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Title: The Power of Movement in Plants
Author: Charles Darwin
Release Date: May, 2004 [EBook #5605]
[Most recently updated: August 14, 2002]
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*** START OF THE PROJECT GUTENBERG EBOOK, THE POWER OF MOVEMENT IN PLANTS ***
This eBook was produced by Sue Asscher.
POWER OF MOVEMENT
POWER OF MOVEMENT
BY CHARLES DARWIN, LL.D., F.R.S.
THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
Brassica oleracea, circumnutation of the radicle, of the arched hypocotyl whilst still buried beneath the ground, whilst rising above the ground and straightening itself, and when erect--Circumnutation of the cotyledons--Rate of movement--Analogous observations on various organs in species of Githago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of several Leguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephrodium, and Selaginella...10-66
GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
Generality of the circumnutating movement--Radicles, their circumnutation of service--Manner in which they penetrate the ground--Manner in which hypocotyls and other organs break through the ground by being arched--Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons--Circumnutation of hypocotyls and epicotyls whilst still buried and arched--Their power of straightening themselves--Bursting of the seed coats--Inherited effect of the arching process in hypo-
gean hypocotyls--Circumnutation of hypocotyls and epicotyls when erect--Circumnutation of cotyledons--Pulvini or joints of cotyledons, duration of their activity, rudimentary in Oxalis corniculata, their development--Sensitiveness of cotyledons to light and consequent disturbance of their periodic movements--Sensitiveness of cotyledons to contact...Page 67-128
SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.
Manner in which radicles bend when they encounter an obstacle in the soil--Vicia faba, tips of radicles highly sensitive to contact and other irritants--Effects of too high a temperature--Power of discriminating between objects attached on opposite sides--Tips of secondary radicles sensitive--Pisum, tips of radicles sensitive--Effects of such sensitiveness in overcoming geotropism--Secondary radicles--Phaseolus, tips of radicles hardly sensitive to contact, but highly sensitive to caustic and to the removal of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus, tip not sensitive to slight contact, highly sensitive to caustic--Quercus, tip highly sensitive to contact--Power of discrimination--Zea, tip highly sensitive, secondary radicles--Sensitiveness of radicles to moist air--Summary of chapter...129-200
THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.
Circumnutation of stems: concluding remarks on--Circumnutation of stolons: aid thus afforded in winding amongst the stems of surrounding plants--Circumnutation of flower stems--Circumnutation of Dicotyledonous leaves--Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sink at night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concluding remarks on the circumnutation of leaves; generally rise in the evening and sink in the morning...201-262
MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC MOVEMENTS.
Circumnutation modified through innate causes or through the action of external conditions--Innate causes--Climbing plants; similarity of their movements with those of ordinary plants; increased amplitude; occasional points of difference--Epinastic growth of young leaves--Hyponastic growth of the hypocotyls and epicotyls of seedlings--Hooked tips of climbing and other plants due to modified circumnutation--Ampelopsis tricuspidata--Smithia Pfundii--Straightening of the tip due to hyponasty--Epinastic growth and circumnutation of the flower peduncles of Trifolium repens and Oxalis carnosa...Page 263-279
MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP OF COTYLEDONS.
Preliminary sketch of the sleep or nyctitropic movements of leaves--Presence of pulvini--The lessening of radiation the final cause of nyctitropic movements--Manner of trying experiments on leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of Mimosa--Concluding remarks on radiation from leaves--Small differences in the conditions make a great difference in the result Description of the nyctitropic position and movements of the cotyledons of various plants--List of species--Concluding remarks--Independence of the nyctitropic movements of the leaves and cotyledons of the same species--Reasons for believing that the movements have been acquired for a special purpose...280-316
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...Page 317-417
MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
Distinction between heliotropism and the effects of light on the periodicity of the movements of leaves--Heliotropic movements of Beta, Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements of tendrils of Bignonia--Of flower peduncles of Cyclamen--Burying of the pods--Heliotropism and apheliotropism modified forms of circumnutation--Steps by which one movement is converted into the other Transversal heliotropismus or diaheliotropism influenced by epinasty, the weight of the part and apogeotropism--Apogeotropism overcome during the middle of the day by diaheliotropism--Effects of the weight of the blades of cotyledons--So called diurnal sleep--Chlorophyll injured by intense light--Movements to avoid intense light...418-448
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...Page 449-492
MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.
Means of observation--Apogeotropism--Cytisus--Verbena--Beta--Gradual conversion of the movement of circumnutation into apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded by heliotropism--Effected by the aid of joints or pulvini--Movements of flower peduncles of Oxalis--General remarks on apogeotropism--Geotropism--Movements of radicles--Burying of seed capsules--Use of process--Trifolium subterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion...493-522
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...Page 523-545
SUMMARY AND CONCLUDING REMARKS.
Nature of the circumnutating movement--History of a germinating seed--The radicle first protrudes and circumnutates--Its tip highly sensitive--Emergence of the hypocotyl or of the epicotyl from the ground under the form of an arch--Its circumnutation and that of the cotyledons--The seedling throws up a leaf bearing stem--The circumnutation of all the parts or organs--Modified circumnutation--Epinasty and hyponasty--Movements of climbing plants--Nyctitropic movements--Movements excited by light and gravitation--Localised sensitiveness--Resemblance between the movements of plants and animals--The tip of the radicle acts like a brain...546-573
THE MOVEMENTS OF PLANTS.
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.* In the case of parts provided with a so called joint, cushion or pulvinus, which consists of an aggregate of small cells that have ceased to increase in size from a very early age, we meet with similar movements; and here, as Pfeffer has shown** and as we shall see in the course of this work, the increased turgescence of the cells on opposite sides is not followed by increased growth. Wiesner denies in certain cases the accuracy of De Vries' conclusion about turgescence, and maintains*** that the increased extensibility of the cell walls is the more important element. That such extensibility must accompany increased turgescence in order that the part may bend is manifest, and this has been insisted on by several botanists; but in the case of unicellular plants it can hardly fail to be the more important element. On the whole we may at present conclude that in-
* Sachs first showed ('Lehrbuch,' etc., 4th edit. p. 452) the intimate connection between turgescence and growth. For De Vries' interesting essay, 'Wachsthumskrümmungen mehrzelliger Organe,' see 'Bot. Zeitung,' Dec. 19, 1879, p. 830.
** 'Die Periodischen Bewegungen der Blattorgane,' 1875.
*** 'Untersuchungen über den Heliotropismus,' Sitzb. der K. Akad. der Wissenschaft. (Vienna), Jan. 1880.
creased growth, first on one side and then on another, is a secondary effect, and that the increased turgescence of the cells, together with the extensibility of their walls, is the primary cause of the movement of circumnutation.*
In the course of the present volume it will be shown that apparently every growing part of every plant is continually circumnutating, though often on a small scale. Even the stems of seedlings before they have broken through the ground, as well as their buried radicles, circumnutate, as far as the pressure of the surrounding earth permits. In this universally present movement we have the basis or groundwork for the acquirement, according to the requirements of the plant, of the most diversified movements. Thus, the great sweeps made by the stems of twining plants, and by the tendrils of other climbers, result from a mere increase in the amplitude of the ordinary movement of circumnutation. The position which young leaves and other organs ultimately assume is acquired by the circumnutating movement being increased in some one direction. the leaves of various plants are said to sleep at night, and it will be seen that their blades then assume a vertical position through modified circumnutation, in order to protect their upper surfaces from being chilled through radiation. The movements of various organs to the light, which are so general throughout the vegetable kingdom, and occasionally from the light, or transversely with respect to it, are all modified
* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts in 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.
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,*--that is, the bending of an organ to or from the light; but it is much more convenient to confine the word heliotropism to bending towards the light, and to designate as apheliotropism bending from the light. There is another reason for this change, for writers, as we have observed, occasionally drop the adjectives positive and negative, and thus introduce confusion into their discussions. Diaheliotropism may express a position more or less transverse to the light and induced by it. In like manner positive geotropism, or bending towards the centre of the earth, will be called by us geotropism; apogeotropism will mean bending in opposition to gravity or from the centre of the earth; and diageotropism, a position more or less transverse to the radius of the earth. The words heliotropism and geotropism properly mean the act of moving in relation to the light or the earth; but in the same manner as gravitation, though defined as "the act of tending to the centre," is often used to express the cause of a body falling, so it will be found convenient occasionally to employ heliotropism and geotropism, etc., as the cause of the movements in question.
The term epinasty is now often used in Germany, and implies that the upper surface of an organ grows more quickly than the
* The highly useful terms of Heliotropism and Geotropism were first used by Dr. A. B. Frank: see his remarkable 'Beiträge zur Pflanzenphysiologie,' 1868.
lower surface, and thus causes it to bend downwards. Hyponasty is the reverse, and implies increased growth along the lower surface, causing the part to bend upwards.*
Methods of Observation.--The movements, sometimes very small and sometimes considerable in extent, of the various organs observed by us, were traced in the manner which after many trials we found to be best, and which must be described. Plants growing in pots were protected wholly from the light, or had light admitted from above, or on one side as the case might require, and were covered above by a large horizontal sheet of glass, and with another vertical sheet on one side. A glass filament, not thicker than a horsehair, and from a quarter to three quarters of an inch in length, was affixed to the part to be observed by means of shellac dissolved in alcohol. The solution was allowed to evaporate, until it became so thick that it set hard in two or three seconds, and it never injured the tissues, even the tips of tender radicles, to which it was applied. To the end of the glass filament an excessively minute bead of black sealing wax was cemented, below or behind which a bit of card with a black dot was fixed to a stick driven into the ground. The weight of the filament was so slight that even small leaves were not perceptibly pressed down. another method of observation, when much magnification of the movement was not required, will presently be described. The bead and the dot on the card were viewed through the horizontal or vertical glass plate (according to the position of the object), and when one exactly covered the other, a dot was made on the glass plate with a sharply pointed stick dipped in thick Indian ink. Other dots were made at short intervals of time and these were afterwards joined by straight lines. The figures thus traced were therefore angular; but if dots had been made every 1 or 2 minutes, the lines would have been more curvilinear, as occurred when radicles were allowed to trace their own courses on smoked glass plates. To make the dots accurately was the sole difficulty, and required some practice. Nor could this be done quite accurately, when the movement was much magnified, such as 30 times and upwards; yet even in this case the general course may be trusted. To test the accuracy of the above method of observation, a filament was fixed to an
* These terms are used in the sense given them by De Vries, 'Würzburg Arbeiten,' Heft ii 1872, p. 252.
inanimate object which was made to slide along a straight edge and dots were repeatedly made on a glass plate; when these were joined, the result ought to have been a perfectly straight line, and the line was very nearly straight. It may be added that when the dot on the card was placed half an inch below or behind the bead of sealing wax, and when the glass plate (supposing it to have been properly curved) stood at a distance of 7 inches in front (a common distance), then the tracing represented the movement of the bead magnified 15 times.
Whenever a great increase of the movement was not required, another, and in some respects better, method of observation was followed. This consisted in fixing two minute triangles of thin paper, about 1/20 inch in height, to the two ends of the attached glass filament; and when their tips were brought into a line so that they covered one another, dots were made as before on the glass plate. If we suppose the glass plate to stand at a distance of seven inches from the end of the shoot bearing the filament, the dots when joined, will give nearly the same figure as if a filament seven inches long, dipped in ink, had been fixed to the moving shoot, and had inscribed its own course on the plate. The movement is thus considerably magnified; for instance, if a shoot one inch in length were bending, and the glass plate stood at the distance of seven inches, the movement would be magnified eight times. It would, however, have been very difficult to have ascertained in each case how great a length of the shoot was bending; and this is indispensable for ascertaining the degree to which the movement is magnified.
After dots had been made on the glass plates by either of the above methods, they were copied on tracing paper and joined by ruled lines, with arrows showing the direction of the movement. The nocturnal courses are represented by straight broken lines. the first dot is always made larger than the others, so as to catch the eye, as may be seen in the diagrams. The figures on the glass plates were often drawn on too large a scale to be reproduced on the pages of this volume, and the proportion in which they have been reduced is always given.* Whenever it could be approximately told how much the movement had been magnified, this is stated. We have perhaps
* We are much indebted to Mr. Cooper for the care with which he has reduced and engraved our diagrams.
introduced a superfluous number of diagrams; but they take up less space than a full description of the movements. Almost all the sketches of plants asleep, etc., were carefully drawn for us by Mr. George Darwin.
As shoots, leaves, etc., in circumnutating bend more and more, first in one direction and then in another, they were necessarily viewed at different times more or less obliquely; and as the dots were made on a flat surface, the apparent amount of movement is exaggerated according to the degree of obliquity of the point of view. It would, therefore, have been a much better plan to have used hemispherical glasses, if we had possessed them of all sizes, and if the bending part of the shoot had been distinctly hinged and could have been placed so as to have formed one of the radii of the sphere. But even in this case it would have been necessary afterwards to have projected the figures on paper; so that complete accuracy could not have been attained. From the distortion of our figures, owing to the above causes, they are of no use to any one who wishes to know the exact amount of movement, or the exact course pursued; but they serve excellently for ascertaining whether or not the part moved at all, as well as the general character of the movement.]
In the following chapters, the movements of a considerable number of plants are described; and the species have been arranged according to the system adopted by Hooker in Le Maout and Decaisne's 'Descriptive Botany.' No one who is not investigating the present subject need read all the details, which, however, we have thought it advisable to give. To save the reader trouble, the conclusions and most of the more important parts have been printed in larger type than the other parts. He may, if he thinks fit, read the last chapter first, as it includes a summary of the whole volume; and he will thus see what points interest him, and on which he requires the full evidence.
Finally, we must have the pleasure of returning our
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.
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