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.