangularly geotropic, and two others were slightly so, and these
+were the three which had been scarcely
+affected by the caustic; the other five were still strictly horizontal.
+After 24 h. 40 m. the three with only slightly discoloured tips were
+bent down rectangularly; the other five were not in the least affected,
+but several of them had grown rather tortuously, though still in a
+horizontal plane. The eight cauterised radicles which had at first a
+mean length of .36 inch, after 9 h. had increased to a mean length of
+.79 inch; and after 24 h. 40 m. to the extraordinary mean length of 2
+inches. There was no plain difference in length between the five well
+cauterised radicles which remained horizontal, and the three with
+slightly cauterised tips which had become abruptly bent down. A few of
+the control radicles were measured after 25 h., and they were on an
+average only a little longer than the cauterised, viz., 2.19 inches. We
+thus see that killing the extreme tip of the radicle of this plant for
+a length of about 0.5 mm., though it stops the geotropic bending of the
+upper part, hardly interferes with the growth of the whole radicle.
+
+In the same box with the 15 control specimens, the rapid geotropic
+bending and growth of which have just been described, there were six
+radicles, about .6 inch in length, extended horizontally, from which
+the tips had been cut off in a transverse direction for a length of
+barely 1 mm. These radicles were examined after 9 h. and again after 24
+h. 40 m., and they all remained horizontal. They had not become nearly
+so tortuous as those above described which had been cauterised. The
+radicles with their tips cut off had grown in the 24 h. 40 m. as much,
+judging by the eye, as the cauterised specimens.
+
+Zea mays.—The tips of several radicles, extended horizontally in damp
+air, were dried with blotting-paper and then touched in the first trial
+during 2 or 3 seconds with dry caustic; but this was too long a
+contact, for the tips were blackened for a length of rather above 1 mm.
+They showed no signs of geotropism after an interval of 9 h., and were
+then thrown away. In a second trial the tips of three radicles were
+touched for a shorter time, and were blackened for a length of from 0.5
+to 0.75 mm.: they all remained horizontal for 4 h., but after 8 h. 30
+m. one of them, in which the blackened tip was only 0.5 mm. in length,
+was inclined at 21° beneath the horizon. Six control radicles all
+became slightly geotropic in 4 h., and strongly so after 8 h. 30 m.,
+with the chief seat of curvature generally between 6 or 7 mm. from the
+apex. In the cauterised specimens, the terminal growing part, 10 mm. in
+length, increased during
+the 8 h. 30 m. to a mean length of 13 mm.; and in the controls to 14.3
+mm.
+
+In a third trial the tips of five radicles (exposed to a temp. of
+70°–71°) were touched with the caustic only once and very slightly;
+they were afterwards examined under the microscope, and the part which
+was in any way discoloured was on an average .76 mm. in length. After 4
+h. 10 m. none were bent; after 5 h. 45 m., and again after 23 h. 30 m.,
+they still remained horizontal, excepting one which was now inclined
+20° beneath the horizon. The terminal part, 10 mm. in length, had
+increased greatly in length during the 23 h. 30 m., viz., to an average
+of 26 mm. Four control radicles became slightly geotropic after the 4
+h. 10 m., and plainly so after the 5 h. 45 m. Their mean length after
+the 23 h. 30 m. had increased from 10 mm. to 31 mm. Therefore a slight
+cauterisation of the tip checks slightly the growth of the whole
+radicle, and manifestly stops the bending of that part which ought to
+bend most under the influence of geotropism, and which still continues
+to increase greatly in length.]
+
+Concluding Remarks.—Abundant evidence has now been given, showing that
+with various plants the tip of the radicle is alone sensitive to
+geotropism; and that when thus excited, it causes the adjoining parts
+to bend. The exact length of the sensitive part seems to be somewhat
+variable, depending in part on the age of the radicle; but the
+destruction of a length of from less than 1 to 1.5 mm. (about 1/20th of
+an inch), in the several species observed, generally sufficed to
+prevent any part of the radicle from bending within 24 h., or even for
+a longer period. The fact of the tip alone being sensitive is so
+remarkable a fact, that we will here give a brief summary of the
+foregoing experiments. The tips were cut off 29 horizontally extended
+radicles of Vicia faba, and with a few exceptions they did not become
+geotropic in 22 or 23 h., whilst unmutilated radicles were always bowed
+downwards in 8 or 9 h. It should be borne in mind that the mere act of
+cutting
+off the tip of a horizontally extended radicle does not prevent the
+adjoining parts from bending, if the tip has been previously exposed
+for an hour or two to the influence of geotropism. The tip after
+amputation is sometimes completely regenerated in three days; and it is
+possible that it may be able to transmit an impulse to the adjoining
+parts before its complete regeneration. The tips of six radicles of
+Cucurbita ovifera were amputated like those of Vicia faba; and these
+radicles showed no signs of geotropism in 24 h.; whereas the control
+specimens were slightly affected in 5 h., and strongly in 9 h.
+
+With plants belonging to six genera, the tips of the radicles were
+touched transversely with dry caustic; and the injury thus caused
+rarely extended for a greater length than 1 mm., and sometimes to a
+less distance, as judged by even the faintest discoloration. We thought
+that this would be a better method of destroying the vegetative point
+than cutting it off; for we knew, from many previous experiments and
+from some given in the present chapter, that a touch with caustic on
+one side of the apex, far from preventing the adjoining part from
+bending, caused it to bend. In all the following cases, radicles with
+uncauterised tips were observed at the same time and under similar
+circumstances, and they became, in almost every instance, plainly bowed
+downwards in one-half or one-third of the time during which the
+cauterised specimens were observed. With Vicia faba 19 radicles were
+cauterised; 12 remained horizontal during 23–24 h.; 6 became slightly
+and 1 strongly geotropic. Eight of these radicles were afterwards
+reversed, and again touched with caustic, and none of them became
+geotropic in 24 h., whilst the reversed control specimens became
+strongly bowed downwards within this time.
+With Pisum sativum, five radicles had their tips touched with caustic,
+and after 32 h. four were still horizontal. The control specimens were
+slightly geotropic in 7 h. 20 m., and strongly so in 24 h. The tips of
+9 other radicles of this plant were touched only on the lower side, and
+6 of them remained horizontal for 24 h., or were upturned in opposition
+to geotropism; 2 were slightly, and 1 plainly geotropic. With Phaseolus
+multiflorus, 15 radicles were cauterised, and 8 remained horizontal for
+24 h.; whereas all the controls were plainly geotropic in 8 h. 30 m. Of
+5 cauterised radicles of Gossypium herbaceum, 4 remained horizontal for
+23 h. and 1 became slightly geotropic; 6 control radicles were
+distinctly geotropic in 7 h. 45 m. Five radicles of Cucurbita ovifera
+remained horizontal in peat-earth during 25 h., and 9 remained so in
+damp air during 8½ h.; whilst the controls became slightly geotropic in
+4 h. 10 m. The tips of 10 radicals of this plant were touched on their
+lower sides, and 6 of them remained horizontal or were upturned after
+19 h., 1 being slightly and 3 strongly geotropic.
+
+Lastly, the tips of several radicles of Vicia faba and Phaseolus
+multiflorus were thickly coated with grease for a length of 3 mm. This
+matter, which is highly injurious to most plants, did not kill or stop
+the growth of the tips, and only slightly lessened the rate of growth
+of the whole radicle; but it generally delayed a little the geotropic
+bending of the upper part.
+
+The several foregoing cases would tell us nothing, if the tip itself
+was the part which became most bent; but we know that it is a part
+distant from the tip by some millimeters which grows quickest, and
+which, under the influence of geotropism, bends most. We have no reason
+to suppose that this part is injured by the death or injury of the tip;
+and it is certain
+that after the tip has been destroyed this part goes on growing at such
+a rate, that its length was often doubled in a day. We have also seen
+that the destruction of the tip does not prevent the adjoining part
+from bending, if this part has already received some influence from the
+tip. As with horizontally extended radicles, of which the tip has been
+cut off or destroyed, the part which ought to bend most remains
+motionless for many hours or days, although exposed at right angles to
+the full influence of geotropism, we must conclude that the tip alone
+is sensitive to this power, and transmits some influence or stimulus to
+the adjoining parts, causing them to bend. We have direct evidence of
+such transmission; for when a radicle was left extended horizontally
+for an hour or an hour and a half, by which time the supposed influence
+will have travelled a little distance from the tip, and the tip was
+then cut off, the radicle afterwards became bent, although placed
+perpendicularly. The terminal portions of several radicles thus treated
+continued for some time to grow in the direction of their
+newly-acquired curvature; for as they were destitute of tips, they were
+no longer acted on by geotropism. But after three or four days when new
+vegetative points were formed, the radicles were again acted on by
+geotropism, and now they curved themselves perpendicularly downwards.
+To see anything of the above kind in the animal kingdom, we should have
+to suppose than an animal whilst lying down determined to rise up in
+some particular direction; and that after its head had been cut off, an
+impulse continued to travel very slowly along the nerves to the proper
+muscles; so that after several hours the headless animal rose up in the
+predetermined direction.
+
+As the tip of the radicle has been found to be the
+part which is sensitive to geotropism in the members of such distinct
+families as the Leguminosae, Malvaceae, Cucurbitaceæ and Gramineæ, we
+may infer that this character is common to the roots of most seedling
+plants. Whilst a root is penetrating the ground, the tip must travel
+first; and we can see the advantage of its being sensitive to
+geotropism, as it has to determine the course of the whole root.
+Whenever the tip is deflected by any subterranean obstacle, it will
+also be an advantage that a considerable length of the root should be
+able to bend, more especially as the tip itself grows slowly and bends
+but little, so that the proper downward course may be soon recovered.
+But it appears at first sight immaterial whether this were effected by
+the whole growing part being sensitive to geotropism, or by an
+influence transmitted exclusively from the tip. We should, however,
+remember that it is the tip which is sensitive to the contact of hard
+objects, causing the radicle to bend away from them, thus guiding it
+along the lines of least resistance in the soil. It is again the tip
+which is alone sensitive, at least in some cases, to moisture, causing
+the radicle to bend towards its source. These two kinds of
+sensitiveness conquer for a time the sensitiveness to geotropism,
+which, however, ultimately prevails. Therefore, the three kinds of
+sensitiveness must often come into antagonism; first one prevailing,
+and then another; and it would be an advantage, perhaps a necessity,
+for the interweighing and reconciling of these three kinds of
+sensitiveness, that they should be all localised in the same group of
+cells which have to transmit the command to the adjoining parts of the
+radicle, causing it to bend to or from the source of irritation.
+
+Finally, the fact of the tip alone being sensitive to
+the attraction of gravity has an important bearing on the theory of
+geotropism. Authors seem generally to look at the bending of a radicle
+towards the centre of the earth, as the direct result of gravitation,
+which is believed to modify the growth of the upper or lower surfaces,
+in such a manner as to induce curvature in the proper direction. But we
+now know that it is the tip alone which is acted on, and that this part
+transmits some influence to the adjoining parts, causing them to curve
+downwards. Gravity does not appear to act in a more direct manner on a
+radicle, than it does on any lowly organised animal, which moves away
+when it feels some weight or pressure.
+
+
+
+
+CHAPTER XII.
+CONCLUDING REMARKS.
+
+
+Nature of the circumnutating movement—History of a germinating seed—The
+radicle first protrudes and circumnutates—Its tip highly
+sensitive—Emergence of the hypocotyl or of the epicotyl from the ground
+under the form of an arch–Its circumnutation and that of the
+cotyledons—The seedling throws up a leaf-bearing stem—The
+circumnutation of all the parts or organs—Modified
+circumnutation—Epinasty and hyponasty—Movements of climbing
+plants—Nyctitropic movements—Movements excited by light and
+gravitation—Localised sensitiveness—Resemblance between the movements
+of plants and animals—The tip of the radicle acts like a brain.
+
+
+It may be useful to the reader if we briefly sum up the chief
+conclusions, which, as far as we can judge, have been fairly well
+established by the observations given in this volume. All the parts or
+organs in every plant whilst they continue to grow, and some parts
+which are provided with pulvini after they have ceased to grow, are
+continually circumnutating. This movement commences even before the
+young seedling has broken through the ground. The nature of the
+movement and its causes, as far as ascertained, have been briefly
+described in the Introduction. Why every part of a plant whilst it is
+growing, and in some cases after growth has ceased, should have its
+cells rendered more turgescent and its cell-walls more extensile first
+on one side and then on another, thus inducing circumnutation is not
+known. It would appear as if the changes in the cells required periods
+of rest.
+
+
+In some cases, as with the hypocotyls of Brassica, the leaves of
+Dionaea and the joints of the Gramineæ, the circumnutating movement
+when viewed under the microscope is seen to consist of innumerable
+small oscillations. The part under observation suddenly jerks forwards
+for a length of .002 to .001 of an inch, and then slowly retreats for a
+part of this distance; after a few seconds it again jerks forwards, but
+with many intermissions. The retreating movement apparently is due to
+the elasticity of the resisting tissues. How far this oscillatory
+movement is general we do not know, as not many circumnutating plants
+were observed by us under the microscope; but no such movement could be
+detected in the case of Drosera with a 2-inch object-glass which we
+used. The phenomenon is a remarkable one. The whole hypocotyl of a
+cabbage or the whole leaf of a Dionaea could not jerk forwards unless a
+very large number of cells on one side were simultaneously affected.
+Are we to suppose that these cells steadily become more and more
+turgescent on one side, until the part suddenly yields and bends,
+inducing what may be called a microscopically minute earthquake in the
+plant; or do the cells on one side suddenly become turgescent in an
+intermittent manner; each forward movement thus caused being opposed by
+the elasticity of the tissues?
+
+Circumnutation is of paramount importance in the life of every plant;
+for it is through its modification that many highly beneficial or
+necessary movements have been acquired. When light strikes one side of
+a plant, or light changes into darkness, or when gravitation acts on a
+displaced part, the plant is enabled in some unknown manner to increase
+the always varying turgescence of the cells on one side; so that the
+ordinary circumnutating movement is
+modified, and the part bends either to or from the exciting cause; or
+it may occupy a new position, as in the so-called sleep of leaves. The
+influence which modifies circumnutation may be transmitted from one
+part to another. Innate or constitutional changes, independently of any
+external agency, often modify the circumnutating movements at
+particular periods of the life of the plant. As circumnutation is
+universally present, we can understand how it is that movements of the
+same kind have been developed in the most distinct members of the
+vegetable series. But it must not be supposed that all the movements of
+plants arise from modified circumnutation; for, as we shall presently
+see, there is reason to believe that this is not the case.
+
+Having made these few preliminary remarks, we will in imagination take
+a germinating seed, and consider the part which the various movements
+play in the life-history of the plant. The first change is the
+protrusion of the radicle, which begins at once to circumnutate. This
+movement is immediately modified by the attraction of gravity and
+rendered geotropic. The radicle, therefore, supposing the seed to be
+lying on the surface, quickly bends downwards, following a more or less
+spiral course, as was seen on the smoked glass-plates. Sensitiveness to
+gravitation resides in the tip; and it is the tip which transmits some
+influence to the adjoining parts, causing them to bend. As soon as the
+tip, protected by the root-cap, reaches the ground, it penetrates the
+surface, if this be soft or friable; and the act of penetration is
+apparently aided by the rocking or circumnutating movement of the whole
+end of the radicle. If the surface is compact, and cannot easily be
+penetrated, then
+the seed itself, unless it be a heavy one, is displaced or lifted up by
+the continued growth and elongation of the radicle. But in a state of
+nature seeds often get covered with earth or other matter, or fall into
+crevices, etc., and thus a point of resistance is afforded, and the tip
+can more easily penetrate the ground. But even with seeds lying loose
+on the surface there is another aid: a multitude of excessively fine
+hairs are emitted from the upper part of the radicle, and these attach
+themselves firmly to stones or other objects lying on the surface, and
+can do so even to glass; and thus the upper part is held down whilst
+the tip presses against and penetrates the ground. The attachment of
+the root-hairs is effected by the liquefaction of the outer surface of
+the cellulose walls, and by the subsequent setting hard of the
+liquefied matter. This curious process probably takes place, not for
+the sake of the attachment of the radicles to superficial objects, but
+in order that the hairs may be brought into the closest contact with
+the particles in the soil, by which means they can absorb the layer of
+water surrounding them, together with any dissolved matter.
+
+After the tip has penetrated the ground to a little depth, the
+increasing thickness of the radicle, together with the root-hairs, hold
+it securely in its place; and now the force exerted by the longitudinal
+growth of the radicle drives the tip deeper into the ground. This
+force, combined with that due to transverse growth, gives to the
+radicle the power of a wedge. Even a growing root of moderate size,
+such as that of a seedling bean, can displace a weight of some pounds.
+It is not probable that the tip when buried in compact earth can
+actually circumnutate and thus aid its downward passage, but the
+circumnutating movement will facilitate the tip entering any lateral
+or oblique fissure in the earth, or a burrow made by an earth-worm or
+larva; and it is certain that roots often run down the old burrows of
+worms. The tip, however, in endeavouring to circumnutate, will
+continually press against the earth on all sides, and this can hardly
+fail to be of the highest importance to the plant; for we have seen
+that when little bits of card-like paper and of very thin paper were
+cemented on opposite sides of the tip, the whole growing part of the
+radicle was excited to bend away from the side bearing the card or more
+resisting substance, towards the side bearing the thin paper. We may
+therefore feel almost sure that when the tip encounters a stone or
+other obstacle in the ground, or even earth more compact on one side
+than the other, the root will bend away as much as it can from the
+obstacle or the more resisting earth, and will thus follow with
+unerring skill a line of least resistance.
+
+The tip is more sensitive to prolonged contact with an object than to
+gravitation when this acts obliquely on the radicle, and sometimes even
+when it acts in the most favourable direction at right angles to the
+radicle. The tip was excited by an attached bead of shellac weighing
+less than 1/200th of a grain (0.33 mg.); it is therefore more sensitive
+than the most delicate tendril, namely, that of Passiflora gracilis,
+which was barely acted on by a bit of wire weighing 1/50th of a grain.
+But this degree of sensitiveness is as nothing compared with that of
+the glands of Drosera, for these are excited by particles weighing only
+1/78740 of a grain. The sensitiveness of the tip cannot be accounted
+for by its being covered by a thinner layer of tissue than the other
+parts, for it is protected by the relatively thick root-cap. It is
+remarkable that although the radicle bends away, when one side of the
+tip is slightly touched
+with caustic, yet if the side be much cauterised the injury is too
+great, and the power of transmitting some influence to the adjoining
+parts causing them to bend, is lost. Other analogous cases are known to
+occur.
+
+After a radicle has been deflected by some obstacle, geotropism directs
+the tip again to grow perpendicularly downwards; but geotropism is a
+feeble power, and here, as Sachs has shown, another interesting
+adaptive movement comes into play; for radicles at a distance of a few
+millimeters from the tip are sensitive to prolonged contact in such a
+manner that they bend towards the touching object, instead of from it
+as occurs when an object touches one side of the tip. Moreover, the
+curvature thus caused is abrupt; the pressed part alone bending. Even
+slight pressure suffices, such as a bit of card cemented to one side.
+therefore a radicle, as it passes over the edge of any obstacle in the
+ground, will through the action of geotropism press against it; and
+this pressure will cause the radicle to endeavour to bend abruptly over
+the edge. It will thus recover as quickly as possible its normal
+downward course.
+
+Radicles are also sensitive to air which contains more moisture on one
+side than the other, and they bend towards its source. It is therefore
+probable that they are in like manner sensitive to dampness in the
+soil. It was ascertained in several cases that this sensitiveness
+resides in the tip, which transmits an influence causing the adjoining
+upper part to bend in opposition to geotropism towards the moist
+object. We may therefore infer that roots will be deflected from their
+downward course towards any source of moisture in the soil.
+
+Again, most or all radicles are slightly sensitive to light, and
+according to Wiesner, generally bend a little
+from it. Whether this can be of any service to them is very doubtful,
+but with seeds germinating on the surface it will slightly aid
+geotropism in directing the radicles to the ground.[1] We ascertained
+in one instance that such sensitiveness resided in the tip, and caused
+the adjoining parts to bend from the light. The sub-aërial roots
+observed by Wiesner were all apheliotropic, and this, no doubt, is of
+use in bringing them into contact with trunks of trees or surfaces of
+rock, as is their habit.
+
+ [1] Dr. Karl Richter, who has especially attended to this subject (‘K.
+ Akad. der Wissenschaften in Wien,’ 1879, p. 149), states that
+ apheliotropism does not aid radicles in penetrating the ground.
+
+
+We thus see that with seedling plants the tip of the radicle is endowed
+with diverse kinds of sensitiveness; and that the tip directs the
+adjoining growing parts to bend to or from the exciting cause,
+according to the needs of the plant. The sides of the radicle are also
+sensitive to contact, but in a widely different manner. Gravitation,
+though a less powerful cause of movement than the other above specified
+stimuli, is ever present; so that it ultimately prevails and determines
+the downward growth of the root.
+
+The primary radicle emits secondary ones which project
+sub-horizontally; and these were observed in one case to circumnutate.
+Their tips are also sensitive to contact, and they are thus excited to
+bend away from any touching object; so that they resemble in these
+respects, as far as they were observed, the primary radicles. If
+displaced they resume, as Sachs has shown, their original
+sub-horizontal position; and this apparently is due to diageotropism.
+The secondary radicles emit tertiary ones, but these, in the case of
+the bean, are not affected by gravitation; consequently they protrude
+in all directions. Thus the general
+arrangement of the three orders of roots is excellently adapted for
+searching the whole soil for nutriment.
+
+Sachs has shown that if the tip of the primary radicle is cut off (and
+the tip will occasionally be gnawed off with seedlings in a state of
+nature) one of the secondary radicles grows perpendicularly downwards,
+in a manner which is analogous to the upward growth of a lateral shoot
+after the amputation of the leading shoot. We have seen with radicles
+of the bean that if the primary radicle is merely compressed instead of
+being cut off, so that an excess of sap is directed into the secondary
+radicles, their natural condition is disturbed and they grow downwards.
+Other analogous facts have been given. As anything which disturbs the
+constitution is apt to lead to reversion, that is, to the resumption of
+a former character, it appears probable that when secondary radicles
+grow downwards or lateral shoots upwards, they revert to the primary
+manner of growth proper to radicles and shoots.
+
+With dicotyledonous seeds, after the protrusion of the radicle, the
+hypocotyl breaks through the seed-coats; but if the cotyledons are
+hypogean, it is the epicotyl which breaks forth. These organs are at
+first invariably arched, with the upper part bent back parallel to the
+lower; and they retain this form until they have risen above the
+ground. In some cases, however, it is the petioles of the cotyledons or
+of the first true leaves which break through the seed-coats as well as
+the ground, before any part of the stem protrudes; and then the
+petioles are almost invariably arched. We have met with only one
+exception, and that only a partial one, namely, with the petioles of
+the two first leaves of Acanthus candelabrum. With Delphinium nudicaule
+the petioles of the two cotyledons are
+completely confluent, and they break through the ground as an arch;
+afterwards the petioles of the successively formed early leaves are
+arched, and they are thus enabled to break through the base of the
+confluent petioles of the cotyledons. In the case of Megarrhiza, it is
+the plumule which breaks as an arch through the tube formed by the
+confluence of the cotyledon-petioles. With mature plants, the
+flower-stems and the leaves of some few species, and the rachis of
+several ferns, as they emerge separately from the ground, are likewise
+arched.
+
+The fact of so many different organs in plants of many kinds breaking
+through the ground under the form of an arch, shows that this must be
+in some manner highly important to them. According to Haberlandt, the
+tender growing apex is thus saved from abrasion, and this is probably
+the true explanation. But as both legs of the arch grow, their power of
+breaking through the ground will be much increased as long as the tip
+remains within the seed-coats and has a point of support. In the case
+of monocotyledons the plumule or cotyledon is rarely arched, as far as
+we have seen; but this is the case with the leaf-like cotyledon of the
+onion; and the crown of the arch is here strengthened by a special
+protuberance. In the Gramineæ the summit of the straight, sheath-like
+cotyledon is developed into a hard sharp crest, which evidently serves
+for breaking through the earth. With dicotyledons the arching of the
+epicotyl or hypocotyl often appears as if it merely resulted from the
+manner in which the parts are packed within the seed; but it is
+doubtful whether this is the whole of the truth in any case, and it
+certainly was not so in several cases, in which the arching was seen to
+commence after the parts had wholly
+escaped from the seed-coats. As the arching occurred in whatever
+position the seeds were placed, it is no doubt due to temporarily
+increased growth of the nature of epinasty or hyponasty along one side
+of the part.
+
+As this habit of the hypocotyl to arch itself appears to be universal,
+it is probably of very ancient origin. It is therefore not surprising
+that it should be inherited, at least to some extent, by plants having
+hypogean cotyledons, in which the hypocotyl is only slightly developed
+and never protrudes above the ground, and in which the arching is of
+course now quite useless. This tendency explains, as we have seen, the
+curvature of the hypocotyl (and the consequent movement of the radicle)
+which was first observed by Sachs, and which we have often had to refer
+to as Sachs’ curvature.
+
+The several foregoing arched organs are continually circumnutating, or
+endeavouring to circumnutate, even before they break through the
+ground. As soon as any part of the arch protrudes from the seed-coats
+it is acted upon by apogeotropism, and both the legs bend upwards as
+quickly as the surrounding earth will permit, until the arch stands
+vertically. By continued growth it then forcibly breaks through the
+ground; but as it is continually striving to circumnutate this will aid
+its emergence in some slight degree, for we know that a circumnutating
+hypocotyl can push away damp sand on all sides. As soon as the faintest
+ray of light reaches a seedling, heliotropism will guide it through any
+crack in the soil, or through an entangled mass of overlying
+vegetation; for apogeotropism by itself can direct the seedling only
+blindly upwards. Hence probably it is that sensitiveness to light
+resides in the tip of the cotyledons of the Gramineæ, and in
+the upper part of the hypocotyls of at least some plants.
+
+As the arch grows upwards the cotyledons are dragged out of the ground.
+The seed-coats are either left behind buried, or are retained for a
+time still enclosing the cotyledons. These are afterwards cast off
+merely by the swelling of the cotyledons. But with most of the
+Cucurbitaceæ there is a curious special contrivance for bursting the
+seed-coats whilst beneath the ground, namely, a peg at the base of the
+hypocotyl, projecting at right angles, which holds down the lower half
+of the seed-coats, whilst the growth of the arched part of the
+hypocotyl lifts up the upper half, and thus splits them in twain. A
+somewhat analogous structure occurs in Mimosa pudica and some other
+plants. Before the cotyledons are fully expanded and have diverged, the
+hypocotyl generally straightens itself by increased growth along the
+concave side, thus reversing the process which caused the arching.
+Ultimately not a trace of the former curvature is left, except in the
+case of the leaf-like cotyledons of the onion.
+
+The cotyledons can now assume the function of leaves, and decompose
+carbonic acid; they also yield up to other parts of the plant the
+nutriment which they often contain. When they contain a large stock of
+nutriment they generally remain buried beneath the ground, owing to the
+small development of the hypocotyl; and thus they have a better chance
+of escaping destruction by animals. From unknown causes, nutriment is
+sometimes stored in the hypocotyl or in the radicle, and then one of
+the cotyledons or both become rudimentary, of which several instances
+have been given. It is probable that the extraordinary manner of
+germination of _Megarrhiza Californica_,
+_Ipomœa leptophylla_ and _pandurata_, and of _Quercus virens_, is
+connected with the burying of the tuber-like roots, which at an early
+age are stocked with nutriment; for in these plants it is the petioles
+of the cotyledons which first protrude from the seeds, and they are
+then merely tipped with a minute radicle and hypocotyl. These petioles
+bend down geotropically like a root and penetrate the ground, so that
+the true root, which afterwards becomes greatly enlarged, is buried at
+some little depth beneath the surface. Gradations of structure are
+always interesting, and Asa Gray informs us that with Ipomœa Jalappa,
+which likewise forms huge tubers, the hypocotyl is still of
+considerable length, and the petioles of the cotyledons are only
+moderately elongated. But in addition to the advantage gained by the
+concealment of the nutritious matter stored within the tubers, the
+plumule, at least in the case of Megarrhiza, is protected from the
+frosts of winter by being buried.
+
+With many dicotyledonous seedlings, as has lately been described by De
+Vries, the contraction of the parenchyma of the upper part of the
+radicle drags the hypocotyl downwards into the earth; sometimes (it is
+said) until even the cotyledons are buried. The hypocotyl itself of
+some species contracts in a like manner. It is believed that this
+burying process serves to protect the seedlings against the frosts of
+winter.
+
+Our imaginary seedling is now mature as a seedling, for its hypocotyl
+is straight and its cotyledons are fully expanded. In this state the
+upper part of the hypocotyl and the cotyledons continue for some time
+to circumnutate, generally to a wide extent relatively to the size of
+the parts, and at a rapid rate. But seedlings profit by this power of
+movement only when it is modified, especially by the action of light
+and
+gravitation; for they are thus enabled to move more rapidly and to a
+greater extent than can most mature plants. Seedlings are subjected to
+a severe struggle for life, and it appears to be highly important to
+them that they should adapt themselves as quickly and as perfectly as
+possible to their conditions. Hence also it is that they are so
+extremely sensitive to light and gravitation. The cotyledons of some
+few species are sensitive to a touch; but it is probable that this is
+only an indirect result of the foregoing kinds of sensitiveness, for
+there is no reason to believe that they profit by moving when touched.
+
+Our seedling now throws up a stem bearing leaves, and often branches,
+all of which whilst young are continually circumnutating. If we look,
+for instance, at a great acacia tree, we may feel assured that every
+one of the innumerable growing shoots is constantly describing small
+ellipses; as is each petiole, sub-petiole, and leaflet. The latter, as
+well as ordinary leaves, generally move up and down in nearly the same
+vertical plane, so that they describe very narrow ellipses. The
+flower-peduncles are likewise continually circumnutating. If we could
+look beneath the ground, and our eyes had the power of a microscope, we
+should see the tip of each rootlet endeavouring to sweep small ellipses
+or circles, as far as the pressure of the surrounding earth permitted.
+All this astonishing amount of movement has been going on year after
+year since the time when, as a seedling, the tree first emerged from
+the ground.
+
+Stems are sometimes developed into long runners or stolons. These
+circumnutate in a conspicuous manner, and are thus aided in passing
+between and over surrounding obstacles. But whether the circumnutating
+movement has been increased for this special purpose is doubtful.
+
+
+We have now to consider circumnutation in a modified form, as the
+source of several great classes of movement. The modification may be
+determined by innate causes, or by external agencies. Under the first
+head we see leaves which, when first unfolded, stand in a vertical
+position, and gradually bend downwards as they grow older. We see
+flower-peduncles bending down after the flower has withered, and others
+rising up; or again, stems with their tips at first bowed downwards, so
+as to be hooked, afterwards straightening themselves; and many other
+such cases. These changes of position, which are due to epinasty or
+hyponasty, occur at certain periods of the life of the plant, and are
+independent of any external agency. They are effected not by a
+continuous upward or downward movement, but by a succession of small
+ellipses, or by zigzag lines,—that is, by a circumnutating movement
+which is preponderant in some one direction.
+
+Again, climbing plants whilst young circumnutate in the ordinary
+manner, but as soon as the stem has grown to a certain height, which is
+different for different species, it elongates rapidly, and now the
+amplitude of the circumnutating movement is immensely increased,
+evidently to favour the stem catching hold of a support. The stem also
+circumnutates rather more equally to all sides than in the case of
+non-climbing plants. This is conspicuously the case with those tendrils
+which consist of modified leaves, as these sweep wide circles; whilst
+ordinary leaves usually circumnutate nearly in the same vertical plane.
+Flower-peduncles when converted into tendrils have their circumnutating
+movement in like manner greatly increased.
+
+We now come to our second group of
+circumnutating movements—those modified through external agencies. The
+so-called sleep or nyctitropic movements of leaves are determined by
+the daily alternations of light and darkness. It is not the darkness
+which excites them to move, but the difference in the amount of light
+which they receive during the day and night; for with several species,
+if the leaves have not been brightly illuminated during the day, they
+do not sleep at night. They inherit, however, some tendency to move at
+the proper periods, independently of any change in the amount of light.
+The movements are in some cases extraordinarily complex, but as a full
+summary has been given in the chapter devoted to this subject, we will
+here say but little on this head. Leaves and cotyledons assume their
+nocturnal position by two means, by the aid of pulvini and without such
+aid. In the former case the movement continues as long as the leaf or
+cotyledon remains in full health; whilst in the latter case it
+continues only whilst the part is growing. Cotyledons appear to sleep
+in a larger proportional number of species than do leaves. In some
+species, the leaves sleep and not the cotyledons; in others, the
+cotyledons and not the leaves; or both may sleep, and yet assume widely
+different positions at night.
+
+Although the nyctitropic movements of leaves and cotyledons are
+wonderfully diversified, and sometimes differ much in the species of
+the same genus, yet the blade is always placed in such a position at
+night, that its upper surface is exposed as little as possible to full
+radiation. We cannot doubt that this is the object gained by these
+movements; and it has been proved that leaves exposed to a clear sky,
+with their blades compelled to remain horizontal, suffered much more
+from the cold than others which were allowed to assume
+their proper vertical position. Some curious facts have been given
+under this head, showing that horizontally extended leaves suffered
+more at night, when the air, which is not cooled by radiation, was
+prevented from freely circulating beneath their lower surfaces; and so
+it was, when the leaves were allowed to go to sleep on branches which
+had been rendered motionless. In some species the petioles rise up
+greatly at night, and the pinnae close together. The whole plant is
+thus rendered more compact, and a much smaller surface is exposed to
+radiation.
+
+That the various nyctitropic movements of leaves result from modified
+circumnutation has, we think, been clearly shown. In the simplest cases
+a leaf describes a single large ellipse during the 24 h.; and the
+movement is so arranged that the blade stands vertically during the
+night, and reassumes its former position on the following morning. The
+course pursued differs from ordinary circumnutation only in its greater
+amplitude, and in its greater rapidity late in the evening and early on
+the following morning. Unless this movement is admitted to be one of
+circumnutation, such leaves do not circumnutate at all, and this would
+be a monstrous anomaly. In other cases, leaves and cotyledons describe
+several vertical ellipses during the 24 h.; and in the evening one of
+them is increased greatly in amplitude until the blade stands
+vertically either upwards or downwards. In this position it continues
+to circumnutate until the following morning, when it reassumes its
+former position. These movements, when a pulvinus is present, are often
+complicated by the rotation of the leaf or leaflet; and such rotation
+on a small scale occurs during ordinary circumnutation. The many
+diagrams showing the movements of sleeping and non-sleeping leaves and
+cotyledons should be compared, and it will be seen that they are
+essentially alike. Ordinary circumnutation is converted into a
+nyctitropic movement, firstly by an increase in its amplitude, but not
+to so great a degree as in the case of climbing plants, and secondly by
+its being rendered periodic in relation to the alternations of day and
+night. But there is frequently a distinct trace of periodicity in the
+circumnutating movements of non-sleeping leaves and cotyledons. The
+fact that nyctitropic movements occur in species distributed in many
+families throughout the whole vascular series, is intelligible, if they
+result from the modification of the universally present movement of
+circumnutation; otherwise the fact is inexplicable.
+
+In the seventh chapter we have given the case of a Porlieria, the
+leaflets of which remained closed all day, as if asleep, when the plant
+was kept dry, apparently for the sake of checking evaporation.
+Something of the same kind occurs with certain Gramineæ. At the close
+of this same chapter, a few observations were appended on what may be
+called the embryology of leaves. The leaves produced by young shoots on
+cut-down plants of Melilotus Taurica slept like those of a Trifolium,
+whilst the leaves on the older branches on the same plants slept in a
+very different manner, proper to the genus; and from the reasons
+assigned we are tempted to look at this case as one of reversion to a
+former nyctitropic habit. So again with Desmodium gyrans, the absence
+of small lateral leaflets on very young plants, makes us suspect that
+the immediate progenitor of this species did not possess lateral
+leaflets, and that their appearance in an almost rudimentary condition
+at a somewhat more advanced age is the result of reversion to a
+trifoliate predecessor. However this may be, the rapid circumnutating
+or
+gyrating movements of the little lateral leaflets, seem to be due
+proximately to the pulvinus, or organ of movement, not having been
+reduced nearly so much as the blade, during the successive
+modifications through which the species has passed.
+
+We now come to the highly important class of movements due to the
+action of a lateral light. When stems, leaves, or other organs are
+placed, so that one side is illuminated more brightly than the other,
+they bend towards the light. This heliotropic movement manifestly
+results from the modification of ordinary circumnutation; and every
+gradation between the two movements could be followed. When the light
+was dim, and only a very little brighter on one side than on the other,
+the movement consisted of a succession of ellipses, directed towards
+the light, each of which approached nearer to its source than the
+previous one. When the difference in the light on the two sides was
+somewhat greater, the ellipses were drawn out into a strongly-marked
+zigzag line, and when much greater the course became rectilinear. We
+have reason to believe that changes in the turgescence of the cells is
+the proximate cause of the movement of circumnutation; and it appears
+that when a plant is unequally illuminated on the two sides, the always
+changing turgescence is augmented along one side, and is weakened or
+quite arrested along the other sides. Increased turgescence is commonly
+followed by increased growth, so that a plant which has bent itself
+towards the light during the day would be fixed in this position were
+it not for apogeotropism acting during the night. But parts provided
+with pulvini bend, as Pfeffer has shown, towards the light; and here
+growth does not come into play any more than in the ordinary
+circumnutating movements of pulvini.
+
+
+Heliotropism prevails widely throughout the vegetable kingdom, but
+whenever, from the changed habits of life of any plant, such movements
+become injurious or useless, the tendency is easily eliminated, as we
+see with climbing and insectivorous plants.
+
+Apheliotropic movements are comparatively rare in a well-marked degree,
+excepting with sub-aërial roots. In the two cases investigated by us,
+the movement certainly consisted of modified circumnutation.
+
+The position which leaves and cotyledons occupy during the day, namely,
+more or less transversely to the direction of the light, is due,
+according to Frank, to what we call diaheliotropism. As all leaves and
+cotyledons are continually circumnutating, there can hardly be a doubt
+that diaheliotropism results from modified circumnutation. From the
+fact of leaves and cotyledons frequently rising a little in the
+evening, it appears as if diaheliotropism had to conquer during the
+middle of the day a widely prevalent tendency to apogeotropism.
+
+Lastly, the leaflets and cotyledons of some plants are known to be
+injured by too much light; and when the sun shines brightly on them,
+they move upwards or downwards, or twist laterally, so that they direct
+their edges towards the light, and thus they escape being injured.
+These paraheliotropic movements certainly consisted in one case of
+modified circumnutation; and so it probably is in all cases, for the
+leaves of all the species described circumnutate in a conspicuous
+manner. This movement has hitherto been observed only with leaflets
+provided with pulvini, in which the increased turgescence on opposite
+sides is not followed by growth; and we can understand why this should
+be so, as the movement is required only for a temporary purpose. It
+would manifestly be
+disadvantageous for the leaf to be fixed by growth in its inclined
+position. For it has to assume its former horizontal position, as soon
+as possible after the sun has ceased shining too brightly on it.
+
+The extreme sensitiveness of certain seedlings to light, as shown in
+our ninth chapter, is highly remarkable. The cotyledons of Phalaris
+became curved towards a distant lamp, which emitted so little light,
+that a pencil held vertically close to the plants, did not cast any
+shadow which the eye could perceive on a white card. These cotyledons,
+therefore, were affected by a difference in the amount of light on
+their two sides, which the eye could not distinguish. The degree of
+their curvature within a given time towards a lateral light did not
+correspond at all strictly with the amount of light which they
+received; the light not being at any time in excess. They continued for
+nearly half an hour to bend towards a lateral light, after it had been
+extinguished. They bend with remarkable precision towards it, and this
+depends on the illumination of one whole side, or on the obscuration of
+the whole opposite side. The difference in the amount of light which
+plants at any time receive in comparison with what they have shortly
+before received, seems in all cases to be the chief exciting cause of
+those movements which are influenced by light. Thus seedlings brought
+out of darkness bend towards a dim lateral light, sooner than others
+which had previously been exposed to daylight. We have seen several
+analogous cases with the nyctitropic movements of leaves. A striking
+instance was observed in the case of the periodic movements of the
+cotyledons of a Cassia; in the morning a pot was placed in an obscure
+part of a room, and all the cotyledons rose up closed; another pot had
+stood in the sunlight, and
+the cotyledons of course remained expanded; both pots were now placed
+close together in the middle of the room, and the cotyledons which had
+been exposed to the sun, immediately began to close, while the others
+opened; so that the cotyledons in the two pots moved in exactly
+opposite directions whilst exposed to the same degree of light.
+
+We found that if seedlings, kept in a dark place, were laterally
+illuminated by a small wax taper for only two or three minutes at
+intervals of about three-quarters of an hour, they all became bowed to
+the point where the taper had been held. We felt much surprised at this
+fact, and until we had read Wiesner’s observations, we attributed it to
+the after-effects of the light; but he has shown that the same degree
+of curvature in a plant may be induced in the course of an hour by
+several interrupted illuminations lasting altogether for 20 m., as by a
+continuous illumination of 60 m. We believe that this case, as well as
+our own, may be explained by the excitement from light being due not so
+much to its actual amount, as to the difference in amount from that
+previously received; and in our case there were repeated alternations
+from complete darkness to light. In this, and in several of the above
+specified respects, light seems to act on the tissues of plants, almost
+in the same manner as it does on the nervous system of animals. There
+is a much more striking analogy of the same kind, in the sensitiveness
+to light being localised in the tips of the cotyledons of Phalaris and
+Avena, and in the upper part of the hypocotyls of Brassica and Beta;
+and in the transmission of some influence from these upper to the lower
+parts, causing the latter to bend towards the light. This influence is
+also
+transmitted beneath the soil to a depth where no light enters. It
+follows from this localisation, that the lower parts of the cotyledons
+of Phalaris, etc., which normally become more bent towards a lateral
+light than the upper parts, may be brightly illuminated during many
+hours, and will not bend in the least, if all light be excluded from
+the tip. It is an interesting experiment to place caps over the tips of
+the cotyledons of Phalaris, and to allow a very little light to enter
+through minute orifices on one side of the caps, for the lower part of
+the cotyledons will then bend to this side, and not to the side which
+has been brightly illuminated during the whole time. In the case of the
+radicles of Sinapis alba, sensitiveness to light also resides in the
+tip, which, when laterally illuminated, causes the adjoining part of
+the root to bend apheliotropically.
+
+Gravitation excites plants to bend away from the centre of the earth,
+or towards it, or to place themselves in a transverse position with
+respect to it. Although it is impossible to modify in any direct manner
+the attraction of gravity, yet its influence could be moderated
+indirectly, in the several ways described in the tenth chapter; and
+under such circumstances the same kind of evidence as that given in the
+chapter on Heliotropism, showed in the plainest manner that
+apogeotropic and geotropic, and probably diageotropic movements, are
+all modified forms of circumnutation.
+
+Different parts of the same plant and different species are affected by
+gravitation in widely different degrees and manners. Some plants and
+organs exhibit hardly a trace of its action. Young seedlings which, as
+we know, circumnutate rapidly, are eminently sensitive; and we have
+seen the hypocotyl of Beta bending
+upwards through 109° in 3 h. 8 m. The after-effects of apogeotropism
+last for above half an hour; and horizontally-laid hypocotyls are
+sometimes thus carried temporarily beyond an upright position. The
+benefits derived from geotropism, apogeotropism, and diageotropism, are
+generally so manifest that they need not be specified. With the
+flower-peduncles of Oxalis, epinasty causes them to bend down, so that
+the ripening pods may be protected by the calyx from the rain.
+Afterwards they are carried upwards by apogeotropism in combination
+with hyponasty, and are thus enabled to scatter their seeds over a
+wider space. The capsules and flower-heads of some plants are bowed
+downwards through geotropism, and they then bury themselves in the
+earth for the protection and slow maturation of the seeds. This burying
+process is much facilitated by the rocking movement due to
+circumnutation.
+
+In the case of the radicles of several, probably of all seedling
+plants, sensitiveness to gravitation is confined to the tip, which
+transmits an influence to the adjoining upper part, causing it to bend
+towards the centre of the earth. That there is transmission of this
+kind was proved in an interesting manner when horizontally extended
+radicles of the bean were exposed to the attraction of gravity for 1 or
+1½ h., and their tips were then amputated. Within this time no trace of
+curvature was exhibited, and the radicles were now placed pointing
+vertically downwards; but an influence had already been transmitted
+from the tip to the adjoining part, for it soon became bent to one
+side, in the same manner as would have occurred had the radicle
+remained horizontal and been still acted on by geotropism. Radicles
+thus treated continued to grow out horizontally for two or three days,
+until a new tip was
+re-formed; and this was then acted on by geotropism, and the radicle
+became curved perpendicularly downwards.
+
+It has now been shown that the following important classes of movement
+all arise from modified circumnutation, which is omnipresent whilst
+growth lasts, and after growth has ceased, whenever pulvini are
+present. These classes of movement consist of those due to epinasty and
+hyponasty,—those proper to climbing plants, commonly called revolving
+nutation,—the nyctitropic or sleep movements of leaves and
+cotyledons,—and the two immense classes of movement excited by light
+and gravitation. When we speak of modified circumnutation we mean that
+light, or the alternations of light and darkness, gravitation, slight
+pressure or other irritants, and certain innate or constitutional
+states of the plant, do not directly cause the movement; they merely
+lead to a temporary increase or diminution of those spontaneous changes
+in the turgescence of the cells which are already in progress. In what
+manner, light, gravitation, etc., act on the cells is not known; and we
+will here only remark that, if any stimulus affected the cells in such
+a manner as to cause some slight tendency in the affected part to bend
+in a beneficial manner, this tendency might easily be increased through
+the preservation of the more sensitive individuals. But if such bending
+were injurious, the tendency would be eliminated unless it was
+overpoweringly strong; for we know how commonly all characters in all
+organisms vary. Nor can we see any reason to doubt, that after the
+complete elimination of a tendency to bend in some one direction under
+a certain stimulus, the power to bend in a directly
+opposite direction might gradually be acquired through natural
+selection.[2]
+
+ [2] See the remarks in Frank’s ‘Die wagerechte Richtung von
+ Pflanzentheilen’ (1870, pp. 90, 91, etc.), on natural selection in
+ connection with geotropism, heliotropism, etc.
+
+
+Although so many movements have arisen through modified circumnutation,
+there are others which appear to have had a quite independent origin;
+but they do not form such large and important classes. When a leaf of a
+Mimosa is touched it suddenly assumes the same position as when asleep,
+but Brucke has shown that this movement results from a different state
+of turgescence in the cells from that which occurs during sleep; and as
+sleep-movements are certainly due to modified circumnutation, those
+from a touch can hardly be thus due. The back of a leaf of Drosera
+rotundifolia was cemented to the summit of a stick driven into the
+ground, so that it could not move in the least, and a tentacle was
+observed during many hours under the microscope; but it exhibited no
+circumnutating movement, yet after being momentarily touched with a bit
+of raw meat, its basal part began to curve in 23 seconds. This curving
+movement therefore could not have resulted from modified
+circumnutation. But when a small object, such as a fragment of a
+bristle, was placed on one side of the tip of a radicle, which we know
+is continually circumnutating, the induced curvature was so similar to
+the movement caused by geotropism, that we can hardly doubt that it is
+due to modified circumnutation. A flower of a Mahonia was cemented to a
+stick, and the stamens exhibited no signs of circumnutation under the
+microscope, yet when they were lightly touched they suddenly moved
+towards the pistil. Lastly, the curling of the extremity of a tendril
+when
+touched seems to be independent of its revolving or circumnutating
+movement. This is best shown by the part which is the most sensitive to
+contact, circumnutating much less than the lower parts, or apparently
+not at all.[3]
+
+ [3] For the evidence on this head, see the ‘Movements and Habits of
+ Climbing Plants,’ 1875, pp. 173, 174.
+
+
+Although in these cases we have no reason to believe that the movement
+depends on modified circumnutation, as with the several classes of
+movement described in this volume, yet the difference between the two
+sets of cases may not be so great as it at first appears. In the one
+set, an irritant causes an increase or diminution in the turgescence of
+the cells, which are already in a state of change; whilst in the other
+set, the irritant first starts a similar change in their state of
+turgescence. Why a touch, slight pressure or any other irritant, such
+as electricity, heat, or the absorption of animal matter, should modify
+the turgescence of the affected cells in such a manner as to cause
+movement, we do not know. But a touch acts in this manner so often, and
+on such widely distinct plants, that the tendency seems to be a very
+general one; and if beneficial, it might be increased to any extent. In
+other cases, a touch produces a very different effect, as with Nitella,
+in which the protoplasm may be seen to recede from the walls of the
+cell; in Lactuca, in which a milky fluid exudes; and in the tendrils of
+certain Vitaceae, Cucurbitaceæ, and Bignoniaceae, in which slight
+pressure causes a cellular outgrowth.
+
+Finally it is impossible not to be struck with the resemblance between
+the foregoing movements of plants and many of the actions performed
+unconsciously by the lower animals.[4] With plants an
+astonishingly small stimulus suffices; and even with allied plants one
+may be highly sensitive to the slightest continued pressure, and
+another highly sensitive to a slight momentary touch. The habit of
+moving at certain periods is inherited both by plants and animals; and
+several other points of similitude have been specified. But the most
+striking resemblance is the localisation of their sensitiveness, and
+the transmission of an influence from the excited part to another which
+consequently moves. Yet plants do not of course possess nerves or a
+central nervous system; and we may infer that with animals such
+structures serve only for the more perfect transmission of impressions,
+and for the more complete intercommunication of the several parts.
+
+ [4] Sachs remarks to nearly the same effect: “Dass sich die lebende
+ Pflanzensubstanz derart innerlich differenzirt, dass einzelne Theile
+ mit specifischen Energien ausgerüstet sind, ähnlich, wie die
+ verschiedenen Sinnesnerven des Thiere” (‘Arbeiten des Bot. Inst. in
+ Würzburg,’ Bd. ii. 1879, p. 282).
+
+
+We believe that there is no structure in plants more wonderful, as far
+as its functions are concerned, than the tip of the radicle. If the tip
+be lightly pressed or burnt or cut, it transmits an influence to the
+upper adjoining part, causing it to bend away from the affected side;
+and, what is more surprising, the tip can distinguish between a
+slightly harder and softer object, by which it is simultaneously
+pressed on opposite sides. If, however, the radicle is pressed by a
+similar object a little above the tip, the pressed part does not
+transmit any influence to the more distant parts, but bends abruptly
+towards the object. If the tip perceives the air to be moister on one
+side than on the other, it likewise transmits an influence to the upper
+adjoining part, which bends towards the source of moisture. When the
+tip is excited by light (though
+in the case of radicles this was ascertained in only a single instance)
+the adjoining part bends from the light; but when excited by
+gravitation the same part bends towards the centre of gravity. In
+almost every case we can clearly perceive the final purpose or
+advantage of the several movements. Two, or perhaps more, of the
+exciting causes often act simultaneously on the tip, and one conquers
+the other, no doubt in accordance with its importance for the life of
+the plant. The course pursued by the radicle in penetrating the ground
+must be determined by the tip; hence it has acquired such diverse kinds
+of sensitiveness. It is hardly an exaggeration to say that the tip of
+the radicle thus endowed, and having the power of directing the
+movements of the adjoining parts, acts like the brain of one of the
+lower animals; the brain being seated within the anterior end of the
+body, receiving impressions from the sense-organs, and directing the
+several movements.
+
+
+
+
+INDEX.
+
+
+
+     A.
+
+     Abies communis, effect of killing or injuring the leading shoot, 187
+     — pectinata, effect of killing or injuring the leading shoot, 187
+     —, affected by Æcidium elatinum, 188
+
+     Abronia umbellata, its single, developed cotyledon, 78
+     —, rudimentary cotyledon, 95
+     —, rupture of the seed coats, 105
+
+     Abutilon Darwinii, sleep of leaves and not of cotyledons, 314
+     —, nocturnal movement of leaves, 323
+
+     Acacia Farnesiana, state of plant when awake and asleep, 381, 382
+     —, appearance at night, 395
+     —, nyctitropic movements of pinnae, 402
+     —, the axes of the ellipses, 404
+     — lophantha, character of first leaf, 415
+     — retinoides, circumnutation of young phyllode, 236
+
+     Acanthosicyos horrida, nocturnal movement of cotyledon 304
+
+     Acanthus candelabrum, inequality in the two first leaves, 79
+     —, petioles not arched, 553
+     — latifolius, variability in first leaves 79
+     — mollis, seedling, manner of breaking through the ground, 78, 79
+     —, circumnutation of young leaf, 249, 269
+     — spinosus, 79
+     —, movement of leaves, 249
+
+     Adenanthera pavonia, nyctitropic movements of leaflets, 374
+
+     Æcidium elatinum, effect on the lateral branches of the silver fir, 188
+
+     Æsculus hippocastanum, movements of radicle, 28, 29
+     —, sensitiveness of apex of radicle, 172–174
+
+     Albizzia lophantha, nyctitropic movements of leaflets, 383
+     —, of pinnae, 402
+
+     Allium cepa, conical protuberance on arched cotyledon, 59
+     —, circumnutation of basal half of arched cotyledon, 60
+     —, mode of breaking through ground, 87
+     —, straightening process, 101
+     — porrum, movements of flower-stems, 226
+
+     Alopecurus pratensis, joints affected by apogeotropism, 503
+
+     Aloysia citriodora, circumnutation of stem, 210
+
+     Amaranthus, sleep of leaves, 387
+     — caudatus, nocturnal movement of cotyledons, 307
+
+     Amorpha fruticosa, sleep of leaflets, 354
+
+     Ampelopsis tricuspidata, hyponastic movement of hooked tips, 272–275
+
+     Amphicarpoea monoica, circumnutation and nyctitropic movements of leaves,
+     365
+     —, effect of sunshine on leaflets, 445
+     —, geotropic movements of, 520
+
+     Anoda Wrightii, sleep of cotyledons, 302, 312
+     —, of leaves, 324
+     —, downward movement of cotyledons, 444
+
+     Apheliotropism, or negative heliotropism, 5, 419, 432
+
+     Apios graveolens, heliotropic movements of hypocotyl, 422–424
+     — tuberosa, vertical sinking of leaflets at night, 368
+
+     Apium graveolens, sleep of cotyledons, 305
+     —, petroselinum, sleep of cotyledons, 304
+
+     Apogeotropic movements effected by joints or pulvini, 502
+
+     Apogeotropism, 5, 494; retarded by heliotropism, 501; concluding remarks
+     on, 507
+
+     Arachis hypogoea, circumnutation of gynophore, 225
+     —, effects of radiation on leaves, 289, 296
+     —, movements of leaves, 357
+     — rate of movement, 404
+     —, circumnutation of vertically dependent young gynophores, 519
+     —, downward movement of the same, 519
+
+     Arching of various organs, importance of, to seedling plants, 87, 88;
+     emergence of hypocotyls or epicotyls in the form of an, 553
+
+     Asparagus officinalis, circumnutation of plumules, 60–62.
+     —, effect of lateral light, 484
+
+     Asplenium trichomanes, movement in the fruiting fronds, 257, n.
+
+     Astragalus uliginosus, movement of leaflets, 355
+
+     Avena sativa, movement of cotyledons, 65, 66.
+     —, sensitiveness of tip of radicle to moist air, 183
+     —, heliotropic movement and circumnutation of cotyledon, 421, 422
+     —, sensitiveness of cotyledon to a lateral light, 477
+     —, young sheath-like cotyledons strongly apogeotropic, 499
+
+     Avena sativa, movements of oldish cotyledons, 499, 500
+
+     Averrhoa bilimbi, leaf asleep, 330
+     —, angular movements when going to sleep, 331–335
+     —, leaflets exposed to bright sunshine, 447
+
+     Azalea Indica, circumnutation of stem, 208
+
+     B.
+
+     Bary, de, on the effect of the Æcidium on the silver fir, 188
+
+     Batalin, Prof., on the nyctitropic movements of leaves, 283; on the sleep
+     of leaves of Sida napoea, 322; on Polygonum aviculare, 387; on the effect
+     of sunshine on leaflets of Oxalis acetosella, 447
+
+     Bauhinia, nyctitropic movements, 373
+     —, movements of petioles of young seedlings, 401
+     —, appearance of young plants at night, 402
+
+     Beta vulgaris, circumnutation of hypocotyl of seedlings, 52
+     —, movements of cotyledons, 52, 53
+     —, effect of light, 124
+     —, nocturnal movement of cotyledons, 307
+     —, heliotropic movements of, 420
+     —, transmitted effect of light on hypocotyl, 482
+     —, apogeotropic movement of hypocotyl, 496
+
+     Bignonia capreolata, apheliotropic movement of tendrils, 432, 450
+
+     Bouché on Melaleuca ericaefolia, 383
+
+     Brassica napus, circumnutation of flower-stems, 226
+
+     Brassica oleracea, circumnutation of seedling, 10
+     —, of radicle, 11
+     —, geotropic movement of radicle, 11
+     —, movement of buried and arched hypocotyl, 13, 14, 15
+     —, conjoint circumnutation of hypocotyl and cotyledons, 16, 17, 18
+     —, of hypocotyl in darkness, 19
+     —, of a cotyledon with hypocotyl secured to a stick, 19, 20
+     —, rate of movement, 20
+     —, ellipses described by hypocotyls when erect, 105
+     —, movements of cotyledons, 115
+     —, — of stem, 202
+     —, — of leaves at night, 229, 230
+     —, sleep of cotyledons, 301
+     —, circumnutation of hypocotyl of seedling plant, 425
+     —, heliotropic movement and circumnutation of hypocotyls, 426
+     —, effect of lateral light on hypocotyls, 479–482
+     —, apogeotropic movement of hypocotyls, 500, 501
+
+     Brassica rapa, movements of leaves, 230
+
+     Brongniart, A., on the sleep of Strephium floribundum, 391
+
+     Bruce, Dr., on the sleep of leaves in Averrhoa, 330
+
+     Bryophyllum (vel Calanchoe) calycinum, movement of leaves, 237
+
+     C.
+
+     Camellia Japonica, circumnutation of leaf, 231, 232
+
+     Candolle, A. de, on Trapa natans, 95; on sensitiveness of cotyledons, 127
+
+     Canna Warscewiczii, circumnutation of plumules, 58, 59
+     —, of leaf, 252
+
+     Cannabis sativa, movements of leaves, 250
+     —, nocturnal movements of cotyledons, 307
+     Cannabis sativa, sinking of the young leaves at night, 444
+
+     Cassia, nyctitropic movement of leaves, 369
+
+     Cassia Barclayana, nocturnal movement of leaves, 372
+     —, slight movement of leaflets, 401
+     — calliantha, uninjured by exposure at night, 289, n.
+     —, nyctitropic movement of leaves, 371
+     — circumnutating movement of leaves, 372
+     — corymbosa, cotyledons sensitive to contact, 126
+     —, nyctitropic movement of leaves, 369
+     — floribunda, use of sleep movements, 289
+     —, effect of radiation on the leaves at night, 294
+     —, circumnutating and nyctitropic movement of a terminal leaflet, 372, 373
+     —, movements of young and older leaves, 400
+     — florida, cotyledons sensitive to contact, 126
+     —, sleep of cotyledons, 308
+     — glauca, cotyledons sensitive to contact, 126
+     —, sleep of cotyledons, 308
+     — laevigata, effect of radiation on leaves, 289, n.
+     — mimosoides, movement of cotyledons. 116
+     —, sensitiveness of, 126
+     —, sleep of, 308
+     —, nyctitropic movement of leaves, 372
+     —, effect of bright sunshine on cotyledons, 446
+     — neglecta, movements of, 117
+     —, effect of light, 124
+     —, sensitiveness of cotyledons, 126
+     — nodosa, non-sensitive cotyledons, 126
+     —, do not rise at night, 308
+     — pubescens, non-sensitive cotyledons, 126
+
+     Cassia pubescens, uninjured by exposure at night, 293
+     —, sleep of cotyledons, 308
+     —, nyctitropic movement of leaves, 371
+     —, circumnutating movement of leaves, 372
+     —, nyctitropic movement of petioles, 400
+     —, diameter of plant at night, 402
+     — sp. (?) movement of cotyledons, 116
+     — tora, circumnutation of cotyledons and hypocotyls, 34, 35, 109, 308
+     —, effect of light, 124, 125
+     —, sensitiveness to contact, 125
+     —, heliotropic movement and circumnutation of hypocotyl, 431
+     —, hypocotyl of seedling slightly heliotropic, 454
+     —, apogeotropic movement of old hypocotyl, 497
+     —, movement of hypocotyl of young seedling, 510
+
+     Caustic (nitrate of silver), effect of, on radicle of bean, 150, 156; on
+     the common pea, 160.
+
+     Cells, table of the measurement of, in the pulvini of Oxalis corniculata,
+     120; changes in, 547
+
+     Centrosema, 365
+
+     Ceratophyllum demersum, movements of stem, 211
+
+     Cereus Landbeckii, its rudimentary cotyledons, 97
+     — speciossimus, circumnutation of stem, 206, 207
+
+     Cerinthe major, circumnutation of hypocotyl, 49
+     —, of cotyledons, 49
+     —, ellipses described by hypocotyls when erect, 107
+     — effect of darkness, 124
+
+     Chatin, M., on Pinus Nordmanniana, 389
+
+     Chenopodium album, sleep of leaves but not of cotyledons, 314, 319
+
+     Chenopodium album, movement of leaves, 387
+
+     Chlorophyll injured by bright light, 446
+
+     Ciesielski, on the sensitiveness of the tip of the radicles, 4, 523
+
+     Circumnutation, meaning explained, 1; modified, 263–279; and heliotropism,
+     relation between, 435; of paramount importance to every plant, 547
+
+     Cissus discolor, circumnutation of leaf, 233
+
+     Citrus aurantium, circumnutation of epicotyl, 28
+     —, unequal cotyledons, 95
+
+     Clianthus Dampieri, nocturnal movement of leaves, 297
+
+     Cobœa scandens, circumnutation of, 270
+
+     Cohn, on the water secreted by Lathraea squamaria, 86, n.; on the movement
+     of leaflets of Oxalis, 447
+
+     Colutea arborea, nocturnal movement of leaflets, 355
+
+     Coniferæ, circumnutation of, 211
+     Coronilla rosea, leaflets asleep, 355
+
+     Corylus avellana, circumnutation of young shoot, emitted from the epicotyl,
+     55, 56
+     —, arched epicotyl, 77
+
+     Cotyledon umbilicus, circumnutation of stolons, 219, 220
+
+     Cotyledons, rudimentary, 94–98; circumnutation of, 109–112; nocturnal
+     movements, 111, 112; pulvini or joints of, 112–122; disturbed periodic
+     movements by light, 123; sensitiveness of, to contact, 125; nyctitropic
+     movements of, 283, 297; list of cotyledons which rise or sink at night,
+     300; concluding remarks on their movements, 311
+
+     Crambe maritima, circumnutation of leaves, 228, 229
+
+     Crinum Capense, shape of leaves, 253
+     —, circumnutation of, 254
+
+     Crotolaria (sp.?), sleep of leaves, 340
+
+     Cryptogams, circumnutation of, 257–259
+
+     Cucumis dudaim, movement of cotyledons, 43, 44
+     —, sleep of cotyledons, 304
+
+     Cucurbita aurantia, movement of hypocotyl, 42
+     —, cotyledons vertical at night, 304
+     —, ovifera, geotropic movement of radicle, 38, 39
+     —, circumnutation of arched hypocotyl, 39
+     —, of straight and vertical hypocotyl, 40
+     —, movements of cotyledons, 41, 42, 115, 124
+     —, position of radicle, 89
+     —, rupture of the seed-coats, 102
+     —, circumnutation of hypocotyl when erect, 107, 108
+     —, sensitiveness of apex of radicle, 169–171
+     —, cotyledons vertical at night, 304
+     —, not affected by apogeotropism, 509
+     —, tips cauterised transversely, 537
+
+     Curvature of the radicle, 193
+
+     Cycas pectinata, circumnutation of young leaf, whilst emerging from the
+     ground, 58
+     —, first leaf arched, 78
+     —, circumnutation of terminal leaflets, 252
+
+     Cyclamen Persicum, movement of cotyledon, 46
+     —, undeveloped cotyledons, 78, 96
+     —, circumnutation of peduncle, 225
+     —, —, of leaf, 246, 247
+     —, downward apheliotropic movement of a flower-peduncle, 433–435
+
+     Cyclamen Persicum, burying of the pods, 433
+
+     Cyperus alternifolius, circumnutation of stem, 212
+     —, movement of stem, 509
+
+     Cytisus fragrans, circumnutation of hypocotyl, 37
+     —, sleep of leaves, 344, 397
+     —, apogeotropic movement of stem, 494–496
+     +
+     D.
+
+     Dahlia, circumnutation of young leaves, 244–246
+
+     Dalea alopecuroides, leaflets depressed at night, 354
+
+     Darkness, effect of, on the movement of leaves, 407
+
+     Darlingtonia Californica, its leaves or pitchers apheliotropic, 450, n.
+
+     Darwin, Charles, on Maurandia semperflorens, 225; on the Swedish turnip,
+     230, n.; movements of climbing plants, 266, 271; the heliotropic movement
+     of the tendrils of Bignonia capreolata, 433; revolution of climbing plants,
+     451; on the curling of a tendril, 570
+     —, Erasmus, on the peduncles of Cyclamens, 433
+     —, Francis, on the radicle of Sinapis alba, 486; on Hygroscopic seeds,
+     489, n.
+
+     Datura stramonium, nocturnal movement of cotyledons, 298
+
+     Delpino, on cotyledons of Chaerophyllum and Corydalis, 96, n.
+
+     Delphinium nudicaule, mode of breaking through the ground, 80
+     —, confluent petioles of two cotyledons, 553
+
+     Desmodium gyrans, movement of leaflets, 257, n.
+     —, position of leaves at night, 285
+     —, sleep of leaves, not of cotyledons, 314
+     —, circumnutation and nyctitropic movement of leaves, 358–360
+     —, movement of lateral leaflets, 361
+     —, jerking of leaflets, 362
+     — nyctitropic movement of petioles, 400, 401
+     —, diameter of plant at night, 402
+     —, lateral movement of leaves, 404
+     —, zigzag movement of apex of leaf, 405
+     —, shape of lateral leaflet, 416
+     —, vespertilionis, 364, n.
+
+     Deutzia gracilis, circumnutation of stem, 205
+
+     Diageotropism, 5; or transverse-geotropism, 520
+
+     Diaheliotropism, 5; or Transversal-Heliotropismus of Frank, 419; influenced
+     by epinasty, 439; by weight and apogeotropism, 440
+
+     Dianthus caryophyllus, 230
+     —, circumnutation of young leaf, 231, 269
+
+     Dicotyledons, circumnutation widely spread among, 68
+
+     Dionoea, oscillatory movements of leaves, 261, 271
+
+     Dionoea muscipula, circumnutation of young expanding leaf, 239, 240
+     —, closure of the lobes and circumnutation of a full-grown leaf, 241
+     —, oscillations of, 242–244
+
+     Diurnal sleep, 419
+
+     Drosera Capensis, structure of first-formed leaves, 414
+     — rotundifolia, movement of young leaf, 237, 238
+     —, of the tentacles, 239
+     —, sensitiveness of tentacles, 261
+     —, shape of leaves, 414
+     —, leaves not heliotropic, 450
+     —, leaves circumnutate largely, 454
+     —, sensitiveness of 570
+
+     Duchartre on Trephrosia cariboea, 354; on the nyctitropic movement of the
+     Cassia, 369
+
+     Duval-Jouve, on the movements of Bryophyllum calycinum, 237; of the narrow
+     leaves of the Gramineæ, 413
+
+     Dyer, Mr. Thiselton, on the leaves of Crotolaria, 340; on Cassia
+     floribunda, 369, n., on the absorbent hairs on the buried flower-heads of
+     Trifolium subterraneum, 517
+
+     E.
+
+     Echeveria stolonifera, circumnutation of leaf, 237
+
+     Echinocactus viridescens, its rudimentary cotyledons, 97
+
+     Echinocystis lobata, movements of tendrils, 266
+     —, apogeotropism of tendrils, 510
+
+     Elfving, F., on the rhizomes of Sparganium ramosum, 189; on the
+     diageotropic movement in the rhizomes of some plants, 521
+
+     Elymus arenareus, leaves closed during the day, 413
+
+     Embryology of leaves, 414
+
+     Engelmann, Dr., on the Quercus virens, 85
+
+     Epinasty, 5, 267
+
+     Epicotyl, or plumule, 5; manner of breaking through the ground, 77; emerges
+     from the ground under the form of an arch, 553
+
+     Erythrina caffra, sleep of leaves, 367
+     — corallodendron, movement of terminal leaflet, 367
+     — crista-galli, effect of temperature on sleep of leaves, 318
+     —, circumnutation and nyctitropic movement of terminal leaflets, 367
+
+     Eucalyptus resinifera, circumnutation of leaves, 244
+
+     Euphorbia jacquineaeflora, nyctitropic movement of leaves, 388
+
+     F.
+
+     Flahault, M., on the rupture of seed-coats, 102–104, 106
+
+     Flower-stems, circumnutation of, 223–226
+
+     Fragaria Rosacea, circumnutation of stolon, 214–218
+
+     Frank, Dr. A. B., the terms Heliotropism and Geotropism, first used by him,
+     5, n.; radicles acted on by geotropism, 70, n.; on the stolons of Fragaria,
+     215; periodic and nyctitropic movements of leaves, 284; on the root-leaves
+     of plants kept in darkness, 443; on pulvini, 485; on natural selection in
+     connection with geotropism, heliotropism, etc., 570
+     —, on Transversal-Heliotropismus, 419
+
+     Fuchsia, circumnutation of stem, 205, 206
+
+     G.
+
+     Gazania ringens, circumnutation of stem, 208 Genera containing sleeping
+     plants, 320, 321
+
+     Geotropism, 5; effect of, on the primary radicle, 196; the reverse of
+     apogeotropism, 512: effect on the tips of radicles, 543
+
+     Geranium cinereum, 304
+     — Endressii, 304
+     — Ibericum, nocturnal movement of cotyledons, 298
+     — Richardsoni, 304
+     — rotundifolium, nocturnal movement of cotyledon, 304, 312
+     — subcaulescens, 304
+
+     Germinating seed, history of a, 548
+
+     Githago segetum, circumnutation of hypocotyl, 21, 108
+     —, burying of hypocotyl, 109
+     —, seedlings feebly illuminated, 124, 128
+     —, sleep of cotyledon, 302
+     —, — leaves 321
+
+     Glaucium luteum, circumnutation of young leaves, 228
+
+     Gleditschia, sleep of leaves, 368
+
+     Glycine hispida, vertical sinking of leaflets, 366
+
+     Glycyrrhiza, leaflets depressed at night, 355
+
+     Godlewski, Emil, on the turgescence of the cells, 485
+
+     Gooseberry, effect of radiation, 284
+
+     Gossypium (var. Nankin cotton), circumnutation of hypocotyl, 22
+     —, movement of cotyledon, 22, 23
+     —, sleep of leaves, 324
+     —, arboreum (?), sleep of cotyledons, 303
+     —, Braziliense, nocturnal movement of leaves, 324
+     —, sleep of cotyledons, 303
+     — herbaceum, sensitiveness of apex of radicle, 168
+     —, radicles cauterised transversely, 537
+     — maritimum, nocturnal movement of leaves, 324
+
+     Gravitation, movements excited by, 567
+
+     Gray, Asa, on Delphinium nudicaule, 80; on Megarrhiza Californica, 81; on
+     the movements in the fruiting fronds of Aesplenium trichomanes, 257; on the
+     Amphicarpoea monoica, 520; on the Ipomœa Jalappa, 557
+
+     Grease, effect of, on radicles and their tips, 182, 185
+
+     Gressner, Dr. H., on the cotyledons of Cyclamen Persicum, 46, 77; on
+     hypocotyl of the same, 96
+
+     Gymnosperms, 389
+
+     H.
+
+     Haberlandt, Dr., on the protuberance on the hypocotyl of Allium, 59; the
+     importance of the arch to seedling plants, 87; sub-aërial and subterranean
+     cotyledons, 110, n.; the arched hypocotyl, 554
+
+     Haematoxylon Campechianum, nocturnal movement of leaves, 368, 369
+
+     Hedera helix, circumnutation of stem, 207
+
+     Hedysarum coronarium, nocturnal movements of leaves, 356
+
+     Helianthemum prostratum, geotropic movement of flower-heads, 518
+
+     Helianthus annuus, circumnutation of hypocotyl, 45
+     —, arching of hypocotyl, 90
+     —, nocturnal movement of cotyledons, 305
+
+     Heliotropism, 5; uses of, 449; a modified form of circumnutation, 490
+
+     Helleborus niger, mode of breaking through the ground, 86
+
+     Hensen, Prof., on roots in worm-burrows, 72
+
+     Henslow, Rev. G., on the cotyledons of Phalaris Canariensis, 62
+
+     Hofmeister, on the curious movement of Spirogyra, 3, 259, n.; of the leaves
+     of Pistia stratiotes, 255; of cotyledons at night, 297; of petals, 414
+     — and Batalin on the movements of the cabbage, 229
+
+     Hooker, Sir J., on the effect of light on the pitchers of Sarracenia, 450
+
+     Hypocotyl, 5; manner of breaking through the ground, 77; emerges under the
+     form of an arch, 553
+
+     Hypocotyls and Epicotyls, circumnutation and other movements when arched,
+     98; power of straightening themselves, 100; rupture of the seed-coats,
+     102–106; illustration of, 106; circumnutation when erect, 107; when in
+     dark, 108
+
+     Hyponasty, 6, 267
+
+     I.
+
+     Iberis umbellata, movement of stem, 202.
+
+     Illumination, effect of, on the sleep of leaves, 398
+
+     Imatophyllum vel Clivia (sp.?), movement of leaves, 255
+
+     Indigofera tinctoria, leaflets depressed at night, 354
+
+     Inheritance in plants, 407, 491
+
+     Insectivorous and climbing plants not heliotropic, 450; influence of light
+     on, 488
+
+     Ipomœa bona nox, arching of hypocotyl, 90
+     —, nocturnal position of cotyledons, 306, 312
+     — coerulea vel Pharbitis nil, circumnutation of seedlings, 47
+     —, movement of cotyledons, 47–49, 109
+     —, nocturnal movements of cotyledons, 305
+     —, sleep of leaves, 386
+     —, sensitiveness to light, 451
+     —, the hypocotyledonous stems heliotropic, 453
+     — coccinea, position of cotyledons at night, 306, 312
+     — leptophylla, mode of breaking through the ground, 83, 84
+     —, arching of the petioles of the cotyledons, 90
+     —, difference in sensitiveness to gravitation in different parts, 509
+     —, extraordinary manner of germination, 557
+
+     Ipomœa pandurata, manner of germination, 84, 557
+     — purpurea (vel Pharbitis hispida), nocturnal movement of cotyledons, 305,
+     312
+     —, sleep of leaves, 386
+     —, sensitiveness to light, 451
+     —, the hypocotyledonous stems heliotropic, 453
+
+     Iris pseudo-acorus, circumnutation of leaves, 253
+
+     Irmisch, on cotyledons of Ranunculus Ficaria, 96
+
+     Ivy, its stems heliotropic, 451
+
+     K.
+
+     Kerner on the bending down of peduncles, 414
+
+     Klinostat, the, an instrument devised by Sachs to eliminate geotropism, 93
+
+     Kraus, Dr. Carl, on the underground shoots of Triticum repens, 189; on
+     Cannabis sativa, 250, 307, 312; on the movements of leaves, 318
+
+     L.
+
+     Lactuca scariola, sleep of cotyledons, 305
+
+     Lagenaria vulgaris, circumnutation of seedlings, 42
+     —, of cotyledons, 43
+     —, cotyledons vertical at night, 304
+
+     Lathraea squamaria, mode of breaking through the ground, 85
+     —, quantity of water secreted, 85, 86, n.
+
+     Lathyrus nissolia, circumnutation of stem of young seedling, 33
+     —, ellipses described by, 107, 108
+
+     Leaves, circumnutation of, 226–262; dicotyledons, 226–252; monocotyledons,
+     252–257; nyctitropism of, 280; their temperature affected by their position
+     at night, 294; nyctitropic or sleep movements, 315, 394; periodicity of
+     their movements inherited, 407; embryology of, 414; so-called diurnal
+     sleep, 445
+
+     Leguminosae, sleep of cotyledons, 308; sleeping species, 340
+
+     Le Maout and Decaisne, 67
+
+     Lepidium sativum, sleep of cotyledons, 302
+
+     Light, movements excited by 418, 563; influence on most vegetable tissues,
+     486; acts on plant as on the nervous system of animals, 487
+
+     Lilium auratum, circumnutation of stem, 212
+     —, apogeotropic movement of stem, 498, 499
+
+     Linnæus, ‘Somnus Plantarum’, 280; on plants sleeping, 320; on the leaves
+     of Sida abutilon, 324; on Œnothera mollissima, 383
+
+     Linum Berendieri, nocturnal movement of cotyledons, 298
+     — usitatissimum, circumnutation of stem, 203
+
+     Lolium perenne, joints affected by apogeotropism, 502
+
+     Lonicera brachypoda, hooking of the tip, 272
+     —, sensitiveness to light, 453
+
+     Loomis, Mr., on the movements in the fruiting fronds of Asplenium
+     trichomanes, 257
+
+     Lotus aristata, effect of radiation on leaves, 292
+     — Creticus, leaves awake and asleep, 354
+     — Gebelii, nocturnal movement of cotyledons, 308
+     —, leaflets provided with pulvini, 353
+     — Jacobæus, movements of cotyledons, 35, 109
+     —, pulvini of, 115
+
+     Lotus Jacobæus, movements at night, 116, 121, 124
+     —, development of pulvini, 122
+     —, sleep of cotyledons, 308, 313
+     —, nyctitropic movement of leaves, 353
+     — major, sleep of leaves, 353
+     — perigrinus, movement of leaflets, 353
+
+     Lunularia vulgaris, circumnutation of fronds, 258
+
+     Lupinus, 340
+     — albifrons, sleep of leaves, 344
+     — Hartwegii, sleep of leaves, 341
+     — luteus, circumnutation of cotyledons, 38, 110
+     —, effect of darkness, 124
+
+     Lupinus, position of leaves when asleep, 341
+     —, different positions of leaves at night, 343
+     —, varied movements of leaves and leaflets, 395
+     — Menziesii, sleep of leaves, 343
+     — mutabilis, sleep of leaves, 343
+     — nanus, sleep of leaves, 343
+     — pilosus, sleep of leaves, 340, 341
+     — polyphyllus, sleep of leaves, 343
+     — pubescens, sleep of leaves by day and night, 342
+     —, position of petioles at night, 343
+     —, movements of petioles, 401
+     — speciosus, circumnutation of leaves, 236
+
+     Lynch, Mr. R., on Pachira aquatica, 95, n.; sleep movements of Averrhoa,
+     330
+
+     M.
+
+     Maranta arundinacea, nyctitropic movement of leaves, 389–391
+     —, after much agitation do not sleep, 319
+
+     Marsilia quadrifoliata, effect of radiation at night, 292
+     —, circumnutation and nyctitropic movement of leaflets, 392–394
+     —, rate of movement, 404
+
+     Martins, on radiation at night, 284, n.
+
+     Masters, Dr. Maxwell, on the leading shoots of the Coniferæ, 211
+
+     Maurandia semperflorens, circumnutation of peduncle, 225
+     Medicago maculata, nocturnal position of leaves, 345
+     — marina, leaves awake and asleep, 344
+
+     Meehan, Mr., on the effect of an Æcidium on Portulaca oleracea, 189
+
+     Megarrhiza Californica, mode of breaking through the ground, 81
+     —, germination described by Asa Gray, 82
+     —, singular manner of germination, 83, 556
+
+     Melaleuca ericaefolia, sleep of leaves, 383
+
+     Melilotus, sleep of leaves, 345
+     — alba, sleep of leaves, 347
+     — coerulea, sleep of leaves, 347
+     — dentata, effect of radiation at night, 295
+     — elegans, sleep of leaves, 347
+     — gracilis, sleep of leaves, 347
+     — infesta, sleep of leaves, 347
+     — Italica, leaves exposed at night, 291
+     —, sleep of leaves, 347
+     — macrorrhiza, leaves exposed at night, 292
+     —, sleep of leaves, 347
+     — messanensis, sleep of leaves on full-grown and young plants, 348, 416
+     — officinalis, effect of exposure of leaves at night, 290, 296
+     —, nocturnal movement of leaves, 346, 347
+     —, circumnutation of leaves, 348
+     —, movement of petioles, 401
+
+     Melilotus parviflora, sleep of leaves, 347
+     — Petitpierreana, leaves exposed at night, 291, 296
+     —, sleep of leaves, 347
+     — secundiflora, sleep of leaves, 347
+     — suaveolens, leaves exposed at night, 291
+     —, sleep of leaves, 347
+     — sulcata, sleep of leaves, 347
+     — Taurica, leaves exposed at night, 291
+     —, sleep of leaves, 347, 415
+
+     Methods of observation, 6
+
+     Mimosa albida, cotyledons vertical at night, 116
+     —, not sensitive to contact, 127
+     —, sleep of cotyledons, 308
+     —, rudimentary leaflets, 364
+     —, nyctitropic movements of leaves, 379, 380
+     —, circumnutation of the main petiole of young leaf, 381
+     —, torsion, or rotation of leaves and leaflets, 400
+     —, first true leaf, 416
+     —, effect of bright sunshine on basal leaflets, 445
+     — marginata, nyctitropic movements of leaflets, 381
+     — pudica, movement of cotyledons, 105
+     —, rupture of the seed-coats, 105
+     —, circumnutation of cotyledons, 109
+     —, pulvini of, 113, 115
+     —, cotyledons vertical at night, 116
+     —, hardly sensitive to contact, 127
+     —, effect of exposure at night, 293
+     —, nocturnal movement of leaves, 297
+     —, sleep of cotyledons, 308
+     —, circumnutation and nyctitropic movement of main petiole, 374–378
+     —, of leaflets, 378
+
+     Mimosa albida, circumnutation and nyctitropic movement of pinnae, 402
+     —, number of ellipses described in given time, 406
+     —, effect of bright sunshine on leaflets, 446
+
+     Mirabilis jalapa and longiflora, nocturnal movements of cotyledons, 307
+     —, nyctitropic movement of leaves, 387
+
+     Mohl, on heliotropism in tendrils, stems, and twining plants, 451
+
+     Momentum-like movement, the accumulated effects of apogeotropism, 508
+
+     Monocotyledons, sleep of leaves, 389
+
+     Monotropa hypopitys, mode of breaking through the ground, 86
+
+     Morren, on the movements of stamens of Sparmannia and Cereus, 226
+
+     Müller, Fritz, on Cassia tora, 34; on the circumnutation of Linum
+     usitatissimum, 203; movements of the flower-stems of an Alisma, 226
+
+     Mutisia clematis, movement of leaves, 246
+     —, leaves not heliotropic, 451
+
+     N.
+
+     Natural selection in connection with geotropism, heliotropism, etc., 570
+
+     Nephrodium molle, circumnutation of very young frond, 66
+     —, of older frond, 257
+     —, slight movement of fronds, 509
+
+     Neptunia oleracea, sensitiveness to contact, 128
+     —, nyctitropic movement of leaflets, 374
+     —, of pinnae, 402
+
+     Nicotiana glauca, sleep of leaves, 385, 386
+     —, circumnutation of leaves, 386
+
+     Nobbe, on the rupture of the seed-coats in a seedling of Martynia, 105
+
+     Nolana prostrata, movement of seedlings in the dark, 50
+     —, circumnutation of seedling, 108
+
+     Nyctitropic movement of leaves, 560
+
+     Nyctitropism, or sleep of leaves, 281; in connection with radiation, 286;
+     object gained by it, 413
+
+     O.
+
+     Observation, methods of, 6
+
+     Œnothera mollissima, sleep of leaves, 383
+
+     Opuntia basilaris, conjoint circumnutation of hypocotyl and cotyledon, 44
+     —, thickening of the hypocotyl, 96
+     —, circumnutation of hypocotyl when erect, 107
+     —, burying of, 109
+
+     Orange, seedling, circumnutation of, 510
+
+     Orchis pyramidalis, complex movement of pollinia, 489
+
+     Oxalis acetosella, circumnutation of flower-stem, 224
+     —, effects of exposure to radiation at night, 287, 288, 296
+     —, circumnutation and nyctitropic movement in full-grown leaf, 326
+     —, circumnutation of leaflet when asleep, 327
+     —, rate of circumnutation of leaflets, 404
+     —, effect of sunshine on leaflets, 447
+     —, circumnutation of peduncle, 506
+     Oxalis acetosella, seed-capsules, only occasionally buried, 518
+     — articulata, nocturnal movements of cotyledons, 307
+     — (Biophytum) sensitiva, rapidity of movement of cotyledons during the
+     day, 26
+     —, pulvinus of, 113
+     —, cotyledons vertical at night, 116, 118
+     — bupleurifolia, circumnutation of foliaceous petiole, 328
+     —, nyctitropic movement of terminal leaflet, 329
+     — carnosa, circumnutation of flower-stem, 223
+     —, epinastic movements of flower-stem, 504
+     —, effect of exposure at night, 288, 296
+     —, movements of the flower-peduncles due to apogeotropism and other
+     forces, 503–506
+     — corniculata (var. cuprea), movements of cotyledons, 26
+     —, rising of cotyledons, 116
+     —, rudimentary pulvini of cotyledons, 119
+     —, development of pulvinus, 122
+     —, effect of dull light, 124
+     —, experiments on leaves at night, 288
+     — floribunda, pulvinus of cotyledons, 114
+     —, nocturnal movement, 118, 307, 313
+     — fragrans, sleep of leaves, 324
+     — Ortegesii, circumnutation of flower-stems, 224
+     —, sleep of large leaves, 327
+     —, diameter of plant at night, 402
+     —, large leaflets affected by bright sunshine, 447
+      — Plumierii, sleep of leaves, 327
+     — purpurea, exposure of leaflets at night, 293
+     — rosea, circumnutation of cotyledons, 23, 24
+
+     Oxalis rosea, pulvinus of, 113
+     —, movement of cotyledons at night, 117, 118, 307
+     —, effect of dull light, 124
+     —, non-sensitive cotyledons, 127
+     — sensitiva, movement of cotyledons, 109, 127, 128
+     —, circumnutation of flower-stem, 224
+     —, nocturnal movement of cotyledons, 307, 312
+     —, sleep of leaves, 327
+     — tropoeoloides, movement of cotyledons at night, 118, 120
+     — Valdiviana, conjoint circumnutation of cotyledons and hypocotyl, 25
+     —, cotyledons rising vertically at night, 114, 115, 117, 118
+     —, non-sensitive cotyledons, 127
+     —, nocturnal movement of cotyledon, 307, 312
+     —, sleep of leaves and not of cotyledons, 315
+     —, movements of leaves, 327
+
+     P.
+
+     Pachira aquatica, unequal cotyledons, 95, n.
+
+     Pancratium littorale, movement of leaves, 255
+
+     Paraheliotropism, or diurnal sleep of leaves, 445
+
+     Passiflora gracilis, circumnutation and nyctitropic movement of leaves,
+     383, 384
+     —, apogeotropic movement of tendrils, 510
+     —, sensitiveness of tendrils, 550
+     Pelargonium zonale, circumnutation of stem, 203
+     —, and downward movement of young leaf, 232, 233, 269
+
+     Petioles, the rising of beneficial to plant at night, 402
+
+     Petunia violacea, downward movement and circumnutation of very young leaf,
+     248, 249, 269.
+
+     Pfeffer, Prof., on the turgescence of the cells, 2; on pulvini of leaves,
+     113, 117; sleep movements of leaves, 280, 283, 284; nocturnal rising of
+     leaves of Malva, 324; movements of leaflets in Desmodium gyrans, 358; on
+     Phyllanthus Niruri, 388; influence of a pulvinus on leaves, 396; periodic
+     movements of sleeping leaves, 407, 408; movements of petals, 414; effect of
+     bright sunshine on leaflets of Robinia, 445; effect of light on parts
+     provided with pulvini, 363
+
+     Phalaris Canariensis, movements of old seedlings, 62
+     —, circumnutation of cotyledons, 63, 64, 108
+     —, heliotropic movement and circumnutation of cotyledon towards a dim
+     lateral light, 427
+     —, sensitiveness of cotyledon to light, 455
+     —, effect of exclusion of light from tips of cotyledons, 456
+     —, manner of bending towards light, 457
+     —, effects of painting with Indian ink, 467
+     —, transmitted effects of light, 469
+     —, lateral illumination of tip, 470
+     —, apogeotropic movement of the sheath-like cotyledons, 497
+     —, change from a straight upward apogeotropic course to circumnutation,
+     499
+     —, apogeotropic movement of cotyledons, 500
+
+     Phaseolus Hernandesii, nocturnal movement of leaves and leaflets, 368
+     —, caracalla, 93
+     —, nocturnal movement of leaves, 368
+     —, effect of bright sunshine on leaflets, 446
+
+     Phaseolus multiflorus, movement of radicles, 29
+     —, of young radicle, 72
+     —, of hypocotyl, 91, 93
+     —, sensitiveness of apex of radicle, 163–167
+     —, to moist air, 181
+     —, cauterisation and grease on the tips, 535
+     —, nocturnal movement of leaves, 368
+     —, nyctitropic movement of the first unifoliate leaves, 397
+     — Roxburghii, effect of bright sunshine on first leaves, 445
+     —, vulgaris, 93
+     —, sleep of leaves, 318
+     —, vertical sinking of leaflets at night, 368
+
+     Phyllanthus Niruri, sleep of leaflets, 388
+     — linoides, sleep of leaves, 387
+
+     Pilocereus Houlletii, rudimentary cotyledons, 97
+
+     Pimelia spectabilis, sleep of leaves, 387
+
+     Pincers, wooden, through which the radicle of a bean was allowed to grow,
+     75
+
+     Pinus austriaca, circumnutation of leaves, 251, 252
+     — Nordmanniana, nyctitropic movement of leaves, 389
+     — pinaster, circumnutation of hypocotyl, 56
+     —, movement of two opposite cotyledons, 57
+     —, circumnutation of young leaf, 250, 251
+     —, epinastic downward movement of young leaf, 270
+
+     Pistia stratiotes, movement of leaves, 255
+
+     Pisum sativum, sensitiveness of apex of radicle, 158
+     —, tips of radicles cauterised transversely, 534
+
+     Plants, sensitiveness to light, 449; hygroscopic movements of, 489
+
+     Plants, climbing, circumnutation of, 264; movements of, 559
+     —, mature, circumnutation of, 201–214
+
+     Pliny on the sleep-movements of plants, 280
+
+     Plumbago Capensis, circumnutation of stem, 208, 209
+
+     Poinciana Gilliesii, sleep of leaves, 368
+
+     Polygonum aviculare, leaves vertical at night, 387
+     — convolvulus, sinking of the leaves at night, 318
+
+     Pontederia (sp.?), circumnutation of leaves, 256
+
+     Porlieria hygrometrica, circumnutation and nyctitropic movements of petiole
+     of leaf, 335, 336
+     —, effect of watering, 336–338
+     —, leaflets closed during the day, 413
+
+     Portulaca oleracea, effect of Æcidium on, 189
+
+     Primula Sinensis, conjoint circumnutation of hypocotyl and cotyledon, 45,
+     46
+
+     Pringsheim on the injury to chlorophyll, 446
+
+     Prosopis, nyctitropic movements of leaflets, 374
+     Psoralea acaulis, nocturnal movements of leaflets, 354
+
+     Pteris aquilina, rachis of, 86
+
+     Pulvini, or joints; of cotyledons, 112–122; influence of, on the movements
+     of cotyledons, 313; effect on nyctitropic movements, 396
+
+     Q.
+
+     Quercus (American sp.), circumnutation of young stem, 53, 54
+     — robur, movement of radicles, 54, 55
+     —, sensitiveness of apex of radicle, 174–176
+
+     Quercus virens, manner of germination, 85, 557
+
+     R.
+
+     Radiation at night, effect of, on leaves, 284–286
+
+     Radicles, manner in which they penetrate the ground, 69–77; circumnutation
+     of 69; experiments with split sticks, 74; with wooden pincers, 75;
+     sensitiveness of apex to contact and other irritants, 129; of Vicia faba,
+     132–158; various experiments, 135–140; summary of results, 143–151; power
+     of an irritant on, compared with geotropism, 151–154; sensitiveness of tip
+     to moist air, 180; with greased tips, 185; effect of killing or injuring
+     the primary radicle, 187–191; curvature of, 193; affected by moisture, 198;
+     tip alone sensitive to geotropism, 540; protrusion and circumnutation in a
+     germinating seed, 548; tip highly sensitive, 550; the tip acts like the
+     brain of one of the lower animals, 573
+     —, secondary, sensitiveness of the tips in the bean, 154; become
+     vertically geotropic, 186–191
+
+     Ramey on the movements of the cotyledons of Mimosa pudica, and Clianthus
+     Dampieri at night, 297
+
+     Ranunculus Ficaria, mode of breaking through the ground, 86, 90
+     —, single cotyledon, 96
+     —, effect of lateral light, 484
+
+     Raphanus sativa, sensitiveness of apex of radicle, 171
+     —, sleep of cotyledons, 301
+
+     Rattan, Mr., on the germination of the seeds of Megarrhiza Californica, 82
+
+     Relation between circumnutation and heliotropism, 435
+
+     Reseda odorata, hypocotyl of seedling slightly heliotropic, 454
+
+     Reversion, due to mutilation, 190
+     Rhipsalis cassytha, rudimentary cotyledons, 97
+
+     Ricinus Borboniensis, circumnutation of arched hypocotyl, 53
+
+     Robinia, effect of bright sunshine on its leaves, 445
+     — pseudo-acacia, leaflets vertical at night, 355
+
+     Rodier, M., on the movements of Ceratophyllum demersum, 211
+
+     Royer, Ch., on the sleep-movements of plants, 281, n.; on the sleep of
+     leaves, 318; the leaves of Medicago maculata, 345; on Wistaria Sinensis,
+     354
+
+     Rubus idæus (hybrid) circumnutation of stem, 205
+     —, apogeotropic movement of stem, 498
+
+     Ruiz and Pavon, on Porlieria hygrometrica, 336
+
+     S.
+
+     SACHS on “revolving nutation,” 1; intimate connection between turgescence
+     and growth, 2, n.; cotyledon of the onion, 59; adaptation of root-hairs,
+     69; the movement of the radicle, 70, 72, 73; movement in the hypocotyls of
+     the bean, etc., 91; sensitiveness of radicles, 131, 145, 198;
+     sensitiveness of the primary radicle in the bean, 155; in the common pea,
+     156; effect of moist air, 180; of killing or injuring the primary radicle,
+     186, 187; circumnutation of flower-stems, 225; epinasty, 268; movements of
+     leaflets of Trifolium incarnatum, 350; action of light in modifying the
+     periodic movements of leaves, 418; on geotropism and heliotropism, 436,
+     n.; on Tropaeolum majus, 453; on the hypocotyls slightly heliotropic, and
+     stems strongly apheliotropic of the ivy, 453; heliotropism of radicles,
+     482; experiments on tips of radicles of bean, 523, 524; curvature of the
+     hypocotyl, 555; resemblance between plants and animals, 571
+
+     Sarracenia purpurea, circumnutation of young pitcher, 227
+
+     Saxifraga sarmentosa, circumn utation of an inclined stolon, 218
+
+     Schrankia aculeata, nyctitropic movement of the pinnae, 381, 403
+     — uncinata, nyctitropic movements of leaflets, 381
+
+     Securigera coronilla, nocturnal movements of leaflets, 352
+
+     Seed-capsules, burying of, 513
+
+     Seed-coats, rupture of, 102–106
+
+     Seedling plants, circumnutating movements of, 10
+     Selaginella, circumnutation of 258
+     — Kraussii (?), circumnutation of young plant, 66
+
+     Sida napoea, depression of leaves at night, 322
+     —, no pulvinus, 322
+     — retusa, vertical rising of leaves, 322
+     — rhombifolia, sleep of cotyledons, 308
+     —, sleep of leaves, 314
+     —, vertical rising of leaves, 322
+     —, no pulvinus, 322
+     —, circumnutation and nyctitropic movements of leaf of young plant, 322
+     —, nyctitropic movement of leaves, 397
+
+     Siegesbeckia orientalis, sleep of leaves, 319, 384
+
+     Sinapis alba, hypocotyl bending towards the light, 461
+     —, transmitted effect of light on radicles, 482, 483, 567
+     —, growth of radicles in darkness, 486
+
+     Sinapis nigra, sleep of cotyledons, 301
+
+     Smilax aspera, tendrils apheliotropic, 451
+
+     Smithia Pfundii, non-sensitive cotyledons, 127
+     —, hyponastic movement of the curved summit of the stem, 274–276
+     —, cotyledons not sleeping at night, 308
+     —, vertical movement of leaves, 356
+     — sensitiva, sensitiveness of cotyledons to contact, 126
+     —, sleep of cotyledons, 308
+
+     Sophora chrysophylla, leaflets rise at night, 368
+
+     Solanum dulcamara, circumnutating stems, 266
+     — lycopersicum, movement of hypocotyl, 50
+     —, of cotyledons, 50
+     —, effect of darkness, 124
+     —, rising of cotyledons at night, 306
+     —, heliotropic movements of hypocotyl, 421
+     —, effect of an intermittent light, 457
+     —, rapid heliotropism, 461
+     — palinacanthum, circumnutation of arched hypocotyl, 51, 100
+     —, of cotyledon, 51
+     —, ellipses described by hypocotyl when erect, 107
+     —, nocturnal movement of cotyledons, 306
+
+     Sparganium ramosum, rhizomes of, 189
+
+     Sphaerophysa salsola, rising of leaflets, 355
+
+     Spirogyra princeps, movements of, 259, n.
+
+     Stahl, Dr., on the effect of Æcidium on shoot, 189; on the influence of
+     light on swarm-spores, 488, n.
+
+     Stapelia sarpedon, circumnutation of hypocotyl, 46, 47
+     —, minute cotyledons, 97
+
+     Stellaria media, nocturnal movement of leaves, 297
+
+     Stems, circumnutation of, 201–214
+
+     Stolons, or Runners, circumnutation of, 214–222, 558
+
+     Strasburger, on the effect of light on spores of Haematococcus, 455, n.;
+     the influence of light on the swarm-spores, 488.
+
+     Strawberry, stolons of the, circumnutate, but not affected by moderate
+     light, 454
+
+     Strephium floribundum, circumnutation and nyctitropic movement of leaves,
+     391, 392
+
+     T.
+
+     Tamarindus Indica, nyctitropic movement of leaflets, 374
+
+     Transversal–heliotropismus (of Frank) or diaheliotropism, 438
+
+     Trapa natans, unequal cotyledons, 95, n.
+
+     Tecoma radicans, stems apheliotropic, 451
+
+     Tephrosia caribaea, 354
+
+     Terminology, 5
+
+     Thalia dealbata, sleep of leaves, 389
+     —, lateral movement of leaves, 404
+
+     Trichosanthes anguina, action of the peg on the radicle, 104
+     —, nocturnal movement of cotyledons, 304
+
+     Trifolium, position of terminal leaflets at night, 282
+     — globosum, with hairs protecting the seed-bearing flowers, 517
+     — glomeratum, movement of cotyledons, 309
+     — incarnatum, movement of cotyledons, 309
+     — Pannonicum, shape of first true leaf, 350, 415
+     Trifolium pratense, leaves exposed at night, 293
+     — repens, circumnutation of flower-stem, 225
+     —, circumnutating and epinastic movements of flower-stem, 276–279
+     —, nyctitropic movement of leaves, 349
+     —, circumnutation and nyctitropic movements of terminal leaflets, 352, 353
+     —, sleep movements, 349
+     — resupinatum, no pulvini to cotyledons, 118
+     —, circumnutation of stem, 204
+     —, effect of exposure at night, 295
+     —, cotyledons not rising at night, 118, 309
+     —, circumnutation and nyctitropic movements of terminal leaflets, 351, 352
+     — strictum, movements of cotyledons at night, 116, 118
+     —, nocturnal and diurnal movements of cotyledons, 309–311, 313
+     —, movement of the left-hand cotyledon, 316
+     — subterraneum, movement of flower-heads, 71
+     —, of cotyledons at night, 116, 118, 309
+     —, circumnutation of flower-stem, 224, 225
+     —, circumnutation and nyctitropic movements of leaves, 350
+     —, number of ellipses in 24 hours, 405
+     —, burying its flower-heads, 513, 514
+     —, downward movement of peduncle, 515
+     —, circumnutating movement of peduncle, 516
+
+     Trigonella Cretica, sleep of leaves, 345
+
+     Triticum repens, underground shoots of, become apogeotropic, 189
+
+     Triticum vulgare, sensitiveness of tips of radicle to moist air, 184
+
+     Tropaeolum majus (?), sensitiveness of apex of radicle to contact, 167
+     —, circumnutation of stem, 204
+     —, influence of illumination on nyctitropic movements, 338–340, 344
+     —, heliotropic movement and circumnutation of epicotyl of a young
+     seedling, 428, 429
+     —, of an old internode towards a lateral light, 430
+     —, stems of very young plants highly heliotropic, of old plants slightly
+     apheliotropic, 453
+     —, effect of lateral light, 484
+     — minus (?), circumnutation of buried and arched epicotyl, 27
+
+     U.
+
+     Ulex, or gorse, first-formed leaf of, 415
+
+     Uraria lagopus, vertical sinking of leaflets at night, 365
+
+     V.
+     Vaucher, on the burying of the flower-heads of Trifolium subterraneum, 513;
+     on the protection of seeds, 517
+
+     Verbena melindres (?), circumnutation of stem, 210
+     —, apogeotropic movement of stem, 495
+
+     Vicia faba, circumnutation of radicle, 29, 30
+     —, of epicotyl, 31–33
+     —, curvature of hypocotyl, 92
+     —, sensitiveness of apex of radicle, 132–134
+     —, of the tips of secondary radicles, 154
+     —, of the primary radicle above the apex, 155–158
+     —, various experiments, 135–143
+     —, summary of results, 143–151
+     —, power of an irritant on, compared with that of geotropism, 151–154
+     Vicia faba, circumnutation of leaves, 233–235
+     —, circumnutation of terminal leaflet, 235
+     —, effect of apogeotropism, 444
+     —, effect of amputating the tips of radicles, 523
+     —, regeneration of tips, 526
+     —, short exposure to geotropic action, 527
+     —, effects of amputating the tips obliquely, 528
+     —, of cauterising the tips, 529
+     —, of grease on the tips, 534
+
+     Vines, Mr., on cell growth, 3
+
+     Vries, De, on turgescence, 2; on epinasty and hyponasty, 6, 267, 268; the
+     protection of hypocotyls during winter, 557; stolons apheliotropic, 108;
+     the nyctitropic movement of leaves, 283; the position of leaves influenced
+     by epinasty, their own weight and apogeotropism, 440; apogeotropism in
+     petioles and midribs, 443; the stolons of strawberries, 454; the joints or
+     pulvini of the Gramineæ, 502
+
+     W.
+
+     Watering, effect of, on Porlieria hygrometrica, 336–338
+
+     Wells, ‘Essay on Dew,’ 284, n.
+
+     Wiesner, Prof., on the circumnutation of the hypocotyl, 99, 100; on the
+     hooked tip of climbing stems, 272; observations on the effect of bright
+     sunshine on chlorophyll in leaves, 446; the effects of an intermittent
+     light, 457; on aërial roots, 486; on special adaptations, 490
+
+     Wigandia, movement of leaves, 248
+
+     Williamson, Prof., on leaves of Drosera Capensis, 414
+
+     Wilson, Mr. A. S., on the movements of Swedish turnip leaves, 230, 298
+
+     Winkler on the protection of seedlings, 108
+
+     Wistaria Sinensis, leaflets depressed at night, 354
+     —, circumnutation with lateral light, 452
+
+     Z.
+
+     Zea mays, circumnutation of cotyledon, 64
+     Zea mays, geotropic movement of radicles, 65
+     —, sensitiveness of apex of radicle to contact, 177–179
+     —, secondary radicles, 179
+     —, heliotropic movements of seedling, 64, 421
+     —, tips of radicles cauterised, 539
+
+     Zukal, on the movements of Spirulina, 259, n.
+
+
+
+
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