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EDWARD NELSON DINGLEY
COLLECTION
PRESENTED BY HIS WIFE
alls
POWER OF MOVEMENT
IN PLANTS
ea
Yo fie’
CHARLES, DARWIN, LL. D., F.R.S.
ASSISTED BY
FRANCIS DARWIN
WITH IELUSTRA TIONS
NPEW > aYeOrReK
Dea so Eel OuN 4 AON Ws CO MyPiA NY
| 1896
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: Authorized Edition.
Mis. Edwarb N. Dingley
July 11 1932
CONTENTS.
TSE OMIONION el.) oc Coes Lee Che A ae ee Page 129
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 ereet—Circumnu-
tation of the cotyledons—Rate of movement—Analogous obser-
vations On various organs in. species of Githago, Gossypium,
Oxalis, Tropzeolum, Citrus, Aisculus, of several Leguminous and
Cucurbitaceous genera, Opuntia, Helianthus, Primula, Cyclamen,
Stapelia, Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus,
Corylus, Pinus, Cycas, Canna, Allium, Asparagus, Phalaris, Zea,
Avena, Nephrodium, and Selaginella .. .. .. .. 10-66
CHAPTER II.
GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF
SEEDLING PLANTS.
Generality of the circumnutating movement—Radicles, their cir-
cumnutation of service—Manner in which they penetrate the
ground—Manner in which hypocotyls and cther organs bieak
through the ground by being arched—Singular manner of ger-
mination in Megarrhiza, &¢.-—Abortion of cotyledons—Circum-
nutation of hypocotyls aud epicotyls whilst still buried and
arched—Their power of straightening themselves— Bursting of
the seed-ecats—tinherited effect of the arching process in hypo-
v1 CONTENTS.
gean hypocotyls—Circumnutation of hypocotyls and epicotyls
when erect—Circumunutation of cotyledons—Puivini or joints of
cotyledons, curation of their activity, rudimentary in Oxsalis
corniculata, their development—Sensitiveness of cotyledons to
light and consequent disturbance of their periodic movements—
Sensitiveness of cotyledons to contact.. .. .. Page 67-128
CHAPTER Tit
SENSITIVENESS OF THE APEX OF THE RADICLE TO CoNTACT AND
TO OTHER IRRITANTS.
Manner in which radicles bend when they encounter an obstacle in
the soil—Vicia faba, tips of radicles highly sensitive to 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 — Pisum, tips of
radicles sensitive—Hffects 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—Tropzolum—Gossy pium—Cucurbita
—Raphanus— Aisculus, 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 .c° oe “ey § 02)! Vand Ser eet eee es
CHAP A Raye
Tue CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF
MaATuRE 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 Dionzea—Leaves of Cannabis sink at night—
Leaves of Gymnosperms—Of Monocotyledons—Cryptogams—
Ccnecluding remarks on the circumnutation of leaves: generally
tise in the evening and sink in the morning .. .. 201-262
CONTENTS. vil
CHL P Tinh” Vv.
MopiIFieD CIRCUMNUTATION: CLIMBING PLANTS; HPINASTIC AND
Hyponastic MovEMENTs.
Circumoutation 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—Epinastie
growth of young leaves—Hyponastic growth of the hypocotyls
and epicotyls of seedlings—Hvoked tips of climbing and other
plants due to modified circumnutation—Ampelopsis tricuspidata
—Smithia Pfundii—Straightening of the tip due to hyponasty—
Epinastic growth and circumnutation of the flower-peduncles of
Trifolium repens and Oxalis carnosa.. .. .. Page 263-279
CHAPTER VI.
MopIrieD CrRCUMNUTATION: SLEEP oR Nyotitropic Movements,
THEIR UsE: SLEEP OF COTYLEDONS,
Preliminary sketch of the sleep or nyctitropic movements of leaves
—Presence of pulvini—The lessening of radiation the final cause
of nictritropic movements—Manuer of trying experiments on
leaves of Oxalis, Arachis, Cassia, Melilotus, Lotus and Mairsilea,
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 cotyledons of various plants—
List of species—Concluding remarks—Independence of the
nyctitropic movements of the leaves and cotyledons of the same
species—Reasons for believing that the movements have been
aeqmired for aspecial purpose. 4. 1 «.«. «: ». 290-316
CHAPTER VII.
Moptriep CiIRcUMNUTATION: NyctTITROPIC OR SLEEP MovEMENTS
oF LEAVES.
Conditions necessary for these movements—List of Genera and
Families, which include sleeping plants—Description of the
movements in the several Genera—Oxalis: leaflets folded at
Vill CONTENTS.
night—Averrhoa: rapid movements of the leaflets—Porlieria :
leaflets close when plant kept very dry—Tropeolum: 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 nizht—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 first true leaves os toe ae ee, Age OIE asin
CEHUACP DER Val:
MopiIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
Distinction between heliotropism and the effects of light on the
periodicity of the movements of leavcs—Heliotropic movements
of Beta, Solanum, Zea, and Avena—Heliotropic movements
towards an obscure light in Apios, Brassica, Phalaris, Tropao-
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-
tatioa—Steps by which one movement is converted into the
other—Transversal-heliotropismus or diaheliotropism influenced
by epinasty, the weight of the part and apogeotropism—A pogeo-
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
Or Ace iia. 2X,
SENSITIVENESS OF PLANTS To LIGHT: ITS TRANSMITTED EFFECTS.
Uses of he ‘iotropism—lInsectivorous and climbing plants not helio-
tropic—Sarne organ heliotropic at one age and not at another—
Extraordinary sensitiveness of some plants to light—The effects
CONTENTS. 1X
of licht do not correspond with its intensity —Efivcts of previous
illumination—T/ime required for the action of light—A fter-cffects
of light—Apogeotropism acts as soon as light fails—Accuracy
with which plants ben to the light—This dependent on the
Ulumination 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 wit!
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 has
been converted into heliotropism or apheliotropism Page 449-492
CHAPTER X.
Mop1IFriEB CIRCUMNUTATION : MOVEMENTS EXCITED BY
GRAVITATION.
Means of observation—A pogeotropism—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—
Gencral remarks on apogeotropism—Geotropism—Movements of
radicles—Burying of seed-capsules—Use of process—Trifolium
subterraneum — Arachis — Amphicarpeea — Diageotropism —
WOMCMISIOlg ey Pasi lone oa Hee’ weet "ey es ees ASOOZe
CHAPTER XL
LOCALISED SENSITIVENESS TO GRAVITATION, AND ITS TRANSMITTED
EFFECTS.
General considerations—Vicia faba, effects of amputating the tips of
the radicles—Regeneration of the tips—Effects of a short ex-
posure of the tips to geotropic action and their subsequent
amputation—Fffects of amputating the tips obliquely—Effects
of cauterising the tips—Effects of grease on the tips—Pisum
x CONTENTS.
sativum, tips of radicles cauterised transversely. and on theit
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
FAGICIES ois) 66s oi Losey \cicauw dis i eee oe eee
CHAP TER XE
SUMMARY AND ConcLuDING REMARKS,
v ature of the circumnutating movement—History of a serminating
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—N yctitropic 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 bra 3.) 3.5 4s 06 eee eee ee
INDEX ee oo eo oo eo oo eo ae eo eo eo 574-593
THE MOVEMENTS OF PLANTS.
INTRODUCTION.
Tue chief object of 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 cercumnutation
and cercumnutate. 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{ 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.
O1 the whole we may at present conclude that in-
* Sachs first showed (‘Lehr- 19, 1879, p. 830.
buch, &c., 4th edit. p. 452) the
intimate connection between tur-
gescence and growth. For De
Vries’ interesting essay, ‘ Wachs-
thumskriimmungen mehrzelliger
Organe,’ see ‘ Bot. Zeitung,’ Dec.
+ ‘Die Periodischen Bewegun-
gen der Blattorgane,’ 1875.
t *Untersuchungen iiber den
Heliotropismus, Sitzb. der K,
Akad. der Wissenschaft. (Vienna),
Jan. 1880.
INTRODUCTION. ce”
creased growth, first oy one side and then on another,
is a secondary effect, and that the increased tur-
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, circumnutate, 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
ereat 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- Naturkunde in Wiirtemberg,’
cussion (‘ Arbeitcn des Bot. Insti-
tuts in Wiirzburg,’ B. II. pp. 142,
143, 1878) on this intricate subject.
Hofmeister’s observations (‘ Jak-
reschrifte des Vereins fur Vaterl.
187+, p. 211) on the curious move-
ments of Spirogyra, a plant con-
sisting of a single row of eells,.are
valuable in relation to this subject.
. 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 ight. 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. 4)
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
be used, must here be given. With seedlings, the stem which
supports the cotyledons (i.e. the organs which represent the first
leaves) has been called by many botanists the hypocotyledonous
stem, but for brevity sake we will speak of it merely as the
hypocotyl: the stem immediately above the cotyledons will be
called the epicotyl or plumule. The radicle can be distinguished
from the hypocotyl only by the presence of root-hairs and the
nature of its covering. The meaning of the word ctircumnu-
tation has already been explained. Authors speak of positive
and negative heliotropism,*—that is, the bending of an organ
to or from the light; but it is much more convenient to confine
the word-hilivtropism to bending towards the light, and -to
designate as a;helivtropism bending from the light. There is
another reason for this chauge, for writers, as we have
observed, occasionally drop the adjectives positive and negative,
and thus introduce confusion into their discussions. Déiahelio-
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 ; apoyeotropism will mean bending in
opposition to gravity or from the centre of the earth; and dia-
jeotropism, &@ 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 epinusty 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 Gcotropism triage zur Pflanzenphysiologie,
were first used by 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 reguire, 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 usedin the ‘ Wiirzburg Arbeiten,” Heft ii
gense 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 our
Mr. Cooper for the care with which diagrams.
2
® INTRODUCTION.
introduzed a superfluous number of diagrams; but they taks
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 ou
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 information on various points.
10 CIRCUMNUTATION OF SEEDLINGS. Ouar. L
CHAPTER I.
Tre CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
Brassica oleracea, circumnutation of the radicle, of the arched hyno-
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-
peolum, Citrus, Aisculus, of several Leguminous and Cucurbita-
ceous genera, Opuntia, Helianthus, Primula, Cyclamen, Stapelia,
Cerinthe, Nolana, Solanum, Beta, Ricinus, Quercus, Corylus, Pinus,
Cycas, Canna, Allium, Asparagus, Phalaris, Zea, Avena, Nephro-
dium, and Selaginella.
Tue 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. Butina future chapter we shall have
to recur to the movements of certain cotyledons which
sleep at night.
Brassica oleracea (Crucifere).—Fuller details will be given
with respect to the movements in this case than in any other,
as space and time will thus ultimately be saved.
fadicle-—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 toll 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
Cuar. 1, BRASSICA. Ha
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 zine
plate. As far as we could
roughly ascertain by measure-
ments made with compasses
on other seeds, the tip alone,
for a length of only +2, to
<35 Of an inch, is acted on
by geotropism. But the trac-
ing shows that the basal part eae
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
Fig. 1.
Brassca oleriacea: circumnutation of
radicle, traced on horizontal glass,
was magnified by the fila-
ment, which was nearly ? inch
in length, it was impossible
from 9 a.m. Jan. 31st to 9 P.M.
Feb. 2nd. Movement of bead at
end of filament magnified about
40 times.
to estimate.
Another seed was treated and observed in the same manner,
but the radicle in this case protruded ‘1 inch, and was not
Fig. 2.
Brassica oleracea: 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. 81st to 7 a.m.
Feb. 2nd; but it continued to move during the whole of the
12 CIRCUMNUTATION OF SEEDLINGS. Cuapr. 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. .
Hypocotyl.—The hypocotyl protrudes through the seed-coats
as a rectangular projection, which grows rapidly into an arch
like the letter U turned upside down q; 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
Cuap. I. BRASSICA, 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
imner 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 sLows that the arched hypocoty] tends at this early
14 CIRCUMNUTATION OF SEEDLINGS. Crap, I
age to circumnutate irregularly. On the first day the greater
movement (from right to left in the figure) was not in the plane
of the vertica] 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
355,
may
Brassica oleracea: circumnutating movement of buried and arched hypo-
cotyl, with the two legs of the arch tied together, traced on horizontal
glass during 333 hours. Movement of the bead of filament magnified
about 26 times, and here reduzed 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 am. on Dee,
Cuap. I. BRASSICA. 15
93rd to 6.45 am. 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
Brassica oleracea: circumtutating 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 times, and here reduced to one-half original
scale.
see that the course was still zigzag, which indicates a tendency
to circumunutation. 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 CIRCUMNUTATION OF SEEDLINGS. Cuap. 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 oleracea: conjoint circumnutation of the hypocotyl and cotyledona
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 10h. 45 m. (see
Fig. 6). It should be noticed that when I looked shortly after
+p.m. the bead was pointing off the glass, but it came on again
at 5.30 Pp.m., and the course during this interval of 1h. 80m. 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 33 ellipses in 10? h.; each being completed on an -
average in 3h. 4m.
On the previous day another seedling had been observed
under similar conditions, excepting that the plant was so
Cuap. I. BRASSICA. 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 aad 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 CIRCUMNUTATION OF SEEDLINGS. Cuap, 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. 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-
Brassica oleracea: conjoint circumnutation ment of both ib eite ole
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 cna ther both circumnutated.
veetical 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 am. they moved a
little back from the light, often crossing and recrossing their
former path in zigzag lines. The sky or 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 12.17p.m. Afterwards it fell a little and madea —
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 mcve
Cuap. I. BRASSICA. 19
vertically up and down all day long, and as there was some
slight lateral movement, they circumnutated.
The cabbage was one of the first plants, the seedlings of which
were observed by us, and we
did not then know how far
the circumnutation of the
different parts was affected
by light. Young seedlings
were therefore kept in 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 13 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
Fig. 9.
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 foliowing morn-
ing. Figure here reduced to one-
half of criginal scale.
the bead was traced during 14h. 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,
20
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.
Rate of movement.—The movements of
the hypocotyls and cotyledons of seedling
cabbages of different ages have now been
sufficiently illustrated. With respect to
the rate, seedlings were placed under the
CIRCUMNUTATION OF SEEDLINGS. Cuap. I,
Fig. 10.
Brassica oleracca : cir-
cumnutation of a
tatyledon, the hypo-
cotyl having been
secured to a stick,
traeed 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.
microscope with the stage removed, and
with a micrometer eye-piece so adjusted
that each division equalled <1, 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 =3,, and sometimes of nearly
siz of aninch. 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
Ouar. 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. ;4, inch) in
1m. 30s. The seedling was then left in darkness for an hour,
and now it required 8m. 6s. to cross one division, that is,
15 m. 380s. 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 5m. 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 12m. 30s. for five divisions. It was then
again left in complete darkness for 1h., and the point now
travelled in the same direction as before, but at the rate of
3m. 18s. for five divisions.
We shall have to recur to the cotyledons of the cabbage in a
future chapter, when we treat of their sleep-movements. The
circumnutation, also, of the leaves of fully-developed plants
will hereafter be described.
Fig. 11.
Githago segetum: circumnutation of hypocotyl, traced on a horizontal
glass, by means of a filament fixed transversely across its summit, from
8.15 A.M. to 12.15 P.M. on the following day. Movement of bead of
filament magnified about 13 times, here reduced to one-half the orizinal
scale.
Githago segetum (Caryophylleze).—A young seedling was dimly
luminated from above, and the circumnutation of the hypo-
22
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 cotten) (Mal-
vaces#).—The circumnutation of a hypo-
cotyl was observed in the hot-house, but
the movement was.so much exaggerated
that the bead twice passed for a time out of
view. It was, however, manifest that two
somewhat irregular ellipses were nearly
completed in 9 h. Another seedling,
CIRCUMNUTATION OF SEEDLINGS. Cuar. 1
Fig. 12.
Gossypium: circumnu-
tation of hypocotyl,
traced on a horizon-
tal glass, from 10.30
A.M. to 9.30 A.M. on
following morning,
by means of a fila-
ment fixed across
its summit. Move-
ment of bead of fila-
ment magnified about
twice; seedling illu-
minated from above.
15 in. in height, was then observed during
23h.; but the observations were not
made at sufficiently short intervals, as
shown by the few dots in Fig. 12, and the
tracing was not now sufficiently enlarged.
Nevertheless there could be no doubt
about the circumnutation of the hypocotyl, which described
in 12h. 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 10.80 a.m. to about 3 p.m.; they then sank till
10 P.M., rising, however, greatly in the latter part of the night
Guar I. GOSSYPIUM. 23
The angles above the horizon at which the cotyledons of another
seedling stood at different hours is recorded in the following
short table :—
Oe 0) 250 wae sa c= 25° above horizon.
” 4.20 ,, 22° ”
eat a0), -, es | eae
ene ek Bet
Ccims "CAO AM, Fos. fs | 28P -
2c STII Abe teeta a lt a ae
Sei pose Ma ston ns... LO° 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 triangies
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 am., 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 shghtly 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.
Oxalis rosea (Oxalideze).—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 long
as it remained nearly horizontal; but in the course of the day 1t
both rose considerably above and fell beneath a horizontal posi-
tion, and then of course the movement was much exaggerated
3
24 CIRCUMNUTATION OF SEEDLINGS. Cuap, 1.
In Fig. 18 ifs course is shown from 6.45 a.m. on June 17th, to
i} 1° 8°30°a.m.
ou
uu
=
s
Jxalis rosea: circumnutation of
cotyledons, the hypocotyl being
secured to a stick; illumina-
ted from above. Figure here
given one-halt of original scale.
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 Fig. 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.
Anotker 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.
Oxalis Valdiviana.—This species is interesting, as the coty-
Cuap. I. OXALIS. 25
ledons rise perpendicularly upwards at night so as to come into
alose contact, instead of sinking vertically downwards, as in the
ease of O. rosea. A glass filament was fixed to a cotyledon,
‘17 of an inch in length, and the hypocotyl was left free. On
Fig. 14. Hen vk
8°12/aan. ae ;
18" 9 6°40'a.m.19th at Ries
:
H H
‘
A Nepal!
A
' ' H
: }7° 22" p.m.
1 ‘
1 ‘
i 1
: ;
1
\
i
9° 28’ i
;
-
5
A
1
i]
i]
H
H
S° p.m
*, i
. ‘
aS
Ny 8°35a.m.
N 19th
Oxalis rosea: conjoint circumnutation of Oaalis Vuldiviana ? conjoint
the cotyledons and hypocotyl, traced circumnutation of a cotyle-
from 8.12 A.M. on June 18th to 7.30 don and the hypocotyl, traced
A.M. 19th. The apex of the cotyledon on vertical glass, during 24
stood only 3$ inches from the vertical hours. Figure here given
glass. Figure here given one-half of one-half of original scale;
original scale. seedling illuminated from
above.
the first day the seedling was placed too far from the vertical
glass; £0 that the tracing was enormously exaggerated and the
movement could not be traced when the cotyledon either rose or
sank much; but it was clearly seen that the cotyledons rose
thrice and fell twice between 8.15 am. and 4.15 p.m. Early on
the following morning (June 19th) the apex of a cotyledon was
26 CIRCUMNUTATION OF SEEDLINGS. Cuap. L
placed only 12 inch from the vertical glass. At 640 a.m. it
stood horizontally ; it then fell till 8.85, and then rose. AlI-
together in the course of 12h. 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 6.3C
A.M. In the present instance, however, the great nocturnal rise
did not commence till 7 p.m.; but this was due to the hypocotyl
having from some unknown cause temporarily bent to the left
side, as is shown in the tracing. ‘To ascertain positively that
the hypocotyl circumnutated, a mark was placed at 8.15 p.m.
behind the two now closed and vertical cotyledons; and the
movement of a glass filament fixed upright to the top of the
hypocotyl was traced until 10.40 p.m. During this time it
moved from side to side, as well as backwards and forwards,
plainly showing circumnutation; but the movement was small
in extent. Therefore Fig. 15 represents fairly well the move-
ments of the cotyledons alone, with the exception of the one
great afternoon curvature to the left.
Oxalis corniculata (var. cuprea).—The cotyledons rise at night
to a variable degree above the horizon, generally about 45°:
those on some seedlings between 2 and 5 days old were found
to be in continued movement all day long; but the movements
were more simple than in the last two species. This may have
partly resulted from their not being sufficiently illuminated
whilst being observed, as was shown by their not beginning 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 223°
to 243°C. One cotyledon rose 70° in 11 m.; another, on a distinct
seedling, fell 80° in 12m. 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 2h. 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 threugh 80° in
121m. The cotyledons of this plant sleep at night by rising
Cuap. I. TROPAOLUM. 27
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) (Tropzolez).—The
cotyledons are hypogean, or never rise above the ground. By
removing the soil a buried epicotyl
or plumule was found, with its
summit arched abruptly 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 11h. 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 Daou: hes © gee
by the basal leg bending back- nutation of buried and arched
wards from the upper part, that is — epicotyl, traced on a horizon-
in adirection opposite tothedepen- _*#! glass, from 9.20 a.m. to
dent tip, in the same manner as rere oa Seer ase
.0) amen magnine a
occurred with the hypocotyl of times.
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.
Fig. 16.
ee re |
28 CIRCUMNUTATION OF SEEDLINGS. Cuap. I,
Citrus aurantium (Orange) (Aurantiacez).—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
ebserved during a period of 44h. 40 m.
Citrus aurantium: circumnutation of epicotyl with a filament fixed trans:
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 105, in figure here given,
and afterwards 36 times, or 18 as here given; seedling illuminated
from above.
Aisculus hippocastanum (Hippocastanez).—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
Cuap. I. VICIA.
29
the tracks varied in breadth. The more perfectly serpentine
tracks made by the radicles of Phaseolus multiflorus and Vicia
faba (presently to be described), render
it almost certain that the radicles of
the present plant cireumnutated.
Phaseolus multiflorus (Leguminose).
—Four smoked glass-plates were ar-
ranged in the same manner as des-
eribed under Aisculus, and the tracks
left by the tips of four radicles of the
present plant, whilst growing down-
wards, were phctographed 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.
Fig. 18.
A. B.
LE:culus hippocastanum : 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 Jength, and
-23 inch in diameter at base.
In B the plate was inclined
65° with the horizon, and
the radicle was a trifle
larger.
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)
(Leguminosee).— Radicle. —Some
beans were allowed to germinate
on bare sand, and after one had
protruded its radicle to a length
of *2 of an inch, it was turned
upside down, so that the radicle,
which was kept in damp air,
now stood upright. A filament,
nearly an inch in length, was
A.
Phaseolus muitiflorus: tracks left
on inclined smoked glass-plates
by tips of radicles in growing
downwards.
inclined at 60°, B inclined at
68° with the horizon.
Fig. 19.
™
B. C.
A and C, plates
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
30 CIRCUMNUTATION OF SEEDLINGS. Cuap. I
abruptly, then made a small loop and then a larger zigzag
eurve. During the night and till 11 4.m. on the following
‘Fig. 20.
enoosre™
Vicia faba: circumnutation of a radicle, at first pointing vertically up-
wards, kept in darkness, traced on a horizontal glass, during 14 houra.
Movement of bead of filament magnified 23 times, here reduced to
one-half of original scale.
morning, the bead moved to a great distance in a nearly straight
line, in the direction indicated by the broken line in the figure.
This resulted from the tip bending quickly downwards, as it
had now become much declined, and had thus gained a position
highly favourable for the action of geotropism.
Fig. 21.
A. EL. C. 10); KE.
Vicia faba: tracks left on inclined smoked glass-plates, by tips of radicles
in growing downwards. Plate C was inclined at 63°, plates A and D
at 71°, plate B at 75°, and plate E at a few degrees beneath the
horizon.
Cuap. 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 A‘sculus 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.
Lpicotyl.—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 14h., and
vertically in 48h. 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 epicoty],
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 Tropeolum and the hypocotyl of the Cabbage. The move-
ments of the tied arches were traced in the usual manner on
B2 CIRCUMNUTATION OF SEEDLINGS. Cuap. 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 50h. (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 50h. that the filament
at the end of our observations was attached at the height of
‘4 inch above the upper surface of the bean, instead of close
to it. If the bean had been properly planted, this part of the
epicotyl would still have been beneath the soil.
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. depth of
hole +1 inch.
the force exerted transversely by these
radicles. Two were so placed as to
penetrate small holes made in litle
sticks, one of which was cut into the
shape here exactly copied (Fig. 55).
The short end of the stick beyond
the hole was purposely split, but not the opposite
Cuap. LI. ACTION OF THE RADICLE. 75
end. As the wood was highly elastic, the split o1
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 imn.e-
diately closed to a width of 2 mm. The stick was
then suspended horizontally by
a fine wire passing through the 4
hole lately filled by the radicle,
and a little saucer was sus-
pended beneath to receive the
weights ; and it required 8 lbs. _ { \ f
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
ereater 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, Wooden pincers, kept closed by
a spiral brass spring, with a
A second stick was tried in the hole (-14 inch in diameter
same manner with almost ex- 724 ‘6 inch in depth) bored
through the narrow closed
actly the same result. part, through which a radicle
- We then followed a better of a bean was allowed to
grow. Temp. 50°-60° F.
plan. Holes were bored near
the narrow end of two wooden clips or pincers (Fig. 56),
kept closed by brass spiral springs. Two radicles in damp
sand were allowed to grow through these holes. The
Fig. 56.
%6 ACTION OF THE RADICLE. Cuap. TI,
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 mch) 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
od 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
eround. 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
Cuap Il. HYPOCOTYLS AND EPICOTYLS. ae
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, &e., rise wp
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
eases, 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
* Thisistheconclusion arrived considered by other botanists as
at by Dr. H. Gressner (‘Bot. the first true leaf is really the
Zeitung, 1874, p. 837), who — second cotyledon, which is greatly
lnaintains that what has been delayed in its development,
78
HYPOCOTYLS, EPICOTYLS, ETC.,
Cuap. IT.
breaks torough the ground as an arch (Fig. 57).
Fig. 57.
Persicum :
figure en-
larged: c, blade of
cotyledon, not yet
expanded, with arched
petiole beginning to
straighten itself; A,
hypocotyl developed
into acorm ; 7,second-
ary radicles.
Cyclamen
seedling,
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,
Fig. 58.
Acanthus mollis: seedling, with the
hypogean cotyledon on the near
side removed and the radicles cut
off: a, blade of first leaf begin-
ping to expand, with petiole still
partially arched; 0, second and
opposite leaf, as yet very imper-
fectly developed; c, hypogean
cotyledon on the opposite side.
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<
Cuar. II. BREAKING THROUGH THE GROUND. ‘19.
tive fact that it is not arched, as it has not to foree
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
Acanthacez 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,
sprnosus, and latefolius 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
Jeaves 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 lamine 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
ot Acanthus the first pair of leaves breaks through
the ground by two widely different methods; and if
See
80 HYPOCOTYLS, EPICOTYLS, ETC., Crap. IE
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. EHxternally 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 pari
(and probably of the blade) splits open one side ot
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.
Ouar. I. BREAKING THROUGH THE GROUND. 81
Megarrhiza Californica.-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 24 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 24 inches, and the
radicle by this time had also become well developed.
The plumule, still enclosed within the tube, was now
* * American Jonrnal of Science,’ vol. xiv. 1877, p. 23.
52 HYPOCOTYLS, EPICOTYLS, ETC. Cuar. If
‘S 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 24} inches of superin-
cumbent soil, still retaming 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 Californica : presented by the line Ge... G.
et ihe oe The germination of the seeds in
reduced to one-half their native Californian home pro-
se Pape ee ceeds in a rather different manner,
the two confluent as we infer from an _ interesting
cee ainsi letter from Mr. Rattan, sent to us
pl, plumule; G....G, by Prof. Asa Gray. The petioles
surface of soil.
protrude from the seeds soon after
the autumnal rains, and penetrate the ground, generally
in a vertical direction, to a depth of from 4 to even
6 inches. They were found in this state by Mr.
Itattan during the Christmas vacation, with the plu.
Chap. UI. 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 shghtly 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 teptophylla.—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, they
BA HYPOCOTYLS, EPICOTYLS, ETC., Cuap. If.
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 38 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 4 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 1s with [pomea pandurata,
the germination of which, as Asa Gray informs us,
resembles that of I. leptophylla.
The following case is interesting in connection with
Crap. IT 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
ereen 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 ot
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
hike 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 Lathrea
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 greatly
Science,’ vol. iv. p. 190. facilitated by the extraordinary
The passage of the flower- quantity of water secreted at this
stem of the Lathrxa through the period of the year by the subter-
86 HYPOCOTYLS, EPICOTYLS, ETC., Cuae, IT,
stem of the parasitic and leafless Monotropa hypopitys.
With Helleborus niger, the flower-stems, which rise up
independently of the leaves, likewise break through
the ground as arches. ‘This is also the case with the
greatly elongated flower-stems, as well as with the
petioles of Epimedium pinnatum. So it is with the
petioles of Ranunculus ficaria, when they have to break
through the ground, but when they arise from the
summit of the bulb above ground, they are from the
first quite straight; and this is a fact which deserves
notice. The rachis of the bracken fern (Pleris aqu-
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 ereat
quantity of sap absorbed in the
early spring by the parasitic roots.
After a long period without any
rain, the earth had become light-
eoloured and very dry, but it was
dark coloured and damp, even in
parts quite wet, for a distance of
ai least six inches all round each
flower-stem.
by glands (described by Cohn,
‘Bericlit. Bot. Sect. der Schile-
sise.en Gesell.” 1876, p. 113)
wi:ich line the longitudinal
channels running through eich
scale-like leaf. A large plant was
dug up, washed so as to remove
tiie 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 liad secreted a
large pool of water. The plite
was wiped dry, and in the course
of the succeeding 7 or 8 hours
The water is secreted
another little pool was secreted,
and after 16 additional hours
several large drops.
Cos
wa = ZBL
o tient Paap IT ot oe
8 Ont
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i
(i
iy
a
wi
He
i
r{
e.
Seo
ii
WV
wee
ai
ain
s
‘G
mea
wy
=e
EE ————————s
Sa =
an
= Be
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ZL.
Ss
Stee
nae
Oxalis rosea: longitudinal section
of a pulvinus on the summ‘t
of the petiole of a cotyledon,
drawn with the camera lucida,
magnified 75 times: p, p, pe-
. tiole; f, fibro-vascular bundle;
b, b, commencement of blade of
cotyledon.
its movement depends on both these causes combined ;t
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
the movements induced by the aid of pulvini and
* «Die Periodische Bewegun-
gen der Blattorgane,’ 1875.
+ Batalin, ‘Flora,’ Oct. 1st, 1873
} Pfeffer, ibid. p. 5.
{14 PULVINI OF COTYLEDONS. Cuap. IL
without such aid, is reduced to the expansion of the
cells not being followed by growth in the first ease,
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 82 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 Ozalis floribunda, the cotyledons continued
for a long time to move vertically down at night, and
the movement apparently depended exclusively on
the pulvini, for their petioles were of nearly the same
length in young, and in old seedlings which had 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
erowth 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
erowth, 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-
Onap. IT. PULVINI OF CO1LYLEDONS. 115
paring the diagrams given in the last chapter. Thus
the movements of the cotyledons of Brassica oleracea
and of Ipomeea cexrulea, 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
Jacobeus 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 pulvyinus. 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 Crucifere, Cucurbitacee,
Githago, and Beta, never last even for a week, to any
conspicuous degree. Pulvinated cotyledons, on the
other hand, continue to rise at night for a much
longer period, even for more than a month, as we
shall now show. But the period no doubt depends
largely on the temperature to which the seedlings are
exposed and their consequent rate of development.
Oxalis Valdiviana.—Some cotyledons which had lately opened
and were horizontal on March 6th at noon, stood at night 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 3°
/
116 PULVINI OF COTYLEDONS. Cuap. 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.
Oxzalis (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.—The cotyledons do not stand vertical at
night, but generally rise to an angle of about 45° above the
horizon. They continued thus to act for 23 days after their
first expansion, by which time two leaves had been formed ;
even after 29 days they still rose moderately above their 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 44 days, the cotyledons were still considerably
raised at night; but those of another seedling, only one day
older, were raised very little.
Mimosa albida.—A seedling was observed during only 12 days,
by which time a leaf had been formed, and the cotyledons were
then quite vertical at night.
Trifolium subterraneum.—A seedling, 8 days old, had its coty-
ledons horizontal at 10.80 a.m. and vertical at 9.15 p.m. After an
interval of two months, by which time the first and second true
leaves had been developed, the cotyledons still performed the
same movement. They had now increased greatly in size, and
had become oval; and their petioles were actually °8 of an inch
in length!
Trifolium strictum.—After 17 days the cotyledons still rose at
night, but were not afterwards observed.
Lotus Jacobeus.—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
Cuap. 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 O. rosea
sink vertically ; yet when sections of their pulvini were
made, no clear difference could be detected between the
corresponding halves of this organ in the two species
which move so differently. With O. rosea, however,
there were rather more cells in the lower than in the
upper half, but this was likewise the case in one speci-
men of O. Valdiviana. The cotyledons of both species
(84 mm. in length) were examined in the morning
whilst extended horizontally, and the upper surface of
the pulvinus of O. rosea was then wrinkled transversely,
showing that it was in a state of compression, and this
might have been expected as the cotyledons sink at
might; with O. 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. Bewegungen,’ 1875, p. 157.
118 PULVINI OF COTYLEDONS. Cuap, IL
the species ‘seen by us are pulvinated; so it is with
the cotyledons of TY. subterraneum and strictum, which
stand vertically at night; whereas those of T. resupt-
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. Ozalis cor-
niculata (var. Atro-purpurea) differs in several respects ;
the cotyledons rise at night to a very variable amount,
rarely more than 45°; and in gone lot of seedlings
(purchased under the name of O. tropxoloides, 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 ease has hitherto, we believe, 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. Trem 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.
Onar. Il. ° PULVINI OF COTYLEDONS. mt!)
When the cotyledons of 0. corniculuta 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 skorter 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
A.
Oxalis corniculata: A and B the almost rudimentary pulvini of the coty-
leduns 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
fw 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 cel!s themselves varied in length in different ;arts of the
9
120 PULVINI OF COTYLEDONS. Cuap, IL
same pulyinus 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
pulvinus was for this species well developed. They offer a
striking contrast with the pulvinus of O. rosea (see former
Fig. 63), or of O. Valdiviana. With the seedlings, falsely called
O. tropeeoloides, 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 O. corniculata are given :—
Secdling 1 day old, with cotyledon 2°3 mm. in length.
Divisions of
Micrometer.f
Average length of cells of pulvinus oof) Sonja leat SSO
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, with the puloimus
quite distinct.
Average length of cells of pulvinus eet ett | oes 6
Length of longest cell below the pulvinus .. .. .. 22
Length of longest cell above the pulvinus .. .. .. 40
Seedling 8 days od, cotyledon 5 mm. in length, with a true leaf
formed but not yet expanded.
Average length of cells of pulvinus os get cae 9
Length of longest cell below the pulvinus .. .. .. 44
Length of longest cell above the pulvinus .. .. «. 70
Secdling 13 days old, cotyledon 4:5 mm. in lengt, with a small
true leaf fully developed.
Average length of cells of pulvinus aot ene Ree Be 7
Length of longest cell below the pulvinus .. .. «. 30
Length of iongest cell above the pulvinus .. .. CO
* Longitudinal sections show pulvinus.
that the forms of the epidermic + Each division cqualled ‘008
sells may be taken as a fairrepre- mm.
sentation of those constituting the
Ounap. IL. PULVINI OF COTYLEDONS 121
We here see that the cells of the pulvinus increase but little
in length with advancing age, in comparison with those of the
petiole both above and below it; but they continue to grow in
width, and keep equal in this respect with the other cells of
the petiole. The rate of growth, however, varies in all parts
of the cotyledons, as may be observed in the measurements of
the 8-days’ old seedling.
The cotyledons of seedlings only a day old rise at night 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 11 days old, the nocturnal
rise was found to depend largely on the action of the pulvinus,
for the petiole at night was curved upwards at this point; and
during the day, whilst the petiole was horizontal, the lower
surface of the pulvinus was wrinkled with the upper surface
tense. Although the cotyledons at an advanced age do not rise
at nivzht toa higher inclination than whilst young, yet they have
to pass through a larger angle (in one instance amounting to
63°) to gain their nocturnal position, as they are generally
deflected beneath the horizon during the day. Even with the
1l1-days’ old seedling the movement did not depend exclusively
on the pulvinus, for the blade where joined to the petiole was
curved upwards, and this must be attributed to unequal growth.
Therefore the periodic movements of the cotyledons of O. 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 Jacobeus.—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. Cuap. IL
he 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
‘O4 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 24 mm. in length,
their petioles do not exhibit any defined zone of small cells,
destitute of chlorophyll and differing in shape exteriorly from
the lower ones. Nevertheless, the cells at the place where a
pulvinus will afterwards be developed are smaller (being on an
average ‘(015 mm. in length) than those in the lower parts of
the same petiole, which gradually become larger in proceeding
downwards, the largest being ‘030 mm. in length. At this early
age the cells of the blade are about ‘027 mm. in length. We
thus see that the pulvinus is formed by the cells in the 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 over a small defined space of the
petiole being almost arrested at an early age. With
Lotus Jacobeus the cells at first increase a little in
length; in Owalis cormiculata 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
Guar. II. DISTURBED PERIODIC MOVEMENTS. 128
with seedlings of Trifolium, Lotus, and Oxalis some of
the species have a well-developed pulvinus, and others
have none, or one in a rudimentary condition. As the
movements caused by the alternate turgescence of
the cells in the two halves of a pulvinus, must be
largely determined by the extensibility and 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 hght; 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.
124 DISTURBED PERIODIC MOVEMENTS. Cuap. I.
For instance, the cotyledons of Beta vulyaris, Solanum lycoper-
sicum, Cerinthe major, and Lupinus 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 Jacobeeus were not affected by 4h. of
complete darkness, but when placed under a double skylight
and thus feebly illuminated, they quite lost their periodical
movements on the third morning. On the other hand, the
cotyledons of Cucurbita ovifera moved in the normal manner
during a whole day in darkness.
Seedlings of Githago segetum were feebly illuminated from
above in the morning before their cotyledons had expanded, and
they remained closed for the next 40h. Other seedlings were
placed in the dark after their cotyledons had opened in the
morning and these did not begin to close until about 4h. had
elapsed. The cotyledons of Oxalis rosca sank vertically down-
wards after being left for 1h. 20m. 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 toopen. 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 2h. to dull sunshine, and the cotyledons
Unap. IL SENSITIVENESS OF COTYLEDONS. 125
remained horizontal; it was then brought back to the table, and
after 50m. 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 6h. The cotyledons
in another pot, similarly treated on another occasion, were open
at 7 A.M. and remained open in the dark for 4h. 30m, 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. tora, when extended
horizontally, were both lightly tapped with a very thin twig for
3m., 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 9m.; 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 ea
126 COTYLEDONS SENSITIVE Cuap, IL
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
80s. or for 1m.; 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. neglecta (likewise from 8S. Brazil) rose in from 5 m. to 15 m. to
various angles between 16° and 34°, after being rubbed during
1m. with a twig. Their sensitiveness is retained to a somewhat
advanced age, for the cotyledons of a little plant of C. neglecta,
34 days old and bearing three true leaves, rose when lightly
pinched between the finger and thumb. ‘Some seedlings were
exposed for 80 m. to a wind (temp. 50° F.) sufficiently strong to
keep the cotyledons vibrating, but this to our surprise did not
cause any movement. The cotyledons of four seedlings of the
Indian C. gluuca were either rubbed with a thin twig for 2m. or
were lightly pinched: one rose 84°; a second only 6°; a third
18°; and a fourth 17°. A cotyledon of C. flortdu similarly
treated rose 9°; one of CO. corymhosa rose 73°, and one of the
very distinct C. mimosoides only 6°. Those of C. pubescens did
not appear to be in the least sensitive; nor were those of C.
nidosa, but these latter are rather thick and fleshy, and do not
rise at night or go to sleep.
Smithia sensitiva.—This plant belongs to a distinct sub-order of
the Leguminose from Cassia. Both cotyledons of an oldish
scedling, with the first true leaf partially unfolded, were rubbed
for 1m. with a fine twig, and in 5m. each rose 32°; thoy
Ouap. II. . TO CONTACT. 127
remained in this position for 15m., but when looked at again
40m. after the rubbing, each had fallen 14°. Both cotyledons of
another and younger seedling were lightly rubbed in the same
manner for 1m., and after an interval of 32 m. each had risen
80°. ‘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 1m. or 2m.; 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 rosea little. It
thus appears that the cotyledons of Mimosa are less sensitive
than those of the previously mentioned plants.*
_ Owalis sensitivaa—The blades and pulvini of two cotyledons,
standing horizontally, were rubbed or rather tickled for 30s.
with a fine split bristle, and in 10m. 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 llm. 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 Ovalis
Valdiviana and rosea were rubbed and did not exhibit any
sensitiveness,
Finally, there secms to exist some relation between
* The sole notice which we _ p. &65), “les cotyledons du M
have met with on the sensitive- pudica tendent & se raprocher par
ness of cotyledons, relates to Mi- leurs faces supérieures lorsqu’on
mesa; for Aug. P. De Candolle _ les irrite.”
says (‘Phys. Vég.,’ 1832, tom. ii.
128 SENSITIVENESS OF COTYLEDONS. Ors 168
the habit of cotyledons rising vertically at night or
going to sleep, and their sensitiveness, especially that
of their pulvini, to a touch; for all the above-named
plants sleep at night. On the other hand, there are
many plants the cotyledons of which sleep, and are
not in the least sensitive. As the cotyledons of
several species of Cassia are easily affected both by
slightly diminished light and by contact, we thought
that these two kinds of sensitiveness might be con-
nected; but this is not necessarily the case, for the
cotyledons of Oxalis sensitiva did not rise when kept
on one occasion for 14 h., and on a second occasion
for nearly 4h., 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 in
any degree.
Ouar, ITL, SENSITIVENESS OF RADICLES. 129
CHAPTER Il.
SENSITIVENESS OF THE APEX OF THE RADICLE TO CONTACT AND TO
OTHER IRRITANTS.
Manner in which radicles bend when they encounter an obstacle in
the soil—Vicia faba, tips of radicles highly sensitive to contact
and other irritants—Effects of too high a temperature—Power of
discriminating between objects attached on opposite sides—Tips of
secondary radicles scnsitive—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-
paolum— Gossy pium—Cucurbita— Raphanus—sculus, 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.
Ix order to see how the radicles of seedlings would
pass over stones, roots, and other obstacles, which they
must incessantly encounter in the soil, germinating
beans (Vicea 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. © Cuap. UL
it, pressing on it with very little force. How far such
abrupt changes in its former course are aided by the
circumnutation of the tip must be left doubtful. Thin
slips of wood were cemented on more or less steeply
inclined glass-plates, at right angles to the radicles
which were gliding down them. Straight lines had
been painted along the growing terminal part of some
of these radicles, before they met the opposing slip
of wood; and the lines became sensibly curved in 2 h.
after the apex had come into contact with the slips.
In one case of a radicle, which was growing rather
slowly, the root-cap, after encountering a rough slip
of wood at right angles, was at first slightly 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 nght 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 sand.
When, as in the above cases, radicles encountered
an obstacle at right angles to their course, the terminal
growing part became curved for a length of between
‘3 and °4 of an inch (8-10 mm.), measured from the
apex. This was well shown by the black lines which
had been previously painted on them. ‘The first and
most obvious explanation of the curvature is, that it
results merely from the mechanical resistance to the
growth of the radicle in its original direction. 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
Cuar. III. SENSITIVENESS OF RADICLES. Tod
their 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
ereatest ease will deflect a radicle: thus, as we have
seen, when the apex of the radicle of the bean
encountered the polished surface of extremely thin
tin-foil laid on soft sand, no impression was left on it
yet the radicle became deflected at right angles. A
second explanation occurred to us, namely, that even
the gentlest pressure might check the growth of the
apex, and in this case growth could continue only on
one side, and thus the radicle would assume a 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 shall
* ¢ Arbeiten Bot. Inst. Wiirzburg,’ Heft iii. 1873, p. 898.
13Z SENSITIVENESS OF THE APEX Cuap. III
hereafter see, Sachs discovered that the radicle a little
above the apex is sensitive, and bends like a tendril
towards the touching object. But when one side of the
apex is pressed by any object, the growing part bends
away from the object; and this seems a beautiful
adaptation for avoiding obstacles in the soil, and, as
we shall see, for following the lines of least resistance.
Many organs, when touched, bend in one fixed direc-
tion, such as the stamens of Berberis, the lobes of
Dioneea, &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 24h.,
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 lght 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 35th of an inch (i.e. about 14 mm.),
or oblong bits of nearly the same size, were found to
Caar. UI. OF THE RADICLE OF THE BEAN. ioe
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 gum-water by itself induces no action. In most
cases the bits of card were touched with an extremely
small quantity of a solution of shellac in spirits of
wine, 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. Hyen extremely minute beads of the shellac
occasionally acted in a slight degree, as will hereafter
be described. But that it was the cards which chiefly
acted in our many trials, was proved by coating one
side of the tip with a little bit of goldbeaters’ skin
(which by itself hardly acts), and then fixing a bit of
card to the skin with shellac which never came into
contact with the radicle: nevertheless the radicle bent
away from the attached card in the ordinary manner.
Some preliminary trials were made, presently to
be described, by which the proper temperature was
determined, and then the following experiments were
made. It should be premised that the beans were
134 SENSITIVENESS OF THE APEX Cuap. III.
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
Fig. 65.,
A. B. C
\
Vicia faba: A, radicle beginning to bend from the attached little square
ef card; B, bent ata 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
Cuar. III OF THE RADICLE OF THE BEAN. lies
were fixed either behind, causing a curvature in direct
opposition to that of Sachs’, or more commonly to the
right or left sides. For the sake of brevity, we will
speak of the bits of card, &¢., 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,
1:8 mm. in length, and 1°5 or only 0°9 mm. in breadth (i.e. ‘071
ot an inch in length and ‘059 or -035 of an inch in breadth) were
fixed with shellac to the tips of the radicles. In the subsequent
trials the little squares were only occasionally measured, but
were of about the same size.
(1.) A young radicle, 4 mm. in length, had-a card fixed 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, 29 h.
from the time of attachment.
(2.) Radicle 5°5 mm. in length, card fixed behind: after 9h.
deflected in the plane of the bean 20° from the perpendicular
and from the card, and in opposition to Sachs’ curvature: after
93 h. no change.
10
136 SENSITIVENESS OF THE APEX (Cuap. IIL
(3.) Radicle 11 mm. in length, card fixed behind: after 9h.
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 23h. the extreme tip was slightly bent to-
wards the card; the general curvature of the radicle remaining
the same.
(4.) Radicle 9 mm. long, card fixed behind and a little
laterally: after 9h. deflected in the plane of the bean only
about 7° or 8° from the perpendicular and from the ecard, in
opposition to Sachs’ curvature. There was in addition a slight
lateral curvature directed partly from the card. After 23h. no
change.
(5.) Radicle 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.) Radicle 9 mm. long, card affixed in front: after 9h. 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 9h. After 23h: no change.
(7.) Radicle 7 mm. long, card affixed to the back: after 9h.
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 22h. 30 m. this part of the radicle
had become straight.
(8.) Radicle 12 mm. long, card affixed almost laterally: after
9h. deflected latera!ly 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 22h. 80m. the extreme tip had become
slightly curved towards the card.
(9.) Card fixed laterally: after 11h. 80m. no effect, the
radicle being still almost vertical.
(10.) Card fixed almost laterally: after 11h. 30m. deflected
90° from the vertical and from the card, in a plane inter-
mediate between that of the bean itself and one at right
Guar. III. OF THE RADICLE OF THE BEAN. 137
angles to it. Radicle consequently partially deflected from
Sachs’ curvature.
(11.) Tip of xadicle protected with goldbeaters’ skin, with a
square of card of the usual dimensions affixed with shellac:
after 11h. 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 11h. no
effect, but after 24h. 40 nf. radicle clearly deflected from the
card. This slow action was probably due to a portion of the
eoldbeaters’ skin having curled round and lghtly touched, the
opposite side of the tip and thus irritated it.
(18.) A radicle of considerable length had a small square of
card fixed with shellac to its apex laterally: after only 7 h. 15m.
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 9h. 15 m. deflected through 90° from the
perpendicular and from the card. After 24h. deflection much
decreased, and after an additional day, reduced to 23° from the
perpendicular.
(16.) Square of card fixed with shellac behind the apex of a
radicle, which from its position having been changed during
growth had become very crooked; but the terminal portion
_ was straight, and this became deflected to about 45° from
the perpendicular and from the card, in opposition to Sachs’
curvature.
(17.) Square of card affixed with shellac: after 8 h. radicle
curved at right angles from the perpendicular and from the
eard. After 15 additional hours curvature much decreased.
(18.) Square of card affixed with shellac: after 8h. no effect;
after 23h. 3m. from time of affixing, radicle much curved from
the square.
(19.) Square of card affixed witn sheliac: after 24h. no effect,
but the radicle had not grown well and seemed sickly.
(20.) Square of card affixed with shellac: after 24h. no effect.
(21, 22.) Squares of card affixed with shellac: after 24h.
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 Cuap. TIL
9h. very slightly curved from the card; after 24h. tip curved
towards card. Refixed new square laterally, atter 9h. 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 te
apex: after 24h. 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 9h. deflection from the perpendicular considerable; after
24h. deflection reduced. Refixed a fresh square with shellac:
after 24h. 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 9h. the deflection from the perpendicular
‘and from the card amounted to nearly a right angle; after 24h.
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 9h. a little deflection,
which after 24h. 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 14h. at right angles
from the card.
(29.) A small square of card was first fixed with thick gum-
water to the apex. It produced a slight effect but soon fell
off. A similar square was now affixed iaterally with shellac:
after 9h. the radicle was deflected nearly 45° from the perpen-
dicular and from the card. Atter 36 additional hours angle of
deflection reduced to about 30°.
(30.) A very small piece, less than 54th of an inch square, of
thin tin-foil fixed with shellac to the apex of a young radicle ;
after 24h. no effect. Tin-foil removed, and a small square of
sanded card fixed with shellac; after 9h. deflection at nearly
tight angles from the perpendicular and from the card. Next
Cuap. III. OF THE RADICIE OF THE BEAN. 139
morning deflection reduced to about 40° from the perpen-
dicular.
(81.) 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 shellae
to it, and after 9h. 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.
ereatly deflected from the perpendicular and from the card.
Curvature extended for a length of ‘22 of an inch from the
apex. After 8 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 15h. deflected at
nearly 90° from the perpendicular and from the card.
(34.) Small oblong of sanded card gummed to apex: after
15h. deflected 90° from the perpendicular and from the card :
in the course of the three following days the terminal porticn
became much contorted and ultimately coiled into a helix.
(35.) Square of card gummed to apex: after 9 h. deflected from
ecard: after 24h. from time of attachment greatly deflected
obliquely and partly in opposition to Sachs’ curvature.
(36.) Small piece of card, rather less than 5th 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.
(87.) Sqtiare 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
eard. 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.
(89.) 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 cciled into a ring.
140 SENSITIVENESS OF THE APEX Cnar. III.
(40.) Square of card gummed to apex: after 9 h. slightly
curved from card; next morning radicle straight, and apex had
grown beyond the card. Refixed another square laterally with
shellac; in 9 h. deflected laterally, but also in the direction of
Sachs’ curvature. After 2 additional days’ curvature 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 24 h. from the time of attachment deflected at 90°
from the perpendicular and from the card; after 9 additional
hours became hooked, with the apex pointing to the zenith. In
d 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
cleflection, 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
olass, 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 afew other
observations. Bits of very thin glass and little squares
ye
Cnar. III. OF THE RADICLE OF THE BEAN. 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 toa
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 @ square of glass or card did
not act, it was removed, and a fresh one affixed, this
being often done thrice to the same radicle. 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
defiection. 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
L42 SENSITIVENESS OF THE APEX Cuap. ITT.
other trial, for it occurred to us that sensitiveness is
easily affected by external conditions, and that radicles
growing naturally in the earth im 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
50° 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. These
radicles therefore presented a striking contrast with
the 44 above described. On March 6th and 7th, when
the temperature of the room varied between 53° and
09° I., eleven germinating beans were tried in the
War. III 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 24h. 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 j,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-
144 SENSITIVENESS OF THE APEX Cuar. TEL,
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.50m. 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
Caar. II] OF THE RADICLE OF THE BEAN. 145
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 im 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 in48 h. Weat 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
eases. ‘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.
146 SENSITIVENESS OF THE APEX Cuap, LIL
after an interval of about 24 or more hours, bent
towards the bit of still attached card,—that is, in a
ilirection exactly opposite to the previously induced
curvature of the whole growing part for a length of
from 7 toS8mm. ‘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 aud 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 due to the shellac
itself. Little squares of goldbeaters’ skin, which is
excessively thin, were damped, and thus made to
adhere to one side of the tips of two radicles; one of
these, after 24 h., produced no effect; nor did the
fmm Tt OF THE RADICLE OF THE BEAN. - 147
other in 8 h., within which time squares of card usually
act; but after 24 h. there was slight deflection.
An oval bead, or rather cake, of dried shellac,
1-01 mm. in length and 0°63 in breadth, caused a
radicle to become deflected at nearly right angles in
the course of only 6 h.; but after 23 h. it had nearly
straightened itself. A very small quantity of dissolved
shellac was spread over a bit-of card, and the tips of
9 radicles were touched laterally with it; only two of
them became shghtly 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
Tooth of a grain; so that a weight of rather _less
than 535th of a grain (0°32 mgs.) sufficed to excite
movement in two out of the nine radicles. Here ©
then we haye 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 55th of an inch. These after being
~ touched with thick gum-water, were placed on the tip
of eleven radicles. Three of hier 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 9h.; but after 24h. 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
L148 SENSITIVENESS OF THE APEX = Cuap. IIL.
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 55th of an inch), were fixed with
shellac on opposite sides of the apices of 12 suspended
radicles. The sanded card was between 0°15 and
0-20 mm. (or between 0:0059 and 0-0079 of an inch),
and the thin paper only 0°045 mm. (or 0:00176 of an
inch) in thickness. In 8 out of the 12 cases there
could be no doubt that the radicle was deflected from
the side to which the card-like paper was attached, and
towards the opposite side, bearing the very thin paper.
This occurred in some instances in 9 h., but in others
not until 24 h. had elapsed. Moreover, some of the
four failures can hardly be considered as really failures :
thus, in one of them, in which the radicle remained
quite straight, the square of thin paper was found,
when both were removed from the apex, to have been
so thickly coated with shellac that it was almost as
stiff as the card: in the second case, the radicle was
bent upwards into a semicircle, but the deflection
was not directly from the side bearing the card, and
this was explained by the two squares having become
cemented laterally together, forming a sort of stiff
gable, from which the radicle was deflected: in the
third case, the square of card had heen fixed by
mistake in front, and though there was deflection
from it, this might have been due to Sachs’ curvature .
Usap. 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° I’. In 7 out of these cases
no effect whatever was produced; in the eighth case
the radicle became slightly deflected from, and in the
ninth case slightly deflected towards, the rubbed side :
but these two latter opposed curvatures were probably
accidental, as radicles do not always grow perfectly
straight downwards. The tips of two other radicles
were rubbed in the same manner for 15 seconds with
a little round twig, two others for 30 seconds, and two
others for 1 minute, but without any effect being 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,
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:
L50 SENSITIVENESS OF THE APEX Cuar 1’J-
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 which had been cut off obliquely by accident,
was curled upwards over the bean, but it was not .
clearly ascertained whether the curvature had been at
first directed from the cut surface. These results led
us to pursue the experiment, and 18 radicles, which
had grown vertically downwards in damp air, had one
side of their conical tips sliced off with a razor. The
tips were allowed just to enter the water in the jars,
and they were exposed to a temperature 14°-16° C.
(07°-61° I'.). The observations were made at dif-
ferent times. Three were examined 12 h. after being
sliced, and were all slightly curved from the cut
surface; and the curvature increased considerably after
an additional 12 h. Hight 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 15 h. 830 m. So that
only 4 out of the 18 radicles were not acted on. To
these 18 cases the 3 previously mentioned ones should
be added. It may, therefore, be concluded that a thin
slice removed by a razor from one side of the conical
apex of the radicle causes irritation, like that from an
attached object, and induces curvature from the injured
surface.
Lastly, dry caustic (nitrate of silver) was employed
to irritate one side of the apex. If one side of the
apex or of the whole terminal growing part of a
radicle, is by any means killed or badly injured, the
other side continues to grow; and this causes the part
Cuar. III. OF THE RADICLE OF THE BEAN. lot
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 them. This was effected by first drying the
tip as far as possible with blotting-paper, though it still
remained somewhat damp, and then touching it once
with quite dry caustic. Seventeen radicles were thus
treated, and were suspended in moist air over water at
a temperature of 58°’ F. They were examined after
an interval of 21 h. or 24 h. The tips of two were
found blackened equally all round, so that they could
tell nothing and were rejected, 15 being left. Of
these, 10 were curved from the side which had been
touched, where there was a minute brown or blackish
mark. Five of these radicles, three of which were
already slightly defiected, were allowed to enter the
water in the jar, and were re-examined after an addi-
tional interval of 27 h. (.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 cf
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.
The power of an Irritant on the apex of the Radicle
* Ciesielski found this to be the -
case (‘ Untersuchungen iiber die
Abwartskriimmung der Wurzel,’
1871, p. 28) after burning with
heated platinum one side of a
radicle. So did we when we
painted longitudinally half of the
whole length of 7 radicles, sus-
11
pended over water, with a thick
layer of grease, which is very
injurious or even fatal to grow-
ing parts; for after 48 hours
five of these radicles were curved
towards the greased side, twe
remaining straight.
L52 SENSITIVENESS OF THE APEX Cnap. Il
of the Bean, compared with that of Geotropism—We
know that when a little square of card or other
object is fixed to one side of the tip of a vertically ©
dependent radicle, the growing part bends from it
often into a semicircle, in opposition to geotropism,
which force is conquered by the effect of the in-
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
lower sides of their tips, so that if the squares
acted, the terminal growing part would curve upwards.
Firstly, eight beans were so placed that their short,
young, horizontally extended radicles would be 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 only5h.! 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, 1} ich 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
Cuar. JI OF THE RADICLE OF THE BEAN. £33
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
upwards and others downwards at the same time—
these opposite movements depending on whether the
radicles, when the squares were first attached to them,
projected vertically down, or were extended 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 young and sensitive radicle is extended
horizontally, with a square attached to the lower side
of the tip, geotropism acts on it at right angles, and,
as we have seen, is then evidently more efficient than
the irritation from the square ; and the power of 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
vertically dependert radicle, and the apex begins to
[54 SENSITIVENESS OF THE RADICLE. Cuar. lil
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 nght angles, but greater than that of geo-
tropism when acting obliquely on it.
Sensitiveness of the tips of the Secondary Radicles of the
Bean to contact—All the previous observations relate
to the main or primary radicle. Some beans suspended
to cork-lds, with their radicles dipping into water, had
developed secondary or lateral radicles, which were
afterwards kept in very damp air, at the proper low
temperature for full sensitiveness. They projected,
as usual, almost horizontally, with only a slight
downward curvature, and retained this position
during several days. Sachs has shown* 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 any
* ¢Arbeiten Bot. Inst., Wiirzburg,’ Heft iv. 1874, p. 605-617.
Cap, II. SENSITIVENESS OF THE RADICLE. Ton
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
satevun).—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 f 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 Sens, 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
* «Arbeiten Bot. Instit., Wiirz- t ‘Die Schutzeinrichtungen de:
burg,’ Heft. iv. 1874, p. 620. Keimpflanze,’ 1877, p. 29.
+ Ibid. Heft iii. 1873, p. 437.
~
136 SENSITIVENESS OF THE Crap, LIL
inch square, or rather less) were attached in the same
manner to one side of the radicle at a distance of 3 or
4mm. above the apex. In our first trial on 15 radicles
no effect was produced. tna second trial on the same
number, three became abruptly curved (but only one
strongly) towards the card within 24h. 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:
sheht permanent depression was left. In such eases
the opposite side continues to grow, and the radicle
necessarily becomes bent towards the injured side.
But when a point 4mm. 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 itselfin 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-
* Sachs, ‘ A beiten Bot. Institut., Wiirzburg,’ Heft iii. p. 488.
ee sees
Cuar. III. UPPER PART OF THE RADICLE. 157
shire 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
eurved 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 Cuap. III
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 on
their tips, and another set curved towards the squares
attached a little higher up. Moreover, the kind of
curvature in the two cases is different. The squares
attached above the apex cause the radicle to bend
abruptly, the part above and beneath remaining nearly
straight; so that here there is little or no transmitted
effect. On the other hand, the squares attached to
the apex affect the radicle for a length of from about
4 to even 8 mm., inducing in most cases a 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 8h. 80 m.; and eight of them became in the course
of 24 h. conspicuously, and the remaining two slightly,
deflected from the perpendicular and from the side
bearing the attached squares. Thus all were acted on;
but it will suffice to describe two conspicuous cases.
In one the terminal portion of the radicle was bent at
right angles (A, Fig. 66) after 24 h.; and in the other
(B) it had by this time become hooked, with the apex
pointing to the zenith. The two kits of card here used
were ‘07 inch in length and ‘04 inch in breadth. Two
other radicles, which after 8 h. 80 m. were moderately
deflected, became straight again after 24h. Anothei
Cuap. III. OF THE RADICLE 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 raiicles, by little squares of card affixed with
shellac to one side of apex: A, bent at right 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 Ist 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 = Cnar. IIL
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 grewslowly,
but 10 out of the 13 became in the course of the three
days very shghtly curved from the squares; the other
3 were not affected; so that this temperature was toc
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 24h., 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 ease after 88 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 eatended 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
Cuap. LIL. OF THE RADICLE 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 iritation caused by the
cards. our of the 13 radicles were a little curved
downwards within 6 or 8h., always reckoning from
the time when the squares were first attached, and
after 23h. three of them pointed vertically down-
wards, and the fourth at an angle of 45° beneath the
horizon. ‘These four radieles therefore did not seem
Fig. 67.
A. B.
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
162 SENSITIVENESS OF THE APEX Gnarp. LL
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 7h. 30m.; 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
Cuap. III. OF THE RADICLE OF PHASEOLUS. 165
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. Jesides the difficulty of
attaching the squares to such finely pointed objects
as were these radicles, the temperature was too high,
—yarying on the first occasion from 72° to 77° F’., and
on the second being almost steadily 78° F.; and this
probably lessened the sensitiveness of the tips. The
result was that after an interval of 8 h. 30 m., 6 of the
22 radicles were bowed upwards (one of them greatly)
in opposition to gravity, and 2 laterally; the 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
Radicle.—Fifty-nine radicles were tried with squares
164 SENSITIVENESS OF THE APEX hap. UT
of various sizes of the same card-like paper, also with
bits of thin glass and rough cinders, affixed with shellac
to one side of the apex. Rather large drops of the
dissolved shellac were also placed on them and allowed
to set into hard beads. The specimens were subjected
to various temperatures between 60° and 72° F., more
commonly at about the latter. But out of this 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 18 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 24h. 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 ~,th of an inch, and
this length became curved at right angles towards the
blackened surface, the curvature afterwards increasing
») several instances until httle hooks were formed.
It was manifest that the blackened side was so much
injured that it could not grow, whilst the opposite
side continued to grow. One alone out of these 12
radicles became curved from the blackened side, the
Cuar. UI. OF THE RADICLE OF PHASEOLUS. L&E
eurvature 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
leneth of nearly 10 mm.; whereas in the first set
166 SENSITIVENESS OF THE APEX Cuap. It
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 Dionza
and Pinguicula.
The effect of cutting off with a razor a thin slice
from one side of the conical apex of 14 young and
short radicles was next tried. Six of them after being
operated on were suspended in damp air; the tips ot
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 witk
Guar. Ill. OF THE RADICLE OF TROPAOLUML 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 23h. 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.
Tropxolum majus: Sensitiveness of the apex of the
Radicle to contact.—Little squares of card were attached
with shellac to one side of the tips of 19 radicles, some
of which were subjected to 78° F., and others to a
much lower temperature. Only 3 became plainly
curved from the squares, 5 slightly, 4 doubtfully,
and 7 not at all. ‘These seeds were, as we believed,
old, so we procured a fresh lot, and now the results
were widely different. Twenty-three were. tried in
the same manner; five of the squares produced no
effect, but three of these cases were no real exceptions,
for in two of them the squares had slipped and were
parallel to the apex, and in the third the shellac was
in excess and had spread equally all round the apex.
One radicle was deflected only slightly from the
perpendicular and from the card; whilst seventeen
were plainly deflected. The angles in several of these
latter cases varied between 40° and 65° from the
perpendicular; and in two of them it amounted after
15h. or 16 h. to about 90°. In one instance a loor
12
168 SENSITIVENESS OF THE APEX Cuap. Itt
was nearly completed in 16h. 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.
Gossypium herbaceum: Sensitiveness of the apex of the
Radicle.—Radicles were experimented on in the same
manner as before, but they proved ill-fitted for our
purpose, as they soon became unhealthy when sus-
pended in damp air. Of 388 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
ett) et ee i in, i ae
a 4
AL;
Guar. 111. OF THE RADICLE OF CUCURBITA. 169
were just touched with dry caustic on one side. After
only 5 h. 10 m. five of them were slightly curved
from the perpendicular and from the side bearing the
little blackish marks. After 8 h. 40 m., 4 out of
these 5 were deflected at angles between 15° and 65°
from the perpendicular. On the other hand, one
which had been slightly curved atter 5 h. 10 m., now
became straight. After 24 h. the curvature in two
cases had considerably increased; also in four other
cases, but these latter radicles had now become so
contorted, some being turned upwards, that it could no
longer be ascertained whether they were still curved
from the cauterised side. The control specimens 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. Thése marks in all
cases were oval or elongated; they were measured in
three instances, and found to be of nearly the same
size, viz. 2 of amm.in length. Bearing this fact in
mind, it should be observed that the length of the
curved part of the radicle, which had become deflected
from the cauterised side in the course of 8 h. 40 m,,
was found to be in three cases 6, 7, and 9 mm.
Cucurbita ovifera: Sensitiveness of the apex of the Ra-
dicle—The tips proved ill-fitted for the attachment of
cards, as they are 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 two occasions, when out of 23
radicles 10 were deflected from the attached squares
170 SENSITIVENESS OF THE APEX Cuap. IIL
of card, and 13 were not acted on. Rather large
squares, though difficult to affix, seemed more efficient
than very small ones.
We were much more successful with caustic; but in
our first trial, 15 radicles were too much cauterised,
and only two became curved from the blackened side ;
the others being either killed on one side, or blackened
equally all round. In our next trial the dried tips
of 11 radicles were touched momentarily with dry
caustic, and after a few minutes were immersed in
water. The elongated marks thus caused were never
black, only brown, and about 4 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 8h. 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
Cuap. If. 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 4 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 ina 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.
Raphanus sativus: Sensitiveness of? the apex of the
Radicle——We here encountered many difficulties in
our trials, both with squares of card and with caustic ;
for when seeds were pinned to a cork-lid, many of the
radicles, to which nothing had been done, grew irre-
eularly, 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
scemed 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 Cuar. Ill
and a third very slightly from the perpendicular and
from the cauterised side.
Aisculus hippocastanum: Sensitiveness of the apex of
the Radicle—Bits of glass and squares of card were
affixed with shellac or gum-water to the tips of 12
radicles of the horse-chestnut ; and when these objects
fell off, they were refixed ; but not in a single instance
was any curvature thus caused. These massive
radicles, one of which was above 2 inches in length
and ‘3 inch in diameter at its base, seemed insensible
to so slight a stimulus as any small attached object.
Nevertheless, when the apex encountered an obstacle
in its downward course, the growing part became 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
he radicles were slightly rubbed once with dry nitrate of silver;
aud 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 blackeued, or in
which the seedling soon became unhealthy.
(1.) The racicle was slightly deflected from the cauterise|
side in one day (i.e. 24 h.); in three days it stood at 60° from
the perpendicular; in four days at 90°; on the fifth day it was
curved up about 40° above the horizon; so that it had passed
through an angle of 130° in the five days, and this was the
greatest amount of curvature observed.
(2.) In two days radicle slightly deflected; after seven days
Cniap, IIT. OF THE RADICLE OF ASCULUS. 178
deflected 69° from the perpendicular and from the cauteriset
side; after eight days the angle amounted to nearly 90°.
(3) After one day slight denen, 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 75°,
which in an additional day increased to 90°.
(4.) After two days slight deflection, which during the nex*
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 ii
amounted to 44°, and on the fourth day to 72° from the perper.-
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 day slight deflection, which on the second day
amounted to 35°, on the fourth day to 50°, and the sixth day
to 63° from the perpendicular and the cauterised side.
(13.) Whole tip blackened, but more on one side than the
other; on the fourth day slightly, and on the sixth day greatly
deflected from the more blackened side; the deflection on ths
tinth day amounted to 90° from the perpendicular.
(14.) Whoie tip blackened in the same manner as in the last
ease: on the second day decided deflection from the more
blackened side, which increased on the seventh day to nearly
90°; on the following day the radicle appeared unhealthy.
(15 ) Here we had the anomalous case of a radicle bending
L74 SENSITIVENESS OF THE APEX Cuap. II
slightly t.wards 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 ld 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
erew 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
Unap. IT. OF THE RADICLE OF QUERCUS. Lie
remarkable cases are worth describing. ‘he radicles
were examined on each successive morning, at nearly
the same hour, that is, after intervals of 24 h.
No. 1. This radicle suffered from a series of accidents, and
acted in an anomalous manner, for the apex appeared at first
insensible and afterwards sensitive to contact. The first square
was attached on Oct. 19th; on the 21st the
radicle was not at all curved, and the square Fig. 68.
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 aiter 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 ih Mehran Anes eA
the terminal growing part had become bent with square of card
away from the square into a hook (see attached to one side
Fig. 68). of apex, oe .
No. 2. Square attached on the 19th; on eee : eens
the 20th radicle slightly deflected from it natural scale.
anu 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
eurvature 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 2Ist; 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
G.e. 26th) the curvature had wholly disappeared and the apex
pointed perpendicularly downwards.
No. 5. Square attached on the 21st; on the 22nd decided
Wd io SENSITIVENESS OF THE APEX Cuap. III.
though slight curvature from the square; on the 28rd the tir
had curved up above the ho izon, and on the 24th 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 Aisculus.
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 60°-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
Cuap. TLL OF THE RADICLE OF Z2BA. L177
ease 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°-
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
and from the perpendicular; 13 out of the 39 forming
hooks with their points directed towards the zenith,
and 8 forming loops. Moreover, 7 other radicles out
of the 68, were slightly and two doubtfully deflected
from the cards. ‘There remain 20 which were not
affected; but 10 of these ought not to be counted; -
for one was diseased, two had their tips quite 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 Cuap. 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 9h. 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 tour 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
eard, 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
Onap. III. OF THE RADICLE OF ZITA. 178
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. 69
Zea mays: radicles areted to bend HN from the little squares of pars
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
180 SENSITIVENESS OF THE APEX Cnap. MIL
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 lhght) on a sand-bath,
which varied between 76° and 82° F. After only
5 h. one appeared to be a little deflected from the
square, and after 20 h. formed a loop. Four others
were considerably curved from the squares after 20 h.,
and three of them became hooked, with their tips
pointing to the zenith,—one after 29 h. and the
two others after 44 h. By this latter time a sixth
radicle had become bent at a right angle from the side
bearing the square. Thus altogether six out of the
ten secondary radicles were acted on, four not being
affected. There can, therefore, be no doubt that the
tips of these secondary radicles are sensitive to slight
contact, and that when thus excited théy 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 Arr.
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
* « Arbeiten des Bot. Institut., in W4rzburg,’ vol. i. 1872, p. 209.
Caar. II. OF THE RADICLE TO MOIST AIR. 181
radicle to bend away from the source of irritatiou.
In our experiments we followed Sachs’ plan, and sieves
with seeds germinating in damp sawdust were sus-
pended so that the bottom was generally iuclined 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
ereased for a length of 2 mm., and two others for a length of
1; 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 Cuap. ITY.
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 8mm. in 19 h.; and two, which had been at first
4 and 6 mm. in length, iucreased in 24 h. to 15 and 20 mm.
The tips of 10 radicles, which likewise grew weil, were coated
with the grease for a length of only 1 mm., and now the result
was somewhat different; for of these 4 curved themselves to
the sieve in from 21 h. to 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
14 to 23 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 atall. 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 (13 mm. in length)
removed, and during the next 154 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 18 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
Cuap. JII. OF THE RADICLE TO MOIST AIR. 1838
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 to8 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,
388 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 sativa.n—The tips of 13 radicles, which projected
between 2 and 4 mm. from the bottom of the sieve. many of
13
1st SENSITIVENESS OF THE APEX Cuapr. TIL
them not quite perpendicularly downwards, were coated with
the black grease for a length of from 1to13 mm. The sieves
were inclined at 30° with the horizon. The greater number of
these radicles were examined after 22 h., and a few after 25 h.,
and within these intervals they had grown so quickly as to have
nearly doubled their lengths. With the ungreased radicles the
chief seat of curvature is at a distance of not less than between
3) and 55 mm., and not more than between 7 and 10 mm. from
the apex. Out of the 13 radicles with greased tips, 4 had not
moved at all towards the sieve; 6 were deflected towards it and
from the perpendicular by angles varying between 10° and 35° ;
and 3 had come into close contact with it. It appears, therefore,
at first sight that greasing the tips of these radicles had checked
but little their bending to the adjoining damp surface. But the
inspection of the sieves on two occasions produced a widely
different impression on the mind; for it was impossible to
behold the radicles with the black greased tips projecting from
the bottom, and all those with ungreased tips, at least 40 to 50
in number, clinging closely to it, and feel any doubt that the
greasing had produced a great effect. On close examination
only a single ungreased radicle could be found which had not
become curved towards the sieve. It is probable that if the
tips had been protected by grease for a length of 2 mm. instead
of from 1 to 1; mm, they would not have been affected by the
moist air and none would have become curved.
Triticum vulgare—Analogous trials were made on 8 radicles
of the common wheat; and greasing their tips produced much
less effect than in the case of the oats. After 22 h., 5 of them
had come into contact with the bottom of the sieve; 2 had
moved towards it 10° and 15°, and one alone remained perpen-
licular. 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 am. We should hardly have thought this case werth
notice, had it not been for the following circumstance. In the
beginning of October, when the temperature was considerably
higher, viz., 12° to 18° C., we found that only a few of the
ungreased radicles became bent towards the sieve; and this
indicates that sensitiveness to moisture in the air is increased
by a low temperature, as we have seen with the radicles of
Vicia faba relatively to objects attached to their tips. But in
the present instance it is possible that a diffcrence in the dryness
Cuar. IJ]. OF THE RADICLE TO MOIST AIR. Les
of the air may have caused the difference, in the results at the
two periods.
Finally, the facts just given with respect to Phaseolus
multiflorus, Vicwa faba, and Avena sativa show, as li
seems to us, that a layer of grease spread for a length
of 14 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
L186 . THE EFFECT OF KILLING OR Cnap. IL.
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 RADIGLES BECOMING VERTICALLY Gro-
TROPIC BY THE DESTRUCTION OR INJURY OF THE
TERMINAL PART OF THE PRIMARY RADICLE.
Sachs has shown that the lateral or secondary
radicles of the bean, and probably of other plants, are
acted on by geotropism in so peculiar a manner, that
they grow out horizontally or a little inclined 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. Hight 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 aftected ; 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.
Caar. III. 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 four cases,
one or more of the secondary radicles, arising from
the thickened part just above the wire, had grown
perpendicularly downwards. In the best case the
primary radicle (the part below the wire being 13 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
adjoming 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
clearly analogous, as Sachs has remarked, to that
which occurs with the shoots of trees, when the leading
one is destroyed and is afterwards replaced by one or
more of the lateral shoots ; for these now grow upright
instead of sub-horizontally. But in this latter case
the lateral shoots are rendered apogeotropic, whereas
with radicles the lateral ones are rendered geotropic.
We are naturally led to suspect that the same cause
acts with shoots as with roots, namely, an increased flow
of sap into the lateral ones. We made some trials with
Abies communis and pectinata, by pinching with wire
the leading and all the lateral shoots excepting one.
But we believe that they were too old when experi-
mented on; and some weve pinched too severely, and
188 THE EFFECT OF KILLING OR Cuap. If
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
yrowth, 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,
Atcidium 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-
gianing to elongate, the shoot developed from it
grows vertically upwards. Such upright shoots after-
* See his valuable article in are culled in German “ Hexen
‘Bot. Zeitung,’ 1867, p. 257, on _ besen,” or “ witch-trooms.”
these monstrous growths, which
Cuap. III. INJURING THE PRIMARY RADICLE. 189
ards produce lateral and horizontal branches; and
they then present a curious appearance, as if a young
fir-tree had grown out of a ball of clay surrounding
the branch. These upright shoots have manifestly
changed their nature and become apogeotropic; for if
they had not been affected by the Aicidium, 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 oleracea are
“normally prostrate or procumbent;” but when they
are attacked by an Aicidium, 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 Sparganiun
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.t
No explanation has hitherto been attempted of such
eases as the foregoing,—namely, of secondary radicles
growing vertically downwards, and of lateral shoots
growing vertically upwards, after the amputation of
* «Proce. Acad. Nat. Sc. Phila- viously observed (‘ Flora,’ 1878,
delphia,” June 16th, 1874, and p. 224) that the underground
July 23rd, 1875. shoots of Triticum repens bend
+ See F. Elfving’s interesting vertically up when the parts above
paper in ‘Arbeiten Bot. Institut., ground are removed, and when
in Wiirzburg,’ vol. ii. 1880, p.489. the rhizomes are kept partly im
Carl Kraus (Triesdorf) had pre- —merscd in water.
19Q0 EFFECT OF KILLING PRIMARY RADICLE. Cuap. 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.
* The facts on which the fol-
lowing conclusions are founded
are given in ‘The Variation of
Animals and Plauts under Domes-
tication, 2nd edit 1875. On the
causes leading to reversion see
chap. xii. vol. ii. and p. 59 chap.
It is indeed pos-
xiv. On pcloric flowers, chap.
Xlil. p.52; and see p. 337 on their
position on the plant. With
respect to seeds, p. 340. On re-
version by means of buds, p. 438
chap. x1 vol. i.
Cuap. III, SUMMARY OF CHAPTER. 191
sible, or even probable, that this tendency to reversion
may have been increased, as it is manifestly of service
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 shown in this chapter, that the tip of the
radicle of the bean is in this sense sensitive to the
contact of any small object attached to one side by
shellac or gum-water; also to a slight touch with dry
caustic, and to a thin slice cut off one side. The
radicles of the pea were tried with attached objects
and caustic, both of which acted. With Phaseolus
multiflorus the tip was hardly sensitive to small squares
of attached card, but was sensitive to caustic and to
slicing. The radicles of Tropeolum 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 Cucurbita ovifera were likewise highly sensitive to
caustic, though only moderately so to contact. fa-
phanus sativus offered a somewhat doubtful case.
With Adsculus the tips were quite indifferent to
bodies attached to them, though sensitive to caustic.
Those of Quercus rubur 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,
Raphanus, and Cucurbita, the tip was so fine and
flexible that it was very difficult to attach any object
to one of its sides. With the radicles of A‘sculus,
the tips were not at all sensitive to small bodies
attached to them; but it does not follow from this
[92 SUMMARY OF CHAPTER. Cuap. III.
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 1mm. 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 faba, Pisum sativum, and Zea mays were alone
observed, and they were found similarly sensitive.
In order that these movements should be properly _
displayed, it appears necessary that the radicles
should grow at their normal rate. If subjected to a
high temperature and made to grow rapidly, the
tips seem either to lose their sensitiveness, or the
upper part to lose the power of bending. So it
appears to be if they grow very slowly from not being
vigorous, or from being kept at too low a temperature ,
also when they are foreed to germinate in the middle
of the winter.
Caas III. SUMMARY OF CHAPTER. 192
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 A’sculus. 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,
ot 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, lke
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
194 SUMMARY OF CHAPTER. Cuap. IIL,
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 adjoming 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, Dionzea and Pinguicula, with which
plants a too powerful stimulus does not excite the
tentacles to become incurved, or the lobes to close, or
the margin to be folded inwards.
With respect to the degree of sensitiveness of the
apex to contact under favourable conditions, we have
seen that with Vicia faba a little square of writing-
paper affixed with shellac sufficed to cause 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 of
dried shellac under 53,th of a grain in 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 g3,th of a grain.
Cuar. III. SUMMARY OF CHAPTER. — 196
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 uiti-
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 Cucurbita ovifera,
196 SUMMARY OF CHAPTER. Cuap. IIL,
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 adjoming 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 larve 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»eiten Bot. Institut., Wiirzburg, Heft iv. 1874, pp. 605-68L
Cuap, III. 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 le
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
erevices 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
eracks, or into burrows made by earth-worms or larve.
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 SUMMARY OF CHAPTER. Cuar. Uf
to be doubled up into a contorted mass. But we have
seen with radicles growing down inclined plates of
glass, that as soon as the tip merely touched a slip of
wood cemented across the plate, the whole terminal
growing part curved away, so that the tip soon stoou
ut 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 Aisculus, 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
* ¢ Arbeiten Bot. Inst. Wurzburg, Heft ili. p. 456.
Crap. IIL SUMMARY OF 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 with a burrowing animal such as a mole,
which wishes to penetrate perpendicularly down into
the ground. By continually moving his head from
side to side, or circumnutating, he will feel any stone
14
200 SUMMARY OF CHAPTER. Cuar. 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 better 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.
Cuar. LY, OIRCUMNUTATION 201
CHAPTER [Y.
Virs CiRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF
MatTuRE PLANTS.
Circumnutation of stems: concluding remarks on—Circumnutation of
stolons: aid thus afforded in winding amongst the stems of sur-
rounding plants—Cireumnutation of flower-stems—Circumnutation
of Dicotyledonous leaves—Sinvular oscillatory movement of leaves
of Dionzea— Leaves of Cannabis sink at night—Leaves of Gymno-
» sperms—Of Monocotvledons—Cryptogams—Concluding remarks
on the circumnutition 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 CIRCUMNUTATION OF STEMS. Cuar. 1V
were sclected 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.) Jberis umbellata (Cruciferze, 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
Fig 70.
Tveris 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 magnifiel
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
{imes. In part of its course an irregular ellipse, or rather
triangle, was completed in 6 h. 80 m. .
(2.) Brassica oleracea (Cruciferee).—A very young plant, bearing
three leaves, of which the longest was only three-quarters of an
inch in length, was placed under a microscope, furnished with
an eye-piece micrometer and the tip of the largest leaf was
Cuap. IV. CIRCUMNUTATION OF STEMS. 203
found to be in constant movement. It crossed five divisions of
the micrometer, that is, ;4,th of an inch, in 6 m. 20s. 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 (Line, Fam. 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.) Pelaryonium zonale (Geraniaces, Fam. 47).—A young
plant, 74 inches in height, was observed in the usual manner ;
but, in ordcr to see the bead at the end of the glass filament
Fig. 71.
4! iam
cs ee
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. oa
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
{he 7th of March to 10.30 p.m. on the 8th, the stem moved
a considerable distance in a zigzag line in the same general
direction. During the night of the 8th it moved to some
distance at right angles to its former course, and next morning
(9th) stood for a time almost still. At noon on the 9th a new
tracing was begun (see Fig. 71), which was continued till 8 a.m.
on the 11th. Between noon on the 9th and 5 p.m. on the 10th
(i.e. in the course of 29 h.), the stem described a circle. This
plant therefore circumnutates, but at a very slow rate, and to a
small extent.
(5.) Tropeolum majus (?) (dwarfed var. called Tom Thumb);
(Geraniacesxe, Fam. 47).—The species of this genus climb by the
204 CIRCUMNUTATION OF STEMS. Cuap. TY
aid of their sensitive petioles, but some of them also twine
round supports ; but even these latter species do not begin to
circumnutate in a conspicuous manner whilst young. The
Fig. 72.
=
mee
=e.
Tropeolum 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 treate1 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-
Fig. 73. 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 (Leguminose,
‘| Fam. 75). — When we
treat of the sleep of
plants, we shall see that
the stems in several
Leguminous genera, for
Trifolium resipinatun ; circumnutation OL ys eee ¢ Hede
stem, traced on vertical glass from 9.30 Instance, aOR 0 ec y-
AM. to 4.36 p.m. Nov. 3rd. Tracing not sarum, Mimosa, Meli-
areaely pazmie’ bs ce ee half “4 lotus, &¢., which are not
e 7 te . :
ie sag pal slog inate ws climbers, circumnutate
in aconspicuousmanner.
We will here give only a single instance (Fig. 73), showing
the cireumnutation of the stem of a large plant of a clover,
Trifolium resupinatum. Tn the course of 7 h. the stem changed
om
Cuar. IV CIRCUMNUTATION 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.
Co Fig. 74,
Ay
X
: \
&ubus (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) (Rosacese, Fam. 76).—As we hap-
pened to have a young plant, 11 inches Fig. 75.
in height and growing vigorously,
which had been: raised from a cross
between the raspberry (/wbus tdeeus)
and a North American Rubus, it was
observed in-the usual manner. During
the morning of March 14th the stem
almost completed a circle, and then
moved far to the right. At 4 p.m. it
reversed its course, and now a fresh
tracing was begun, which was con-
tinued during 403 h., and is given in
Fig. 74. We here have weli-marked
circumnutation. ;
(8.) Deutzia gracilis (Saxifrages,
Fam. 77).—A shoot on a bush about
18 inches in height was observed. The
bead changed its course greatly eleven p,.4,;q gracilis: circumnu-
times in the course of 10h. 30m. tation of stem, kept in
(Fig. 75), and there could be no _ darkness, traced on hori-
E be ie : zontal glass, from 8.30
doubt about the circumnutation of the Pe ee Shan March 20ch:
stein. ; Movement of bead origin-
9.) Fuchsia (greenhouse var, with ally magnified about 20
g uf $
large flowers, probably a hybrid) (Ona- nae reduced to
erarieee, Fam. 100).—A young plant, ua tats
15 ‘inches in height, was observed during nearly 48 h. The
206 CIRCUMNUTATION OF STEMS. Cuap. IV.
accompanying figure (Fig. 76) gives the necessary particulars,
and shows that the stem circumnutated, though rather
slowly.
Fuchsia (garden var.): circumnutation of stem, kept in darkness, traced on
horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of
bead originally magnified about 40 times, here reduced to half scale.
(10.) Cereus speciocissimus (garden var., sometimes called
Phyllocactus multiflorus) (Cactew, Fam. 109).— This plant
which was growing vigorously from having been removed a ~
few days before from the greenhouse to the hot-house, was
observed with especial interest, as it seemed so little probable
that the stem would circumnutate. The branches are flat, or
flabelliform; but some of them are triangular in section, with
the three sides hollowed out. A branch of this latter shape,
9 inches in length and 12 in diameter, was chosen for observa-
tion, as less likely to cireumnutate than a flabelliform branch.
The movement of the bead at the end of the glass filament,
affixed to the summit of the branch, was traced (A, Fig. 77)
from 9.23 a.m. to 4.380 p.m. on Nov. 28rd, 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 oftencr, 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, cireumnutated; but the
Cuap. LV. CIRCUMNUTATION OF STEMS. 207
extreme amount of movement during the time was very small,
probably rather less than the 5th of an inch.
Fig. 77.
Sain. 25 ih
ee
ver2us 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.
B.
(11.) Hedera ‘elix (Arvaliacee, 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, aud after an interval of 15 days the stems were
again observed during 2 days and their movements traced, aud
208 CIRCUMNUTATION OF STEMS. Cuap. IV
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.
- 6°45' aan. 22%"
SF am.215t
10° 37° m.21F
Gazania rinyens: circumnutation of stem traced from 9 A.M. March 21st .
to 6 P.M. on 22nd; plant kept in darkness. Movement of bead at the
close of the observations magnified 34 times, here reduced to half the
original scale.
main lines may be observed running at nearly right angles to
two other main lines; but these are interrupted by small
loops.
(13.) Azalea Indica (Ericineze, Fam. 128).—A bush 21 inches
in height was selected for observation, and the circumnutation
of its leading shoot was traced during 26 h. 40 m, as shown
in the following figure (Fig. 79).
(14.) Plumbago Cupensis (Plumbagines, 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 48 h. 40 m. (see Fig. 80).
During the first 2 h. it followed nearly the same diréction as
before. and then changed it a little; during the night it
moved at nearly right angles to its previous course. Next
Cuar. IV CIRCUMNUTATION 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.
eo
Azalea Indica; circumnutation
of stem, illuminated from PS
above, traced on horizontal if HEE
glass, from 9.30 A.M. March
9th to 12.10 p.M. on the 10th.
But on the morning of the Plumbago Capensis: cireumnu-
10th only four dots were
made between 8.30 A.M.
and 12.10 P.M., both hours
included, so that the circum-
tation of tip of a lateral
branch, traced on horizontal
glass, from 7.20 P.M. on
March 7th to 3 P.M. on the
nutation is not fairly repre- 9th. Movement of bead
sented in this part of the magnified 13 times. Plant.
diagram. Movement of the feebly illuminated from
bead here magnified about above.
oO times.
circumnutate over the same small space, which was only about
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
maenified.
(15.) Aloysia citrivdora (Verbenaccee, Fam. 173).—The follow-
ing figure (Fig. 81) gives the movements of a shoot during
210 CIRCUMNUTATION OF STEMS. Cuap. IY.
31 h. 40 m., and ehows that it cireumnutated. The bush was
15 inches in height.
Fig. 81,
>
rege
Aloysia citriodora: circumnutation of stem, traced from 8.20 a.m. on March
22nd te 4p.M.on 23rd. Plant keptin darkness. Movement magnified
about 40 times.
(16.) Verbena melindres (?) (a scarlet-flowered herbaceous vat.)
(Verbenaceze).—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
13 inches. A glass filament, with a bead at the end, was fixed
Fig. 82.
“am. 7
6'50'ame*
Verbena melindres: circumnutation of stem in darkness, traced on vertical
glass, from 5.30 P.M. on June 5th to 11 a.m. June 7th. Movement of
bead magnified 9 times,
upright to the tip, and its movements were traced during
41 h. 30 m. on a vertical glass (Fig. 82). Under these cireum-
stances the lateral movements were chiefly shown: but as the
lines from side to side are not on the same level, the shaot
Cuap. IV. CIRCUMNUTATION OF STEMS. DY
must have moved in a plane at right angles to that of the lateral
movement, that is, it must have circumnutated. On the next day
(6th) the shoot moved in the course of 16 h. four times to the right,
and four times to the left; and this apparently represents the
formation of four ellipses, so that each was completed in 4 h.
(17.) Ceratophyllum demersuwm (Ceratophyllee, Fam. 220).—An
interesting account of the movements of the stem of this water-
plant has been published by M. E. Rodier.2. 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 3h. 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. Rodier 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. Rodier says: “Il est alors facile de voir que
le mouvement de flexion se produit d’abord dans les mérithalles
supérieurs, qu'il se propage ensuite, en s'amoindrissant du huut
en bas; tandis qu'au contraire le mouvement de redressement
commence par la partie inférieure pour se terminer & la partie
supérieure qui, quelquefois, peu de temps avant de se relever
tout & fait, forme avec l’axe un angle trés aigu.”
(18) Conifercee.—Dr. Maxwell Masters states (‘ Journal Linn
Soc.,’ Dec. 2nd, 1879) that the leading shoots of many Conifers
during the season of their active growth exhibit very remark-
able movements of revolving nutation, that is, they cireumnu-
tate. We may feel sure that the lateral shoots whilst growing
would exhibit the same movement if carefully observed.
* ‘Comptes Rendus,’ April 20th. 1877. Also a second unutice
published separately in Bourdcaux, Noy. 12th, 1877.
912 CIRCUMNUTATION OF STEMS. Cuap. LV
19.) Lilium auratum (Fam. Liliacee).—The. circumnutation
Lilium aurctun: cireumnutation 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 18h. 15m. are represented by a long broken line. Diagram reduced
to half original scale. .
of the stem of a plant 24 inches in height is represented in the
above figure (Fig. ¢3).
Fig. 48.
-
--
-
--
-
Oyperus alternifolus : circumnutation of stem, illuminated from above,
traced on horizontal glass,.from 9.45 A.M. March 9th to 9 p.m. on 10th
‘The stem grew so rapidly whilst being observed, that it was not possible
to estimate how much its movements were magnified in the tracing.
(20.) Cyperus alternifolius (Fam. Cyperaceze.)— A glass
Cuar. 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
30 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 toa 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 cireumnutating
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 CIRCUMNUTATION OF STOLONS. Cuap. LV
fore, sooner or later, bend to all sides; but after a
stem has bent in any one direction, it commonly
bends back at first in nearly, though not quite, the
opposite direction; and this gives the tendency to
the formation of ellipses, which are generally narrow,
but not so narrow as those described by stolons and
leaves. On the other hand, the figures sometimes
approach in shape to circles. Whatever the figure
may be, the course pursued is often interrupted by
zigzags, small triangles, loops, or ellipses. A stem
may describe a single large ellipse one day, and
two on the next. With different plants the 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
rics (ay.
CIRCUMNUTATION OF STOLONS OR RUNNERS.
Stolons consist of much elongated, flexible branches,
which run along the surface of the ground and form
roots at a distance from the parent-plant. They are
therefore of the same homological nature as stems;
and the three following cases may be added to the
twenty previously given cases.
Fragaria (cultivated garden var.): Rosacee.—A plant growing
in.a pot had emitted a long stolon; this was supported by a
stick, so that it projected for the length of several inches 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 12h. 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 following
Crap. 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.
2 645'am19%
10° pm<
IP45 MN
1s
7°45' am19™
Fraga 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 14h. 30m.
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 acted on by geotropism, but only
Naturliche wagerechte Richtung after a considerable interval of
von Pflanzentheilen,’ 1870, p.20) time.
that the stolons of this plant are
15
216 CIRCUMNUTATION OF STOLONS. Cuap. 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-
Fig. 86.
Tam.20. i
nrp-m.20"\h Sal
+ 8°.m.21%
Fragaria : circumnutation of the same stolon
as in the last figure, observed in the same
manner, and traced from 8 A.M. May 19th
to 8 A.M. 21st.
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 sue-
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
the 20th and 8 A.m. on the
J1st a distance of *82 inch.
A younger and shorter
stolon was supported so
that it’ projected at about
45° above the horizon, and its movement was traced by the
same orthogonal method. On the first day the apex soon
rose above the field of vision. By the next morning it had
sunk, and the course pursued was
now traced during 14h.
30 m. (Fig. 87). The amount of movement was almost the same,
Guar (V. CIRCUMNUTATION OF STOLONS. ap WF
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 10.80 p.m., the
Fig. 87.
1° 10'a.m.19"
LAT
Ny 20' pm
Fy 7°- 30.0.
cols ay
@——. .
2 LIN.
Fragavia: civcumnutation 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 Vries 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 CIRCUMNUTATION OF STOLONS. Cuap IV,
gn a cloudy day, became distinctly curved towards the light, and
were therefore heliotropic. Close in front of the tips of tne
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 stil
straight. But to this subject we shall recur under Saxifraga.
Saxifraga sarmentosa (Saxifragee).—A plant in a suspended
pot had emitted long branched stolons, which depended like
Fig. 88. |
Sazifraga sarmentosa: circumnutation of an inclined stolon, traced in
darkness on a horizontal glass, from 7.45 A.M. April 18th to 9 A.M. on
9th. Movement of end of stolon magnified 2°2 times.
threads on all sides. ‘Two were tied upso as to stand vertically,
and their upper ends became gradually bent downwards, but se
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 174 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
Crap. 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, with
their longer axes somewhat differently directed, were almost
completed in the first 15h. 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
eurved 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. S }
% 3 H
BE, 3S Vy
Ly
=)
S b6°40'a.m 1444
oO th 9
10°35" ‘p.m.16" wS
é?
eS
10°45'p.m.14dh¢
Dianthus caryophylius: 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, 82 inches from the vertical glass, so tracing not
greatly magnified. The leaf was 53 incheslong. Temp. 153°-172° 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 23 days.
As th2 leaf continued to move all the time to the left, it is
evident that the zigzag line represents many circumnutations.
(6.) Camellia Japonica (Camelliacee, Fam. 32).—A youngish
leaf, which together with its petiole was 2% 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
CIRCUMNUTATION OF LEAVES.
Cuap. 1V
petiole very short, much movement could not be expected
Camellia Japonica: cir-
cumnutation of leaf,
traced from 6.40
A.M. June 14th to
6.50 "A.M tosh:
Apex of leaf 12
inches from the ver-
tical glass, so figure
considerably mag-
nified. Temp. 16°
163° C.
Nevertheless, the apex changed its course
completely seven times in the course of
113 h., but moved to only a very small
distance. On the next day the movement
of the apex was traced during 26 h, 20 m.
(as shown in Fig. 97), and was nearly of
the same nature, but rather less complex.
The movement seems to be periodical, for
on both days the leaf circumnutated in the
forenoon, fell in the afternoon (on the first
day until between 3 and 4 p.m., and on the
second day until 6 P.m.), and then rose,
falling again during the night or early
morning.
In the chapter on the Sleep of Plants
we shall see that the leaves in several Malvaceous genera sink
Fig. 98.
9°. 30’ a.m.14th
¢ 10° 45.p.m.
y
BR H
6°50'a.mi6eh 6°50" a.m. 15
ey
YG
6°.30'p.m..16! |
10°.35'p.m.15¢
Pelargonium zonale: circumnutation and downward movement of young
leaf, traced from 9.30 A.M. June 14th to 6.30 p.m. 16th. Apex of leaf
94 inches from the vertical glass, so figure moderately magnified.
Temp. 15°-163° C.
at night; and as they often do not then occupy a vertical
position, especially if they have not been well illuminated during
Cuap. 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 zonale (Geraniacex, Fam. 47).—A young
leaf, 1¢ 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 discolur (Ampelides, Fam. 67).—A leaf, not nearly
full-grown, the third from the apex of Fig. 99.
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 faba (Leguminose, 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 514 h. The filament fell oe
all morning (July 2nd) till 3 p.m., and Dae eee ae
then rose greatly till 10.35 p.m.; but tation of leaf, traced
the rise this day was so great, com- ae 10.35 a.m. May
pared with that which subsequently hie i a £3 eee
occurred, that it was probably due in from the vertical glass.
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.
-
234 CIRCUMNUTATION OF LEAVES. Cuap. 1V
As the evening rise and the early morning fall were unusually
large, the angle of the petiole above the horizon was measured
at the two periods, and the leaf was found to have risen 19°
Fig. 100.
6245'a.m. 4th
7? 15'p. m.2nd
Vicia fubu: circumnutation of leaf, traced from 7.15 p.m. July 2nd te
10.15 a.m. 4th. Apex of the two terminal leaflets 73 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 22° 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
Cuap. TV. DICOTYLEDONS. . pe,
of the two terminal leaflets, which were 1°4 inch in length; and
the movements of one of them were traced during 48 h. (see
Fig. 101). ~The course pursued is closely analogous to that of
the whole leaf. The zigzag line between 8.30 a.m. and 3.30 P.M.
on the second day represents 5 very small ellipses, with their
Fig. 101.
10°30'am.6%
10°40'a.m. 4%
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 63 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
CIRCUMNUTATION OF LEAVES.
Cuapr. IV
(10.) Acacia retinoides (Leguminose).—The movement of a
young phyllode, 23 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 cireumnutation is shown
during only 21h. 30m. During part of
this time (viz., 14 h. 30 m.) the phyllode
Fig 102,
described a figure re-
om 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 pm., but there was
no evidence on either
day of a regular pe-
riodic movement.
(11.) Lupinus spe-
ciosus (Leguminose).
—Plants were raised
from seed purchased under this name.
This is one of the species in this large
genus, the leaves of which do not sleep
at night. The petioles rise direct from
the ground, and are from 5 to 7 inches
in length. A filament was fixed to the
midrib of one of the longer leaflets, and
the movement of the whole leaf was traced,
as shown in Fig. 103. In the course of
6 h. 80 m. the filament went four times up
and three times down. A new tracing
was then begun (not here given), and
during 123 h. the leaf moved eight times
up and seven times down; so that it
described 75 ellipses in this time, and
this is an extraordinary rate of movement.
The summit of the petiole was then secured
Acacia retinoides : cir-
cumnutation of a
young phyllode,
traced from 10.45
A.M. July 18th to
S15 - Ao. = 19th.
Apex of phyllode 9
inches from the
vertical glass; temp.
163°-172° C.
Fig. 103,
Lupinus speciosus: cit-
cumnutation of leaf
traced on vertical
glass, from 10.15 A.M.
to 5.45 P.M.5 1.
during 6 h. 30 m.
to a stick, and the separate leaflets were found to be continnally
circumnutating.
Cuap. IV. DICOTYLEDONS. 28
(12.) Echeveria stolunifera (Crassulacese, 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- Fig. 104. y
nutation could be detected. A fila-
ment was fixed to a young upwardly
inclined leaf, ‘75 inch in Jength 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 “cheveria stolonifera : circum-
scale nutation of leaf, traced
from 8.20 a.M. June 25th
(18.) Bryophyllum (vel Culanche) to 8.45 am. 28th. Apex
calycinum (Crassulacese).— Duval- of leaf 12} inches from the
Jouve (‘Bull. Soc. Bot. de France, — $1455, $0 ae eee
Feb. 14th, 1868) measured the dis- 930940
tance between the tips of the upper
pair of feaves 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 :—
1
1
'
1
1
i]
1
{
‘
'
8 A.M. 2 P.M. 7 P.M.
Nov. 16. PROM. Ss. vayoe peor, (4.4 <1 sen)
b)) 19 ° ° ° 48 9 e e ° 60 ” e . e 48 mm.
Da MY adie ee hae ae Ho ES Bales OA PUG RIIS" ies
We see from this Table that the leaves stood considerably
further apart at 2 p.m. than at either 8 a.m. or 7 p.m.; and this
shows that they rise a little in the evening and fall or open
in the forenoon.
(14.) Drosera rotundifolia (Droseracese, Fam. 85).—The move-
ments of a young leaf, having a long petiole but with its tentacles
(or gland-bearing hairs) as yet unfolded, were traced during
47h.15m. The figure (Fig. 105) shows that it circumnutated
largely, chiefly in a vertical direction, making two ellipses each
238 Cuap. 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
two occasions. We therefore
thought that the movement
was periodic; but on obsery-
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
CIRCUMNUTATION OF LEAVES.
Fig. 105.
O15 pm7'!
\10°40'pm:8?
9°15 a.m.7% |
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.
little in a slightly zigzag line.
On the following morning, at
7 AM., a drop of a solution
of carbonate of ammonia (2
er. to 1 oz. of water) was
placed on the disc, and this
blackened the glands and in-
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 3pm. 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 lead
Cuap. 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
wscertained.
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 =4, 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 <4, of an inch in
20 m., or ;4,5 of inch in 1 h. 40 m.; but as it likewise moved
from side to side to an extent of above <1, of inch, the move-
ment was probably one of modified circumnutation. >555 of an inch. During the next 73 h. it was looked at
repeatedly, and during this whole time it moved only another
zoos 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 muscipela (Droseracese).—It should be premised
that the leaves at an early stage of their development have the
two lobes pressed closely together. These are at first directed
back towards the centre of the plant; but they gradually rise up
and soon stand at right angles to the petiole, and ultimately in
nearly a straight line with it. A young leaf, which with the
240 CIRCUMNUTATION OF LEAVES. Cuap. IV
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 2h. On
Fig. 106. the following day (Sept. 25th) its move-
ments were traced during 22 h.; and we
see in Fig. 106 that it moved in the same
general direction, due to the straightening
of the leaf, but in an extremely zigzag line.
This line represents several drawn-out or
} modified ellipses. There can therefore be
A no doubt that this young leaf cireumnu-
/ 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
14h. 2) m. there was hardly any change
in the position of the filament. We may
ey therefore infer that an old and only
byte moderately sensitive leaf does not circum- .
nutate plainly; but we shall soon see
Donea Ha re that it by no means follows that such
cumnutation a : -
spony a) exiia mee a leaf is absolutely motionless. We may
leaf, traced on a hori- further infer that the stimulus from a
zontal glass in dark- touch does not re-excite plain circumnu-
ness, from noon Sept. tation
24th to 10 A.M. 25th.
Apex of leaf 134 Another full-grown leaf had a filameni
inches from the glass, attached externally along one side of the
so tracing consider- midrib and parallel to it, so that the fila-
abi eeei aed, ment would move if the lobes closed. Ii
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
of carbonate of ammonia is placed on the lobes, they close so
slowly that generally 24 h. is required for the completion of the
act. The above leaf was first observed for 2 h. 30 m., and did
not circumnutate, but it onght to have been observed for a
Cnap. LY. 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 or 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. 30m. 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.
48pm 40°45’ pm.254
; 5pm. Po a.m26"
\ G50.m.25 thy
Dionawa muscipula ; closure of the lobes and circumnutation of a full-grown
leaf, whilst absorbing an infusion of raw meat, traced in darkness, from
7.15 P.M. Sept. 24th to 9 a.m. 26th. Apex of leaf 84 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 stated in botanical works that the lobes close
or sleep at night; but this is an error. To test the statement,
very long glass filaments were fixed inside the two lobes of
three leaves, and the distances between their tips were measured
in the middle of the day and at night; but no difference could
be detected.
The previous observations relate to the movements of the
whole leaf, but the lobes move independently of the petiole, and
3
242 CIRCUMNUTATION OF LEAVES. Cuap. IY.
seem to be continually opening and shutting to a very small
extent. A nearly full-grown leaf (afterwards proved to be
highly sensitive to contact) stood almost horizontally, so that
by driving a long thin pin through the foliaceous petiole close
to the blade, it was rendered motionless. ‘The plant, with
a little triangle of paper attached to one of the marginal spikes,
was placed under a microscope with an eye-piece micrometer,
each division of which equalled =3, of an inch. The apex of
the paper-triangle was now seen to be in constant slight move-
ment; for in 4h. it crossed nine divisions, or 85 of an inch,
and after ten additional hours it moved back and had crossed
=35 In an opposite direction. The plant was kept in rather
too cool a place, and on the following day it moved rather less,
namely, =i, in 3 h., and <2, 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. =4, of
an inch) in 80m. 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 Dionza present the same
phenomenon, which is a wonderful one, as viewed under a low
power (2-inch object-glass), with an eye-piece micrometer of
which each division (3, of an inch) appeared as a rather wide
space. The young unexpanded leaf, of which the cireumnutating
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
Cuap. 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 +25 or zs, and in
one instance of +455 of an inch. After each jerk forwards, the
apex drew itself backwards with comparative slowness for part
of the distance which had just been gained; and then after a
very short time made another jerk forwards. Four 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 +45 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 1h. 30m., it was again oscillating.
The plant was taken back into the hot-house, and on the
following morning was seen to be oscillating, though not very
vigorously. Another old but healthy leaf, which was not in the
least sensitive to a touch, was likewise observed during two
days in the hot-house, and the attached filament made many
little jerks forwards of about +2,, or only +45 of an inch.
Finally, to ascertain whether the lobes independently of the
_ petiole oscillated, the petiole of an old leaf was cemented close
to the blade with shellac to the top of a little stick driven into
the soil. But before this was done the leaf was observed, and
found to be vigorously oscillating or jerking; and after it had
been cemented to the stick, the oscillations of about —2,. of
i900
an inch stil] continued. On the following day a little infusion
244 CIRCUMNUTATION OF LEAVES. Cuap. IV
of raw meat was placed on the leaf, which caused the lobes te
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 4,5 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, cr of a small animal
struggling to escape from some constraint.
(16.) Hucalyptus resinifera (Myrtaceze, 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
ies day it was observed more fre-
Eucalyptus resinifera : circumnu- : ote
tation of a leaf, traced, A, from quently; and two tracings were
6.40 A.M. to1 P.M. June 8th; made (see A and B, Fig. 108), as
Fae ae ar ee ie a a single one would have been too
bth Aper of lent 14] ches complicated. The apex changed
figures considerably magnified, its course 13 times in the course
of 16h., chiefly up and down, but
with some lateral movement. ‘The actual amount of movement
in any one direction was small.
(17.) Daklia (garden var.) (Composite, Fam. 122).—A fine young
Fig. 108.
Saar. IV: DICOTYLEDONS. 245
leaf 53 inches in length, produced by a young plant 2 feet high,
growing vigorously in a large pot, was directed at an angie ot
about 45° beneath the horizon. On June 18th the leaf descended
from 10 a.m. till 1135 a.m. (see Fig. 109); it then ascended
greatly till 6 p.w., this ascent being probably due to the light
Fig. 109.
2640 0.m19%
?°8'10'c.m
20%
>----
owe enone aese momen nar=
Dahha: circumnutation of leaf, traced from 10 A.M. June 18th to 8.10 a.m.
20th, but with a break of Lh. 40 m. on the morning of the 19th, as,
owing to the glass filament pointing too much to one side, the pot had
to ke 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 42 in a horizontal line.
coming only from above. It zigzagged between 6 P.m. and
10.35 p.m., and ascended a little during the night. It should be
remarked that the vertical distances in the lower part of the
diagram are much exaggerated, as the leaf was at first deflected
beneath the horizon, and after it had sunk downwards, the
filament pointed in a very oblique line towards the glass. Next
246 CIRCUMNUTATION OF LEAVES. Cuap. IV.
day the leaf descended from 8.20 a.m. till 7.15 p.., then zigzagged
and ascended greatly during the night. On the morning of the
20th the leaf was probably beginning to descend, though the
short line in the diagram is horizontal. The actual distances
travelled by the apex of the leaf were considerable, but could
not be calculated with safety. From the course pursued on the
second day, when the plant had accommodated itself to the light
from above, there cannot be much doubt that the leaves undergo
a daily periodic movement, sinking during the day and rising
at night.
(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 * which has been published, namely, that the leaves
sink at night and rise during the day. The leaves which
behaved in this manner had been kept for some days in a
northern room and had not been sufficiently illuminated. A
plant therefore was left undisturbed in the hot-house, and three
leaves had their angles measured at noon and at 10 p.m. All
three were inclined a little beneath the horizon at noon, but one
stood at night 2°, the second 21°, and the third 10° higher than
in the middle of the day; so that instead of sinking they rise
a little at night.
(19.) Cyclumen Persicwum (Primulaces, 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.
Cuar. IV. DICOTYLEDONS. 247
(20,) Allamanda Schottii (Apocynee, Fam. 144).—The young
leaves of this shrub are elongated, with the blade bowed so much
Fig. 110
6°45 ama”
& 50 ams’
eT a
9°40'a.m. oh
\
f \
| \
65pm 4h
Cyclamen Persicum: circumnutation of leaf, traced from 6.45 A.M June 22d
to 6.40 A.M. 5th. Apex 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 the
petiole—of a young leaf: 42 inches in length, stood at 2.50 p.m on
aly)
248 CIRCUMNUTATION OF LEAVES. Cuap. IV
Dec. 5th at an angle of 13° beneath the horizon, but by 9.30 p.m.
Fig. 111.
te
Petunia violacea: 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 tne
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 lea? 7 inches
from the vertical glass. Temp.
generally 173° C.
the blade had straightened itself
so much, which implies the
raising of the apex, that the
chord now stood at 387° above the
horizon, and had therefore risen
50°. On the next day similar
angular measurements of the
same leaf were made; and at
noon the chord stood 36° be-
neath the horizon, and 9.80 p.m.
33° above it, so had risen 394°.
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.) Wiyandia (Hydroleacee,
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
Guar. IV. DICOTYLEDONS. 249
(22.) Petunia violacea (Solanewe, Fam. 157).—A very young
leaf, only 2 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. 111), 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 tc become more and Mie tte
more horizontal as it grew older. The
angles which two older leaves formed
together, were measured in the even-
ing and about noon on 8 successive
days, and each night the angle de-
creased a little, though irregularly.
(23.) Acanthus mollis (Acanthacez,
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. A
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 Acanthus mollis: circumnuta-
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
the second day. ‘The leaves of an
allied species (A. spinosus) certainly
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 to one-
haif of original scale.
Temp. 15°-163° C.
tose every night ; and the rise between noon and 10.15 p.m,
when measured on one occasion, was 10°.
This rise was chiefly
250 CIRCUMNUTATION OF LEAVES. Crap, TV:
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 (Cannabinew, 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 8am. May 29th, and at
10.30 p.m. were considerably declined. On a subsequent day two
leaves stood at 2p.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 (Coniferse, 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 cireumnu-
tating. The same little plant, when grown to a height of 5 inches,
was again observed during four days. A filament was fixed
transversely to the apex of a leaf, one inch in length, and which
had already diverged considerably from its originally upright
position. It continued to diverge (see A, Fig. 118), and to
descend from 11.45 a.m. July 3lst to 6.40 a.m. Aug. Ist. On
August Ist it circumnutated about the same small space, and
again descended at night. Next morning the pot was moved
nearly one inch to the right, and a new tracing was begun (B),
From this time, viz., 7 A.M. August 2nd to 8.20 a.m. on the 4th
* We were led to observe this Flcra, 1879, p.66. We regret that
plant by Dr. Carl Kraus’ paper, we cannot fully understand parts
‘ Beitrage zur Kentniss der Bewe- _ of this paper.
gungen Wacksender Laubblatter,’
Cuap. IV. DICOTYLEDONS. © 251
the leaf manifestly cireumnutated. 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
W45'a.m., 318"
Fig. 113.
meEnn-mon=
A. B.
es
6°40 'cm.30
6°40’ amt1st 8° 20’ a.m.4
°
Pinus pinaster: circumnutation of young leaf, traced from 11.45 a.m.
July 31st to 8.20 a.m. Aug. 4th. At 7 A.M. Aug. 2nd the pot was
moved an inch to one side, so that the tracing consists of two figures.
Apex of leaf 143 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 CIRCUMNUTATION OF LEAVES. Cuap. IV.
quite fully grown, produced by a lateral shoot, on a young tree
3 feet in height, were observed during 29 h. (July 3lst), in the
same manner as the leaves of the previous species. Both these
leaves certainly circumnuta‘ed, making
Fig. 114. within the above period two, or two and
a half, small, irregular ellipses.
(26.) Cycas pectinata (Cycades, Fam
224).— A young leaf, 113 inches in
length, of which the leaflets had only
recently become uncurled, was observed
during 47h. 30m. The main petiole
was secured to a stick at the base of the
two terminal leaflets. To one of the
latter, 3% inches in length, a filament
was fixed; the leaflet was much bowed
downward, but as the terminai 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.
Gycas pectinata: circum-
nutation of one of the
terminal leaflets, traced es ae
ham 830 a he CIRCUMNUTATION OF LEAVES:
22nd to 8 a.m. June MoNOCOTYLEDONS.
24th. Apex of leaflet é beat
7% inches from the ver- (27.) Canna Warscewiczti (Cannacem,
tical glass, so tracing Fam. 2).—The movements of a young
not greatly magnified, Jeaf 8 inches in length and 33 in
and here reduced to :
one-third of original breadth, produced by a vigorous young
scale; temp. 19°-21°C, plant, were observed during 45 h,
50m.,as shown in Fig. 115. The pot
was slided about an inch to the right on the morning of the
llth, 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 5p.m., and ascended during the rest of the evening and ©
CuHap, IV.
part of the night.
MONOCOTYLEDONS. 253
On the evening of the 11th it circumnutated
on a small scale for some time about the same spot.
Fig. 115.
A. . ,
Canna Warscewiczti: 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 (Iridee, Fam. 10).—The movements
of a young leaf, rising 13 inches above the ven 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 8h. 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 (Amaryllides,
Fam. 11).—The leaves of this plant
are remarkable for their great length
and narrowness: one was measured
and found to be 53 inches long and
only 1-4 broad at the base.
Fig, 116,
¢
:
?
A
¢
s
7
Tris 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.
Whilst quite young they stand up
almost vertically to the height of about a fvot;
afterwards
254 CIRCUMNUTATION OF LEAVES. Cazaap. IV.
their tips begin to bend over, and subsequenily hang vertically
down, and thus continue to grow. A rather young leaf was
selected, of which the dependent tapering point was as yet only
52 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
23 days. During the first day (22nd) the tip travelled laterally
far to the left, perhaps in consequence of the plant having been
Fig. 117.
Crinum capense: circumnutation of dependent tip of young leaf, traced on
a bell-glass, from 10.30 P.M. May 22nd to 10.15 A.M. 25th. Figure not
greatly magnified.
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 3Pp.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,
Cuap. 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 cireumnutation,
was 34 inches in the course of 204 h. During the next 24h. it
travelled 23 inches. The circumnutating movement, therefore,
of this young leaf was strongly marked.
(30.) Paneratium littorale (Amaryllidex).—The movements,
much magnified, of a leaf, 9 inches in length and inclined at
about 45° above the horizon, were traced during two days. On
the first day it changed its course completely, upwards and
downwards and laterally, 9 times in 12 h.; and the figure traced
apparently represented five ellipses. On the second day it was
observed seldomer, and was therefore not seen to change its
course so often, viz., only 6 times, but in the same complex
manner as before. The movements were small in extent, but
there could be no doubt about the circumnutation of the leaf.
(381.) Imatophyllum vel Clivia (sp. ?) (Amaryllidez).—A long
elass 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.
_ (82.) Pistia stratiotes (Aroidee, 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 183° and 283°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,’ 1867, p. 327.
256 CIRCUMNUTATION OF LEAVES. Cuar. IV °
5 and 6p.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
dP.M.; at 540 pM. 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.
Pontede.ta (sp. ?): cireumnutat‘on of leaf, traced from 4.50 p.m. July 2nd
to 10.15 A.M. 4th. Apex of leaf 163 inches from the vertical glass, so
tracing greatly magnified. Temp. about 17° C., and therefore rather
too low.
Brazil) (Pontederiacexw, Fam. 46).—A filament was fixed across
the apex of a moderately young leaf, 73 inches in height, and
its movements were traced during 423 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
Cuar. IV. CIRCUMNUTATION OF CRYPTOGAMS. 207
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 moile (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 24h. We see in Fig. 119 that it
Fig. 119.
Nephrodium molle: circumnutation of rachis, traced from 9.15 A.M. May
28th to 9 A.M. 29th. Figure here given two-thirds of original scale.
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. Loomis 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 Asplentwm
trechomanes. They move.almost
as rapidly as the little leaflets
of Desmedium gyrans, alternately
backwards and forwards through
from 20 to 40 degrees, ina plane at
right angles to that of the frond.
The apex of the frond describes “ a
Jong and very narrow ellipse,’’ so
that it cireumnutates. But the
movement differs from ordinary
258 CIRCUMNUTATION OF CRYPTOGAMS. Cuap. IV
In the chapter on the Sleep of Plants the conspicuous circum-
nutation of Marsilea quadrt/oliata (Marsileacese, Fam. 4) will be
described.
It has also been shown in Chap. I. that a very young Sela-
ginella (uycopodiacex, 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 (Hepatice, Fam. 11, Muscales)—
The earth in an old flower-pot was
coated with this plant, bearing
gemme. A highly inclined frond,
which projected 8 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-
-
Fig. 120.
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 glass-plate in front,
that the movement of the end was
: ee: magnified about 40 times: Never-
Lunularia vulgaris: cireumnuta- = :
tion of a frond, traced from theless, we are convinced that onr
9 a.m. Oct 25th to 8 A.M.27th. 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,
circumnutation as it occurs only _ sufficient to excite motion for #
when the plant is exposed to the few minutes.”
light; even artificial light “is
Cuarp. IV. CIRCUMNUTATION OF LEAVES. 259
was highly inclined, and the pot stood in front of a north-east
window. During the five first days the frond moved downwards
or became less inclined; and the long line which was traced
was strongly zigzag, with loops occasionally formed or nearly
formed; and this indicated circumnutation. Whether the sink-
ing was due to epinastic growth, or apheliotropism, we do not
kuow. As the sinking was slight on the fifth day, a new tracing
was begun on the sixth day (Oct. 25th), and was continued
for 47 h.; itis here given (Fig.120). Another tracing was made
on the next day (27th) and the frond was found to be still cir-
curnutating, for during 14h. 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
4) 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
minute account of the curious, but less regular though constant,
movements of Spirogyra: during 24 h. the filament moved 4
times to the left and 3 times to the right, and he refers to a
movement at right angles to the above. The tip moved at the
rate of about 0'lmm. in five minutes. He compares the move-
ment with the nutation of the higher plants.}| 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.
CoNCLUPING REMARKS ON THE CIRCUMNUTATION
oF LEAVES.
‘The circumnutating movements of young leaves in
303 genera, belonging to 25 families, widely distributed
* ©Ueber die Bewegungen der 1880, vol. iii. p. 220) that the
Faden der Spirogyra princeps: movements of Spirulina, a mem-
Jahreshefte des Vereins fiir vater- _ ber of the Oscillatoriew, are closely
landische Naturkunde in Wiirt- analogous ‘*‘to the well-known
temberg,’ 1874, p. 211. rotation of growing shoots and
t Zukalalsoremarks(asquoted tendrils.”
in ‘Journal R. Mivroscop. Soc.,’
260 CIRCUMNUTATION OF LEAVES. Cuap. IV.
amongst ordinary and gymnospermous Dicotyledons
and amongst Monocotyledons, together with several
Cryptogams, have now been described. It would,
therefore, not be rash to assume that the growing
leaves of all plants circumnutate, as we have seen
reason to conclude is the case with cotyledons. The
seat of movement generally lies in the petiole, but
sometimes both in the petiole and blade, or in the
blade alone. The extent of the movement differed much
in different plants; but the distance passed over was
never great, except with Pistia, which ought perhaps
to have been included amongst sleeping plants. The
angular movement of the leaves was only occasionally
measured ; it commonly varied from only 2° (and 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
Cuap. IV. CIRCUMNUTATION OF LEAVES. 261
the next day. The course followed was curvilinear or
straight, or slightly or strongly zigzag, and little loops
or triangles were often formed. ay} SHOTIPUOT) *YI8TS 1496
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B30 - MODIFIED CIRCUMNUTATION. Cuap, VI
Averrhoa bilimbi (Oxalide).—It has long beer. 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 differ
essentially from the species of Oxalis. They differ, however, as
Mr. R. I. Lynch f has lately shown, in their spontaneous move-
ments being strongly marked. In the case of A. bilimbi, it isa
wonderful spectacle to behold on a warm sunny day the leaflets
one after the other sinking rapidly downwards, and again
ascending slowly. Their movements rival those of Desmodiwm
gyrans. At night the leaflets hang vertically down; and now
Fig. 132,
i
egao ——_ ~ SS
on Sh)
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QM Wis y
ak 4 4
SV WW,
Averrhox bilimbi: leaf asleep; drawing reduced.
thoy are motionless, 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 temperature,
the petiole of the leaf to be observed pointing straight at
the observer, being separated from him by a vertical pane of
glass. The petiole was secured so that the basal joint, or pul-
vinus, of one of the lateral leaflets was at the centre of a gradu-
ated are 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. Soce.,’ vol. xvi. 1877, p. 231.
Cuap. VII. SLEEP OF LEAVES. dol
midrib. This filament acted as an index; and as the leaf rose
and fell, rotating about its basal joint, its angular movement
Averrhoa 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 are. In order
B32. MODIFIED CIRCUMNUTATION. Cuar. VII
to avoid errors of parallax, all readings were made by looking
through a small ring painted on the vertical glass, in a line
with the joint of the leaflet and the centre of the graduated are.
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 de-
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 pM. the leaflet
formed an angle of 85° with the vertical, or was only 5° below
the hor:zontal; but in order that the diagram might get inte
our page, the leaflet is represented falling from 75° instead
of 85°. Shortly after 6 pM. it hung vertically down, and had
attained its nocturnal position. Between 6.10 and 6.85 p.m. it
performed a number of minute oscillations of about 2° each,
occupying periods of 4 or 5m. The complete state of rest of
the leaflet which ultimately followed is not snown 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 25m. A blind was then pulled up so that the plant was
brightly illuminated (BR in Fig. 134), and within a minute it
began to fall, and ultimately fell 47°, as shown in the diagram.
This descent was performed by six descending steps, precisely
similar to those by which the nocturnal fall is effected. The
plant was then again shaded (SH), and a long slow rise occurred
until another series of falls commenced at BR’, when the sun
was again admitted. In this experiment cool air was allowed
to enter by the windows being opened at the same time that
he 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 1s
* In all the diagrams 1 mm.in ment. In Figs. 133 and 134 the
the horizontaldirection represents temperature is represented (along
one minute of time. Each mm. _ tie ordinates) in the scale of 1
in the vertical direction repre- mm. to each 0°'1°C. In Fig.
sents one degree of angular move- 135 each mm. equals 0: 2° F.
Cuap. VII. SLEEP OF LEAVES. Bae
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
Fig. 134.
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Averrhoa bilimbi: 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 akove
diagram. Thus, when the temperature was between 31° end
MODIFIED CIRCUMNUTATION. CHAP.
Fig. 135.
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Cuap. Vil. SLEEP OF LEAVES. 333
32° C., 14 oscillations of a few degrees occurred in 19m. On
the other hand, an oscillation may be much slower ; thus a leaflet
was observed (temperature 25° C.) to
rise during 40 m. before it fell and Fig. 136.
completed its oscillation.
Povrlieria hygrometrica (Zy gophylles)
—The leaves of this plant (Chilian
form) are from 1 to 13 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 ona bush from Chili,
which was sent to us from Kew, bore
many leaflets, whilst those on plants
in the Botanic Garden at Wirzburg
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 horizontaily
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 vight vertically depressed,
but only highly inclined. In one
instance we found a branch which had Policria hygrometrica: cir-
grown perpendicularly downwards, Cu™Butation and nycti-
: : : tropic movements of pe-
and the petioles on it moved in the same tile of leaf, traced from
direction relatively to the branch as 9.35 a.m. July 7th to
just stated, and therefore moved up- ae seopes ae ae
wards. On horizontal branches the 5 yo. ae =e ae
younger petioles likewise move at night glass. Temp. 192°-203°C,
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
330 MODIFIED CIRCUMNUTATION. Cuap. 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 earih 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 contatt 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 Leguminosa,
but are separated by an open furrow; nor could they exactly
coincide, as they stand alternately with respect to one another.
The circumuutation of the petiole of a leaf # of an inch in
length, on an upright branch, was observed during 36 h.,
and is shown in the preceding diagram (Fig. 186). On the
first morning, the leaf fell a little and then rose until 1pm,
and this was probably due to its being now illuminated through
a skylight from above; it then circumnutated on a very small
scale round the same spot until about 4 p.m., when the great
evening fall commenced. During the latter part of the night or
very early on the next morning the leaf rose again. On the
second day it fell during the morning till 1 p.m., and_ this no
doubt is its normal habit. From 1 to 4 p.m. it rose in a zigzag
line, and soon afterwards the great evening fall commenced. It
thus completed a double oscillation during the 24 h.
The specific name given to this plant by Ruiz and Pavon, 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 Wiirzburg, there was a plant in a pot out of doors
which was daily watered, and another in the open ground which
was never watered. After some hot and dry weather there was
a great difference in the state of the leaflets on these two plants;
those on the unwatered plant in the open ground remaining half,
* «Systema Veg. Flore Peru-
vianze et Chilensis,’ tom. i. p. 93,
1798. We cannvt understand the
account given by the authors of
the behaviow: of this plant in its
native home There is much
.
about its power of foretell ng
changes in the weather; and it
appears as if the brightness of the
sky largely d.termined the oper
ing and closing of the leatlets.
Cuap. VIL. SLEEP OF LEAVES. ool
or even quite, closed during the day. But twigs cut from this
bush, with their ends standing in water, or wholly immersed in
it, or kept in damp air under a bell-glass, opened their leaves
though exposed to a blazing sun; whilst those on the plant
in the ground remained closed. The leaves on this same plant,
after some heavy rain, remained open for two days; they then
became half closed during two days, and after an additional
day were quite closed. This plant was now copiously 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 egondition 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
ithe 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
aroad. On the 29th, when the bush was shaken, some Icaves
fell off, and the remaining ones were unable to sleep at night.
Tt was therefore moderately watered, as well as syringed, late in
the evening. On the next morning (80th) the bush looked as fresh
as ever, and at night the leaves went to sleep. It may be added
thut a small branch while growing on the bush was enclosed,
by means of a curtain of bladder, during 18 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 CIRCUMNUTATION. Cuar. VU
least, and did not close at all during the hottest days. Another
trial was made with the same bush on August 2nd and 6th (the soil
appearing at this latter date extremely dry), for it was exposed
out of doors during the whole day to the wind, but the leaflets
showed no signs of closing. ‘The Chilian form therefore differs
widely from the one at Wiirzburg, in not closing its leaflets
when suffering from the want of water; and it can live for a
surprisingly long time without water.
Tropeolum majus (?) (cultivated var.) (Tropseolese).—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 _
inherited than that of sinking or sleeping at night. The move-
ments are caused by the bending of an upper part of the petiole,
between 3 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. 38rd 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,
becume 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
Cuap. VIL. SLEEP OF LEAVES. 309
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 10.80 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 3h. 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. 40 m. and thus illuminated (though not
brightly, as the sky was cloudy during the whoie 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. 45m. 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 25m., 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;
340 MODIFIED CIRCUMNUTATION. Crap. VIL.
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 same leaves faced
the light as formerly, and these now went to sleep in the usual
manner. We will only add that with some young seedlings
kept in the greenhouse, the blades of the first pair of true leaves
(the cotyledons being hypogean) stood during the day almost
horizontally and at night almost vertically.
A few observations were subsequently made on the 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.
Leguminose.—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 ite
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. 187), of a leaf of Z. pilosus, as seen during the
day from vertically above, and of another leaf asleep with the
leaflets inclined downwards. As in this position they are
crowded together, and as they do not become folded like those
in the genus Oxalis, they cannot occupy a vertically dependent
position ; but they are often inclined at an angle of 50° beneath
the horizon. In this species, whilst the leaflets are sinking,
the petioles rise up, in two instances when the angles were
measured to the extent of 23°. The leaflets of L. sub-carnosus and
arboreus, Which were horizontal during the day, sank down at
night in nearly the same manner; the former to an angle of 38°,
and the latter of 36°, beneath the horizon; but their peticles
Cur. VI. = SLEEP OF LEAVES. 341
did not move in any plainly perceptible degree. It-is, however,
quite possible, as we shall presently see, that if a large number .
of plants of the three foregoing and of the following species
Fig. 137.
Ie
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 ZL. Hartwegit 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 ZL. 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 theirown 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 CIRCUMNUTATION. Cuap. VI".
base of each leaflet is developed (at least in the case of L. luteus)
_into a pulvinus. The result is that all the leaflets on the
same leaf stand at night more or less highly inclined, or even
quite vertically, forming in this latter case a vertical star. This
occurs with the leaves of a species purchased under the name of
Fig. 138
C,
Lupinus pubescens: A, leaf viewed laterally during the day; B, same leaf
at night; C, another leaf with the leaflet forming a vertical star at
night. Figures reduced.
I. pubescens ; and in the accompanying figures we see at A (Fig.
138) the leaves in their diurnal position; and at B the same
plant at night with the two upper leaves having their leaflets
almost vertical. At C another leaf, viewed laterally, is shown
with the leaflets quite vertical. It is chiefly or exclusively the
youngest leaves which form at night vertical stars. But there
Cuap. VIL 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 al/ their leaflets sloping downwards, as in our first
class of cases. It is also a remarkable fact, that although all the
plants produced from the same lot of seeds were identical in
appearance, yet some individuals at night had the leaflets of all
their leaves arranged so as to form more or less highly inclined
stars; others had them all sloping downwards and never forming
a star; and others, again, retained them either in a horizontal
position or raised them a little. _
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 80°. The
petiole of another leaf, the leaflets of which occupied 1 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 theiz
leaflets sloping downwards at 46° beneath the horizon, but
their petioles had hardly moved. Again, L.lut-ws 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 mght 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, Menziesti, speciosus,
23
344 MODIFIED CIRUUMNUTATION. Cuap. VIL
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 Tropeevlum majus, the
leaves must be well illuminated during the day in order to s!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 arbireus. It will suffice to say that the
leaflets of the latter exhibited a double oscillation in the course
of 24h.; for they fell from the early morning until 1015 4.m.,
then rose and zigzagged greatly till 4p.m., after which hour the
great nocturnal fall commenced. By 8 a.m. on the following
morning the leaflets had risen to their proper height. We have
seen in the fourth chapter, that the leaves of Lupinus speciosus,
which do not sleep, circumnutate to an extraordinary extent,
making many ellipses in the course of the day.
Cytisus (Tribe 2), Trigonella and Medicago (Tribe 3).—Only
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 Oytisus
fragrans rose at night, on one occasion 23° and on another 33°.
The three leaflets als) bend upwards, and at the same time
Omar. VIL 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 much that they press against the stem; and on looking
down on one of these j)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.
Trigonella Cretica resembles a Melilotus in its sleep, which will
be immediately described. According to M. Royer,* the leaves
of Medicago maculata rise up at night, and “se renversent un
peu de maniére 4 presenter obliquement au ciel leur face in-
ferieure.” A drawing is here given (Fig. 1389) of the leaves
of M. marina awake and asleep; and this would almost serve
for Cytisus fragrans in the same two states.
Melilotus (Tribe 3).—The species in this genus sleep in a
remarkable manner. The three leaflets of each leaf twist through
an angle of 90°, so that their blades stand vertically at night
with one lateral edge presented to the zenith (Fig. 140). We
shall best understand the other and more complicated 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 cf
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.’ (Sth series), ix. 1868, p. 368.
346 MODIFIED CIRCUMNUTATION. Cuap. VIL
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
Fig. 140.
Melilotus officinalis: A, leaf during the daytime. B, another leaf asleep.
C, a leaf asleep as viewed from vertically above; but in this case the
terminal leaflet did not happen to be in such close contact with the
lateral one, as is usual.
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 and bend in the same direction; for
Cuar. VII. SLEEP OF LEAVES. 347
oue 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, infesta, dentutu, gracilis, sulcata, elegans, ceerulea,
petitpicrreana, mucrorrhiza, Itulies, secundiflura, and Taurica,
slee} 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 I. petitpierreaia and
secundiflora the terminal leaflet was rarely seen to bend to one
side. In young plants of .\/. /‘alica it bent in the usual manner,
but with old plants in full flower, growing in the same pot and
observed at the same hour, viz., 8.30 p.m., none of the terminal
leaflets on several scores of leaves had bent to one side, though
they stood vertically ; nor nad the two lateral leaflets, though
standing vertically, moved towards the terminal one. At
1030 p.m, and again one hour after midnight, the terminal
leaflets had become very slightly bent to one side, and the
lateral leaflets had moved a very little towards the terminal one,
so that the posit on of the leaflets even at this late hour was far
from the ordinary one. Again, with M. Jaurica the terminal
leaflets were never seen to bend towards either of the two lateral
leaflets, though these, whilst becoming vertical, had bent towards
the terminal one. The sub-petiole of the terminal leaflet in
this species is of unusual length, and if the leaflet had bent to
one side, its upper surface could have come into contact only
with the apex of either lateral leaflet; and this, perhaps, is the
meaning of the loss of the lateral movement.
The cotyledons do not sleep at night. ‘The first leaf consists of
a single orbicular kaflet, which twists at night so that the blade
stands vertically. It is a remarkable fact that with /. Taurica,
and in a somewhat less degree with M. macrorrhiza and _ petit-
pierrcana, all the many small and young leaves produced during
348 MODIFIED CIRCUMNUTATION. Cuap. VIL
the early spring from shoots on some cut-down plants in the
greenhouse, slept in a tutally 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. mcs:anensis, the leaves of
which, on full-grown plants between 2 and 8 feet in height,
sleep like the foregoing small leaves and like those of a Trifolium.
We were so much surprised at this latter case that, until the
flowers and fruit were examined, we thought that the seeds of
some Trifolium had been sown by mistake instead of those of a
Melilotus. It appears therefore probable that MM. 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
4p.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 stnall scale, of the main petiole which moved most, and of
the sub-petiole of the terminal leaflet. The main petiole of a
leaf having been secured to a stick, close to the base of the sub-
petiole of the terminal leaflet, the latter described two small
ellipses between 10.30 a4.m.,and2pm. At 7.15 p.m., after ths
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 10.85 p.m., after which hour ee were
no longer observed.
As M. messunensis sleeps in an anomalous manner, unlike that
of any other species in the genus, the circumnutation of a
terminal leaflet, with the stem secured, was traced during two
days. On each morning the leaflet fell, until about noon, and
then began to rise very slowly; but on the first day the rising
movement was interrupted between 1 and 3 p.m. by the formation
of a laterally extended ellipse, and on the second day, at the
game time, by two smaller ellipses. The rising movement then
Cuar. VIL SLEEP OF LEAVES. 349
recommenced, and bcame 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 7. repens having an upright petiole, and with
the three leaflets expanded horizontally, the two lateral leaflets
will be seen in the evening to twist and approach each other,
until their upper surfaces come into contact. At the same time
they bend downwards in a plane at right angles to that of their
former position, until their midribs form an angle of about 45°
with the upper part of the petiole. ‘This peculiar change of
position requires a considerable amount of torsion in the pul-
vinus. The t.rminal leaflet merely rises up without any twist-
Fig. 141.
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
termiua] Jeaflet 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 casc the terminal
leaflet stands at night horizontally (as in Fig. 141), with its
lower surface fully exposed tothe 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 CIRCUMNUTATION. Cuap. VIL
With 7. Punnonicum the first true leaf was generally unifoliate,
but sometimes trifoliate, or again partially lobed and in an
intermediate condition.
Circumnutation.—Sachs described in 1868* the spontaneous
up and down movements of the leaflets of 7. ‘ncarnatum, when
kept in darkness. Pfeffer made many observations on the
similar movements in 7. pratense.t He states that the terminal
leaflet of this species, observed at different times, passed through
angles of from 80° to 120° in the course of from 13 to4h. We
observed the movements of 7. subterranewm, resupinatum, and
repens.
Trifolium subterranervm.—A petiole was secured close to the
base of the three leaflets, and the movement of the terminal
leaflet was traced during 263 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 38 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 continned
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 httle 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.38 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!ongation 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,’ 1863, p. 497.
t ‘Die Period. Bewegungen, 1875, pp. 35, 52.
Cuap. VII. SLEEP OF LEAVES. 351
Trifolium resupinatum.—A plant left entirely fre. was placed
Fig. 142.
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
7.00 Am. 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,
?
), traced from
al glass, and movement, as here shown
Plant illuminated from above; temp. 16°-17° C.
Z inches from the vertic
circumnutation and nyctitropic movement of terminal leaflet (*68 inch in length
1 scale.
6.45 a.m. July 4th to 9.15 a.m. Sth. Apex of leaf 3
magnitied 53 times, reduced to one-half ot origina
Trifolium subterraneum :
o0e MODIFIED CIRCUMNUTATION. Cuap. VIL
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 TZ. subterraneum.
But we should remember
that it was shown in the
fourth chapter that the
stem circumnutates, as no
doubt does the main petiole
and the sub-petioles; sa
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
ae NE . began to go to sleep after
rend uyctitrepie movements of the ter. 2: 15 70., andi dials
minal 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. Ist (Fig. 144), the leaflet began to rise, but
the movement was slow on both days, both before and after
this hour, until4 pm. The rapid evening and nocturnal rise
then commenced. Thus in this species the course during 24h.
consists of a single great ellipse; in 7. reswpinatum 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.
Securigera coronilla (Tribe 4).—The leaflets, which stand
opposite one another and are numerous, rise up at night, come
into close contact, ard bend backwards at a moderate. angle
towards the base of the petiole.
Cuar VII.
SLEEP OF LEAVES.
dod
Lotus (Tribe 4).—The nyctitropic movements of 10 species
in this genus were observed, and found to be the same,
main petiole rises a little at night, and
the three leaflets rise till they become
vertical, and at the same time approach
cach other. This was conspicuous with
I. Jacobeus, 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 leaficts of Z. Gebelit, 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. perigrinus (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.
The
Fig. 144,
CaN
\
1
i
1
'
'
1
Y
{
t
1
!
t
I
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.
It occasionally stood still for about 20 m. during the day, and
sometimes zigzageged a little.
The movement of one of the
basal, stipule-like leaflets was likewise traced in the same
manner and at the same time, and its course was Closely similar
to that of the terminal leaflet.
In Tribe 5 of Bentham and Hooker, the sleep-movements
of species in 12 genera have been observed by ourselves and
304 MODIFIED CIRCUMNUTATION. Coar VU
others, but only in Robinia with any care. /soralea acaulis
raises its three leaflets at night; whilst Amorpha fruticosu,*
Dalea alopzcuroid.s, and Indigofera tinctoria depress them.
Duchartre ¢ states that Zephrosia caribeea is the sole example
of “ folicles couchées le long du petiole et vers la base;” but a
Fig. 145.
A. B.
Lotus C: eticus: 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. Wéistaria Sinensis, according to
Royer,t “ abaisse les folioles qui par une disposition bizarre
sont inclinées dans la méme feuille, les supérieures vers le
* Ducharte, ‘lémentg de + ‘Ann. des Sciences, Nats,
B.tanique, 1867, p. 349. Bot. (Sth series), ix. 1868.
+ Ibid., p. 347.
Cuap. VII. SLEEP OF LEAVES. 355
sommet, les inférieures vers la base du petiole commun;” 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
house bush) is a hybrid raised in as C. Herbertiana,
France, and that it cames very
370 MODIFIED CIRCUMNUTATION. Cuap. 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 durmg day; B, same flant at night.
Both figures copied from fhotographs.,
ln one instance we found that the midrib of a terminal
leaflet formed at night an angle of 36°, with a line dropped
Onap. VIL. SLEEP OF LEAVES. one
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 calliantha.—The leaves bear a large number of leaflets,
which move at night in nearly the same manner as just
described; but the petioles apparently do not rise, and one
which was carefully observed certainly fell 3°.
Cassia pubescens.— The chief difference in the nyctitropic
Fig. 155.
ne WY
\
Cassia pubescens: A, upper part of plant during the day; B, same p ant
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;
EIA MODIFIED CIRCUMNUTATION. Cuar VIL
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 mimosvides.— 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. a
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,
calliantha, and pubescens were observed, each during three or four ~
days; they were essentially alike, those of the last-named species
being the simplest. The petiole of C. floribunda was secured to
a stick at the base of the two terminal leaflets, and a filament
was fixed along the midrib of one of them. Its movements were
traced from, 1P.m. on August 18th to 8.30 a.m. 17th; but those
during the last 2 h. are alone given in Fig. 156. From 8 a.m. or
each day (by which hour the leaf had assumed its diurnal posi-
tion) to 2 or 38 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
Cuap. VIL. SLEEP OF LEAVES. 373
perpenticularly down, the movement of its apex was traced
until 10.80 p.m.; and during this whole time it swayed from
side to side, completing more than one ellipse.
Bauhinia (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
leaves are large and deeply
notched at their ends. At
nivht the two halves rise
up and close completely
together, like the opposite
leaflets of many Legumi-
nose. 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 80°; another
rose 34°. Whilst the two
halves of the leaf are closing,
the midrib at first sinks ;
vertically downwards and a
afterwards bends _back- ;
wards, so as to pass close A
inch in length)
Apex of leaflet 54 inches from the
5
6
z inches long, Temp. 16°-173°C. Figure reduced to one-half
circuninutation and nyctitropic movement of a terminal leaflet (1
Main petiole 3
traced from-8.30 A.M. to same hour on following morning.
along one side of its own 3
upwardly inclined petiole; i Seo =
the midrib being thus di- ie ae
rected towards the stem or oy = Er z
axis of the plant. The angle aoe mo: 2
which the midrib formed 3 ae
with the horizon was mea- y 8
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°
3/4 MODIFIED CIRCUMNUTATION. Cuap. VIL
Owing to this movement—to the leaves being folded—and to
the petioles rising, the whole plant is as much more compact at
night than during the day, as a fastigiate Lombardy poplar is
compared with any other species of poplar. It is remarkable
that when our plants had grown a little older, viz., to a height
of 2 or 3 feet, the petioles did not rise at night, and the midribs
of the folded leaves were no longer bent back along one side of
the petiole. We have noticed in some other genera that the
petioles of very young plants rise much more at night then do
those of older plants.
Tamarindus Indica (Tribe 16).—The leaflets approach or
rneet 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 Hematoxylon (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 pinne 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 pinne 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 inthe morning. The stem is continually cireumnu-
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
12h. 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.
Cnar. VII. SLEEP OF LEAVES. ote
A filament had been fixed on the previous evei.ing, 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 ona
vertical glass in the hot-house under a high temperature. In
the figure given (Fig. 157), the
first dot was made at 8.30 a.m.
August 2nd, and the last at 7
p.m.on the 3rd. During 12 h. on
the first day the petiole moved
thrice downwards and twice
upwards. Within the same
length of time on the second
day, it moved five times down-
wards and four times upwards.
As the ascending and descend-
ing lines do not coincide, the
petiole manifestly circumnu-
tates; the great evening fall 7°25m.a7%\--
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.
Aiter 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. (ie. after
‘midnight) standing almost up-
right, and much more highly
inclined than it was at 10.40
P.M. When observed again at Mimosa pudica : circumnutation and
A aor, it had begun to fall,and Regueoplemevement of main pe
continued falling till 6.15 a.m.,
after which hour it zigzagged and again cireumnutated. Similar
observations were made on another petiole, with nearly the
same result.
On two other occasions the movement of the main petiole
25
Fig. 157.
8°S0'a.m.27\..\. \g
376 MODIFIED CIRCUMNUTATION. Cuap. 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 23 ellipses in 69 m.
On the second occasion, when the temperature was 81°—86° F.,
it made rather more than 8 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 hetween 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 Am. 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-
nutating 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 pinne move independently
of the main petiole. The petiole of a leaf was fixed to a cork
support, close to the point whence the four pinne diverge, with
a short fine filament cemented longitudinally to one of the twe
terminal pinne, and a graduated semicircle was placed close
beneath it. By looking vertically down, its angular or lateral
movements could be measured with accuracy. Between noon
and 4.15 rm. 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,
Duar. VII. SLEEP OF LEAVES. oii
in one instance ‘to the extent of 16°. This pinna, therefore,
circumnutated. Later in the evening the four pinnz approach
each other, and the one which was observed moved inwards
59° between noon and 6.45 p.m. Ten observations were mace
in the course of 2h. 20m. (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
iu 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 of
a little stick driven firmly into the ground, immediately beneath
a pair of leaflets, to the midribs of both of which excessively
fine glass filaments were attached. This treatment did not
injure the leaflets, for they went to sleep in the usual manner,
and long retained their sensitiveness. The movements of one
of them were traced during 49 h., as shown in Fig. 158. On the
first day the leaflet sank down till 11.30 a.m., and then rose
till late in the evening in a zigzag line, indicating 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 uc signs of sleeping, and were only slightly sensitive to
a touch. On the following day the stem was cemented to a
B18 MODIFIED CIRCUMNUTATION. Cuap. VIL
stick, and the movements of two leaves were traced on a verticai
glass during 72h. The plants were still kept in the dark, ex-
cepting that at each observation, which lasted 3 or 4 minutes,
Fig. 158.
720'p.m
40°20
10°40'a mis
W30' Ui
Mimosa pudica: cireumnutation 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,
‘Cuap, VIL. SLEEP OF LEAVES. 379
although the proper order of their movements in relation to the
day and night was wholly lost. Thus, one leaf descended during
the first two nights (ie. 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 ;3, of an inch in
one second; and this would have equalled 4 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 ® 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 2 of the natural size, present some
Fig, 159.
Mimosa albidus: leaf seen from vertioally above.
interesting peculiarities. It consists of a long petiole bearing
only two pinne (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) te
us that this Peruvian plant (which be “the species or variety which
was sent to us from Kew) is con- most commonly represents the
sidered by Mr. Bentham (‘Traus. _ sensitiva of our gardens.”
B80 MODIFIED CIRCUMNUTATION. Cuap. VIL
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 funetion, for they
are sensitive, extremely heliotropic, circumnutate at nearly the
same rate as the fully developed leaflets, and assume when
asleep exactly the same position. With /. pudica the inner
leaflets at the base and between the pinne are likewise much
shortened and obliquely truncated ; this fact was well seen in
some seedlings of M. pudlica, in which the third leaf above the
cotyledons bore only two pinne, each with only 3 or 4 pairs of
leaflets, of which the inner basal one was less than half as long
as its feilow; so that the whole leaf resembled pretty closely
that of VW. 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
wareins, 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. =2, of an
inch, whilst the inner one is only $58 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 mequality of size. The con-
clusion that the pinne of the parent-form of I. 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 WM. 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 pinne 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
sutside of the united terminal leaflets, and thus all eight leaflets
Cuar. VIL. SLEEP OF LEAVES. 381
(the rudimentary ones included) form together a single vertical
packet. The two pinnew 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 2? 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 4, 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 pinne, each bearing many small leaflets. These, when
the plant is asleep, are directed forwards and become imbricated.
The angle between the two terminal pinns was diminished at
night, in one case by 15°; and they sank almost vertically down-
wards. The hinder pairs of pinne 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
zinne, 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 pinne of Schrankia aculeata 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
pinna and become imbricated, and the pinne then look like bits
of dangling string. The following remarks and measuremeuts
DS2 MODIFIED CIRCUMNUTATION. Cuap, VIL
do not fully apply to the small leaf here figured. The pinnz
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 pinne of one specimen formed
together an angle of 100° during the day, and at night of only
y8°, so each had moved 31° forwards. The penultimate pinnz
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 pinnee 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 pinne 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 14h ; it was strongly zigzag, and apparently
Cuap. VIL SLEEP OF LEAVES. 383
represented five ellipses, with their longer axes differently
directed.
Albizzia lophantha (Tribe 23)—-The leaflets at night come into
contact with one another, and are directed towards the apex of
the pinna. The pinne 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 Bouché (‘ Bot.
Zeit., 1874, p. 859) the leaves sleep at night, in nearly the same
manner as those of certain species of Pimelia.
(nothera mollissima (Onagraries).—According to Linneus
(‘Somnus Plantarum’), the leaves rise up vertically at night.
Passiflora gracilis (Passifloracee).—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
15th 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
Bot MODIFIED CIRCUMNUTATION. Cupeeyala,
leaf had risen greatly, and continued to rise till 7.50 a.m., after
which hour it redescended. It should be observed that the lines
traced on this second morning would have coincided with and
confused those previously traced, had not the pot been slided
a very little to the left. In the evening (14th) a mark was
placed behind the filament attached to the apex of the leaf, and
its movement was carefully traced from 5 p.m. to 10.15 Pm.
Fig. 161.
ae
ANG,
Ss oS \
ae NG
™~.
“ss, —
Passiflora gracilis: circumnutation and nyctitropic movement of leaf
traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 a.m. 14th
Figure reduced to two-thirds of original scale.
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 orientalis (Composite).—Some seedlings were
raised in the middle of winter and kept in the hot-house; they
flowered, but did not grow well, and their leaves never showed
any signs of sleep. The leaves on other seedlings raised in May
were horizontal at noon (June 22nd), and depended at a consi:
Cuapr. VIL. SLEEP OF LEAVES. O8n
derable angle beneath the horizon at 10 p.m. In the case of four
youngish leaves, which were from 2 to 25 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 L1
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.
Nicotiana glavca: 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
sleeping. Professor Pfeffer informs us that the leaves of
another species (S. Jorullensis ?) hang vertically down at night.
386 MODIFIED CIRCUMNUTATION. Cuar. VILL
I) omea cerulea and purpurea (Convolvulaces). —The leaves on
very young plants, afoot or two in height, are depressed at night
Fig. 163.
>
i
10°35’ pm.12
erga
'44'p.m.t0"
aaa
ck le eS ie ee ree
S10 'amiIgh
3*p.m.lo” B'pm.ig®
Nicotiana tabacum: circumnutation and nyc-
titropic movement of a leat (53 inches in
length), traced on a vertical glass, from
3 P.M. Julv 10th to 8.10 a.m. 13th. Apex
of leaf 4 inches from glass. Temp. 173%°-
183° C. Figure reduced to one-half
original scale.
to between 68° and 80°
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 (Solanez).— The
young leaves of both
these species sleep by
bendinh vertically up-
wards. Figures of two
shoots of WN. glauca,
awake 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
Gasp VIL. SLEEP OF LEAVES. 387
have their longer axes differently directed from the cells of tha
parenchyma, and may therefore be considered as forming a sort
of pulvinus. A young plant of N. tabacum was selected, and
the circumnutation of the fifth leaf above the cotyledons was
observed during three days. On the first morning (July 10th)
the leaf fell from 9 to 10 A.m., which is its normal course, but
rose during the remainder of the day; and this no doubt was
due to its being illuminated exclusively from above; for properly
the evening rise does not commence until 3 or 4 p.m. In the
figure as given on p. 386 (Fig. 163) the first dot was made at
3 P.M.; and the tracing was continued for the following 65 h.
When the leaf pointed to the dot next above that marked 3 p.m.
it stood horizontally. The tracing is remarkable only from its
simplicity and the straightness of the lines. The leaf each day
described a single great ellipse; for it should be observed that
the ascending and descending lines do not coincide. On the
evening of the llth the leaf did not descend quite so low as
usual, and it now zigzagged alittle. 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 (Nyctaginese).—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 (Polygonez).— Professor Batalin informs
us that the young leaves rise up vertically at night. This is
likewise the case, according to Linnzeus, with several specics
of Amaranthus (Amaranthaceze); and we observed asleep move-
ment of this kind in one member of the genus. Again, with
Chenopodium album (Chenopodies), the upper young leaves ot
some seedlings, about 4 inches in height, were horizontal or
sub-horizontal during the day, and at 10 p.m. on March 7th
were quite, or almost quite, vertical. Other seedlings raised in
the greenhouse during the winter (Jan. 28th) were observed day
and night, and no difference could be perceived in the position
of their leaves. According to Bouché (‘ Bot. Zeitung, 1874,
p. 359) the leaves of Pimelia linoides and spectabilis (Thymeles)
slecp at night.
388 MODIFIED CIRCUMNUTATION. — Cuap. VIL
Euphorbia jacquinieflora (Euphorbiacee). — Mr. Lynch
called our attention to the fact that the young leaves of this
plant sleep by depending vertically. The third leaf from the
summit (March 11th) was inclined during the day 30° beneath
the horizon, and at night hung vertically down, as did some of
the still younger leaves. It rose up to its former level on the
following morning. The fourth and fifth leaves from the summit
stood horizontally during the day, and sank down at night only
38°. The sixth leaf did not sensibly alter its position. The
sinking movement is due to the downward curvature of the
petiole, no part of which exhibits any structure like that of
a pulvinus. Early on the morning of June 7th a filament was
fixed longitudinally to a young leaf (the third from the summit,
and 22 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 sc
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.
Phyllanthus Niruri (Kuphorbiaceee).— The leaflets of this
plant sleep, as described by Pfeffer,* in a remarkable manner,
apparently like those of Cassia, for they sink downwards at
night and twist round, so that their lower surfaces are turned
* «Dic Period. Beweg.,’ p. 159.
Cuarv. VIL. SLEEP OF LEAVES 389
outwards. They are furnished, as might have been expected
from this complex kind of movement, with a pulvinus,
GYMNOSPERMS.
Pinus Nordmanniana (Conifere).—M. Chatin states* that the
lsaves, 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 méme temps, ce mouvement d’érection
est accompagneé d’un mouvement de torsion imprimé a la partie
basilaire de la feuille, et pouvant souvent parcourir un arc de
90 degrés.” As the lower surfaces of the leaves are white,
whilst the upper are dark green, the tree presents a widely
different appearance by day and night. The leaves on a small
tree in a pot did not exhibit with us any nyctitropic move-
ments. We have seen in a former chapter that the leaves of
Pinus pinaster and Austriuca are continually circumnutating.
MONOCOTYLEDONS.
Thalia dealbata (Cannaceze).—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 18+ inches in length and 63 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 10P.m., was
10$ 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 4PM. 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. | Cnuap. 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 10.80 a.m. they rose, and then
fell greatly till 1.37pm. But by 3p.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 dces 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 no}
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.) (Aroidez),.
—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 6p.m., 48°; at 7.20 p.m.,69°; and at
8 30 p.m., 68°; so it had now begun to rise; at 10.15 p.m. it stood
at 65°, and on the following early morning at 11° beneath the
horizon. The circumnutation of another young leaf (with its
petiole only 3¢ 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 cansed
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
secn that in another genus of the Aroidex, namely, Pistia, the
ee
Guar Vil. SLEEP OF LEAVES. 391
leaves rise so much at night that they may almost be said to
Bleep.
Strephium jloribundum™ (Graminez).— The oval leaves are
provided with a pulvinus, and are extended horizontally or
declined a little beneath the horizon during the day. Those
on the upright culms simply rise up vertically at night, so
that their tips are directed towards the zenith. (Fig. 164.)
Fig. 164.
Strephium floribundum: culms with leaves during the day, and when asleep
atnight. 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 6P.m.,
descending again late in the night or in the very early morning.
* A. Brongniart first observed a Soc. Bot. de France,’ tom. vii
that the leaves of this plant and _1860, p. 470.
of Marsilea sleep: see ‘ Bull. de
26
392 MODIFIED CIRCUMNUTATION. Cuap. VIL
On the second day the descending line zigzagged slightly. Ag
Fig. 165.
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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 82 inches from the vertical
giass; plant illuminated from
above. Temp. 233°-243° C.
usual, the ascending and de-
scending lines did not coincide.
On another occasion, when the
temperature was a little higher,
viz., 24°-263° C., a leaf was
observed 17 times between 8.50
AM. and 12.16 p.m.; it changed
its course by as much as a
rectangle six times in this in-
terval of 8 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 quadrifoliata (Mar-
sileacees).—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 (C); 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
Cuap VII SLEEP OF LEAVES. 393
during 24h. (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
ox
A. B. U,
HMarsilea quadrifoliata: A, leaf during the day, seen from vertically above
B, leaf beginning to go to sleep, seen laterally; C, the same asleep.
Figures reduced to one-half of natural scale.
Fig. 166.
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 4.m. on the following morning, the leaf
was awaking, and the filament pointed above the vertical glass,
Fig. 167.
8:45 m7”
LS5O'pm.
Marsilea quadrifoliata : circumnutation and nyetitropic 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 am. 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 contast with
394 MODIFIED CIRCUMNUTATION. Cuap. VIL
its fellow. The movement of another leaflet, when asleep,
was traced between 6 p.m. and 10.85 P.m., and it clearly cir-
eumnutated, for if 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
JAM. to 3 P.M. at ahigh but varying temperature (viz., between
72° and 85° 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 Nyctitropie
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 bend at night vertically downwards with
the terminal pair directed considerably backwards, but .
they also rotate on their own axes, so that their lower
surfaces are turned outwards. The terminal leaflet
of Mehlotus likewise rotates, by which movement one
of its lateral edges is directed upwards, and at the
same time it moves either to the left or to the right,
until its upper surface comes into contact with that of
the lateral leaflet on the same side, which has hke-
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 througl. an angle of 90° upwards, and of others
Car. VIL SUMMARY ON SLEEP OF LEAVES. DVO
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
B96 MODIFIED CIRCUMNUTATION: Crap. VIT
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 bardly 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
pulyini. 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
Ouap. VII SUMMARY ON SLEEP OF LEAVES. - 3897
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 Trafoliwm
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
Pfeffer has remarked, is almost confined to leaves thus
provided. a
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 Aox-
burgh 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 rhombeefolia, 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. Cuap. Vii,
must extend the same conclusion to a large number of
sleeping plants; for the most complicated and thu
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
Cuav. 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 taan the lower; and
here we have an exception to our rule. But in other
species of this genus the leaflets succeed in placing
themselves vertically ; this, however, is effected by a
very unusual movement, namely, by the leaflets on
the opposite sides of the same leaf moving in opposite
directions.
It is again a very common rule that when leaflets
come into close contact with one another, they do so
by their upper surfaces, which are thus best protected.
In some cases this may be the direct result of their
rising vertically ; but it is obviously for the 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 Icafiets
are pressed together, and are thus better protected
400 MODIFIED CIRCUMNUTATION. Cuapr. VIL
than the upper surfaces; but this depends merely on
each leaflet becoming folded at night so as to be able
to sink vertically downwards. ‘he 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 nyctitropic 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-
Cuar. VII. SUMMARY ON SLEEP OF LEAVES. 401
carpea, 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
ereatly. 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 officonalis
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 Phaseolez 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. ven on the same
plant of Lupinus pubescens some of the petioles rose 30°,
others only 6°, and others sank 4° at night. ‘lhe
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 aj parently indicate that the movements
£0275": MODIFIED CIRCUMNUTATION. Cuap, YII
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-
modiwm gyrans. So it was in a very striking manner
with young plants of Bauhinia, and with Ozalis
Ortegesw.
We are led to an analogous conclusion with respect
to the movements of the secondary petioles of certain
pinnate leaves. The pinne 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 Albizziu lophantha the pinne close
together in the same manner. Although the pinnex
of Acacia Farnesiana do not converge much, they
sink downwards. ‘Those of Neptwnza oleracea likewise
Cuap. VIL 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 pinnz are depressed at night.
Now in these three latter cases, though the pinne
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, Amphicarpeea,
two species of Erythrina, a Cassia, Passiflora, Euphorbia
and Marsilea; and the leaves after they had gone to
sleep, were found to be in constant movement. When,
however, opposite. leaflets come into close contact with
one another or with the stem at night, they are, as we
believe, mechanically prevented from moving, but this
point was not sufficiently investigated.
When the movements of sleeping leaves are traced
during twenty-four hours, the ascending and descend-
ing lines do not coincide, except occasionally and by
accident for a short space; so that with many plants a
404 MODIFIED CIRCUMNUTATION. Caap. VII
single large ellipse is described during each twenty-four
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 Desmodium gyrans and Thaiia
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.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 hovrs; 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: Oxats 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 8h. 80 m.; and Arachis hypogza, in 4 h. 50 m.
But the number of ellipses described within a given
time depends largely on the state of the plant and
ou the conditions to which it is exposed. It often hap-
pens that a single ellipse may be described during one
Cuav. VII. SUMMARY ON SLEEP OF LEAVES. 405
day, and two on the next. Erythrina corallodendron
made four ellipses on the first day of observation
and only a single one on the third, apparently owing
to having been kept not sufficiently illuminated and
perhaps not warm enough. But there seems likewise
to be an innate tendency in different species of the
same genus to make a different number of ellipses in
the twenty-four hours: the leaflets of Trifolowm repens
_made only one; those of T. resupinatum two, and those
of TY. subterraneum three in this time. Again, the
leaflets of Oxalis Plumeru made a single ellipse; those
of O. bupleurifolia, two; those of O. Valdiviana, two or
three; and those of 0. PROSE at least five in the
tw enty-four hours.
The line followed by the apex of a leaf or leaflet,
whilst describing one or more ellipses during the day,
is often zigzag, either throughout its whole course or
only during the morning or evening: Robinia offered
an instance of zigzaggine 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 1s shown in two parts of the diagram (Fig. 150)
of the movements of Desmodiwm gyrans. Strephiwm
floribundum, observed under a high temperature,
106 MODIFIED CIRCUMNUTATION. Cuar. VIL
made several little triangles at the rate of 43 m.
for each. Mimosa pudica, similarly observed, de-
scribed three little ellipses in 67 m.; and the apex
of a leaflet crossed 54, 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-
modium 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 Dionza, 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 Amphicarpea, and is probably
comnion 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 hght and
darkness, that nothing farther need be said on this
* ‘Dic Periodischen Bewegungen der Blattorgane, 1875, p. 30. a
passim.
OnaP. 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 aud 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 lhght 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 inter from the fact of several plants (Tropeolum,
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 the after-
ance; he attributes (‘ Die Period.
Bewegungen,’ pp. 30-56) the
periodicity when prolonged for
a day or two in d.rkness, to
27
effects of light and darkness.
But we are unable to follow his
train of reasoning. There does
not seem to be any more reason fo3
105 MODIFIED CIRCUMNUTATION. .Cuap. VI
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
tule, 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-
minosee (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
atiributing such movc ments to this
cause than, for instance, the in-
herited habit of winter and
summer wheat to grow best at
different seasons; fr this habit
is lost after a few years, like the
movements of leavis in darkness
after a few days. No doubt some
eff.ct must be produced on the
seeds by the long-continucd culti-
vation of the parent-plants under
difterent climates, but no one pro-
bably would eall this the ‘‘ Nach-
wirkung ” of the climates.
* Pfeffer, ibid., p. 46.
Cuap. 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 Leguminose come the Mal-
vacee, 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 Leguminose 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. Cuap. VIL
case of a sleeping leaf, we see that it makes a single
ellipse in the twenty-four hours, which resembles one
described by a non-sleeping leaf in every respect, except
that itis 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
Cuar. VII SUMMARY ON SLEEP OF LEAVES. Al]
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
Trifoluum subterraneum, remembering that the curved
broken lines at the top ought to be prolonged much
higher up, he will see that the great rise in the evening
and the great fall in the morning together form a
large ellipse like one of those described during the
daytime, differing only im size. Or, he may look at
the diagram (Fig. 103, p. 236) of the 34 ellipses
described in the course of 6 h. 35 m. by a leaf of
Lupinus spectosus, 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
migzag line, often making in their course minute loops,
triangles, &., 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, im
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
pinnze 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 expeaded
412 MODIFIED CIRCUMNUTATION. Cuap. 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-
eide with the period of the day proper for this
movement.
The most complex of all the movements performed
by sleeping plants, is that when leaves or leaflets,
after describing in the daytime several vertically
directed ellipses, rotate greatly on their axes in the
evening, by which twisting movement they occupy
a wholly different position at night to what they do
during the day. For instance, the terminal leaflets
of Cassia not only move vertically downwards in the
evening, but twist round, so that their lower surfaces
face outwards. Such movements are wholly, or almost
wholly, confined to leaflets provided with a pulvinus.
But this torsion is not a new kind of movement
introduced solely for the purpose of sleep; for it
has been shown that some leaflets whilst describing
their ordinary ellipses during the daytime rotate
slightly, causing their blades to face first to one side
and then to another. Although we can see how the
slight periodical movements of leaves in a vertical
plane could be easily converted into the greater yet
simple nyctitropic movements, we do not at present
know by what graduated steps the more complex
movements, effected by the torsion of the pulvini,
have been acquired. A probable explanation could
be given in each case only after a close investigation
of the movements in all the allied forms.
From the facts and considerations now advanced we
may conclude that nyctitropism, or the sleep of leaves
Cuap. VIL. 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 ght 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
oxtreme 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 aequire 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
Porlierva 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
Gramines, 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. (Bot.),’ 1875, tom. i. pp. 32¢ -329.
114 STRUCTURE OF Cuap. VII
There is another movement, which since the time —
of Linneeus has generally been called sleep, namely,
that of the petals of the many flowers which close at
night. ‘These mavements have been ably investigated
by Pfeffer, who has shown (as was first observed by
Hofmeister) that they are caused or regulated more
by temperature than by the alternations of hght and
darkness. Although they cannot fail to protect the
organs of reproduction from radiation at night, this
does not seem to be their chief funetion, but rather
the protection of the organs from eold winds, and
especially from rain, durmg 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 pedunele, serves in many
eases 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.
Embryology of Leaves.—A few facts have been in-
cidentally given in this chapter on what may be ealled
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. Capenses, 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 Schutzmittcl des Pollens,’ 1873, pp. 30-39,
Cuap. VIL. FIRST-FORMED J.EAVES. 415
the gorse, or Ulex, is not narrow and spinose like the
older leaves. On the other hand, with many Legumi-
nous planis, for instance, Cassia, Acacia lophantha, &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 Trzfoliwum 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 leafiet. Here, then, we have the
rare opportunity of seeing a structure proper to a more
advanced age, in the act of gradually encroaching on
and replacing an earlier or embryological condition.
The genus Melilotus is closely allied to Trifolium, and
the first leaf bears only a single leaflet, which at night
rotates on its axis so as to present one lateral edge to
the zenith. Hence it sleeps like the terminal leaflet
of a mature plant, as was observed in 15 species, and
wholly unlike the corresponding leaflet of Trifolium,
which simply bends upwards. It is therefore a curious
fact that in one of these 15 species, viz., M. Taurica (and
in a lesser degree in two others), leaves arising from
young shoots, produced on plants which had been cut
down and kept in pots during the winter in the green-
house, slept like the leaves of a Trifolium, whilst the
leaves on the fully-grown branches on these same
plants afterwards slept normally like 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
416 STRUCTURE OF . Cpap Walle
as an embryologica] habit, probably the result of Meli-.
lotus being descended trom some form which slept like
a Trifolium. ‘This view is partially supported by the
leaves on old and young branches of another species,
M. Messanensis (not included in the above 15 species),
always sleeping like those of a Trifolium.
The first true leaf of Mimosa albida consists of a
simple petiole, often bearing three pairs of leaflets, ali
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 pinne, 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
aud 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
Jeads 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,
Cnap. VII. FIRST-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 extraoidinary
porer of gyration.
_
je MODIFIED CIRCUMNUTATION. Cuap. VIIL
CHAPTER VIIL
Mopiriev CrrcuMNUTATION: MovEMENTS EXCITED BY LIGHT.
Distinction 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 light in Apios, Brassica, Phalaris, Tropzolum, 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 diahelivtropism, influenced by epinasty, the
weight of the part and apogeotropism —Apogeotropism overcome
during the middle of the day by diahelictropism—Hftects of the
weight of the blades of cotyledons—So-ealled diurnal sleep—Chloro-
phyll injured by intense light—Movements to avoid intense light.
Sacus 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
* ‘Physiologie Veg’ (French Translation), 1868, pp. 42, 517, &e.
Cuar. VII. 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
well-marked degree. There is a third and large sub-
cluss 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 thatif a part of a plant, whilst still growing, did not
cireumnutate—though such a supposition is most im-
probable—it could not bend towards the light; but, as
a matter of fact, heliotropism seems always to consist
of modified cireumnutation. 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 hght, their
420 MODIFIED CIRCUMNUTATION. Cuar VIII
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 moye-
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-
Fig. 168.
Beta vulga ‘is: cireumnu-
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 ofthe
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,
tion of highly sensitive seedlings,
which were unintentionally illu-
minated rather obliquely, or only
at successive intervals of time.
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 whicl: 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
Cuap. VIII. HELIOTROPISM. 421
only slightly zigzag. Although not a single ellipse was even
approximately formed, we inferred from the zigzag lines—and,
as it proved, correctly—that their hypocotyls were 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. Ifwe 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 lyco-
persicum 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 Soe a ee ae
similar manner, as described in our first ee ieeen ce
chapter (p. 64), for they bowed themselves. nutation of sheath-like
during the whole day towards one side, cotyledon (13 inch in
making, however, in their course some height) traced on hori-
: zontal glass from 8 A.M.
conspicuous flexures. Before we knew 4, 109.93 p.m. Oct, loth,
how greatly ordinary circumnutation was
modified by a lateral light, soine 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 e.tz.
Fig. 169.
OV 5EL.
A
422
MODIFIED CIRCUMNUTATION.
Cuap. VIIL
overcast with extraordinarily dark thunder-clouds, and it was
interesting to note how plainly the cotyledons circumnutated
during this interval.
!
——— Oo - -
Fig. 170.
Aptos graveolens :
heliotropic movement of hypocotyl (-45 of inch in height) towards a moderately bright
Figure reduced to
lateral light, traced on a horizontal glass from 8.30 a.m. to 11.30 a.m. Sept. 18th.
one-third of original scale.
The foregoing observations are of some
value, from having bien made when we were
not attending to heliotropism; and they led
us to experiment on several kinds of seed-
lings, by exposing them to a dim lateral light,
so as to observe the gradations between
ordinary circumnutation and heliotropism.
Seedlings in pots were placed in front of,
and about a yard from, a north-east window;
on each side and over the pots black boards
were placed; in the rear the pots were open
to the diffused light of the room, which
had a second north-east and a north-west
window. By hanging up one or more blinds
before the window where the seedlings stood,
it was easy to dim the light, so that very
little more entered on this side than on the
opposite one, which received the diffused
light of the room. Late in the evening the
blinds were successively removed, and as the
plants had been subjected during the day to
a very obscure light, they continued to bend
towards the window later in the evening than
would otherwise have occurred. Most of the
seedlings were selected because they were
known to be highly sensitive to light, and
some because they were but little sensitive,
or had become so from having grown old.
The movements were traced in the usual
manner on a horizontal glass cover; a fine
glass filament with little triangles of paper
having been cemented in an upright position
to the hypocotyls. ‘Whenever the stem or
hypocotyl became much bowed towards the
light, the latter part of its course had to
be traced on a vertical glass, parallel to the
window, and at right angles to the horizontal
glass cover.
Aptos graveolens.—The hypocotyl bends ina few hours rectan-
Cuar. VIIL. 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 3h., 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 window, and then returned in a nearly parallel
line; that is, it almost completed between 12.48 and 2 Pm.
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
23
424. MODIFIED CIRCUMNUTATION. Cuap. VIEL
similar to the previous one. In these two cases there could be
no doubt that the ordinary circumnutating movement of the
hypocotyl was modified and rendered heliotropic.
Fig. 171.
12°48"
10°50 a4
Apios graveolens: heliotropic movement and circumnutation of the hypo-
coty|s of two seedlings towards a dim lateral light, traced on a horizontal
glass during the day. The broken lines show their return nocturnal
courses. Height of hypocotyl of A °5, and of B ‘55 inch. Figure reduced
to one-half of original scale.
Brassica oleracea.—The ‘hypocotyl of the cabbage, when not
disturbed by a lateral light, circumnutates in a complicated
Cuap. VIII, HELIOTROPISM. 425
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, ine. But when
the lateral light is very dim its course is extremely tortuous, and
evidently consists of modified cirecumnutation. 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
Pig, 172.
—
i:
Brassica oleracea ordinary circumnutating mcvement of the hypocoty) 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, te bend at right angles towards the window,
so that in the evening (after 4.28 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
426 MODIFIED CIRCUMNUTATION. Crap. 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 thna
Fig. 173.
SIS pm.
Lp mr,
7 pm.
sn m.
6°25!
Brassica oleracea : heliotropic movement and circumnutation of a hypocoty!]
towards a very dim lateral light, traced during 11 hours, ona 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. the hypocotyl moved obliquely from the light, and this,
Cuap, VIII. HELIOTROPISM. 421
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 uutil 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 Fig. 174.
began to circumnutate
about the same spot. The
contrast between the two
figures (172 and 173)
would have been more
striking, if they had been
originally drawn on the
same scale, and had been
equally reduced. But the
movements shown in Fig.
172 were at first more mag-
nified, and have been re-
duced to only one-haif of
the original scale; whereas
those in Fig. 173 were at
first less magnified, and
have been reduced to a
one-third scale. A tracing
made at the same time
with the last of the {
6°30’
movements of a second
hypocotyl, presented a
closely analogous appear-
ance; but it did not bend
quite somuch towards the
light, and it circumnu- 93)
tated rather more plainly Phalaris Canariensis : heliotropic movement
"and circumnutation of a rather old coty-
Phalaris Canartensis,— ledon, towards a dull lateral light, traced
Thesheath-likecotyledons ona horizontal glass from 8.15 a.m. Sept.
of this monocotyledonous 16th to 7.45 A.M. 17th. Figure reduced
plant were selected for to one-third of original scale.
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°15", Sep.16.%
£28 MODIFIED CIRCUMNUTATION. Cuap. Vill,
for the first 45 m. in a zigzag line; it then felt the full influence
of the light, and travelled towards it for the next 2h. 30m. in an
almost straight line. The tracing has not been given, as it was
almost identical with that of Apios under similar cireum-
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 rather old and not very sensitive
cotyledon, 1°9 inch in height, which became much bowed,
but was never rectangularly bent towards the light. From
11 A.m., when the sky became rather duller, until 6.30 p.m., the
zigzageing 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 toa
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.
Tropeolum majus.—Some very young seeijlings, bearing only
two leaves, and therefore not as yet arrived at the climbing
stage of growth, were first tried before a north-cast 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 janction were nearly straight, or only very slightly zigzag,
as in the other parts of the f.gures. After the epicotyls had
bowed themselves to the full extent towards the light, ellipses
of considerable size were described in the usual manner.
Cuap. VIII, HELIOTROPISM. 429
After having seen how the epicotyls moved 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 9.18 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.
eo 45p. 7h
40:40 p.m,
748'a.m
Tropeolum 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
the blinds were taken away, one by one. The course pursued
by an epicotyl under these circumstances is shown in Fig. 175.
During the whole day, until 6.45 pm., 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” MODIFIED CIRCUMNUTATION. Cuap. VIII
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 littie 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 smail
loops stillformed. If the light had
4 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 Tropzolum are aphe-
liotopic; we therefore placed a
plant, 11% inches high, in a box,
blackened within, but open on one
side in front of a north-east window
Tropeolum majus: heliotropic without any blind. A filament was
movement and circumnuta- fixed to the third internode from
Uae ne aa Tight, traced the summit on one plant, and to
on a horizontal glass from 8 the fourth internode of another.
A.M. Noy. 2nd to 10.204.M. These internodes were either not
Nov. 4th. Broken linesshow 414 enough, or the light was not suf-
the nocturnal course. ; : : ‘
ficiently bright, to induce aphelio-
tropism, for both plants ben! slowly towards, instead of from
the window during four days. The course, during two days of
the first-mentioned internode, is given in Fig. 176 ; and we see
that it either cireumnutated 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,
Fig. 176.
5
-“—<« =
o-
oo”, 4
= >
Cuap. VILL. HELIOTROPISM. 431
which, whilst young, is so extremely sensitive to peak worth
giving.
Cassia tora. — The cotyledons of this plant are extremely
sensitive to light, whilst the
hypocotyls are much _ less
sensitive than those of most
other seedlings, as we had
often observed with surprise.
It seemed therefore worth
while to trace their 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
vlind 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
inthe morning. Theaccom-
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 am. to 12.23 p.m., toward
Fig. 177. 6723 mz th
10°10 pm
Stam.7 3
Cassia tora: heliotropic movement and
circumnutation of a hypocotyl (14
inch in height) traced ona horizontal
glass from 8 A.M. to 10.10 P.M. Oct.
7th. Also its circumnutation in
darkness from 7 a.m. Oct. 8th to 7.45
A.M. Oct. 9th.
the light in a zigzag line, then turned abruptly to the left
and afterwards described a small ellipse. Another irregular
432 MODIFIED CIRCUMNUTATION. Cuap. VIIL
eliipse was completed between 3 p.m. and about 5.30 P.m.,
the hypocotyl still bending towards the light. The hypocotyl
Fig. 178.
/
G
f
Bignonia capreolata: aphe-
liotropic movement of a
tendril, traced on a hori-
zontal glass from 6.45
AM. July 19th to 10 a.m.
20th. Movements
days the peduncles were considerably bowed downwards. We
are thus Jed 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 mover
Guise. Vill: .PHELIOTROPISIL 435
ment, greatly magnified, was traced on a horizontal glass during
67h. 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
circumnautation, 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 24h.
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 vetween Crireumnutation and Heliotropism.—
Any one who will look at the foregoing diagrams,
showing the movements of the stems of various plants
towards a lateral and more or less dimmed light, will
be forced to admit that ordinary circumnutation and
heliotropism graduate into one another. When a
plant is exposed to a dim lateral light and continues
during the whole day bending towards it, receding
late in the evening, the movement unquestionably is
one of heliotropism. Now, in the case of Tropzolum
(Fig. 175) the stem or epicotyl obviously circumnu-
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 ot
Cassia (Fig. 177) the comparison of the movement ot
the hypocotyl, when exposed to adim 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
4186 RELATION BETWEEN Cuap. VID
and in many others, it was interesting to notice how
gradually the stems began to circumnutate as the
light waned in the evening. We have therefore many
kinds of gradations from a movement towards the light,
which must be considered as one of circumnutation
very slightly modified and still consisting of ellipses
or circles,—though a movement more or less strongly
zigzag, with loops or ellipses occasionally formed,—to
a nearly straight, or even quite straight, heliotropiec
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 ight, 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 his paper, ‘Ucber ortho- — theile’ (‘ Arbeiten des Bot. Inst
trope und plugiotrope Pflanzen- in Wiirzburg,’ Band ii. Heft ii
Cuar. VIII. CIRCUMNUTATION AND HELIO: ROPISM. 437
The result is that the course is rendered more or less
zigzag and unequal in rate. Lastly, when the light
is very bright all lateral movement is lost; and the
whole energy of the plant is expended in rendering
the circumnutating movement rectilinear and rapid in
one direction alone, namely, towards the light.
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
438 MODIFIED CIRCUMNUTATION. Cuap. 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 circumnutation ; 1t 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 hght and a course consisting of a series of loops
or ellipses, becomes perfectly intelligible. Finally,
the conversion of circumnutation into heliotropism or
apheliotropism, is closely analogous to what takes place
with sleeping plants, which during the daytime 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.
Transversal-heliotropismus (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 natiirliche Wagerechte Frage tiber Transversal-Geo-und
Richtung von Pflanzenth-ilen, Heliotropismus,” ‘ Bot. Zeitung,
18'’0 See also some interesting 1873, p. 17 et seq.
articles by the same author, * Zur
Cuar. 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 cireumnutation. 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 CIRCUMNUTATION. Cyuap, VIII
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
* «Arbeiten des Bot. Instituts in Wiirzburg,’ Heft. ii. 1872, pp.
¥23-277. ;
Cuar. VIII. DIAHELIOTROPISM. 441
above conditions of illumination. All these movements
evidently depend in some manner on the obliquity of
the light, but cannot be called heliotropic, as this
implies bending towards the light; whereas the coty-
ledon nearest to the hght 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 down 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 st bject.
442 MODIFIED CIRCUMNUTATION. Cuar. VIE
than can the nyctitropic movements of cotyledons
and leaves. In the latter case they place themselvez
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 of
nap. VILL. DIAHELIOTROPISM. 443
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,
end there can hardly be a doubt that this is the daily
alternations of heht 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 toso 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
nat. Wagerechte Richtung von
Pilanzentheilen,’ 1870, p. 46) the
root-leaves of many plants, kept
in darkness, rise up and even be-
come vertical; and so it is in some
cases with shoots. (See Rauwen-
hoff, ‘Archives Neérlandaises,’
tom. xii. p. 82.) These movements
indicate apogeotropism ; but when
organs have been long kept in the
dark, the amount of water and of
mineral matter which they con-
tain is so much altered, and their
regular growth is so much dis-
turbed, that it is perhaps rash to
infer from their movements what
would occur under normal con-
ditions. (See Godlewski, ‘ Bot
Zeitung, Feb. 14th, 1879.)
144 MODIFIED CIRCUMNUTATION. Cuapr. VIIL
the action of apogeotropism was yuite 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 80° 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 Wrightw, of one variety of
(yossypium, and of several species of Ipomcea, remain
horizontal in the evening whilst they are very young;
as they grow a little older they curve a little down-
wards, and when targe and heavy sink so much that
they come under our definition of sleep. In the case of
Cuap. VIII. PARAHELIOTROPISM. 445
the Anoda and of some species of Ipomeea, it was proved
that the downward movement did not depend on thie
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.—tThis is another class of movements, dependent
on the action of light, which supports to some extent
the belief that the movements above described are
only indirectly due to its action. We refer to the
movements of leaves and cotyledons which when
moderately illuminated are diaheliotropic; but which
change their positions and present their edges to the
light, when the sun shines brightly on them. These
movements have sometimes been called diurnal sleep,
but they differ wholly with respect to the object
gained from those properly called nyctitropic; and in
some cases the position occupied during the day is the
reverse of that during the night.
It has long been known* that when the sun shines brightly
on the leaflets of Robinia, they rise up and present their edges
to the light; whilst their position at night is vertically down-
wards. We have observed the same movement, when the
sun shone brightly on the leaflets of an Australian Acacia.
Those of Amphicarpea 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 2h. to
* Pfeffer zives the names and dates of several ancient writers in hia
‘Die Periodischen Bewegungen,’ 1875, p. 62.
446 MODIFIED CIRCUMNUTATION. Cuap, VIEL
a bright-sun, they stood at 56° above the horizon; they were
then protected from the rays of the sun, but were left well
illuminated from above, and after 30 m. they had fallen 40°, for
they now stood at only 16° above the horizon. Some young
plants of Phaseolus Hernandesii had been exposed to the same
bright sunlight, and their broad, unifoliate, first leaves now
stood up almost or quite vertically, as did many of the leaflets
on the trifoliate secondary leaves; but some of the leaflets had
twisted round on their own axes by as much as 90° without
rising, so as to present their edges to the sun. The leaflets on
the same leaf sometimes behaved in these two different manners,
but always with the result of being less intensely illuminated.
These plants were then protected from the sun, and were looked
at after 13h.; 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 453° 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.
Frum 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* beheves 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 Robinia, by fixing them in such a position that they could
not escape being intensely illuminated, whilst others were
allowed to place themselves obliquely; and the former began to
suffer from the light in the course of two days.
In the cases above given, the leaflets move either upwards
* ‘Die Naturlicher EHinrich- the ection of concentrated light
tungen zum Schutze des Chloro-
phylls,” &c., 1876. Pringsheim
has recently cbserved under tlie
microscope the destruction of
ehJorophyll in a few minutes by
from the sun, in the presence of
oxygen. See, also, Stahl on the
protection of chlorophyll from
intense light, in ‘ Bot. Zeitung,
1880.
Cuap. VIII. PARAHELIOTROPISM. 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 U. Ortegesii. A similar movement may fre-
quently be observed with the leaflets of Averrhoa bilimbi (a
member of the Oxalidz); and a leaf is here represented (Fig.
180) on which the sun had shone. A diagram (Fig. 184) was
given in the last chapter, representing the oscillations by which
a leaflet rapidly descended under these circumstances; and the
movement may be seen closely to resemble that (Fig. 188) bv
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 acetoselia 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. Yetthe 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 Robinia,
and the downward movements of those of Oxalis, have
been proved to be highly beneficial to these plants
when subjected to bright sunshine, it seems probable
that they have been acquired for the special purpose
of avoiding too intense an illumination. As it would
have been very troublesome in all the above cases to
448 MODIFIED CIRCUMNUTATION. Cuar. VIEL
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-
fied circumnutation ; but this certainly was the case
with the Averrhoa, and probably with the other species,
az their leaves were continually circumnutating.
Cuar. Lx. SENSITIVENESS TO LIGHT. 449
CHAPTER [X.
SENSITIVENEsS OF PLANTS TO LIGHT: ITS TRANSMITTED EFFEOTS,
Uses of heliotropism—lInsectivorous and climbing plants not heliotropic
—Same organ helicvtropic at one age and not at another—Extra-
erdinary sensitiveness of some plants to light—The effects of light de
not correspond with its intensity— Effects of previous illumination
—Time required for the action of light—After-effects of light—
Apogeotropism acts as soon as light fails—Accuracy with which
plants bend to the light—This dependent on the illumination of
one whole side of the part—Localised sensitiveness to light and its
transmitted eftects—Cotyledons of | 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 Avena, curvature of basal part due to the illumination of
upper part—S.:milar results with the hypocotyls of Brassica and
Beta—Radicles of Sinapis apheliotropic, due to tlie sensitiveness of
their tips—Concluding remarks and summary of chapter—Means
by which circumnutation has been converted into heliotropism or
apheliotropism.
No one can look at the plants growing on a bank or
on the borders of a thick wood, and doubt that the
young stems and leaves place themselves so that the
leaves may be well illuminated. They are thus enabled
to decompose carbonic acid. But the sheath-like coty-
ledons of some Graminee, 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, into the light and air. This view
450 SENSITIVENESS TO LIGHT. Cuap. IX.
is strengthened by the favt that with Phalaris and
Avena the first true leaf, which is bright green and ne
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 upwards,
against any overlying obstacle.
Heliotropism prevails so extensively among the
higher plants, that there are extremely few, of which
some part, either the stem, flower-peduncle, petiole,
or leaf, does not bend towards a lateral light.
Drosera rotundifolia is one of the few plants the
leaves of which exhibit no trace of helotropism. Nor
could we see any in Dionza, 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 capreolata
* According to F. Kuriz(‘Ver- tonia Californica are strongly
handl. des Bot. Vereins der Pro- apheliotropic. We failed to detect
vinz Brandenburg,’ Bd. xx. 1878) this movement in a plant which
the leaves or pitchers of Darling- | we possessed for a short time.
~
@nap iN. SENSITIVENESS TO LIGHT. 451
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 Mutisra
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 Ipomea
cerulea and four of I. purpwrea, growing in separate
pots, were placed on a bright day before a north-east
window in a room otherwise darkened, with the tips
of their revolving stems fronting the window. When
the tip of each plant pointed directly from the window,
and when again towards it, the times were recorded.
This was continued from 6.45 a.m. till a little after
2PM. on June 17th. After a few observations we
concluded that we could safely estimate the time
* Some erroneous statements number of observitions, for we did
are unfortunately given on this not then know at how unequal
subject, in ‘The Movements and a rate the stems and tendrils of
Habits of Climbing Plants, 1875, climbing plants sometimes travel
pp. 28, 32, 40,and 53. Conclusions in different parts of the same re-
were drawn from an insufficient volution.
452 SENSITIVENESS TO LIGHT. Crap. TX
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 ar average in 73°95 minutes; and 22 semicircles
from the hight 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°38 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 expos
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 /ess time for its performance thar
did the first semicircle to the light; and this resuit,
Cirap. IX. SENSITIVENESS ‘10 LIGHT. 453
if not accidental, appears to indicate that the shootw
retain a trace of an original apheliotropic teadercy.
With Lonicera brachypoda 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 stems of Ipomoea
cerulea 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 stems of very young
plants of Tropzolum majus are highly heliotropic, whilst
those of older plants, according to Sachs, are slightly
apheliotropic. In all these cases the heliotropism of
the very young stems serves to expose the cotyledons,
or when the cotyledons are hypogean the first true
leaves, fully to the light; and the loss of this power
by the older stems, or their becoming apheliotropie,
is connected with their habit of climbing.
Most seedling plants are strongly heliotropic, and
454 SENSITIVENESS TO LIGHT. Cnap. 1X
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 cireumnutate largely and
are not at all, or only slightly, heliotropic. Of such
cases Drosera rotundifolia offers an excellent instance.
The stolons of the strawberry circumnutate almost
like the stems of climbing plants, and they are not at
all affected by a moderate hght; but when exposed
late in the summer to a somewhat brighter ight they
were slightly heliotropic; in sunlight, according to
De Vries, they are apheliotropic. Clmbing 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 odorata. With
respect to the degree of sensitiveness of the more
sensitive kinds, it was shown in the last chapter that
seedlings of several species, placed before a north-east
window protected by several blinds, and exposed in
the rear to the diffused light of the room, moved
with unerring certainty towards the window, although
Ui eee ee ee se eS ee ee ee | a seaieiatiia daa eecle
a”
Char. IX. SENSITIVENESS TO LIGHT. 4.56
it was impossible to judge, excepting by the shadow
cast by an upright pencil on a white card, on which
side most light entered, so that the excess on one side
must have been extremely small.
A pot with seedlings of Phalaris Canariensis, which
had been raised in darkness, was placed in a com-
pletely darkened room, at 12 feet from a very small
lamp. After 3 h. the cotyledons were doubtfully
curved towards the lght, 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 teet; 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 lhght.*
We next tried how small a beam of light would act ;
ns this bears on light serving as a guide to seedlings
whilst they emerge through fissured or encumbered
ground. —
|»
ie
Lilium
Figure reduced to one-half of the original scale.
Rubus ideus (hybrid): apogeotropic movement of stem, traced on a vertical glass during 3 days and 5 nights, from
10.40 A.M. March 18th to 8 A.M. 21st.
tropism, under circumstances to be specified
in each instance.
Rubus ideus (hybrid).—A young plant, 1]
“Inches in height, growing in a pot, was placed
horizontally; and the upward movement was
traced during nearly 70 h.; but the plant,
though growing vigorously, was not highly
sensitive to apogeotropism, or it was not
capable of quick movement, for during the
above time it rose only 67°. We may see in
the diagram (Fig. 184) that during the first
day of 12 h. it rose in a nearly straight line.
When placed horizontally, it was evidently
circumnutating, for it rose at first a little,
notwithstanding the weight of the stem, and
then sank down; so that it did not start on
its permanently upward course until 1 h.
25 m. had elapsed. On the second day, by
which time it had risen considerably, and
when apogeotropism acted on it with somewhat
less power, its course during 15> h. was clearly
zigzag, aud the rate of the upward movement
was not equable. During the third day, also
of 153 h., when apogeotropism acted on it
with still less power, the stem plainly circum-
nutated, for it moved during this day 3 times
up and 3 times down, 4 times to the left and
4 to the right. But the course was so complex
that it could hardly be traced on the glass.
We can, however, see that the successively
formed irregular ellipses rose higher and
higher. Apogeotropism continued to act on
the fourth morning, as the stem was sfill
rising, though it now stood only 23° from the
perpendicular. In this diagram the several
stages may be followed by which an almost
rectilinear, upward, apogeotropic course first
becomes zigzag, and then changes into a
circumnutating movement, with most of the
successively formed, irregular ellipses directed
upwards.
auratum.—A plant 28 inches in height was placed
\
(wap, X
APOGEOTROPISM.
499
horizontally, and the upper part of the stem rose 58° in 46 h..
in the manner shown in the accom-
panying diagram (Fig.185). We here
see that during the whole of the
second day of 153 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.
Phalaris Canariensis—A cotyledon
of this plant (1°3 inch in height) has
already been described as rising in
4 h. 30 m. from 40° beneath the 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 4h. 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. (ce. 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-
nutation, instead of an abrupt change,
as in the former case.
Avena sativa.—The 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
Lilium auratum «
Fig. 185.
Apoged=
tropic movement of stein,
traced on a vertical glass
during 2 days and 2
nights, from 10.40 a.m.
March 18th to § A.M.
20th. Figure reduced to
one-half of the original
scale.
straight. An oldish cotyledon, from which the first leaf began ta
ste 10) MODIFIED CIRCUMNUTATION, Cuar. X
Fig, 186,
ITO pi
*
e
2° IMem, *
Phaiarts Canariensis: apogeotropic move-
ment of cotyledon, traced on a vertical
and horizontal glass, from 9.10 A.m. Sept.
19th to9 AM. 20th. Figure here re-
duced to one-fifth of original scale.
protrude whilst the fol-
lowing observations were
being made, was placed
at 10° beneaththe horizon,
and it rose only 59° in
24h. It behaved rather
differently from any other
plant, observed by us, for
during the first 43 h. it
rose in a line not far from
straight; during the next
63 h. it cireumnutated,
that is, it descended and
again ascended in a
strongly murked 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 43 h.,
ordinary circumnutation
almost completely con-
quered for a time apogeo-
tropism.
Brassica 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 24 inches, and
was therefore rather old
aud 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. 50]
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-
mert 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 circum-
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 Felio-
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
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 Tropeolum majus (see former
Fig. 175) moved towards the dim
evening light in a slightly zigzag
Fig. 187.
f
-->.
4
Brassica oleracea: apogeotropic
movement of hypocotyl, traced
on vertical glass, from 9.20
A.M. Sept. 12th to 8.30 a.m.
isth. 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.
line until 6.45 p.m., it then returned on its course until
502 MODIFIED CIRCUMNUTATION. Cuar. &
10.40 p.m., during which time it zigzagged and described an
ellipse of considerable size. The hypocotyl of Brassica oleracea
(see former Fig. 173) moved in a straight line to the light until
5.15 p.m., and then from the light, making in its backward
course a great rectangular bend, and then returned for a short
distance towards the former source of the light; no 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 pulvint.
—Movements of this kind are well known to occur in the
Graminesx, 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 Gramines; for if so, the apward curvature of their stems,
when extended horizontally or laid prostrate, would be explained
in accordance with our view—namely, that apogeotropism
results from modified cireumnutation. After these joints have
curved upwards, they are fixed in their new position by increased
growth along their lower sides.
Lolium perenne.—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 throvgh the action of apogeotropism (in the
manner described by De Vries) after the haulm had been
fastened down for 24 h. in a horizontal position. The pot was
* This structure has becn re- die Aufrichtung des gelagerter
cently described by De Vries in Getreides, in ‘ Landwirthschaft-
an interesting article, ‘Ueber liche.Jahrbiicher,’ 1880, p. 473.
Cuap. X. APOGEOTROPISM. 503
so placed that the end of the filament stood beneath the 2-inch
object glass of a microscope with an eye-piece micrometer, each
division of which equalled =3, 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 (;45 inch) in 2h. Occasionally it moved forwards by
jerks, some of which were ;4;5 inch in length, and then slowly
retreated a little, afterwards again jerking forwards. These
oscillations were exactly like those described under Brassica
and Dionea, but they occurred only occasionally. We may
therefore conclude that this moderately old joint was continually
circumnutating on a small scale.
Alopecurus pratensis.—A young plant, 11 inches in height, with
the flower-head protruded, but with the florets not yet expanded,
had a glass filament fixed close above the second joint, at a
height of only 2 inches above the ground. The basal internode,
2 inches in length, was cemented to a stick to prevent any
possibility of its circumnutating. The extremity of the filament,
which projected about 50° above the horizon, was often observed
during 24 h. in the same manner as in the last case. Whenever
looked at, it was always in movement, and it crossed 30 divisions
of the micrometer (4 inch) in 33 h.; but it sometimes moved
at a quicker rate, for at one time it crossed 5 divisions in 13 h.
Tne 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 ;;455 Of an mch. 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
mair 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
sare spot, as we have seen (Fig 91) in the fourth chapter.
But soon after the flowers have begun to wither the sub-
33
50-4 MODIFIED CIRCUMNUTATION. Cap. X
peduncles bend downwards, and this is due to epinasty; fer
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 angie 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 cireum-
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 56 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 am. 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 cireumnutating 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 cccurs in darkness, and
in whatever position the main peduncle may have been secured,
Caar. X&. APOGEOTROPISM.
005
the upward curvature cannot be due to heliotropism or hypo-
uasty, but to apogeotropism.
----G-- thy
Fig. 188.
A C
Ny
x
_
1
Pxalis carnosa: movements of flower-peduncle, traced on a vertical giass
A, epinastic downward movement; B, circumnutation whilst depends
ine vertically ; C, subsequent upward movement, due to apogeotropism
and hvponasty combined
D06 MODIFIED CIRCUMNUTATION. Cuap. 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
course was traced (see C, Fig. 188) during 53 h., by which time
it had become nearly upright. The course is seen to be strongly
zigzag, together with some little loops. We may therefore 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. Butas 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 Ozalis
acetosella, the capsules are said sometimes to bury themselves
under loose leaves or moss on the ground, but this cannot occur
with those of O. carnosa, as the woody stem is too high.
Oxalis ucetosella—The peduncles are furnished with a joint in
Fig.-189, .
Qealis acetosella : course pursued by the upper part of a peduncle, whilst
rising, traced from 11 A.M. June Ist to9 a.m. 3rd. Figure here re«
duced to one-half wf the original scale.
the middle, so that the lower part answers to the main peduncle,
Guar. X. APOGEOTROPISM. 507
and the upper part to one of the sub-peduncles of O. ca7nosa,
The upper part bends downwards, after the flower has begun
to wither, and the whole peduncle then forms a hook; that
this bending is due to epinasty we may infer from the case of
O. carnosa. When the pod is nearly ripe, the upper part
straightens itself and becomes erect; and this is due to 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, cireum-
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 this 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. Cuar. X
tinually circLmnutating, 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 3h. 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 3h. 8 m.,
and a cotyledon of Phalaris an angle of 130° in 4 h.
30m. On the other hand, the stem of a herbaceous
Cuar. X. APOGEOTROPISM. 508
Verbena rose 90° in about 24 h.; that of Rubus 67°,
in 70h.; that of Cytisus 70°, in 72h.; that of a young
American Oak only 37°, in 72h. The stem of a
young Cyperus aliernifolius 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. ‘I'wo fronds of a fern, Nephrodiwm molle, both
of them yuung 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 Lpomcea
lertophylla. 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 Cucurbita ovifera, placed horizontally,
moved upwards during 7 h. in a straight line, until it
stood at 40° above the horizon; it then began to 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 Cark.
In this position the tendril circumnutated and made
510 MODIFIED CIRCUMNUTATION. Cuar. 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 Passiflora gracilis are likewise apogeotropic. Two
branches were tied down so that their tendrils pointed
many degrees beneath the horizon. One was observed
for 8 h., during which time it rose, describing two
circles, one above the other. ‘The other tendril rose
in a moderately straight line during the first 4 h.,
making however one small loop in its course; it then
stood at about 45° above the horizon, where it cireum-
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 remarkable; 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 tora, similarly placed, became vertical in
12h.; that of an older seedling, 14 inch in height,
* For details see ‘ The Movements and Habits of Climbing Plants,
1875, p. 131,
Cuap. X. APOGEOTROPISM. ayy |
became so in 28h.; and that of another still older
one, 14 inch in height, remained horizontal during
two days, but distinctly circumnutated during this
whole time.
When the cotyledons of Phalaris or Avena are laid
horizontally, the uppermost part first bends upwards,
_and then the lower part; consequently, after the lower
part has become much curved upw ards, 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. ,
374-378
—, of leaflets, 378
, effect of bright sunshine on:
NEPTUNIA.
Mimosa albida, circumnutation and
nyctitropic movement of pinns,
402
, number of ellipses described
in given time, 406
, effect of bright sunshine on
leaflets, 446
Mirabilis jalapa and longiflora.
nocturnal movements of cotyle-
dons, 307
» yctitropic movement of
. leaves, 387
Mohl, on heliotropism in ten-
drils, stems, and twining plants,
451
Momentum-like movement, the ac-
cumulated effects
tropi m, 508
Monocotyledons, sleep of leaves,
389
Monotropa hypopitys, mode of
brcaking through the ground, 8€
Morren, on the movements of
stamens of Sparmannia and
Cereus, 226
Miller, Fritz, on Cassia tora, 34;
on the circumnutation of Linum
usitatissimum, 203; movements
of the flower-stems of an Alisma,
226
Mutisia clematis,
leaves, 246
, leaves not heliotropic, 451
of apogeo-
movement of
N.
Natural selection in connectior
with geotropism, heliotropism,
&e., 570
Nephrodium molle, circumnutation
of very young frond, 65
, of older frond, 257
-——, slight movement of fronds
509
Neptunia oleracea, sensitiveness to
conta t, 128
, nyctitropic movement of leaf.
lets. 37+
, of pinne, 402
INDEX.
NICOTIANA.
Nicotiana glauca, sleep of leaves,
385, 386
—, circumnutation of leaves,
386
Nobbe, on the rupture of the seed-
coats in a seedling of Martynia,
105
Nolana prostrata, movement of seed-
lings in the dark, 50
, circumnutation of seedling,
108
Nyctitropic movement of leaves,
560
Nyctitropism, or sleep of leaves,
281; in connection with radia-
tion, 286; object gained by it,
413
0.
Observati n, methods of, 6
Cnothera mollissima, sleep of leaves,
383
Opuntia basilaris, conjoint cireum-
nutation of hypocotyl and coty-
ledon, 44
—, thickening of the hypocotyl,
96
——, circumnutation of hypocotyl
when ercet, 107
——, burying of, 109
Orange, seedling, circumnutation
of, 510
Orchis pyramidalis, complex move-
ment of pollinia, 489
Oxalis acetosella, circumnutation of
flower-stem, 224
——., effect: of exposure to radia-
tion at night, 287, 288, 296
, circumnutation and nycti-
tropi: movement in full-grown
leaf, 326
——, circumnutation of leaflet when
asleep, 327
-—, rate of circumnutation of
leaflets, 404
—--—, effect of sunshine on leaflets,
447
——, circumnutation of peduncle,
506
585
OXALIS.
Oxalis acetosella, seed-capsules, only
occasionally buried, 518
—— articulatu, nocturnal muve-
ments of cotyledons, 307
(Biophytum) sensitiva, ra
pidity of movement of cotyledons
during the day, 26
——, pulvinus of, 113
——,, cotyledons vertical at night,
NAldG, FES
bupleurtfolia, circumnutation
of foliaceous petiole, 328
, nyctitropic movement of ter-
minal leaflet, 329
carnosa, circumnutation of
fluwer-stem, 223
, epinastic movements of flower-
stem, 504
——, effect of exposure at night,
288, 296
, movements of the flower-pe-
duncles due to apozcotropism
and other forces, 503-506
corniculata (var. cuprea),
movements of cotyledons, 26
, rising of cotyledons, 116
——, rudimentary pulvini of coty-
ledons, 119
—, development of pulvinus,
122
. effect of dull light, 124
——,, experiments on leaves at night,
288
—— floribunda, pulvinus of coty-
ledons, 114
—. nocturnal movement, 118,
307, 313 .
—— fragrans, sleep of leaves,
324
Ortegest#, circumnutation of
flower stems, 224
——, sleep of large leaves, 327
——, diameter of plant at nighé,
402
, large leaflets affected by bright
sunshine, 447
— Plumicerii, sleep of leaves, 327
purpurea, exposure of leaflets
at night, 293
rosea, ciicumnutation of coty
leduns, 23, 24
586
INDEX.
OXALIS.
Oxalis rosea, pulvinus of, 113
——, movement of cotyledons at
night. 117, 118, 307
—, effect of dull light, 124
—, non-sensitive cotyledons,
127
sensitiva, movement of coty-
ledons, 109, 127, 128
, circumnutation of flower-stem,
224
, nocturnal movement of coty-
ledons, 307, 312
, Sleep of leaves, 327
—— tropeoloides, movement of co-
tyledons at night, 118, 120
Valdiviana, conjoint circum-
nutation of cotyledons and hypo-
cotyl, 25
, cotyledons rising vertically at
niglit, 114, 115, 117, 118
——,, non-sensitive cotyledons, 127
, nocturnal movement of coty-
ledon, 307, 312
——, sleep of leaves and not of co-
tyledons, 315
——, movements of leaves, 327
P,
Pachira aquatica, unequal cotyle-
dons, 95, n.
Pancratium littorale, movement of
leaves, 255
Paraheliotropism, or diurnal slcep
of leaves, 445
Passiflora gracilis, circumnutation
and nyctitropic movement of
leaves, 383, 384
, apogeotropic movement of
tendrils, 510
——.,, sensitiveness of tendrils, 550
Pelargonium zonale, circumnutation
of stem, 203
——,and downward movement of
young leaf, 232, 233, 269
Petioles, the, rising of, beneficial to
plant at night, 402
Petunia violacea, downward move-
PHASEOLUS.
ment and circumnutation of very
young leaf, 248, 249, 269.
Pfeffer, Prof., on the turgescence of
the cells, 2; on pulvini of leaves,
113, 117; sleep movements of
leaves, 280, 283, 284; nocturnal
rising of leaves of Malva, 324;
movements of leaflets in Desmo-
dium gyrans, 358; on Phyllan- _
thus Niruri, 388; influence of a
pulvinus on leaves, 396; periodic
movements of sleeping leaves,
407, 408; movements of petals,
414; effect of bright sunshine on
leaflets of Robinia, 445; effect of
light on parts provided with pul-
vini, 363
Phalaris Canariensés, movements of
old seedlings, 62
, cireumnutation of cotyledons,
63, 64, 108
——,, heliotropic movement and cir-
cumnutation of cotyledon towards
a dim lateral light, 427
, sensitiveness of cotyledon to
light, 455
» effect of exclusion of light
from tips of cotyledons, 456
, manner of bending towards
light, 457
, effects of painting with Indian
ink, 467
, transmitted effects of light,
——, lateral illumination of tip,
470
——, apogeotropic movement of the
sheath-like cotyledons, 497
——, change from a straight upe
ward apogeotropic course to cir-
cumnutation, 499
——, apogeotropic movement of
cotyledons, 500
Phaseolus Hernandesii, nocturnal
movement of leaves and leaficts,
368
caracalla, 93
, nocturnal movement of leaves,
368
——., effect of bright sunshine on
leaflets, 446
INDEX,
PHASEOLUS,
Phaseolus multiflorus, movement of
radicles, 29
—, of young radicle, 72
, of hypocotyl, 91, 93
—, sensitiveness of apex of radicle,
163-167
—,, to moist air, 181
—, cauterisation and grease on
the tips, 535
—, nocturnal movement of leaves,
368
—-, nyctitropic movement of the
first unifoliite leaves, 397
Roxburghii, effect of bright
sunshine on first leaves, 445
— , vulgaris. 93
, sleep of leaves, 318
——, vertical sinking of leaflets at
night, 368
Phyllanthus Niruri, sleep of leaf-
lets, 388
— linoides,
387
sleep of leaves,
Pilocereus Houlletit, rudimentary”
cotyledons, 97
Pimelia spectabilis, sleep of leaves,
387
Pincers, wooden, through which
the radicle of a bean was allowed
to grow, 75
Pinus austriaca, cireumnutation of
leaves, 251, 252
— Nordmanniana, nyctitropic
movement of leaves, 389
— pinaster, circumnutation of
hypoevtyl, 56
——, movement of two opposite
cotyledons, 57
——., circumnutation of young leaf,
250, 251
——, epinastic downward move-
ment of young leaf, 270
Fistia stratiotes, movement of
leaves, 255
Pisum sativum, sensitiveness of
apex of radicle, 158
——., tips of radicles cauterised
transversely, 534
Plants, sensitiveness to light,
449; hygroscopic movements of,
89
587
QUERCUS.
Plants, climbing, circumnutation of,
264; movements of, 559
—, mature, circumnutation of.
201-214
Pliny on the sleep-movements of
plants, 280
Plumbago Capensis, circumnutation
of stem, 208, 209
Poinciana Gilliesti, sleep of leaves,
368
Polygonum aviculare, leaves vertical
at night, 387
convolvulus, sinking of the
leaves at night, 318
Pontederta (sp.?), circumnutation
of leaves, 256
Porlieria hygrometrica, cireum-
nutation an‘! nyctitropic move-
ments of petiole of leaf, 335,
336
, effect of watering, 336-338
—, leaflets closed during the diy,
413
Portulaca oleracea, efiect of Aici-
dium on, 189
Primula Sinensiés, conjoint circum-
nutation of hypocotyl and coty-
ledon, 45, 46
Pringsheim on the injury to chloro-
phyll, 446
Prosopis, nyctitropic movements of
leaflets, 374
Psoralea acaulis, nocturnal move-
ments of leaflets, 354
Pteris aquilina, rachis of, 86
Pulvini, or joints; of cotyledons,
112-122; influence of, on the
movements of cotyledons, 313;
effect on nyctitropic movements,
396
Q.
Quercus (American sp.), circumnus
tation of young stem, 53, 54
robur, movement of radicles,
o4, 55
sensitiveness of apex of
radicle, 174-176
INDEX.
QUERCUS.
Quercus virens, manner of germina-
tion, 85, 557
R.
Radiation at night. effect of, on
leaves, 284-286
Radicles, manner in which they
penetrate the ground. 69-77 ; cir-
cumnutation of, 69; experiments
with split sticks, 74: with
wooden pincers, 75 ; sensitiveness
of apex to contact and other irri-
tants, 129; of Vicia faba, 132-
158; various experiments, 135-
140; summary of results, 143-151;
power of an irrifant on, com-
pared with geotropism, 151-154 ;
sensitiveness of tip to moist
air, 180; with greased tips,
185; effect of killing or injuring
the primary radicle, 187-191;
curvature of, 193; affected by
moisture, 198; tip alone sensitive
to geotropism, 549; protrusion
and circumnutation ina gc: rmina-
ting seed, 548; tip highly sen-
sitive, 550; the tip acts like the
brain of one of the lower animals,
573
——, secondary, sensitiveness of
the tips in the bean, 15+; become
vertically geotropic, 186-191
Ramey on the movements of the
cotyledons of Mimosa pudica,
and Clianthus Dampieri at night,
297
Ranunculus Ficaria, mode of
breaking through the ground,
86, 90
, single cotyledon, 96
, effect of lateral light, 484
Htaphanus sativa, s nsitiveness of
apex of radicle, 171
——,, sleep of cotyledons, 301
Rattan, Mr., on the g-rmination of
the seeds of Megarrhiza Califor-
nica, 82 ;
Relation between circumnutation
and heliotropism, 435
SACHS.
Reseda odorata, hypocotyl of seed:
ling slightly heliotropiec, 454
Reversion, due to mutilation, 190
Rhipsalis cassytha, rudimentary co=
tyledons, 97
Ricinus Borboniensis, circumnuta:
tion of arched }:ypocotyl, 53
Robinia, effect of bright sunshine
on its leaves, 445
pseudo-acacia, leaflets vertical
at night, 355
Rodier, M., on the movements of
Ceratophyllum demersum, 211
Royer, Ch., on the sleep-movements
of plants, 281, n.; on the sleep of
lcaves, 318: the leaves of Medi-
cago maculata, 345; on Wistaria
Sinensis, 354
Rubus ideus (hybrid) cireumnuta-
tion of stem, 205
, apogeotropic movement of
stem, 498
Ruiz and Pavon, on Porlieria hy-
grometrica, 336
s.
Sacus on “ revolving nutation,” 1;
intimate connection between tur-
gescence and growth, 2,n.; coty-
ledon of the onion, 59; adapta-
tion of root-hairs, 69 ; the move-
ment of the rad:cle, 70, 72, 73;
movement in the hypocotyls of
the bean, &c., 91; sensitiveness
of radieles, 131, 145, 198; sensi-
tiveness of the primary radicle
in the bean, 155; in the com-
mon pea, 156; effect of moist
air, 180; of killing or injuring
the primary radicle, 186, 187;
circumnutation of flower-stems,
225; epinasty, 268; movements
of leaflets of Trifolium incar-
natum, 350; action of light in |
modifying the periodic move-
ments of leaves, 418; on geotro-
pism and heliotropism, 4386, n.;
on Tropeolum majus, 458°
INDEX.
SARRACENIA.
on the hypocotyls slightly helio-
tropic, and stems strongly aphe-
liotropic of the ivy, 453; he-
liotropism of radicles, 482; ex-
periments on tips of radicles
of bean, 523, 524; curvature of
the hypocotyl, 555 ; resemblance
between plants and animals,
571
Sarracenia purpurea, circumnuta-
tion of young pitcher, 227
Saxifraga sarmentosa, circum-
nutation of an inelined stolon,
218
Schrankia aculeata,
movement of the pinna, 381,
403
uncinata, nyctitropic move-
ments of leaflets. 381
Securigera coronilla, nocturnal
movements of leaflets, 352
Seed-capsules, burying of, 513
Seed-coats, rupture of, 102-106
Seedling plants, circumnutating
movements of, 10
Selaginella, circumuutation of, 258
Kraussti (?), cireumnutation of
young plant, 66
Sida napea, depression of leaves at
night, 322
——, no pulvinus, 322
retusa, vertical rising of leaves,
322
—— rhombifolia, sleep of cotyledons,
308
——,, sleep of leaves, 314
——,, vertical rising of leaves, 322
, no pulvinus, 322
, circumnutation and _ nycti-
tropic movements of leaf of young
plant, 322
, nyctitropic
leaves, 397
Siegesbeckia orientalis, sleep of
leaves, 319, 384
Sinapis alba, liypocotyl bending to-
wards the light, 461
——, transmitted effect of light on
radicles, 482, 483, 567
-—, growth of radicles in dark-
ness, 486
movement of
nyctitropic —
589
STAPELIA,
Sinapis ntgra, sleep of cotyledons,
301
Smilax aspera, tendrils aphelio-
tropic, 451
Smithia Pfundit,
cotyledons, 127
, hyponastic movement of the
curved summit of the stem, 274-
276
» cotyledons not sleeping at
night, 308
, vertical movement of leaves,
356
—— sensitiva, sensitiveness of coty-
ledons to contact, 126
, sleep of cotyledons, 308
Sophora chrysophylla, leaflets rise at
nicht, 368
Solanum dulcamara, circumnuta-
ting stems, 266
lycopersicum, movement of
hypocotyl, 50
——,, of cotyledons, 50
——.,, effect of darkness, 124
, rising of cotyledons at night.
6
non - sensitive
—, heliotropic movements of
hypocotyl, 421
—, effect of an intermittent light,
457 :
—,, rapid heliotropism, 461
palinacanthum, circumnu-
tation of arched hypocotyl, 51,
100
——,, of cotyledon, 51
, ellipses described by hypo-
cotyl when erect, 107
—, nocturnal movement of coty-
ledons, 306
Spargantum ramosum, rhizomes of,
189
Spherophysa
leaflets, 355
Spirogyra princeps, movements of,
»n.
Stahl, Dr., on the effect of ei-
dium on shoot, 189; on the in-
fluence of light on swarm-spores,
488, n.
Stapelia sarpedon, circumnutation
of hypocotyl, 46, 47
salsola, rising of
590
INDEX.
STAPELIA,
Stapelia sarpedon,
ledons, 97
Stellaria media, nocturnal move-
ment of leaves, 297
Stemeg, circumnutation of, 201-214
Stolons, or Runners, circumnuta-
tion of, 214-222, 558
Strasburger, on the effect of light
on spores of Heematoccus, 455, 7. ;
the influence of light on the
swarm-spores, 488
Strawberry, stolons of the, circum-
nutate, but not affected by mode-
rate light, 454
Strephium floribundum, circumnu-
tation and nyctitropic movement
of leaves, 391, 392
minute coty-
ts
Tamarindus Indica, nyctitropic
movement of leaflets, 374
Transversal - heliotropismus (of
Frank) or diaheliotropism, 438
Trapa natans, unequal cotyledons,
95, n.
Tecoma radicans, stems aphelio-
tropic, 451
Tephrosia caribea, 354
Terminology, 5
Thalia dealbata, sleep of leaves,
389
— , lateral movement of leaves,
404
Trichosanthes anguina, action of the
peg on the radicle, 104
, nocturnal movement of coty-
ledons, 304
Trifolium, position of terminal leaf-
lets at night, 282
—— globosum, with hairs protecting
the seed-bearing flowers, 517
glomeratum, movement of
cotyledons, 309
—— incarnatum, movement of
cotyledons, 309
—— Pannonicum, shape of first
true leaf, 350, 415
TRITICUM.
Trifolium pratense, leaves exposed
at night, 295
repens, circumnutation of
flower-stem, 225
——,, circumnutating and epinastic
movements of flower-stem, 276-
279
—, nyctitropic movement of
leaves, 349
—, circumnutation and nycti-
tropic movements of terminal
leaflets, 352, 353
——,, sleep movements, 349
resupinatum, no pulyini ts
cotyledons, 118
, circumnutation of stem, 204
—, effect of exposure at night,
295
—, cotyledons not rising at
night, 118, 309
, circumnutation and nycti-
tropic movements of terminal
leaflets, 351, 352
strictum, movements of coty-
ledons at night, 116, 118
——, nocturnal and diurnal move-
ments of cotyledons, 309-311,
313
,» movement of the left-hand
cotyledon, 316
subterraneum,
flower-heads, 71
—, of cotyledons at night, 116,
118, 309
——,, circumnutation of flower-stem,
224, 225
—, circumnutation and nycti-
tropic movements of leaves, 350
,number of ellipses in 24
hours, 405
——., burying its flower heads, 513,
514
——, downward movement of pe-—
dunele, 515
——., circumnutating movement of
peduncle, 516
Trigonella Cretica, sleep of leaves,
345
Triticum repens, undergrounJ
shoots of, become apogeotropis,
189
————
movement of
INDEX.
eee Ft ee
TRITICUM.
Triticum vulgare, sensitiveness of
tips of radicle to moist air, 184
Tropeolum majus (?), sensitiveness
of apex of radicle to contact, 167
——, circunmutation of stem, 204
——, influence of illumination on
nyctitropic movements, 338-340,
344
——, heliotropic movement and
circumnutation of epicotyl of a
young seedling, 428, 429
——, of an old internode towards a
lateral light, 430
——, stems of very young plants
highly heliotropic, of old plants
slightly apheliotropic, 453
—, effect of lateral light, 484
—— minus (?), circumnutation of
buried and arched epicotyl, 27
U.
Ulex, or gorse, first-formed leaf of,
415
Uraria lagopus, vertical sinking of
leaflets at night, 369
Vaucher, on the burying of the
flower-heads of Trifoliwm sub-
terraneum, 513; on the protec-
tion of seeds, 517
Verbena melindres (?), circumnuta-
tion of stem, 210
, apogeotropic movement of
stem, 4995
V.
Vicia faba, circumnutation of ra-
dicle, 29, 30
, of epicotyl, 31-33
——, curvature of hypocotyl, 92
——, sensitiveness of apex of ra-
dicle, 182-134
——, of the tips of secondary ra-
dic-es, 154
——, of the primary radicle above
the apex, 155-158
——, various experiments, 135-143
——, summary of results, 143-151
——, power of an irritant on, com-
t
591
WILSON.
pared with that of geotropism,
151-154
Vicia faba, circumnutation of leaves,
233-235
——, circumnutat.on of terminal
leaflet, 235
‘—_—, effect of apogeotropism, 444
—, effect of amputating the tips
of radicles, 523
, regeneration of tips, 526
——, short exposure to geotropic
action, 527
—, effects of amputating the tips
obliquely, 528
, of cauterising the tips, 529
, of grease on the tips, 534
Vines, Mr., on cell growth, 3
Vries, De, on turgescence, 2; on
epinasty and hyponasty, 6, 267,
268; the protection of hypo-
cotyls during winter, 557 ; stoluns
apheliotropic, 108; the nycti-
tropic movement of leaves, 283;
the position of leaves influeucced
by epinasty, their own weight and
apogeotropism, 440; apozeotro-
pism in petioles and midribs, 443;
the stolons of strawberries, 45+ ;
the joints or pulvini of the Gra-
minex, 002
W.
Watering, effect of, on Porlieria
hygrometrica, 336-338
Wells, ‘Essay on Dew,’ 284, n.
Wiesner, Prof., on the circumnuta-
tion of the hypocotyl, 99, 100;
on the hooked tip of climbing
stems, 272; observations on the
effect of bright sunshine on
chlorophyll in leaves, 446; the
effects of an intermittent light,
457; on aérial roots, 486; on
special adaptations, 490
Wigandia, movement of leaves, 248
Williamson, Prof., on leaves of
Drosera Capensis, 414
Wilson, Mr. A. S., on the move-
ments of Swedish turnip leaves,
230, 298
D92 INDEX.
WINELER. . ZUKAL.
Wiukler on the protection of seed- | Zea Mays, geotropie movement cz
lings, 108 radicles, 65
Wistaria Sinensis, leaficts depressed , sensitiveness of apex of ra-
at nixht, 354 dicle to contact, 177-179
——, circumnutation with lateral , secondary radicles, 179
light, 452 —, heliotropic movements of
seedling, 64, 421
Z ——, tips of radicles cauterised,
539
Zea Mays, circumnutation of coty- | Zukal, on the movements of Spiru-
ledon, 64 lina, 259, nN.
THE END.
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