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EDWARD NELSON DINGLEY
COLLECTION
PRESENTED BY HIS WIFE
THE
POWER OF MOVEMENT
IN PLANTS
BY
HARLES DARWIN, LL. D., F.R.S.
ASSISTED BY
FRANCIS DARWIN
WITH ILLUSTRATIONS
NEW YORK
D. APPLETON AND COMPANY
1896
QK77I
I23G
Authorized Edition.
C f .rcrr
Mr§. Edwarb N. Dingle*
July 11 1932
CONTENTS
Lnteoduction Page 1-9
CHAPTER I.
The Circumnutating Movements of Seedling Plants.
Brassica oleracea, circumnutation of the radicle, of the arched hypo-
cotyl whilst still buried beneath the ground, whilst rising above
the ground and straightening itself, and when erect — Circumnu-
tation of the cotyledons — Kate of movement — Analogous obser-
vations on various organs in species of Githago, Gossypium,
Oxalis, Tropseolum, Citrus, iEsculus, of several Leguminous and
Cucurbitaceous genera, Opuntia, Helianthus, Primura, Cyclamen,
Stapelia, Cerinthe, Nolana, Solanum, Beta, Eicinus, Quercus,
Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea,
A vena, Nephrodium, and Selaginella 10-66
CHAPTER II.
General Considerations on the Movements and Growth of
Seedling Plants.
Generality of the circumnutating movement — Eadicles, their cir-
cumnutation of service — Manner in which they penetrate the
ground — Manner in which hypocotyls and other organs bieak
through the ground by being arched — Singular manner of ger-
mination in Megarrhiza, &c— Abortion of cotyledons — Circum-
nutation of hypocotyls and epicotyls whilst still buried and
arched — Their power of straightening themselves— Bursting of
the seed-ccats — Inherited effect of the arching process in hypo-
VI CONTENTS.
gean hypocotyls — Circumnutation of hypocotyls and epicotyl?
when erect — Circumnutation of cotyledons — Pulvini or joints of
cotyledons, duration of their activity, rudimentary in Oxalis
corniculata, their development — Sensitiveness of cotyledons to
light and consequent disturbance of their periodic movements —
Sensitiveness of cotyledons to contact Page 67-128
CHAPTER III.
Sensitiveness of the Apex of the Radicle to Contact and
to other Irritants.
Maimer in which radicles bend when they encounter an obstacle in
the soil — Yicia faba, tips of radicles highly sensitive to con-
tact and other irritants — Effects of too high a temperature —
Power of discriminating between objects attached on opposite
sides — Tips of secondary radicles sensitive — Fisum, 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 — Tropseolurn — Gossypium-^-Cucurbita
— Raphanus— ^E^culus, tip not sensitive to slight contact, highly
sensitive to caustic — Quercus, tip highly sensitive to contact —
Power of discrimination — Zea, tip highly sensitive, secondary
radicles — Sensitiveness of radicles to moist air — Summary of
chapter 129-200
CHAPTER IY.
The Circoinutating 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 — Circum-
nutation of Dicotyledonous leaves — Singular oscillatory move-
ment }f leaves of Dionsea — Leaves of Cannabis sink at night —
Leaves of Gymnosperms — Of Monocotyledons — Cryptogams —
Concluding remarks on the circumnutation of leaves : generally
rise in the evening and sink in the morning .. .. 201-262
CONTENTS. V11
CHAPTER V.
modified circumnutation .* climbing plants; epinastic and
Hyponastic Movements.
Circumautation modified through innate causes or through the action
of external conditions — Innate causes — Climbing plants ; simi-
larity of their movements with those of ordinary plants; in-
creased 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 Pi'undii — Straightening of the tip due to hyponasty —
Epinastic growth and circumnutation of the flower-peduncles of
Trifolium repens and Oxalis carnosa Page 263-279
CHAPTER VI.
Modified Circumnutation: Sleep or Nyctitropic Movements,
their Use: Sleep of Cotyledons.
Preliminary sketch of the sleep or nyctitropic movements of leaves
— Presence of pulvini — The lessening of radiation the final cause
of nictri tropic movements — Manner of trying experiments on
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea,
and on the cotyledons of Mimosa — Concluding remarks on radia-
tion from leaves — Small differences in the conditions make a
great difference in the result — Description of the nyctitropic
position and movements of the cot}ledons of various plants —
List of species — Concluding remarks — Independence of the
nyctitropic movements of the leaves and cotyledons of the same
species — Reasons for believing that the movements have been
acquired for a special purpose 280-316
CHAPTER VII.
Modified Circumnutation : Nyctitropic or Sleep Movements
of Leaves.
Conditions necessary for these movements — List of Genera and
Families, which include sleeping plants — Description of tho
movements in the several Genera — Oxalis : leaflets folded at
Vill CONTENTS.
night — Averrhoa: rapid movements of the leaflets — Porlieria'.
leaflets close when plant kept very dry — Tropa?olum : 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 nuht — Mimosa pudica : compounded move-
ments of leaves, effect of darkness — Mimosa albida, reduced
leaflets of — Schrankia : downward movement of the pinna? —
Marsilea : the only cryptogam known to sleep — Concluding
remarks and summary — Nyctitropism consists of modified cir-
cumnutation, regulated by the alternations of light and darkness
— Shape of fiist true leaves Page 317-417
CHAPTEE 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, Tropseo-
lum, and Cassia — Apheliotropic movements of tendrils of Big-
nonia — Of flower-peduncles of Cyclamen — Burying of the pods
— Heliotropism and apheliotropism modified forms of circumnu-
tation — Steps by which one movement is converted into the
other — Transversal-heliotropismus or diaheliotropism influenced
by epinasty, the weight of the part and apogeotropism — Apogeo-
tropism overcome during the middle of the day by diaheliotro-
pism — Effects of the weight of the blades of cotyledons — So-
called diurnal sleep — Chlorophyll injured by intense light —
Movements to avoid intense light 418-448
CHAPTER IX.
Sensitiveness of Plants to Light : its transmitted effects.
Uses of he'iotropism — Insectivorous and climbing plants not helio-
tropic — Same organ heliotropic at one age and not at another —
Extraordinary sensitiveness of some plants to light — The effects
CONTENTS. IS
of light do not correspond with its intensity — E fleets 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 benfl. to the light — This dependent on the
illumination of one whole side of the part — Localised sensitive-
ness 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 aphelio-
tropic, due to the sensitiveness of their tips — Concluding remarks
and summary of chapter — Means by which circumnutation han
been converted into heliotropism or apheliotropism Page 449-492
CHAPTER X.
Modified Circumnutation : Movements excited by
Gravitation.
Means of observation — Apogeotropism — Cytisus — Verbena — Beta
— Gradual conversion of the movement of circumnutation into
apogeotropism in Rubus, Lilium, Phalaris, Avena, and Brassica
— Apogeotropism retarded by heliotropism — Effected by the aid
of joints or pulvini — Movements of flower- peduncles of Oxalis —
General remarks on apogeotropism — Geotropism — Movements of
radicles — Burying of seed-capsules — Use of process — Trifolium
subterraneum — Arachis — Amphicarpasa — Diageotropism —
Conclusion 493-522
CHAPTER XL
Localised Sensitiveness to Gravitation, and its Transmitter
Effects.
General considerations — Vicia faba, effects of amputating the tips of
the radicles — Regeneration of the tips — Effects of a short ex-
posure of the tips to geotropic action and their subsequent
amputation — Kffects of amputating the tips obliquely — Effects
of cauterising the tips — Effects of grease on the tips — Pisum
CONTENTS.
sativum, tips of radicles cauterised transversely, and on theii
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 — Con-
cluding remarks and summary of chapter — Advantages of the
sensibility to geotropism being localised in the tips of the
radicles Page 23-545
CHAPTER XII.
Summary and Concluding Remarks.
Nature of the circum nutating 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 grcund under the form of an arch — Its circumnutation
and that of the cotyledons — The seedling throws up a leaf-
bearing stem — The circumnutation of all the parts or organs —
Modified circumnutation — Epinasty and hyponasty — Movements
of climbing plants — Nyctitropic movements — Movements excited
by light and gravitation — Localised sensitiveness — Resemblance
between the movements of plants and animals — The tip of the
radicle acts like a brain 546-573
Index 574-593
THE MOVEMENTS OF PLANTS.
INTRODUCTION.
The chief object o£ the present work is to describe
and connect together several large classes of move-
ment, 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 point-
ing 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
2 INTRODUCTION.
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 move-
ments was believed to be due to the increased growth
of the side which becomes for a time convex ; that this
side does temporarily grow more quickly than the
concave side has been well established ; but De Vries
has lately shown that such increased growth follows
a previously increased state of turgescence on the
convex side.* In the case of parts provided with a
so-called joint, cushion or pulvinus, which consists of
an aggregate of small cells that have ceased to
increase in size from a very early age, we meet with
similar movements; and here, as Pfeffer has shown f
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' con-
clusion about turgescence, and maintains :f 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.
Oi the whole we may at present conclude that in-
* Sachs first showed "Lehr- 19, 1879, p. 830.
buch,' &c, 4th edit. p. 452) the f 'D;e Periodiscben Bewegun-
intimate connection between tur- gen der Blattorgane,' 1875.
g< scence and growth. For De I 'Untersuchungen iiber den
Vries* interesting essay, ' Wachs- Heliotropismus,' Sitzb. der K,
thumskrurnmungen mehrzelliger Akad. derWissenschaft. (Vienna).
Organe,' see • Bot. Zeitung,' Dec. Jan. 1880.
INTRODUCTION. ' O*
creased growth, first ov one side and then on another,
is a secondary effect, and that the increased tnr-
gescence of the cells, together with the extensibility
of their walls, is the primary cause of the movement of
circumnutation.*
In the course of the present volume it will be shown
that apparently every growing part of every plant is
continually circumnutating, though often on a small
scale. Even the stems of seedlings before they have
broken through the ground, as well as their buried
radicles, circum nutate, as far as the pressure of the
surrounding earth permits. In this universally pre-
sent movement we have the basis or groundwork for
the acquirement, according to the requirements of the
plant, of the most diversified movements. Thus, the
great sweeps made by the stems of twining plants,
and by the tendrils of other climbers, result from
a mere increase in the amplitude of the ordinary
movement of circumnutation. The position which
young leaves and other organs ultimately assume
is acquired by the circumnutating movement being
increased in some one direction. The leaves of
various plants are said to sleep at night, and it will
be seen that their blades then assume a vertical
position through modified circumnutation, in order
to protect their upper surfaces from being chilled
through radiation. The movements of various organs
to the light, which are so general throughout the
vegetable kingdom, and occasionally from the light,
or transversely with respect to it, are all modified
* See Mr. Vines excellent dis- Naturkimde in Wurtemberg,'
cussion (' Arbeiton des Bot. Insti- 1874, p. 211) on the curious move-
tuts in Wi'uzburg,' B. II. pp 142, ments of Spirogyra, a plant con-
143, 1878) on this intricate subject. sisting of a single row of cells,.aro
Hofrneister's observations ('J;ih- valuable in relation to this subject,
reschrifte des Vereins fur Yaterl.
4 INTRODUCTION.
forms of circumnutation ; as again are the equally
prevalent movements of stems, &c, towards the zenith,
and of roots towards the centre of the earth. In
accordance with these conclusions, a considerable diffi-
culty in the way of evolution is in part removed, for
it might have been asked, how did all their 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 direc-
tion, 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 influ-
ence to the lower part, causing it to bend. If there-
fore 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 seed-
lings, the tip is sensitive to various stimuli, espe-
cially 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
INTRODUCTION. 5
attraction of gravity being transmitted from the ex-
cited 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
he 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 epicc-fyl or plumule. The radich can be distinguished
from the hypocotyl only by the presence of root-hairs and the
nature of its covering. The meaning of the word circumnu-
tation has already been explained. Authors speak of positive
and negative licliotropism,* — that is, the bending of an organ
to or from the light ; but it is much more convenient to confine
the word htliotropism 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. Diahelio-
tropism may express a position more or less transverse to
the light and induced by it. In like manner positive geotro-
pism, or bending towards the centre of the earth, will be
called by us geotropism ; upoyeotropi^m will mean bending in
opposition to gravity or from the centre of the earth; and dia-
jeotropism, 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, &c, as the cause of the
movements in question.
The term epinasty is now often used in Germany, and implies
that the upper surface of an organ grows more quickly than the
* The highly useful terms of Frank : see his remarkable ' Bei-
Heliotrop'sm and Gtotropism ti'age zur Pflauzenphysiologie,
were first used bv Dr. A. B. 1868.
6 INTRODUCTION.
lower surface, and thus causes it to bend downwards. Hypo-
nasty is the reverse, and implies increased growth along the
lower surface, causing the part to bend upwards.*
M>thods 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 obser-
vation, 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 after-
wards joined by straight lines. The figures thus traced were
therefore angular; but if dots had been made every 1 or
2 minutes, the lines would have been more curvilinear, as
occurred when radicles were allowed to trace their own
courses on smoked glass-plates. To make the dots accurately
was the sole difficulty, and required some practice. Nor could
this be done quite accurately, when the movement was much
magnified, such as 30 times and upwards; yet even in this
case the general course may be trusted. To test the accuracy
of the above method of observation, a filament was fixed to an
* These terms are used in the ' Wiirzburg Aibeiten,' Heft ii.
•ease given them by De Vries, 1872, p. 252.
INTRODUCTION. 7
inanimate object which was made to siide 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 ■£$ 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 dis-
tance 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 move-
ment would be magnified eight times. It would, however, have
been very difficult to have ascertained in each case how great
a length of the shoot was bending; and this is indispensable
for ascertaining the degree to which the movement is magnified.
After dots had been made on the glass-plates by either of
the above methods, they were copied on tracing paper and
joined by ruled lines, with arrows showing the direction of the
movement. The nocturnal courses are represented by straight
broken lines. The first dot is always made larger than the
others, so as to catch the eye, as may be seen in the diagrams.
The figures on the glass-plates were often drawn on too large
a scale to be reproduced on the pages of this volume, and the
proportion in which they have been reduced is always given.*
Whenever it could be approximately told how much the move-
ment had been magnified, this is stated. We have perhaps
* We are much indebted to he has reduced and engraved out
Mr. Cooper fur the care with which diagrams.
8 INTRODUCTION.
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, &c, were carefully drawn
for us by Mr. George Darwin.
As shoots, leaves, &c, 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 con-
siderable number of plants are described ; and the
species have been arranged according to the system
adopted by Hooker in Le Maout and Decaisne's * De-
scriptive 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 oiu
INTRODUCTION. 9
sincere thanks to Sir Joseph Hooker and to Mr. W.
Thiselton Dyer for their great kindness, in not only
sending us plants from Kew, but in procuring others
from several sources when they were required for our
observations ; also, for naming many species, and giving
us in formation on various points.
10 CIRCUMNUTATION OF SEEDLINGS. Chap, t
CHAPTER I.
The Circtjmntjtattng Movements of Seedling Plants.
Brassica oleracea, circumnutation of the radicle, of the arched hypo-
cotyl 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, Tro-
pseolum, Citrus, JEsculus, of several Leguminous and Cucurbita-
ceous genera, Opuntia, Helianthus, Primula, Cyclamen, StapeLa,
Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus,
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, A vena, Nephro-
dium, and Selagiuella.
The following chapter is devoted to the circum-
nutating movements of the radicles, hypocotyls, and
cotyledons of seedling plants ; and, when the coty-
ledons 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 oltracea (Cruciferce). — Puller 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 move-
ment 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 re-
mained there all the time, the movement exhibited would have
Chap. I,
BEASSIOA.
11
^>
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 measure- Fi S« l •
ments made with compasses
on other seeds, the tip alone,
for a length of only y§o to
y§o of an inch, is acted on
by geotropism. But the trac-
ing shows that the basal part
of the radicle continued to
circumnutate irregularly dur-
ing 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 fila-
ment, which was nearly f inch
in length, it was impossible
to estimate.
Another seed was treated and observed in the same manner,
but the radicle in this case protruded -1 inch, and was not
Brassca oleriacea : 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.
Fig. 2.
\
Brassica okracea : circumnutating and geotropic movement of radicle,
traced on horizontal glass during 46 hours.
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
12 CTRCUMNUTATION OF SEEDLINGS. Chap. 1.
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 curva-
ture. 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 move-
ments of the radicles of other plants, that they may be largely
trusted.
Hyipocotyl. — 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 sur-
rounding 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
Chap. I. BKASS1CA. 13
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 circum-
nutating, whilst it was straightening itself by growth along its
inner or concave surface.
A somewhat different method of observation was next followed :
Fig. 3.
Brassica oleracea : circumnutating movement of buried and arched hypo-
cotyl (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.
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 hypo-
cotyl, 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
14 CIKCUMNUTATION OF SEEDLINGS. Chap. I
age to circumnutate irregularly. On the first day the greatei
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. 4.
Brassica oleracea : circumnutating movement of buried and arched hypo-
cotyl, with the two legs of the arch tied together, traced on horizontal
glass during 33J hours. Movement of the bead of filament magnified
about 26 times, and here reduied to one-half original scale.
Before the above observations were made, some arched hypo-
cotyls buried at the depth of a quarter of an inch were un-
covered ; 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 circumnu-
tation. 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.
Chap. I. BRASSICA. 15
23rd to 6.45 a.m. on Dec. 25th. No doubt the natural move-
ment 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 fila-
ment 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
Fig. 5.
Brassica oleracea : circummtating movement of the crown of a buried and
arched hypocotyl, with the two legs tied together, traced on a hori-
zontal glass during 23 hours. Movement of the bead of the filament
magnified about 58 time?, and here reduced to one-half original
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.
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 circum-
nutation 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
16 CIECUMNUTATION OF SEEDLINGS. Chap. I.
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 ver-
tical 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
Fig. 6.
Brassica olcracea; conjoint circumnutation of the hypocotyl and cotyledons
during 10 hours 45 minutes. Figure here reduced to one-half original
scale.
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 de-
scribed 3s ellipses in 101 h.; each being completed on an
average in 3 h. 4 m.
On the previous day another seedling had been observed
under similar conditions, excepting that the plant was so
Chap. I.
BKASS1CA.
17
placed that a line joining the two "cotyledons pointed towards
the window ; and the filament was attached to the smaller coty-
ledon on the side furthest from the window. Moreover, the
plant was now for the first time placed in this position. Tho
cotyledons bowed themselves greatly towards the light from 8 to
10.50 a.m., when the first dot was made (Fig. 7). During the
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.
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.
By 9.25 a.m. on this second day the same cotyledon had
18
CIKCUMNUTATION OF SEEDLINGS. Chap. 1
begun to fall, and a dot was made on a fresh glass. The move-
ment 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
Fig. 8.
Brassica oleracea : conjoint circumnutation
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.
As the filaments were
fixed in the three last
cases to one of the coty-
ledons, and as the hypo- '
cotyl was left free, the
tracings show the move-
ment of both organs con-
of the hypocotyl and cotyledons during joined ; and we now
8 hours. Figure here reduced to one- wished to ascertain whe-
third of the original scale, as traced en a ther both circumriu tated.
vertical glass.
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 morn-
ing, 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
Chap. I.
BRASSICA.
19
vertically up and down all day long, and as there was some
slight lateral movement, they circnmnutated.
The cabbage was one of the first plants, the seedlings of which
were observed by us, and we 9
did not then know how far
the circumnutation of the
different parts was affected
by light. Young seedlings
were therefore kept in com-
plete 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 hy-
pocotyl was If 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,
Brassica oleracea : circumnutation of
hypocotyl, in darkness, traced on a
horizontal glass, by means of a fila-
ment with a bead fixed across its
summit, between 9.15 A.M. and
8.30 a.m. on the following morn-
ing. Figure here reduced to one-
half of original scale.
20
CIRCUMNUTATION OF SEEDLINGS. Chap. I.
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 observa-
tion. Therefore the cotyledons certainly
circumnutated, though the chief move-
ment was up and down in a vertical
plane.
Bate 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 ^i_ inch ; the
plants were illuminated by light passing
through a solution of bichromate of potas-
sium so as to eliminate heliotropism.
Under these circumstances it was interest-
ing to observe how rapidly the circum-
nutating apex of a cotyledon passed across
the divisions of the micrometer. Whilst
travelling in any direction the apex generally oscillated back-
wards and forwards to the extent of ^ and sometimes of nearly
2I0 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, -^ 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 divi-
sions 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
h-assica oleracea : cir-
cumnutation of a
cotyledon, the hypo-
cotyl having been
secured to a stick,
traoed on a horizon-
tal glass, in dark-
ness, from 8.15 A.M.
to 10.30 P.M. Move-
ment of the bead of
the filament magni-
fied 13 times.
Ouap. I. GITHAGO. 21
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. y^ 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 move-
ment 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 lh., 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.
Bithago 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
Uluminated from above, and the circumnutation of the hypo-
22 CIKCUMNUTATION OF SEEDLINGS. CuAr. 1
cotyl 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 coty-
ledons, but as they close together unless kept exposed to a mode-
rately bright light, and as the hypocotyl is extremely heliotropic,
the necessary arrangements were too
troublesome. We shall recur to the noc-
turnal or sleep-movements of the cotyle-
dons in a future chapter.
Gossypium (var. Nankin cotton) (Mal-
vaceae). — The circumnutation of a hypo-
cotyl was observed in the hot-house, but
Gossypium: arcumnu- the movement was so much exaggerated
trac°ed on aTriSJ that the bead twice P aSSed for a time 0ut ° f
tal glass, from 10.30 view. It was, however, manifest that two
a.m. to 9.30 a.m. on somewhat irregular ellipses were nearly
following morning, comp i et ed in 9 h. Another seedling,
by means of a fila- .. , . r . , . , , , , , .
ment fixed across *-* m - m beight, was then observed durmg
its summit. Move- 23 h. ; but the observations were not
ment of bead of fila- ma a e a t sufficiently short intervals, as
TSLTSSSfSZ shOT ™ h ? the few dots in K s- 12 > and the
minated from above, 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 coty-
ledons rose from 10.30 a.m. to about 3 p.m. ; they then sank tiD
10 p.m., rising, however, greatly in the latter part of the night
Chap I. GOSSYriUM. 23
The angles above the horizon at which the cotyledons of another
seedling stood at different hours is recorded in the following
6hort table : — ■
Oct. 20 2.50 p.m 25° above horizon.
» 4.20 , 22°
„ 5.20 „ 15°
„ 10.40 „ 8 1
Oct. 21 8.40 A.M 28° „
„ 11.15 „ 35°
„ 9.11 r.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 move-
ments 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 circumnuta-
tion. 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.
(Jxalis 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 ir ch
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 loDg
as it remained nearly horizontal ; but in the course of the day it
both rose considerably above and fell beneath a horizontal posi-
tion, and then of course the movement was much exaggerated.
24
CIECUMXUTATION OF SEEDLINGS.
Chap. I.
In Fig. 13 its course is shown from 6.45 a.m. on June 17th, to
7.40 a.m. on the following morn-
ing ; 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 verti-
cally ; 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 con-
tinued to rise until 8 a.m., when
they stood almost horizontally.
Their movement was traced dur-
ing the whole of this day and
until the next morning; but a
tracing is not given, as it was
closely similar to Eig. 13, except-
ing that the lines were more
zigzag. The cotyledons moved
7 times, either upwards or down-
wards; and at about 4 p.m. the
great nocturnal sinking move-
ment commenced.
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 cotyle-
don moved 7 times upwards or
downwards (Fig. 14). The noc-
turnal sinking movement, which
is merely a great increase of one
of the diurnal oscillations, com-
menced about 4 p.m.
Oxales VaJdiviana.-— This species is interesting, as the coty-
fi°30'a.m.
U
Oxalis rosea: circumnutation of
cotyledons, the hypocotyl being
secured to a stick ; illumina-
ted from above. Figure here
given one-halt of original scale.
Chap. I.
OXALIS.
25
ledons rise perpendicularly upwards at night so as to come into
close contact, instead of sinking vertically downwards, as in the
case of 0. rosea. A glass filament was fixed to a cotyledon,
17 of an inch in length, and the hypocotyl was left free. On
Fig. 14.
8°12W.
^ f 6°40'a.m.l9 f >
i.
Fig. 15.
9*28'
5°45'
7°22 r p.fli.
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| inches from the vertical
glass. Figure here given one-half of
original scale.
Oxalis Valdiviana : conjoint
circumnutation of a cotyle-
don and the hypocotyl, traced
on vertical glass, during 24
hours. Figure here given
one-half of original scale ;
seedling illuminated from
above.
the first day the seedling was placed too far from the vertical
glass j S3 that the tracing was enormously exaggerated and tho
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
Ihe following morning (June 19th) the apex of a cotyledon was
26 CIRCUMNUTATION OF SEEDLINGS. Chap. 1
placed only If inch from the vertical glass. At 6.40 a.m. it
stood horizontally; it then fell till 8.35, and then rose. Al-
together 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 coty-
ledons 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, 1he 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 move-
ments of the cotyledons alone, with the exception of the one
great afternoon curvature to the left.
Oxalis corniculata (var. cupred).— 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 tc
rise until very late in the evening.
Oxalis (Biophytum) sensitiva. — The cotyledons are highly re-
markable 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 22 i°
to 24s ° 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 rn. The cotyledons of this plant sleep at night by rising
Chap. I.
TEOP^OLUM.
27
Fig. 16.
vertically and coming into close contact. This upward move-
ment 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.
Tropxolum 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
Bummit arched abruptly down-
wards, 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 con-
cave surface of the arch, was caused
by the basal leg bending back-
wards from the upper part, that is
in a direction opposite to the depen-
dent 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 move-
ments of a young plant bearing a few leaves and of a mature
plant, will hereafter be described.
Tropceolum minus (?) : circum-
nutation of buried and arched
epicotyl, traced on a horizon-
tal glass, from 9.20 a.m. to
8. 15 P.M. Movement of bead
of filament magnified 27
times.
28
CIRCUMNUTATION OF SEEDLINGS. Chap, t
Citrus aurantinm (Orange) (Aurantiacese). — 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.
Fie. 17.
Citrus aurantium: circumnutation of epicotyl with a filament fixed traus*
versely 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 10^, in figure here given,
and afterwards 36 times, or 18 as here given ; seedling illuminated
from above.
JEsculus 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 down-
ward course pressed with unequal force on the plates, as
Chap. I
VICIA.
29
Fig. 18.
JEsculus h'ppocastanum : out«
lines of tracks left on in-
clined 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
larger.
was a trifle
the tracks varied in breadth. The more perfectly serpentine
tracks made by the radicles of Phaseolus multijlorus and Vicia
faba (presently to be described), render
it almost certain that the radicles of
the present plant circumnutated.
Phaseolus multijlorus (Leguminosse).
—Four smoked glass-plates were ar-
ranged in the same manner as des-
cribed under iEsculus, and the tracks
left by the tips of fonr radicles of the
present plant, whilst growing down-
wards, were photographed as trans-
parent 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 direc-
tions — 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 ascer-
tained by measurement with an
eye-piece micrometer.
Vicia faba (Common Bean)
(Leguminosse). — 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
npside 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
Fig. 19.
i
Phaseolus multijlorus : 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.
30
CIRCUMNUTATION OF SEEDLINGS. Chap. 1
abruptly, then made a small loop and then a larger zigzag
curve. During the night arid till 11a.m. on the following
Fig. 20.
Vicia faba: circumnutation of a radicle, at first pointing vertically up-
wards, kept in darkness, traced on a horizontal glass, during 14 hour.*.
Movement of bead of filament magnified 23 times, here reduced to
one-half of original scale.
morning, the head 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. 21.
0. D. E.
ficia 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.
Chap. I. VICIA. 31
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 .ZEsculus 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 circumnuta-
tion. 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 protuber-
ance 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 re-
moved 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 move-
ments of the tied arches were traced in the usual manner on
32 CIRCUMNUTATION OF SEEDLINGS. Chap. L
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 con-
clude 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 some-
what 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
Fig. 22.
Vicia faba: circumnutation of young epicotyl, traced in darkness during
50 hours on a horizontal glass. Movement of bead of filament mag-
nified 20 times, here reduced to one-half of original scale.
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 planttd, this part of the
epicotyl would still have been beneath the soil.
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
Chap. I.
LATHYRUS.
33
during 38 h. (Fig. 23). We here again have plain evidence of
continued circunmutation. Had the bean been properly planted,
the part of the epicotyl to which the filament was attached, the
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.
movement of which is here shown, would probably have just
risen above the surface of the ground.
Lathyrus nissolia (Leguminosse). — This plant was selected for
observation from being an abnormal form with grass-like leaves.
Fig. 24.
Lathyrus n'ssolia: 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 re-
duced to one-half of original scale.
The cotyledons are hypogean, and the epicotyl breaks through
the ground in an arched form. The movements of a stem, 12
inch in height, consisting of three internodes, the lower one
almost wholly subterranean, and the upper one bearing a short,
34
CIKCUMNUTATION OF SEEDLINGS.
Chap. I.
narrow leaf, is shown during 2i 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 de-
scribed 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.
Cassia tora* (Leguminosae). — A seedling was placed before a
Fig. 25
ram. riO>a.m.B&
9°a.m
Cteril 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.
* Seeds of tl.is plant, which
grew near the sea-side, were sent
to us by Fritz Miiller from S.
Brazil. The seedlings did not
flourish or flower well with us;
they were sent to Kew, and were
pronounced not to be distinguish-
able from C. tora.
Ohap. I. LOTUS. 35
north-east window ; it bent very little towards it, as the bypo-
cotyl 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 com-
menced about 4.30 p.m.
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 Jacobosus (Leguminosse). — 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 i'n
this manner for a long time even after the development of some
of the true leaves. With seedlings, 3 inches in height, and bear-
ing five or six leaves, they roso at night about 45°. They con-
tinued to act thus for about an additional fortnight. Subse-
quently they remained horizontal at Dight, though still green,
36 CIKCUMNUTATICXN" OF SEEDLINGS. Chap. 1.
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 (1£ 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 move-
ment. 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 17 £° 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
r.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
Chap. L CYTISUS. 37
rise did not coincide with that of the great diurnal fall, so that
narrow ellipses were described, as is the usual rule with circum-
nutating organs. The cotyledons are provided with a pulvinus,
and its development will hereafter be described.
Mimosa pudica (Leguminosse). — The cotyledons rise up verti-
cally 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 iti
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 morn-
ing 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, more-
over, to be protected from lateral light, and perhaps were not
sufficiently illuminated. Under these circumstances the coty-
ledons moved simply downwards from 7 a.m. till 2 p.m., after
which hour and during a large part of the night they con-
tinued 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.
Cytisusfra grans (Leguminosae). — Only a few observations were
made on this plant. The hypocotyl circumnutated to a con-
siderable 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
38 CIKCUMNUTATION OF SEEDLINGS. Cbap. 1.
move both up and down and laterally ; but they do not rise up
at night in a conspicuous manner.
Lupinus luteus (Leguminosse). — 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 daik. 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 14g hours
was only fa of an inch ; it sometimes travelled at the rate of
fa of an inch in an hour.
Cucurbita ovifera (Cucurbitaceae). — Radicle : a seed which had
Fig. 26.
A-
Cucurbita ovf era: course followed by a radicle in bending geotropicnlly
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.
germinated on damp sand was fixed so that the slightly curved
radicle, which was only - 07 inch in length, stood almost vertically
Char I.
CUCUKBITA.
39
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 -15° 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 fila-
ment, 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 littlo
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.
Four radicles were allowed to grow downwards over plates
of smoked glass, inclined at 70° to the horizon, under the
Fig. 27.
A. B.
Cucurbita ovifera: tracks
left by tips of radicles
in growing downwards
over smoked glass-
plates, inclined at 70°
to the horizon.
C
Fig. 28.
--<-- '
J
Cucurbita ovifcra : circumnuta-
tion 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 move-
ment of the bead magnified
20 times, here reduced to one-
half of original scale.
same conditions as in the cases of JEsculus, Phaseolus, and
Vicia. Facsimiles are here given (Fig. 27) of two of these
tracbs ; 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
4
w
CIRCUMXUTATION OF SEEDLINGS.
Chap, t
Fie;. 29.
pressed alternately with greater and less force on them. There
must, therefore, havo 1™en 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 im-
portant 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.
Hypocotyl. — A seed lying on damp sand was firmly fixed by
two crossed wires and by its own growing radicle. The cotyle-
dons 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 cotyle-
dons. This part would ultimately
form the upper end of the hypo-
cotyl, 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; ard this
movement would aid in dragging
them out of the seed-coats, which
are held down by a special struc-
ture 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 observa-
tions apply to the leg of the arch next to the cotyledons, but
Cucurbit a oti'j
circumnuta-
tion of straight and verti-
cal hypocotyl, with filament
fastened transversely across
its upper end, traced in dark-
ness 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 4^
times.
Uhap. I. CUCUEBITA. 41
the other leg adjoining the radicle likewise circumnutated at on
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 cotyle-
dons 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 8745 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
vortically and were almost closed. At 7 a.m. on. the third morn-
ing 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 fioni this hour till 3 p.m. the
£2 CIKCUMNUTATION OF SEEDLINGS. Cha*. J,
cotyledon fell; it then rose rapidly till 9 p.m., then verj
gradually till 3 a.m. February 19th,. after which hour it sank
gradually till 4.30 p.m. ; but the downward movement was inter-
rupted by one slight rise or oscillation about 1.30 p.m. Aftei
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 aurardia. — 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 sur-
face of the soil, the tied arch changed its course greatly nine
times in the course of 14J h. It swept a large, extremely irre-
gular, 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 per-
manently reflexed. These cotyledons, at 7 a.m. on the following
morning, were again well expanded.
Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbi-
taceae). — 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 26 h°. 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 thre6
days in a closely similar manner. The cotyledons, therefore.
Uhap. J.
CUCUKLITA.
43
Fig.
10° 95 p.m.
12th '
G"38 r a.m.\
13^
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.
In order to ascertain more ac-
curately the nature of these move-
ments, 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 17£° 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 13 th (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 r.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 morn-
ing 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 coty-
ledons became more widely open at
,7°35'}
.10° a. m*
10° 3 0rp.i
Vl 14t/i
Lagcnaria vulgaris: circumnu-
tation of a cotyledon, 1§
inch in length, apex only 4f
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.
noon on each succeeding day ; and
that they rose considerably each night, though not acquiring
a vertical position, except during the first two nights.
(Jucumis dudaim (Cucurbitaceae). — Two seedlings had opened
34
CIRCUMNUTATION OF SEEDLINGS.
Chap. I.
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.
Opuntia basilaris (Cactese). — A seedling was carefully ob-
served, because considering its
appearance and the nature of the
mature plant, it seemed very un-
likely that either the hypocotyl or
cotyledons would circumnutate to
an appreciable extent. The coty-
ledons 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 dia-
meter. The tracing (Fig. 31) shows
the combined movement of the
Fig. 31.
I
Opuntia basilaris : conjoint cir-
cumnutation of hypocotyl
and cotyledon ; filament hypocotyl and of one of the COty-
fixed longitudinally to coty- ^ {mm 44& Qn M 2gth
ledon, and movement traced ' J
during 66 h. on horizontal to 11 A.M. on the 31st. On the 29th
glass. Movement of the ter- a nearly perfect ellipse was com-
pleted. On the 30th the hypocotyl
moved, from some unknown cause,
minal bead magnified about
30 times, here reduced to one-
third scale. Seedling kept in
hot-house, feebly illuminated in the same general direction in a
from above. 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 cf the bead at the end of the
Uhak L
PRIMULA.
45
filament was, as far as could be ascertained, about "14 of an
inch.
lldianthus annuus (Composite). — The upper part of the
hypocotyl moved during the
day-time in the course I r ig- 32 -
shown in the annexed figure
(Fig. 32). As the line runs
in various directions, cross-
ing itself several times,
the movement may be con-
sidered as one of circumnu-
tation. The extreme actual
distance travelled was at
least *1 of an inch. The
movements of the cotyle-
dons of two seedlings were
observed; one facing a north-
east window, and the other
so feebly illuminated from
above 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-clay, 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 (Primulacere). — 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 clown to a certain extent both during the
day and night. The movements of the same seedling were traced
HelianfJius annuus : eircumnutation 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 follow-
ing morning. Movement of bead
magnified 21 times, here reduced to
one-half of original scale.
m
CIRCUMNUTATION OF SEEDLINGS. Chap. 1
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 (Primulaceao). — This plant is generally sup-
posed to produce only a single cotyledon, but Dr. H. G-ressner *
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 hjpocotyl or epicotyl of any ordinary dicotyle-
donous 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 24 2 h. on a
horizontal glass, magnified 50
\ Z
34.
times ; and in this interval it
described two irregular small
circles; it therefore circum nu-
tates, though on an extremely
small scale.
Stapelia mrpedon (Ascle-
piadese). — This plant, when
mature, resembles a cactus.
The flattened hypocotyl is
fleshy, enlarged in the upper
part, and bears two rudimen-
tary cotyledons. It breaks
through the ground in an arched form, with the rudimentary
cotyledons closed or in contact. A filament was affixed almost
\H "Vr^;
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
* 'Bot. Zeitung,' 1S74, p. 837.
Ohai. I. IPOMCEA. 47
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, anl as this was effected by a zigzag course, it probably
circumnutated ; but with hardly any other seedliDg observed
by us was this movement so obscurely shown.
' Ipomoea ccerulea vel Pharbitis nil (Convolvulacese). — 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 maimer.^ In
this particular instance the fifth internode (including the hypo-
cotyl) was the first which plainly circumnutated and twined
round a stick. We therefore wished to learn whether circum-
nivtation 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 re-
markably 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 3? 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 with-
out the aid of marks.
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 mid-
night (12.30 p.m.) it had risen a very little, and considerably bj
Movements and Habits of Climbing Plants/ p. 33, 1875.
48
CISCUMNUTATION OF SEEDLINGS. Chap. I
3.45 am. When again looked
Fig. 35'.
\5°j>.m.
tf'.io'.ujn 21 s ?
IO~30.jp.Ta.21
[pomcea ccsrulea : circumnutation of
cotyledon, tracod 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 mag-
nified ; temp. 20° C.
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. aud 10.30
p.m.) thirteen times.
The cotyledons of this seed-
ling 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 seed-
lings 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 be-
tween 30° and 35°; but three
stood at about 50° and one at
even 70° beneath the horizon.
The blades of all these cotyle-
dons had attained almost their
full size, viz. from 1 to li 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 tire
same manner— they do not sink
Chap. T. CERINTHE. 49
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 seed-
ling were almost paralysed by having been similarly kept during
two whole days. The cotyledons of several other species if
Ipomcea likewise sink downwards late in the evening.
Cerinthe major (Boragineae). — The circumnutation of the
hypocotyl of a young seedling with the cotyledons hardly
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.
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 i n 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.
w>
CIRCUMXUTATIOX OF SEEDLINGS.
Chap. I
Fig. 37.
NoJana prostrata (NolanesB). — 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
g^oth of an inch. The apex of one of the cotyledons crossed
rather obliquely four divisions in 13 minutes ; it was also sink-
ing, 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 dark-
ness, it crossed two divisions in 4 m. 15 s., there-
fore at a quicker rate. In the afternoon, after a
longer interval in the dark, the apex was motion-
less, 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 circumnu ra-
ting.
Solarium ly copers icum (Solaneae) — The move-
ments of the hypocotyls of two seedling to-
matoes were observed during seven hours, and
there could be no doubt that both circumnu-
tated. They were illuminated from above, but
by an accident a little light entered on one side,
across its sum- and in the accompanying figure (Fig. 37) it
mit, traced on
horizontal glass,
from 10 a.m. to
5 p.m. Oct. 24th.
Illuminated ob-
liquely from
above. Move-
ment of bead
Solanum lycoper-
sicum : circum-
nu tation of hy-
pocotyl, with
filament fixed
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 move-
ments of the cotyledons were also traced both
on vertical and horizontal glasses ; their angles
with the horizon were likewise measured at
magnified about various hours. They fell from 8.30 a.m. (October
35 times, here 17th) to about noon ; then moved laterally in a
reduced to one- z jp. za g u ne anc [ a t about 4 p.m. began to rise ;
third of original & & ' °
scale- 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.
Uhap. l solanum. 61
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 up-
wards. 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 com-
mencement of the straightening of the hypocotyl. In order to
prevent this latter movement, the two legs of an arch, the
Fig. 38.
Solanum palinacanthum: eircumnutatiou 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.
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.
The cotyledons of some seedlings in the hot-house were hori-
zontal 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
52
CTRCUMNUTATION OF SEEDLINGS. Ciiak 1
Fig. 39.
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 (Chenopodese). — 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 trac-
ings 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 move-
ment of the hypocotyl during 9 h. under
these circumstances. A second seedhng
was similarly observed at the same time,
and the tracing had the same peculiar
character, due to the hypocotyl often mov-
ing and returning in nearly parallel lines.
The movement of a third hypocotyl differed
greatly.
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 verti-
cally 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 ft was still dark, two of them
Betatu'garis: circum-
nutatioa of hypo-
cotyl, with filament
fixed obliquely a-
cross 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.
Ohap. I. RICINUS AND QUEECUS. 53
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 occa-
sion) w r ere again completely closed. On the morning of the
29th they were open, but at night only one of the four w r as
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 Borhouiensis (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.) (Cupuliferas). — 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 m a conspicuous manner, changing its
course many times and travelling in all directions during the
43 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
54
ClliCUMNUTATION OF SEEDLINGS. Chap. L
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.
Qwrcus (American sp.) : circumnutation of young stem, traced on hori-
zontal glass, from 12.50 P.M. Feb. 22nd to 12.50 P.M. 24th. -Movement
of bead gieatly 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 move-
ments 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
Chap. L QUEECUS AND COEYLUS. 55
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
ladicle of the oak circumnutated whilst growing downwards.
Fig. 41.
\
Quercus rohur : 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.
Cdrylus aveJIana (Cory lacese).— 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 illumi-
nated 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. Con-
sequently a long line was traced on the horizontal glass ; and
5
Ml
CIKCUMNUTATION OF SEEDLINGS.
Chap. 1
Fisc. 42.
this was in parts straight and in parts decidedly zigzag,
indicating circnmnutation.
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 pro-
jected 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 circum-
nutating course pursued is shown
in the accompanying figure (Fig.
42). The actual distance traversed
from side to side was about '04 of
an inch.
Finns pinaster (Conifers). — 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,
\
Corylus avellana: circumnuta-
tion 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 ol
bead magnified about 27
times.
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 abcu*
35 times.
Chaf. I. PINUS AND CYCAS. 57
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.
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
Ficr. 44.
A.
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.
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 longitu-
dinally 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
dui-ing the day.
58 CIRCUMNUTATION OF SEEDLINGS. Chap I
Cycas-vectinata (Cycadeae). — 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 2i 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) slows how large, complex, and rapid were the circ urn-
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.
nutating movements. The extreme distance from side to side
which it passed over amounted to between *6 and *7 of an
inch.
fauna Warscewiczii (Cannacese).— A seedling with the plu-
mule 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 incom-
plete irregular circles or ellipses in the course of 48 hours. The
plumule is straight ; and this was the first instance observed
Cuap. L
ALLIUM.
59
by us of the part that first breaks through the ground not
being arched.
Fig, 46.
Canna Warscevciczii : circumnutation of plumule with filament affixed
obliquely to outer sheath-like leaf, traced in darkness onhorizontal glass
from 8.45 a.m. Nov. 9th to 8.10 a.m. 11th. Movement of bead mag-
nified 6 times.
Allium cepa (Liliaceae).— -The narrow green leaf, which pro-
trudes from the seed of the common onion as a cotyledon,*
breaks through the grouud 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,f and answers the same end as the knife-like white
crest on the summit of the straight cotyledon of the Graminese.
* This is the expression used
by Sachs in his ' Text-book of
Botany.'
t Haberlandt has briefly de-
scribed ('Die Schutzeinrichtun-
gen . . . Keimpflanze,' 1877, p. 77)
this curious structure and the
pitrpose 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.
60
CIKCUMNUTATION OF SEEDLINGS.
Chap. I.
Fig. 47.
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 ox
never becoming perfectly straight, these cotyledons differ remark-
ably 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.
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) indi-
cates 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.
Asparagus officinalis (Asparagese). — ■
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 depth 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 lb. 15m. the plumule moved to
the right, and daring 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
Allium cepa : circumnu-
tation 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.
Chap. 1
ASPABAGU3.
61
tlie 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 move-
Ficr. 48.
Asparagus officinalis : circumnutation of plumules with tips whitened and
marks placed beneath, traced ou 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.
ments 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 green-
house on one side than on any other. All five tracings re-
sembled 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
62 CIRCUMNUTATION" OF SEEDLINGS. Chap. I
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 3^ ellipses, had not the stem been
drawn to one side until 1 p.m., after which hour it moved back-
wards. Ou 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 -jL-th of an inch. The seedling of which the movements
are shown in Fig. 48, B, was If 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.
Phalaris Canarie.nsis (Gramineae). — "With the Graminese 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 authori-
ties to be a cotyledon, which term we will use without venturing
to express any opinion on the subject.* 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.
The movements of three rather old seedlings, about li inch
in height, shortly before the protrusion of the leaves, were first
traced. They were illuminated exclusively from above; for, ns
will hereafter, be shown, they are excessively sensitive to thd
* We are indebted to the Rev. this subject, together with r&
G. H^nslow for an abstract of the ferences.
dews which have been held on
CUAP. I,
PHALAEIS.
08
Fig. 49
Phalaris Conan'ensis : circumnu-
tation 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.
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.
A seedling which had just broken
through the ground and projected
only •Jgth 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 observa-
tion ; 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.
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 micro-
Phalaris Canariensis : circumnu-
tation of a very young coty-
ledon, 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-fov.rth of original scale.
64
CIRCUMNUTATION OF SEEDLINGS.
Chap. L
scope with a micrometer eye-piece, so arranged that each
division equalled -^oth of an inch. After an interval of 30 in.
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. 50 s. The seedling
was again observed after an intervalof 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 ^tli 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 circum-
nutate 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.
Zea mays (Grarninepe). — A glass filament was fixed obliquely
to the summit of a cotyledon,
Fig. 51. rising - 2 of an inch above the
ground ; but by the third morn-
ing it had grown to exactly
thrice this height, so that the
distance of the bead from the
mark below was greatly in-
creased, consequently the trac-
ing (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 precau-
tion, 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
Zea mays : circumnutation of cotyle-
don, traced on horizontal glass, from
8.30 A.M. Feb. 4th to 8 a.h. 6th.
Movement of bead magnified on an
average about 25 times.
Chap. I. PHALAKIS 65
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 down-
wards 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 dif-
ferent parts, or in little bridges of soot being-
left, that the apex had come alternately into Fig. 52.
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 ,-, L ,
, .. , ... , Zea mays : track
delicate radicles. left on incliued
Avena saliva (Gramineze). — A cotyledon, lh smoked glass-
inch in height, was placed in front -of a north- P late b ^ t |P
east window, and the movement of the apex growLgdown-
was traced on a horizontal glass during two wards,
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 during 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.
66
CIKCUMNUTATION OF SEEDLINGS. Chap, t
A filament was next fixed to a cotyledon only £ 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 5j hours.
Nephrodium molle (Filices). — A seedling fern of this species
*'ig. 53.
came up by chance in a flower-
pot 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 circumnuta-
tion 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.
Selaginella Kraussii (?) (Lycopodiaceae).
— A very young plant, only '4 of an inch
young plant, kept in j n height, had sprung up in a pot in the
8*45*1*1*1? lVlm *ot-house. Al1 extremely fine glass fila-
Oct. 31st. ment 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
ihat this little plant circumnutated
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.
Fig. 54.
Selaginella Kraussii (?) :
circumnutation of
Chap. II. CIEOUMNUTATION OF SEEDLINGS. 67
CHAPTER II.
General Con^iperat'ons on the Movements and Growth o»
Seedling Plants.
Generality of the cireumimtating movement — Eadicles, their circum-
nutation 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 Megar.
rhiza, &c. — Abortion of cotyledons— Circumnutation of hypocotyls
and epicotyls whilst still buried and arched — Their power of
straightening themselves — Bursting of the seed-co r tts— Inherited
effect of the arching process in hypogean hypocotyls— Circumnuta-
tion of hypocotyls and epicotyls when erect — Cii cumnutation of
cotyledons — Pulvini or joints of cotyledons, duration of their
activity, rudimentary in Oxalis corniculata, their development —
Sensitiveness of cotyledons to light and consequent disturbance of
their periodic movements — Sensitiveness of cotyledons to contact.
The cireumimtating movements of the several parts
or organs of a considerable number of seedling plants
have been described in the last chapter. A list is here
appended of the Families, Cohorts, Sub-classes, &c,
to which they belong, arranged and numbered ac-
cording to the classification adopted by Hooker.*
Any one who will consider this list will see that the
young plants selected for observation, fairly represent
the whole vegetable series excepting the lowest
cryptogams, and the movements of some of the lattei
when mature will hereafter be described. As all the
seedlings which were observed, including Conifers.
Cycads and Ferns, which belong to the most ancient
* As given in the ' General System of Botany.' by Le Maout and
Decaisne, 1873.
68
CffiCUMXUTATIOX OF SEEDLINGS. Ce^p. H,
types amongst plants, were continually circumnu-
tating, we may infer that this kind of movement is
common to every seedling species.
Sub-Kingdom I. — Phaeiiogamous Plants.
Class I.— Dicotyledons.
Sub-class I
Familv.
14. Cruciferce.
26. Caryophyllece.
36 Malvacece.
41. Oxalidece.
49. Tropoeolece.
52. Aurantiaccce.
70. Hippocastanece.
75. Leguminosce.
106. Cucurbitacece.
109. Cactece.
122. Composites.
135. Primulacece.
145. Asclepiadece.
151. Convolvulaccce.
154. Borraginece.
156. Isolanece.
157. Solanece.
181. Chenopodiece.
202. Euphorbiaceoe.
211. Cupuliferce.
212. Corylaceas.
—Angiosperms.
Cohort.
II. Parietales.
IV. CARYOPHTLLALEi
VI. Maxyales.
VII. Geraniales.
Ditto
Ditto
X. Sapindales.
XI. ROSALES.
XII. PASSIFLO RALES.
XIV. FlCOIDALES.
XVII. ASTRALES.
XX. Primulales.
XXII. Gentianales.
xxiii. polemoxialks.
Ditto
Ditto
XXIV. Solaxales.
xxvii. chenopodiale&
xxxii. euphorbiales.
xxxvi. querxales
Ditto
Sub-class II. — Gymnosperms.
223. Coniferce.
224. Cycadeae.
Class II.-— Monocotyledons.
2. Cannaceoe,
34. Liliacecc.
41. Asparagccc.
55. GramincGC.
II. Amojiales.
XI. LlLIALES.
Ditto
XV. Glumales.
Sub-Kingi>om II. — Cryptcgamic Plants.
1. Filices. I. FlLICALES.
6. LycopodiacccB. Ditto
Chap. II. ACTION OF THE RADICLE. 69
Radicles. — In all the germinating seeds observed
by us, the first change is the protrusion of the
radicle, which immediately bends downwards and
endeavours to penetrate the ground. In order to
effect this, it is almost necessary that the seed should
be pressed clown so as to offer some resistance, unless
indeed the soil is extremely loose ; for otherwise the
seed is lifted up, instead of the radicle penetrating
the surface. But seeds often get covered by earth
thrown up by burrowing quadrupeds or scratching
birds, by the castings of earth-worms, by heaps of
excrement, the decaying branches of trees, &c, and
will thus be pressed clown ; and they must often fall
into cracks when the ground is dry, or into holes.
Even with seeds lying on the bare surface, the first
developed root-hairs, by becoming attached to stones
or other objects on the surface, are able to hold down
the upper part of the radicle, whilst the tip pene-
trates the ground. Sachs has shown* how well and
closely root-hairs adapt themselves by growth to the
most irregular particles in the soil, and become firmly
attached to them. This attachment seems to be
effected by the softening or liquefaction of the outer
surface of the wall of the hair and its subsequent
consolidation, as will be on some future occasion
more fully described. This intimate union plays an
important part, according to Sachs, in the absorption
of water and of the inorganic matter dissolved in it.
The mechanical aid afforded by the root-hairs in pene-
trating the ground is probably only a secondary
service.
The tip of the radicle, as soon as it protrudes from
the seed-coats, begins to circumnutate, and the whole
* ' Physiologie Vegetate,' 1S6S, pp. 109, 205.
70 ACTION OF THE EADICLE. Chap. II
growing part continues to do so, probably for as long
as growth continues. This movement of the radicle
has been described in Brassica, iEsculus, Phaseolus,
Yicia, Cucurbita, Quercus and Zea. The probability
of its occurrence was inferred by Sachs,* from radicles
placed vertically upwards being 'acted on by geotro-
pism (which we likewise found to be the case), for if
they had remained absolutely perpendicular, the attrac-
tion of gravity could not have caused them to bend to
any one side. Circumnutation was observed in the above
specified cases, either by means of extremely fine fila-
ments of glass affixed to the radicles in the manner
previously described, or by their being allowed to
grow downwards over inclined smoked glass-plates, on
which they left their tracks. In the latter cases the
serpentine course (see Figs. 19, 21, 27, 41) showed
unequivocally that the apex had continually moved
from side to side. This lateral movement was small
in extent, being in the case of Phaseolus at most
about 1 mm. from a medial line to both sides. But
there was also movement in a vertical plane at right
angles to the inclined glass-plates. This was shown
by the tracks often being alternately a little broader
and narrower, due to the radicles having alternately
pressed with greater and less force on the plates.
Occasionally little bridges of soot were left across the
tracks, showing that the apex had at these spots been
lifted up. This latter fact was especially apt to occui
* ' Ueber das Wachsthum der had previously remarked (' Bei
Wurzeln : Arbeiten des bot. In- tr'age zur Pflanzenphysiologie,
3tituts in Wiirzburg,' Heft iii. 1868, p. 81) on the fact of radicles
1873, p. 460. This memoir, be- placed vertically upwards being
sides its intrinsic and great in- acted on by geotropism, and ho
rarest, deserves to be studied as a explained it by the supposition
model of careful investigation, that their growth was not equa]
and we shall have occasion to on all sides,
refer to it repeatedly. Dr. Frank
Ckap. II. ACTION OF THE RADICLE. 71
when the radicle instead of travelling straight down
the glass made a semicircular bend ; but Fig. 52
shows that this may occur when the track is rectilinear.
The apex by thus rising, was in one instance able to
surmount a bristle cemented across «an inclined glass-
plate ; but slips of wood only ^ of an inch in thickness
always caused the radicles to bend rectangularly to
one side, so that the apex did not rise to this small
height in opposition to geotropism.
In those cases in which radicles with attached fila-
ments were placed so as to stand up almost vertically,
they curved downwards through the action of geotro-
pism, circumnutating at the same time, and their
courses were consequently zigzag. Sometimes, how-
ever, they made great circular sweeps, the lines being
likewise zigzag.
Eadicles closely surrounded by earth, even when
this is thoroughly soaked and softened, may perhaps
be quite prevented from circumnutating. Yet we
should remember that the circumnutating sheath-like
cotyledons of Phalaris, the hypocotyls of Solanum,
and the epicotyls of Asparagus formed round them-
selves little circular cracks or furrows in a superficial
layer of damp argillaceous sand. They were also
able, as well as the hypocotyls of Brassica, to form
straight furrows in damp sand, whilst circumnutating
and bending towards a lateral light. In a future
chapter it will be shown that the rocking or circum-
nutating movement of the flower-heads of Trifolium
subterraneum aids them in burying themselves. It is
therefore probable that the circumnutation of the tip
of the radicle aids it slightly in penetrating the
ground ; and it may be observed in several of the
previously given diagrams, that the movement is
more strongly pronounced in radicles when they first
6
72 ACTION OF THE RADICLE. Chap. II
protrude from the seed than at a rather later period j
but whether this is an accidental or an adaptive
coincidence we do not pretend to decide. Never-
theless, when young radicles of Phaseolus multiflorvs
were fixed vertically close over damp sand, in the
expectation that as soon as they reached it they
would form circular furrows, this did not occur, — a
fact which may be accounted for, as we believe, by
the furrow being filled up as soon as formed by the
rapid increase of thickness in the apex of the radicle.
Whether or not a radicle, when surrounded by soft-
ened earth, is aided in forming a passage for itself
by circumnutating, this movement can hardly fail
to be of high importance, by guiding the radicle
along a line of least resistance, as will be seen in the
next chapter when we treat of the sensibility of the
tip to contact. If, however, a radicle in its down-
ward growth breaks obliquely into any crevice, or a
hole left by a decayed root, or one made by the
larva of an insect, and more especially by worms, the
circumnutating movement of the tip will materially
aid it in following such open passage ; and we have
observed that roots commonly run down the old
burrows of worms.*
When a radicle is placed in a horizontal or inclined
position, the terminal growing part, as is well known,
bends down towards the centre of the earth; and
Sachs f has shown that whilst thus bending, the growth
of the lower surface is greatly retarded, whilst that
* See, also, Prof. Hensen's state- rows made by worms,
ments (' Zeitschiift fur Wissen, f ' Arbeiten des bot. Inst.
Zool.,' B. xxviii. p. 354, 1S77) to Wiirzburg,' vol. i. 1873, p. 4b* 1.
the same effect. He goes so far See also p. 397 for the length of
as to believe that roots are able the growing part, and p. 451 on
to penetrate the ground to a great the force of geotropi^m.
rlepth only by means of the bur-
Chav. tt. action of the kadicle. 73
of the upper surface continues at the normal rate,
or may be even somewhat increased. He has further
shown by attaching a thread, running over a pulley,
to a horizontal radicle of large size, namely, that
of the common bean, that it was able to pull up a
weight of only one gramme, or 15*4 grains. We may
therefore conclude that geotropism does not give a
radicle force sufficient to penetrate the ground, but
merely tells it (if such an expression may be used)
which course to pursue. Before we knew of Sachs'
more precise observations we covered a flat surface of
damp sand with the thinnest tin-foil which we could
procure ("02 to '03 mm., or '00012 to -00079 of an inch
in thickness), and placed a radicle close above, in such
a position that it grew almost perpendicularly down-
wards. When the apex came into contact with the
polished level surface it turned at right angles and
glided over it without leaving any impression ; yet
the tin-foil was so flexible, that a little stick of soft
wood, pointed to the same degree as the end of the
radicle and gently loaded with a weight of only a
quarter of an ounce (120 grains) plainly indented the
tin-foil.
Kadicles are able to penetrate the ground by the
force due to their longitudinal and transverse growth ;
the seeds themselves being held down by the weight
of the superincumbent soil. In the case of the bean
the apex, protected by the root-cap, is sharp, and
the growing part, from 8 to 10 mm. in length, is
much more rigid, as Sachs has proved, than the part
immediately above, which has ceased to increase in
length. We endeavoured to ascertain the downward
pressure of the growing part, by placing germinating
beans between two small metal plates, the upper one
of which was loaded with a known weight: and the
74 ACTION OF THE KADICLE. Chap. T£
radicle was then allowed to grow into a narrow hole iu
wood, 2 or 3 tenths of an inch in depth, and closed at
the bottom. The wood was so cut that the short space
of radicle between the month of the hole and the
bean could not bend laterally on three sides ; but it
was impossible to protect the fourth side, close to
the bean. Consequently, as long as the radicle con-
tinued to increase in length and remained straight,
the weighted bean would be lifted up after the tip
bad reached the bottom of the shallow hole. Beans
thus arranged, surrounded by damp sand, lifted up a
quarter of a pound in 24 h. after the tip of the
radicle had entered the hole. With a greater weight
the radicles themselves always became bent on the one
unguarded side; but this probably would not haye
occurred if they had been closely surrounded on all
sides by compact earth. There was, however, a
possible, but not probable, source of error in these
trials, for it was not ascertained whether the beans
themselves go on swelling for several days after they
have germinated, and after having been treated in
the manner in which ours had been ;
*_^_. namely, being first left for 24 h. in
water, then allowed to germinate in
— J very damp air, afterwards placed over
autiine of piece of the hole and almost surrounded by
stick (reduced to d d { j d fe
one-half natural r
size) with a hole We succeeded better in ascertaining
the°mdicie ^of'a tne force exerted transversely by these
bean grew. Thick- radicles. Two were so placed as to
ness of stick at , . n i i i ■ tmi
narrow* end -08 penetrate small holes made in little
inch, at broad end sticks, one of which was cut into the
hole'-i inch. shape here exactly copied (Fig. 55).
The short end of the stick beyond
the hole was purposely split, but not the opposite
CUAP. II.
ACTION OF THE RADICLE.
76
Fig. 56.
end. As the wood was highly elastic, the split 01
fissure closed immediately after being made. After
six days the stick and bean were dug out of the damp
sand, and the radicle was found to be much enlarged
above and beneath the hole. The fissure, which was
at first quite closed, was now open to a width of
4 mm. ; as soon as the radicle was extracted, it imme-
diately closed to a width of 2 mm. The stick was
then suspended horizontally by
a fine wire passing through the
hole lately filled by the radicle,
and a little saucer was sus-
pended beneath to receive the
weights ; and it required 8 lbs.
8 ozs. to open the fissure to the
width of 4 mm. — that is, the
width before the root was ex-
tracted. But the part of the
radicle (only *1 of an inch in
length) which was embedded in
-the hole, probably exerted a
greater transverse strain even
than 8 lbs. 8 ozs., for it had split
the solid wood for a length of
rather more than a quarter of
an inch (exactly "275 inch), and
this fissure is shown in Fig. 55.
A second stick was tried in the
same manner with almost ex-
actly the same result.
We then followed a better
plan. Holes were bored near
the narrow end of two wooden clips or pincers (Fig. 56),
kept closed by brass spiral springs. Two radicles in damp
«and were allowed to grow through these holes. The
Wooden pincers, kept closed by
a spiral brass spring, with a
hole (*14 inch in diameter
and "6 inch in depth) bored
through the narrow closed
part, through which a radicle
of a bean was allowed to
grow. Temp. 50°-60° F.
76 ACTION OF THE RADICLE. Chap 7T.
pincers rested on glass-plates to lessen the friction from
the sand. The holes were a little larger (viz. *14 inch)
and considerably deeper (viz. *6 inch) than in the
trials with the sticks ; so that a greater length of a
rather thicker radicle exerted a transverse strain.
After 13 days they were taken up. The distance of
two dots (see the figure) on the longer ends of the
pincers was now carefully measured ; the radicles were
then extracted from the holes, and the pincers of
course closed. They were then suspended horizontally
in the same manner as were the bits of sticks, and a
weight of 1500 grams (or 3 lbs. 4 ozs.) was necessary
with one of the pincers to open them to the same
extent as had been effected by the transverse growth
of the radicle. As soon as this radicle had slightly
opened the pincers, it had grown into a flattened form
and had escaped a little beyond the hole ; its diameter
in one direction being 4 - 2 mm., and at right angles
3*5 mm. If this escape and flattening could have
been prevented, the radicle would probably have
exerted a greater strain than the 3 lbs. 4 ozs. With
the other pincers the radicle escaped still further
out of the hole ; and the weight required to open
them to the same extent as had been effected by the
radicle, was only 600 grams.
With these facts before us, there seems little diffi-
culty in understanding how a radicle penetrates the
ground. The apex is pointed and is protected by
the root-cap ; the terminal growing part is rigid, and
increases in length with a force equal,, as far as our
observations can be trusted, to the pressure of at least
a quarter of a pound, probably with a much greater
force when prevented from bending to any side by the
surrounding earth. Whilst thus increasing in length
it increases in thickness, pushing away the damp
CiiAP II. HYPOCOTYLS AND EFICOTYLS. 77
earth on all sides, with a force of above 8 pounds in
one case, of 3 pounds in another case. It was impos-
sible to decide whether the actual apex exerts, relatively
to its diameter, the same transverse strain as the parts
a little higher up ; but there seems no reason to doubt
that this would be the case. The growing part there-
fore does not act like a nail when hammered into a
board, but more like a wedge of wood, which whilst
slowly driven into a crevice continually expands at
the same time by the absorption of water; and a
wedge thus acting will split even a mass of rock.
Manner in which Hypocotyls, Epicotyls, &c, rise up
and break through the ground. — After the radicle has
penetrated the ground and fixed the seed, the hypo-
cotyls of all the dicotyledonous seedlings observed by
us, which lift their cotyledons above the surface, break
through the ground in the form of an arch. When
the cotyledons are hypogean, that is, remain buried in
the soil, the hypocotyl is hardly developed, and the
epicotyl or plumule rises in like manner as an arch
through the ground. In all, or at least in most of such
cases, the downwardly bent apex remains for a time
enclosed within the seed-coats. With Corylus avel-
lena the cotyledons are hypogean, and the epicotyl
is arched; but in the particular case described in
the last chapter its apex had been injured, and it
grew laterally through the soil like a root; and in
consequence of this it had emitted two secondary
shoots, which likewise broke through the ground as
arches.
Cyclamen does not produce any distinct stem, and
only a single cotyledon appears at first ; * its petiole
* This is the conclusion arrived considered by other botflnisfs as
at by Dr. H. Gressner ('Bot. the first true leaf is really the
Zeitung,' 1874, p. 837), who second cotyledon, which is greatly
maintains that what has been delayed in its development.
78
HYPOCOTYLS, EPICOTYLS, ETC., Ciiap. it
Cyclamen
seedling,
larged :
Persicum :
figure en-
c, blade of
cotyledon, not yet
expanded, with arched
petiole beginning to
straighten itself; h,
hypocotyl developed
into a corm ; r, second-
ary radicles.
bleaks through the ground as an arch (Fig. 57).
Fig. 57. Abronia also has only a single fully
developed cotyledon, but in this
case it is the hypocotyl which first
emerges and is arched. Abronia
umbellata, however, presents this
peculiarity, that the enfolded blade
of the one developed cotyledon
(with the enclosed endosperm)
whilst still beneath the surface has
its apex upturned and parallel to
the descending leg of the arched
hypocotyl ; but it is dragged
out of the ground by the con-
tinued growth of the hypocotyl,
with the apex pointing downward.
With Cycas pectinata the cotyledons are hypogean,
and a true leaf first breaks
through the ground with
its petiole forming an
arch.
In the genus Acanthus
the cotyledons are likewise
hypogean. In A. mollis,
a single leaf first breaks
through the ground with
its petiole arched, and with
the opposite leaf much less
developed, short, straight,
of a yellowish colour, and
with the petiole at first not
half as thick as that of the
other. The undeveloped
leaf is protected by stand-
ing beneath its arched fel-
low; and it is an instruc-
Acanthus mollis: seedling, with the
hypogean cotyledon on the near
side removed and the radicles cut
off: a, blade of first leaf begin-
ning to expand, with petiole still
partially arched ; b, second and
opposite leaf, as yet very imper-
fectly developed : c, hypogean
cotvlcdon on the opposite side.
Ohai>. II. BREAKING THEOUGH THE GKOUND. 79
five fact that it is not arched, as it has not to force
for itself a passage through the ground. In the accom-
panying sketch (Fig. 58) the petiole of the first leaf
has already partially straightened itself, and the blade
is beginning to unfold. The small second leaf ulti-
mately grows to an equal size with the first, but this
process is effected at very different rates in different
individuals : in one instance the second leaf did not
appear fully above the ground until six weeks after the
first leaf. As the leaves in the whole family of the
Acanthacese stand either opposite one another or in
whorls, and as these are of equal size, the great in-
equality between the first two leaves is a singular fact.
We can see how this inequality of development and
the arching of the petiole could have been gradually
acquired, if they were beneficial to the seedlings by
favouring their emergence ; for with A. candelabrum,
sjmwsus, and latifolius there was great variability in the
inequality between the two first leaves and in the
arching of their petioles. In one seedling of A. can-
delabrum the first leaf was arched and nine times as
long as the second, which latter consisted of a mere
little, yellowish- white, straight, hairy style. In other
seedlings the difference in length between the two
] eaves was as 3 to 2, or as 4 to 3, or as only "76 to
• 62 inch. In these latter cases the first and taller leaf
was not properly arched. Lastly, in another seedling-
there was not the least difference in size between the
two first leaves, and both of them had their petioles
straight ; their laminae were enfolded and pressed
against each other, forming a lance or wedge, by
which means they had broken through the ground.
Therefore in different individuals of this same species
of Acanthus the first pair of leaves breaks through
the ground by two widely different methods ; and ii
80 HYPOCOTYLS, EPICOTTLS, ETC., Chap, a
either had proved decidedly advantageous or disad-
vantageous, one of them no doubt would soon have
prevailed.
Asa Gray has described * the peculiar manner of ger-
mination of three widely different plants, in which the
hypocotyl is hardly at all developed. These were there-
fore observed by us in relation to our present subject.
Delphinium nudicaule. — The elongated petioles of
the two cotyledons are confluent (as are sometimes
their blades at the base), and they break through the
ground as an arch. They thus resemble in a most
deceptive manner a hypocotyl. At first they are
solid, but after a time become tubular ; and the basal
part beneath the ground is enlarged into a hollow
chamber, within which the young leaves are developed
without any prominent plumule. Externally root-
hairs are formed on the confluent petioles, either a
little above, or on a level with, the plumule. The
first leaf at an early period of its growth and whilst
within the chamber is quite straight, but the petiole
soon becomes arched; and the swelling of this part
(and probably of the blade) splits open one side of
the chamber, and the leaf then emerges. The slit
was found in one case to be 3*2 mm. in length, and
it is seated on the line of confluence of the two
petioles. The leaf when it first escapes from the
chamber is buried beneath the ground, and now an
upper part of the petiole near the blade becomes
arched in the usual manner. The second leaf comes
out of the slit either straight or somewhat arched, but
afterwards the upper part of the petiole, — certainly in
some, and we believe in all cases, — arches itself whilst
forcing a passage through the soil.
Botanical Text-Book,' 1879, p. 22.
Ohap. II. BREAKING THROUGH THE GROUND. 81
Megarrhiza Calif ornica. — The cotyledons of this
Gourd never free themselves from the seed-coats and
are hypogean. Their petioles are completely con-
fluent, forming a tube which terminates downwards
in a little solid point, consisting of a minute radicle
and hypocotyl, with the likewise minute plumule
enclosed within the base of the tube. This structure
was well exhibited in an abnormal specimen, in which
one of the two cotyledons failed to produce a petiole,
whilst the other produced one consisting of an open
semicylinder ending in a sharp point, formed of the
parts just described. As soon as the confluent
petioles protrude from the seed they bend down, as
they are strongly geotropic, and penetrate the ground.
The seed itself retains its original position, either
on the surface or buried at some depth, as the case
may be. If, however, the point of the confluent
petioles meets with some obstacle in the soil, as
appears to have occurred with the seedlings described
and figured by Asa Gray,* the cotyledons are lifted
up above the ground. The petioles are clothed with
root-hairs like those on a true radicle, and they
likewise resemble radicles in becoming brown when
immersed in a solution of permanganate of potassium.
Our seeds were subjected to a high temperature, and
in the course of three or four days the petioles pene-
trated the soil perpendicularly to a depth of from
2 to 2J inches ; and not until then did the true
radicle begin to grow. In one specimen which was
closely observed, the petioles in 7 days after their
first protrusion attained a length of 2 J inches, and the
radicle by this time had also become well developed.
The plumule, still enclosed within the tube, was now
American Journal of Science,' vol. xiv. 1877, p. 21.
82
HYPOCOTYLS, EPICOTYLS, ETC.
Chap. It
Fig. 58, A.
"o* inch in length, and was quite straight ; but from
having increased in thickness it had just begun to
split open the lower part of the petioles on one side,
along the line of their confluence. By the following
morning the upper part of the plumule had arched
itself into a right angle, and the
convex side or elbow had thus been
forced out through the slit. Here
then the arching of the plumule
plays the same part as in the case of
the petioles of the Delphinium. As
the plumule continued to grow, the
tip became more arched, and in
the course of six days it emerged
through the 2J inches of superin-
cumbent soil, still retaining its
arched form. After reaching the
surface it straightened itself in the
usual manner. In the accompany-
ing figure (Fig. 58, A) we have a
sketch of a seedling in this ad-
vanced state of development; the
surface of the ground being re-
Meqarrhiza CaKfomica : presented by the line G Gr.
sketch of seedling, The germination of the seeds in
copied from Asa dray, ©
reduced to one-half their native Californian home pro-
S'm' sVtatst;: ceeds in a rather different manner,
the two confluent as we infer from an interesting
pocoiyi ; a A nd n radic^; letter from Mr. Rattan, sent to us
pi, plumule ; G G, \,y p r of. Asa Gray. The petioles
surface of soil. ,1^*1 i e±.
protrude from the seeds soon alter
the autumnal rains, and penetrate the ground, generally
in a vertical direction, to a depth of from 4 to even
8 inches. They were found in this state by Mr.
Rattan during the Christmas vacation, with the pin-
Chap. II BREAKING THROUGH THE GROUND. 83
mules still enclosed within the tubes ; and he remarks
that if the plumules had been at once developed and
had reached the surface (as occurred with our seeds
which were exposed to a high temperature), they
would surely have been killed by the frost. As it is
they lie dormant at some depth beneath the surface,
and are thus protected from the cold ; and the root-
hairs on the petioles would supply them with sufficient
moisture. We shall hereafter see that many seedlings
are protected from frost, but by a widely different
process, namely, by being drawn beneath the surface
by the contraction of their radicles. We may, how-
ever, believe that the extraordinary manner of germi-
nation of Megarrhiza has another and secondary
advantage. The radicle begins in a few weeks to
enlarge into a little tuber, which then abounds with
starch and is only slightly bitter. It would therefore
be very liable to be devoured by animals, were it not
protected by being buried whilst young and tender, at a
depth of some inches beneath the surface. Ultimately
it grows to a huge size.
Ipomoea hptophylla. — In most of the species of this
genus the hypocotyl is well developed, and breaks
through the ground as an arch. But the seeds of the
present species in germinating behave like those of
Megarrhiza, excepting that the elongated petioles of
the cotyledons are not confluent. After they have
protruded from the seed, they are united at their
lower ends with the undeveloped hypocotyl and un-
developed radicle, which together form a point only
about '1 inch in length. They are at first highly
geotropic, and penetrate the ground to a depth of
rather above half an inch. The radicle then begins
to grow. On four occasions after the petioles had
grown for a short distance vertically downwards, the\
84 HYPOCOTYLS, EPICOTYLS, ETC., Chap. II.
were placed in a horizontal position in damp air in the
dark, and in the course of 4 hours they again became
curved vertically downwards, having passed through
90° in this time. But their sensitiveness to geotropism
lasts for only 2 or 3 days; and the terminal part
alone, for a length of between *2 and "4 inch, is thus
sensitive. Although the petioles of our specimens
did not penetrate the ground to a greater depth than
about J inch, yet they continued for some time to grow
rapidly, and finally attained the great length of about
3 inches. The upper part is apogeotropic, and there-
fore grows vertically upwards, excepting a short
portion close to the blades, which at an early period
bends downwards and becomes arched, and thus
breaks through the ground. Afterwards this portion
straightens itself, and the cotyledons then free them-
selves from the seed-coats. Thus we here have in
different parts of the same organ widely different kinds
of movement and of sensitiveness ; for the basal part
is geotropic, the upper part apogeotropic, and a portion
near the blades temporarily and spontaneously arches
itself. The plumule is not developed for -some little
time ; and as it rises between the bases of the parallel
and closely approximate petioles of the cotyledons,
which in breaking through the ground have formed an
almost open passage, it does not require to be arched and
is consequently always straight. Whether the plumule
remains buried and dormant for a time in its native
country, and is thus protected from the cold of winter,
we do not know. The radicle, like that of the Megar-
rhiza, grows into a tuber-like mass, which ultimately
attains a great size. So it is with Ipomaea pandurata,
the germination of which, as Asa Gray informs us,
resembles that of I. leptophylla.
The following case is interesting in connection with
Chap. II BREAKING THROUGH THE GROUND. 85
the root-like nature of the petioles. The radicle of a
seedling was cut off, as it was completely decayed,
and the two now separated cotyledons were planted.
They emitted roots from their bases, and continued
green and healthy for two months. The blades of
both then withered, and on removing the earth the
bases of the petioles '(instead of the radicle) were
found enlarged into little tubers. Whether these
would have had the power of producing two in-
dependent plants in the following summer, we do not
know.
In Quercus virens, according to Dr. Engelmann,*
both the cotyledons and their petioles are confluent.
The latter grow to a length "of an inch or even
more;" and, if we understand rightly, penetrate the
ground, so that they must be geotropic. The nutri-
ment within the cotyledons is then quickly transferred
to the hypocotyl or radicle, which thus becomes
developed into a fusiform tuber. The fact ol
tubers being formed by the foregoing three widely
distinct plants, makes us believe that their protection
from animals at an early age and whilst tender, is one
at least of the advantages gained by the remark-
able elongation of the petioles of the cotyledons,
together with their power of penetrating the ground
like roots under the guidance of geotropism.
The following cases may be here given, as they bear
on our present subject, though not relating to seed-
lings. The flower-stem of the parasitic Lathrsea
squamaria, which is destitute of true leaves, breaks
through the ground as an arch ;f so does the flower-
* ' Transact. St. Louis Acad. ground cannot fail to be great)}
Science,' vol. iv. p. 190. facilitated by the extraordinary
t The passage of the flower- quantity of water secreted at this
etem of the Lathrcea through the period of the year by the subter*
86
HYPOCOTYLS, EPICOTYLS, ETC., Chap. II,
stein of the parasitic and leafless Monotropa hypopitys.
With Helleborus niger, the flower-stems, which rise up
independently of the leaves, likewise break through
the ground as arches. This is also the case with the
greatly elongated flower-stems, as well as with the
petioles of Epimedium pinnatum. So it is with the
petioles of Ranunculus ficaria, when they have to break
through the ground, but when they arise from the
summit of the bulb above ground, they are from the
first quite straight ; and this is a fact which deserves
notice. The rachis of the bracken fern (Pteris aqui-
lina), and of some, probably many, other ferns, like-
wise rises above ground under the form of an arch.
No doubt other analogous instances could be found by
careful search. In all ordinary cases of bulbs, rhizomes,
ranean scale-like leaves : not that
there is any reason to suppose
that the secretion is a special
adaptation for this purpose : it
probably follows from the areat
quantity of sap absorbed in the
early spring by the parasitic roots.
After a long period without any
rain, the earth had become light-
coloured and very dry, but it was
dark coloured and damp, even in
parts quite wet. for a distance of
al least six inches all round eacli
flower-stem. The water is secreted*
by glands (described by Cohn,
' Bericht. Bot. Sect, der Schle-
bi>cen Gesell.,' 1876, p. 113)
which line the longitudinal
channels running through e ich
scale-like leaf. A large plant was
dug up, washed so as to remove
t'.e earth, left for some time to
drain, and then placed in the
evening on a dry glass-plate,
covered with a bell-glass, and by
next morning it bad secreted a
large pool of water. The pLte
was wiped dry, and in the course
of the succeeding 7 or 8 hours
another little pool was secreted,
and after 16 additional hours
several large drops. A smaller
plant was washed and placed in a
large jar, which was left inclined
for an hour, by whicli time no
more water drained off. The jar
was then placed upright and
closed : after 23 hours twodrachms
of water were collected from the
bottom, and a little more after 25
additional hours. The flower-
stems were now cut off, for tbey
do not secrete, and the subter-
ranean part of the plant was found
to weigh 106-8 grams (1611
grains), and the water seer, ted
during the 48 . hours weighed
11 - 9 grams (1^3 grains). — that is,
one-ninth of the whole weight of
the plant, excluding the flower-
stems. We should remember that
plants in a state of nature would
probably secrete in 48 hours much
more than the above large amount,
for their roots would continue all
the time absorbing sap from the
plant on which they were para'
sitic.
Chap. II. BREAKING THROUGH THE GROUND. 87
root-stocks, &c, buried beneath the ground, the surface
is broken by a cone formed by the young imbricated
leaves, the combined growth of which gives them force
sufficient for the purpose.
With germinating monocotyledonous seeds, of
which, however, we did not observe a large number,
the plumules, for instance, those of Asparagus and
Canna, are straight whilst breaking through the ground.
With the Graminese, the sheath-like cotyledons are
likewise straight ; they, however, terminate in a sharp
crest, which is white and somewhat indurated ; and this
structure obviously facilitates their emergence from
the soil : the first true leaves escape from the sheath
through a slit beneath the chisel-like apex and at
right angles to it. In the case of the onion (Allium
ceipa) we again meet with an arch ; the leaf-like coty-
ledon being abruptly bowed, when it breaks through
the ground, with the apex still enclosed within the
seed-coats. The crown of the arch, as previously
described, is developed into a white conical pro-
tuberance, which we may safely believe to be a
special adaptation for this office.
The fact of so many organs of different kinds. —
hypocotyls and epicotyls, the petioles of some coty-
ledons and of some first leaves, the cotyledons of
the onion, the rachis of some ferns, and some flower-
stems — being all arched whilst they break through
the ground, shows how just are Dr. Haberlandt's *
remarks on the importance of the arch to seedling
plants. He attributes its chief importance to the
upper, young, and more tender parts of the hypocotyl
* • Die Sohutzeinrichtungen in though our observations lead us
dnr Eutwickelung der Keim- to differ on some points from ihu
julanze,' 1877. We have learned author.
Diiich from this interesting essay,
7
88 HYPOCOTYLS, EPICOTYLS, ETC., Cuap II.
or epicotyl, being thus saved from abrasion and
pressure whilst breaking through the ground. But
we think that some importance may be attributed to
the increased force gained by the hypocotyl, epicotyl,
or other organ by being at first arched ; for both legs of
the arch increase in length, and both have points of
resistance as long as the tip remains enclosed within
the seed-coats ; and thus the crown of the arch is
pushed up through the earth with twice as much force
as that which a straight hypocotyl, &c, could exert.
As soon, however, as the upper end has freed- itself,
all the work has to be done by the basal leg. In
the case of the epicotyl of the common bean, the
basal leg (the apex having freed itself from the seed-
coats) grew upwards with a force sufficient to lift a,
thin plate of zinc, loaded with 12 ounces. Two more
ounces were added, and the 14 ounces were lifted up
to a very little height, and then the epicotyl yielded
and bent to one side.
With respect to the primary cause of the arching
process, we long thought in the case of many seedlings
that this might be attributed to the manner in which
the hypocotyl or epicotyl was packed and curved
within the seed-coats ; and that the arched shape thus
acquired was merely retained until the parts in question
reached the ^surface of the ground. But it is doubtful
whether this is the whole of the truth in any case.
For instance, with the common bean, the epicotyl or
plumule is bowed into an arch whilst breaking through
the seed-coats as shown in Fig. 59 (p. 92). The
plumule first protrudes as a solid knob (e in A), which
after twenty-four hours' growth is seen (e in B) to be
the crown of an arch. Nevertheless, with several
beans which germinated in damp air, and had other*
wise been treated in an unnatural manner, little
Chav. II. BREAKING THROUGH THE GROUND. 89
plumules were developed in the axils of the petioles
of both cotyledons, and these were as perfectly arched
as the normal plumule ; yet they had not been sub-
jected to any confinement or pressure, for the seed-
coats were completely ruptured, and they grew in the
open air. This proves that the plumule has an innate
or spontaneous tendency to arch itself.
In some other cases the hypocotyl or epicotyl pro-
trudes from the seed at first only slightly bowed ; but
the bowing afterwards increases independently of any
constraint. The arch is thus made narrow, with the
two legs, which are sometimes much elongated, parallel
and close together, and thus it becomes well fitted
for breaking through the ground.
With many kinds of plants, the radicle, whilst still
enclosed within the seed and likewise after its first pro-
trusion, lies in a straight line with the future hypocotyl
and with the longitudinal axis of the cotyledons. This
is the case with Cucurbita ovifera ; nevertheless, in
whatever position the seeds were buried, the hypocotyl
always came up arched in one particular direction.
Seeds were planted in friable peat at a depth of aboi. t
an inch in a vertical position, with the end from which
the radicle protrudes downwards. Therefore all the
parts occupied the same relative positions which
they would ultimately hold after the seedlings had
risen clear above the surface. Notwithstanding this
fact, the hypocotyl arched itself; and as the arch
grew upwards through the peat, the buried seeds were
turned either upside down, or were laid horizontally,
being afterwards dragged above the ground. Ulti-
mately the hypocotyl straightened itself in the usual
manner ; and now after all these movements the
several parts occupied the same position relatively to
one another and to the centre of the earth, which they
90 HYPOCOTYLS, EPICOTYLS, ETC., Chap. IT
had done when the seeds were first buried. But it may
be argued in this and other such cases that, as the
hypocotyl grows up through the soil, the seed will
almost certainly be tilted to one side ; and then
from the resistance which it must offer during its
further elevation, the upper part of the hypocotyl will
be doubled down and thus become arched. This view
seems the more probable, because with Ranunculus
ficaria only the petioles of the leaves which forced
a passage through the earth were arched ; and not
those which arose from the summits of the bulbs above
the ground. Nevertheless, this explanation does not
apply to the Cucurbita, for when germinating seeds
were suspended in damp air in various positions by
pins passing through the cotyledons, fixed to the
inside of the lids of jars, in which case the hypo-
cotyls were not subjected to any friction or constraint,
yet the upper part became spontaneously arched.
This fact, moreover, proves that it is not the weight
of the cotyledons which causes the arching. Seeds
of Helianthus annuus and of two species of Ipomcea
(those of I. bona nox being for the genus large
and heavy) were pinned in the samo manner,
and the hypocotyls became spontaneously arched ;
the radicles, which had been vertically dependent,
assumed in consequence a horizontal position. In
the case of Ipomoea leptophylla it is the petioles of the
cotyledons which become arched whilst rising through
the ground ; and this occurred spontaneously when
the seeds were fixed to the lids of jars.
It may, however, be suggested with some degree of
probability that the arching was aboriginally caused
by mechanical compulsion, owing to the confinement
of the parts in question within the seed-coats, or to
friction whilst they were being dragged upwards. Bat
Chap. II. BREAKING THROUGH THE GROUND. 91
if this is so, we must admit from the cases just given,
that a tendency in the upper part of the several
specified organs to bend downwards and thus to be-
come arched, has now become with many plants firmly
inherited. The arching, to whatever cause it may be
clue, is the result of modified circumnutation, through
increased growth along the convex side of the part ;
such growth being only temporary, for the part always
straightens itself subsequently by increased growth
along the concave side, as will hereafter be described.
It is a curious fact that the hypocotyls of some
plants, which are but little developed and which
never raise their cotyledons above the ground, never-
theless inherit a slight tendency to arch themselves,
although this movement is not of the least use to
them. We refer to a movement observed by Sachs
in the hypocotyls of the bean and some other Legumi-
nosae, and which is shown in the accompanying figure
(Fig. 59), copied from his Essay.* The hypocotyl
and radicle at first grow perpendicularly downwards,
as at A, and then bend, often in the course of 24 hours,
into the position shown at B. As we shall here-
after often have to recur to this movement', we will, for
brevity sake, call it " Sachs' curvature." At first sight
it might be thought that the altered position of the
radicle in B was wholly due to the outgrowth of the
epicotyl (e), the petiole (p) serving as a hinge ; and
it is probable that this is partly the cause ; bat the
hypocotyl and upper part of the radicle themselves
become slightly curved.
The above movement in the bean was repeatedly
seen by us ; but our observations were made chiefly on
l*haseolus multiflorus, the cotyledons of which are like-
Arheiten <U'a bot. Iustit. Wiirzburg,' vol. i. 1873, p. 108.
92
HYPOCOTYLS, EPICOTYLS, ETC.
Chap. IX
wise hypogean. Some seedlings with well-developed
radicles were first immersed in a solution of perman-
ganate of potassium ; and, judging from the changea
of colour (though these were not very clearly defined),
the hypocotyl is about *3 inch in length. Straight,
thin, black lines of this length were now drawn from
the bases of the short petioles along the hypocotyls
F g. 59
Vici" faba: germinating seeds, suspended in damp air: A, with radicle
growing perpendicularly downwards ; B, the same bean after 24 hours
and after the radicle has curved itself; r, radicle; h, short hypocotyl ;
<?, epicotyi appearing as a knob in A and as an arch in B ; p, petiole of
the cotyledon, the latter enclosed within the seed-coats.
of 23 germinating seeds, which were pinned to the
lids of jars, generally with the hilum downwards, and
with their radicles pointing to the centre of the
earth. After an interval of from 2-i to 48 hours the
black lines on the hypocotyls of 16 out of the 23
seedlings became distinctly curved, but in very
various degrees (namely, with radii between 20 and
Chap. II. BREAKING THROUGH THE GROUND. 03
80 mm. on Sachs' cyclometer) in the same relative
direction as shown at B in Fig. 59. As geotropism
will obviously tend to check this curvature, seven
seeds were allowed to germinate with proper pre-
cautions for their growth in a klinostat,* by which
means geotropism was eliminated. The position of the
hypocotyls was observed during four successive days,
and they continued to bend towards the hilum and
lower surface of the seed. On the fourth day the/
were deflected by an average angle of 63° from a lino
perpendicular to the lower surface, and were therefore
considerably more curved than the hypocotyl and
radicle in the bean at B (Fig. 59), though in the same
relative direction.
It will, we presume, be admitted that all leguminous
plants with hypogean cotyledons are descended from
forms which once raised their cotyledons above the
ground in the ordinary manner ; and in doing so, it is
certain that their hypocotyls would have been abruptly
arched, as in the case of every other dicotyledonous
plant. This is especially clear in the case of Phaseolus,
for out of five species, the seedlings of which we
observed, namely, P. multiflorus, caracalla, vulgaris,
Hemandesii and Boxburghii (inhabitants of the Old
and New Worlds), the three last-named species have
well-developed hypocotyls which break through the
ground as arches. Now, if we imagine a seedling of
the common bean or of P. multiflorus, to behave as its
progenitors once did, the hypocotyl (A, Fig. 59), in
whatever position the seed may have been buried;
would become so much arched that the upper part
would be doubled down parallel to the lower part ; and
* An instrument devised by on which the plant under observa-
Suc.s, consisting essentially of a tion is supported : see ' Wiirzburg
slowly revolving horizontal axis. Arbeiten,' 1879, p. 209.
94
RUDIMENTARY COTYLEDONS.
Chap. 11
this is exactly the kind of curvature which actually
occurs in these two plants, though to a much less
degree. Therefore we can hardly doubt that their
short hypocotyls have retained by inheritance a ten-
dency to curve themselves in the same manner as they
did at a former period, when this movement was highly
important to them for breaking through the ground,
though now rendered useless by the cotyledons being
hypogean. Kudimentary structures are in most cases
highly variable, and we might expect that rudimentary
or obsolete actions would be equally so ; and Sachs'
curvature varies extremely in amount, and sometimes
altogether fails. This is the sole instance known to
us of the inheritance, though in a feeble degree, of
movements which have become superfluous from
changes which the species has undergone.
Rudimentary Cotyledons. — A few remarks on this
subject may be here interpolated. It is well known
Fi 60 that some dicotyle-
donous plants produce
only a single cotyle-.
don ; for instance, cer-
tain species of Ranun-
culus, Corydalis, Chse-
rophyllum ; and we
will here endeavour to
show that the loss of
one or both cotyle-
dons is apparently due
to a store of nutri-
ment being laid up in
some other part, as in
the hypocotyl or one
of the two cotyledons, or one of the secondary radicles.
Pilrus aumntium: two young seedlings:
c, larger cotyledon ; c', smaller cotyle-
don ; h, thickened hypocotyl ; r, radicle.
In A the epicotyl is still arched, in B it
has become erect.
Chap. II
RUDIMENTARY COTYLEDONS
95
Fig. 61.
With tlie orange {Citrus avrantium) the cotyledons are
hypogean, and one is larger than the other, as may
be seen in A (Fig. 60). In B the inequality is rather
greater, and the stem has grown between the points
of insertion of the two petioles, so that they do not
stand opposite to one another; in another case the
separation amounted to one-fifth of an inch. The
smaller cotyledon of one seedling
was extremely thin, and not half
the length of the larger one, so that
it was clearly becoming rudimen-
tary.* In all these seedlings the
Jiypocotyl was enlarged or swollen.
With Abronia umbellata one of
the cotyledons is quite rudimen-
tary, as may be seen (c) in Fig. 61.
In this specimen it consisted of a
little green flap, -gV^h inch in
length, destitute of a petiole and
covered with glands like those on
the fully developed cotyledon (c).
At first it Stood Opposite to the Abronia umbellata . seed
larger cotyledon ; but as the petiole
of the latter increased in length
and grew in the same line with
the hypocotyl (h), the rudiment
appeared in older seedlings as if
seated some way down the hypocotyl. With Abronia
armaria there is a similar rudiment, which in one
ling twice natural size:
c, cotyledon ; c', rudi-
mentary cotyledon ; h,
enlarged hypocotyl,
with a heel or projec-
tion (//) at the lower
end : r, radicle
* Tn Pachira aquatica, as de-
scribed by Mr. R. I. Lynch
('Journal Linn. Soc. Bot.' vol.
xvii. 1878, p. 147), one of the
hypogean cotyledons is of im-
mense size; the other is small
ami soon falls off; the pair do not
always stand opposite. In another
and very different water-plant,
Trapa nutans, one of the cotyle-
dons, filled with farinaceous
matter, is much larger than the
other, which is scarcely visible,
as is stated by Aug. de Candolle,
' Phvsiologie Veg.' torn. ii. p. 834,
1832.
96 KUDIMENTAKY COTYLEDONS. Chap. II
specimen was only -r^o-th and in another -gVth incli in
length; it ultimately appeared as if seated halfway
down the hypocotyl. In both these species the hypo-
cotyl is so much enlarged, especially at a very early
age, that it might almost be called a corrn. The lower
end forms a heel or projection, the use of which will
hereafter be described.
In Cyclamen Persicum the hypocotyl, even whilst still
within the seed, is enlarged into a regular corm,* and
only a single cotyledon is at first developed (see former
Fig. 57.) With Ranunculus ficaria two cotyledons are
never produced, and here one of the secondary radicles
is developed at an early age into a so-called bulb.f
Again, certain species of Chserophyllum and Corydalis
produce only a single cotyledon ;| in the former the
hypocotyl, and in the latter the radicle is enlarged,
according to Irmisch, into a bulb.
In the several foregoing cases one of the cotyledons
is delayed in its development, or reduced in size, or
rendered rudimentary, or quite aborted ; but in other
cases both cotyledons are represented by mere rudi-
ments. With Opuntia basilar is this is not the case,
for both cotyledons are thick and large, and the
hypocotyl shows at first no signs of enlargement ; but
afterwards, when the cotyledons have withered and dis-
articulated themselves, it becomes thickened, and from
its tapering form, together with its smooth, tough,
brown skin, appears, when ultimately drawn down to
some depth into the soil, like a root. On the other
* Dr. H. Gressner, 'Bot. Zei- Vauchers account ('Hist. Phys.
tung.' 1874, p. 824. des Plan tes d'Europe,' torn i. 1841,
t Irmisch, 'Beitr'age zur Mor- p. 149) of the germination of the
phologie der Pflanzen,' 1854, pp. seeds of several species of Cory-
II, 12; 'Bot. Zeituug,' 1874, p. dalis, that the bulb or tubernile
805. begins to be formed at an ex-
X Delpino, 'Rivisia Botanica,' tremely early age.
1877, p. 21. It is evident from
Ckm\ II. EUDIMENTARY COTYLEDONS. 97
band, with several other Cactese, the hypocotyl is from
the first much enlarged, and both cotyledons are
almost or quite rudimentary. Thus with Cereus Land-
hechii two little triangular projections, representing the
cotyledons, are narrower than the hypocotyl, which is
pear-shaped, with the point downwards. In Ehipsalis
cassytha the cotyledons are represented by mere points
on the enlarged hypocotyl. In Echinocactus viridescens
the hypocotyl is globular, with two little prominences
on its summit. In Pilocereus Hoidletii the hypocotyl,
much swollen in the upper part, is merely notched on
the summit ; and each side of the notch evidently repre-
sents a cotyledon. Stapelia sarpedon, a member of the
very distinct family of the Asclepiadea3, is fleshy like
a cactus ; and here again the upper part of the flattened
hypocotyl is much thickened and bears two minute coty-
ledons, which, measured internally, were only *15 inch
in length, and in breadth not equal to one-fourth of the
diameter of the hypocotyl in its narrow axis ; yet these
minute cotyledons are probably not quite useless, for
when the hypocotyl breaks through the ground in the
form of an arch, they are closed or pressed against one
another, and thus protect the plumule. They after-
wards open.
From the several cases now given, which refer to
widely distinct plants, we may infer that there is some
close connection between the reduced size of one or
both cotyledons and the formation, by the enlargement
of the hypocotyl or of the radicle, of a so-called bulb.
But it may be asked, did the cotyledons first tend to
abort, or did a bulb first begin to be formed? As
all dicotyledons naturally produce two well-developed
cotyledons, whilst the thickness of the hypocotyl and
of the radicle differs much in different plants, it seems
probable that these latter organs first became from
98 CIECUMNUTATING MOVEMENTS OF Chap. IT.
some cause thickened — in several instances apparently
in correlation with the fleshy nature of the mature
plant — so as to contain a store of nutriment sufficient
for the seedling, and then that one or both cotyledons,
from being superfluous, decreased in size. It is not
surprising that one cotyledon alone should sometimes
have been thus affected, for with certain plants, for
instance the cabbage, the cotyledons are at first of
unequal size, owing apparently to the manner in which
they are packed within the seed. It does not, how-
ever, follow from the above connection, that whenever
a bulb is formed at an early age, one or both coty-
ledons will necessarily become superfluous, and conse-
quently more or less rudimentary. Finally, these
cases offer a good illustration of the principle of com-
pensation or balancement of growth, or, as Goethe
expresses it, " in order to spend on one side, Nature
is forced to economise on the other side."
Circumnutation and other movements of Hi^ocotyls
and Eincotyls, whilst still arched and buried beneath
the ground, and whilst breaking through it. — According
to the position in which a seed may chance to
have been buried, the arched hypocotyl or epicotyl
will begin to protrude in a horizontal, a more or
less inclined, or in a vertical plane. Except when
already standing vertically upwards, both legs of the
arch are acted on from the earliest period by apo-
geotropism. Consequently they both bend upwards,
until the arch becomes vertical. During the whole of
this process, even before the arch has broken through
the ground, it is continually trying to circumnutate
to a slight extent ; as it likewise does if it happens at
first to stand vertically up, — all which cases have
been observed and described, more or less fully, in
the last chapter. After the arch has grown to some
Chap. II HYPOCOTYLS, ETC., WHILST ARGUED. 09
height upwards, the basal part ceases to circurnmitate,
>vhilst the upper part continues to do so.
That an arched hypocotyl or epicotyl, with the two
legs fixed in the ground, should be able to cir-
cumnutate, seemed to us, until we had read Prof.
Wiesner's observations, an inexplicable fact. He has
shown* in the case of certain seedlings, whose tips
are bent downwards (or which nutate), that whilst the
posterior side of the upper or dependent portion grows
quickest, the anterior and opposite side of the basal
portion of the same internode grows quickest ; these
two portions being separated by an indifferent zone,
where the growth is equal on all sides. There may
even be more than one indifferent zone in the same
internode ; and the opposite sides of the parts above
and below each such zone grow quickest. This pecu-
liar manner of growth is called by Wiesner "un-
dulatory nutation." Circumnutation depends on one
side of an organ growing quickest (probably preceded
by increased turgescence), and then another side,
generally almost the opposite one, growing quickest.
Now if we look at an arch like this f| and suppose
the whole of one side — we will say the whole convex
side of both legs — to increase in length, this would
not cause the arch to bend to either side. But if the
outer side or surface of the left leg were to increase
in length the arch would be pushed over to the right,
and this would be aided by the inner side of the
right leg increasing in length. If afterwards the
process were reversed, the arch would be pushed over
to the opposite or left side, and so on alternately, — ■
that is, it would circumnutate. As an arched hypo*
* 'Die undulireude Nutation Also published separately se«
dor Internodien,' Akad. der Wis- p. 32.
senc/t. (Vienna), Jan. 17th, 1878.
iOO CIRCUMNUTAT1NG MOVEMENTS OF Chap. II
cotyl, with the two legs fixed in the ground, certainly
circumnutates, and as it consists of a single internode,
we may conclude that it grows in the manner de-
scribed by Wiesner. It may be added, that the crown
of the arch _ does not grow, or grows very slowly, foi
it does not increase much in breadth, whilst the arch
itself increases greatly in height.
The circumnutating movements of arched hypo-
cotyls and epicotyls can hardly fail to aid them in
breaking through the ground, if this be damp and
soft ; though no doubt their emergence depends
mainly on the force exerted by their longitudinal
growth. Although the arch circumnutates only to a
slight extent and probably with little force, yet it is
able to move the soil near the surface, though it may
not be able to do so at a moderate depth. A pot with
seeds of Solanum palinacantkum, the tall arched hypo-
cotyls of which had emerged and were growing rather
slowly, was covered with fine argillaceous sand kept
damp, and this at first closely surrounded the bases of
the arches ; but soon a narrow open crack was formed
round each of them, which could be accounted for
only by their having pushed away the sand on all
sides ; for no such cracks surrounded some little sticks
and pins which had been driven into the sand. It
has already been stated that the cotyledons of Phalaris
and Avena, the plumules of Asparagus and the hypo-
cotyls of Brassica, were likewise able to displace the
same kind of sand, either whilst simply circumnu-
tating or whilst bending towards a lateral light.
As long as an arched hypocotyl or epicotyl remains
buried beneath the ground, the two legs cannot sepa-
rate from one another, except to a slight extent from
the yielding of the soil; but as soon as the arch
rises above the ground, or at an earlier period if
Crap. II. HYPOCOTYLS, ETC., WHILST ARCHED. 10]
(lie pressure of the surrounding earth be artificially
removed, the arch immediately begins to straighten
itself. This no doubt is due to growth along the
whole inner surface of both legs of the arch ; such
growth being checked or prevented, as long as the two
legs of the arch are firmly pressed together. When the
earth is removed all round an arch and the two legs
are tied together at their bases, the growth on the
under side of the crown causes it after a time to
become much flatter and broader than naturally
occurs. The straightening process consists of a mo-
dified form of circumnutation, for the lines described
during this process (as with the hypocotyl of Brassica,
and the epicotyls of Vicia and Corylus) were often
plainly zigzag and sometimes looped. After hypo-
cotyls or epicotyls have emerged from the ground,
they quickly become perfectly straight. No trace is
left of their former abrupt curvature, excepting in the
case of Allium cepa, in which the cotyledon rarely
becomes quite straight, owing to the protuberance
developed on the crown of the arch.
The increased growth along the inner surface of the
arch which renders it straight, apparently begins in
the basal leg or that which is united to the radicle ;
for this leg, as we often ojbserved, is first bowed back-
wards from the other leg. This movement facilitates
the withdrawal of the tip of the epicotyl or of the
cotyledons, as the case may be, from within the seed-
coats and from the ground. But the cotyledons often
emerge from the ground still tightly enclosed within
the seed-coats, which apparently serve to protect them.
The seed-coats are afterwards ruptured and cast off by
the swelling of the closely conjoined cotyledons, and not
by any movement or their separation from one another.
Nevertheless, in some few cases, especially with the
102 RUPTURE OF THE SEED-COATS. Crap. II.
Cucurbitacere, the seed-coats are ruptured by a curious
contrivance, described by M. Flahault.* A heel or
peg is developed on one side of the summit of the
radicle or base of the hypocotyl ; and this holds down
the lower half of the seed-coats (the radicle being
fixed into the ground) whilst the continued growth of
the arched hypocotyl forces up-
wards the upper half, and tears
asunder the seed-coats at one end,
and the cotyledons are then easily
withdrawn. The accompanying
figure (Fig. 62) will render this
description intelligible. Forty-
one seeds of Cucurhita ovifera
were laid on friable peat and were
covered by a layer about an inch
in thickness, not much pressed
down, so that the cotyledons in
being dragged up were subjected
to very little friction, yet forty of
them came up naked, the seed-
Cucurhita ovifera : germi- i i xx i • i • xi x
nating seed, showing the coats being left buried in the peat.
heel or peg projecting r rjhis was certainly dueto the action
on one side from summit ,, , „ -, ..
of radicle and holding of the peg, for when it was pre-
down lower tip of seed- ven ted from acting, the cotyledons,
coats, which have been in xi
partially ruptured by as we snail presently see, were
the growth of the arched nfte& up still enclosed in their
hypocotyl. L
seed-coats. Ihey were, however,
cast off in tne course of two or three days by the
swelling of the cotyledons. Until this occurs light is
excluded, and the cotyledons cannot decompose car-
bonic acid; but no one probably would have thought
that the advantage thus gained by a little earlier cast*
* 'Bull. Soc. Cot. de France/ torn. xxiv. 1877, p. 201.
CHAr. II. EUPTUEE OF THE SEED-COATS. 103
ing off of the seed-coats would be sufficient to account
for the development of the peg. Yet, according to
M. Flahault, seedlings which have been prevented
from casting their seed-coats whilst beneath the
ground, are inferior to those which have emerged with
their cotyledons naked and ready to act.
The peg is developed with extraordinary rapidity ;
for it could only just be distinguished in two seed-
lings, having radicles '35 inch in length, but after an
interval of only 24 hours was well developed in
both. It is formed, according to Flahault, by the
enlargement of the layers of the cortical parenchyma
at the base of the hypocotyl. If, however, we judge
by the effects of a solution of permanganate of
potassium, it is developed on the exact line of
junction between the hypocotyl and radicle; for
the flat lower surface, as well as the edges, were
coloured brown like the radicle: whilst the upper
slightly inclined surface was left uncoloured like the
hypocotyl, excepting indeed in one out of 33 im-
mersed seedlings in which a large part of the upper sur-
face was coloured brown. Secondary roots sometimes
spring from the lower surface o'f the peg, which thus
seems in all respects to partake of the nature of the
radicle. The peg is always developed on the side which
becomes concave by the arching of the hypocotyl;
and it would be of no service if it were formed on any
other side. It is also always developed with the flat
lower side, which, as just stated, forms a part of the
radicle, at right angles to it, and in a horizontal plane.
This fact was clearly shown by burying some of the
thin flat seeds in the same position as in Fig. 62,
excepting that they were not laid on their flat broad
sides, but with one edge downwards. Nine seeds
were thus planted, and the peg was developed in tha
8
104 RUPTURE OF THE SEED-COATS. Chap, JL
same position, relative] y to the radicle, as in the
figure; consequently it did not rest on the flat tip
of the lower half of the seed-coats, but was inserted
like a wedge between the two tips. As the arched
hypocotyl grew upwards it tended to draw up the
whole seed, and the peg necessarily rubbed against
both tips, but did not hold either down. The result
was, that the cotyledons of five out of the nine seeds
thus placed were raised above the ground still enclosed
within their seed-coats. Four seeds were buried with
the end from which the radicle protrudes pointing
vertically downwards, and owing to the peg being
always developed in the same position, its apex alone
came into contact with, and rubbed against the tip on
one side ; the result was, that the cotyledons of all
four emerged still within their seed-coats. These cases
show us how the peg acts in co-ordination with the
position which the flat, thin, broad seeds would almost
always occupy when naturally sown. When the tip
of the lower half of the seed-coats was cut off, Flahault
found (as we did likewise) that the peg could not act,
since it had nothing to press on, and the cotyledons
were raised above the ground with their seed-coats not
cast off. Lastly, nature shows us the use of the peg ;
for in the one Cucurbitaceous genus known to us, in
which the cotyledons are hypogean and do not cast
their seed-coats, namely, Megarrhiza, there is no
\restige of a peg. This structure seems to be present
in most of the other genera in the family, judging from
Fkhault's statements ; we found it well-developed and
properly acting in Trichosanthes angnina, in which we
hardly expected to find it, as the cotyledons are some-
what thick and fleshy. Few cases can be advanced
of a structure better adapted for a special purpose
than the present one.
Chap. II. RUPTURE OF THE SEED-COATS. 105
With Mimosa pudica the radicle protrudes from a
email hole in the sharp edge of the seed ; and on its
summit, where united with the hypocotyl, a transverse
ridge is developed at an early age, which clearly aids
in splitting the tough seed-coats ; but it does not aid
in casting them off, as this is subsequently effected by
the swelling of the cotyledons after they have been
raised above the ground. The ridge or heel therefore
acts rather differently from that of Cucurbita. Its
lower surface and the edges were coloured brown by
the permanganate of potassium, but not the upper
surface. It is a singular fact that after the ridge has
done its work and has escaped from the seed-coats,
it is developed into a frill all round the summit of the
radicle.*
At the base of the enlarged hypocotyl of Abronia
umbellata, where it blends into the radicle, there is a
projection or heel which varies in shape, but its out-
line is too angular in our former figure (Fig. 61). The
radicle first protrudes from a small hole at one end of
the tough, leathery, winged fruit. At this period the
upper part of the radicle is packed within the fruit
parallel to the hypocotyl, and the single cotyledon is
doubled back parallel to the latter. The swelling of
these three parts, and especially the rapid development
of the thick heel between the hypocotyl and radicle
at the point where they are doubled, ruptures the
tough fruit at the upper end and allows the arched
hypocotyl to emerge ; and this seems to be the function
of the heel. A seed was cut out of the fruit and
* Our attention was called to fit the junction of the radicle and
this case by a brief statement by hypocotyl. This seed possesses a
Nobbe in his ' Handbuch der very hard and tough coat, and
Samenkunde,' 1876. p. 215, where would be likely to require aid in
a figure is also given of a seedling bursting and freeing the cotyle-
of Martvnia with a heel or ridge dons.
/ 06 RUPTUKE OF THE SEED-COATS. Chap. IL
allowed to germinate in damp air, and now a thin
flat disc was developed all round the base of the
hypocotyl and grew to an extraordinary breadth, like
the frill described under Mimosa, but somewhat broader.
Flahault says that with Mirabilis, a member of the
same family with Abronia, a heel or collar is developed
all round the base of the hypocotyl, but more on one
side than on the other; and that it frees the coty-
ledons from their seed-coats. We observed only old
seeds, and these were ruptured by the absorption of
moisture, independently of any aid from the heel and
before the protrusion of the radicle ; but it does not
follow from our experience that fresh and tough fruits
would behave in a like manner.
In concluding this section of the present chapter it
may be convenient to summarise, under the form of an
illustration, the usual movements of the hypocotyls
and epicotyls of seedlings, whilst breaking through the
ground and immediately afterwards. We may suppose
a man to be thrown down on his hands and knees, and
at the same time to one side, by a load of hay falling
on him. He would first endeavour to get his arched
back upright, wriggling at the same time in all
directions to free himself a little from the surrounding
pressure ; and this may represent the combined effects
of apogeotropism and circumnutation, when a seed is so
buried that the arched hypocotyl or epicotyl protrudes
at first in 'a horizontal or inclined plane. The man,
still wriggling, would then raise his arched back as
high as he could ; and this may represent the growth
and continued circumnutation of an arched hypocotyl
or epicotyl, before it has reached the surface of the
ground. As soon as the man felt himself at all free, he
would raise the upper part of his body, whilst still on
Chat. II. CIRCUMNUTATION OF HYPOCOTYLS, ETC. 107
Lis knees and still wriggling ; and this may represent
the bowing backwards of the basal leg of the arch,
which in most cases aids in the withdrawal of the
cotyledons from the buried and ruptured seed-coats,
and the subsequent straightening of the whole hypo-
cotyl or epicotyl — circumnutation still continuing.
Circumnutatiow of Hijfocotyh and Epicotyls, when
erect. — The hypocotyls, epicotyls, and first shoots of the
many seedlings observed by us, after they had become
straight and erect, circumnutated continuously. The
diversified figures described by them, often during two
successive clays, have been shown in the woodcuts in
the last chapter. It should be recollected that the
dots were joined by straight lines, so that the figures
are angular; but if the observations had been made
every few minutes the lines would have been more
or less curvilinear, and irregular ellipses or ovals, or
perhaps occasionally circles, would have been formed.
The direction of the longer axes of the ellipses made
during the same day or on successive days generally
changed completely, so as to stand at right angles to
one another. The number of ^irregular ellipses or
circles made within a given time differs much with
different species. Thus with Brassica oleracea, Cerinthe
major, and Cueurhita ovifera about four such figures
were completed in 12 h. ; whereas with Solatium palina-
canthum and Opuntia basilaris, scarcely more than one.
The figures likewise differ greatly in size ; thus they
were very small and in some degree doubtful in Stapelia,
and large in Brassica, &c. The ellipses described by
Lathjrus nissolia and Brassica were narrow, whilst
those made by the Oak were broad. The figures are
often complicated by small loops and zigzag lines.
As most seedling plants before the development
of true leaves are of low, sometimes very low stature,
108 CIECUMNUTATION OF HYPOCOTYLS, ETC. Chap. IL
the extreme amount of movement from side to side
of their circumnutating stems was small; that of
the hypocotyl of Githago segetum was about *2 of an
inch, and that of Cucurbita ovifera about '28. A
very young shoot of Lathy rus nissolia moved about
•14, that of an American oak *2, that of the common
nut only *04, and a rather tall shoot of the Asparagus
•11 of. an inch. The extreme amount of movement
of the sheath-like cotyledon of Phalaris Canariensis
was '3 of an inch ; but it did not move very quickly,
the tip crossing on one occasion five divisions of the
micrometer, that is, xJo* n °f an incn > in 22 m. 5 s. A
seedling Nolana prostrata travelled the same distance
in 10 m. 38 s. Seedling cabbages circumutated much
more quickly, for the tip of a cotyledon crossed
T <j -th of an inch on the micrometer in 3 m. 20 s. ; and
this rapid movement, accompanied by incessant oscil-
lations, was a wonderful spectacle when beheld under
the microscope.
The absence of light, for at least a clay, does not
interfere in the least with the circumnutation of the
hypocotyls, epicotyls, or young shoots of the various
dicotyledonous seedlings observed by us ; nor with that
of the young shoots of some monocotyledons. The
circumnutation was indeed much plainer in darkness
than in light, for if the light was at all lateral the
stem bent towards it in a more or less zigzag course.
Finally, the hypocotyls of many seedlings are drawn
during the winter into the ground, or even beneath it
so that they disappear. This remarkable process,
which apparently serves for their protection, has
been fully described by De Vries.* He shows that
* ' Bot. Zeitung,' 1879, p. 649. burg,' Jahrg. xvi. p. 16, as quoted
See also Winkler in ' Verhandl. by Haberlandt, ' Schutzeinriohuu-
des Bot. Veieins der P. Brandon- gen der Keimpflanze,' 1877, p. 52
Chap. II. CIRCUMNUTATION OF COTYLEDONS. 109
it is effected by the contraction of the parenchyma-
cells of the root. But the hypocotyl itself in some
cases contracts greatly, and although at first smooth
becomes covered with zigzag ridges, as we observed
with Githago segetum. How much of the drawing
down and burying of the hypocotyl oiOjpuntia hasilaris
was due to the contraction of this part and how much
to that of the radicle, we did not observe.
Circumnutation of Cotyledons. — With all the dico-
tyledonous seedlings described in the last chapter, the
cotyledons were in constant movement, chiefly in a ver-
tical plane, and commonly once up and once down in
the course of the 24 hours. But there were many excep-
tions to such simplicity of movement ; thus the cotyle-
dons of Ipomoea cserulea moved 13 times either upwards
or downwards in the course of 16 h. 18 m. Those of
Oxalis rosea moved in the same manner 7 times in the
course of 24 h. ; and those of Cassia tora described 5
irregular ellipses in 9 h. The cotyledons of some
individuals of Mimosa pudica and of Lotus Jacohseus
moved only once up and clown in 24 h., whilst those of
others performed within the same period an additional
small oscillation. Thus with different species, and
with different individuals of the same species, there
were many gradations from a single diurnal move-
ment to oscillations as complex as those of the
Ipomcea and Cassia. The opposite cotyledons on the
same seedling move to a certain extent independently
of one another. This was conspicuous with those of
Oxalis sensitiva, in which one cotyledon might be
seen during the daytime rising up until it stood
vertically, whilst the opposite one was sinking down.
Although the movements of cotyledons were gene-
rally in nearly the same vertical plane, yet their
upward and downward courses never exactly coin*
110 CIECUMXTJTATIOX OF COTYLEDONS. Chap. II
cided ; so that ellipses, more or less narrow, were
described, and the cotyledons may safely be said tc
have circnmnntated. Nor could this fact be accounted
for by the mere increase in length of the cotyledons
through growth, for this by itself would not induce
any lateral movement. That there was lateral rnove-
ment in some instances, as with the cotyledons of the
cabbage, was evident ; for these, besides moving up
and down, changed their course from right to left 12
times in 14 h. 15 m. With Solanum lycopersicum the
cotyledons, after falling in the forenoon, zigzagged
from side to side between 12 and 4 P.M., and then
commenced rising. The cotyledons of Lupinus luteus
are so thick (about '08 of an inch) and neshy,* that
they seemed little likely to move, and were there-
fore observed with especial interest; they certainly
moved largely up and down, and as the line traced was
zigzag there was some lateral movement. The nine
cotyledons of a seedling Pinus pinaster plainly circum-
nutated ; and the figures described approached more
nearly to irregular circles than to irregular ovals or
ellipses. The sheath-like cotyledons of the Gra-
minese circuinnutate, that is, move to all sides, as
plainly as do the hypocotyls or epicotyls of any dico-
tyledonous plants. Lastly, the very young fronds of
a Fern and of a Selaginella circumnutated.
In a large majority of the cases which were care-
fully observed, the cotyledons sink a little downwards
in the forenoon, and rise a little in the afternoon or
evening. They thus stand rather more highly inclined
during the night than during the mid-day, at which
* The cotyledons, though bright & c , 1S77, p. 95), on the gradations
green, resemble to a certain ex- in the Legun.inosae between sub-
tent hypogean ones; see the in- aerial and subterranean cotvle
fceresting discussion by Haber- dons,
landt ('Die Schutzcinriehtungen,'
Caap. II. CIRCUMNUTATION OF COTYLEDONS. Ill
time they are expanded almost horizontally. The
circumnutating movement is thus at least partially
periodic, no doubt in connection, as we shall hereafter
see, with the daily alternations of light and daikuess.
The cotyledons of several plants move up so much at
night as to stand nearly or quite vertically; and in
this latter case ihey come into close contact with one
another. On the other hand, the cotyledons of a
few plants sink almost or quite vertically down at
night ; and in this latter case they clasp the upper
part of the hypocotyl. In the same genus Oxalis the
cotyledons of certain species stand vertically up, and
those of other species vertically down, at night. In
all such cases the cotyledons may be said to sleep,
for they act in the same manner as do the leaves of
many sleeping plants. This is a movement for a
special purpose, and will therefore be considered in a
future chapter devoted to this subject.
In order to gain some rude notion of the proportional
number of cases in which the cotyledons of dico-
tyledonous plants (hypogean ones being of course
excluded) changed their position in a conspicuous
manner at night, one or more species in several
genera were cursorily observed, besides those described
in the last chapter. Altogether 153 genera, included
in as many families as could be procured, were thus
observed by us. The cotyledons were looked at in
the middle of the day and again at night ; and those
were noted as sleeping which stood either vertically
or at an angle of at least 60 ° above or beneath the
horizon. Of such genera there were 26 ; and in 21 of
them the cotyledons of some of the species rose, and
in only 6 sank at night ; and some of these latter
cases are rather doubtful from causes to be explained
in the chapter on the sleep of cotyledons. When
112 PULVINI OF COTYLEDONS. Chap. II
cotyledons which at noon were nearly horizontal, stood
at ni^ht at more than 20° and less than 60° above the
horizon, they were recorded, as " plainly raised ;" and
of such genera there were 38. We did not meet with
any distinct instances of cotyledons periodically sink-
ing only a few degrees at night, although no doubt
such occur. We have now accounted for 64 genera
out of the 153, and there remain 89 in which the
cotyledons did not change their position at night by
as much as 20°— that is, in a conspicuous manner
which could easily be detected by the unaided eye and
by memory ; but it must not be inferred from this
statement that these cotyledons did not move at all,
for in several cases a rise of a few degrees was re-
corded, when they were carefully observed. The
number 89 might have been a little increased, for the
cotyledons remained almost horizontal at night in
some species in a few genera, for instance, Trifo-
lium and Geranium, which are included amongst the
sleepers, such genera might therefore have been added
to the 89. Again, one species of Oxalis generally
raised its cotyledons at night more than 20° and less
than 60° above the horizon ; so that this genus might
have been included under two heads. But as several
species in the same genus were not often observed,
such double entries have been avoided.
In a future chapter it will be shown that the leaves
of many plants which do not sleep, rise a few degrees in
the evening and during the early part of the night ;
and it will be convenient to defer until then the
consideration of the periodicity of the movements of
cotyledons.
On the Pulvini or Joints of Cotyledons. — With several
of the seedlings described in this and the last chapter,
the summit of the petiole is developed into a pulvinus,
Chap. II.
PULVINI OF COTYLEDONS.
113
cushion, or joint (as this organ has been variously
called), like that with which many leaves are provided.
It consists of a mass of small cells usually of a pale
colour from the absence of chlorophyll, and with its
outline more or less convex, as shown in the annexed
figure. In the case of Oxalis
sensitiva two-thirds of the
petiole, and in that of Mi-
mosa pudica, apparently the
whole of the short sub-
petioles of the leaflets have
been converted into pulvini.
With pulvinated leaves (i.e.
those provided with a pul-
vinus) their periodical move-
ments depend, according to
Pfeffer,* on the cells of the
pulvinus alternately expand-
ing more quickly on one side
than on the other; whereas
the similar movements of
leaves not provided with pul-
vini, depend on their growth
being alternately more rapid
on one side than on the
other.f As long as a leaf
provided with a pulvinus is
young and continues to grow,
its movement depends on both these causes combined ;J
and if the view now held by many botanists be sound,
namely, that growth is always preceded by the expan-
sion of the growing cells, then the difference between
Oxalis rosea : longitudinal section
of a pulvinus on the summit
of the petiole of a cotyledon,
drawn with the camera lucida,
magnified 75 times : p, p, pe-
. tiole ; /, fibro-yascular bundle ;
b, b, commencement of blade ol
cotyledon.
Die Periodische Bewegun-
gcn der BLittorgane,' 1875.
t Batalin, 'Flora,' Oct. 1st, 1873
% Pfeffer, ibid. p. 5.
Ill PULVINI OF COTYLEDONS. Chap. IL
without such aid, is reduced to the expansion of the
cells not being followed by growth in the first case,
and being so followed in the second case.
Dots were made with Indian ink along the midrib
of both pulvinated cotyledons of a rather old seedling
of Oxalis Valdiviana ; their distances were repeatedly
measured with an eye-piece micrometer during 8f days,
and they did not exhibit the least trace of increase.
It is therefore almost certain that the pulvinus itself
was not then growing. Nevertheless, during this
whole time and for ten days afterwards, these coty-
ledons rose vertically every night. In the case of
some seedlings raised from seeds purchased under the
name of Oxalis Jforibunda, the cotyledons continued
for a long time to move vertically clown at night, and
the movement apparently depended exclusively on
the pulvini, for their petioles were of nearly the same
length in young, and in old seedlings which had pro-
duced true leaves. With some species of Cassia, on
the other hand, it was obvious without any measure-
ment that the pulvinated cotyledons continued to
increase greatly in length during some weeks ; so that
here the expansion of the cells of the pulvini and the
growth of the petiole were probably combined m
causing their prolonged periodic movements. It was
equally evident that the cotyledons of many plants,
not provided with pulvini, increased rapidly in length ;
and their periodic movements no doubt were exclu-
sively due to growth.
In accordance with the view that the periodic
movements of all cotyledons depend primarily on the
expansion of the cells, whether or not followed by
growth, we can understand the fact that there is but
little difference in the kind or form of movement
in the two sets of cases. This may be seen by com-
CtoAP. II. PULVINI OF COIYLEDONS. 115
paring the diagrams given in the last chapter. Thus
the movements of the cotyledons of Brassica oleraeea
and of Ipomoea cserulea, which are not provided with
pulvini, are as complex as those of Oxalis and Cassia
which are thus provided. The pulvinated cotyledons
of some individuals of Mimosa pudica and Lotus
Jacobseus made only a single oscillation, whilst those
of other individuals moved twice up and down in the
course of 24 hours ; so it was occasionally with the
cotyledons of Cucurbita ovifera, which are destitute of
a pulvinus. The movements of pulvinated cotyledons
are generally larger in extent than those without a
pulvinus; nevertheless some of the latter moved
through an angle of 90°. There is, however, one
important difference in the two sets of cases; the
nocturnal movements of cotyledons without pulvini,
for instance, those in the Cruciferse, Cucurbitacese,
Githago, and Beta, never last even for a week, to any
conspicuous degree. Pulvinated cotyledons, on the
other hand, continue to rise at night for a much
longer period, even for more than a month, as we
shall now show. But the period no doubt depends
largely on the temperature to which the seedlings are
exposed and their consequent rate of development.
Oxalis Valdioiana. — Some cotyledons which had late! y opened
and were horizontal on March 6th at noon, stood at night ver-
tically up ; on the 13th the first true leaf was formed, and was
embraced at night by the cotyledons; on April 9th, after an in-
terval of 35 days, six leaves were developed, and yet the coty-
ledons rose almost vertically at night. The cotyledons of
another seedling, which when first observed had already pro-
duced a leaf, stood vertically at night and continued to do so for
11 additional days. After 16 days from the first observation
two leaves were developed, and the cotyledons were still greatly
raised at night. After 21 days the cotyledons during the day
were deflected beneath the horizon, but at night were raised 4 j°
L1G PULVINI OF COTYLEDONS. Chap. IL
above it. After 24 days from the first observation (begun after
a true leaf had been developed) the cotyledons ceased to rise at
night.
Oxalis (Biophytum) sensitiva. — The cotyledons of several seed-
lings, 45 days after their first expansion, stood nearly vertical at
night, and closely embraced either one or two true leaves which
by this time had been formed. These seedlings had been kept
in a very warm house, and their development had been rapid.
Oxalis corniculuta.—Tke cotyledons do not stand vertical at
night, but generally rise to an angle of about 45° above the
horizon. They continued thus to act for 23 days after their
first expansion, by which time two leaves had been formed ;
even after 29 days they still rose moderately above their hori-
zontal or downwardly deflected diurnal position.
Mimosa pudica, — The cotyledons were expanded for the first
time on Nov. 2nd, and stood vertical at night. On the 15th the
first leaf was formed, and at night the cotyledons were vertical.
On the 28th they behaved in the same manner. On Dec. 15th,
that is after 41 days, the cotyledons were still considerably
raised at night; but those of another seedling, only one day
older, were raised very little.
Mimosa albida. — A seedling was observed during only 12 days,
by which time a leaf had been formed, and the cotyledons were
then quite vertical at night.
Trifolium subterranpum. — A seedling, 8 days old, had its coty-
ledons horizontal at 10.30 a.m. and vertical at 9.15 p.m. After an
interval of two months, by which time the first and second true
leaves had been developed, the cotyledons still performed the
same movement. They had now increased greatly in size, and
had become oval; and their petioles were actually "8 of an inch
in length !
Trifolium strict um. — After 17 days the cotyledons still rose at
night, but were not afterwards observed.
Lotus Jacobceus. — The cotyledons of some seedlings having
well-developed leaves rose to an angle of about 45° at night;
and even after 3 or 4 whorls of leaves had been formed, the co-
tyledons rose at night considerably above their diurnal hori-
zontal position.
Cassia mimosoides. — The cotyledons of this Indian species,
14 days after their first expansion, and when a leaf had been
formed, stood during the day horizontal, and at night vertical.
Cassia sp ? (a large S. Brazilian tree raised from seeds sent us
Chap. II. PULVINI OF COTYLEDONS. 117
by F. Miiller).— The cotyledons, after 16 days from their first
expansion, had increased greatly in size with two leaves just
formed. They stood horizontally during the day and vertically
at night, but were not afterwards observed.
Cassia neglecta (likewise a S. Brazilian species). — A seedling,
34 days after the first expansion of its cotyledons, was between 3
and 4 inches in height, with 3 well-developed leaves ; and the
cotyledons, which during the day were nearly horizontal, at night
stood vertical, closely embracing the young stem. The cotyle-
dons of another seedling of the same age, 5 inches in height,
with 4 well-developed leaves, behaved at night in exactly the
same manner.
It is known * that there is no difference in structure
between the upper and lower halves of the pulvini of
leaves, sufficient to account for their upward or down-
ward movements. In this respect cotyledons offer an
unusally good opportunity for comparing the structure
of the two halves ; for the cotyledons of Oxalis Valdi-
viana rise vertically at night, whilst those of 0. rosea
sink vertically ; yet when sections of their pulvini were
made, no clear difference could be detected between the
corresponding halves of this organ in the two species
which move so differently. With. 0. rosea, however,
there were rather more cells in the lower than in the
upper half, but this was likewise the case in one speci-
men of 0. Valdiviana. The cotyledons of both species
(3J mm. in length) were examined in the morning
whilst extended horizontally, and the upper surface of
the pulvinus of 0. rosea was then wrinkled transversely,
showing that it was in a state of compression, and this
might have been expected as the cotyledons sink at
night ; with 0. Valdiviana it was the lower surface
which was wrinkled, and its cotyledons rise at night.
Trifolium is a natural genus, and the leaves of all
* Pfeffer, * Die Period. Bewegungeri,' 1S75, p. 157.
1.18 TULVINI OF COTYLEDONS. Chap. II
the species seen by us are pulvinated ; so it is with
the cotyledons of T. subterraneum and strictum, which
stand vertically at night ; whereas those of T. resupi-
natum exhibit not a trace of a pulvinus, nor of any
nocturnal movement. This was ascertained by mea-
suring the distance between the tips of the cotyledons
of four seedlings at mid-day and at night. In this
species, however, as in the others, the first-formed leaf,
which is simple or not trifoliate, rises up and sleeps
like the terminal leaflet on a mature plant.
In another natural genus, Oxalis, the cotyledons of
0. Valdiviana, rosea, floribunda, articulata, and sensitiva
are pulvinated, and all move at night into an upward
or downward vertical position. In these several species
the pulvinus is seated close to the blade of the coty-
ledon, as is the usual rule with most plants. Oxalis cor-
niadata (var. Atro-pwpiirea) differs in several respects ;
the cotyledons rise at night to a very variable amount,
rarely more than 45°; and in one lot of seedlings
(purchased under the name of 0. tropseoloides, but
certainly belonging to the above variety) they rose
only from 5° to 15° above the horizon. The pulvinus
is developed imperfectly and to an extremely variable
degree, so that apparently it is tending towards abor-
tion. No such case has hitherto, we belie ye, been
described. It is coloured green from its cells con-
taining chlorophyll ; and it is seated nearly in the
middle of the petiole, instead of at the upper end as
in all the other species. The nocturnal movement is
effected partly by its aid, and partly by the growth of
the upper part of the petiole as in the case of plants
destitute of a pulvinus. Fr^m these several reasons
and from our having partially traced the develop-
ment of the pulvinus from an early age, the case
seems worth describing in some detail.
Chap. II.
PULVINI OF COTYLEDONS.
119
When the cotyledons of 0. comimluia were dissected out of a
seed from which they would soon have naturally emerged, no
trace of a pulvinus could be detected ; and all the cells forming
the short petiole, 7 in number in a longitudinal row, were of nearly
equal size. In seedlings one or two days old, the pulvinus was
so indistinct that we thought at first that it did not exist ; but
in the middle of the petiole an ill-defined transverse zone of cells
could be seen, which were much shorter than those both above
and below, although of the same breadth with them. They
presented the appearance of having been just formed by the
transverse division of longer cells ; and there can be little doubt
that this had occurred, for the cells in the petiole which had
Fig. 6*.
Oxalts cornlculata: A and B the almost rudimentary pulvini of the coty-
ledons of two rather old seedlings, viewed as transparent objects.
Magnified 50 times.
been dissected out of the seed averaged in length 7 divisions
of the micrometer (each division equalling -003 mm ), and were
a little longer than those forming a well-developed pulvinus,
which varied between 4 and 6 of these same divisions. After a
fsw additional days the ill -defined zone of cells becomes distinct,
and although it does not extend across the whole width of the
petiole, and although the cells are of a green colour from contain-
ing chlorophyll, yet they certainly constitute a pulvinus, which,
as we shall presently see, acts as one. These small cells were
arranged in longitudinal rows, and varied from 4 to 7 in number ;
and the cells themselves varied in length in different parts of the
9
120 PULVIXI OF COTYLEDONS. Chap. II
sante pulvinus and in different individuals. In the accompany-
ing figures, A and B (Fig. 64), we have views of the epidermis *
in the middle part of the petioles of two seedlings, in which the
pulvimis was for this species well developed. They offer a
striking contrast with the pulvhms of 0. rosea (see former
Fig. 63), or of 0. Valdiviana. With the seedlings, falsely called
0. trojjceoloides, the cotyledons of which rise very little at night,
the small cells were still fewer in number and in parts formed
a single transverse row, and in other parts short longitudinal
rows of only two or three. Nevertheless they sufficed to attract
the eye, when the whole petiole was viewed as a transparent
object beneath the microscope. In these seedlings there could
hardly be a doubt that the pulvinus was becoming rudimentary
and tending to disappear; and this accounts for its great
variability in structure and function.
In the following Table some measurements of the cells in
fairly well-developed pulvini of 0. corniculata are given : —
Seedling 1 day old, v:ith cotyledon 23 mm. in length.
Divisions of
Micrometer.f
Average length of cells of pulvinus 6 to 7
Length of longest cell below the pulvinus 13
Length of longest cell above the pulvinus 20
Seedling 5 d ys old, cotyledon 3*1 mm. in length, wi.h the pulvinus
quite distinct.
Average length of cells of pulvinus 6
Length of longest cell below the pulvinus 22
Leng;h of longest cell above the pulvinus 40
Seedling 8 days o 7 d, cotyledon 5 mm. in length, with a true leaf
formed but not yet expanded.
Average length of cells of pulvinus 9
Length of longest cell below the pulvinus 44
Length of longest cell above the pulvinus 70
Seedling 13 days old, cotyledon 4-5 mm. in length, with a small
true leaf fully developed.
Average length of cells of pulvinus 7
Length of longest cell below the pulvinus .. .. .. 30
Length of longest cell above the pulvinus t0
* Longitudinal sections show pulvinus.
that the forms of the epidermic f Each division equalled '0011
cells may be taken as a fair repre- mm.
Bcntation of those constituting the
Chap. II. PULVINI OF COTYLEDONS 121
We here see that the cells of the pulviims increase but little
in length with advancing age, in comparison with those of the
petiole both above and below it ; but they continue to grow in
width, and keep equal in this respect with the other cells of
the petiole. The rate of growth, however, varies in all parts
of the cotyledons, as may be observed in the measurements of
the 8-days' old seedling.
The cotyledons of seedlings only a day old rise at night con-
siderably, sometimes as much as afterwards; but there was
much variation in this respect. As the pulvinus is so indistinct
at first, the movement probably does not then depend on the
expansion of its cells, but on periodically unequal growth in
the petiole. By the comparison of seedlings of different known
ages, it was evident that the chief seat of growth of the petiole
was in the upper part between the pulvinus and the blade ;
and this agrees with the fact (shown in the measurements above
given) that the cells grow to a greater length in the upper than
in the lower part. With a seedling 1 1 days old, the nocturnal
rise was found to depend largely on the action of the pulvinus,
for the petiole at night was curved upwards at this point ; and
during the day, whilst the petiole was horizontal, the lower
surface of the pulvinus was wrinkled with the upper surface
tense. Although the cotyledons at an advanced age do not rise
at night to a higher inclination than whilst young, yet they have
to pass through a larger angle (in one instance amounting to
(53°) to gain their nocturnal position, as they are generally
deflected beneath the horizon during the day. Even with the
11-days' old seedling the movement did not depend exclusively
on the pulvinus, for the blade where joined to the petiole was
curved upwards, and this must be attributed to unequal growth.
Therefore the periodic movements of the cotyledons of 0. corni-
culata depend on two distinct but conjoint actions, namely, the
expansion of the cells of the pulvinus and on the growth of
the upper part of the petiole, including the base of the blade.
Lotus Jacobceus. : — The seedlings of this plant present a case
parallel to that of Oxalis corniculata in some respects, and in
others unique, as far as we have seen. The cotyledons during
the first 4 or 5 days of their life do not exhibit any plain noc-
turnal movement ; but afterwards they stand vertically or
almost vertically up at night. There is, however, some degree of
variability in this respect, apparently dependent on the season
and on the degree to which they have been illuminated during
122 PULVINI OF COTYLEDONS. Chap. IL
Llie day. With older seedlings, having cotyledons 4 mm. in
length, which rise considerably at night, there is a well -deve-
loped pulvinus close to the blade, colourless, and rather nar-
rower than the rest of the petiole, from which it is abruptly
separated. It is formed of a mass of small cells of an average
length of -021 mm. ; whereas the cells in the lower part of the
petiole are about *06 mm., and those in the blade from # 034 to
•04 mm. in length. The epidermic cells in the lower part of the
petiole project conically, and thus differ in shape from those
over the pulvinus.
Turning now to very young seedlings, the cotyledons of which
do not rise at night and are only from 2 to 2^ mm. in length,
their petioles do not exhibit any denned zone of small cells,
destitute of chlorophyll and differing in shape exteriorly from
the lower ones. Nevertheless, the cells at the place where a
pulvinus will afterwards be developed are smaller (being on an
average "015 mm. in length) than thoss in the lower parts of
the same petiole, which gradually become larger in proceeding
downwards, the largest being -030 mm. in length. At this ear]y
age the cells of the blade are about *027 mm. in length. We
thus see that the pulvinus is formed by the cells in the upper-
most part of the petiole, continuing for only a short time to
increase in length, then being arrested in their growth, accom-
panied by the loss of their chlorophyll grains ; whilst the cells
in the lower part of the petiole continue for a long time to
increase in length, those of the epidermis becoming more conical.
The singular fact of the cotyledons of this plant not sleeping at
first is therefore due to the pulvinus not being developed at an
early age.
We learn from these two cases of Lotus and Oxalis,
that the development of a pulvinus follows from the
growth of the cells oyer a small defined space of the
petiole being almost arrested at an early age. With
Lotus Jacobseus the cells at first increase a little in
length ; in Oxalis corniculata they decrease a little,
owing to seli-division. A mass of such small cells
forming a pulvinus, might therefore be either acquired
or lost without any special difficulty, by different
species in the same natural genus : and we know that
Chap. II. DISTURBED PERIODIC MOVEMENTS. 123
with seedlings of Trifoliurn, Lotus, and Oxalis some of
the species have a well-developed pulvinus, and others
have none, or one in a rudimentary condition. As the
movements caused by the alternate turgescence of
the cells in the two halves of a pulvinus, must be
largely determined by the extensibility and subse-
quent contraction of their walls, we can perhaps under-
stand why a large number of small cells will be more
efficient than a small number of large cells occupying
the same space. As a pulvinus is formed by the
arrestment of the growth of its cells, movements de-
pendent on their action may be long-continued withou
any increase in length of the part thus provided;
and such long-continued movements seem to be one
chief end gained by the development of a pulvinus.
Long-continued movement would be impossible in any
part, without an inordinate increase in its length, if the
turgescence of the cells was always followed by growth.
Disturbance of the Periodic Movements of Cotyledons by
Light. — The hypocotyls and cotyledons of most seed-
ling plants are, as is well known, extremely heliotropic ;
but cotyledons, besides being heliotropic, are affected
paratonically (to use Sachs' expression) by light ; that
is, their daily periodic movements are greatly and
quickly disturbed by changes in its intensity or by
its absence. It is not that they cease to circumnutate
in darkness, for in all the many cases observed by us
they continued to do so; but the normal order of
their movements in relation to the alternations of day
and night is much disturbed or quite annulled. This
holds good with species the cotyledons of which rise
or sink so much at night that they may be said to
sleep, as well as with others which rise only a little.
But different species are affected in very different
degrees by changes in the light.
L24: DISTUEBED PEEIODIC MOVEMENTS. Chap. II.
For instance, the cotyledons of Beta vulgaris, Solarium lycoper-
sicum, Ctrinthe major, and Lvpinus luteus, when placed in dark-
ness, moved down during the afternoon and early night, instead
of rising as they would have done if they had been exposed tc
the light. All the individuals of the Solanum did not behave
in the same manner, for the cotyledons of one circumnutated
about the same spot between 2.30 and 10 p.m. The cotyledons
of a seedling of Oxalis corniculata, which was feebly illuminated
from above, moved downwards during the first morning in the
normal manner, but on the second morning it moved upwards.
The cotyledons of Lotus Jacobceus were not affected by 4 h. of
complete darkness, but when placed under a double skylight
and thus feebly illuminated, they quite lost their periodical
movements on the third morning. On the other hand, the
cotyledons of Cucurbita ovifera moved in the normal manner
during a whole day in darkness.
Seedlings of Gitliago segetum were feebly illuminated from
above in the morning before their cotyledons had expanded, and
they remained closed for the next 40 h. Other seedlings were
placed in the dark after their cotyledons had opened in the
morning and these did not begin to close until about 4 h. had
elapsed. The cotyledons of Oxalis rosra sank vertically down-
wards after being left for 1 h. 20 m. in darkness ; but those of
some other species of Oxalis were not affected by several hours
of darkness. The cotyledons of several species of Cassia are
eminently susceptible to changes in the degree of light to which
they are exposed : thus seedlings of an unnamed S. Brazilian
species (a large and beautiful tree) were brought out of the hot-
house and placed on a table in the middle of a room with two
north-east and one north-west window, so that they were fairly
well illuminated, though of course less so than in the hot-house,
the day being moderately bright ; and after 36 m. the cotyledons
which had been horizontal rose up vertically and closed together
as when asleep ; after thus remaining on the table for 1 h. 13 m.
they began to open. The cotyledons of young seedlings of another
Brazilian species and of C. neglecta, treated in the same manner,
behaved similarly, excepting that they did not rise up quite so
much : they again became horizontal after about an hour.
Here is a more interesting case : seedlings of Cassia tora in
two pots, which had stood for some time on the table in the
room just described, had their cotyledons horizontal. One pot
was now exposed for 2 h. to dull sunshine, and the cotyledons
Ohap. it. sensitiy eness of cotyledons. 125
remained horizontal ; it was then brought back to the table, and
after 50 m. the cotyledons had risen 68° above the horizon.
The other pot was placed during the same 2 h. behind a screen
in the room, where the light was very obscure, and the cotyledons
rose 63° above the horizon ; the pot was then replaced on the
table, and after 50 m. the cotyledons had fallen 33°. These two
pots with seedlings of the same age stood close together, and
were exposed to exactly the same amount of light, yet the coty-
ledons in the one pot were rising, whilst those in the other
pot were at the same time sinking. This fact illustrates in a
striking manner that their movements are not governed by the
actual amount, but by a change in the intensity or degree of
the light. A similar experiment was tried with two sets of seed-
lings, both exposed to a dull light, but different in degree, and
the result was the same. The movements of the cotyledons of this
Cassia are, however, determined (as in many other cases) largely
by habit or inheritance, independently of light; for seedlings
which had been moderately illuminated during the day, were
kept all night and on the following morning in complete dark-
ness ; yet the cotyledons were partially open in the morning
and remained open in the dark for about 6 h. The cotyledons
in another pot, similarly treated on another occasion, were open
at 7 a.m. and remained open in the dark for 4 h. 30 m., after
which time they began to close. Yet these same seedlings, when
brought in the middle of the day from a moderately bright
into only a moderately dull light raised, as we have seen, their
cotyledons high above the horizon.
Sensitiveness of Cotyledons to contact. — This subject does not
possess much interest, as it is not known that sensitiveness of this
kind is of any service to seedling plants. We have observed cases
in only four genera, though we have vainly observed the coty-
ledons of many others. The genus Cassia seems to be pre-eminent
in this respect : thus, the cotyledons of C. torn, when extended
horizontally, were both lightly tapped with a very thin twig for
3 m., and in the course of a few minutes they formed together
an angle of 90°, so that each had risen 45°. A single cotyledon
of another seedling was tapped in a like manner for 1 m., and it
rose 27° in 9 m. ; and after eight additional minutes it had risen
10° more ; the opposite cotyledon, which was not tapped, hardly
moved at all. The cotyledons in all these cases became hori-
zontal again in less than half an hour. The pulvinus is the most
sensitive part, for on slightly pricking three cotyledons with a
1.26 COTYLEDONS SENSITIVE Chap. IX
pin in this part, they rose up vertically ; but the blade was found
also to be sensitive, care having been taken that the pulvinus
was not touched. Drops of water placed quietly on these coty-
ledons produced no effect, but an extremely fine stream of water,
ejected from a syringe, caused them to move upwards. When
a pot of seedlings was rapidly hit with a stick and thus jarred,
the cotyledons rose slightly. When a minute drop of nitric
acid was placed on both pulvini of a seedling, the cotyledons
rose so quickly that they could easily be seen to move, and
almost immediately afterwards they began to fall; but the
pulvini had been killed and became brown.
The cotyledons of an unnamed species of Cassia (a large tree
from S. Brazil) rose 31° in the course of 26 m. after the pulvini
and the blades had both been rubbed during 1 m. with a twig ;
but when the blade alone was similarly rubbed the cotyledons
rose only 8°. The remarkably long and narrow cotyledons, of a
third unnamed species from S. Brazil, did not move when their
blades were rubbed on six occasions with a pointed stick for
30 s. or for 1 m. ;'but when the pulvinus was rubbed and slightly
pricked with a pin, the cotyledons rose in the course of a few
minutes through an angle of 60°. Several cotyledons of
C. neghda (likewise from S. Brazil) rose in from 5 m. to 15 m. to
various angles between 16° and 34°, after being rubbed during
1 m. with a twig. Their sensitiveness is retained to a somewhat
advanced age, for the cotyledons of a little plant of 0. riegleda,
34 days old and bearing three true leaves, rose when lightly
pinched between the finger and thumb. -Some seedlings were
exposed for 30 m. to a wind (temp. 50° F.) sufficiently strong to
keep the cotyledons vibrating, but this to our surprise did not
cause any movement. The cotyledons of four seedlings of the
Indian ft gluuca were either rubbed with a thin twig for 2 m. or
were lightly pinched : one rose 34° ; a second only 6° ; a third
13°; and a fourth 17°. A cotyledon of C. floridu similarly
treated rose 9° ; one of ft corymhosa rose 7h°, and one of the
very distinct C. mimosoides only 6°. Those of C. pubescens did
not appear to be in the least sensitive ; nor were those of ft
ntdosa, but these latter are rather thick and fleshy, and do not
rise at night or go to sleep.
Smithia sen sit iva.— This plant belongs to a distinct sub-order of
the Leguminosse from Cassia. Both cotyledons of an oldish
seedling, with the first true leaf partially unfolded, were rubbed
for 1 m. with a fine twig, and in 5 m. each rose 32°; thoj
Chap. II. , TO CONTACT. 127
remained in this position for 15 m., bnt when looked at again
40 m. after the rubbing, each had fallen 14°. Both cotyledons of
another and younger seedling were lightly rubbed in the same
manner for 1 m., and after an interval of 32 m. each had risen
30°. They were hardly at all sensitive to a fine jet of water.
The cotyledons of S. Pfundii, an African water plant, are thick
and fleshy ; they are not sensitive and do not go to sleep.
Mimosa pudica and albida. — The blades of several cotyledons
of both these plants were rubbed or slightly scratched with a
needle during 1 m. or 2 m. ; but they did not move in the least.
When, however, the pulvini of six cotyledons of M. pudica were
thus scratched, two of them were slightly raised. In these two
cases perhaps the pulvinus was accidentally pricked, for on
pricking the pulvinus of another cotyledon it rose a little. It
thus appears that the cotyledons of Mimosa are less sensitive
than those of the previously mentioned plants.*
Oxcilis sensitiva. — The blades and pulvini of two cotyledons,
standing horizontally, were rubbed or rather tickled for 30 s.
with a fine split bristle, and in 10 m. each had risen 48°;
when looked at again in 35 m. after being rubbed they had
risen 4° more ; after 30 additional minutes they were again hori-
zontal. On hitting a pot rapidly with a stick for 1 m., the coty-
ledons of two seedlings were considerably raised in the course
of 11 m. A pot was carried a little distance on a tray and thus
jolted ; and the cotyledons of four seedlings were all raised in
10 m. ; after 17 m. one had risen 56°, a second 45°, a third almost
90°, and a fourth 90°. After an additional interval of 40 m. three
of them had re-expanded to a considerable extent. These obser-
vations were made before we were aware at what an extraordi-
narily rapid rate the cotyledons circumnutate, and are therefore
liable to error. Nevertheless it is extremely improbable that the
cotyledons in the eight cases given, should all have been rising
at the time when they were irritated. The cotyledons of Oocalis
Valdiviana and rosea were rubbed and did not exhibit any
sensitiveness.
Finally, there seems to exist some relation between
* The sole notice which wo p. 865), " les cotyledons du M
have met with on the sensitive- pudica tendent a se raprocher pal
ness of cotyledons, relates to Mi- leurs faces supe'rienres lorsqu'oB
mesa; for Au°\ P. De Candolle les ii'rite.''
says (' Phys. Ve'g.,' 1832, torn. ii.
128 SENSITIVENESS OF COTYLEDONS. Chap IL
the liabit of cotyledons rising vertically at night or
going to sleep, and their sensitiveness, especially that
of their pulvini, to a touch ; for all the above-named
plants sleep at night. On the other hand, there are
many plants the cotyledons of which sleep, and are
not in the least sensitive. As the cotyledons of
several species of Cassia are easily affected both by
slightly diminished light and by contact, we thought
that these two kinds of sensitiveness might be con-
nected ; but this is not necessarily the case, for the
cotyledons of Oxalis sensitiva did not rise when kept
on one occasion for 1J h., and on a second occasion
for nearly 4 h., in a dark closet. Some other coty-
ledons, as those of Githago segetum, are much affected
by a feeble light, but do not move when scratched by
a needle. That with the same plant there is some
relation between the sensitiveness of its cotyledons
and leaves seems highly probable, for the above de-
scribed Smithia and Oxalis have been called sensitiva,
owing to their leaves being sensitive ; and though the
leaves of the several species of Cassia are not sensitive
to a touch, yet if a branch be shaken or syringed
with water, they partially assume their nocturnal de-
pendent position. But the relation between the sen-
sitiveness to contact of the cotyledons and of the
leaves of the same plant is not very close, as may be
inferred from the cotyledons of Mimosa pudica being
only slightly sensitive, whilst the leaves are well
known to be so in the highest degree. Again, the
leaves of Neptunia oleracea are very sensitive to a
touch, whilst the cotyledons do not appear to be so iu
any degree.
Chap. III. SENSITIVENESS OF RADICLES. 120
CHAPTEK 111.
Sensitiveness of the Apex of the Radicle to Contact and to
othek 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 — Tro-
paoolum — Gossypiuin— Cucurbita — Eaphanus — iEsculus, tip not
sensitive to slight contact, highly sensitive to caustic — Quercus,
tip highly sensitive to contact — Power of discrimination — Zea
tip highly sensitive, secondary radicles — Sensitiveness of radicles
to moist air — Summary of chapter.
In order to see how trie radicles of seedlings would
pass over stones, roots, and other obstacles, which they
must incessantly encounter in the soil, germinating
beans (Vicia faba) were so placed that the tips of the
radicles came into contact, almost rectangularly or
at a high angle, with underlying plates of glass. In
other cases the beans were turned about whilst their
radicles were growing, so that they descended nearly
vertically on their own smooth, almost flat, broad upper
surfaces. The delicate root-cap, when it first touched
any directly opposing surface, was a little flattened
transversely ; the flattening soon became oblique, and
in a few hours quite disappeared, the apex now point-
ing at right angles, or at nearly right angles, to its
former course. The radicle then seemed to glide in
its new direction over the surface which had opposed
130 SENSITIVENESS OF RADICLES. Chap. II L
it, pressing on it with very little force. How. far such
abrupt changes in its former course are aided by the
circum nutation of the tip must be left doubtful. Thin
slips of wood were cemented on more or less steeply
inclined glass-plates, at right angles to the radicles
which were gliding down them. Straight lines had
been painted along the growing terminal part of some
of these radicles, before they met the opposing slip
of wood ; and the lines became sensibly curved in 2 h.
after the apex had come into contact with the slips.
In one case of a radicle, which was growing rather
slowly, the root-cap, after encountering a rough slip
of wood at right angles, was at first slightly flat-
tened transversely : after an interval of 2 h. 30 m.
the flattening became oblique ; and after an addi-
tional 3 hours the flattening had wholly disappeared,
and the apex now pointed at right angles to its former
course. It then continued to grow in its new direc-
tion alongside the slip of wood, until it came to the
end of it, round which it bent rectangularly. Soon
afterwards when coming 'to the edge of the plate of
glass, it was again bent at a large angle, and de-
scended perpendicularly into the damp sancl.
When, as in the above cases, radicles encountered
an obstacle at right angles to their course, the terminal
growing part became curved for a length of between
•3 and *4 of an inch (8-10 mm.), measured from the
apex. This was well shown by the black lines which
had been previously painted on them. The first and
most obvious explanation of the curvature is, that it
results merely from the mechanical resistance to the
growth of the radicle in its original direction. Never-
theless, this explanation did not seem to us satisfactory.
The radicles did not present the appearance of having
been subjected to a sufficient pressure to account for
Chat. III. SENSITIVENESS OF RADICLES. 131
tiieir curvature ; and Sachs has shown * that the
growing part is more rigid than the part immediately
above which has ceased to grow, so that the latter
might have been expected to yield and become curved
as soon as the apex encountered an unyielding object ;
whereas it was the stiff growing part which became
curved. Moreover, an object which yields with the
greatest ease will deflect a radicle : thus, as we have
seen, when the apex of the radicle of the beau
encountered the polished surface of extremely thin
tin-foil laid on soft sand, no impression was left on it
yet the radicle became deflected at right angles. A
second explanation occurred to us, namely, that even
the gentlest pressure might check the growth of the
apex, and in this case growth could continue only on
one side, and thus the radicle would assume a rectan-
gular form ; but this view leaves wholly unexplained
the curvature of the upper part, extending for a length
of 8-10 mm.
We were therefore led to suspect that the apex
was sensitive to contact, and that an effect was trans-
mitted from it to the upper part of the radicle, which
was thus excited to bend away from the touching object.
As a little loop of fine thread hung on a tendril or
on the petiole of a leaf-climbing plant, causes it to
bend, we thought that any small hard object affixed
to the tip of a radicle, freely suspended and growing
in damp air, might cause it to bend, if it were sensitive,
and yet would not offer any mechanical resistance to
its growth. Full details will be given of the experi-
ments which were tried, as the result proved remark-
able. The fact of the apex of a radicle being sensitive
to contact has never been observed, though, as we slial 1
Arbeiten Bot. Inst. Wiirztmrg,' Heft iii. 1S73, p. 398.
132 SENSITIVENESS OF THE APEX Chap. Ill
hereafter see, Sacks discovered that the radicle a little
above the apex is sensitive, and bends like a tendril
towards the touching object. But when one side of the
apex is pressed by any object, the growing part bends
away from the object; and this seems a beautiful
adaptation for avoiding obstacles in the soil, and, as
we shall see, for following the lines of least resistance.
Many organs, when touched, bend in one fixed direc-
tion, such as the stamens of Berberis, the lobes of
Dionsea, &c. ; and many organs, such as tendrils, whe-
ther modified leaves or flower-peduncles, and some few
stems, bend towards a touching object ; but no case,
we believe, is known of an organ bending away from
a touching object.
Sensitiveness of the Apex of the Radicle of Vicia faba.
— Common beans, after being soaked in water for 24 h.,
were pinned with the hilum downwards (in the manner
followed by Sachs), inside the cork lids of glass-vessels,
which were half filled with water ; the sides and the
cork were well moistened, and light was excluded.
As soon as the beans had protruded radicles, some to a
length of less than a tenth of an inch, and others to
a length of several tenths, little squares or oblongs of
card were affixed to the short sloping sides of their
conical tips. The squares therefore adhered obliquely
with reference to the longitudinal axis of the radicle ;
and this is a very necessary precaution, for if the bits
of card accidentally became displaced, or were drawn
by the viscid matter employed, so as to adhere parallel
to the side of the radicle, although only a little way
above the conical apex, the radicle did not bend in
the peculiar manner which we are here considering.
Squares of about the ^\jtk of an inch (i.e. about 1 J mm.),
or oblong bits of nearly the same size, were found to
Chap. III. OF THE RADICLE OF THE BEAN. 133
be the most convenient and effective. We employed
at first ordinary thin card, such as visiting cards, or
bits of very thin glass, and various other objects ; but
afterwards sand-paper was chiefly employed, for it was
almost as stiff as thin card, and the roughened surface
favoured its adhesion. At first we generally used very
thick gum-water; and this of course, under the cir-
cumstances, never dried in the least ; on the contrary,
it sometimes seemed to absorb vapour, so that the bits
of card became separated by a layer of fluid from the
tip. When there was no such absorption and the card
was not displaced, it acted well and caused the radicle
to bend to the opposite side. I should state that
thick gam-water by itself induces no action. In most
cases the bits of card were touched with an extremely
small quantity of a solution of shellac in spirits of
wine, whieh had been left to evaporate until it was
thick ; it then set hard in a few seconds, and fixed the
bits of card well. When small drops of the shellac
were placed on the tips without any card, they set into
hard little beads, and these acted like any other hard
object, causing the radicles to bend to the opposite
side. Even extremely minute beads of the shellac
occasionally acted in a slight degree, as will hereafter
be described. But that it was the cards which chiefly
acted in our many trials, was proved by coating one
side of the tip with a little bit of goldbeaters' skin
(which by itself hardly acts), and then fixing a bit of
card to the skin with shellac which never came into
contact with the radicle : nevertheless the radicle bent
away from the attached card in the ordinary manner.
Some preliminary trials were made, presently to
be described, by which the proper temperature was
determined, and then the following experiments were
made. It should he piemised that the beans were
134
SENSITIVENESS OF THE APEX Chap. Ill,
always fixed to the cork-lids, for the convenience of
manipulation, with the edge from which the radicle
and plumule protrudes, outwards ; and it must be
remembered that owing to what we have called Sachs'
curvature, the radicles, instead of growing perpendi-
cularly downwards, often bend somewhat, even as much
Vhia faba ; A, radicle beginning to bend from the attached little square
of card ; B, bent at a rectangle ; C, bent into a circle or loop, with the
tip beginning to bend downwards through the action of geotropism.
as about 45° inwards, or under the suspended bean.
Therefore when a square of card was fixed to the apex
in front, the bowing induced by it coincided with Sachs'
curvature, and could be distinguished from it only by
being more strongly pronounced or by occurring more
quickly. To avoid this source of doubt, the squares
Chai\ III. OF THE EADICLE OF THE BEAN. 136
wore fixed either behind, causing a curvature in direct
opposition to that of Sachs', or more commonly to the
right or left sides. For the sake of brevity, we will
speak of the bits of card, &c, as fixed in front, or
behind, or laterally. As the chief curvature of the
radicle is at a little distance from the apex, and as
the extreme terminal and basal portions are nearly
straight, it is possible to estimate in a rough manner
the amount of curvature by an angle ; and when it is
said that the radicle became deflected at any angle
from the perpendicular, this implies that the apex was
turned upwards by so many degrees from the down-
ward direction which it would naturally have followed,
and to the side opposite to that to which the card was
affixed. That the reader may have a clear idea of the
kind of movement excited by the bits of attached
card, we append here accurate sketches of three ger-
minating beans thus treated, and selected out of
several specimens to show the gradations in the
degrees of curvature. We will now give in detail a
series of experiments, and afterwards a summary of
the results.
In the first 12 trials, little squares or oblongs of sanded card,
3/8 mm. in length, and 1*5 or only 0*9 mm. in breadth (i.e. "071
ot an inch ia length and '059 or "035 of an inch in breadth) were
fixed with shellac to the tips of the radicles. In the subsequent
trials the little squares were only occasionally measured, but
were of about the same size.
(1.) A young radicle.. 4 mm. in length, had-a card fixed be-
hind : after 9 h. deflected in the plane in which the bean is
flattened, 50° from the perpendicular and from the card, and in
opposition to Sachs' curvature : no change next morning, 23 h.
from the time of attachment.
(2.) Radicle 5'5 mm. in length, card fixed behind : after 9 h.
deflected in the plane of the bean 20 c from the perpendicular
and from the card, and in opposition to Sachs' curvature : after
23 h. no change.
10
L36 SENSITIVENESS OF THE APEX Chap. Ill
(3.) Eadicle 11 mm. in length, card fixed behind: after 9 h.
deflected in the plane of the bean 40° from the perpendicular
and from the card, and in opposition to Sachs' curvature. The
tip of the radicle more curved than the upper part, but in the
same plane. After 23 h. the extreme tip was slightly bent to-
wards the card ; the general curvature of the radicle remaining
the same.
(4.) Eadicle 9 mm. long, card fixed behind and a little
laterally: after i9h. deflected in the plane of the bean only
about 7° or 8° from the perpendicular and from the card-, in
opposition to Sachs' curvature. There was in addition a slight
lateral curvature directed partly from the card. After 23 h. no
change.
(5.) Eadicle 8 mm. long, card affixed almost laterally : after
9h. deflected 30° .from the perpendicular, in the plane of the
bean and in opposition to Sachs' curvature ; also deflected in a
plane at right angles to the above one, 20° from the perpen-
dicular : after 23 h. no change.
(6.) Eadicle 9 mm, long, card affixed in front : after 9 h. de-
flected in the plane of the bean about 40° from the vertical,
away from the card and in the direction of Sachs' curvature.
Here therefore we have no evidence of the card being the
cause of the deflection, except that a radicle never moves
spontaneously, as far as we have seen, as much as 40° in the
course of 9 h. After 23 h. no change.
(7.) Eadicle 7 mm. long, card affixed to the back : after 9 h.
the terminal part of the radicle deflected in the plane of the
bean 20° from the vertical, away from the card and in opposition
to Sachs' curvature. After 22 h. 30 m. this part of the radicle
had become straight.
(8.) Eadicle 12 mm. long, card affixed almost laterally : after
9 h. deflected laterally in a plane at right angles to that of the
bean between 40° and 50° from the vertical and from the card.
In the plane of the bean itself the deflection amounted to 8° or
9° from the vertical and from the card, in opposition to Sachs'
curvature. After 22 h. 30 m. the extreme tip had become
slightly curved towards the card.
(9.) Card fixed laterally: after 11 h. 30m. no effect, the
radicle being still almost vertical.
(10.) Card fixed almost laterally: after 11 h. 30m. deflected
90° from the vertical and from the card, in a plane inter-
mediate between that of the bean itself and one at rigtt
Chap. III. OF THE EADICLE OF THE BEAN. 187
angles to it. Radicle consequently partially deflected from
ISachs' curvature.
(11.) Tip of jradicle protected with goldbeaters' skin, with a
square of card of the usual dimensions affixed with shellac:
after 11 h. greatly deflected in the plane of the bean, in the
direction of Sachs' curvature, but to a much greater degree and
in less time than ever occurs spontaneously.
(12.) Tip of radicle protected as in last case : after 11 h. no
effect, but after 24 h. 40 nf. radicle clearly deflected from the
card. This slow action was probably due to a portion of the
goldbeaters' skin having curled round and lightly touched, the
opposite side of the tip and thus irritated it.
(13.) A radicle of considerable length had a small square of
card fixed with shellac to its apex laterally : after only 7 h. 15 m.
a length of "4 of an inch from the apex, measured along the
middle, was considerably curved from the side bearing the card.
(14.) Case like the last in all respects, except that a length of
only *25 of an inch of the radicle was thus deflected.
(15.) A small square of card fixed with shellac to the apex of
a young radicle; after 9 h. 15 m. deflected through 90° from the
perpendicular and from the card. After 24 h. deflection much
decreased, and after an additional day, reduced to 23° from the
perpendicular.
(10.) Square of card fixed with shellac behind the apex of a
radicle, which from its position having been changed during
growth had become very crooked; but the terminal portion
was straight, and this became deflected to about 45° from
the perpendicular and from the card, in opposition to Sachs'
curvature.
(17.) Square of card affixed with shellac : after 8 h. radicle
curved at right angles from the perpendicular and from the
card. After 15 additional hours curvature much decreased.
(18.) Square of card affixed with shellac: after 8h. no effect;
after 23 h. 3 m. from time of affixing, radicle much curved from
the square.
(19.) Square of card affixed with shellac : after 24 h. no effect,
but the radicle had not grown well and seemed sickly.
(20.) Square of card affixed with shellac : after 24 h. no effect.
(21, 22.) Squares of card affixed with shellac : after 24 h.
radicles of both curved at about 45° from the perpendicular and
from the cards.
(23.) Square of card fixed with shellac to young radicle : after
138 SENSITIVENESS OF THE APEX Chap. Ill
9k. very slightly curved from the card; after 24 h. tip curved
towards card. Eefixed new square laterally, alter 9 h. distinctly
curved from the card, and after 24 h. curved at right angles frcm
the perpendicular and from the card.
(24.) A rather large oblong piece of card fixed with shellac to
apex ; after 24 h. no effect, but the card was found not to be
touching the apex. A small square was now refixed with
shellac ; after 16 h. slight deflection from the perpendicular
and from the card. After an additional day the radicle became
almost straight.
(25.) Square of card fixed laterally to apex of young radicle;
after 9 h. deflection from the perpendicular considerable ; after
24 h. deflection reduced. Eefixed a fresh square with shellac:
after 24 h. deflection about 40° from the perpendicular and from
the card.
(26.) A very small square of card fixed with shellac to apex of
young radicle : after 9 h. the deflection from the perpendicular
'and from the card amounted to nearly a right angle; after 24 h.
deflection much reduced ; after an additional 24 h. radicle almost
straight.
(27.) Square of card fixed with shellac to apex of young
radicle : after 9 h. deflection from the card and from the perpen-
dicular a right angle ; next morning quite straight. Refixed
a square laterally with shellac ; after 9 h. a little deflection,
which after 24 h. increased to nearly 20° from the perpendicular
and from the card.
(23.) Square of card fixed with shellac; after 9 h. some
deflection; next morning the card dropped off; refixed it with
shellac ; it again became loose and was refixed ; and now on the
third trial the radicle was deflected after 14 h. at right a ogles
from the card.
(29.) A small square of card was first fixed with thick gum-
water to the apex. It produced a slight effect but soon fell
off. A similar square was now affixed laterally with shellac :
after 9 h. the radicle was deflected nearly 45° from the perpen-
dicular and from the card. Alter 36 additional hours angle of
deflection reduced to about 30°.
(30.) A very small piece, less than -^th of an inch square, of
thin tin-foil fixed with shellac to the apex of a young radicle ;
after 24 h. no effect. Tin-foil removed, and a small square of
sanded card fixed with shellac ; after 9 \\ deflection at nearly
right angles from the perpendicular and from the card. Next
Chap. III. OF THE RADIC1 E OF THE BEAN. 139
morning deflection reduced to about 40° from the perpen-
dicular.
(31.) A splinter of thin glass gummed to apex, after 9 h. no
effect, but it was then found not to be touching the apex of the
radicle. Next morning a square of card was fixed with shellac
to it, and after 9 h. radicle greatly deflected from the card.
After two additional days the deflection had decreased and was
only 35° from the perpendicular.
(32.) Small square of sanded card, attached with thick gum-
water laterally to the apex of a long straight radicle: after 9 h.
greatly deflected from the perpendicular and from the card.
Curvature extended for a length of *22 of an inch from the
apex. After 3 additional hours terminal portion deflected at
right angles from the perpendicular. Next morning the curved
portion was '36 in length.
(33.) Square of card gummed to apex : after 15 h. deflected at
nearly 90° from the perpendicular and from the card.
(34.) Small oblong of sanded card gummed to apex: after
15 h. deflected 90° from the perpendicular and from the card :
in the course of the three following days the terminal portion
became much contorted and ultimately coiled into a helix.
(35.) Square of card gummed to apex: after 9 h. deflected from
card: after 24 h. from time of attachment greatly deflected
obliquely and partly in opposition to Sachs' curvature.
(36.) Small piece of card, rather less than ^th of an inch
square, gummed to apex : in 9 h. considerably deflected from
card and in opposition to Sachs' curvature ; after 24 h. greatly
deflected in the same direction. After an additional day the
extreme tip was curved towards the card.
(37.) Square of card, gummed to apex in front, caused after
8 h. 30 m. hardly any effect ; refixed fresh square laterally, after
15 h. deflected almost 90° from the perpendicular and from the
card. After 2 additional days deflection much reduced.
(38.) Square of card gummed to apex : after 9 h. much deflec-
tion, which after 24 h. from time of fixing increased to nearly
90°. After an additional day terminal portion was curled into
a loop, and on the following day into a helix.
(39.) Small oblong piece of card gummed to apex, nearly in
front, but a little to one side; in 9 h. slightly deflected in the
direction of Sachs' curvature, but rather obliquely, and to
side opposite to card. Next day more curved in the same
direction, and after 2 additional days ceiled into a ring.
110 SENSITIVENESS OF THE APEX Chap. III.
(10.) Square of card gummed to apex: after 9 h. slightly
curved from card ; uext morning radicle straight, and apex had
grown beyond the card. Eefixed another square laterally with
shellac ; in 9 h. deflected laterally, but also in the direction of
Sachs' curvature. After 2 additional days' curvature consider-
ably increased in the same direction.
(41.) Little square of tin-foil fixed with gum to one side of
apex of a young and short radicle : after 15 h. no effect, but
tin-foil had become displaced. A little square of card was now
gummed to one side of apex, which after 8 h. 40 m. was slightly
deflected; in 21 h. from the time of attachment deflected at 90°
from the perpendicular and from the card ; after 9 additional
hours became hooked, with the apex pointing to the zenith. In
3 days from the time of attachment the terminal portion of the
radicle formed a ring or circle.
(42.) A little square of thick letter-paper gummed to the
apex of a radicle, which after 9 h. was deflected from it. In
24 h. from time when the paper was affixed the deflection much
increased, and after 2 additional days it amounted to 50° from
the perpendicular and from the paper.
(43.) A narrow chip of a quill was fixed with shellac to the
apex of a radicle. After 9 h. no effect; after 24 h. moderate
deflection, but now the quill had ceased to touch the apex.
Removed quill and gummed a little square of card to apex,
which after 8 h. caused slight deflection. On the fourth day
from the first attachment of any object, the extreme tip was
curved towards the card.
(44.) A rather long and narrow splinter of extremely thin
glass, fixed with shellac to apex, it caused in 9 h. slight
deflection, which disappeared in 24 h. ; the splinter was then
found not touching the apex. It was twice refixed, with nearly
similar results, that is, it caused slight deflection, which soon
disappeared. On the fourth day from the time of first attach-
ment the tip was bent towards the splinter.
From these experiments it is clear that the apex of
the radicle of the bean is sensitive to contact, and
that it causes the upper part to bend away from the
touching object. But before giving a summary of the
results, it will be convenient briefly to give a few othei
observations. Bits of very thin glass and little square?
Chap. III. OF THE RADICLE OF THE BEA3. 141
of common card were affixed with thick gum- water to
the tips of the radicles of seven beans, as a pre-
liminary trial. Six of these were plainly acted on,
and in two cases the radicles became coiled up into
complete loops. One radicle was curved into a semi-
circle in so short a period as 6 h. 10 m. The
seventh radicle which was not affected was apparently
sickly, as it became brown on the following day ; so
that it formed no real exception. Some of these trials
were made in the early spring during cold weather in
a sitting-room, and others in a greenhouse, but the
temperature was not recorded. These six striking
cases almost convinced us. that the apex was sensitive,
but of course we determined to make many more trials.
As we had noticed that the radicles grew much more
quickly when subjected to considerable heat, and as
we imagined that heat would increase their sensitive-
ness, vessels with germinating beans suspended in
damp air were placed on a chimney-piece, where they
were subjected during the greater part of the day to a
temperature of between 69° and 72° F. ; some, how-
ever, were placed in the hot-house where the tempera-
ture was rather higher. Above two dozen beans were
thus tried ; and when a square of glass or card did
not act, it was removed, and a fresh one affixed, this
beins; often done thrice to the same radicle. There-
fore between five and six dozen trials were altogether
made. But there was moderately distinct deflection
from the perpendicular and from the attached object
in only one radicle out of this large number of cases.
In five other cases there was very slight and doubtful
deflection. We were astonished at this result, and
concluded that we had made some inexplicable mis-
take in the first six experiments. But before finally
relinquishing the subject, we resolved to make one
142 SENSITIVENESS OF THE APEX Chap. III.
other trial, for it occurred to us that sensitiveness is
easily affected by external conditions, and that radicles
growing naturally in the earth in the early spring
would not be subjected to a temperature nearly so
high as 70° F. We therefore allowed the radicles
of 12 beans to grow at a temperature of between
55° and 60° F. The result was that in every one of
these cases (included in the above-described experi-
ments) the radicle was deflected in the course of a few
hours from the attached object. All the above re-
corded successful trials, and some others presently to
be given, were made in a sitting-room at the tempera-
tures just specified. It therefore appears that a tem-
perature of about, or rather above, 70° F. destroys
the sensitiveness of the radicles, either directly, or
indirectly through abnormally accelerated growth ;
and this curious fact probably explains why Sachs,
who expressly states that his beans were kept at a
high temperature, failed to detect the sensitiveness of
the apex of the radicle.
But other causes interfere with this sensibility.
Eighteen radicles were tried with little squares of
sanded card, some affixed with shellac and some with
gum- water, during the few last days of 1878, and few
first days of the next year. They were kept in a room
at the proper temperature during the day, but were
probably too cold at night, as there was a hard frost at
the time. The radicles looked healthy but grew very
slowly. The result was that only 6 out of the 18
were deflected from the attached cards, and this only
to a slight degree and at a very slow rate. T«hese
radicles therefore presented a striking contrast with
the 41 above described. On March 6th and 7th, when
the temperature of the room varied between 53° and
59° F., eleven germinating beans were tried in the
Ohap. [II OF THE RADICLE OF' THE BEAN. 143
same manner, and now every one of the radicles
became curved away from the cards, though one was
only slightly deflected. Some horticulturists believe
that certain kinds of seeds will not germinate pro-
perly in the middle of the winter, although kept at a
right temperature. If there really is any proper period
for the germination of the bean, the feeble degree of
sensibility of the above radicles may have resulted
from the trial having been made in the middle of the
winter, and not simply from the nights being too cold.
Lastly, the radicles of four beans, which from some
innate cause germinated later than all the others of
the same lot, and which grew slowly though appearing
healthy, were similarly tried, and even after 24 h. they
were hardly at all deflected from the attached cards
We may therefore infer that any cause which renders
the growth of the radicles either slower or more rapid
than the normal rate, lessens or annuls the sensibility
of their tips to contact. It deserves particular atten-
tion that when the attached objects failed to act, there
was no bending of any kind, excepting Sachs' curva-
ture. The force of our evidence would have been
greatly weakened if occasionally, though rarely, the
radicles had become curved in any direction inde-
pendently of the attached objects. In the foregoing
numbered paragraphs, however, it may be observed
that the extreme tip sometimes becomes, after a con-
siderable interval of time, abruptly curved towards the
bit of card ; but this is a totally distinct phenomenon;
as will presently be explained.
Summary of the Results of the foregoing Experiments
on the Radicles of Vicia faba. — Altogether little squares
(about ^th of an inch), generally of sanded paper
as stiff as thin card (between *15 and -20 mm. in
thickness), sometimes of ordinary card, or little frag-
L44 SENSITIVENESS OF THE APEX Chap. Ill
ments of very thin glass, &c, were affixed at different
times to one side of the conical tips of 55 radicles.
The 11 last-mentioned cases, but not the preliminary
ones, are here included. The squares, &c, were most
commonly affixed with shellac, but in 19 cases with
thick gum-water. When the latter was used, the
squares were sometimes found, as previously stated,
to be separated from the apex by a layer of thick
fluid, so that there was no contact, and conse-
quently no bending of the radicle ; and such few
cases were not recorded. But in every instance in
which shellac was employed, unless the square fell
off very soon, the result was recorded. In several
instances when the squares became displaced, so as
to stand parallel to the radicle, or were separated by
fluid from the apex, or soon fell off, fresh squares
were attached, and these cases (described under the
numbered paragraphs) are here included. Out of
55 radicles experimented on under the proper tempe-
rature, 52 became bent, generally to a considerable
extent from the perpendicular, and away from the
side to which the object was attached. Of the three
failures, one can be accounted for, as the radicle
became sickly on the following day; and a second
was observed only during 11 h. 30 m. As in several
cases the terminal growing part of the radicle continued
for some time to bend from the attached object, it
formed itself into a hook, with the apex pointing to
the zenith, or even into a ring, and occasionally into a
spire or helix. It is remarkable that these latter cases
occurred more frequently when objects were attached
with thick gum-water, which never became dry, than
when shellac was employed. The curvature was often
well-marked in from 7 h. to 11 h. ; and in one instance
a semicircle was formed in 6 h. 10 m. from the time
Chap. III. OF THE KADIOLE OF THE BEAN. Il5
of attachment. But in order to see the phenomenon
as well displayed as in the above described cases, it is
indispensable that the bits of card, &c, should be
made to adhere closely to one side of the conical
apex ; that healthy radicles should be selected and
kept at not too high or too low a temperature, and
apparently that the trials should not be made in the
middle of the winter.
In ten instances, radicles which had curved away
from a square of card or other object attached to their
tips, straightened themselves to a certain extent, or
even completely, in the course of from one to two days
from the time of attachment. This was more espe-
cially apt to occur when the curvature was slight.
But in one instance (No. 27) a radicle which in 9 h.
had been deflected about 90° from the perpendicular,
became quite straight in 24 h. from the period of
attachment. With No. 26, the radicle was almost
straight in 48 h. We at first attributed the straighten-
ing process to the radicles becoming accustomed to a
slight stimulus, in the same manner as a tendril or
sensitive petiole becomes accustomed to a very light
loop of thread, and unbends itself though the loop
remains still suspended ; but Sachs states * that radicles
of the bean placed horizontally in damp air after
curving downwards through geotropism, straighten
themselves a little by growth along their lower or
concave sides. Why this should occur is not clear ;
but perhaps it likewise occurred in the above ten
cases. There is another occasional movement which
must not be passed over : the tip of the radicle, for a
length of from 2 to 3 mm., was found in six instances,
Arbeiten Bot. Instit., Wiirzburg,' Heft iii. p. 456.
140 SENSITIVENESS OF THE APEX Chap, lit
after an interval of about 24 or more hours, bent
towards the bit of still attached card, — that is, in a
direction exactly opposite to the previously induced
curvature of the whole growing part for a length of
from 7 to 8 mm. This occurred chiefly when the first
curvature was small, and when an object had been
affixed more than once to the apex of the same radicle.
The attachment of a bit of card by shellac to one
side of the tender apex may sometimes mechanically
prevent its growth ; or the application of thick gum-
water more than once to the same side may injure it ;
and then checked growth on this side with continued
growth on the opposite and unaffected side would
account for the reversed curvature of the apex.
Various trials were made for ascertaining, as far
as we could, the nature and degree of irritation to
which the apex must be subjected, in order that the
terminal growing part should bend away, as if to
avoid the cause of irritation. We have seen in the
numbered experiments, that a little square of rather
thick letter-paper gummed to the apex induced,
though slowly, considerable deflection. Judging from
several cases in which various objects had been affixed
with gum, and had soon become separated from the
apex by a layer of fluid, as well as from some trials
in which drops of thick gum-water alone had been
applied, this fluid never causes bending. We have
also seen in the numbered experiments that narrow
splinters of quill and of very thin glass, affixed with
shellac, caused only a slight degree of deflection, and
this may perhaps have been clue to the shellac
itself. Little squares of goldbeaters' skin, which is
excessively thin, were damped, and thus made to
adhere to one side of the tips of two radicles ; one of
these, after 24 h., produced no effect ; nor did the
Ohap. III. OF THE RADICLE OF THE BEAN. L47
other in 8 h., within which time squares of card usually
act ; but after 24 h. there was slight deflection.
An oval bead, or rather cake, of dried shellac,
1 • 01 mm. in length and * 63 in breadth, caused a
radicle to become deflected at nearly right angles in
the course of only 6 h. ; but after 23 h. it had nearly
straightened itself. A very small quantity of dissolved
shellac was spread over a bit of card, and tbe tips of
9 radicles were touched laterally with it ; only two of
them became slightly deflected to the side opposite
to that bearing the speck of dried shellac, and they
afterwards straightened themselves. These specks
were removed, and both together weighed less than
T ^th of a grain; so that a weight of rather Jess
than 2-^o th of a grain (0"32 mgs.) sufficed to excite
movement in two out of the nine radicles. Here
then we have apparently reached nearly the minimum
weight which will act.
A moderately thick bristle (which on measurement
was found rather flattened, being 0*33 mm. in one
diameter, and 0"20 mm. in the other) was cut into
lengths of about ^th. of an inch. These after being
touched with thick gum-water, were placed on the tip
of eleven radicles. Three of them were affected ; one
being deflected in 8 h. 15 m. to an angle of about 90°
from the perpendicular ; a second to the same amount
when looked at after 9 h. ; but after 24 h. from the
time of first attachment the deflection had decreased
to only 19° ; the third was only slightly deflected
after 9 h., and the bit of bristle was then found not
touching the apex ; it was replaced, and after 15
additional hours the deflection amounted to 26° from
the perpendicular. The remaining eight radicles
were not at all acted on by the bits of bristle, so that
we here appear to have nearly reached the minimum
148 SENSITIVENESS OF THE APEX Chap. III.
of size of an object which will act on the radicle of
the bean. But it is remarkable that when the bits of
bristle did act, that they should have acted so quickly
and efficiently.
As the apex of a radicle in penetrating the ground
must be pressed on all sides, we wished to learn
whether it could distinguish between harder or more
resisting, and softer substances. A square of the sanded
paper, almost as stiff as card, and a square of extremely
thin paper (too thin for writing on), of exactly the
same size (about ^V^h °f an lncn )> were fixed with
shellac on opposite sides of the apices of 12 suspended
radicles. The sanded card was between 0*15 and
0-20 mm. (or between 0-0059 and 0-0079 of an inch),
and the thin paper only 0*045 mm. (or 0-00176 of an
inch) in thickness. In 8 out of the 12 cases there
could be no doubt that the radicle was deflected from
the side to which the card-like paper was attached, and
towards the opposite side, bearing the very thin paper.
This occurred in some instances in 9 h., but in others
not until 24 h. had elapsed. Moreover, some of the
four failures can hardly be considered as really failures :
thus, in one of them, in which the radicle remained
quite straight, the square of thin paper was found,
when both were removed from the apex, to have been
so thickly coated with shellac that it was almost as
stiff as the card : in the second case, the radicle was
bent upwards into a semicircle, but the deflection
was not directly from the side bearing the card, and
this was explained by the two squares having become
cemented laterally together, forming a sort of stiff
gable, from which the radicle was deflected : in the
third case, the square of card had been fixed by
mistake in front, and though there was deflection
from it, this might have been due to Sachs' curvature ,
Chap. III. OF THE RADICLE OF THE BEAN. 149
in the fourth case alone no reason could be assigned
why the radicle had not been at all deflected. These
experiments suffice to prove that the apex of the
radicle possesses the extraordinary power of discri-
minating between thin card and very thin paper, and
is deflected from the side pressed by the more re-
sisting or harder substance.
Some trials were next made by irritating the tips
without any object being left in contact with them.
Nine radicles, suspended over water, had their tips
rubbed, each six times with a needle, with sufficient
force to shake the whole bean ; the temperature was
favourable, viz. about 63° F. In 7 out of these cases
no effect whatever was produced ; in the eighth case
the radicle became slightly deflected from, and in thp
ninth case slightly deflected towards, the rubbed side :
but these two latter opposed curvatures were probably
accidental, as radicles do not always grow perfectly
straight downwards. The tips of two other radicles
were rubbed in the same manner for 15 seconds with
a little round twig, two others for 30 seconds, and two
others for 1 minute, but without any effect being pro-
duced. We may therefore conclude from these 15
trials that the radicles are not sensitive to temporary
contact, but are acted on only by prolonged, though
very slight, pressure.
We then tried the effects of cutting off a very thin
slice parallel to one of the sloping sides of the apex 5
as we thought that the wound would cause prolonged
irritation, which might induce bending towards the
opposite side, as in the case of an attached object.
Two preliminary trials were made : firstly, slices were
cut from the radicles of 6 beans suspended in damp
air, with a pair of scissors, which, though sharp,
probably caused considerable crushing, and no curva-
[50 SENSITIVENESS OF THE APEX Chap 1HT.
ture followed. Secondly, thin slices were cut with a
razor obliquely off the tips of three radicles similarly
suspended ; and after 44 h. two were found plainly
bent from the sliced surface ; and the third, the whole
apex of w T hich had been cut off obliquely by accident,
was curled upwards over the bean, but it was not
clearly ascertained whether the curvature had been at
first directed from the cut surface. These results led
us to pursue the experiment, and 18 radicles, w 7 hich
had grown vertically downwards in damp air, had one
side of their conical tips sliced off with a razor. The
tips were allowed just to enter the water in the jars,
and they were exposed to a temperature 14°-16° C.
(57°-61° F.). The observations were made at dif-
ferent times. Three were examined 12 h. after beimr
sliced, and were all slightly curved from the cut
surface ; and the curvature increased considerably after
an additional 12 h. Eight were examined after 19 h. :
four after 22 h. 30 m. ; and three after 25 h. The
final result was that out of the 18 radicles thus tried,
13 were plainly bent from the cut surface after the
above intervals of time ; and one other became so
after an additional interval of 13 h. 30 m. So that
only 4 out of the 18 radicles were not acted on. To
these 18 cases the 3 previously mentioned ones should
be added. It may, therefore, be concluded that a thin
slice removed by a razor from one side of the conical
apex of the radicle causes irritation, like that from an
attached object, and induces curvature from the injured
surface.
Lastly, dry caustic (nitrate of silver) was employer!
to irritate one side of the apex. If one side of the
apex or of the whole terminal growing part of a
radicle, is by any means killed or badly injured, the
other side continues to grow ; and this causes the pan
Chap. III. OF THE RADICLE OF THE BEAN. 151
to bend over towards the injured side.* But i: the
following experiments we endeavoured, generally with
success, to irritate the tips on one side, without ladly
injuring thern. This was effected by first drying the
tip as far as possible with blotting-paper, though it still
remained somewhat damp, and then touching it once
with quite dry caustic. Seventeen radicles were thus
treated, and were suspended in moist air over water at
a temperature of 58 J F. They were examined after
an interval of 21 h. or 24 h. The tips of two were
found blackened equally all round, so that they could
tell nothing and were rejected, 15 being ]eft. Of
these, 10 were curved from the side which had been
touched, where there was a minute brown or blackish
mark. Five of these radicles, three of which were
already slightly deflected, were allowed to enter the
water in the jar, and were re-examined after an addi-
tional interval of 27 h. (i.e. in 48 h. after the appli-
cation of the caustic), and now four of them had
become hooked, being bent from the discoloured side
with their points directed to the zenith ; the fifth
remained unaffected and straight. Thus 11 radicles
out of the 15 were acted on. But the curvature of
the four just described was so plain, that they alone
would have sufficed to show that the radicles of the
bean bend away from that side of the apex which has
been slightly irritated by caustic.
TJie power of an Irritant on the apex of the Radicle
* Ciesielski found this to be the pended over water, with a thick
case (' Untersuchungen iiber die layer of grease, which is very
Abwartskrummung der Wurzel,' injurious or even fatal to grow-
1871, p. 28) after burning with ing parts; for after 48 hours
heated platinum one side of a five of these radicles were curved
radicle. So did we when we towards the greased side, two
painted longitudinally half of the remaining straight.
* T hole length of 7 radicles, sus-
11
L52 SENSITIVENESS OF THE APEX Chap. Ill
of the Bean, compared ivith tJiat of Geotrojrism. — We
know that when a little square of card or other
object is fixed to one side of the tip of a vertically
dependent radicle, the growing part bends from it
often into a semicircle, in opposition to geotropism,
which force is conquered by the effect of the irri-
tation from the attached object. Radicles were there-
fore extended horizontally in damp air, kept at
the proper low temperature for full sensitiveness,
and squares of card were affixed with shellac on the
loiver sides of their tips, so that if the squares
acted, the terminal growing part would curve upwards.
Firstly, eight beans were so placed that their short,
young, horizontally extended radicles would be simul-
taneously acted on both by geotropism and by Sachs'
curvature, if the latter came into play ; and they all
eight became bowed downwards to the centre of the
earth in 20 h., excepting one which was only slightly
acted on. Two of them were a little bowed downwards
in only 5 h. ! Therefore the cards, affixed to the lower
sides of their tips, seemed to produce no effect ; and
geotropism easily conquered the effects of the irritation
thus caused. Secondly, 5 oldish radicles, 1J- inch in
length, and therefore less sensitive than the above-
mentioned young ones, were similarly placed and
similarly treated. From what has been seen on many
other occasions, it may be safely inferred that if they
had been suspended vertically they would have bent
away from the cards ; and if they had been extended
horizontally, without cards attached to them, they
would have quickly bent vertically downwards through
geotropism ; but the result was that two of these
radicles were still horizontal after 23 h. ; two were
curved only slightly, and the fifth as much as 40°
beneath the horizon. Thirdly, 5 beans were fastened
Chap. J II. OF THE EADICLE OF THE BEAN. 153
with their flat surfaces parallel to the cork-lid, so that
Sachs' curvature would not tend to make the hori-
zontally extended radicles turn either upwards or
downwards, and little squares of card were affixed as
before, to the lower sides of their tips. The result
was that all five radicles were bent down, or towards
the centre of the earth, after only 8 h. 20 m. At
the same time and within the same jars, 3 radicles of
the same age, with squares affixed to one side, were
suspended vertically ; and after 8 h. 20 m. they were
considerably deflected from the cards, and therefore
curved upwards in opposition to geotropism. In these
latter cases the irritation from the squares had over-
powered geotropism ; whilst in the former cases, in
which the radicles were extended horizontally, geo-
tropism had overpowered the irritation Thus within
the same jars, some of the radicles were curving
upw r ards and others downwards at the same time—
these opposite movements depending on whether the
radicles, when the squares were first attached to them,
projected vertically down, or were extended horizon-
tally. This difference in their behaviour seems at first
inexplicable, but can, we believe, be simply explained
by the difference between the initial power of the two
forces under the above circumstances, combined with
the well-known principle of the after-effects of a sti-
mulus. When a vouno- and sensitive radicle is extended
horizontally, with a square attached to the lower side
of the tip, geotropism acts on it at right angles, and,
as we have seen, is then evidently more efficient than
the irritation from the square ; and the power of geo-
tropism will be strengthened at each successive period
by its previous action — that is, by its after-effects.
On the other hand, when a square is affixed to a
rertically dependent radicle, and the apex begins to
1 54 SENSITIVENESS OF THE EADICLE. Chap. Ill
curve upwards, this movement will be opposed by geo-
tropism acting only at a very oblique angle, and the
irritation from the card will be strengthened by its
previous action. We may therefore conclude that the
initial power of an irritant on the apex of the radicle
of the bean, is less than that of geotropism when
acting at right angles, but greater than that of geo-
tropism when acting obliquely on it.
Sensitiveness of the tips of the Secondary Radicles of the
Bsan to contact. — All the previous observations relate
to the main or primary radicle. Some beans suspended
to cork-lids, with their radicles dipping into water, had
developed secondary or lateral radicles, wilich were
afterwards kept in very damp air, at the proper low
temperature for full sensitiveness. They projected,
as usual, almost horizontally, with only a slight
downward curvature, and retained this position
during several davs. Sachs has shown* that these
secondary roots are acted on in a peculiar manner by
geotropism, so that if displaced they reassume their
former sub -horizontal position, and do not bend verti-
cally downwards like the primary radicle. Minute
squares of the stiff sanded paper were affixed by
means of shellac (but in some instances with thick
gum-water) to the tips of 39 secondary radicles of
different ages, generally the uppermost ones. Most
of the squares were fixed to the lower sides of the apex,
so that if they acted the radicle would bend upwards ;
but some were fixed laterally, and a few on the upper
side. Owing to the extreme tenuity of these radicles,
it was very difficult to attach the square to the
actual apex. Whether owing to this or some other
circumstance, only nine of the squares induced an y
Arbeitec Eot. Inst., Wiirzburg,' Heft iv. 1874, p. 605-617.
Chap. Ill SENSITIVENESS OF THE EADICLE. 155
curvature. The curvature amounted in some cases to
about 45° above the horizon, in others to 90°, and then
the tip pointed to the zenith. In one instance a
distinct upward curvature was observed in 8 h. 15 m.,
but usually not until 24 h. had elapsed. Although
only 9 out of 39 radicles were affected, yet the curva-
ture was so distinct in several of them, that there could
be no doubt that the tip is sensitive to slight contact,
and that the growing part bends away from the touch-
ing object. It is possible that some secondary radicles
are more sensitive than others ; for Sachs has proved *
the interesting fact that each individual secondary
radicle possesses its own peculiar constitution.
Sensitiveness to contact of the Primary Radicle, a little
above the apex, in the Bean ( Vicia faba) and Pea (Pisum
sativum). — The sensitiveness of the apex of the radicle
in the previously described cases, and the consequent
curvature of the upper part from the touching object
or other source of irritation, is the more remarkable,
because Sachs | has shown that pressure at the distance
of a few millimeters above the apex causes the radicle
to bend, like a tendril, towards the touching object.
By fixing pins so that they pressed against the radicles
of beans suspended vertically in damp air, we saw this
kind of curvature ; but rubbing the part with a twig
or needle for a few minutes produced no effect. Haber-
landt remarks,:]: that these radicles in breaking through
the seed-coats often rub and press against the ruptured
edges, and consequently bend round them. As little
squares of the card-like paper affixed with shellac to
the tips were highly efficient in causing the radicles
to bend away from them, similar pieces (of about -^th
* 'Arbeileti Bot. Instit., Wiirz- \ 'DieScliutzeinrichtungendej
burg,' Heft. iv. 1874, p. 620. Keimpflauze,' 1877, p. 25.
t Ibid. Heft iii. 1873, p. 437.
)56 SENSITIVENESS OF THE Chap. Ill
inch square, or rather less) were attached in the same
manner to one side of the radicle at a distance of 3 or
4 mm. above the apex. In our first trial on 15 radicles
no effect was produced. In a second trial on the same
number, three became abruptly curved (but only one
strongly) towards the card within 24 h. From these
cases we may infer that the pressure from a bit of card
affixed with shellac to one side above the apex, is hardly
a sufficient irritant ; but that it occasionally causes the
radicle to bend like a tendril towards this side.
We next tried the effect of rubbing several radicles
at a distance of 4 mm. from the apex for a few seconds
with lunar caustic (nitrate of silver) ; and although the
radicles had been wiped dry and the stick of caustic
was dry, yet the part rubbed was much injured and a
slight permanent depression was left. In such cases
the opposite side continues to grow, and the radicle
necessarily becomes bent towards the injured side.
But when a point 4 mm. from the apex was momen-
tarily touched with dry caustic, it was only faintly
discoloured, and no permanent injury was caused. This-
was shown by several radicles thus treated straighten-
ing themselves after one or two days ; yet at first they
became curved towards the touched side, as if they had
been there subjected to slight continued pressure.
These cases deserve notice, because when one side of
the apex was just touched with caustic, the radicle, as
we have seen, curved itself in an opposite direction, that
is, away from the touched side.
The radicle of the common pea at a point a little
above the apex is rather more sensitive to continued
pressure than that of the bean, and bends towards the
pressed side.* We experimented on a variety (York-
Saclis, ' A beiten Bot. Institut, Wiirzburg,' Heft iii. p. 438.
CHAr. III. UPPER PART OF THE RADICLE. 15?
sJiire Hero) which has a much wrinkled tough skin,
too large for the included cotyledons ; so that out of
30 peas which had been soaked for 24 h. and allowed
to germinate on damp sand, the radicles of three were
unable to escape, and were crumpled up in a strange
manner within the skin ; four other radicles were
abruptly bent round the edges of the ruptured skin
against which they had pressed. Such abnormalities
would probably never, or very rarely, occur with forms
developed in a state of nature and subjected to natural
selection. One of the four radicles just mentioned in
doubling backwards came into contact with the pin
by which the pea was fixed to the cork-lid ; and now it
bent at right angles round the pin, in a direction quite
different from that of the first curvature due to contact
with the ruptured skin ; and it thus afforded a good
illustration of the tendril-like sensitiveness of the
radicle a little above the apex.
Little squares of the card-like paper were next
affixed to radicles of the pea at 4 mm. above the apex,
in the same manner as with the bean. Twenty-eight
radicles suspended vertically over water were thus
treated on different occasions, and 13 of them became
curved towards the cards. The greatest degree of
curvature amounted to 62° from the perpendicular;
but so large an angle was only once formed. On one
occasion a slight curvature was perceptible after 5 h.
45 m., and it was generally well-marked after 14 h.
There can therefore be no doubt that with the pea,
irritation from a bit of card attached to one side of the
radicle above the apex suffices to induce curvature.
Squares of card were attached to one side of the tips
of 11 radicles within the same jars in which the above
trials were made, and five of them became plainly,
and one slightly, curved away from this side. Other
158 SENSITIVENESS OF THE APEX Chap. Ill
analogous cases will be immediately described. The
fact is here mentioned because it was a striking spec-
tacle, showing the difference in the sensitiveness of
the radicle in different parts, to behold in the same
jar one set of radicles curved away from the squares ou
their tips, and another set curved towards the squares
attached a little higher up. Moreover, the kind of
curvature in the two cases is different. The squares
attached above the apex cause the radicle to bend
abruptly, the part above and beneath remaining nearly
straight ; so that here there is little or no transmitted
effect. On the other hand, the squares attached to
the apex affect the radicle for a length of from about
4 to even 8 mm., inducing in most cases a sym-
metrical curvature ; so that here some influence is
transmitted from the apex for this distance along the
radicle.
Pisum sativum (var. Yorkshire Hero) : Sensitiveness of
the apex of the Radicle. — Little squares of the same card-
like paper were affixed (April 24th) with shellac to
one side of the apex of 10 vertically suspended radicles :
the temperature of the water in the bottom of the jars
was 60°-61° F. Most of these radicles were acted on
in 8 h. 30 m. ; and eight of them became in the course
of 24 h. conspicuously, and the remaining two slightly,
deflected from the perpendicular and from the side
bearing the attached squares. Thus all were acted on ;
but it will suffice to describe two conspicuous cases.
Tn one the terminal portion of the radicle was bent at
right angles (A, Fig. 66) after 24 h. ; and in the other
(B) it had by this time become hooked, with the apex
pointing to the zenith. The two bits of card here used
were *07 inch in length and *04 inch in breadth. Two
other radicles, which after 8 h. 30 m. were moderately
deflected, became straight again after 24 h. Another
Chap. III.
OF THE EADICLE OF THE PEA.
159
trial was made in the same manner with 15 radicles ;
but from circumstances, not worth explaining, they
were only once and briefly examined after the short
Fig. 66.
B.
Pisum sativum ; deflection produced within 24 hours in the growth of
vertically dependent radicles, by little squares of card affixed with
shellac to one side of apex : A, bent at light angles ; B, hooked.
interval of 5 h. 30 m. ; and we merely record in our
notes " almost all bent slightly from the perpendicular,
and away from the squares ; the deflection amounting
in one or two instances to nearly a rectangle." These
two sets of cases, especially the first one, prove that
the apex of the radicle is sensitive to slight contact
and that the upper part bends from the touching
object. Nevertheless, on June 1st and 4th, 8 other
radicles were tried in the same manner at a tempera-
ture of 58°-60° F., and after 24 h. only 1 was decidedly
bent from the card, 4 slightly, 2 doubtfully, and 1 not
in the least. The amount of curvature was unaccount-
ably small ; but all the radicles which were at all bent,
were bent away from the cards.
We now tried the effects of widely different tempera-
tures on the sensitiveness of these radicles with squares
160 SENSITIVENESS OF THE APEX Chap. Ill
of card attached to their tips. Firstly, 13 peas, most
of them having very short and young radicles, were
placed in an ice-box, in which the temperature rose
during three days from 44° to 47° F. They grew slowly,
but 10 out of the 13 became in the course of the three
days very slightly curved from the squares ; the other
3 were not affected ; so that this temperature was too
low for any high degree of sensitiveness or for much
movement. Jars with 13 other radicles were next
placed on a chimney-piece, where they were subjected
to a temperature of between 68° and 72° F., and
after 24 n., 4 were conspicuously curved from the
cards, 2 slightly, and 7 not at all ; so that this tem-
perature was rather too high. Lastly, 12 radicles
were subjected to a temperature varying between
72° and 85° F., and none of them were in the least
affected by the squares. The above several, trials,
especially the first recorded one, indicate that the
most favourable temperature for the sensitiveness of
the radicle of the pea is about 60° F.
The tips of 6 vertically dependent radicles were
touched once with dry caustic, in the manner described
under Vicia faba. After 24 h. four of them were bent
from the side bearing a minute black mark ; and the
curvature increased in one case after 38 h., and in
another case after 48 h., until the terminal part pro-
jected almost horizontally. The two remaining ra-
dicles were not affected.
With radicles of the bean, when extended horizontally
in damp air, geotropism always conquered the effects
of the irritation caused by squares of card attached to
the lower sides of their tips. A similar experiment
was tried on 13 radicles of the pea ; the squares being
attached with shellac, and the temperature between
58°~60° F. The result was somewhat different ; for
Chap. III.
OF THE KADICLE OF THE PEA.
161
these radicles are either less strongly acted on by
geotropism, or, what is more probable, are more sen-
sitive to contact. After a time geotropism always
prevailed, but its action was often delayed ; and in
three instances there was a most curious struggle
between geotropism and the irritation caused by the
cards. Eour of the 13 radicles were a little curved
downwards within 6 or 8 h., always reckoning from
the time when the squares were first attached, and
after 23 h. three of them pointed vertically down-
wards, and the fourth at an angle of 45° beneath the
horizon. These four radicles therefore did not seem
Fig. 67.
Pisum sativum: a radicle extended horizontally in damp air with a little
square of card affixed to the lower side of its tip, causing it to bend
upwards in opposition to geotropism. The deflection of the radicle
after 21 hours is shown at A, and of the same radicle after 45 hours at
B, now forming a loop.
to have been at all affected by the attached squares.
Four others were not acted on by geotropism within
the first 6 or 8 h., but after 23 h. were much bowed
down. Two others remained almost horizontal for
23 h., but afterwards were acted on. So that in these
latter six cases the action of geotropism was much
delayed. The eleventh radicle was slightly curved
down after 8 h., but when looked at again after 23 h.
the terminal portion was curved upwards ; if it had
lf>2 SENSITIVENESS OF THE APEX Chap. III.
been longer observed, the tip no doubt would have
been found again curved down, and it would have
formed a loop as in the following case. The twelfth
radicle after 6 h. was slightly curved downwards ; but
when looked at again after 21 h., this curvature had
disappeared and the apex pointed upwards ; after 30 h.
the radicle formed a hook, as shown at A (Fig. 67) ;
which hook after 45 h. was converted into a loop (B).
The thirteenth radicle after 6 h. was slightly curved
downwards, but within 21 h. had curved considerably
up, and then down again at an angle of 45° beneath
the horizon, afterwards becoming perpendicular. In
these three last cases geotropism and the irritation
caused by the attached squares alternately prevailed
in a highly remarkable manner ; geotropism being
ultimately victorious.
Similar experiments were not always quite so suc-
cessful as in the above cases. Thus 6 radicles, horizon-
tally extended with attached squares, were tried on
June 8th at a proper temperature, and after 7 h. 30 m.
none were in the least curved upwards and none were
distinctly geotropic ; whereas of 6 radicles without any
attached squares, which served as standards of com-
parison or controls, 3 became slightly and 3 almost
rectangularly geotropic within the 7 h. 30 m. ; but
after 23 h. the two lots were equally geotropic. On
July 10th another trial was made with 6 horizontally
extended radicles, with squares attached in the same
manner beneath their tips ; and after 7 h. 30 m., 4 were
slightly geotropic, 1 remained horizontal, and 1 was
curved upwards in opposition to gravity or geotropism.
This latter radicle after 48 h. formed a loop, like that
at B (Fig. 67).
An analogous trial was now made, but instead of
attaching squares of card to the lower sides of the
Chap. in. OF THE KADICLE OF PIIASEOLUS. 16o
tips, these were touched with dry caustic. The details
of the experiment will be given -in the chapter on
Geotropism, and it will suffice here to say that 10
peas, with radicles extended horizontally and not cau-
terised, were laid on and under damp friable peat ;
these, which served as standards or controls, as well as
10 others which had been touched on the upper side
with the caustic, all became strongly geotropic in 24 h.
Nine radicles, similarly placed, had their tips touched
on the lower side with the caustic ; and after 24 h.,
3 were slightly geotropic, 2 remained horizontal, and
4 were bowed upwards in opposition to gravity and to
geotropism. This upward curvature was distinctly
visible in 8 h. 45 m. after the lower sides of the tips
had been cauterised.
Little squares of card were affixed with shellac on
two occasions to the tips of 22 young and short
secondary radicles, which had been emitted from the
primary radicle whilst growing in water, but were now
suspended in damp air. Besides the difficulty of
attaching the squares to such finely pointed objects
as were these radicles, the temperature was too high,
— varying on the first occasion from 72° to 77° F., and
on the second being almost steadily 78° F. ; and this
probably lessened the sensitiveness of the tips. The
result was that after an interval of 8 h. 30 m,, 6 of the
122 radicles were bowed upwards (one of them greatly)
in opposition to gravity, and 2 laterally ; the remain-
ing 14 were not affected. Considering the unfavour-
able circumstances, and bearing in mind the case of
the bean,, the evidence appears sufficient to show that
the tips of the secondary radicles of the pea are
sensitive to slight contact.
Phaseolus multiflorus : Sensitiveness of the apex of the
161 SENSITIVENESS OF THE APEX Chap. Ill
of various sizes of the same card-like paper, also with
bits of thin glass and rough cinders, affixed with shellac
to one side of the apex. Eather large drops of the
dissolved shellac were also placed on them and allowed
to set into hard beads. The specimens were subjected
to various temperatures between 60° and 72° F., more
commonly at about the latter. But out of this con-
siderable number of trials only 5 radicles were plainly
bent, and 8 others slightly or even doubtfully, from
the attached objects ; the remaining 46 not being at
all affected. It is therefore clear that the tips of the
radicles of this Phaseolus are much less sensitive to
contact than are those of the bean or pea. We
thought that they might be sensitive to harder
pressure, but after several trials we could not devise
any method for pressing harder on one side of the
apex than on the other, without at the same time
offering mechanical resistance to its growth. We
therefore tried other irritants.
The tips of 13 radicles, dried with blotting-paper,
were thrice touched or just rubbed on one side
with dry nitrate of silver. They were rubbed thrice,
because we supposed from the foregoing trials, that
the tips were not highly sensitive. After 24 h. the
tips were found greatly blackened ; 6 were blackened
equally all round, so that no curvature to any one
side could be expected ; 6 were much blackened on
one side for a length of about T ' th of an inch, and
this length became curved at right angles towards the
blackened surface, the curvature afterwards increasing
in several instances until little hooks were formed.
] t was manifest that the blackened side was so much
injured that it could not grow, whilst the opposite
side continued to grow. One alone out of these 13
radicles became curved from the blackened side, the
Chap. in. OF THE RADICLE OF PHASEOLUS. 16S
curvature extending for some little distance above
the apex.
After the experience thus gained, the tips of six
almost dry radicles were once touched with the dry
caustic on one side ; and after an interval of 10 m.
were allowed to enter water, which was kept at a
temperature of 65°-67° F. The result was that after
an interval of 8 h. a minute blackish speck could
just be distinguished on one side of the apex of five
of these radicles, all of which became curved towards
the opposite side — in two cases at about an angle
of 45° — in two other cases at nearly a rectangle — and
in the fifth case at above a rectangle, so that the apex
was a little hooked ; in this latter case the black mark
was rather larger than in the others. After 24 h.
from the application of the caustic, the curvature of
three of these radicles (including the hooked one) had
diminished ; in the fourth it remained the same, and
in the fifth it had increased, the tip being now hooked.
It has been said that after 8 h. black specks could
be seen on one side of the apex of five of the six
radicles ; on the sixth the speck, which was extremely
minute, was on the actual apex and therefore central ;
and this radicle alone did not become curved. It was
therefore again touched on one side with caustic, and
after 15 h. 30 m. was found curved from the perpen-
dicular and from the blackened side at an angle of 34°,
which increased in nine additional hours to 54°.
It is therefore certain that the apex of the radicle
of this Phaseolus is extremely sensitive to caustic,
more so than that of the bean, though the latter is
far more sensitive to pressure. In the experiments
just given, the curvature from the slightly cauterised
side of the tip, extended along the radicle for a
length of nearly 10 mm. ; whereas in the first set
166 SENSITIVENESS OF THE APEX Chap. Ill
of experiments, when the tips of several were greatly
blackened and injured on one side, so that their growth
was arrested, a length of less than 3 mm. became
curved towards the much blackened side, owing to the
continued growth of the opposite side. This differ-
ence in the results is interesting, for it shows that too
strong an irritant does not induce any transmitted
effect, and does not cause the adjoining, upper and
growing part of the radicle to bend. We have analo-
gous cases with Drosera, for a strong solution of car-
bonate of ammonia when absorbed by the glands, or
too great heat suddenly applied to them, or crushing
them, does not cause the basal part of the tentacles
to bend, whilst a weak solution of the carbonate, or a
moderate heat, or slight pressure always induces such
bending. Similar results were observed with Dionsea
and Pinguicula.
The effect of cutting off with a razor a thin slice
from one side of the conical apex of 14 young and
short radicles was next tried. Six of them after beiug
operated on were suspended in damp air ; the tips of
the other eight, similarly suspended, were allowed to
enter water at a temperature of about 65° F. It was
recorded^ in each case which side of the apex had
been sliced off, and when they were afterwards
examined the direction of the curvature was noted,
before the record was consulted. Of the six radicles
in damp air, three had their tips curved after an
interval of 10 h. 15 m. directly away from the sliced
surface, whilst the other three were not affected and
remained straight ; nevertheless, one of them after
13 additional hours became slightly curved from the
sliced surface. Of the eight radicles with their tips
immersed in water, seven were plainly curved away
from the sliced surfaces after 10 h. 15 m. ; and with
CkAP. III. OF THE KADICLE OF TKOP^EOLUM. 167
respect to the eighth which remained quite straight,
too thick a slice had been accidentally removed, so
that it hardly formed a real exception to the general
result. When the seven radicles were looked at
again, after an interval of 23 h. from the time of
slicing, two had become distorted ; four were deflected
at an angle of about 70° from the perpendicular and
from the cut surface ; and one was deflected at nearly
90°, so that it projected almost horizontally, but with
the extreme tip now beginning to bend downwards
through the action of geotropism. It is therefore
manifest that a thin slice cut off one side of the conical
apex, causes the upper growing part of the radicle of
this Phaseolus to bend, through the transmitted effects
of the irritation, away from the sliced surface.
Tropseolum majus : Sensitiveness of the apex of the
Radicle to contact. — Little squares of card were attached
with shellac to one side of the tips of 19 radicles, some
of which were subjected to 78 J F., and others to a
much lower temperature. Only 3 became plainly
curved from the squares, 5 slightly, 4 doubtfully,
and 7 not at all. These seeds were, as we believed,
old, so we procured a fresh lot, and now the results
were widely different. Twenty-three were tried in
the same manner ; five of the squares produced no
effect, but three of these cases were no real exceptions,
for in two of them the squares had slipped and were
parallel to the apex, and in the third the shellac was
in excess and had spread equally all round the apex.
One radicle was deflected only slightly from the
perpendicular and from the card ; whilst seventeen
were plainly deflected. The angles in several of these
latter cases varied between 40° and 65° from the
perpendicular ; and in two of them it amounted after
15 h. or 16 h. to about 90°. In one instance a loop
12
168 SENSITIVENESS OF THE APEX Chap. Ill
was nearly completed in 16 h. There can, therefore,
be no doubt that the apex is higl.ly sensitive to slight
contact, and that the upper part of the radicle bends
away from the touching object.
Gossijpium herbaceum : Sensitiveness of the apex of the
Badicle. — Radicles were experimented on in the same
manner as before, but they proved ill-fitted for our
purpose, as they soon became unhealthy when sus-
pended in damp air. Of 38 radicles thus suspended,
at temperatures varying from 66° to 69° F., with
squares of card attached to their tips, 9 were plainly
and 7 slightly or even doubtfully deflected from the
squares and from the perpendicular; 22 not being
affected. We thought that perhaps the above tempera-
ture was not high enough, so 19 radicles with attached
squares, likewise suspended in damp air, were subjected
to a temperature of from 74° to 79° F., but not one of
them was acted on, and they soon became unhealthy.
Lastly, 19 radicles were suspended in water at a tem-
perature from 70° to 75° F., with bits of glass or
squares of the card attached to their tips by means of
Canada-balsam or asphalte, which adhered rather better
than shellac beneath the water. The radicles did not
keep healthy for long. The result was that 6 were
plainly and 2 doubtfully deflected from the attached
objects and the perpendicular; 11 not being affected.
The evidence consequently is hardly conclusive,
though from the two sets of cases tried under a
moderate temperature, it is probable that the radicles
are sensitive to contact ; and would be more so under
favourable conditions.
Fifteen radicles which had germinated in friable peat
were suspended vertically over water. Seven of them
served as controls, and they remained quite straight
during 24 h. The tips of the other eight radicles
Ohap. III. OF THE KADICLE OF CUCUKBITA. 169
were just touched with dry caustic on one side. Aftei
only 5 h. 10 m. five of thern were slightly curved
from the perpendicular and from the side beariug the
little blackish marks. After 8 h. 40 m.. 4 out of
these 5 were deflected at angles between 15° and 65°
from the perpendicular. On the other hand, one
which had been slightly curved after 5 h. 10 in., now
became straight. After 24 h. the curvature in two
cases had considerably increased ; also in four other
cases, but these latter radicles had now become so
contorted, some being turned upwards, that it could no
longer be ascertained whether they were still curved
from the cauterised side. The control specimens ex-
hibited no such irregular growth, and the two sets
presented a striking contrast. Out of the 8 radicles
which had been touched with caustic, two alone were
not affected, and the marks left on their tips by the
caustic were extremely minute. These marks in all
cases were oval or elongated ; they were measured in
three instances, and found to be of nearly the same
size, viz. § of a mm. in length. Bearing this fact in
mind, it should be observed that the length of the
curved part of the radicle, which had become deflected
from the cauterised side in the course of 8 h. 40 m.,
was found to be in three cases 6, 7, and 9 mm.
Cucurbit a ovifera : Sensitiveness of the cqyex of the Ba~
dicle. — The tips proved ill-fitted for the attachment of
cards, as they ars extremely fine and flexible. More-
over, owing to the hypocotyls being soon developed
and becoming arched, the whole radicle is quickly
displaced and confusion is thus caused. A large
number of trials were made, but without any definite
result, excepting on tw^o occasions, when out of 23
radicles 10 were deflected from the attached squares
170 SENSITIVENESS OF THE APEX Chap. Ill
of card, and 13 were not acted on. Bather large
Bquares, though difficult to affix, seemed more efficient
than very small ones.
We were much more successful with caustic ; but in
our first trial, 15 radicles were too much cauterised,
and only two became curved from the blackened side ;
the others being either killed on one side, or blackened
equally all round. In our next trial the dried tips
of 11 radicles were touched momentarily with dry
caustic, and after a few minutes were immersed in
water. The elongated marks thus caused were never
black, only brown, and about \ mm. in length, or
even less. In 4 h. 30 m. after the cauterisation, 6 of
them were plainly curved from the side with the
brown mark, 4 slightly, and 1 not at all. The latter
proved unhealthy, and never grew ; and the marks on
2 of the 4 slightly curved radicles were excessively
minute, one being distinguishable only with the aid
of a lens. Of 10 control specimens tried in the same
jars at the same time, not one was in the least curved.
In 8 h. 40 m. after the cauterisation, 5 of the radicles
out of the 10 (the one unhealthy one being omitted)
were deflected at about 90°, and 3 at about 45° from
the perpendicular and from the side bearing the
brown mark. After 24 h. all 10 radicles had in-
creased immensely in length ; in 5 of them the curva-
ture was nearly the same, in 2 it had increased, and
in 3 it had decreased. The contrast presented by the
10 controls, after both the 8 h. 40 m. and the 24 h.
intervals, was very great ; for. they had continued to
grow vertically downwards, excepting two which, from
some unknown cause, had become somewhat tortuous.
In the chapter on Geotropism we shall see that
10 radicles of this plant were extended horizontally on
and beneath damp friable peat, under which conditions
Chap. III. OF THE RADICLE OF RAPHANUS. 171
they grow better and more naturally than in damp
air; and their tips were slightly cauterised on the
lower side, brown marks about J mm. in length
being thus caused. Uncauterised specimens similarly
placed became much bent downwards through geo-
tropism in the course of 5 or 6 hours. After 8 h.
only 3 of the cauterised ones were bowed downwards,
and this in a slight degree ; 4 remained horizontal ;
and 3 were curved upwards in opposition to geo-
tropism and from the side bearing the brown mark.
Ten other specimens had their tips cauterised at the
same time and in the same degree, on the upper
side ; and this, if it produced any effect, would tend
to increase the power of geotropism ; and all these
radicles were strongly bowed downwards after 8 h.
From the several foregoing facts, there can be no
doubt that the cauterisation of the tip of the radicle
of this Cucurbita on one side, if done lightly enough,
causes the whole growing part to bend to the opposite
side.
Baphanus sativus : Sensitiveness of the apex of tlie
Radicle. — We here encountered many difficulties in
our trials, both with squares of card and with caustic ;
for when seeds were pinned to a cork-lid, many of the
radicles, to which nothing had been done, grew irre-
gularly, often curving upwards, as if attracted by the
damp surface above ; and when they were immersed
in water they likewise often grew irregularly. We
did not therefore dare to trust our experiments with
attached squares of card ; nevertheless some of them
seemed to indicate that the tips were sensitive to
contact. Our trials with caustic generally failed from
the difficulty of not injuring too greatly the extremely
fine tips. Out of 7 radicles thus tried, one became
bowed after 22 h. at an angle of 60°, a second at 40°
172 SENSITIVENESS OF THE APEX Chap. Ill
and a third very slightly from the perpendicular and
from the cauterised side.
Mseulus hippoeastanum : Sensitiveness of the apex oj
the Radicle. — Bits of glass and squares of card were
affixed with shellac or gum-water to the tips of 12
radicles of the horse-chestnut ; and when these objects
fell off, they were reflxed ; but not in a single instance
was any curvature thus caused. These massive
radicles, one of which was above 2 inches in length
and *3 inch in diameter at its base, seemed insensible
to so slight a stimulus as any small attached object.
Nevertheless, when the apex encountered an obstacle
in its downward course, the growing part became sc
uniformly and symmetrically curved, that its appear-
ance indicated not mere mechanical bending, but
increased growth along the whole convex side, due to
the irritation of the apex.
That this is the correct view may be inferred from
the effects of the more powerful stimulus of caustic.
The bending from the cauterised side occurred much
slower than in the previously described species, and it
will perhaps be worth while to give our trials in
detail.
The seeds germinated in sawdust, and one side of the tips of
the radicles were slightly rubbed once with dry nitrate of silver;
and after a few minutes were allowed to dip into water. They
were subjected to a rather varying temperature, generally
between 52° and 58° F. A few cases have not been thought
worth recording, in which the whole tip was blackened, or in
which the seedling soon became unhealthy.
(1.) The radicle was slightly deflected from the cauterised
side in one day (i.e. 24 fa.) ; in three days it stood at 60° from
the xjerpendicular ; in four days at 90° ; on the fifth day it was
curved up about 40° above the horizon ; so that it had passed
through an angle of 130° in the five days, and this was the
greatest amount of curvature observed.
(2.) In two days radicle slightly deflected ; after seven days
Chap. III. OF THE KADICLE OF .ESCULUS. 173
deflected 69° from the perpendicular and from the cauterised
side; after eight days the angle amounted to nearly 90°.
(3.) After one day slight deflection, but the cauterised mark
was so faint that the same side was again touched with caustic.
In four days from the first touch deflection amounted to 7b°,
which in an additional day increased to 90°.
(4.) After two days slight deflection, which during the next
three days certainly increased but never became great ; the
radicle did not grow well and died on the eighth day.
(5.) After two days very slight deflection ; but this on the
fourth day amounted to 56° from the perpendicular and from
the cauterised side.
(6.) After three days doubtfully, but after four days certainly
deflected from the cauterised side. On the fifth day deflection
amounted to 45° from the perpendicular, and this on the seventh
day increased to about 90°.
(7.) After two days slightly deflected ; on the third day the
deflection amounted to 25° from the perpendicular, and this
did not afterwards increase.
(8.) After one day deflection distinct; on the third day i*
amounted to 44°, and on the fourth day to 72° from the perpen-
dicular and the cauterised side.
(9.) After two days deflection slight, yet distinct ; on the
third day the tip was again touched on the same side with
caustic and thus killed.
(10.) After one day slight deflection, which after six days
increased to 50° from the perpendicular and the cauterised side.
(11.) After one day decided deflection, which after six days
increased to 62° from the perpendicular and from the cauterised
side.
(12.) After one clay slight deflection, which on the second day
amounted to 35°, on the fourth day to 50°, and the sixth day
to 63° from the perpendicular and the cauterised side.
(13.) Whole tip blackened, but more on one side than the
other ; on the fourth day slightly, and on the sixth day greatly
deflected from the more blackened side ; the deflection on the
rinth day amounted to 90° from the perpendicular.
(14.) Whole tip blackened in the same manner as in the last
case ; on the second day decided deflection from the more
blackened side, which increased on the seventh day to nearlj
90° ; on the following day the radicle appeared unhealthy.
^15 ) Here we had the anomalous case of a radicle bending
174: SENSITIVENESS OF THE APEX Chap. Ul
slightly towards the cauterised side on the first day, and con-
tinuing to do so for the next three days, when the deflection
amounted to about 90° from the perpendicular. The cause
appeared to lie in the tendril-like sensitiveness of the upper part
of the radicle, against which the point of a large triangular flap
of the seed-coats pressed with considerable force ; and this
irritation apparently conquered that from the cauterised apex.
These several cases show beyond doubt that the
irritation of one side of the apex, excites the upper
part of the radicle to bend slowly towards the opposite
side. This fact was well exhibited in one lot of five
seeds pinned to the cork-lid of a jar ; for when after
6 days the lid was turned upside down and viewed
from directly above, the little black marks made by the
caustic were now all distinctly visible on the upper
sides of the tips of the laterally bowed radicles.
A thin slice was shaved off with a razor from one
side of the tips of 22 radicles, in the manner described
under the common bean ; but this kind of irritation
did not prove very effective. Only 7 out of the 22
radicles became moderately deflected in from 3 to 5
days from the sliced surface, and several of the others
grew irregularly. The evidence, therefore, is far from
conclusive.
Quercus robur: Sensitiveness of the apex of the Radicle.
— The tips of the radicles of the common oak are fully
as sensitive to slight contact as are those of any plant
examined by us. They remained healthy in damp air
for 10 days, but grew slowly. Squares of the card-
like paper were fixed with shellac to the tips of 15
radicles, and ten of these became conspicuously bowed
from the perpendicular and from the squares ; two
slightly, and three not at all. But two of the latter
were not real exceptions, as they were at first very
short, and hardly grew afterwards. Some of the more
L'dap. III. OF THE EADICLE OF QUEKCUS.
175
remarkable cases are worth describing. The radicles
were examined on each successive morning, at nearly
the same hour, that is, after intervals of 24 h.
Fig. 68.
No. 1. This radicle suffered from a series of accidents, and
acted in an anomalous manner, for the apex appeared at first
insensible and afterwards sensitive to contact. The first square
was attached on Oct. 19th ; on the 21st the
radicle was not at all curved, and the square
was accidentally knocked off; it was refixed
on the 22nd. and the radicle became slightly
carved from the square, but tne curvature
disappeared on the 23rd, when the square
was removed and refixed. No curvature en-
sued, and the square was again accidentally
knocked off, and refixed. On the morning of
the 27th it was washed off by having reached
the water in the bottom of the jar. The
square was refixed, and on the 29th, that
is, ten days alter the first square had been
attached, and two days after the attachment
of the last square, the radicle had grown to
the great length of 3 2 inches, and now
the terminal growing part had become bent
away from the square into a hook (see
Fig. 68).
No. 2. Square attached on the 19th ; on
the 20th radicle slightly deflected from it
and from the perpendicular; on the 21st
deflected at nearly right angles ; it remained during the next
two days in this position, but on the 25th the upward curva-
ture was lessened through the action of geotropism, and still
more so on the 26th.
No. 8. Square attached on the 19th; on the 21st a trace of
curvature from the square, which amounted on the 22nd to
about 40°, and on the 23rd to 53° from the perpendicular.
No. 4. Square attached on the 21st; on the 22nd trace of
curvature from the square ; on the 23rd completely hooked
with the point turned up to the zenith. Three days afterwards
(i.e. 26th) the curvature had wholly disappeared and the apex
pointed perpendicularly downwards.
No. 5. Square attached on the 21st ; on the 22nd decided
}uercus robur : radicle
with square of card
attached to one side
of apex, causing it
to become hooked.
Drawing one-half
natural scale.
176 SENSITIVENESS OF THE APEX Chap. Ill
though slight curvature from the square ; on the 23rd the tip
had curved up above the ho izon, and on the 21th was hooked
with the apex pointing almost to the zenith, as in Fig. 68.
No. 6. Square attached on the 2ist ; on the 22nd slightly
curved from the square; 23rd more curved; 25th consider-
ably curved ; 27th all curvature lost, and the radicle was now
directed perpendicularly downwards.
No. 7. Square attached on the 21st ; on the 22nd a trace of
curvature from the square, which increased next day, and on
the 24th amounted to a right angle.
It is, therefore, manifest that the apex of the radicle
of the oak is highly sensitive to contact, and retains
its sensitiveness during several days. The movement
thus induced was, however, slower than in any of the
previous cases, with the exception of that of ^Esculus.
As with the bean, the terminal growing part, after
bending, sometimes straightened itself through the
action of geotropism, although the object still remained
attached to the tip.
The same remarkable experiment was next tried,
as in the case of the bean ; namely, little squares of
exactly the same size of the card-like sanded paper
and of very thin paper (the thicknesses of which have
been given under Vicia faba) were attached with
shellac on opposite sides (as accurately as could be
done) of the tips of 13 radicles, suspended in damp
air, at a temperature of 65°-66° F. The result was
striking, for 9 out of these 13 radicles became plainly,
and 1 very slightly, curved from the thick paper
towards the side bearing the thin paper. In two of
these cases the apex became completely hooked after
two days ; in four cases the deflection from the per-
pendicular and from the side bearing the thick paper,
amounted in from two to four days to angles of 90°,
72°, 60°, and 49°, but in two other cases to only 18°
and 15°. It should, however, be stated that in the
Uhap. III. OF THE KADICLE OF ZEA. 177
case in which the deflection was 49°, the two squares
had accidentally come into contact on one side of the
apex, and thus formed a lateral gable ; and the deflec-
tion was directed in part from this gable and in part
from the thick paper. In three cases alone the radicles
were not affected by the difference in thickness of the
squares of paper attached to their tips, and conse-
quently did not bend away from the side bearing the
stiffer paper.
Zea mays : Sensitiveness of the apex of the Radicle to
contact — A large number of trials were made on this
plant, as it was the only monocotyledon on which we
experimented. An abstract of the results will suffice.
In the first place, 22 germinating seeds were pinned to
cork-lids without any object being attached to their
radicles, some being exposed to a temperature of 65 J -~
66° F., and others to between 74° and 79° ; and none of
them became curved, though some were a little inclined
to one side. A few were selected, which from having
germinated on sand were crooked, but when suspended
in damp air the terminal part grew straight down-
wards. This fact having been ascertained, little squares
of the card-like paper were affixed with shellac, on
several occasions, to the tips of 68 radicles. Of these
the terminal growing part of 39 became within 24 h.
conspicuously curved away from the attached squares
md from the perpendicular ; 13 out of the 39 forming
Looks with their points directed towards the zenith,
md 8 forming loops. Moreover, 7 other radicles out
)f the 68, were slightly and two doubtfully deflected
:om the cards. There remain 20 which were not
iffected ; but 10 of these ought not to be counted ;
for one was diseased, two had their tips quite sur
rounded by shellac, and the squares on 7 had slipped
so as to stand parallel to the apex, instead of obliquely
178 SENSITIVENESS OF THE APEX Chap. III.
on it. There were therefore only 10 out of the 68
which certainly were not acted on. Some of the
radicles which were experimented on were young and
short, most of them of moderate length, and two or
three exceeded three inches in length. The curva-
ture in the above cases occurred within 24 h., but it
was often conspicuous within a much shorter period.
For instance, the terminal growing part of one radicle
was bent upwards into a rectangle in 8 h. 15 m., and
of another in 9 h. , On one occasion a hook was
formed in 9 h. Six of the radicles in a jar containing
nine seeds, which stood on a sand-bath, raised to
a temperature varying from 76° to 82° F., became
hooked, and a seventh formed a complete loop, when
first looked at after 15 hours.
The accompanying figures of four germinating seeds
(Fig. 69) show, firstly, a radicle (A) the apex of which
has become so much bent away from the attached
square as to form a hook. Secondly (B), a hook
converted through the continued irritation of the
card, aided perhaps by geotropism, into an almost
complete circle or loop. The tip in the act of forming
a loop generally rubs against the upper part of the
radicle, and pushes off the attached square ; the loop
then contracts or closes, but never disappears ; and
the apex afterwards grows vertically downwards, being-
no longer irritated by any attached object. This
frequently occurred, and is represented at C. The
jar above mentioned with the six hooked radicles and
another jar were kept for two additional days, for the
sake of observing how the hooks would be modified.
Most of them became converted into simple loops,
like that figured at C ; but in one case the apex did
not rub against the upper part of the radicle and thus
remove the card ; and it consequently made, owing
Ojiap. III.
OF THE RADICLE OF Zr&.
J 79
to the continued irritation from the card, two complete
loops, that is, a helix of two spires; which afterwards
became pressed closely together. Then geotropism
prevailed and caused the apex to grow perpendicularly
downwards. In another case, shown at (D), the apex
Fig. (39
.C D.
Zea mays: radicles excited to bend away from the little squares of card
attached to one side of their tips.
in making a second turn or spire, passed through the
first loop, which was at first widely open, and in
doing so knocked off the card ; it then grew perpen-
dicularly downwards, and thus tied itself into a knot,
which soon became tight !
Secondary Radicles of Zea. — A short time after the
first radicle has appeared, others protrude from the
1 80 SENSITIVENESS OF THE APEX Chap. ITL
seed, but not laterally from the primary one. Ten of
these secondary radicles, which were directed obliquely
downwards, were experimented on with very small
squares of card attached with shellac to the lower
sides of their tips. If therefore the squares acted, the
radicles would bend upwards in opposition to gravity.
The jar stood (protected from light) on a sand-bath,
which varied between 76° and 82° F. After only
5 h. one appeared to be a little deflected from the
square, and after 20 h. formed a loop. Four others
were considerably curved from the squares a r ter 20 h.,
and three of them became hooked, with their tips
pointing to the zenith, — one after 29 h. and the
two others after 44 h. By this latter time a sixth
radicle had become bent at a right angle from the side
bearing the square. Thus altogether six out of the
ten secondary radicles were acted on, four not being
affected. There can, therefore, be no doubt that the
tips of these secondary radicles are sensitive to slight
contact, and that when thus excited they cause the
upper part to bend from the touching object; but
generally, as it appears, not in so short a time as in
the case of the first-formed radicle.
Sensitiveness of the tip of the Radicle to
Moist Ant.
Sachs made the interesting discovery, a few years
ago, that the radicles of many seedling plants bend
towards an adjoining damp surface.* We shall here
endeavour to show that this peculiar form of sensitive-
ness resides in their tips. The movement is directly
the reverse of that excited by the irritants hitherto
considered, which cause the growing part of the
* ' ArLeiten des Bot. Institute in Wurztmrg,' vol. i. 1872, p. 209.
Ohai. III. OF THE RADICLE TO MOIST AIR. 181
radicle to bend away from the source of irritation.
In our experiments we followed Sachs' plan, and sieves
with seeds germinating in damp sawdust were sus-
pended so that the bottom was generally inclined at
40° with the horizon. If the radicles had been acted
on solely by geotropism, they would have grown out
of the bottom of the sieve perpendicularly down-
wards ; but as they were attracted by the adjoining
damp surface they bent towards it and were deflected
50° from the perpendicular. For the sake of ascertain-
ing whether the tip or the whole growing part of the
radicle was sensitive to the moist air, a length of from
1 to 2 mm. was coated in a certain number of cases
with a mixture of olive-oil and lamp-black. This
mixture was made in order to give consistence to the
oil, so that a thick layer could be applied, which
would exclude, at least to a large extent, the moist air,
and would be easily visible. A greater number of
experiments than those which were actually tried
would have been necessary, had not it been clearly
established that the tip of the radicle is the part which
is sensitive to various other irritants.
Phaseolus multiflorus. — Twenty-nine radicles, to which no-
thing had been done, growing out of a sieve, were observed
at the same time with those which had their tips greased,
and for an equal length of time. Of the 29, 24 curved them-
selves so as to come into close contact with the bottom of the
sieve. The place of chief curvature was generally at a distance
of 5 or 6 mm. from the apex. Eight radicles had their tips
greased for a length of 2 mm., and two others for a length of
U mm. ; they were kept at a temperature of 15°-16° C. After
intervals of from 19 h. to 24 h. all were still vertically or
almost vertically dependent, for some of them had moved
towards the adjoining damp surface by about 10°. They had
therefore not been acted on, or only slightly acted on, by the
damper air on one side, although the whole upper part was
freely exposed. After 48 h. three of these radicles became
182 SENSITIVENESS OF THE APEX Cijap. III.
considerably curved towards the sieve ; and the absence of curva-
ture in some of the others might perhaps be accounted for by
their not having grown very well. But it should be observed
that during the first 19 h. to 24 h. all grew well ; two of them
having increased 2 and 3 mm. in length in 11 h. ; five others
increased 5 to 8 mm. in 19 h. ; and two, which had been at first
4 and 6 mm. in length, iL creased in 24 h. to 15 and 20 mm.
The tips of 10 radicles, which likewise grew well, were coated
with the grease for a length of only 1 mm., and now the result
was some^v hat different ; for of these 4 curved themselves to
the sieve in from 21 h. to 24 h., whilst 6 did not do so.
Five of the latter were observed for an additional day, and now
all excepting one became curved to the sieve.
The tips of 5 radicles were cauterised with nitrate of silver,
and about 1 mm. in length was thus destroyed. They were
observed for periods varying between 11 h. and 24 h., and were
found to have grown well. One of them had curved until it
came into contact with the sieve ; another was curving towards
it ; whilst the remaining three were still vertically dependent.
Of 7 not cauterised radicles observed at the same time, all had
come into contact with the sieve.
The tips of 11 radicles were protected by moistened gold-
beaters' skin, which adheres closely, for a length varying from
lh to 2 2 mm. After 22 h. to 24 h, 6 of these radicles were
clearly bent towards or had come into contact with the sieve ;
2 were slightly curved in this direction, and 3 not at all. All
had grown well. Of 14 control specimens observed at the same
time, all excepting one had closely approached the sieve. It
appears from these cases that a cap of goldbeaters' skin checks,
though only to a slight degree, the bending of the radicles to
an adjoining damp surface. Whether an extremely thin sheet
of this substance when moistened allows moisture from the air
to pass through it, we do not know. One case indicated that
the caps were sometimes more efficient than appears from the
above results; for a radicle, which after 23 h. had only
slightly approached the sieve, had its cap (U mm. in length)
removed, and during the next 15 1 h. it curved itself abruptly
towards the source of moisture, the chief seat of curvature
being at a distance of 2 to 3 mm. from the apex.
Vicia faba. — The tips of 13 radicles were coated with the
grease for a length of 2 mm. ; and it should be remembered
that with these radicles the seat of chief curvature is about
Chap. III. OF THE RADICLE TO MOIST AIR. 183
4 or 5 mm. from the apex. Four of them were examined after
22 h., three after 26 h., and six after 36 h., and none had
been attracted towards the damp lower surface of the sieve.
In another trial 7 radicles were similarly treated, and 5 of them
still pointed perpendicularly downwards after 11 h., whilst
2 were a little curved towards the sieve; by an accident they
were not subsequently observed. .In both these trials the
radicles grew well ; 7 of them, which were at first from 4 to
11 mm. in length, were after 11 h. between 7 and 16 mm. ;
3 which were at first from 6 to 8 mm. after 26 h. were 11*5
to 18 mm. in length ; and lastly, 4 radicles which were at first
5 to 8 mm. after 46 h. were 18 to 23 mm. in length. The
control or ungreased radicles were not invariably attracted
towards the bottom of the sieve. But on one occasion 12 out of
13, which were observed for periods between 22 h. and 36 h.,
were thus attracted. On two other occasions taken together,
38 out of 40 were similarly attracted. On another occasion
only 7 out of 14 behaved in this manner, but after two more
days the proportion of the curved increased to 17 out of 23.
On a last occasion only 11 out of 20 were thus attracted. If
we add up these numbers, we find that 78 out of 96 of the
control specimens curved themselves towards the bottom of the
sieve. Of the specimens with greased tips, 2 alone out of the
20 (but 7 of these were not observed for a sufficiently long
time) thus curved themselves. We can, therefore, hardly doubt
that the tip for a length of 2 mm. is the part which is sensitive
to a moist atmosphere, and causes the upper part to bend
towards its source.
The tips of 15 radicles were cauterised with nitrate of silver,
and they grew as well as those above described with greased
tips. After an interval of 24 h., 9 of them were not at all
curved towards the bottom of the sieve ; 2 were curved towards
it at angles of 20° and 12° from their former vertical position,
and 4 had come into close contact with it. Thus the destruc-
tion of the tip for a length of about 1 mm. prevented the curva-
ture of the greater number of these radicles to the adjoining
damp surface. Of 24 control specimens, 23 were bent to the
sieve, and on a second occasion 15 out of 16 were similarly
curved in a greater or less degree. These control trials are
included in those given in the foregoing paragraph.
Avena sati.va. — The tips of 13 radicles, which projected
between 2 and 4 mm. from the bottom of the sieve, many of
13
181 SENSITIVENESS OF THE APEX Chap. TIL
them not quite perpendicularly downwards, were coated with
the black grease for a length of from 1 to li mm. The sieves
were inclined at 30° with the horizon. The greater number of
these radicles were examined after 22 h., and a few after 25 h.,
and within these intervals they had grown so quickly as to have
nearly doubled their lengths. With the ungreased radicles the
chief seat of curvature is at a distance of not less than between
35 and 55 mm., and not more than between 7 and 10 mm. from
the ajDex. Out of the 13 radicles with greased tips, 4 had not
moved at all towards the sieve ; 6 were deflected towards it and
from the perpendicular by angles varying between 10° and 35° ;
and 3 had come into close contact with it. It appears, therefore,
at first sight that greasing the tips of these radicles had checked
but little their bending to the adjoining damp surface. But the
inspection of the sieves on two occasions produced a widely
different impression on the mind; for it was impossible to
behold the radicles with the black greased tips projecting from
the bottom, and all those with ungreased tips, at least 40 to 50
in number, clinging closely to it, and feel any doubt that the
greasing had produced a great effect. On close examination
only a single ungreased radicle could be found which had not
become curved towards the sieve. It is probable that if the
tips had been protected by grease for a length of 2 mm. instead
of from 1 to Is mm., they would not have been affected by the
moist air and none would have become curved.
Triticum v/dgare. — Analogous trials were made on 8 radicles
of the common wheat ; and greasing their tips produced much
less effect than in the case of the oats. After 22 h., 5 of them
had come into contact with the bottom of the sieve; 2 had
moved towards it 10° and 15°, and one alone remained perpen-
dicular. Not one of the very numerous ungreased radicles
failed to come into close contact with the sieve. These trials
were made on Nov. 28th, when the temperature was only 4° - 8 C.
at 10 a.m. We should hardly have thought this case worth
notice, had it not been for the following circumstance. In the
beginning of October, when the temperature was considerably
higher, viz., 12° to 13° C, we found that only a few of the
ungreased radicles became bent towards the sieve ; and this
indicates that sensitiveness to moisture in the air is increased
by a low temperature, as we have seen with the radicles of
Vicia faba relatively to objects attached to their tips. But iD
the present instance it is possible that a difference in the dryness
Chap. III. OF THE RADICLE TO MOIST AIR. ISt
of the air may have caused the difference in the results at the
two periods.
Finally, the facts just given with respect to Phaseohts
multijiorus, Vicia fala, and Avena sativa show, as ii
seems to us, that a layer of grease spread for a length
of li to 2 mm. over the tip of the radicle, or the
destruction of the tip by caustic, greatly lessens or
quite annuls in the upper and exposed part the power
of bending towards a neighbouring source of moisture.
We should bear in mind that the part which bends
most, lies at some little distance above the greased or
oauterised tip ; and that the rapid growth of this part,
proves that it has not been injured by the tips having
been thus treated. In those cases in which the radicles
with greased tips became curved, it is possible that the
layer of grease was not sufficiently thick wholly to ex-
clude moisture, or that a sufficient length was not thus
protected, or, in the case of the caustic, not destroyed.
When radicles with greased tips are left to grow for
several days in damp air, the grease is drawn out into
the finest reticulated threads and dots, with narrow
portions of the surface left clean. Such portions
would, it is probable, be able to absorb moisture, and
thus we can account for several of the radicles with
greased tips having become curved towards the sieve
after an interval of one or two days. On the whole,
we may infer that sensitiveness to a difference in the
amount of moisture in the air on the two sides of a
radicle resides in the tip, which transmits some influ-
ence to the upper part, causing it to bend towards the
source of moisture. Consequently, the movement is
the reverse of that caused by objects attached to one
side of the tip, or by a thin slice being cut off, or by
being slightly cauterised. In a future chapter it
will be shown that sensitiveness to the attraction of
186 THE EFFECT OF KILLING OR Chap. 111.
gravity likewise resides in the tip; so that it is the
tip which excites the adjoining parts of a horizontally
extended radicle to bend towards the centre of the
earth.
Secondary Kadicles becoming vertically Geo-
tropic by the destruction or injury of the
Terminal Part of the Primary Eadicle.
Sachs has shown that the lateral or secondary
radicles of the bean, and probably of other plants, are
acted on by geotropism in so peculiar a manner, that
they grow out horizontally or a little inclined down-
wards ; and he has further shown* the interesting fact,
that if the end of the primary radicle be cut off, one
of the nearest secondary radicles changes its nature
and grows perpendicularly downwards, thus replacing
the primary radicle. We repeated this experiment,
and planted beans with amputated radicles in friable
peat, and saw the result described by Sachs ; but
generally two or three of the secondary radicles grew
perpendicularly downwards. We also modified the
experiment, by pinching young radicles a little way
above their tips, between the arms of a U-shaped
piece of thick leaden wire. The part pinched was
thus flattened, and was afterwards prevented from
growing thicker. Five radicles had their ends cut
off, and served as controls or standards. Eight were
pinched ; of these 2 were pinched too severely and
their ends died and dropped off ; 2 were not pinched
enough and were not sensibly affected ; the remaining
4 were pinched sufficiently to check the growth ot
the terminal part, but did not appear otherwise injured.
When the U-shaped wires were removed, after an
Arbeiten Bot. Institut., Wiirzburg,' Heft iv. 1874, p. 622.
Obxv. Ill, INJURING THE PRIMARY RADICLE. 187
interval of 15 days, the part beneath the wire was
found to be very thin and easily broken, whilst the
part above was thickened. Now in these fonr cases,
one or more of the secondary radicles, arising from
the thickened part just above the wire, had grown
perpendicularly downwards. In the best case the
primary radicle (the part below the wire being 1J inch
in length) was somewhat distorted, and was not half
as long as three adjoining secondary radicles, which
had grown vertically, or almost vertically, downwards.
Some of these secondary radicles adhered together or
had become confluent. We learn from these four cases
that it is not necessary, in order that a secondary
radicle should assume the nature of a primary one,
that the latter should be actually amputated ; it is
sufficient that the flow of sap into it should be
checked, and consequently should be directed into the
adjoining secondary radicles ; for this seems to be
the most obvious result of the primary radicle being
pinched between the arms of a U-shaped wire.
This change in the nature of secondary radicles is
dearly analogous, as Sachs has remarked, to that
which occurs with the shoots of trees, when the leading
one is destroyed and is afterwards replaced by one or
more of the lateral shoots ; for these now grow upright
instead of sub-horizontally. But in this latter case
the lateral shoots are rendered apogeotropic, whereas
with radicles the lateral ones are rendered geotropic.
We are naturally led to suspect that the same cause
acts with shoots as with roots, namely, an increased flow
of sap into the lateral ones. We made some trials with
Abies communis and peetinata, by pinching with wire
the leading and all the lateral shoots excepting one.
Rut we believe that they were too old when experi-
mented on ; and some were pinched too severely, and
188 THE EFFECT OF KILLING OR Chap, in
some not enough. Only one case succeeded, namely
with the spruce-fir. The leading shoot was not killed,
but its growth was checked ; at its base there were
three lateral shoots in a whorl, two of which were
pinched, one being thus killed ; the third was left
untouched. These lateral shoots, when operated on
(July 14th) stood at an angle of 8° above the horizon ;
by Sept. 8th the unpinched one had risen 35° ; by
Oct. 4th it had risen 46°, and by Jan. 26th 48°, and
it had now become a little curved inwards. Part
of this rise of 48° may be attributed to ordinary
growth, for the pinched shoot rose 12° within the same
period. It thus follows that the unpinched shoot
stood, on Jan. 26th, 56° above the horizon, or 34°
from the vertical; and it was thus obviously almost
ready to replace the slowly growing, pinched, lead-
ing shoot. Nevertheless, we feel some doubt about
this experiment, for we have since observed with
spruce-firs growing rather unhealthily, that the lateral
shoots near the summit sometimes become highly
inclined, whilst the leading shoot remains apparently
sound.
A widely different agency not rarely causes shoots
which naturally would have grown out horizontally to
grow up vertically. The lateral branches of the Silver
Fir (A. pectinata) are often affected by a fungus,
JEcidium elatinum, which causes the branch to enlarge
into an oval knob formed of hard wood, in one of
which we counted 24 rings of growth. According to
De Bary,* when the mycelium penetrates a bud be-
ginning to elongate, the shoot developed from it
grows vertically upwards. Such upright shoots after-
* See his valuable article in are culled in Gorman " He xen
'Bot. Zeitung,' 18(37, p. 257, on besen," or " witch-brooms."
these monstrous growths, which
Chap. III. INJURING THE PRIMARY RADICLE. 189
-ards produce lateral and horizontal branches; and
they then present a curious appearance, as if a youug
fir- tree had grown out of a ball of clay surrounding
the branch. These upright shoots have manifestly
changed their nature and become apogeotropic ; for if
they had not been affected by the iEcidium, they
would have grown out horizontally like all the other
twigs on the same branches. This change can hardly
be due to an increased flow of sap into the part ; but
the presence of the mycelium will have greatly dis-
turbed its natural constitution.
According to Mr. Meehan,* the stems of three
species of Euphorbia and of Portulaca oleraeea are
" normally prostrate or procumbent ;" but when they
are attacked by an iEciclium, they " assume an erect
habit." Dr. Stahl informs us that he knows of several
analogous cases ; and these seem - to be closely related
to that of the Abies. The rhizomes of Sparganium
ramosum grow out horizontally in the soil to a con-
siderable length, or are diageotropic ; but F. Elfving
found that when they were cultivated in water
their tips turned upwards, and they became apogeo-
tropic. The same result followed when the stem of the
plant was bent until it cracked or was merely much
bowed.f
No explanation has hitherto been attempted of such
cases as the foregoing, — namely, of secondary radiclss
growing vertically downwards, and of lateral shoots
growing vertically upwards, after the amputation of
* ' Proc. Acad. Nat. Sc. Phil a- viously observed (' Flora,' 1878,
dfclphia,' June 16th, 1874, and p. 324) that the underground
July 23rd, 1875. shoots of Triticum repens bend
t See F. Elfving's interesting vertically up when the parts above
paper in 'Arbeiten Bot. Institut., ground are removed, aud when
in Wurzburg,' vol. ii. 1880, p. 489. tho rhizomes are kept partly im
Carl Kraus (Triesdorf) had pre- mcrsul in water.
I 90 EFFECT OF KILLING PRIMARY KADICLE. Chap. IIL
the primary radicle or of the leading shoot. The
following considerations give us, as we believe, the
clue. Firstly, any cause which disturbs the con-
stitution * is apt to induce reversion ; such as the
crossing of two distinct races, or a change of con-
ditions, as when domestic animals become feral.
But the case which most concerns us, is the frequent
appearance of peloric flowers on the summit of a stem,
or in the centre of the inflorescence, — parts which, it is
believed, receive the most sap ; for when an irregular
flower becomes perfectly regular or peloric, this may
be attributed, at least partly, to reversion to a primi-
tive and normal type. Even the position of a seed at
the end of the capsule sometimes gives to the seedling
developed from it a tendency to revert. Secondly,
reversions often occur by means of buds, independently
of reproduction by seed ; so that a bud may revert to
the character of a former state many bud-generations
ago. In the case of animals, reversions may occur in
the individual with advancing age. Thirdly and
lastly, radicles when they first protrude from the seed
are always geotropic, and plumules or shoots almost
always apogeotropic. If then any cause, such as an
increased flow of sap or the presence of mycelium,
disturbs the constitution of a lateral shoot or of a
secondary radicle, it is apt to revert to its primordial
state ; and it becomes either, apogeotropic or geotropic,
as the case may be, and consequently grows either
vertically upwards or downwards. It is indeed pos-
* The facts on which the fol- xiv. On pyloric flowers, chap,
lowing conclusions are founded xid. p. 32 ; and see p. 337 on theii
are given in ' The Variation of position on the plant. With
Animals and Plants under Domes- respect to seeds, p. 340. On re-
tiration,' 2nd edit 1875. On the version by means of buds, p. 438
causes leading to reversi >n see chap, xi vol. i.
chnp. xii. vol. ii. and p. 59 chap.
Chap. III. SUMMARY OF CHAPTER. 191
sible, or even probable, that this tendency to reversion
may have been increased, as it is manifestly of servicu
to the plant.
Summary of Chapter.
A part or organ may be called sensitive, when its
irritation excites movement in an adjoining part. Now
it has been show 7 n in this chapter, that the tip of the
radicle of the bean is in this sense sensitive to the
contact of any small object attached to one side by
shellac or gum-water ; also to a slight touch with dry
caustic, and to a thin slice cut off one side. The
radicles of the pea were tried with attached objects
and caustic, both of which acted. With Pliaseolus
multiflorus the tip was hardly sensitive to small squares
of attached card, but was sensitive to caustic and to
slicing. The radicles of Tropaeolum were highly sen-
sitive to contact ; and so, as far as we could judge,
were those of Gossypium herbaceum, and they were
certainly sensitive to caustic. The tips of the radicles
of Gucurbita ovifera were likewise highly sensitive to
caustic, though only moderately so to contact. Ea-
phanus sativus offered a somewhat doubtful case.
With iEsculus the tips were quite indifferent to
bodies attached to them, though sensitive to caustic.
Those of Quercus nibur and Zea mays were highly sen-
sitive to contact, as were the radicles of the latter
to caustic. In several of these cases the difference in
sensitiveness of the tip to contact and to caustic was,
as we believe, merely apparent ; for with Gossypium,
Kaphanus, and Cucurbita, the tip was so fine and
flexible that it was very difficult to attach any object
to one of its sides. With the radicles of iEsculus,
the tips were not at all sensitive to small bodies
attached to them ; but it does not follow from this
L92 SUMMARY OF CHAPTER. Chap. Ill
fact that they would not have been sensitive to some-
what greater continued pressure, if this could have
been applied.
The peculiar form of sensitiveness which we are
here considering, is confined to the tip of the radicle
for a length of from 1 mm. to 1 • 5 mm. When this
part is irritated by contact with any object, by caustic,
or by a thin slice being cut off, the upper adjoining
part of the radicle, for a length of from 6 or 7 to
even 12 mm., is excited to bend away from the side
which has been irritated. Some influence must there-
fore be transmitted from the tip along the radicle for
this length. The curvature thus caused is generally
symmetrical. The part which bends most apparently
coincides with that of the most rapid growth. The
tip and the basal part grow very slowly and they
bend very little.
Considering the widely separated position in the
vegetable series of the several above-named genera,
we may conclude that the tips of the radicles of all, or
almost all, plants are similarly sensitive, and transmit
an influence causing the upper part to bend. With
respect to the tips of the secondary radicles, those of
Vicia fdba, Pisum sativum, and Zea mays were alone
observed, and they were found similarly sensitive.
In order that these movements should be properly
displayed, it appears necessary that the radicles
should grow at their normal rate. If subjected to a
high temperature and made to grow rapidly, the
tips seem either to lose their sensitiveness, or the
upper part to lose the power of bending. So it
appears to be if they grow very slowly from not being
vigorous, or from being kept at too low a temperature ,
also when they are forced to germinate in the middle
of the winter.
Chai III. SUMMARY OF CHAPTER. 193
The curvature of the radicle sometimes occurs
within from 6 to 8 hours after the tip has been irritated,
and almost always within 24 h., excepting in the
case of the massive radicles of iEsculus. The curva-
ture often amounts to a rectangle, — that is, the ter-
minal part bends upwards until the tip, which is but
little curved, projects almost horizontally. Occa-
sionally the tip, from the continued irritation of the
attached object, continues to bend up until it forms a
hook with the point directed towards the zenith, or
a loop, or even a spire. After a time the radicle
apparently becomes accustomed to the irritation, as
occurs in the case of tendrils, for it again grows down-
wards, although the bit of card or other object may
remain attached to the tip.
It is evident that a small object attached to the free
point of a vertically suspended radicle can offer no
mechanical resistance to its growth as a whole, for the
object is carried downwards as the radicle elongates,
or upwards as the radicle curves upwards. Nor can
the growth of the tip itself be mechanically checked
by an object attached to it by gum- water, which
remains all the time perfectly soft. The weight of
the object, though quite insignificant, is opposed
to the upward curvature. We may therefore conclude
that it is the irritation due to contact which excites
the movement. The contact, however, must be pro-
longed, for the tips of 15 radicles were rubbed for a
,! hort time, and this did not cause them to bend. Here
then we have a case of specialised sensibility, like
that of the glands of Drosera ; for these are ex-
quisitely sensitive to the slightest pressure if prolonged,
but not to two or three rough touches.
When the tip of a radicle is lightly touched on one
side with dry nitrate of silver, the injury caused is
L9i SUMMARY OF CHAPTER. Chap. III.
very slight, and the adjoining upper part bends away
from the cauterised point, with more certainty in most
cases than from an object attached on one side. Here
it obviously is not the mere touch, but the effect
produced by the caustic, which induces the tip to
transmit some influence to the adjoining part, causing
it to bend away. If one side of the tip is badly
injured or killed by the caustic, it ceases to grow,
whilst the opposite side continues growing ; and the
result is that the tip itself bends towards the injured
side and often becomes completely hooked ; and it is
remarkable that in this case the adjoining upper part
does not bend. The stimulus is too powerful or the
shock too great for the proper influence to be trans-
mitted from the tip. We have strictly analogous cases
with Drosera, Dionaea and Pinguicula, with which
plants a too powerful stimulus does not excite the
tentacles to become incurved, or the lobes to close, or
the margin to be folded inwards.
With respect to the degree of sensitiveness of the
apex to contact under favourable conditions, we have
seen that with Vicia fdba a little square of writing-
paper affixed with shellac sufficed to cause move-
ment; as did on one occasion a square of merely
damped goldbeaters' skin, but it acted very slowly.
Short bits of moderately thick bristle (of which mea-
surements have been given) affixed with gum-water
acted in only three out of eleven trials, and beads oi
dried shellac under 20 o^ n °f a g ram m weight acted
only twice in nine cases ; so that here we have
nearly reached the minimum of necessary irrita-
tion. The apex, therefore, is much less sensitive to
pressure than the glands of Drosera, for these are
affected by far thinner objects than bits of bristle
and by a very much less weight than 2 ^ 5 th of a grain.
Chap. III. SUMMARY OF CHAPTER. 195
But the most interesting evidence of the delicate
sensitiveness of the tip of the radicle, was afforded by
its power of discriminating between equal-sized squares
of card-like and very thin paper, when these were
attached on opposite sides, as was observed with the
radicles of the bean and oak.
When radicles of the bean are extended horizon-
tally with squares of card attached to the lower sides of
their tips, the irritation thus caused was always con-
quered by geotropism, which then acts under the most
favourable conditions at right angles to the radicle.
But when objects were attached to the radicles of and
of the above-named genera, suspended vertically, the
irritation conquered geotropism, which latter power
at first acted obliquely on the radicle ; so that the
immediate irritation from the attached object, aided
by its after-effects, prevailed and caused the radicle
to bend upwards, until sometimes the point was
directed to the zenith. We must, however, assume
that the after-effects of the irritation of the tip by an
attached object come into play, only after movement
has been excited. The tips of the radicles of the pea
seem to be more sensitive to contact than those of the
bean, for when they were extended horizontally with
squares of card adhering to their lower sides, a most
curious struggle occasionally arose, sometimes one
and sometimes the other force prevailing, but ulti-
mately geotropism was always victorious ; neverthe-
less, in two instances the terminal part became so
much curved upwards that loops were subsequently
formed. With the pea, therefore, the irritation from
an attached object, and from geotropism when acting
at right angles to the radicle, are nearly balanced
forces. Closely similar results were observed with the
horizontally extended radicles of Cuourbita ovifera^
196 SUMMARY OF CHAPTER. Chap. Ill,
when their tips were slightly cauterised on the lower
side.
Finally, the several co-ordinated movements by
which radicles are enabled to perform their proper
functions are admirably perfect. In whatever direc-
tion the primary radicle first protrudes from the seed,
geotropism guides it perpendicularly downwards ; and
the capacity to be acted on by the attraction of
gravity resides in the tip. But Sachs has proved*
that the secondary radicles, or those emitted by the
primary one, are acted on by geotropism in such a
manner that they tend to bend only obliquely down-
wards. If they had been acted on like the primary
radicle, all the radicles would have penetrated the
ground in a close bundle. We have seen that if
the end of the primary radicle is cut off or in-
jured, the adjoining secondary radicles become geo-
tropic and grow vertically downwards. This power
must often be of great service to the plant, when the
primary radicle has been destroyed by the larvae of
insects, burrowing animals, or any other accident. The
tertiary radicles, or those emitted by the secondary
ones, are not influenced, at least in the case of the
bean, by geotropism ; so they grow out freely in all
directions. From this manner of growth of the various
kinds of radicles, they are distributed, together with
their absorbent hairs, throughout the surrounding soil,
as Sachs has remarked, in the most advantageous
manner ; for the whole soil is thus closely searched.
Geotropism, as was shown in the last chapter,
excites the primary radicle to bend downwards with
very little force, quite insufficient to penetrate the
ground. Such penetration is effected by the pointed
Ar'ieiten Bot. lnstitut., Wurzburg,' Heft iv. 1874, pp. 605-631
Chap. in. SUMMARY OF CHAPTER. 197
apex (protected by the root-cap) being pressed down
by the longitudinal expansion or growth of the ter-
minal rigid portion, aided by its transverse expan-
sion, both of which forces act powerfully. It is,
however, indispensable that the seeds should be at
first held down in some manner. When they lie
on the bare surface they are held down by the attach-
ment of the root-hairs to any adjoining objects ; and
this apparently is effected by the conversion of
their outer surfaces into a cement. But many seeds
get covered up by various accidents, or they fall into
crevices or holes. With some seeds their own weight
suffices.
The circumnutating movement of the terminal grow-
ing part both of the primary and secondary radicles
is so feeble that it can aid them very little in pene-
trating the ground, excepting when the superficial
layer is very soft and damp. But it must aid them
materially when they happen to break obliquely into
cracks, or into burrows made by earth-worms or larvae.
This movement, moreover, combined with the sen-
sitiveness of the tip to contact, can hardly fail to be
of the highest importance ; for as the tip is always
endeavouring to bend to all sides it will press on all
sides, and will thus be able to discriminate between
the harder and softer adjoining surfaces, in the same
manner as it discriminated between the attached
squares of card-like and thin paper. Consequently it
will tend to bend from the harder soil, and will thus
follow the lines of least resistance. So it will be if it
meets with a stone or the root of another plant in the
soil, as must incessantly occur. If the tip were not
sensitive, and if it did not excite the upper part of the
root to bend away, whenever it encountered at right
angles some obstacle in the ground, it would be liable
198 SUMMAKY OF CHAPTER. Chap. IJI
to be doubled up into a contorted mass. But we have
seen with, radicles growing down inclined plates oi
glass, that as soon as the tip merely touched a slip oi
wood cemented across the plate, the whole terminal
growing part curved away, so that the tip soon stood
at right angles to its former direction ; and thus it
would be with an obstacle encountered in the ground,
as far as the pressure of the surrounding soil would
permit. We can also understand why thick and strong
radicles, like those of ^Esculus, should be endowed
with less sensitiveness than more delicate ones ; for
the former would be able by the force of their growth
to overcome any slight obstacle.
After a radicle, which has been deflected by some
stone or root from its natural downward course,
reaches the edge of the obstacle, geotropism will direct
it to grow again straight downward ; but we know that
geotropism acts with very little force, and here another
excellent adaptation, as Sachs has remarked,* comes
into play. For the upper part of the radicle, a little
above the apex, is, as we have seen, likewise sensitive ;
and this sensitiveness causes the radicle to bend like a
tendril towards the touching object, so that as it rubs
over the edge of an obstacle, it will bend downwards ;
and the curvature thus induced is abrupt, in which
respect it differs from that caused by the irritation of
one side of the tip. This downward bending coincides
with that due to geotropism, and both will cause the
root to resume its original course.
As radicles perceive an excess of moisture in the air
on one side and bend towards this side, we may infer
that they will act in the same manner with respect to
moisture in the earth. The sensitiveness to moisture
* ' Aibeiten Bot. Inst. Wiirzburg.' Heft iii. p. 456.
Chap. IIL SUMMARY UF CHAPTER. 199
resides in the tip, which determines the bending of
the upper part. This capacity perhaps partly accounts
for the extent to which drain-pipes often become
choked with roots.
Considering the several facts given in this chapter,
we see that the course followed by a root through
the soil is governed by extraordinarily complex and
diversified agencies, — by geotropism acting in a
different manner on the primary, secondary, and ter-
tiary radicles, — by sensitiveness to contact, different in
kind in the apex and in the part immediately above
the apex, and apparently by sensitiveness to the
varying dampness of different parts of the soil.
These several stimuli to movement are all more
powerful than geotropism, when this acts obliquely
on a radicle, which has been deflected from its perpen-
dicular downward course. The roots, moreover, of
most plants are excited by light to bend either to or
from it ; but as roots are not naturally exposed to the
light it is doubtful whether this sensitiveness, which is
perhaps only the indirect result of the radicles being
highly sensitive to other stimuli, is of any service to
the plant. The direction which the apex takes at each
successive period of the growth of a root, ultimately
determines its whole course ; it is therefore highly
important that the apex should pursue from the first
the most advantageous direction ; and we can thus
understand why sensitiveness to geotropism, to contact
and to moisture, all reside in the tip, and why the tip
determines the upper growing part to bend either
from or to the exciting cause. A radicle may be
compared w r ith a burrowing animal such as a mole,
which wishes to penetrate perpendicularly down into
the ground. By continually moving his head from
side to side, or circumnutating, he will feel any stone
14
200 SUMMARY OF CHArTEK. Chat.' IIL
or other obstacle, as well as any difference in the
hardness of the soil, and he will turn from that side ;
if the earth is damper on one than on the other side
he will turn thitherward as a Letter hunting-ground.
Nevertheless, after each interruption, guided by the
sense of gravity, he will be able to recover his down-
ward course and to burrow to a greater depth.
Chap. IV. GIRCUMNUTATIOH 201
CHAPTER IV.
The Circtjmnttatixg Movements of the severa_ parts of
Mature Plants.
Circumnutation of stems : concluding remarks on— Circumnutation of
stolons: aid thus afforded, in winding amongst the stems of sur-
rounding plants — Circumnutation of flower-stems — Circumnutation
of Dicotyledonous leaves — Singular oscillatory movement of leaves
of Diontea— Leaves of Cannabis sink at night — Leaves of Gymno-
sperms — Of Monocotyledons — Cryptogams — Concluding remarks
on the circumnut ition of leaves : generally rise in the evening and
sink in the morning.
We have seen in the first chapter that the stems of all
seedlings, whether hypocotyls or epicotyls, as well as
the cotyledons and the radicles, are continually cir-
cumnutating — that is, they grow first on one side and
then on another, such growth being probably preceded
by increased turgescence of the cells. As it was
unlikely that plants should change their manner of
growth with advancing age, it seemed probable that
the various organs of all plants at all ages, as long as
they continued to grow, would be found to circum-
nutate, though perhaps to an extremely small extent.
As it was important for us to discover whether this
was the case, we determined to observe carefully a
certain number of plants which were growing vigor-
ously, and which were not known to move in any
manner. We commenced with stems. Observations
of this kind are tedious, and it appeared to us that it
would be sufficient to observe the stems in about a
score of genera, belonging to widely distinct families
and inhabitants of various countries. Several plants
202 CIKCUMNUTATION OF STEMS. Chap. IV
were selected which, from being woody, or for other
reasons, seemed the least likely to circumnutate. The
observations and the diagrams were made in the
manner described in the Introduction. Plants in pots
were subjected to a proper temperature, and whilst
being observed, were kept either in darkness or were
feebly illuminated from above. They are arranged
in the order adopted by Hooker in Le Maout and
Decaisne's ' System of Botany.' The number of the
family to. which each genus belongs is appended, as
this serves to show the place of each in the series.
(1.) Iberia umbellata (Cruciferse, Fam. 14).— The movement of
the stem of a young plant, 4 inches in height, consisting of
four internodes (the hypocotyl included) besides a large bud
Fis: 70.
Iberis umbellata : circumnutation of stem of young plant, traced from
8.30 a.m. Sept. 13th to same hour on following morning. Distance of
summit of stem beneath the horizontal glass 7*6 inches. Diagram
reduced to half of original size. Movement as here shown magnifie I
between 4 and 5 times.
on the summit, was traced, as here shown, during 24 h.
(Fig. 70). As far as we could judge the uppermost inch alone
of the stem circumnutated, and this in a simple manner. The
movement was slow, and the rate very unequal at different
times. In part of its course an irregular ellipse, or rather
triangle, was completed in 6 h. 30 in.
(2.) Brassica oleracea (Ouciferse). — A very young plant, bearing
three leaves, of which the longest was only three-quarters of arj
inch in length, was placed under a microscope, furnished with
an eye-piece micrometer and the tip of the largest leaf was
Chap. IV. CIKCUMNUTATION OF STEMS. 203
found to be in constant movement. It crossed five di visions of
the micrometer, that is, y^h of an inch, in 6 m. 20 s. There
could hardly be a doubt that it was the stem which chiefly
moved, for the tip did not get quickly out of focus ; and this
would have occurred had the movement been confined to the
leaf, which moves up or down in nearly the same vertical plane.
(3.) Linum usitatissimum (Linese, Earn. 39). — The stems of this
plant, shortly before the flowering period, are stated by Fritz
Miiller ('Jenaische Zeitschrift,' B. v. p. 137) to revolve, or
circumnutate.
(4.) Felargonium zonah (Geraniaceae, Fam. 47).— A young
plant, 7h inches in height, was observed in the usual manner ;
but, in order to see the bead at the end of the glass filament
Fig. 71
lO'20p.m.Q A
/\fl*wJ#
<10'3Q'pjnJO th
Pelargonium zonale: circumnutation of stem of young plant, feebly illu-
minated from above. Movement of bead magnified about 11 times ;
traced on a horizontal glass from noon on March 9th to 8 A.M. on
the 11th.
and at the same time the mark beneath, it was necessary to cut
off three leaves on one side. We do not know whether it was
owing to this cause, or to the plant having previously become
bent to one side through heliotropism, but from the morning of
the 7th of March to 10.30 p.m. on the 8th, the stem moved
a considerable distance in a zigzag line in the same general
direction. During the night of the 8th it moved to some
distance at right angles to its former course, and next morning
(9th) stood for a time almost still. At noon on the 9th a new
tracing was begun (see Fig. 71), which was continued till 8 a.m.
ou the 11th. Between noon on the 9th and 5 p.m. on the 10th
(i.e. in the course of 29 h.), the stem described a circle. This
plant therefore circumnutates, but at a very slow rate, and to a
small extent.
(5.) Tropceolum majus (?) (dwarfed var. called Tom Thumb) ;
(Geraniacesc, Fam. 47). — The species of this genus climb by the
204
CIRCUMNUTATION OF STEMS.
CUAP- 17
aid of their sensitive petioles, but some of them also twint
round supports; but even these latter species do not begin to
circumnutate in a conspicuous manner whilst young. The
Fig. 72.
">
Trojxzolum majus(?): circumnutation of stem of young plant, traced on a
horizontal glass from 9 A.M. Dec. 26th to 10 a.m. on 27th. Movement
of bead magnified about 5 times, and here reduced to half of original
scale.
variety here treate 1 of has a rather thick stem, and is so dwarf
that apparently it does not climb in any manner. We there-
fore wished to ascertain whether the stem of a young plant,
consisting of two in-
ternodes, together 3*2
inches in height, cir-
cumnutated. It was
observed during 25 h.,
and we see in Fig. 72
that the stem moved in
a zigzag course, indicat-
ing circumnutation.
(6.) Trifolium resupi-
natum (LegmninosaB,
Fam. 75). — When we
treat of the sleep of
plants, we shall see that
the stems in several
Leguminous genera, for
instance, those of Hedy-
sarum, Mimosa, Meli-
lotus, &c, which are not
climbers, circumnutate
in a conspicuous manner.
We will here give only a single instance (Fig. 73), showing
the circumnutation of the stem of a large plant of a clover,
Trifolium resupinatum. In the course of 7 h. the stem changed
Trifolium rcs'ipinatun ; circumnutation of
stein, traced on vertical glass from 9.30
a.m. to 4.30 p.m. Nov. 3rd. Tracing not.
greatly rnagmfiel, reduced to half of
original size. Plant feebly illuminated
from above.
Chap. IV
CIRCUMNUTATIOX OF STEMS.
205
its course greatly eight times and completed three irregular
circles or ellipses. It therefore circumnutated rapidly. Some
of the lines run at right angles to one another.
Fig. 74.
(bus (hyboid) : circumnutation of stem, traced on horizontal glass, from
4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magnified, re-
duced to half of original size. Plant illuminated feebly from above.
(7.) Rubus idceus (hybrid) (KosaceaB, Fam. 76). — As we hap-
Fig. 75.
pened to have a young plant, 11 inches
in height and growing vigorously,
which had been raised from a cross
between the raspberry (Rubus idceus)
and a North American Eubus, it was
observed in the usual manner. During
the morning of March 14th the stem
almost completed a circle, and then
moved far to the right. At 4 p.m. it
reversed its course, and now a fresh
tracing was begun, which was con-
tinued during 40! h., and is given in
Fig. 74. We here have well-marked
circumnutation.
(8.) JDeutzia gracilis (Saxifragese,
Fam. 77). — A shoot on a bush about
18 inches in height was observed. The
bead changed its course greatly eleven
times in the course of 10 h. 30 m.
(Fig. 75), and there could be no
doubt about the circumnutation of the
stem.
(9.) Fuchsia (greenhouse var,., with
large flowers, probably a hybrid) (Ona-
grariese, Fam. 100). — A young plant,
15 inches in height, was observed during nearly 48 h.
Deutzia gracilis ; circumnu-
tation of stem, kept in
darkness, traced on hori-
zontal glass, from 8.30
A.M. to 7 P.M.March 20th.
Movement of bead origin-
ally magnified about 20
times, here reduced to
half scale.
The
206 CIRCUMNUTATION OF STEMS. Chap. IV.
accompanying figure (Fig. 76) gives the necessary particulars,
and shows that the stem circumnutated, though rather
Blowly.
Fuchsia (garden var.) : circum nutation of stem, kept in darkness, traced CD
horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of
bead originally magnified about 40 times, here reduced to half scale.
(10.) Cereus sptciocissimns (garden var., sometimes called
Phyllocactus multiflorus) (Cactese, Fam. 109). — This plant
which was growing vigorously from haviDg been removed a
few clays before from the greenhouse to the hot- house, was
observed with especial interest, as it seemed so little probable
that the stem would circumnutate. The branches are flat, or
fiabelliform ; but some of them are triangular in section, with
the three sides hollowed out. A branch of this latter shape,
9 inches in length and 1 i in diameter, was chosen for observa-
tion, as less likely to circumnutate than a fiabelliform branch.
The movement of the bead at the end of the glass filament,
affixed to the summit of the branch, was traced (A, Fig. 77)
from 9.23 a.m. to 4.30 p.m. on Nov. 23rd, during which time it
changed its course greatly six times. On the 24th another
tracing was made (see B), and the bead on this day changed its
course oftener, making in 8 h. what may be considered as four
ellipses, with their longer axes differently directed. The position
of the stem and its commencing course on the following
morning are likewise shown. There can be no doubt that this
branch, though appearing quite rigid, circumnutated; but the
Chap. IV
CIBCUMNUTATION OF STEMS.
207
extreme amount of movement during the time was very small,
probably rather less than the J^th of an inch.
Fi K . 77.
8T30\
^\4.°30'p.m.
Jf2$a.m.
A
S°a.m..2&
'Jereus speciocissimus ; circumnutation of stem, illuminated from above,
traced on a horizontal glass, in A from 9 a.m. to 4.30 P.M. on Nov.
23rd ; and in B from 8.30 a.m. on the 24th to 8 A.M. on the 25th.
Movement of the bead in B magnified about 38 times.
(11.) Hedtra "lelix (Araliacese, Fam. 114).— The stem is known
to be apheliotropic, and several seedlings growing in a pot in
the greenhouse became bent in the middle of the summer at
right angles fiom the light. On Sept. 2nd some of these stems
were tied up so as to stand vertically, and were placed before
a north-east window; but to our surprise they were now
decidedly heliotropic, for during 4 days they curved them-
selves towards the light, and their course being traced on a
horizontal glass, was strongly zigzag. During the 6 succeed-
ing days they circumnutated over the same small space at a
slow rate, but there could be no doubt about their circumnuta-
tion. The plants were kept exactly in the same place before the
window, and after an interval of 15 days the stems were
again observed during 2 days and their movements traced, and
208
CIBCUMNUTATION OF STEMS.
Chap. iy
they were found to be still circumnutating, but on a yet smaller
scale.
(12.) Gazania ringens (Composite, Fam. 122).— The circum-
nutation of the stem of a young plant, 7 inches in height, as
measured to the tip of the highest leaf, was traced during
33 h , and is shown in the accompanying figure (Fig. 78). Two
Fig. 78.
SM.nt.8ST
P.ji.m.l
Z'am.2Z nd '
'io° 3fp.m.2f
Gazania rinjens : circunmutation of stem traced from 9 A.M. March 21st
to 6 P.M. on 22nd; plant kept in darkness. Movement of bead at the
close of the observations magnified 3-i times, here reduced to half the
original scale.
main lines may be observed running at nearly right angles to
two other main lines; but these are interrupted by small
loops.
(13.) Azalea Indlca (Ericincse, Fam. 128). — A bush 21 inches
in height was selected for observation, and the circunmutation
of its leading shoot was traced during 26 h. 40 m , as shown
in the following figure (Fig. 79).
(14.) Plumbago C<<perisis (Plumbaginess, Fam. 134).— A small
lateral branch which projected from a tall freely growing bush,
at an angle of 35° above the horizon, was selected for obser-
vation. For the first 11 h. it moved to a considerable distance
in a nearly straight line to one side, owing probably to its
having been previously deflected by the light whilst standing in
the greenhouse. At 7.20 p.m. on March 7th a fresh tracing was
begun and continued for the next 43 h. 40 m. (see Fig. 80).
During the first 2 h. it followed nearly the same direction as
before, and then changed it a little; during the night it
moved at nearly right angles to its previous course. Next
Chap. IV
CIECUMNUTATION OF STEMS
209
day (8th) it zigzagged greatly, and on the 9th moved irregu-
larly round and round a small circular space. By 3 p.m. on
the 9th the figure had become so complicated that no more dots
could be made ; but the shoot continued during the evening of
the 9th, the whole of the 10th, and the morning of the 11th to
Fig. 79.
Fig. 80.
Azalea Indica : circumnutatiou
of stem, illuminated from
abore, traced on horizontal
glass, from 9.30 A.M. March
9th to 12.10 P.M. on the 10th.
But on the morning of the
10th only four dots were
made between 8.30 A.M.
and 12.10 P.M., both hours
included, so that the cireum-
nutation is not fairly repre-
sented in this part of the
diagram. Movement of the
bead here magnified about
30 times.
Plumbago Capensis : circumnu-
tatiou of tip of a lateral
branch, traced on horizontal
glass, from 7.20 P.M. on
March 7th to 3 p.m. on the
9th. Movement of bead
magnified 13 times. Plant,
feebly illuminated from
above.
circumnutate over the same small space, which was only aboui
the ^th of an inch ('97 mm.) in diameter. Although this
branch circumnutated to a very small extent, yet it changed its
course frequently. The movements ought to have been more
magnified.
(15.) Aloyua citriodora (Verbenacefle, Fain. 173). — The follow-
ing figure (Fig. 81) gives the movements of a shoot during
210
CIKCUMNUTATION OF STEMS.
Chap. IV.
31 h. 40 m., and shows that it circumnutated. The bu*h was
15 inches in height.
Fig. 81.
Aloy&ia ciiriodora : circumnutation of stem, traced from 8.20 A.M. on March
22nd to 4 P.M. on 23rd. Plant kept in darkness. Movement magnified
about 40 times.
(16.) Verbena melindres (?) (a scarlet-flowered herbaceous var.)
(Verbenaceae). — A shoot 8 inches in height had been laid hori-
zontally, for the sake of observing its apogeotropism, and the
terminal portion had grown vertically upwards for a length of
1| inches. A glass filament, with a bead at the end, was fixed
'a.m. 7*!*
Fig. 82 .
5°3(f]>.m.5? h
6'50'a.m.6 l
Verbena melindres: circumnutation of stem in d
glass, from 5.30 p.m. on June 5th to 11 a.m.
bead magnified 9 times.
irkness, traced on vertical
June 7th. Movement of
upright to the tip, and its movements were traced during
41 h. 30 m. on a vertical glass (Fig. 82). Under these circum-
stances the lateral movements were chiefly shown ; but as the
lines from side to side are not on the same level, the shoot
Chap. IV. CIKCUMNUTATION OF STEMS. 211
must have moved in a plane at right angles to that of the lateral
movement, that is, it must have circumnutated. On the next day
(6th) the shoot moved in the course of 16 h. four times to the right,
and four times to the left; and this apparently represents the
formation of four ellipses, so that each was completed in 4 h.
(17.) Ceratophyttum demersum (Ceratophyilese, Fam. 220). — An
interesting account of the movements of the stem of this water-
plant has been published by M. E. Eodier,£. The movements are
confined to the young internodes, becoming less and less lower
down the stem ; and they are extraordinary from their amplitude.
The stems sometimes moved through an angle of above 2.0° in
6 h., and in one instance through 220° in 3 h. They generally
bent from right to left in the morning, and in an opposite direc-
tion in the afternoon ; but the movement was sometimes tempo-
rarily reversed or quite arrested. It was not affected by light.
It does not appear that M. Eodier made any diagram on a hori-
zontal plane representing the actual course pursued by the
apex, but he speaks of the " branches executing round their
axes of growth a movement of torsion." From the particulars
above given, and remembering in the case of twining plants and
of tendrils, how difficult it is not to mistake their bending to all
points of the compass for true torsion, we are led to believe that
the stems of this Ceratophyllum circumnutate, probably in the
shape of narrow ellipses, each completed in about 26 h. The
following statement, however, seems to indicate something
different from ordinary circumnutation, but we cannot fully
understand it. M. Eodier says : " II est alors facile de voir que
le mouvement de flexion se produit d'abord dans les merithalles
superieurs, qu'il se propage ensuite, en s'amoindrissant du huut
en bas; tandis quau contraire le mouvement de redressement
commence par la par tie inferieure pour se terminer a la partie
superieure qui, quelquefois, peu de temps avant de se relcver
tout a fait, forme avec l'axe un angle tres aigu."
(18 ) Coniferce. — Dr. Maxwell Masters states (' Journal Linn
Soc.,' Dec. 2nd, 1879) that the leading shoots of many Coniferre
during the season of their active growth exhibit very remark-
able movements of revolving nutation, that is, they circumnu-
tate. We may feel sure that the lateral shoots whilst growing
would exhibit the same movement if carefully observed.
* 'Coraptes Eendus,' April 30th. 1877. Also a second nuticte
published separately in Bouidt-aux, Nov. 12th, 1877.
212 CIBCUMNUTATION OF STEMS. Chap. IV
(19.) Lillum auratum (Fam. Liliacese). — The- circummitation
Fig. 83.
6p.m]5Z--
XjgpOL
8>am.U th
Lillum aumtun: eireumnutation of a stem in darkness, traced on a horizontal
glass, from 8 A.M. on March 14th to 8.35 A.M. on 16th. But it should
be noted that our observations were interrupted between 6 P.M. on the
14th and 12.15 P.M. on 15th, and the movements during this interval
of 18 h. 15 m. are represented by a long broken line. Diagram reduced
to half original scale.
of the stem of a plant 2 1 inches in height is represented in the
above figure (Fig. bS).
Fig. 48.
Oyperus rtlternifohus : eireumnutation of stem, illuminated from above,
traced on horizontal glass,. from 9.45 A.M. March 9th to 9 P.M. on 10th
The stem grew so rapidly whilst being observed, that it was not possible
to estimate how much its movements were magnified in the tracing.
(20.) Cyperus alternifoTius (Fam. Cyperacese.) — A glass
Chap. IV. CIRCUMNUTATION OF STEMS. 213
filament, with a bead at the end, was fixed across the summit
of a young stem 10 inches in height, close beneath the crown of
elongated leaves. On March 8th, between 12.20 and 7.20 p.m.,
the stem described an ellipse, open at one end. On the follow-
ing day a new tracing was begun (Fig. 84), which plainly shows
that the stem completed three irregular figures in the course of
35 h. 15 m.
Concluding Remarks on the Circumnutation of Stems.—
Any one who will inspect the diagrams now given, and
will bear in mind the widely separated position of the
plants described in the series, — remembering that we
have good grounds for the belief that the hypocotyls
and epicotyls of all seedlings circumnutate, — not
forgetting the number of plants distributed in the
most distinct families which climb by a similar move-
ment, — will probably admit that the growing stems
of all plants, if carefully observed, would be found
to circumnutate to a greater or less extent. When
we treat of the sleep and other movements of plants,
many other cases of circumnutating stems will be
incidentally given. In looking at the diagrams, we
should remember that the stems were always growing,
so that in each case the circumnutating apex as it
rose will have described a spire of some kind. The
dots were made on the glasses generally at intervals
of an hour, or hour and a half, and were then joined
by straight lines. If they had been made at intervals
of 2 or 3 minutes, the lines would have been more
curvilinear, as in the case of the tracks left on the
smoked glass-plates by the tips of the circumnutating
radicles of seedling plants. The diagrams generally
approach in form to a succession of more or less
irregular ellipses or ovals, with their longer axes
directed to different points of the compass during the
same day or on succeeding days. The stems there-
214 CIECUMNUTATION OF STOLONS. Chap. IV
fore, sooner or later, bend to all sides; but after a
stern has bent in any one direction, it commonly
bends back at first in nearly, though not quite, the
opposite direction ; and this gives the tendency to
the formation of ellipses, which are generally narrow,
but not so narrow as those described by stolons and
leaves. On the other hand, the figures sometimes
approach in shape to circles. Whatever the figure
may be, the course pursued is often interrupted by
zigzags, small triangles, loops, or ellipses. A stem
may describe a single large ellipse one day, and
two on the next. With different plants the com-
plexity, rate, and amount of movement ..differs
much. The stems, for instance, of Iberis and Azalea
described only a single large ellipse in 24 h. ;
whereas those of the Deutzia made four or five deep
zigzags or narrow ellipses in 11 \ h., and those of the
Trifolium three triangular or quadrilateral figures
in 7 h.
ClRCUMNUTATION OF STOLONS OR KuKNERS.
Stolons consist of much elongated, flexible branches,
which run along the surface of the ground and form
roots at a distance from the parent-plant. They are
therefore of the same homological nature as stems ;
and the three following cases may be added to the
twenty previously given cases.
Frafjaria (cultivated garden var.) : Rosacea;. — A plant growirg
m a pot had emitted a long stolon ; this was supported by a
stick, so that it projected for the length of several inches hori-
zontally. A glass filament bearing two minute triangles of
paper was affixed to the terminal bud, which was a little up-
turned ; and its movements were traced during 21 h., as shown
in Fig. 85. In the course of the first 12 h. it moved twice up
and twice down in somewhat zigzag lines, and no doubt tra-
velled in the same manner during the night. On the followiug
Chap. IV. CIRCUMNUTATION OF STOLONS.
215
morning after an interval of 20 h. the apex stood a little higher
than it did at first, and this shows that the stolon had not been
Fig. 85.
9 6°45 l am.l9 1 ^
JO°pm
Jff/lS'a.m
48*
3 r4S , amJ9 t
Frag a ia : circumnutation of stolon, kept in darkness, traced on vertical
glass, from 10.45 A.M. May 18th to 7.45 A.M. on 19th.
acted on within this time by geotropism;* nor had its own
weight caused it to bend downwards.
On the following morning (19th) the glass filament was
detached and refixed close behind the bud, as it appeared pos-
sible that the circumnutation of the terminal bud and of the
adjoining part of the stolon might be different. The movement
was now traced during two consecutive days (Fig. 86). During
the first day the filament travelled in the course of 14 h. 30 m.
five times up and four times down, besides some lateral move-
ment. On the 20th the course was even more complicated, and
can hardly be followed in the figure ; but the filament moved in
16 h. at least five times up and five times down, with very little
* Dr. A. B. Frank states (• Die
Naturliche wagerechte Richtung
von Pflanzentheilen,' 1S70, p. 20)
that the stolons of this plant are
15
acted on by geotropism, but only
after a considerable interval o(
time.
216
CIRCUMNUTATION OF STOLONS. Chap. IV
lateral deflection. The first and last dots made on this second
day, viz., at 7 a.m. and 11 p.m., were close together, showing
that the stolon had not fallen or risen. Nevertheless, by com-
paring its position on
the morning of the 19th
and 21st, it is obvious
that the stolon had sunk ;
and this may be attri-
buted to slow bending
down either from its own
weight or from geotro-
pism.
During a part of the 20th
an orthogonal tracing was
made by applying a cube
of wood to the vertical
glass and bringing the
apex of the stolon at suc-
cessive periods into a line
with one edge; a dot
being made each time on
the glass. This tracing
therefore represented very
nearly the actual amount
of movement of the apex ;
and in the course of 9 h.
the distance of the ex-
treme dots from one an-
other was '45 inch. By
the same method it was
ascertained that the apex
moved between 7 a.m. on
8°a.m.2l? the 20th and 8 a.m. on the
21st a distance of '82 inch.
A younger and shorter
stolon was supported so
that it" projected at about
movement was traced by the
the first day the apex soon
rose above the field of vision. By the next morning it had
sunk, and the course pursued was now traced during 14 h.
30 m. (Fig. 87). The amount of movement was almost the same,
Fvagaria : circumnutation of the same stolon
as in the last figure, observed in the same
manner, and traced from 8 a.m. May 19th
to 8 a.m. 21st.
45° above the horizon, and its
same orthogonal method. On
Chap IV. CIKCUMNUTATION OF STOLONS. 217
from side to side as up and down ; and differed in this respect
remarkably from the movement in the previous cases. During
the latter part of the day, viz., between 3 and 10.30 p.m., the
Fig. 87.
iCtam
T20j).m
Fragwia : circum nutation of another and younger stolon, traced from
8 a.m. to 10.30 p.m. Figure reduced to one-half of original scale.
actual distance travelled by the apex amounted to 1*15 inch;
and in the course of the whole day to at least 2*67 inch. This
is an amount of movement almost comparable with that of
some climbing plants. The same stolon was observed on the
following day, and now it moved in a somewhat less complex
manner, in a plane not far from vertical. The extreme amount
of actual movement was 1*55 inch in one direction, and "6 inch
in another direction at right angles. During neither of these
days did the stolon bend downwards through geotropism or its
own weight.
Four stolons still attached to the plant were laid on damp
sand in the back of a room, with their tips facing the north-east
windows. They were thus placed because De Tries says * that
they are apheliotropic when exposed to the light of the sun ; but
we could not perceive any effect from the above feeble degree of
illumination. We may add that on another occasion, late in the
summer, some stolons, placed upright before a south-west window
Arbeiten Bot. Inst., Wiirzburg,' 1872, p. 434.
218 CIKCUMNUTATION OF STOLONS. Chap IV.
on a cloudy day, became distinctly curved towards the light, and
were therefore heliotropic. Close in front of the tips of the
prostrate stolons, a crowd of very thin sticks and the dried
haulms of grasses were driven into the sand, to represent the
crowded stems of surrounding plants in a state of nature. This
was done for the sake of observing how the growing stolons
would pass through them. They did so easily in the course of
6 days, and their circumnutation apparently facilitated their
passage. When the tips encountered sticks so close together
that they could not pass between them, they rose up and passed
over them. The sticks and haulms were removed after the
passage of the four stolons, two of which were found to have
assumed a permanently sinuous shape, and two were stiU
straight. But to this subject we shall recur under Saxifraga.
Saxifraga sarmentosa (Saxifrages). — A plant in a suspended
pot had emitted long branched stolons, which depended like
Fig. 88.
I.
Saxifraga sarmentosa: circumnutation of an inclined stolon, traoed in
darkness on a horizontal glass, from 7.45 A.M. April 18th to 9 A.M. on
9th. Movement of end of stolon magnified 2-2 times.
threads on all sides. Two were tied up so as to stand vertically,
and their upper ends became gradually bent downwards, but so
slowly in the course of several days, that the bending was pro-
bably due to their weight and not to geotropism. A glass fila-
ment with little triangles of paper was fixed to the end of one of
these stolons, which was 17J inches in length, and had already
become much bent down, but still projected at a considerable
angle above the horizon. It moved only slightly three times
from side to side and then upwards; on the following day
Chap. IV. CIRCUMNUTATION OF STOLONS. 219
the movement was even less. As this stolon was so long we
thought that its growth was nearly completed, so we tried
another which was thicker and shorter, viz., 10} inches in length.
It moved greatly, chiefly upwards, and" changed its course five
times in the course of the day. During the night it curved sc
much upwards in opposition to gravity, that the movement
could no longer be traced on the vertical glass, and a horizontal
one had to be used. The movement was followed during the
next 25 h., as shown in Fig' 88. Three irregular ellipses, ivith
their longer axes somewhat differently directed, were almost
completed in the first 15 h. The extreme actual amount of
movement of the tip during the '25 h. was '75 inch.
Several stolons were laid on a flat surface of damp sand, in the
same manner as with those of the strawberry. The friction of
the sand did not interfere with their circumnutation ; nor could
we detect any evidence of their being sensitive to contact. In
order to see how in a state of nature they would act, when
encountering a stone or other obstacle on the ground, short
pieces of smoked glass, an inch in height, were stuck upright
into the sand in front of two thin lateral branches. Their tips
scratched the smoked surface in various directions ; one made
three upward and two downward lines, besides a nearly hori-
zontal one; the other curled quite away from the glass; but
ultimately both surmounted the glass and pursued their original
course. The apex of a third thick stolon swept up the glass in a
curved line, recoiled and again came into contact with it ; it then
moved to the right, and after ascending, descended vertically ;
ultimately it passed round one end of the glass instead of over it.
Many long pins were next driven rather close together into
the sand, so as to form a crowd in front of the same two thin
lateral branches; but these easily wound their way through
the crowd. A thick stolon was much delayed in its passage ;
at one place it was forced to turn at right angles to its former
course; at another place it conld not pass through the pins,
and the hinder part became bowed ; it then curved upwards
and passed through an opening between the upper part of some
pins which happened to diverge ; it then descended and finally
emerged through the crowd. This stolon was rendered perma-
nently sinuous to a slight degree, and was thicker where sinuous
than elsewhere, apparently from its longitudinal growth having
been checked.
Cotyledon umbilicus (Crassulaceae).— A plant growing in a pan
220
CIRCUMNUTATION OF STOLONS. Chap. IV
of damp moss had emitted 2 stolons, 22 and 20 inches in length
One of these was supported, so that a length of 4£ inches pro*
iected in a straight and horizontal line, and the movement
of the apex was traced. The first dot was made at 9.10 a.m.
Fig. 89.
fJVl5 , a.nb.2S f !'
f,G°10 1 a.m.37( h
ijfa.7ti.27 t ? 1
5"SC?jMn.26 e £
10°35'p.m.2&h
Cotyledon umbilicus: circumnutation of stolon, traced from 11.15 A.M
Aug. 25th to 11 a.m. 27th. Plant illuminated from above. TV
terminal internocle was *25 inch in length, the penultimate 2 -25, ana
the third 3 inches in length. Apex of stolon stood at a distance of
5*75 inches from the vertical glass ; but it was not possible to ascertain
how much the tracing was magnified, as it was not known how great
a length of the internode circumnutated.
the terminal portion soon began to bend downwards and con-
tinned to do so nntil noon. Therefore a straight line, very
nearly as long as the whole figure here given (Fig. 89), was first
traced on the glass ; but the upper part of this line has not been
copied in the diagram. The curvature occurred in the middle
Chap. IV.
CIKCUMNUTATION OF STOLONS.
221
of the penultimate internode; and its chief seat was at the
distance of lj inch from the apex; it appeared due to the
weight of the terminal portion, acting on the more flexible
part of the internode, and not to geotropism. The apex after
thus sinking down from 9.10 a.m. to noon, moved a little to the
left; it then rose up and circumnutated in a nearly vertical
plane until 10.35 p.m. On the following day (26th) it was ob-
Fig. 90.
J°U' a.m.25 l b
W3£part.25f!\ o ,
6°m'a.m.2Q$
^20 r jp.m.26 t !
Cotyledon umbilicus: circumnutation and downward movement of another
ttolon, traced on vertical glass, from 9.11 a.m. Aug. 25th to 11 A.m. 27th.
Apex close to glass, so that figure but little magnified, and here reduced
to two-thirds of original size.
served from 6.40 a.m. to 5.20 p.m., and within this time it moved
twice up and twice down. On the morning of the 27th the apex
stood as high as it did at 11.30 a.m. on the 25th. • Nor did it
sink down during the 28th, but continued to circumnutate about
the same place.
Another stolon, which resembled the last in almost every
222 CIRCUMNUTATION OF STOLONS. Chap. IV.
respect, was observed during the same two days, but only two
inches of the terminal portion was allowed to project freely and
horizontally. On the 25th it continued from 9.10 a.m. to 1.30 r.M.
to bend straight downwards, apparently owing to its weight
(Fig. 90); but after this hour until 10.35 p.m. it zigzagged.
This fact deserves notice, for we here probably see the combined
effects of the bending down from weight and of circumnutation.
The stolon, however, did not circumnutate when it first began
to bend down, as may be observed in the present diagram, and
as was still more evident in the last case, when a longer portion
of the stolon was left unsupported. On the following day
(26th) the stolon moved twice up and twice down, but still con-
tinued to fall ; in the evening and during the night it travelled
from some unknown cause in an oblique direction.
We see from these three cases that stolons or
runners circumnutate in a very complex manner. The
lines generally extend in a vertical plane, and this
may probably be attributed to the effect of the weight
of the unsupported end of the stolon ; but there is
always some, and occasionally a considerable, amount
of lateral movement. The circumnutation is so great
in amplitude that it may almost be compared with
that of climbing plants. That the stolons are thus
aided in passing over obstacles and in winding between
the stems of the surrounding plants, the observations
above given render almost certain. If they had not
circumnutated, their tips would have been liable to
have been doubled up, as often as they met with
obstacles in their path ; but as it is, they easily avoid
them. This must be a considerable advantage to the
plant in spreading from its parent-stock ; but we are
far from supposing that the power has been gained
by the stolons for this purpose, for circumnutation
seems to be of universal occurrence with all growing
parts; but it is not improbable that the amplitude
of the movement may have been specially increased
for this purpose.
Chai IV. CIECUMNUTATION OF FLO WEE -STEMS. 223
ClRCUMNUTATION OF FlOWEE-STEMS.
We did not think it necessary to make any special
observations on the circumnutation of flower-sterns,
these being axial in their nature, like stems or stolons ;
but some were incidentally made whilst attending
to other subjects, and these we will here briefly give.
A few observations have also been made by other
botanists. These taken together suffice to render it
probable that all peduncles and sub-peduncles cir-
cumnutate whilst growing.
Oxalis caraosa.-— The peduncle which springs from the thick
and woody stem of this plant bears three or four suo-peduncles.
Fig. 91.
Oxalis camosa : flower-stem, feebly illuminated from above, its circumnuta
tion traced from 9 A.M. April 13th to 9 A.M. 15th. Summit of flowei
8 inches beneath the horizontal glass. Movement probably magnified
about 6 times.
A filament with little triangles of paper was fixed within the
calyx of a flower which stood upright. Its movements were
observed for 48 h. ; during the first half of this time the flower
was fully expanded, and during the second half withered. The
figure here given (Fig. 91) represents 8 or 9 ellipses. Although
the main peduncle circumnutated, and described one large and
224 CIRCUMNUTATION OF FLOWER-STEMS Chat, I\
two smaller ellipses in the course of 24 h., yet the chief seat oi
movement lies in the sub-peduncles, which ultimately bend
vertically downwards, as will be described in a future chapter.
The peduncles of Oxalis acetosella likewise bend downwards, and
afterwards, when the pods are nearly mature, upwards ; and this
is effected by a circumnutating movement.
It may be seen in the above figure that the flower-stem of
O.carnosa circumnutated during two days about the same spot.
On the other hand, the flower-stem of (J. sensitiva undergoes a
strongly marked, daily, periodical change of position, when kept
at a proper temperature. In the middle of the day it stands
vertically up, or at a high angle ; in the afternoon it sinks, and
in the evening projects horizontally, or almost horizontally,
rising again during the night. This movement continues from
the period when the no wers are in bud to when, as we believe,
the pods are mature : and it ought perhaps to have been included
amongst the so-called sleep-movements of plants. A tracing
was not made, but the angles were measured at successive periods
during one whole day; and these showed that the movement
was not continuous, but that the peduncle oscillated up and
down. We may therefore conclude that it circumnutated. At
the base of the peduncle there is a mass of small cells, forming
a well- developed pulvinus, which is exteriorly coloured purple
and hairy. In no other genus, as far as we kno w, is the peduncle
furnished with a pulvinus. The peduncle of 0. Ortegesii behaved
differently from that of 0. sensitiva, for it stood at a less angle
above the horizon in the middle of the day, than in the morning
or evening. By 10.20 p.m. it had risen greatly. During the
middle of the day it oscillated much up and down.
Trifolium sublerraneum. — A filament was fixed vertically to
the uppermost part of the peduncle of a yourjg and upright
flower-head (the stem of the plant having been secured to a
stick); and its movements were traced during 36 h. Within
this time it described (see Fig. 92) a figure which represents four
ellipses; but during the latter part of the time the peduncle
began to bend downwards, and after 10.30 p.m. on the 24th it
curved so rapidly down, that by 6.45 a.m. on the 25th it stood
only 19° above the horizon. It went on circumnu fating in nearly
the same position for two dajs. Even after the flower-heads
have buried themselves in the ground they continue, as will
hereafter be shown, to circumnutate. It will also be seen in the
uext chapter that the sub-peduncles of the separate flowers of
Chap. IV. CIKCUMNUTATION OF FLOWER-STEMS. 225
Trifolium repens circumnutate in a complicated course during
Beveral days. I may add that the gynophore of Arnolds hypogaa,
Fig. 92.
'40'a.m 23*5*
\Hf.30:p,m.24^
Trifolium subterraneum : main flower-peduncle, illuminated from above,
circumnutation traced on horizontal glass, from 8.40 A.M. July 23rd
to 10.30 P.M. 24th.
which looks exactly like a peduncle, circumnutates whilst growing
vertically downwards, in order to bury the young pod in the
ground.
The movements of the flowers of Cyclamen Persicum were not
observed ; but the peduncle, whilst the pod is forming, increases
much in length, and bows itself down by a circumnutating
movement. A young peduncle of Maurandia semperflorens,
li inch in length, was carefully observed during a whole day,
and it made 4 \ narrow, vertical, irregular and short ellipses,
each at an average rate of about 2 h. 25 m. An adjoining
peduncle described during the same time similar, though fewer,
.* According to Sachs f the flower-stems, whilst growing,
* ' The Movements and Habits
of Climbing Plants,' 2nd edit.,
1875, p. 68.
t ' Text-Book of Botany,' 1875,
226 CIRCUMNUTATION OF LEAVES. Chap. IV.
of many plants, for instance, those of Brassica napus, revolve or
circumnutate ; those of Allium porrum bend from side to side,
and, if this movement had been traced on a horizontal glass,
no doubt ellipses would have been formed. Fritz Miiller has
described * the spontaneous revolving movements of the flower-
stems of an Alisma, which he compares with those of a climbing
plant.
We made no observations on the movements of the different
parts of flowers. Morren, however, has observed f in the
stamens of Sparmannia and Cereus a " frenJssement spontane,"
which, it may be suspected, is a circumnutating movement.
The circumnutation of the gynostemium of Stylidium, as de-
scribed by Gad,J is highly remarkable, and apparently aids in
the fertilisation of the flowers. The gynostemium, whilst spon-
taneously moving, comes into contact with the viscid labellum,
to which it adheres, until freed by the increasing tension of the
parts or by being touched.
We have now seen that the flower-stems of plants
belonging to such widely different families as the
Cruciferse, Oxalidge, LeguminosaB, Primulaceae, Scro-
phularinese, Alismacese, and Liliacese, circumnutate ;
and that there are indications of this movement in
many other families. With these facts before us,
bearing also in mind that the tendrils of not a few
plants consist of modified peduncles, we may admit
without much doubt that all growing flower-stems
circumnutate.
Circumnutation of Leaves : Dicotyledons.
Several distinguished botanists, Hofmeister, Sachs,
Pfeffer, De Vries, Batalin, Millardet, &c, have ob-
p. 766. Linnreus and Treviranus plies circumnutation.
(according to Pfeffer, ' Die Pe- * ' Jenaische Zeitsch./ B. v.
riodischen Bewegungen/ &c.„ p. p. 133.
162) state that the flower-stalks f ' N. Mem. de l'Acad. E. de
of many plants occupy different Bruxelles,' torn. xiv. 1841, p. 3.
positions by night and day, and % ' Sitzungbericht des bot. Ve-
we shall s< e in the chapter on reins der P. Brandenburg,' xxi
the Sleep of Plants that this im- p. 84.
Chap IV.
DICOTYLEDONS.
227
served, and some of them with the greatest care, the
periodical movements of leaves ; bat their attention
has been chiefly, though not exclusively, directed to
those which move largely and are commonly said to
sleep at night. From considerations hereafter to be
given, plants of this nature are here excluded, and
will be treated of separately. As we wished to ascer-
tain whether all young and growing leaves circumnu-
tated, we thought that it would be sufficient if we
observed between 30 and 40 genera, widely distributed
throughout the vegetable series, selecting some un-
usual forms and others on woody plants. All the
plants were healthy and grew in pots. They were
illuminated from above, but the light perhaps was not
always sufficiently bright, as many of them were ob-
served under a skylight of ground-glass. Except in a
few specified cases, a fine glass filament with two minute
triangles of paper was fixed to the leaves, and their
movements were traced on a
vertical glass (when not stated
to the contrary) in the manner
already described. I may repeat
that the broken lines represent
the nocturnal course. The stem
was always secured to a stick,
close to the base of the leaf
under observation. The ar-
rangement of the species, with
the number of the Family ap-
pended, is the same as in the
case of stems.
Fig. 93.
Sarracenia purpurea : circum-
nutation of young pitcher,
traced from 8 A.M. July 3rd
to 10.15 a.m. 4th. Temp.
17°-1 8° C. Apex of pitcher
20 inches from glass, so
movement greatly mag-
nified.
(1.) Sarracenia purpurea (Sarra-
ceneae, Fam. 11).— A young leaf, or
pitcher, 8^ inches in height, with the bladder swollen, but with
the hood not as yet open, had a filament fixed transversely
228
CIKCUMNUTATION OF LEAVES.
Chap. IV.
across its apex ; it was observed for 4S h., and during the whole
of this time it circumnutated in a nearly similar manner, but
to a very small extent. The tracing given (Fig. 93) relates
only to the movements during the first 26 h.
(2.) Glaucium luteum (Papave-
racese, Fam. 12).— A young plant,
bearing only 8 leaves, had a fila-
ment attached to the youngest leaf
but one, which was 3 inches in
length, including the petiole. The
circumnutating movement was
traced during 47 h. On both days
the leaf descended from before 7 a.m.
until about 11 a.m., and then
ascended slightly during the rest
of the day and the early part of
the night. During the latter part
of the night it fell greatly. It did
not ascend so much during the
second as during the first day, and
it descended considerably lower on
the second night than on the first.
This difference was probably due
to the illumination from above
having been insufficient during the
two days of observation. Its course
during the two days is shown in
Fig. 94.
(3.) Crambe maritima (Cruciferse,
Fam. 14). — A leaf 9s inches in length
on a plant not growing vigorously
was first observed. Its apex was
in constant movement, but this
could hardly be traced, from being
so small in extent. The apex, how-
ever, certainly changed its course at
least 6 times in the course of 14 h.
A more vigorous young plant, bear-
ing only 4 leaves, was then selected,
and a filament was affixed to the
midrib of the third leaf from the base, which, with the petiole, was
6 inches in length. The leaf stood up almost vertically, but the^tip
Glaucium luteum : circumnuta-
tion of young leaf, traced
from 9.30 a.m. June 14th
to 8.30 A.M. 16th. Tracing
not much magnified, as apex
of leaf stood only 5£ inches
from the glass.
Chap. IT.
DICOTYLEDONS.
229
was deflected, so that the filament projected almost horizontally,
and its movements were traced during 48 h. on a vertical glass,
as shown in the accompanying figure (Fig. 95). We here plainly
see that the leaf was con- •
Fig. 95.
6°40'a.m.25 i h
S£p.m.83&
M>°20'2.m.24&
7'. 50' a.m. 23^
tinually circumnutatmg ;
but the proper periodicity
of its movements was dis-
turbed by its being only
dimly illuminated from
above through a double
skylight. We infer that
this was the case, because
two leaves on plants grow-
ing out of doors, had their
angles above the horizon
measured in the middle
of the day and at 9 to
about 10 p.m. on succes-
sive nights, and they
were found at this latter
hour to have risen by an
average angle of 9° above
their mid-day position :
on the following morning
they fell to their former
position. Now it may be
observed in the diagram
that the leaf rose during
the second night, so that
it stood at 6.40 a.m. higher
than at 10.20 p.m. on the
preceding night ; and this
may be attributed to the
leaf adjusting itself to the
dim light, coming exclu-
sively from above.
(4.) Brassica oleracea (Cruciferae).— Hofmeister and Batalin *
state that the leaves of the cabbage rise at night, and fall by
day. We covered a young plant, bearing 8 leaves, under a large
bell-glass, placing it in the same position with respect to the
£3fam.84$\
3°j>,m2<tfb
Crambe maritima : circumnutation of leaf,
disturbed by being insufficiently illumi-
nated from above, traced from 7.50 A.M.
June 23rd to 8 A.M. 25th. Apex of leaf
15J inches from the vertical glass, so that
the tracing was much magnified, but is
here reduced to one-fourth of original scale.
* ' Flora,' 1873, p. 437
230 CIECUMNUTATION OF LEAVES. Chap. rV
light in which it had long remained, and a filament was fixed
at the distance of '4 of an inch from the apex of a young leaf
nearly 4 inches in length. Its movements were then traced
during three days, but the tracing is not worth giving. The
leaf fell during the whole morning, and rose in the evening and
during the early part of the night. The ascending and descend-
ing lines did not coincide, so that an irregular ellipse was formed
each 24 h. The basal part of the midrib did not move, as was
ascertained by measuring at successive periods the angle which
it formed with the horizon, so that the movement was confined
to the terminal portion of the leaf, which moved through an
angle of 11° in the course of 24 h., and the distance travelled by
the apex, up and down, was between ■ 8 and • 9 of an inch.
In order to ascertain the effect of darkness, a filament was
fixed to a leaf 5^ inches in length, borne by a plant which after
forming a head had produced a stem. The leaf was inclined
44° above the horizon, and its movements were traced on a
vertical glass every hour by the aid of a taper. During the
first day the leaf rose from 8 a.m. to 10.40 p.m. in a slightly
zigzag course, the actual distance travelled by the apex being
• 67 of an inch. During the night the leaf fell, whereas it ought
to have risen ; and by 7 a.m. on the following morning it had
fallen ■ 23 of an inch, and it continued falling until 9.40 a.m. It
then rose until 10.50 p.m., but the rise was interrupted by one
considerable oscillation, that is, by a fall and re-ascent. During
the second night it again fell, but only to a very short distance,
and on the following morning re-ascended to a very short
distance. Thus the normal course of the leaf was greatly
disturbed, or rather completely inverted, by the absence of
light ; and the movements were likewise greatly diminished in
amplitude.
We may add that, according to Mr. A. Stephen Wilson,* the
young leaves of the Swedish turnip, which is a hybrid between
B. oleracea and rapa, draw together in the evening so much
" that the horizontal breadth diminishes about 30 per cent, of
the daylight breadth." Therefore the leaves must rise con-
siderably at night.
(5.) Dianihus caryophyllus (Caryophyllese, Fain. 26). — The
* 'Trans. Bot. Soc. Edinburgh,' see Darwin, 'Animals and Plants
vol. xiii. p. 32. With resptct to under Domestication,' 2nd edit.
the origin of the Swedish turnip, vol. i. p. 344. ■
Chap. IV.
DICOTYLEDONS.
231
terminal shoot of a young plant, growing very vigorously, was
selected for observation. The young leaves at first stand up
vertically and close together, but they soon bend outwards and
downwards, so as to become horizontal, and often at the same
time a little to one side. A filament was fixed to the tip of a
young leaf whilst still highly inclined, and the first dot was
made on the vertical glass at 8.30 a.m. June 13th, but it curved
downwards so quickly that by 6.40 a.m. on the following
morning it stood only a little above the horizon. In Fig. 96
96
flO'-j
l5'p.mJ3$
JO°35 , p.mie^
I /
§ {6°4o'cum.U&
I(f45p.m.l4*tp>*
Dianthus caryophyllus : circumnutation of young leaf, traced from 10.15
P.M. June 13th to 10.35 p.m. 16th. Apex of leaf stood, at the close of
our observations, 8f inches from the vertical glass, so tracing not
greatly magnified. The leaf was h\ inches long. Temp. 15^°-17£° C.
the long, slightly zigzag line representing this rapid downward
course, which was somewhat inclined to the left, is not given ;
but the figure shows the highly tortuous and zigzag course,
together with some loops, pursued during the next 2| days.
As ths leaf continued to move all the time to the left, it is
evident that the zigzag line represents many circumnutations.
(6.) Camellia Japonica (Camelliacese, Fam. 32). — A youngish
leaf, which together with its petiole was 21 inches in length and
which arose from a side branch on a tall bush, had a filament
attached to its apex. This leaf sloped downwards at an angle
of 40° beneath the horizon. As it was thick and rigid, and its
16
232
CIKCUMNUTATION OF LEAVES.
Chap. 11
petiole very short;
Ficr. 97.
much movement could not be expected
Nevertheless, the apes changed its course
completely seven times in the course of
lis h., but moved to only a very small
distance. On the next day the movement
of the apex was traced during 26 h. 20 m.
(as shown in Fig. 97), and was nearly ol
the same nature, but rather less complex.
The movement seems to be periodical, for
on both days the leaf circumnutated in the
forenoon, fell in the afternoon (on the first
day until between 3 and 4 p.m., and on the
second day until 6 p.m.), and then rose,
falling again during the night or early
morning.
In the chapter on the Sleep of Plants
we shall see that the leaves in several Malvaceous genera sink
Fig. 98.
°. 30'ajn.uth
Camellia Japonica : cir-
cumnutation of leaf,
traced from 6.40
A.M. June 14th to
6.50 A.M. loth.
Apex of leaf 12
inches from the ver-
tical crlass, so figure
considerably mag-
nified. Temp. 16°-
16§° C.
B*50'a.mJ8 f b
\m a .45\p.
(S'.SO'a.m.l^
10 .25'p.m.ioty
Pelargonium zonnle : circumnutation and downward movement of young
leaf, traced from 9.30 a.m. June 14th to 6.30 p.m. 16th. Apex of leaf
9| inches from the vertical glass, so figure moderately magnified.
Temp. 15°-16£° C.
at night; and as they often do not then occupy a vertical
position, especially if they have not been well illuminated during
Chap. IV.
DICOTYLEDONS.
233
the day, it is doubtful whether some of these cases ought not
to have been included in the present chapter.
(7.) Pelargonium zonule (Geraniacea), Fam. 47). — A young
leaf, lj inch in breadth, with its petiole 1 inch long, borne on
a young plant, was observed in the usual manner during 61 h. ;
and its course is shown in the preceding figure (Fig. 98),
During the first day and night the leaf moved downwards, but
circumnutated between 10 a.m. and 4.30 p.m. On the second
day it sank and rose again, but between 10 a.m. and 6 p.m. it
circumnutated on an extremely small scale. On the third day
the circumnutation was more plainly marked.
(8.) Cissus discolor (Ampelideae, Fam. 67). — A leaf, not nearly
Fig. 99.
full-grown, the third from the apex of
a shoot on a cut-down plant, was
observed during 31 h. 30 m. (see Fig.
99). The day was cold (15°-16° C),
and if the plant had been observed in
the hot-house, the circumnutation,
though plain enough as it was, would
probably have been far more con-
spicuous.
(9.) Vicia fdba (Leguminosas, Fam.
75). — A young leaf, 3'1 inches in
length, measured from base of petiole to
end of leaflets, had a filament affixed
to the midrib of one of the two ter-
minal leaflets, and its movements were
traced during 5H h. The filament fell
all morning (July 2nd) till 3 p.m., and
then rose greatly till 10.35 p.m. ; but
the rise this day was so great, com-
pared with that which subsequently
occurred, that it was probably due in
part to the plant being illuminated
from above. The latter part of the course on July 2nd is alone
given in the following figure (Fig. 100). On the next day
(July 3rd) the leaf again fell in the morning, then circumnu-
tated in a conspicuous manner, and rose till late at night ; but
the movement was not traced after 7.15 p.m., as by that time the
filament pointed towards the upper edge of the glass. During
the latter part of the night or early morning it again fell in the
same manner as before.
Cissus disccfor ; circumnu-
tation of leaf, traced
from 10.35 A.M. May
28th to 6 p.m. 29th.
Apex of leaf 8| inches
from the vertical glass.
234
CIBCUMXUTATION OF LEAVES.
Chap. IV
As tlio evening rise and the early morning fall were unusually
large, the angle of the petiole above the horizon was measured
at the two periods, and the leaf was found to have risen 10 e
Fig. 100.
6*48 f a.m.3Z*
6Z45'am.4fa
".15' a.m. 4$
Vicia f"bi: circumnutation of leaf, traced from 7.15 p.m. July 2nd tc
10.15 A.M. 4th. Apex of the two terminal leaflets 1\ inches from the
vertical glass. Figure here reduced to two-thirds of original scale.
Temp. 17°-18° C.
between 12.20 p.m. and 10.45 p.m., and to have fallen 23° 30
between the latter hour and 10.20 a.m. on the following morning.
The main petiole was now secured to a stick close to the base
Chap. IV.
DICOTYLEDONS.
235
of tlie two terminal leaflets, which were 1 4 inch in length ; and
the movements of one of them were traced during 48 h. (see
Fig. 101). 'The course pursued is closely analogous to that of
the whole leaf. The zigzag line between 8.30 a.m. and 3.30 p.m.
on the second day represents 5 very small ellipses, with their
Fig. 101.
JO°J5'jp.m/.
10 a 30'a.m.6 e ! i
lO^O'a.m.
Vicia faba: circumnutation of one of the two terminal leaflets, the main
petiole having been secured, traced from 10.40 A.M. July 4th to 10.30 A.M>
6th. Apex of leaflet 6| inches from the vertical glass. Tracing here
reduced to one-half of original scale. Temp. 16°-18° C.
longer axes differently directed. From these observations it
follows that both the whole leaf and the terminal leaflets undergo
a well-marked daily periodical movement, rising in the evening
and falling during the latter part of the night or early morning;
whilst in the middle of the day they generally circumnutate
round the same small space.
236
CIBCUMNUTATION OF LEAVES.
Cuaf. rv
(10.) Acacia retinoides (Leguminosae).— The movement of a
young phyllode, 2$ inches in length, and inclined at a consider-
able angle above the horizon, was traced
during 45 h. 30 m. ; but in the figure here
given (Fig. 102), its circumnutation is shown
during only 21 h. 30 m. During part of
this time (viz., 14 h. 30 m.) the phyllode
described a figure re-
presenting 5 or 6
small ellipses. The
actual amount of
movement in a ver-
tical direction was *3
inch. The phyllode
rose considerably be-
tween 1.30 p.m. and
4 p.m., but there was
no evidence on either
day of a regular pe-
riodic movement.
(11.) Lupinus spe-
ciosus (Leguminosee).
— Plants were raised
from seed purchased under this name.
This is one of the species in this large
genus, the leaves of which do not sleep
at night. The petioles rise direct from
the ground, and are from 5 to 7 inches
in length. A filament was fixed to the
midrib of one of the longer leaflets, and
the movement of the whole leaf was traced,
as shown in Tig. 103. In the course of
6 h. 30 m. the filament went four times up
and three times down. A new tracing
was then begun (not here given), and
during 12 1 h. the leaf moved eight times
up and seven times down; so that it
described 1\ ellipses in this time, and
this is an extraordinary rate of movement.
The summit of the petiole was then secured
to a stick, and the separate leaflets were found to be continually
circumnutating.
Acacia retinoides : cir-
cumnutation of a
young phyllode,
traced from 10.45
A.M. July 18th to
8.15 a.m. 19th.
Apex of phyllode 9
inches from the
vertical glass; temp.
16£°-17|° C
Lvpinus specioaus: cir-
cumnutation of leaf
traced on vertical
glass, from 10.15 a.m.
to 5.45 p.m.; i.e.,
during 6 h. 30 ui.
Chap. IV.
DICOTYLEDONS.
237
(12.) Echeveria stohnifera (Crassulaceae, Fam. 84). — The older
leaves of this plant are so thick and fleshy, and the young ones
so short and broad, that it seemed
very improbable that any circum-
nutation could be detected. A fila-
ment was fixed to a young upwardly
inclined leaf, "75 inch in length and
•28 in breadth, which stood on the
outside of a terminal rosette of leaves,
produced by a plant growing very
vigorously. Its movement was traced
during 3 days, as here shown (Fig.
104). The course was chiefly in an
upward direction, and this may be
attributed to the elongation of the
leaf through growth ; but we see that
the lines are strongly zigzag, and that
occasionally there was distinct cir-
cumnutation, though on a very small
scale.
(13.) Bryophyllum (vel Culanchct)
calycinum (Crassulaceae). — Duval-
Jouve ('Bull. Soc. Bot. de France,'
Feb. 14th, 1868) measured the dis-
tance between the tips of the upper
pair of leaves on this plant, with the result shown in the following
Table. It should be noted that the measurements on Dec. 2nd
were made on a different pair of leaves : —
Echeveria stohnifera : circum-
nutation of leaf, traced
from 8.20 A.M. June 25th
to 8.45 A M. 28th. Apex
of leaf 121 inches from the
glass, so that the movement
was much magnified; temp.
23°-24J C.
8 A.M.
2 r.M.
7 P.M.
Nov. 16 .
. 15 mm.
. 25 mm.
. • (?)
„ 19 •
. . 48 „ .
. . 60 „ .
. 48 mm
Dec. 2 .
. . 22 „ .
. . 43 „ .
. . 28 „
We see from this Table that the leaves stood considerably
further apart at 2 p.m. than at either 8 am. or 7 p.m. ; and this
shows that they rise a little in the evening and fall or open
in the forenoon.
(14.) Drosera rotundlfolia (Droseracea3, Fam. 85). — The move-
ments of a young leaf, having a long petiole but with its tentacles
(oi gland-bearing hairs) as yet unfolded, were traced during
47 h. 15 m. The figure (Fig. 105) shows that it circumnutated
largely, chiefly in a vertical direction, making two ellipses each
238
CIRCUMNUTATION OF LEAVES.
Chap. IV.
day. On both days the leaf began to descend after 12 or
1 o'clock, and continued to do so all night, though to a
very unequal distance on the
Fig. 105.
9'J^a.mJ
two occasions. We therefore
thought {hat the movement
was periodic; but on observ-
ing three other leaves during
several successive days and
nights, we found this to be an
error; and the case is given
merely as a caution. On the
third morning the above leaf
occupied almost exactly the
same position as on the first
morning ; and the tentacles
by this time had unfolded
sufficiently to project at right
angles to the blade or disc.
The leaves as they grow
older generally sink more
and more downwards. The
movement of an oldish leaf,
the glands of which were
still secreting freely, was
traced for 24 h., during which
time it continued to sink a
little in a slightly zigzag line.
On the following morning, at
7 a.m., a drop of a solution
of carbonate of ammonia (2
gr. to 1 oz. of water) was
placed on the disc, and this
blackened the glands and in-
Drosera rotundifolia : circumnutation
of young leaf, with filament fixed
to back of blade, traced from 9.15
a.m. June 7th to 8.30 A.M. June
9th. Figure here reduced to one-
half original scale.
duced inflection of many of the tentacles. The weight of the
drop caused the leaf at first to sink a little ; but immediately
afterwards it began to rise in a somewhat zigzag course, and
continued to do so till 3 p.m. It then circumnutated about
the same spot on a very small scale for 21 h. ; and during the
next 21 h. it sank in a zigzag line to nearly the same level
which it had held when the ammonia was first administered.
By this time the tentacles had re-expanded, and the glands had
recovered their proper colour. We thus learn that an old leal
Ohap. IV. DICOTYLEDONS. 239
circumnutates on a small scale, at least whilst absorbing car-
bonate of ammonia ; for it is probable that this absorption may
stimulate growth and thus re-excite circumnutation. Whether
the rising of the glass filament which was attached to the back
of the leaf, resulted from its margin becoming slightly inflected
(as generally occurs), or from the rising of the petiole, was not
ascertained.
In order to learn whether the tentacles or gland-bearing hairs
circumnutate, the back of a young leaf, with the innermost
tentacles as yet incurved, was firmly cemented with shellac
to a flat stick driven into compact damp argillaceous sand.
The plant was placed under a microscope with the stage re-
moved and with an eye- piece micrometer, of which each
division equalled -^ of an inch. It should be stated that as
the leaves grow older the tentacles of the exterior rows bend
outwards and downwards, so as ultimately to become deflected
considerably beneath the horizon. A tentacle in the second
row from the margin was selected for observation, and was
found to be moving outwards at a rate of -^ of an inch in
20 m., or ^ of inch in 1 h. 40 m. ; but as it likewiso moved
from side to side to an extent of above gi_ f inch, the move-
ment was probably one of modified circumnutation. A tentacle
on an old leaf was next observed in the same manner. In
15 m. after being placed under the microscope it had moved
about y^qq of an inch. Daring the next lh h. it was looked at
repeatedly, and during this whole time it moved only another
y^qq of an inch ; and this small movement may have been due
to the settling of the damp sand (on which the plant rested),
though the sand had been firmly pressed down. We may there-
fore conclude that the tentacles when old do not circumnutate ;
yet this tentacle was so sensitive, that in 23 seconds after its
gland had been merely touched with a bit of raw meat, it began
to curl inwards. This fact is of some importance, as it appa-
rently shows that the inflection of the tentacles from the stimulus
of absorbed animal matter (and no doubt from that of contact
with any object) is not due to modified circumnutation.
(15.) Dioncea musciptla (Droseracece). — It should be premised
that the leaves at an early stage of their development have the
two lobes pressed closely together. These are at first directed
back towards the centre of the plant ; but they gradually rise up
and soon stand at right angles to the petiole, and ultimately in
nearly a straight line with it. A young leaf, which with t,h«
240
CTRCUMNUTAT10N OF LEAVES.
Chap. IV
Fig. 106.
petiole was only 1*2 inch in length, had a filament fixed exter-
nally along the midrib of the still closed lobes, which projected
at right angles to the petiole. In the evening this leaf com-
pleted an ellipse in the course of 2 h. On
the following day (Sept. 25th) its move-
ments were traced during 22 h. ; and we
see in Eig. 106 that it moved in the same
general direction, due to the straightening
of the leaf, but in an extremely zigzag line.
This line represents several drawn-out or
modified ellipses. There can therefore be
no doubt that this young leaf circumnu-
tated.
A rather old, horizontally extended
leaf, with a filament attached along the
under side of the midrib, was next
observed during 7 h. It hardly moved,
but when one of its sensitive hairs
was touched, the blades closed, though
not very quickly. A new dot was now
made on the glass, but in the course of
14 h. 2>J m. there was hardly any change
in the position of the filament. We may
therefore infer that an old and only
moderately sensitive leaf does not circum-
nutate plainly ; but we shall soon see
that it by no means follows that such
a leaf is absolutely motionless. We may
further infer that the stimulus from a
touch does not re-excite plain circumnu-
tation.
Another full-grown leaf had a filament
attached externally along one side of the
midrib and parallel to it, so that the fila-
ment would move if the lobes closed. It
should be first stated that, although a touch on one of the sensi-
tive hairs of a vigorous leaf causes it to close quickly, often
almost instantly, yet when a bit of damp meat or some solution
af carbonate of ammonia is placed on the lobes, they close so
slowly that generally 24 h. is required for the completion of the
act. The above leaf was first observed for 2 h. 30 m., and did
not circumnutate, but it ought to have been observed for a
Doncea muscipuia : cir-
cumnutation of a
young and expanding
leaf, traced on a hori-
zontal glass in dark-
ness, from noon Sept.
24th to 10 A.M. 25th.
Apex of leaf 13£
inches from the glass,
so tracing consider-
ably magnified.
Chap. IV. DICOTYLEDONS 241
longer period ; although, as we have seen, a young leaf com-
pleted a fairly large ellipse in 2 h. A drop of an infusion of
raw meat was then placed on the leaf, and within 2 h. the glass
filament rose a little ; and this implies that the lobes had begun
to close, and perhaps the petiole to rise. It continued to rise
with extreme slowness for the next 8 h. 30 m. The position of
the pot was then (7.15 p.m., Sept. 24th) slightly changed and
an additional drop of the infusion given, and a new tracing
was begun (Fig. 107). By 10.50 p.m. the filament had risen
only a little more, and it fell during the night. On the follow-
ing morning the lobes were closing more quickly, and by 5 p.m.
it was evident to the eye that they had closed considerably ; by
8.48 p.m. this was still plainer, and by 10.45 p.m. the marginal
spikes were interlocked. The leaf fell a little during the night,
and next morning (25th) at 7 am. the lobes were completely
shut. The course pursued, as may be seen in the figure, was
Fig. 107.
Dionasa mu^cipida ; closure of the lobes and circumnutation of a full-grown
leaf, whilst absorbing an infusion of raw meat, traced in darkness, from
7.15 p.m. Sept. 24th to 9 a.m. 26th. Apex of leaf 8 J inches from the
vertical glass. Figure here reduced to two-thirds of original scale.
strongly zigzag, and this indicates that the closing of the lobes
was combined with the circumnutation of the whole leaf,
and there cannot be much doubt, considering how motionless
the leaf was during 2 h. 30 m. before it received the infusion,
that the absorption of the animal matter had excited it to
circumnutate. The leaf was occasionally observed for the next
four days, but was kept in rather too cool a place ; nevertheless,
it continued to circumnutate to a small extent, and the lobes
remained closed.
It is sometimes staled in botanical works that the lobes close
or sleep at night ; but this is an error. To test the statement,
very long glass filaments were fixed inside the two lobes of
thiee leaves, and the distances between their tips were measured
in the middle of the day and at night ; but no difference could
be detected.
The previous observations relate to the movements of the
whole leaf, but the lobes move independently of the petiole, and
242 C1KCUMNUTATI0N OF LEAVES. Chap. IV.
seem to be continually opening and shutting to a very small
extent. A nearly full-grown leaf (afterwards proved to be
highly sensitive to contact) stood almost horizontally, so thai
by driving a long thin pin through the foliaceous petiole close
to the blade, it was rendered motionless. The plant, with
a little triangle of paper attached to one of the marginal spikes,
was placed under a microscope with an eye-piece micrometer,
each division of which equalled -^ of an iuch. The apex oi
the paper-triangle was now seen to be in constant slight move-
ment ; for in 4 h. it crossed nine divisions, or -^ of an inch,
and after ten additional hours it moved back and had crossed
5§o in an opposite direction. The plant was kept in rather
too cool a place, and on the following day it moved rather less,
namely, -gfe in 3 h., and t %q in an opposite direction during the
next 6 h. The two lobes, therefore, seem to be constantly
closing or opening, though to a very small distance; for we must
remember that the little triangle of paper affixed to the marginal
spike increased its length, and thus exaggerated somewhat the
movement. Similar observations, with the important difference
that the petiole was left free and the plant kept under a high
temperature, were made on a leaf, which was healthy, but so old
that it did not close when its sensitive hairs were repeatedly
touched, though judging from other cases it would have slowly
closed if it had been stimulated by animal matter. The apex of
the triangle was in almost, though not quite, constant movement,
sometimes in one direction and sometimes in an opposite one ;
and it thrice crossed five divisions of the micrometer (i.e. y^ of
an inch) in 30 m. This movement on so small a scale is hardly
comparable with ordinary circumnutation ; but it may perhaps
be compared with the zigzag lines and little loops, by which the
larger ellipses made by other plants are often interrupted.
In the first chapter of this volume, the remarkable oscillatory
movements of the circumnutating hypocotyl of the cabbage
have been described. The leaves of Dionsea present the same
phenomenon, which is a wonderful one, as viewed under a low
power (2-inch object-glass), with an eye-piece micrometer oi
which each division (^^ of an inch) appeared as a rather wide
space. The young unexpanded leaf, of which the circumnutating
movements were traced (Fig. 106), had a glass filament fixed
perpendicularly to it; and the movement of the apex was
observed in the hot-house (temp. 84° to 86° F.), with light
admitted only from above, and with any lateral currents of air
Chat. IV. DICOTYLEDONS. 243
excluded. The apex sometimes crossed one or two divisions of
the micrometer at an imperceptibly slow rate, but generally it
moved onwards by rapid starts or jerks of y^^ or yoVo* an d in
one instance of toVo °f an inch. After each jerk forwards, the
apex drew itself backwards with comparative slowness for part
of the distance which had just been gained ; and then after a
very short time made another jerk forwards. Four conspi-
cuous jerks forwards, with slower retreats, were seen on one
occasion to occur in exactly one minute, besides some minor
oscillations. As far as we could judge, the advancing and
retreating lines did not coincide, and if so, extremely minute
ellipses were each time described. Sometimes the apex remained
quite motionless for a short period. Its general course during
the several hours of observation was in two opposite directions,
so that the leaf was probably circumnutating.
An older leaf with the lobes fully expanded, and which was
afterwards proved to be highly sensitive to contact, was next
observed in a similar manner, except that the plant was exposed
to a lower temperature in a room. The apex oscillated forwards
and backwards in the same manner as before ; but the jerks for-
ward were less in extent, viz. about y^^ inch ; and there were
longer motionless periods. As it appeared possible that the
movements might be due to currents of air, a wax taper was
held close to the leaf during one of the motionless periods, but
no oscillations were thus caused. After 10 m., however, vigorous
oscillations commenced, perhaps owing to the plant having been
warmed and thus stimulated. The candle was then removed and
before long the oscillations ceased ; nevertheless, when looked at
again after an interval of 1 h. 30 m., it was again oscillating.
The plant was taken back into the hot-house, and on the
following morning was seen to be oscillating, though not very
vigorously. Another old but healthy leaf, which was rot in the
least sensitive to a touch, was likewise observed during two
days in the hot-house, and the attached filament made many
little jerks forwards of about To ^q or only yJ^ of an inch.
Finally, to ascertain whether the lobes independently of the
petiole oscillated, the petiole of an old leaf was cemented close
to the blade with shellac to the top of a little stick driven into
the soil. But before this was done the leaf was observed, and
found to be vigorously oscillating or jerking ; and after it had
been cemented to the stick, the oscillations of about 10 2 00 of
an inch stil] continued. On the following day a little infusion
244
CIKCUMNUTATION OF LEAVES. Chap. IT
of raw meat was placed on the leaf, which caused the lobes tc
close together very slowly in the course of two days ; and the
oscillations continued during this whole time and for the next
two days. After nine additional days the leaf began to open
and the margins were a little everted, and now the apex of the
glass filament remained for long periods motionless, and then
moved backwards and forwards for a distance of about y^o of
an inch, slowly, without any jerks. Nevertheless, after warming
the leaf with a taper held close to it, the jerking movement
recommenced.
This same leaf had been observed 2| months previously, and
was then found to be oscillating or jerking. We may therefore
infer that this kind of movement goes on night and day for a
very long period ; and it is common to young unexpanded leaves
and to leaves so old as to have lost their sensitiveness to a
touch, but which were still capable of absorbing nitrogenous
matter. The phenomenon when well displayed, as in the young
leaf just described, is a very interesting one. It often brought
before our minds the idea of effort, or of a small animal
struggling to escape from some constraint.
(16.) Eucalyptus resinifera (Myrtacese, Fam. 94). — A young leaf,
two inches in length together with
the petiole, produced by a lateral
shoot from a cut-down tree, was
observed in the usual manner.
The blade had not as yet as-
sumed its vertical position. On
June 7th only a few observations
were made, and the tracing merely
showed that the leaf had moved
three times upwards and three
downwards. On the following
day it was observed more fre-
quently; and two tracings were
made (see A and B, Fig. 108), as
a single one would have been too
complicated. The apex changed
its course 13 times in the course
of 16 h., chiefly up and down, but
The actual amount of movement
Fig. 108.
Eucalyptus resinifera : circumnu-
tation of a leaf, traced, A, from
6.40 A.M. to 1 P.M. June 8th ;
B, from 1 p.m. 8th to 8.30 A.M.
9th. Apex of leaf 14| inches
from the horizontal glass, so
figures considerably magnified.
with some lateral movement.
in any one direction was small.
(17.) Dallia (garden var.) (Composite, Fam. 122).
-A fine young
'flAP. IV.
DICOTYLEDONS.
245
leaf 5| inches in length, produced by a young plant 2 feet high,
growing vigorously in a large pot, was directed at an angle ol
about 45° beneath the horizon. On June 18th the leaf descended
from 10 a.m. till 11 35 a.m. (see Fig. 109) ; it then ascended
greatly till 6 p.m., this ascent being probably due to the light
Fig. 109.
J*f35'pmJ8*
'.Sffa/nm
Dahlia : circumnutation of leaf, traced from 10 A.M. June 18th to 8.10 a.m.
20th, but with a break of 1 h. 40 m. on the morning of the 19th, as,
owing to the glass filament pointing too much to one side, the pot had
to be slightly moved ; therefore the relative position of the two tracings
is somewhat arbitrary. The figure here given is reduced to one-fifth of
the original scale. Apex of leaf 9 inches from the glass in the line
of its inclination, and 4| in a horizontal line.
coming only from above. It zigzagged between 6 r.M. and
10.35 p.m., and ascended a little during the night. It should be
remarked that the vertical distances in the lower part of the
diagram are much exaggerated, as the leaf was at first deflected
beneath the horizon, and after it had sunk downwards, the
filament pointed in a very oblique line towards the glass. Next
246 CIRCUMNUTATION OF LEAVES. Chap. IV.
day the leaf descended from 8.20 a.m. till 7.15 p.m., then zigzagged
and ascended greatly during the night. On the morning of tbe
20th the leaf was probably beginning to descend, though the
short line in the diagram is horizontal. The actual distances
travelled by the apex of the leaf were considerable, but could
not be calculated with safety. From the course pursued on the
second day, when the plant had accommodated itself to the light
from above, there cannot be much doubt that the leaves undergo
a daily periodic movement, sinking during the day and rising
at night.
(18.) Mutisia clematis (Composite). — The leaves terminate in
tendrils and circumnutate like those of other tendril-bearers ;
but this plant is here mentioned, on account of an erroneous
statement * whioh has been published, namely, that the leaves
sink at night and rise during the day. The leaves which
behaved in this manner had been kept for some days in a
northern room and had not been sufficiently illuminated. A
plant therefore was left undisturbed in the hot-house, and three
leaves had their angles measured at noon and at 10 p.m. All
three were inclined a little beneath the horizon at noon, but one
stood at night 2°, the second 21°, and the third 10° higher than
in the middle of the day; so that instead of sinking they rise
a little at night.
(19.) Cyclamen Persicum (Primulacese, Fam. 135). — A young
leaf, 1*8 of an inch in length, petiole included, produced by an
old root-stock, was observed during three days in the usual
manner (Fig. 110). On the first day the leaf fell more than after-
wards, apparently from adjusting itself to the light from above.
On all three days it fell from the early morning to about 7 p.m.,
and from that hour rose during the night, the course being
slightly zigzag. The movement therefore is strictly periodic.
It should be noted that the leaf would have sunk each evening
a little lower down than it did, had not the glass filament rested
between 5 and 6 p.m. on the rim of the pot. The amount of
movement was considerable ; for if we assume that the whole
leaf to the base of the petiole became bent, the tracing would
be magnified rather less than five times, and this would give
to the apex a rise and fall of half an inch, with some lateral
movement. This amount, however, would not attract attention
without the aid of a tracing or measurement of some kind.
The Movements and Habits of Climbing Plants," 1875, p. 118.
Chap. IV.
DICOTYLEDONS.
247
(20.) Allamanda ScLottii (Apocynese, Fam. 144). — The young
leaves of tliis shrub are elongated, with the blade bowed so much
Fig. 110
7J3.1
Cyclamen Persicum : circumnutation of leaf, traced from 6.45 A.M June 2ad
to 6.40 A.M. 5th. Apes of leaf 7 inches from the vertical glass.
downwards as almost to form a semicircle. The chord — that
is, a line drawn from the apex of the blade to the base of th6
petiole — of a young leaf. 4| inches in length, stood at 2.50 p.m od
17
248
CIRCUMNUTATION OF LEAVES.
Chap. I\>
Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 p.m.
the blade had straightened itself
Fl S- 111 so much, which implies the
t raising of the apex, that the
V chord now stood at 37° above the
J^ horizon, and had therefore risen
50°. On the next day similar
angular measurements of the
same leaf were made; and at
noon the chord stood 36° be-
neath the horizon, and 9.30 p.m.
3|° above it, so had risen 39£°.
The chief cause of the rising
movement lies in the straighten-
ing of the blade, but the short
petiole rises between 4° and 5°.
On the third night the chord
stood at 35° above the horizon,
and if the leaf occupied the
same position at noon, as on
the previous day, it had risen
71°. With older leaves no such
change of curvature could be
detected. The plant was then
brought into the house and
kept in a north-east room, but
at night there was no change
in the curvature of the young
leaves; so that previous expo-
sure to a strong light is appa-
rently requisite for the periodi-
cal change of curvature in the
blade, and for the slight rising
of the petiole.
(21.) Wigandia (Hydroleacese,
Fam. 149).— Professor Pfeffer
informs us that the leaves of this
plant rise in the evening ; but as
we do not know whether or not
the rising is great, this species
ought perhaps to be classed
amongst sleeping plants
Petunia violacca : downward move-
ment and circumnutation of a
very young leaf, traced from 10
A.M. June 2nd to 9.20 A.M. June
6th. N.B. — At 6.40 a.m. on the
5th it was necessary to move the
pot a little, and a new tracing
was begun at the point where
two dots are not joined in the
diagram. Apex of leaf 7 inches
from the veitical glass. Temp,
generally 17J°C.
Ohap. IV.
DICOTYLEDONS.
249
Fig. 112.
(22.) Pttunia violacea (Solanese, Fam. 157).— A very young
leaf, only I inch in length, highly inclined upwards, was observed
for four days. During the whole of this time it bent outwards
and downwards, so as to become more and more nearly hori-
zontal. The strongly marked zigzag line in the figure on p. 248
(Fig. Ill), shows that this was effected by modified circum-
nutation ; and during the latter part of the time there was much
ordinary circumnutation on a small scale. The movement in
the diagram is magnified between 10 and 11 times. It exhibits
a clear trace of periodicity, as the leaf rose a little each evening ;
but this upward tendency appeared to be almost conquered by
the leaf striving to become more and
more horizontal as it grew older. The
angles which two older leaves formed
together, were measured in the even-
ing and about noon on 3 successive
days, and each night the angle de-
creased a little, though irregularly.
(23.) Acanthus mollis (Acanthaceae,
Fam. 168). — The younger of two
leaves, 2[ inches in length, petiole
included, produced by a seedling
plant, was observed during 47 h.
Early on each of the three morn-
ings, the apex of the leaf fell ; and
it continued to fall till 3 p.m., on
the two afternoons when observed.
After 3 p.m. it rose considerably, and
continued to rise on the second night
until the early morning. But on
the first night it fell instead of rising,
and we have little doubt that this
was owing to the leaf being very
young and becoming through epi-
nastic growth more and more hori-
zontal; for it may be seen in the
diagram (Fig. 112), that the leaf stood
on a higher level on the first than on
fcne second day. The leaves of an
allied species (A. spinosus) certainly
rose every night ; and the rise between noon and 10.15 p.m.,
when measured on one occasion, was 10°. This rise was chiefly
Acanthus mollis: circumnuta-
tion of young leaf, traced
from 9.20 A.M. June 14th
to 8.30 A.M. 16th. Apex
of leaf 11 inches from the
vertical glass, so movement
considerably magnified.
Figure here reduced toone-
haif of original Kcale.
Temp. 15°-16J°C.
250 CIRCUMNUTATION OF LEAVES. Chap. IV.
or exclusively due to the straightening of the blade, and not to
the movement of the petiole. We may therefore conclude that
the leaves of Acanthus circumnutate periodically, falling in the
morning and rising in the afternoon and night.
(24.) Cannabis sativa (Cannabinea?, Fam. 195). — We have
here the rare case of leaves moving downwards in the evening,
but not to a sufficient degree to be called sleep.* In the early
morning, or in the latter part of the night, they move upwards.
For instance, all the young leaves near the summits of several
stems stood almost horizontally at 8 a.m. May 29th, and at
10.30 p.m. were considerably declined. On a subsequent day two
leaves stood at 2 p.m. at 21° and 12° beneath the horizon, and at
10 p.m. at 38° beneath it. Two other leaves on a younger plant
were horizontal at 2 p.m., and at 10 p.m. had sunk to 36° beneath
the horizon. With respect to this downward movement of the
leaves, Kraus believes that it is due to their epinastic growth.
He adds, that the leaves are relaxed during the day, and tense
at night, both in sunny and rainy weather.
(25.) Pinus pinuster (Coniferae, Fam. 223). — The leaves on the
summits of the terminal shoots stand at first in a bundle almost
upright, but they soon diverge and ultimately become almost
horizontal. The movements of a young leaf, nearly one inch in
length, on the summit of a seedling plant only 3 inches high,
were traced from the early morning of June 2nd to the evening
of the 7th. During these five days the leaf diverged, and its apex
descended at first in an almost straight line ; but during the two
latter days it zigzagged so much that it was evidently circumnu-
tating. The same little plant, when grown to a height of 5 irjche^,
was again observed during four days. A filament was fixed
transversely to the apex of a leaf, one inch in length, and which
had already diverged considerably from its originally upright
position. It continued to diverge (see A, Fig. 113), and to
descend from 11.45 a.m. July 31st to 6.40 a.m. Aug. 1st. On
August 1st it circumnutated about the same small spaco, and
again descended at night. Next morning the pot was moved
nearly one inch to the right, and a new tracing was begun (B),
From tbis time, viz., 7 a.m. August 2ud to 8.20 a.m. on the 4th
* We were led to observe this Flcra, 1879, p. G6. We regret that
plant by Dr. Carl Kraus' paper, we cannot fully understand parti.
' Beitr'age zur Kentniss der Bcwe- of ibis paper,
gungen Wachsender Laubblatter,'
Chap. IV.
DICOTYLEDONS.
251
the leaf manifestly circumnutatcd. It does not appear from the
diagram that the leaves move periodically, for the descending
course during the first two nights, was clearly due to epinastic
1
Fig. 113.
6*4iO'a.mJ*. t
6"40'cf,m.3l^
8'20'a.m.4
-7°a.m.£ v A
Pinus pinaster: circumnutation of young leaf, traced from 11.45 a.m.
July JUst to 8.20 A.M.' Aug. 4th. At 7 A.M. Aug. 2nd the pot was
moved an inch to one side, so that the tracing consists of two figures.
Apex of leaf 14| inches from the vertical glass, so movements much
magnified.
growth, and at the close of our observations the leaf was not
nearly so horizontal as it would ultimately become.
Pinus austriaca. — Two leaves, 3 inches in length, but not
252
CIBCUMNUTATIGN OF LEAVES. Chap. IV.
quite fully grown, produced by a lateral shoot, on a young tree
3 feet in height, were observed during 29 h. (July 31st), in the
same manner as the leaves of the previous species. Both these
leaves certainly circumnutafed, making
Fi S- 114 - within the above period two, or two and
V a half, small, irregular ellipses.
(26.) Cycas pectinata (Cycadese, Fam
224). — A young leaf, 11$ inches in
length, of which the leaflets had only
recently become uncurled, was observed
during 47 h. 30 m. The main petiole
was secured to a stick at the base of the
two terminal leaflets. To one of the
4 latter, 31 inches in length, a filament
was fixed ; the leaflet was much bowed
downward, but as the terminal part was
upturned, the filament projected almost
horizontally. The leaflet moved (see
Fig. 114) largely and periodically, for it
fell until about 7 p.m. and rose during
the night, falling again next morning
after 6.40 a.m. The descending lines
are in a marked manner zigzag, and so
probably would have been the ascending
lines, if they had been traced throughout
the night.
i
Oycas pectinata : circura-
nutation of one of the
terminal leaflets, traced
from 8.30 a.m. June
22nd to 8 A.M. June
24th. Apex of leaflet
7| inches from the ver-
tical glass, so tracing
not greatly magnified,
and here reduced to
one-third of original
scale; temp. 19°-21°C.
clrcumnutation of leaves
Monocotyledons.
(27.) Canni Warscewiczii (Cannacese,
Fam. 2). — The movements of a young
leaf, 8 inches in length and 3£ in
breadth, produced by a vigorous young
plant, were observed during 45 h.
50 in., as shown in Fig. 115. The pot
was slided about an inch to the right on the morning of the
11th, as a single figure would have been too complicated; but
the two figures are continuous in time. The movement is
periodical, as the leaf descended from the early morning until
about 5 p M., and ascended during the rest of the evening and
Chap. IV.
MONOCOTYLEDONS.
253
part of the night. On the evening of the 11th it circuinnutated
on a small scale for some time about the same spot.
Fig. 115.
A. B.
Canna Warscewiczii • circumnutation of leaf, traced (A) from 11.30 A.M
June 10th to 6.40 A.M. 11th ; and (B) from 6.40 A.M. 11th to 8.40 A.M.
12th. Apex of leaf 9 inches from the vertical glass.
(28.) Iris pseudo-acorus (Iridese, Fam. 10). — The movements
of a young leaf, rising 13 inches above the water in which the
plant grew, were traced as shown in the
figure (Fig. 116), during 27 h. 30 m.
It manifestly circumnutated, though
only to a small extent. On the second
morning, between 6.40 a.m. and 2 p.m.
(at which latter hour the figure here
given ends), the apex changed its course
five times. During the next 8 h. 40 m. it
zigzagged much, and descended as far
as the lowest dot in the figure, making
in its course two very small ellipses ;
but if these lines had been added to
the diagram it would have been too
complex.
(29.) Crinum Capense (Amaryllideas,
Fam. 11). — The leaves of this plant
are remarkable for their great length
and narrowness: one was measured
and found to be 53 inches long and
only T4 broad at the base. Whilst quite young they stand up
almost vertically to the height of about a foot; afterwards
Fig. 116.
7
Iris pseudo-acorus ; circum-
nutation of leaf, traced
from 10.30 A.M. May 28th
to 2 P.M. 29th. Tracing
continued to 11 P.M., but
not here copied. Apex
of leaf 12 inches beneath
the horizontal glass, so
figure considerably mag-
nified. Temp. 15°-16°C.
254 CIKCUMNUTATION OF LEAVES. Chap. IV.
their tips begin to bend over, and subsequenily bang vertically
down, and thus continue to grow. A rather young leaf was
selected, of which the dependent tapering point was as yet only
5 1 inches in length, the upright basal part being 20 inches high,
though this part would ultimately become shorter by being
more bent over. A large bell-glass was placed over the plant,
with a black dot on one side ; and by bringing the dependent
apex of the leaf into a line with this dot, the accompanying
figure (Fig. 117) was traced on the other side of the bell, during
2£ days. During the first day (22nd) the tip travelled laterally
far to the left, perhaps in consequence of the plant having been
Fig. 117.
J2°p.m.23 T : d
Crinum capense : circumnutation of dependent tip of young leaf, traced „..
a bell-glass, from 10.30 p.m. May 22nd to 10.15 A.M. 25th. Figure not
greatly magnified.
disturbed ; and the last dot made at 10.30 p.m. on this day is
alone here given. As we see in the figure, there can be no
doubt that the apex of this leaf circumnutated.
A glass filament with little triangles of paper was at the
same time fixed obliquely across the tip of a still younger leaf,
which stood vertically up and was as yet straight. Its move-
ments were traced from 3 p.m. May 22nd to 10.15 a.m. 25th.
The leaf was growing rapidly, so that the apex ascended greatly
during this period ; as it zigzagged much it was clearly circum-
nutating, and it apparently tended to form one ellipse each
day. The lines traced during the night were much more vertical
than those traced during the day ; and this indicates that the
tracing would have exhibited a nocturnal rise and a diurnal
fall, if the leaf had not grown so quickly. The movement of
this same leaf after an interval of six days (May 31st), by which
time the tip had curved outwards into a horizontal position
Chap. IV. MONOCOTYLEDONS. 255
and had thus made the first step towards becoming dependent,
was traced orthogonically by the aid of a cube of wood (in the
manner before explained) ; and it was thus ascertained that the
actual distance travelled by the apex, and due to cir cum nutation,
was 3| inches in the course of 20£ h. During the next 24 h. it
travelled 2s inches. The circumnutating movement, therefore,
of this young leaf was strongly marked.
(30.) Pancratium littorale (Amaryllidese). — The movements,
much magnified, of a leaf, 9 inches in length and inclined at
about 45° above the horizon, were traced during two days. On
the first day it changed its course completely, upwards and
downwards and laterally, 9 times in 12 h. ; and the figure traced
apparently represented five ellipsea. On the second day it was
observed seldomer, and was therefore not seen to change its
course so often, viz., only 6 times, but in the same complex
manner as before. The movements were small in extent, but
there could be no doubt about the circumnutation of the lea£
(31.) Imatopliyllum. vel Olivia (sp. ?) (Amaryllidese). — A long
glass filament was fixed to a leaf, and the angle formed by it
with the horizon was measured occasionally during three suc-
cessive days. It fell each morning until between 3 and 4 p.m.,
and rose at night. The smallest angle at any time above the
horizon was 48°, and the largest 50°; so that it rose only 2°
at night ; but as this was observed each day, and as similar
observations were nightly made on another leaf on a distinct
plant, there can be no doubt that the leaves move periodically,
though to a very small extent. The position of the apex when
it stood highest was *8 of an inch above its lowest point.
(32.) Pistia stratiotes (Aroideae, Fam. 30). — Hofmeister
remarks that the leaves of this floating water-plant are more
highly inclined at night than by day.* We therefore fastened
a fine glass filament to the midrib of a moderately young
leaf, and on Sept. 19th measured the angle which it formed
with the horizon 14 times between 9 a.m. and 11.50 p.m. The
temperature of the hot-house varied during the two days of
observation between 181° and 232° C. At 9 a.m. the filament
stood at 32° above the horizon ; at 3.34 p.m. at 10° and at
11.50 p.m. at 55° ; these two latter angles being the highest and
the lowest observed during the day, showing a difference of 45°.
The rising did not become strongly marked until between
Die Lehre von der Pflanzenzelle,' 18G7, p. 327.
256 CIRCUMNUTATION OF LEAVES. Chap. IV
5 and 6 p.m. On the next day the leaf stood at only 10° above
the horizon at 8.25 a.m., and it remained at about 15° till past
3 p.m.; at 5.40 p.m. it was 23°, and at 9.30 p.m. 58°; so that
the rise was more sudden this evening than on the previous
one, and the difference in the angle amounted to 48°. The
movement is obviously periodical, and as the leaf stood on the
first night at 55°, and on the second night at 58° above the
horizon, it appeared very steeply inclined. This case, as we
shall see in a future chapter, ought perhaps to have been
included under the head of sleeping plants.
(33.) Pontederia (sp. ?) (from the highlands of St. Catharina,
Fig. 118.
Pontederia (sp. ?) : circumnutat:on of leaf, traced from 4.50 P.M. July 2nd
to 10.15 A.M. 4th. Apex of leaf 16J inches from the vertical glass, so
tracing greatly magnified. Temp, about 17° C, and therefore rather
too low.
Brazil) (Pontederiaceae, Fam. 46). — A filament was fixed across
the apex of a moderately young leaf, 71 inches in height, and
its movements were traced during 42 k h. (see Fig. 118). On
the first evening, when the tracing was begun, and during the
night, the leaf descended considerably. On the next morning
it ascended in a strongly marked zigzag line, and descended
again in the evening and during the night. The movement,
therefore, seems to be periodic, but some doubt is thrown on
this conclusion, because another leaf, 8 inches in height,
appearing older and standing more highly inclined, behaved
differently. During the first 12 h. it circumnutated over a
Chap. IV. CIBCUMNUTATION OF CEYPTOGAMS.
257
small space, but during the night and the whole following day
it ascended in the same general direction; the ascent being
effected by repeated up and down well-pronounced oscillations.
Cryptogams.
(34.) Nephrodium molle (Filices, Fam. 1). — A filament was
fixed near the apex of a young frond of this Fern, 17 inches
in height, which was not as yet fully uncurled ; and its move-
ments were traced during 24 h. We see in Fig. 119 that it
Fig. 119.
Nephrodium molle: circumnutation of rachis, traoed from 9.15 A.M. May
28th to 9 A.M. 29th. Figure here given two-thirds of original scale.
plainly circumnutated. The movement was not greatly magnified
as the frond was placed near to the vertical glass, and would
probably have been greater and more rapid had the day been
warmer. For the plant was brought out of a warm greenhouse
and observed under a skylight, where the temperature was
between 15° and 16° C. We have seen in Chap. I. that a frond of
this Fern, as yet only slightly lobed and with a rachis only "23
inch in height, plainly circumnutated.*
* Mr. Loomi'S and Prof. Asa
Gray have described (' Botanical
Gazette,' 1880, pp. 27, 43), an
extremely curious case of move-
ment in the fronds, but only in
the fruiting fronds, of Asplenium
trtchomanes. They move almost
as rapidly as the little leaflets
of Desmodium gyran?, alternately
backwards and forwards through
from 20 1 o 40 degrees, in a plane at
right angles to that of the frond.
The apex of the frond describes *' a
long and very narrow ellipse,'' so
that it ciroumnutates. But the
movement differs from ordinary
258 CIKCUMNUTATION OF CRYPTOGAMS. Chap. 17
Fig. 120.
In the chapter on the Sleep of Plants the conspicuous circum-
nutation of Marsilea quadiijoliata (Marsileacese, Fam. 4) will be
described.
It has also been shown in Chap. I. that a very young Sela-
ginella (Lycopodiaceae, Fam. 6), only "4 inch in height, plainly
circumnutated; we may therefore conclude that older plants,
whilst growing, would do the same.
(35.) Lunularia vulgaris (Hepaticse, Fam. 11, Muscales).—
The earth in an old flower-pot was
coated with this plant, bearing
gemmae. A highly inclined frond,
which projected "3 inch above the
soil and was - 4 inch in breadth, was
selected for observation. A glass
hair of extreme tenuity, 75 inch
in length, with its end whitened,
was cemented with shellac to the
frond at right angles to its breadth ;
and a white stick with a minute
black spot was driven into the soil
close behind the end of the hair.
The white end could be accurately
brought into a line with the black
spot, and dots could thus be suc-
cessively made on the vertical
glass-plate in front. Any move-
ment of the frond would of course
be exhibited and increased by the
long glass hair ; and the black spot
was placed so close behind the end
of the hair, relative'y to the dis-
tance of the g^ass-plate in front,
that the movement of the end was
magnified about 40 times: Never-
theless, we are convinced that onr
tracing gives a fairly faithful re-
presentation of the movements of
the frond. In the intervals between each observation, the plant
was covered by a small bell-glass. The frond, as already stated,
Lunularia vulgaris: circumnuta-
tion of a frond, traced from
9 A.M. Oct 25th to 8 A.M. 27th.
rircnmnntation as it occurs only
when the plant is exposed to the
light; even artificial light "is
sufficient to excite motion for a
few minutes."
Chap. IV. CIRCUMNUTATION OF LEAVES. 259
tvas highly inclined, and the pot stood in front of a north-east
window. During the five first days the frond moved downwards
or became less inclined; and the long line which was traced
was strongly zigzag, with loops occasionally formed or nearly
formed ; and this indicated circumnutation. Whether the sink-
ing was due to epinastic growth, or apheliotropism, we do not
know. As the sinking was slight on the fifth day, a new tracing
was begun on the sixth day (Oct. 25th), and was continued
for 47 b. ; it is here given (Fig. 120). Another tracing was made
on the next day (27th) and the frond was found to be still cir-
cumnutating, for during 14 h. 30 m. it changed its course com-
pletely (besides minor changes) 10 times. It was casually
observed for two more days, and was seen to be continually
moving.
The lowest members of the vegetable series, the Thallogens,
apparently circumnutate. If an Oscillaria be watched under
the microscope, it may be seen to describe circles about every
40 seconds. After it has bent to one side, the tip first begins
to bend back to the opposite side and then the whole filament
curves over in the same direction. Hofmeister* has given a
minu te account of the curious, but less regular though constant,
movements of Spirogyra: during 2j h. the filament moved 4
times to the left and 3 times to the right, and he refers to a
movement at right angles to the above. The tip moved at the
rate of about 0T mm. in five minutes. He compares the move-
ment with the nutation of the higher plants.f We shall hereafter
see that heliotropic movements result from modified circum-
nutation, and as unicellular Moulds bend to the light we may
infer that they also circumnutate.
Concluding Kemarks on the Circumnutation
of Leaves.
The circumnutating movements of young leaves in
33 genera, belonging to 25 families, widely distributed
* ' Ueber die Bewegungen der 1880, vol. hi. p. 320) that the
Faden der Spirogyra princeps: movements of Spirulina, a mem-
Jahreskefte des Veieins fur vater- ber of the Oscillutorie*, are closely
landische Naturkunde iu "Wiirt- analogous "to the well-kuown
tember°:,' 1874, p. 211. rotation of growiug shoots aud
t Zukal also remarks (as.quoted tendrils."
in 'Journal R. Mkroscop. Soe.,'
260 CIECUMNUTATION OF LEAVES. Chap. IV.
amongst ordinary and gyninospermous Dicotyledons
and amongst Monocotyledons, together with several
Cryptogams, have now been described. It would,
therefore, not be rash to assume that the growing
leaves of all plants circumnutate, as we nave seen
reason to conclude is the case with cotyledons. The
seat of movement generally lies in the petiole, but
sometimes both in the petiole and blade, or in the
blade alone. The extent of the movement differed much
in different plants ; but the distance passed over was
never great, except with Pistia, which ought perhaps
to have been included amongst sleeping plants. The
angular movement of the leaves was only occasionally
measured ; it commonly varied from only 2° (and pro-
bably even less in some instances) to about 10°; but
it amounted to 23° in the common bean. The move-
ment is chiefly in a vertical plane, but as the ascending
and descending lines never coincided, there was always
some lateral movement, and thus irregular ellipses
were formed. The movement, therefore, deserves tc
be called one of circumnutation ; for all circumnuta-
ting organs tend to describe ellipses, — that is, growth
on one side is succeeded by growth on nearly but not
quite the opposite side. The ellipses, or the zigzag
lines representing drawn-out ellipses, are generally
very narrow ; yet with the Camellia, their minor axes
were half as long, and with the Eucalyptus more than
half as long as their major axes. In the case of Cissus,
parts of the figure more nearly represented circles than
ellipses. The amount of lateral movement is therefore
sometimes considerable. Moreover, the longer axes
of the successively formed ellipses (as with the Bean,
Cissus, and Sea-kale), and in several instances the
zigzag lines representing ellipses, were extended in
very different directions during the same day or on
Uhap. IV. CIECUMNUTATION OF LEAVES. 2G1
the next day. The course followed was curvilinear or
straight, or slightly or strongly zigzag, and little loops
or triangles were often formed. A single large irregular
ellipse may be described on one day, and two smaller
ones by the same plant on the next day. With Drosera
two, and with Lupinus, Eucalyptus and Pancratium,
several were formed each day.
The oscillatory and jerking movements of the leaves
of Dionaea, which resemble those of the hypocotyl of
the cabbage, are highly remarkable, as seen under the
microscope. They continue night and day for some
months, and are displayed by young unexpanded leaves,
and by old ones which have lost their sensibility to a
touch, but which, after absorbing animal matter, close
their lobes. We shall hereafter meet with the same
kind of movement in the joints of certain Gramineae,
and it is probably common to many plants while cir-
cumnutating. It is, therefore, a strange fact that no
such movement could be detected in the tentacles of
Drosera rotundifolia, though a member of the same
family with Dionsea ; yet the tentacle which was ob-
served was so sensitive, that it began to curl inwards
in 23 seconds after being touched by a bit of raw meat.
One of the most interesting facts with respect to
the circumnutation of leaves is the periodicity of their
movements ; for they often, or even generally, rise a
little in the evening and early part of the night, and
sink again on the following morning. Exactly the
same phenomenon was observed in the case of coty^
iedons. The leaves in 16 genera out of the 33 which
were observed behaved in this manner, as did probably
2 others. Nor must it be supposed that in the remain-
ing 15 genera there was no periodicity in their move-
ments ; for 6 of them were observed during too short
a period for any judgment to be formed on this head
262 CIECUMNUTATTON OF LEAVES. Chap. IV
and 3 were so young that their epinastic growth
which serves to bring them down into a horizontal
position, overpowered every other kind of movement
In only one genus, Cannabis, did the leaves sink in
the evening, and Kraus attributes this movement to
the prepotency of their epinastic growth. That the
periodicity is determined by the daily alternations
of light and darkness there can hardly be a doubt, as
will hereafter be shown. Insectivorous plants are
very little affected, as far as their movements are con-
cerned, by light ; and hence probably it is that their
leaves, at least in the cases of Sarracenia, Drosera, and
Diona?a, do not move periodically. The upward move-
ment in the evening is at first slow, and with different
plants begins at very different hours ; — with Glaucium
as early as 11 a.m., commonly between 3 and 5 p.m.,
but sometimes as late as 7 p.m. It should be observed
that none of the leaves described in this chapter
(except, as we believe, those of Lupinus speciosus)
possess a pulvinus; for the periodical movements of
leaves thus provided have generally been amplified
into so-called sleep-movements, with which we are not
here concerned. The fact of leaves and cotyledons
frequently, or even generally, rising a little in the
evening and sinking in the morning, is of interest as
giving the foundation from which the specialised sleep-
movements of many leaves and cotyledons, not pro-
vided with a pulvinus, have been developed. The
above periodicity should be kept in mind, by any one
considering the problem of the horizontal position of
leaves and cotyledons during the day, whilst illumi-
nated from above.
Chap. V MODIFIED OIKCUMNUTATION. 2fri
CHAPTER V.
Modified Circumnutation: Climbing Planjs ; Epinastic and
Hyponastic Movements.
Circumnntation 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 ampli-
tude ; occasional points of difference— Epinastic growth of young
leaves — Hyponastic growth of the hypoeotyls and epieotyls of seed-
lings — Hooked tips of climbing and other plants due to modified
circumuutation — Ampelopsis tricuspidata — Smithia Pfundii —
Straightening of the tip due to hyponasty — Epinastic growth and
circunmutation of the flower-peduncles of Trifolium repens and
Oxalis carnosa.
The radicles, hypocotyls and epieotyls of seedling
plants, even before they emerge from the ground, and
afterwards the cotyledons, are all continually circum-
nutating. So it is with the stems, stolons, flower-
peduncles, and leaves of older plants. We may, there-
fore, infer with a" considerable degree of safety that all
the growing parts of all plants circumnutate. Although
this movement, in its ordinary or unmodified state,
appears in some cases to be of service to plants,
either directly or indirectly — for instance, the circum-
nutation of the radicle in penetrating the ground, or
that of the arched hypocotyl and epicotyl in breaking
through the surface — yet circumnutation is so general,
or rather so universal a phenomenon, that we cannot
suppose it to have been gained for any special pur-
pose. We must believe that it follows in some un-
known way from the manner in which vegetable tissues
grow.
18
264 MODIFIED CIRCUMNUTATION. Chap. V
We shall now consider the many cases in which
3ircumnutation has been modified for various special
purposes ; that is, a movement already in progress is
temporarily increased in some one direction, and tem-
porarily diminished or quite arrested in other direc-
tions. These cases may be divided in two sub-classes ;
in one of which the modification depends on innate or
constitutional causes, and is independent of external
conditions, excepting in so far that the proper ones for
growth must be present. In the second sub-class the
modification depends to a large extent on external
agencies, such as the daily alternations of light and
darkness, or light alone, temperature, or the attraction
of gravity. The first small sub-class will be considered
in the present chapter, and the second sub-class in the
remainder of this volume.
The Circumnutation of Climbing Plants.
The simplest case of modified circumnutation is that
offered by climbing plants, with the exception of
those which climb by the aid of motionless hooks or
of rootlets : for the modification consists chiefly in the
greatly increased amplitude of the movement. This
would follow either from greatly increased growth over
a small length, or more probably from moderately in-
creased growth spread over a considerable length of the
moving organ, preceded by turgescence, and acting suc-
cessively on all sides. The circumnutation of climbers
is more regular than that of ordinary plants ; but in
almost every other respect there is a close similarity
between their movements, namely, in their tendency
to describe ellipses directed successively to all points
of the compass — in their courses being often inter-
rupted oy zigzag lines, triangles, loops, or small
Chap V. CLIMBING PLANTS. 265
ellipses — in the rate of movement, and in different
species revolving once or several times within the same
length of time. In the same internode, the move-
ments cease first in the lower part and then slowly
upwards. In both sets of cases the movement mav be
modified in a closely analogous manner by geotropism
and by heliotropism ; though few climbing plants are
jieliotropic. Other points of similarity might be
pointed out.
That the movements of climbing plants consist of
ordinary circumnutation, modified by being increased
in amplitude, is well exhibited whilst the plants are
very young ; for at this early age they move like other
seedlings, but as they grow older their movements
gradually increase without undergoing any other
change. That this power is innate, and is not excited
by any external agencies, beyond those necessary for
growth and vigour, is obvious. No one doubts that
this power has been gained for the sake of enabling
climbing plants to ascend to a height, and thus to
reach the light. This is effected by two very different
methods; first, by twining spirally round a support
but to do so their stems must be long and flexible ;
and, secondly, in the case of leaf-climbers and tendril-
bearers, by bringing these organs into contact with a
support, which is then seized by the aid of their
sensitiveness. It may be here remarked that these
latter movements have no relation, as far as we can
judge, with circumnutation. In other cases the tips
of tendrils, after having been brought into contact with
a support, become developed into little discs which
adhere firmly to it.
We have said that the circumnutation of climbing
plants differs from that of ordinary plants chiefly by
its greater amplitude. But most leaves circumnutate
266 MODIFIED CIKCUMN CITATION. Chap. V,
in an almost vertical plane, and therefore describe very
narrow ellipses, whereas the many kinds of tendrils
which consist of metamorphosed leaves, make much
broader ellipses or nearly circular figures ; and thus
they have a far better chance of catching hold of a
support on any side. The movements of climbing
plants have also been modified in some few other
special ways. Thus the circumnutating stems of Sol-
nanum dulcamara can twine round a support only
when this is as thin and flexible as a string or thread.
The twining stems of several British plants cannot
twine round a support when it is more than a few
inches in thickness ; whilst in tropical forests some
can embrace thick trunks ;* and this great difference
in power depends on some unknown difference in
their manner of circumnutation. The most remarkable
special modification of this movement which we have
observed is in the tendrils of Echlnoci/stis lobata ; these
are usually inclined at about 45° above the horizon,
but they stiffen and straighten themselves so as to
stand upright in a part of their circular course, namely,
when they approach and have to pass over the summit
of the shoot from which they arise. If they had not
possessed and exercised this curious power, chey would
infallibly have struck against the summit of the shoot
and been arrested in their course. As soon as one of
these Tendrils with its three branches begins to stiffen
itself and rise up vertically, the revolving motion
becomes more rapid ; and as soon as it has passed
over the point of difficulty, its irotion coinciding
with that from its own weight, causes it to fall into its
previously inclined position so quickly, that the apex
can be seen travelling like the hand of a gigantic clock
'The Movements and Habits of Climbing Plants,' p. 36.
iTttap. V. EPINASTY AND HYPONASTY. 267
A large number of ordinary leaves and leaflets and
a few flower-peduncles are provided with pulvini ; but
this is not the case with a single tendril at present
known. The cause of this difference probably lies in
the fact, that the chief service of a pulvinus is to
prolong the movement of the part thus provided after
growth has ceased ; and as tendrils or other climbing-
organs are of use only whilst the plant is increasing
in height or growing, a pulvinus which served to
prolong their movements would be useless.
It was shown in the last chapter that the stolons or
runners of certain plants circumnutate largely, and
that this movement apparently aids them in finding a
passage between the crowded stems of adjoining plants.
If it could be proved that their movements had been
modified and increased for this special purpose, they
ought to have been included in the present chapter ;
but as the amplitude of their revolutions is not so
conspicuously different from that of ordinary plants,
as in the case of climbers, we have no evidence on
this head. We encounter the same doubt in the case
of some plants which bury their pods in the ground.
This burying process is certainly favoured by the
circumnutation of the flower-peduncle ; but we do not
know whether it has been increased for this special
purpose.
Epinasty— Hyponasty.
The term epinasty is used by De Vries * to express
greater longitudinal growth along the upper than
* 'Arbciten des Bot. Inst., two terms as first used by Sobim-
in Wiirzburg,' Heftii. 1872, p. 223. per, and they have been' adopted
Df- Yries has slightly modified in this sente by Sacks,
(p. 252) the meaning of the above
268 MODIFIED CIRCUMNUTATION. Cuaf. V
along the lower side of a part, which is thus caused tc
bend downwards ; and hyponasty is used for the reversed
process, by which the part is made to bend upwards.
These actions come into play so frequently that the
use of the above two terms is highly convenient. The
movements thus induced result from a modified form
of circumnutation ; for, as we shall immediately see,
an organ under the influence of epinasty does not
generally move in a straight line downwards, or under
that of hyponasty upwards, but oscillates up and down
with some lateral movement : it moves, however, in a
preponderant manner in one direction. This shows
that there is some growth on all sides of the part, but
more on the upper side in the case of epinasty, and
more on the lower side in that of hyponasty, than on
the other sides. At the same time there may be in
addition, as De Yries insists, increased growth on one
side due to geotropism, and on another side due to
heliotropism ; and thus the effects of epinasty or of
hyponasty may be either increased or lessened.
He who likes, may speak of ordinary circumnutation
as being combined with epinasty, hyponasty, the effects
of gravitation, light, &c. ; but it seems to us, from
reasons hereafter to be given, to be more correct to
say that circumnutation is modified by these several
agencies. We will therefore speak of circumnutation,
which is always in progress, as modified by epinasty,
hyponasty, geotropism, or other agencies, whether
internal or external.
One of the commonest and simplest cases of epinasty is that
offered by leaves, which at an early age are crowded together
round the buds, and diverge as they grow older. Sachs first
remarked that this was due to increased growth along the upper
side of the petiole and blade ; and De Vries has now shown in
more detail that the movement is thus caused, aided slightly by
Chap. V. E PIN AST Y AND HYPONASTY. 269
tue weight of the leaf, and resisted as he believes by apogeo-
tropism, at least after the leaf has somewhat diverged. In our
observations on the circumnntation of leaves, some were selected
which were rather too young, so that they continued to diverge
or sink downwards whilst their movements were being traced.
This may be seen in the diagrams (Figs. 98 and 112, pp. 232
and 249) representing the circumnutation of the young leaves ot
Acanthus mollis and Pelargonium zonale. Similar cases were ob-
served with Drosera. The movements of a young leaf, only f inch
in length, of Petunia violacea were traced during four days, and
offers a better instance (Fig. Ill, p. 248), as it diverged during
the whole of this time in a curiously zigzag line with some of the
angles sharply acute, and during the latter days plainly circum-
nutated. Some young leaves of about the same age on a plant
of this Petunia, which had been laid horizontally, and on another
plant which was left upright, both being kept in complete dark-
ness, diverged in the same manner for 48 h., and apparently
were not affected by apogeotropism ; though their stems were in
a state of high tension, for when freed from the sticks to which
they had been tied, they instantly curled upwards.
The leaves, whilst very young, on the leading shoots of the
Carnation (Dianthus caryophyllus) are highly inclined or vertical ;
and if the plant is growing vigorously they diverge so quickly
that they become almost horizontal in a day. But they move
downwards in a rather oblique line and continue for some time
afterwards to move in the same direction, in connection, we pre-
sume, with their spiral arrangement on the stem. The course
pursued by a young leaf whilst thus obliquely descending was
traced, and the line was distinctly yet not strongly zigzag ; the
larger angles formed by the successive lines amounting only to
135°, 154°, and 163°. The subsequent lateral movement (shown
in Fig. 96, p. 231) was strongly zigzag with occasional circum-
nutations. The divergence and sinking of the young leaves
of this plant seem to be very little affected by geotropism or
heliotropism ; for a plant, the leaves of which were growing
rather slowly (as ascertained by measurement) was laid hori-
zontally, and the opposite young leaves diverged from one
another symmetrically in the usual manner, without any up-
turning in the direction of gravitation or towards the light.
The needle-like leaves of Pinus pinaster form a bundle whilst
young ; afterwards they slowly diverge, so that those on the up-
right shoots become horizontal. The movements of one such
270
MODIFIED CIRCUMNUTATIOX.
Chap. V
young leaf was traced during 4 \ days, and the tracing here given
(Fig. 121) shows that it descended at first in a nearly straight
line, but afterwards zigzagged,
Fig. 121.
Vhvj*, frhister : epinastic downward
movernent of a young leaf, pro-
duced by a young plant in a pot,
traced on a vertical glass under a
8Ky light, from 6.45 a.m. June 2nd
to' 10.40 P.M. Gth.
making one or two little loops.
The diverging and descend-
ing movements of a rather
older leaf were also traced
(see former Fig. 113, p. 251) :
it descended during the first
day and night in a some-
what zigzag line ; it then cir-
eumnutated round a small
space and again descended.
By this time the leaf had
nearly assumed its final posi-
tion, and now plainly circuni-
nutated. As in the case of the
Carnation, the leaves, whilst
very young, do not seem to be
much affected by geotropism
or heliotropism, for those on a
young plant laid horizontally,
and those on another plant
left upright, both kept in the
dark, continued to diverge in
the usual manner without
bending to either side.
With. Cuhcea scandens, the
young leaves, as they succes-
sively diverge from the lead-
ing shoot which, is bent to
one side, rise up so as to pro-
ject vertically, and they retain
this position for some time
whilst the tendril is revolving.
The diverging and ascending
movements of the petiole of
one such a leaf, were traced on
a vertical glass under a sky-
light ; and the course pursued
was in most parts nearly
straight, but there were twc
Chap. V. EPINASTY AND HYPONASTY. 271
well-marked zigzags (one of theni forming an angle of 112°) ,
and this indicates circumnutation.
The still closed lobes of a young leaf of Dionrea projected at
right angles to the petiole, and were in the act of slowly rising.
A glass filament was attached to the under side of the midrib,
and its movements were traced on a vertical glass. It circum-
nutated once in the evening, and on the next day rose, as already
described (see Fig. 106, p. 240), by a number of acutely zigzag
lines, closely approaching in character to ellipses. This move-
ment no doubt was due to epinasty, aided by apogeotropism,
for the closed lobes of a very young leaf on a plant which had
been placed horizontally, moved into nearly the same line with
the petiole, as if the plant had stood upright ; but at the same
time the lobes curved laterally upwards, and thus occupied an
unnatural position, obliquely to the plane of the foliaceous petiole.
As the hypocotyls and epicotyls of some plants protrude from
the seed-coats in an arched form, it is doubtful whether the
arching of these parts, which is invariably present when they
break through the ground, ought always to be attributed to
epinasty; but when they are at first straight and afterwards
become arched, as often happens, the arching is certainly due to
epinasty. As long as the arch is surrounded by compact earth
it must retain its form; but as soon as it rises above the
surface, or even before this period if artificially freed from the
surrounding pressure, it begins to straighten itself, and this no
doubt is mainly due to hyponasty. The movement of the
upper and lower half of the arch, and of the crown, was occa-
sionally traced ; and the course was more or less zigzag, showing
modified circumnutation.
With not a few plants, especially climbers, the summit of the
shoot is hooked, so that the apex points vertically downwards.
In seven genera of twining plants * the hooking, or as it has been
called by Sachs, the nutation of the tip, is mainly due to an
exaggerated form of circumnutation. That is, the growth is so
great along one side that it bends the shoot completely over to
the opposite side, thus forming a hook; the longitudinal line or
zone of growth then travels a little laterally round the shoot,
and the hook points in a slightly different direction, and so
onwards until the hook is completely reversed. Ultimately it
* ' The Movements and Habits of Climbing Plants,' 2nd edit. p. 13.
'SI 2 MODIFIED CIECUMNUTATION. Chap. V.
comes back to the point whence it started. This was ascertained
by painting narrow lines with Indian ink along the convex
surface of several hooks, and the line was found slowly to be-
come at first lateral, then to appear along the concave surface,
and ultimately back again on the convex surface. In the case of
Lonictra brachypoda the hooked terminal part of the revolving
shoot straightens itself periodically, but is never reversed ; that
is, the periodically increased growth of the concave side of the
hook is sufficient only to straighten it, and not to bend it over
to the opposite side. The hooking of the tip is of service to
twining plants by aiding them to catch hold of a support, and
afterwards by enabling this part to embrace the support much
more closely than it could otherwise have done at first, thus
preventing it, as we often observed, from being blown away by a
strong wind. Whether the advantage thus gained by twining
plants accounts for their summits being so frequently hooked,
we do not know, as this structure is not very rare with plants
which do not climb, and with some climbers (for instance, Vitis,
Ampelopsis, Cissus, &c.) to whom it does not afford any assist-
ance in climbing.
With respect to those cases in which the tip remains always
bent or hooked towards the same side, as in the genera just
named, the most obvious explanation is that the bending is due
to continued growth in excess along the convex side. Wiesner,
however, maintains * that in all cases the hooking of the tip is
the result of its plasticity and weight, — a conclusion which from
what we have already seen with several climbing plants is
certainly erroneous. Nevertheless, we fully admit that the
weight of the part, as well as geotropism, &c, sometimes come
into play.
Ampelopsis tricu^pidata. — This plant climbs by the aid of
adhesive tendrils, and the hooked tips of the shoots do not
appear to be of any service to it. The hooking depends chiefly,
as far as we could ascertain, on the tip being affected by epinasty
and geotropism ; the lower and older parts continually straight-
ening themselves through hyponasty and apogeotropism. We
believe that the weight of the apex is an unimportant element,
because on horizontal or inclined shoots the hook is often
extended horizontally or even faces upwards. Moreover shoots
frequently form loops instead of hooks; and in this case the
Sitzb. der k. Akad. dcr Wissensch.,' Yiennn, Jan. 1880, p. 1G.
Chap. Y.
EFINASTY AND HYPONASTY.
273
extreme- part, instead of hang-
ing vertically down as would
follow if weight was the efficient
cause, extends horizontally or
even points upwards. A shoot,
which terminated in a rather
open hook, was fastened in
a highly inclined downward
position, so that the concave
side faced upwards, and the
result was that the apex at first
curved upwards. This ap-
parently was due to epinasty
and not to apogeotropism, for
the apex, soon after passing
the perpendicular, curved so
rapidly downwards that we
could not doubt that the move-
ment was at least aided by
geotropism. In the course of
a few hours the hook was thus
converted into a loop with the
apex of the shoot pointing
straight downwards. The
longer axis of the loop was at
first horizontal, but after-
wards became vertical. During
this same time the basal part
of the hook (and subsequently
of the loop) curved itself slowly
upwards ; and this must have
been wholly due to apogeo-
tropism in opposition to hypo-
nasty. The loop was then
fastened upside down, so that
its basal half would be simul-
taneously acted on by hypo-
nasty (if present) and by apo-
geotropism; and now it curved
itself so greatly upwards in
the course of only 4h. that
there could hardly be a doubt
that both forces were acting
Fi S- I 22 - *Wt.^
/
ld°50'a«ril4 ^f^V/>«<&S^
10*.l&JX7)lJ£fr
t9j>.m.'
^fSrjS'ctm.
8Wa.ml3
4
Ampelopsis tricuspidata : hvponnstic
movement of hooked tip of leading
shoot, traced from 8.10 A.M. July
13th to 8 A.M. 15th. Apex of shoot
5g inches from the vertical glass.
Plant illuminated through a sky-
light. Temp. 17t c -19°C. Diagram
reduced to one-third of ori^ina"; scale-
274
MODIFIED CIRCUMNUTATION.
CiiAr. V.
Kg. 123. JMPIgjJ.
B'aAn.lO^
•Jl°miv.
fr&aanJS*
6'd& } cx.m.l3t?L
C?a.ml2':
a
Smithia Pfnndh: hyponastic movement
of the curved summit of astern, whilst
straightening itself, traced from 9
A.M. July 10th to 3 P.M. 13th. Apex
9£ inches from the vertical glass.
Diagram reduced to one-fifth of
original scale. Plant illuminated
through skylight ; temp. 17£°-19° C.
together. At the same time
the loop became open and
was thus reconverted into a
hook, and this apparently
was effected by the geotropic
movement of the apex in
opposition to epinasty. In
the case of Ampelopsis hede-
racea, weight plays, as far as
we could judge, a more im-
portant part in the hooking
of the tip.
In order to ascertain
whether the shoots of A. tri-
cuspiduta in straightening
themselves under the com-
bined action of hyponasty and
apogeotropism moved in a
simple straight course, or
whether they circumnutated,
glass filaments were fixed to
the crowns of four hooked
tips standing in their natural
position ; and the movements
of the filaments were traced
on a vertical glass. All four
tracings resembled each other
in a general manner ; but we
will give only one (see Fig.
122, p. 273). Tha filament
rose at first, which shows
that the hook was straighten-
ing itself; it then zigzagged,
moving a little to the left
between 9.25 a.m. and 9 p.m.
From this latter hour on the
13th to 10.50 a.m. on the fol-
lowing morning (14th) the
hook continued to straighten
itself, and then zigzagged a
short distance to the right.
But from 1 p.m. to 10.40 p.m.
on the 14th the movement
iJiiai'. V. EPINASTY AND HYPONASTY. 275
was reversed and the shoot became more hooked. During
the night, after 10.40 p.m. to 8.15 a.m. on the 15th, the hook
again opened or straightened itself. By this time the glass
filament had become so highly inclined that its movements could
no longer be traced with accuracy; and by 1.30 p.m. on this same
day, the crown of the former arch or hook had become perfectly
straight and vertical. There can therefore be no doubt that the
straightening of the hooked shoot of this plant is effected by
the circumnutation of the arched portion — that is, by growth
alternating between the upper and lower surface, but prepon-
derant on the lower surface, with some little lateral movement.
We were enabled to trace the movement of another straight-
ening shoot for a longer period (owing to its slower growth and
to its having been placed further from the vertical glass), namely,
from the early morning on July 13th to late in the evening of the
16th. During the whole daytime of the 14th, the hook straight-
ened itself very little, but zigzagged and plainly circumnutated
about nearly the same spot. By the 16th it had become nearly
straight, and the tracing was no longer accurate, yet it was
manifest that there was still a considerable amount of movement
both up and down and laterally; for the crown whilst con-
tinuing to straighten itself occasionally became for a short time
more curved, causing the filament to descend twice during the
day.
ISmithia Pfandii. — The stiff terminal shoots of this Legu-
minous water-plant from Africa project so as to make a rectangle
with the stem below ; but this occurs only when the plants are
growing vigorously, for when kept in a cool place, the summits
of the stems become straight, as they likewise did at the close
of the growing season. The direction of the rectangularly bent
part is independent of the chief source of light. But from
observing the effects of placing plants in the dark, in which
case several shoots became in two or three days upright or nearly
upright, and when brought back into the light again became
rectangularly curved, we believe that the bending is in part
due to apheliotropism, apparently somewhat opposed by apogeo-
tropism. On the other hand, from observing the effects of tying
a shoot downwards, so that the rectangle faced upwards, we are
led to believe that the curvature is partly due to epinasty. As
the rectangularly bent portion of an upright stem grows older,
the lower part straightens itself; and this is effected through
hyponasty. He who has read Sachs' recent Essay on the vertical
276 MODIFIED CIECUMNUTATION. Chap. V
and inclined positions of the parts of plants* will see how diffi-
cult a subject this is, and will feel no surprise at our expressing
ourselves doubtfully in this and other such cases.
A plant, 20 inches in height, was secured to a stick close
beneath the curved summit, which formed rather less than a
rectangle with the stem below. The shoot pointed away from the
observer ; and a glass filament pointing towards the vertical glass
on which the tracing was made, was fixed to the convex surface of
the curved portion. Therefore the descending lines in the figure
represent the straightening of the curved portion as it grew
older. The tracing (Fig. 123, p. 274) was begun at 9 a.m. on
July 10th; the filament at first moved but little in a zigzag line,
but at 2 p.m. it began rising and continued to do so till 9 p.m. ;
and this proves that the terminal portion was being more bent
downwards. After 9 p.m. on the 10th an opposite movement
commenced, and the curved portion began to straighten itself,
?nd this continued till 11.10 a.m. on the 12th, but was interrupted
by some small oscillations and zigzags, showing movement in
different directions. After 11.10 a.m. on the 12th tins part of
the stem, still considerably curved, circumnutated in a con-
spicuous manner until nearly 3 p.m. on the 13th; but during all
this time a downward movement of the filament prevailed,
caused by the continued straightening of the stem. By the
afternoon of the 13th, the summit, which had originally been
deflected more than a right angle from the perpendicular, had
grown so nearly straight that the tracing could no longer be
continued on the vertical glass. There can therefore be no
doubt that the straightening of the abruptly curved portion of
the growing stem of this plant, which appears to be wholly due
to hyponasty, is the result of modified circumnutation. "We
will only add that a filament was fixed in a different manner
across the curved summit of another plant, and the same general
kmd of movement was observed.
Trifolium reruns. — In many, but not in all the species of Tri-
folium, as the separate little flowers wither, the sub-peduncles
bend downwards, so as to depend parallel to the upper part of
the main peduncle. In Tr. subtcrraneum the main peduncle
curves downwards for the sake of burying its capsules, and in
this species the sub-peduncles of the separate flowers bend
* ' Uebei Orthotrope unci PI a- ten dee Bot. Inst., in Wiirzburg, 1
gioti-ope PilanzeutUL'ile;' 'Arbei- Heft ii. 1S79, p. 22ti.
CJtiAP. V.
EPINASTY AND HYPONASTY.
2?
Fig. 124.
>nci
8?pt.f!<
teWama*!
Trifeliwm repens : circumnu-
tating and epinastic move-
ments of the sub-peduncle
of a single flower, traced
on a vertical glass under
a skylight, in A from 11.30
A.M. Aug. 27th to 7 A.M.
30th ; in B from 7 a.m.
Aug. 30th to a little after
6 p.m. Sept. 8th.
rn8 l
278 MODIFIED CIRCUMNUTATION. CHAr. V.
upwards, so as to occupy the same position relatively to the
upper part of the main peduncle as in tr. re-pens. This fact
alone would render it probable that the movements of the sub-
peduncles in Tr. repens were independent of geotropism. Never
theless, to make sure, some flower-heads were tied to little sticks
upside down and others in a horizontal position ; their sub-
peduncles, however, all quickly curved upwards through the
action of heliotropism. We therefore protected some flower-
heads, similarly secured to sticks, from the light, and although
some of them rotted, many of their sub-peduncles turned very
slowly from their reversed or from their horizontal positions,
so as to stand in the normal manner parallel to the upper part
of the main peduncle. These facts show that the movement is
independent of geotropism or apheliotropism ; it must there-
be attributed to epinasty, which however is enecked, at least as
long as the flowers are young, by heliotropism. Most of the
above flowers were never fertilised owing to the exclusion of
bees ; they consequently withered very slowly, and the movements
of the sub-peduncles were in like manner much retarded.
To ascertain the nature of the movement of the sub-peduncle,
whilst bending downwards, a filament was fixed across the
summit of the calyx of a not fully expanded and almost upright
flower, nearly in the centre of the head. The main peduncle
was secured to a stick close beneath the head. In order to see
the marks on the glass filament, a few flowers had to be cut
away on the lower side of the head. The flower under obser-
vation at first diverged a little from its upright position, so as
to occupy the open space caused by the removal of the adjoining
flowers. This required two days, after which time a new tracing-
was begun (Fig. 124). In A we see the complex circumnutating
course pursued from 11.30 a.m. Aug. 26th to 7 a.m. on the
30th. The pot was then moved a very little to the right, and
the tracing (B) was continued without interruption from 7 a.m,
Aug. 30th to after 6 p.m. Sept. 8th. It should be observed that
on most of these days, only a single dot was made each morning
at the same hour. Whenever the flower was observed carefully,
as on Aug. 30th and Sept. 5th and 6 th, it was found to be cir-
cumnutating over a small space. At last, on Sept. 7th, it
began to bend downwards, and continued to do so until after
6 p.m. on the 8th, and indeed until the morning of the 9th, when
its movements could no longer be traced on the vertical glass.
It was carefully observed during the whole of the 8th, and by
Csap. V. EPINASTY AND HYFONASTY. 279
10.30 p.m. it had descended to a point lower down by two- thirds
of the length of the figure as here given ; but from want of space
the tracing has been copie 1 in B, only to a little after 6 p.m. On
the morning of the 9th the flower was withered, and the sub-
peduncle now stood at an angle of 57° beneath the horizon. If
the flower had been fertilised it would have withered much
sooner, and have moved much more quickly. We thus see that
the sub-peduncle oscillated up and down, or circumnutattd,
during its whole downward epinastic course.
The sub-peduncles of the fertilised and withered flowers
of Oxalis carnosa likewise bend downwards through epinasty,
as will be shown in a future chapter; and theii downward
course is strongly zigzag, indicating circumnutation.
The number of instances in which various organs
move through epinasty or hyponasty, often in com-
bination with other forces, for the most diversified
purposes, seems to be inexhaustibly great ; and from
the several cases which have been here given, we may
safely infer that such movements are due to modified
circumnutation.
19
280 MODIFIED CIRCUMNUTATION. Chap. VI
CHAPTER VI.
Modified Circumntjtation : Sleep or Nyctitropic Movements,
their Use: Sleep op Cotyledons.
Preliminary sketch of the sleep or nyctitropic movements of leaves —
Presence of pulvini — The lessening of radiation the final cause of
nyctitropic movements — Manner of trying experiments on leaves of
Oxalis, Arachis, Cassia, Melilotus, Lotus and Marsilea, and on the
cotyledons of Mimosa — Concluding remarks on radiation from leaves
— Small differences in the conditions make a great difference in the
result — Description of the nyctitropic position and movements of
the cotyledons of various plants — List of species — Concluding
remarks — Independence of the nyctitropic movements of the leaves
and cotyledons of the same species — Eeasons for believing that the
movements have been acquired for a special purpose.
The so-called sleep of leaves is so conspicuous a
phenomenon that it was observed as early as the
time of Pliny ;* and since Linnaeus published his
famous Essay, i Somnus Plantarum,' it has been the
subject of several memoirs. Many flowers close at
night, and these are likewise said to sleep ; but we
are not here concerned with their movements, for
although effected by the same mechanism as in the
case of young leaves, namely, unequal growth on the
opposite sides (as first proved by Pfeffer), yet they differ
essentially in being excited chiefly by changes of
temperature instead of light ; and in being effected, as
far as we can judge, for a different purpose. Hardly
any one supposes that there is any real analogy
* Pfeffer has given a clear and riodischen Bewegungcn der Blat-
interesting sketch of the history torgarie,' 1875, p. 1G3
Df this subject in his 'Die Pe-
Chap. VI. SLEEP MOVEMENTS. 281
between the sleep of animals and that of plants,*
whether of leaves or flowers. It seems, therefore,
advisable to give a distinct name to the so-called
sleep-movements of plants. These have also generally
been confounded, under the term " periodic," with the
slight daily rise and fall of leaves, as described in the
fourth chapter ; and this makes it all the more desir-
able to give some distinct name to sleep-movements.
.Nyctitropism and nyctitropic, i.e. night-turning, may
be applied both to leaves and flowers, and will be
occasionally used by us ; but it would be best to con-
fine the term to leaves. The leaves of some few plants
move either upwards or downwards when the sun shines
intensely on them, and this movement has sometimes
been called diurnal sleep ; but we believe it to be of
an essentially different nature from the nocturnal
movement, and it will be briefly considered in a
future chapter.
The sleep or nyctitropism of leaves is a large
subject, and we think that the most convenient plan
will be first to give a brief account of the position
which leaves assume at night, and of the advantages
apparently thus gained. Afterwards the more re-
markable cases will be described in detail, with
respect to cotyledons in the present chapter, and to
leaves in the next chapter. Finally, it will be shown
that these movements result from circumnutation,
much modified and regulated by the alternations of
day and night, or light and darkness ; but that they
are also to a certain extent inherited.
Leaves, when they go to sleep, move either upwards
or downwards, or in the case of the leaflets of coni-
* Ch. Eoyer must, however, be Nat.' (5th series), Bot. vol, ix
excepted ; see ' Annales des Sc. 1868, p. 378.
282 MODIFIED ClllCUMNUTATION. Cuap. VI
pound leaves, forwards, that is, towards the apex of the
leaf, or backwards, that is, towards its base ; or, again,
they may rotate on their own axes without moving
either upwards or downwards. But in almost every
case the plane of the blade is so placed as to stand
nearly or quite vertically at night. Therefore the apex,
or the base, or either lateral edge, may be directed
towards the zenith. Moreover, the upper surface of
each leaf, and more especially of each leaflet, is often
brought into close contact with that of the opposite
one ; and this is sometimes effected by singularly
complicated movements. This fact suggests that the
upper surface requires more protection than the lower
one. For instance, the terminal leaflet in Trifolium,
after turning up at night so as to stand vertically,
often continues to bend over until the upper surface is
directed downwards whilst the lower surface is fully
exposed to the sky ; and an arched roof is thus
formed over the two lateral leaflets, which have their
upper surfaces pressed closely together. Here we have
the unusual case of one of the leaflets not standing
vertically, or almost vertically, at night.
Considering that leaves in assuming their nycti-
tropic positions often move through an angle of
90°; that the movement is rapid in the evening;
that in some cases, as Ave shall see in the next
chapter, it is extraordinarily complicated ; that with
certain seedlings, old enough to bear true leaves,
the cotyledons move vertically upwards at night,
whilst at the same time the leaflets move ver-
tically downwards ; and that in the same genus
the leaves or cotyledons of some species move
upwards, whilst those of other species move down-
wards ; — from these and other such facts, it is hardly
possible to doubt that plants must derive some
Chap. VI SLEEP MOVEMENTS. 283
great advantage from such remarkable powers oi
movement.
The nyctitropic movements of leaves and cotyledons
are effected in two ways,* firstly, by means of pulvini
which become, as PfefTer has shown, alternately more
turgescent on opposite sides ; and secondly, by in-
creased growth along one side of the petiole or
midrib, and then on the opposite side, as was first
proved by Batalin.t But as it has been shown by
De Yries t that in these latter cases increased growth
is preceded by the increased turgescence of the cells,
the difference between the above two means of move-
ment is much diminished, and consists chiefly in the
turgescence of the cells of a fully developed pulvinus,
not being followed by growth. When the move-
ments of leaves or cotyledons, furnished with a pul-
vinus and destitute of one, are compared, they are seen
to be closely similar, and are apparently effected for
the same purpose. Therefore, with our object in view,
it does not appear advisable to separate the above two
sets of cases into two distinct classes. There is, how-
ever, one important distinction between them, namely,
that movements effected by growth on the alternate
sides, are confined to young growing leaves, whilst those
effected by means of a pulvinus last for a long time.
We have already seen well-marked instances of this
latter fact with cotyledons, and so it is with leaves, as
has been observed by PfefTer and by ourselves. The
long endurance of the nyctitropic movements when
effected by the aid of pulvini indicates, in addition tc
the evidence already advanced, the functional irnnort-
* This distinction was first Dassen in 1837.
pointed out (according to Pfeffer, t ' Flora,' 1873, p. 433.
'Die Periodischen Bewegungen X 'Bet. Zeitung/ 1879, Dec
der Blattorgane,' 1875, p. 161) by 19 th, p. 830.
284 MODIFIED CIRCUMN CITATION. Chap. VI
ance of such movements to the plant. There is another
difference between the two sets of cases, namely, that
there is never, or very rarely, any torsion of the
leaves, excepting when a pulvinus is present ; * but
this statement applies only to periodic and nyctitropic
movements, as may be inferred from other cases given
by Frank.f
The fact that the leaves of* many plants place
themselves at night in widely different positions from
what they hold during the day, but with the one
point in common, that their upper surfaces avoid
facing the zenith, often with the additional fact that
they come into close contact with opposite leaves or
leaflets, clearly indicates, as it seems to us, that the
object gained is the protection of the upper sur-
faces from being chilled at night by radiation. There
is nothing improbable in the upper surface needing
protection more than the lower, as the two differ in
function and structure. All gardeners know that
plants suffer from radiation. It is this and not
cold winds which the peasants of Southern Europe
fear for their olives.J Seedlings are often protected
from radiation by a very thin covering of straw ; and
fruit-trees on walls by a few fir-branches, or even by a
fishing-net, suspended over them. There is a variety
of the gooseberry,§ the flowers of which from being
produced before the leaves, are not protected by
them from radiation, and consequently often fail to
yield fruit. An excellent observer || has remarked
* Pfeffer, ' Die Period. Beweg. Dew/ remarks that an exposed
der Blattore;ane,' 1875, p. 159. thermometer rises as soon as even
f 'Die Nat. Wagerechte Rich- a fleecy cloud, high in the sky,
tung von Pflanzentheilen,' 1870, passes over the zenith.
p. 52. § 'Loudon's Gardener's Mag.,'
% Martins in 'Bull. Soe. Bot. vol. iv. 1828, p. 112.
de France,' torn. xix. 1872. [| Mr. Rivers in ■ Gardener's
Wells, in his famous ' Essay on Chron.,' 1866, p. 732.
Chap. VI. USE OF SLEEP MOVEMENTS. 28£
that one variety of the cherry has the petals of its
flowers much curled backwards, and after a severe
frost all the stigmas were killed ; whilst at the same
time, in another variety with incurved petals, the
stigmas were not in the least injured.
This view that the sleep of leaves saves them from
being chilled at night by radiation, would no doubt
have occurred to Linnreus, had the principle of radia-
tion been then discovered ; for he suggests in many
parts of his * Somnus Plantarum ' that the position of
the leaves at night protects the young stems and
buds, and often the young inflorescence, against cold
winds. We are far from doubting that an additional
advantage may be thus gained ; and we have observed
with several plants, for instance, Desmodium gyrans,
that whilst the blade of the leaf sinks vertically down at
night, the petiole rises, so that the blade has to move
through a greater angle in order to assume its vertical
position than would otherwise have been necessary ; but
with the result that all the leaves on the same plant
are crowded together as if for mutual protection.
We doubted at first whether radiation would affect
in any important manner objects so thin as are many
cotyledons and leaves, and more especially affect dif-
ferently their upper and lower surfaces ; for although
the temperature of their upper surfaces would un-
doubtedly fall when freely exposed to a clear sky, yet
we thought that they would so quickly acquire by
conduction the temperature of the surrounding air,
that it could hardly make any sensible difference
to them, whether they stood horizontally and radiated
into the open sky, or vertically and radiated chiefly
in a lateral direction towards neighbouring plants and
other objects. We endeavoured, therefore, to ascer-
tain something on this head by preventing the leaves
286 MODIFIED CIECUMNUTATION. Chap. VI
of several plants from going to sleep, and by exposing
to a clear sky when the temperature was beneath
the freezing-point, these, as well as the other leaves
on the same plants which had already assumed their
nocturnal vertical position. Our experiments show
that leaves thus compelled to remain horizontal at
night, suffered much more injury from frost than
those which were allowed to assume their normal
vertical position. It may, however, be said that
conclusions drawn from such observations are not
applicable to sleeping plants, the inhabitants of
countries where frosts do not occur. But in every
country, and at all seasons, leaves must be exposed to
nocturnal chills through radiation, which might be in
some degree injurious to them, and which they would
escape by assuming a vertical position.
In our experiments, leaves were prevented from
assuming their nyctitropic position, generally by
being fastened with the finest entomological pins
(which did not sensibly injure them) to thin sheets
of cork supported on sticks. But in some instances
they were fastened down by narrow strips of card,
and in others by their petioles being passed through
slits in the cork. The leaves were at first fastened
close to the cork, for as this is a bad conductor, and as
the leaves were not exposed for long periods, we thought
that the cork, w r hich had been kept in the house, would
very slightly warm them ; so that if they were injured
by the frost in a greater degree than the free vertical
leaves, the evidence would be so much the stronger
that the horizontal position was injurious. But we
found that when there was any slight difference in the
result, which could be detected only occasionally, the
leaves which had been fastened closely down suffered
rather more than those fastened with very long and
Chap. VI. USE OF SLEEP MOVEMENTS. 287
thin pins, so as to stand from J to £ inch above the
cork. This difference in the result, which is in itself
cnrious as showing what a very slight difference in
the conditions influences the amount of injury in-
flicted, may be attributed, as we believe, to the sur-
rounding warmer air not circulating freely beneath the
closely pinned leaves and thus slightly warming them.
This conclusion is supported by some analogous facts
hereafter to be given.
We will now describe in detail the experiments
which were tried. These were troublesome from our
not being able to predict how much cold the leaves of
the several species could endure. Many plants had
every leaf killed, both those which were secured in
a horizontal position and those which were allowed to
sleep — that is, to rise up or sink down vertically.
Others again had not a single leaf in the least in-
jured, and these had to be re-exposed either for a
longer time or to a lower temperature.
Oxalis acetosella. — A very large pot, thickly covered with
between 300 and 400 leaves, had been kept all winter in the
greenhouse. Seven leaves were pinned horizontally open,
and were exposed on March 16th for 2 h. to a clear sky, the
temperature on the surrounding grass being — 4° C. (24° to
25° F.). Next morning all seven leaves were found quite
killed, so were many of the free ones which had previously
gone to sleep, and about 100 of them, either dead or browned
and injured, were picked off. Some leaves showed that they
had been slightly injured by not expanding during the whole
of the next day, though they afterwards recovered. As all the
leaves which were pinned open were killed, and only about a
third or fourth of the others were either killed or injured, we
had some little evidence that those which were prevented from
assuming their vertically dependent position suffered most.
The following night (17th) was clear and almost equally cold
(— 3° to — 4° O. on the grass), and the pot was again exposed
but this time for only 30 m. Eight leaves had been pinned out,
288 MODIFIED CIKCUMNUTATION. Chap. \I
and in the morning two of them were dead, whilst not a single
other leaf on the many plants was even injured.
On the 23rd the pot was exposed for 1 h. 30 m., the tempera-
ture on the grass being only - 2° C, and not one leaf was
injured: the pinned open leaves, however, all stood from
2 to I of an inch above the cork.
On the 24th the pot was again placed on the ground and
exposed to a clear sky for between 35 m. and 40 m. By a mis-
take the thermometer was left on an adjoining sun-dial 3 feet
high, instead of being placed on the grass ; it recorded 25° to
26° F. (-33° to - 3-8° C), but when looked at after 1 h. had
fallen to 22° F. (- 5*5° C); so that the pot was perhaps exposed
to rather a lower temperature than on the two first occasions.
Eight leaves had been pinned out, some close to the cork and
some above it, and on the following morning five of them (i.e.
63 per cent.) were found killed. By counting a portion of the
leaves we estimated that about 250 bad been allowed to go to
sleep, and of these about 20 were killed (i.e. only 8 per cent.),
and about 30 injured.
Considering these cases, there can be no doubt that the
leaves of this Oxalis, when allowed to assume their normal
vertically dependent position at night, suffer much less from
frost than those (23 in number) which had their upper surfaces
exposed to the zenith.
Oxalis carnosa. — A plant of this Chilian species was exposed
for 30 m. to a clear sky, the thermometer on the grass standing
at — 2° C , with some of its leaves pinned open, and not one leaf
on the whole bushy plant was in the least injured. On the
16th of March another plant was similarly exposed for 30 m.,
when the temperature on the grass was only a little lower, viz ,
— 3° to - 4° C. Six of the leaves had been pinned open, and
next morning five of them were found much browned. The
plant was a large one, and none of the free leaves, which
were asleep and depended vertically, were browned, excepting
four very young ones. But three other leaves, though not
browned, were in a rather flaccid condition, and retained their
nocturnal position during the whole of the following day. In
this case it was obvious that the leaves which w-ere exposed hori-
zontally to the zenith suffered most. This same pot was after-
wards exposed for 35-^tO m. on a slightly colder night, and
every leaf, both the pinned open and the free ones, was killed
It may be added that two pots of O. comiculuta (var. Atro
Chap. VI. USE OF SLEEP MOVEMENTS. 289
purpurea) were exposed for 2 h. and 3 h. to a clear sky with the
temp, on grass — 2° C, and none of the leaves, whether free or
pinned open, were at all injured.
Arachis hypogcea. — Some plants in a pot were exposed at night
for 30 m. to a clear sky, the temperature on the surrounding
grass being — 2° C, and on two nights afterwards they were again
exposed to the same temperature, but this time during 1 h. 30 m.
On neither occasion was a single leaf, whether pinned open or
free, injured ; and this surprised us much, considering its native
tropical African home. Two plants were next exposed (March
16th) for 30 m. to a clear sky, the temperature of the surrounding-
grass being now lower, viz., between — 3° and — 4° C, and all
four pinned-open leaves were killed and blackened. These two
plants bore 22 other and free leaves (excluding some very young
bud-like ones) and only two of these were killed and three some-
what injured ; that is, 23 per cent, were either killed or injured,
whereas all four pinned open leaves were utterly killed.
On another night two pots with several plants were exposed
for between 35 m. and 40 m. to a clear sky, and perhaps to a
rather lower temperature, for a thermometer on a dial, 3 feet
high, close by stood at - 3'3° to - 3'8° C. In one pot three
leaves were pinned open, and all were badly injured; of the
44 free leaves, 26 were injured, that is, 59 per cent. In the
other pot 3 leaves were pinned open and all were killed; four
other leaves were prevented from sleeping by narrow strips of
stiff paper gummed across them, and all were killed ; of 24 free
leaves, 10 were killed, 2 much injured, and 12 unhurt; that is,
50 per cent, of the free leaves were either killed or much in-
jured. Taking the two pots together, we may say that rather
more than half of the free leaves, which were asleep, were either
killed or injured, whilst all the ten horizontally extended leaves,
which had been prevented from going to sleep, were either killed
or much injured.
Cassia floribunda. — A bush was exposed at night for 40 m. to
a clear sky, the temperature on the surrounding grass being
— 2° C, and not a leaf was injured.* It was again exposed on
* Cassia Ixvigata was exposed injured. But when C. laevigata
to a clear sky for 35 ra., and G. was exposed for 1 h., the temp.
calliantha (a Guiana species) for on the surrounding grass being
(30 m., the temperature on the between — 3° and — 4° C, everj
surrounding grass being — 2° C, leaf was killed.
and neither were in the least
290 MODIFIED CTRCUMNUTATION. Chap. VI.
another night for lh., when the temperature of the grass was
— 4° C. ; and now all the leaves on a large bush, whether pinned
fiat open or free, were killed, blackened, and shrivelled, with
the exception of those on one small branch, low down, which
was very slightly protected by the leaves on the branches
above. Another tall bush, with four of its large compound
leaves pinned out horizontally, was afterwards exposed (temp.
of surrounding grass exactly the same, viz., — 4° C), but only
for 30 m. On the fo'lowing morning every single leaflet on
these four leaves was dead, with both their upper and lower
surfaces completely blackened. Of the many free leaves on the
bush, only seven were blackened, and of these only a single one
(which was a younger and more tender leaf than any of the
pinned ones) had both surfaces of the leaflets blackened. The
contrast in this latter respect was well shown by a free leaf, which
stood between two pinned-open ones; for these latter had the
lower surfaces of their leaflets as black as ink, whilst the inter-
mediate free leaf, though badly injured, still retained a plain
tinge of green on the lower surface of the leaflets. This bush
exhibited in a striking manner the evil effects of the leaves not
being allowed to assume at night their normal dependent posi-
tion ; for had they all been prevented from doing so, assuredly
every single leaf on the bush would have been utterly killed by
this exposure of only 30 m. The leaves whilst sinking down-
wards in the evening twist round, so that the upper surface is
turned inwards, and is thus better protected than the outwardly
turned lower surface. Nevertheless, it was always the upper
surface which was more blackened than the lower, whenever
any difference could be perceived between them ; but whether this
was due to the cells near the upper surface being more tender,
or merely to their containing more chlorophyll, we do not know.
Mdilotus officinalis. — A large pot w r ith many plants, which
had been kept during the winter in the greenhouse, was exposed
during 5 h. at night to a slight frost and clear sky. Four
leaves had been pinned out, and these died after a few days ;
but so did many of the free leaves. Therefore nothing certain
could be inferred from this trial, though it indicated that the
Horizontally extended leaves suffered most. Another large pot
with many plants was next exposed for 1 h., the temperature on
the surrounding grass being lower, viz., — 3° to — 4° C. Ten
leaves had been pinned out, and the result was striking, for
on the following morning all these were found much injured or
Chap. VI. USE OF SLEEP MOVEMENTS. 291
killed, and none of the many free leaves on the several plants
were at all injured, with the doubtful exception of two or
three very young ones.
Melilotus Italica. — Six leaves were pinned out horizontally,
three with their upper and three with their lower surfaces turned
to the zenith. The plants were exposed for 5 h. to a clear sky,
the temperature on ground being about — 1° C. Next morning
the six pinned-open leaves seemed more injured even than the
younger and more tender free ones on the same branches. The
exposure, however, had been too long, for after an interval of
some days many of the free leaves seemed in almost as bad a
condition as the pinned-out ones. It was not possible to decide
whether the leaves with their upper or those with their lower
surfaces turned to the zenith had suffered most.
Melilotus suaveoleits. — Some plants with 8 leaves pinned out
were exposed to a clear sky during 2 h., the temperature on the
surrounding grass being — 2° C. Next morning 6 out of these
8 leaves were in a flaccid condition. There were about 150 free
leaves on the plant, and none of these were injured, except 2 or 3
very young ones. But after two days, the plants having been
brought back into the greenhouse, the 6 pinned-out leaves all
recovered.
Melilotus Taurica. — Several plants were exposed for 5 h- during
two nights to a clear sky and slight frost, accompanied by some
wind; and 5 leaves which had been pinned out suffered more
than those both above and below on the same branches which
had gone to sleep. Another pot, which had likewise been kept
in the greenhouse, was exposed for 35-40 m. to a clear sky,
the temperature of the surrounding grass being between — 3° and
— 4° C. Nine leaves had been pinned out, and all of these were
killed. On the same plants there were 210 free leaves, which
had been allowed to go to sleep, and of these about 80 were
killed, i.e. only 38 per cent.
Melilotus Fef/itjoitrnana. — The plants were exposed to a clear
sky for 35-40 m. : temperature on surrounding grass — 3° to
— 4° C. Six leaves had been pinned out so as to stand about
k inch above the cork, and four had been pinned close to it.
These 10 leaves were all killed, but the closely pinned ones
suffered most, as 4 of the 6 which stood above the cork still
retained small patches of a green colour. A considerable
number, but not nearly all, of the free leaves, were killed or
much injured, whereas all the pinned out ones were killed.
292 MODIFIED CIKCUMNUTATION. Ciiai- VI.
Melilotus macrorrJiiza. — The plants were exposed in the same
manner as in the last case. Six leaves had been pinned out
horizontally, and five of them were killed, that is, 83 per cent.
We estimated that there were 200 free leaves on the plants, and
of these about 50 were killed and 20 badly injured, so that abou!
85 per cent, of the free leaves were killed or injured.
Lotus aristata. — Six plants were exposed for nearly 5 h. to a
clear sky ; temperature on surrounding grass - 1'5° C. Four
leaves had been pinned out horizontally, and 2 of these suffered
more than those above or below on the same branches, which
had been allowed to go to sleep. It is rather a remarkable fact
that some plants of Lotus Jacohosus, an inhabitant of so hot a
country as the Cape Verde Islands, were exposed one night to a
clear sky, with the temperature of the surrounding grass — 2° C,
and on a second night for 30 m. with the temperature of
the grass between — 3° and — 4° C, and not a single leaf, either
the pinned-out or free ones, was in the least injured.
Marsilea quadrifoliata. — A large plant of this species — the
only Cryptogamic plant known to sleep— with some leaves pinned
open, was exposed for 1 h. 35 m. to a clear sky, the temperature
on the surrounding ground being — 2° C, and not a single leaf
was injured. After an interval of some days the plant was again
exposed for 1 h. to a clear sky, with the temperature on the
surrounding ground lower, viz., — 4° C. Six leaves had been
pinned out horizontally, and all of them were utterly killed.
The plant had emitted long trailing stems, and these had been
wrapped round with a blanket, so as to protect them from the
frozen ground and from radiation; but a very large number
of leaves were left freely exposed, which had gone to sleep,
and of these only 12 were killed. After another interval, th6
plant, with 9 leaves pinned out, was again exposed for 1 h., the
temperature on the ground being again — 4° C. Six of the leaves
were killed, and one which did not at first appear injured after-
wards became streaked with brown. The trailing branches, which
rested on the frozen ground, had one-half or three-quarters of their
leaves killed, but of the many other leaves on the plant, which
alone could be fairly compared with the pinned-out ones, none
appeared at first sight to have been killed, but on careful search
12 were found in this state. After another interval, the plant
with 9 leaves pinned out, was exposed for 35-40 m. to a clear
sky and to nearly the same, or perhaps a rather lower, tempera-
ture (for the thermometer by an accident had been left on a
Chap. VI. USE OF SLEEP MOVEMENTS. 293
Bun-dial close by), and. 8 of these leaves were killed. Of the free
leaves (those on the trailing branches not being considered), a
good many were killed, but their number, compared with the
uninjured ones, was small. Finally, taking the three trials
together, 24 leaves, extended horizontally, were exposed to the
zenith and to unobstructed radiation, and of these 20 were
killed and 1 injured ; whilst a relatively very small proportion
of the leaves, which had been allowed to go to sleep with their
leaflets vertically dependent, were killed or injured.
The cotyledons of several plants were prepared for trial, but
the weather was mild and we succeeded only in a single instance
in having seedlings of the proper age on nights which were
clear and cold. The cotyledons of 6 seedlings of Mimosa pudica
were fastened open on cork, and were thus exposed for 1 h. 45 m,
to a clear sky, with the temperature on the surrounding ground
at 29° F. ; of these, 3 were killed. Two other seedlings, after
their cotyledons had risen up and had closed together, were
bent over and fastened so that they stood horizontally, with the
lower surface of one cotyledon fully exposed to the zenith, and
both were killed. Therefore of the 8 seedlings thus tried 5, or
more than half, were killed. Seven other seedlings, with their
cotyledons in their normal nocturnal position, viz., vertical and
closed, were exposed at the same time, and of these only 2 were
killed.* Hence it appears, as far as these few trials tell anything,
that the vertical position at night of the cotyledons of Mimosa
pudica protects them to a certain degree from the evil effects of
radiation and cold.
Concluding Remarks on the Radiation from Leaves
at Night. — We exposed on two occasions during the
summer to a clear sky several pinned-open leaflets
of Trifolium pratense, which naturally rise at night,
and of Oxalis purpurea, which naturally sink at night
(the plants growing out of doors), and looked at
* We were surprised that It may be added that seedlings of
young seedlings of so tropical a the Indian Cassia pubescens were
plant as Mimosa pudica were able exposed fur 1 b.. 30 m. to a clear
to resist, as well as they did, ex- sky, with the temp, on the sur-
posure for ] hr. 45 m. to a clear rounding ground at — 2° C, and
fciky, the temperature on the sur- they were not in the least injured
rounding ground being 29° P.
294 MODIFIED CIBCUMNUTATIOy. Chat. VI
theni early on several successive mornings, after they
had assumed their diurnal positions. The difference
in the amount of clew on the pinned-open leaflets
and on those which had gone to sleep was generally
conspicuous ; the latter beiug sometimes absolutely
dry, whilst the leaflets which had been horizontal
were coated Avith large beads of dew. This shows how
much cooler the leaflets fully exposed to the zenith
must have become, than those which stood almost
vertically, either upwards or downwards, during the
nio-ht.
From the several cases above given, there can be no
doubt that the position of the leaves at night affects
their temperature through radiation to such a degree,
that when exposed to a clear sky during a frost, it is a
question of life and death. We may therefore admit
as highly probable, seeing that their nocturnal posi-
tion is so well adapted to lessen radiation, that the
object gained by their often complicated sleep move-
ments, is to lessen the degree to which they are
chilled at night. It should be kept in mind that
it is especially the upper surface which is thus pro-
tected, as it is never directed towards the zenith, and
is often brought into close contact with the upper
surface of an opposite leaf or leaflet.
We failed to obtain sufficient evidence, whether
the better protection of the upper surface has been
gained from its being more easily injured than the
lower surface, or from its injury being a greater evi]
to the plant. That there is some difference in consti-
tution between the two surfaces is shown by the follow-
ing cases. Cassia floribunda was exposed to a clear sky
on a sharp frosty night, and several leaflets which
had assumed their nocturnal dependent position with
their lower surfaces turned outwards so as to be
Chap. VI. USE OF SLEEP MOVEMENTS. 295
exposed obliquely to the zenith, nevertheless had these
lower surfaces less blackened than the upper surfaces
which were turned inwards and were in close contact
with those of the opposite leaflets. Again, a pot
full of plants of Trifolium resupinatum, which had
been kept in a warm room for three days, was turned
out of doors (Sept. 21st) on a clear and almost frosty
night. Next morning ten of the terminal leaflets were
examined as opaque objects under the microscope.
These leaflets, in going to sleep, either turn vertically
upwards, or more commonly bend a little over the
lateral leaflets, so that their lower surfaces are more
exposed to the zenith than their upper surfaces.
Nevertheless, six of these ten leaflets were distinctly
yellower on the upper than on the lower and more
exposed surface. In the remaining four, the result
was not so plain, but certainly whatever difference
there was leaned to the side of the upper surface
having suffered most.
It has been stated that some of the leaflets experi-
mented on were fastened close to the cork, and others
at a height of from J to f of an inch above it ; and
that whenever, after exposure to a frost, any difference
could be detected in their states, the closely pinned
ones had suffered most. We attributed this difference
to the air, not cooled by radiation, having been pre-
vented from circulating freely beneath the closely
pinned leaflets. That there was really a difference in
the temperature of leaves treated in these two dif-
ferent methods, was plainly shown on one occasion ;
for after the exposure of a pot with plants of Melilotus
dentaia for 2 h. to a clear sky (the temperature on the
surrounding grass being — 2° C), it was manifest that
more dew had congealed into hoar-frost on the closely
pinned leaflets, than on those which stood horizontally
296 MODIFIED CIKCUMNUTATION. Chap. VL
a little above the cork. Again, the tips of some few
leaflets, which had been pinned close to the cork, pro-
ectecl a little beyond the edge, so that the air could
circulate freely round them. This occurred with six
leaflets of Oi'alis acetosella, and their tips certainly
suffered rather less than the rest of the same leaflets ;
lor on the following morning they were still slightly
green. The same result followed, even still more
clearly, in two cases with leaflets of Melilotus officinalis
which projected a little beyond the cork; and in two
other cases some leaflets which were pinned close to
the cork were injured, whilst other free leaflets on
the same leaves, which had not space to rotate and
assume their proper vertical position, were not at all
injured.
Another analogous fact deserves notice : we observed
on several occasions that a greater number of free
leaves were injured on the branches which had been
kept motionless by some of their leaves having been
pinned to the corks, than on the other branches. This
was conspicuously the case with those of Melilotus
Peiitjnerreana, but the injured leaves in this instance
were not actually counted. With Aracliis lujpogma, a
young plant with 7 stems bore 22 free leaves, and of
these 5 were injured by the frost, all of which were on
two stems, bearing four leaves pinned to the cork-
supports. With Oxalis carnosa, 7 free leaves were
injured, and every one of them belonged to a cluster
of leaves, some of which had been pinned to the cork.
We could account for these cases only by supposing
that the branches which were quite free had been
slightly waved about by the wind, and that their
leaves had thus been a little warmed by the sur-
rounding warmer air. If we hold our hands motion
less before a hot fire, and then wave them about, we
Chap. VI. SLEEP OF COTYLEDONS. 297
immediately feel relief; and this is evidently an
analogous, though reversed, ease. These several facts
— in relation to leaves pinned close to or a little above
the cork-supports — to their tips projecting beyond it —
and to the leaves on branches kept motionless — seem
to us curious, as showing how a difference, apparently
trifling, may determine the greater or less injury of
the leaves. We may even infer as probable that the
less or greater destruction during a frost of the leaves
on a plant which does not sleep, may often depend on
the greater or less degree of flexibility of their petioles
and of the branches which bear them.
Nyctitropic or Sleep Movements of Cotyledons.
We now come to the descriptive part of our work,
and will begin with cotyledons, passing on to leaves
in the next chapter. We have met with only two
brief notices of cotyledons sleeping. Hofmeister,*
after stating that the cotyledons of all the observed
seedlings of the Caryophyllese (Alsineae and Silenese)
bend upwards at night (but to what angle he does not
state), remarks that those of Stellaria media rise up so
as to touch one another ; they may therefore safely be
said to sleep. Secondly, according to Ramey, f the
cotyledons of Mimosa joudica and of Cliantlius Dam-
pier i rise up almost vertically at night and approach
each other closely. It has been shown in a previous
chapter that the cotyledons of a large number of
plants bend a little upwards at night, and we here
have to meet the difficult question at what inclination
may they be said to sleep? According to the view
ahich we maintain, no movement deserves to be called
* ' Die Lehre von der Pflnnzenzelle,' 18G7, p. 327.
t * Adansoiiia' March 10th, 18G9.
k 298 MODIFIED OIRCUMNUTATION. Chap. VI
nyctitropic, unless it has been acquired for the sake of
lessening radiation ; but this could be discovered only
by a long series of experiments, showing that the
leaves of each species suffered from this cause, if pre-
vented from sleeping. We must therefore take an
arbitrary limit. If a cotyledon or leaf is inclined at
60° above or beneath the horizon, it exposes to the
zenith about one-half of its area ; consequently the
intensity of its radiation will be lessened by about
half, compared with what it would have been if the
cotyledon or leaf had remained horizontal. This
degree of diminution certainly would make a great
difference to a plant having a tender constitution.
We will therefore speak of a cotyledon and hereafter
of a leaf as sleeping, only when it rises at night to
an angle of about 60°, or to a still higher angle, above
the horizon, or sinks beneath it to the same amount.
Not but that a lesser diminution of radiation may be
advantageous to a plant, as in the case of Datura
stramonium, the cotyledons of which rose from 31° at
noon to 55° at night above the horizon. The Swedish
turnip may profit by the area of its leaves being
reduced at night by about 30 per cent., as estimated
by M r. A. S. Wilson ; though in this case the angle
through which the leaves rose was not observed. On
the other hand, when the angular rise of cotyledons or
of leaves is small, such as less than 30°, the diminution
of radiation is so slight that it probably is of no sig-
nificance to the plant in relation to radiation. For
instance, the cotyledons of Geranium Ihericum rose at
night to 27° above the horizon, and this would lessen
radiation by only 11 per cent. : those of Linum Beren-
dieri rose to 33°, and this would lessen radiation by
16 per cent.
There are, however, some other sources of doubt witD
Chap. VI. SLEEP OF COTYLEDONS. 299
respect to the sleep of cotyledons. In certain cases,
the cotyledons whilst young diverge during the day to
only a very moderate extent, so that a small rise at
night, which we know occurs with the cotyledons of
many plants, would necessarily cause them to assume
a vertical or nearly vertical position at night ; and in
this case it would be rash to infer that the movement
was effected for any special purpose. On this account
we hesitated long whether we should introduce several
Cucurbitaceous ,plants into the following list ; but from
reasons, presently to be given, we thought that they
had better be at least temporarily included. . This
same source of doubt applies in some few other cases ;
for at the commencement of our observations we did
not always attend sufficiently to whether the cotyle-
dons stood nearly horizontally in the middle of the day.
With several seedlings, the cotyledons assume a highly
inclined position at night during so short a period of
their life, that a doubt naturally arises whether this
can be of any service to the plant. Nevertheless, in
most of the cases given in the following list, the coty-
ledons may be as certainly said to sleep as may the
leaves of any plant. In two cases, namely, with the
cabbage and radish, the cotyledons of which rise almost
vertically during the few first nights of their life, it
was ascertained by placing young seedlings in the
klinostat, that the upward movement was not due to
apogeotropism.
The names of the plants, the cotyledons of which
stand at night at an angle of at least 60 D with the
horizon, are arranged in the appended list on the same
system as previously followed. The numbers of the
Families, and with the LeguminosaB the numbers of
the Tribes, have been added to show how widely
the plants in question are distributed throughout the
MODIFIED CIKCUMNUTATION.
Chap. VL
dicotvledonous series. A few remarks will have to
be made about many of the plants in the list. In
doing so, it will be convenient not to follow strictly
any systematic order, but to treat of the Oxalidae
and the Legummosae at the close ; for in these
two Families the cotyledons are generally provided
with a pulvinus, and their movements endure for a
much longer time than those of the other plants in
the list.
List of Seedling Plants, the cotyledons of which rise or sink at
night to an angle of at least 60° above or beneath the horizon.
Brassica oleracea. Cruciferse (Fam.
14).
napus (as we are informed
by Prof. Pfefter).
Eaphauus sativus. Cruciferse.
Githago segetum. Caryophylleae
(Fam. 26).
Stellaria media (according to Hof-
meister, as quoted). Caryophyl-
leae.
Anoda Wrightii. Malvaceae (Fam.
36).
Gossypium (var. Nankin cotton).
Malvaceae.
Oxalis rosea. Oxalidae (Fam. 41).
floribunda.
articulata.
Valdiviana.
sensitiva.
Geranium rotundifolium
Gera-
Legu-
niaceae (Fam. 47).
Trifolium subterraneum.
minosae (Fam. 75, Tribe 3).
strictum.
leucanthemum.
Lotus ornithopopoides. Legumi-
nosae (Tribe 4).
peregrinus.
Jacobaeus,
Clianthus Dampieri. Legumi-
nosae (Tribe 5) — according to M.
Ramey.
Smithia sensitiva. Leguminosa>
(Tribe 6).
Haematoxvlon Campechianum. Le-
guminosae (Tribe 13) — accord-
ing to Mr. K. I. Lynch.
Cassia mimosoides. Leguminosae
(Tribe 14).
glauca.
florida.
corymbosa.
pubeseens.
tora.
neglecta.
3 other Brazilian unnamed
?)•
species.
Bauhinia (sp.
(Tribe 15).
Neptunia oleracea.
(Tribe 20).
Mimosa pudica.
(Tribe 21).
albida.
Cucurbita ovifera.
Leguminosa 1
Leguminosae
Leguminosae
Cucurbitaceae
(Fam. 106).
aurantia.
Lagenaria vulgaris. Cucurbitaceae.
Cucumis dudaim. Cucurbitaceae.
Apium petroselinum. Umbelliferae
(Fam. 113).
graveolens.
Lactuca scariola. Compositae (Fam.
122).
Helianthus annuus (?). Compositae,
Ipomcea cserulea. Convoh ulacea
(Fam. 151).
purpurea.
bona-nox.
coccinea.
Chap. VI. SLEEP OF COTYLEDONS. 301
List of Seedling Plants (continued).
Mirabilis longiflora.
Beta vulgaris. Polygoneae (Fanci.
179).
Amaranthus caudatus. Amaran-
thacese (Fam. 18u).
Cannabis sativa (?). Cannabineae
(Fam. 195).
Rolanum lycopersicum. Solanese
(Fam. 157).
Mimulus, (sp. ?) Scrophularinese
(Fam. 159) — from information
given us by Prof. Pfeffer.
Mirabilis jalapa. Nyctagineaj
(Fam. 177).
Brassica olerarea (Crucifcrse). —It was shown in the first chapter
that the cotyledons of the common cabbage rise in the evening
and stand vertically np at night with their petioles in contact.
But as the two cotyledons are of unequal height, they frequently
interfere a little with each other's movements, the shorter one
often not standing quite vertically. They awake early in the
morning; thus at 6.45 a.m. on Nov. 27th, whilst it was still
dark, the cotyledons, which had been vertical and in contact on
the previous evening, were reflexed, and thus presented a very
different appearance. It should be borne in mind that seedlings
in germinating at the proper season, would not be subjected to
darkness at this hour in the morning. The above amount of
movement of the cotyledons is only temporary, lasting with plants
kept in a warm greenhouse from four to six days ; how long it
would last with seedlings growing out of doors we do not know.
Raphanus sativus.—lri the middle of the day the blades of
the cotyledons of 10 seedlings stood at right angles to their
hypocotyls, with their petioles a little divergent ; at night the
blades stood vertically, with their bases in contact and with
their petioles parallel. Next morning, at 6.45 a.m., whilst it
was still dark, the blades were horizontal. On the following-
night they were 'much raised, but hardly stood sufficiently ver-
tical to be said to be asleep, and so it was in a still less degree
on the third night. Therefore the cotyledons of this plant (kept
in the greenhouse) go to sleep for even a shorter time than
those of the cabbage. Similar observations were made, but only
during a single day and night, on 13 other seedlings likewise
raised in the greenhouse, with the same result.
The petioles of the cotyledons of 11 young seedlings of
Sinapis nigra were slightly divergent at noon, and the blades
stood at right angles to the hypocotyls ; at night the petioles
were in close contact, and the blades considerably raised,
with their bases in contact, but only a few stood sufficiently
upright to be called asleep. On the following morning,
302 MODIFIED CIKCUMNUTATION. Chat. VI.
the petioles diverged before it was light. The hypocotyl in
slightly sensitive, so that if rubbed with a needle it bends
towards the rubbed side. In the case of Lepidium sativum, the
petioles of the cotyledons of young seedlings diverge during
the day and converge so as to touch each other during the
night, by which means the bases of the tripartite blades are
brought into contact ; but the blades are so little raised that
they cannot be said to sleep. The cotyledons of several other
cruciferous plants were observed, but they did not rise sufficiently
during the night to be said to sleep.
Githago segetum (Caryophyllese). — On the first day after the
cotyledons had burst through the seed-coats, they stood at noon
at an angle of 75° above the horizon ; at night they moved
upwards, each through an angle of 15° so as to stand quite
vertical and in contact with one another. On the second day
they stood at noon at 59° above the horizon, and again at
night were completely closed, each having risen 31°. On the
fourth day the cotyledons did not quite close at night. The
first and succeeding pairs of young true leaves behaved in
exactly the same manner. We think that the movement in this
case may be called nycti tropic, though the angle passed through
was small. The cotyledons are very sensitive to light and will
not expand if exposed to an extremely dim one.
Anoda Wrightii (Malvacese). — The cotyledons whilst moderately
young, and only from -2 to *3 inch in diameter, sink in the
evening from their mid-day horizontal position to about 35°
beneath the horizon. But when the same seedlings were older
and had produced small true leaves, the almost orbicular
cotyledons, now *55 inch in diameter, moved vertically downwards
at night. This fact made us suspect that their sinking might
be due merely to their weight ; but they were not in the least
flaccid, and when lifted up sprang back through elasticity into
their former dependent position. A pot with some old seedlings
was turned upside down in the afternoon, before the noc-
turnal fall had commenced, and at night they assumed in op-
position to their own weight (and to any geotropic action) an
upwardly directed vertical position. When pots were thus
reversed, after the evening fall had already commenced, the
sinking movement appeared to be somewhat disturbed ; but all
their movements were occasionally variable without any apparent
cause. This latter fact, as well as that of the young cotyledons
not sinking nearly so much as the older ones, deserves notice.
Chai-. VI. SLEEP OF COTYLEDONS. 303
Although the movement of the cotyledons endured for a long
time, no pulvinus was exteriorly visible; but their growth
continued for a long time. The cotyledons appear to be only
slightly heliotropic, though the hypocotyl is strongly so.
Gassy pium arboi -turn (?) (var. Nankin cotton) (Malvaceae). — The
cotyledons behave in nearly the same manner as those of the
Anoda. On June 15th the cotyledons of two seedlings were
'65 inch in length (measured along the midrib) and stood hori-
zontally at noon ; at 10 p.m. they occupied the same position
and had not fallen at all. On June 23rd, the cotyledons of one
of these seedlings were 11 inch in length, and by 10 p.m. they
had fallen from a horizontal position to 62° beneath the horizon.
The cotyledons of the other seedling were 1*3 inch in length, arid
a minute true leaf had been formed; they had fallen at 10 p.m.
to 70° beneath the horizon. On June 25th, the true leaf of this
latter seedling was *9 inch in length, and the cotyledons occu-
pied nearly the same position at night. By July 9th the cotyle-
dons appeared very old and showed signs of withering; but they
stood at noon almost horizontally, and at 10 p.m. hung down
vertically.
Gossypium herbiceum. — It is remarkable that the cotyledons of
this species behave differently from those of the last. They were
observed during 6 weeks from their first development until
they had grown to a very large size (still appearing fresh and
green), viz. 2£ inches in breadth. At this age a true leaf had
been formed, which with its petiole was 2 inches long. During
the whole of these 6 weeks the cotyledons did not sink at night ;
yet when old their weight was considerable and they were borne
by much elongated petioles. Seedlings raised from some seed
sent us from Naples, behaved in the same manner ; as did those
of a kind cultivated in Alabama and of the Sea-island cotton.
To what species these three latter forms belong we do not know.
"We could not make out in the case of the Naples cotton, that
the position of the cotyledons at night was influenced by the
soil being more or less dry ; care being taken that they were
not rendered flaccid by being too dry. The weight of the large
cotyledons ot the Alabama and Sea-island kinds caused them to
hang somewhat downwards, when the pots in which they grew
were left for a time upside down. It should, however, be
observed that these three kinds were raised in the middle of
the winter, which sometimes greatly interferes with the proper
nyctitropic movements of leaves and cotyledons.
304 MODIFIED CIRCUMNUTATIOtf. Chap. VI.
Cucurbitacece. — The cotyledons of Cucurbita aurantia and ovi-
fera, and of Lagenaria vulgaris, stand from the 1st to the 3rd day
of their life at about 60° above the horizon, and at night rise up
so as to become vertical and in close contact with one another.
"With Cucumis dudaim- they stood at noon at 45° above the hori-
zon, and closed at night. The tips of the cotyledons of all these
species are, however, reflexed, so that this part is fully exposed
to the zenith at night ; and this fact is opposed to the belief
that the movement is of the same nature as that of sleeping
plants. After the first two or three days the cotyledons
diverge more during the day and cease to close at night.
Those of Trichosanthes anguina are somewhat thick and fleshy,
and did not rise at night ; and they could perhaps hardly be
expected to do so. On the other hand, those of Acanthosicyos
horrida * present nothing in their appearance opposed to their
moving at night in the same manner as the preceding species ;
yet they did not rise up in any plain manner. This fact leads
to the belief that the nocturnal movements of the above-named
species has been acquired for some special purpose, which may
be to protect the young plumule from radiation, by the close
contact of the whole basal portion of the two cotyledons.
Geranium rotundifolium (Geraniacere). — A single seedling came
up accidentally in a pot, and its cotyledons were observed to
bend perpendicularly downwards during several successive
nights, having been horizontal at noon. It grew into a fine
plant but died before flowering : it was sent to Kew and pro-
nounced to be certainly a Geranium, and in all probability the
above-named species. This case is remarkable because the
cotyledons of G. cinereurn, Endressii, Ibericum, JRicJiardsoni, and
subcaultscens were observed during some weeks in the winter,
and they did not sink, whilst those of G. Ibricum rose 27° at
night.
Apium petroselinum (Umbelliferas). — A seedling had its coty-
ledons (Nov. 22nd) almost fully expanded during the day ; by
8.30 p.m. they had risen considerably, and at 10.30 p.m. were
almost closed, their tips being only -3-^0 of an inch apart. Ou
the following morning (23rd) the tips were T 5 ^ of an inch apart,
* This plant, from Dammara climber ; it 1ms been described
Land in S. Africa, is remarkable in 'Transact. Linn. Soc.,' xxvii
from being the ono known mem- p. 30.
ber of the Family which is not a
Chap. VI. SLEEP OF COTYLEDONS. 305
or more than seven times as much. On the next night the
cotyledons occupied nearly the same position as before. On the
morning of the 24th they stood horizontally, and at night were
60° above the horizon ; and so it was on the night of the 25th.
But four days afterwards (on the 29th), when the seedlings
were a week old, the cotyledons had ceased to rise at night to
any plain degree.
Apium graveolens. — The cotyledons at noon were horizontal,
and at 10 p.m. stood at an angle of 61° above the horizon.
Lactuca scariula (Compositse). — The cotyledons whilst young
stood sub-horizontally during the day, and at night rose so as
to be almost vertical, and some were quite vertical and closed ;
but this movement ceased when they had grown old and large,
after an interval of 11 days.
Helianthus annuus (Composite). — This case is rather doubtful ;
the cotyledons rise at night, and on one occasion they stood at
73° above the horizon, so that they might then be said to have
been asleep.
Ipomcea ccerulea vel Pharhitis nil (Convolvulacese). — The coty-
ledons behave in nearly the same manner as those of the Anoda
and Nankin cotton, and like them grow to a large size. Whilst
young and small, so that their blades were from *5 to *6 of an
inch in length, measured along the middle to the base of the
central notch, they remained horizontal both during the middle
of the day and at night. As they increased in size they began
to sink more and more in the evening and early night ; and
when they had grown to a length (measured in the above
manner) of from 1 to 1*25 inch, they sank between 55° and 70°
beneath the horizon. They acted, however, in this manner only
when they had been well illuminated during the day. Never-
theless, the cotyledons have little or no power of bending
towards a lateral light, although the hypocotyl is strongly helio-
tropic. They are not provided with a pulvinus, but continue
to grow for a long time.
Ipomcea purpurea (vel Pharhitis Jiispidd). — The cotyledons
behave in all respects like those of /. ccerulea. A seedling with
cotyledons "75 inch in length (measured as before) and 1-65
inch in breadth, having a small true leaf developed, was placed
at 5.30 p.m. on a klinostat in a darkened box, so that neither
weight nor geotropism could act on them. At 10 p.m. one coty-
ledon stood at 77° and the other at 82° beneath the horizon.
Before being placed in the klinostat they stood at 15° and 29°
306 MODIFIED CIRCUMNUTATION. Chap. VL
beneath the horizeu. The nocturnal position depends chiefly
on the curvature of the petiole close to the blade, but the whole
petiole becomes slightly curved downwards. It deserves notice
that seedlings of this and the last-named species were raised at
the end of February and another lot in the middle of March,
and the cotyledons in neither case exhibited any nyctitropic
movement.
Jpomoca bona-nox. — The cotyledons after a few days grow to
an enormous size, those on a young seedling being 3± inches
in breadth. They were extended horizontally at noon, and at
10 p.m. stood at 63° beneath the horizon. Five days after-
wards they were 4| inches in breadth, and at night one stood at
64° and the other 48° beneath the horizon. Though the blades
are thin, yet from their great, size and from the petioles being
long, we imagined that their depression at night might be
determined by their weight ; but when the pot was laid hori-
zontally,, they became curved towards the hypocotyl, which
movement could not have been in the least aided by their
weight, at the same time they were somewhat twisted upwards
through apogeotropism. . Nevertheless, the weight of the coty-
ledons is so far influential, that when on another night the pot
was turned upside down, they were unable to rise and thus to
assume their proper nocturnal position.
Ipom'ta cocci nea. — The cotyledons whilst young do not sink
at night, but when grown a little older, but still only *4 inch in
length (measured as before) and '82 in breadth, they became
greatly depressed. In one case they were horizontal at noon,
and at 10 p.m. one of them stood at 64° and the other at 47°
beneath the horizon. The blades are thin, and the petioles,
which become much curved down at night, are short, so that
here weight can hardly have produced any effect. With all the
above species of Ipomcea, when the two cotyledons on the same
seedling were unequally depressed at night, this seemed to
depend on the position which they had held during the day
with reference to the light.
Solai i um lycopersicum (Solaneas). — The cotyledons rise so
much at night as to come nearly in contact. Those of S. pal in a-
car/thum were horizontal at noon, and by 10 p.m., had risen only
'27° 30' ; but on the following morning before it was light they
stood at 59° above the horizon, and in the afternoon of the same
d;iy were again horizontal. The behaviour of the cotyledons of
this latter species seems, therefore, to be anomalous.
Chap. VI. SLEEP OF COTYLEDONS. 307
Mirabilis jalapa and Ion gi flora (Nyctagineae).— The cotyledons,
which are of unequal size, stand horizontally during the middle
of the day, and at night rise up vertically and come into close
coutact with one another. But this movement with M. longijiora
lasted for only the three first nights.
Beta vulgaris (Polygoneae). — A large number of seedlings were
observed on three occasions. During the day the cotyledons
sometimes stood sub-horizontally, but more commonly at an
angle of about 50° above the horizon, and for the first two or
three nights they rose up vertically so as to be completely
closed. During the succeeding one or two nights they rose
only a Lttle, and afterwards hardly at all.
Amaranthus caudatus (Amaranthacese). — At noon the coty-
ledons of many seedlings, which had just germinated, stood at
about 45° above the horizon, and at 10.15 p.m. some were nearly
and others quite closed. On the following morning they were
again well expanded or open.
Cannabis sativa (Cannabineae). — We are very doubtful whether
this plant ought to be here included. The cotyledons of a large
number of seedlings, after being well illuminated during the
day, were curved downwards at night, so that the tips of some
pointed directly to the ground, but the basal part did not appear
to be at all depressed. On the following morning they were
again flat and horizontal. The cotyledons of many other seed-
lings were at the same time not in any way affected. Therefore
this case seems very different from that of ordinary sleep, and
probably comes under the head of epinasty, as is the case with
the leaves of this plant according to Kraus. The cotyledons are
heliotropic, and so is the hypocotyl in a still stronger degree.
Oxalis. — We now come to cotyledons provided with a pulvinus,
all of which are remarkable from the continuance of the nocturnal
movements during several days or even weeks, and apparently
after growth has ceased. The cotyledons of 0. rosea, floribunda
and articulata sink vertically down at night and clasp the upper
part of the hypocotyl. Those of 0. Vahiiviana and sensitiva, on
the contrary, rise vertically up, so that their upper surfaces come
into close contact; and after the young leaves are developed these
are clasped by the cotyledons. As in the daytime they stand hori-
zontally, or are even a little deflected beneath the horizon, they
move in the evening through an angle of at least 90°. Theii
complicated circumnutating movements during the day have
308 MODIFIED CIRCUMNUTATION. Chap. VI
been described in the first chapter. The experiment was a
superfluous one, but pots with seedlings of 0. rosea and floribunda
were turned upside down, as soon as the cotyledons began to
show any signs of sleep, and this made no difference in their
movements.
Leguminosce. — It may be seen in our list that the cotyledons
of several species in nice genera, widely distributed through-
out the Family, sleep at night ; and this probably is the case
with many others. The cotyledons of all these species are pro-
dded with a pulvinus ; and the movement in all is continued
during many days or weeks. In Cassia the cotyledons of the
ten species in the list rise up vertically at night and come
into close contact with one another. We observed that those
of G. florida opened in the morning rather later than those of
G. glauca and pubesce?is. The movement is exactly the same
in C. mimosoides as in the other species, though its subsequently
developed leaves sleep in a different manner. The cotyledons
of an eleventh species, namely, 0. nodosa, are thick and fleshy,
and do not rise up at night. The circumnutation of the coty-
ledons during the day of G. tora has been described in the first
chapter. Although the cotyledons of Smithia sensitiva rose from
a horizontal position in the middle of the day to a vertical one
at night, those of S. Pfundii, which are thick and fleshy, did not
sleep. When Mimosa pudica and albida have been kept at a
sufficiently high temperature during the day, the cotyledons
come into close contact at night ; otherwise they merely rise up
almost vertically. The circumnutation of those of M. pudica
has been described. The cotyledons of a Bauhinia from St.
Catharina in Brazil stood during the day at an angle of about
5L»° above the horizon, and at night rose to 77°; but it is pro-
bable that they would have closed completely, if the seedlings
had been kept in a warmer place.
Lotus. — In three species of Lotus the cotyledons were observed
to sleep. Those of L. Jacobceus present the singular case of not
rising at night in any conspicuous manner for the first 5 or
6 days of their life, and the pulvinus is not well developed at
this period. Afterwards the sleeping movement is well dis-
played, though to a variable degree, and is long continued.
We shall hereafter meet with a nearly parallel case with the
leaves of Sida rhombifo 7 <a. The cotyledons of L. Gebelii are
only slightly raised at night, and differ much in this respec'
from the three species in our list.
Chap VI. SLEEP OF COTYLEDONS. 309
Trifulium. — The germination of 21 species was observed. In
most of them the cotyledons rise hardly at all, .or only slightly,
at night ; but those of T. glomeratum, striatum and incm natum
rose from 45° to 55° above the horizon. With T. subterraneum,
kucanthemum and striatum, they stood up vertically ; and with
T. strictum the rising movement is accompanied, as we shall see,
by another movement, which makes us believe that the rising
is truly nyctitropio. We did not carefully examine the coty-
ledons of all the species for a pulvinus, but this organ was
distinctly present in those of T. subterraneum and strictum ; whilst
there was no trace of a pulvinus in some species, for instance, in
T. resupinatum, the cotyledons of which do not rise at night.
Trifolium subterraneum. — The blades of the cotyledons on the
first day after germination (Nov. 21st) were not fully expanded,
being inclined at about 35° above the horizon; at night they
rose to about 75°. Two days afterwards the blades at noon
were horizontal, with the petioles highly inclined upwards;
and it is remarkable that the nocturnal movement is almost
wholly confined to the blades, being effected by the pulvinus at
their bases ; whilst the petioles retain day and night nearly the
same inclination. On this night (Nov. 23rd), and for some few
succeeding nights, the blades rose from a horizontal into a
vertical position, and then became bowed inwards at about an
average angle of 10° ; so that they had passed through an angle
of 100°. Their tips now almost touched one another, their
bases being slightly divergent. The two blades thus formed
a highly inclined roof over the axis of the seedling. This
movement is the same as that of the terminal leaflet of the
tripartite leaves of many species of Trifolium. After an interval
of 8 days (Nov. 29th) the blades were horizontal during the
day, and vertical at night, and now they were no longer bowed
inwards. They continued to move in the same manner for the
following two months, by which time they had increased greatly
in size, their petioles being no less than * 8 of an inch in length,
and two true leaves had by this time been developed.
Trifulium strictum. — On the first day after germination the
cotyledons, which are provided with a pulvinus, stood at noon
horizontally, and at night rose to only about 45° above the
horizon. Four days afterwards the seedlings were again ob-
served at night, and now the blades stood vertically and were
in contact, excepting the tips, which were much deflexed, so
that they faced the zenith. At this age the petioles are curved
310
MODIFIED CIKCUMNUTATION.
Chap. VI.
upwards, and at night, when the bases of the blades are in con-
tact, the two petioles together form a vertical ring surrounding
the plumule. The cotyledons continued to act in Dearly the same
manner for 8 or 10 days from the period of germination ; but
the petioles had by this time become straight and had increased
much in length. After from 12 to 14 days the first simple true
leaf was formed, and during the ensuing fortnight a remarkable
movement was repeatedly observed. At I. (Fig. 125) we have
a sketch, made in the middle of the day, of a seedling about
a fortnight old. The two cotyledons, of which Re is the
light, and Lc the left one, stand directly opposite one another,
Trifolium strictum ; diurnal and nocturnal positions of the two cotyledons
and of the first leaf. I. Seedling viewed obliquely from above, during:
the day: Be, right cotyledon; Lc, left cotyledon; F, first true leaf.
II. A rather younger seedling, viewed at night: Be, right cotyledon
raised, but its position not otherwise changed ; Lc, left cotyledon raised
and laterally twisted; F, first leaf raised and twisted so as to face the
left twisted cotyledon. III. Same seedling viewed at night from the
opposite side. The back of the first leaf, F, is here shown instead of
the front, as in II.
and the first true leaf (F) projects at right angles to them. At
night (see II. and III.) the right cotyledon (Be) is greatly
raised, but is not otherwise changed in position. The left
cotyledon (Lc) is likewise raised, but it is also twisted, so that
its blade, instead of exactly facing the opposite one, now stands
at nearly right angles to it This nocturnal twisting movement
is effected not by means of the pulvinus, but by the twisting of
the whole length of the petiole, as could be seen by the curved
line of its upper concave surface. At the same time the true
leaf (F) rises up, so as to stand vertically, or it even passes the
vertical and is inclined a little inwards. It also twists a little,
by which means the upper surface of its blade fronts, and
almost comes into contact with, the upper surface of the twisted
Chap. VI. SLEEP OF COTYLEDONS. 311
left cotyledon. This seems to be the object gained by these
singular movements. Altogether 20 seedlings were examined on
successive nights, and in 19 of them it was the left cotyledon
alone which became twisted, with the true leaf always so twisted
that its upper surface approached closely and fronted that of the
left cotyledon. In only one instance was the right cotyledon
twisted, with the true leaf twisted towards it ; but this seedling
was in an abnormal condition, as the left cotyledon did not rise
up properly at night. This whole case is remarkable, as with
the cotyledons of no other plant have we seen any nocturnal
movement except vertically upwards or downwards. It is the
more remarkable, because we shall meet with an analogous case
in the leaves of the allied genus Melilotus, in which the ter-
minal leaflet rotates at night so as to present one edge to the
zenith and at the same time bends to one side, so that its upper
surface comes into contact with that of one of the two now ver-
tical lateral leaflets.
Concluding Bemarks on the Nycfitropic . Movements of
Cotyledons. — The sleep of cotyledons (though this is a
subject which has been little attended to), seems to be
a more common phenomenon than that of leaves. We
observed the position of the cotyledons during the day
and night in 153 genera, widely distributed through-
out the dicotyledonous series, but otherwise selected
almost by hazard ; and one or more species in 26 of
these genera placed their cotyledons at night so as
to stand vertically or almost vertically, having gene-
rally moved through an angle of at least 60°. If we
lay on one side the Leguminosee, the cotyledons of
which are particularly liable to sleep, 140 genera
remain ; and out of these, the cotyledons of at least one
species in 19 genera slept. Now if we were to select
by hazard 140 genera, excluding the Leguminosae, and
observed their leaves at night, assuredly not nearly
so many as 19 would be found to include sleeping
species. We here refer exclusively to the plants
observed by ourselves.
21
312 MODIFIED CIRCUMNTJTATION. Chap. VI
In our entire list of seedlings, there are 30 genera,
belonging to 16 Families, the cotyledons of which in
some of the species rise or sink in the evening or
early night, so as to stand at least 60° above or be-
neath the horizon. In a large majority of the genera,
namely, 24, the movement is a rising one ; so that
the same direction prevails in these nyctitropic move-
ments as in the lesser periodic ones described in the
second chapter. The cotyledons move downwards
during the early part of the night in only 6 of the
genera ; and in one of them, Cannabis, the curving
down of the tip is probably due to epinasty, as Kraus
believes to be the case with the leaves. The down-
ward movement to the amount of 90 J is very decided
in Oxalis Valdiviana and sensitive*,, and in Geranium
rotundifolium. It is a remarkable fact that with Anoda
Wriglitii, one species of Gossypium and at least 3
species of Ipomoea, the cotyledons whilst young and
light sink at night very little or not at all ; although
this movement becomes well pronounced as soon as
they have grown large and heavy. Although the
downward movement cannot be attributed to the
weight of the cotyledons in the several cases which
were investigated, namely, in those of the Anoda,
Ipomoea purpurea and bona-nox, nor in that of I coc-
cinea, yet bearing in mind that cotyledons are con-
tinually circumnutating, a slight cause might at first
have determined whether the great nocturnal move-
ment should be upwards or downwards. We may
therefore suspect that in some aboriginal member of
the groups in question, the weight of the cotyledons
first determined the downward direction. The fact of
the cotyledons of these species not sinking down much
whilst they are young and tender, seems opposed to
the belief that the greater movement when they are
Chap. VI. SLEEP OF COTYLEDONS. 313
grown older, has been acquired for the sake of pro-
tecting them from radiation at night ; but then we
should remember that there are many plants, the
leaves of which sleep, whilst the cotyledons do not ;
and if in some cases the leaves are protected from cold
at night whilst the cotyledons are not protected, so in
other cases it may be of more importance to the species
that the nearly full-grown cotyledons should be better
protected than the young ones.
In all the species of Oxalis observed by us, the coty-
ledons are provided with pulvini ; but this organ has
become more or less rudimentary in 0. corniculata,
and the amount of upward movement of its cotyledons
at night is very variable, but is never enough to be
called sleep. We omitted to ascertain whether the
cotyledons of Geranium rotundi folium possess pulvini.
In the Leguminosse all the cotyledons which sleep, as
far as we have seen, are provided with pulvini. But
with Lotus Jacobmus, these are not fully developed
during the first few days of the life of the seedling,
and the cotyledons do not then rise much at night.
With Trifolium striatum the blades of the cotyledons
rise at night by the aid of their pulvini ; whilst the
petiole of one cotyledon twists half-round at the same
time, independently of its pulvinus.
As a general rule, cotyledons which are provided
with pulvini continue to rise or sink at night during
a much longer period than those destitute of this organ.
In this latter case the movement no doubt depends on
alternately greater growth on the upper and lower side
of the petiole, or of the blade, or of both, preceded
probably by the increased turgesc'ence of the growing
cells. Such movements generally last for a very
short period — for instance, with Brassica and Githago
for 4 or 5 nights, with Beta for 2 or 3, and with
314 MODIFIED CIKCUMNUTATION. Chap VL
Raplianus for only a single night. There are, however,
some strong exceptions to this rule, as the cotyledons
of Gossypium, Anoda and Ipomoaa do not possess pul-
vini, yet continue to move and to grow for a long time.
We thought at first that when the movement lasted for
only 2 or 3 nights, it could hardly be of any service
to the plant, and hardly deserved to be called sleep ;
but as many quickly-growing leaves sleep for only a
few nights, and as cotyledons are rapidly developed
and soon complete their growth, this doubt now seems
to us not well-founded, more especially as these move-
ments are in many instances so strongly pronounced.
We may here mention another point of similarity
between sleeping leaves and cotyledons, namely, that
some of the latter (for instance, those of Cassia and
Githago) are easily affected by the absence of light ;
and they then either close, or if closed do not open ;
whereas others (as with the cotyledons of Oxalis) are
very little affected by light. In the next chapter it
will be shown that the nyctitropic movements both
of cotyledons and leaves consist of a modified form of
circumnutation.
As in the Leguniinosse and Oxalidae, the leaves and
the cotyledons of the same species generally sleep, the
idea at first naturally occurred to us, that the sleep
of the cotyledons was merely an early development of
a habit proper to a more advanced stage of life. But
no such explanation can be admitted, although there
seems to be some connection, as might have been
expected, between the two sets of cases. For the
leaves of many plants sleep, whilst their cotyledons do
not do so — of which fact Vesmodium gyrans offers a
good instance, as likewise do three species of Nico-
tiana observed by us ; also Sida rhombifolia, Abutilon
Darwinii, and C/ienopodium album. On the other
Cuai>. VI. SLEEP OF COTYLEDONS. 315
hand, the cotyledons of some plants sleep and not the
leaves, as with the species of Beta, Brassica, Geranium,
Apium, Solanum, and Mirabilis, named in our list.
Still more striking is the facfc that, in the same genus,
the leaves of several or of all the species may sleep,
but the cotyledons of only some of them, as occurs
with Trifolium, Lotus, Gossypium, and partially with
Oxalis. Again, when both the cotyledons and the
leaves of the same plant sleep, their movements may
be of a widely dissimilar nature : thus with Cassia the
cotyledons rise vertically up at night, whilst their
leaves sink down and twist round so as to turn their
lower surfaces outwards. With seedlings of Oxalis
Valdiviana, having 2 or 3 well-developed leaves, it
was a curious spectacle to behold at night each leaflet
folded inwards and hanging perpendicularly down-
wards, whilst at the same time and on the same plant
the cotyledons stood vertically upwards.
These several facts, showing the independence of
the nocturnal movements of the leaves and cotyledons
on the same plant, and on plants belonging to the
same genus, lead to the belief that the cotyledons have
acquired their power of movement for some special
purpose. Other facts lead to the same conclusion,
such as the presence of pulvini, by the aid of which
the nocturnal movement is continued during some
weeks. In Oxalis the cotyledons of some species
move vertically upwards, and of others vertically
downwards at night ; but this great difference within
the same natural genus is not so surprising as it
may at first appear, seeing that the cotyledons of all
the species are continually oscillating up and down
during the day, so that a small cause might determine
whether they should rise or sink at night. Again, the
peculiar nocturnal movement of the left-hand coty-
818 MODIFIED CIKCUMNUTATION. CiiAr VT,
ledon of Trifolium, striatum, in combination with that
of the first true leaf. Lastly, the wide distribution in
the dicotyledonous series of plants with cotyledons
which sleep. Reflecting on these several facts, our
conclusion seems justified, that the nyctitropic move-
ments of cotyledons, by which the blade is made to
stand either vertically or almost vertically upwards
or downwards at night, has been acquired, at least
in most cases, for some special purpose ; nor can w&
doubt that this purpose is the protection of the upper
surface of the blade, and perhaps of the central bud
or plumule, from radiation at night.
Chai. vil modified ciecumnutation. 317
CHAPTER VIL
MoDiriED Circtmnuta^ion : Nyctitropic or Sleep MoVEBrENlS 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 — Porlieua: leaflets close when
plant kept very dry — Tropseolum : 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 de-
veloped 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 dark-
ness — 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 circum nutation, regulated by the alternations of light
and darkness — Shape of first true leaves.
We now come to the nyctitropic or sleep move-
ments of leaves. It should be remembered that we
confine this term to leaves which place their blades
at night either in a vertical position or not more than
30° from the vertical, — that is, at least 60° above or
beneath the horizon. In some few cases this w
effected by the rotation of the blade, the petiole not
being either raised or lowered to any considerable
extent. The limit of 30° from the vertical is obviously
an arbitrary one, and has been selected for reasons
previously assigned, namely, that when the blade
approaches the perpendicular as nearly as this, ouly
half a3 much of the surface is exposed at night to the
318 MODIFIED C1RCUMNUTATIQN. Chap. Vli
zenith and to free radiation as when the blade is
horizontal. Nevertheless, in a few instances, leaves
which seem to be prevented by their structure from
moving to so great an extent as 60° above or beneath
the horizon, have been included amongst sleeping
plants.
It should be premised that the nyctitropic move-
ments of leaves are easily affected by the conditions
to which the plants have been subjected. If the ground
is kept too dry, the movements are much delayed
or fail : according to Dassen,* even if the air is
very dry the leaves of Impatiens and Malva are
rendered motionless. Carl Kraus has also lately
insisted f on the great influence which the quantity of
water absorbed has on the periodic movements of
leaves ; and he believes that this cause chiefly deter-
mines the variable amount of sinking of the leaves of
Polygonum convolvulus at night ; and if so, their move-
ments are not in our sense strictly nyctitropic. Plants
in order to sleep must have been exposed to a proper
temperature : Erythrina crista-galli, out of doors and
nailed against a wall, seemed in fairly good health,
but the leaflets did not sleep, whilst those on another
plant kept in a warm greenhouse were all vertically de-
pendent at night. In a kitchen-garden the leaflets of
Phaseolus vulgaris did not sleep during the early part
of the summer. Ch. Koyer says,:j: referring I suppose
to the native plants in France, that they do not sleep
when the temperature is below 5° C. or 41° F. In
the case of several sleeping plants, viz., species of
* Dassen, ' Tijdschrift vor. Na- Bot.' (5th series), ix. 1868, p. 345.
fcurlijke Gesch. en Physiologic,' f ' Beitriage zur Kentuiss der
1837, vol. iv. p. 106. See also Bewegungen,' &c, in * Flora,'
Ch. Royer on the importance of a 1879, pp. 42, 43, 67, &c.
proper state of turgescence of the J ' Annal. des So. Nat. Bot.'
cells, in 'Annal. det Sc. Nat. (5th Series), ix. 1868 p.H66.
Chap. ML SLEEP OF LEAVES. 319
Tropseolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia,
and probably other genera, it is indispensable that
the leaves should be well illuminated during the day
in order that they may assume at night a vertical
position ; and it was probably owing to this cause
that seedlings of Chenopodium album and Siegesbeckia
orientalis, raised by us during the middle of the winter,
though kept at a proper temperature, did not sleep.
Lastly, violent agitation by a strong wind, during a
few minutes, of the leaves of Maranta arundinacea
(which previously had not been disturbed in the hot-
house), prevented their sleeping during the two next
nights.
We will now give our observations on sleeping
plants, made in the manner described in the Intro-
duction. The stem of the plant was always secured
(when not stated to the contrary) close to the base of
the leaf, the movements of which were being observed,
so as to prevent the stem from circumnutating. As
the tracings were made on a vertical glass in front of
the plant, it was obviously impossible to trace its
course as soon as the leaf became in the evening
greatly inclined either upwards or downwards ; it
must therefore be understood that the broken lines
in the diagrams, which represent the evening and
nocturnal courses, ought always to be prolonged to a
much greater distance, either upwards or downwards,
than appears in them. The conclusions which may be
deduced from our observations will be given near the
end of this chapter.
In the following list all the genera which include
sleeping plants are given, as far as known to us. The
same arrangement is followed as in former cases, and
the number of the Family is appended. This list
possesses some interest, as it shows that the habit of
320
MODIFIED CIRCUMNUTATION.
Chat VII.
sleeping is common to some few plants throughout
the whole vascular series. The greater number of the
genera in the list have been observed by ourselves
with more or less care ; but several are given on the
authority of others (whose names are appended in the
list), and about these we have nothing more to say.
No doubt the list is very imperfect, and several genera
might have been added from the ' Somnus Plantarum'
by Linnaeus ; but we could not judge, iD some of his
cases, whether the blades occupied at night a nearly
vertical position. He refers to some plants as sleeping,
for instance, Lathyrus odoratus and Viciafaba, in which
we could observe no movement deserving to be called
sleep, and as no one can doubt the accuracy of Linnseus,
we are left in doubt.
List of Genera, including species the leaves of which sleep
Class I. DICOTYLEDONS.
Sub-class I. Angiosperms.
Family.
Caryophylleae (26;
Genus.
Githago
Stellaria (Batalin).
Portulaca (Ch."l
Royer). J
Sida.
Abutilon.
Malva (Linnseusl
and Pfeffer). J
Hibiscus (Lin-1
naeus). J
Anoda.
Gossypium.
Ayenia (Linnaeus).
Iriumfetta (Lin-l
nams). /
Linum (Batalin).
Oxalis.
Averrhoa.
Porlieria.
Guiacum.
Impatieus (Lin
na;us, Pfo Fer
Batalin)
Portulaceae (27).
Malvaceae (36).
Sterculacese (37).
Tiliaceae (38).'
Lineae (39).
Oxalidse (41).
Zygophylleae (45).
Balsaminese (48).
Sub-class I. Angiospeiuis — continued.
Genus. \ Family.
Tropaeolum. Tropaeoleae (49).
Crotolaria (Thisel-\ Leguminosae (75)
ton Dyer),
Lupinus.
Cytisus.
Trigonella.
Medicago.
Melilotus.
Trifolium.
Securigera.
Lotus.
Psoralea.
Amorpha
chartre).
Daslea.
Indigofera.
Tephrosia.
Wistaria.
Robinia.
Sphaerophysa.
Colutea.
Astragalus.
Glycyrvhiza.
Coronilla.
Hcdysarum.
v
(Du-
Tribe II.
Tr. 'ill.
55
Tr.'lV.
Tr."\ r .
Tr. VI
Chap. Vll.
SLEEP OF LEAVES.
321
List of Genera (continued).
Cl^ss 1. DICOTYLEDONS (continued).
Sub-class I. Angiospeums.
Genus.
Onobrychis.
Smithia.
Arachis.
Desmodium.
Urania.
Vicia.
Oentrosema.
Amphicarpaea.
Glycine.
Erythrina.
Apios.
Phaseolus.
Sophora.
Oaesalpinia.
Haematoxylon.
Gleditsclna (Du-
chartre).
Poinciana.
Cassia.
Bauhinia.
Tamarind up.
Adenanthera.
Prosopis.
Neptunia.
Mimosa.
Schrankia.
Acacia.
Albizzia.
Family.
Leguminosse (75)
Tr. VI.
Tr. VII.
Tr. VIII.
Sub-class I. Angiospfims (continued),
Tr. X.
Tr. XII [.
XIV.
XV.
XVI.
Tr. XX.
Tr. XXII.
Tr. XXIII.
Melaleuca(Bouche). Myrtaccae (94),
Genus.
iEnothera (Lin
nams).
PassiHora.
Siegesbeckia.
Ipomoea.
Nicotiana.
Mirabilis.
Polygonum (Ba-
talin).
Amaranthus.
Chenopodium.
Pimelia (Bouche).
Euphorbia.
Phyllanthus(Pfef-
fer). •
""I
Family.
Onagrarieae (100).
Passifloracese (105 s
Composite (122).
jConvolvulaceae
\ (151).
Solanea? (157).
Nyctagineae (177).
Polygoneac (179).
(Amaranthaceaj
\ (180).
Chenopodiese (181)
Thymeteae (188).
Euphorbiaceae (202^
Sub-class II. Gymmosperms.
Abies (Chatiu).
Class 11. MONOCOTYLEDONS.
Thalia.
Cannaceae (21).
Maranta.
J?
Colocasia.
Aroideao (30).
Strephium.
Gramineae (55).
Class III. ACOTYLEDOXS.
Marsilea. Marsileaceae (4).
Githago segetum (Caryophyllese). — The first leaves produced
by young seedlings, rise up and close together at night. On a
rather older seedlicg, two young leaves stood at noon at 55°
above the horizon, and at night at 86°, so each had risen 31°.
The angle, however, was less in some cases. Similar observations
were occasionally made on young leaves (for the older ones moved
very little) produced by nearly full-grown plants. Batalin
says (' Flora/ Oct. 1st, 1873, p. 437) that the young leaves of
Stellaria close up so completely at night that they form together
great buds.
Sida (Malvaceae). — The nyctitropic movements of the leaves
in this genus are remarkable in some respects. Batalin informs
322
MODIFIED CIKCUMNUTATION.
Chap. YII.
us (see also 'Flora/ Oct. 1st, 1873, p. 437) that those oi
S. napcea fall at night, but
Fig. 126
6"40 , a.m.20 t .
\lO'fy.m.3S*
6°d5'cf.m.30 t i\
8°S0'a.nr,.30 t ? t
~9°l£'a,m.28 e ;
tk
.39^1
Eirfa rhomhifolia : circumnu^ation and
to what angle he cannot
remember. The leaves of
S. rhomhifolia and retusa, on
the other hand, rise up
vertically, and' are pressed
against the stem. We have
therefore here within the
same genus, directly op-
posite movements. Again,
the leaves of S. rhomhifolia
are furnished with a pul-
vinus, formed of a mass of
small cells destitute of chlo-
rophyll, and with their
longer axes perpendicular
to the axis of the petiole.
As measured along this
latter line, these cells are
only 1th of the length of
those of the petiole; but
instead of being abruptly
separated from them (as is
usual with the pulvinus in
most plants), they graduate
into the larger cells of the
petiole. On the other hand,
<S. napcea, according to Ba-
talin, does not possess a
pulvinus; and he informs
us that a gradation may lie
traced in the several species
of the genus between these
nyctitropic (or sleep) movements of two states of the petiole.
!>ida rhomhifolia presents
another peculiarity, of which
we have seen no other in-
stance with leaves that
sleep: for those on very
young plants, though they
rise somewhat in the evening, do not go to sleep, as we observed
leaf on a young plant, 9J inches
high ; filament fixed to midrib of
nearly full-grown leaf, 2| inches in
length ; movement traced under a sky-
light. Apex of leaf 5| inches from
the vertical glass, so diagram not
greatly enlarged.
Chap. ML SLEEP OF LEAVES. 323
on several occasions ; whilst those on rather older plants sleep
in a conspicuous manner. For instance, a leaf ( - 85 of an inch
in length) on a very young seedling 2 inches high, stood at noon
9° above the horizon, and at 10 p.m. at 28°, so it had risen only
19°; another leaf (1'4 inch in length) on a seedling of the
same height, stood at the same two periods at 7° and 32°, and
therefore had risen 25°. These leaves, which moved so little,
had a fairly well-developed pulvinus. After an interval of some
weeks, when the same seedlings were 2g and 3 inches in height,
some of the young leaves stood up at night quite vertically, and
others were highly inclined ; and so it was with bushes which
were fully grown and were flowering.
The movement of a leaf was traced from 9.15 a.m. on
May 28th to 8.30 a.m. on the 30th. The temperature was too
low (15° — 16° C), and the illumination hardly sufficient; con-
sequently the leaves did not become quite so highly inclined at
night, as they had done previously and as they did subse-
quently in the hot-house ; but the movements did not apjoear
otherwise disturbed. On the first day the leaf sank till
5.15 p.m. ; it then rose rapidly and greatly till 10.5 p.m., and
only a little higher during the rest of the night (Fig. 126).
Early on the next day (29th) it fell in a slightly zigzag line
rapidly until 9 a.m., by which time it had reached nearly the
same place as on the previous morning. During the remainder
of the day it fell slowly, and zigzagged laterally. . The evening
rise began after 4 p.m. in the same manner as before, and on
the second morning it again fell rapidly. The ascending and
descending lines do not coincide, as may be seen in the diagram.
On the 30th a new tracing was made (not here given) on a
rather enlarged scale, as the apex of the leaf now stood 9 inches
from the vertical glass. In order to observe more carefully the
course pursued at the time when the diurnal fall changes into
the nocturnal rise, dots were made every half-hour between
4 p.m. and 10.30 p.m. This rendered the lateral zigzagging
movement during the evening more conspicuous than in the
diagram given, but it was of the same nature as there shown.
The impression forced on our minds was that the leaf was
expending superfluous movement, so that the great nocturnal
rise might not occur at too early an hour.
Abutilon Darwinii (Malvaceae). — The leaves on some very
young plants stood almost horizontally during the day, and
hung down vertically at night. Very fine jjlants kept in a
324 MODIFIED CIRCUMNUTATION. Chap. VFT.
large hall, lighted only from the roof, did not sleep at night,
for in order to do so the leaves must be well illuminated during
the day. The cotyledons do not sleep. Linnaeus says that the
leaves of his Sida abutilon sink perpendicularly down at night,
though the petioles rise. Prof. Pfeffer informs us that the
leaves of a Malva, allied to M. sylvestris, rise greatly at night;
and this genus, as well as that of Hibiscus, are included by
Linnaeus in his list of sleeping plants.
Anoda Wrightii (Malvaceae). — The leaves, produced by very
young plants, when grown to" a moderate size, sink at night
either almost vertically down or to an angle of about 45° beneath
the horizon ; for there is a considerable degree of variability in
the amount of sinking at night, which depends in part on the
degree to which they have been illuminated during the day.
But the leaves, whilst quite young, do not sink down at night,
and this is a very unusual circumstance. The summit of the
petiole, where it joins the blade, is developed into a pulvinus,
and this is present in very young leaves which do not sleep ;
though it is not so well defined as in older leaves.
Gossypium (var. Nankin cotton, Malvaceae). — Some young
leaves, between 1 and 2 inches in length, borne by two seedlings
6 and 7 \ inches in height, stood horizontally, or were raised a
little above the horizon at noon on July 8th and 9th ; but by
10 p.m. they had sunk down to between 68° and 90° beneath
the horizon. When the same plants had grown to double
the above height, their leaves stood at night almost or quite
vertically dependent. The leaves on some large plants of
67. maritimum and Brazilense, which were kept in a very badly
lighted hot-house, only occasionally sank much downwards
at night, and hardly enough to be called sleep.
Oxalis (Oxalidae).— In most of the species in this large genus
the three leaflets sink vertically down at night; but as their
sub-petioles are short the blades could not assume this position
from the want of space, unless they were in some manner ren-
dered narrower; and this is effected by their becoming more
or less folded (Fig. 127). The angle formed by the two halves
of the same leaflet was found to vary in different individuals of
several species between 92° and 150°; in three of the best
folded leaflets of 0. fragrans it was 76°, 74°, and 54°. The
angle is often different in the three leaflets of the same leaf.
As the leaflets sink down at night and become folded, their
lower surfaces are brought near together (see B), or even into
CllAV. VII.
SLEEP OF LEAVES.
325
close contact; and from this circumstance it might be thought
that the object of the folding was the protection of their lower
surfaces. If this had been the case, it would have formed
a strongly marked exception to the rule, that when there is any
difference in the degree of protection from radiation of the two
surfaces of the leaves, it is always the upper surface which is
the best protected. But that the folding of the leaflets, and
consequent mutual approximation of their lower surfaces,
serves merely to allow them to sink down vertically, may be
Fig. 127.
A. B.
Oxalis acetoseUa: A, leaf seen from vertically above; B, diagram of leaf
asliep, also seen from vertically above.
inferred from the fact that when the leaflets do not radiate
from the summit of a common petiole, or, again, when there is
plenty of room, from the sub-petioles not being very short, the
leaflets sink down without becoming folded. This occurs with
the leaflets of 0. sensitiva, Plumierii, and bupleurifolia.
There is no use in giving a long list of the many species
which sleep in the above described manner. This holds good
with species having rather fleshy leaves, like those of 0. carnosa,
or large leaves like those of 0. Or tf genii, or four leaflets like
those of 0. variabilis. There are, however, some species which
show no signs of sleep, viz., 0. peniaphylla, enneaphylla, hirta,
and rubella. We will now describe the nature of the movements
in some of the species.
Oxalis acetoseUa. — The movement of a leaflet, together with
that of the main petiole, are shown in the following dia-
gram (Fig. 128), traced between 11 a.m. on October 4th and
7.45 a.m. on the 5th. After 5.30 p.m. on the 4th the leaflet sank
rapidly, and at 7 p.m. depended vertically. Fw some time
before it assumed this latter position, its movements could, of
course, no longer be traced on the vertical glass, and the
broken line in the diagram ought to be extended much further
326
MODIFIED CIRCUMNUTATiON
Fig. 128.
JSbun A l h
Chap. VTI,
\Q a A5 y a.m.5 [
Oxalis acetosella: circumnutation and
nyctitropic movements of a nearly-
full-grown leaf, with filament at-
tached to the midrib of one of the
leaflets; traced on vertical glass dur-
ing 20 h. 45 m.
down in this and all other
cases. By 6.45 a.m. on the
following morning it had
risen considerably, and con-
tinued to rise for the next
hour; but, judging from
other observations, it would
soon have begun to fall again.
Between 11 a.m. and 5.30 p.m.
the leaflet moved at least four
times up and four times
down before the great noc-
turnal fall commenced; it
reached its highest point at
noon. Similar observations
were made on two other
leaflets, with nearly the same
results. Sachs and Pfeffer
have also described briefly*
the autonomous movements
of the leaves of this plant.
On another occasion the
petiole of a leaf was secured
to a little stick close beneath
the leaflets, and a filament
tipped with a bead of sealing-
wax was affixed to the mid-
rib of one of them, and a
mark was placed close behind.
At 7 p.m., when the leaflets
were asleep, the filament de-
pended vertically down, and
the movements of the bead
were then traced till 10.40
p.m., as shown in the fol-
lowing diagram (Fig. 129).
We here see that the leaflet
moved a little from side to
side, as well as a little up
and down, whilst asleep.
* Sachs in 'Flora,' 1863, p. 470, &c. ; Pfeffer, 'Die Period. Bewe-
Kungen,' &c, 1875, p. 53.
Chap. VII. SLEEP OF LEAVES. 327
Oxalis Vdldiviana. — The leaves resemble those of the last
species, and the movements of two leaflets (the main petioles of
both having been secured) were
traced during two days ; but the Fig- 129.
tracings are not given, as they
resembled that of 0. acetosella, with
the exception that the up and
down oscillations were not so fre-
quent during the day, and there Oxalis acetosella: circumnuta-
was more lateral movement, so that * ion °/ leaflet wh + en 1 asle «P 5
, ... -. .-, n traced on vertical glass
broader ellipses were described. during 3 h. 40 m.
The leaves awoke early in the morn-
ing, for by 6.45 a.m. on June 12th and 13th they had not only
risen to their full height, but had already begun to fall, that is,
they were circumnutating. We have seen in the last chapter
that the cotyledons, instead of sinking, rise up vertically at
night.
Oxalis Ortegesii. — The large leaves of this plant sleep like
those of the previous species. The main petioles are long, and
that of a young leaf rose 20° between noon and 10 p.m., whilst
the petiole of an older leaf rose only 13°. Owing to this rising
of the petioles, and the vertical sinking of the large leaflets,
the leaves become crowded together at night, and the whole
plant then exposes a much smaller surface to radiation than
during the day.
Oxalis Plumierii. — In this species the three leaflets do not
surround the summit of the petiole, but the terminal leaflet
projects in the line of the petiole, with a lateral leaflet on each
side. They all sleep by bending vertically downwards, but
do not become at all folded. The petiole is rather long, and,
one having been secured to a stick, the movement of tfre terminal
leaflet was traced during 45 h. on a vertical glass. It moved
in a very simple manner, sinking rapidly after 5 p.m., and
rising rapidly early next morning. During the middle of the day
it moved slowly and a little laterally. Consequently the ascend-
ing and descending lines did not coincide, and a single great
ellipse was formed each day. There was no other evidence of
circumnutation, and this fact is of interest, as we shall here-
after see.
Oxalis sensitiva. — The leaflets, as in the last species, bend
vertically down at night, without becoming folded. The much
elongated main petiole rises considerably in the evening, but in
22
32S
MODIFIED CIKCUMNTJTATION.
Chap. VII
Fig. 130.
some very young plants the rise did not commence until late
at night. We have seen that the cotyledons, instead of sink-
ing like the leaflets, rise up vertically at night.
Oxalis bupleurifolia. — This species
is rendered remarkable by the petioles
being foliaceous, like the phyllcdes
of .many Acacias. The leaflets are
small, of a paler green and more
tender consistence than the folia-
ceous petioles. The leaflet which was
observed was * 55 inch in length, and
was borne by a petiole 2 inches long
and '3 inches broad. It may be
suspected that the leaflets are on the
road to abortion or obliteration, as
has actually occurred with those of
another Brazilian species, 0. msci-
formis. Nevertheless, in the present
species the nyctitropic movements
are perfectly performed. The folia-
ceous petiole was first observed
during 48 h., and found to be in
continued circumnutation, as shown
in the accompanying figure (Fig.
130). It rose during the day and
early part of the night, and fell
during the remainder of the night
and early morning; but the move-
ment was not sufficient to be called
sleep. The ascending and descend-
ing lines did not coincide, so that an
ellipse was formed each day. There
was but little zigzagging; if the
filament had been fixed longitudi-
nally, we should probably have seen
that there was more lateral move-
ment than appears in the diagram.
A terminal leaflet on another leaf was next observed (the
petiole being secured), and its movements are shown in
Fig. 131. During the day the leaflets are extended horizon-
tally, and at night depend vertically ; and as the petiole rises
daring the day the leaflets have to bend down in the evening
Oxalis bupleurifolia: circum-
autation of foliaceous pe-
tiole, filament fixed ob-
liquely across end of petiole ;
movements traced on ver-
tical glass from 9. A.M. June
26th to 8.50 A.M. 28th.
Apex of leaflet 4£ inches
from the glass, so movement
not much magnified. Plant
9 inches high, illuminated
fmm above. Temp. 23|°-
24£° C.
Chap. VII
SLEEP OE LEAVES.
32§
more than 90°, so as to assume at night their vertical position.
On the first day the leaflet simply moved up and down ; on the
Fig. 131.
T3 &0
Sag
.e -2 to
S3 «
'a. £ rt
s 1 S
•2 -*> M
1
second day it plainly circunmutated between 8 a.m. and 4.30 p.m.
after which hour the great evening fall commenced.
330
MODIFIED CIECUMNUTATION.
Chap. VII
Averrhoa bilimhi (Oxalidse). — It has long been known,* firstly,
that the leaflets in this genus sleep ; secondly, that they move
spontaneously during the day ; and thirdly, that they are sensi-
tive to a touch; but in none cf these respects do they diffei
essentially from the species of Oxalis. They differ, however, as
Mr. E. I. Lynch f has lately shown, in their spontaneous move-
ments being strongly marked. In the case of A. Ulirribi, it is a
wonderful spectacle to behold on a warm sunny day the leaflets
one after the other sinking rapidly downw r ards, and again
ascending slowly. Their movements rival those of Desmodium
gyrans. At night the leaflets hang vertically down ; and now
Fig. 132.
Averrho2 bitimbi : loaf asleep ; drawing reduced.
they are motiouless, but this may be due to the opposite ones
being pressed together (Fig. 132). The main petiole is in con-
stant movement during the day, but no careful observations were
made on it. The following diagrams are graphic representa-
tions of the variations in the angle, which a given leaflet makes
with the vertical. The observations were made as follows.
The plant growing in a pot was kept in a high temperaturo,
the petiole of the leaf to be observed pointing straight at
the observer, being separated from him by a vertical pane of
glass. The petiole was secured so that the basal joint, or pul-
vinus, of one of the lateral leaflets was at the centre of a gradu-
ated arc placed close behind the leaflet. A fine glass filament
was fixed to the leaf, so as to project like a continuation of the
* Dr. Bruce, 'Philosophical Trans.,' 1785, p. 356.
'Journal Linn. Sue.,' vol. xvi. 1877, p. 231.
Chap. VII.
SLEEP OF LEAVES.
331
midrib. This filament acted as an index ; and as the leaf rose
a v nd fell, rotating about its basal joint, its angular movement
Fie. 133.
Averrkoa bilimbi: angular movements of a leaflet during its evening
descent, when going to sleep. Temp. 78°-81° F.
could be recorded by reading off at short intervals of time the
position of the glass filament on the graduated arc. In order
332 MODIFIED CIRCUMNUTATION. Chap. VII
to avoid errors of parallax, all readings were made by looking
through a small ring painted on the vertical glass, in a lino
with the joint of the leaflet and the centre of the graduated arc.
In the following diagrams the ordinates represent the angles
which the leaflet made with the vertical at successive instants.*
It follows that a fall in the curve represents an actual dropping
of the leaf, and that the zero line represents a vertically do-
pendent position. Fig. 133 represents the nature of the move-
ments which occur in the evening, as soon as the leaflets begin
to assume their nocturnal position. At 4.55 p m. the leaflet
formed an angle of 85° with the vertical, or was only 5° below
the horizontal ; but in order that the diagram might get into
our page, the leaflet is represented falling from 75° instead
of 85°. Shortly after 6 p m. it hung vertically down, and had
attained its nocturnal position. Between 6.10 and 6.35 p.m. it
performed a number of minute oscillations of about 2° each,
occupying periods of 4 or 5 m. The complete state of rest of
the leaflet which ultimately followed is not shown in the dia-
gram. It is manifest that each oscillation consists of a gradual
rise, followed by a sudden fall. Each time the leaflet fell, it
approached nearer to the nocturnal position than it did on the
previous fall. The amplitude of the oscillations diminished,
while the periods of oscillation became shorter.
In bright sunshine the leaflets assume a highly inclined de-
pendent position. A leaflet in diffused light was observed rising
for 25 m. A blind was then pulled up so that the plant was
brightly illuminated (BR in Fig. 134), and within a minute it
began to fall, and ultimately fell 47°, as shown in the diagram.
This descent was performed by six descending steps, precisely
similar to those by which the nocturnal fall is effected. The
pkmt was then again shaded (SH), and a long slow rise occurred
until another series of falls commenced at BR', when the sun
was again admitted. In this experiment cool air was allowed
to enter by the windows being opened at the same time that
the blinds were pulled up, so that in spite of the sun shining
on the plant the temperature was not raised.
The effect of an increase of temperature in diffused light is
* In all the diagrams 1 mm. in ment. In Figs. 133 and 134 the
the horizontal direction represents temperature is represented (along
one minute of time. Each mm. the ordinates) in the scale of 1
in the vertical direction repre- mm. to each , l°O. In Fig.
sents one degree of angular move- 135 each mm. equals 0'2° F.
Chap. VII. SLEEP OF LEAVES. 333
shown in Fig. 135. The temperature began to rise at 11.35
a.m. (in consequence of the fire being lighted), but by 12.42 a
marked fall had occurred. It may be seen in the diagram that
when the temperature was highest there were rapid oscillations
Averrhoa hiUmbi : angular movements of leaflet during a change from
bright illumination to shade ; temperature (broken line) remaining
nearly the same.
of small amplitude, the mean position of the leaflet being at the
time nearer the vertical. When the temperature began to fall,
the oscillations became slower and larger, and the mean position
of the leaf again approached the horizontal. The rate of oscil-
lation was sometimes, quicker than is represented in the above
diagram. Thus, when the temperature was between 31° rnd
334
MODIFIED CIRCUMNUTATION.
Fig. 135.
Chap. VII.
Chap. VII.
SLEEP OF LEAVES.
33i
Fig. 136.
32° C, 14 oscillations of a few degrees occurred in 19 m. On
the other hand, an oscillation may be much slower ; thus a leaflcJ
was observed (temperature 25° C.) to
rise during 40 m. before it fell and
completed its oscillation.
Porlieria hygrometrica (Zygophylleae)
— The leaves of this plant (Chilian
form) are from 1 to lj inches in length,
and bear as many as 16 or 17 small
leaflets on each side, which do not
stand opposite one another. They are
articulated to the petiole, and the
petiole to the branch by a pulvinus.
We must premise that apparently two
forms are confounded under the same
name : the leaves on a bush from Chili,
which was sent to us from Kew, bore
many leaflets, whilst those on plants
in the Botanic Garden at "Wiirzburg
bore only 8 or 9 pairs ; and the whole
character of the bushes appeared some-
what different. We shall also see that
they differ in a remarkable physio-
logical peculiarity. On the Chilian
plant the petioles of the younger leaves
on upright branches, stood horizontally
during the day, and at night sank
down vertically so as to depend parallel
and close to the branch beneath. The
petioles of rather older leaves did not
become at night vertically depressed,
but only highly inclined. In one
instance we found a branch which had Polierla hygrometrica : oir-
grown perpendicularly downwards, cumnutation and nycti-
1 ,, ,. , ., t . ,, tropic movements ot pe-
and the petioles on it moved in the same t iole of leaf, traced from
direction relatively to the branch as 9.35 a.m. July 7th to
just stated, and therefore moved up-
wards. On horizontal branches the
younger petioles likewise move at night
in the same direction as before, that is,
towards the branch, and are consequently then extended hori-
zontally; but it is remarkable that the older petioles on the
about midnight on the
8th. Apex of leaf 7 J
inches from the vertical
glass. Temp.l9£°-20j°C.
336 MODIFIED CIRCUMNUTATION. Chap. VII.
same branch, though moving a little in the same direction, also
bend downwards ; they thus occupy a somewhat different posi-
tion, relatively to the centre of the earth and to the branch, from
that of the petioles on the upright branches. With respect to
the leaflets, they move at night towards the apex of the petiole
until their midribs stand nearly parallel to it; and they then
lie neatly imbricated one over the other. Thus half of the upper
surface of each leaflet is in close contact with half of the lower
surface of the one next in advance ; and all the leaflets, except-
ing the basal ones, have the whole of their upper surfaces and
half of their lower surfaces well protected. Those on the oppo-
site sides of the same petiole do not come into close contact
at night, as occurs with the leaflets of so many Leguminosas,
but are separated by an open furrow; nor could they exactly
coincide, as they stand alternately with respect to one another.
The circumnutation of the petiole of a leaf f of an inch in
length, on an upright branch, was observed during 36 h.,
and is shown in the preceding diagram (Fig. 136). On the
first morning, the leaf fell a little and then rose until 1 p.m ,
and this was probably due to its being now illuminated through
a skylight from above; it then circumnutated on a very small
scale round the same spot until about 4 p.m., when the great
evening fall commenced. During the latter part of the night or
very early on the next morning the leaf rose again. On the
second day it fell during the morning till 1 p.m., and this no
doubt is its normal habit. From 1 to 4 p.m. it rose in a zigzag
line, and soon afterwards the great evening fall commenced. It
thus completed a double oscillation during the 2-4 h.
The specific name given to this plant by Ptuiz and Pavon, indi-
cates that in its native arid home it is affected in some manner
by the dryness or dampness of the atmosphere.* In the Botanic
Garden at Wtirzburg, there was a plant in a pot out of doors
which was daily watered, and another in the open ground which
was never watered. After some hot and dry weather there was
a great difference in the state of the leaflets on these two plants;
those on the un watered plant in the open ground remaining half,
* ' Systema Vcg. Florse Peru- about its power of foretell ng
vianse et Cliilensis,' torn. i. p. 95, changes in the weather; and it
1708. We cannot understand the appe.irs as if the brightness of the
account given by the authors of sky largely determined the opeu
the behuviuu- of this plant in its ing and closing of the leaflets,
native home There is much
Ch^. VII. SLEEP OF LEAVES. 337
or even quite, closed during the day. But twigs cut from tins
bush, with their ends standing in water, or wholly immersed in
it, or kept in damp air under a bell-glass, opened their leaves
though exposed to a blazing sun; whilst those on the plant
in the ground remained closed. The leaves on this same plant,
after some heavy rain, remained open for two days; they then
became half closed duriDg two days, and after an additional
day were quite closed. This plant was now copiously wateied,
and on the following morning the leaflets were fully ex-
panded. The other plant growing in a pot, after having been
exposed to heavy rain, was placed before a window in the Labo-
ratory, with its leaflets open, and they remained so during the
daytime for 48 h. ; but after an additional day were half closed.
The plant was then watered, and the leaflets on the two following
days remained open. On the third day they were again half
closed, but on being again watered remained open during the
two next days. From these several facts we may conclude that
the plant soon feels the want of water ; and that as soon as this
occurs, it partially or quite closes its leaflets, which in their
then imbricated condition expose a small surface to evaporation.
It is therefore probable that this sleep-like movement, which
occurs only when the ground is dry, is an adaptation against
the loss of moisture.
A bush about 4 feet in height, a native of Chili, which was
thickly covered with leaves, behaved very differently, for during
the day it never closed its leaflets. On July 6th the earth ir.
the small pot in which it grew appeared extremely dry, and
it was given a very little water. After '21 and 22 days (on
the 27th and 28th), during the whole of which time the plant
did not receive a drop of water, the leaves began to droop, but
they showed no signs of closing during the day. It appeared
almost incredible that any plant, except a fleshy one, could
have kept alive in soil so dry, which resembled the dust on
a road. On the 29th, when the bush was shaken, some leaves
fell off, and the remaining ones were unable to sleep at night.
It was therefore moderately watered, as well as syringed, late in
the evening. On the next morning (30th) the bush looked as fresh
as ever, and at night the leaves went to sleep. It may be added
that a small branch while growing on the bush was enclosed,
by means of a curtain of bladder, during 13 days in a large
bottle half full of quicklime, so that the air within must have been
intensely dry ; yet the leaves on this branch did not suffer in the
338 MODIFIED CIKCUMNUTATION. Chap. V1J
least, and did not close at all during the hottest days. Anothei
trial was made with the same bush on August 2nd and 6th (the soil
appearing at this latter date extremely dry), for it was exposed
out of doors during the whole day to the wind, but the leaflets
showed no signs of closing. The Chilian form therefore differs
widely from the one at Wiirzburg, in not closing its leaflets
when suffering from the want of water ; and it can live for a
surprisingly long time without water.
Tropceolum majus (?) (cultivated var.) (Tropasoleae). — Several
plants in pots stood in the greenhouse, and the blades of
the leaves which faced the front-lights were during the day
highly inclined and at night vertical ; whilst the leaves on
the back of the pots, though of course illuminated through
the roof, did not become vertical at night. We thought, at first,
that this dfference in their positions was in some manner
due to heliotropism, for the leaves are highly heliotropic. The
true explanation, however, is that unless they are well illu-
minated during at least a part of the .day they do not sleep at
night; and a little difference in the degree of illumination deter-
mines whether or not they shall become vertical at night. We
have observed no other so well-marked a case as this, of the
influence of previous illumination on nyctitropic movements.
The leaves present also another peculiarity in their habit of
rising or awaking in the morning, being more strongly fixed or
iuherited than that of sinking or sleeping at night. The move-
ments are caused by the bending of an upper part of the petiole,
between £ and 1 inch in length ; but the part close to the blade,
for about £ of an inch in length, does not bend and always
remains at right angles to the blade. The bending portion does
not present any external or internal difference in structure
from the rest of the petiole. We will now give the experiments
on which the above conclusions are founded.
A large pot with several plants was brought on the morning
of Sept. 3rd out of the greenhouse and placed before a north-east
window, in the same position as before with respect to the light,
as far as that was possible. On the front of the plants, 24 leaves
were marked with thread, some of which had their blades hori-
zontal, but the greater number were inclined at about 45°,
beneath the horizon; at night all these, without exception,
became vertical. Early on the following morning (4th) they
reassumed their former positions, and at night again became
vertical. On the 5th the shutters were opened at 6.15 a.m., and
Chap. VII. SLEEP OF LEAVES. 339
by 8.18 a.m., after the leaves had been illuminated for 2 h. 3 m.,
and had acquired their diurnal position, they were placed in a
dark cupboard. They were looked at twice during the day and
thrice in the evening, the last time at 10 30 p.m., and not one had
become vertical. At 8 a.m. on the following morning (6th) they
still retained the same diurnal position, and were now replaced
before the north-east window. At night all the leaves which
had faced the light had their petioles curved and their blades
vertical ; whereas none of the leaves on the back of the plants,
although they had been moderately illuminated by the diffused
light of the room, were vertical. They were now at night placed
in the same dark cupboard ; at 9 a.m. on the next morning (7th)
all those which had been asleep had reassumed their diurnal
position. The pot was then placed for 3 h. in the sunshine, so
as to stimulate the plants ; at noon they were placed before the
same north-east window, and at night the leaves slept in the
usual manner and awoke on the following morning. At noon on
this day (8th) the plants, after having been left before the north-
east window for 5 h. 45 m. and thus illuminated (though not
brightly, as the sky was cloudy during the whole time), were
replaced in the dark cupboard, and at 3 p. m. the position of the
leaves was very little, if at all, altered, so that they are not
quickly affected by darkness ; but by 10.15 p.m. all the leaves
which had faced the north-east sky during the 5h. 45 m. of
illumination stood vertical, whereas those on the back of the
plant retained their diurnal position. On the following morning
(9th) the leaves awoke as on the two former occasions in the dark,
and they were kept in the dark during the whole day ; at night
a very few of them became vertical, and this was the one in-
stance in which we observed any inherited tendency or habit in
this plant to sleep at the proper time. That it was real sleep
was shown by these same leaves reassuming their diurnal posi-
tion on the following morning (10th) whilst still kept in the
dark.
The pot was then (9.45 a.m. 10th) replaced, after having been
kept for 36 h. in darkness, before the north-east window ; and at
night the blades of all the leaves (excepting a few on the back of
the plants) became conspicuously vertical.
At 6.45 a.m. (11th) after the plants had been illuminated on the
same side as before during only 25 m., the pot was turned round,
so that the leaves which had faced the light now faced the
interior of the room, and not one. of these went to sleep at night;
BIO MODIFIED CIKCUMXUTATIOK. Chap. VII.
whilst some, but not many, of those which had formerly stood
facing the back of the room and which had never before been
well illuminated or gone to sleep, now assumed a vertical posi-
tion at night. On the next day (12th) the plant was turned
round into its original position, so that the saTne leaves faced
the light as formerly, and these now went to sleep in the usual
manner. We will only add that with some young seedlings
kept in the greenhouse, the blades of the first pair of true leaves
(the cotyledons being hypogean) stood during the day almost
horizontally and at night almost vertically.
A few observations were subsequently made on the circum-
nutation of three leaves, whilst facing a north-east window ; but
the tracings are not given, as the leaves moved somewhat
towards the light. It was, however, manifest that they rose
and fell more than once during the daytime, the ascending and
descending lines being in parts extremely zigzag. The nocturnal
fall commenced about 7 p.m., and the leaves had risen consider-
ably by 6.45 a.m. on the following morning.
Leguminosce. — This Family includes many more genera with
sleeping species than all the other families put together. The
number of the tribes to which each genus belongs, according to
Bentham and Hooker's arrangement, has been added.
Crotolaria (sp. ?) (Tribe 2). — This plant is monophyllous, and
we are informed by Mr. T. Thiselton Dyer that the leaves rise
up vertically at night and press against the stem.
Lupinus (Tribe 2). — The palmate or digitate leaves of the
species in this large genus sleep in three different manners.
One of the simplest, is that all the leaflets become steeply in-
clined downwards at night, having been during the day ex-
tended horizontally. This is shown in the accompanying
figures (Fig. 137), of a leaf of L. pilosus, as seen during the
day from vertically above, and of another leaf asleep with the
leaflets inclined downwards. As in this position they are
crowded together, and as they do not become folded like those
in the genus Oxalis, they cannot occupy a vertically dependent
position ; but they are often inclined at an angle of 50° beneath
the horizon. In this species, whilst the leaflets are sinking,
the petioles rise up, in two instances when the angles were
measured to the extent of 23°. The leaflets of L. sub-camosus and
arboreus, which were horizontal during the day, sank down at
night in nearly the same manner ; the former to an angle of 38°,
and the latter of 36°, beneath the horizon; but their peticles
Chap. VII. SLEEP OF LEAVES. 341
did not move in any plainly perceptible degree. It is, however,
quite possible, as we sliall presently see, that if a large number
of plants of the three foregoing and of the following species
Fig. 137.
A.
Lupinus pilosus : A, leaf seen from vertically above in daytime; B, leaf
asleep, seen laterally at night.
were to be observed at all seasons, some of the leaves would be
found to sleep in a different manner.
In the two following species the leaflets, instead of moving
downwards, rise at night. With L. Eartwegii some stood at
noon at a mean angle of 36° above the horizon, and at night
at 51°, thus forming together a hollow cone with moderately
steep sides. The petiole of one leaf rose 14° and of a second
11° at night. "With L. luteus a leaflet rose from 47° at noon to
65° above the horizon at night, and another on a distinct leaf
rose from 45° to 69°. The petioles, however, sink at night to
a small extent, viz., in three instances by 2°, 6°, and 9° 30'.
Owing to this movement of the petioles, the outer and longer
leaflets have to bend up a little more than the shorter and inner
ones, in order that all should stand symmetrically at night.
We shall presently see that some leaves on the same individual
plants of L. luteus sleep in a very different manner.
We now come to a remarkable position of the leaves
when asleep, which is common to several species of Lupines.
On the same leaf the shorter leaflets, which generally face the
centre of the plant, sink at night, whilst the longer ones
on the opposite side rise; the intermediate and lateral ones
merely twisting on their own axes. But there is some variability
with respect to which leaflets rise or fall. As might have been
expected from such diverse and complicated movements, the
342
MODIFIED C1KCUMNUTATION.
Chap. VI t .
base of each leaflet is developed (at least in the case of L. luteus)
into a pulvimis. The result is that all the leaflets on the
same leaf stand at night more or less highly inclined, or even
quite vertically, forming in this latter case a vertical star. This
occurs with the leaves of a species purchased under the name of
Fig. 138
Lupinns pubescens: A, leaf viewed laterally during the day; B, same leaf
at night ; C, another leaf with the leaflet forming a vertical star at
night. Figures reduced.
//. pubescens ; and in the accompanying figures we see at A (Fig.
138) the leaves in their diurnal position; and at B the same
plant at night with the two npper leaves having their leaflets
almost vertical. At C another leaf, viewed laterally, is shown
with the leaflets quite vertical. It is chiefly or exclusively the
youngest leaves which form at night vertical stars. But there
Chap VII. SLEEP OF LEAVES. 343
is much variability in the position of the leaves at night on the
same plant ; some remaining with their leaflets almost horizontal,
others forming more or less highly inclined or vertical stars, and
some with all their leaflets sloping downwards, as in our first
class of cases. It is also a remarkable fact, that although all the
plants produced from the same lot of seeds were identical m
appearance, yet some individuals at night had the leaflets of all
their leaves arranged so as to form more or less highly inclined
stars ; others had them all sloping downwards and never forming
a star ; and others, again, retained them either in a horizontal
position or raised them a little.
We have as yet referred only to the different positions of tne
leaflets of L. pubescens at night ; but the petioles likewise differ
in their movements. That of a young leaf which formed a
highly inclined star at night, stood at noon at 42° above the
horizon, and during the night at 72°, so had risen 30°. The
petiole of another leaf, the leaflets of which occupied a similar
position at night, rose only 6°. On the other hand, the petiole
of a leaf with all its leaflets sloping down at night, fell at this
time 4°. The petioles of two rather older leaves were subse-
quently observed ; both of which stood during the day at exactly
the same angle, viz., 50° above the horizon, and one of these rose
7°-8°, and the other fell 3°— 4° at night.
We meet with cases like that of L. pubescens with some other
species. On a single plant of L. mutabilis some leaves, which
stood horizontally during the day, formed highly inclined stars
at night, and the petiole of one rose 7°. Other leaves which
likewise stood horizontally during the day, had at night all theii
leaflets sloping downwards at 46° beneath the horizon, but
their petioles had hardly moved. Again, L. hit us offered a still
more remarkable case, for on two leaves, the leaflets which stood
at noon at about 45° above the horizon, rose at night to 65° and
69°, so that they formed a hollow cone with steep sides. Four
leaves on the same plant, which had their leaflets horizontal at
noon, formed vertical stars at night; and three other leaves
equally horizontal at noon, had all their leaflets sloping down-
wards at night. So that the leaves on this one plant assumed
at night three different positions. Though we cannot account
for this fact, we can see that such a stock might readily give
birth to species having widely different nyctitropic habits.
Little more need be said about the sleep of the species of Lu-
pinus; several, namely, L. polyphyllus, nanus, Menziesii, speciosus t
23
344
MODIFIED CIRoUMtfUTATlON.
Chap. VII
and albifrons, though observed out of doors and in the green-
house, did not change the position of their leaves sufficiently at
night to be said to sleep. • From observations made on two
sleeping species, it appears that, as with Tropceolum majus, the
leaves must be well illuminated during the day in order to p!eep
at night. For several plants, kept all day in a sitting-room
with north-east windows, did not sleep at night ; but when the
pots were placed on the following day out of doors, and were
brought in at night, they slept in the usual manner. The trial
was repeated on the following day and night with the same
result.
Some observations were made on the circumnutation of the
leaves of L. luteus and arb>reus. It will suffice to say that the
leaflets of the latter exhibited a double oscillation in the course
of 24 h. ; for they fell from the early morning until 10 15 a.m.,
then rose and zigzagged greatly till 4 p.m., after which hour the
great nocturnal fall commenced. By 8 a.m. on the following
morning the leaflets had risen to their proper height. We have
seen in the fourth chapter, that the leaves of Lupinus spcciosus,
which do not sleep, circumnutate to an extraordinary extent,
making many ellipses in the course of the day.
Cytistis (Tribe 2), Trigonella and Medicago (Tribe 3). — Only
Fig. 139.
A. B.
Medicago marina ; A. leaves during the day ; B, leaves asleep at night.
a few observations were made on these three genera. The
petioles on a young p'ant, about a foot in height, of Cyfisus
fragrans rose at night, on one occasion 23° and on another 33°.
The three leaflets als> bend upwards, and at the same time
Chap. VII. SLEEP OF LEAVES. 345
approach each other, so that the base of the central leaflet
overlaps the bases of the two lateral leaflets. They bend
up so nmch that they press against the stem; and on looking
down on one of these 3 oung plants from vertically above, the
lower surfaces of the leaflets are visible; and thus their upper
surfaces, in accordance with the general rule, are best protected
from radiation. Whilst the leaves on these young plants were
thus behaving, those on an old bush in full flower did not sleep
at night.
Tiigonella Cretica resembles a Melilotus in its sleep, which will
be immediately described. According to M. Eoyer,* the leaves
of Medicago maculata rise up at night, and " se renversent un
peu cle maniere a presenter obliquement au ciel leur face in-
ferieure." A drawing is here given (Fig. 139) of the leaves
of M. marina awake and asleep ; and this would almost serve
for Oytisus fragrans in the same two states.
Melilotus (Tribe 3).— The species in this gemis sleep in a
remarkable manner. The three leaflets of each leaf twist through
an angle of 90°, so that their blades stand vertically at night
with one lateral erlge presented to the zenith (Fig. 140). We
shall best understand the other and more complicated move-
ments, if we imagine ourselves always to hold the leaf with the
tip of the terminal leaflet pointed to the north. The leaflets in
becoming vertical at night could of course twist so that their
upper surfaces should face to either side ; but the two lateral
leaflets always twist so that this surface tends to face the north,
but as they move at the same time towards the terminal leaflet,
the upper surface of the one faces about N.N.W., and that of
the other N.N.E. The terminal leaflet behaves differently, for
it twists to either side, the upper surface facing sometimes east
and sometimes west, but rather more commonly west than east.
The terminal leaflet also moves in another and more remarkable
manner, for whilst its blade is twisting and becoming vertical,
the whole leaflet bends to one side, and invariably to the side
towards which the upper surface is directed; so that if this
surface faces the west the whole leaflet bends to the west, until
it comes into contact with the upper and vertical surface of
the western lateral leaflet. Thus the upper surface of the
terminal and of one of the two lateral leaflets is well protected.
The fact of the terminal leaflet twisting indifferently to either
Annales des Sc. Nat. Bot.' (5th series), ix. 1868, p. 368.
346
MODIFIED CIRCUMNUTATION.
Chap. VII
side and afterwards bending to the same side, seemed to us so
remarkable, that we endeavoured to discover the cause. We
imagined that at the commencement of the movement it might
be determined by one of the two halves of the leaflet being
a little heavier than the other. Therefore bits of wood were
gummed on one side of several leaflets, but this produced no
effect; and they continued to twist in the same direction as
Fisr. 140.
Melilotus officinalis : A, leaf during the daytime. B, another leaf asleep.
C, a leaf asleep as viewed from vertically above ; but in this case the
terminal leaflet did not happen to be in such close contact with the
lateral one, as is usual.
they had previously done. In order to discover whether the
same leaflet twisted permanently in the same direction, black
threads were tied to 20 leaves, the terminal leaflets of which
twisted so that their upper surfaces faced west, and 14 white
threads to leaflets which twisted to the east. These were ob-
served occasionally during 14 days, and they all continued, with
a single exception:, to twist aud bend in the same direction; for
Chap. YJI. SLEEP OF LEAVES. 347
one leaflet, which had originally faced east, was observed after
9 days to face west. The seat of both the twisting and bending
movement is in the pulvinus of the sub-petioles.
We believe that the leaflets, especially the two lateral ones,
in performing the above described complicated movements
generally bend a little downwards ; but we are not sure of this,
for, as far as the main petiole is concerned, its nocturnal move-
ment is largely determined by the position which the leaf
happens to occupy during the day. Thus one main petiole was
observed to rise at night 59°, whilst three others rose only 7°
and 9°. The petioles and sub-petioles are continually circum-
nutating during the whole 24 h., as we shall presently see.
The leaves of the following 15 species, M. officinalis, suaveolens,
parviflora, alba, inftsta, dtntata, gracilis, sulcata, elegans, ccerulea,
petitpurreatia, macrorrhiza, Itulici, secundiflora, and Taurica,
sleep in nearly the same manner as just described; but the
bending to one side of the terminal leaflet is apt to fail unless
the plants are growing vigorously. With M.petitpierreara and
sccundiflora the terminal leaflet was rarely seen to bend to one
side. In young plants of M. Halica it bent in the usual manner,
but with old plants in full flower, growing in the same pot and
observed at the same hour, viz., 8.30 p.m., none of the terminal
leaflets on several scores of leavt s had bent to one side, though
they stood vertically ; nor nad the two lateral leaflets, though
standing vertically, moved towards the terminal one. At
10 30 p.m , and again one hour after midnight, the terminal
leaflets had become very slightly bent to one side, and the
lateral leaflets had mo ?ed a very little towards the terminal one,
so that the posit on of the leaflets even at this late hour was far
from the ordinary one. Again, with M. Taurica the terminal
leaflets were never seen to bend towards either of the two lateral
leaflets, chough these, whilst becoming vertical, had bent towards
the terminal one. The sub-petiole of the terminal leaflet in
this species is of unusual length, and if the leaflet had bent to
one side, its upper surface could have come into contact only
with the apex of either lateral leaflet ; and this, perhaps, is the
meaning of the loss of the lateral movement.
The cotyledons do not sleep at night. The first leaf consists of
a single orbicular haflet, which twists at night so that the blade
stands vertically. It is a remarkable fact that with M. Taurica,
and in a somewhat less degree with M. macrorrhiza and petit-
piemana, all the many small and young leaves produced during
318 MODIFIED CIRCUMNUTATION. Chap. VU
the early spring from shoots on some cut-down plants in the
greenhouse, slept in a totally different manner from the normal
one ; for the three leaflets, instead of twisting on their own axes
so as to present their lateral edges to the zenith, turned upwards
and stood vertically with their apices pointing to the zenith.
They thus assumed nearly the same position as in the allied
genus Trifolium ; and on the same principle that embryological
characters reveal the lines of descent in the animal kingdom, so
the movements of the small leaves in the above three species of
Melilotus, perhaps indicate that this genus is descended from
a form which was closely allied to and slept like a Trifolium.
Moreover, there is one species, M. me sinensis, the leaves of
which, on full-grown plants between 2 and 3 feet in height,
sleep like the fort-going small leaves and like those of a Trifolium.
We were so much surprised at this latter case that, until the
flowers and fruit were examined, we thought that the seeds of
some Trifolium had been sown by mistake instead of those of a
Melilotus. It appears therefore probable that M. messanensis
has either retained or recovered a primordial habit.
The circumnutation of a leaf of M. officinalis was traced,
the stem being left free; and the apex of the terminal leaflet
described three laterally extended ellipses, between 8 a.m. and
4 p.m.; after the latter hour the nocturnal twi-ting movement
commenced. It was afterwards ascertained that the above
movement was compounded of the circumnutation of the stem
on a small scale, of the main petiole which moved most, and of
the sub-petiole of the to-minal leaflet. The main petiole of a
leaf having been secured to a stick, close to the base of the sub-
petiole of the terminal leaflet, the latter described two small
ellipses between 10.30 a.m., and 2 p.m. At 7.15 p.m., after th's
same leaflet (as well as another) had twisted themselves into
their vertical nocturnal position, they began to rise slowly, and
continued to do so until 10.35 p.m., after which hour they were
no longer observed.
As M. messanensis sleeps in an anomalous manner, unlike that
of any other species in the genus, the circumnutation of a
terminal leaflet, with the stem secured, was traced during two
days. On each morning the leaflet fell, until about noon, and
then began to rise very slowly; but on the first clay the rising
movement was interrupted between 1 and 3 p.m. by the formation
of a laterally extended ellipse, and on the second day, at the
game time, by two smaller ellipses. The rising movement then
Chap. VII. SLEEP OF LEAVES. 349
recommenced, and btcame rapid late in the evening, when
the leaflet was beginning to go to sleep. The awaking or
sinking movement had already commenced by 6.45 a.m on both
mornings.
Trifolium (Tribe 3). — The nyctitropic movements of 11
species were observed, and were found to be closely similar. If
we select a leaf of T. rcpens having an upright petiole, and with
the three leaflets expanded horizontally, the two lateral leaflets
will be seen in the evening to twist and approach each other,
until their upper surfaces come into contact. At the same time
they bend downwards in a plane at right angles to that of their
former position, until their midribs form an angle of about 45°
with the upper part of the petiole. This peculiar change of
position requires a considerable amount of torsion in the pul-
vinus. The terminal leaflet merely rises up without any twist-
Fi 2 . 141.
A. B.
Trifolium repens: A, leaf during the day; B, leaf asleep at night.
ing, and bends over until it rests on and forms a roof over the
edges of the now vertical and united lateral leaflets. Thus the
termiual leaflet always passes through an angle of at least 90°,
generally of 130° or 140°, and not rarely — as was often observed
with T. s bterraneum — of 180°. In this latter case the terminal
leaflet stands at night horizontally (as in Fig. 141), with its
lower surface fully exposed to the zenith. Besides the difference
in the angles, at which the terminal leaflets stand at night in
the individuals of the same species, the degree to which the
lateral leaflets approach each other often likewise differs.
We have seen that the cotyledons of some species and not of
others rise up vertically at night. The first true leaf is generally
unifoliate and orbicular ; it always rises, and either stands verti-
cally at night or more commonly bends a little over so as to expose
the lower surface obliquely to the zenith, in the same manner
as does the terminal leaflet of the mature leaf. But it does not
twist itself like the corresponding first simple leaf of Melilotus.
350 MODIFIED CIKCUMNUTATION. Chap. VII
With T. Pannonicum the first true leaf was generally unifoliate,
but sometimes trifoliate, or again partially lobed and in an
intermediate condition.
Oircumnutation. — Sachs described in 1863* the spontaneous
up and down movements of the leaflets of T. incarnatum, when
kept in darkness. Pfeffer made many observations on the
similar movements in T. pratensej He states that the terminal
leaflet of this species, observed at different times, passed through
angles of from 30° to 120° in the course of from Is to 4h. We
observed the movements of T. subterraneum, resiipinatum, and
repens.
TrifoMvm subterraneum. — A petiole was secured close to the
base of the three leaflets, and the movement of the terminal
leaflet was traced during 26£ h., as shown in the figure on the
next page
Between 6.45 a.m. and 6 p.m. the apex moved 3 times up
and 3 times down, completing 3 ellipses in 11 h. 15 m. The
ascending and descending lines stand nearer to one another
than is usual with most plants, yet there was some lateral
motion. At 6 p.m. the great nocturnal rise commenced, and
on the next morning the sinking of the leaflet was continued
until 8.30 a.m., after which hour it circumnutated in the manner
just described. In the figure the great nocturnal rise and
the morning fall are greatly abbreviated, from the want of
space, and are merely represented by a short curved line. The
leaflet stood horizontally when at a point a little beneath the
middle of the diagram; so that during the daytime it oscillated
almost equally above and beneath a horizontal position. At
8.30 a.m. it stood 48° beneath the horizon, and by 11.30 a.m. it
had risen 50° above the horizon ; so that it passed through 98°
in 3 h. By the aid of the tracing we ascertained that the
distance travelled in the 3 h. by the apex of this leaflet was
1*03 inch. If we look at the figure, and prolong upwards in
our mind's eye the short curved broken line, which repre-
sents the nocturnal course, we see that the latter movement is
merely an exaggeration or pro^ngation of one of the diurnal
ellipses. The same leaflet had been observed on the previous
day, and the course then pursued was almost identically the
same as that here described.
* 'Flora,' 18H3, p. 497.
t ' Die Period. Bewegungen,' 1875, pp. 35, 52.
Chap. VII.
SLEEP OF LEAVES.
351
Fig. 142.
Trifolium resnpinatum. — A plant left entirely
before a north-east win-
dow, in such a position
that a terminal leaflet
projected at right angles
to the source of the light,
the sky being uniformly
clouded all day. The
movements of this leaflet
were traced during two
days, and on both were
closely similar. Those
executed on the second
day are shown in Fig.
143. The obliquity of
the several lines is due
partly to the manner in
which the leaflet was
viewed, and partly to its
having moved a little to-
wards the light. From
7.50 a.m. to 8.40 a.m. the
leaflet fell, that is, the
awakening movement was
continued. It then rose
and moved a little late-
rally towards the light.
At 12.30 it retrograded,
and at 2.30 resumed its
original course, having
thus completed a small
ellipse during the middle
of the day. In the even-
ing it rose rapidly, and
by 8 a.m. on the following
morning had returned to
exactly the same spot as
on the previous morning.
The line representing the
nocturnal course ought
to be extended much
higher up, and is here
abbreviated into a short,
fre«. was placed
352
MODIFIED CIRCUMNUTATION.
Chap. VIL
H
curved, broken line. The terminal leaflet, therefore, of this
species described during the daytime only a single additional
ellipse, instead of two ad-
Fig. 143. ditional ones, as in the
case of T. subterrat/eum.
But we should remember
that it was shown in the
fourth chapter that the
stem circumnutates, as no
doubt does the main petiole
and the sub-petioles; so
that the movement repre-
sented in fig. 143 is a com-
pounded one We tried
to observe the movements
of a leaf kept during the
day in darkness, but it
,., .. ~ • j • began to go to sleep after
infolium resupinatum : circumnutation ~ ° f
and nyetitropic movements of the ter- <* n - J- 5 m -> anci tnis was
minaf leaflet during 24 hours. well pronounced after 4 h.
30 m.
Trifolium repens. — A stem was secured close to the base of
a moderately old leaf, and the movement of the terminal leaflet
was observed during two days. This case is interesting solely
from the simplicity of the movements, in contrast with those of
the two preceding species. On the first day the leaflet fell
between 8 a.m. and 3 p.m., and on the second between 7 a.m.
and 1 p.m. On both days the descending course was somewhat
zigzag, and this evidently represents the circumnutating move-
ment of the two previous species during the middle of the day.
After 1 p.m., Oct. 1st (Fig. 144), the leaflet began to rise, but
the movement was slow on both days, both before and after
this hour, until 4 p.m. The rapid evening and nocturnal rise
then commenced. Thus in this species the course during 24 h.
consists of a single great ellipse ; in T. resupinatum of two
ellipses, one of which includes the nocturnal movement and is
much elongated; and in T. subterrane^m of three ellipses, of
which the nocturnal one is likewise of great length.
Secur igera coronilla (Tribe 4). — The leaflets, which stand
opposite one another and are numerous, rise up at night, come
into close contact, a»«d bend backwards at a moderate angle
towards the base of the petiole.
Chap VII.
SLEEP OF LEAVES.
353
Fig. 144.
Lotus (Tribe 4).— The nyctitropic movements of 10 species
in this genus were observed, and found to be the same. The
main petiole rises a little at night, and
the three leaflets rise till they become
vertical, and at the same time approach
each other. This was conspicuous with
L. Jacobceus, in which the leaflets are
almost linear. In most of the species
the leaflets rise so much as to press
against the stem, and not rarely they
become inclined a little inwards with
their lower surfaces exposed obliquely
to the zenith. This was clearly the
case with L. major, as its petioles are
unusually long, and the leaflets are thus
enabled to bend further inwards. The
young leaves on the summits of the
stems close up at night so much, as
often to resemble large buds. The
stipule-like leaflets, which are often of
large size, rise up like the other leaflets,
and press against the stem (Fig. 145).
All the leaflets of L. Gebelii, and pro-
bably of the other species, are provided
at their bases with distinct pulvini, of
a yellowish colour, and formed of very
small cells. The circumnutation of a
terminal leaflet of L. perigrinvs (with
the stem secured) was traced during
two days, but the movement was so
simple that it is not worth while to
give the diagram. The leaflet fell
slowly from the early morning till
about 1 p.m. It then rose gradually
at first, but rapidly late in the evening.
It occasionally stood still for about 20 m. during the day, and
sometimes zigzagged a little. The movement of one of the
basal, stipule-like leaflets was likewise traced in the same
manner and at the same lime, and its course was closely similar
to that of the terminal leaflet.
In Tribe 5 of Bentham and Hooker, the sleep-movements
of species in 12 genera have been observed by ourselves and
Trifolium repens : circum-
nutation and nyctitropic
movements of a nearly
full - grown terminal
leaflet, traced on a ver-
tical glass from 7 A.M.
Sept. 30th to 8 A.M. Oct.
1st. Nocturnal course,
represented by curved
broken line, much ab-
breviated.
354
MODIFIED CIKCUMNUTATION.
Chat VII
others, but only in Robinia with any care. Vsoralea acaulu
raises its three leaflets at night ; whilst Amorpha fruticosa*
Balea alop.curoid.s, and Indigofera tinctoria depress them.
Duchartre f states that Ttphrosia caribcea is the sole example
>f " folioles couchees le long du petiole et vers la base ; " but a
Fig. 145.
A. B.
Lotus deticus : A, stem with leaves awake during the day ; B, with leaves
asleep at night. SS, stipule-like leaflets.
similar movement occurs, as we have already seen, and shall
again see in other cases. Wistaria Sinensis, according to
Eoyer.J " abaisse les folioles qui par une disposition bizarre
sont inclinees dans la meme feuille, les sup6rieures vers le
* Ducharte, ' Elements
BAanique, 1867, p. 31"J.
t Ibid., p. 347.
de
X 'Ann. dee Sciences, Nats,
Bot.' C5th series), ix. 1868.
Chap. VII.
SLEEP OF LEAVES.
355
sommet, les inferieures vers la base du petiole cornmun ; " but
the leaflets on a young plant observed by us in -the green-
house merely sank vertically downwards at night. The leaflets
are raised in Sphcerophysa salsola, Colutea arborea, and Astra-
galus uliginosus, but are depressed, according to Linnaeus, in
Glycyrrhiza. The leaflets of Robinia pst-vdo-acacia likewise sink
vertically down at night, but the petioles rise a little, viz., in
one case 3°, and in another 4°. The circumnutating move-
ments of a terminal leaflet on a rather old leaf were traced
during two days, and were simple. The leaflet fell slowly, in a
slightly zigzag line, from 8 a.m. to 5 p.m., and then more
rapidly ; by 7 a.m. on the following morning it had risen to its
diurnal position. There was only one peculiarity in the move-
ment, namely, that on both days there was a distinct though
small oscillation up and down between 8.80 and 10 a.m., and
this would probably have been more strongly pronounced if
the leaf had been younger.
Cor on ilia rosea (Tribe 6). — The leaves bear 9 or 10 pairs of
opposite leaflets, which during the day stand horizontally, with
Fig. 146.
Coronilla rosea : leaf asleep.
their midribs at right angles to the petiole. At night they rise
up, so that the opposite leaflets come nearly into contact, and
those on the younger leaves into close contact. At the same
time they bend back towards the base of the period, until their
midribs form with it angles of from 40° to 50° in a vertical
plane, as here figured (Fig. 146). The leaflets, however, some-
times bend ?o much back that their midribs become parallel to
and lie on the petiole. They thus occupy a reversed position
to what they do in several Leguminosse, for instance, in Mimosa
556
MODIFIED CIECUMNUTATION.
Chaf. VII.
pudica ; but, from standing further apart, they do not overlap
one another nearly so much as in this latter plant. The main
petiole is curved slightly downwards during the day, but
straightens itself at night. In three cases it rose from 3° above
the horizon at noon, to 9° at 10 p.m. ; from 11° to 33° ; and from
5° to 33° — the amount of angular movement in this latter case
amounting to 28°. In several other species of Coronilla the
leaflets showed only feeble movements of a similar kind.
Jdedysarum coronarium (Tribe 6). — The small lateral leaflets
on plants growing out of doors rose up vertically at night, but
the large terminal one became only moderately inclined. The
petioles apparently did not rise at all.
Smithia Pfundii (Tribe 6). — The leaflets rise up vertically,
and the main petiole also rises considerably.
Araehis hypogcea (Tribe 6;.— The shape of a leaf, with its two
pairs of leaflets, is shown at A (Fig. 147) ; and a leaf asleep,
Fig. 147.
Araehis hypogcea: A, leaf during the day, seen from vertically above ; B,
leaf asleep, seen laterally; copied from a photogr.iph. Figures much
reduced.
traced from a photograph (made by the aid of aluminium
light), is given at B. The two terminal leaflets twist round at
night until their blades stand vertically, and approach each
other until they meet, at the same time moving a little upwards
and backwards. The two lateral leaflets meet each other in the
same manner, but move to a greater extent forwards, that is, in
a contrary direction to the two terminal leaflets, which they
partially embrace. Thus all four leaflets form together a single
packet, with their edges directed to the zenith, and with their
lower surfaces turned outwards. On a plant which was not
growing vigorously the closed leaflets seemed too heavy for the
Chap. VII.
SLEEP OF LEAVES.
357
petioles to support them in a vertical position, so that each
night the main petiole became twisted, and all the packets were
extended horizontally, with the lower surfaces of the leaflets on
one side directed to the zenith in a most anomalous manner.
This fact is mentioned solely as a caution, as it surprised us
greatly, until we discovered that it was an anomaly. The
petioles are inclined upwards during the day, but sink at night,
so as to stand at about right angles with the stem. The amount
of sinking was measured only on one occasion, and found to be
39°. A petiole was secured to a stick at the base of the two
terminal leaflets, and the circumnutating movement of one of
these leaflets was traced from 6.40 a.m. to 10.40 p.m., the plant
being illuminated from above. The temperature was 17° — 17^° C,
and therefore rather too low. During the 16 h. the leaflet moved
thrice up and thrice down, and as the ascending and descend-
ing lines did not coincide, three ellipses were formed.
Desmodium gyrans (Tribe 6). — A large and full-grown leaf of
this plant, so famous for the spontaneous
movements of the two little lateral leaflets,
is here represented (Fig. 148). The large
terminal leaflet sleeps by sinking vertically
down, whilst the petiole rises up. The coty-
ledons do not sleep, but the first -formed leaf
sleeps equally well as the older ones. The
appearance presented by a sleeping branch
and one in the day-time, copied from two
photographs, are shown at A and B (Fig.
149), and we see how at night the leaves are
crowded together, as if for mutual pro-
tection, by the rising of the petioles. The
petioles of the younger leaves near the sum-
mits of the shoots rise up at night, so as to
stand vertical and parallel to the stem ;
whilst those on the sides were found in four
cases to have risen respectively 461°, 36°,
20°, and 19*5° above the inclined positions
which they had occupied during the day.
For instance, in the first of these four cases
the petiole stood in the day at 23°, and at
night at 695° above the horizon. In the
evening the rising of the petioles is almost
completed before the leaflets sink perpendicularly downwards.
Desmodium qyrans:
leaf seen from
above, reduced
to one-half na-
tural size. The
minute stipulei
unusually large
358
MODIFIED CIRC [IMPUTATION
Chap. VU.
Circumvutation. — The circumnutating movements of four
young shoots were observed during 5 h. 15 m. ; and in this time
each completed an oval figure of small size. The main petiole
also circumnutates rapidly, for in the course of 31m. (temp,
91° F.) it changed its course by as much as a rectangle six times,
describing a figure which apparently, represented two ellipses.
Fig. 149
Desmodium gyrans : A, stem during the day ; B, stem with leaves asleep.
Copied from a photograph ; figures reduced.
The movement of the terminal leaflet by means of its sub-
petiole or pulvinus is quite as rapid, or even more so, than that
of the main petiole, and has much greater amplitude. Pfeffer
has reen* these leaflets move through an angle of 8° in the
course of from 10 to 30 seconds.
A fine, nearly full-grown leaf on a young plant, 8 inches in
height, with the stem secured to a stick at the base of the leaf,
vas observed from 8.30 a.m. June 22nd to 8 a.m. June 24th
Die Period. Beweg.,' p. 35.
Chap. VII. SLEEP OF LEAVES. 359
In the diagram given on the next page (Fig. 150), the two
curved broken lines at the base, which represent the nocturnal
courses, ought to be prolonged far downwards. On the first
day the leaflet moved thrice down and thrice up, and to a con-
siderable distance laterally ; the course was also remarkably
crooked. The dots were generally made every hour; if they
had been made every few minutes all the lines would have been
zigzag to an extraordinary degree, with here and there a loop
formed. We may infer that this would have been the caee,
because five dots were made in the course of 31m. (between
12.34 and 1.5 p.m.), and we see in the upper part of the diagram
how crooked the course here is: if only the first and last dots
had been joined we should have had a straight line. Exactly
the same fact may be seen in the lines representing the course
between 2.24 p.m. and 3 p.m., when six intermediate dots were
made ; and again at 4.46 and 4.50. But the result was widely
different after 6 p.m., — that is, after the great nocturnal descent
had commenced ; for though nine dots were then made in the
course of 32 m , when these were joined (see Figure) the line thus
formed was almost straight. The leaflets, therefore, begin to
descend in the afternoon by zigzag lines, but as soon as the
descent becomes rapid their whole energy is expended in thus
moving, and their course becomes rectilinear. After the leaflets
are completely asleep they move very little or not at all.
Had the above plant been subjected to a higher temperature
than 67° — 70° F., the movements of the terminal leaflet would
probably have been even more rapid and wider in extent than
those shown in the diagram ; for a plant was kept for some time
in the hot-houee at from 92° — 93° F., and in the course of 35 m.
the apex of a leaflet twice descended and once ascended, travelling
over a space of 1*2 inch in a vertical direction and of "82 inch in
a horizontal direction. Whilst thus moving the leaflet also
rotated on its own axis (and this was a point to which no atten-
tion had been before paid), for the plane of the blade differed by
41° after an interval of only a few minutes. Occasionally the
leaflet stood still for a short time. There was no jerking move-
ment, which is so characteristic of the little lateral leaflets. A
sudden and considerable fall of temperature causes the terminal
leaflet to sink downwards ; thus a cut-off leaf was immersed in
water at 95° F., which was slowly raised to 103° F., and after-
wards allowed to sink to 70° F., and the sub-petiole of the ter-
minal leaflet then curved downwards. The water was afterwards
24
660 MODIFIED CIRCUMNUTATION.
Fig. 1.50.
Chap YH
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Chap. VII. SLEEP OF LEAVES 361
raised to 120° F., and the sub-petiole straightened itself. Similar
experiments with leaves in water were twice repeated, with
nearly the same result. It should be added, that water raised
to even 122° F. does not soon kill a leaf. A plant was placed
in darkness at 8.37 a.m., and at 2 p.m. (i.e. after 5 h. 23 m.), though
the leaflets had sunk considerably, they had by no means ac-
quired their nocturnal vertically dependent position. Pfeffer, on
the other hand, says * that this occurred with him in from f h.
to 2 h. ; perhaps the difference in our results may be due to
the plant on which we experimented being a very young and
vigorous seedling.
The Movements of the little Lateral Leaflets. — These have been so
often described, that we will endeavour to be as brief as possible
in giving a few new facts and conclusions. The leaflets some-
times quickly change their position by as much as nearly 180° ;
and their sub-petioles can then be seen to become greatly curved.
They rotate on their own axes, so that their upper surfaces are
directed to all points of the compass. The figure described by
the apex is an irregular oval or ellipse. They sometimes re-
main stationary for a period. In these several respects there is
no difference, except in rapidity and extent, between their move-
ments and the lesser ones performed by the large terminal
leaflet whilst making its great oscillations. The movements of
the little leaflets are much influenced, as is well known, by
temperature. This was clearly shown by immersing leaves with
motionless leaflets in cold water, which was slowly raised to
103° F., and the leaflets then moved quickly, describing about a
dozen little irregular circles in 40 m. By this time the water
had Income much cooler, and the movements became slower or
almost ceased ; it was then raised to 100° F., and the leaflets
again began to move quickly. On another occasion a tuft of
fine leaves was immersed in water at 53° F., and the leaflets
were of course motionless. The water was raised to 99°, and
the leaflets soon began to move ; it was raised to 105°, and the
movements became much more rapid ; each little circle or oval
being completed in from 1 m. 30 s. to 1 m. 45 s. There was,
however, no jerking, and this fact may perhaps be attributed to
the resistance of the water.
Sachs stat»*s that the leaflets do not move until the surround-
ing air is as high as 71° — 72° F., and this agrees with our
* * Die Period. Beweg.,' p. 39.
362 MODIFIED CIKCUMNUTATION. Chap. VII.
experience on full-grown, or nearly full-grown, plants. But the
leaflets of young seedlings exhibit a jerking movement at much
lower temperatures. A seedling was kept (April 16th) in a room
for half the day where the temperature was steady at 64° F.,
and the one leaflet which it bore was continually jerking, but
not so rapidly as in the hot-house. The pot was taken in the
evening into a bed-room where the temperature remained at
62° during nearly the whole night; at 10 and 11p.m. and at
1 a.m. the leaflet was still jerking rapidly ; at 3.30 a m. it was not
seen to jerk, but was observed during only a short time. It was,
however, now inclined at a much lower angle than that occupied
at 1 a.m. At 6.30 a.m. (temp. 61° F.) its inclination was still
less than before, and again less at 6.45 a.m. ; by 7.40 A m. it had
risen, and at 8.30 a.m. was again seen to jerk. This leaflet,
therefore, was moving during the whole night, and the move-
ment was by jerks up to 1 a.m. (and possibly later) and again at
8.30 am., though the temperature was only 61° to 62° F. "We
must therefore conclude that the lateral leaflets produced by
young plants differ somewhat in constitution from those on
older plants.
In the large genus Desmodium by far the greater number
of the species are trifoliate ; but some are unifoliate, and even
the same plant may bear uni- and trifoliate leaves. In most
of the species the lateral leaflets are only a little smaller than
the terminal one. Therefore the lateral leaflets of D. gyrans
(see former Fig. 148) must be considered as almost rudi-
mentary. They are also rudimentary in function, if this ex-
pression may be used ; for they certainly do not sleep like the
full-sized terminal leaflets. It is, however, possible that the
sinking down of the leaflets between 1 a.m. and 6.45 a.m , as
above described, may represent sleep. It is well known that
the leaflets go on jerking during the early part of the night;
but my gardener observed (Oct. 13th) a plant in the hot-house
between 5 and 5.30 a.m., the temperature having been kept up
to 82° F., and found that all the leaflets were inclined, but he
saw no jerking movement until 6.55 a.m., by which time the
terminal leaflet had risen and was awake. Two days after-
wards (Oct. 15th) the same plant was observed by him at
4.47 a.m. (temp. 77° F.), and he found that the large terminal
leaflets were awake, though not quite horizontal ; and the only
cause which we could assign for this anomalous wakefulness was
that the plant had been kept for experimental purposes during
Chap. VII. SLEEP OF LEAVES. 363
the previous day at an unusually high temperature ; the little
lateral leaflets were also jerking at this hour, but whether
there was any connection between this latter fact and the sub-
horizontal position of the terminal leaflets we do not know.
Anyhow, it is certain that the lateral leaflets do not sleep like
the terminal leaflets; and in so far they may be scid to be
in a functionally rudimentary condition They are in a similar
condition in relation to irritability; for if a plant be shaken
or syringed, the terminal leaflets sink down to about 45° be-
neath the horizon ; but we could never detect any effect thus
produced on the lateral leaflets; yet we are not prepared to
assert positively that rubbing or pricking the pulvinus produces
no effect.
As in the case of most rudimentary organs, the leaflets are
variable in size ; they often depart from their normal position
and do not stand opposite one another; and one of the two is
frequently absent. This absence appeared in some, but not in
all the cases, to be due to the leaflet having become completely
confluent with the main petiole, as might be inferred from the
presence of a slight ridge along its upper margin, and from the
course of the vessels. In one instance there was a vestige of
the leaflet, in the shape of a minute point, at the further end of the
ridge. The frequent, sudden, and complete disappearance of one
or both of the rudimentary leaflets is a rather singular fact ; but
it is a much more surprising one that the leaves which are first
developed on seedling plants are not provided with them. Thus,
on one seedling the seventh leaf above the cotyledons was the
first which bore any lateral leaflets, and then only a single one.
On another seedling, the eleventh leaf first bore a leaflet ; of the
nine succeeding leaves five bore a single lateral leaflet, and
Jour bore none at all ; at last a leaf, the twenty-first above the
cotyledons, was provided with two rudimentary lateral leaflets.
Irom a widespread analogy in the animal kingdom, it might
have been expected that these rudimentary leaflets would have
been better developed and more regularly present on very young
than on older plants. But bearing in mind, firstly, that long-
lost characters sometimes reappear late in life, and secondly,
that the species of Desmodium are generally trifoliate, but that
some are unifoliate, the suspicion arises that D. gyrans is
descended from a unifoliate species, and that this was descended
from a trifoliate one ; for in this case both the absence of the
little lateral leaflets. on very young seedlings, and their sub-
364 MODIFIED CTRCUMNUTATION Chap. VII
sequent appearance, may be attributed to reversion to more 01
less distant progenitors.*
No one supposes that the rapid movements of the lateral
leaflets of D. gyrans are of any use to the plant; and why
they should behave in this manner is quite unknown. We
imagined that their power of movement might stand in some
relation with their rudimentary condition, and therefore ob-
served the almost rudimentary leaflets of Mimosa albida vel
sensitiva (of which a drawing will hereafter be given, Fig. 159) ;
but they exhibited no extraordinary movements, and at night
they went to sleep like the full-sized leaflets. There is, how-
ever, this remarkable difference in the two cases ; in Desmo-
dium the pulvinus of the rudimentary leaflets has not been
reduced in length, in correspondence with the reduction of the
blade, to the same extent as has occurred in the Mimosa ; and it
is on the length and degree of curvature of the pulvinus that the
amount of movement of the blade depends. Thus, the average
length of the pulvinus in the large terminal leaflets of Desmo-
dium is 3 mm., whilst that of the rudimentary leaflets is 2'86 mm. ;
so that they differ only a little in length. But in diameter they
differ much, that of the pulvinus of the little leaflets being only
0*3 mm. to 0*4 mm.; whilst that of the terminal leaflets is
1-33 mm. If we now turn to the Mimosa, we find that the
average length of the pulvinus of the almost rudimentary
leaflets is only - 466 mm., or rather more than a quarter of the
length of the pulvinus of the full-sized leaflets, namely, 1 • 66 mm.
In this small reduction in length of the pulvinus of the rudi-
mentary leaflets of Desmodium, we apparently have the proxi-
mate cause of their great and rapid circumnutating movement,
in contrast with that of the almost rudimentary leaflets of the
Mimosa. The small size and weight of the blade, and the little
resistance opposed by the air to its movement, no doubt also come
into play ; for we have seen that these leaflets if immersed in
water, when the resistance would be much greater, were pre-
vented from jerking forwards. Why, during the reduction of
the lateral leaflets of Desmodium, or during their reappearance
— if they owe their origin to reversion — the pulvinus should
have been so much less affected than the blade, whilst with the
* Desmodium vezpertilionis is rudimentary lateral leaflets. Du-
closely allied to D. gyrans, and chartre, 'Eie'meutsde Botanique,
it seems only occasionally to bear 1867, p. 353.
Chap. VII.
SLEEP OF LEAVES.
365
Mimosa the pulvinus has been greatly reduced, we do not
know. Nevertheless, it deserves notice that the reduction of
the leaflets in these two genera has apparently been effected by
a different process and for a different end ; for with the Mimosa
the reduction of the inner and basal leaflets was necessary from
the want of space; but no such necessity exists with Desmo-
dium, and the reduction of its lateral leaflets seems to have
been due to the principle of compensation, in consequence of
the great size of the terminal leaflet.
Uraria (Tribe 6) and Centro&ema (Tribe 8). — The leaflets of
Uraria lay opus and the leaves of a Centrosema from Brazil
both sink vertically down at night. In the latter plant the
petiole at the same time rose 16 £°.
Amphkarpcea monoica (Tribe 8). — The leaflets sink down ver-
tically at night, and the petioles likewise fall considerably.
Amphicarpcea monoica : circumnutation and nyctitropic movement of leaf
during 48 h. ; its apex 9 inches from the vertical glass. Figure reduced
to one-third of original scale. Plant illuminated from above; temp.
I7i°-18i° C.
A petiole, which was carefully observed, stood during the day
25° above the horizon and at night 32° below it ; it therefore
fell 57°. A filament was fixed transversely across the terminal
leaflet of a fine young leaf (2 J inches in length including the
'SS6 MODIFIED CIRCUMNUTATION. Chap. VII.
petiole), and the movement of the whole leaf was traced on a
vertical glass. This was a bad plan in some respects, because
the rotation of the leaflet, independently of its rising or falling,
raised and depressed the filament ; but it was the best plan for
our special purpose of observing whether the leaf moved much
after it had gone to sleep. The plant had twined closely round
a thin stick, so that the circumnutation of the stem was pre-
vented. The movement of the leaf was traced during 48 h.,
from 9 a.m. July 10th to 9 a.m. July 12th. In the figure given
(Fig. 151) we see how complicated its course was on both days :
during the second day it Changed its course greatly 13 times.
The leaflets began to go to sleep a little after 6 p.m., and by
7.15 p.m. hung vertically down and were completely asleep;
but on both nights they continued to move from 7.15 p.m.
to 10.40 and 10.50 p.m., quite as much as during the day ; and
this was the point which we wished to ascertain. "We see in
the figure that the great sinking movement late in the evening
does not differ essentially from the circumnutation during
the day.
Glycine hispida (Tribe 8). — The three leaflets sink vertically
down at night.
Erythrina (Tribe 8). — Five species were observed, and the
leaflets of all sank vertically down at night ; with E. caffra and
with a second unnamed species, the petioles at the same time
rose slightly. The movements of the terminal leaflet of E. crista-
galli (with the main petiole secured to a stick) were traced
from 6.40 a.m., June 8th, to 8 a.m. on the 10th. In order to
observe the nyctitropic movements of this plant, it is necessary
that it should have grown in a warm greenhouse, for out of
doors in our climate it does not sleep. We see in the tracing
(Fig. 152) that the leaflet oscillated twice up and down between
early morning and noon ; it then fell greatly, afterwards rising
till 3 p.m. At this latter hour the great nocturnal fall com-
menced. On the second day (of which the tracing is not given)
there was exactly the same double oscillation before noon, but
only a very small one in the afternoon. On the third morning
the leaflet moved laterally, which was due to its beginning to
assume an oblique position, as seems invariably to occur with
the leaflets of this species as they grow old. On both nights after
the leaflets were asleep and hung vertically down, they continued
to move a little both up and down, and from side to side.
Erythrina caffra. — A filament was fixed transversely across
Chap. VII.
SLEEP OF LEAVES.
367
a terminal leaflet, as we wished
to observe its movements when
asleep. The plant was placed
in the morning of June lUth
under a skylight, where the
light was not bright ; and we
do not know whether it was
owing to this cause or to the
plant having been disturbed,
but the leaflet hung vertically
down all day; nevertheless it
circumnutated in this posi-
tion, describing a figure which
represented two irregular el-
lipses. On the next day it
circumnutated in a greater
degree, describing four irre-
gular ellipses, and by 3 p.m.
had risen into a horizontal po-
sition. By 7.15 p.m. it was
asleep and vertically depen-
dent, but continued to circum-
nutate as long as observed,
until 11 p.m.
Erythrina corallodendron. —
The movements of a terminal
leaflet were traced. During
the second day it oscillated
four times up and four times
down between 8 a.m. and 4
p.m., after which hour the great
nocturnal fall commenced. On
the third day the movement
was equally great in ampli-
tude, but was remarkably
simple, for the leaflet rose in
an almost perfectly straight
line from 6.50 a.m. to 3 p.m.,
and then sank down in an
equally straight line until
vertically dependent and
6'40a.m.8 i tt
2'j>.m.>
G°J5'a.m.9 t t
3 Q
Erythrina crista-galli : circumnuta-
tion and nyctitropic movement
of terminal leaflet, 3f inches in
length, traced during 25 h. ; apex
of leaf 3 J inches from the vertical
glass. Figure reduced to one-half
of original scale. Plant illumi-
nated from above: temp. 17A°-
18£° C.
#68 MODIFIED CIRCUMNUTATION. Chap. TO.
Apios tuherosa (Tribe 8). — The leaflets sink vertically dowc
at night.
Phaseolus vulgaris (Tribe 8). — The leaflets likewise sink verti-
cally down at night. In the greenhouse the petiole of a young
leaf rose 16°, and that of an older leaf 10° at night. With
plants growing ont of doors the leaflets apparently do not sleep
until somewhat late in the season, for on the nights of July 11th
and 12th none of them were asleep ; whereas on the night of
August 15th the same plants had most of their leaflets verti-
cally dependent and asleep. With Ph. caiacalla and Eernan-
dtsii, the primary unifoliate leaves and the leaflets of the
secondary trifoliate leaves sink vertically down at night. This
holds good with the secondary trifoliate leaves of Ph. Eox-
ourghii, but it is remarkable that the primary unifoliate leaves,
which are much elongated, rise at night from about 20° to
about 60° above the horizon. With older seedlings, however,
having the secondary leaves just developed, the primary leaves
stand in the middle of the day horizontally, or are deflected
a little beneath the horizon. In one such case the primary
leaves rose from 26° beneath the horizon at noon, to 20° above
it at 10 p.m.; whilst at this same hour the leaflets of the
secondary leaves were vertically dependent. Here, then, we
have the extraordinary case of the primary and secondary
leaves on the same plant moving at the same time in opposite
directions.
We have now seen that the leaflets in the six genera of Pha-
seoleae observed by us (with the exception of the primary leaves
of Phaseolus Roxburyhii) all sleep in the same manner, namely,
by sinking vertically down. The movements of the petioles
were observed in only three of these genera. They rose in
Centrosema and Phaseolus, and sunk in Amphicarpsea.
Sophora chrysojahylla (Tribe 10). — The leaflets rise at night,
and are at the same time directed towards the apex of the leaf,
as in Mimosa pudica.
Ccesalpinia, ffcematoxylon, Oltditschia, Poinciana. — The leaflets
of two species of Csesalpinia (Tribe 13) rose at night. With
Hozmatoxylon Campechianum (Tribe 13) the leaflets move for-
wards at night, so that their midribs stand parallel to the
petiole, and their now vertical lower surfaces are turned out-
wards (Fig. 153). The petiole sinks a little. In Gleditschia, if
we understand correctly Duchartre's description, and in Poin*
Chai\ VII.
SLEEP OF LEAVES.
3G9
ciana Gilliesii (both belonging to Tribe 13), the leaves behave
in the same manner.
Fig. 153.
Hcematoxylon Campechianum : A, branch during daytime ; B, branch with
leaves asleep, reduced to two-thirds of natural scale.
Cassia (Tribe 14). — The nyctitropic movements of the leaves
in many species in this genus are closely alike, and are highly
complex. They were first briefly described by Linnseus, and since
by Duchartre. Our observations were made chiefly on 0. flori-
bunda * and corymbosa, but several other species were casually
observed. The horizontally extended leaflets sink down verti-
cally at night ; but not simply, as in so many other genera, for
each leaflet rotates on its own axis,, so that its lower surface
faces outwards. The upper surfaces of the opposite leaflets are
thus brought into contact with one another beneath the petiole,
and are well protected (Fig. 154). The rotation and other move-
ments are effected by means of a well-developed pulvinus at the
base of each leaflet, as could be plainly seen when a straight
narrow black line had been painted along it during the day.
The two terminal leaflets in the daytime include rather less than
a right angle; but their divergence increases greatly whilst they
* I am informed by Mr. Dyer
that Mr. Bentham believes that
C. floribunda (a common green-
house bush) is a hybrid raited in
France, and that it cunes very
near to C, Icevigatn. It is no doubt
the same as the form described by
Lindley (' Bot. Keg.,' Tab. 1422;
*»« C. Nerhertiana.
370
MODIFIED CIKCUMNUTATION.
Chap. VIL
sink downwards and rotate, so that they stand laterally at night,
as may be seen in the figure. Moreover, they move somewhat
backwards, so as to point towards the base of the petiole.
Fig. 154.
Cassia corymbcsa: A, plant during day; B, same jlant at night.
Both figures copied from photographs.
lu one instance we found that the midrib of a terminal
leaflet formed at night an angle of 36°, with a line dropped
Chap. VII.
SLEEP OF LEAVES.
371
perpendicularly from the end of the petiole. The second pair
of leaflets likewise moves a little backwards, but less than the
terminal pair; and the third pair moves vertically downwards,
or even a little forwards. Thus all the leaflets, in those species
which bear only 3 or 4 pairs, tend to form a single packet, with
their upper surfaces in contact, and their lower surfaces turned
outwards. Lastly, the main petiole rises at night, but with
leaves of different ages to very different degrees, namely, some
rose through an angle of only 12°, and others as much as 41°.
Cassia call iautha.— The leaves bear a large number of leaflets,
which move at night in nearly the same manner as just
described; but the petioles apparently do not rise, and one
which was carefully observed certainly fell 3°.
Cassia pubeaceits. — The chief difference in the nyctitropic
Fig. 155.
Caseia pubescens; A, upper part of plant during the day ; B, same pant
at night. Figures reduced from photographs.
movements of this species, compared with those of the former
species, consists in the leaflets not rotating nearly so much;
372 MODIFIED CIRCUMNUTATION. Chap VII
therefore their lower surfaces face but little outwards at night.
The petioles, which during the day are inclined only a little
above the horizon, rise at night in a remarkable manner, and
stand neirly or quite vertically. This, together with the
dependent position of the leaflets, makes the whole plant won-
derfully compact at night. In the two foregoing figures, copied
from photographs, the same plant is represented awake and
asleep (Fig. 155), and we see how different is its appearance.
Cassia rnimosuides — At night the numerous leaflets on each
leaf rotate on their axes, and their tips move towards the apex
of the leaf; they thus become imbricated with their lower
surfaces directed upwards, and with their midribs almost
parallel to the petiole. Consequently, this species differs from
all the others seen by us, with the exception of the following
one, in the leaflets not sinking down at night. A petiole, the
movement of which was measured, rose 8° at night.
Cassia Barclayana. — The leaflets of this Australian species are
numerous, very narrow, and almost linear. At night they rise up
a little, and also move towards the apex of the leaf. For instance,
two opposite leaflets which diverged from one another during
the day at an angle of 104°, diverged at night only 72° ; so that
each had risen 16° above its diurnal position. The petiole of a
young leaf rose at night 34°, and that of an older leaf 19°.
Owing to the slight movement of the leaflets and the consider-
able movement of the petiole, the bush presents a different
appearance at night to what it does by day; yet the leaves can
hardly be said to sleep.
The circumnutating movements of the leaves of C. floribunda,
calliantlia, and pubesceas were observed, each during three or four
days ; they were essentially alike, those of the last-named species
being the simplest. The petiole of C. floribunda was secured to
a stick at the base of the two terminal leaflets, and a filament
was fixed along the midrib of one of them. Its movements were
traced from 1 p.m. on August 13th to 8.30 a.m. 17th ; but those
during the last 2 h. are alone given in Fig. 156. From 8 a.m. on
each day (by which hour the leaf had assumed its diurnal posi-
tion) to 2 or 3 p.m., it either zigzagged or circumnutated over
nearly the same small space; at between 2 and 3 p.m. the great
evening fall commenced. The lines representing this fall and
the early morning rise are oblique, owing to the peculiar manner
in which the leaflets sleep, as already described. After the
leaflet was asleep at 6 p.m., and whilst the glass filament hung
Chap. VII
SLEEP OF LEAVES.
373
£^~3
? £ 2
V -SB
41
^
a «o
^ br,co
nr
S bo E
perpenlicularly down, the movement of its apex was traced
until 10.30 p.m.; and during this whole time it swayed from
side to side, completing more than one ellipse.
Bauhiuia (Tribe 15).— Fig. 156
The nyctitropic movements
of four species were alike,
and were highly peculiar.
A plant raised from seed
sent us from South Brazil
by Fritz Miiller, was more
especially observed. The
loaves are large and deeply
notched at their ends. At
night the two halves rise
up and close completely
together, like the opposite
leaflets of many Legumi-
nosse. With very young
plants the petioles rise con-
siderably at the same time ;
one, which was inclined at
noon 45° above the hori-
zon, at night stood at 75° ;
it thus rose 30°; another
rose 34°. Whilst the two
halves of the leaf are closing,
the midrib at first sinks
vertically downwards and
afterwards bends back-
wards, so as to pass close
along one side of its own
upwardly inclined petiole;
the midrib being thus di-
rected towards the stem or
axis of the plant. The angle
which the midrib formed
with the horizon was mea- // js
sured in one case at dif-
ferent hours: at noon it stood horizontally; late in the even-
ing it depended vertically; then rose to the opposite side, and
at 10.15 p.m. stood at only 27° beneath the horizon, being
directed towards the stem. It had thus travelled through 153°
"Z £ bo
o o 5
a O g
III
5 5 "3 <S
S3
^ O c3 S
r2 S U *
5 " c <**
374 MODIFIED CIRCUMNUTATIOiN. Chap. VII.
Owing to this movement — to the leaves being folded — and to
the petioles rising, the whole plant is as much more compact at
night than during the day, as a fastigiate Lombardy poplar is
compared with any other species of poplar. It is remarkable
that when our plants had grown a little older, viz., to a hoight
of 2 or 3 feet, the petioles did not rise at night, and the midribs
of the folded leaves were no longer bent back along one side of
the petiole. We have noticed in some other genera that the
petioles of very young plants rise much more at night than do
those of older plants.
Tamarindus Indica (Tribe 16). — The leaflets approach or
meet each other at night, and are all directed towards the apex
of the leaf. They thus become imbricated with their midribs
parallel to the petiole. The movement is closely similar to
that of Hsematoxylon (see former Fig. 153), but more striking
from the greater number of the leaflets.
Adenanthera, Prosopis, and Neptunia (Tribe 20). — With Ade-
nanthera paronia the leaflets turn edgeways and sink at night.
In Prosopis they turn upwards With Neptunia oleracea the
leaflets on the opposite sides of the same pinna come into
contact at night and are directed forwards. The pinnse them-
selves move downwards, and at the same time backwards or
towards the stem of the plant. The main petiole rises.
Mimosa pudica (Tribe 20). — This plant has been the subject of
innumerable observations; but there are some points in rela-
tion to our subject which have not been sufficiently attended
to. At night, as is well known, the opposite leaflets come into
contact and point towards the apex of the leaf ; they thus be-
come neatly imbricated with their upper surfaces protected. The
four pinnse also approach each other closely, and the whole leaf
is thus rendered very compact. The main petiole sinks down-
wards during the day till late in the evening, and rises until
very early in the morning. The stem is continually circumnu-
tating at a rapid rate, though not to a wide extent. Some very
young plants, kept in darkness, were observed during two days,
and although subjected to a rather low temperature of 57° — 59° F.,
the stem of one described four small ellipses in the course ot
12 h. We shall immediately see that the main petiole is like-
wise continually circumnutating, as is each separate pinna and
each separate leaflet. Therefore, if the movement of the apex
of any one leaflet were to be traced, the course described would
be compounded of the movements of four separate parts.
Chap. VII.
SLEEP OF LEAVES.
375
A filament had been fixed on the previous eveiing, longi-
tudinally to the main petiole of a nearly full-grown, highly-
sensitive leaf (four inches in length), the stem having been
secured to a stick at its base ; and a tracing was made on a
vertical glass in the hot-bouse under a high temperature. In
the figure given (Fig. 157), tbe
first dot was made at 8.30 a.m.
August 2nd, and the last at 7
p.m. on the 3rd. During 12 h. on
the first day the petiole moved
thrice downwards and twice
upwards. Within the same
length of time on the second
day, it moved five times down-
wards and four times upwards.
Fig. 157.
8°30'a.m.2^-
I'SftunAR*
ing lines do not coincide, the
petiole manifestly circumnu-
tates; the great evening fall
and nocturnal rise being an
exaggeration of one of the cir •
cumnutations. It should, how-
ever, be observed that the pe-
tiole fell much lower down in
the evenings than could be
seen on the vertical glass or is
represented in the diagram.
After 7 p.m. on the 3rd (when
the last dot in Fig. 157 was
made) the pot was carried into
a bed-room, and the petiole was
found at 12.50 a.m. (i.e. after
midnight) standing almost up-
right, and much more highly
inclined than it was at 10.40
p.m. When observed again at
4 a.m. it had begun to fall, and
continued falling till 6.15 a.m.,
after which hour it zigzagged and again circumnutatecl. Similar
observations were made on another petiole, with nearly the
same result.
On two other occasions the movement of the main petiole
25
e'p.m.2™ *
Mimosa pudica : circunmutation and
nventropic movement of main pe-
tiole, traced during 34 h. 30 m.
376 MODIFIED CIKCUMNUTATION. Chap. VII
was observed every two or three minutes, the plants being kept
at a rather high temperature, viz., on the first occasion at
77° — 81° F., and the filament then described 2\ ellipses in 69 m.
On the second occasion, when the temperature was 81° — 86° F.,
it made rather more than 3 ellipses in 67 m. Therefore,
Fig. 157, though now sufficiently complex, would have been in-
comparably more so, if dots had been made on the glass every
2 or 3 minutes, instead of every hour or half-hour. Although
the main petiole is continually and rapidly describing small
ellipses during the day, yet after the great nocturnal rising
movement has commenced, if dots are made every 2 or 3
minutes, as was done for an hour between 9.30 and 10.30 p.m.
(temp. 84° F.), and the dots are then joined, an almost abso-
lutely straight line is the result.
To show that the movement of the petiole is in all proba-
bility due to the varying turgescence of the pulvinus, and not
to growth (in accordance with the conclusions of Pfeffer), a very
old leaf, with some of its leaflets yellowish and hardly at all
sensitive, was selected for observation, and the plant was kept
at the highly favourable temp, of 80° F. The petiole fell from
8 a.m. till 10.15 a.m., it then rose a little in a somewhat zigzag
line, often remaining stationary, till 5 p.m., when the great
evening fall commenced, which was continued till at least
10 p.m. By 7 a.m. on the following morning it had risen to the
same level as on the previous morning, and then descended in
a zigzag line. But from 10.30 a.m. till 4.15 p.m. it remained
almost motionless, all power of movement being now lost. The
petiole, therefore, of this very old leaf, which must have long
ceased growing, moved periodically ; but instead of circum-
nntating several times during the day, it moved only twice
down and twice up in the course of 24 h., with the ascending
and descending lines not coincident.
It has already been stated that the pinnae move independently
of the main petiole. The petiole of a leaf was fixed to a cork
support, close to the point whence the four pinnae diverge, with
a short fine filament cemented longitudi Dally to one of the two
terminal pinnae, and a graduated semicircle was placed close
beneath it. By looking vertically down, its angular or lateral
movements could be measured w T ith accuracy. Between noon
and 4.15 r m. the pinna changed its position to one side by only
7°; but not continuously in the same direction, as it moved
four times to one side, and three times to the opposite side,
Chap. VII. SLEEP OF LEAVES. 377
in one instance to the extent of 16°. This pinna, therefore,
circumnutated. Later in the evening the four pinnae approach
each other, and the one which was observed moved inwards
59° between noon and 6.45 p.m. Ten observations were made
in the course of 2 h. 20 m. (at average intervals of 14 m.)
between 4.25 and 6.45 p.m. ; and there was now, when the leat
was going to sleep, no swaying from side to side, but a steady
inward movement. Here therefore there is in the evening the
same conversion of a circumnutating into a steady movement
in one direction, as in the case of the main petiole.
It has also been stated that each separate leaflet circum-
nutates. A pinna was cemented with shellac on the summit ot
a little stick driven firmly into the ground, immediately beneath
a pair of leaflets, to the midribs of both of which excessively
fine glass filaments were attached. This treatment did not
injure the leaflets, for they went to sleep in the usual manner,
and long retained their sensitiveness. The movements of one
of them were traced during 49 h., as shown in Fig. 158. On the
first day the leaflet sank down till 11.30 a.m., and then rose
till late in the evening in a zigzag line, indicating circum-
nutation. On the second day, when more accustomed to its
new state, it oscillated twice up and twice down during the
24 h. This plant was subjected to a rather low temperature,
viz., 62° — 64° F. ; had it been kept warmer, no doubt the move-
ments of the leaflet would have been much more rapid and
complicated. It may be seen in the diagram that the ascending
and descending lines do not coincide; but the large amount of
lateral movement in the evening is the result of the leaflets
bending towards the apex of the leaf when going to sleep.
Another leaflet was casually observed, and found to be con-
tinually circumnutating during the same length of time.
The circumnutation of the leaves is not destroyed by their
being subjected to moderately long continued darkness ; but the
proper periodicity of their movements is lost. Some very young
seedlings were kept during two days in the dark (temp. 57° — 59°
F.), except when the circumnutation of their stems was occa-
sionally observed ; and on the evening of the second day the
leaflets did not fully and properly go to sleep. The pot was
then placed for three days in a dark cupboard, under nearly the
same temperature, and at the close of this period the leaflets
showed no signs of sleeping, and were only slightly sensitive to
a touch. On the following day the stem was cemented to a
378
MODIFIED CIECUMNUTATION.
Chap. Vll
stick, and the movements of two leaves were traced on a vertical
glass during 72 h. The plants were still kept in the dark, ex-
cepting that at each observation, which lasted 3 or 4 minutes,
UttilS**
jOHS&aM
Mimosa pudica; circumnutation and nyctitropic movement of a leaflet
(with pinna secured), traced on a vertical glass, from 8 A.M. Sept. 14th
to 9 A.M. 16th.
they were illuminated by two candles. On the third day the
leaflets still exhibited a vestige of sensitiveness when forcibly
pressed, but in the evening they showed no signs of sleep.
Nevertheless, their petioles continued to circumnutate distinctly,
Chap. VII.
SLEEP OF LEAVES.
379
although the proper order of their movements in relation to tht
day and night was wholly lost. Thus, one leaf descended during
the first two nights (i.e. between 10 p.m. and 7 a.m. next morn-
ing) instead of ascending, and on the third night it moved
chiefly in a lateral direction. The second leaf behaved in an
equally abnormal manner, moving laterally during the first
night, descending greatly during the second, and ascending to
an unusual height during the third night.
With plants kept at a high temperature and exposed to the
light, the most rapid circumnutating movement of the apex
of a leaf which was observed, amounted to -^ of an inch in
one second; and this would have equalled £ of an inch in a
minute, had not the leaf occasionally stood still. The actual
distance travelled by the apex (as ascertained by a measure
placed close to the leaf) was on one occasion nearly | of an inch
in a vertical direction in 15 m. ; and on another occasion f of an
inch in 60 m. ; but there was also some lateral movement.
Mimosa albida* — The leaves of this plant, one of which is here
figured (Fig. 159) reduced to § of the natural size, present some
Fig. 159.
Mimosa albida ; leaf seen from vertically above.
interesting peculiarities. It consists of a long petiole bearing
only two pinnae (here represented as rather more divergent
than is usual), each with two pairs of leaflets. But the inner
* Mr. Thistleton Dyer informs Linn. Soc.,' vol. xxx. p. 390) to
us that this Peruvian plant (which' be " the species or variety which
was sent to us from Kew) is con- most commonly represents the M
sidered by Mr. Bentham (' Trans. sensitiva of our gardens."
380 MODIFIED CD3CUMNUTATI0N. Chap. VII.
basal leaflets are greatly reduced in size, owing probably to the
want of space for their full development, so that they may be
considered as almost rudimentary. They vary somewhat in
size, and both occasionally disappear, or only one. Neverthe-
less, they are not in the least rudimentary in function, for they
are sensitive, extremely heliotropic, circum nutate at nearly the
same rate as the fully developed leaflets, and assume when
asleep exactly the same position. JWith $■• pudica the inner
leaflets at the base and between the pinnce are likewise much
shortened and obliquely truncated ; this fact was well seen in
some seedlings of M. pudica, in which the third leaf above the
cotyledons bore only two pinnae, each with only 3 or 4 pairs of
leaflets, of which the inner basal one was less than half as long
as its fellow; so that the whole leaf resembled pretty closely
that of M. albida. In this latter species the main petiole termi-
nates in a little point, and on each side of this there is a pair
of minute, flattened, lancet-shaped projections, hairy on their
margins, which drop off and disappear soon after the leaf is
fully developed. There can hardly be a doubt that these little
projections are the last and fugacious representatives of an
additional pair of leaflets to each pinna; for the outer one is
twice as broad as the inner one, and a little longer, viz. -^ of an
inch, whilst the inner one is only f=£ long. Now if the basal
pair of leaflets of the existing leaves were to become rudimen-
tary, we should expect that the rudiments would still exhibit
some trace of their present great inequality of size. The con-
clusion that the pinnae of the parent-form of M. albida possessed
at least three pairs of leaflets, instead of, as at present, only two,
is supported by the structure of the first true leaf; for this
consists of a simple petiole, often bearing three pairs of leaflets.
This latter fact, as well as the presence of the rudiments, both
lead to the conclusion that M. albida is descended from a form
the leaves of which bore more than two pairs of leaflets. The
second leaf above the cotyledons resembles in all respects the
leaves on fully developed plants.
When the leaves go to sleep, each leaflet twists half round,
so as to present its edge to the zenith, and comes into close
contact with its fellow. The pinnse also approach each other
closely, so that the four terminal leaflets come together. The
large basal leaflets (with the little rudimentary ones in contact
with them) move inwards and forwards, so as to embrace the
outside of the united terminal leaflets, and thus all eight leaflets
Chm\ VII. SLEEP OF LEAVES. 381
(the rudimentary ones included) form together a single vertical
packet. The two pinnae at the same time that they approach
each other sink downwards, and thus instead of extending hori-
zontally in the same line with the main petiole, as during the
day, they depend at night at about 45°, or even at a greater
angle, beneath the horizon. The movement of the main petiole
seems to be variable ; we have seen it in the evening 27° lower
than during the day ; but sometimes in nearly the same position.
Nevertheless, a sinking movement in the evening and a rising
one during the night is probably the normal course, for this
was well-marked in the petiole of the first-formed true leaf.
The circumnutation of the main petiole of a young leaf was
traced during 21 days, and was considerable in extent, but less
complex than that of M. pudica. The movement was much
more lateral than is usual with circumnutating leaves, and this
was the sole peculiarity which it presented. The apex of
one of the terminal leaflets was seen under the microscope to
travel -^ of an inch in 3 minutes.
Mimosa marginata. — The opposite leaflets rise up and approach
each other at night, but do not come into close contact, except in
the case of very young leaflets on vigorous shoots. Full-grown
leaflets circumnutate during the day slowly and on a small scale.
Schrankia uncinata (Tribe 20). — A leaf consists of two or three
pairs of pinnae, each bearing many small leaflets. These, when
the plant is asleep, are directed forwards and become imbricated.
The angle between the two terminal pinnae was diminished at
night, in one case by 15°; and they sank almost vertically down-
wards. The hinder pairs of pinnae likewise sink downwards,
but do not converge, that is, move towards the apex of the leaf.
The main petiole does not become depressed, at least during the
evening. In this latter respect, as well as in the sinking of the
} innae, there is a marked difference between the nyctitropic
movements of the present plant and of Mimosa pudica. It
should, however, be added that our specimen was not in a very
vigorous condition. The pinnae of Schrankia acuhata also sink
at night.
Acacia Farnesiana (Tribe 22). — The different appearance pre-
sented by a bush of this plant when asleep and awake is won-
derful. The same leaf in the two states is shown in the following
figure (Fig. 160). The leaflets move towards the apex of the
pmna and become imbricated, and the pinnae then look like bits
of dangling string. The following remarks and measurements
382
MODIFIED CIRCUMNUTATION.
Chap. Vll
do not fully apply to the small leaf here figured. The pinnae
move forwards and at the same time sink downwards, whilst
the main petiole rises considerably. With respect to the degree
of movement : the two terminal pinnae of one specimen formed
together an angle of 100° during the day, and at flight of only
38°, so each had moved 31° forwards. The penultimate pinnae
during the day formed together an angle of 180°, that is, they
stood in a straight line opposite one another, and at night each
had moved 65° forwards. The basal pair of pinnae were directed
Fig. 160.
A. B.
Acacia Farnesiana; A, leaf during the day; B, the same leaf at night.
during the day, each about 21° backwards, and at night 38°
forwards, so each had moved 59° forwards. But the pinnae at
the same time sink greatly, and sometimes hang almost perpen-
dicularly downwards. The main petiole, on the other hand,
rises much : by 8.30 p.m. one stood 34° higher than at noon,
and by 6.40 a.m. on the following morning it was still higher
by 10°; shortly after this hour the diurnal sinking move-
ment commenced. The course of a nearly full-grown leaf was
traced during 14 h ; it was strongly zigzag, and apparently
CW. YIl. SLEEP OF LEAVES. 38d
represented five ellipses, with their longer axes differently
directed.
Albizzia lopliantha (Tribe 23). — The leaflets at night come into
contact with one another, and are directed towards the apex of
the pinna. The pinnae approach one another, but remain in the
same plane as during the day ; and in this respect they differ
much from those of the above Schrankia and Acacia. The main
petiole rises but little. The first-formed leaf above the coty-
ledons bore 11 leaflets on each side, and these slept like those
on the subsequently formed leaves ; but the petiole of this first
leaf was curved downwards during the day and at night
straightened itself, so that the chord of its arc then stood 16°
higher than in the day-time.
Melaleuca ericcefolia (Myrtacese). — According to Bouche (' Bot.
Zeit.,' 1874, p. 359) the leaves sleep at night, in nearly the same
manner as those of certain species of Pimelia.
(Enothtra mollissima (Onagrarieae). — According to Linnaeus
( c Somnus Plantarum '), the leaves rise up vertically at night.
Passiflora gracilis (Passifloracae). — The young leaves sleep by
their blades hanging vertically downwards, and the whole length
of the petiole then becomes somewhat curved downwards.
Externally no trace of a pulvinus can be seen. The petiole of
the uppermost leaf on a young shoot stood at 10.45 a.m. at 33°
above the horizon ; and at 10.30 p.m., when the blade was verti-
cally dependent, at only 15°, so the petiole had fallen 18°. That
of the next older leaf fell only 7°. From some unknown cause
the leaves do not always sleep properly. The stem of a plant,
which had stood for some time before a north-east window, was
secured to a stick at the base of a young leaf, the blade of
which was inclined at 40° below the horizon. From its position
the leaf had to be viewed obliquely, consequently the vertically
ascending and descending movements appeared when traced
oblique. On the first day (Oct. 12th) the leaf descended in a
zigzag line until late in the evening; and by 8.15 a.m. on the
13th had risen to nearly the same level as on the previous
morning. A new tracing was now begun (Fig. 161). The
leaf continued to rise until 8.50 a.m., then moved a little to the
right, and afterwards descended. Between 11 a.m. and 5 p.m. it
circumnutated, and after the latter hour the great nocturnal
fall commenced. At 7.15 p.m. it depended vertically. The
dotted line ought to have been prolonged much lower down in
the figure. By 6.50 a.m. on the following morning (14th) the
381
MODIFIED CIKCUMNUTATION.
Chap. VII.
leaf had risen greatly, and continued to rise till 7.50 a.m., aftei
which hour it redescended. It should be observed that the lines
traced on this second morning would have coincided with and
confused those previously traced, had not the pot been slided
a very little to the left. In the evening (14th) a mark was
placed behind the filament attached to the apex of the leaf, and
its movement was carefully traced from 5 p.m. to 10.15 p.m.
Fig. 161.
Passiflora gracilis: circumnutation and nyctitropic movement of leaf
traced on vertical glass, from 8.20 A.M. Oct. i3th to 10 A.M. 14th
Figure reduced to two-thirds of original scale.
Between 5 and 7.15 p.m. the leaf descended in a straight line;
and at the latter hour it appeared vertically dependent. But
between 7.15 and 10.15 p.m. the line consisted of a succession
of steps, the cause of which we could not understand ; it was,
however, manifest that the movement was no longer a simple
descending one.
Siegesbeckia orien talis (Compositse). — Some seedlings were
raised in the middle of winter and kept in the hot-house ; they
flowered, but did not grow well, and their leaves never showed
any signs of sleep. The leaves on other seedlings raised in May
wore horizontal at noon (June 22nd), and depended at a consi*
Chap. VII.
SLEEP OF LEAVES.
38, f
derable angle beneath the horizon at 10 p.m. In the case of four
youngish leaves, which were from 2 to 2j inches in length,
these angles were found to be 50°, 56°, 60°, and 65°. At the
end of August, when the plants had grown to a height of 10 to LI
inches, the younger leaves were so much curved downwards at
night that they might truly be said to be asleep. This is one
Fig. 162.
Nicotianct gla»ca : shoots with leaves expanded during the day, and asleep
at night. Figures copied from photographs, and reduced.
of the species which must be well illuminated during the day
in order to sleep, for on two occasions when plants were kept
all day in a room with north-east windows, the leaves did not
sleep at night. The same cause probably accounts for the
leaves on our seedlings raised in the dead of the winter not
Bleeping. Professor Pfeffer informs us that the leaves oi
another species (S. Joruttensis ?) hang vertically down at night.
B86
MODIFIED OIECUMN UTATIOX.
Chap. VII
iun5jp«kia^
W°JA'p.m.lO l .
1/ oman cmrulea said purpurea (Convolvulaceae). —The leases on
very young plants, a foot or two in height, are depressed at night
to between 68° and 80°
%- 163 - beneath the horizon;
and some hang quite
vertically downwards.
On the following morn-
ing they again rise into
a horizontal position.
The petioles become
at night downwardly
curved, either through
their entire length or in
the upper part alone ;
and this apparently
causes the depression
of the blade. It seems
necessary that the
leaves should be well
illuminated during the
day in order to sleep,
for those which stood
on the back of a plant
before a north-east
window did not sleep.
Nicotiana tabacum
(var. Virginian) and
glauca (Solaneae).— The
young leaves of both
these species sleep by
bendinh vertically up-
wards. Figures of two
shoots of N. ftauca,
aw^ake and asleep (Fig.
162), are given on p
385 : one of the shoots,
from which the photo-
graphs were taken, was
accidentally bent to one
side.
At the base of the petiole of N. tabacum, on the outside, there
is a mass of cells, which are rather smaller than elsewhere, and
$£M'a.mJ3 f ! 1
>5°15'ii.L
J1L"
»/&j
JB l
3'p.i
Nicotiana tabacum : circumnutation and nyc-
titropic movement of a leaf (5^- inches in
length), traced on a vertical glass, from
3 P.M. July 10th to 8.10 A.M. 13th. Apex
of leaf 4 inches from glass. Temp. 17£°-
I83 C. Figure reduced to one-half
original scale.
Chap. VII. SLEEP OF LEAVES. 387
have their longer axes differently directed from the ceUs of the
parenchyma, and may therefore be considered as forming a sort
of pnlvinus. A young plant of N. tabacum was selected, and
the circummitation of the fifth leaf above the cotyledons was
observed during three days. On the first morning (July 10th)
the leaf fell from 9 to 10 a.m., which is its normal course, but
rose during the remainder of the day; and this no doubt was
due to its being illuminated exclusively from above; for properly
the evening rise does not commence until 3 or 4 p.m. In the
figure as given on p. 386 (Fig. 163) the first dot wasjnade at
3 p.m. ; and the tracing was continued for the following 65 h.
When the leaf pointed to the dot next above that marked 3 p.m.
it stood horizontally. The tracing is remarkable only from its
simplicity and the straightness of the lines. The leaf each day
described a single great ellipse ; for it should be observed that
the ascending and descending lines do not coincide. On the
evening of the 11th the leaf did not descend quite so low as
usual, and it now zigzagged a little. The diurnal sinking move-
ment had already commenced each morning by 7 a.m. The broken
lines at the top of the figure, representing the nocturnal vertical
position of the leaf, ought to be prolonged much higher up.
Mirabilis longiflora and jalapa (Nyctagine?e). — The first pair
of leaves above the cotyledons, produced by seedlings of both
these species, were considerably divergent during the day, and
at night stood up vertically in close contact with one another.
The two upper leaves on an older seedling were almost horizontal
by day, and at night stood up vertically, but were not in close
contact, owing to the resistance offered by the central bud.
Polygonum aviculare (Polygoneae). — Professor Batalin informs
us that the young leaves rise up vertically at night. This is
likewise the case, according to Linnseus, with several species
of Amaranthus (Amaranthacese) ; and we observed a sleep move-
ment of this kind in one member of the genus. Again, with
Clienopodium album (Chenopodiese), the upper young leaves of
some seedlings, about 4 inches in height, were horizontal or
sub-horizontal during the day, and at 10 p.m. on Mqxch 7th
were quite, or almost quite, vertical. Other seedlings raised in
the greenhouse during the winter (Jan. 28th) were observed day
and night, and no difference could be perceived in the position
of their leaves. According to Bouche (' Bot. Zeitung/ 1874,
p. 359; the leaves of Pimdia linoides and spectabilis (Thymeleae)
sleep at night.
388 MODIFIED CIRCUMNUTATION. Chap. VII.
Euphorbia jacquiniceflora (Euphorbiacese). — Mr. Lynch
called our attention to the fact that the yonng leaves of this
plant sleep by depending vertically. The third leaf from the
summit (March 1] th) was inclined during the day 30° beneath
the horizon, and at night hung vertically down, as did some of
the still younger leaves. It rose up to its former level on the
following morning. The fourth and fifth leaves from the summit
stood horizontally during the day, and sank down at night only
38°. The sixth leaf did not sensibly alter its position. The
sinking movement is due to the downward curvature of the
petiole, no part of which exhibits any structure like that of
a pulvinus. Early on the morning of June 7th a filament was
fixed longitudinally to a young leaf (the third from the summit,
and 2f inches in length), and its movements were traced on
a vertical glass during 72 h., the plant being illuminated from
above through a skylight. Each day the leaf fell in a nearly
straight line from 7 a.m. to 5 p.m., after which hour it was so
much inclined downwards that the movement could no longer
be traced ; and during the latter part of each night, or early in
the morning, the leaf rose. It therefore circumnutated in a
very simple manner, making a single large ellipse every 24 h.,
for the ascending and descending lines did not coincide. On
each successive morning it stood at a less height than on the
previous one, and this was probably due, partly to the increasing
age of the leaf, and partly to the illumination being insufficient ;
for although the leaves are very slightly heliotropic, yet, accord-
ing to Mr. Lynch's and our own observations, their inclination
during the day is determined by the intensity of the light. On
the third day, by which time the extent of the descending
movement had much decreased, the line traced was plainly
much more zigzag than on any previous day, and it appeared
as if some of its powers of movement were thus expended. At
10 p.m. on June 7th, when the leaf depended vertically, its move-
ments were observed by a mark being placed behind it, and the
end of the attached filament was seen to oscillate slowly and
slightly from side to side, as well as upwards and downwards.
PhyUanthus Mruri (Euphorbiacese). — The leaflets of this
plant sleep, as described by Pfeffer,* in a remarkable manner,
apparently like those of Cassia, for they sink downwards at
uight and twist round, so that their lower surfaces are turned
Die Period. Bewcg.,' p. 159.
Chap. VII. SLEEP OF LEAVES 389
outwards. They are furnished, as might have been expected
from this complex kind of movement, with a pulvinus.
Gymnospekms.
Pinus Nordmatunana (Coniferse). — M. Chatin states * that the
leaves, which are horizontal during the day, rise up at night, so
as to assume a position almost perpendicular to the branch from
which they arise ; we presume that he here refers to a horizontal
branch. He adds : " En meme temps, ce mouvement direction
est accompagne d'un mouvement de torsion imprime a la partie
basilaire de la feuille, et pouvant sou vent parcourir un arc do
90 degres." As the lower surfaces of the leaves are white,
whilst the upper are dark green, the tree presents a widely
different appearance by day and night. The leaves on a small
tree in a pot did not exhibit with us any nyctitropic move-
ments. We have seen in a former chapter that the leaves of
Pinus pinaster and Austriaca are continually circumnutating.
Monocotyledons.
Thalia dealbata (Cannaceae). — The leaves of this plant sleep
by turning vertically upwards ; they are furnished with a well-
developed pulvinus. It is the only instance known to us of
a very large leaf sleeping. The blade of a young leaf, which
was as yet only 13i inches in length and 62 in breadth,- formed
at noon an angle with its tall petiole of 121°, and at night stood
vertically in a line with it, and so had risen 59°. The actual
distance travelled by the apex (as measured by an orthogonic
tracing) of another large leaf, between 7.30 a.m. and 10 p.m., was
101 inches. The circumnutation of two young and dwarfed
leaves, arising amongst the taller leaves at the base of the plant,
was traced on a vertical glass during two days. On the first day
the apex of one, and on the second day the apex of the other leaf,
described between 6.40 a.m. and 4 pm. two ellipses, the longer
axes of which were extended in very different directions from the
lines representing the great diurnal sinking and nocturnal rising
movement.
Maranta arundinacea (Cannacese). — The blades of the leaves,
which are furnished with a pulvinus, stand horizontally during
* ' Comptes Rendus,' Jan. 1876, p. 171.
390 MODIFIED CIRCUMNUTATION. Chap. VII
the day or between 10° and 20° above the horizon, and at night
vertically upwards. They therefore rise between 70° and 90° at
night. The plant was placed at noon in the dark in the hot-
house, and on the following day the movements of the leaves
were traced. Between 8.40 and 10 30 a.m. they rose, and then
fell greatly till 1.37 p.m. But by 3 p.m. they had again risen a
little, and continued to rise during the rest of the afternoon and
night ; on the following morning they stood at the same level as
on the previous day. Darkness, therefore, during a day and a
half does not interfere with the periodicity of their, movements.
On a warm but stormy evening, the plant whilst being brought
into the house, had its leaves violently shaken, and at night not
one went to sleep. On the next morning the plant was taken
back to the hot-house, and again at night the leaves did not
sleep ; but on the ensuing night they rose in the usual manner
between 70° and 80°. This fact is analogous with what we
have observed with climbing plants, namely, that much agitation
checks for a time their power of circumnutation ; but the effect
in this instance was much more strongly marked and prolonged.
Colocasia antiquorum {Caladium esculentum, Hort.) (Aroideae).
— The leaves of this plant sleep by their blades sinking in the
evening, so as to stand highly inclined, or even quite vertical'y
with their tips pointing to the ground. They are not provided
with a pulvinus. The blade of one stood at noon 1° beneath the
' horizon; at 4.20 p.m., 20° ; at 6 p.m., 43° ; at 7.20 p.m., 69° ; and at
8 30 p.m., 68° ; so it had now begun to rise ; at 10.15 p.m. it stood
at 65°, and on the following early morning at 11° beneath the
horizon. The circumnutation of another young leaf (with its
petiole only 3j inches, and the blade 4 inches in length), was
traced on a vertical glass during 48 h. ; it was dimly illuminated
through a skylight, and this seemed to disturb the proper perio-
dicity of its movements. Nevertheless, the leaf fell greatly
during both afternoons, till either 7.10 pm. or 9 p.m., when it
rose a little and moved laterally. By an early hour on both
mornings, it had assumed its diurnal position. The well-marked
lateral movement for a short time in the early part of the night,
was the only interesting fact which it presented, as this caused
the ascending and descending lines not to coincide, in accord-
ance with the general rule with circumnutating organs. The
movements of the leaves of this plant are thus of the most
simple kind; and the tracing is not worth giving. We have
seen that in another genus of the Aroideas, namely, Pistia, the
Chap. VII.
SLEEP OF LEAVES.
191
leaves rise so much at night that they may almost be said to
Strephium floribundum* (Graminese). — The oval leaves are
provided with a pulvinus, and are extended horizontally or
declined a little beneath the horizon during the day. Those
on tho upright culms simply rise up vertically at night, so
that their tips are directed towards the zenith. (Fig. 164.)
Fig. 164.
Strephium floribundum : culms with leaves during the day, and when asl<?ej:
at night. Figures reduced.
Horizontally extended leaves arising from much inclined or
almost horizontal culms, move at night so that their tips
point towards the apex of the culm, with one lateral margin
directed towards the zenith; and in order to assume this
position the leaves have to twist on their own axes through an
angle of nearly 90°. Thus the surface of the blade always stands
vertically, whatever may be the position of the midrib or of the
leaf as a whole.
The circumnutation of a young leaf (2 ■ 3 inches in length) was
traced during 48 h. (Fig. 165). The* movement was remarkably
simple; the leaf descended from before 6.40 a.m. until 2 or
2.50 p.m., and then rose so as to stand vertically at about 6 p.m.,
descending again late in the night or in the very early morning.
* A. Brongniart first observed
that the leaves of this plant and
of Marsilea sleep : see ' Bull, de
26
la Soc. Bot. de France/ torn, vii
I860, p. 470.
392
MODIFIED CIRCUMNUTATION.
Chap. VII
On the second day the descending line zigzagged slightly. As
jr lg 165# usual, the ascending and de-
scending lines did not coincide.
) On another occasion, when the
/ temperature was a little higher,
f viz., 24°-26d° C, a leaf was
observed 17 times between 8.50
a.m. and 12.16 p.m. ; it changed
its course by as much as a
rectangle six times in this in-
terval of 3 h. 26 m., and de-
scribed two irregular triangles
and a half. The leaf, therefore,
on this occasion circumnutated
.rapidly and in a complex
manner.
ACOTYLEDONS.
Marsilea quadrifoliaia (Mar-
siieaceas). — The shape of a leaf,
expanded horizontally during
the day, is shown at A (Fig. 166).
Each leaflet is provided with
a well-developed pulvinus.
When the leaves sleep, the two
terminal leaflets rise up, twist
half round and come into con-
tact with one another (B), and
are afterwards embraced by the
two lower leaflets (G); so that
the four leaflets with their lower
surfaces turned outwards form
a vertical packet. The curva-
ture of the summit of the petiole
of the leaf figured asleep, is
merely accidental. The plant
was brought into a room, where
the temperature was only a little
above 60° F., and the movement
of one of the leaflets (the petiole
having been secured) was traced
Strephium floribundum : circumnu-
tation and nyctitropic movement
of a leaf, traced from 9 A.M. June
26th to 8.45 A.M. 27th ; filament
fixed along the midrib. Apex of
leaf 8.J inches from the vertical
glass ; plant illuminated from
above. Temp. 23£°-24£° C.
Chap Ml
SLEEP OF LEAVES.
393
during 24 h. (Fig. 167). The leaf fell from the early morning
till 1.50 p.m., and then rose till 6 p.m., when it was asleep. A
Fig. 166.
A. B. C.
Marsilca quadrifoliaia: A, leaf during the day, seen from vertically above
B, leaf beginning to go to sleep, seen laterally ; C, the same asleep.
Figures reduced to one-half of natural scale.
vertically dependent glass filament was now fixed to one of the
terminal and inner leaflets; and part of the tracing in Fig. 167,
after 6 p.m., shows that it continued to sink,. making one zigzag,
until 10.40 p.m. At 6.45 a.m. on the following morning, the leaf
was awaking, and the filament pointed above the vertical glass,
6° p.m.
J.G11/ nm.
Marsilea quadrifoliaia : circumnutation and nyctitropic movement of leaflet
traced on vertical glass, during nearly 24 h. Figure reduced to two-
thirds of original scale. Plant kept at rather too low a temperature.
but by 8.25 A m. it occupied the position shown in the figure.
The diagram differs greatly in appearance from most of those
previously given; and this is due to the leaflet twisting and
moving laterally as it approaches and comes into contact with
394 MODIFIED CIRCUMNTJTATION. Chap. VII
its fellow. The movement of another leaflet, when asleep,
was traced between 6 p.m. and 10.35 p.m., and it clearly cir-
eumnutated, for it continued for two hours to sink, then rose,
and then sank still lower than it was at 6 p.m. It may be
seen in the preceding figure (167) that the leaflet, when the
plant was subjected to a rather low temperature in the house
descended and ascended during the middle of the day in a
somewhat zigzag line; but when kept in the hot-house from
9 a.m. to 3 p.m. at a high but varying temperature (viz., between
72° and 83° F.) a leaflet (with the petiole secured) circumnutated
rapidly, for it made three large vertical ellipses in the course of
the six hours. According to Brongniart, Marsilea pubescens sleeps
like the present species. These plants are the sole cryptogamic
ones known to sleep.
Summary and Concluding Remarks on the Nijctitrojpic
or Sleep-movements of Leaves. — That these movements
are in some manner of high, importance to the plants
which exhibit them, few will dispute who have ob-
served how complex they sometimes are. Thus with
Cassia, the leaflets which are horizontal during the
day not only bencl at night vertically downwards with
the terminal pair directed considerably backwards, but
they also rotate on their own axes, so that their lower
surfaces are turned outwards. The terminal leaflet
of Melilotus likewise rotates, by which movement one
of its lateral edges is directed upwards, and at the
same time it moves either to the left or to the right,
until its upper surface comes into contact with that ot
the lateral leaflet on the same side, which has like-
wise rotated on its own axis. With Arachis, all four
leaflets form together during the night a single
vertical packet ; and to effect this the two anterior
leaflets have to move upwards and the two posterior
ones forwards, besides all twisting on their own axes.
In the genus Sida the leaves of some species move at
night through an angle of 90° upwards, and of others
Chap. VII. SUMMARY ON SLEEP OF LEAVES. 395
through the same angle downwards. We have seen a
similar difference in the nyctitropic movements of the
cotyledons in the genus Oxalis. In Lupinus, again,
the leaflets move either upwards or downwards ; and
in some species, for instance L. luteus, those on one
side of the star-shaped leaf move up, and those on -the
opposite side move down ; the intermediate ones rota-
ting on their axes ; and by these varied movements, the
whole leaf forms at night a vertical star instead of a
horizontal one, as during the day. Some leaves and
leaflets, besides moving either upwards or downwards,
become more or less folded at night, as in Bauhinia
and in some species of Oxalis. The positions, indeed,
which leaves occupy when asleep are almost infinitely
diversified ; they may point either vertically upwards
or downwards, or, in the case of leaflets, towards the
apex or towards the base of the leaf, or in any inter-
mediate position. They often rotate at least as much
as 90° on their own axes. The leaves which arise
from upright and from horizontal or much inclined
branches on the same plant, move in some few cases
in a different manner, as with Porlieria and Strephium.
The whole appearance of many plants is wonderfully
changed at night, as may be seen with Oxalis, and
still more plainly with Mimosa. A bush of Acacia
Farnesiana appears at night as if covered with little
dangling bits of string instead of leaves. Excluding
a few genera not seen by ourselves, about which we
are in doubt, and excluding a few others the leaflets of
which rotate at night, and do not rise or sink much,
there are 37 genera in which the leaves or leaflets rise,
often moving at the same time towards the apex or
towards the base of the leaf, and 32 genera in which
they sink at night.
The nyctitropic movements of leaves, leaflets, and
3D 6 MODIFIED CIRCUMNUTATION". Chap. VII
petioles are effected in two different ways ; firstly, by
alternately increased growth on their opposite sides,
preceded by increased turgescence of the cells; and
secondly by means of a pulvinus or aggregate of small
cells, generally destitute of chlorophyll, which become
alternately more turgescent on nearly opposite sides ;
and this turgescence is not followed by growth except
during the early age of the plant. A pulvinus seems
to be formed (as formerly shown) by a group of cells
ceasing to grow at a very early age, and therefore does
not differ essentially from the surrounding tissues.
The cotyledons of some species of Trifolium are pro-
vided with a pulvinus, and others are destitute of one,
and so it is with the leaves in the genus Sida. We
see also in this same genus gradations in the state of
the development of the pulvinus ; and in Nicotiana
we have what may probably be considered as the
commencing development of one. The nature of the
movement is closely similar, whether a pulvinus is
absent or present, as is evident from many of the
diagrams given in this chapter. It deserves notice
that when a pulvinus is present, the ascending and
descending lines hardly ever coincide, so that ellipses
are habitually described by the leaves thus provided,
whether they are young or so old as to have quite
ceased growing. This fact of ellipses being described,
shows that the alternately increased turgescence of
the cells does not occur on exactly opposite sides of the
pulvinus, any more than the increased growth which
causes the movements of leaves not furnished with
pulvini. When a pulvinus is present, the nyctitropic
movements are continued for a very much longer
period than when such do not exist. This has been
amply proved in the case of cotyledons, and Pfeffer
has given observations to the same effect with respect
Chap. VII SUMMARY ON SLEEP OF LEAVES. 39 1
to leaves. We have seen that a leaf of Mimosa
pudica continued to move in the ordinary manner,
though somewhat more simply, until it withered and
died. It may be added that some leaflets of Trifolium
pratense were pinned open during 10 days, and on the
first evening after being released they rose up and
slept in the usual manner. Besides the long con-
tinuance of the movements when effected by the aid
of a pulvinus (and this appears to be the final cause
of its development), a twisting movement at night, as
PfefTer has remarked, is almost confined to leaves thus
provided.
It is a very general rule that the first true leaf,
though it may differ somewhat in shape from the
leaves on the mature plant, yet sleeps like them ; and
this occurs quite independently of the fact whether or
not the cotyledons themselves sleep, or whether they
sleep in the same manner. But with Phaseolus Box-
bur gliii the first unifoliate leaves rise at night almost
sufficiently to be said to sleep, whilst the leaflets of
the secondary trifoliate leaves sink vertically at night.
On young plants of Sida rhomhuefolia, only a few
inches in height, the leaves did not sleep, though on
rather older plants they rose up vertically at night.
On the other hand, the leaves on very young plants of
Cytisus fragrans slept in a conspicuous manner, whilst
on old and vigorous bushes kept in the greenhouse,
the leaves did not exhibit any plain nyctitropic move-
ment. In the genus Lotus the basal stipule-like
leaflets rise up vertically at night, and are provided
with pulvini.
As already remarked, when leaves or leaflets change
their position greatly at night and by complicated
movements, it can hardly be doubted that these must
be in some manner beneficial to the plant. If so, we
398 MODIFIED CIRCUMNUTATION. Chap. V\l.
must extend the same conclusion to a large number of
sleeping plants; for the most complicated and tho
simplest nyctitropic movements are connected together
by the finest gradations. But owing to the causes spe-
cified in the beginning of this chapter, it is impossible
in some few cases to determine whether or not certain
movements should be called nyctitropic. Generally,
the position which the leaves occupy at night indi-
cates with sufficient clearness, that the benefit thus
derived, is the protection of their upper surfaces from
radiation into the open sky, and in many cases the
mutual protection of all the parts from cold by their
being brought into close approximation. It should be
remembered that it was proved in the last chapter, that
leaves compelled to remain extended horizontally at
night, suffered much more from radiation than those
which were allowed to assume their normal vertical
position.
The fact of the leaves of several plants not sleeping
unless they have been well illuminated during the
day, made us for a time doubt whether the pro-
tection of their upper surfaces from radiation was in
all cases the final cause of their well-pronounced
nyctitropic, movements. But we have no reason to
suppose that the illumination from the open sky,
during even the most clouded day, is insufficient for
this purpose ; and we should bear in mind that leaves
which are shaded from being seated low down on the
plant, and which sometimes do not sleep, are likewise
protected at night from full radiation. Nevertheless,
we do not wish to deny that there may exist cases in
which leaves change their position considerably at
night, without their deriving any benefit from such
movements.
Although with sleeping plants the blades almost
Chap. VII. SUMMARY ON SLEEP OF LEAVES. 399
always assume at night a vertical, or nearly vertical
position, it is a point of complete indifference whether
the apex, or the base, or one of the lateral edges, is
directed to the zenith. It is a rule of wide generality,
than whenever there is any difference in the degree of
exposure to radiation between the upper and the lower
surfaces of leaves and leaflets, it is the upper which is
the least exposed, as may be seen in Lotus, Cytisus,
Trifolium, and other genera. In several species of
Lupinus the leaflets do not, and apparently from
their structure cannot, place themselves vertically at
night, and consequently their upper surfaces, though
highly inclined, are more exposed than the lower ; and
here we have an exception to our rule. But in other
species of this genus the leaflets succeed in placing
themselves vertically ; this, however, is effected by a
very unusual movement, namely, by the leaflets on
the opposite sides of the same leaf moving in opposite
directions.
It is again a very common rule that when leaflets
come into close contact with one another, they do so
by their upper surfaces, which are thus best protected.
In some cases this may be the direct result of their
vising vertically ; but it is obviously for the pro-
tection of the upper surfaces that the leaflets of
Cassia rotate in so wonderful a manner whilst sinking
downwards ; and that the terminal leaflet of Melilotus
rotates and moves to one side until it meets' the lateral
leaflet on the same side. When opposite leaves or
leaflets sink vertically down without any twisting,
their lower surfaces approach each other and some-
times come into contact; but this is the direct and
inevitable result of their position. With many species
of Oxalis the lower surfaces of the adjoining leaflets
are pressed together, and are thus better protected
400 MODIFIED CIRCUMNUTATION. Chap. vu.
than the upper surfaces ; but this depends merely ou
each leaflet becoming folded at night so as to be able
to sink vertically downwards. The torsion or rotation
of leaves and leaflets, which occurs in so many cases,
apparently always serves to bring their upper surfaces
into close approximation with one another, or with
other parts of the plant, for their mutual protection.
We see this best in such cases as those of Arachis^
Mimosa albida, and Marsilea, in which all the leaflets
form together at night a single vertical packet. If
with Mimosa pudica the opposite leaflets had merely
moved upwards, their upper surfaces would have come
into contact and been well protected ; but as it is,
they all successively move towards the apex of the
leaf; and thus not only their upper surfaces are pro-
tected, but the successive pairs become imbricated and
mutually protect one another as well as the petioles.
This imbrication of the leaflets of sleeping plants is a
common phenomenon.
The nycti tropic movement of the blade is gene-
rally effected by the curvature of the uppermost part
of the petiole, which has often been modified into a
pulvinus ; or the whole petiole, when short, may be
thus modified. But the blade itself sometimes curves
or moves, of which fact Bauhinia offers a striking
instance, as the two halves rise up and come intc
close contact at night. Or the blade and the upper
part of the petiole may both move. Moreover, the
petiole as a whole commonly either rises or sinks at
night. This movement is sometimes large : thus the
petioles of Cassia pubescens stand only a little above
the horizon during the day, and at night rise up
almost, or quite, perpendicularly. The petioles of the
younger leaves of Desmodium gyrans also rise up ver-
tically at night. On the other hand, with Amphi-
Chap. VII. SUMMARY ON SLEEP OF LEAVES. 401
earpsea, the petioles of some leaves sank down as
much as 57° at night; with Arachis they sank 39°,
and then stood at right angles to the stem. Gene-
rally, when the rising or sinking of several petioles on
the same plant was measured, the amount differed
greatly. This is largely determined by the age of the
leaf: for instance, the petiole of a moderately old leaf
of Desmodium gyrans rose only 46°, whilst the young
ones rose up vertically ; that of a young leaf of Cassia
floribunda rose 41°, whilst that of an older leaf rose
only 12°. It is a more singular fact that the age of
the plant sometimes influences greatly the amount of
movement ; thus with some young seedlings of a Bau-
hinia the petioles rose at night 30° and 34°, whereas
those on these same plants, when grown to a height
of 2 or 3 feet, hardly moved at all. The position of
the leaves on the plant as determined by the light,
seems also to influence the amount of movement
of the petiole; for no other cause was apparent
why the petioles of some leaves of Melilotus officinalis
rose as much as 59°, and others only 7° and 9° at
night.
In the case of many plants, the petioles move at
night in one direction and the leaflets in a directly
opposite one. Thus, in three genera of Phaseolese the
leaflets moved vertically downwards at night, and the
petioles rose in two of them, whilst in the third they
sank. Species in the same genus often differ widely
in the movements of their petioles. Even on the same
plant of Lupinus pubescens some of the petioles rose 3o° ?
others only 6°, and others sank 4° at night. The
leaflets of Cassia Barclayana moved so little at night
that they could not be said to sleep, yet the petioles
of some young leaves rose as much as 34°. These
several facts apparently indicate that the movements
102 MODIFIED CIKCUMNUTATION. Chap. VII
of the petioles are not performed for any special pur-
pose; though a conclusion of this kind is generally
rash. When the leaflets sink vertically down at night
and the petioles rise, as often occurs, it is certain that
the upward movement of the latter does not aid the
leaflets in placing themselves in their proper posi-
tion at night, for they have to move through a
greater angular space than would otherwise have been
necessary.
Notwithstanding what has just been said, it may be
strongly suspected that in some cases the rising of
the petioles, when considerable, does beneficially serve
the plant by greatly reducing the surface exposed to
radiation at night. If the reader will compare the
two drawings (Fig. 155, p. 371) of Cassia pubescens,
copied from photographs, he will see that the dia-
meter of the plant at night is about one-third of
what it is by day, and therefore the surface exposed
to radiation is nearly nine times less. A similar
conclusion may be deduced from the drawings (Fig.
149, p. 358) of a branch awake and asleep of Des-
modium gyrans. So it was in a very striking manner
with young plants of Bauhinia, and with Oxalis
Ortegesii.
We are led to an analogous conclusion with respect
to the movements of the secondary petioles of certain
pinnate leaves. The pinnae of Mimosa pudica con-
verge at night; and thus the imbricated and closed
leaflets on each separate pinna are all brought close
together into a single bundle, and mutually protect
one another, with a somewhat smaller surface exposed
to radiation. With Albizzia lophantha the pinnae close
together in the same manner. Although the pinnae
of Acacia Famesiana do not converge much, they
sink downwards. Those of Neptunia oleracea likewise
Chap. VII. SUMMARY ON SLEEP OF LEAVES. 403
move downwards, as well as backwards, towards the
base of the leaf, whilst the main petiole rises. With
Schrankia, again, the pinnae are depressed at night.
Now in these three latter cases, though the pinna)
do not mutually protect one another at night, yet
after having sunk down they expose, as does a
dependent sleeping leaf, much less surface to the
zenith and to radiation than if they had remained
horizontal..
Any one who had never observed continuously a
sleeping plant, would naturally suppose that the leaves
moved only in the evening when going to sleep, and
in the morning when awaking ; but he would be quite
mistaken, for we have found no exception to the rule
that leaves which sleep continue to move during the
whole twenty -four hours ; they move, however, more
quickly when going to sleep and when awaking than
at other times. That they are not stationary during
the day is shown by all the diagrams given, and by
the many more which were traced. It is troublesome
to observe the movements of leaves in the middle of
the night, but this was done in a few cases ; and
tracings were made during the early part of the night
of the movements, in the case of Oxalis, Amphicarpa3a,
two species of Erythrina, a Cassia, Passiflora, Euphorbia
and Marsilea ; and the leaves after they had gone to
s-leep, were found to be in constant movement. When,
however, opposite leaflets come into close contact with
one another or with the stem at night, they are, as we
believe, mechanically prevented from moving, but this
point was not sufficiently investigated.
When the movements of sleeping leaves are traced
during twenty -four hours, the ascending and descend-
ing lines do not coincide, except occasionally and by
accident for a shoi t space ; so that with many plants a
i04 MODIFIED CIECUMNUTATION. Chap. VII
single large ellipse is described during each twenty-foui
hours. Such ellipses are generally narrow and ver-
tically directed, for the amount of lateral movement is
small. That there is some lateral movement is shown
by the ascending and descending lines not coinciding,
and occasionally, as with Besmodium gyrans and Thalia
dealbata, it was strongly marked. In the case of Meli-
lotus the ellipses described by the terminal leaflet
during the day are laterally extended, instead of ver-
tically, as is usual ; and this fact v evidently stands in
relation with the terminal leaflet moving laterally
when it goes to sleep. With the majority of sleeping
plants the leaves oscillate more than once up and
down in the twenty-four hoi?rs ; so that frequently two
ellipses, one of moderate size, and one of very large size
which includes the nocturnal movement, are described
within the twenty-four hours. For instance, a leaf
which stands vertically up during the night will sink
in the morning, then rise considerably, again sink in
the afternoon, and in the evening reascend and assume
its vertical nocturnal position. It will thus describe,
in the course of the twenty-four hours, two ellipses of
unequal sizes. Other plants describe within the same
time, three, four, or five ellipses. Occasionally the
longer axes of the several ellipses extend in different
directions, of which Acacia Farnesiana offered a good
instance. The following cases will give an idea of the
rate of movement : Oxalis acetosella completed two
ellipses at the rate of 1 h. 25 m. for each ; Marsilea
quadrifoliata, at the rate of 2 h.; Trifolium subterraneum,
one in 3 h. 30 m. ; and Arachis liypogxa, in 4 h. 50 m.
But the number of ellipses described within a given
time depends largely on the state of the plant and
on the conditions to which it is exposed. It often hap-
pens that a single ellipse may be described during one
Chap. VII. SUMMAKY ON SLEEP OF LEAVES. 405
clay, and two on the next. Erythrina corallodendron
made four ellipses on the first day of observation
and only a single one on the third, apparently owing
to having been kept not sufficiently illuminated and
perhaps not warm enough. But there seems likewise
to be an innate tendency in different species of the
same genus to make a different number of ellipses in
the twenty-four hours : the leaflets of Trifolium repens
made only one ; those of T. resupinatum two, and those
of T. subterraneum three in this time. Again, the
leaflets of Oxalis Plumierii made a single ellipse ; those
of 0. bupleurifolia, two ; those of 0. Valdiviana, two or
three; and those of 0. aeetosella, at least five in the
twenty-four hours.
The line followed by the apex of a leaf or leaflet,
whilst describing one or more ellipses during the day,
is often zigzag, either throughout its whole course or
only during the morning or evening : Eobinia offered
an instance of zigzagging confined to the morning,
and a similar movement in the evening is shown in
the diagram (Fig. 126) given under Sida. The amount
of the zigzag movement depends largely on the plant
being placed under highly favourable conditions. But
even under such favourable conditions, if the dots which
mark the position of the apex are made at consider-
able intervals of time, and the dots are then joined,
the course pursued will still appear comparatively
simple, although the number of the ellipses will be
increased ; but if dots are made every two or three
minutes and these are joined, the result often is that
all the lines are strongly zigzag, many small loops,
triangles, and other figures being also formed. This
fact is shown in two parts of the diagram (Fig. 150)
of the movements of Desmodium gyrans. Strepliium
floribundum, observed under a high temperature,
106 MODIFIED CIRCUMNUTATION. Chap. VII
made several little triangles at the rate of 43 m.
for each. Mimosa pudiea, similarly observed, de-
scribed three little ellipses in 67 m. ; and the apex
of a leaflet crossed -^q of an inch in a second, or
0'12 inch in a minute. The leaflets of Averrhoa
made a countless number of little oscillations when
the temperature was high and the sun shining. The
zigzag movement may in all cases be considered as
an attempt to form small loops, which are drawn out
by a prevailing movement in some one direction. The
rapid gyrations of the little lateral leaflets of Des-
modiuni belong to the same class of movements,
somewhat exaggerated in rapidity and amplitude.
The jerking movements, with a small advance and
still smaller retreat, apparently not exactly in the
same line, of the hypocotyl of the cabbage and of
the leaves of Dionsea, as seen under the microscope,
all probably come under this same head. We may
suspect that we here see the energy which is freed
during the incessant chemical changes in progress in
the tissues, converted into motion. Finally, it should
be noted that leaflets and probably some leaves, whilst
describing their ellipses, often rotate slightly on their
axes ; so that the plane of the leaf is directed first to
one and then to another side. This was plainly seen
to be the case with the large terminal leaflets of Des-
modium, Erythrina and Arnphicarpsea, and is probably
common to all leaflets provided with a pulvinus.
With "espect to the periodicity of the movements of
sleeping leaves, Pfeffer * has so clearly shown that
this depends on the daily alternations of light and
darkness, that nothing farther need be said on this
* 'Die Perio'Iisclien Bewcgungen der Blattorgnne,' 1875, p. 30, ei
•passim.
Ohap. VII. SUMMARY ON SLEEP OF LEAVES. 407
head. But we may recall the behaviour of Mimosa
in the North, where the sun does not set, and the
complete inversion of the daily movements by artificial
light and darkness. It has also been shown by us,
that although leaves subjected to darkness for a mode-
rately long time continue to circumnutate, yet the
periodicity of their movements is soon greatly dis-
turbed, or quite annulled. The presence of light or
its absence cannot be supposed to be the direct cause
of the movements, for these are wonderfully diversified
even with the leaflets of the same leaf, although all
have of course been similarly exposed. The move-
ments depend on innate causes, and are of an adaptive
nature. The alternations of light and darkness
merely give notice to the leaves that the period has
arrived for them to move in a certain manner. We
may infer from the fact of several plants (Tropreoluni,
Lupinus, &c.) not sleeping unless they have been well
illuminated during the day, that it is not the actual
decrease of light in the evening, but the contrast
between the amount at this hour and during the early
part of the day, which excites the leaves to modify
their ordinary mode of circumnutation.
As the leaves of most plants assume their proper
diurnal position in the morning, although light be
excluded, and as the leaves of some plants continue to
move in the normal manner in darkness during at
least a whole day, we may conclude that the periodi-
city of their movements is to a certain extent in-
herited.* The strength of such inheritance differs
* Pfeffer denies such inherit- " Nachwirkung," or Ihe after-
p.nce; he attributes (' Die Period. effects of light and darkness.
Bewegungen,' pp. 30-56) the But we are unable to follow his
periodicity when prolonged for train of reasoning. There does
a day or two in d..rkness, to not seem to be anymore reason foi
27
108 MODIFIED CIKCUMNUTATION. . Chap. VH
much in different species, and seems never to be rigid ;
for plants have been introduced from all parts of the
world into our gardens and greenhouses ; and if their
movements had been at all strictly fixed in relation to
the alternations of day and night, they would have
slept in this country at very different hours, which
is not the case. Moreover, it has been observed that
sleeping plants in their native homes change their
times of sleep with the changing seasons. *
We may now turn to the systematic list (p. 320),
This contains the names of all the sleeping plants
known to us, though the list undoubtedly is very
imperfect. It may be premised that, as a general
rule, all the species in the same genus sleep in
nearly the same manner. But there are some ex-
ceptions; in several large genera including many
sleeping species (for instance, Oxalis), some do not
sleep. One species of Melilotus sleeps like a Tri-
folium, and therefore very differently from its con-
geners ; so does one species of Cassia. In the genus
Sida, the leaves either rise or fall at night ; and with
Lupinus they sleep in three different methods. Re-
turning to the list, the first point which strikes us, is
that there are many more genera amongst the Legu-
minosae (and in almost every one of the Leguminous
tribes) than in all the other families put together;
and we are tempted to connect this fact with the great
attributing such movements to this effect must be produced on the
cause than, for instance, the in- seeds by the long-continued culti-
herited habit of winter and vation of the parent-plants under
summer wheat to grow best at different climates, but no one pro-
different seasons; fir this habit bably would call this the " Naeh«
is lost after a few years, like the wirkung " of the climates,
movements of leavi s in darkness * Pfefifer, ibid., p. 46.
after a few days. No doubt some
Crap. VII. SUMMARY ON SLEEP OF LEAVES. 40S
mobility of the stems and leaves in this family, as
shown by the large number of climbing species which
it contains. Next to the Leguminosae come the Mal-
vaceae, together with some closely allied families. But
by far the most important point in the list, is that we
meet with sleeping plants in 28 families, in all Ihe
great divisions of the Phanerogamic series, and in one
Cryptogam. Now, although it is probable that with
the Leguminosse the tendency to sleep may have been
inherited from one or a few progenitors and possibly
so in the cohorts of the Malvales and Chenopodiales,
yet it is manifest that the tendency must have been
acquired by the several genera in the other families,
quite independently of one another. Hence the ques-
tion naturally arises, how has this been possible ?
and the answer, we cannot doubt, is that leaves owe
their nyctitropic movements to their habit of cir-
cumnutating, — a habit common to all plants, and
everywhere ready for any beneficial development or
modification.
It has been shown in the previous chapters that the
leaves and cotyledons of all plants are continually
moving up and down, generally to a slight but some-
times to a considerable extent, and that they describe
either one or several ellipses in the course of twenty-
four hours ; they are also so far affected by the alter-
nations of day and night that they generally, or
at least often, move periodically to a small extent ;
and here we have a basis for the development of the
greater nyctitropic movements. That the movements
of leaves and cotyledons which do not sleep come
within the class of circumnutating movements cannot
be doubted, for they are closely similar to those of
hypocotyls, epicotyls, the stems of mature plants, and
of various other organs. Now, if we take the simplest
410 MODIFIED CIRCUMNUTATION. Chap. VII.
case of a sleepiDg leaf, we see that it makes a single
ellipse in the twenty-four hours, which resembles one
described by a non-sleeping leaf in every respect, except
that it is much larger. In both cases the course pursued
is often zigzag. As all non-sleeping leaves are inces-
santly circumnutating, we must conclude that a part
at least of the upward and downward movement of one
that sleeps, is due to ordinary circumnutation ; and it
seems altogether gratuitous to rank the remainder of
the movement under a wholly different head. With
a multitude of climbing plants the ellipses which they
describe have been greatly increased for another pur-
pose, namely, catching hold of a support. With these
climbing plants, the various circumnutating organs have
been so far modified in relation to light that, differently
from all ordinary plants, they do not bend towards it.
With sleeping plants the rate and amplitude of the
movements of the leaves have been so far modified in
relation to light, that they move in a certain direction
with the waning light of the evening and with the
increasing light of the morning more rapidly, and to
a greater extent, than at other hours
But the leaves and cotyledons of many non-sleeping
plants move in a much more complex manner than in
the cases just alluded to, for they describe two, three,
or more ellipses in the course of a day. Now, if a
plant of this kind were converted into one that slept,
one side of one of the several ellipses which each
leaf daily describes, would have to be greatly increased
in length in the evening, until the leaf stood ver-
tically, when it would go on circumnutating about the
same spot. On the following morning, the side of
another ellipse would have to be similarly increased
in length, so as to bring the leaf back again into its
diurnal position, when it would again circumnutate
CHAr. VII SUMMARY ON SLEEP OF LEAVES. 411
until the evening. If the reader will lock, for in-
stance, at the diagram (Fig. 142, p. 351), representing
the nyctitropic movements of the terminal leaflet of
Trifolium subterranewn, remembering that the curved
broken lines at the top ought to be prolonged much
higher up, he will see that the great rise in the evening
and the great fall in the morning together form a
large ellipse like one of those described during the
daytime, differing only in size. Or, he may look at
the diagram (Fig. 103, p. 236) of the 3£ ellipses
described in the course of 6 h. 35 m. by a leaf of
Lupinus speciosus, which is one of the species in this
genus that does not sleep ; and he will see that by
merely prolonging upwards the line which was already
rising late in the evening, and bringing it down
again next morning, the diagram would represent the
movements of a sleeping plant.
With those sleeping plants which describe several
ellipses in the daytime, and which travel in a strongly
zigzag line, often making in their course minute loops,
triangles, &c, if as soon as one of the ellipses begins
in the evening to be greatly increased in size, dots are
made every 2 or 3 minutes and these are joined, the
line then described is almost strictly rectilinear, in
strong contrast with the lines made during the day-
time. This was observed with Desmodium gyrans and
Mimosa pudica. With this latter plant, moreover, the
pinnae converge in the evening by a steady move-
ment, whereas during the day they are continually
converging and diverging to a slight extent. In all
such cases it was scarcely possible to observe the
difference in the movement during the day and even-
ing, without being convinced that in the evening the
plant saves the expenditure of force by not moving
laterally, and that its whole energy is now expended
112 MODIFIED CIRCUMNUTATION. Chap. VII
in gaining quickly its proper nocturnal position by
a direct course. In several other cases, for instance,
when a leaf after describing during the day one or
more fairly regular ellipses, zigzags much in the
evening, it appears as if energy was being expended,
so that the great evening rise or fall might coin-
cide with the period of the day proper for this
movement.
The most complex of all the movements performed
by sleeping plants, is that when leaves or leaflets,
after describing in the daytime several vertically
directed ellipses, rotate greatly on their axes in the
evening, by which twisting movement they occupy
a wholly different position at night to what they do
during the day. For instance, the terminal leaflets
of Cassia not only move vertically downwards in the
evening, but twist round, so that their lower surfaces
face outwards. Such movements are wholly, or almost
wholly, confined to leaflets provided with a pulvinus.
But this torsion is not a new kind of movement
introduced solely for the purpose of sleep; for it
has been shown that some leaflets whilst describing
their ordinary ellipses during the daytime rotate
slightly, causing their blades to face first to one side
and then to another. Although we can see how the
slight periodical movements of leaves in a vertical
plane could be easily converted into the greater yet
simple nycti tropic movements, we do not at present
know by what graduated steps the more complex
movements, effected by the torsion of the pulvini,
have been acquired. A probable explanation could
be given in each case only after a close investigation
of the movements in all the allied forms.
From the facts and considerations now advanced we
may conclude that nyctitropism, or the sleep of leaves
Chap. VII. MODIFIED CIRCUMNUTATION. 413
and cotyledons, is merely a modification of their ordi-
nary circumnutating movement, regulated in its period
and amplitude by the alternations of light and dark-
ness. The object gained is the protection of the upper
surfaces of the leaves from radiation at night, often
combined with the mutual protection of the several
parts by their close approximation. In such cases as
those of the leaflets of Cassia — of the terminal leaflets
of Melilotus — of all the leaflets of Arachis, Marsilea,
&c. — we have ordinary circumnutation modified to the
extreme extent known to us in any of the several great
classes of modified circumnutation. On this view of
the origin of nyctitropism we can understand how it
is that a few plants, widely distributed throughout the
Vascular series, have been able to acquire the habit of
placing the blades of their leaves vertically at night,
that is, of sleeping, — a fact otherwise inexplicable.
The leaves of some plants move during the day in
a manner, which has improperly been called diurnal
sleep ; for when the sun shines brightly on them, they
direct their edges towards it. To such cases we shall
recur in the following chapter on Heliotropism. It
has been shown that the leaflets of one form of
Porlieria hygrometrica keep closed during the day, as
long as the plant is scantily supplied with water, in
the same manner as when asleep ; and this apparently
serves to check evaporation. There is only one other
analogous case known to us, namely, that of certain
Gramineae, which fold inwards the sides of their narrow
leaves, when these are exposed to the sun and to a
dry atmosphere, as described by Duval- Jouve.* We
have also observed the same phenomenon in Elymus
arenareus.
* » Annal. des Sc. Nat. (Bpt.),' 1875. torn. i. pp. 32c -329.
114 STKUCTURE OF Chap. VII
There is another movement, which since the time
of Linnaeus has generally been called sleep, namely^
that of the petals of the many flowers which close at
night. These mo Fements have been ably investigated
by PfefTer, who has shown (as was first observed by
Hofmeister) that they are caused or regulated more
by temperature than by the alternations of light and
darkness. Although they cannot fail to protect the
organs of reproduction from radiation at night, this
does not seem to be their chief function, but rather
the protection of the organs from cold winds, and
especially from rain, during the day. The latter
seems probable, as Kerner * has shown that a widely
different kind of movement, namely, the bending down
of the upper part of the peduncle, serves in many
cases the same end. The closure of the flowers will
also exclude nocturnal insects which may be ill-adapted
for their fertilisation, and the well-adapted kinds at
periods when the temperature is not favourable for
fertilisation. Whether these movements of the petals
consist, as is probable, of modified circumnutation we
do not know.
Emhnjology of Leaves. — A few facts have been in-
cidentally given in this chapter on what may be called
the embryology of leaves. With most plants the
first leaf which is developed after the cotyledons,
resembles closely the leaves produced by the mature
plant, but this is not always the case. The first
leaves produced by some species of Drosera, for instance
by D. Capensis, differ widely in shape from those
borne by the mature plant, and resemble closely the
eaves of D. rotundifolia, as was shown to us by Prof.
Williamson of Manchester. The first true leaf of
Die Schutzmittt 1 des Pollens,' 1873, pp. 30-39.
Chap. VII. FIRST-FORMED LEAVES. 415
the gorse, or Ulex, is not narrow and spinose like the
older leaves. On the other hand, with many Legumi-
nous plants, for instance, Cassia, Acacia lophantka, &c,
the first leaf has essentially the same character as the
older leaves, excepting that it bears fewer leaflets. In
Trifolium the first leaf generally bears only a single
leaflet instead of three, and this differs somewhat in
shape from the corresponding leaflet on the older leaves.
Now, with Trifolium Pannonicum the first true leaf on
some seedlings was unifoliate, and on others completely
trifoliate ; and between these two extreme states there
were all sorts of gradations, some seedlings bearing
a single leaflet more or less deeply notched on one
or both sides, and some bearing a single additional
and perfect lateral leaflet. Here, then, we have the
rare opportunity of seeing a structure proper to a more
advanced age, in the act of gradually eucroaching on
and replacing an earlier or embryological condition.
The genus Melilotus is closely allied to Trifolium, and
the first leaf bears only a single leaflet, which at night
rotates on its axis so as to present one lateral edge to
the zenith. Hence it sleeps like the terminal leaflet
of a mature plant, as was observed in 15 species, and
wholly unlike the corresponding leaflet of Trifolium,
which simply bends upwards. It is therefore a curious
fact that in one of these 15 species, viz., M. Taurica (and
in a lesser degree in two others), leaves arising from
young shoots, produced on plants which had been cut
down and kept in pots during the winter in the green-
house, slept like the leaves of a Trifolium, whilst the
leaves on the fully-grown branches on these same
plants afterwards slept normally like tl ose of a Meli-
lotus. If young shoots rising from the ground may
be considered as new individuals, partaking to a certain
extent of the nature of seedlings, then the peculiar
manner in which their leaves slept may be considered
4:16 STRUCTURE OF Chap. VII.
as an erubryological habit, probably the result of Meli-
lotus being descended from some form which slept like
a Trifolimn. This view is partially supported by th6
leaves on old and young branches of another species,
M. Messanensis (not included in the above 15 species);
always sleeping like those of a Trifolium.
The first true leaf of Mimosa alb i da consists of a
simple petiole, often bearing three pairs of leaflets, all
of which are of nearly equal size and of the same
shape : the second leaf differs widely from the first,
and resembles that on a mature plant (see Fig. 159,
p. 379), for it consists of two pinnae, each of which
bears two pairs of leaflets, of which the inner basal
one is very small. But at the base of each pinna
there is a pair of minute points, evidently rudiments
of leaflets, for they are of unequal sizes, like the two
succeeding leaflets. These rudiments are in one sense
embryological, for they exist only during the youth of
the leaf, falling off and disappearing as soon as it is
fully grown.
With Desmodium gyrans the two lateral leaflets are
very much smaller than the corresponding leaflets in
most of the species in this large genus ; they vary
also in position and size ; one or both are sometimes
absent ; and they do not sleep like the fully-developed
leaflets. They may therefore be considered as almost
rudimentary ; and in accordance with the general prin-
ciples of embryology, they ought to be more constantly
and fully developed on very young than on old plants.
But this is not the case, for they were quite absent
on some young seedlings, and did not appear until
from 10 to 20 leaves had been formed. This fact
leads to the suspicion that D. gyrans is descended
through a unifoliate form (of which some exist) from
a trifoliate species ; and that the little lateral leaflets
reappear through reversion. However this may be,
Chap. VII. FIKST-FORMED LEAVES. 417
the interesting fact of the pulvini or organs of move-
ment of these little leaflets, not having been reduced
nearly so much as their blades — taking the large
terminal leaflet as the standard of comparison — gives
us probably the proximate cause of their extraoidiuaiy
pov or of gyration.
4J8 MODIFIED CIRCUMNUTATION. Chap. VI1L
CHAPTEE VIII.
Modified Ciroumnutation : Movements excited by Light.
Dintinction between heliotropism and the effects of light on the perio-
dicity of the movements of leaves — Heliotropic movements of Beta,
Solanum, Zea, and Avena — Heliotropic movements towards an
obscure lkjht in Apios, Brassica, Phalaris, Tropseolum, 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 t lie
weight of the blades of cotyledons — So-called diurnal sleep — Chloro-
phyll injured by intense light — Movements to avoid interne light.
Sachs first clearly pointed out the important dif-
ference between the action of light in modifying the
periodic movements of leaves, and in causing them to
bend towards its source.* The latter, or heliotropic
movements are determined by the direction of the light,
whilst periodic movements are affected by changes in
its intensity and not by its direction. The periodicity
of the circumnutating movement often continues for
some time in darkness, as we have seen in the last
chapter ; whilst heliotropic bending ceases very quickly
when the light fails. Nevertheless, plants which have
ceased through long-continued darkness to move pe-
riodically, if re-exposed to the light are still, according
to Sachs, heliotropic.
Apheliotropism, or, as usually designated, negative
* 'Physiologic Veg' (French Translation), 1868, pp.42, 517, &c.
Chap. VIII. MOVEMENTS EXCITED BY LIGHT. 419
heliotropism, implies that a plant, when unequally
illuminated on the two sides, bends from the light,
instead of, as in the last sub-class of cases, towards it ;
but apheliotropism is comparatively rare, at least in a
w^U-marked degree. There is a third and large sub-
class of cases, namely, those of " Transversal- Helio-
tropismus " of Frank, which we will here call diahelio-
tropism. Parts of plants, under this influence, place
themselves more or less transversely to the direction
whence the light proceeds, and are thus fully illumi-
nated. There is a fourth sub-class, as far as the final
cause of the movement is concerned ; for the leaves of
some plants when exposed to an intense and injurious
amount of light direct themselves, by rising or sinking
or twisting, so as to be less intensely illuminated.
Such movements have sometimes been called diurnal
sleep. If thought advisable, they might be called
paraheliotropic, and this* term would correspond with
our other terms.
It will be shown in the present chapter that all the
movements included in these four sub-classes, con-
sist of modified circumnutation. We do not pretend to
say that if a part of a plant, whilst still growing, did not
circumnutate — though such a supposition is most im-
probable j — it could not bend towards the light ; but, as
a matter of fact, heliotropism seems always to consist
of modified circumnutation. Any kind of movement
in relation to light will obviously be much facilitated
by each part circumnutating or bending successively
in all directions, so that an already existing movement
has only to be increased in some one direction, and to
be lessened or stopped in the other directions, in order
that it should become heliotropic, apheliotropic, &c,
as the case may be. In the next chapter some obser-
vations on the sensitiveness of plants to light, their
420
MODIFIED CIKCUMNUTATION.
Chap VIU
rate of bending towards it, and the accuracy with
which they point towards its source, &c, will be
given. Afterwards it will be shown — and this seems
to us a point of much interest — that sensitiveness to
light is sometimes confined to a small part of the
plant; and that this part when stimulated by light,
transmits an influence to distant parts, exciting them
to bend.
Heliotropism. — When a plant which is strongly
heliotropic (and species differ much in this respect)
is exposed to a bright lateral light, it bends quickly
towards it, and the course pursued by the stem is
quite or nearly straight. But if the light is much
dimmed, or occasionally interrupted, or admitted in
only a slightly oblique direction,
the course pursued is more or less
zigzag ; and as we have seen and
shall again see, such zigzag move-
ment results from the elongation or
drawing out of the ellipses, loops,
&c, which the plant would have de-
scribed, if it had been illuminated
from above. On several occasions
we were much struck with this fact,
whilst observing the circumnuta-
tion of highly sensitive seedlings,
which were unintentionally illu-
minated rather obliquely, or only
at successive intervals of time.
Fig. 168.
Beta vulga -''s : circumnu-
tation of hypocotyl, de-
flected by the light
being slightly lateral,
traced on a horizontal
glass from 8.30 A.M. to
5.30 p.m. Direction of the
lighted taper by which
it was illuminated,
shown by a line joining
the first and penultimate
dots. Figure reduced to
one-third of the original
scale.
For instance, two young seedlings of
Beta vulgaris were placed in the middle
of a room with north-east windows, and
were kept covered up, except during
each observation which lasted for only a minute or two ; but the
result was that their hypocotyls bowed themselves to the side,
whence some light occasionally entered, in lines which were
Chap. VIII.
HELIOTROPISM.
421
only slightly zigzag. Although not a single ellipse was eTen
approximately formed, we inferred from the zigzag lines — and,
as it proved, correctly — that their hypocotyls were circumnuta-
ting, for on the following day these same seedlings were placed
in a completely darkened room, and were observed each time by
the aid of a small wax taper held almost
directly above them, and their movements
were traced on a horizontal glass above ;
and now their hypocotyls clearly circum-
nutated (Fig. 168, and Fig. 39, formerly
given, p. 52); yet they moved a short
distance towards the side where the taper
was held up. If we look at these diagrams,
and suppose that the taper had been held
more on one side, and that the hypocotyls,
still circumnutating, had bent themselves
within the same time much more towards
the light, long zigzag lines would ob-
viously have been the result.
Again, two seedlings of Solanum Jy co-
pers i cum were illuminated from above,
but accidentally a little more light entered
on one than on any other side, and their
hypocotyls became slightly bowed towards
the brighter side ; they moved in a zigzag-
line and described in their course two little
triangles, as seen in Fig. 37 (p. 50), and
in another tracing not given. The sheath-
like cotyledons of Zea mays behaved, under
nearly similar circumstances, in a nearly 8°a.m ;
similar manner, as described in our first ^nasativa: heliotrope
movement and circum-
nutation of sheath-like
cotyledon (1J inch in
height) traced on hori-
zontal glass from 8 A.M.
to 10.25 P.M. Oct. Kith.
chapter (p. 64), for they bowed themselves
during the whole day towards one side,
making, however, in their course some
conspicuous flexures. Before we knew
how greatly ordinary circumnutation was
modified by a lateral light, some seedling oats, with rather old
and therefore not highly sensitive cotyledons, were placed in
front of a north-east window, towards which they bent all day in
a strongly zigzag course. On the following day they continued
to bend in the same direction (Fig. 169), but zigzagged much
less. The sky, however, became between 12.40 and 2.35 P.Bt.
122 MODIFIED CIRCUMNUTATION. Chap. VIII.
overcast with extraordinarily dark thunder-clouds, and it was
interesting to note how plainly the cotyledons circumnutated
during tins interval.
The foregoing observations are of some
*ig. 170. value, from having then made when we were
1° not attending to heliotropism ; and they led
; % us to experiment on several kinds of seed-
*£ lings, by exposing them to a dim lateral light,
| S so as to observe the gradations between
\ 2 ordinary circumnutation and heliotropism.
j.Sf- Seedlings in pots were placed in front of,
s and about a yard from, a north-east window ;
\ ^ on each side and over the pots black boards
: - 1 were placed ; in the rear the pots were open
3 U, to the diffused light of the room, which
: rr - had a second north-east and a north-west
\ q window. By hanging up one or more blinds
- o before the window where the seedlings stood,
; J; it was easy to dim the light, so that very
; o little more entered on this side than on the
3 j opposite one, which received the diffused
\ < light of the room. Late in the evening the
J'S blinds were successively removed, and as the
I -g g plants had been subjected during the day to
g. 2 a very obscure light, they continued to bend
c m towards the window later in the evening than
g «S would otherwise have occurred. Most of the
£-3 seedlings were selected because they were
g g known to be highly sensitive to light, and
% •£ some because they were but little sensitive,
2 ^ . or had become so from having grown old.
•gLfl^ The movements were traced in the usual
S^Z manner on a horizontal glass cover; a fine
~ I B glass filament with little triangles of paper
A *^*S having been cemented in an upright position
§£"3 to the hypocotyls. Whenever the stem or
"S ^ "£ hypocotyl became much bowed towards the
§ "* '% light, the latter part of its course had to
^-f S be traced on a vertical glass, parallel to the
•S "* window, and at right angles to the horizontal
^ glass cover.
Apios graveolens. — The hypocotyl bends in a few hours rectan-
i
Chap VIII. HELIOTROPISM. 423
gularly towards a bright lateral light. In order to ascertain
how straight a course it would pursue when fairly well illumi-
nated on one side, seedlings were first placed before a south-west
window on a cloudy and rainy morning ; and the movement of
two hypocotyls were traced for 3 h., during which time they
became greatly bowed towards the light. One of these tracings
is given on p. 422 (Fig. 170), and the course may be seen to be
almost straight. But the amount of light on this occasion was
superfluous, for two seedlings were placed before a north-east
window, protected by an ordinary linen and two muslin blinds,
yet their hypocotyls moved towards this rather dim light in
only slightly zigzag lines; but after 4p.m., as the light waned,
the lines became distinctly zigzag. One of these seedlings,
moreover, described in the afternoon an ellipse of considerable
size, with its longer axis directed towards the window.
We now determined that the light should be made dim
enough, so we began by exposing several seedlings before a
north-east window, protected by one linen blind, three muslin
blinds, and a towel. But so little light entered that a pencil
cast no perceptible shadow on a white card, and the hypocotyls
did not bend at all towards the window. During this time,
from 8.15 to 10.50 a.m., the hypocotyls zigzagged or circum-
nutated near the same spot, as may be seen at A, in Fig. 171.
The towel, therefore, was removed at 10.50 a.m., and replaced
by two muslin blinds, and now the light passed through
one ordinary linen and four muslin blinds. When a pencil
was held upright on a card close to the seedlings, it cast a
shadow (pointing from the window; which could only just
be distinguished. Yet this very slight excess of light on
one side sufficed to cause the hypocotyls of all the seedlings
immediately to begin bending in zigzag lines towards the
window. The course of one is shown at A (Fig. 171): after
moving towards the window from 10.50 a.m. to 12.48 p.m. it
bent from the wiudow, and then returned in a nearly parallel
line; that is, it almost completed between 12.48 and 2 p.m.
a narrow ellipse. Late in the evening, as the light waned,
the hypocotyl ceased to bend towards the window, and circum-
nutated on a small scale round the same spot ; during the night
it moved considerably backwards, that is, became more upright,
through the action of apogeotropism. At B, we have a tracing
of the movements of another seedling from the hour (10.50 a.m.)
when the towel was removed ; and it is in all essential respects
424
MODIFIED CIECUMNUTATION.
Chap. V1U
similar to the previous one. In these two cases there could be
no doubt that the ordinary circurnnutating movement of the
hypocotyl was modified and rendered heliotropic.
Fig. 171.
12.48,'*
10°. 50 a*
815 f a.m*
10>~50 a.m.
Apios grareolens : heliotropic movement and circumnutation of the hypo-
cotyls of two seedlings towards a dim lateral light, traced on a horizontal
glass during the day. The broken lines show their return nocturnal
courses. Height of hypocotyl of A -5, and of B "55 inch. Figure reduceJ
to one-half of original scale.
Brassica oleracea. — The 'hypocotyl of the cabbage, when not
disturbed by a lateral light, circumnutates in a complicated
Chap. VIII.
HELIOTROriSM.
42^
manner over nearly the same space, and a figure formerly given
is here reproduced (Fig. 172). If the hypocotyl is exposed to
a moderately strong lateral light it moves quickly towards this
side, travelling in a straight, or nearly straight, line. But when
the lateral light is very dim its course is extremely tortuous, and
evidently consists of modified circumnutation. Seedlings were
placed before a north-east window, protected by a linen and
muslin blind and by a towel. The sky was cloudy, and when-
ever the clouds grew a little lighter an additional muslin blind
was temporarily suspended. The light from the window was
Fig. 172.
Brassica olcracea ordinary circumnutating increment of the hj-pocoty! of
a seedling plant.
thus so much obscured that, judging by the unassisted eye, the
seedlings appeared to receive more light from the interior
of the room than from the window; but this was not really
the case, as was shown by a very faint shadow cast by a pencil
on a card. Nevertheless, this extremely small excess of light
on one side caused the hypocotyls, which in the morning had
stood upright, to bend at right angles towards the window,
so that in the evening (after 4.23 p.m.) their course had to be
traced on a vertical glass parallel to the window. It should be
stated that at 3.30 p.m., by which time the sky had become
darker, the towel was removed and replaced by an additional
muslin blind, which itself was removed at 4 p.m., the other two
126
MODIFIED CIRCUMNUTATION.
Chap. VilL
blinds being left suspended. In Fig. 173 the course pursued,
between 8.9 a.m. and 7.10 p.m., by one cf the hypocotyls thiia
>8°25'
Brasska oleracea : heliotropic movement and circumnutation of a hypocotyl
towards a very dim lateral light, traced during 11 hours, on a horizontal
glass in the morning, and on a vertical glass in the evening. Figure
reduced to one-third of the original scale.
exposed is shown. It may be observed that during the first
16 m. tl:e hypocotyl moved obliquely from the light, and this,
Chap. VIII.
HELIOTEOPISM.
127
no doubt, was due to its then circumnutating in this direction
Similar cases were repeatedly observed, and a dim light rarely
or never produced any effect until from a quarter to three-
quarters of an hour had elapsed. After 5.15 p.m., by which
time the light had become
obscure, the hypocotyl
began to circumnutate
about the same spot. The
contrast between the two
figures (172 and 173)
would have been more
striking, if they had been
originally drawn on the
same scale, and had been
equally reduced. But the
movements shown in Fig.
1 72 were at first more mag-
nified, and have been re-
duced to only one-half of
the original scale; whereas
those in Tig. 173 were at
first less magnified, and
have been reduced to a
one-third scale. A tracing
made at the same time
with the last of the
movements of a second
hypocotyl, presented a
closely analogous appear-
ance ; but it did not bend
quite so much towards the
light, and it circumnu-
tated rather more plainly.
Phalaris Canariensis. —
The sheath-like cotyledons
of this monocotyledonous
plant were selected for
trial, because they are very sensitive to light and circumnutate
well, as formerly shown (see Fig. 49, p. 63). Although we felt
no doubt about the result, some seedlings were first placed
before a south-west window on a moderately bright morning, and
the movements of one were traced. As is so common, it moved
8?lS 1 a.m.Sepl6. t . h
Phalaris Canariensis: heliotropic movement
and circumnutation of a rather old coty-
ledon, towards a dull lateral light, traced
on a horizontal glass from 8.15 A.M. Sept.
16th to 7.45 A.M. 17th. Figure reduced
to one-third of original scale.
128 MODIFIED CIKCUMNUTATION. Chap. VIII,
for the first 45 m. in a zigzag line ; it then felt the fnll influence
of the light, and travelled towards it for the next 2 h. 30 m. in an
almost straight line. The tracing has not been given, as it was
almost identical with that of Apios under similar circum-
stances (Fig. 170). By noon it had bowed itself to its full
extent ; it then circumnutated about the same spot and described
two ellipses ; by 5 p.m. it had retreated considerably from the
light, through the action of apogeotropism. After some pre-
liminary trials for ascertaining the right degree of obscurity,
some seedlings were placed (Sept. 16th) before a north-east
window, and light was admitted through an ordinary linen
and three muslin blinds. A pencil held close by the pot now
cast a very faint shadow on a white card, pointing from the
window. In the evening, at 4.30, and again at 6 p.m., some of
the blinds were removed. In Fig. 174 we see the course pursued
under these circumstances by a rathsr old and not very sensitive
cotyledon, 1*9 inch in height, which became much bowed,
but was never rectangularly bent towards the light. From
11 a.m., when the sky became rather duller, until 6.30 p.m., the
zigzagging was conspicuous, and evidently consisted of drawn-
out ellipses. After 6.30 p.m. and during the night, it retreated
in a crooked line from the window. Another and younger seed-
ling moved during the same time much more quickly and to a
much greater distance, in an only slightly zigzag line towards
the light ; by 11 a.m. it was bent almost rectangularly in this
direction, and now circumnutated about the same place.
Tropceolum majus. — Some very young see llings, bearing only
two leaves, and therefore not as yet arrived at the climbing
stage of growth, were first tried before a north-east window
without any blind. The epicotyls bowed themselves towards
the light so rapidly that in little more than 3 h. their tips
pointed rectangularly towards it. The lines traced were either
nearly straight or slightly zigzag ; and in this latter case we
see that a trace of circumnutation was retained even under the
influence of a moderately bright light. Twice whilst these
epicotyls were bending towards the window, dots were made
every 5 or 6 minutes, in order to detect any trace of lateral
movement, but there was hardly any ; and the lines formed by
their junction were nearly straight, or only very slightly zigzag,
as in the other pirts of the igures. After the epicotyls had
bowed themselves to the full extent towards the light, ellipses
of considerable size were described in the usual manner.
Chap. VIII.
HELIOTKOPISM.
429
After haying seen how the epicotyls moTed towards a mode
rately bright light, seedlings were placed at 7.48 a.m. (Sept. 7th)
before a north-east window, covered by a towel, and shortly
afterwards by an ordinary linen blind, but the epicotyls still
moved towards the window. At 9.13 a.m. two additional muslin
blinds were suspended, so that the seedlings received very little
more light from the window than from the interior of the room
The sky varied in brightness, and the seedlings occasionally
Fig. 175.
6M\
i0°40'jo.m.
JG>dO'p.m,
7°48'a.trf
Tropceolum majus : heliotropic movement and circumnutation of the epicotyl
of a young seedling towards a dull lateral light, traced on a horizontal
glass from 7.48 A.M. to 10.40 p.m. Figure reduced to one-half of the
original scale.
received for a short time less light from the window than from
the opposite side (as ascertained by the shadow cast), and then
one of the blinds was temporarily removed. In the evening
tho blinds were taken away, one by one. The course pursued
by an epicotyl under these circumstances is shown in Fig. 175.
During the whole day, until 6.45 p.m., it plainly bowed itself
towards the light ; and the tip moved over a considerable space.
After 6.45 p.m. it moved backwards, or from the window, till
430
MODIFl ED CIKCUMNUTATION.
Chap. VIII
Fig. 176.
10.40 p.m., when the last dot was made. Here, then, we have
a distinct heliotropic movement, effected by means of six
elongated figures (which if dots had been made every few
minutes would have been more or less elliptic) directed towards
the light, with the apex of each suc-
cessive ellipse nearer to the window
than the previous one. Now, if the
light had been only a little brighter,
the epicotyl would have bowed itself
more to the light, as we may safely
conclude from the previous trials ;
there would also have been less
lateral movement, and the ellipses or
other figures would have been drawn
out into a strongly marked zigzag
line, with probably one or two small
loops still formed. If the light had
been much brighter, we should have
had a slightly zigzag line, or one
quite straight, for there would have
been more movement in the direc-
tion of the light, and much less from
side to side.
Sachs states that the older inter-
nodes of this Tropseolum are aphe-
liotopic; we therefore placed a
plant, 111 inches high, in a box,
blackened within, but open on one
side in front of a north-east window
without any blind. A filament was
fixed to the third internode from
the summit on one plant, and to
the fourth internode of another.
These internodes were either not
old enough, or the light was not suf-
ficiently bright, to induce aphelio-
tropism, for both plants ben^ slowly towards, instead of from
the window during four days. The course, during two da\ s of
the first-mentioned internode, is given in Fig. 176 ; and we see
that it either circumnutated on a small scale, or travelled in a
zigzag line towards the light. We have thought this case of
feeble heliotropism in one of the older internodes of a plant,
Tropceolum majus : heliotropic
movement and circumnuta-
tion of an old internode to-
wards a lateral light, traced
on a horizontal glass from 8
A.M. Nov. 2nd to 10.20 A.M.
Nov. 4th. Broken lines show
the nocturnal course.
Chap. VI EI.
HELIOTROPISM.
431
which, whilst young, is so extremely sensitive to light, worth
giving.
Cassia tora. — The cotyledons of this plant are extremely
sensitive to light, whilst the
mnufjpmT
Ch
hypocotyls are much less
sensitive than those of most
other seedlings, as we had
often observed with surprise.
It seemed therefore worth
while to trace their move-
ments. They were exposed
to a lateral light before a
north-east window, which
was at first covered merely
by a muslin blind, but as
the sky grew brighter about
11 a.m., an additional linen
olind was suspended. After
4 p.m. one blind and then the
other was removed. The
seedlings were protected on
each side and above, but were
open to the diffused light
of the room in the rear. Up-
right filaments were fixed to
the hypocotyls of two seed-
lings, which stood vertically
in the morning. The accom-
panying figure (Fig. 177)
shows the course pursued by
one of them during two days ;
but it should be particularly
noticed that during the
second day the seedlings were
kept in darkness, and they
then circumnutated round
nearly the same small space.
On the first day (Oct. 7th)
the hypocotyl moved from
8 a.m. to 12.23 p.m., toward
the light in a zigzag line, then turned abruptly to the left
and afterwards described a small ellipse. Another irregular
Qy.mJ
tl
8 a.m.'/ -.4
Cassia tora: heliotropic movement and
circumnntation of a hypocotyl (1|
inch in height) traced on a horizontal
glass from 8 A.M. to 10.10 P.M. Oct.
7th. Also its circumnntation in
darkness from 7 A.M. Oct. 8th to 7.45
A.M. Oct. 9th.
132
MODIFIED CIRCUMNUTATION. Chap. VIII
ellipse was completed between 3 p.m. and abont 5.30 p.m.,
the hypocotyl still bending towards the light. The hypocotyl
Fig. 178.
\
Bignonia capreolata : aphe-
liotropic movement of a
tendril, traced on a hori-
zontal glass from 6.45
A.M. July 19th to 10 a.m.
20th. Movements as
originally traced, little
magnified, here reduced
to two-thirds of the
original scale.
stationary ; they
was straight and upright in the morn-
ing, but by 6 p.m. its upper half was
bowed towards the light, so that the
chord of the arc thus formed stood at
an angle of 20° with the perpendicular.
After 6 p.m. its course was reversed
through the action of apogeotropism,
and it continued to bend from the
window during the night, as shown by
the broken line, On the next day it
was kept in the dark (excepting when
each observation was made by the aid
of a taper), and the course followed
from 7 a,m on the 8th to 7.45 a.m. on
the 9th is here likewise shown. The
difference between the two parts of the
figure (177), namely, that described
during the daytime on the 7th, when
exposed to a rather dim lateral light,
and that on the 8th in darkness, is
striking. The difference consists in the
lines during the first day having been
drawn out in the direction of the light.
The movements of the other seedling,
traced under the same circumstances,
were closely similar.
Apheliotropism. — We succeeded in
observing only two cases of aphelio-
tropism, for these are somewhat rare ;
and the movements are generally so
slow that they would have been very
troublesome to trace.
Bignonia capreolata. — No organ of
any plant, as far as we have seen, bends
away so quickly from the light as do
the tendrils of this Bignonia. They
are also remarkable from circum-
nutating much less regularly than
most other tendrils, often remaining
depend on apheliotropism for corning into
(jiiap. VIII. AFHELIOTROPISM. 438
contact with the trunks of trees.* The stem of a young plant
was tied to a stick at the base of a pair of fine tendrils, which
projected almost vertically upwards; and it was placed in
front of a north-east window, being protected on all other sides
from the light. The first dot was made at 6.45 a.m., and by
7.35 a.m. both tendrils felt the full influence of the light, for
they moved straight away from it until 9.20 a.m., when they
circumnutated for a time, still moving, but only a little, from
the light (see Fig. 178 of the left-hand tendril). After 3 p.m.
they again moved rapidly away from the light in zigzag lines.
By a late hour in the evening both had moved so far, that
they pointed in a direct line from the light. During the night
they returned a little in a nearly opposite direction. On the
following morning they again moved from the light and con-
verged, so that by the evening they had become interlocked,
still pointing from the light. The right-hand tendril, whilst
converging, zigzagged much more than the one figured. Both
tracings showed that the apheliotropic movement was a modi-
fied form of circumnutation.
Cyclamen Persicum. — Whilst this plant is in flower the peduncles
stand upright, but their uppermost part is hooked so that the
flower itself hangs downwards. As soon as the pods begin to
swell, the peduncles increase much in length and slowly curve
downwards, but the short, upper, hooked part straightens itself.
Ultimately the pods reach the ground, and if this is covered
with moss or dead leaves, they bury themselves. We have often
seen saucer-like depressions formed by the pods in damp sand
or sawdust ; and one pod ( • 3 of inch in diameter) buried itself
in sawdust for three-quarters of its length. f We shall have
occasion hereafter to consider the object gained by this burying
process. The peduncles can change the direction of their cur-
vature, for if a pot, with plants having their peduncles already
bowed downwards, be placed horizontally, they slowly bend
at right angles to their former direction towards the centre of
the earth. We therefore at first attributed the movement to
geotropism ; but a pot which had lain horizontally with the pods
* • The Movements and Habits tanic Garden,' Canto., iii. p. 126),
of Climbing Plants,' 1875, p. 97. the pods forcibly penetrate the
t The peduncles of several earth. See also Grenier and
other species of Cyclamen twist Godron, * Fl ;>re de France,' torn ii
themselves into a spire, and ao p. 459.
uurding to Erasmus Darwin C Bo-
134
MODIFIED CIKCUMNTJTATIOK
Chap. VIII.
all painting to the ground, was reversed, being still kept hori-
zontal, so that the pods now pointed directly upwards ; it was
then placed in a dark cupboard, but the pods still pointed up-
wards after four days and nights. The pot, in the same position,
was next brought back into the light, and after two days there
was some bending downwards of the peduncles, and on the fourth
day two of them pointed to the centre of the earth, as did the
others after an additional day or two. Another plant, in a pot
which had always stood upright, was left in the dark cupboard
for six days ; it bore 3 peduncles, and only one became within this
Fig. 179.
Cyclamen Persicum: downward apheliotropic movement of a flower-peduncle,
greatly magnified (about 47 times ?), traced on a horizontal glass from
1 p.m. Feb. 18th to 8 A.M. 21st.
time at all bowed downwards, and that doubtfully. The weight,
therefore, of the pods is not the cause of the bending down.
This pot was then brought back into the light, and after three
days the peduncles were considerably bowed downwards. We
are thus led to infer that the downward curvature is due to
apheliotropism ; though more trials ought to have been made.
In order to observe the nature of this movement, a peduncle
bearing a large pod which had reached and rested on the
ground, was lifted a little up and secured to a stick. A filament
was fixed across the pod with a mark beneath, and its move*
CiiAr. VIII. i.PHELIOTBOPIStf. 435
ment, greatly magnified, was traced on a horizontal glass during
67 h. The plant was illuminated during the day from above. A
copy of the tracing is given on p. 434 (Fig. 179) ; and there can
be no doubt that the descending movement is one of modified
circumn«utation, but on an extremely small scale. The observa-
tion was repeated on another pod, which had partially buried
itself in sawdust, and which was lifted up a quarter of an inch
above the surface; it described three very small circles in 24 h.
Considering the great length and thinness of the peduncles
and the lightness of the pods, we may conclude that they
would not be able to excavate saucer-like depressions in sand
or sawdust, or bury themselves in moss, &c, unless they were
aided by their continued rocking or circumnutating move-
ment.
Relation hetiveen Gir cum nutation and Heliotrojnsm. —
Any one who will look at the foregoing diagrams,
showing the movements of the stems of various plants
towards a lateral and more or less dimmed light, will
be forced to admit that ordinary circumnutation and
heliotropism graduate into one another. When a
plant is exposed to a dim lateral light and continues
during the whole day bending towards it, receding
late in the evening, the movement unquestionably is
one of heliotropism. Now, in the case of Tropaeolura
(Fig. 175) the stem or epicotyl obviously chcumnu-
tated during the whole day, and yet it continued at
the same time to move heliotropically ; this latter
movement being effected by the apex of each succes-
sive elongated figure or ellipse standing nearer to
the light than the previous one. In the case of
Cassia (Fig. 177) the comparison of the movement ot
the hypocotyl, when exposed to a dim lateral light and
to darkness, is very instructive ; as is that between
the ordinary circumnutating movement of a seedling
Brassica (Figs. 172, 173), or that of Phalaris (Figs.
49,~174), and their heliotropic movement towards a
window protected by blinds. In both these cases
±3(3 RELATION BETWEEN Chap. VIII
and in many others, it was interesting to notice Low
gradually the stems began to circumnutate as the
light waned in the evening. We have therefore many
kinds of gradations from a movement towards the light,
which must be considered as one of circumnutation
very slightly modified and still consisting of ellipses
or circles, — though a movement more or less strongly
zigzag, with loops or ellipses occasionally formed, — to
a nearly straight, or even quite straight, heliotropic
course.
A plant, when exposed to a lateral light, though
this may be bright, commonly moves at first in a
zigzag line, or even directly from the light ; and
this no doubt is due to its circumnutating at the
time in a direction either opposite to the source of
the light, or more or less transversely to it. As soon,
however, as the direction of the circumnutating move-
ment nearly coincides with that of the entering light,
the plant bends in a straight course towards the light,
if this is bright. The course appears to be rendered
more and more rapid and rectilinear, in accordance with
the degree of brightness of the light — firstly, by the
longer axes of the elliptical figures, which the plant
continues to describe as long as the light remains very
dim, being directed more or less accurately towards
its source, and by each successive ellipse being de-
scribed nearer to the light. Secondly, if the light
is only somewhat dimmed, by the acceleration and
increase of the movement towards it, and by the
retardation or arrestment of that from the light, some
lateral movement being still retained, for the light
will interfere less with a movement at right angles
to its direction, than with one in its own direction.*
" In bis paper, ' Ut her oitlio- tleile' (' Avbeiten des Bot. Inst
trope und plagiotrope Pfianzen- in Wurzburg,' Band ii. Heft ii
Chap. VIII. CIRCUMNUTATION AND HELIOS ROPISM. 437
The result is that the course is rendered more or less
zigzag aud unequal in rate. Lastly, when the light
is very bright all lateral movement is lost ; and the
whole energy of the plant is expended in renderiug
the circumnutating movement rectilinear and rapid in
oue direction alone, namely, towards the light.
The common view seems to be that heliotropism is
a quite distinct kind of movement from circumnuta-
tion ; and it may be urged that in the foregoing
diagrams we see heliotropism merely combined with,
or superimposed on, circumnutation. But if so, it must
be assumed that a bright lateral light completely
stops circumnutation, for a plant thus exposed moves
in a straight line towards it, without describing any
ellipses or circles. If the light be somewhat obscured,
though amply sufficient to cause the plant to bend
towards it, we have more or less plain evidence of still-
continued circumnutation. It must further be assumed
that it is only a lateral light which has this extraor-
dinary power of stopping circumnutation, for we know
that the several plants above experimented on, and
all the others which were observed by us whilst grow-
ing, continue to circumnutate, however bright the light
may be, if it comes from above. Nor should it be
forgotten that in the life of each plant, circumnuta-
tion precedes heliotropism, for hypocotyls, epicotyls,
and petioles circumnutate before they have broken
through the ground and have ever felt the influence of
light.
We are therefore fully justified, as it seems to us, in
believing that whenever light enters laterally, it is the
1879), Sachs has discussed the the organs of plants stand with
manner in which geotropism and respect to the direction of the
heliotropism are affected by dif- incident force,
ferences in the angles at which
£38 MODIFIED CIECUMNUTATION. Chap. VIIL
movement of circumnutation which gives rise to, or is
converted into, heliotropism and apheliotropism. On
this view we need not assume against all analogy that
a lateral light entirely stops circumnntation ; it merely
excites the plant to modify its movement for a time
in a beneficial manner. The existence of every pos-
sible gradation, between a straight course towards a
lateral light and a conrse consisting of a series of loops
or ellipses, becomes perfectly intelligible. Finally,
the conversion of circnmnntation into heliotropism or
apheliotropism, is closely analogous to what takes place
with sleeping plants, which during the daytime de-
scribe one or more ellipses, often moving in zigzag lines
and making little loops ; for when they begin in the
evening to go to sleep, they likewise expend all their
energy in rendering their course rectilinear and rapid.
In the case of sleep-movements, the exciting or regu-
lating cause is a difference in the intensity of the
light, coming from above, at different periods of the
twenty-four hours ; whilst with heliotropic and aphe-
liotropic movements, it is a difference in the intensity
of the light on the two sides of the plant.
Tramvcrsal-lieliotropismus {of Frank *) or Diahelio-
tropism. — The cause of leaves placing themselves
more or less transversely to the light, with their
upper surfaces directed towards it, has been of late
the subject of much controversy. We do not here
refer to the object of the movement, which no doubt
is that their upper surfaces may be fully illuminated,
but the means by which this position is gained.
Hardly a better or more simple instance can be given
* 'Die natiirlioliK Wugercchte Frnge liber Transversal Geo-und
ltichtung von Pnanzenth< ilen,' Heliotropismus," ' Bot. Zeitung,
18 '0 See also suue interesting 1873, p. 17 et seq.
aiticles by the same author, " Zux
Chap. VIII. DIAHELIOTROPISM. 439
of diaheliotropism than that offered by many seed-
lings, the cotyledons of which are extended hori-
zontally. When they first burst from their seed-coats
they are in contact and. stand, in various positions,
often vertically upwards ; they soon diverge, and this
is effected by epinasty, which, as we have seen, is a
modified form of circumnutation. After they have
diverged to their full extent, they retain nearly the
same position, though brightly illuminated all day
long from above, with their lower surfaces close to the
ground and thus much shaded. There is therefore a
great contrast in the degree of illumination of their
upper and lower surfaces, and if they were heliotropic
they would bend quickly upwards. It must not, how-
ever, be supposed that such cotyledons are immovably
fixed in a horizontal position. When seedlings are
exposed before a window, their hypocotyls, which are
highly heliotropic, bend quickly towards it, and the
upper surfaces of their cotyledons still remain ex-
posed at right angles to the light ; but if the hypo-
cotyl is secured so that it cannot bend, the cotyledons
themselves change their position. If the two are
placed in the line of the entering light, the one
furthest from it rises up and that nearest to it often
sinks down ; if placed transversely to the light, they
twist a little laterally ; so that in every case they
endeavour to place their upper surfaces at right angles
to the light. So it notoriously is with the leaves on
plants nailed against a wall, or grown in front of a
window. A moderate amount of light suffices to in-
duce such movements ; all that is necessary is that the
light should steadily strike the plants in an oblique
direction. With respect to the above twisting move-
ment of cotyledons, Frank has given many and much
more striking instances in the case of the leaves on
29
440 MODIFIED CIKCUMNUTATION. Chap. Vm
branches which had been fastened in various positions
or turned upside down.
In our observations on the cotyledons of seedling
plants, we often felt surprise at their persistent hori-
zontal position during the day, and were convinced
before we had read Frank's essay, that some special
explanation was necessary. De Vries has shown*
that the more or less horizontal position of leaves is
in most cases influenced by epinasty, by their own
weight, and by apogeotropism. A young cotyledon
or leaf after bursting free is brought down into its
proper position, as already remarked, by epinasty,
which, according to De Vries, long continues to act
on the midribs and petioles. Weight can hardly be
influential in the case of cotyledons, except in a few
cases presently to be mentioned, but must be so with
large and thick leaves. With respect to apogeotropism,
De Vries maintains that it generally comes into play,
and of this fact we shall presently advance some
indirect evidence. But over these and other constant
forces we believe that there is in many cases, but we
do not say in all, a preponderant tendency in leaves
and cotyledons to place themselves more or less trans-
versely with respect to the light.
In the cases above alluded to of seedlings exposed
to a lateral light with their hypocotyls secured, it is
impossible that epinasty, weight and apogeotropism,
either in opposition or combined, can be the cause of
the rising of one cotyledon, and of the sinking of the
other, since the forces in question act equally on both :
and since epinasty, weight and apogeotropism all act
in a vertical plane, they cannot cause the twisting of
the petioles, which occurs in seedlings under the
* 'Arbeikm des Bot. Institute in Wiirz'mrg,' Heft. ii. 1872, pp.
223-277.
Chap. VIII. DIAHELIOTROPISM. 441
above conditions of illumination. All thest movements
evidently depend in some manner on the obliquity of
the light, but cannot be called heliotropic, as this
implies bending towards the light ; whereas the coty-
ledon nearest to the light bends in an opposed direc-
tion or downwards, and both place themselves as nearly
as possible at right angles to the light. The move-
ment, therefore, deserves a distinct name. As coty-
ledons and leaves are continually oscillating up and
down, and yet retain all day long their proper position
with their upper surfaces directed transversely to the
light, and if displaced reassume this position, dia-
heliotropism must be considered as a modified form of
circumnutation. This was often evident when the
movements of cotyledons standing in front of a window
were traced. We see something analogous in the case
of sleeping leaves or cotyledons, which after oscillating
up and down during the whole day, rise into a vertical
position late in the evening, and on the following
morning sink clown again into their horizontal or dia-
heliotropic position, in direct opposition to heliotro-
pism. This return into their diurnal position, which
often requires an angular movement of 90°, is analo-
gous to the movement of leaves on displaced branches,
which recover their former positions. It deserves
notice that any force such as apogeotropism, will act
with different degrees of power* in the different posi-
tions of those leaves or cotyledons which oscillate
largely up and down during the day ; and yet they
recover their horizontal or diaheliotropic position.
We may therefore conclude that diaheliotropic
movements cannot be fully explained by the direct
action of light, gravitation, weight, &c, any more
* See former note, in reference to Sachs' remarks on this sr bject.
±42 MODIFIED CIKCUMNUTATION. Ch*i\ VIII
than can the nyctitropic movements of cotyledons
and leaves. In the latter case they place themselves
so that their upper surfaces may radiate at night
as little as possible into open space, with the upper
surfaces of the opposite leaflets often in contact. These
movements, which are sometimes extremely complex,
are regulated, though not directly caused, by the alter-
nations of light and darkness. In the case of diahelio-
tropism, cotyledons and leaves place themselves so
that their upper surfaces may be exposed to the light,
and this movement is regulated, though not directly
caused, by the direction whence the light proceeds. In
both cases the movement consists of circumnutation
modified by innate or constitutional causes, in the
same manner as with climbing plants, the circumnu-
tation of which is increased in amplitude and rendered
more circular, or again with very young cotyledons
and leaves which are thus brought down into a hori-
zontal position by epinasty.
We have hitherto referred only to those leaves and
cotyledons which occupy a permanently horizontal
position ; but many stand more or less obliquely, and
some few upright. The cause of these differences of
position is not known ; but in accordance with Wiesner's
views, hereafter to be given, it is probable that some
leaves and cotyledons would suffer, if they were fully
illuminated by standing at right angles to the light.
We have seen in the second and fourth chapters
that those cotyledons and leaves which do not alter
their positions at night sufficiently to be said to sleep,
commonly rise a little in the evening and fall again
on the next morning, so that they stand during the
night at a rather higher inclination than during the
middle of the day. It is incredible that a rising
movement of 2° or 3°, or even of 10° or 20°, can be oi
Chap. VLLI. DIAHELIOTKOPISM. 4'1'J
any service to the plant, so as to have been specially
acquired. It must be the result of some periodical
change in the conditions to which they are subjected,
and there can hardly be a doubt that this is the daily
alternations of light and darkness. De Vries states in
the paper before referred to, that most petioles and
midribs are apogeotropic ;* and apogeotropism would
account for the above rising movement, which is com-
mon to so many widely distinct species, if we suppose it
to be conquered by diaheliotropism during the middle
of the day, as long as it is of importance to the plant
that its cotyledons and leaves should be fully exposed
to the light. The exact hour in the afternoon at which
they begin to bend slightly upwards, and the extent of
the movement, will depend on their degree of sen-
sitiveness to gravitation and on their power of resist-
ing its action during the middle of the day, as well as
on the amplitude of their ordinary circumnutating
movements ; and as these qualities differ much in dif-
ferent species, we might expect that the hour in the
afternoon at which they begin to rise would differ
much in different species, as is the case. Some other
agency, however, besides apogeotropism, must come
into play, either directly or indirectly, in this upward
movement. Thus a young bean (Vicia faba), growing
in a small pot, was placed in front of a window in a
klinostat ; and at night the leaves rose a little, although
* According to Frank ('Die organs have been long kept in the
nat. Wagerechte Eichtung von da-k, the amount of water and of
Prtanzentheilen.' 1870, p. 46) the mineral matter which they con-
root-leaves of many pLants, kept tain is so much altered, and their
in darkness, rise up and even be- regular growth is so much dis-
come vertical ; and so it is in some turbed, that it is perhaps rash to
cases with shoots. (See Eauwen- infer from their movements what
hoff, 'Archives Neerlandaises,' would occur under normal con-
tom. xii. p. 32.) These movements ditions. (See Godlewski, 'Bot
indicate apogeotiopism ; but when Zeitung,' Feb. 14th, 1879.)
144 MODIFIED CIRCUMNUTATION. Chap. VIII
the action of apogeotropism was quite eliminated.
Nevertheless, they did not rise nearly so much at
night, as when subjected to apogeotropism. Is it
not possible, or even probable, that leaves and coty-
ledons, which have moved upwards in the evening
through the action of apogeotropism during countless
generations, may inherit a tendency to this movement ?
We have seen that the hypocotyls of several Legu-
minous plants have from a remote period inherited a
tendency to arch themselves ; and we know that the
sleep-movements of leaves are to a certain extent
inherited, independently of the alternations of light
and darkness.
In our observations on the circumnutation of those
cotyledons and leaves which do not sleep at night, we
met with hardly any distinct cases of their sinking
a little in the evening, and rising again in the morn-
ing, — that is, of movements the reverse of those just
discussed. We have no doubt that such cases occur,
inasmuch as the leaves of many plants sleep by
sinking vertically downwards. How to account for the
few cases which were observed must be left doubtful.
The young leaves of Cannabis sativa sink at night
between 30° and 40° beneath the horizon ; and Kraus
attributes this to epinasty in conjunction with the
absorption of water. Whenever epinastic growth is
vigorous, it might conquer diaheliotropism in the
evening, at which time it would be of no import-
ance to the plant to keep its leaves horizontal.
The cotyledons of Anoda Wrightii, of one variety of
Grossypium, and of several species of Ipomoea, remain
horizontal in the evening whilst they are very young ;
as they grow a little older they curve a little down-
wards, and when large and heavy sink so much that
they come under our definition of sleep. In the case of
Chap. VIII. PARAHELIOTROPISM. 445
the Anoda and of some species of Ipomoea, it was proved
that the downward movement did not depend on the
weight of the cotyledons ; but from the fact of the move-
ment being so much more strongly pronounced after
the cotyledons have grown large and heavy, we may
suspect that their weight aboriginally played some part
in determining that the modification of the circum-
nutating movement should be in a downward direction.
The so-called Diurnal Sleep of Leaves, or Parahelio-
tropism. — This is another class of movements, dependent
on the action of light, which supports to some extent
the belief that the movements above described are
only indirectly due to its action. We refer to the
movements of leaves and cotyledons which when
moderately illuminated are diahelio tropic ; but which
change their positions and present their edges to the
light, when the sun shines brightly on them. These
movements have sometimes been called diurnal sleep,
but they differ wholly with respect to the object
gained from those properly called nyctitropic ; and in
some cases the position occupied during the day is the
reverse of that during the night.
It has long been known* that when the sun shines brightly
on the leaflets of Eobinia, they rise up and present their edges
to the light ; whilst their position at night is vertically down-
wards. We have observed the same movement, when the
sun shone brightly on the leaflets of an Australian Acacia.
Those of Am/phicarpma monoica turned their edges to the sun ;
and an analogous movement of the little almost rudimentary
basal leaflets of Mimosa albida was on one occasion so rapid that
it could be distinctly seen through a lens. The elongated, uni-
foliate, first leaves of Phaseolus Roxburghii stood at 7 a.m. at 20°
above the horizon, and no doubt they afterwards sank a little
lower. At noon, after having been exposed for about 2 h. to
* Pfeffer gives the names and dates of sevoral ancient writers in his
Die Periodischen Bewegungen,' 1875, p. 62.
446 MODIFIED CIRCUMNUTATION. Chap. VHT.
a bright sun, they stood at 56° above the horizon ; they were
then protected from the rays of the sun, but were left well
illuminated from above, and after 30 m. they had fallen 40°, for
they now stood at only 16° above the horizon. Some young
plants of Fhaseolus Hernandesii had been exposed to the same
bright sunlight, and their broad, unifoliate, first leaves now
stood up almost or quite vertically, as did many of the leaflets
on the trifoliate secondary leaves ; but some of the leaflets had
twisted round on their own axes by as much as 90° without
rising, so as to present their edges to the sun. The leaflets on
the same leaf sometimes behaved in these two different manners,
but always with the result of being less intensely illuminated.
These plants were then protected from the sun, and were looked
at after lh h. ; and now all the leaves and leaflets had re-
assumed their ordinary sub-horizontal positions. The copper-
coloured cotyledons of some seedlings of Cassia mimosoides were
horizontal in the morning, but after the sun had shone on
them, each had risen 45^° above the horizon. The movement
in these several cases must not be confounded with the sudden
closing of the leaflets of Mimosa pudica, which may sometimes
be noticed when a plant which has been kept in an obscure
place is suddenly exposed to the sun ; for in this case the light
seems to act, as if it were a touch.
From Prof. Wiesner's interesting observations, it is probable
that the above movements have been acquired for a special
purpose. The chlorophyll in leaves is often injured by too
intense a light, and Prof. Wiesner * believes that it is protected
by the most diversified means, such as the presence of hairs,
colouring matter, &c, and amongst other means by the leaves
presenting their edges to the sun, so that the blades then
receive much less light. He experimented on the young leaflets
of Eobinia, by fixing them in such a position that they could
not escape being intensely illuminated, whilst others were
allowed to place themselves obliquely ; and the former began to
suffer from the light in the course of two days.
In the cases above given, the leaflets move either upwards
* 'Die N'aturlichen Einrich- the action of concentrated light
tungen zum Schutze des Chloro- from the sun, in the presence of
phylls,' &c, 1876. Pringsheim oxygen. See, also, Stahl on th6
lias recently observed under the protection of chlorophyll from
microscope the destruction of intense light, in 'Bot. Zeitung,
chlorophyll in a few minutes by 1880.
Chap. VIII. PARAHELIOTROriSM. 447
or twist laterally, so as to place their edges in the direction of the
sun's light; but Cohn long ago observed that the leaflets of
Oxalis bend downwards when fully exposed to the sun. We
witnessed a striking instance of this movement in the very
large leaflets of 0. Urteyesii. A similar movement may fre-
quently be observed with the leaflets of Averrhoa bilimbi (a
member of the Oxalidse) ; and a leaf is here represented (Fig.
180) on which the sun had shone. A diagram (Fig. 134) was
given in the last chapter, representing the oscillations by which
a leaflet rapidly descended under these circumstances ; and the
movement may be teen closely to resemble that (Fig. 133) bv
Fig-. 180.
Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine*
but the leaflets are sometimes more depressed than is here shown.
Figure much reduced.
which it assumed its nocturnal position. It is an interesting
fact in relation to our present subject that, as Prof. Batalin
informs us in a letter, dated February, 1879, the leaflets of
Oxalis acetosella may be daily exposed to the sun during many
weeks, and they do not suffer if they are allowed to depress
themselves ; but if this be prevented, they lose their colour and
wither in two or three days. Yet the duration of a leaf is about
two months, when subjected only to diffused light; and in this
case the leaflets never sink downwards during the day
As the upward movements of the leaflets of Eobinia,
and the downward movements of those of Oxalis, have
been proved to be highly beneficial to these plants
when subjected to bright sunshine, it seems probable
that they have been acquired for the special purpose
of avoiding too intense an illumination. As it would
have been very troublesome in all the above cases to
148 MODIFIED CIRCUMNUTATIOK Chap. Via
have watched for a fitting opportunity and to have
traced the movement of the leaves whilst they were
fully exposed to the sunshine, we did not ascertain
whether paraheliotropism always consisted of modi-
tied circumnutation ; but this certainly was the case
with the Averrhoa, and probably with the other species,
©3 their leaves were continually circumnutatin^.
Chat. IX. SENSITIVENESS TO LIGHT. 440
CHAPTEK IX.
Sensitiveness of Plants to Light: its transmitted effects.
Uses of heliotropism — Insectivorous and climbing plants not heliotropic
— Same orjjan helii ■tropic at one age and not at another — Extra-
ordinary sensitiveness of some plants to light — The effects of light dc
not correspond with its intensity— Effects of previous illumination
— Time required for the action of light— After-effects of light —
ApogeotropLm 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 1 halaris, manner of bending —
Results of the exclusion of light from their tips — Effects trans-
mitted beneath the surface of the ground — Lateral illumination of
the tip determines the direction of the curvature of the base— Coty-
ledons of A vena, 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
apheliotropibrn.
No one can look at the plants growing on a bank or
on the borders of a thick wood, and doubt that the
young stems and leaves place themselves so that the
leaves may be well illuminated. They are thus enabled
to decompose carbonic acid. But the sheath-like coty-
ledons of some Graminese, for instance, those of Pha-
laris, are not green and contain very little starch ;
from which fact we may infer that they decompose
little or no carbonic acid. Nevertheless, they are ex-
tremely heliotropic ; and this probably serves them in
another way, namely, as a guide from the buried seeds
through fissures in the ground or through overlying
masses of vegetation, iuto the light and air. This view
£50 SENSITIVENESS TO LIGHT. Chap. IX.
is strengthened by the fact that with Phalaris and
Avena the first trne leaf, which is bright green and nc
doubt decomposes carbonic acid, exhibits hardly a
trace of heliotropism. The heliotropic movements of
many other seedlings probably aid them in like
manner in emerging from the ground ; for apogeo-
tropism by itself would blindly guide them v.pwards,
against any overlying obstacle.
Heliotropism prevails so extensively among the
higher plants, that there are extremely few, of which
some part, either the stem, flow T er-peduncle, petiole,
or leaf, does not bend towards a lateral light.
Drosera rotundifolia is one of the few T plants the
leaves of which exhibit no trace of heliotropism. Nor
could we see any in Dionsea, though the plants were
not so carefully observed. Sir J. Hooker exposed the
pitchers of Sarracenia for some time to a lateral light,
but they did not bend towards it.* We can understand
the reason why these insectivorous plants should not
be heliotropic, as they do not live chiefly by decom-
posing carbonic acid ; and it is much more important
to them that their leaves should occupy the best
position for capturing insects, than that they should
be fully exposed to the light.
Tendrils, which consist of leaves or of other organs
modified, and the stems of twining plants, are, as
Mohl long ago remarked, rarely heliotropic ; and here
again we can see the reason why, for if they had
moved towards a lateral light they would have been
drawn away from their supports. But some tendrils are
apheliotropic, for instance those of Bignonia cajpreolata
* According to F. Kurtz (' Ver- tonia Calif omica are strongly
handl. di s Bot. Vereins der Pro- apheliotropic. We failed to detect
vinz Brandenburg,' Bd. xx. 1JS78) this movement in a plant which
the leaves or pitchers of Darling- we possessed for a short time.
Chap, IX. SENSITIVENESS TO LIGHT. '151
and of Smilax aspera ; and the stems of some plants
which climb by rootlets, as those of the Ivy and Tecoma
radicans, are likewise apheliotropic, and they thus find
a support. The leaves, on the other hand, of most
climbing plants are heliotropic ; but we could detect
no signs of any such movement in those of Mutiaia
clematis.
As heliotropism is so widely prevalent, and as
twining plants are distributed throughout the whole
vascular series, the apparent absence of any tendency
in their stems to bend towards the light, seemed to
us so remarkable a fact as to deserve further in-
vestigation, for it implies that heliotropism can be
readily eliminated. When twining plants are exposed
to a lateral light, their stems go on revolving or cir-
cumnutating about the same spot, without any evident
deflection towards the light ; but we thought that
we might detect some trace of heliotropism by com-
paring the average rate at which the stems moved to
and from the light during their successive revolutions.*
Three young plants (about a foot in height) of Ipomoea
cmrulea and four of I. purpurea, growing in separate
pots, were placed on a bright day before a north-east
window in a room otherwise darkened, with the tips
of their revolving stems fronting the window. When
the tip of each plant pointed directly from the window,
and when again towards it, the times w T ere recorded.
This was continued from 6.45 a.m. till a little after
2 p.m. on June 17th. After a few observations we
concluded that we could safely estimate the time
* Some erroneous statements number of observations, for we did
are unfortunately given on this not then know at how unequal
subject, in 'The Movements and a rate the stems and tetidrils of
Habits of Climb in % PJants,' 1875, climbing plants rometimes travel
pp. 28, 32, 40, and 53. Conclusions in different parts of the same re-
were drawn from an insufficient volution.
£52 SENSITIVENESS TO LIGHT. Chap. IX.
taken by each semicircle, within a limit of error of at
most 5 minutes. Although the rate of movement in
different parts of the same revolution varied greatly,
yet 22 semicircles to the light were completed, each
on an average in 73*95 minutes; and 22 semicircles
from the light each in 73'5 minutes. It may, there-
fore, be said that they travelled to and from the light
at exactly the same average rate ; though probably
the accuracy of the result was in part accidental. In
the evening the stems were not in the least deflected
towards the window. Nevertheless, there appears to
exist a vestige of heliotropism, for with 6 out of the
7 plants, the first semicircle from the light, described
in the early morning after they had been subjected to
darkness during the night and thus probably rendered
more sensitive, required rather more time, and the first
semicircle to the light considerably less time, than the
average. Thus with all 7 plants, taken together, the
mean time of the first semicircle in the morning from
the light, was 76*8 minutes, instead of 73*5 minutes,
which is the mean of all the semicircles during the
day from the light ; and the mean of the first semi-
circle to the light was only 63*1, instead of 73*95
minutes, which was the mean of all the semicircles
during the day to the light.
Similar observations were made on Wistaria Sinensis,
and the mean of 9 semicircles from the light was
117 minutes, and of 7 semicircles to the light 122
minutes, and this difference does not exceed the pro-
bable limit of error. During the three days of expo-
sure, the shoot did not become at all bent towards the
window before which it stood. In this case the first
semicircle from the light in the early morning of each
day, required rather less time for its performance thar
did the first semicircle to the light; and this result,
Ciikv. IX. SENSITIVENESS 'JO LIGHT. 453
if not accidental, appears to indicate that the shoota
retain a trace of an original apheliotropic tendency.
With Lonicera brachijpoda the semicircles from and to
the light differed considerably in time ; for 5 semi-
circles from the light required on a mean 202*4
minutes, and 4 to the light, 229*5 minutes ; but the
shoot moved very irregularly, and under these circum-
stances the observations were much too few.
It is remarkable that the same part on the same
plant may be affected by light in a widely different
manner at different ages, and as it appears at different
seasons. The hypocotyledonous steins of Ipomoea
cserulea and purpurea are extremely heliotropic, whilst
the stems of older plants, only about a foot in height,
are, as we have just seen, almost wholly insensible to
light. Sachs states (and we have observed the same
fact) that the hypocotyls of the Ivy (Hedera helix) are
slightly heliotropic ; whereas the stems of plants grown
to a few inches in height become so strongly aphelio-
tropic, that they bend at right angles away from the
light. Nevertheless, some young plants which had
behaved in this manner early in the summer again
became distinctly heliotropic in the beginning of
September ; and the zigzag courses of their stems, as
they slowly curved towards a north-east window, were
traced during 10 days. The steins of very young
plants of Tropseolum majus are highly heliotropic, whilst
those of older plants, according to Sachs, are slightly
apheliotropic. In all these cases the heliotropism of
the very young steins serves to expose the cotyledons,
or when the cotyledons are hypogean the first true
leaves, fully to the light ; and the loss of this power
by the older stems, or their becoming apheliotropic,
is connected with their habit of climbing.
Most seedling plants are strongly heliotropic, and
154 SENSITIVENESS TO LIGHT. Chap. IX
it is no doubt a great advantage to them in their
struggle for life to expose their cotyledons to the
light as quickly and as fully as possible, for the sake
of obtaining carbon. It has been shown in the first
chapter that the greater number of seedlings circum-
nutate largely and rapidly ; and as heliotropism con-
sists of modified circumnutation, we are tempted to
look at the high development of these two powers in
seedlings as intimately connected. Whether there are
any plants which circumnutate slowly and to a small
extent, and yet are highly heliotropic, we do not
know ; but there are several, and there is nothing
surprising in this fact, which circumnutate largely and
are not at all, or only slightly, heliotropic. Of such
cases Brosera rotundifolia offers an excellent instance,
The stolons of the strawberry circumnutate almost
like the stems of climbing plants, and they are not at
all affected by a moderate light ; but when exposed
late in the summer to a somewhat brighter light they
were slightly heliotropic ; in sunlight, according to
De Vries, they are apheliotropic. Climbing plants
circumnutate much more widely than any other plants,
yet they are not at all heliotropic.
Although the stems of most seedling plants are
strongly heliotropic, some few are but slightly helio-
tropic, without our being able to assign any reason.
This is the case with the hypocotyl of Cassia tora, and
we were struck with the same fact with some other
seedlings, for instance, those of Reseda odor at a. With
respect to the degree of sensitiveness of the more
sensitive kinds, it was shown in the last chapter that
seedlings of several species, placed before a north-east
window protected by several blinds, and exposed in
the rear to the diffused light of the room, moved
with unerring certainty towards the window, although
Chai\ IX. SENSITIVENESS TO LIGHT. 455
it was impossible to judge, excepting by the shadow
cast by an upright pencil on a white card, on which
side most light entered, so that the excess on one side
must have been extremely small.
A pot with seedlings of Phalaris Ganariensis, which
had been raised in darkness, was placed in a com-
pletely darkened room, at 12 feet from a very small
lamp. After 3 h. the cotyledons were doubtfully
curved towards the light, and after 7 h. 40 m. from
the first exposure, they were all plainly, though
slightly, curved towards the lamp. Now, at this dis-
tance of 12 feet, the light was so obscure that we could
not see the seedlings themselves, nor read the large
Roman figures on the white face of a watch, nor see a
pencil line on paper, but could just distinguish a line
made with Indian ink. It is a more surprising fact
that no visible shadow was cast by a pencil held
upright on a white card; the seedlings, therefore,
were acted on by a difference in the illumination of
their two sides, which the human eye could not dis-
tinguish. On another occasion even a less degree of
light acted, for some cotyledons of Phalaris became
slightly curved towards the same lamp at a distance
of 20 feet; at this distance we could not see a cir-
cular dot 2*29 mm. (-09 inch) in diameter made with
Indian ink on white paper, though we could just see a
dot 3*56 mm. (*14 inch) in diameter; yet a dot of
the former size appears large when seen in the light.*
We next tried how small a beam of light would act ;
as this bears on light serving as a guide to seedlings
whilst they emerge through fissured or encumbered
ground. A pot with seedlings of Phalaris was cove-ed
* Strasburger says (' Wirkung Haematococcus moved to a light
des Lichtes auf Sciiwarrasporen,' which only just sufficed to alio*
1878, p, 52), that the spores of middle-sized type to be read.
30
±56 SENSITIVENESS TO LIGHT. Ctiap. IS.
by a tin-vessel, having on one side a circular hole
123 mm. in diameter (i.e. a little less than the -n-^-tk oi
an inch) ; and the box was placed in front of a paraffin
lamp and on another occasion in front of a window ;
and both times the seedlings were manifestly bent
after a few hours towards the little hole.
A more severe trial was now made ; little tubes of
very thin glass, closed at their upper ends and coated
with black varnish, were slipped over the cotyledons
of Phalaris (which had germinated in darkness) and
just fitted them. Narrow stripes of the varnish had
been previously scraped off one side, through which
alone light could enter ; and their dimensions were
afterwards measured under the microscope. As a
control experiment, similar unvarnished and trans-
parent tubes were tried, and they did not prevent the
cotyledons bending towards the light. Two cotyledons
were placed before a south-west window, one of which
was illuminated by a stripe in the varnish, only *004
inch (0*1 mm.) in breadth and *016 inch (0*4 mm.) in
length ; and the other by a stripe '008 inch in breadth
and '06 inch in length. The seedlings were examined
after an exposure of 7 h. 40 m., and were found to be
manifestly bowed towards the light. Some other coty-
ledons were at the same time treated similarly, ex-
cepting that the little stripes were directed not to the
sky, but in such a manner that they received only the
diffused light from the room ; and these cotyledons did
not become at all bowed. Seven other cotyledons were
illuminated through narrow, but comparatively long,
cleared stripes in the varnish — namely, in breadth
between "01 and "026 inch, and in length between "15
and '3 inch ; and these all became bowed to the side,
by which light entered through the stripes, whether
these were directed towards the sky or to one side oi
Chap. IX. SENSITIVENESS TO LIGHT. 457
the room. That light passing through a hole only
004 inch in breadth by *016 in length, should induce
curvature, seems to us a surprising fact.
Before we knew how extremely sensitive the coty-
ledons of Phalaris were to light, we endeavoured to
trace their circumnutation in darkness by the aid ( f
a small wax taper, held for a minute or two at each
observation in nearly the same position, a little on the
left side in front of the vertical glass on which the
tracing was made. The seedlings were thus observed
seventeen times in the course of the day, at intervals of
from half to three-quarters of an hour ; and late in the
evening we were surprised to find that all the 29 coty-
ledons were greatly curved and pointed towards the
vertical glass, a little to the left where the taper had
been held. The tracings showed that they had tra-
velled in zigzag lines. Thus, an exposure to a feeble
light for a very short time at the above specified
intervals, sufficed to induce well-marked heliotropism.
An analogous case was observed with the hypocotyls
of Solanum lycopersicum. We at first attributed this
result to the after-effects of the light on each occasion ;
but since reading Wiesner's observations,* which will
be referred to in the last chapter, we cannot doubt that
an intermittent light is more efficacious than a con-
tinuous one, as plants are especially sensitive to any
contrast in its amount.
The cotyledons of Phalaris bend much more slowly
towards a very obscure light than towards a bright
one. Thus, in the experiments with seedlings placed
in a dark room at 12 feet from a very small lamp, they
were just perceptibly and doubtfully curved towards it
after 3 h., and only slightly, yet certainly, after 4 h
* l Sitz. der k. Akad. der Wis sen sell.* (Vienna), Jan. 1880, p. 12.
i58 SENSITIVENESS TO LIGHT. Chap. IX
After 8 h. 40 m. the chords of their arcs were deflected
from the perpendicular by an average angle of only
16°. Had the light been bright, they would have
become much more curved in between 1 and 2 h.
Several trials were made with seedlings placed at
various distances from a small lamp in a dark room ;
but we will give only one trial. Six pots were placed
at distances of 2, 4, 8, 12, 16, and 20 feet from the
lamp, before which they were left for 4 h. As light
decreases in a geometrical ratio, the seedlings in the
2nd pot received |th, those in the 3rd pot -iVth,
those in the 4th gVth, those in the 5th gVth, and those
in the 6th T^-o-th of the light received by the seedlings in
the first or nearest pot. Therefore it might have been
expected that there would have been an immense differ-
ence in the degree of their heliotropic curvature in the
several pots ; and there was a well-marked difference
between those which stood nearest and furthest from
the lamp, but the difference in each successive pair of
pots was extremely small. In order to avoid prejudice,
we asked three persons, who knew nothing about the
experiment, to arrange the pots in order according to
the degree of curvature of the cotyledons. The first
jDerson arranged them in proper order, but doubted
long between the 12 feet and 16 feet pots ; yet these
two received light in the proportion of 36 to 64. The
second person also arranged them properly, but
doubted between the 8 feet and 12 feet pots, which
received light in the proportion of 16 to 36. The
third person arranged them in wrong order, and
doubted about four of the pots. This evidence shows
conclusively how little the curvature of the seedlings
differed in the successive pots, in comparison with the
great difference in the amount of light which they
received ; and it should be noted that there was no
Chap. IX. SENSITIVENESS TO LIGHT. 459
excess of superfluous light, for the cotyledons became
but little and slowly curved even in the nearest pot.
Close to the 6th pot, at the distance of 20 feet from
the lamp, the light allowed us just to distinguish
a dot 3*56 mm. (*14 inch) in diameter, made with
Jndian ink on white paper, but not a dot 2*29 mm.
(•09 inch) in diameter.
The degree of curvature of the cotyledons of Phalaris
within a given time, depends not merely on the
amount of lateral light which they may then receive,
but on that which they have previously received from
above and on all sides. Analogous facts have been
given with respect to the nyctitropic and periodic
movements of plants. Of two pots containing seedlings
of Phalaris which had germinated in darkness, one was
still kept in the dark, and the other was exposed (Sept.
26th) to the light in a greenhouse during a cloudy day
and on the following bright morning. On this morn-
ing (27th), at 10.30 a.m., both pots were placed in a
box, blackened within and open in front, before a
north-east window, protected by a linen and muslin
blind and by a towel, so that but little light was
admitted, though the sky was bright. Whenever the
pots were looked at, this was done as quickly as pos-
sible, and the cotyledons were then held transversely
with respect to the light, so that their curvature could
not have been thus increased or diminished. After
50 m. the seedlings which had previously been kept
in darkness, were perhaps, and after 70 m. were cer-
tainly, curved, though very slightly, towards the
window. After 85 m. some of the seedlings, which
had previously been illuminated, were perhaps a little
affected, and after 100 m. some of the younger ones
were certainly a little curved towards the light. At
this time (i.e. after 100 m.) there was a plain difference
160 SENSITIVENESS TO LIGHT. Chap. IX
in the curvature of the seedlings in the two pots.
After 2 h. 12 m. the chords of the arcs of four of
the most strongly curved seedlings in each pot were
measured, and the mean angle from the perpendicular
of those which had previously been kept in darkness
was 19°, and of those which had previously been illu-
minated only 7°. Nor did this difference diminish
during two additional hours. As a check, the seed-
lings in both pots were then placed in complete dark-
ness for two hours, in order that apogeotropism should
act on them ; and those in the one pot which were
little curved became in this time almost completely
upright, whilst the more curved ones in the other pot
still remained plainly curved.
Two days afterwards the experiment was repeated,
with the sole difference that even less light was
admitted through the window, as it was protected by a
linen and muslin blind and by two towels ; the sky,
moreover, was somewhat less bright. The result was
the same as before, excepting that everything occurred
rather slower. The seedlings which had been pre-
viously kept in darkness were not in the least curved
after 54 m., but were so after 70 m. Those which had
previously been illuminated were not at all affected
until 130 m. had elapsed, and then only slightly.
After 145 m. some of the seedlings in this latter pot
were certainly curved towards the light ; and there
was now a plain difference between the two pots. After
3 h. 45 m. the chords of the arcs of 3 seedlings in
each pot were measured, and the mean angle from the
perpendicular was 16° for those in the pot which had
previously been kept in darkness, and only 5° lbi
those which had previously been illuminated.
The curvature of the cotyledons of Phalaris towards
a lateral light is therefore certainly influenced by the
Chap. IX. SENSITIVENESS TO LIGHT. 40 J
degree to which they have been previously illu-
minated. We shall presently see that the influence
of light on their bending continues for a short time
after the light has been extinguished. These facts, as
well as that of the curvature not increasing or de-
creasing in nearly the same ratio with that of the
amount of light which they receive, as shown in the
trials with the plants before the lamp, all indicate
that light acts on them as a stimulus, in somewhat
the same manner as on the nervous system of animals,
and not in a direct manner on the cells or cell-walls
which by their contraction or expansion cause the
curvature.
It has already been incidentally shown how slowly
the cotyledons of Phalaris bend towards a very dim
light ; but when they were placed before a bright
paraffin lamp their tips were all curved rectangularly
towards it in 2 h. 20 m. The hypocotyls of Solanum
ly coper sicum had bent in the morning at right angles
towards a north-east window. At 1 p.m. (Oct. 21st) the
pot was turned round, so that the seedlings now pointed*
from the light, but by 5 p.m. they had reversed their
curvature and again pointed to the light. They had
thus passed through 180° in 4 h., having in the
morning previously passed through about 90°. But the
reversal of the first half of the curvature will have
been aided by apogeotropism. Similar cases were
observed with other seedlings, for instance, with those
of Sinapis alba.
We attempted to ascertain in how short a time
light acted on the cotyledons of Phalaris, but this
was difficult on account of their rapid circumnutating
movement; moreover, they differ much in sensibility,
fcceording to age; nevertheless, some of our observa-
tions are worth giving. Pots with seedlings we e
i62 SENSITIVENESS TO LIGHT. Chap. IX.
placed under a microscope provided with an eye-piece
micrometer, of which each division equalled -g-J-oth of an
inch (0'051 mm.) ; and they were at first illuminated
by light from a paraffin lamp passing through a solu-
tion of bichromate of potassium, which does not induce
heliotropism. Thus the direction in which the coty-
ledons were circumnutating could be observed inde-
pendently of any action from the light ; and they could
be made, by turning round the pots, to circumnutate
transversely to the line in which the light would strike
them, as soon as the solution was removed. The fact
that the direction of the circumnutating movement
might change at any moment, and thus the plant
might bend either towards or from the lamp indepen-
dently of the action of the light, gave an element of
uncertainty to the results. After the solution had
been removed, five seedlings which were circumnutat-
ing transversely to the line of light, began to move
towards it, in 6, 4, 7J, 6, and 9 minutes. In one of
these cases, the apex of the cotyledon crossed five
of the divisions of the micrometer (i.e. j^oth of an
inch, or 0*254 mm.) towards the light in 3 m. Of two
seedlings which were moving directly from the light at
the time when the solution was removed, one began to
move towards it in 13 m., and the other in 15 m.
This latter seedling was observed for more than an
hour and continued to move towards the light ; it
crossed at one time 5. divisions of the micrometer
(0*254 mm.) in 2 m. 30 s. In all these cases, the
movement towards the light was extremely unequal in
rate, and the cotyledons often remained almost sta-
tionary for some minutes, and two of them retrograded
a little. Another seedling which was circumnutating
transversely to the line of light, moved towards it in
4 m. after the solution was removed ; it then remained
Cbai\ IX. SENSITIVENESS TO LIGHT. 468
almost stationary for 10 m. ; then crossed 5 divisions
of the micrometer in 6 m.; and then 8 divisions in
11 m. This unequal rate of movement, interrupted
by pauses, and at first with occasional retrogressions,
accords well with our conclusion that heliotropism
consists of modified circumnutation.
In order to observe how long the after-effects of
light lasted, a pot with seedlings of Phalaris, which
had germinated in darkness, was placed at 10.40 a.m.
before a north-east window, being protected on all
other sides from the light; and the movement of a
cotyledon was traced on a horizontal glass. It cir-
cumnutated about the same space for the first 24 m.,
and during the next 1 h. 33 m. moved rapidly towards
the light. The light was now (i.e. after 1 h. 57 m.)
completely excluded, but the cotyledon continued
bending in the same direction as before, certainly for
more than 15 m., probably for about 27 m. The doubt
arose from the necessity of not looking at the seed-
lings often, and thus exposing them, though momen-
tarily, to the light. This same seedling was now kept
in the dark, until 2.18 p.m., by which time it had
reacquired through apogeotropism its original upright
position, when it was again exposed to the light from
a clouded sky. By 3 p.m. it had moved a very short
distance towards the light, but during the next 45 in.
travelled quickly towards it. After this exposure of
1 h. 27 m. to a rather dull sky, the light was again
completely excluded, but the cotyledon continued to
bend in the same direction as before for 14 m. within
a very small limit of error. It was then placed in
the dark, and it now moved backwards, so that after
1 h. 7 m. it stood close to where it had started from at
2.18 p.m. These observations show that the coty-
ledons of Phalaris, after being exposed to a lateral
464 SENSI1IVENESS TO LIGHT. Chap. IX.
light, continue to bend in the same direction for
between a quarter and half an hour.
In the two experiments just given, the cotyledons
moved backwards or from the window shortly after
being subjected to darkness ; and whilst tracing the
circumnutation of various kinds of seedlings exposed
to a lateral light, we repeatedly observed that late in
the evening, as the light waned, they moved from it.
This fact is shown in some of the diagrams given in
the last chapter. We wished therefore to learn whether
this was wholly due to apogeotropism, or whether an
organ after bending towards the light tended from
any other cause to bend from it, as soon as the light
failed. Accordingly, two pots of seedling Phalaris
and one pot of seedling Brassica were exposed for 8 h.
before a paraffin lamp, by which time the cotyledons
of the former and the hypocotyls of the latter were bent
rectangularly towards the light. The pots were now
quickly laid horizontally, so that the upper parts of
the cotyledons and of the hypocotyls of 9 seedlings
projected vertically upwards, as proved by a plumb-line.
In this position they could not be acted on by apo-
geotropism, and if they possessed any tendency to
straighten themselves or to bend in opposition to their
former heliotropic curvature, this would be exhibited,
for it would be opposed at first very slightly by apogeo-
tropism. They were kept in the dark for 4 h., during
which time they were twice looked at ; but no uniform
bending in opposition to their former heliotropic
curvature could be detected. We have said uniform
bending, because they circumnutated in their new
position, and after 2 h. were inclined in different
directions (between 4° and 11°) from the perpendicular.
Their directions were also changed after two additions]
hours, and again on the following morning. We may
Chap. IX. SENSITIVENESS TO LIGHT. 4H5
therefore conclude that the bending back of plants
from a light, when this becomes obscure or is extin-
guished, is wholly due to apogeotropism.*
In our various experiments we were often struck
with the accuracy with which seedlings pointed to a
light although of small size. To test this, many seed-
lings of Phalaris, which had germinated in darkness in
a very narrow box several feet in length, were placed
in a darkened room near to and in front of a lamp
having a small cylindrical wick. The cotyledons at
the two ends and in the central part of the box, would
therefore have to bend in widely different directions
in order to point to the light. After they had become
rectangularly bent, a long white thread was stretched
by two persons, close over and parallel, first to one and
then to another cotyledon ; and the thread was found
in almost every case actually to intersect the small
circular wick of the now extinguished lamp. The
deviation from accuracy never exceeded, as far as we
could judge, a degree or two. This extreme accuracy
seems at first surprising, but is not really so, for an
upright cylindrical stem, whatever its position may
be with respect to the light, would have exactly half
its circumference illuminated and half in shadow ; and
as the difference in illumination of the two sides is
the exciting cause of heliotropism, a cylinder would
naturally bend with much accuracy towards the light.
The cotyledons, however, of Phalaris are not cylin-
drical, but oval in section; and the longer axis was
to the shorter axis (in the one which was measured)
as 100 to 70. Nevertheless, no difference could be
* It appears from a reference heliotropically is at the same time
in Wiesner (' Die Undulirende striving, through apogeotropism,
Nutation der Internodien,' p. 7), to raise itself into a vertical posi-
that H. Mailer of Thurgau found tion.
that a stem which is bending
Itff) SENSITIVENESS TO LIGHT. Chap. IX
detected in the accuracy of their bending, whether
they stood with their broad or narrow sides facing
the light, or in any intermediate position; and so it
was with the cotyledons 01 Avena sativa, which are
likewise oval in section. Now, a little reflection will
show that in whatever position the cotyledons may
stand, there will be a line of greatest illumination,
exactly fronting the light, and on each side of this
line an equal amount of light will be received ; but
if the oval stands obliquely with respect to the light,
this will be diffused over a wider surface on one side
of the central line than on the other. We may there-
fore infer that the same amount of light, whether
diffused over a wider surface or concentrated on a
smaller surface, produces exactly the same effect ; for
the cotyledons in the long narrow box stood in all
sorts of positions with reference to the light, yet all
pointed truly towards it.
That the bending of the cotyledons to the light
depends on the illumination of one whole side or on
the obscuration of the whole opposite side, and not on
a narrow longitudinal zone in the line of the light
being affected, was shown by the effects of painting
longitudinally with Indian ink one side of five coty-
ledons of Phalaris. These were then placed on a table
near to a south-west window, and the painted half was
directed either to the right or left. The result was that
instead of bending in a direct line towards the window,
they were deflected from the window and towards the
unpainted side, by the following angles, 35°, 83°, 31°,
43 3 , and 39°. It should be remarked that it was hardly
possible to paint one-half accurately, or to place all
the seedlings which are oval in section in quite the
same position relatively to the light; and this will
account for the differences in the angles. Five coty«
Chap. IX. SENSITIVENESS TO LIGHT. 4(57
ledons of Avena were also painted in the same manner,
but with greater care; and they were laterally de-
flected from the line of the window, towards the
unpainted side, by the following angles, 44°, 44°, 55°,
51°, and 57°. This deflection of the cotyledons from
the window is intelligible, for the whole unpainted
side must have received some light, whereas the oppo-
site and painted side received none ; but a narrow
zone on the unpainted side directly in front of the
window will have received most light, and all the
hinder parts (half an oval in section) less and less light
in varying degrees ; and we may conclude that the
angle of deflection is the resultant of the action of the
light over the whole of the unpainted side.
It should have been premised that painting with
Indian ink does not injure plants, at least within
several hours ; and it could injure them only by stop-
ping respiration. To ascertain whether injury was thus
soon caused, the upper halves of 8 cotyledons of Avena
were thickly coated with transparent matter, — 4 with
gum, and 4 with gelatine ; they were placed in the
morning before a window, and by the evening they
were normally bowed towards the light, although the
coatings now consisted of dry crusts of gum and
gelatine. Moreover, if the seedlings which were painted
longitudinally with Indian ink had been injured on
the painted side, the opposite side would have gone
on growing, and they would consequently have become
bowed towards the painted side ; whereas the curvature
was always, as we have seen, in the opposite direction,
or towards the unpainted side which was exposed to
the light. We witnessed the effects of injuring longi-
tudinally one side of the cotyledons of Avena and
Phalaris ; for before we knew that grease was highly
injurious to them, several were painted down one side
£68 TRANSMITTED EFFECTS OF LIGHT. Chap. IX
with a mixture of oil and larnp-black, and were then
exposed before a window ; others similarly treated were
afterwards tried in darkness. These cotyledons soon
became plainly bowed towards the blackened side,
evidently owing to the grease on this side having
checked their growth, whilst growth continued on the
opposite side. But it deserves notice that the curva-
ture differed from that caused by light, which ulti-
mately becomes abrupt near the ground. These
seedlings did not afterwards die, but were much injured
and grew badly.
Localised Sensitiveness to Light, and its
transmitted effects.
Phalaris Canariensis. — Whilst observing the accu-
racy with which the cotyledons of this plant became
bent towards the light of a small lamp, we were
impressed with the idea that the uppermost part deter-
mined the direction of the curvature of the lower part.
When the cotyledons are exposed to a lateral light,
the upper part bends first, and afterwards the bending
gradually extends down to the base, and, as we shall
presently see, even a little beneath the ground.
This holds good with cotyledons from less than
*1 inch (one was observed to act in this manner which
was only *03 in height) to about '5 of an inch in
height ; but when they have grown to nearly an inch
in height, the basal part, for a length of '15 to *2 of
an inch above the ground, ceases to bend. As with
young cotyledons the lower part goes on bending,
after the upper part has become well arched towards
a lateral light, the apex would ultimately point to
the ground instead of to the light, did not the upper
part reverse its curvature and straighten itself, as
Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 469
soon as the upper convex surface of the bowed-
down portion received more light than the lower
concave surface. The position ultimately assumed by
young and upright cotyledons, exposed to light enter-
ing obliquely from above through a window, is shown
in the accompanying figure (Fig. 181) ; and here it
may be seen that the whole upper part has become
very nearly straight. When the cotyledons were
exposed before a bright lamp, standing on the same
level with them, the upper part, which was at first
Fie. 181.
Phalaris Canariensis : cotyledons after exposure in a box open on one side
in front of a south-west window during 8 h. Curvature towards the
light accurately traced. The short horizontal lines show the level ol
the ground.
great] y arched towards the light, became straight and
strictly parallel with the surface of the soil in the
pots; the basal part being now rectangularly bent.
All this great amount of curvature, together with the
subsequent straightening of the upper part, was often
effected in a few hours.
After the uppermost part has become bowed a little to the
light, its overhanging weight must tend to increase the curva-
ture of the lower part; but any such effect was shown in several
ways to be quite insignificant. When little caps of tin-foil
(hereafter to be described) were placed on the summits of the
cotyledons, though this must have added considerably to their
weight, the rate or amount of bending was not thus increased.
But the best evidence was afforded by placing pots with seedlings
of Phalaris before a lamp in such a position, that the cotyledons
were horizontally extended and projected at right angles to the
line of light. . In the course of 5§ h. they were directed towards
the light with their bases bent at right angles ; ai i this abrupt
470 TRANSMITTED EFFECTS OF LIGHT. Chap. IX.
curvature could uot have been aided in the least by the weight
cf the upper part, which acted at right angles to the plane of
curvature.
It will be shown that when the upper halves of the coty-
ledons of Phalaris and Avena were enclosed in little pipes of
tin-foil or of blackened glass, in which case the upper part was
mechanically prevented from bending, the lower and unenclosed
part did not bend when exposed to a lateral light; and it
occurred to us that this fact might be due, not to the exclusion
of the light from the upper part, but to some necessity of the
bending gradually travelling down the cotyledons, so that
unless the upper part first became bent, the lower could not
bend, however much it might be stimulated. It was necessary
for our purpose to ascertain whether this notion was true, and it
was proved false; for the lower halves of several cotyledons
became bowed to the light, although their upper halves were
enclosed in little glass tubes (not blackened), which prevented,
as far as we could judge, their bending. Nevertheless, as the
part within the tube might possibly bend a very little, fine rigid
rods or flat splinters of thin glass were cemented with shellac to
one side of the upper part of 15 cotyledons; and in six cases
they were in addition tied on with threads. They were thus
forced to remain quite straight. The resnlt was that the lower
halves of all became bowed to the light, but generally not in so
great a degree as the corresponding part of the free seedlings
in the same pots ; and this may perhaps be accounted for by
some slight degree of injury having been caused by a consider-
able surface having been smeared with shellac. It may be
added, that when the cotyledons of Phalaris and Avena are
acted on by apogeotropism, it is the upper part which begins
first to bend; and when this part was rendered rigid in the
manner just described, the upward curvature of the basal part
was not thus prevented.
To test our belief that the upper part of the cotyledons of
Phalaris, when exposed to a lateral light, regulates the bending
of the lower part, many experiments were tried ; but most of our
first attempts proved useless from various causes not worth
specifying. Seven cotyledons had their tips cut off for lengths
varying between *1 and T6 of an inch, and these, when left
exposed all day to a lateral light, remained upright. In another
set of 7 cotyledons, the tips were cut off for a length of only
about *05 of an inch (1-27 mm.) and these became bowed towards
Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 471
a lateral light, but not nearly so much as the many other seed-
lings in the same pots. This latter case shows that cutting off
the tips does not by itself injure the plants so seriously as to
prevent heliotropism ; but we thought at the time, that such
injury might follow when a greater length was cut off, as in the
tirst set of experiments. Therefore, no more trials of this kind
were made, which we now regret ; as we afterwards found that
when the tips of three cotyledons were cut off for a length of
2 inch, and of four others for lengths of *14, *12, T, and *07
inch, and they were extended horizontally, the amputation did
not interfere in the least with their bending vertically upwards,
through the action oi apogeotropism, like unmutilated speci-
mens. It is therefore extremely improbable that the amputation
of the tips for lengths of from T to TI inch, could from the
injury thus caused have prevented the lower part from bending
towards the light.
We next tried the effects of covering the upper part of the
cotyledons of Phalaris with little caps which were impermeable
to light ; the whole lower part being left fully exposed before a
south-west window or a bright paraffin lamp. Some of the caps
were made of extremely thin tin-foil blackened within; these
had the disadvantage of occasionally, though rarely, being too
heavy, especially when twice folded. The basal edges could be
pressed into close contact with the cotyledons ; though this
again required care to prevent injuring them. Nevertheless,
any injury thus caused could be detected by removing the caps,
and trying whether the cotyledons were then sensitive to light.
Other caps were made of tubes of the thinnest glass, which
when painted black served well, with the one great disadvantage
that the lower ends could not be closed- But tubes were used
which fitted the cotyledons almost closely, and black paper was
placed on the soil round each, to check the upward reflection of
light from the soil. Such tubes were in one respect far better
than caps of tin-foil, as it was possible to cover at the same
time some cotyledons with transparent and others with opaque
tubes ; and thus our experiments could be controlled. It should
be kept in mind that young cotyledons were selected for trial,
and that these when not interfered with become bowed down
to the ground towards the light.
We will begin with the glass-tubes. The summits of nine
sotyledons, differing somewhat in height, wero enclosed for
rather less than half their lengths in uncoloured or transparent
31
472 TEANSMITTED EFFECTS OF LIGHT. Chav. IX
tubes ; and these were then exposed before a south-west window
on a bright day for 8 h. All of them became strongly curved
towards the light, in the same degree as the many other free
seedlings in the same pots ; so that the glass-tubes certainly did
not prevent the cotyledons from bending towards the light.
Nineteen other cotyledons were, at the same time, similarly
enclosed in tubes thickly painted with Indian ink. On five of
them, the paint, to our surprise, contracted after exposure
to the sunlight, and very narrow cracks were formed, through
which a little light entered ; and these five cases were rejected.
Of the remaining 14 cotyledons, the lower halves of which had
been fully exposed to the light for the whole time, 7 continued
quite straight and upright ; 1 was considerably bowed to the
light, and 6 were slightly bowed, but with the exposed bases of
most of them almost or quite straight. It is possible that some
light may have been reflected upwards from the soil and entered
the bases of these 7 tubes, as the sun shone brightly, though
bits of blackened paper had been placed on the soil round
them. Nevertheless, the 7 cotyledons which were slightly
bowed, together with the 7 upright ones, presented a most re-
markable contrast in appearance with the maoy other seedlings
in the same pots to which nothing had been done. The
blackened tubes were then removed from 10 of these seedlings,
and they were now exposed before a lamp for 8 h. : 9 of them
became greatly, and 1 moderately, curved towards the light,
proving that the previous absence of any curvature in the
basal part, or the presence of only a slight degree of curvature
there, was due to the exclusion of light from the upper part.
Similar observations were made on 12 younger cotyledons
with their upper halves enclosed within glass-tubes coated with
black varnish, and with their lower halves fully exposed to
bright sunshine. In these younger seedlings the sensitive zone
seems to extend rather lower down, as was observed on some
other occasions, for two became almost as much curved towards
the light as the free seedlings; and the remaining ten were
slightly curved, although the basal part of several of them,
which normally becomes more curved than any other part,
exhibited hardly a trace of curvature. These 12 seedlings
taken together differed greatly in their degree of curvature from
all the many other seedlings in the same pots.
Better evidence of the efficiency of the blackened tubes was
incidentally afforded by some experiments hereafter to be given,
Chap. IX TKANSMITTED EFFECTS OF LIGHT. 478
in which the upper halves of 14 cotyledons were enclosed ir
tubes from which an extremely narrow stripe of the black
varnish had been scraped off. These cleared stripes were
not directed towards the window, but obliquely to one side
of the room, so that only a very little light could act on the
upper halves of the cotyledons. These 14 seedlings remained
during eight hours of exposure before a south-west window on
a hazy day quite upright; whereas all the other many free
seedlings in the same pots became greatly bowed towards the
light.
We will now turn to the trials with caps made of very thin
tin-foil. These were placed at different times on the summits of
24 cotyledons, and they extended down for a length of between
T5 and 2 of an inch. The seedlings were exposed to a lateral
light for periods varying between 6 h. cSO m. and 7 h. 45 m.,
which sufficed to cause all the other seedlings in the same pots
to become almost rectangularly bent towards the light. They
varied in height from only "04 to 1T5 inch, but the greater
number were about 75 inch. Oi" the 24 cotyledons with their
summits thus protected, 3 became much bent, but not in the
direction of the light, and as they did not s-traighten themselves
through apogeotropism during the following night, either the
caps were too heavy or the plants themselves were in a weak
condition; and these three cases may be excluded. There
are left for consideration 21 cotyledons ; of these 17 remained
all the time quite upright ; the other 4 became slightly inclined
to the light, but not in a degree comparable with that of the
many free seedlings in the same pots. As the glass-tubes, when
unpainted, did not prevent the cotyledons from becoming
greatly bowed, it cannot be supposed that the caps o f very
thin tin-foil did so, except through the exclusion of the light.
To prove that the plants had not been injured, the caps were
removed from 6 of the upright seedlings, and these were exposed
before a paraffin lamp for the same length of time as before
and they now all became greatly curved towards the light.
As caps between '15 and '2 of an inch in depth were thus
proved to be highly efficient in preventing the cotyledons from
bending towards the light, 8 other cotyledons were protected
with caps between only 06 and T2 in depth. Of these, two
remained vertical, one was considerably and five slightly curved
towards the light, but far less so than the free seedlings in the
same pots.
i74 TKANSMITTED EFFECTS OF LIGHT. Chap. IX
Another trial was made in a clLTerent manner, namely, It?
handaging with strips of tin-foil, about *2 in breadth, the npper
part, but not the actual summit, of. eight moderately young
seedlings a little over half an inch in height. The summits and
the basal parts were thus left fully exposed to a lateral light
during 8 h. ; an upper intermediate zone being protected.
With four of these seedlings the summits were exposed for
a length of '05 inch, and in two of them this part became
curved towards the light, but the whole lower part remained
quite upright; whereas the entire length of the other two
seedlings became slightly curved towards the light. The
summits of the four other seedlings were exposed for a length
of '04 inch, and of these one remained almost upright, whilst
the other three became considerably curved towards the light.
The many free seedlings in the same pots were all greatly
curved towards the light.
From these several sets of experiments, including those with
the glass-tubes, and those when the tips were cut off, we may
infer that the exclusion of light from the upper part of the
cotyledons of Phalaris prevents the lower part, though fully
exposed to a lateral light, from becoming curved. The summit
for a length of "04 or "05 of an inch, though it is itself sensitive
and curves towards the light, has only a slight power of causing
the lower part to bend. Nor has the exclusion of light from the
summit for a length of *1 of an inch a strong influence on the
curvature of the lower part. On the other hand, an exclusion
for a length of between *15 and "2 of an inch, or of the whole
upper half, plainly prevents the lower and fully illuminated
part from becoming curved in the manner (see Fig. 181) which
invariably occurs when a free cotyledon is exposed to a lateral
light. With very young seedlings the sensitive zone seems to
extend rather lower down relatively to their height than in older
seedlings. We must therefore conclude that when seedlings
are freely exposed to a lateral light some influence is trans-
mitted from the upper to the lower part, causing the latter to
bend.
This conclusion is supported by what may be seen to occur
on a small scale, especially with young cotyledons, without any
artificial exclusion of the light; for they bend beneath the earth
where no light can enter. Seeds of Phalaris were covered
with a layer one-fourth of an inch in thickness of very fine
sand, consisting of extremely minute grains of silex coated with
Chap. IX. TKANSMITTED EFFECTS OF LIGHT. 475
oxide of iron. A layer of this sand, moistened to the same
degree as that over the seeds, was spread over a glass-plate ; and
when the layer was '05 of an inch in thickness (carefully mea-
sured) no light from a bright sky could be seen to pass through
it, unless it was viewed through a ]ong blackened tube, and
then a trace of light could be detected, but probably much too
little to affect any plant. A layer *1 of an inch in thickness was
quite impermeable to light, as judged by the eye aided by the tube.
It may be worth adding that the layer, when dried, remained
equally impermeable to light. This sand yielded to very slight
pressure whilst kept moist, and in this state did not contract
or crack in the least. In a first trial, cotyledons which had
grown to a moderate height were exposed for 8 h. before a paraffin
lamp, and they became greatly bowed. At their bases on the
shaded side opposite to the light, well-defined, crescentic, open
furrows were formed, which (measured under a microscope with
a micrometer) were from 02 to 03 of an inch in breadth, and
these had evidently been left by the bending of the buried bases
of the cotyledons towards the light. On the side of the light
the cotyledons were in close contact with the sand, which was a
very little heaped up. By removing with a sharp knife the
sand on one side of the cotyledons in the line of the light, the
bent portion and the open furrows were found to extend down
to a depth of about -1 of an inch, where no light could enter.
The chords of the short buried arcs formed in four cases angles
of 11°, 13 3 , 15°, and 18°, with the perpendicular. By the
following morning these short bowed portions had straightened
themselves through apogeotropism.
In the next trial much younger cotyledons were similarly
treated, but were exposed to a rather obscure lateral light.
After some hours, a bowed cotyledon, - 3 inch in height, had an
open furrow on the shaded side ■ 04 inch in breadth ; another
cotyledon, only *13 inch in height, had left a furrow *02 inch in
breadth. But the most curious case was that of a cotyledon which
had just protruded above the ground and was only ' 03 inch in
height, and this was found to be bowed in the direction of the
light to a depth of "2 of an inch beneath the surface. From
what we know of the impermeability of this sand to light, the
upper illuminated part in these several cases must have deter-
mined the curvature of the lower buried portions. But an
apparent cause of doubt may be suggested : as the cotyledons
are continually circumnutating, they tend to form a minut€
§76 TRANSMITTED EFFECTS OF LIGHT. Chak IX
crack or furrow all round their bases, which would admit a
little light on all sides; but this would not happen when thej
were illuminated laterally, for we know that they quickly bend
towards a lateral light, and they then press so firmly against the
sand on the illuminated side as to furrow it, and this would
effectually exclude light on this side. Any light admitted on
the opposite and shaded side, where an open furrow is formed,
would tend to counteract the curvature towards the lamp or
other source of the light. It may be added, that the use of fine
moist sand, which yields easily to pressure, was indispensable
in the above experiments ; for seedlings raised in common soil,
not kept especially damp, and exposed for 9 h. 30 m. to a strong
lateral light, did not form an open furrow at their bases on the
shaded side, and were not bowed beneath the surface.
Perhaps the most striking proof of the action of the upper
on the lower part of the cotyledons of Phalaris, when laterally
illuminated, was afforded by the blackened glass-tubes (before
alluded to) with very narrow stripes of the varnish scraped
off on one side, through which a little light was admitted.
The breadth of these stripes or slits varied between *01 and
•02 inch (*25 and *51 mm.). Cotyledons with their upper
halves enclosed in such tubes were placed before a south-west
window, in such a position, that the scraped stripes did not
directly face the window, but obliquely to one side. The seed-
lings were left exposed for 8 h., before the close of which time
the many free seedlings in the same pots had become greatly
bowed towards the window. Under these circumstances, the
whole lower halves of the cotyledons, which had their summits
enclosed in the tubes, were fully exposed to the light of the
sky, whilst their upper halves received exclusively or chiefly
diffused light from the room, and this only through a very
narrow slit on one side. Now, if the curvature of the lower
part had been determined by the illumination of this part, all
the cotyledons assuredly would have become curved towards
the window; but this was far from being the case. Tubes
of the kind just described were placed on several occasions
over the upper halves of 27 cotyledons ; 14 of them remained
all the time quite vertical; so that sufficient diffused light
did not enter through the narrow slits to produce any effect
whatever; and they behaved in the same manner as if their
upper halves had been enclosed in completely blackened tubes.
The lower halves of the 13 other cotvledons became bowed
Dhap. IX. TRANSMITTED EFFECTS OF LIGHT. 477
not directly in the line of the window, but obliquely towards
it ; one pointed at an angle of only 18°, but the remaining 12
at angles varying between 45° and 62° from the line of the
window. At the commencement of the experiment, pins had
been laid on the earth in the direction towards which the slits in
the varnish faced ; and in this direction alone a small amount
of diffused light entered. At the close of the experiment, 7 ot
the bowed cotyledons pointed exactly in the line of the pins,
and 6 of them iu a line between that of the pins and that of the
window. This intermediate position is intelligible, for any light
from the sky which entered obliquely through the slits would
be much more efficient than the diffused light which entered
directly through them. After the 8 h. exposure, the contrast
in appearance between these 13 cotyledons and the many other
seedlings in the same pots, which were all (excepting the above
14 vertical ones) greatly bowed in straight and parallel lines
towards the window, was extremely remarkable. It is therefore
certain that a little weak light striking the upper halves of the
cotyledons of Phalaris, is far more potent in determining the
direction of the curvature of the lower halves, than the full
illumination of the latter during the whole time of exposure.
In confirmation of the above results, the effect of thickly
painting with Indian ink one side of the upper part of three coty-
ledons of Phalaris, for a length of ■ 2 inch from their tips, may be
worth giving. These were placed so that the unpainted surface
was directed not towards the window, but a little to one side ;
and they all became bent towards the unpainted side, and from
the line of the window by angles amounting to 31°, 35°, and 83°.
The curvature in this direction extended down to their bases,
although the whole lower part was fully exposed to the light
from the window.
Finally, although there can be no doubt that the illumination
of the upper part of the cotyledons of Phalaris greatly affects
the power and manner of bending of the lower part, jet some
observations seemed to render it probable that the simultaneous
stimulation of the lower part by light greatly favours, or is
almost necessary, for its well-marked curvature ; but our experi-
ments were not conclusive, owing to the difficulty of excluding
light from the lower halves without mechanically preventing
their curvature.
Avena sativa. — The cotyledons of this plant become quickly
oo wed towards a lateral light, exactly like those of Phalaris.
1:78 TRANSMITTED EFFECTS OF LIGHT. Chap. IX
Experiments similar to the foregoing ones were tried, and \vc
will give the results as briefly as possible. They are somewhat
less conclusive than in the case of Phalaris, and this may
possibly be accounted for by the sensitive zone varying in exten-
sion, in a species so long cultivated and variable as the common
Oat. Cotyledons a little under three-quarters of an inch in
height were selected for trial : six had their summits protected
from light by tin-foil caps, *25 inch in depth, and two others by
caps *3 inch in depth. Of these 8 cotyledons, five remained
upright during 8 hours of exposure, although their lower parts
were fully exposed to the light all the time; two were very slightly,
and one considerably, bowed towards it. Caps only • 2 or • 22 inch
in depth were placed over 4 other cotyledons, and now only one
remained upright, one was slightly, and two considerably bowed
to the light. In this and the following cases all the free seedlings
in the same pots became greatly bowed to the light.
Our next trial was made with short lengths of thin and
fairly transparent quills; for glass-tubes of sufficient diameter
to go over the cotyledons would have been too heavy. Firstly,
the summits of 13 cotyledons were enclosed in unpainted
quills, and of these 11 became greatly and 2 slightly bowed
to the light ; so that the mere act of enclosure did not prevent
the lower part from becoming bowed. Secondly, the summits
of 11 cotyledons were enclosed in quills *3 inch in length, painted
so as to be impermeable to light; of these, 7 did not be-
come at all inclined towards the light, but 3 of them were
slightly bent more or less transversely with respect to the line
of light, and these might perhaps have been altogether ex-
cluded; one alone was slightly bowed towards the light.
Painted quills, "25 inch in length, were placed over the summits
of 4 other cotyledons ; of these, one alone remained upright, a
second was slightly bowed, and the two others as much bowed
to the light as the free seedlings in the same pots. These two
latter cases, considering that the caps were ' 25 in length, are
inexplicable.
Lastly, the summits of 8 cotyledons were coated with flexible
and highly transparent gold-beaters' skin, and all became as
much bowed to the light as the free seedlings. The summits of
9 other cotyledons were similarly coated with gold-beaters' skin,
which was then painted to a depth of between '25 and - 3 inch,
so as to be impermeable to light; of these 5 remained upright,
and •! were well bowed to the light, almost or quite as well as
Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 47ft
the free seedlings. These latter four cases, as well as the two
in the last paragraph, offer a strong exception to the rule that
the illumination of the upper part determines the curvature of
the lower part. Nevertheless, 5 of these 8 cotyledons remained
quite upright, although their lower halves were fully illuminated
all the time; and it would almost be a prodigy to find five free
seedlings standing vertically after an exposure for several hours
to a lateral light.
The cotyledons of Avena, like those of Phalaris, when growing
in soft, damp, fine sand, leave an open crescentric furrow on the
shaded side, after bending to a lateral light ; and they become
bowed beneath the surface at a depth to which, as we know,
light cannot penetrate. The arcs of the chords of the buried
bowed portions formed in two cases angles of 20° and 21° with
the perpendicular. The open furrows on the shaded side were,
in four cases, *008, '016, "024, and '024 of an inch in breadth.
Brassica oleracea (Common Eed). — It will Ijere be shown that
the upper half of the hypocotyl of the cabbage, when illuminated
by a lateral light, determines the curvature of the lower half.
It is necessary to experimeutise on young seedlings about half
an inch or rather less in height, for when grown to an inch and
upwards the basal part ceases to bend. We first tried painting
'ihe hypocotyls with Indian ink, or cutting off their summits for
various lengths ; but these experiments are not worth giving,
though they confirm, as far as they can be trusted, the results
of the following ones. These were made by folding gold-beaters'
skin once round the upper halves of young hypocotyls, and
painting it thickly with Indian ink or with black grease. As
a control experiment, the same transparent skin, left unpainteel,
was folded round the upper halves of 12 hypocotyls ; and these
all became greatly curved to the light, excepting one r which was
only moderately curved. Twenty other young hypocotyls had
the skin round their upper halves painted, whilst their lower
halves were left quite uncovered. These seedlings were then
exposed, generally for between 7 and 8 h., in a box blackened
within and open in front, either before a south-west window or
a paraffin lamp. This exposure was amply sufficient, as was
shown by the strongly-marked heliotropism of all the free seed-
lings in the same pots; nevertheless, some were left exposed
to the light for a much longer time. Of the 20 hwnocotyls
thus treated, 14 remained quite upright, and 6 became slightly
bowed to the light ; but 2 of these latter cases were not really
180 TRANSMITTED EFFECTS OF LIGHT. Chap. IX.
exceptions, for on removing the skin the paint was found im-
perfect and was penetrated by many small transparent spaces
on the side which faced the light.. Moreover,, in two other cases
the painted skin did not extend quite halfway down the hypo-
cotyl. Altogether there was a wonderful contrast in the several
pots between these 20 hypocotyls and the other many free
seedlings, which were all greatly bowed down to their bases m
the direction of the light, some being almost prostrate on the
ground.
The most successful trial on any one day (included in the
above results) is worth describing in detail. Six young seed-
lings were selected, the hypocotyls of which were nearly "45 inch,
excepting one, which was " 6 inch in height, measured from the
bases of their petioles to the ground. Their upper halves,
judged as accurately as could be done by the eye, were folded
once round with gold-beaters' skin, and this was painted
thickly with Indian ink. They were exposed in an otherwise
darkened room before a bright paraffin lamp, which stood on
a level with the two pots containing the seedlings. They
were first looked at after an interval of 5 h. 10 m., and five
of the protected hypocotyls were found quite , erect, the sixth
beiug very slightly inclined to the light ; whereas all the many
L.*ee seedlings in the same two pots were greatly bowed
to the light. They were again examined after a continuous
exposure to the light of 20 h. 35 m. ; and now the contrast
between the two sets was wonderfully great ; for the free seed-
lings had their hypocotyls extended almost horizontally in the
direction of the light, and were curved down to the ground;
whilst those with the upper halves protected by the painted
skin, but with their lower halves fully exposed to the light, still
remained quite upright, with the exception of the one which
retained the same slight inclination to the light which it had
before. This latter seedling was found to have been rather
badly painted, for on the side facing the light the red colour
of the hypocotyl could be distinguished through the paint.
We next tried nine older seedlings, the hypocotyls of which
varied between 1 and 1*6 inch in height. The gold-beaters'
skin round their upper parts was painted with black grease to
a depth of only ■ 3 inch, that is, from less than a third to a fourth
or fifth of their total heights. They were exposed to the light
for 7 h. 15 m. ; and the result showed that the whole of the
sensitive zone, which determines the curvature of the lower
Chap. IX. TRANSMITTED EFFECTS OF LIGHT. 481
part, was not protected from the action of the light ; for all 9
became curved towards it, 4 of them very slightly, 3 moderately,
and 2 almost as much as the unprotected seedlings. Neverthe-
less, the whole 9 taken together differed plainly in their degree
of curvature from the many free seedlings, and from some
which were wrapped in unpainted skin, growing in the same
two pots.
Seeds were covered with about a quarter of an inch of the fine
sand described under Phalaris ; and when the hypocotyls had
grown to a height of between '4 and *55 inch, they were exposed
during 9 h. before a paraffin lamp, their bases being at first
closely surrounded by the damp sand. They all became bowed
down to the ground, so that their upper parts lay near to and
almost parallel to the surface of the soil. On the side of the
light their bases were in close contact with the sand, which was
here a very little heaped up; on the opposite or shaded side
there were open, crescentic cracks or furrows, rather above * 01
of an inch in width ; but they were not so sharp and regular
as those made by Phalaris and Avena, and therefore could not
be so easily measured under the microscope. The hypocotyls
were found, when the sand was removed on one side, to be
curved to a depth beneath the surface in three cases of at least
•1 inch, in a fourth case of '11, and in a fifth of *15 inch. The
chords of the arcs of the short, buried, bowed portions formed
angles of between 11° and 15° with the perpendicular. From
what we have seen of the impermeability of this sand to light,
the curvature of the hypocotyls certainly extended down to a
depth where no light could enter; and the curvature must
have been caused by an influence transmitted from the upper
illuminated part.
The lower halves of five young hypocotyls were surrounded by
unpainted gold-beaters' skin, and these, after an exposure of 8 h.
before a paraffin lamp, ail became as much bowed to the light
as the free seedlings. The lower halves of 10 other young
hypocotyls, similarly surrounded with the skin, were thickly
painted with Indian ink; their upper and unprotected halves
became well curved to the light, but their lower and protected
halves remained vertical in all the cases excepting one, and on
this the layer of paint was imperfect. This result seems to
prove that the influence transmitted from the upper part is
not sufficient to cause the lower part to bend, unless it be at
the same time illuminated; but there remains the doubt, as in
482 TRANSMITTED EFFECTS OF LIGHT. Chap. IX
the case of Phalaris, whether the skin covered with a rathoi
thick crust of dry Indian ink did not mechanically prevent!
their curvature.
Beta vulgaris. — A few analogous experiments were tried on
this plant, which is not very well adapted for the purpose, as the
basal part of the hypocotyl, after it has grown to above half an
inch in height, does not bend much on exposure to a lateral
light. Four hypocotyls were surrounded close beneath their
petioles with strips of thin tin-foil, ■ 2 inch in breadth, and they
remained upright all day before a paraffin lamp ; two others
were surrounded with strips '15 inch in breadth, and one of
these remained upright, the other becoming bowed ; the band-
ages in two other cases were only "1 inch in breadth, and both
of these hypocotyls became bowed, though one only slightly,
towards the light. The free seedlings in the same pots were
all fairly well curved towards the light; and during the follow-
ing night became nearly upright. The pots were now turned
round and placed before a window, so that the opposite sides
of the seedlings were exposed to the light, towards which all
the unprotected hypocotyls became bent in the course of 7 h.
Seven out of the 8 seedlings with bandages of tin-foil remained
upright, but one which had a bandage only - 1 inch in breadth,
became curved to the light. On another occasion, the upper
halves of 7 hypocotyls were surrounded with painted gold-
beaters' skin ; of these 4 remained upright, and 3 became a little
curved to the light: at the same time 4 other seedlings sur-
rounded with unpainted skin, as well as the free ones in the
same pots, all became bowed towards the lamp, before which
they had been exposed during 22 hours.
lladicles of Sinapis alba. — The radicles of some plants are
indifferent, as far as curvature is concerned, to the action of
light ; whilst others bend towards and others from it.* Whether
these movements are of any service to the plant is very doubtful,
at least in the case of subterranean roots ; they probably result
from the radicles being sensitive to contact, moisture, and gravi-
tation, and as a consequence to other irritants which are never
naturally encountered. The radicles of Sinapis alba, when
immersed in water and exposed to a lateral light, bend from it,
or are apheliotropic. They become bent for a length of aboui
4 mm. from their tips. To ascertain whether this movement
* Sachs, 'Physiologie Yegetale,' 1868, p. 44.
itaAr. IS. TRANSMITTED EFFECTS OF LIGHT. 483
generally occurred, 41 radicles, which had germinated in damp
sawdust, were immersed in water and exposed to a lateral light;
and they all, with two doubtful exceptions, became curved from
the light. At the same time the tips of 54 other radicles,
similarly exposed, were just touched with nitrate of silver.
They were blackened for a length of from - 05 to "07 mm., and
probably killed ; but it should be observed that this did not
check materially, if at all, the growth of the upper part; for
several, which were measured, increased in the course of only
8-9 h. by 5 to 7 mm. in length. Of the 54 cauterised radicles
one case was doubtful, 25 curved themselves from the light in
the normal manner, and 28, or more than half, were not in the
least apheliotropic. There was a considerable difference, which
we cannot account for, in the results of the experiments tried
towards the end of April and in the middle of September.
Fifteen radicles (part of the above 54) were cauterised at the
former period and were exposed to sunshine, of which 12 failed
to be apheliotropic, 2 were still apheliotropic, and 1 was doubt-
ful. In September, 39 cauterised radicles were exposed to a
northern light, being kept at a proper temperature ; and now
23 continued to be apheliotropic in the normal manner, and
only 16 failed to bend from the light. Looking at the aggregate
results at both periods, there can be no doubt that the de-
struction of the tip for less than a millimeter in length destroyed
in more than half the cases their power of moving from the
light. It is probable that if the tips had been cauterised for
the length of a whole millimeter, all signs of apheliotropism
would have disappeared. It may be suggested that although
the application of caustic does not stop growth, yet enough may
be absorbed to destroy the power of movement in the upper
part; but this suggestion must be rejected, for we have seen
and shall again see, that cauterising one side of the tip of various
kinds of radicles actually excites movement. The conclusion
.seems inevitable that sensitiveness to light resides in the tip
of the radicle of Sinapis alba ; and that the tip when thus
stimulated transmits some influence to the upper part, causing
it to bend. The case in this respect is parallel with that of
the radicles of several plants, the tips of which are sensitive to
contact and to other irritants, and, as will be shown in tha
eleventh chapter, to gravitation.
i£4 CONCLUDING EEMAEKS AND Chap. IX.
Concluding Kemarks and Summary of Chapter.
We do not know whether it is a general rule with
seedling plants that the illumination of the upper
part determines the curvature of the lower part. But
as this occurred in the four species examined by us,
belonging to such distinct families as the Gramineee,
Cruciferse, and Chenopodeae, it is probably of common
occurrence. It can hardly fail to be of service to seed-
lings, by aiding them to find the shortest path from
the buried seed to the light, on nearly the same
principle that the eyes of most of the lower crawling
animals are seated at the anterior ends of their bodies.
It is extremely doubtful whether with fully developed
plants the illumination of one part ever affects the
curvature of another part. The summits of 5 young
plants of Asparagus officinalis (varying in height be-
tween l'l and 2*7 inches, and consisting of several
short internodes) were covered with caps of tin-foil
from 03 to 035 inch in depth ; and the lower un-
covered parts became as much curved towards a lateral
light, as were the free seedlings in the same pots.
Other seedlings of the same plant had their summits
painted with Indian ink with the same negative result.
Pieces of blackened paper were gummed to the edges
and over the blades of some leaves on young plants of
Trojoseolum majus and Ranunculus ficaria ; these were
then placed in a box before a window, and the petioles
of the protected leaves became curved towards the
light, as much as those of the unprotected leaves.
The foregoing cases with respect to seedling plants
have been fully described, not only because the trans-
mission of any effect from light is a new physiological
fact, bufbecause we think it tends to modify somewhat
the current views on heliotropic movements. Until
Chap. IX. „ SUMMARY OF CHAPTER. 48S
lately such movements were believed to result simply
from increased growth on the shaded side. At preseiil
it is commonly admitted * that diminished light in-
creases the turgescence of the cells, or the extensibility
of the cell-walls, or of both together, on the shaded
side, and that this is followed by increased growth.
But Pfeffer has shown that a difference in the tur-
gescence on the two sides of a pulvinus, — that is, an
aggregate of small cells which have ceased to grow at
an early age, — is excited by a difference in the amount
of light received by the two sides; and that move-
ment is thus caused without being followed by in-
creased growth on the more turgescent side.t All
observers apparently believe that light acts directly
on the part which bends, but we have seen with the
above described seedlings that this is not the case.
Their lower halves were brightly illuminated for hours,
and yet did not bend in the least towards the light,
though this is the part which under ordinary circum-
stances bends the most. It is a still more striking
fact, that the faint illumination of a narrow stripe on
one side of the upper part of the cotyledons of Phalaris
determined the direction of the curvature of the lower
part ; so that this latter part did not bend towards the
bright light by which it had been fully illuminated,
* Emil Godlewski has given 63, 123, &c. Frank has also
('Rot. Z.itung,' 1879, Nos. 6-9) insisted ('Die ^aturliche w'a-
au excellent account (p. 120) of gerechte Richtung von Pflau-
tlie present state of the question. zentheilen,' 1870, p. 53) on the
See also Vines in ' Arbeiten des important part which the pulvini
Rot. Inst, in Wurzburg,' 1878, R. of the leaflets of compound Laves
ii. pp. 114-147. Hugo de Vries play in placing tie leaflets in a
has recently published a still proper po,-ition wi'h respect to the
more important article on this light. This holds ^ood, especially
subject : ' Rot. Zeitung,' Dec. 19th with the leave^of climbing plants,
and 26th, 1879. which are carried, into all sorts
t ' Die Periodischen Rewegun- of positions, ill -adapted for the
gen der Blattorgane,' 1875, pp. 7, action of the light.
£86 CONCLUDING EEMAEKS AND Chap. IX.
but obliquely towards one side where only a little
light entered. These results seem to imply the pre-
sence of some matter in the upper part which is acted
on by light, and which transmits its effects to the
lower part. It has been shown that this transmission
is independent of the bending of the upper sensitive
part. We have an analogous case of transmission in
Drosera, for when a gland is irritated, the basal and
not the upper or intermediate part of the tentacle
bends. The flexible and sensitive filament of Dionaea
likewise transmits a stimulus, without itself bending ;
as does the stem of Mimosa.
Light exerts a powerful influence on most vege-
table tissues, and there can be no doubt that it
generally tends to check their growth. But when the
two sides of a plant are illuminated in a slightly
different degree, it does not necessarily follow that
tne bending towards the illuminated side is caused by
changes in the tissues of the same nature as those
which lead to increased growth in darkness. We
know at least that a part may bend from the light,
and yet its growth may not be favoured by light.
This is the case with the radicles of Sinapis alba, which
are plainly apheliotropic ; nevertheless, they grow
quicker in darkness than in light.* So it is with
many aerial roots, according to Wiesner ;| but there
are other opposed cases. It appears, therefore, that
light does not determine the growth of apheliotropic
parts in any uniform manner.
We should bear in mind that the power of bending
to the light is highly beneficial to most plants. There
* Francis Darwin, ' Uber das Heft iii., 1880, p. 521.
Wachvtlium negativ heliotiopi- f ' Sitzb. der k. AUad. der "Wia
seller Wurzeln': ' Arbeiten des sensch' (Vitnua), 1880, p. 12.
But. Inst, in Wurzbui",' B. ii .
Ohaf. IX. SUMMARY OF CHAPTER. 487
is therefore no improbability in this power having been
specially acquired. In several respects light seems to
act on plants in nearly the same manner as it does
on animals by means of the nervous system.* With
seedlings the effect, as we have just seen, is trans-
mitted from one part to another. An animal may be
excited to move by a very small amount of light ; and
it has been shown that a difference in the illumination
of the two sides of the cotyledons of Phalaris, which
could not be distinguished by the human eye, sufficed
to cause them to bend. It has also been shown that
there is no close parallelism between the amount of
light which acts on a plant and its degree of curva-
ture; it was indeed hardly possible to perceive any
difference in the curvature of some seedlings of Phalaris
exposed to a light, which, though dim, was very much
brighter than that to which others had been exposed.
The retina, after being stimulated by a bright light,
feels the effect for some time ; and Phalaris continued
to bend for nearly half an hour towards the side which
had been illuminated. The retina cannot perceive
a dim light after it has been exposed to a bright one ;
and plants which had been kept in the daylight
during the previous day and morning, did not move
so soon towards an obscure lateral light as did others
which had been kept in complete darkness.
Even if light does act in such a manner on the
growing parts of plants as always to excite in them
a tendency to bend towards the more illuminated
side — a supposition contradicted by the foregoing-
experiments on seedlings and by all apheliotropic
* Sachs has made some striking See his paper ' I7?ber ortho-trope
remarks to the same effect with und plagiotrope Pflanzentheile,
respect to the various stimuli 4 Arb. cles. Bot. Inst, in Wiirzburg-
o-hich excite movement in plants. 1879 B. ii. p. 282.
32
188 CONCLUDING KEMAKKS AND Chap. IX.
organs — yet the tendency differs greatly m different
species, and is variable in degree in the individuals of
the same species, as may be seen in almost any pot
of seedlings of a long cultivated plant.* There is
therefore a basis for the modification of this tendency
to almost any beneficial extent. That it has been
modified, we see in many cases : thus, it is of more
importance for insectivorous plants to place their
leaves in the best position for catching insects than
to turn their leaves to the light, and they have
no such power. If the stems of twining plants were
to bend towards the light, they would often be drawn
away from their supports ; and as we have seen they
do not thus bend. As the stems of most other plants
are heliotropic, we may feel almost sure that twining
plants, which are distributed throughout the whole
vascular series, have lost a power that their non-
climbing progenitors possessed. Moreover, with Ipo-
moea, and probably all other twiners, the stem of the
young plant, before it begins to twine, is highly helio-
tropic, evidently in order to expose the cotyledons or
the first true leaves fully to the light. With the Ivy the
stems of seedlings are moderate] y heliotropic, whilst
those of the same plants when grown a little older
* Str;isburger has shown in his the light. Some individuals, more-
interesting work (' Wirkung des over, appear to be indifferent to
Lichtes . . . an f Sell warm spore n,' the light; and those of different
1878), that the movement of the species behave very differently,
swarm-spores of various lowly The brighter the light, the
organised plants to a lateral light straighter is their course. They
is influenced by their stage of exhibit also for a s-hort time the
development, by the temperature after-effects of light. In all these
to which they are subjected, by r< spt cts they reemble the higher
the degree of illumination under plants. See, also, Stahl, * Ueber
which they have been raised, and den einfluss der Lichts auf die
by other unknown causes ; so that Bewegungs - erscheinungeu der
the swarm-spores of the same Schwarmsporen ' Verb. d. phys.-
speoies may move across the field med. Geselsshalft in Wiirzburg,
of the microscope either to or from B. xii. 1878.
Cu A v. IX. SUMMARY OF CHAPTER. 489
are apheliotropic. Some tendrils which consist of
modified leaves — organs in all ordinary cases strongly
diaheliotropic — have been rendered apheliotropic, and
their tips crawl into any dark crevice.
Even in the case of ordinary heliotropic movements,.
it is hardly credible that they result directly from
the action of the light, without any special adaptation.
We may illustrate what we mean by the hygroscopic
movements of plants : if the tissues on one side of an
organ permit of rapid evaporation, they will dry
quickly and contract, causing the part to bend to this
side. Now the wonderfully complex movements of
the pollinia of Orchis pyramidalis, by which they clasp
the proboscis of a moth and afterwards change their
position for the sake of depositing the pollen-masses
on the double stigma — or again the twisting move-
ments, by which certain seeds bury themselves in
the ground* — follow from the manner of drying of
the parts in question ; yet no one will suppose that
these results have been gained without special adapta-
tion. Similarly, we are led to believe in adaptation
when we see the hypocotyl of a seedling, which contains
chlorophyll, bending to the light ; for although it thus
receives less light, being now shaded by its own coty-
ledons, it places them — the more important organs — in
the best position to be fully illuminated. The hypo-
cotyl may therefore be said to sacrifice itself for the
good of the cotyledons, or rather of the whole plant.
But if it be prevented from bending, as must some-
times occur with seedlings springing up in an en-
tangled mass of vegetation, the cotyledons themselves
bend so as to face the light ; the one farthest off rising
* Francis "Darwin, * On the Fly- actions Linn. See.,' series ii. vol. i.
groscopic Mechanism,' &c, 'Trans- p. 149, 1876.
£90 CONCLUDING REMAKES AND Chap. IX
up, and that nearest to the light sinking down, 01
both twisting laterally.* We may, also, suspect iiiat
the extreme sensitiveness to light of the upper part
of the sheath-like cotyledons of the Graminese, and
their power of transmitting its effects to the lower
part, are specialised arrangements for finding the
shortest path to the light. With plants growing on
a bank, or thrown prostrate by the wind, the manner
in which the leaves move, even rotating on their own
axes, so that their upper surfaces may be again directed
to the light, is a striking phenomenon. Such facts
are rendered more striking when we remember that
too intense a light injures the chlorophyll, and that
the leaflets of several Leguminosse when thus exposed
bend upwards and present their edges to the sun, thus
escaping injury. On the other hand, the leaflets of
Averrhoa and Oxalis, when similarly exposed, bend
downwards.
It was shown in the last chapter that heliotropism
is a modified form of circumnutation ; and as every
growing part of every plant circumnutates more or less,
we can understand how it is that the power of bending
to the light has been acquired by such a multitude
of plants throughout the vegetable kingdom. The
manner in which a circumnutating movement — that
is, one consisting of a succession of irregular ellipses
or loops — is gradually converted into a rectilinear
course towards the light, has been already explained.
First, we have a succession of ellipses with their
longer axes directed towards the light, each of which
* Wiesrier has made remarks to tracted from B. lxxvii. (187s)
Dearly the same effect with respect Sitb. der k. Akud. der Wisseusch
to leaves : * Die imdulirende Nu- Wien.
tation der Intornodieu,' p. G, ex-
Chap. IX. SUMMARY OF CHAPTER. 49.1
is described nearer and nearer to its source ; then the
loops are drawn ont into a strongly pronounce d zigzag
line, with here and there a small loop still formed.
At the same time that the movement towards the light
is increased in extent and accelerated, that in the
opposite direction is lessened and retarded, and. at last
stopped. The zigzag movement to either side is
likewise gradually lessened, so that finally the course
becomes rectilinear. Thus under the stimulus of a
fairly bright light there is no useless expenditure of
force.
As with plants every character is more or less
variable, there seems to be no great difficulty in be-
lieving that their circumnutating movements may
have been increased or modified in any beneficial
manner by the preservation of varying individuals.
The inheritance of habitual movements is a necessary
contingent for this process of selection, or the survival
of the fittest ; and we have seen good reason to believe
that habitual movements are inherited by plants. In
the case of twining species the circumnutating move-
ments have been increased in amplitude and rendered
more circular ; the stimulus being here an internal
or innate one. With sleeping plants the movements
have been increased in amplitude and often changed
in direction ; and here the stimulus is the alternation
of light and darkness, aided, however, by inheritance.
In the case of heliotropism, the stimulus is the unequal
illumination of the two sides of the plant, and this
determines, as in the foregoing cases, the modifica-
tion of the circumnutating movement in such a manner
that the organ bends to the light. A plant which
has been rendered heliotropic by the above means,
might readily lose this tendency, judging from the
cases already given, as soon as it became useless 01
±92 CONCLUDING KEMAEKS. Chap. IX.
injurious. A species which has ceased to be helio-
tropic might also be rendered apheliotropic by the
preservation of the individuals which tended to cir-
cumnutate (though the cause of this and most other
variations is unknown) in a direction more or less
opposed to that whence the light proceeded. In like
manner a plant might be rendered diaheliotropie.
Chap X MOVEMENTS EXCITED BY GKAVITATION. 493
CHAPTER X.
Modified Circumnutation : Movements excited by Gkavitatio:?.
Means of observation - -Apogeotropism — Cytisus — Verbena — Beta —
Gradual conversion of the movement of circumnutation into apogeo-
tropis.ni in Kubus, Lilium, Phalaris, Avena, and Bra^sica — Apogeo-
tropisni retarded by heliotropism — Effected by the aid of joints
or pulviui — Movements of flower-peduncles of Oxalis — GJeneral
remarks on apogeotropism — Geotropism — Movements of rarlicles —
Burying of seed-capsules — Use of process — Trifolium subterraneum
— Arachis — Amphicarpsea — Dia geotropism — Conclusion.
Our object in the present chapter is to show that
geotropism, apogeotropism, and diageotropism are mo-
dified forms of circumnutation. Extremely fine fila-
ments of glass, bearing two minute triangles of paper,
were fixed to the summits of young stems, frequently
to the hypocotyls of seedlings, to flower-peduncles,
radicles, &c, and the movements of the parts were
then traced in the manner already described on
vertical and horizontal glass-plates. It should be
remembered that as the stems' or other parts become
more and more oblique with respect to the glasses, the
figures traced on them necessarily become more and
more magnified. The plants were protected from light,
excepting whilst each observation was being made, and
then the light, which was always a dim one, was
allowed to enter so as to interfere as little as possible
with the movement in progress ; and we did not detect
any evidence of such interference.
When observing the gradations between circumnu
494 MODIFIED CIKC UMNUTATION. Chap X.
tation and heliotropisrn, we had the great advantage of
being able to lessen the light ; bnt with geotropisni
analogous experiments were of course impossible.
We could, however, observe the movements of stemH
placed at first only a little from the perpendicular, in
which case geotropism did not act with nearly so much
power, as when the stems were horizontal and at right
angles to the force. Plants, also, were selected which
were but feebly geotropic or apogeotropic, or had
become so from having grown rather old. Another
plan was to place the stems at first so that they pointed
30 or 40 degrees beneath the horizon, and then apo-
geotropism had a great amount of work to do before
the stem was rendered upright ; and in this case
ordinary circumnutation was often not wholly oblite-
rated. Another plan was to observe in the evening
plants which during the day had become greatly
curved heliotropically ; for their stems under the gra-
dually waning light very slowly became upright through
the action of apogeotropism ; and in this case modified
circumnutation was sometimes well displayed.
Apogeotropism. — Plants were selected for observation almost
by chance, excepting that they were taken from widely different
families. If the stem of a plant which is even moderately
sensitive to apogeotropism be placed horizontally, the upper
growing part bends quickly upwards, so as to become perpen-
dicular; and the line traced by joining the dots successively
made on a glass -plate, is generally almost straight. For in-
stance, a young Cytims fragrans, 12 inches in height, was placed
so that the stem projected 10° beneath the horizon, and its
course was traced during 72 h. At first it bent a very little
downwards (Fig. 182), owing no doubt to the weight of the
stem, as this occurred with most of the other plants observed,
though, as they were of course circumnutating, the short down-
ward lines were often oblique. After three-quarters of an hour
the stem began to curve upwards, quickly during the first two
hours, but much more slowly during the afternoon and night,
ClIAP. X.
APOGEOTKOPISM.
495
Fig. 182. J
T
%
{W'
and on the following day. During the second night it foil
a little, and circumnutated
during the following day; but it
also moved a short distance to
the right, which was caused by
a little light haying been ac-
cidentally admitted on this side.
The stem was now inclined
60 3 above the horizon, and had
therefore risen 70°. "With time
allowed it would probably have
become upright, and no doubt
would have continued circum-
nutating. The sole remarkable
feature in the figure here given
is the straightness of the course
pursued. The stem, however,
did not move upwards at an
equable rate, and it sometimes
stood almost or quite still.
Such periods probably represent
attempts to circumnutate in a
direction opposite to apogeo-
tropism.
The herbaceous stem of a
Verbena melindres (?) laid hori-
zontally, rose in 7 h. so much
that it could no longer be
observed on the vertical glass
which stood in front of the plant.
The long line which was traced
was almost absolutely straight.
After the 7 h. it still continued
to rise, but now circumnutated Cy ^/^ an : s L apog , e °n™ pic '
slightly. On the following day
it stood upright, and circum-
nutated regularly, as shown in
Tig. 82, given in the fourth
chapter. The stems of several
other plants which were highly
sensitive to apogeotropism rose
up in almost straight lines, and
move
ment of stem from 10° beneath t'
60° above horizon, traced on vei
tical glass, from 8.30 a.m. Marcf
12th to 10.30 p.m. 13th. The sub-
sequent circumnutating movement
is likewise shown up to 6.45 A.M.
on the 15th. Nocturnal course
represented, as usual, by a broken
liDe. Movement not greatly mag-
nified, and tracing reduced to two-
thirds of original scale.
496
MODIFIED CTRCUMNUTATION.
Chap. X
then suddenly began to circumnutate. A partially etiolated
and somewhat old hypocotyl
Fig. 183. M2:s8'p.w..
12:3
Btp.m.
7.°a.m.29 l
8°55'am.28 r ? £
Seta wilwris: apogcotropic movement
ofhypocotyl from 19° beneath horizon
to a vertical position, with subsequent
circunmutation, traced on a vertical
and on a horizontal glass-plate, from
8.28 A.M. Sept. 28th to 8.40 A.M. 29th.
Figure reduced toone-thirdof original
seal ;.
of a seedling cabbage (21
inches in height) was so
sensitive that when placed
at an angle of only 23° from
the perpendicular, it became
vertical in 33 minutes. As
it could not have been
strongly acted upon by
apogeotropism in the above
slightly inclined position,
we expected that it would
have circumnutated, or at
least have moved in a zig-
zag course. Accordingly,
dots were made every 3
minutes; but, when these
were joined, the line was
nearly straight. After this
hypocotyl had become up-
right it still moved onwards
for half an hour in the same
general direction, but in a
zigzag manner. During the
succeeding 9 h. it circum-
nutated regularly, and de-
scribed 3 large ellipses. In
this case apogeotropism,
although acting at a very
unfavourable angle, quite
overcame the ordinary cir-
cumnutating movement.
The hypocotyls of Beta
vulgaris are highly sensitive
to apogeotropism. One was
placed so as to project 19°
beneath the horizon ; it fell
at first a very little (see
Fig. 183), no doubt owing
to its weight : but as it was
circumnutating the line was
Chap. X. APOGEOTROPISM. 41)7
oblique During the next 3 h. 8 m. it rose in a nearly straight
line, passing through an angle of 109°, and then (at 12.3 p.m.)
stood upright. It; continued for 55 m. to move in the same
general direction beyond, the perpendicular, but in a zigzag-
course. It returned also in a zigzag line, and then circumnu-
tated regularly, describing three large ellipses during the
remainder of the day. It should be observed that the ellipses
in this figure are exaggerated in size, relatively to the length of
the upward straight line, owing to the position of the vertical
and horizontal glass-plates. Another and somewhat old hypo-
cotyl was placed so as to stand at only 31° from the perpen-
dicular, in which position apogeotropism acted on it with little
force, and its course accordingly was slightly zigzag.
The sheath-like cotyledons of Phalaris Canadensis are ex-
tremely sensitive to apogeotropism. One was placed so as to
project 40° beneath the horizon. Although it was rather old
and 1*3 inch in height, it became vertical in 4 h. 30 m., having
passed through an angle of 130° in a nearly straight line. It then
suddenly began to circumnutate in the ordinary manner. The
cotyledons of this plant, after the first leaf has begun to pro-
trude, are but slightly apogeotropic, though they still continue
to circumnutate. One at this stage of development was placed
horizontally, and did not become upright even after 13 h., and its
course was slightly zigzag. So, again, a rather old hypocotyl
of Cassia tora (li inch in height) required 28 h. to become up-
right, and its course was distinctly zigzag ; whilst younger hypo-
cotyls moved much more quickly and in a nearly straight line.
When a horizontally placed stem or other organ rises in a
zigzag line, we may infer from the many cases given in our
previous chapters, that we have a modified form of circumnu-
tation ; but when the course is straight, there is no evidence
of circumnutation, and any one might maintain that this latter
movement had been replaced by one of a wholly distinct kind.
This view seems the more probable when (as sometimes
occurred with the hypocotyls of Brassica and Beta, the stems of
Cucurbita, and the cotyledons of Phalaris) the part in question
after bending up in a straight course, suddenly begins to circum-
nutate to the full extent and in the usual manner. A fairly
good instance of a sudden change of this kind — that is, from a
nearly straight upward movement to one of circumnutation —
is shown in Fig. 183 ; but more striking instances were occa-
sionally observed with Beta, Brassica, and Phalaris.
We will now describe a few cases in which it may be
£98 MODIFIED CTRCUMNUTATION. Chap. X
seen how gradually circumnutation becomes changed into apogeo-
tropism, nnder circumstances to be specified
lg ' in each instance.
Eubus idceus (hybrid). — A young plant, 11
g 'inches in height, growing in a pot, was placed
g horizontally; and the upward movement was
i3, traced during nearly 70 h. ; but the plant,
though growing vigorously, was not highly
re -5 sensitive to apogeotropism, or it was not
«* * capable of quick movement, for during the
£> a above time it rose only 67°. We may see in
2 r° the diagram (Fig. 184) that during the first
w> ° day of 12 h. it rose in a nearly straight line,
•g 5 When placed horizontally, it was evidently
« ° circumnutating, for it rose at first a little,
J S notwithstanding the weight of the stem, and
~ g then sank down ; so that it did not start on
•J o its permanently upward course until 1 h.
£ Z 25 m. had elapsed. On the second day, by
rt g which time it had risen considerably, and
° g when apogeotropism acted on it with somewhat
o 2 less power, its course during 152 h. was clearly
3 5/5 zigzag, and the rate of the upward movement
S "^ was not equable. During the third day, also
w -£ of 15! h., when apogeotropism acted on it
° « with still less power, the stem plainly circum-
§ * nutated, for it moved during this day 3 times
£ ao up and 3 times down, 4 times to the left and
| S 4 to the right. But the course was so complex
g 5 that it could hardly be traced on the glass.
I"" -1 We can, however, see that the successively
§ | formed irregular ellipses rose higher and
§Jg higher. Apogeotropism continued to act on
* ^ the fourth morning, as the stem was still
ri 1 ^ rising, though it now stood only 23° from the
"f^o perpendicular. In this diagram the several
w* -1 stages may be followed by which an almost
I | rectilinear, upward, apogeotropic course first
' becomes zigzag, and then changes into a
[ J circumnutating movement, with most of the
» c=i successively formed, irregular ellipses directed
upwards.
Lilium aitratum. — A plant 23 inches in height was placed
Chap. X
APOGEOTROPISM.
499
Fig. 185.
horizontally, and the upper part of the stem rose 58° in 16 h..
in the manner shown in the accom-
panying diagram (Fig. 185). We here
see that during the whole of the
second day of 15* h., the stem plainly
circumnutated whilst bending upwards
through apogeotropism. It had still
to rise considerably, for when the last
dot in the figure was made, it stood
32° from an upright position.
PJialaris Canariemis. — A cotyledon
of this plant (1"3 inch in height) has
already been described as rising in
4 h. 30 m. from 40° beneath the hori-
zon into a vertical position, passing
through an angle of 130° in a nearly
straight line, and then abruptly be-
ginning to circumnutate. Another
somewhat old cotyledon of the same
height (but from which a true leaf
had not yet protruded), was similarly
placed at 40° beneath the horizon. For
the first 4 h. it rose in a nearly straight
course (Fig. 186), so that by 1.10 p.m.
it was highly inclined, and now apo-
geotropism acted on it with much less
power than before, and it began to
zigzag. At 4.15 p.m. (i.e. in 7 h. from
the commencement) it stood vertically,
and afterwards continued to circum-
nutate in the usual manner about the
same spot. Here then we have a
graduated change from a straight up-
ward apogeotropic course into circum- Lilium auratum : apoge->
nutation, instead of an abrupt change, tropic movement of stem,
as in the former case.
Avena sativa.—Tlie sheath-like coty-
ledons, whilst young, are strongly apo-
geotropic ; and some which were placed
at 45° beneath the horizon rose 90° in
7 or 8 h. in lines almost absolutely
straight. An oldish cotyledon, from which the first leaf began to
traced on a vertical glass
during 2 days and 2
nights, from 10.40 A.M.
March 18th to 8 A.M.
20th. Figure reduced to
one-half of the original
scale.
500
MODIFIED CIRCUMXUTATIOX.
Chat. X
Fig. 186.
Phalans Canariensis ; ipogeotropic move-
ment of cotyledon, traced on a vertical
and horizontal glass, from 9. 10 A.M. Sept.
19th to 9 a M. 20th. Figure here re-
duced to one-fifth of original ?cale.
protrude whilst the fol-
lowing observations were
being made, was placed
at 10° beneath the horizon,
and it rose only 59° in
24 h. It behaved rather
differently from any other
plant, observed by us, for
during the first 4^ h. it
rose in a line not far from
straight ; during the next
6a h. it circumnutatecl,
that is, it descended and
again ascended in a
strongly marked zigzag
course; it then resumed
its upward movement in
a moderately straight line,
and, with time allowed,
no doubt would have be-
come upright. In this
case, after the first 4 J h.,
ordinary circumnutation
almost completely con-
quered for a time apogeo-
tropisrn.
Brass tea oleracea. — The
hypocotyls of several
young seedlings placed
horizontally, rose up ver-
tically in the course of 6
or 7 h. in nearly straight
lines. A seedling which
had grown in darkness to
a height of 2i inches, and
was therefore rather old
and not highly sensitive,
was placed so that the
hypocotyl projected at be-
tween 30° and 40° beneath
the horizon. The upper
part alone became curved
Chap. X.
APOGEOTROPISM.
501
upwards, and rose during the first 3h. 10 m. in a nearly straight
line (Fig. 187); but it was not
possible to trace the upward move-
ment on the vertical glass for the
first 1 h. 10 m., so that the nearly
straight line in the diagram ought
to have been much longer. During
the next 11 h. the hypocotyl circuni-
nutated, describing irregular figures,
each of which rose a little above
the one previously formed. During
the night and following early morn-
ing it continued to rise in a zigzag
course, so that apogeotropism was
still acting. At the close of our ob-
servations, after 23 h. (represented
by the highest dot in the diagram)
the hypocotyl was still 32° from
the perpendicular. There can be
little doubt that it would ulti-
mately have become upright by
describing an additional number
of irregular ellipses, one above the
other.
Apogeotropism retarded by TJelio-
tropism. — When the stem of any
plant bends during the day towards
a lateral light, the movement is
opposed by apogeotropism ; but as
the light gradually wanes in the
evening the latter power slowly
gains the upper hand, and draws Brass
the stem back into a vertical
position. Here then we have a
good opportunity for observing how
apogeotropism acts when very
nearly balanced by an opposing
force. For instance, the plumule
of Tropceolum majus (see former
J?ig. 175) moved towards the dim
evening light in a slightly zigzag
b'ne until 6.45 p.m., it then returned on its course until
ica oleracea: apogeotropic
movement of hypocotyl, traced
on vertical glass, from 9.20
a.m. Sept. 12th to 8.30 a.m.
loth. The upper part of the
figure is more magnified than
the lower part. If the whole
course had been traced, the
straight upright line would
have been much longer. Figure
here reduced to one-third of
the original scale.
502 MODIFIED CIKCUMNUTATION. Chap. X
10.40 p.m., during which time it zigzagged and described an
sllipse of considerable size. The hypocotyl of Brassica oleracea
(see former Fig. 173) moved in a straight line to the light until
5.15 p.m., and then from the light, making in its backward
course a great rectangular bend, and then returned for a short
distance towards the former source of the light ; no observa-
tions were made after 7.10 p.m., but during the night it re-
covered its vertical position. A hypocotyl of Cassia tora moved
in the evening in a somewhat zigzag line towards the failing-
light until 6 p.m., and was now bowed 20° from the perpendi-
cular ; it then returned on its course, making before 10.30 p.m.
four great, nearly rectangular bends and almost completing an
ellipse. Several other analogous cases were casually observed,
and. in all of them the apogeotropic movement could be seen to
consist of modified circumnutation.
Apogeotropic Movements effected by the aid of joints or pidvini.
— Movements of this kind are well known to occur in the
Graniinese, and are effected by means of the thickened bases
of their sheathing leaves; the stem within being in this part
thinner than elsewhere.* According to the analogy of all other
pulvini, such joints ought to continue circumnutating for a
long period, after the adjoining parts have ceased to grow. We
therefore wished to ascertain whether this was the case with
the Graminese ; for if so, the upward curvature of their stems,
when extended horizontally or laid prostrate, would be explained
in accordance with our view — namely, that apogeotropism
results from modified circumnutation. After these joints have
curved upwards, they are fixed in their new position by increased
growth along their lower sides.
Lolium prrenTie. — A young stem, 7 inches in height, consist-
ing of 3 internodes, with the flower-head not yet protruded,
was selected for observation. A long and very thin glass fila-
ment was cemented horizontally to the stem close above the
second joint, 3 inches above the ground. This joint was subse-
quently proved to be in an active condition, as its lower side
swelled much through the action of apogeotropism (in the
manner described by De Vries) after the haulm had been
fastened down for 2-1 h. in a horizontal position. The pot was
* This structure lias been re- die Aufrichtung des gelagerten
cently dts<ribed by De Vries in Getreides,' in ' Landwirthschaft-
an interesting article, 'Ueber liclie. Jahrliiiclier,' 1880, p. 473.
CaiP. X. APOGEOTROPISM. 503
bo placed that the end of the filament stood beneath the 2-inch
object glass of a microscope with an eye-piece micrometer, each
division of which equalled ^ of an inch. The end of the fila-
ment was repeatedly observed during 6 h., and was seen to be
in constant movement ; and it crossed 5 divisions of the micro-
meter (yig inch) in 2 h. Occasionally it moved forwards bv
jerks, some of which were yoVo mcn m length, and then slowly
retreated a little, afterwards again jerking forwards. These
oscillations were exactly like those described under Brassica
and Dionsea, but they occurred only occasionally. We may
therefore conclude that this moderately old joint was continually
circumnutating on a small scale.
Alopecurus pratensis.—A young plant, 11 inches in height, with
the flower-head protruded, but with the florets not yet expanded,
had a glass filament fixed close above the second joint, at a
height of only 2 inches above the ground. The basal internode,
2 inches in length, was cemented to a stick to prevent any
possibility of its circumnutatiug. The extremity of the filament,
which projected about 50° above the horizon, was often observed
during 24 h. in the same manner as in the last case. Whenever
looked at, it was always in movement, and it crossed 30 divisions
of the micrometer (^ inch) in 3| h. ; but it sometimes moved
at a quicker rate, for at one time it crossed 5 divisions in Is h.
The pot had to be moved occasionally, as the end of the filament
travelled beyond the field of vision ; but as far as we could
judge it followed during the daytime a semicircular course ;
and it certainly travelled in two different directions at right
angles to one another. It sometimes oscillated in the same
manner as in the last species, some of the jerks forwards being
as much as y^ of an inch. We may therefore conclude that
the joints in this and the last species of grass long continue to
circumnutate ; so that this movement would be ready to be
converted into an apogeotropic movement, whenever the stem
was placed in an inclined or horizontal position.
Movements of the Flower-peduncles of Oxalis carnosa, due to
apogeotropism and other fortes. — The movements of the main
peduncle, and of the three or four sub-peduncles which each
main peduncle of this plant bears, are extremely complex, and
are determined by several distinct causes. Whilst the flowers
are expanded, both kinds of peduncles circumnutate about the
same spot, as we have seen (Fig 91) in the fourth chapter.
But soon after the flowers have begun to wither the sub-
501 MODIFIED CIECUMNUTATION. Chap. X
peduncles bend downwards, and this is due to epinasty; foi
on two occasions when pots were laid horizontally, the sub-
peduncles assumed the same position relatively to the main
peduncle, as would have been the case if they had remained
upright; that is, each of them formed with it an angle of
about 40°. If they had been acted on by geotropism or aphelio-
tropism (for the plant was illuminated from above), they would
have directed themselves to the centre of the earth. A main
peduncle was secured to a stick in an upright position, and one
of the upright sub-peduncles which had been observed circum-
nutating whilst the flower was expanded, continued to do so for
at least 24 h. after it had withered. It then began to bend
downwards, and after 36 h. pointed a little beneath the horizon.
A new figure was now begun (A, Fig. 188), and the sub-peduncle
was traced descending in a zigzag line from 7.20 p.m. on the 19th
to 9 a.m. on the 22nd. It now pointed almost perpendicularly
downwards, and the glass filament had to be removed and
fastened transversely across the base of the young capsule.
"We expected that the sub-peduncle would have been motionless
in its new position ; but it continued slowly to swing, like a
pendulum, from side to side, that is, in a plane at right angles
to that in which it had descended. This circumnutating move-
ment was observed from 9 a.m. on 22nd to 9 a.m. 24th, as shown
at B in the diagram. We were not able to observe this par-
ticular sub-peduncle any longer; but it would certainly have
gone on circumnutating until the capsule was nearly ripe (which
requires only a short time), and it would then have moved
upwards.
The upward movement (C, Fig. 188) is effected in part by the
whole sub-peduncle rising in the same manner as it had pre-
viously descended through epinasty — namely, at the joint where
united to the main peduncle. As this upward movement
occurred with plants kept in the dark and in whatever position
the main peduncle was fastened, it could not have been caused
by heliotropism or apogeotropism, but by hyponasty. Besides
this movement at the joint, there is another o: a very different
kind, for the sub-peduncle becomes upwardly bent in the middle
part. If the sub-peduncle happens at the time to be inclined
much downwards, the upward curvature is so great that the
whole forms a hook. The upper end bearing the capsule, thus
always places itself upright, and as this recurs in darkness, and
in whatever position the main peduncle may have been secured,
Chap. X.
APOGEOTROPISM.
505
fche upwai'd curvature cannot be due to heliotropism or hypo-
nasty, but to apogeotropism.
;
Fig. 188.
%
Oxalh carnosa : movements of flower-peduncle, traced on a vertical g/iass
A, apinastic downward movement ; B, circumnutation whilst depend*
int; rertically ; C, subsequent upward movement, due to apogeotropism
and hvponastv combined
506 MODIFIED CIRCUMNUTATION. Chap. X.
In order to trace this upward movement, a filament was fixed
to a sub-peduncle bearing a capsule nearly ripe, which was
beginning to bend upwards by the two means just described. Its
coarse was traced (see C, Fig. 188) during 53 h., by which time
it had become nearly upright. The course is seen to be strongly
zigzag, together with some little loops. We may therefore con-
clude that the movement consists of modified circumnutation.
The several species of Oxalis probably profit in the following
manner by their sub-peduncles first bending downwards and
then upwards. They are known to scatter their seeds by the
burs'ing of the capsule; the walls of which are so extremely
thin, like silver paper, that they would easily be permeated by
rain. But as soon as the petals wither, the sepals rise up and
enclose the young capsule, forming a perfect roof over it as
soon as the sub-peduncle has bent itself downwards. By its
subsequent upward movement, the capsule stands when ripe
at a greater height above the ground by twice the length of the
sub-peduncle, than it did when dependent, and is thus able
to scatter its seeds to a greater distance. The sepals, which
enclose the ovarium whilst it is young, present an additional
adaptation by expanding widely when the seeds are ripe, so as
not to interfere with their dispersal. In the case of Oaalis
aretosella, the capsules are said sometimes to bury themselves
under loose leaves or moss on the ground, but this cannot occur
with those of 0. carnosa, as the woody stem is too high.
Oxalis acetosella. — The peduncles are furnished with a joint in
Fig.-189. .
Oxalis acetosella : course pursued by the upper part of a peduncle, whilst
rising, traced from 11 a.m. June 1st to 9 a.m. 3rd. Figure here re-
duced to one-half of the original scale.
the middle, so that the lower part answers to the main peduncle,
Chap. X. APOGEOTROPISM. 507
and the upper part to one of the sub-peduncles of 0. cai nosa.
The upper part bends downwards, after the flower has begun
to wither, and the whole peduncle then forms a hook; that
this bending is due to epinasty we may infer from the case of
0. carnosa. When the pod is nearly ripe, the upper part
straightens itself and becomes erect ; and this is due to hypo-
nasty or apogeotropism, or both combined, and not to helio-
tropism, for it occurred in darkness. The short, hooked part of
the peduncle of a cleistogamic flower, bearing a pod nearly ripe,
was observed in the dark during three days. The apex of the
pod at first pointed perpendicularly down, but in the course of
three days rose 90°, so that it now projected horizontally. The
course during the two latter days is shown in Fig. 189; and
it may be seen how greatly the peduncle, whilst rising, circum-
nutated. The lines of chief movement were at right angles
to the plane of the originally hooked part. The tracing was
not continued any longer ; but after two additional days, the
peduncle with its capsule had become straight and stood
upright.
Concluding Remarks on Apogeotropism. — When apo-
geotropism is rendered by any means feeble, it acts,
as shown in the several foregoing cases, by increasing
the always present circumnutating movement in a
direction opposed to gravity, and by diminishing that
in the direction of gravity, as well as that to either
side. The upward movement thus becomes unequal
in rate, and is sometimes interrupted by stationary
periods. Whenever irregular ellipses or loops are still
formed, their longer axes are almost always directed
in the line of gravity, in an analogous manner as
occurred with heliotropic movements in reference to
the light. As apogeotropism acts more and more
energetically, ellipses or loops cease to be formed, and
the course becomes at first strongly, and then less and
less zigzag, and finally rectilinear. From thip grada-
tion in the nature of the movement, and more especially
from all growing parts, which alone (except when pul-
vini are present) are acted on by apogeotropism, con-
508 MODIFIED CIRCUMNUTATION. Chap. X,
fcinually circLinnutating, we may conclude that even
a rectilinear course is merely an extremely modified
form of circumnutation. It is remarkable that a stem
or other organ which is highly sensitive to apogeo-
tropism, and which has bowed itself rapidly upwards
in a straight line, is often carried beyond the vertical,
as if by momentum. It then bends a little backwards
to a point round which it finally circumnutates. Two
instances of this were observed with the hypocotyls of
Beta vulgaris, one of which is shown in Fig. 183, and
two other instances with the hypocotyls of Brassica.
This momentum-like movement probably results from
the accumulated effects of apogeotropism. For the
sake of observing how long such after-effects lasted,
a pot with seedlings of Beta was laid on its side in the
dark, and the hypocotyls in 3 h. 15 m. became highly
inclined. The pot, still in the dark, was then placed
upright, and the movements of the two hypocotyls were
traced ; one continued to bend in its former direction,
now in opposition to apogeotropism, for about 37 m.,
perhaps for 48 m. ; but after 61 m. it moved in an
opposite direction. The other hypocotyl continued
to move in its former course, after being placed
upright, for at least 37 m.
Different species and different parts of the same
species are acted on by apogeotropism in very dif-
ferent degrees. Young seedlings, most of which cir-
cumnutate quickly and largely, bend upwards and
become vertical in much less time than do any older
plants observed by us; but whether this is due to
their greater sensitiveness to apogeotropism, or merely
to their greater flexibility we do not know. A hypo-
cotyl of Beta traversed an angle of 109° in 3 h. 8 m.,
and a cotyledon of Phalaris an angle of 130° in 4 h.
30 m. On the other hand, the stem of a herbaceous
Chap. X. A.POGEOTROPISM. 509
Verbena rose 90° in about 24 h. ; that of Eubus 67°,
in 70 h. ; that of Oytisus 70°, in 72 h. ; that of a young
American Oak only 37°, in 72 h. The stem of a
young Gyperus altemifolius rose only 11° in 96 h. ;
the bending being confined to near its base. Though
the sheath-like cotyledons of Phalaris are so extremely
sensitive to apogeotropism, the first true leaves which
protrude from them exhibited only a trace of this
action. Two fronds of a fern, Nephrodium molle, both
of them young and one with the tip still inwardly
curled, were kept in a horizontal position for 46 h.,
and during this time they rose so little that it was
doubtful whether there was any true apogeotropic
movement.
The most curious case known to us of a difference
in sensitiveness to gravitation, and consequently of
movement, in different parts of the same organ, is that
offered by the petioles of the cotyledons of Ipomoea
leptophylla. The basal part for a short length where
united to the undeveloped hypocotyl and radicle is
strongly geotropic, whilst the whole upper part is
strongly apogeotropic. But a portion near the blades
of the cotyledons is after a time acted on by epinasty
and curves downwards, for the sake of emerging in the
form of an arch from the ground; it subsequently
straightens itself, and is then again acted on by apo-
geotropism.
A branch of Gucurbita ovifera, placed horizontally,
moved upwards during 7 h. in a straight line, until it
stood at 40° above the horizon ; it then began to cir-
cumnutate, as if owing to its trailing nature it had no
tendency to rise any higher. Another upright branch
was secured to a stick, close to the base of a tendril,
and the pot was then laid horizontally in the c'ark.
In this position the tendril eircumnutated and mad#
510 MODIFIED CIRCUMNUTATION. Chap. X.
several large ellipses during 14 h., as it likewise did
on the following day; but during this whole time it
was not in the least affected by apogeotropism. On the
other hand, when branches of another Cucurbitaceous
plant, Echinocytis lobata, were fixed in the dark so that
the tendrils depended beneath the horizon, these began
immediately to bend upwards, and whilst thus moving
they ceased to circumnutate in any plain manner;
but as soon as they had become horizontal they re-
commenced to revolve conspicuously.* The tendrils
of Passifiora gracilis are likewise apogeotropic. Two
branches were tied down so that their tendrils pointed
many degrees beneath the horizon. One was observed
for 8 h., during which time it rose, describing two
circles, one above the other. The other tendril rose
in a moderately straight line during the first 4 h.,
making however one small loop in its course ; it then
stood at about 45° above the horizon, where it circum-
nutated during the remaining 8 h. of observation.
A part or organ which whilst young is extremely
sensitive to apogeotropism ceases to be so as it grows
old ; and it is remarkablej as showing the independence
of this sensitiveness and of the circumnutating move-
ment, that the latter sometimes continues for a time
after all power of bending from the centre of the earth
has been lost. Thus a seedling Orange bearing only
3 young leaves, with a rather stiff stem, did not curve
in the least upwards during 24 h. whilst extended
horizontally ; yet it circumnutated all the time over
a small space. The hypocotyl of a young seedling
of Cassia tor a, similarly placed, became vertical in
12 h. ; that of an older seedling, 1£ inch in height,
* For details see • The Movements and Habits of Climbing Plants,
1875, p. 131.
Chap. X. APOGEOTEOPISM. 511
became so in 28 h. ; and that of another still older
one, 1J inch in height, remained horizontal d lring
two days, but distinctly circumnutated during this
whole time.
When the cotyledons of Phalaris or Avena are laid
horizontally, the uppermost part first bends upwards,
and then the lower part ; consequently, after the lower
part has become much curved upwards, the upper part
is compelled to curve backwards in an opposite direc-
tion, in order to straighten itself and to stand ver-
tically ; and this subsequent straightening process is
likewise due to apogeotropism. The upper part of
8 young cotyledons of Phalaris were made rigid by
being cemented to thin glass rods, so that this part
could not bend in the least ; nevertheless, the basal
part was not prevented from curving upward. A stem
or other organ which bends upwards through apogeo-
tropism exerts considerable force; its own weight,
which has of course to be lifted, was sufficient in
almost every instance to cause the part at first to bend
a little downwards ; but the downward course was
often rendered oblique by the simultaneous circum-
nutating movement. The cotyledons of Avena placed
horizontally, besides lifting their own weight, were
able to furrow the soft sand above them, so as to leave
little crescentic open spaces on the lower sides of their
bases; and this is a remarkable proof of the force
exerted.
As the tips of the cotyledons of Phalaris and Avena
bend upwards through the action of apogeotropism
before the basal part, and as these same tips when
excited by a lateral light transmit some influence to
the lower part, causing it to bend, we thought that
the same rule might hold good with apogeotropism.
Consequently, the tips of 7 cotyledons of Phalaris w ere
512 MODIFIED CIECUMNUTATION. Chap X
cut off for a length in three cases of *2 inch and in
the four other cases of *14, -12, *1, and *07 inch. But
these cotyledons, after being extended horizontally,
bowed themselves upwards as effectually as the un-
niutilated specimens in the same pots, showing that
sensitiveness to gravitation is not confined to their tips.
Geotkopism.
This movement is directly the reverse of apogeo-
tropism. Many organs bend downwards through epi-
nasty or apheliotropism or from their own weight ; but
we have met with very few cases of a downward move-
ment in sub-aerial organs due to geotropism. We
shall, however, give one good instance in the following
section, in the case of Trifolium siibterraneum, and
probably in that of Aracliis liypogsea.
On the other hand, all roots which penetrate the
ground (including the modified root-like petioles of
Megarrhiza and Ipomoea leptophylla) are guided in their
downward course by geotropism ; and so are many
aerial roots, whilst others, as those of the Ivy, appear
to be indifferent to its action. In our first chapter the
movements of the radicles of several seedlings were
described. We may there see (Fig. 1) how a radicle
of the cabbage, when pointing vertically upwards so
as to be very little acted on by geotropism, circum-
nutated ; and how another (Fig. 2) which was at first
placed in an inclined position bowed itself downwards
in a zigzag line, sometimes remaining stationary for a
time. Two other radicles of the cabbage travelled
downwards in almost rectilinear courses. A radicle of
the bean placed upright (Fig. 20) made a great sweep
and zigzagged ; but as it sank downwards and was
more strongly acted on by geotropism, it moved in an
Chap. X. GEOTEOPISM. 513
almost straight course. A radicle of Cucurbita, directed
upwards (Fig. 26), also zigzagged at first, and de-
scribed small loops ; it then moved in a straight line.
Nearly the same result was observed with the radicles
of Zea mays. But the best evidence of the intimate
connection between circumnutation and geotropism
was afforded by the radicles of Phaseolus, Vicia, and
Quercus, and in a less degree by those of Zea and
^Esculus (see Figs. 18, 19, 21, 41, and 52) ; for when
these were compelled to grow and slide down highly
inclined surfaces of smoked glass, they left distinctly
serpentine tracks.
Tlie Burying of Seed-capsules: Trifolium subterraneum. — The
flower-heads of this plant are remarkable from producing only
3 or 4: perfect flowers, which are situated exteriorly. All the
other many flowers abort, and are modified into rigid points,
with a bundle of vessels running up their centres. After a time
5 long, elastic, claw-like projections, which represent the divi-
sions of the calyx, are developed on their summits. As soon as
the perfect flowers wither they bend downwards, supposing the
peduncle to stand upright, and they then closely surround its
upper part. This movement is due to epinasty, as is likewise
the case with the flowers of T. repens. The imperfect central
flowers ultimately follow, one after the other, the same course.
Whilst the perfect flowers are thus bending down, the whole
peduncle curves downwards and increases much in length,
until the flower-head reaches the ground. Vaucher * says that
when the plant is so placed that the heads cannot soon reach
the ground, the peduncles grow to the extraordinary length of
from 6 to 9 inches. In whatever position the branches may be
placed, the upper part of the peduncle at first bends vertically
upwards through heliotropism ; but as soon as the flowers
begin to wither the downward curvature of the whole peduncle
commences. As this latter movement occurred in complete
darkness, and with peduncles arising from upright and from
dependent branches, it cannot be due to apheliotropism or to
epinasty, but must be attributed to geotropism. Nineteen
* 'Hist Phys. des Plantes d'Europe,' torn. ii. 1811, p. 106.
514 MODIFIED CIKCUMNUTATION Chap. X.
upright flower-heads, arising from branches in all sorts of posi-
tions, on plants growing in a warm greenhouse, were marked
with thread, and after 24 h. six of them were vertically depen-
dent; these therefore had travelled through 180° in this time.
Ten were extended sub-horizontally, and these had moved
through about 90°. Three very young peduncles had as yet
moved only a little downwards, but after an additional 24 h.
were greatly inclined.
At the time when the flower-heads reach the ground, the
younger imperfect flowers in the centre are still pressed closely
together, and form a conical projection ; whereas the perfect and
imperfect flowers on the outside are upturned and closely sur-
round the peduncle. They are thus adapted to offer as little
resistance, as the case admits of, in penetrating the ground,
though the diameter of the flower-head is still considerable.
The means by which this penetration is effected will presently
be described. The flower-heads are able to bury themselves in
common garden mould, and easily in sand or in fine sifted
cinders packed rather closely. The depth to which they pene-
trated, measured from the surface to the base of the head, was
between i and § inch, but in one case rather above 0'6 inch.
With a plant kept in the house, a head partly buried itself in
sand in 6 h. : after 3 days only the tips of the reflexed calyces
were visible, and after 6 days the whole had disappeared. But
with plants growing out of doors we believe, from casual obser-
vations, that they bury themselves in a much shorter time.
After the heads have buried themselves, the central aborted
flowers increase considerably in length and rigidity, and
become bleached. They gradually curve, one after the other,
upwards or towards the peduncle, in the same manner as
did the perfect flowers at first. In thus moving, the long claws
on their summits carry with them some earth. Hence a flower-
head which has been buried for a sufficient time, forms a rather
large ball, consisting of the aborted flowers, separated from one
another by earth, and surrounding the little pods (the product
of the perfect flowers) which lie close round the upper part of
the peduncle. The calyces of the perfect and imperfect flowers
are clothed with simple and multicellular hairs, which have the
power of absorption ; for when placed in a weak solution of
carbonate of ammonia (2 gr. to 1 oz. of water) their proto-
plasmic contents immediately became aggregated and afterwards
displayed the usual slow movements. This clover generally
Chap. X. GEOTROPISM. 515
grows in dry soil, but whether the power of absorption by the
hairs on the buried flower-heads is of any importance to them
we do not know. Only a few of the flower-heads, which from
their position are not able to reach the ground and bury them-
selves, yield seeds ; whereas the buried ones never failed, as far
as we observed, to produce as many seeds as there had been
perfect flowers.
We will now consider the movements of the peduncle whilst
Fig. 190.
JI°(tm.22 n . d
7 c am.22f?
TrifoHum subterraneum : downward movement of peduncle from 19°beneath
the horizon to a nearly vertically dependent position, traced from
1 I A.M. July 22nd to the morning of 25th. Glass filament fixed
transversely across peduncle, at base of flower-head.
curving down to the ground. We have seen in Chap. IV.,
Fig. 92, p. 225, that an upright young flower-head circumnu-
tated conspicuously; and that this movement continued after
the peduncle had begun to bend downwards. The same
peduncle was observed when inclined at an angle of 19° abc ?e
the horizon, and it circumnutated during two da} s. Anothei
516
MODIFIED CIRCUMNUTATION.
Chap. X.
which was already curved 36° beneath the horizon, was observed
from 11 a.m. July 22nd to the 27th, by which latter date it
had become vertically dependent. Its course during the first
1.2 h. is shown in Fig. 190, and its position on the three
succeeding mornings until the 25th.
when it was nearly vertical. During
the first day the peduncle clearly
circumnutated, for it moved 4 times
down and 3 times up; and on each
succeeding day, as it sank downwards,
the same movement continued, but
was only occasionally observed and
was less strongly marked. It should
m: cir- be stat ^ d that these peduncles were
cumoutating movement of observed under a double skylight in
peduncle, whilst the flower-
head was burying itself in
the house, and that they generally
moved downwards very much more
traced from 8 A.M. July
26th to 9 A.M. on 27th.
Glass filament fixed trans-
vei-sely across peduncle,
'near flower- head.
Fig. 192.
sand, with the reflexed tips
of the calvx still visible ; slowly than those on plants growing
out of doors or in the greenhouse.
The movement of another vertically
dependent peduncle with the flower-
head standing half an inch above the
ground, was traced, and again when
it first touched the ground; in both cases irregular ellipses
were described every 4 or 5 h. A peduncle on a plant which
had been brought into the house,
moved from an upright into a ver-
tically dependent position in a
single clay; and here the course
during the first 12 h. was nearly
straight, but with a few well-rnarkt- d
zigzags which betrayed the essential
nature of the" movement. Lastly,,
the circumnntation of a peduncle
was traced during 51 h. whilst in
the act of burying itself obliquely
in a little heap of sand. After it had buried itself to such a
depth that the tips of the sepals were alone visible, the above
figure (Fig. 191) was traced during 25 h. When the flower-
head had completely disappeared beneath the. sand, another
tracing was made during 11 h. 45 m. (Fig. 192) ; and here again
we see that the peduncle was circumnutating.
Trijo'ium suhterraneum : move-
ment of same peduncle, with
flower-head completely buried
beneath the sand ; traced from
8 A.M. to 7. 1 5 P.M. on July 29th.
Chap. X. GEOTPOPISM. 517
Any one who will observe a flower-head burying itself, will be
convinced that the rocking novement, due to the continued
circumnutation of he peduncle, plays an important part in the
act. Considering that the flower-heads are very light, that the
peduncles are long, thin, and flexible, and that they arise from
flexible branches, it is incredible that an objc ct as blunt as one
of these flower-heads could penetrate the ground by means of
the growing force of the peduncle, unless it were aided by the
locking movement. After a flower-liead has penetrated the
ground to a small depth, another and efficient ageicy comes into
play; the central rigid aborted flowers, each terminating in five
long claws, curve up towards the peduncle; and in doing so
can hardly fail to drag the head down to a greater depth, aided
as this action is by the circumnutating movement, which con-
tinues after the flower-head has completely buried itself. The
aborted flowers thus act something like the hands of the mole,
which force the. earth backwards and the body forwards.
It is well known that the seed-capsules of various widely
distinct plants either bury themselves in the ground, or are
produced from imperfect flowers developed beneath the surface.
Besides the present case, two other well-marked instances will
be immediately, given. It is probable that one chief good thus
gained is the protection of the seeds from animals which prey on
them. In'the case of T. suiter raneum, the seeds are not only
concealed by being buried, but are likewise protected by being
closely surrounded by the rigid, aborted flowers. We may the
mure confidently infer that protection is here aimed at, because
the seeds of several species in this same genus are protected in
other ways ;* namely, by the swelling and closure of the calyx,
or by the persistence and bending down of the standard-petal, &c.
But the most curious instance is that of T. globosum, in which
the upper flowers are sterile, as in T. subterraneum, but are here
developed into large brushes of hairs which envelop and protect
the seed-bearing flow r ers. Nevertheless, in all these cases the
capsules, w-ith their seeds, may profit, as Mr. T. Thiselton Dyer
has remarked,! by their being kept somewhat damr and the
advantage of such dampness perhaps throws light on the pre-
sence of the absorbent hairs on the buried flower-heads of T. sub-
terraneum. According to Mr. Bentham, as quoted by Mr. Dyer,
* Vancher, 'Hist. Phys. des t See his interesting article in
Plantes d'Europe,' torn. ii. p. 110. ' Nature,' April 4th. 1878, p. 44(3
518 MODIFIED CIRCUMNUTATION. Chap. X.
the prostrate habit of Hdianthemum prostratum " brings the
capsules in contact with the surface of the ground, postpones
their maturity, and so favours the seeds attaining a larger size."
The capsules of Cyclamen and of Oxulis ucetosdla are only occa-
sionally buried, and this only beneath dead leaves or moss. If
it be an advantage to a plant that its capsules should be kept
damp and cool by being laid on the ground, we have in these
latter cases the first step, from which the power of penetrating
the ground, with the aid of the. always present movement of
circumnutation, might afterwards have been gained.
Arachis hyp< gcea. — The flowers which bury themselves, rise
from stiff branches a few inches above the ground, and stand
upright. After they have fallen off, the gynophore, that is the
part which supports the ovarium, grows to a great length, even
to 3 or 4 inches, and bends perpendicularly downwards. It
resembles closely a peduncle, but has a smooth and pointed
apex, which contains the ovules, and is at first not in the least
enlarged. The apex after reaching the ground penetrates it, in
one case observed by us to a depth of 1 inch, and in another
to 0*7 inch. It there becomes developed into a large pod.
Flowers which are seated too high on the plant for the gyno-
phore to reach the ground are said * never to produce pods.
The movement of a young gynophore, rather under an inch
in length and vertically dependent, was traced during 46 h. by
means of a glass filament (with sights) fixed transversely a
iittle above the apex. It plainly drcumnutated (Fig. 193)
whilst increasing in length and growing downwards. It was
then raised up, so as to be extended almost horizontally, and
the terminal part curved itself downwards, fo'lowing a nearly
straight course during 12 h., but with one attempt to circum-
nutate, as shown in Fig. 194. After 24 h. it had become nearly
vertical. Whether the exciting cause of the downward move-
ment is geotropism or apheliotropism was not ascertained ; but
probably it is not apheliotropism, as all the gynophores grew
straight down towards the ground, whilst the light in the hot-
house entered from one side as well as from above. Another
and older gynophore, the apex of which had nearly reached the
ground, was observed during 3 days in the same manner as the
first-mentioned short one; and it was found to be always circum-
nutatiug. During the first 34 h. it described a figure which
* ' Gard. Chronicle,' 1857, p. 566.
Chap. X.
GEOTROPISM.
519
represented four ellipses. Lastly, a long gynophore, the apex of
which had buried itself to the depth of about half an inch, was
Fig. 194
Pig. 193
Arachis h>/pogce<x : circum-
nutation of vertically-
dependent young gyno-
phore, traced on a ver-
tical glass from 10 A.M.
July 31st to 8 A.M. Aug.
2nd.
Arachis hypojcea : down-
ward movement of same
young gynophore, after
being extended horizon-
tally; traced on a vertical
glass from 8.30 A.M. to
8.30 p.m. Aug. 2nd.
pulled up and extended horizontally: it quickly began to curve
downwards in a zigzag line ; but on the following day the ter-
- ' 34.
520 MODIFIED CIRCUMNUTATION. Chap. X.
minal bleached portion was a little shrivelled. As the gyno-
phores are rigid and arise from stiff branches, and as they
terminate in sharp smooth points, it is probable that they could
penetrate the ground by the mere 
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