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This etext was produced from a book in the possession of
Sue Asscher

THE
POWER OF MOVEMENT
IN
PLANTS.

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

ASSISTED BY
FRANCIS DARWIN.

WITH ILLUSTRATIONS.

'SECOND THOUSAND.'

LONDON:
JOHN MURRAY, ALBEMARLE STREET.
1880.

'The right of Translation is reserved.'

***

From 'The Life and Letters of Charles Darwin,' ed. F. Darwin:

In 1880 I published, with [my son] Frank's assistance, our 'Power of
Movement in Plants.' This was a tough piece of work. The book bears
somewhat the same relation to my little book on 'Climbing Plants,'
which 'Cross-Fertilisation' did to the 'Fertilisation of Orchids;'
for in accordance with the principle of evolution it was impossible
to account for climbing plants having been developed in so many
widely different groups unless all kinds of plants possess some
slight power of movement of an analogous kind. This I proved to be
the case; and I was further led to a rather wide generalisation, viz.
that the great and important classes of movements, excited by light,
the attraction of gravity, etc., are all modified forms of the
fundamental movement of circumnutation. It has always pleased me to
exalt plants in the scale of organised beings; and I therefore felt
an especial pleasure in showing how many and what admirably well
adapted movements the tip of a root possesses.

***

CONTENTS.

-----

INTRODUCTION

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, 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 -- 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

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
nictritropic 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.

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 -- Amphicarpaea -- 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.

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.

INDEX.

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 has
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 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.****

*[footnote] Sachs first showed ("Lehrbuch," etc., 4th edit. p. 452)
the intimate connection between turgescence and growth. For De
Vries' interesting essay, 'Wachsthumskrummungen mehrzelliger Organe,'
see 'Bot. Zeitung,' Dec. 19, 1879, p. 830.

**[footnote] 'Die Periodischen Bewegungen der Blattorgane,' 1875.

***[footnote] 'Untersuchungen uber den Heliotropismus,' Sitzb. der K.
Akad. der Wissenschaft. (Vienna), Jan. 1880.

****[footnote] See Mr. Vines' excellent discussion ('Arbeiten des
Bot. Instituts in Wurzburg,' B. II. pp. 142, 143, 1878) on this
intricate subject. Hofmeister's observations ('Jahreschrifte des
Vereins fur Vaterl. Naturkunde in Wurtemberg,' 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,* -- 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.

*[footnote] The highly useful terms of Heliotropism and Geotropism
were first used by Dr. A. B. Frank: see his remarkable 'Beitrage 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.*

*[footnote] These terms are used in the sense given them by De Vries,
'Wurzburg 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.* 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.