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The Power of Movement in Plants

Charles Darwin

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Title: The Power of Movement in Plants

Author: Charles Darwin

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[page i.]

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THE

POWER OF MOVEMENT

PLANTS.

[page ii.]

[page iii.]

THE

POWER OF MOVEMENT

PLANTS.

BY CHARLES DARWIN, LL.D., F.R.S.

ASSISTED BY

FRANCIS DARWIN.

[page iv.]

[page v.]

CONTENTS.

-----

INTRODUCTION...Page 1-9.

CHAPTER I.

THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.

Brassica oleracea, circumnutation of the radicle, of the arched hypocotyl

whilst still buried beneath the ground, whilst rising above the ground and

straightening itself, and when erect--Circumnutation of the cotyledons--

Rate of movement--Analogous observations on various organs in species of

Githago, Gossypium, Oxalis, Tropaeolum, Citrus, Aesculus, of several

Leguminous and Cucurbitaceous genera, Opuntia, Helianthus, Primula,

Cyclamen, Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus,

ads:

Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena,

Nephrodium, and Selaginella...10-66

CHAPTER II.

GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.

Generality of the circumnutating movement--Radicles, their circumnutation

of service--Manner in which they penetrate the ground--Manner in which

hypocotyls and other organs break through the ground by being arched--

Singular manner of germination in Megarrhiza, etc.--Abortion of cotyledons-

-Circumnutation of hypocotyls and epicotyls whilst still buried and arched-

-Their power of straightening themselves--Bursting of the seed-coats--

Inherited effect of the arching process in hypo-

[page vi.]

gean hypocotyls--Circumnutation of hypocotyls and epicotyls when erect--

Circumnutation of cotyledons--Pulvini or joints of cotyledons, duration of

their activity, rudimentary in Oxalis corniculata, their development--

Sensitiveness of cotyledons to light and consequent disturbance of their

periodic movements--Sensitiveness of cotyledons to contact...Page 67-128

CHAPTER III.

SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO OTHER IRRITANTS.

Manner in which radicles bend when they encounter an obstacle in the soil--

Vicia faba, tips of radicles highly sensitive to contact and other

irritants--Effects of too high a temperature--Power of discriminating

between objects attached on opposite sides--Tips of secondary radicles

sensitive--Pisum, tips of radicles sensitive--Effects of such sensitiveness

in overcoming geotropism--Secondary radicles--Phaseolus, tips of radicles

hardly sensitive to contact, but highly sensitive to caustic and to the

removal of a slice--Tropaeolum--Gossypium--Cucurbita--Raphanus--Aesculus,

tip not sensitive to slight contact, highly sensitive to caustic--Quercus,

tip highly sensitive to contact--Power of discrimination--Zea, tip highly

sensitive, secondary radicles--Sensitiveness of radicles to moist air--

Summary of chapter...129-200

CHAPTER IV.

THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS.

Circumnutation of stems: concluding remarks on--Circumnutation of stolons:

aid thus afforded in winding amongst the stems of surrounding plants--

Circumnutation of flower-stems--Circumnutation of Dicotyledonous leaves--

Singular oscillatory movement of leaves of Dionaea--Leaves of Cannabis sink

at night--Leaves of Gymnosperms--Of Monocotyledons--Cryptogams--Concluding

remarks on the circumnutation of leaves; generally rise in the evening and

sink in the morning...201-262

[page vii.]

CHAPTER V.

MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC

MOVEMENTS.

Circumnutation modified through innate causes or through the action of

external conditions--Innate causes--Climbing plants; similarity of their

movements with those of ordinary plants; increased amplitude; occasional

points of difference--Epinastic growth of young leaves--Hyponastic growth

of the hypocotyls and epicotyls of seedlings--Hooked tips of climbing and

other plants due to modified circumnutation--Ampelopsis tricuspidata--

Smithia Pfundii--Straightening of the tip due to hyponasty--Epinastic

growth and circumnutation of the flower-peduncles of Trifolium repens and

Oxalis carnosa...Page 263-279

CHAPTER VI.

MODIFIED CIRCUMNUTATION: SLEEP OR NYCTITROPIC MOVEMENTS, THEIR USE: SLEEP

OF COTYLEDONS.

Preliminary sketch of the sleep or nyctitropic movements of leaves--

Presence of pulvini--The lessening of radiation the final cause of

nyctitropic movements--Manner of trying experiments on leaves of Oxalis,

Arachis, Cassia, Melilotus, Lotus and Marsilea and on the cotyledons of

Mimosa--Concluding remarks on radiation from leaves--Small differences in

the conditions make a great difference in the result - Description of the

nyctitropic position and movements of the cotyledons of various plants--

List of species--Concluding remarks--Independence of the nyctitropic

movements of the leaves and cotyledons of the same species--Reasons for

believing that the movements have been acquired for a special

purpose...280-316

CHAPTER VII.

MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.

Conditions necessary for these movements--List of Genera and Families,

which include sleeping plants--Description of the movements in the several

Genera--Oxalis: leaflets folded at

[page viii.]

night--Averrhoa: rapid movements of the leaflets--Porlieria: leaflets close

when plant kept very dry--Tropaeolum: leaves do not sleep unless well

illuminated during day--Lupinus: various modes of sleeping--Melilotus:

singular movements of terminal leaflet--Trifolium--Desmodium: rudimentary

lateral leaflets, movements of, not developed on young plants, state of

their pulvini--Cassia: complex movements of the leaflets--Bauhinia: leaves

folded at night--Mimosa pudica: compounded movements of leaves, effect of

darkness--Mimosa albida, reduced leaflets of--Schrankia: downward movement

of the pinnae--Marsilea: the only cryptogam known to sleep--Concluding

remarks and summary--Nyctitropism consists of modified circumnutation,

regulated by the alternations of light and darkness--Shape of first true

leaves...Page 317-417

CHAPTER VIII.

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.

Distinction between heliotropism and the effects of light on the

periodicity of the movements of leaves--Heliotropic movements of Beta,

Solanum, Zea, and Avena--Heliotropic movements towards an obscure light in

Apios, Brassica, Phalaris, Tropaeolum, and Cassia--Apheliotropic movements

of tendrils of Bignonia--Of flower-peduncles of Cyclamen--Burying of the

pods--Heliotropism and apheliotropism modified forms of circumnutation--

Steps by which one movement is converted into the other--

Transversal-heliotropismus or diaheliotropism influenced by epinasty, the

weight of the part and apogeotropism--Apogeotropism overcome during the

middle of the day by diaheliotropism--Effects of the weight of the blades

of cotyledons--So called diurnal sleep--Chlorophyll injured by intense

light--Movements to avoid intense light...418-448

CHAPTER IX.

SENSITIVENESS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFECTS.

Uses of heliotropism--Insectivorous and climbing plants not heliotropic--

Same organ heliotropic at one age and not at another--Extraordinary

sensitiveness of some plants to light--The effects

[page ix.]

of light do not correspond with its intensity--Effects of previous

illumination--Time required for the action of light--After-effects of

light--Apogeotropism acts as soon as light fails--Accuracy with which

plants bend to the light--This dependent on the illumination of one whole

side of the part--Localised sensitiveness to light and its transmitted

effects--Cotyledons of Phalaris, manner of bending--Results of the

exclusion of light from their tips--Effects transmitted beneath the surface

of the ground--Lateral illumination of the tip determines the direction of

the curvature of the base--Cotyledons of Avena, curvature of basal part due

to the illumination of upper part--Similar results with the hypocotyls of

Brassica and Beta--Radicles of Sinapis apheliotropic, due to the

sensitiveness of their tips--Concluding remarks and summary of chapter--

Means by which circumnutation has been converted into heliotropism or

apheliotropism...Page 449-492

CHAPTER X.

MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY GRAVITATION.

Means of observation--Apogeotropism--Cytisus--Verbena--Beta--Gradual

conversion of the movement of circumnutation into apogeotropism in Rubus,

Lilium, Phalaris, Avena, and Brassica--Apogeotropism retarded by

heliotropism--Effected by the aid of joints or pulvini--Movements of

flower-peduncles of Oxalis--General remarks on apogeotropism--Geotropism--

Movements of radicles--Burying of seed-capsules--Use of process--Trifolium

subterraneum--Arachis--Amphicarpaea--Diageotropism--Conclusion...493-522

CHAPTER XI.

LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED EFFECTS.

General considerations--Vicia faba, effects of amputating the tips of the

radicles--Regeneration of the tips--Effects of a short exposure of the tips

to geotropic action and their subsequent amputation--Effects of amputating

the tips obliquely--Effects of cauterising the tips--Effects of grease on

the tips--Pisum

[page x.]

sativum, tips of radicles cauterised transversely, and on their upper and

lower sides--Phaseolus, cauterisation and grease on the tips--Gossypium--

Cucurbita, tips cauterised transversely, and on their upper and lower

sides--Zea, tips cauterised--Concluding remarks and summary of chapter--

Advantages of the sensibility to geotropism being localised in the tips of

the radicles...Page 523-545

CHAPTER XII.

SUMMARY AND CONCLUDING REMARKS.

Nature of the circumnutating movement--History of a germinating seed--The

radicle first protrudes and circumnutates--Its tip highly sensitive--

Emergence of the hypocotyl or of the epicotyl from the ground under the

form of an arch--Its circumnutation and that of the cotyledons--The

seedling throws up a leaf-bearing stem--The circumnutation of all the parts

or organs--Modified circumnutation--Epinasty and hyponasty--Movements of

climbing plants--Nyctitropic movements--Movements excited by light and

gravitation--Localised sensitiveness--Resemblance between the movements of

plants and animals--The tip of the radicle acts like a brain...546-573

INDEX...574-593

[page 1]

THE MOVEMENTS OF PLANTS.

INTRODUCTION.

THE chief object of the present work is to describe and connect together

several large classes of movement, common to almost all plants. The most

widely prevalent movement is essentially of the same nature as that of the

stem of a climbing plant, which bends successively to all points of the

compass, so that the tip revolves. This movement has been called by Sachs

"revolving nutation;" but we have found it much more convenient to use the

terms circumnutation and circumnutate. As we shall have to say much about

this movement, it will be useful here briefly to describe its nature. If we

observe a circumnutating stem, which happens at the time to be bent, we

will say towards the north, it will be found gradually to bend more and

more easterly, until it faces the east; and so onwards to the south, then

to the west, and back again to the north. If the movement had been quite

regular, the apex would have described a circle, or rather, as the stem is

always growing upwards, a circular spiral. But it generally describes

irregular elliptical or oval figures; for the apex, after pointing in any

one direction, commonly moves back to the opposite side, not, however,

returning along the same line. Afterwards other irregular ellipses or ovals

are successively described, with their longer

[page 2]

axes directed to different points of the compass. Whilst describing such

figures, the apex often travels in a zigzag line, or makes small

subordinate loops or triangles. In the case of leaves the ellipses are

generally narrow.

Until recently the cause of all such bending movements was believed to be

due to the increased growth of the side which becomes for a time convex;

that this side does temporarily grow more quickly than the concave side has

been well established; but De Vries has lately shown that such increased

growth follows a previously increased state of turgescence on the convex

side.* In the case of parts provided with a so-called joint, cushion or

pulvinus, which consists of an aggregate of small cells that have ceased to

increase in size from a very early age, we meet with similar movements; and

here, as Pfeffer has shown** and as we shall see in the course of this

work, the increased turgescence of the cells on opposite sides is not

followed by increased growth. Wiesner denies in certain cases the accuracy

of De Vries' conclusion about turgescence, and maintains*** that the

increased extensibility of the cell-walls is the more important element.

That such extensibility must accompany increased turgescence in order that

the part may bend is manifest, and this has been insisted on by several

botanists; but in the case of unicellular plants it can hardly fail to be

the more important element. On the whole we may at present conclude that

in-

* Sachs first showed ('Lehrbuch,' etc., 4th edit. p. 452) the intimate

connection between turgescence and growth. For De Vries' interesting essay,

'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.

[page 3]

creased growth, first on one side and then on another, is a secondary

effect, and that the increased turgescence of the cells, together with the

extensibility of their walls, is the primary cause of the movement of

circumnutation.*

In the course of the present volume it will be shown that apparently every

growing part of every plant is continually circumnutating, though often on

a small scale. Even the stems of seedlings before they have broken through

the ground, as well as their buried radicles, circumnutate, as far as the

pressure of the surrounding earth permits. In this universally present

movement we have the basis or groundwork for the acquirement, according to

the requirements of the plant, of the most diversified movements. Thus, the

great sweeps made by the stems of twining plants, and by the tendrils of

other climbers, result from a mere increase in the amplitude of the

ordinary movement of circumnutation. The position which young leaves and

other organs ultimately assume is acquired by the circumnutating movement

being increased in some one direction. the leaves of various plants are

said to sleep at night, and it will be seen that their blades then assume a

vertical position through modified circumnutation, in order to protect

their upper surfaces from being chilled through radiation. The movements

of various organs to the light, which are so general throughout the

vegetable kingdom, and occasionally from the light, or transversely with

respect to it, are all modified

* See Mr. Vines' excellent discussion ('Arbeiten des Bot. Instituts in

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.

[page 4]

forms of circumnutation; as again are the equally prevalent movements of

stems, etc., towards the zenith, and of roots towards the centre of the

earth. In accordance with these conclusions, a considerable difficulty in

the way of evolution is in part removed, for it might have been asked, how

did all these diversified movements for the most different purposes first

arise? As the case stands, we know that there is always movement in

progress, and its amplitude, or direction, or both, have only to be

modified for the good of the plant in relation with internal or external

stimuli.

Besides describing the several modified forms of circumnutation, some other

subjects will be discussed. The two which have interested us most are,

firstly, the fact that with some seedling plants the uppermost part alone

is sensitive to light, and transmits an influence to the lower part,

causing it to bend. If therefore the upper part be wholly protected from

light, the lower part may be exposed for hours to it, and yet does not

become in the least bent, although this would have occurred quickly if the

upper part had been excited by light. Secondly, with the radicles of

seedlings, the tip is sensitive to various stimuli, especially to very

slight pressure, and when thus excited, transmits an influence to the upper

part, causing it to bend from the pressed side. On the other hand, if the

tip is subjected to the vapour of water proceeding from one side, the upper

part of the radicle bends towards this side. Again it is the tip, as stated

by Ciesielski, though denied by others, which is sensitive to the

attraction of gravity, and by transmission causes the adjoining parts of

the radicle to bend towards the centre of the earth. These several cases of

the effects of contact, other irritants, vapour, light, and the

[page 5]

attraction of gravity being transmitted from the excited part for some

little distance along the organ in question, have an important bearing on

the theory of all such movements.

[Terminology.--A brief explanation of some terms which will be used, must

here be given. With seedlings, the stem which supports the cotyledons (i.e.

the organs which represent the first leaves) has been called by many

botanists the hypocotyledonous stem, but for brevity sake we will speak of

it merely as the hypocotyl: the stem immediately above the cotyledons will

be called the epicotyl or plumule. The radicle can be distinguished from

the hypocotyl only by the presence of root-hairs and the nature of its

covering. The meaning of the word circumnutation has already been

explained. Authors speak of positive and negative heliotropism,*--that is,

the bending of an organ to or from the light; but it is much more

convenient to confine the word heliotropism to bending towards the light,

and to designate as apheliotropism bending from the light. There is another

reason for this change, for writers, as we have observed, occasionally drop

the adjectives positive and negative, and thus introduce confusion into

their discussions. Diaheliotropism may express a position more or less

transverse to the light and induced by it. In like manner positive

geotropism, or bending towards the centre of the earth, will be called by

us geotropism; apogeotropism will mean bending in opposition to gravity or

from the centre of the earth; and diageotropism, a position more or less

transverse to the radius of the earth. The words heliotropism and

geotropism properly mean the act of moving in relation to the light or the

earth; but in the same manner as gravitation, though defined as "the act of

tending to the centre," is often used to express the cause of a body

falling, so it will be found convenient occasionally to employ heliotropism

and geotropism, etc., as the cause of the movements in question.

The term epinasty is now often used in Germany, and implies that the upper

surface of an organ grows more quickly than the

* The highly useful terms of Heliotropism and Geotropism were first used by

Dr. A. B. Frank: see his remarkable 'Beiträge zur Pflanzenphysiologie,'

1868.

[page 6]

lower surface, and thus causes it to bend downwards. Hyponasty is the

reverse, and implies increased growth along the lower surface, causing the

part to bend upwards.*

Methods of Observation.--The movements, sometimes very small and sometimes

considerable in extent, of the various organs observed by us, were traced

in the manner which after many trials we found to be best, and which must

be described. Plants growing in pots were protected wholly from the light,

or had light admitted from above, or on one side as the case might require,

and were covered above by a large horizontal sheet of glass, and with

another vertical sheet on one side. A glass filament, not thicker than a

horsehair, and from a quarter to three-quarters of an inch in length, was

affixed to the part to be observed by means of shellac dissolved in

alcohol. The solution was allowed to evaporate, until it became so thick

that it set hard in two or three seconds, and it never injured the tissues,

even the tips of tender radicles, to which it was applied. To the end of

the glass filament an excessively minute bead of black sealing-wax was

cemented, below or behind which a bit of card with a black dot was fixed to

a stick driven into the ground. The weight of the filament was so slight

that even small leaves were not perceptibly pressed down. another method of

observation, when much magnification of the movement was not required, will

presently be described. The bead and the dot on the card were viewed

through the horizontal or vertical glass-plate (according to the position

of the object), and when one exactly covered the other, a dot was made on

the glass-plate with a sharply pointed stick dipped in thick Indian-ink.

Other dots were made at short intervals of time and these were afterwards

joined by straight lines. The figures thus traced were therefore angular;

but if dots had been made every 1 or 2 minutes, the lines would have been

more curvilinear, as occurred when radicles were allowed to trace their own

courses on smoked glass-plates. To make the dots accurately was the sole

difficulty, and required some practice. Nor could this be done quite

accurately, when the movement was much magnified, such as 30 times and

upwards; yet even in this case the general course may be trusted. To test

the accuracy of the above method of observation, a filament was fixed to an

* These terms are used in the sense given them by De Vries, 'Würzburg

Arbeiten,' Heft ii 1872, p. 252.

[page 7]

inanimate object which was made to slide along a straight edge and dots

were repeatedly made on a glass-plate; when these were joined, the result

ought to have been a perfectly straight line, and the line was very nearly

straight. It may be added that when the dot on the card was placed

half-an-inch below or behind the bead of sealing-wax, and when the

glass-plate (supposing it to have been properly curved) stood at a distance

of 7 inches in front (a common distance), then the tracing represented the

movement of the bead magnified 15 times.

Whenever a great increase of the movement was not required, another, and in

some respects better, method of observation was followed. This consisted in

fixing two minute triangles of thin paper, about 1/20 inch in height, to

the two ends of the attached glass filament; and when their tips were

brought into a line so that they covered one another, dots were made as

before on the glass-plate. If we suppose the glass-plate to stand at a

distance of seven inches from the end of the shoot bearing the filament,

the dots when joined, will give nearly the same figure as if a filament

seven inches long, dipped in ink, had been fixed to the moving shoot, and

had inscribed its own course on the plate. The movement is thus

considerably magnified; for instance, if a shoot one inch in length were

bending, and the glass-plate stood at the distance of seven inches, the

movement would be magnified eight times. It would, however, have been very

difficult to have ascertained in each case how great a length of the shoot

was bending; and this is indispensable for ascertaining the degree to which

the movement is magnified.

After dots had been made on the glass-plates by either of the above

methods, they were copied on tracing paper and joined by ruled lines, with

arrows showing the direction of the movement. The nocturnal courses are

represented by straight broken lines. the first dot is always made larger

than the others, so as to catch the eye, as may be seen in the diagrams.

The figures on the glass-plates were often drawn on too large a scale to be

reproduced on the pages of this volume, and the proportion in which they

have been reduced is always given.* Whenever it could be approximately told

how much the movement had been magnified, this is stated. We have perhaps

* We are much indebted to Mr. Cooper for the care with which he has reduced

and engraved our diagrams.

[page 8]

introduced a superfluous number of diagrams; but they take up less space

than a full description of the movements. Almost all the sketches of plants

asleep, etc., were carefully drawn for us by Mr. George Darwin.

As shoots, leaves, etc., in circumnutating bend more and more, first in one

direction and then in another, they were necessarily viewed at different

times more or less obliquely; and as the dots were made on a flat surface,

the apparent amount of movement is exaggerated according to the degree of

obliquity of the point of view. It would, therefore, have been a much

better plan to have used hemispherical glasses, if we had possessed them of

all sizes, and if the bending part of the shoot had been distinctly hinged

and could have been placed so as to have formed one of the radii of the

sphere. But even in this case it would have been necessary afterwards to

have projected the figures on paper; so that complete accuracy could not

have been attained. From the distortion of our figures, owing to the above

causes, they are of no use to any one who wishes to know the exact amount

of movement, or the exact course pursued; but they serve excellently for

ascertaining whether or not the part moved at all, as well as the general

character of the movement.]

In the following chapters, the movements of a considerable number of plants

are described; and the species have been arranged according to the system

adopted by Hooker in Le Maout and Decaisne's 'Descriptive Botany.' No one

who is not investigating the present subject need read all the details,

which, however, we have thought it advisable to give. To save the reader

trouble, the conclusions and most of the more important parts have been

printed in larger type than the other parts. He may, if he thinks fit, read

the last chapter first, as it includes a summary of the whole volume; and

he will thus see what points interest him, and on which he requires the

full evidence.

Finally, we must have the pleasure of returning our

[page 9]

sincere thanks to Sir Joseph Hooker and to Mr. W. Thiselton Dyer for their

great kindness, in not only sending us plants from Kew, but in procuring

others from several sources when they were required for our observations;

also, for naming many species, and giving us information on various points.

[page 10]