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.